Evaluation of conditions favorable for the enhanced stability of magnetite suspensions using visible spectroscopy

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The impact of various factors such as sonication power and time, use of pH modifiers, and surface preparation on the temporal evolution of transmittance of nanofluids was investigated by analyzing the vector distances of 350- and 650-nm spectra for each time. Nanofluids were prepared by dispersing magnetite nanoparticles in deionized water using an ultrasonic homogenizer in line with the factorial experimental design that generated 32 runs. The stability of nanofluid was evaluated via direct observation, visible spectroscopy, and Zeta potential measurement. Results indicated that the combination of unwashed nanoparticles, pH-modified nanoparticles using NaOH, and those sonicated at 400 W for 60 min enhanced the stability of nanofluids, resulting in a homogeneous and stable dispersion. These findings offer valuable insights into optimize the synthesis conditions of magnetite nanofluids with potential applications in fields such as magnetic hyperthermia and controlled drug delivery. Nanofluid stability magnetite visible spectroscopy Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 1. Introduction Nanofluids, an emerging class of materials, are obtained by suspending nanoparticles in a base fluid. Due to nanoparticle suspension, nanofluids exhibit enhanced thermal and physical properties than conventional fluids [ 1 ]. They also have high thermal conductivity and flow stability, making them ideal for use in the fields of medicine, energy, and electronics [ 2 ]. Magnetic nanofluids can be manipulated using magnetic fields and are used for treatments such as magnetic hyperthermia—an oncology treatment where magnetic nanoparticles (MNPs) selectively heat tumor tissues [ 3 ]. Ferrofluids are nanofluids formed by dispersing MNPs, typically magnetite (Fe 3 O 4 ), in base fluids such as water or oils. These nanoparticles have excellent magnetic field response and biocompatibility; therefore, their fluid properties can be dynamically modified under an external magnetic field [ 4 ]. In biomedical applications such as magnetic hyperthermia nanoparticles must be homogeneously dispersed to prevent therapeutic performance degradation resulting from particle agglomeration [ 5 ]. Ferrofluids are used in the medical field for cancer treatment such as in magnetic hyperthermia, wherein tumor cells are destroyed by the heat generated from nanoparticles under alternating magnetic fields [ 6 ]. Ferrofluids are used for controlled drug delivery, wherein nanoparticles are targeted to specific regions of the body [ 7 ]. In the industrial sector, ferrofluids are used in refrigeration systems owing to their high thermal conductivity as well as in magnetic seals and electronic devices to ensure their stability and control by precisely manipulating the magnetic field [ 8 ]. The thermal and magnetic properties of MNPs provide ferrofluids with versatility and wide applicability [ 9 ]. However, their effectiveness depends on the stability of suspended nanoparticles, which is their ability to remain uniformly dispersed in base fluids without settling or agglomerating [ 10 ], [ 11 ]. Nanoparticles can agglomerate due to phenomena such as gelation and flocculation that can considerably damage their functionality. In particular, MNPs must be uniformly dispersed for guaranteeing homogeneous heat distribution in magnetic hyperthermia [ 12 ]. In terms of biosafety, nanoparticle agglomeration may increase the likelihood of cellular toxicity and potentially elicit adverse immune responses within the organism [ 7 ]. Moreover, the homogeneity of ferrofluids used for magnetic hyperthermia enhances the predictive capacity of models employed for determining attainable theoretical temperatures under specified magnetic field intensities and inversion frequencies. Several strategies have been developed to synthesize ferrofluids with enhanced stability to effectively retain their properties such as mixing procedures, charge stabilization, and the use of surfactants [ 13 ], [ 14 ]. Nanofluids can be synthesized using various techniques such as mechanical stirring and ultrasonic agitation. Mechanical agitation is accessible and economical but generates insufficient amounts of energy to disintegrate nanoparticle aggregates. Therefore, mechanical agitation cannot be used for forming an initial suspension before employing other techniques such as particle washing and synthesis [ 15 ]. In contrast, ultrasonic agitation—whether using a probe (20–25 kHz) or an ultrasonic bath (40–45 kHz)—can effectively disintegrate aggregates and uniformly disperse nanoparticles due to cavitation effects [ 11 ]. However, some ultrasound energy dissipates as heat and considerably increases the system temperature. Moreover, power and stirring time significantly impact the nanofluid stability [ 16 ]. The Derjaguin–Landau–Verwey–Overbeek (DLVO) theory describes the kinetics of sedimentation in colloidal systems. It postulates that when the van der Waals potential dominates the electrostatic repulsion potential, particles tend to agglomerate and cause sedimentation [ 17 ]. The binding of functional groups to the nanoparticle surface or the presence of ions in the solution can alter the electrostatic repulsion conditions and move the system away from the isoelectric point. Consequently, the van der Waals forces negatively impact the nanoparticle stability [ 18 ]. The homogeneity and stability of nanofluids can be improved by adjusting the pH and using coatings [ 19 ]. Surface synthesis of nanoparticles is crucial for ensuring nanofluid stability. The residues on the nanoparticle surface can be removed using low concentration of strong acids, thereby rendering them more receptive to interaction with functional groups. As a result, the resulting nanofluid is stabilized due to enhanced electrostatic repulsion [ 20 ]. The stability of suspensions resulting from the electrostatic charges on the nanoparticle surface can be determining via Zeta potential measurements. However, this approach does not consider factors such as particle size or concentration and does not provide information on the temporal evolution of suspension behavior. Temporal nanofluid stability can be rapidly assessed via UV/vis spectroscopy to determine the dispersion and sedimentation tendency of nanoparticles over time [ 21 ]. Nanofluids exhibit low transmittance when the nanoparticles are well dispersed, indicating uniform distribution [ 22 ]. However, transmittance increases as the nanoparticles agglomerate and form large aggregates, thereby impacting the homogeneity of suspension. Therefore, surfactants are used and parameters such as pH and ultrasonication are altered to ensure low transmittance and high stability by promoting repulsion between particles and preventing agglomerate formation [ 23 ]. The effect of different treatments and formulations on the temporal stability of nanofluids can be qualitatively compared via transmittance spectroscopy. Although this approach reliably and indirectly measures the quality of dispersion, it does not yield absolute metrics and cannot be used to quantitatively determine the system stability. In this study, conditions favoring the stability of magnetite nanoparticles synthesized via the coprecipitation of iron salts were evaluated for enhancing their efficiency in various applications and ensuing a homogeneous nanoparticle dispersion over time. To this end, the impact of variables such as power and sonication time, pH modification, and surface preparation on the time-dependent transmittance of nanofluids was investigated to determine the optimal conditions for ensuring their stability. Several techniques were employed for evaluating the stability of suspensions. These evaluations were correlated with the vector analysis of UV/vis spectroscopy results for obtaining quantitative stability indicators that described the temporal behavior of suspensions. 2. Methodology 2.1 Materials MNPs were synthesized using these reagents: iron (III) chloride hexahydrate (2FeCl₃·6H₂O, 97%, Sigma-Aldrich), iron (II) chloride tetrahydrate (FeCl₂·4H₂O, 99%, Sigma-Aldrich), and a sodium hydroxide solution (NaOH, 10% w/v, Sigma-Aldrich). Hydrochloric acid (37% wt, Sigma-Aldrich) and ethanol (99.5% v/v, PanReac) were used for surface preparation and subsequent washes, respectively. 