Effect of process conditions on the stability and physicochemical properties of green coffee oil Pickering emulsions stabilized with ZnO nanoparticles for quercetin-enhanced sunscreen formulations

Effect of process conditions on the stability and physicochemical properties of green coffee oil Pickering emulsions stabilized with ZnO nanoparticles for quercetin-enhanced sunscreen formulations | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Effect of process conditions on the stability and physicochemical properties of green coffee oil Pickering emulsions stabilized with ZnO nanoparticles for quercetin-enhanced sunscreen formulations César Uriel Rodríguez-Fuentes, Ana Guadalupe Castillo-Olmos, Julieta del Carmen Villalobos-Espinosa, and 4 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8613997/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 13 You are reading this latest preprint version Abstract Solar ultraviolet (UV) radiation is the primary etiological factor in the development of several cutaneous malignancies, including carcinomas. In this context, the use of sunscreen formulations usually helps to prevent and reduce UV skin damage. The aim of this work was to explore the impact of the process conditions on some relevant physicochemical properties in sunscreens with a formulation based on quercetin Pickering emulsions stabilized with ZnO particles. Four formulations were prepared by controlling the speed and homogenization time using green coffee oil as the external phase and a mixture of stabilizers, water, and polyethylene glycol as the dissolution media. The stability of the emulsified systems was analyzed in terms of time after 28 days of storage by optical microscopy and digital image analysis to determine the mean particle size. The Turbiscan Stability Index (TSI), Sun Protection Factor (SPF), rheological behavior, and antioxidant activity were also evaluated. The system with the highest physical stability, minimal changes in rheological properties, and superior stability during storage time with respect to breakage phenomena was obtained at 15,000 rpm/2 min, with a SPF of ~ 40 and exhibiting one of the highest antioxidant capacities compared to other treatments. This stability was constant during the evaluation period. With these results we established the optimal conditions for the potential development of sunscreens with desirable attributes to reduce the harmful effects of UV radiation in addition to establishing the encapsulation conditions of bioactive compounds and facilitating its scale-up. Pickering emulsion sunscreen quercetin rheological behavior Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 1 INTRODUCTION Exposure to ultraviolet (UV) radiation from sunlight is a critical risk factor in skin carcinogenesis (Matsumara and Ananthaswamy 2004). In that sense, the use of sunscreen is essential for preventing and reducing UV damage such as burns, photoaging, and abnormal pigmentation (Dale Wilson et al., 2012; Mesa-Arango et al., 2017). Sunscreens exert their protective effects through different mechanisms of action, including: (i) act as a chemical barrier, by including compounds capable of absorbing damaging wavelengths, and (ii) set up a physical barrier to reflect and disperse potentially dangerous light (Manaia et al., 2013; Osterwalder et al., 2014). Most sunscreen formulations are primarily composed of synthetic drugs exerting their protective effects against photoirritation, photosensitization, and dermatitis (Geoffrey et al., 2019; Serpone et al., 2007). However, the development of formulations that include biocompatible compounds of natural origin is a desirable attribute in the constant search for strategies to protect human health from the harmful effects of UV radiation. Several research groups have conducted investigations on the protective effects of plant oils present in flowers, olives, coconut, green coffee, and other plants (Baldisserotto et al., 2018; Gönülalan et al., 2017; Kaur & Saraf, 2010; J. Marto, A. Ascenso, et al., 2016; Joana Marto et al., 2016; J. Marto, L. F. Gouveia, B. G. Chiari, et al., 2016; J. Marto, L. F. Gouveia, L. Gonçalves, et al., 2016; Sopyan et al., 2019; Wagemaker et al., 2011). Among these natural sources, green coffee oil stands out for its sun protection factor of ~ 5, along with antioxidant and moisture-retaining properties. This natural oil is characterized by being a complex mixture of antioxidant substances with a high concentration of triacylglycerides, diterpenic esters, fatty acids, tocopherols, and sterols, which act as UV radiation absorbers (Nosari et al., 2015; Speer & Kölling-Speer, 2006; Wagemaker et al., 2016; Wagemaker et al., 2011). Incorporating this kind of natural oil into a cosmetic dermal product has the potential to create a matrix that transports antioxidant compounds (Baldisserotto et al., 2018; J. Marto, L. F. Gouveia, B. G. Chiari, et al., 2016; Wagemaker et al., 2016). The sun protection products include creams, sprays, lotions, gels, and emulsions. Emulsions have been widely used due to their properties for enhancing the bioactivity of diverse chemical compounds, achieving stable systems with better compound distribution throughout the product, thus facilitating the incorporation of bioactive compounds with photoprotective activity (Geoffrey et al., 2019). Among emulsions, there is a classification known as Pickering emulsions, which are colloidal dispersions of two immiscible liquids that stabilize themselves through the incorporation of solid particles or nanoparticles in the interfacial region, forming a protective barrier around the droplets. This configuration provides greater rigidity and reduces the contact area between the phases (Jiang et al., 2020; Zembyla et al., 2020). Remarkably, the key property of this nanotechnology lies in the fine control of the degree of wetting of the nanoparticles at the oil-water interface, which reduces their interfacial free energy and enhances emulsion stability (Tsujii, 2017). However, to functionalize such a system, strict control over conditions of their formation is crucial, as these depend on several parameters, including particle size, distribution, and stability (Guzmán et al., 2022; Yukuyama et al., 2016). Pickering emulsions, also known as surfactant-free emulsions, utilize solid nanoparticles that can serve a dual function: as emulsifiers and as physical barriers. Thus, they allow the reduction in the use of synthetic components (Bordes et al., 2021). Nevertheless, typical sunscreen formulations, composed of Pickering emulsions, utilize micrometer-scale particle sizes, which affect the rheological properties of the product during application, resulting in a white cast upon skin deposition (Ahmad Zaki et al., 2018; Bordes et al., 2021; Guzmán et al., 2022). In contrast, nanoparticles with a minimum particle size of 100 nm avoid skin permeation, improve the product’s appearance after application, and enhance their optical properties, such as the ability to refract incident light (Lu et al., 2018; Smijs & Pavel, 2011). Additionally, from a rheological perspective, nanoparticles provide stability and structure to the product without compromising its spreadability on the skin (Lee et al., 2021; Joana Marto et al., 2016). Among the solid particles used in sunscreen formulations, TiO 2 , ZnO, and aluminum starch are widely used. Due to the low toxicity of nanoparticles and their high efficacy in protecting the skin, ZnO particles have been widely preferred choice in the formulation of diverse sunscreen products in combination with other nanomaterials, such as fatty acids, silica derivatives and polymers, which reduce or eliminate the potential production of free radicals and improve adherence to the skin preventing its permeability (Kandil et al., 2024; Lee et al., 2021; Lu et al., 2018; Smijs & Pavel, 2011). Emulsions enable the encapsulation of a wide range of natural compounds, such as flavonoids, a group of plant phenolic derivatives with potential nutraceutical, pharmaceutical, medicinal, and cosmetic applications, including their addition to some sunscreen systems (Petruk et al., 2018). Quercetin (2-(3,4-dihydroxyphenyl)-3,5,7-trihydroxychromen-4-one) is a flavonoid with anti-inflammatory, anticarcinogenic, and antioxidant properties. Additionally, it can increase the sun protection factor (SPF) when combined with physical barriers (Choquenet et al., 2008). However, quercetin and other bioactive compounds are prone to degradation, exhibiting relatively low bioavailability. Hence, nanosystems are attractive for protecting them and optimizing their absorption and biological activities (Fogaça et al., 2020; Fonseca et al., 2023). Hatahet et al. (2017) reported that an acceptable performance of substances in terms of activity could be achieved by using lipid nanosystems, water in oil (W/O) micro- and nanoemulsions, as well as collagen matrices. The use of W/O Pickering emulsions not only increases the bioavailability of quercetin but also allows the creation of a stable system in which inorganic compounds can be introduced to enhance the functionality of the final product (Guzmán et al., 2022). Green coffee oil has been reported as a carrier for compounds like melatonin. However, changing the bioactive compound can affect the stability and functional properties of the final product, making each evaluation specific to the molecular cargo. Due to the influence of the formation process in emulsion systems on the encapsulation of bioactive compounds, the aim of this study was to evaluate the effect of Pickering emulsion formation and stabilization process conditions on key physicochemical properties of sunscreens, including quercetin as cargo and ZnO nanoparticles as molecular stabilizers. 2 MATERIALS AND METHODS 2.1 Materials All reagents and solvents used were of analytical or HPLC grade. Green coffee oil was purchased from CREACOS (Guadalajara, Mexico). Zinc oxide nano coated with triethoxycaprylylsilane (ZnO) (Z Cote® HP1) was purchased from BASF (Ludwigshafen, Germany), glycerol and polyethylene glycol (PEG) from Sigma-Aldrich® (St. Louis, MO, USA), and xanthan gum from Albert and Ferran Adrià® (Barcelona, Spain). The bioactive compound used was Quercetin ≥ 95% (Sigma-Aldrich®, St. Louis, MO, USA). 2.2 Characterization of ZnO nanoparticles 2.2.1 Particle size distribution, morphology and optical properties The morphology of ZnO nanoparticles was examined using a Tescan® MIRA3 scanning electron microscope (Brno, Czech Republic) operated at 10.0 kV in high-vacuum mode with a secondary electron detector at a working distance of 9.39 mm. The sample was mounted on carbon tape, and the obtained micrographs were processed using MIRA3 software version 4.2.19.1 (Tescan®, Brno, Czech Republic) in TIFF format. Particle size was determined using dynamic light scattering (DLS) on a Zetasizer Nano ZS equipment (Malvern Panalytical, UK), following the methodology of Assis Dias Alves et al. (2024). The nanoparticles were suspended in ethyl acetate to obtain a final concentration of 0.2 mg/mL and subjected to ultrasonic treatment for 1 hour. Subsequently, 1 mL of the suspension was transferred to a glass cuvette for measurement. The optical properties were evaluated through the absorption spectrum of ZnO nanoparticles coated with triethoxycaprylylsilane dispersed in ethyl acetate, obtained using a UV-Vis spectrophotometer (UV-Vis Agilent 8453, Agilent Technologies, Santa Clara, USA) in the range of 200 to 600 nm. The same nanoparticle concentration as used for size analysis was employed (0.2 mg/mL). 2.2.3 Contact angle determination For the determination of the contact angle, the glass slide methodology was used with modifications (Nowak et al., 2013). To prepare the surface covered with nanoparticles, double-sided tape was placed on a microscope slide, followed by the deposition of a uniform layer of ZnO nanoparticles. This layer was lightly compressed to ensure material fixation, with excess particles removed. With the prepared surface, 5 µL of distilled water was placed on the nanoparticles using a 26 S syringe. Subsequently, a digital microscope (Celestron Handheld Digital Microscope Pro, Celestron LLC, CA, USA) was used to capture the micrograph, utilizing Celestron Microcapture Pro 2.5 software (Celestron LLC, CA, USA). The micrographs were saved in JPG format and processed using digital image analysis following the method reported by Stalder et al. (2010). For digital image analysis, ImageJ software version 1.53t (NIH, USA) was employed, utilizing the drop analysis plugin with the LB-ASDA function. Parameters were adjusted so that the contour outlined by the software matched the droplet’s contour in the image, and so the contact angle was obtained. The procedure was performed in triplicate with different samples, and the results are presented as mean ± SD. 2.3 Preparation of emulsions Emulsions were prepared according to the procedure described by J. Marto, L. F. Gouveia, B. G. Chiari, et al. (2016), with modifications in the dispersed phase and the type of nanoparticles used. The final composition is shown in Table 1 . In compliance with Regulation 1223/2009 of the European Parliament and Council, the INCI names and the function of the ingredients in the formulation are listed alongside their respective percentages. Table 1 Composition of Pickering emulsion sunscreen Component (INCI Name) Function Quantitative composition (%, w/w) Continuous phase 69.64 Coffea Arabica (Coffee) Seed Oil Anti-Aging Agent / Antioxidant / Emollient / Moisturizing Agent 46.12 Zinc Oxide (and) Triethoxycaprylylsilane UV Filter / Skin protector 23.52 Disperse phase 30.36 Polyethylene glycol Solvent / Emulsion stabilizer 10.00 Glycerin Humectant / Viscosity controller 6.65 Aqua Solvent 13.36 Xanthan gum Gel forming / Viscosity controller / Emulsion stabilizer 0.30 Quercetin Antioxidant 0.05 Zinc oxide and green coffee oil were homogenized for the continuous phase using magnetic stirring PC-420D (Corning®, NY, USA) at 1,100 rpm for 30 min. Then, ZnO was dispersed with higher energy using a rotor-stator homogenizer Ultraturrax® T25 (IKA®, Staufen, Germany) at 13,000 rpm for 1 min to ensure that most particles came into contact with the oil phase (Bordes et al., 2021). The dispersed phase was prepared by dissolving the bioactive compound in PEG using an ultrasonic bath Aquawave 9376® (1.75 L, 115 V, 50/60 HZ, 500 W) (Barnstead Lab-Line, IA, USA) at room temperature (25 ºC) for 5 min. Next, purified water, xanthan gum, and glycerol, which had been previously mixed manually, were added. Phases were mixed using a rotor-stator homogenizer while varying the working conditions in terms of time (min) and speed (rpm) according to Table 2 , with cycles consisting of 1 min of work followed by 1 min of rest. The obtained emulsions exhibited a pH ranging from 6 to 6.5 and were physically characterized. The effect of the processing conditions on their physicochemical properties was evaluated. Additionally, their potential for encapsulating bioactive compounds was assessed to develop a system that could be considered functional as a sunscreen. Table 2 Operating conditions for Pickering emulsion sunscreen Treatment Speed (rpm) Time (min) S13T2 13,000 2 S13T4 13,000 4 S15T2 15,000 2 S15T4 15,000 4 Note: Equipment used for emulsification was a high shear rotor – stator homogenizer Ultraturrax® T25 The analyses were conducted on the emulsions both on the preparation day and after 28 days of storage in dark conditions at 25°C and 36.5% relative humidity, using a crystal cell. 2.4 Particle size distribution Particle size distribution was assessed using a DM2000 LED optical microscope (Leica Microsystems®, Wetzlar, Germany) at 100 \(\:\times\:\) magnification. Micrographs were captured using Leica Application Suite V4.9 software. Each sample was prepared by dispersing 0.25 g of each emulsion in 1 mL of green coffee oil, and 5 µL of the final dispersion was placed on a glass slide. The micrographs were saved in TIFF format using the automatic settings provided by the software and subsequently analyzed through digital image analysis. Emulsions were evaluated at both 0 and 28 days after storage (Alade et al., 2021; Prestes et al., 2016). 