Fluorescent probes based on Ag NPs@N/GQDs and molecularly imprinted polymer for sensitive detection of noradrenaline in bananas

Fluorescent probes based on Ag NPs@N/GQDs and molecularly imprinted polymer for sensitive detection of noradrenaline in bananas | 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 Fluorescent probes based on Ag NPs@N/GQDs and molecularly imprinted polymer for sensitive detection of noradrenaline in bananas Yaru Wang, Shuhuai Li, Xionghui Ma, Chaohai Pang, Yuwei Wu, Mingyue Wang, and 2 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-3647535/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 09 Jan, 2024 Read the published version in Journal of Fluorescence → Version 1 posted 7 You are reading this latest preprint version Abstract Fluorescence intensity and selective recognition ability are crucial factors in determining the analytical techniques for fluorescent probes. In this study, a core-shell fluorescent material composed of silver nanoparticles@nitrogen-doped graphene quantum dots (Ag NPs@N/GQDs) was synthesised using mango leaves as the raw material through a thermal cracking method, resulting in strong fluorescence luminescence intensity. By employing noradrenaline as a template molecule and utilising a surface molecular imprinting technique, a molecularly imprinted membrane (MIP) was formed on the surface of the fluorescent material, which was subsequently eluted to obtain a highly specific fluorescent probe capable of recognising noradrenaline. The probe captured various concentrations of noradrenaline using the MIP, causing a decrease in the probe fluorescence intensity. Then a method for detecting trace amounts of noradrenaline was established. This method exhibited a linear range from 0.5 –700 pM with a detection limit of 0.154 pM. The proposed method was achievemently implemented in banana samples. Satisfactory recoveries were confirmed at four different concentrations. The method presented a relative standard deviation (RSD%) of less than 5.0%. Fluorescent probe Graphene quantum dots Silver nanoparticles Molecular imprinted polymer Noradrenaline Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Introduction The banana (Musa spp.) is globally recognised as the most extensively traded fresh fruit and is a dietary staple for approximately 500 million individuals worldwide. Owing to the presence of noradrenaline and 5-hydroxytryptamine, which inhibit lipid peroxidation reactions, bananas have the potential to delay ageing and prevent cardiovascular diseases[1]. Analysis of the noradrenaline and other micronutrient contents in bananas can provide valuable data support for investigating their nutritional functions, extracting essential components, and facilitating genetic breeding programmes. High-performance liquid chromatography (HPLC)[2,3] and electrochemical methods[4,5] are predominantly employed for detecting noradrenaline in bananas; however, these techniques require necessitate addressing practical challenges such as the time-consuming procedures associated with HPLC and enhancing the stability and reproducibility of the electrochemical process. Consequently, novel approaches that offer increased sensitivity, selectivity, stability, rapidity, and simplicity must be explored with regard to detecting noradrenaline. Fluorescent probes employ fluorescent substances as indicators and bind to specific molecules to detect target molecules[6,7]. Optical sensors are characterised by their exceptional selectivity, rapid response time, heightened sensitivity, and straightforward operation. Traditional fluorescent probes primarily consist of organic dyes[8,9] and metal quantum dots[10,11]. However, conventional fluorescent probes generally exhibit limited biocompatibility, toxicity, and restricted quantum yields, which hinder their application[12]. Moreover, traditional fluorescent probes lack specificity towards target molecules and require further enhancement regarding their anti-interference capabilities. Therefore, the development of fluorescent probes with excellent biocompatibility, high quantum dot yield, low toxicity,and selective recognition of target molecules remains a significant challenge in current fluorescence analysis technology. There has been growing interest in novel fluorescent nanomaterials, such as graphene quantum dots (GQDs). GQDs consist of multiple layers of graphene layers with transverse dimensions smaller than 100 nm and exhibit pronounced quantum-limited domain and edge effects[13-15]. Consequently, they possess distinctive physical properties, including robust photoluminescence,which are customisable for specific applications through size and shape manipulation. Compared to conventional quantum dots made of metals or silicon, GQDs are biocompatible, photostable, and inherit exceptional thermal, electrical, and mechanical characteristics of graphene[16]. Tang et al.[17] synthesised dual-emission F,N-doped fluorescent graphene quantum dot sensors, which were utilised for the detection of doxycycline in milk, pork, and water samples with a low detection limit of 40 nM. Han et al.[18] developed fluorescent GQD sensors based on a hydration process using citric acid and urea as carbon sources. They successfully prepared nitrogen-doped graphene quantum dots (N-GQDs) through an efficient one-step hydrothermal synthesis using hydrated citric acid and urea as carbon and nitrogen sources. The N-GQDs exhibited a high fluorescence quantum yield of 88.20% and fluorescence lifetime of 12.29 ns. A fluorescent probe was designed based on the excellent fluorescence properties, water solubility, and low cytotoxicity of N-GQDs for the simultaneous detection of L-glutamic acid and L-aspartic acid using an aspartic acid method with a remarkable detection limit of 2.18 μg/mL. However, the selective recognition ability of these GQDs relies on the introduction of recognition groups during the synthesis process. Their immunity to interference when detecting targets in complex matrices is not sufficiently strong; thus, further improvement to enhance their recognition ability is crucial[19]. Incorporating specific recognition elements during the synthesis of fluorescent probes, such as GQDs, is an effective strategy for enhancing their selective recognition ability.Among these, aptamers and molecularly imprinted membranes have garnered significant attention because of their robust specificity for target molecules. Aptamers[20,21] are short oligonucleotide sequences or peptides obtained through in vitro screening that exhibit high affinity and specificity for the corresponding ligands. Molecularly imprinted membranes[22,23] are polymers designed to match a spatial structure and binding sites of molecules, thereby enabling specific binding to target molecules. Liang et al.[24] synthesised nitrogen-doped GQDs and combined them with aptamers as fluorescent probes, thereby improving their recognition capability and successfully applying them to the specific detection of Golgi protein 73. However, the complexity, time-consuming nature, and high cost associated with aptamer synthesis hinder its widespread application in fluorescent probes[25]. Contrarily, molecularly imprinted polymers (MIP) have gained popularity owing to their simple and convenient synthesis process and low cost. Mantashloo et al.[26] prepared GQDs after magnetisation by thermally decomposing glucose precursors, followed by coating a layer of MIPs on them; this approach was employed to enhance the selective recognition ability of GQDs for quercetin. Thus, MIPs are promising ideal recognition elements for fluorescent probes. In this study, mango leaves were utilised as the raw material to synthesise Ag NPs@N/GQDs through thermal cleavage of N-doped GQDs (N/GQDs) and the subsequent generation of silver nanoparticles (AgNPs) on their surfaces via reduction. Moreover, a MIP was synthesised using noradrenaline as a template molecule by imprinting it onto the surface of the Ag NPs@N/GQDs. A 50% formic acid solution (V formic acid :V water = 1:1) was employed to elute the template molecules to obtain Ag NPs@N/GQDs@MIP fluorescent probes that retained the recognition site for noradrenaline and exhibited specific recognition. By utilising the recognition site on the probe as a switch, different concentrations of noradrenaline can be adsorbed, effectively quenching of the fluorescence intensity of the probe and establishing a novel method for detecting noradrenaline. The incorporation of silver nanoparticles enhanced the fluorescence performance of the GQDs, resulting in a highly sensitive fluorescence intensity for the probe; moreover, integration with the MIP significantly improved its ability to specifically recognise noradrenaline. Experimental Reagents and instruments Sulfuric acid, nitric acid, sodium hydroxide, ethylenediamine, AgNO 3 , (3-aminopropyl)triethoxysilane, tetraethoxysilane, and noradrenaline standards were purchased from Aladdin Reagent Co. Ltd. in Shanghai, China. The reagents used in the experiments were analytically pure, and the water used was deionised