2.2 Synthesis Fe 3 O 4 nanoparticles were synthesized via ultrasound-assisted coprecipitation, following the following (Eq. 1 ): $$\:2Fe{Cl}_{3}\cdot\:6{H}_{2}O+Fe{Cl}_{2}\cdot\:4{H}_{2}O+8NaOH\:\to\:\:{Fe}_{3}{O}_{4}+20{H}_{2}O+8Na{Cl}_{2}Fe{Cl}_{3}·6H₂O\:+\:FeCl₂·4H₂O\:+\:8NaOH$$ 1 This reaction was standardized for preparing 2 g of Fe 3 O 4 . Specifically, 4.814 g of FeCl₃·6H₂O (Fe³⁺ solution) and 1.734 g of FeCl₂·4H₂O (Fe²⁺ solution) were dissolved separately in 100 mL of deionized water. This Fe²⁺ solution was added to Fe³⁺ solution under constant stirring. The mixture was then transferred to a custom-made reactor designed to accommodate a Cole Palmer 500 W probe sonicator and a nitrogen bubbling system. The sonicator was operated in continuous mode at 50% amplitude and 90°C temperature. At this stage, 27.6 mL of NaOH at 10% was added using a syringe pump set to a flow rate of 50 mL/h. The total energy supplied to the system was 109,366 J over ~ 2 h. The solution color changed to black upon adding NaOH, indicating possible magnetite formation. Magnetite was recovered after five washing cycles with deionized water, followed by magnetic separation. The resulting powders were dried in a muffle furnace at 60°C for 24 h. 2.3 Characterization of nanoparticles X-ray diffraction (XRD) was performed using a Malvern-PANalytical Model Empyean diffractometer equipped with a Cu source (λ = 1.541874 Å) and operated at 30 kV and 10 mA, with a step size of 0.02° and a time per step of 118 s. Morphological analysis of nanoparticles was performed using a scanning transmission electron microscope manufactured by Thermo Fisher Scientific (model Scios 2 LoVac). The samples were prepared via ultrasonic dispersion in anhydrous ethanol, followed by deposition on a support film to obtain high-precision images. These images were then processed using the ImageJ software to determine the particle size distribution. To determine saturation magnetization and superparamagnetic behavior of nanoparticles, magnetization versus applied field curves were constructed in a Quantum Design brand vibrating sample magnetometer at room temperature. 2.4 Nanofluid preparation Nanofluids were prepared by dispersing 25 mg of as-synthetized nanoparticles in 50 ml of deionized water using a TU-1000E4 ultrasonic homogenizer (XIAN TOPTION INSTRUMENT CO., China) with a frequency of 25 kHz. A factorial experiment was conducted to investigate the effects of washing with HCl, pH modification with NaOH, and variations in power and sonication time on nanofluid preparation. Figure 1 shows the experimental design. To evaluate the impact of surface treatment on nanofluid stability, half of the experiments included washing the nanoparticles with a 1:20 HCl solution mixed with deionized water. In this experiment, 200 mg of synthesized nanoparticles was sonicated for 20 min in 100 ml of acidic solution using a power output of 400 W. Then, four consecutive washes were performed with ethanol to remove residual acid solution. Figure 2 shows the surface preparation process. To analyze the impact of electrostatic charge stabilization, the nanofluid pH was adjusted in half of the formulations by adding 40 µl of 1 M NaOH into deionized water before adding nanoparticles. The remaining suspensions generated upon adding the nanoparticles was maintained at a natural pH. The influence of sonication power on the nanofluid stability was assessed by applying powers of 200 and 400 W. The impact of sonication time on the nanofluid stability was also investigated by extracting 3 ml nanofluid samples at 15-, 30-, 60-, and 120-min intervals from the outset of sonication. The factors evaluated were combined in a factorial design with the levels described in Fig. 1 , and four nanofluid formulations were prepared. Each formulation was tested under specific preparation and agitation conditions, resulting in a total of 32 experimental runs (Table 1 ). Table 1 Formulations used for experimental runs Formulation Surface preparation Nanofluid preparation Agitation power (W) Agitation time (min) 1 HCl washed pH 9 with NaOH 200 15 30 60 120 400 15 30 60 120 2 Natural pH 200 15 30 60 120 400 15 30 60 120 3 Unwashed pH 9 with NaOH 200 15 30 60 120 400 15 30 60 120 4 Natural pH 200 15 30 60 120 400 15 30 60 120 2.5 Evaluation of nanofluid stability Three principal methodologies were used to assess the stability of nanofluid formulations: direct observation, visible spectroscopy, and Zeta potential measurement. Figure 3 shows the experimental design of spectrophotometry. For quantitative analysis, a Vis A721 spectrophotometer that can measure wavelengths of 350–1020 nm with a resolution of ± 2 nm was used. The transmitted light spectra of nanofluids were recorded at specific wavelengths at the outset and at 5-min intervals for the first hour, after which the intervals were extended to 10 min for the subsequent hour; 19 measurements in total were conducted. The temporal evolution of stability was evaluated by determining the rate of change of vector distances between a reference spectrum and the spectra obtained at different times. From the transmitted light spectra, 19 vectors were generated for each sample and each vector corresponded to a specific measurement time. The transmittance values were then used as vector components, and the vector distance with respect to initial spectrum was calculated to analyze the temporal variations in transmittance. The rate of change of transmittance was used as a quantitative indicator of nanofluid stability, wherein lower values corresponded to enhanced stability of nanofluids. The Zeta potential and particle sizes of four fluid formulation using the Malvern Instruments Zetasizer Nano Series. The suspensions were derived via sonication at 400 W for 120 min. 3. Results and analysis Figure 4 shows the XRD patterns of powders synthesized via ultrasound-assisted coprecipitation. The diffraction peaks corresponding to the (200), (311), (400), (422), (511), (440), and (533) planes were successfully identified, which matched the standard pattern for magnetite (PDF card 01-075-0449). The crystallite size was estimated using the Debye–Scherrer equation (Eq. 2 ): $$\:{D}_{p}=\frac{K\lambda\:}{\beta\:cos\theta\:},$$ 2 where Dp is the crystallite size, β is the full width at half maximum of the diffraction peak, λ is the X-ray wavelength of 1.5406 Å, K is the Scherrer constant, and θ is the Bragg angle. The estimated crystallite size of magnetite nanoparticles was 23.54 nm. Figure 5 shows the scanning transmission electron microscopy images at 25,000x magnification, which revealed the presence of magnetite nanoparticles with predominantly granular morphologies. Image analysis was performed using an open-source software, and average particle size was estimated as 19.24 ± 6.8 nm; this finding was consistent with the crystallite size obtained via XRD measurements. Figure 6 shows the M–H magnetization curve for Fe 3 O 4 nanoparticles. The curve shows minimal coercivity and the absence of a hysteresis loop that indicate superparamagnetic behavior. The measured saturation magnetization was 67.4 emu/g, lower than that reported in the literature for bulk magnetite [ 24 ]. This was because of high surface-to-volume ratio of the nanoparticles, which affected the orientation of magnetic moments due to super-exchange and dipolar interactions. To identify favorable conditions for stability in applied treatments, transmittance values obtained via spectroscopy at wavelengths from 350 to 750 nm were compared. Figure 7 shows the corresponding results for each nanofluid formulation, where the solid and dashed lines represent the transmittance values measured immediately after the interruption of sonication and the values recorded 120 min later, respectively. As shown in Fig. 7 , the dispersion and stability of nanofluids exhibited different behaviors depending on the preparation conditions, stirring power, and surface state of nanoparticles. These behaviors were classified into three main categories. Formulations with unwashed nanoparticles and a natural pH exhibited low initial transmittance values, indicating an effective initial dispersion with a high number of individual particles in suspension. After 120 min, the transmittance reached its maximum value across the majority of spectrum, indicating a significant degree of particle sedimentation. This indicated that although good initial dispersion was achieved, the nanofluids exhibited poor stability due to the formation of clusters and their subsequent precipitation. The unwashed nanoparticle formulations with NaOH