2.5 Digital image analysis The average particle size D[3:2] and Feret diameter was carried out using ImageJ 1.53t software (NIH, USA), following the methodology of Cano-Sarmiento et al. (2014) with some modifications. The contrast and brightness of the micrographs were manually adjusted to ensure optimal visualization of the particles. Subsequently, the images were converted to 8-bit format, and the threshold level was automatically adjusted to generate a binary image. The Binary: Outline function was used to outline the contours of the particles, and finally, the Analyze Particles option was applied. For all treatments, at least three areas per image were analyzed at both 0 and 28 days of storage. Figure 1 illustrates the steps and modifications in the micrographs for digital image analysis, from which the average particle size was estimated. 2.6 Physical stability of emulsions The Turbiscan Stability Index (TSI) was measured to assess the presence of instability phenomena. The TSI value was determined following the methodology described by Li et al. (2020) with a Turbiscan Lab® Expert instrument (Formulaction, Toulouse, France). It was performed through multiple light scattering technique, where a sample of each emulsion was placed in cylindrical glass cells. The measurements were conducted at 25°C for 28 days. TSI values close to zero indicate higher emulsion stability (Kang et al., 2011). 2.7 Rheological characterization Pickering emulsions were analyzed for rheological behavior using a rheometer MCR 92 and RheoCompass 1.30 software (Anton Paar, Graz, Austria) with a 25 mm diameter plate-plate geometry equipped with a Peltier temperature control system set at 25°C, and a gap of 0.5 mm between each plate. 2.7.1 Rotational test Emulsions were subjected to flow curve tests over a shear rate range of 0.1 to 100 1/s, and shear stress vs. shear rate data were plotted for each treatment. These data were then fitted to the Herschel-Bulkley (HB) model (Eq. 1 ) to characterize the behavior and structural changes that occurred in the emulsion at time zero and after 28 days of storage (Cefali et al., 2019; Chiari-Andréo et al., 2021; Liu et al., 2021; Sheng et al., 2018). $$\:\sigma\:={\sigma\:}_{0}+k\bullet\:{\gamma\:}^{n}$$ 1 where \(\:\sigma\:\) represents the shear stress (Pa), \(\:{\sigma\:}_{0}\) is the yield stress (Pa), \(\:k\) is the consistency index \(\:\left(\text{P}\text{a}\bullet\:{\text{s}}^{n}\right)\) , \(\:\gamma\:\) is the shear rate (s − 1 ), and \(\:n\) is the flow behavior index (dimensionless). 2.7.2 Oscillatory test Amplitude sweeps were conducted for each treatment over a shear strain range of 0.01 to 100% at a constant angular frequency of 10 rad/s to determine the linear viscoelastic region (LVR). This region is characterized by constant storage \(\:\left(G{\prime\:}\right)\) and loss moduli \(\:\left(G{\prime\:}{\prime\:}\right)\) (Dabbaghi et al., 2021). From the LVR, a specific strain value was selected and fixed in frequency sweep tests to analyze the structural changes inherent to the system, rather than those induced by equipment-generated stresses (Geremias-Andrade et al., 2016). Additionally, a variation in angular frequency from 1 to 100 rad/s was performed to evaluate \(\:G{\prime\:}\) and \(\:G{\prime\:}{\prime\:}\) related to the conformation of the emulsion structure and the product's consistency characteristics (Ma et al., 2023). 2.8 In vitro sun protection factor The Sun Protection Factor (SPF) was determined using the standards proposed by Ahmad Zaki et al. (2018) for sunscreens containing ZnO nanoparticles. One gram of the emulsion was dissolved and made up to 100 mL with 40% ethanol. The solution was subjected to an ultrasonic bath Aquawave 9376 (1.75 L, 115 V, 50/60 HZ, 500 W, Barnstead Lab-Line, IA, USA) for 5 min at 25°C and filtered through cotton, discarding the first 10 mL. An aliquot of 5 mL was diluted and made up to 50 mL with the same solvent, resulting in a solution with a concentration of 1 mg/mL. The same procedure was done with the commercial 25% ZnO 50 SPF labeled sunscreen as a reference. The absorbance was measured using a UV-Vis spectrophotometer Agilent 8453 (Agilent Technologies, CA, USA) with a 1 cm glass cuvette and 40% ethanol as the blank. Data were obtained at wavelengths from 290 to 320 nm, in 5 nm increments, and were substituted into Eq. 2 (Mansur et al., 1986) to determine the SPF value. $$\:\text{S}\text{P}\text{F}=\text{C}\text{F}\text{*}{\sum\:}_{320}^{290}\text{E}\text{E}\left({\lambda\:}\right)\text{*}\text{I}\left({\lambda\:}\right)\text{*}\text{A}\text{b}\text{s}\left({\lambda\:}\right)$$ 2 where \(\:\lambda\:\) represents the wavelength, \(\:EE\left(\lambda\:\right)\) is the erythema effect spectrum, \(\:I\left(\lambda\:\right)\) is the solar spectrum’s intensity, \(\:Abs\left(\lambda\:\right)\) is the sunscreen’s absorbance, \(\:CF\) is a correction factor (= 10) and \(\:SPF\) is the sun protection factor. This equation is a correlation between the absorbance at each wavelength and the equivalent to the intensity at the same wavelength using constant values of EE x I reported elsewhere (Sayre et al., 1979). 2.9 Inhibition of ABTS •+ radicals The free radical ABTS •+ (2,2’-azino-bis(3-ethylbenzothiazoline-6-sulfonate) solution was prepared according to Espino-Manzano et al. (2020). Separate solutions of 7 mM ABTS + and 2.45 mM potassium persulfate (K 2 S 2 O 8 ) were prepared in distilled water, mixed in a 0.5:1 ratio, and stored at 25°C in the dark for 12 h. Subsequently, the resulting solution was diluted with ethanol to achieve an absorbance of 0.700 ± 0.02 at \(\:\lambda\:=734\) nm. Sample preparation was carried out using the procedure described for SPF evaluation, with a final concentration of 1 mg/mL of the emulsion in 40% ethanol. The samples were stored in amber vials at room temperature (25°C) to prevent compound degradation due to light exposure. The method of Medina-Pérez et al. (2020) was used to evaluate the antioxidant activity, with modifications in the volume of ABTS •+ used. Briefly, 200 µL of the sample was added to 1800 µL of the ABTS •+ radical solution and incubated in the dark at room temperature (25°C) for 6 min. The same procedure was carried out with 40% ethanol to obtain a blank. Absorbance was measured at 734 nm using a UV-Vis spectrophotometer Agilent 8453 (Agilent Technologies, CA, USA), and the obtained data were substituted into Eq. 3 (Espino-Manzano et al., 2020) to determine the percentage of free radical inhibition. $$\:Inhibition\:\%=\left(\frac{{A}_{0}-{A}_{A}}{{A}_{0}}\right)*100$$ 3 where \(\:{A}_{0}\) is the absorbance value of the blank and \(\:{A}_{A}\) is the absorbance value of the sample. 2.10 Statistical analysis Statistical analysis of the obtained results was conducted using analysis of variance (ANOVA) and Tukey's mean comparison with Graph Pad Prism® 8 software (GraphPad Software, Inc., San Diego, CA). Curve fitting was performed using OriginLab 2021 (9.8.0.200) software (OriginLab Corporation, Northampton, Massachusetts). A significant alpha level of p < 0.05 was used ( n = 3). 3 RESULTS AND DISCUSSION 3.1 Characterization of ZnO nanoparticles Zinc oxide nanoparticles are widely used in the formulation of inorganic physical sunscreens due to their superior protective properties compared to organic compounds. Additionally, they offer advantages such as the absence of skin irritation or sensitivity, low skin penetration, and broad-spectrum coverage. However, as particle diameter decreases, the surface-to-volume ratio increases. Consequently, nanoparticles may exhibit higher reactivity than traditional materials, enhancing their utility in biomedical applications but also increasing the risk of potential health and environmental hazards (Stern & McNeil, 2007). In this context, it is crucial to understand their characterization in terms of size, morphology, contact angle, and optical properties before incorporating them into final sunscreen products (Khan et al., 2023; Smijs & Pavel, 2011). Coatings such as triethoxycaprylylsilane and silica derivatives have been employed to reduce ion generation and migration from the material to the skin (Kandil et al., 2024; Ramasamy et al., 2014). However, these nanofunctionalized particles can alter important physical characteristics, including morphology, particle size, and contact angle, thereby modifying the properties of the formulations obtained. The zinc oxide exhibits multiple morphologies, with the most commonly observed being rod-shaped (Fig. 2 ). This observation aligns with the information provided by the supplier in their technical datasheet. Furthermore, dynamic light scattering (DLS) analysis revealed an average particle size of 134.8 ± 1.2 nm and a D90 percentile of 243 ± 17.4 nm. According to nanomaterial usage regulations, the particle sizes of ZnO comply with the established requirement of being greater than 100 nm (Ferreira et al., 2023). The advantages of using nanoscale zinc oxide in cosmetic formulations, such as sunscreens, include its enhanced surface area and intrinsic properties, such as a high refractive index and absorption in the UV-A (315 – 400 nm) and UV-B (280 – 315 nm) regions (Khan et al., 2023; Song et al., 2011). As shown in Fig. 3 , ZnO coated with triethoxycaprylylsilane exhibits strong absorption from 250 to 400 nm. In comparison, bulk ZnO has a maximum absorption at 375 nm. Due to its nanoscale size, coating, and multiple morphologies, a shift and broadening of this absorption range are observed, which can be beneficial, as it indicates enhanced photoprotective activity through increased absorption across the UV spectrum (Agarwal et al., 2019; Singh et al., 2012). 3.2 Contact angle In both conventional and Pickering emulsions, the region between the two liquid phases contains a mixture of oil, water, and emulsifying agents (i.e., solid particles or surfactant molecules). The properties of this region determine the emulsion type (W/O or O/W), with wettability being crucial for particle-stabilized systems. Wettability is determined by the contact angle (Yousufi et al., 2023), which indicates the particles' affinity for one of the two phases. According to Finkle's empirical rule, particles with contact angles below 90° are predominantly hydrophilic and stabilize O/W emulsions, whereas angles greater than 90° are more hydrophobic and stabilize W/O emulsions (Gonzalez Ortiz et al., 2020; Ribeiro et al., 2023). In this study, zinc oxide particles coated with triethoxycaprylylsilane showed a contact angle of 121.6 ± 1.6°, indicating a stronger tendency to form emulsions with a dispersed aqueous phase. This behavior is attributed to the coating, which contains polar molecules with some affinity for oil. This characteristic is advantageous for developing topical products, as a continuous oil phase exhibits greater affinity and adhesion to the skin, enhancing its water resistance and prolonging its retention on the skin (Assis Dias Alves et al., 2024; J. Marto, L. F. Gouveia, B. G. Chiari, et al., 2016). 3.3 Particle size distribution In emulsion studies, several reports highlight the influence of process parameters, such as speed, temperature, and processing time, on particle size and their stability (Surjit Singh et al., 2022; Touzouirt et al., 2018; Tsabet & Fradette, 2015). Specifically, speed influences droplet breakage and size reduction, with high speeds imparting significant energy that can lead to smaller sizes and greater homogeneity, albeit at the expense of temperature rises that may degrade organic compounds such as quercetin (Chaaban et al., 2017; Hariyatno et al., 2021; Yulianingsih & Gohtani, 2020). Processing time is crucial for Pickering emulsions, as it facilitates proper component contact and, with longer times, allows better particle arrangement around the continuous phase (Hariyatno et al., 2021). For topical products, to ensure sizes above 100 nm and to avoid ethical issues and genotoxicity testing, an in-depth study of the emulsion particles is recommended (Kang et al., 2011). In all samples analyzed in this study, the mean particle size D[3:2] was greater than 100 nm (0.44–0.53 µm) at zero-day. After 28 days of storage, increases of 19%, 21%, 20%, and 11% were observed in the mean particle size for samples S13T2, S13T4, S15T2, and S15T4, respectively (Table 3 ). Meanwhile, percentile D90 shows a global increase of around ~ 0.1 µm at the same evaluated time. Table 3 Mean particle size D[3:2] and percentile D90 of Pickering emulsions*. Treatment Day 0 Day 28 D[3:2] (µm) D90 (µm) D[3:2] (µm) D90 (µm) S13T2 0.67 ± 0.08ª A 0.81 ± 0.12ª A 0.80 ± 0.06 aA 0.95 ± 0.06 aA S13T4 0.65 ± 0.04ª A 0.83 ± 0.03ª A 0.79 ± 0.04 bA 0.94 ± 0.08 aA S15T2 0.67 ± 0.08ª A 0.84 ± 0.09ª A 0.81 ± 0.06 aA 0.91 ± 0.04 aA S15T4 0.69 ± 0.08ª A 0.89 ± 0.14ª A 0.77 ± 0.05 aA 0.87 ± 0.04 bA *: Averages ± standard deviation ( p < 0.05). Equal letters (upper at the same time, and lower for the same treatment) indicate no significant difference. For Pickering emulsions, some authors (Chaaban et al., 2017; Guzmán et al., 2022; Zembyla et al., 2020) indicate that once the coating layer around the dispersed phase is formed correctly, it is challenging to break owing that the contact area between the two liquids has been reduced or even eliminated, and the water-particle-oil contact replaces the direct water-oil contact. Additionally, the material’s configuration used can enhance stability and interactions with the continuous phase. This is because systems that are less prone to coalescence are produced. However, the coating may have areas of poor coverage due to uneven distribution of the interface (de Carvalho-Guimarães et al., 2022; Zembyla et al., 2020). Even so, this possibility is minimized by the nanometer scale of the solid nanoparticles, which regulate droplet size and arrangement while reducing potential pores or oil-water contact areas where instability might occur (Fig. 4 ). Although emulsions were prepared under different operating conditions, the treatments did not show a significant difference ( p > 0.05) at both the initial (day 0) and at final time (day 28) (Table 3 ). However, the particle size showed a trend toward increasing during storage, although the emulsion instability was not visible macroscopically. Furthermore, the optical microscopy analysis technique requires sample dilution, which can break up agglomerates or redistribute nanoparticles. Overcoming these limitations requires the use of other techniques to evaluate the physical properties of these microsystems in suspension. 