water (18.2 MΩ). The morphology and microstructure of the probe materials were observed using a scanning electron microscope (SEM; Carl Zeiss AG, Germany), and their crystal structures were characterised using X-ray powder diffraction (XRD; Ultima IV polycrystalline X-ray diffractometer, Rigaku Co., Ltd., Japan). The elements on the surfaces of the probes were characterised and analysed using X-ray photoelectron spectroscopy (XPS; ESCALAB 250 X-ray photoelectron spectrometer, Thermo Scientific, Inc., USA). Optical properties of the probe materials were measured using an F-7000 fluorescence spectrometer (Hitachi). Ag NPs@N/GQDs synthesis The synthesis method was based on an existing approach, which was further enhanced[27]. Details of the synthesis are shown in the Supporting Material. Ag NPs@N/GQDs/MIP synthesis Noradrenaline solution (10 mL; 0.1 mM), Ag NPs@N/GQDs (20 mg) and (3-aminopropyl)triethoxysilane (APTES) (100 μL) were added to a 50 mL flask, and the reaction was stirred at room temperature (25℃) for 20 min. Further,100 μL of tetraethoxysilane (TEOS) and 150 μL of NH 3 ▪H 2 O (w/v 25%) were added and stirred at room temperature(25℃) for 8 h. The reaction product was obtained by centrifugation at 10000 rpm, the oligomers and unreacted monomers generated during the reaction process were washed off with deionised water, and the reaction was repeated three times. Next, the template noradrenaline in the MIP was eluted with 20 mL of 50% formic acid solution (v/v formic acid : water = 1:1) for 5 min. Finally, the product obtained from the reaction was redissolved in 10 mL of PBS buffer (0.1 M, pH = 7.4) and stored at 4℃ for further use. The Ag NPs@N/GQDs/NIP was synthesized in the same way, but with no addition of noradrenaline. Fluorescence detection methods First, 600 μL Ag NPs@N/GQDs/MIP probe solution was weighed, and different concentrations of noradrenaline were added. The fluorescence intensity before and after the addition of noradrenaline was detected after the full reaction with 0.1 M pH = 7.4 PBS to 2 mL for 5 min. The excitation wavelength Ex = 480 nm, emission spectrum range was 480–700 nm, excitation wavelength slit was 5 nm, emission spectrum slit was 2.5 nm, and photomultiplier tube voltage was set at 700 V. The fluorescence intensity of noradrenaline was measured before and after its addition. Sample pre-treatment First,0.1 g of the banana sample was weighed, and 1 mL of 80% methanol solution was added. Next, the mixture was agitated using a vortex shaker for 5 min, followed by centrifugation at 8000 rpm and 4 ℃ for 1 h.The supernatant was collected and dried using a nitrogen blowing apparatus. Next, it was reconstituted with 500 μL of Ag NPs@N/GQDs/MIP probe solution and subjected to another round of centrifugation at 8000 rpm and 4 ℃ for an additional 10 min. Finally, it was stored in the refrigerator at 4 ℃ until ready for testing. Results and discussion Characterisation of Ag NPs@N/GQDs The transmission electron microscopy (TEM) images showed a uniform distribution and relatively consistent size of the synthesised N/GQDs, indicating excellent dispersion (Fig. 2A). The particle size distribution plot in Fig. 2B shows that the synthesised N/GQDs ranged from 3 to 10 nm, with an average size of approximately 7 nm. When Ag NPs were generated by reducing the surface area of the N/GQDs, nucleoshell spherical nanoparticles formed with a core composed of N/GQDs (Fig. 2C). These particles were uniformly dispersed, and their particle size distribution ranged from 10 to 20 nm (Fig. 2D), with an average particle size of approximately 15 nm. The structure of the synthesised Ag NPs@N/GQDs was characterised and analysed, as depicted in Fig. 2E. Therefore, the XRD pattern that the synthesised Ag NPs@N/GQDs exhibit a distinct peak at 2θ=24.6°, which is low and narrow, indicating a significant particle size corresponding to the (002) crystal plane in the graphitic carbon structure[28]. Sharp peaks observed at 2θ=37.9°, 44.1°, 64.6°, and 77.1° correspond to the (111), (200), (220), and (311) crystal faces of Ag NPs respectively[29]. Furthermore, an elemental composition analysis of the Ag NPs@N/GQDs was conducted using the XPS data (Fig. 2F). Peaks C(1s) and N(1s) appear at 284.08 eV and 394.08 eV, respectively, originating from GQDs[30]; whereas the peaks of Ag(3d 3/2 ) and Ag(3d 5/2 ) appeared at 368.08 eV and 374.08 eV, which were both from AgNPs[31]. Therefore, the Ag NPs@N/GQDs were successfully synthesised. MIP characterisation The MIP on the probe was characterised using XPS and Fourier-transform infrared (FTIR) spectroscopy. Fig. 3A shows a significant reduction in the C(1s) peak at 284.08 eV, O(1s) at 394.08 eV, and N(1s) peak at 401.08 eV in the MIP after eluting noradrenaline, because it mainly comprises the elements C, H, O, and N. Further characterisation of the MIP was performed using FTIR spectroscopy. As shown in Fig. 2B, absorption peaks corresponding to the amino stretching vibration regions were observed for noradrenaline at 3448 cm –1 and 3418 cm –1 ,while the characteristic absorption peaks of the benzene ring skeleton were observed at 960 cm –1 , 943 cm –1 , 638 cm –1 , and 599 cm –1 . These peaks were also present in Ag NPs@N/GQDs/MIP but disappeared upon the removal of noradrenaline through elution from Ag NPs@N/GQDs/MIP. Therefore, the MIP was successfully synthesised. Fluorescence properties of Ag NPs@N/GQDs/MIP probes The optical properties of the fluorescent probes were examined using UV-Vis absorption and fluorescence spectroscopy. As shown in Fig.S1A (Supplementary Material), the Ag NPs@N/GQDs/MIP aptamer exhibited strong absorption at 495 nm. Additionally, Fig.S1B shows that the material has a maximum excitation wavelength near 480 nm (curve a). This wavelength was used to excite the probe, which resulted in a maximum emission wavelength of approximately 533 nm (curve b). The larger Stokes shift effectively prevented overlap between the excitation and emission spectra. Furthermore, the emission spectrum of the probe exhibited high intensity, symmetry, and a narrow peak shape, indicating excellent fluorescence performance. The quantum yield of a fluorescent substance is a crucial parameter that affects fluorescent probes. On the basis of the integrated fluorescence intensity (i.e. the area included in the corrected fluorescence spectrum) of the two dilute solutions of the fluorescence sample to be measured, the reference fluorescence reference material with known quantum yield under the same excitation conditions, and the absorbance of the incident light (UV–visible light) with the same excitation wavelength, the quantum yield of the tested fluorescent specimen can be calculated as[32] Here, Φ u and Φ s represent the fluorescence quantum yields of the substance under measurement and the reference standard, respectively; F u and F s denote the integrated fluorescence intensities of the substance being measured and the reference substance; A u and A s indicate the absorbance of incident light at the excitation wavelengths of the substance under measurement and the reference substance (A=εbc). In this study, Rhodamine B (RhB) solution was utilised as a reference fluorescence standard. According to the literature, RhB has a reported quantum yield of 0.89[33]; the relevant parameters shown in Table S1. Mechanism of Ag NPs@N/GQDs/MIP probe signal reduction by norepinephrine With the addition of different concentrations of noradrenaline, the probe captured increasing amounts of noradrenaline using the MIP. As shown in Fig. 5A, the fluorescence intensity of the Ag NPs@N/GQDs/MIP decreased, indicating that noradrenaline effectively suppressed the fluorescence intensity of the probe. When the template molecule norepinephrine was readded, the Ag NPs@N/GQDs/MIP was able to interact with it, leading to fluorescence quenching. We suggest that an electron transfer between the N/GQDs and norepinephrine is responsible for this phenomenon. The mechanism of fluorescence quenching was studied through UV-visible and fluorescence spectroscopy. As shown in Fig. 5B, according to previous reports, the fluorescence quenching mechanism may mainly involve electron or energy transfer from Ag NPs@N/GQDs/MIP to norepinephrine. The maximum norepinephrine adsorption was 453 nm (curve a), which was close to the band gap of the Ag NPs@N/GQDs/MIP at an excitation wavelength of 480 nm (curve b). These results indicate that the electrons in the conduction bands of the Ag NPs@N/GQDs/MIP could be transferred to the lowest unoccupied orbital of norepinephrine. In addition, the propanil absorption peaks exhibited evident red shifts after APTES addition (curve c). This indicates that analytes may act as electron acceptors to trigger a non-radiative decay electron process[34]. The maximum emission of the Ag NPs@N/GQDs/MIP occurred at 533 nm, exhibiting a narrow and symmetrical line width (curve e). There is no spectral overlap between the emission spectrum of the Ag NPs@N/GQDs/MIP and the absorption spectra of norepinephrine. Hence, we do not think that the fluorescence quenching