sonicated at 200 W and washed nanoparticle formulations (independent of the preparation method or stirring power) exhibited high initial transmittance values at 650–750 nm. This indicated that a low number of individual particles were dispersed in suspension, probably due to cluster formation. However, the variation between the initial and final transmittance values was minimal, indicating good temporal stability with slight particle sedimentation. This could also be due to less interaction among nanoparticles, probably due to surface preparation or specific chemical conditions. Formulations with unwashed nanoparticles prepared at a modified pH with NaOH and sonicated at 400 W exhibited optimal performance, with low initial transmittance values. This indicated that several nanoparticles were highly dispersed in the suspension. Moreover, the variation in transmittance values after 120 min was minimal, suggesting that the nanoparticles remained stable during analysis. Thus, the preparation conditions enabled high initial dispersion and adequate stabilization of the suspended particles, confirming that these conditions were favorable for achieving good dispersion and temporal stability. The temporal evolution in the transmittance of the nanofluid formulations across each category at the end of 120-min evaluation are shown in Fig. 8 . As shown in Fig. 8 , the UW + Nat pH + 400 W formulation appeared transparent after 120 min and precipitates are formed at the bottom of the cuvette. The UW + Nat pH + 200 W formulation showed large clusters in suspension. The UW + NaOH + 400 W formulation initially exhibited a reddish hue at the 15-min preparation stage, which gradually diminished with increasing sonication time, and turned darker after 60 and 120 min of sonication; this color remained constant throughout the observation period. The HCl + NaOH + 400 W formulation retained a reddish hue throughout the sonication, which is representative of the other samples with similar characteristics of surface preparation and pH. The findings for the HCl + NaOH + 400 W formulation corresponded with the high transmittance levels reported at 650–750 nm. Transmittance vectors for each time point were defined from the transmittance values determined for each wavelength. The vector distance at time t was calculated as the distance between the transmittance vector corresponding to t and the initial transmittance vector (at t = 0). As a result, the variation in the vector distance with the temporal changes in the nanofluid stability was determined. The vector components corresponding to wavelengths of 700 and 750 nm were excluded from subsequent analysis due to low variability. This was because some samples reached maximum transmittance values from the outset of measurements, which did not accurately reflect their temporal stability. Figure 9 a shows the vector distance at t = 120 min, and Fig. 9 b shows the rate of change of vector distance in the temporal observation window. The data demonstrate a linear fit with an R 2 value of ≥ 0.95. Figure 9 a illustrates the diminution of transmittance vector distance with increasing sonication time, indicating a reduction in both natural pH and NaOH usage. Notably, NaOH treatments exhibit superior outcomes in comparison to natural pH, signifying the beneficial effect of alkaline conditions on system stability. In addition, the data show that the addition of 400 watts of ultrasonic power results in a greater reduction in transmittance vector distance, especially under NaOH conditions. This suggests that increasing power can enhance the efficiency of the treatment. On the other hand, Fig. 9 b indicates that the rate of vector change, which is higher in the initial treatments, tends to stabilize with prolonged sonication times. This suggests that after 60 minutes, the benefits from the addition of power may reach a point where they are saturated. This phenomenon is particularly evident in the UW + NaOH + 400W treatment, which is identified as the optimal condition by achieving the lowest vector distance and adequate rate of change, thereby optimizing stability and efficiency.Finally, the analysis indicates that extending sonication up to 120 minutes does not yield significant additional benefits, underscoring the importance of limiting treatment time to 60 minutes to maximize energy savings without compromising results. Therefore, the results obtained after 60 min of sonication can be analyzed further as the nanofluid stability did not improve considerably beyond this sonication time. Therefore, Fig. 10 only shows the sequences of vector distances obtained for the formulations prepared with 60 min of sonication. Figure 10 shows lower vector distances of the UW + NaOH + 400 W formulation during analysis, which gradually reduced toward the end of evaluation. Figure 11 shows the stability results against the derived Zeta potential and particle size distribution. Figure 11 shows that the hydrodynamic diameter of nanoparticles decreased when the pH was modified of 6 to 9 without surface treatment. In contrast, nanoparticles subjected to surface modification with HCl washing showed comparable hydrodynamic diameters at natural pH and modified pH with NaOH. Zeta potential analysis revealed that all the cases exhibited favorable conditions for charge stability in the suspension, with potentials exceeding 30 mV. The results associated with the UW + Nat pH formulation could not be adequately interpreted because of low stability of suspension. 4. Conclusions The stability of magnetite nanoparticle suspensions was considerably influenced by their synthesis conditions. The use of HCl for washing reduced surface impurities, whereas pH modification with NaOH is conducive to the optimization of electrostatic stabilization. The combination of sonication at a power of 400 W for 60 min ensured uniform dispersion and enhanced temporal stability of nanoparticles, thereby preventing agglomerations and sedimentation. Spectrophotometric analysis of transmittance and vector distance calculated with respect to the initial time proved to be an effective tool for evaluating the nanoparticle dispersion and nanofluid stability. Using these methods, variations in the homogeneity of suspensions were quantitatively measured and the conditions favoring stable nanoparticle distribution were accurately identified. The optimization of nanofluid synthesis conditions is essential for biomedical and industrial applications. To ensure their efficient performance in magnetic hyperthermia, drug delivery, and other applications, homogeneous nanoparticle dispersion and magnetic manipulability are essential to ensure the efficacy and safety of ferrofluids. The findings of this study represent a significant advance in the design of functional suspensions of magnetite nanoparticles. Declarations Acknowledgements The authors express their profound gratitude to Universidad Nacional de Colombia, which provided the financial support for this study through the "Convocatoria Nacional para el Fomento de Alianzas Estratégicas Interdisciplinarias" program (Project 57864). Author contributions Laura Álvarez-Gil: Conceptualization, Methodology, Investigation, Writing – original draft, Validation, Writing – review & editing, Formal analysis, Visualization. Gloria Soto-Calle: Investigation, Validation, Writing – original draft, Visualization. Alex Lopera: Visualization, Validation, Writing – original draft, Writing – review & editing, Formal analysis. Alcides Becerra: Investigation, Validation. Sandra Navarro : Investigation, Validation. Néstor Ricardo Rojas-Reyes: Supervision, Writing – review & editing. Declaration of interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Funding This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. Data statement Data will be made available on request. References H. Ben Bacha, N. Ullah, A. Hamid, and N. A. 