3.4 Turbiscan stability index Emulsions, being thermodynamically unstable systems, are prone to instability phenomena such as sedimentation, creaming, coalescence, flocculation, and Ostwald ripening, which can lead to phase separation (McClements & Jafari, 2018). The encapsulation of bioactive compounds is crucial for preserving the physicochemical properties of emulsions and ensuring the proper release and protection of compounds during storage (Hatahet et al., 2017). Although Pickering emulsions are less susceptible to coalescence and Ostwald ripening, improper distribution of solid particles at the interface can promote instability phenomena (Zembyla et al., 2020). Here, TSI was used to monitor and characterize the physical stability of the four emulsions over 28 days (Fig. 5 ). The TSI value is a parameter that quantifies the degree of instability in the emulsion by comparing backscattering values (Zalewska et al., 2019). Since the composition remained constant, changes in TSI over time allowed observation of the effect of processing conditions on the development of a functional product. Figure 5 depicts the TSI values plotted over time for each treatment. No significant changes in emulsion stability were observed up to 4 h except for S13T4 vs S15T2 (p < 0.05). As time progressed on day 2, significant differences in TSI values became evident among all treatments, with TSI values of 4.0, 2.0, 3.45, and 2.95 for S13T2, S13T4, S15T2, and S15T4 samples, respectively (p < 0.05). This gradual onset of differences can be attributed to the intrinsic properties of the emulsion, such as its viscosity and structural conformation, which can slow down the movement of droplets within the emulsion, thereby enhancing stability (Santos et al., 2020; Yulianingsih & Gohtani, 2020). From day 3 to day 21, no significant differences were observed between the TSI values of S13T2 and S15T2 (p > 0.05) (Fig. 5 ). To note, the S13T2 and S15T2 were generated at two different speeds, 13,000 rpm and 15,000 rpm, respectively. The change in mixing speed from 13,000 to 15,000 rpm did not significantly affect the emulsion behavior during short mixing times (2 min). However, at longer mixing times (4 min), this change in speed was decisive for the system’s behavior, suggesting that prolonged exposure enhances interactions between components, including van der Waals and electrostatic forces. These properties can contribute to either stability or instability in those systems, depending on the balance and control of materials selected during the formulation process (Frelichowska et al., 2010; McClements & Jafari, 2018). The speed parameter modulates the shear force in the homogenization zone, where higher speeds yielding high shear forces; however, their use is limited by the associated temperature increase. At low speeds, both temperature increase and shear force are lower, therefore longer processing times must be used to compensate this effect; as shown in S13T2 treatment, where the low speed and short processing time affected the distribution of ZnO nanoparticles around the droplets, as a result, there was a higher presence of instability phenomena such as flocculation, coalescence, and Ostwald ripening, i.e. the thermodynamic process where small oil droplets are dispersed in an aqueous medium (Frelichowska et al., 2010; Gonzalez Ortiz et al., 2020; Mulia et al., 2019). Regarding S15T4 treatment, a TSI value increase was observed compared to treatments S15T2 and S13T4, suggesting that the amount of energy required to form the emulsion was exceeded. The employed conditions resulted in a phenomenon referred to as over-processing, in which the energy density applied to the system is exceptionally high, favoring phenomena of coalescence due to the constant rupture and formation of droplets, along with a minor migration rate of nanoparticles to the interfacial region than the rate of their formation (Páez-Hernández et al., 2019). On day 28, both S13T4 and S15T4 treatments had the lowest TSI values, with final 4.4 and 4.8, respectively. Both treatments were more stable compared to others (6.5 and 5.9, TSI values for S13T2 and S15T2, respectively), even though they had the largest increase in particle size (57%) (Fig. 5 ). Overall, our results indicate that, in all treatments, the combination of speed and time indeed modifies the stability value. 3.5 Rheological characterization 3.5.1 Flow curves Characterizing the flow behavior of O/W emulsions enables the study of packing systems in relation to product filling, skin assimilation, and ease of product extraction (Zhang et al., 2022). The viscosity of these emulsions depends on shear rate, a typical characteristic of non-Newtonian fluids, as evidenced by the flow curves (Fig. 6 ). This behavior may be related to the hydrophobicity of the materials used. In this sense, Hohl et al. (2016) suggested that the hydrophobicity of the particles, rather than their surface properties, has a greater influence on the stability of the emulsified structure. Additionally, flow curves help understand the physical behavior of fluids under stress. Given that for topically applied products, an excellent spreadability is a desirable attribute (Guzmán et al., 2022), all emulsions obtained in this work showed an acceptable shear-thinning behavior (Fig. 6 ). According to Bordes et al. (2021) and J. Marto, L. F. Gouveia, B. G. Chiari, et al. (2016), shear-thinning behavior enables better application and distribution on the skin, creating a homogeneous film that ensures proper deposition of particles. This enhances their UV protection capacity by forming a single zone for reflection and refraction of UV rays. The emulsions data curve was fitted to the Herschel-Bulkley (HB) rheological model ( R 2 > 0.99) to obtain parameter values that allow a clear comparison and definition of rheological behavior (Bordes et al., 2021). This model represents the behavior of non-Newtonian fluids that exhibit a yield stress before flow begins, as is typical of thixotropic fluids, and is widely used in pharmaceutical and cosmetic products (Herschel & Bulkley, 1926; J. Marto, A. Ascenso, et al., 2016). Table 4 shows the obtained values. Table 4 The Herschel- Buckley model parameters for Pickering emulsions. Treatment \(\:{\varvec{\sigma\:}}_{0}\) (Pa) \(\:\varvec{\kappa\:}\) (Pa s n ) n (-) Day 0 S13T2 25.07 ± 0.26 aA 7.59 ± 0.13 aB 0.514 ± 0.003 aB S13T4 21.31 ± 0.22 aB 6.59 ± 0.10 aC 0.534 ± 0.003 aA S15T2 25.03 ± 0.38 aA 6.94 ± 0.18 aBC 0.520 ± 0.005 aAB S15T4 25.27 ± 0.84 aA 9.11 ± 0.46 aA 0.474 ± 0.009 aC Day 28 S13T2 30.60 ± 0.41 bA 4.69 ± 0.17 bB 0.583 ± 0.007 bA S13T4 26.26 ± 0.33 bB 5.00 ± 0.15 bAB 0.544 ± 0.005 bC S15T2 25.24 ± 0.31 aC 5.09 ± 0.14 bA 0.552 ± 0.005 bBC S15T4 29.51 ± 0.31 bD 4.892 ± 0.14 bAB 0.560 ± 0.005 bC Means ± standard deviation (Tukey, p < 0.05). Different capital letters indicate significant differences within the same storage time. Different lowercase letters indicate significant differences within the same treatment across storage times. At day 0, the S15T4 treatment exhibited the highest yield stress (25.26 Pa), while the S13T4 treatment was more fluid with \(\:{\sigma\:}_{0}\:\) = 21.31 Pa. For all the samples studied, the yield stress increased after 28 days of storage at 22%, 23%, 0.8%, and 17% for S13T2, S13T4, S15T2, and S15T4, respectively. However, the S15T2 emulsion exhibited the lowest yield stress after 28 days of storage with no statistically significant difference throughout storage time. Similar values for Pickering emulsion formulations used in topical products were reported by Marto et al. (J. Marto, A. Ascenso, et al., 2016; J. Marto, L. F. Gouveia, B. G. Chiari, et al., 2016; J. Marto, L. F. Gouveia, L. Gonçalves, et al., 2016) and Chiari-Andréo et al. (2021), despite the use of different solid particles. Considering the particle size generated by each treatment, slight variations in particle size led to changes in the flow curves, with the smallest particles producing the lowest yield points and consistency values with the least change over time (J. Marto, L. F. Gouveia, L. Gonçalves, et al., 2016). Despite the particle sizes generated being smaller (0.65–0.69 µm) than those reported by J. Marto, L. F. Gouveia, B. G. Chiari, et al. (2016) for Pickering emulsions (5.54–8.87 µm) with green coffee oil stabilized with particles of TiO 2 , similar values and trends in flow curves are observed when using different types of solid particles. In this case, the similarity can be attributed to the type of emulsion, as factors such as concentration, particle interactions, and the formation of clusters of these particles increase viscosity and create a complex structure in the interfacial region, promoting greater stability, as previously reported (Franco et al., 1998; Mijovic et al., 2013; Shankar Banerjee et al., 2017). After storage, all systems show a decrease in the shear stress required to generate fluid movement (Table 4 ). Those effects may result from structural changes in the emulsion packing, the formation of aligned structures between ZnO nanoparticles, and the presence of some texture modifiers such as xanthan gum (Santos et al., 2020; Trujillo-Cayado et al., 2016). Interestingly, this decrease does not affect the emulsion’s spreadability, as the previously described behavior remains unchanged. 3.5.2 Oscillatory test (Frequency and amplitude sweep) The amplitude sweep test of each emulsion shows that they have a linear viscoelastic region (LVR) between shear deformations of 0.01% to 0.1%, indicating the range of values where the moduli are independent of the shear stress (Miri, 2011). The strain γ = 0.05% was used in all frequency sweep tests. The emulsion frequency sweep curves showed that the elastic and viscous moduli varied minimally over the tested range. In the frequency sweep tests, the moduli that make up the fluids can be observed (Fig. 7 ) where \(\:G{\prime\:}\) corresponds to the storage modulus or elastic part, and \(\:G{\prime\:}{\prime\:}\) indicates the loss modulus or viscous part. By analyzing the arrangement of both moduli and their values, it is possible to determine the behavior of the structure, its stability, and consistency (Wong et al., 2021). According to Fig. 5 , all treatments showed higher values of \(\:G{\prime\:}\) than \(\:G{\prime\:}{\prime\:}\) , indicating a gel-like behavior in which the structure resists rupture and exhibits a more solid consistency. Hohl et al. (2016), Wei and Huang (2020), and Xu et al. (2021) associate this phenomenon with the presence of solid particles, which increase the system's viscosity and form a structural network that prevents loss of plasticity and enhances its stability. The frequency sweeps of the different systems showed a decrease in both moduli across all treatments (Fig. 7 ). Some authors, including Franco et al. (1998), Isaac et al. (2013), and Fazilati et al. (2021), have attributed this decrease to a restructuring that the emulsion undergoes during this period due to the interaction between the phases and particles, leading to a repacking of the system by rearranging the droplets in the emulsion. However, this repacking did not alter the flow behavior of the systems, so their ability to flow and be dispersed on the skin remained constant during storage. 3.6 In vitro sun protection factor The four emulsions containing 23.63% ZnO exhibited SPF values > 40, which is considered high protection by the European Commission (Communities, 2006). Ahmad Zaki et al. (2018) reported SPF values ranging from 40.37 to 40.16 for sunscreens with similar ZnO nanoparticle concentrations, values comparable to those obtained for a commercial sunscreen with 25% ZnO labeled as 50 FPS (56.38 ± 1.96). These values can be attributed to the extensive contact area provided by the nanosized particles relative to their volume, resulting in greater optical interaction in the UVA and UVB regions. Other authors have also reported this phenomenon in sunscreens containing ZnO or TiO 2 nanoparticles smaller than 100 nm (Ghamarpoor et al., 2023; Singh & Nanda, 2014). SPF values were comparable across all emulsions (Fig. 8 ), suggesting that variations in time or processing conditions did not significantly affect the photoprotective capacity of each formulation. This behavior can be attributed to the use of ZnO nanoparticles, since the optical properties of the physical barrier are the primary source of protection, providing optimized SPF values (Chauhan et al., 2022; Elbrolesy et al., 2023; Singh & Nanda, 2014). However, the operating conditions used in our formulations were insufficient to modify SPF; therefore, further research is necessary to verify that these conditions do not affect this factor. 3.7 Inhibition of ABTS •+ radicals Exposure to solar UV radiation facilitates the generation of free radicals, a factor that determines their harmful effects on human skin damage (de Jager et al., 2017). To evaluate the antioxidant potential of the emulsified systems, a spectroscopic assay was used to measure in vitro , the elimination of the radical cation ABTS • . At time 0, all emulsions exhibited similar inhibition by 16–20%, except for the S15T4 emulsion, which had a significantly lower inhibition activity by 10% (p < 0.05). After storage (day 28), the ABTS •+ radical inhibition of S13T2 emulsion significantly decays to 12% (p 0.05) (Fig. 9 ). Since the emulsion is composed of various components, with quercetin serving as a standout antioxidant, changes in its activity could be related to potential impacts on the speeds and times applied during its preparation (Hatahet et al., 2017). These potential effects include inadequate or uneven distribution of the wall material for encapsulation as well as an increase in temperature associated with the applied speeds, which could degrade the bioactive component (Chaaban et al., 2017; Tan & McClements, 2021). Figure 9 shows that treatment S13T2 had the greatest decrease in ABTS + radical inhibition. This behavior is attributed to poor encapsulation and the formation of a ZnO coating around the bioactive compound, influenced by the speed-time correlation (Wong et al., 2021; Yulianingsih & Gohtani, 2020). Conversely, treatment S15T4 showed the lowest inhibition rates, suggesting that increases in speed (rpm) and prolonged processing times may affect the antioxidant capacity of those systems. However, it is important to note that the emulsion obtained from the S15T4 system conserves its ABTS + radical inhibition activity after 28 days of storage. In contrast, the S13T4 and S15T2 treatments did not show a statistically significant difference over time (p > 0.05), suggesting that the compound may provide effective protection and enhanced structural conformation stability. This behavior could be associated with the relationship between speed and time that incorporates energy into the system, suggesting remarkable chemical stability that preserves quercetin’s bioactivity. For a quercetin concentration of 0.5 µg/mL, comparable to the one used in this study to evaluate antioxidant capacity using ABTS + , previous studies, by Uzma et al. (2022) and Chittasupho et al. (2022), reported antioxidant activity values of ~ 10–15%. These values were lower than those obtained in this study for the S13T4 and S15T2 treatments, which were 19% and 20%, respectively. The observed increase could be attributed to a synergistic effect of quercetin and the antioxidant activity of the other components used in those emulsions. In this context, green coffee oil is known for its free radical inhibitory effects due to its chemical composition (Wagemaker et al., 2016; Wagemaker et al., 2011). Additionally, ZnO nanoparticles have been highlighted for their antioxidant properties, further enhancing free radical inhibition (Dias et al., 2023; Esquivel & Jiménez, 2012; Ostovar et al., 2023; Ramesh et al., 2022). 