is caused by the energy transfer mechanism. These results also imply that the electron transfer from the Ag NPs@N/GQDs/MIP to norepinephrine could be the main optosensing turn-off mechanism[35]. During the fluorescence response, the electrons in the N/GQDs were excited by the UV spectrum energy. Subsequently, when the excited electrons returned to their ground state, the N/GQDs emitted fluorescence. Correspondingly, after norepinephrine was added, it was specifically adsorbed onto the imprinting cavities. The Meisenheimer complex was formed through the strong interactions between the –NH 2 of the functional monomer APTES and –OH of norepinephrine. Therefore, the electrons of the N/GQDs were transferred to the complex, resulting in fluorescence quenching. In addition, the fluorescence quenching mechanism can be explained by molecular orbital theory. As shown in Fig. S2A, an electron of the N/GQDs is excited from the valence band (ground state) to the conduction band after accepting the UV photon. Afterward, the excited electron returns to the valence band, and the N/GQDs produce the fluorescence signal. In addition, in the presence of the template norepinephrine, a hydrogen bonding interaction exists between the amino groups of the N/GQDs and norepinephrine. The interaction force is so strong that it can cause the electron to be transferred between the norepinephrine and N/GQDs. The excited electron is able to transition directly into the LUMO of the complex. It would then return to the ground state, generating no fluorescence signals, because the energy level of the complex would be higher than that of the N/GQDs[36]. This explains the phenomenon of fluorescence quenching (Fig. S2B). Experimental optimisation conditions The effects of elution time, buffer pH and reaction time on the detection of noradrenaline by the Ag NPs@N/GQDs/MIP aptamer probes were investigated. As shown in Fig. S3A, as the elution time increased, increasing amounts of noradrenaline were was removed from the MIP, the fluorescence intensity of probe was continuously enhanced. After 5 min, the probe fluorescence increased to the maximum and remained unchanged thereafter, indicating that the probe had been completely eluted. As shown in Fig. S3B, the ratio of the decrease in the fluorescence value △IF (△IF=F 0 –F 1 , where F 0 represents the fluorescence intensity before adding 1.0 nM noradrenaline and F 1 represents the fluorescence intensity after) exhibited different changes as the pH increased from 6.4, but reached its maximum at pH=7.4. Therefore, PBS (pH 7.4) was selected as the optimal buffer. Additionally, the effect of the reaction time of noradrenaline with the fluorescent probe on the quenching effect was investigated. As shown in Fig. S3C, as the reaction proceeded, an increasing amount of noradrenaline was adsorbed onto the probe by MIP, and the probe fluorescence intensity continuously decreased until the reaction progressed for a minimum of 5 min and remained constant thereafter. Therefore, a reaction time of 5 min was selected as the optimal reaction time. Fluorescence response of Ag NPs@N/GQDs/MIP probes to noradrenaline The fluorescence intensities of the Ag NPs@N/GQDs/MIP probes were measured before and after adding different concentrations of noradrenaline under the optimal experimental conditions. The decrease in the fluorescence intensity value (△IF=F 0 –F 1 ) was calculated, and a calibration curve was plotted. As shown in Fig. 5A, the fluorescence intensity of the probe increased continuously with increasing noradrenaline concentration.In the range of 0.5 –700 pM, there was a good linear relationship between the logarithm of noradrenaline concentration (ln(c)) and the logarithm of the decrease in fluorescence intensity (ln(△IF)). The linear regression equation was ln(△IF) = 0.505 ln(c) (pM) + 5.27 (Fig. 5B), with an r-value of 0.9991 and a detection limit of 0.154 pM (D.L.= KS b /a, K = 3). This method exhibited a higher sensitivity for detecting noradrenaline than previously reported methods ( Table S2). Selectivity of Ag NPs@N/GQDs/MIP probes The selectivity of the Ag NPs@N/GQDs/MIP probes was evaluated by studying their response in the presence of dopamine, epinephrine, catechol, vitamin B6, vitamin B2, chlorpyrifos, and methyl parathion. As shown in Fig. 6, because there are no molecularly imprinted recognition sites, the Ag NPs@N/GQDs/NIP cannot recognize norepinephrine and adsorb it to the surface of the probe, so it does not decrease the fluorescence intensity of the probe. Similarly, the aforementioned interfering substances are not recognized and adsorbed by the NIP, and the fluorescence intensity of probe does not decrease. In contrast, because the molecularly imprinted sites in the Ag NPs@N/GQDs/MIP can identify and adsorb norepinephrine, the fluorescence intensity of the probe is decreased. However, because the interfering substances cannot enter the imprinted holes or be recognized and adsorbed by the MIP, the fluorescence of the probe does not change. This result demonstrates that the probes were able to recognize norepinephrine with excellent specificity. Reproducibility and stability of Ag NPs@N/GQDs/MIP probes The reproducibility and stability of the Ag NPs@N/GQDs/MIP probes for noradrenaline detection were investigated. Five Ag NPs@N/GQDs/MIP probes were subjected to the addition of 1.0 nM noradrenaline under identical conditions, and the fluorescence intensities before and after the addition were measured to calculate ΔIF. The results demonstrated a relative standard deviation (RSD) of 2.73% for the ΔIF values from five experiments, indicating excellent reproducibility (Fig. S4). Furthermore, the prepared Ag NPs@N/GQDs/MIP fluorescent probe was exposed to 1.0 nM noradrenaline, and its fluorescence intensity was monitored at intervals of 5 min. As shown in Fig. S5, after a reaction time of 30 min,the fluorescence intensity of the probe reached 92.2% of its intensity after the completion of the reaction (5 min), demonstrating remarkable stability. Real sample analysis A fluorescent probe was used to detect the banana samples, and a spiked recovery test was conducted. Table 1 presents the results of the study. The method exhibited recoveries ranging from 93.6% to 112.0%, with an RSD valueless than 5.0%. Therefore, the current fluorescent probe method can be used effectively to detect actual samples and yield satisfactory results. Table 1. Results of sample assay and recovery analysis. Samples This method (nM, n=5) RSD % Added (nM) Total found (nM, n=5) RSD % Recoveries % Banana Ⅰ 1.85 4.97 1.50 3.27 3.44 94.7 15.00 15.89 4.90 93.6 Banana Ⅱ 2.83 3.76 1.00 3.95 3.76 112.0 10.00 12.32 4.32 94.9 Conclusion In this study, an Ag NPs@N/GQDs/MIP fluorescent composite nanoprobe was synthesised and utilised for the ultra-sensitive detection of noradrenaline in bananas. Incorporating Ag NPs enhanced the fluorescence performance of the N/GQDs, resulting in a stable and intense fluorescence emission spectrum with a high probe quantum yield. Furthermore, by introducing MIP, the probe exhibited excellent selective recognition of the target molecule, noradrenaline, effectively eliminating interference. Therefore, this probe demonstrated remarkable sensitivity, selectivity, and ease of operation for detecting noradrenaline residues in bananas. Declarations Acknowledgements We would like to thank Prof. Dr. Lianming Zhang for his consultancy in the nanomaterials synthesis studies. Author Contributions Yaru Wang: conceptualization, methodology, formal analysis and investigation, writing–original draft preparation. Shuhuai Li: conceptualization, resources, writing–review and editing, supervision, project administration, funding acquisition. Xionghui Ma: data curation, methodology, writing–review and editing. Chaohai Pang: investigation, writing–review and editing. Yuwei Wu: investigation, writing–review and editing. Mingyue Wang: conceptualization, resources, writing–review and editing, supervision, project administration, funding acquisition. Bei Li: investigation, writing–review and editing. Sixin Liu: conceptualization, resources, supervision, project administration. Data Availability Not applicable. Funding This research was supported by China Agriculture Research System of MOF and MARA (CARS-31), Chinese Academy of Tropical Agricultural Sciences for Science and Technology Innovation Team of National Tropical Agricultural Science Center (CATASCXTD202314), the Key Laboratory of Tropical Fruits and Vegetables Quality and Safety for State Market Regulation (ZX-2023002). Ethical Approval This study does not contain any studies with human participants or animals performed by any of the authors. Consent to Participate Not applicable. Consent for Publication Not applicable. Conflict of Interest There are no conflicts to declare. 