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Sun, Z. Ren, R. Wang, W. Chen, and C. Chen, “Magnetite hollow spheres: Solution synthesis, phase formation and magnetic property,” Journal of Nanoparticle Research , vol. 13, no. 1, pp. 213–220, Jan. 2011, doi: 10.1007/s11051-010-0020-5. Additional Declarations No competing interests reported. Supplementary Files GraphicalAbstract.jpeg Cite Share Download PDF Status: Under Review Version 1 posted Editorial decision: Revision requested 06 Jun, 2025 Reviews received at journal 05 Jun, 2025 Reviews received at journal 29 May, 2025 Reviewers agreed at journal 15 May, 2025 Reviewers agreed at journal 15 May, 2025 Reviewers invited by journal 02 May, 2025 Editor assigned by journal 06 Apr, 2025 Submission checks completed at journal 06 Apr, 2025 First submitted to journal 31 Mar, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-6345991","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":439201580,"identity":"24a75ba4-5fe9-4fe0-887b-ea07bdd14c63","order_by":0,"name":"Laura Álvarez-Gil","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA8ElEQVRIiWNgGAWjYBACPgYGxgMQJjOItgDiBPxa2IAYqoUNpFSCJC08BkRqYT/84DBPzT05+f4136QLaiQY+NlzDBh+VODRwpNmcJjnWLEx44y326RnHJNgkOx5Y8DYcwaPFgkGg4Mz2BISmyXObpPmAXFv5BgwM7bh08L+4eCMfwn1bRJnnknz/JNgsCeshcfgwMe2hAQe/h42ad42oC0ShLTw5BQc+NiXYDhDgs3YmrdPggdoW8FBfH7hZz++8UHCtwR5+f7DD2/zfLOR429P3vgAX4ghgEQCmOIBEQeI0QC0j0h1o2AUjIJRMPIAALgXSK7V6tKWAAAAAElFTkSuQmCC","orcid":"","institution":"Universidad Nacional de Colombia","correspondingAuthor":true,"prefix":"","firstName":"Laura","middleName":"","lastName":"Álvarez-Gil","suffix":""},{"id":439201581,"identity":"732a7a8c-3cb2-4113-ae72-f0e12121a90a","order_by":1,"name":"Gloria Soto-Calle","email":"","orcid":"","institution":"Universidad Nacional de Colombia","correspondingAuthor":false,"prefix":"","firstName":"Gloria","middleName":"","lastName":"Soto-Calle","suffix":""},{"id":439201582,"identity":"e60e6303-6092-4502-be6d-104b20313b36","order_by":2,"name":"Alex Lopera","email":"","orcid":"","institution":"Universidad Nacional de Colombia– Sede Medellín","correspondingAuthor":false,"prefix":"","firstName":"Alex","middleName":"","lastName":"Lopera","suffix":""},{"id":439201583,"identity":"e7120cc2-4c37-4574-b2d2-a7af8f5d499b","order_by":3,"name":"Alcides Becerra","email":"","orcid":"","institution":"Universidad Nacional de Colombia","correspondingAuthor":false,"prefix":"","firstName":"Alcides","middleName":"","lastName":"Becerra","suffix":""},{"id":439201584,"identity":"e91883b1-1530-4e9e-b945-6bc543e4e52c","order_by":4,"name":"Sandra Navarro","email":"","orcid":"","institution":"Cecoltec Services","correspondingAuthor":false,"prefix":"","firstName":"Sandra","middleName":"","lastName":"Navarro","suffix":""},{"id":439201585,"identity":"ff385067-55f1-47b7-8e72-39a5df9edb38","order_by":5,"name":"Néstor Ricardo Rojas-Reyes","email":"","orcid":"","institution":"Universidad Nacional de Colombia","correspondingAuthor":false,"prefix":"","firstName":"Néstor","middleName":"Ricardo","lastName":"Rojas-Reyes","suffix":""}],"badges":[],"createdAt":"2025-03-31 15:08:09","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6345991/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6345991/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":80599493,"identity":"f3813f3d-d335-42a7-a7a4-6b386742c3f3","added_by":"auto","created_at":"2025-04-15 05:04:03","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":462828,"visible":true,"origin":"","legend":"\u003cp\u003eExperimental design for nanofluid preparation. Created in BioRender (\u003ca href=\"https://biorender.com/m01s612\"\u003ehttps://BioRender.com/m01s612\u003c/a\u003e)\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-6345991/v1/c939da6e97e24ad7d0cecb55.png"},{"id":80600382,"identity":"d94e6708-0b82-46b9-8a61-3648e652db9a","added_by":"auto","created_at":"2025-04-15 05:12:03","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":349167,"visible":true,"origin":"","legend":"\u003cp\u003eSurface preparation. Created in BioRender (\u003ca href=\"https://biorender.com/q14l070\" target=\"_blank\"\u003ehttps://BioRender.com/q14l070\u003c/a\u003e)\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-6345991/v1/a3ed202d20fa52f583b60625.png"},{"id":80599498,"identity":"1bb22612-c5e3-451e-88db-3a9e6badbfbb","added_by":"auto","created_at":"2025-04-15 05:04:03","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":572493,"visible":true,"origin":"","legend":"\u003cp\u003eVis spectrophotometer used for nanofluid stability characterization. Created in BioRender (\u003ca href=\"https://biorender.com/i19u372\"\u003ehttps://BioRender.com/i19u372\u003c/a\u003e)\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-6345991/v1/7871b963a1e1211fed3570d4.png"},{"id":80601639,"identity":"fb8cf3ba-aa23-4bc0-9666-ec0c21623062","added_by":"auto","created_at":"2025-04-15 05:36:03","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":130310,"visible":true,"origin":"","legend":"\u003cp\u003eX-ray diffraction patterns of synthesized powders\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-6345991/v1/a86dfdbef4b2fb9c5c18fada.png"},{"id":80599502,"identity":"11830be0-89e7-4970-9d73-39632689aeb0","added_by":"auto","created_at":"2025-04-15 05:04:03","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":916719,"visible":true,"origin":"","legend":"\u003cp\u003eScanning transmission electron microscopy (STEM) image\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-6345991/v1/021817b78a5a589ec9b37f2b.png"},{"id":80599500,"identity":"b6a20209-d224-478a-b763-e3de37f23a7d","added_by":"auto","created_at":"2025-04-15 05:04:03","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":154733,"visible":true,"origin":"","legend":"\u003cp\u003eM–H magnetization curve for Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4 \u003c/sub\u003enanoparticles\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-6345991/v1/bc44f89fad5b87b3fc8db308.png"},{"id":80601641,"identity":"68e6551c-6a67-42ee-a10d-8879bc62e5b3","added_by":"auto","created_at":"2025-04-15 05:36:03","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":1229829,"visible":true,"origin":"","legend":"\u003cp\u003eVariation in the transmittance intensity for various nanofluid formulations\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-6345991/v1/fb064557b78d2cb5a3e35bc0.png"},{"id":80599503,"identity":"27a12312-67c0-45b0-ade7-5e1802d35b91","added_by":"auto","created_at":"2025-04-15 05:04:03","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":1515620,"visible":true,"origin":"","legend":"\u003cp\u003ePhotographs of Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e nanofluid formulations 120 min after suspension during sonication\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-6345991/v1/5ec64068c90e32d39fd82fdd.png"},{"id":80600391,"identity":"4ad2d42f-d400-4469-a965-9ff25b12d167","added_by":"auto","created_at":"2025-04-15 05:12:03","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":351712,"visible":true,"origin":"","legend":"\u003cp\u003eStability indicators based on transmittance between 350 and 650 nm. a) Final transmittance vector distance. b) Transmittance vector change rate.\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-6345991/v1/b7245cef546d07c71cd0ad22.png"},{"id":80599531,"identity":"6573fe88-d5e6-4b05-a5d9-8dda229fe4d8","added_by":"auto","created_at":"2025-04-15 05:04:04","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":317984,"visible":true,"origin":"","legend":"\u003cp\u003eTransmittance vector distances of formulation sonicated for 60 min\u003c/p\u003e","description":"","filename":"10.png","url":"https://assets-eu.researchsquare.com/files/rs-6345991/v1/60e6ffe1a8fc21fa1b692379.png"},{"id":80599507,"identity":"bbb9e5f7-8c6f-41a7-8dbb-f30d56216264","added_by":"auto","created_at":"2025-04-15 05:04:03","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":178998,"visible":true,"origin":"","legend":"\u003cp\u003eZeta potentials and hydrodynamic diameters of nanoparticles after 120-min sonication at 400 W\u003c/p\u003e","description":"","filename":"11.png","url":"https://assets-eu.researchsquare.com/files/rs-6345991/v1/7ce5cbff2c892f02267f87bd.png"},{"id":80599491,"identity":"e9aebe59-03d3-487a-aa5e-55bbad6cb031","added_by":"auto","created_at":"2025-04-15 05:04:03","extension":"jpeg","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":130942,"visible":true,"origin":"","legend":"","description":"","filename":"GraphicalAbstract.