4 CONCLUSIONS The Pickering emulsions studied in this work had particle sizes greater than 100 nm and SPF values of ~ 40, suggesting greater optical interaction in the UV-A and UV-B regions. These values were preserved throughout 28 days of storage, demonstrating the viability of these systems for use in pharmaceutical and cosmetical applications. Regarding the Turbiscan Stability Index, the results show that homogenization time influenced the physical stability of the emulsions, with the S13T4 and S15T2 systems being the most stable in terms of physicochemical properties. In particular, the S15T4 emulsion exhibited the lowest ABTS •+ inhibition values. Furthermore, over time, rheological properties showed shear-thinning behavior, with yield points within the preferred range for topical applications. Additionally, the HB model adequately described the flow behavior of the samples. In conclusion, among the emulsions tested, S15T2 had the best physicochemical and viscoelastic properties. It showed a lower increase in particle size, maintained yield stress, and higher antioxidant capacity and SPF values over 28 days of storage. This stability is associated with improved encapsulation and nanoparticle distribution at the interfacial region, which could be attributed to the emulsion’s structural integrity, making it a suitable vehicle for developing systems with great potential in pharmacology/cosmetology. At this point, the interactions between Pickering emulsions and the skin stratum layers remain to be determined to discard any associated toxicity of those formulations. It is also necessary to study quercetin's bioactivity in other encapsulated nanocarriers to evaluate its therapeutic potential for different skin conditions. Declarations ACKNOWLEDGEMENTS The authors express their gratitude to Anton Paar for the donation of the rheometer. César Uriel Rodríguez-Fuentes and Ana Guadalupe Castillo-Olmos acknowledge the Consejo Nacional de Humanidades, Ciencia y Tecnología (CONAHCYT) for their master's and doctoral scholarships, respectively. Autor Contribution C.U.R.F.: conceptualization, writing the original draft, data curation, investigation; A.G.C.O.: writing original draft and data curation; J.C.V.E: revision and editing; E.T.M.: editing and graphics; D.B.: revision, graphical treatment of results and discussion; Z.J.H.E.: editing and graphics; C.C.S.: conceptualization, supervision, investigation, funding acquisition, editing. All authors have read and agreed to the published version of the manuscript Funding This research was financed by Tecnológico Nacional de México (TecNM). Data Availability The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request. Consent for publication All authors have approved the final draft and submission of manuscript. Conflicts of Interest The authors declare there are no conflicts of interest. Research Involving Humans and Animals Statement Not applicable. Ethical Approval Not applicable. 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Cano-Sarmiento","email":"data:image/png;base64,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","orcid":"","institution":"Tecnológico Nacional de México/Instituto Tecnológico de Veracruz","correspondingAuthor":true,"prefix":"","firstName":"Cynthia","middleName":"","lastName":"Cano-Sarmiento","suffix":""}],"badges":[],"createdAt":"2026-01-15 23:38:07","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8613997/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8613997/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":102182147,"identity":"ff998529-50c3-4fb9-940e-a8dc27bf0d56","added_by":"auto","created_at":"2026-02-09 07:27:58","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":780353,"visible":true,"origin":"","legend":"\u003cp\u003ea) Typical micrograph taken from the emulsion droplets and its processing for digital analysis. b) Crop of selected area. c) Contrast modification. d) Binarization. e) Outlined particles\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-8613997/v1/d054397096f613e5f5d9fcb5.png"},{"id":102182142,"identity":"10c6cc0d-7eb6-4a2b-8bf3-0811ad277857","added_by":"auto","created_at":"2026-02-09 07:27:58","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":605367,"visible":true,"origin":"","legend":"\u003cp\u003eSEM micrograph of zinc oxide coated with triethoxycaprylylsilane (Z-COTE HP1, BASF®)\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-8613997/v1/3185a734dbf94fc35ca25cbd.png"},{"id":102182141,"identity":"94a555a1-126f-442d-9c8d-9a1deab9a33c","added_by":"auto","created_at":"2026-02-09 07:27:58","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":112398,"visible":true,"origin":"","legend":"\u003cp\u003eUV-Vis absorbance spectra of ZnO coated with triethoxycaprylylsilane\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-8613997/v1/9e56953ba6846ba441532f8f.png"},{"id":102296820,"identity":"76a6b3fe-b218-45f0-824f-6e754a3d08b2","added_by":"auto","created_at":"2026-02-10 10:22:00","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":244243,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic representation of emulsion components and their distribution in droplet conformation: a) Desirable formation of an interfacial coarse. b) Deficient, non-complete interfacial coverage.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-8613997/v1/ab230ed97e4b054310dcea83.png"},{"id":102296834,"identity":"9c235048-6255-44a3-b2d9-fb926a67a985","added_by":"auto","created_at":"2026-02-10 10:22:06","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":361777,"visible":true,"origin":"","legend":"\u003cp\u003eTurbiscan stability index (TSI) of Pickering emulsions elaborated by combinations of speed (13, 000 – 15,000 rpm) and time (2 – 4 min). TSI was measured for 28 days for storage stability. Two – way ANOVA test followed by Tukey´s multiple comparison. **** \u003cem\u003ep\u003c/em\u003e\u0026lt;0.0001. ***\u003cem\u003ep\u003c/em\u003e\u0026lt;0.001, **\u003cem\u003ep\u003c/em\u003e\u0026lt;0.01, *\u003cem\u003ep\u003c/em\u003e\u0026lt;0.05\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-8613997/v1/d71f8fea20e6200a1f571c26.png"},{"id":102182145,"identity":"b364b82b-83d5-4514-8389-13c7514a6a70","added_by":"auto","created_at":"2026-02-09 07:27:58","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":171553,"visible":true,"origin":"","legend":"\u003cp\u003eFlow curves (average, n=3) of emulsions at days 0 and 28, showing shear-thinning behavior and shear rate dependence. Variations in yield stress and consistency are influenced by particle size and hydrophobicity.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-8613997/v1/57b612725b1e094905c1f16c.png"},{"id":102296985,"identity":"fd7857dc-21c3-4bf6-849f-a915b25f6e62","added_by":"auto","created_at":"2026-02-10 10:24:36","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":147738,"visible":true,"origin":"","legend":"\u003cp\u003eFrequency sweep (averages, n=3) of emulsions. Predominance of \u0026nbsp;G' over G'' indicates a gel-like behavior, influenced by particle interactions and structural rearrangements over time.\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-8613997/v1/288e1d5ddaac4071968ce443.png"},{"id":102296622,"identity":"3b4625fb-e67a-424f-ba29-6b8b8014b625","added_by":"auto","created_at":"2026-02-10 10:20:24","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":57232,"visible":true,"origin":"","legend":"\u003cp\u003eSun protection factor of emulsions on days 0 and 28. (Tukey, \u003cem\u003ep\u003c/em\u003e\u0026lt;0.05). Lowercase letters compare the same treatment over storage time; uppercase letters compare all treatments at each condition. Equal letters indicate no statistically significant difference.\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-8613997/v1/73f009a7d7e62d54862e18a0.png"},{"id":102182148,"identity":"21f2411f-571c-4999-818f-7846de6952be","added_by":"auto","created_at":"2026-02-09 07:27:58","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":53569,"visible":true,"origin":"","legend":"\u003cp\u003eABTS\u003csup\u003e\u003cstrong\u003e•+\u003c/strong\u003e\u003c/sup\u003e\u003cem\u003e \u003c/em\u003eInhibition percentage of emulsions elaborated by different treatments on days 0 and 28. Lowercase letters compare the same treatment over time; uppercase letters compare different treatments at the same time. Means with the same letter are not significantly different (Tukey, \u003cem\u003ep\u003c/em\u003e\u0026lt;0.05).\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-8613997/v1/41cd2dfe2919c296b17686f4.png"},{"id":102299191,"identity":"5fd942cc-c245-48fa-8991-761371e03156","added_by":"auto","created_at":"2026-02-10 11:03:29","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":4074830,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8613997/v1/57f702ef-4d96-4b1e-bf43-58ca5e4a3809.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Effect of process conditions on the stability and physicochemical properties of green coffee oil Pickering emulsions stabilized with ZnO nanoparticles for quercetin-enhanced sunscreen formulations","fulltext":[{"header":"1 INTRODUCTION","content":"\u003cp\u003eExposure to ultraviolet (UV) radiation from sunlight is a critical risk factor in skin carcinogenesis (Matsumara and Ananthaswamy 2004). In that sense, the use of sunscreen is essential for preventing and reducing UV damage such as burns, photoaging, and abnormal pigmentation (Dale Wilson et al., 2012; Mesa-Arango et al., 2017). Sunscreens exert their protective effects through different mechanisms of action, including: (i) act as a chemical barrier, by including compounds capable of absorbing damaging wavelengths, and (ii) set up a physical barrier to reflect and disperse potentially dangerous light (Manaia et al., 2013; Osterwalder et al., 2014). Most sunscreen formulations are primarily composed of synthetic drugs exerting their protective effects against photoirritation, photosensitization, and dermatitis (Geoffrey et al., 2019; Serpone et al., 2007). However, the development of formulations that include biocompatible compounds of natural origin is a desirable attribute in the constant search for strategies to protect human health from the harmful effects of UV radiation.\u003c/p\u003e \u003cp\u003eSeveral research groups have conducted investigations on the protective effects of plant oils present in flowers, olives, coconut, green coffee, and other plants (Baldisserotto et al., 2018; G\u0026ouml;n\u0026uuml;lalan et al., 2017; Kaur \u0026amp; Saraf, 2010; J. Marto, A. Ascenso, et al., 2016; Joana Marto et al., 2016; J. Marto, L. F. Gouveia, B. G. Chiari, et al., 2016; J. Marto, L. F. Gouveia, L. Gon\u0026ccedil;alves, et al., 2016; Sopyan et al., 2019; Wagemaker et al., 2011). Among these natural sources, green coffee oil stands out for its sun protection factor of ~\u0026thinsp;5, along with antioxidant and moisture-retaining properties. This natural oil is characterized by being a complex mixture of antioxidant substances with a high concentration of triacylglycerides, diterpenic esters, fatty acids, tocopherols, and sterols, which act as UV radiation absorbers (Nosari et al., 2015; Speer \u0026amp; K\u0026ouml;lling-Speer, 2006; Wagemaker et al., 2016; Wagemaker et al., 2011). Incorporating this kind of natural oil into a cosmetic dermal product has the potential to create a matrix that transports antioxidant compounds (Baldisserotto et al., 2018; J. Marto, L. F. Gouveia, B. G. Chiari, et al., 2016; Wagemaker et al., 2016). The sun protection products include creams, sprays, lotions, gels, and emulsions. Emulsions have been widely used due to their properties for enhancing the bioactivity of diverse chemical compounds, achieving stable systems with better compound distribution throughout the product, thus facilitating the incorporation of bioactive compounds with photoprotective activity (Geoffrey et al., 2019). Among emulsions, there is a classification known as Pickering emulsions, which are colloidal dispersions of two immiscible liquids that stabilize themselves through the incorporation of solid particles or nanoparticles in the interfacial region, forming a protective barrier around the droplets. This configuration provides greater rigidity and reduces the contact area between the phases (Jiang et al., 2020; Zembyla et al., 2020). Remarkably, the key property of this nanotechnology lies in the fine control of the degree of wetting of the nanoparticles at the oil-water interface, which reduces their interfacial free energy and enhances emulsion stability (Tsujii, 2017). However, to functionalize such a system, strict control over conditions of their formation is crucial, as these depend on several parameters, including particle size, distribution, and stability (Guzm\u0026aacute;n et al., 2022; Yukuyama et al., 2016).\u003c/p\u003e \u003cp\u003ePickering emulsions, also known as surfactant-free emulsions, utilize solid nanoparticles that can serve a dual function: as emulsifiers and as physical barriers. Thus, they allow the reduction in the use of synthetic components (Bordes et al., 2021). Nevertheless, typical sunscreen formulations, composed of Pickering emulsions, utilize micrometer-scale particle sizes, which affect the rheological properties of the product during application, resulting in a white cast upon skin deposition (Ahmad Zaki et al., 2018; Bordes et al., 2021; Guzm\u0026aacute;n et al., 2022). In contrast, nanoparticles with a minimum particle size of 100 nm avoid skin permeation, improve the product\u0026rsquo;s appearance after application, and enhance their optical properties, such as the ability to refract incident light (Lu et al., 2018; Smijs \u0026amp; Pavel, 2011). Additionally, from a rheological perspective, nanoparticles provide stability and structure to the product without compromising its spreadability on the skin (Lee et al., 2021; Joana Marto et al., 2016). Among the solid particles used in sunscreen formulations, TiO\u003csub\u003e2\u003c/sub\u003e, ZnO, and aluminum starch are widely used. Due to the low toxicity of nanoparticles and their high efficacy in protecting the skin, ZnO particles have been widely preferred choice in the formulation of diverse sunscreen products in combination with other nanomaterials, such as fatty acids, silica derivatives and polymers, which reduce or eliminate the potential production of free radicals and improve adherence to the skin preventing its permeability (Kandil et al., 2024; Lee et al., 2021; Lu et al., 2018; Smijs \u0026amp; Pavel, 2011).\u003c/p\u003e \u003cp\u003eEmulsions enable the encapsulation of a wide range of natural compounds, such as flavonoids, a group of plant phenolic derivatives with potential nutraceutical, pharmaceutical, medicinal, and cosmetic applications, including their addition to some sunscreen systems (Petruk et al., 2018). Quercetin (2-(3,4-dihydroxyphenyl)-3,5,7-trihydroxychromen-4-one) is a flavonoid with anti-inflammatory, anticarcinogenic, and antioxidant properties. Additionally, it can increase the sun protection factor (SPF) when combined with physical barriers (Choquenet et al., 2008). However, quercetin and other bioactive compounds are prone to degradation, exhibiting relatively low bioavailability. Hence, nanosystems are attractive for protecting them and optimizing their absorption and biological activities (Foga\u0026ccedil;a et al., 2020; Fonseca et al., 2023). Hatahet et al. (2017) reported that an acceptable performance of substances in terms of activity could be achieved by using lipid nanosystems, water in oil (W/O) micro- and nanoemulsions, as well as collagen matrices. The use of W/O Pickering emulsions not only increases the bioavailability of quercetin but also allows the creation of a stable system in which inorganic compounds can be introduced to enhance the functionality of the final product (Guzm\u0026aacute;n et al., 2022). Green coffee oil has been reported as a carrier for compounds like melatonin. However, changing the bioactive compound can affect the stability and functional properties of the final product, making each evaluation specific to the molecular cargo.\u003c/p\u003e \u003cp\u003eDue to the influence of the formation process in emulsion systems on the encapsulation of bioactive compounds, the aim of this study was to evaluate the effect of Pickering emulsion formation and stabilization process conditions on key physicochemical properties of sunscreens, including quercetin as cargo and ZnO nanoparticles as molecular stabilizers.\u003c/p\u003e"},{"header":"2 MATERIALS AND METHODS","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Materials\u003c/h2\u003e \u003cp\u003eAll reagents and solvents used were of analytical or HPLC grade. Green coffee oil was purchased from CREACOS (Guadalajara, Mexico). Zinc oxide nano coated with triethoxycaprylylsilane (ZnO) (Z Cote\u0026reg; HP1) was purchased from BASF (Ludwigshafen, Germany), glycerol and polyethylene glycol (PEG) from Sigma-Aldrich\u0026reg; (St. Louis, MO, USA), and xanthan gum from Albert and Ferran Adri\u0026agrave;\u0026reg; (Barcelona, Spain). The bioactive compound used was Quercetin\u0026thinsp;\u0026ge;\u0026thinsp;95% (Sigma-Aldrich\u0026reg;, St. Louis, MO, USA).