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Dyes Pigments 118: 9-17. https://doi.org/10.1016/j.dyepig.2015.02.021 Nawara K, Rana A, Panda PK, Waluk J (2018) Versatile approach for reliable determination of both high and low values of luminescence quantum yields. Anal Chem 90: 10139-10143. https://doi.org/10.1021/acs.analchem.8b02751 Sha QL, Deng JR, Zhang HS, Luo XG, Wu FS (2023) Organic nanomaterials from self-assembly of BODIPY-benzothiadiazole conjugate for PDT/PTT synergistic therapy. J Porphyr Phthalocya 27: 852-860. https://doi.org/10.1142/s1088424623500621 Liu J, Chen H, Lin Z, Lin JM (2010) Preparation of surface imprinting polymer capped Mn-doped ZnS quantum dots and their application for chemiluminescence detection of 4-nitrophenol in tap water. Anal Chem 82: 7380-7386. https://doi.org/10.1021/ac101510b Zhang L, Chen L (2016) Fluorescence probe based on hybrid mesoporous silica/quantum dot/molecularly imprinted polymer for detection of tetracycline. ACS Appl Mater Interfaces 8: 16248-16256. https://doi.org/10.1021/acsami.6b04381 Tu RY, Liu BH, Wang ZY, Gao DM, Wang F, Fang QL, Zhang ZP (2008) Amine-capped ZnS-Mn 2+ nanocrystals for fluorescence detection of trace TNT explosive. Anal Chem 80: 3458-65. https://doi.org/10.1021/ac800060f Additional Declarations No competing interests reported. Supplementary Files supportinformation1120.doc Cite Share Download PDF Status: Published Journal Publication published 09 Jan, 2024 Read the published version in Journal of Fluorescence → Version 1 posted Editorial decision: Revision requested 07 Dec, 2023 Reviews received at journal 05 Dec, 2023 Reviewers agreed at journal 30 Nov, 2023 Reviewers invited by journal 28 Nov, 2023 Submission checks completed at journal 23 Nov, 2023 Editor assigned by journal 23 Nov, 2023 First submitted to journal 22 Nov, 2023 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. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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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-3647535","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":252645296,"identity":"c950843f-5c61-40ea-a6e7-d20b22d16c56","order_by":0,"name":"Yaru Wang","email":"","orcid":"","institution":"Hainan University","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Yaru","middleName":"","lastName":"Wang","suffix":""},{"id":252645297,"identity":"e45b4b5d-963a-4908-941b-8bf2e53cce7f","order_by":1,"name":"Shuhuai Li","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA70lEQVRIiWNgGAWjYBACAzBZIAFk8IC4Njz8/A3EaDGAa0mTkZxxgCgtDFAtDIdtDBoS8GsxZz977DGPgYW9uUTuMYkfBed5DBgOMH74mINbi2VPXroxj4FE4s4ZeWmSPQa3ecyZG5glZ27D47ADOWbSQC0JBjdyzCR4gFosGw6wMfPi03L+DViLPUiL5B+DczwGBxIIaLkBsYVxA4RxgBgt79IN5wD9suHMG2NrGYNkHskZB5vx++V87rEHbyrq7A2O5xjefPPHzp6fv/ngh494tDAw8LAx8aCKMDbgUw/WwviDgJJRMApGwSgY4QAA1D9MxK0SrWwAAAAASUVORK5CYII=","orcid":"","institution":"Chinese Academy of Tropical Agricultural Sciences","correspondingAuthor":true,"submittingAuthor":false,"prefix":"","firstName":"Shuhuai","middleName":"","lastName":"Li","suffix":""},{"id":252645298,"identity":"d87f7f8a-40e8-432d-8e56-fcc96277dea0","order_by":2,"name":"Xionghui Ma","email":"","orcid":"","institution":"Chinese Academy of Tropical Agricultural Sciences","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Xionghui","middleName":"","lastName":"Ma","suffix":""},{"id":252645301,"identity":"297fb9b4-95d0-4063-b427-fdb45253cc69","order_by":3,"name":"Chaohai Pang","email":"","orcid":"","institution":"Chinese Academy of Tropical Agricultural Sciences","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Chaohai","middleName":"","lastName":"Pang","suffix":""},{"id":252645304,"identity":"0c29764b-c6eb-4bb9-814d-7c71a27aecb2","order_by":4,"name":"Yuwei Wu","email":"","orcid":"","institution":"Chinese Academy of Tropical Agricultural Sciences","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Yuwei","middleName":"","lastName":"Wu","suffix":""},{"id":252645307,"identity":"c9cd0c7c-7ea4-492b-ac86-aad194c2f2c9","order_by":5,"name":"Mingyue Wang","email":"","orcid":"","institution":"Chinese Academy of Tropical Agricultural Sciences","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Mingyue","middleName":"","lastName":"Wang","suffix":""},{"id":252645310,"identity":"b1300b9a-358d-45a6-ab6a-39eb3a753fa4","order_by":6,"name":"Bei Li","email":"","orcid":"","institution":"Hainan Institute for Food Control","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Bei","middleName":"","lastName":"Li","suffix":""},{"id":252645312,"identity":"f0af4cc5-9d32-4978-8514-3e47216cb139","order_by":7,"name":"Sixin Liu","email":"","orcid":"","institution":"Hainan University","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Sixin","middleName":"","lastName":"Liu","suffix":""}],"badges":[],"createdAt":"2023-11-22 08:29:13","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-3647535/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-3647535/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s10895-023-03565-w","type":"published","date":"2024-01-09T15:02:05+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":47215632,"identity":"faf93784-7717-458b-97e0-b4f5f3ce1a5c","added_by":"auto","created_at":"2023-11-28 17:55:51","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":209935,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic illustrating noradrenaline detection by the newly developed fluorescence probe.\u003c/p\u003e","description":"","filename":"Figure1.png","url":"https://assets-eu.researchsquare.com/files/rs-3647535/v1/8f6c0792bb70d608df0f1ed5.png"},{"id":47216043,"identity":"32c30254-5020-4b5d-9fe4-7f519e1af855","added_by":"auto","created_at":"2023-11-28 18:03:51","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":367279,"visible":true,"origin":"","legend":"\u003cp\u003e(A) TEM image of N/GQDs; (B) Statistical particle size distribution of N/GQDs; (C) TEM image of Ag NPs@N/GQDs; (D) Statistical particle size distribution of Ag NPs@N/GQDs; (E) XRD spectrogram of Ag NPs@N/GQDs; (F) XRS spectrogram of Ag NPs@N/GQDs.\u003c/p\u003e","description":"","filename":"Figure2.png","url":"https://assets-eu.researchsquare.com/files/rs-3647535/v1/03596805e15e449a95d1e8d2.png"},{"id":47215631,"identity":"c1145548-b6c7-45ce-a61e-bd3a239b6fdb","added_by":"auto","created_at":"2023-11-28 17:55:51","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":226833,"visible":true,"origin":"","legend":"\u003cp\u003e(A) XPS spectroscopy of MIP on the probe: a. MIP; b. MIP after noradrenalinewas removed; (B) FTIR spectroscopy of noradrenaline and MIP.\u003c/p\u003e","description":"","filename":"Figure3.png","url":"https://assets-eu.researchsquare.com/files/rs-3647535/v1/c380cf83bcb5b85c8cdb3586.png"},{"id":47215629,"identity":"14f4b40b-911b-45ba-af37-d174894e18a8","added_by":"auto","created_at":"2023-11-28 17:55:51","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":60613,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003e(A) \u003c/strong\u003eEffect of noradrenalineconcentration on the fluorescence intensity of Ag NPs@N/GQDs/MIP.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e(B) \u003c/strong\u003eUV-vis absorption spectra and fluorescence spectra of probe system: a. UV-vis absorption spectra of norepinephrine; b. UV-vis absorption spectra of Ag NPs@N/GQDs/MIP; c. UV-vis absorption spectra of norepinephrine with APTES added ; d. UV-vis absorption spectra of APTES; e. Fluorescence spectrum of Ag NPs@N/GQDs/MIP.\u003c/p\u003e","description":"","filename":"Figure4.png","url":"https://assets-eu.researchsquare.com/files/rs-3647535/v1/fd4c34bbdf372d22f2f999e2.png"},{"id":47215630,"identity":"ba635a2a-e37e-449b-b0c8-ed686e292a5f","added_by":"auto","created_at":"2023-11-28 17:55:51","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":307906,"visible":true,"origin":"","legend":"\u003cp\u003e(A) Fluorescence spectra of Ag NPs@N/GQDs/MIP at different concentrations of noradrenaline (a–p): (0.5, 2.5, 5, 10, 20, 30, 40, 50, 75, 100, 200, 250, 400, 500, 600, and 700) pM; (B) Calibration curve.\u003c/p\u003e","description":"","filename":"Figure5.png","url":"https://assets-eu.researchsquare.com/files/rs-3647535/v1/e9ca9c4752b8b15b927d2e8b.png"},{"id":47215627,"identity":"28efce08-66b9-4635-9ed8-1053a9a51d1e","added_by":"auto","created_at":"2023-11-28 17:55:51","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":27553,"visible":true,"origin":"","legend":"\u003cp\u003eFluorescence response of Ag NPs@N/GQDs/NIP and Ag NPs@N/GQDs/MIP probe to 1.0 nM noradrenaline, and 0.1μM mol/L interfering substances.