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6345991/v1/3a3890227a1462453eb0aa93.jpeg"}],"financialInterests":"No competing interests reported.","formattedTitle":"Evaluation of conditions favorable for the enhanced stability of magnetite suspensions using visible spectroscopy","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eNanofluids, an emerging class of materials, are obtained by suspending nanoparticles in a base fluid. Due to nanoparticle suspension, nanofluids exhibit enhanced thermal and physical properties than conventional fluids [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. They also have high thermal conductivity and flow stability, making them ideal for use in the fields of medicine, energy, and electronics [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. Magnetic nanofluids can be manipulated using magnetic fields and are used for treatments such as magnetic hyperthermia\u0026mdash;an oncology treatment where magnetic nanoparticles (MNPs) selectively heat tumor tissues [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eFerrofluids are nanofluids formed by dispersing MNPs, typically magnetite (Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e), in base fluids such as water or oils. These nanoparticles have excellent magnetic field response and biocompatibility; therefore, their fluid properties can be dynamically modified under an external magnetic field [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. In biomedical applications such as magnetic hyperthermia nanoparticles must be homogeneously dispersed to prevent therapeutic performance degradation resulting from particle agglomeration [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eFerrofluids are used in the medical field for cancer treatment such as in magnetic hyperthermia, wherein tumor cells are destroyed by the heat generated from nanoparticles under alternating magnetic fields [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. Ferrofluids are used for controlled drug delivery, wherein nanoparticles are targeted to specific regions of the body [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. In the industrial sector, ferrofluids are used in refrigeration systems owing to their high thermal conductivity as well as in magnetic seals and electronic devices to ensure their stability and control by precisely manipulating the magnetic field [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. The thermal and magnetic properties of MNPs provide ferrofluids with versatility and wide applicability [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. However, their effectiveness depends on the stability of suspended nanoparticles, which is their ability to remain uniformly dispersed in base fluids without settling or agglomerating [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e], [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. Nanoparticles can agglomerate due to phenomena such as gelation and flocculation that can considerably damage their functionality. In particular, MNPs must be uniformly dispersed for guaranteeing homogeneous heat distribution in magnetic hyperthermia [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. In terms of biosafety, nanoparticle agglomeration may increase the likelihood of cellular toxicity and potentially elicit adverse immune responses within the organism [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. Moreover, the homogeneity of ferrofluids used for magnetic hyperthermia enhances the predictive capacity of models employed for determining attainable theoretical temperatures under specified magnetic field intensities and inversion frequencies.\u003c/p\u003e \u003cp\u003eSeveral strategies have been developed to synthesize ferrofluids with enhanced stability to effectively retain their properties such as mixing procedures, charge stabilization, and the use of surfactants [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e], [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. Nanofluids can be synthesized using various techniques such as mechanical stirring and ultrasonic agitation. Mechanical agitation is accessible and economical but generates insufficient amounts of energy to disintegrate nanoparticle aggregates. Therefore, mechanical agitation cannot be used for forming an initial suspension before employing other techniques such as particle washing and synthesis [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. In contrast, ultrasonic agitation\u0026mdash;whether using a probe (20\u0026ndash;25 kHz) or an ultrasonic bath (40\u0026ndash;45 kHz)\u0026mdash;can effectively disintegrate aggregates and uniformly disperse nanoparticles due to cavitation effects [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. However, some ultrasound energy dissipates as heat and considerably increases the system temperature. Moreover, power and stirring time significantly impact the nanofluid stability [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe Derjaguin\u0026ndash;Landau\u0026ndash;Verwey\u0026ndash;Overbeek (DLVO) theory describes the kinetics of sedimentation in colloidal systems. It postulates that when the van der Waals potential dominates the electrostatic repulsion potential, particles tend to agglomerate and cause sedimentation [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. The binding of functional groups to the nanoparticle surface or the presence of ions in the solution can alter the electrostatic repulsion conditions and move the system away from the isoelectric point. Consequently, the van der Waals forces negatively impact the nanoparticle stability [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. The homogeneity and stability of nanofluids can be improved by adjusting the pH and using coatings [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eSurface synthesis of nanoparticles is crucial for ensuring nanofluid stability. The residues on the nanoparticle surface can be removed using low concentration of strong acids, thereby rendering them more receptive to interaction with functional groups. As a result, the resulting nanofluid is stabilized due to enhanced electrostatic repulsion [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. The stability of suspensions resulting from the electrostatic charges on the nanoparticle surface can be determining via Zeta potential measurements. However, this approach does not consider factors such as particle size or concentration and does not provide information on the temporal evolution of suspension behavior.\u003c/p\u003e \u003cp\u003eTemporal nanofluid stability can be rapidly assessed via UV/vis spectroscopy to determine the dispersion and sedimentation tendency of nanoparticles over time [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. Nanofluids exhibit low transmittance when the nanoparticles are well dispersed, indicating uniform distribution [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. However, transmittance increases as the nanoparticles agglomerate and form large aggregates, thereby impacting the homogeneity of suspension. Therefore, surfactants are used and parameters such as pH and ultrasonication are altered to ensure low transmittance and high stability by promoting repulsion between particles and preventing agglomerate formation [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. The effect of different treatments and formulations on the temporal stability of nanofluids can be qualitatively compared via transmittance spectroscopy. Although this approach reliably and indirectly measures the quality of dispersion, it does not yield absolute metrics and cannot be used to quantitatively determine the system stability.\u003c/p\u003e \u003cp\u003eIn this study, conditions favoring the stability of magnetite nanoparticles synthesized via the coprecipitation of iron salts were evaluated for enhancing their efficiency in various applications and ensuing a homogeneous nanoparticle dispersion over time. To this end, the impact of variables such as power and sonication time, pH modification, and surface preparation on the time-dependent transmittance of nanofluids was investigated to determine the optimal conditions for ensuring their stability. Several techniques were employed for evaluating the stability of suspensions. These evaluations were correlated with the vector analysis of UV/vis spectroscopy results for obtaining quantitative stability indicators that described the temporal behavior of suspensions.\u003c/p\u003e"},{"header":"2. Methodology","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Materials\u003c/h2\u003e \u003cp\u003eMNPs were synthesized using these reagents: iron (III) chloride hexahydrate (2FeCl₃\u0026middot;6H₂O, 97%, Sigma-Aldrich), iron (II) chloride tetrahydrate (FeCl₂\u0026middot;4H₂O, 99%, Sigma-Aldrich), and a sodium hydroxide solution (NaOH, 10% w/v, Sigma-Aldrich). Hydrochloric acid (37% wt, Sigma-Aldrich) and ethanol (99.5% v/v, PanReac) were used for surface preparation and subsequent washes, respectively.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Synthesis\u003c/h2\u003e \u003cp\u003eFe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e nanoparticles were synthesized via ultrasound-assisted coprecipitation, following the following (Eq.