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Characterization of ZnO nanoparticles\u003c/h2\u003e \u003cdiv id=\"Sec5\" class=\"Section3\"\u003e \u003ch2\u003e2.2.1 Particle size distribution, morphology and optical properties\u003c/h2\u003e \u003cp\u003eThe morphology of ZnO nanoparticles was examined using a Tescan\u0026reg; MIRA3 scanning electron microscope (Brno, Czech Republic) operated at 10.0 kV in high-vacuum mode with a secondary electron detector at a working distance of 9.39 mm. The sample was mounted on carbon tape, and the obtained micrographs were processed using MIRA3 software version 4.2.19.1 (Tescan\u0026reg;, Brno, Czech Republic) in TIFF format.\u003c/p\u003e \u003cp\u003eParticle size was determined using dynamic light scattering (DLS) on a Zetasizer Nano ZS equipment (Malvern Panalytical, UK), following the methodology of Assis Dias Alves et al. (2024). The nanoparticles were suspended in ethyl acetate to obtain a final concentration of 0.2 mg/mL and subjected to ultrasonic treatment for 1 hour. Subsequently, 1 mL of the suspension was transferred to a glass cuvette for measurement.\u003c/p\u003e \u003cp\u003eThe optical properties were evaluated through the absorption spectrum of ZnO nanoparticles coated with triethoxycaprylylsilane dispersed in ethyl acetate, obtained using a UV-Vis spectrophotometer (UV-Vis Agilent 8453, Agilent Technologies, Santa Clara, USA) in the range of 200 to 600 nm. The same nanoparticle concentration as used for size analysis was employed (0.2 mg/mL).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section3\"\u003e \u003ch2\u003e2.2.3 Contact angle determination\u003c/h2\u003e \u003cp\u003eFor the determination of the contact angle, the glass slide methodology was used with modifications (Nowak et al., 2013). To prepare the surface covered with nanoparticles, double-sided tape was placed on a microscope slide, followed by the deposition of a uniform layer of ZnO nanoparticles. This layer was lightly compressed to ensure material fixation, with excess particles removed. With the prepared surface, 5 \u0026micro;L of distilled water was placed on the nanoparticles using a 26 S syringe. Subsequently, a digital microscope (Celestron Handheld Digital Microscope Pro, Celestron LLC, CA, USA) was used to capture the micrograph, utilizing Celestron Microcapture Pro 2.5 software (Celestron LLC, CA, USA). The micrographs were saved in JPG format and processed using digital image analysis following the method reported by Stalder et al. (2010).\u003c/p\u003e \u003cp\u003eFor digital image analysis, ImageJ software version 1.53t (NIH, USA) was employed, utilizing the drop analysis plugin with the LB-ASDA function. Parameters were adjusted so that the contour outlined by the software matched the droplet\u0026rsquo;s contour in the image, and so the contact angle was obtained. The procedure was performed in triplicate with different samples, and the results are presented as mean\u0026thinsp;\u0026plusmn;\u0026thinsp;SD.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Preparation of emulsions\u003c/h2\u003e \u003cp\u003eEmulsions were prepared according to the procedure described by J. Marto, L. F. Gouveia, B. G. Chiari, et al. (2016), with modifications in the dispersed phase and the type of nanoparticles used. The final composition is shown in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. In compliance with Regulation 1223/2009 of the European Parliament and Council, the INCI names and the function of the ingredients in the formulation are listed alongside their respective percentages.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eComposition of Pickering emulsion sunscreen\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"3\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eComponent (INCI Name)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eFunction\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eQuantitative composition\u003c/p\u003e \u003cp\u003e(%, w/w)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eContinuous phase\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e69.64\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cem\u003eCoffea Arabica\u003c/em\u003e (Coffee) Seed Oil\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eAnti-Aging Agent / Antioxidant / Emollient / Moisturizing Agent\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e46.12\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eZinc Oxide (and) Triethoxycaprylylsilane\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eUV Filter / Skin protector\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e23.52\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eDisperse phase\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003e30.36\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePolyethylene glycol\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSolvent / Emulsion stabilizer\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e10.00\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eGlycerin\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eHumectant / Viscosity controller\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e6.65\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAqua\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSolvent\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e13.36\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eXanthan gum\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eGel forming / Viscosity controller / Emulsion stabilizer\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.30\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eQuercetin\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eAntioxidant\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.05\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eZinc oxide and green coffee oil were homogenized for the continuous phase using magnetic stirring PC-420D (Corning\u0026reg;, NY, USA) at 1,100 rpm for 30 min. Then, ZnO was dispersed with higher energy using a rotor-stator homogenizer Ultraturrax\u0026reg; T25 (IKA\u0026reg;, Staufen, Germany) at 13,000 rpm for 1 min to ensure that most particles came into contact with the oil phase (Bordes et al., 2021).\u003c/p\u003e \u003cp\u003eThe dispersed phase was prepared by dissolving the bioactive compound in PEG using an ultrasonic bath Aquawave 9376\u0026reg; (1.75 L, 115 V, 50/60 HZ, 500 W) (Barnstead Lab-Line, IA, USA) at room temperature (25 \u0026ordm;C) for 5 min. Next, purified water, xanthan gum, and glycerol, which had been previously mixed manually, were added.\u003c/p\u003e \u003cp\u003ePhases were mixed using a rotor-stator homogenizer while varying the working conditions in terms of time (min) and speed (rpm) according to Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, with cycles consisting of 1 min of work followed by 1 min of rest. The obtained emulsions exhibited a pH ranging from 6 to 6.5 and were physically characterized. The effect of the processing conditions on their physicochemical properties was evaluated. Additionally, their potential for encapsulating bioactive compounds was assessed to develop a system that could be considered functional as a sunscreen.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eOperating conditions for Pickering emulsion sunscreen\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"3\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTreatment\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSpeed\u003c/p\u003e \u003cp\u003e(rpm)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eTime\u003c/p\u003e \u003cp\u003e(min)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eS13T2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e13,000\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eS13T4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e13,000\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eS15T2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e15,000\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eS15T4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e15,000\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003ctfoot\u003e \u003ctr\u003e\u003ctd colspan=\"3\"\u003eNote: Equipment used for emulsification was a high shear rotor \u0026ndash; stator homogenizer Ultraturrax\u0026reg; T25\u003c/td\u003e\u003c/tr\u003e \u003c/tfoot\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eThe analyses were conducted on the emulsions both on the preparation day and after 28 days of storage in dark conditions at 25\u0026deg;C and 36.5% relative humidity, using a crystal cell.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e2.4 Particle size distribution\u003c/h2\u003e \u003cp\u003eParticle size distribution was assessed using a DM2000 LED optical microscope (Leica Microsystems\u0026reg;, Wetzlar, Germany) at 100\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\times\\:\\)\u003c/span\u003e\u003c/span\u003e magnification. Micrographs were captured using Leica Application Suite V4.9 software. Each sample was prepared by dispersing 0.25 g of each emulsion in 1 mL of green coffee oil, and 5 \u0026micro;L of the final dispersion was placed on a glass slide. The micrographs were saved in TIFF format using the automatic settings provided by the software and subsequently analyzed through digital image analysis. Emulsions were evaluated at both 0 and 28 days after storage (Alade et al., 2021; Prestes et al., 2016).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e2.5 Digital image analysis\u003c/h2\u003e \u003cp\u003eThe average particle size D[3:2] and Feret diameter was carried out using ImageJ 1.53t software (NIH, USA), following the methodology of Cano-Sarmiento et al. (2014) with some modifications. The contrast and brightness of the micrographs were manually adjusted to ensure optimal visualization of the particles. Subsequently, the images were converted to 8-bit format, and the threshold level was automatically adjusted to generate a binary image. The \u003cem\u003eBinary: Outline\u003c/em\u003e function was used to outline the contours of the particles, and finally, the \u003cem\u003eAnalyze Particles\u003c/em\u003e option was applied. For all treatments, at least three areas per image were analyzed at both 0 and 28 days of storage. Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e illustrates the steps and modifications in the micrographs for digital image analysis, from which the average particle size was estimated.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e2.6 Physical stability of emulsions\u003c/h2\u003e \u003cp\u003eThe Turbiscan Stability Index (TSI) was measured to assess the presence of instability phenomena. The TSI value was determined following the methodology described by Li et al. (2020) with a Turbiscan Lab\u0026reg; Expert instrument (Formulaction, Toulouse, France). It was performed through multiple light scattering technique, where a sample of each emulsion was placed in cylindrical glass cells. The measurements were conducted at 25\u0026deg;C for 28 days. TSI values close to zero indicate higher emulsion stability (Kang et al., 2011).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e2.7 Rheological characterization\u003c/h2\u003e \u003cp\u003ePickering emulsions were analyzed for rheological behavior using a rheometer MCR 92 and RheoCompass 1.30 software (Anton Paar, Graz, Austria) with a 25 mm diameter plate-plate geometry equipped with a Peltier temperature control system set at 25\u0026deg;C, and a gap of 0.5 mm between each plate.\u003c/p\u003e \u003cdiv id=\"Sec12\" class=\"Section3\"\u003e \u003ch2\u003e2.7.1 Rotational test\u003c/h2\u003e \u003cp\u003eEmulsions were subjected to flow curve tests over a shear rate range of 0.1 to 100 1/s, and shear stress vs. shear rate data were plotted for each treatment. These data were then fitted to the Herschel-Bulkley (HB) model (Eq.\u0026nbsp;\u003cspan refid=\"Equ1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) to characterize the behavior and structural changes that occurred in the emulsion at time zero and after 28 days of storage (Cefali et al., 2019; Chiari-Andr\u0026eacute;o et al., 2021; Liu et al., 2021; Sheng et al., 2018).\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:\\sigma\\:={\\sigma\\:}_{0}+k\\bullet\\:{\\gamma\\:}^{n}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\sigma\\:\\)\u003c/span\u003e\u003c/span\u003e represents the shear stress (Pa), \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\sigma\\:}_{0}\\)\u003c/span\u003e\u003c/span\u003e is the yield stress (Pa), \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:k\\)\u003c/span\u003e\u003c/span\u003e is the consistency index \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\left(\\text{P}\\text{a}\\bullet\\:{\\text{s}}^{n}\\right)\\)\u003c/span\u003e\u003c/span\u003e, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\gamma\\:\\)\u003c/span\u003e\u003c/span\u003e is the shear rate (s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:n\\)\u003c/span\u003e\u003c/span\u003e is the flow behavior index (dimensionless).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section3\"\u003e \u003ch2\u003e2.7.2 Oscillatory test\u003c/h2\u003e \u003cp\u003eAmplitude sweeps were conducted for each treatment over a shear strain range of 0.01 to 100% at a constant angular frequency of 10 rad/s to determine the linear viscoelastic region (LVR). This region is characterized by constant storage \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\left(G{\\prime\\:}\\right)\\)\u003c/span\u003e\u003c/span\u003e and loss moduli \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\left(G{\\prime\\:}{\\prime\\:}\\right)\\)\u003c/span\u003e\u003c/span\u003e (Dabbaghi et al., 2021). From the LVR, a specific strain value was selected and fixed in frequency sweep tests to analyze the structural changes inherent to the system, rather than those induced by equipment-generated stresses (Geremias-Andrade et al., 2016). Additionally, a variation in angular frequency from 1 to 100 rad/s was performed to evaluate \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:G{\\prime\\:}\\)\u003c/span\u003e\u003c/span\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:G{\\prime\\:}{\\prime\\:}\\)\u003c/span\u003e\u003c/span\u003e related to the conformation of the emulsion structure and the product's consistency characteristics (Ma et al., 2023).\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003e2.8 \u003cem\u003eIn vitro\u003c/em\u003e sun protection factor\u003c/h2\u003e \u003cp\u003eThe Sun Protection Factor (SPF) was determined using the standards proposed by Ahmad Zaki et al. (2018) for sunscreens containing ZnO nanoparticles. One gram of the emulsion was dissolved and made up to 100 mL with 40% ethanol. The solution was subjected to an ultrasonic bath Aquawave 9376 (1.75 L, 115 V, 50/60 HZ, 500 W, Barnstead Lab-Line, IA, USA) for 5 min at 25\u0026deg;C and filtered through cotton, discarding the first 10 mL. An aliquot of 5 mL was diluted and made up to 50 mL with the same solvent, resulting in a solution with a concentration of 1 mg/mL. The same procedure was done with the commercial 25% ZnO 50 SPF labeled sunscreen as a reference.\u003c/p\u003e \u003cp\u003eThe absorbance was measured using a UV-Vis spectrophotometer Agilent 8453 (Agilent Technologies, CA, USA) with a 1 cm glass cuvette and 40% ethanol as the blank. Data were obtained at wavelengths from 290 to 320 nm, in 5 nm increments, and were substituted into Eq.\u0026nbsp;\u003cspan refid=\"Equ2\" class=\"InternalRef\"\u003e2\u003c/span\u003e (Mansur et al., 1986) to determine the SPF value.