\u003c/p\u003e","description":"","filename":"Figure6.png","url":"https://assets-eu.researchsquare.com/files/rs-3647535/v1/34578043819da12e33df011e.png"},{"id":49628697,"identity":"4b9a0747-140b-41c9-8811-db0ac71a14e1","added_by":"auto","created_at":"2024-01-15 15:09:02","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1510174,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-3647535/v1/b1e40099-04d3-4300-8973-01455757a277.pdf"},{"id":47215633,"identity":"28d402ba-13f0-4b77-be4f-91931369a0a0","added_by":"auto","created_at":"2023-11-28 17:55:51","extension":"doc","order_by":9,"title":"","display":"","copyAsset":false,"role":"supplement","size":278264,"visible":true,"origin":"","legend":"","description":"","filename":"supportinformation1120.doc","url":"https://assets-eu.researchsquare.com/files/rs-3647535/v1/af20f8901e4a1942e262043a.doc"}],"financialInterests":"No competing interests reported.","formattedTitle":"Fluorescent probes based on Ag NPs@N/GQDs and molecularly imprinted polymer for sensitive detection of noradrenaline in bananas","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThe banana (Musa spp.) is globally recognised as the most extensively traded fresh fruit and is a dietary staple for approximately 500 million individuals worldwide. Owing to the presence of noradrenaline and 5-hydroxytryptamine, which inhibit lipid peroxidation reactions, bananas have the potential to delay ageing and prevent cardiovascular diseases[1]. Analysis of the noradrenaline and other micronutrient contents in bananas can provide valuable data support for investigating their nutritional functions, extracting essential components, and facilitating genetic breeding programmes. High-performance liquid chromatography (HPLC)[2,3] and electrochemical methods[4,5] are predominantly employed for detecting noradrenaline in bananas; however, these techniques require necessitate addressing practical challenges such as the time-consuming procedures associated with HPLC and enhancing the stability and reproducibility of the electrochemical process. Consequently, novel approaches that offer increased sensitivity, selectivity, stability, rapidity, and simplicity must be explored with regard to detecting noradrenaline.\u003c/p\u003e\n\u003cp\u003eFluorescent probes employ fluorescent substances as indicators and bind to specific molecules to detect target molecules[6,7]. Optical sensors are characterised by their exceptional selectivity, rapid response time, heightened sensitivity, and straightforward operation. Traditional fluorescent probes primarily consist of organic dyes[8,9] and metal quantum dots[10,11]. However, conventional fluorescent probes generally exhibit limited biocompatibility, toxicity, and restricted quantum yields, which hinder their application[12]. Moreover, traditional fluorescent probes lack specificity towards target molecules and require further enhancement regarding their anti-interference capabilities. Therefore, the development of fluorescent probes with excellent biocompatibility, high quantum dot yield, low toxicity,and selective recognition of target molecules remains a significant challenge in current fluorescence analysis technology.\u003c/p\u003e\n\u003cp\u003eThere has been growing interest in novel fluorescent nanomaterials, such as graphene quantum dots (GQDs). GQDs consist of multiple layers of graphene layers with transverse dimensions smaller than 100 nm and exhibit pronounced quantum-limited domain and edge effects[13-15]. Consequently, they possess distinctive physical properties, including robust photoluminescence,which are customisable for specific applications through size and shape manipulation. Compared to conventional quantum dots made of metals or silicon, GQDs are biocompatible, photostable, and inherit exceptional thermal, electrical, and mechanical characteristics of graphene[16]. Tang et al.[17] synthesised dual-emission F,N-doped fluorescent graphene quantum dot sensors, which were utilised for the detection of doxycycline in milk, pork, and water samples with a low detection limit of 40 nM. Han et al.[18] developed fluorescent GQD sensors based on a hydration process using citric acid and urea as carbon sources. They successfully prepared nitrogen-doped graphene quantum dots (N-GQDs) through an efficient one-step hydrothermal synthesis using hydrated citric acid and urea as carbon and nitrogen sources. The N-GQDs exhibited a high fluorescence quantum yield of 88.20% and fluorescence lifetime of 12.29 ns. A fluorescent probe was designed based on the excellent fluorescence properties, water solubility, and low cytotoxicity of N-GQDs for the simultaneous detection of L-glutamic acid and L-aspartic acid using an aspartic acid method with a remarkable detection limit of 2.18 \u0026mu;g/mL. However, the selective recognition ability of these GQDs relies on the introduction of recognition groups during the synthesis process. Their immunity to interference when detecting targets in complex matrices is not sufficiently strong; thus, further improvement to enhance their recognition ability is crucial[19].\u003c/p\u003e\n\u003cp\u003eIncorporating specific recognition elements during the synthesis of fluorescent probes, such as GQDs, is an effective strategy for enhancing their selective recognition ability.Among these, aptamers and molecularly imprinted membranes have garnered significant attention because of their robust specificity for target molecules. Aptamers[20,21] are short oligonucleotide sequences or peptides obtained through in vitro screening that exhibit high affinity and specificity for the corresponding ligands. Molecularly imprinted membranes[22,23] are polymers designed to match a spatial structure and binding sites of molecules, thereby enabling specific binding to target molecules. Liang et al.[24] synthesised nitrogen-doped GQDs and combined them with aptamers as fluorescent probes, thereby improving their recognition capability and successfully applying them to the specific detection of Golgi protein 73. However, the complexity, time-consuming nature, and high cost associated with aptamer synthesis hinder its widespread application in fluorescent probes[25]. Contrarily, molecularly imprinted polymers (MIP) have gained popularity owing to their simple and convenient synthesis process and low cost. Mantashloo et al.[26] prepared GQDs after magnetisation by thermally decomposing glucose precursors, followed by coating a layer of MIPs on them; this approach was employed to enhance the selective recognition ability of GQDs for quercetin. Thus, MIPs are promising ideal recognition elements for fluorescent probes.\u003c/p\u003e\n\u003cp\u003eIn this study, mango leaves were utilised as the raw material to synthesise Ag NPs@N/GQDs through thermal cleavage of N-doped GQDs (N/GQDs) and the subsequent generation of silver nanoparticles (AgNPs) on their surfaces via reduction. Moreover, a MIP was synthesised using noradrenaline as a template molecule by imprinting it onto the surface of the Ag NPs@N/GQDs. A 50% formic acid solution (V \u003csub\u003eformic acid\u003c/sub\u003e:V\u003csub\u003e\u0026nbsp;water\u003c/sub\u003e = 1:1) was employed to elute the template molecules to obtain Ag NPs@N/GQDs@MIP fluorescent probes that retained the recognition site for noradrenaline and exhibited specific recognition. By utilising the recognition site on the probe as a switch, different concentrations of noradrenaline can be adsorbed, effectively quenching of the fluorescence intensity of the probe and establishing a novel method for detecting noradrenaline. The incorporation of silver nanoparticles enhanced the fluorescence performance of the GQDs, resulting in a highly sensitive fluorescence intensity for the probe; moreover, integration with the MIP significantly improved its ability to specifically recognise noradrenaline.\u003c/p\u003e"},{"header":"Experimental","content":"\u003cp\u003e\u003cstrong\u003eReagents and instruments\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eSulfuric acid, nitric acid, sodium hydroxide, ethylenediamine, AgNO\u003csub\u003e3\u003c/sub\u003e, (3-aminopropyl)triethoxysilane, tetraethoxysilane, and noradrenaline standards were purchased from Aladdin Reagent Co. Ltd. in Shanghai, China. The reagents used in the experiments were analytically pure, and the water used was deionised water (18.2 M\u0026Omega;).\u003c/p\u003e\n\u003cp\u003eThe morphology and microstructure of the probe materials were observed using a scanning electron microscope (SEM; Carl Zeiss AG, Germany), and their crystal structures were characterised using X-ray powder diffraction (XRD; Ultima IV polycrystalline X-ray diffractometer, Rigaku Co., Ltd., Japan). The elements on the surfaces of the probes were characterised and analysed using X-ray photoelectron spectroscopy (XPS; ESCALAB 250 X-ray photoelectron spectrometer, Thermo Scientific, Inc., USA). Optical properties of the probe materials were measured using an F-7000 fluorescence spectrometer (Hitachi).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAg NPs@N/GQDs synthesis\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe synthesis method was based on an existing approach, which was further enhanced[27]. Details of the synthesis are shown in the Supporting Material.