\u0026nbsp;\u003cspan refid=\"Equ1\" class=\"InternalRef\"\u003e1\u003c/span\u003e):\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:2Fe{Cl}_{3}\\cdot\\:6{H}_{2}O+Fe{Cl}_{2}\\cdot\\:4{H}_{2}O+8NaOH\\:\\to\\:\\:{Fe}_{3}{O}_{4}+20{H}_{2}O+8Na{Cl}_{2}Fe{Cl}_{3}\u0026middot;6H₂O\\:+\\:FeCl₂\u0026middot;4H₂O\\:+\\:8NaOH$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eThis reaction was standardized for preparing 2 g of Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e. Specifically, 4.814 g of FeCl₃\u0026middot;6H₂O (Fe\u0026sup3;⁺ solution) and 1.734 g of FeCl₂\u0026middot;4H₂O (Fe\u0026sup2;⁺ solution) were dissolved separately in 100 mL of deionized water. This Fe\u0026sup2;⁺ solution was added to Fe\u0026sup3;⁺ solution under constant stirring. The mixture was then transferred to a custom-made reactor designed to accommodate a Cole Palmer 500 W probe sonicator and a nitrogen bubbling system.\u003c/p\u003e \u003cp\u003eThe sonicator was operated in continuous mode at 50% amplitude and 90\u0026deg;C temperature. At this stage, 27.6 mL of NaOH at 10% was added using a syringe pump set to a flow rate of 50 mL/h. The total energy supplied to the system was 109,366 J over ~\u0026thinsp;2 h. The solution color changed to black upon adding NaOH, indicating possible magnetite formation.\u003c/p\u003e \u003cp\u003eMagnetite was recovered after five washing cycles with deionized water, followed by magnetic separation. The resulting powders were dried in a muffle furnace at 60\u0026deg;C for 24 h.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Characterization of nanoparticles\u003c/h2\u003e \u003cp\u003eX-ray diffraction (XRD) was performed using a Malvern-PANalytical Model Empyean diffractometer equipped with a Cu source (λ\u0026thinsp;=\u0026thinsp;1.541874 \u0026Aring;) and operated at 30 kV and 10 mA, with a step size of 0.02\u0026deg; and a time per step of 118 s. Morphological analysis of nanoparticles was performed using a scanning transmission electron microscope manufactured by Thermo Fisher Scientific (model Scios 2 LoVac). The samples were prepared via ultrasonic dispersion in anhydrous ethanol, followed by deposition on a support film to obtain high-precision images. These images were then processed using the ImageJ software to determine the particle size distribution. To determine saturation magnetization and superparamagnetic behavior of nanoparticles, magnetization versus applied field curves were constructed in a Quantum Design brand vibrating sample magnetometer at room temperature.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.4 Nanofluid preparation\u003c/h2\u003e \u003cp\u003eNanofluids were prepared by dispersing 25 mg of as-synthetized nanoparticles in 50 ml of deionized water using a TU-1000E4 ultrasonic homogenizer (XIAN TOPTION INSTRUMENT CO., China) with a frequency of 25 kHz. A factorial experiment was conducted to investigate the effects of washing with HCl, pH modification with NaOH, and variations in power and sonication time on nanofluid preparation. Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e shows the experimental design.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo evaluate the impact of surface treatment on nanofluid stability, half of the experiments included washing the nanoparticles with a 1:20 HCl solution mixed with deionized water. In this experiment, 200 mg of synthesized nanoparticles was sonicated for 20 min in 100 ml of acidic solution using a power output of 400 W. Then, four consecutive washes were performed with ethanol to remove residual acid solution. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e shows the surface preparation process.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo analyze the impact of electrostatic charge stabilization, the nanofluid pH was adjusted in half of the formulations by adding 40 \u0026micro;l of 1 M NaOH into deionized water before adding nanoparticles. The remaining suspensions generated upon adding the nanoparticles was maintained at a natural pH.\u003c/p\u003e \u003cp\u003eThe influence of sonication power on the nanofluid stability was assessed by applying powers of 200 and 400 W. The impact of sonication time on the nanofluid stability was also investigated by extracting 3 ml nanofluid samples at 15-, 30-, 60-, and 120-min intervals from the outset of sonication.\u003c/p\u003e \u003cp\u003eThe factors evaluated were combined in a factorial design with the levels described in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, and four nanofluid formulations were prepared. Each formulation was tested under specific preparation and agitation conditions, resulting in a total of 32 experimental runs (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e).\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\u003eFormulations used for experimental runs\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"8\"\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=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFormulation\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSurface preparation\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eNanofluid preparation\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eAgitation power (W)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"4\" nameend=\"c8\" namest=\"c5\"\u003e \u003cp\u003eAgitation time (min)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\" morerows=\"3\" rowspan=\"4\"\u003e \u003cp\u003eHCl washed\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003epH 9 with NaOH\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e200\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e120\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e400\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e120\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eNatural pH\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e200\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e120\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e400\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e120\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\" morerows=\"3\" rowspan=\"4\"\u003e \u003cp\u003eUnwashed\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003epH 9 with NaOH\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e200\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e120\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e400\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e120\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eNatural pH\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e200\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e120\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e400\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e120\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e2.5 Evaluation of nanofluid stability\u003c/h2\u003e \u003cp\u003eThree principal methodologies were used to assess the stability of nanofluid formulations: direct observation, visible spectroscopy, and Zeta potential measurement. Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e shows the experimental design of spectrophotometry. For quantitative analysis, a Vis A721 spectrophotometer that can measure wavelengths of 350\u0026ndash;1020 nm with a resolution of \u0026plusmn;\u0026thinsp;2 nm was used. The transmitted light spectra of nanofluids were recorded at specific wavelengths at the outset and at 5-min intervals for the first hour, after which the intervals were extended to 10 min for the subsequent hour; 19 measurements in total were conducted.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe temporal evolution of stability was evaluated by determining the rate of change of vector distances between a reference spectrum and the spectra obtained at different times. From the transmitted light spectra, 19 vectors were generated for each sample and each vector corresponded to a specific measurement time. The transmittance values were then used as vector components, and the vector distance with respect to initial spectrum was calculated to analyze the temporal variations in transmittance. The rate of change of transmittance was used as a quantitative indicator of nanofluid stability, wherein lower values corresponded to enhanced stability of nanofluids.