\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$$\\:\\text{S}\\text{P}\\text{F}=\\text{C}\\text{F}\\text{*}{\\sum\\:}_{320}^{290}\\text{E}\\text{E}\\left({\\lambda\\:}\\right)\\text{*}\\text{I}\\left({\\lambda\\:}\\right)\\text{*}\\text{A}\\text{b}\\text{s}\\left({\\lambda\\:}\\right)$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\lambda\\:\\)\u003c/span\u003e\u003c/span\u003e represents the wavelength, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:EE\\left(\\lambda\\:\\right)\\)\u003c/span\u003e\u003c/span\u003e is the erythema effect spectrum, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:I\\left(\\lambda\\:\\right)\\)\u003c/span\u003e\u003c/span\u003e is the solar spectrum\u0026rsquo;s intensity, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:Abs\\left(\\lambda\\:\\right)\\)\u003c/span\u003e\u003c/span\u003e is the sunscreen\u0026rsquo;s absorbance, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:CF\\)\u003c/span\u003e\u003c/span\u003e is a correction factor (=\u0026thinsp;10) and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:SPF\\)\u003c/span\u003e\u003c/span\u003e is the sun protection factor. This equation is a correlation between the absorbance at each wavelength and the equivalent to the intensity at the same wavelength using constant values of EE x I reported elsewhere (Sayre et al., 1979).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec15\" class=\"Section2\"\u003e \u003ch2\u003e\u003cb\u003e2.9 Inhibition of ABTS\u003c/b\u003e\u003csup\u003e\u003cb\u003e\u0026bull;+\u003c/b\u003e\u003c/sup\u003e \u003cb\u003eradicals\u003c/b\u003e\u003c/h2\u003e \u003cp\u003eThe free radical ABTS\u003csup\u003e\u003cb\u003e\u0026bull;+\u003c/b\u003e\u003c/sup\u003e (2,2\u0026rsquo;-azino-bis(3-ethylbenzothiazoline-6-sulfonate) solution was prepared according to Espino-Manzano et al. (2020). Separate solutions of 7 mM ABTS\u0026thinsp;+\u0026thinsp;and 2.45 mM potassium persulfate (K\u003csub\u003e2\u003c/sub\u003eS\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e8\u003c/sub\u003e) were prepared in distilled water, mixed in a 0.5:1 ratio, and stored at 25\u0026deg;C in the dark for 12 h. Subsequently, the resulting solution was diluted with ethanol to achieve an absorbance of 0.700\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02 at \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\lambda\\:=734\\)\u003c/span\u003e\u003c/span\u003e nm. Sample preparation was carried out using the procedure described for SPF evaluation, with a final concentration of 1 mg/mL of the emulsion in 40% ethanol. The samples were stored in amber vials at room temperature (25\u0026deg;C) to prevent compound degradation due to light exposure.\u003c/p\u003e \u003cp\u003eThe method of Medina-P\u0026eacute;rez et al. (2020) was used to evaluate the antioxidant activity, with modifications in the volume of ABTS\u003csup\u003e\u003cb\u003e\u0026bull;+\u003c/b\u003e\u003c/sup\u003e used. Briefly, 200 \u0026micro;L of the sample was added to 1800 \u0026micro;L of the ABTS\u003csup\u003e\u003cb\u003e\u0026bull;+\u003c/b\u003e\u003c/sup\u003e radical solution and incubated in the dark at room temperature (25\u0026deg;C) for 6 min. The same procedure was carried out with 40% ethanol to obtain a blank. Absorbance was measured at 734 nm using a UV-Vis spectrophotometer Agilent 8453 (Agilent Technologies, CA, USA), and the obtained data were substituted into Eq.\u0026nbsp;\u003cspan refid=\"Equ3\" class=\"InternalRef\"\u003e3\u003c/span\u003e (Espino-Manzano et al., 2020) to determine the percentage of free radical inhibition.\u003cdiv id=\"Equ3\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ3\" name=\"EquationSource\"\u003e\n$$\\:Inhibition\\:\\%=\\left(\\frac{{A}_{0}-{A}_{A}}{{A}_{0}}\\right)*100$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e3\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{A}_{0}\\)\u003c/span\u003e\u003c/span\u003eis the absorbance value of the blank and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{A}_{A}\\)\u003c/span\u003e\u003c/span\u003e is the absorbance value of the sample.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec16\" class=\"Section2\"\u003e \u003ch2\u003e2.10 Statistical analysis\u003c/h2\u003e \u003cp\u003eStatistical analysis of the obtained results was conducted using analysis of variance (ANOVA) and Tukey's mean comparison with Graph Pad Prism\u0026reg; 8 software (GraphPad Software, Inc., San Diego, CA). Curve fitting was performed using OriginLab 2021 (9.8.0.200) software (OriginLab Corporation, Northampton, Massachusetts). A significant alpha level of \u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05 was used (\u003cem\u003en\u003c/em\u003e\u0026thinsp;=\u0026thinsp;3).\u003c/p\u003e \u003c/div\u003e"},{"header":"3 RESULTS AND DISCUSSION","content":"\u003cdiv id=\"Sec18\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Characterization of ZnO nanoparticles\u003c/h2\u003e \u003cp\u003eZinc oxide nanoparticles are widely used in the formulation of inorganic physical sunscreens due to their superior protective properties compared to organic compounds. Additionally, they offer advantages such as the absence of skin irritation or sensitivity, low skin penetration, and broad-spectrum coverage. However, as particle diameter decreases, the surface-to-volume ratio increases. Consequently, nanoparticles may exhibit higher reactivity than traditional materials, enhancing their utility in biomedical applications but also increasing the risk of potential health and environmental hazards (Stern \u0026amp; McNeil, 2007). In this context, it is crucial to understand their characterization in terms of size, morphology, contact angle, and optical properties before incorporating them into final sunscreen products (Khan et al., 2023; Smijs \u0026amp; Pavel, 2011). Coatings such as triethoxycaprylylsilane and silica derivatives have been employed to reduce ion generation and migration from the material to the skin (Kandil et al., 2024; Ramasamy et al., 2014). However, these nanofunctionalized particles can alter important physical characteristics, including morphology, particle size, and contact angle, thereby modifying the properties of the formulations obtained.\u003c/p\u003e \u003cp\u003eThe zinc oxide exhibits multiple morphologies, with the most commonly observed being rod-shaped (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). This observation aligns with the information provided by the supplier in their technical datasheet. Furthermore, dynamic light scattering (DLS) analysis revealed an average particle size of 134.8\u0026thinsp;\u0026plusmn;\u0026thinsp;1.2 nm and a D90 percentile of 243\u0026thinsp;\u0026plusmn;\u0026thinsp;17.4 nm. According to nanomaterial usage regulations, the particle sizes of ZnO comply with the established requirement of being greater than 100 nm (Ferreira et al., 2023).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe advantages of using nanoscale zinc oxide in cosmetic formulations, such as sunscreens, include its enhanced surface area and intrinsic properties, such as a high refractive index and absorption in the UV-A (315\u003cb\u003e\u0026ndash;\u003c/b\u003e400 nm) and UV-B (280\u003cb\u003e\u0026ndash;\u003c/b\u003e315 nm) regions (Khan et al., 2023; Song et al., 2011). As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, ZnO coated with triethoxycaprylylsilane exhibits strong absorption from 250 to 400 nm. In comparison, bulk ZnO has a maximum absorption at 375 nm. Due to its nanoscale size, coating, and multiple morphologies, a shift and broadening of this absorption range are observed, which can be beneficial, as it indicates enhanced photoprotective activity through increased absorption across the UV spectrum (Agarwal et al., 2019; Singh et al., 2012).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec19\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Contact angle\u003c/h2\u003e \u003cp\u003eIn both conventional and Pickering emulsions, the region between the two liquid phases contains a mixture of oil, water, and emulsifying agents (i.e., solid particles or surfactant molecules). The properties of this region determine the emulsion type (W/O or O/W), with wettability being crucial for particle-stabilized systems. Wettability is determined by the contact angle (Yousufi et al., 2023), which indicates the particles' affinity for one of the two phases. According to Finkle's empirical rule, particles with contact angles below 90\u0026deg; are predominantly hydrophilic and stabilize O/W emulsions, whereas angles greater than 90\u0026deg; are more hydrophobic and stabilize W/O emulsions (Gonzalez Ortiz et al., 2020; Ribeiro et al., 2023). In this study, zinc oxide particles coated with triethoxycaprylylsilane showed a contact angle of 121.6\u0026thinsp;\u0026plusmn;\u0026thinsp;1.6\u0026deg;, indicating a stronger tendency to form emulsions with a dispersed aqueous phase. This behavior is attributed to the coating, which contains polar molecules with some affinity for oil. This characteristic is advantageous for developing topical products, as a continuous oil phase exhibits greater affinity and adhesion to the skin, enhancing its water resistance and prolonging its retention on the skin (Assis Dias Alves et al., 2024; J. Marto, L. F. Gouveia, B. G. Chiari, et al., 2016).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec20\" class=\"Section2\"\u003e \u003ch2\u003e3.3 Particle size distribution\u003c/h2\u003e \u003cp\u003eIn emulsion studies, several reports highlight the influence of process parameters, such as speed, temperature, and processing time, on particle size and their stability (Surjit Singh et al., 2022; Touzouirt et al., 2018; Tsabet \u0026amp; Fradette, 2015). Specifically, speed influences droplet breakage and size reduction, with high speeds imparting significant energy that can lead to smaller sizes and greater homogeneity, albeit at the expense of temperature rises that may degrade organic compounds such as quercetin (Chaaban et al., 2017; Hariyatno et al., 2021; Yulianingsih \u0026amp; Gohtani, 2020). Processing time is crucial for Pickering emulsions, as it facilitates proper component contact and, with longer times, allows better particle arrangement around the continuous phase (Hariyatno et al., 2021). For topical products, to ensure sizes above 100 nm and to avoid ethical issues and genotoxicity testing, an in-depth study of the emulsion particles is recommended (Kang et al., 2011).\u003c/p\u003e \u003cp\u003eIn all samples analyzed in this study, the mean particle size D[3:2] was greater than 100 nm (0.44\u0026ndash;0.53 \u0026micro;m) at zero-day. After 28 days of storage, increases of 19%, 21%, 20%, and 11% were observed in the mean particle size for samples S13T2, S13T4, S15T2, and S15T4, respectively (Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). Meanwhile, percentile D90 shows a global increase of around ~\u0026thinsp;0.1 \u0026micro;m at the same evaluated time.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eMean particle size D[3:2] and percentile D90 of Pickering emulsions*.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"6\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eTreatment\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e \u003cp\u003eDay 0\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"3\" nameend=\"c6\" namest=\"c4\"\u003e \u003cp\u003eDay 28\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eD[3:2] (\u0026micro;m)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eD90 (\u0026micro;m)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eD[3:2] (\u0026micro;m)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"2\" nameend=\"c6\" namest=\"c5\"\u003e \u003cp\u003eD90 (\u0026micro;m)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eS13T2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.67\u0026thinsp;\u0026plusmn;\u0026thinsp;0.08\u0026ordf;\u003csup\u003eA\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.81\u0026thinsp;\u0026plusmn;\u0026thinsp;0.12\u0026ordf;\u003csup\u003eA\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.80\u0026thinsp;\u0026plusmn;\u0026thinsp;0.06\u003csup\u003eaA\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.95\u0026thinsp;\u0026plusmn;\u0026thinsp;0.06\u003csup\u003eaA\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"1\" nameend=\"c6\" namest=\"c6\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eS13T4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.65\u0026thinsp;\u0026plusmn;\u0026thinsp;0.04\u0026ordf;\u003csup\u003eA\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.83\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03\u0026ordf;\u003csup\u003eA\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.79\u0026thinsp;\u0026plusmn;\u0026thinsp;0.04\u003csup\u003ebA\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.94\u0026thinsp;\u0026plusmn;\u0026thinsp;0.08\u003csup\u003eaA\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"1\" nameend=\"c6\" namest=\"c6\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eS15T2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.67\u0026thinsp;\u0026plusmn;\u0026thinsp;0.08\u0026ordf;\u003csup\u003eA\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.84\u0026thinsp;\u0026plusmn;\u0026thinsp;0.09\u0026ordf;\u003csup\u003eA\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.81\u0026thinsp;\u0026plusmn;\u0026thinsp;0.06\u003csup\u003eaA\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.91\u0026thinsp;\u0026plusmn;\u0026thinsp;0.04\u003csup\u003eaA\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"1\" nameend=\"c6\" namest=\"c6\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eS15T4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.69\u0026thinsp;\u0026plusmn;\u0026thinsp;0.08\u0026ordf;\u003csup\u003eA\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.89\u0026thinsp;\u0026plusmn;\u0026thinsp;0.14\u0026ordf;\u003csup\u003eA\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.77\u0026thinsp;\u0026plusmn;\u0026thinsp;0.05\u003csup\u003eaA\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.87\u0026thinsp;\u0026plusmn;\u0026thinsp;0.04\u003csup\u003ebA\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"1\" nameend=\"c6\" namest=\"c6\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e*: Averages\u0026thinsp;\u0026plusmn;\u0026thinsp;standard deviation (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05). Equal letters (upper at the same time, and lower for the same treatment) indicate no significant difference.\u003c/p\u003e \u003cp\u003eFor Pickering emulsions, some authors (Chaaban et al., 2017; Guzm\u0026aacute;n et al., 2022; Zembyla et al., 2020) indicate that once the coating layer around the dispersed phase is formed correctly, it is challenging to break owing that the contact area between the two liquids has been reduced or even eliminated, and the water-particle-oil contact replaces the direct water-oil contact. Additionally, the material\u0026rsquo;s configuration used can enhance stability and interactions with the continuous phase. This is because systems that are less prone to coalescence are produced. However, the coating may have areas of poor coverage due to uneven distribution of the interface (de Carvalho-Guimar\u0026atilde;es et al., 2022; Zembyla et al., 2020). Even so, this possibility is minimized by the nanometer scale of the solid nanoparticles, which regulate droplet size and arrangement while reducing potential pores or oil-water contact areas where instability might occur (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAlthough emulsions were prepared under different operating conditions, the treatments did not show a significant difference (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026gt;\u0026thinsp;0.05) at both the initial (day 0) and at final time (day 28) (Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). However, the particle size showed a trend toward increasing during storage, although the emulsion instability was not visible macroscopically. Furthermore, the optical microscopy analysis technique requires sample dilution, which can break up agglomerates or redistribute nanoparticles. Overcoming these limitations requires the use of other techniques to evaluate the physical properties of these microsystems in suspension.