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAg NPs@N/GQDs/MIP synthesis\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNoradrenaline solution (10 mL; 0.1 mM), Ag NPs@N/GQDs (20 mg) and (3-aminopropyl)triethoxysilane (APTES) (100 \u0026mu;L) were added to a 50 mL flask, and the reaction was stirred at room temperature (25℃) for 20 min. Further,100 \u0026mu;L of tetraethoxysilane (TEOS) and 150 \u0026mu;L of NH\u003csub\u003e3\u003c/sub\u003e▪H\u003csub\u003e2\u003c/sub\u003eO (w/v 25%) were added and stirred at room temperature(25℃) for 8 h. The reaction product was obtained by centrifugation at 10000 rpm, the oligomers and unreacted monomers generated during the reaction process were washed off with deionised water, and the reaction was repeated three times. Next, the template noradrenaline in the MIP was eluted with 20 mL of 50% formic acid solution (v/v \u003csub\u003eformic acid\u003c/sub\u003e: \u003csub\u003ewater\u003c/sub\u003e = 1:1) for 5 min. Finally, the product obtained from the reaction was redissolved in 10 mL of PBS buffer (0.1 M, pH = 7.4) and stored at 4℃ for further use. The Ag NPs@N/GQDs/NIP was synthesized in the same way, but with no addition of noradrenaline.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFluorescence detection methods\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eFirst, 600 \u0026mu;L Ag NPs@N/GQDs/MIP probe solution was weighed, and different concentrations of noradrenaline were added. The fluorescence intensity before and after the addition of noradrenaline was detected after the full reaction with 0.1 M pH = 7.4 PBS to 2 mL for 5 min. The excitation wavelength Ex = 480 nm, emission spectrum range was 480\u0026ndash;700 nm, excitation wavelength slit was 5 nm, emission spectrum slit was 2.5 nm, and photomultiplier tube voltage was set at 700 V. The fluorescence intensity of noradrenaline was measured before and after its addition.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSample pre-treatment\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eFirst,0.1 g of the banana sample was weighed, and 1 mL of 80% methanol solution was added. Next, the mixture was agitated using a vortex shaker for 5 min, followed by centrifugation at 8000 rpm and 4 ℃ for 1 h.The supernatant was collected and dried using a nitrogen blowing apparatus. Next, it was reconstituted with 500 \u0026mu;L of Ag NPs@N/GQDs/MIP probe solution and subjected to another round of centrifugation at 8000 rpm and 4 ℃ for an additional 10 min. Finally, it was stored in the refrigerator at 4 ℃ until ready for testing.\u003c/p\u003e"},{"header":"Results and discussion","content":"\u003cp\u003e\u003cstrong\u003eCharacterisation of Ag NPs@N/GQDs\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe transmission electron microscopy (TEM) images showed a uniform distribution and relatively consistent size of the synthesised N/GQDs, indicating excellent dispersion (Fig. 2A). The particle size distribution plot in Fig. 2B shows that the synthesised N/GQDs ranged from 3 to 10 nm, with an average size of approximately 7 nm. When Ag NPs were generated by reducing the surface area of the N/GQDs, nucleoshell spherical nanoparticles formed with a core composed of N/GQDs (Fig. 2C). These particles were uniformly dispersed, and their particle size distribution ranged from 10 to 20 nm (Fig. 2D), with an average particle size of approximately 15 nm. The structure of the synthesised Ag NPs@N/GQDs was characterised and analysed, as depicted in Fig. 2E. Therefore, the XRD pattern that the synthesised Ag NPs@N/GQDs exhibit a distinct peak at 2θ=24.6°, which is low and narrow, indicating a significant particle size corresponding to the (002) crystal plane in the graphitic carbon structure[28]. Sharp peaks observed at 2θ=37.9°, 44.1°, 64.6°, and 77.1° correspond to the (111), (200), (220), and (311) crystal faces of Ag NPs respectively[29]. Furthermore, an elemental composition analysis of the Ag NPs@N/GQDs was conducted using the XPS data (Fig. 2F). Peaks C(1s) and N(1s) appear at 284.08 eV and 394.08 eV, respectively, originating from GQDs[30]; whereas the peaks of Ag(3d\u003csub\u003e3/2\u003c/sub\u003e) and Ag(3d\u003csub\u003e5/2\u003c/sub\u003e) appeared at 368.08 eV and 374.08 eV, which were both from AgNPs[31]. Therefore, the Ag NPs@N/GQDs were successfully synthesised.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eMIP\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003echaracterisation\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe MIP on the probe was characterised using XPS and Fourier-transform infrared (FTIR) spectroscopy. Fig. 3A shows a significant reduction in the C(1s) peak at 284.08 eV, O(1s) at 394.08 eV, and N(1s) peak at 401.08 eV in the MIP after eluting noradrenaline, because it mainly comprises the elements C, H, O, and N. Further characterisation of the MIP was performed using FTIR spectroscopy. As shown in Fig. 2B, absorption peaks corresponding to the amino stretching vibration regions were observed for noradrenaline at 3448 cm\u003csup\u003e–1\u003c/sup\u003e and 3418 cm\u003csup\u003e–1\u003c/sup\u003e,while the characteristic absorption peaks of the benzene ring skeleton were observed at 960 cm\u003csup\u003e–1\u003c/sup\u003e, 943 cm\u003csup\u003e–1\u003c/sup\u003e, 638 cm\u003csup\u003e–1\u003c/sup\u003e, and 599 cm\u003csup\u003e–1\u003c/sup\u003e. These peaks were also present in Ag NPs@N/GQDs/MIP but disappeared upon the removal of noradrenaline through elution from Ag NPs@N/GQDs/MIP. Therefore, the MIP was successfully synthesised.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFluorescence properties of Ag NPs@N/GQDs/MIP probes\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe optical properties of the fluorescent probes were examined using UV-Vis absorption and fluorescence spectroscopy. As shown in Fig.S1A (Supplementary Material), the Ag NPs@N/GQDs/MIP aptamer exhibited strong absorption at 495 nm. Additionally, Fig.S1B shows that the material has a maximum excitation wavelength near 480 nm (curve a). This wavelength was used to excite the probe, which resulted in a maximum emission wavelength of approximately 533 nm (curve b). The larger Stokes shift effectively prevented overlap between the excitation and emission spectra. Furthermore, the emission spectrum of the probe exhibited high intensity, symmetry, and a narrow peak shape, indicating excellent fluorescence performance. The quantum yield of a fluorescent substance is a crucial parameter that affects fluorescent probes. On the basis of the integrated fluorescence intensity (i.e. the area included in the corrected fluorescence spectrum) of the two dilute solutions of the fluorescence sample to be measured, the reference fluorescence reference material with known quantum yield under the same excitation conditions, and the absorbance of the incident light (UV–visible light) with the same excitation wavelength, the quantum yield of the tested fluorescent specimen can be calculated as[32]\u003c/p\u003e\n\u003cp\u003e\u003cimg src=\"data:image/png;base64,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\"\u003e\u003cbr\u003e\u003c/p\u003e\n\u003cp\u003eHere, Φ\u003csub\u003eu\u003c/sub\u003e and Φ\u003csub\u003es\u003c/sub\u003e represent the fluorescence quantum yields of the substance under measurement and the reference standard, respectively; F\u003csub\u003eu\u003c/sub\u003e and F\u003csub\u003es\u003c/sub\u003e denote the integrated fluorescence intensities of the substance being measured and the reference substance; A\u003csub\u003eu\u003c/sub\u003e and A\u003csub\u003es\u003c/sub\u003e indicate the absorbance of incident light at the excitation wavelengths of the substance under measurement and the reference substance (A=εbc). In this study, Rhodamine B (RhB) solution was utilised as a reference fluorescence standard. According to the literature, RhB has a reported quantum yield of 0.89[33]; the relevant parameters shown in Table S1.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eMechanism of Ag NPs@N/GQDs/MIP probe signal reduction by norepinephrine\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWith the addition of different concentrations of noradrenaline, the probe captured increasing amounts of noradrenaline using the MIP. As shown in Fig. 5A, the fluorescence intensity of the Ag NPs@N/GQDs/MIP decreased, indicating that noradrenaline effectively suppressed the fluorescence intensity of the probe. When the template molecule norepinephrine was readded, the Ag NPs@N/GQDs/MIP was able to interact with it, leading to fluorescence quenching. We suggest that an electron transfer between the N/GQDs and norepinephrine is responsible for this phenomenon. The mechanism of fluorescence quenching was studied through UV-visible and fluorescence spectroscopy. As shown in Fig. 5B, according to previous reports, the fluorescence quenching mechanism may mainly involve electron or energy transfer from Ag NPs@N/GQDs/MIP to norepinephrine. The maximum norepinephrine adsorption was 453 nm (curve a), which was close to the band gap of the Ag NPs@N/GQDs/MIP at an excitation wavelength of 480 nm (curve b). These results indicate that the electrons in the conduction bands of the Ag NPs@N/GQDs/MIP could be transferred to the lowest unoccupied orbital of norepinephrine. In addition, the propanil absorption peaks exhibited evident red shifts after APTES addition (curve c). This indicates that analytes may act as electron acceptors to trigger a non-radiative decay electron process[34]. The maximum emission of the Ag NPs@N/GQDs/MIP occurred at 533 nm, exhibiting a narrow and symmetrical line width (curve e). There is no spectral overlap between the emission spectrum of the Ag NPs@N/GQDs/MIP and the absorption spectra of norepinephrine. Hence, we do not think that the fluorescence quenching is caused by the energy transfer mechanism.