\u003c/p\u003e \u003cp\u003eThe Zeta potential and particle sizes of four fluid formulation using the Malvern Instruments Zetasizer Nano Series. The suspensions were derived via sonication at 400 W for 120 min.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results and analysis","content":"\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e shows the XRD patterns of powders synthesized via ultrasound-assisted coprecipitation. The diffraction peaks corresponding to the (200), (311), (400), (422), (511), (440), and (533) planes were successfully identified, which matched the standard pattern for magnetite (PDF card 01-075-0449). The crystallite size was estimated using the Debye\u0026ndash;Scherrer equation (Eq.\u0026nbsp;\u003cspan refid=\"Equ2\" class=\"InternalRef\"\u003e2\u003c/span\u003e):\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$$\\:{D}_{p}=\\frac{K\\lambda\\:}{\\beta\\:cos\\theta\\:},$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere Dp is the crystallite size, β is the full width at half maximum of the diffraction peak, λ is the X-ray wavelength of 1.5406 \u0026Aring;, K is the Scherrer constant, and θ is the Bragg angle. The estimated crystallite size of magnetite nanoparticles was 23.54 nm.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e shows the scanning transmission electron microscopy images at 25,000x magnification, which revealed the presence of magnetite nanoparticles with predominantly granular morphologies. Image analysis was performed using an open-source software, and average particle size was estimated as 19.24\u0026thinsp;\u0026plusmn;\u0026thinsp;6.8 nm; this finding was consistent with the crystallite size obtained via XRD measurements.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e shows the M\u0026ndash;H magnetization curve for Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e nanoparticles. The curve shows minimal coercivity and the absence of a hysteresis loop that indicate superparamagnetic behavior. The measured saturation magnetization was 67.4 emu/g, lower than that reported in the literature for bulk magnetite [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. This was because of high surface-to-volume ratio of the nanoparticles, which affected the orientation of magnetic moments due to super-exchange and dipolar interactions.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo identify favorable conditions for stability in applied treatments, transmittance values obtained via spectroscopy at wavelengths from 350 to 750 nm were compared. Figure\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e shows the corresponding results for each nanofluid formulation, where the solid and dashed lines represent the transmittance values measured immediately after the interruption of sonication and the values recorded 120 min later, respectively.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e, the dispersion and stability of nanofluids exhibited different behaviors depending on the preparation conditions, stirring power, and surface state of nanoparticles. These behaviors were classified into three main categories. Formulations with unwashed nanoparticles and a natural pH exhibited low initial transmittance values, indicating an effective initial dispersion with a high number of individual particles in suspension. After 120 min, the transmittance reached its maximum value across the majority of spectrum, indicating a significant degree of particle sedimentation. This indicated that although good initial dispersion was achieved, the nanofluids exhibited poor stability due to the formation of clusters and their subsequent precipitation.\u003c/p\u003e \u003cp\u003eThe unwashed nanoparticle formulations with NaOH sonicated at 200 W and washed nanoparticle formulations (independent of the preparation method or stirring power) exhibited high initial transmittance values at 650\u0026ndash;750 nm. This indicated that a low number of individual particles were dispersed in suspension, probably due to cluster formation. However, the variation between the initial and final transmittance values was minimal, indicating good temporal stability with slight particle sedimentation. This could also be due to less interaction among nanoparticles, probably due to surface preparation or specific chemical conditions.\u003c/p\u003e \u003cp\u003eFormulations with unwashed nanoparticles prepared at a modified pH with NaOH and sonicated at 400 W exhibited optimal performance, with low initial transmittance values. This indicated that several nanoparticles were highly dispersed in the suspension. Moreover, the variation in transmittance values after 120 min was minimal, suggesting that the nanoparticles remained stable during analysis. Thus, the preparation conditions enabled high initial dispersion and adequate stabilization of the suspended particles, confirming that these conditions were favorable for achieving good dispersion and temporal stability.\u003c/p\u003e \u003cp\u003eThe temporal evolution in the transmittance of the nanofluid formulations across each category at the end of 120-min evaluation are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e, the UW\u0026thinsp;+\u0026thinsp;Nat pH\u0026thinsp;+\u0026thinsp;400 W formulation appeared transparent after 120 min and precipitates are formed at the bottom of the cuvette. The UW\u0026thinsp;+\u0026thinsp;Nat pH\u0026thinsp;+\u0026thinsp;200 W formulation showed large clusters in suspension. The UW\u0026thinsp;+\u0026thinsp;NaOH\u0026thinsp;+\u0026thinsp;400 W formulation initially exhibited a reddish hue at the 15-min preparation stage, which gradually diminished with increasing sonication time, and turned darker after 60 and 120 min of sonication; this color remained constant throughout the observation period. The HCl\u0026thinsp;+\u0026thinsp;NaOH\u0026thinsp;+\u0026thinsp;400 W formulation retained a reddish hue throughout the sonication, which is representative of the other samples with similar characteristics of surface preparation and pH. The findings for the HCl\u0026thinsp;+\u0026thinsp;NaOH\u0026thinsp;+\u0026thinsp;400 W formulation corresponded with the high transmittance levels reported at 650\u0026ndash;750 nm.\u003c/p\u003e \u003cp\u003eTransmittance vectors for each time point were defined from the transmittance values determined for each wavelength. The vector distance at time t was calculated as the distance between the transmittance vector corresponding to t and the initial transmittance vector (at t\u0026thinsp;=\u0026thinsp;0). As a result, the variation in the vector distance with the temporal changes in the nanofluid stability was determined. The vector components corresponding to wavelengths of 700 and 750 nm were excluded from subsequent analysis due to low variability. This was because some samples reached maximum transmittance values from the outset of measurements, which did not accurately reflect their temporal stability. Figure\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003ea shows the vector distance at t\u0026thinsp;=\u0026thinsp;120 min, and Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003eb shows the rate of change of vector distance in the temporal observation window. The data demonstrate a linear fit with an R\u003csup\u003e2\u003c/sup\u003e value of \u0026ge;\u0026thinsp;0.95.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003ea illustrates the diminution of transmittance vector distance with increasing sonication time, indicating a reduction in both natural pH and NaOH usage. Notably, NaOH treatments exhibit superior outcomes in comparison to natural pH, signifying the beneficial effect of alkaline conditions on system stability. In addition, the data show that the addition of 400 watts of ultrasonic power results in a greater reduction in transmittance vector distance, especially under NaOH conditions. This suggests that increasing power can enhance the efficiency of the treatment. On the other hand, Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003eb indicates that the rate of vector change, which is higher in the initial treatments, tends to stabilize with prolonged sonication times. This suggests that after 60 minutes, the benefits from the addition of power may reach a point where they are saturated. This phenomenon is particularly evident in the UW\u0026thinsp;+\u0026thinsp;NaOH\u0026thinsp;+\u0026thinsp;400W treatment, which is identified as the optimal condition by achieving the lowest vector distance and adequate rate of change, thereby optimizing stability and efficiency.Finally, the analysis indicates that extending sonication up to 120 minutes does not yield significant additional benefits, underscoring the importance of limiting treatment time to 60 minutes to maximize energy savings without compromising results.