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec21\" class=\"Section2\"\u003e \u003ch2\u003e3.4 Turbiscan stability index\u003c/h2\u003e \u003cp\u003eEmulsions, being thermodynamically unstable systems, are prone to instability phenomena such as sedimentation, creaming, coalescence, flocculation, and Ostwald ripening, which can lead to phase separation (McClements \u0026amp; Jafari, 2018). The encapsulation of bioactive compounds is crucial for preserving the physicochemical properties of emulsions and ensuring the proper release and protection of compounds during storage (Hatahet et al., 2017). Although Pickering emulsions are less susceptible to coalescence and Ostwald ripening, improper distribution of solid particles at the interface can promote instability phenomena (Zembyla et al., 2020). Here, TSI was used to monitor and characterize the physical stability of the four emulsions over 28 days (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). The TSI value is a parameter that quantifies the degree of instability in the emulsion by comparing backscattering values (Zalewska et al., 2019). Since the composition remained constant, changes in TSI over time allowed observation of the effect of processing conditions on the development of a functional product.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e depicts the TSI values plotted over time for each treatment. No significant changes in emulsion stability were observed up to 4 h except for S13T4 vs S15T2 (p\u0026thinsp;\u0026lt;\u0026thinsp;0.05). As time progressed on day 2, significant differences in TSI values became evident among all treatments, with TSI values of 4.0, 2.0, 3.45, and 2.95 for S13T2, S13T4, S15T2, and S15T4 samples, respectively (p\u0026thinsp;\u0026lt;\u0026thinsp;0.05). This gradual onset of differences can be attributed to the intrinsic properties of the emulsion, such as its viscosity and structural conformation, which can slow down the movement of droplets within the emulsion, thereby enhancing stability (Santos et al., 2020; Yulianingsih \u0026amp; Gohtani, 2020). From day 3 to day 21, no significant differences were observed between the TSI values of S13T2 and S15T2 (p\u0026thinsp;\u0026gt;\u0026thinsp;0.05) (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). To note, the S13T2 and S15T2 were generated at two different speeds, 13,000 rpm and 15,000 rpm, respectively. The change in mixing speed from 13,000 to 15,000 rpm did not significantly affect the emulsion behavior during short mixing times (2 min). However, at longer mixing times (4 min), this change in speed was decisive for the system\u0026rsquo;s behavior, suggesting that prolonged exposure enhances interactions between components, including van der Waals and electrostatic forces. These properties can contribute to either stability or instability in those systems, depending on the balance and control of materials selected during the formulation process (Frelichowska et al., 2010; McClements \u0026amp; Jafari, 2018). The speed parameter modulates the shear force in the homogenization zone, where higher speeds yielding high shear forces; however, their use is limited by the associated temperature increase. At low speeds, both temperature increase and shear force are lower, therefore longer processing times must be used to compensate this effect; as shown in S13T2 treatment, where the low speed and short processing time affected the distribution of ZnO nanoparticles around the droplets, as a result, there was a higher presence of instability phenomena such as flocculation, coalescence, and Ostwald ripening, i.e. the thermodynamic process where small oil droplets are dispersed in an aqueous medium (Frelichowska et al., 2010; Gonzalez Ortiz et al., 2020; Mulia et al., 2019). Regarding S15T4 treatment, a TSI value increase was observed compared to treatments S15T2 and S13T4, suggesting that the amount of energy required to form the emulsion was exceeded. The employed conditions resulted in a phenomenon referred to as over-processing, in which the energy density applied to the system is exceptionally high, favoring phenomena of coalescence due to the constant rupture and formation of droplets, along with a minor migration rate of nanoparticles to the interfacial region than the rate of their formation (P\u0026aacute;ez-Hern\u0026aacute;ndez et al., 2019). On day 28, both S13T4 and S15T4 treatments had the lowest TSI values, with final 4.4 and 4.8, respectively. Both treatments were more stable compared to others (6.5 and 5.9, TSI values for S13T2 and S15T2, respectively), even though they had the largest increase in particle size (57%) (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). Overall, our results indicate that, in all treatments, the combination of speed and time indeed modifies the stability value.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec22\" class=\"Section2\"\u003e \u003ch2\u003e3.5 Rheological characterization\u003c/h2\u003e \u003cdiv id=\"Sec23\" class=\"Section3\"\u003e \u003ch2\u003e3.5.1 Flow curves\u003c/h2\u003e \u003cp\u003eCharacterizing the flow behavior of O/W emulsions enables the study of packing systems in relation to product filling, skin assimilation, and ease of product extraction (Zhang et al., 2022). The viscosity of these emulsions depends on shear rate, a typical characteristic of non-Newtonian fluids, as evidenced by the flow curves (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e). This behavior may be related to the hydrophobicity of the materials used. In this sense, Hohl et al. (2016) suggested that the hydrophobicity of the particles, rather than their surface properties, has a greater influence on the stability of the emulsified structure. Additionally, flow curves help understand the physical behavior of fluids under stress. Given that for topically applied products, an excellent spreadability is a desirable attribute (Guzm\u0026aacute;n et al., 2022), all emulsions obtained in this work showed an acceptable shear-thinning behavior (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e). According to Bordes et al. (2021) and J. Marto, L. F. Gouveia, B. G. Chiari, et al. (2016), shear-thinning behavior enables better application and distribution on the skin, creating a homogeneous film that ensures proper deposition of particles. This enhances their UV protection capacity by forming a single zone for reflection and refraction of UV rays.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe emulsions data curve was fitted to the Herschel-Bulkley (HB) rheological model (\u003cem\u003eR\u003c/em\u003e\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;\u0026gt;\u0026thinsp;0.99) to obtain parameter values that allow a clear comparison and definition of rheological behavior (Bordes et al., 2021). This model represents the behavior of non-Newtonian fluids that exhibit a yield stress before flow begins, as is typical of thixotropic fluids, and is widely used in pharmaceutical and cosmetic products (Herschel \u0026amp; Bulkley, 1926; J. Marto, A. Ascenso, et al., 2016). Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e shows the obtained values.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab4\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eThe Herschel- Buckley model parameters for Pickering emulsions.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eTreatment\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\varvec{\\sigma\\:}}_{0}\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e \u003cp\u003e(Pa)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\varvec{\\kappa\\:}\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e \u003cp\u003e(Pa s\u003csup\u003en\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003en\u003c/p\u003e \u003cp\u003e(-)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"3\" rowspan=\"4\"\u003e \u003cp\u003eDay 0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eS13T2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e25.07\u0026thinsp;\u0026plusmn;\u0026thinsp;0.26\u003csup\u003eaA\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e7.59\u0026thinsp;\u0026plusmn;\u0026thinsp;0.13\u003csup\u003eaB\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.514\u0026thinsp;\u0026plusmn;\u0026thinsp;0.003\u003csup\u003eaB\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eS13T4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e21.31\u0026thinsp;\u0026plusmn;\u0026thinsp;0.22\u003csup\u003eaB\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e6.59\u0026thinsp;\u0026plusmn;\u0026thinsp;0.10\u003csup\u003eaC\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.534\u0026thinsp;\u0026plusmn;\u0026thinsp;0.003\u003csup\u003eaA\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eS15T2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e25.03\u0026thinsp;\u0026plusmn;\u0026thinsp;0.38\u003csup\u003eaA\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e6.94\u0026thinsp;\u0026plusmn;\u0026thinsp;0.18\u003csup\u003eaBC\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.520\u0026thinsp;\u0026plusmn;\u0026thinsp;0.005\u003csup\u003eaAB\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eS15T4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e25.27\u0026thinsp;\u0026plusmn;\u0026thinsp;0.84\u003csup\u003eaA\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e9.11\u0026thinsp;\u0026plusmn;\u0026thinsp;0.46\u003csup\u003eaA\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.474\u0026thinsp;\u0026plusmn;\u0026thinsp;0.009\u003csup\u003eaC\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"3\" rowspan=\"4\"\u003e \u003cp\u003eDay 28\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eS13T2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e30.60\u0026thinsp;\u0026plusmn;\u0026thinsp;0.41\u003csup\u003ebA\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e4.69\u0026thinsp;\u0026plusmn;\u0026thinsp;0.17\u003csup\u003ebB\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.583\u0026thinsp;\u0026plusmn;\u0026thinsp;0.007\u003csup\u003ebA\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eS13T4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e26.26\u0026thinsp;\u0026plusmn;\u0026thinsp;0.33\u003csup\u003ebB\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e5.00\u0026thinsp;\u0026plusmn;\u0026thinsp;0.15\u003csup\u003ebAB\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.544\u0026thinsp;\u0026plusmn;\u0026thinsp;0.005\u003csup\u003ebC\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eS15T2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e25.24\u0026thinsp;\u0026plusmn;\u0026thinsp;0.31\u003csup\u003eaC\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e5.09\u0026thinsp;\u0026plusmn;\u0026thinsp;0.14\u003csup\u003ebA\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.552\u0026thinsp;\u0026plusmn;\u0026thinsp;0.005\u003csup\u003ebBC\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eS15T4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e29.51\u0026thinsp;\u0026plusmn;\u0026thinsp;0.31\u003csup\u003ebD\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e4.892\u0026thinsp;\u0026plusmn;\u0026thinsp;0.14\u003csup\u003ebAB\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.560\u0026thinsp;\u0026plusmn;\u0026thinsp;0.005\u003csup\u003ebC\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colspan=\"5\" nameend=\"c5\" namest=\"c1\"\u003e \u003cp\u003eMeans\u0026thinsp;\u0026plusmn;\u0026thinsp;standard deviation (Tukey, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05). Different capital letters indicate significant differences within the same storage time. Different lowercase letters indicate significant differences within the same treatment across storage times.\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eAt day 0, the S15T4 treatment exhibited the highest yield stress (25.26 Pa), while the S13T4 treatment was more fluid with \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\sigma\\:}_{0}\\:\\)\u003c/span\u003e\u003c/span\u003e= 21.31 Pa. For all the samples studied, the yield stress increased after 28 days of storage at 22%, 23%, 0.8%, and 17% for S13T2, S13T4, S15T2, and S15T4, respectively. However, the S15T2 emulsion exhibited the lowest yield stress after 28 days of storage with no statistically significant difference throughout storage time. Similar values for Pickering emulsion formulations used in topical products were reported by Marto et al. (J. Marto, A. Ascenso, et al., 2016; J. Marto, L. F. Gouveia, B. G. Chiari, et al., 2016; J. Marto, L. F. Gouveia, L. Gon\u0026ccedil;alves, et al., 2016) and Chiari-Andr\u0026eacute;o et al. (2021), despite the use of different solid particles. Considering the particle size generated by each treatment, slight variations in particle size led to changes in the flow curves, with the smallest particles producing the lowest yield points and consistency values with the least change over time (J. Marto, L. F. Gouveia, L. Gon\u0026ccedil;alves, et al., 2016).\u003c/p\u003e \u003cp\u003eDespite the particle sizes generated being smaller (0.65\u0026ndash;0.69 \u0026micro;m) than those reported by J. Marto, L. F. Gouveia, B. G. Chiari, et al. (2016) for Pickering emulsions (5.54\u0026ndash;8.87 \u0026micro;m) with green coffee oil stabilized with particles of TiO\u003csub\u003e\u003cb\u003e2\u003c/b\u003e\u003c/sub\u003e, similar values and trends in flow curves are observed when using different types of solid particles. In this case, the similarity can be attributed to the type of emulsion, as factors such as concentration, particle interactions, and the formation of clusters of these particles increase viscosity and create a complex structure in the interfacial region, promoting greater stability, as previously reported (Franco et al., 1998; Mijovic et al., 2013; Shankar Banerjee et al., 2017).\u003c/p\u003e \u003cp\u003eAfter storage, all systems show a decrease in the shear stress required to generate fluid movement (Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). Those effects may result from structural changes in the emulsion packing, the formation of aligned structures between ZnO nanoparticles, and the presence of some texture modifiers such as xanthan gum (Santos et al., 2020; Trujillo-Cayado et al., 2016). Interestingly, this decrease does not affect the emulsion\u0026rsquo;s spreadability, as the previously described behavior remains unchanged.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec24\" class=\"Section3\"\u003e \u003ch2\u003e3.5.2 Oscillatory test (Frequency and amplitude sweep)\u003c/h2\u003e \u003cp\u003eThe amplitude sweep test of each emulsion shows that they have a linear viscoelastic region (LVR) between shear deformations of 0.01% to 0.1%, indicating the range of values where the moduli are independent of the shear stress (Miri, 2011). The strain γ\u0026thinsp;=\u0026thinsp;0.05% was used in all frequency sweep tests. The emulsion frequency sweep curves showed that the elastic and viscous moduli varied minimally over the tested range.\u003c/p\u003e \u003cp\u003eIn the frequency sweep tests, the moduli that make up the fluids can be observed (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e) where \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:G{\\prime\\:}\\)\u003c/span\u003e\u003c/span\u003e corresponds to the storage modulus or elastic part, and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:G{\\prime\\:}{\\prime\\:}\\)\u003c/span\u003e\u003c/span\u003e indicates the loss modulus or viscous part. By analyzing the arrangement of both moduli and their values, it is possible to determine the behavior of the structure, its stability, and consistency (Wong et al., 2021). According to Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e, all treatments showed higher values of \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:G{\\prime\\:}\\)\u003c/span\u003e\u003c/span\u003e than \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:G{\\prime\\:}{\\prime\\:}\\)\u003c/span\u003e\u003c/span\u003e, indicating a gel-like behavior in which the structure resists rupture and exhibits a more solid consistency. Hohl et al. (2016), Wei and Huang (2020), and Xu et al. (2021) associate this phenomenon with the presence of solid particles, which increase the system's viscosity and form a structural network that prevents loss of plasticity and enhances its stability.