\u003c/p\u003e\n\u003cp\u003eThese results also imply that the electron transfer from the Ag NPs@N/GQDs/MIP to norepinephrine could be the main optosensing turn-off mechanism[35]. During the fluorescence response, the electrons in the N/GQDs were excited by the UV spectrum energy. Subsequently, when the excited electrons returned to their ground state, the N/GQDs emitted fluorescence. Correspondingly, after norepinephrine was added, it was specifically adsorbed onto the imprinting cavities. The Meisenheimer complex was formed through the strong interactions between the –NH\u003csub\u003e2\u003c/sub\u003e of the functional monomer APTES and –OH of norepinephrine. Therefore, the electrons of the N/GQDs were transferred to the complex, resulting in fluorescence quenching.\u003c/p\u003e\n\u003cp\u003eIn addition, the fluorescence quenching mechanism can be explained by molecular orbital theory. As shown in Fig. S2A, an electron of the N/GQDs is excited from the valence band (ground state) to the conduction band after accepting the UV photon. Afterward, the excited electron returns to the valence band, and the N/GQDs produce the fluorescence signal. In addition, in the presence of the template norepinephrine, a hydrogen bonding interaction exists between the amino groups of the N/GQDs and norepinephrine. The interaction force is so strong that it can cause the electron to be transferred between the norepinephrine and N/GQDs. The excited electron is able to transition directly into the LUMO of the complex. It would then return to the ground state, generating no fluorescence signals, because the energy level of the complex would be higher than that of the N/GQDs[36]. This explains the phenomenon of fluorescence quenching (Fig. S2B).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eExperimental optimisation conditions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe effects of elution time, buffer pH and reaction time on the detection of noradrenaline by the Ag NPs@N/GQDs/MIP aptamer probes were investigated. As shown in Fig. S3A, as the elution time increased, increasing amounts of noradrenaline were was removed from the MIP, the fluorescence intensity of probe was continuously enhanced. After 5 min, the probe fluorescence increased to the maximum and remained unchanged thereafter, indicating that the probe had been completely eluted. As shown in Fig. S3B, the ratio of the decrease in the fluorescence value △IF (△IF=F\u003csub\u003e0\u003c/sub\u003e–F\u003csub\u003e1\u003c/sub\u003e, where F\u003csub\u003e0\u0026nbsp;\u003c/sub\u003erepresents the fluorescence intensity before adding 1.0 nM noradrenaline and F\u003csub\u003e1\u003c/sub\u003e represents the fluorescence intensity after) exhibited different changes as the pH increased from 6.4, but reached its maximum at pH=7.4. Therefore, PBS (pH 7.4) was selected as the optimal buffer. Additionally, the effect of the reaction time of noradrenaline with the fluorescent probe on the quenching effect was investigated. As shown in Fig. S3C, as the reaction proceeded, an increasing amount of noradrenaline was adsorbed onto the probe by MIP, and the probe fluorescence intensity continuously decreased until the reaction progressed for a minimum of 5 min and remained constant thereafter. Therefore, a reaction time of 5 min was selected as the optimal reaction time.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFluorescence response of Ag NPs@N/GQDs/MIP probes to noradrenaline\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe fluorescence intensities of the Ag NPs@N/GQDs/MIP probes were measured before and after adding different concentrations of noradrenaline under the optimal experimental conditions. The decrease in the fluorescence intensity value (△IF=F\u003csub\u003e0\u003c/sub\u003e–F\u003csub\u003e1\u003c/sub\u003e) was calculated, and a calibration curve was plotted. As shown in Fig. 5A, the fluorescence intensity of the probe increased continuously with increasing noradrenaline concentration.In the range of 0.5 –700 pM, there was a good linear relationship between the logarithm of noradrenaline concentration (ln(c)) and the logarithm of the decrease in fluorescence intensity (ln(△IF)). The linear regression equation was ln(△IF) = 0.505 ln(c) (pM) + 5.27 (Fig. 5B), with an r-value of 0.9991 and a detection limit of 0.154 pM (D.L.= KS\u003csub\u003eb\u003c/sub\u003e/a, K = 3). This method exhibited a higher sensitivity for detecting noradrenaline than previously reported methods ( Table S2).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSelectivity of Ag NPs@N/GQDs/MIP probes\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe selectivity of the Ag NPs@N/GQDs/MIP probes was evaluated by studying their response in the presence of dopamine, epinephrine, catechol, vitamin B6, vitamin B2, chlorpyrifos, and methyl parathion. As shown in Fig. 6, because there are no molecularly imprinted recognition sites, the Ag NPs@N/GQDs/NIP cannot recognize norepinephrine and adsorb it to the surface of the probe, so it does not decrease the fluorescence intensity of the probe. Similarly, the aforementioned interfering substances are not recognized and adsorbed by the NIP, and the fluorescence intensity of probe does not decrease. In contrast, because the molecularly imprinted sites in the Ag NPs@N/GQDs/MIP can identify and adsorb norepinephrine, the fluorescence intensity of the probe is decreased. However, because the interfering substances cannot enter the imprinted holes or be recognized and adsorbed by the MIP, the fluorescence of the probe does not change. This result demonstrates that the probes were able to recognize norepinephrine with excellent specificity.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eReproducibility and stability of Ag NPs@N/GQDs/MIP probes\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe reproducibility and stability of the Ag NPs@N/GQDs/MIP probes for noradrenaline detection were investigated. Five Ag NPs@N/GQDs/MIP probes were subjected to the addition of 1.0 nM noradrenaline under identical conditions, and the fluorescence intensities before and after the addition were measured to calculate ΔIF. The results demonstrated a relative standard deviation (RSD) of 2.73% for the ΔIF values from five experiments, indicating excellent reproducibility (Fig. S4). Furthermore, the prepared Ag NPs@N/GQDs/MIP fluorescent probe was exposed to 1.0 nM noradrenaline, and its fluorescence intensity was monitored at intervals of 5 min. As shown in Fig. S5, after a reaction time of 30 min,the fluorescence intensity of the probe reached 92.2% of its intensity after the completion of the reaction (5 min), demonstrating remarkable stability.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eReal sample analysis\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eA fluorescent probe was used to detect the banana samples, and a spiked recovery test was conducted. Table 1 presents the results of the study. The method exhibited recoveries ranging from 93.6% to 112.0%, with an RSD valueless than 5.0%. Therefore, the current fluorescent probe method can be used effectively to detect actual samples and yield satisfactory results.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTable 1.\u0026nbsp;\u003c/strong\u003eResults of sample assay and recovery analysis.\u003c/p\u003e\n\u003ctable border=\"1\" cellspacing=\"0\" cellpadding=\"0\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd width=\"14.583333333333334%\"\u003e\n \u003cp\u003eSamples\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"16.666666666666668%\"\u003e\n \u003cp\u003eThis method\u003c/p\u003e\n \u003cp\u003e(nM, n=5)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.458333333333334%\"\u003e\n \u003cp\u003eRSD\u003c/p\u003e\n \u003cp\u003e%\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"15.625%\"\u003e\n \u003cp\u003eAdded\u003c/p\u003e\n \u003cp\u003e(nM)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"21.875%\" style=\"width: 20.6529%;\"\u003e\n \u003cp\u003eTotal found\u003c/p\u003e\n \u003cp\u003e(nM, n=5)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.333333333333334%\" style=\"width: 8.9142%;\"\u003e\n \u003cp\u003eRSD\u003c/p\u003e\n \u003cp\u003e%\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.458333333333334%\"\u003e\n \u003cp\u003eRecoveries\u003c/p\u003e\n \u003cp\u003e%\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"14.583333333333334%\" rowspan=\"2\"\u003e\n \u003cp\u003eBanana Ⅰ \u0026nbsp; \u0026nbsp;\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"16.666666666666668%\" rowspan=\"2\"\u003e\n \u003cp\u003e1.85\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.458333333333334%\" rowspan=\"2\"\u003e\n \u003cp\u003e4.97\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"15.625%\"\u003e\n \u003cp\u003e1.50\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"21.875%\" style=\"width: 