\u003c/p\u003e \u003cp\u003eTherefore, the results obtained after 60 min of sonication can be analyzed further as the nanofluid stability did not improve considerably beyond this sonication time. Therefore, Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e only shows the sequences of vector distances obtained for the formulations prepared with 60 min of sonication.\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e shows lower vector distances of the UW\u0026thinsp;+\u0026thinsp;NaOH\u0026thinsp;+\u0026thinsp;400 W formulation during analysis, which gradually reduced toward the end of evaluation. Figure\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e shows the stability results against the derived Zeta potential and particle size distribution.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e shows that the hydrodynamic diameter of nanoparticles decreased when the pH was modified of 6 to 9 without surface treatment. In contrast, nanoparticles subjected to surface modification with HCl washing showed comparable hydrodynamic diameters at natural pH and modified pH with NaOH. Zeta potential analysis revealed that all the cases exhibited favorable conditions for charge stability in the suspension, with potentials exceeding 30 mV. The results associated with the UW\u0026thinsp;+\u0026thinsp;Nat pH formulation could not be adequately interpreted because of low stability of suspension.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"4. Conclusions","content":"\u003cp\u003eThe stability of magnetite nanoparticle suspensions was considerably influenced by their synthesis conditions. The use of HCl for washing reduced surface impurities, whereas pH modification with NaOH is conducive to the optimization of electrostatic stabilization. The combination of sonication at a power of 400 W for 60 min ensured uniform dispersion and enhanced temporal stability of nanoparticles, thereby preventing agglomerations and sedimentation.\u003c/p\u003e \u003cp\u003eSpectrophotometric analysis of transmittance and vector distance calculated with respect to the initial time proved to be an effective tool for evaluating the nanoparticle dispersion and nanofluid stability. Using these methods, variations in the homogeneity of suspensions were quantitatively measured and the conditions favoring stable nanoparticle distribution were accurately identified.\u003c/p\u003e \u003cp\u003eThe optimization of nanofluid synthesis conditions is essential for biomedical and industrial applications. To ensure their efficient performance in magnetic hyperthermia, drug delivery, and other applications, homogeneous nanoparticle dispersion and magnetic manipulability are essential to ensure the efficacy and safety of ferrofluids. The findings of this study represent a significant advance in the design of functional suspensions of magnetite nanoparticles.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors express their profound gratitude to Universidad Nacional de Colombia, which provided the financial support for this study through the \"Convocatoria Nacional para el Fomento de Alianzas Estratégicas Interdisciplinarias\" program (Project 57864).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eLaura Álvarez-Gil:\u003c/strong\u003e Conceptualization, Methodology, Investigation, Writing – original draft, Validation, Writing – review \u0026amp; editing, Formal analysis, Visualization. \u003cstrong\u003eGloria Soto-Calle:\u0026nbsp;\u003c/strong\u003eInvestigation, Validation, Writing – original draft, Visualization. \u003cstrong\u003eAlex Lopera:\u003c/strong\u003e Visualization, Validation, Writing – original draft, Writing – review \u0026amp; editing, Formal analysis. \u003cstrong\u003eAlcides Becerra:\u003c/strong\u003e Investigation, Validation. \u003cstrong\u003eSandra Navarro\u003c/strong\u003e: Investigation, Validation. \u003cstrong\u003eNéstor Ricardo Rojas-Reyes:\u0026nbsp;\u003c/strong\u003eSupervision, Writing – review \u0026amp; editing.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDeclaration of interest\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData statement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eData will be made available on request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eH. 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Chen, \u0026ldquo;Magnetite hollow spheres: Solution synthesis, phase formation and magnetic property,\u0026rdquo; \u003cem\u003eJournal of Nanoparticle Research\u003c/em\u003e, vol. 13, no. 1, pp. 213\u0026ndash;220, Jan. 2011, doi: 10.1007/s11051-010-0020-5.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":false,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"colloid-and-polymer-science","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":" Learn more about [Colloid and Polymer Science](https://www.springer.com/journal/396) ","snPcode":"396","submissionUrl":"https://mc.manuscriptcentral.com/cps","title":"Colloid and Polymer Science","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Nanofluid, stability, magnetite, visible spectroscopy","lastPublishedDoi":"10.21203/rs.3.rs-6345991/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6345991/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eIn this study, the conditions favorable for enhanced stability of magnetite nanoparticles synthesized via the coprecipitation of iron salts were evaluated for enhancing their effectiveness in various applications and ensuring homogeneous nanoparticle dispersion over time. The impact of various factors such as sonication power and time, use of pH modifiers, and surface preparation on the temporal evolution of transmittance of nanofluids was investigated by analyzing the vector distances of 350- and 650-nm spectra for each time. Nanofluids were prepared by dispersing magnetite nanoparticles in deionized water using an ultrasonic homogenizer in line with the factorial experimental design that generated 32 runs. The stability of nanofluid was evaluated via direct observation, visible spectroscopy, and Zeta potential measurement. Results indicated that the combination of unwashed nanoparticles, pH-modified nanoparticles using NaOH, and those sonicated at 400 W for 60 min enhanced the stability of nanofluids, resulting in a homogeneous and stable dispersion. These findings offer valuable insights into optimize the synthesis conditions of magnetite nanofluids with potential applications in fields such as magnetic hyperthermia and controlled drug delivery.\u003c/p\u003e","manuscriptTitle":"Evaluation of conditions favorable for the enhanced stability of magnetite suspensions using visible spectroscopy","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-04-15 05:03:58","doi":"10.21203/rs.3.rs-6345991/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-06-06T09:14:05+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-06-05T10:42:51+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-05-29T07:52:57+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"173914581422661191104602770280762104416","date":"2025-05-15T06:10:55+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"279463967420772129259006420464805673748","date":"2025-05-15T04:11:36+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-05-02T07:47:59+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-04-07T02:31:08+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-04-07T02:29:44+00:00","index":"","fulltext":""},{"type":"submitted","content":"Colloid and Polymer Science","date":"2025-03-31T14:52:50+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"colloid-and-polymer-science","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":" Learn more about [Colloid and Polymer Science](https://www.springer.com/journal/396) ","snPcode":"396","submissionUrl":"https://mc.manuscriptcentral.com/cps","title":"Colloid and Polymer Science","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"b6134051-4775-4bf9-b501-a1ce11694d4a","owner":[],"postedDate":"April 15th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2025-07-14T10:53:08+00:00","versionOfRecord":[],"versionCreatedAt":"2025-04-15 05:03:58","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-6345991","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6345991","identity":"rs-6345991","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}