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe frequency sweeps of the different systems showed a decrease in both moduli across all treatments (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e). Some authors, including Franco et al. (1998), Isaac et al. (2013), and Fazilati et al. (2021), have attributed this decrease to a restructuring that the emulsion undergoes during this period due to the interaction between the phases and particles, leading to a repacking of the system by rearranging the droplets in the emulsion. However, this repacking did not alter the flow behavior of the systems, so their ability to flow and be dispersed on the skin remained constant during storage.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec25\" class=\"Section2\"\u003e \u003ch2\u003e3.6 \u003cem\u003eIn vitro\u003c/em\u003e sun protection factor\u003c/h2\u003e \u003cp\u003eThe four emulsions containing 23.63% ZnO exhibited SPF values\u0026thinsp;\u0026gt;\u0026thinsp;40, which is considered high protection by the European Commission (Communities, 2006). Ahmad Zaki et al. (2018) reported SPF values ranging from 40.37 to 40.16 for sunscreens with similar ZnO nanoparticle concentrations, values comparable to those obtained for a commercial sunscreen with 25% ZnO labeled as 50 FPS (56.38\u0026thinsp;\u0026plusmn;\u0026thinsp;1.96). These values can be attributed to the extensive contact area provided by the nanosized particles relative to their volume, resulting in greater optical interaction in the UVA and UVB regions. Other authors have also reported this phenomenon in sunscreens containing ZnO or TiO\u003csub\u003e\u003cb\u003e2\u003c/b\u003e\u003c/sub\u003e nanoparticles smaller than 100 nm (Ghamarpoor et al., 2023; Singh \u0026amp; Nanda, 2014).\u003c/p\u003e \u003cp\u003eSPF values were comparable across all emulsions (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e), suggesting that variations in time or processing conditions did not significantly affect the photoprotective capacity of each formulation. This behavior can be attributed to the use of ZnO nanoparticles, since the optical properties of the physical barrier are the primary source of protection, providing optimized SPF values (Chauhan et al., 2022; Elbrolesy et al., 2023; Singh \u0026amp; Nanda, 2014). However, the operating conditions used in our formulations were insufficient to modify SPF; therefore, further research is necessary to verify that these conditions do not affect this factor.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec26\" class=\"Section2\"\u003e \u003ch2\u003e3.7 Inhibition of ABTS\u003csup\u003e\u0026bull;+\u003c/sup\u003e radicals\u003c/h2\u003e \u003cp\u003eExposure to solar UV radiation facilitates the generation of free radicals, a factor that determines their harmful effects on human skin damage (de Jager et al., 2017). To evaluate the antioxidant potential of the emulsified systems, a spectroscopic assay was used to measure \u003cem\u003ein vitro\u003c/em\u003e, the elimination of the radical cation ABTS\u003csup\u003e\u003cb\u003e\u0026bull;\u003c/b\u003e\u003c/sup\u003e. At time 0, all emulsions exhibited similar inhibition by 16\u0026ndash;20%, except for the S15T4 emulsion, which had a significantly lower inhibition activity by 10% (p\u0026thinsp;\u0026lt;\u0026thinsp;0.05). After storage (day 28), the ABTS\u003csup\u003e\u003cb\u003e\u0026bull;+\u003c/b\u003e\u003c/sup\u003e radical inhibition of S13T2 emulsion significantly decays to 12% (p\u0026thinsp;\u0026lt;\u0026thinsp;0.05). On the other hand, S13T4 and S15T2 formulations display the same level of activities as day 0 (p\u0026thinsp;\u0026gt;\u0026thinsp;0.05) (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eSince the emulsion is composed of various components, with quercetin serving as a standout antioxidant, changes in its activity could be related to potential impacts on the speeds and times applied during its preparation (Hatahet et al., 2017). These potential effects include inadequate or uneven distribution of the wall material for encapsulation as well as an increase in temperature associated with the applied speeds, which could degrade the bioactive component (Chaaban et al., 2017; Tan \u0026amp; McClements, 2021).\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e shows that treatment S13T2 had the greatest decrease in ABTS\u003csup\u003e\u003cb\u003e+\u003c/b\u003e\u003c/sup\u003e radical inhibition. This behavior is attributed to poor encapsulation and the formation of a ZnO coating around the bioactive compound, influenced by the speed-time correlation (Wong et al., 2021; Yulianingsih \u0026amp; Gohtani, 2020). Conversely, treatment S15T4 showed the lowest inhibition rates, suggesting that increases in speed (rpm) and prolonged processing times may affect the antioxidant capacity of those systems. However, it is important to note that the emulsion obtained from the S15T4 system conserves its ABTS\u003csup\u003e\u003cb\u003e+\u003c/b\u003e\u003c/sup\u003e radical inhibition activity after 28 days of storage.\u003c/p\u003e \u003cp\u003eIn contrast, the S13T4 and S15T2 treatments did not show a statistically significant difference over time (p\u0026thinsp;\u0026gt;\u0026thinsp;0.05), suggesting that the compound may provide effective protection and enhanced structural conformation stability. This behavior could be associated with the relationship between speed and time that incorporates energy into the system, suggesting remarkable chemical stability that preserves quercetin\u0026rsquo;s bioactivity.\u003c/p\u003e \u003cp\u003eFor a quercetin concentration of 0.5 \u0026micro;g/mL, comparable to the one used in this study to evaluate antioxidant capacity using ABTS\u003csup\u003e\u003cb\u003e+\u003c/b\u003e\u003c/sup\u003e, previous studies, by Uzma et al. (2022) and Chittasupho et al. (2022), reported antioxidant activity values of ~\u0026thinsp;10\u0026ndash;15%. These values were lower than those obtained in this study for the S13T4 and S15T2 treatments, which were 19% and 20%, respectively. The observed increase could be attributed to a synergistic effect of quercetin and the antioxidant activity of the other components used in those emulsions. In this context, green coffee oil is known for its free radical inhibitory effects due to its chemical composition (Wagemaker et al., 2016; Wagemaker et al., 2011). Additionally, ZnO nanoparticles have been highlighted for their antioxidant properties, further enhancing free radical inhibition (Dias et al., 2023; Esquivel \u0026amp; Jim\u0026eacute;nez, 2012; Ostovar et al., 2023; Ramesh et al., 2022).\u003c/p\u003e \u003c/div\u003e"},{"header":"4 CONCLUSIONS","content":"\u003cp\u003eThe Pickering emulsions studied in this work had particle sizes greater than 100 nm and SPF values of ~\u0026thinsp;40, suggesting greater optical interaction in the UV-A and UV-B regions. These values were preserved throughout 28 days of storage, demonstrating the viability of these systems for use in pharmaceutical and cosmetical applications. Regarding the Turbiscan Stability Index, the results show that homogenization time influenced the physical stability of the emulsions, with the S13T4 and S15T2 systems being the most stable in terms of physicochemical properties. In particular, the S15T4 emulsion exhibited the lowest ABTS\u003csup\u003e\u003cb\u003e\u0026bull;+\u003c/b\u003e\u003c/sup\u003e inhibition values. Furthermore, over time, rheological properties showed shear-thinning behavior, with yield points within the preferred range for topical applications. Additionally, the HB model adequately described the flow behavior of the samples.\u003c/p\u003e \u003cp\u003eIn conclusion, among the emulsions tested, S15T2 had the best physicochemical and viscoelastic properties. It showed a lower increase in particle size, maintained yield stress, and higher antioxidant capacity and SPF values over 28 days of storage. This stability is associated with improved encapsulation and nanoparticle distribution at the interfacial region, which could be attributed to the emulsion\u0026rsquo;s structural integrity, making it a suitable vehicle for developing systems with great potential in pharmacology/cosmetology. At this point, the interactions between Pickering emulsions and the skin stratum layers remain to be determined to discard any associated toxicity of those formulations. It is also necessary to study quercetin's bioactivity in other encapsulated nanocarriers to evaluate its therapeutic potential for different skin conditions.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eACKNOWLEDGEMENTS\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors express their gratitude to Anton Paar for the donation of the rheometer. C\u0026eacute;sar Uriel Rodr\u0026iacute;guez-Fuentes and Ana Guadalupe Castillo-Olmos acknowledge the Consejo Nacional de Humanidades, Ciencia y Tecnolog\u0026iacute;a (CONAHCYT) for their master\u0026apos;s and doctoral scholarships, respectively.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAutor Contribution\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eC.U.R.F.: conceptualization, writing the original draft, data curation, investigation; A.G.C.O.: writing original draft and data curation; J.C.V.E: revision and editing; E.T.M.: editing and graphics; D.B.: revision, graphical treatment of results and discussion; Z.J.H.E.: editing and graphics; C.C.S.: conceptualization, supervision, investigation, funding acquisition, editing.\u0026nbsp;All authors have read and agreed to the published version of the manuscript\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis research was financed by Tecnol\u0026oacute;gico Nacional de M\u0026eacute;xico (TecNM).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData Availability\u003c/strong\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent for publication\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll authors have approved the final draft and submission of manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConflicts of Interest\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare there are no conflicts of interest.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eResearch Involving Humans and Animals Statement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthical Approval\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eInformed Consent\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eAgarwal, S., Jangir, L. 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Preparation and Optimization of O/W Emulsions Stabilized by Triglycerol Monolaurate for Curcumin Encapsulation. \u003cem\u003eMolecules\u003c/em\u003e,\u003cem\u003e\u0026nbsp;27\u003c/em\u003e(24). https://doi.org/10.3390/molecules27248861\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"discover-nano","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"narl","sideBox":"Learn more about [Discover Nano](https://www.springer.com/journal/11671)","snPcode":"11671","submissionUrl":"https://submission.nature.com/new-submission/11671/3","title":"Discover Nano","twitterHandle":"@SpringerOpen","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Discover Series","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"Pickering emulsion, sunscreen, quercetin, rheological behavior","lastPublishedDoi":"10.21203/rs.3.rs-8613997/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8613997/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eSolar ultraviolet (UV) radiation is the primary etiological factor in the development of several cutaneous malignancies, including carcinomas. In this context, the use of sunscreen formulations usually helps to prevent and reduce UV skin damage. The aim of this work was to explore the impact of the process conditions on some relevant physicochemical properties in sunscreens with a formulation based on quercetin Pickering emulsions stabilized with ZnO particles. Four formulations were prepared by controlling the speed and homogenization time using green coffee oil as the external phase and a mixture of stabilizers, water, and polyethylene glycol as the dissolution media. The stability of the emulsified systems was analyzed in terms of time after 28 days of storage by optical microscopy and digital image analysis to determine the mean particle size. The Turbiscan Stability Index (TSI), Sun Protection Factor (SPF), rheological behavior, and antioxidant activity were also evaluated. The system with the highest physical stability, minimal changes in rheological properties, and superior stability during storage time with respect to breakage phenomena was obtained at 15,000 rpm/2 min, with a SPF of ~\u0026thinsp;40 and exhibiting one of the highest antioxidant capacities compared to other treatments. This stability was constant during the evaluation period. With these results we established the optimal conditions for the potential development of sunscreens with desirable attributes to reduce the harmful effects of UV radiation in addition to establishing the encapsulation conditions of bioactive compounds and facilitating its scale-up.\u003c/p\u003e","manuscriptTitle":"Effect of process conditions on the stability and physicochemical properties of green coffee oil Pickering emulsions stabilized with ZnO nanoparticles for quercetin-enhanced sunscreen formulations","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-02-09 07:27:49","doi":"10.21203/rs.3.rs-8613997/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2026-02-16T14:42:07+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-02-13T02:16:10+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-02-12T06:16:30+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"314369832954922778838241893150191216267","date":"2026-02-09T14:47:27+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"144437135732799320867802473748302878901","date":"2026-02-05T01:14:20+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"331281370772730583229657217978709697350","date":"2026-02-04T14:51:15+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"129778544666270089325248127285990459854","date":"2026-02-04T07:45:12+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"226088707205961462085085461829500351403","date":"2026-02-04T05:50:56+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-02-03T17:14:12+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"","date":"2026-02-01T16:46:54+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-01-27T07:35:50+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2026-01-24T21:41:04+00:00","index":"","fulltext":""},{"type":"submitted","content":"Discover Nano","date":"2026-01-24T21:35:08+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"discover-nano","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"narl","sideBox":"Learn more about [Discover Nano](https://www.springer.com/journal/11671)","snPcode":"11671","submissionUrl":"https://submission.nature.com/new-submission/11671/3","title":"Discover Nano","twitterHandle":"@SpringerOpen","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Discover Series","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"ed53c8c4-0787-495d-8ae5-4d8c6bc604e8","owner":[],"postedDate":"February 9th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2026-05-04T10:24:51+00:00","versionOfRecord":[],"versionCreatedAt":"2026-02-09 07:27:49","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8613997","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8613997","identity":"rs-8613997","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}