20.6529%;\"\u003e\n \u003cp\u003e3.27\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.333333333333334%\" style=\"width: 8.9142%;\"\u003e\n \u003cp\u003e3.44\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.458333333333334%\"\u003e\n \u003cp\u003e94.7\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"27.272727272727273%\"\u003e\n \u003cp\u003e15.00\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"38.18181818181818%\" style=\"width: 20.6529%;\"\u003e\n \u003cp\u003e15.89\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"14.545454545454545%\" style=\"width: 8.9142%;\"\u003e\n \u003cp\u003e4.90\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"20%\"\u003e\n \u003cp\u003e93.6\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"14.583333333333334%\" rowspan=\"2\"\u003e\n \u003cp\u003eBanana Ⅱ\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"16.666666666666668%\" rowspan=\"2\"\u003e\n \u003cp\u003e2.83\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.458333333333334%\" rowspan=\"2\"\u003e\n \u003cp\u003e3.76\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"15.625%\"\u003e\n \u003cp\u003e1.00\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"21.875%\" style=\"width: 20.6529%;\"\u003e\n \u003cp\u003e3.95\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.333333333333334%\" style=\"width: 8.9142%;\"\u003e\n \u003cp\u003e3.76\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.458333333333334%\"\u003e\n \u003cp\u003e112.0\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"27.272727272727273%\"\u003e\n \u003cp\u003e10.00\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"38.18181818181818%\" style=\"width: 20.6529%;\"\u003e\n \u003cp\u003e12.32\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"14.545454545454545%\" style=\"width: 8.9142%;\"\u003e\n \u003cp\u003e4.32\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"20%\"\u003e\n \u003cp\u003e94.9\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e"},{"header":"Conclusion","content":"\u003cp\u003eIn this study, an Ag NPs@N/GQDs/MIP fluorescent composite nanoprobe was synthesised and utilised for the ultra-sensitive detection of noradrenaline in bananas. Incorporating Ag NPs enhanced the fluorescence performance of the N/GQDs, resulting in a stable and intense fluorescence emission spectrum with a high probe quantum yield. Furthermore, by introducing MIP, the probe exhibited excellent selective recognition of the target molecule, noradrenaline, effectively eliminating interference. Therefore, this probe demonstrated remarkable sensitivity, selectivity, and ease of operation for detecting noradrenaline residues in bananas.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe would like to thank Prof. Dr. Lianming Zhang for his consultancy in the nanomaterials synthesis studies.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eYaru Wang: conceptualization, methodology, formal analysis and investigation, writing\u0026ndash;original draft preparation. Shuhuai Li: conceptualization, resources, writing\u0026ndash;review and editing, supervision, project administration, funding acquisition. Xionghui Ma: data curation, methodology, writing\u0026ndash;review and editing. Chaohai Pang: investigation, writing\u0026ndash;review and editing. Yuwei Wu: investigation, writing\u0026ndash;review and editing. Mingyue Wang: conceptualization, resources, writing\u0026ndash;review and editing, supervision, project administration, funding acquisition. Bei Li: investigation, writing\u0026ndash;review and editing. Sixin Liu: conceptualization, resources, supervision, project administration.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData Availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis research was supported by China Agriculture Research System of MOF and MARA (CARS-31), Chinese Academy of Tropical Agricultural Sciences for Science and Technology Innovation Team of National Tropical Agricultural Science Center (CATASCXTD202314), the Key Laboratory of Tropical Fruits and Vegetables Quality and Safety for State Market Regulation (ZX-2023002).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthical Approval\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis study does not contain any studies with human participants or animals performed by any of the authors.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent to Participate\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent for Publication\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConflict of Interest\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThere are no conflicts to declare.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eMuhammad FA, Waseem K, Sidra A, Muhammad AK, Muhammad Z, Safura K, Khalid AM, Afifa A, Shahida AS (2022) Bioactive profile and functional food applications of banana in food sectors and health: a review. 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Anal Chem 80: 3458-65. https://doi.org/10.1021/ac800060f\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":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"journal-of-fluorescence","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"jofl","sideBox":"Learn more about [Journal of Fluorescence](https://www.springer.com/journal/10895)","snPcode":"10895","submissionUrl":"https://submission.nature.com/new-submission/10895/3","title":"Journal of Fluorescence","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Fluorescent probe, Graphene quantum dots, Silver nanoparticles, Molecular imprinted polymer, Noradrenaline","lastPublishedDoi":"10.21203/rs.3.rs-3647535/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-3647535/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"Fluorescence intensity and selective recognition ability are crucial factors in determining the analytical techniques for fluorescent probes. In this study, a core-shell fluorescent material composed of silver nanoparticles@nitrogen-doped graphene quantum dots (Ag NPs@N/GQDs) was synthesised using mango leaves as the raw material through a thermal cracking method, resulting in strong fluorescence luminescence intensity. By employing noradrenaline as a template molecule and utilising a surface molecular imprinting technique, a molecularly imprinted membrane (MIP) was formed on the surface of the fluorescent material, which was subsequently eluted to obtain a highly specific fluorescent probe capable of recognising noradrenaline. The probe captured various concentrations of noradrenaline using the MIP, causing a decrease in the probe fluorescence intensity. Then a method for detecting trace amounts of noradrenaline was established. This method exhibited a linear range from 0.5 –700 pM with a detection limit of 0.154 pM. The proposed method was achievemently implemented in banana samples. Satisfactory recoveries were confirmed at four different concentrations. The method presented a relative standard deviation (RSD%) of less than 5.0%.","manuscriptTitle":"Fluorescent probes based on Ag NPs@N/GQDs and molecularly imprinted polymer for sensitive detection of noradrenaline in bananas","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2023-11-28 17:55:46","doi":"10.21203/rs.3.rs-3647535/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2023-12-07T11:45:01+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2023-12-05T12:28:52+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"e92e7453-c6b1-4bad-b325-c782fef74243","date":"2023-11-30T17:58:51+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2023-11-28T12:21:03+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2023-11-24T04:55:37+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2023-11-24T04:55:37+00:00","index":"","fulltext":""},{"type":"submitted","content":"Journal of Fluorescence","date":"2023-11-22T08:13:23+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"journal-of-fluorescence","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"jofl","sideBox":"Learn more about [Journal of Fluorescence](https://www.springer.com/journal/10895)","snPcode":"10895","submissionUrl":"https://submission.nature.com/new-submission/10895/3","title":"Journal of Fluorescence","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"1bd0b886-9e69-4196-9fe9-1859920c15ea","owner":[],"postedDate":"November 28th, 2023","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2024-01-15T15:05:35+00:00","versionOfRecord":{"articleIdentity":"rs-3647535","link":"https://doi.org/10.1007/s10895-023-03565-w","journal":{"identity":"journal-of-fluorescence","isVorOnly":false,"title":"Journal of Fluorescence"},"publishedOn":"2024-01-09 15:02:05","publishedOnDateReadable":"January 9th, 2024"},"versionCreatedAt":"2023-11-28 17:55:46","video":"","vorDoi":"10.1007/s10895-023-03565-w","vorDoiUrl":"https://doi.org/10.1007/s10895-023-03565-w","workflowStages":[]},"version":"v1","identity":"rs-3647535","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-3647535","identity":"rs-3647535","version":["v1"]},"buildId":"FbvkV6FR0MCFSLy54lSbu","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}