Preparation of Molecularly Imprinted Cysteine Modified Zinc Sulfide Quantum Dots Based Sensor for Rapid Detection of Dopamine Hydrochloride

By combining surface molecular imprinting technology with cysteine-modified ZnS quantum dots, an elegant, molecularly imprinted cysteine-modified Mn2+: ZnS QDs (MIP@ZnS QDs) based fluorescence sensor was successfully developed. The constructed fluorescence sensor is based on a molecularly imprinted polymer (MIP) coated on the surface cysteine-modified ZnS quantum dots and used for rapid fluorescence detection of dopamine hydrochloride. The MIP@ZnS quantum dots possess the advantages of rapid response, high sensitivity, and selectivity for the detection of dopamine hydrochloride molecules. Experimental results show that the adsorption equilibrium time of MIP@ZnS QDs for dopamine hydrochloride molecules is 12 min, and it can selectively capture and bind dopamine in the sample with an imprinting factor of 29.5. The fluorescence quenching of MIP@ZnS QDs has a good linear (R2 = 0.9936) with the concentration of dopamine hydrochloride ranged from 0.01 to 1.0 μM, and the limit of detection is 3.6 nM. In addition, The MIP@ZnS QDs demonstrate good recyclability and stability and are successfully employed for detection of dopamine hydrochloride in urine samples with recoveries was 95.2% to 103.8%. The proposed MIP@ZnS QDs based fluorescent sensor provides a promising approach for food safety detection and drug analysis.


Introduction
Dopamine hydrochloride (3-hydroxytyramine hydrochloride, DA-HCL) is considered a central neurotransmitter which plays an important role in cardiovascular, central nervous system, renal function regulation, and so on [1,2]. As a β-stimulant, dopamine hydrochloride has the function of accelerating metabolism and inhibiting lipogenesis. It is illegally added to foodborne animal feed as a "clenbuterol" drug, causing great concern for food safety supervision [3,4]. Such substances can accumulate in the human body through contaminated food, causing acute poisoning symptoms such as palpitation, arrhythmia, nausea and vomiting, inducing Parkinson's disease, hypertension and gastrointestinal dysfunction [5][6][7]. At present, the detection methods of dopamine hydrochloride include high performance liquid chromatography (HPLC), HPLC-mass spectrometry, solid phase extraction-HPLC, enzyme-linked immunosorbent assay, electrochemistry, chemical/biological sensing, and so on [8][9][10][11][12][13]. However, many of these detection techniques require complex sample pretreatment processes or high-precision instruments, requiring professional and technical personnel to operate, and long detection cycles. Therefore, it is important to develop a low-cost, rapid, convenient, and highly selective method for detecting dopamine hydrochloride.
Quantum dots (QDs) are nanoscale semiconductors with quantum confinement of electrons and holes [14,15]. Quantum dots are highly valuable in biological analysis and sensing due to their unique characteristics, such as quantum size effect, size-dependent

Preparation of MIPs@ZnS QDs
The fabrication process of the dopamine hydrochloride fluorescent sensor is depicted in Figure 1. First, Mn 2+ : ZnS QDs were used as the signal transduction element and solid substrate nanomaterials. The ZnS quantum dots have narrow emission bands, and the Mn 2+ doped in ZnS QDs can further improve the fluorescence quantum yield and photochemical stability of quantum dots [41]. In addition, Mn 2+ doped ZnS QDs has water dispersibility and low cytotoxicity. The cysteine was modified on Mn 2+ : ZnS QDs to enhance the fluorescence stability and dispersion in aqueous. Moreover, cysteine modification can provide carboxylic acid group (-COOH) on the surface of QDs, which promotes the binding of monomers with template molecules and makes it easier to form recognition cavies. The cysteine-modified Mn 2+ : ZnS QDs can also be used as assistant monomers contributing to form imprinting cavies. Second, methacrylic acid was chosen as a functional monomer and EGDMA as a crosslinker; the dopamine hydrochloride molecules were imprinted on cysteine-modified Mn 2+ : ZnS QDs though sol-gel reaction. The constructed MIP@ZnS QDs has a core-shell structure. The MIP layer on the surface of MIP@ZnS QDs can not only selectively capture the template molecule (dopamine hydrochloride), but also prevent the interaction between other interfering substances and the quantum dot. The MIP shell on the QDs can effectively reduce the influence caused by the interfering substances. Finally, the MIP@ZnS QDs were eluted by acetic acid-methanol solvent to remove imprinted template molecules until the fluorescence intensity of MIP@ZnS QDs was not changed and similar to that of NIP. The formed imprinting sites was complementary to dopamine hydrochloride molecules in shape, size and chemical structure. The developed molecularly imprinted quantum dot can be specifically combined with dopamine hydrochloride molecules through a molecularly imprinted layer, and the fluorescence of the quantum dot is quenched because of the energy transfer effect of electrons between the quantum dot and dopamine hydrochloride molecules.

Characterization
Fluorescence spectra indicated that the maximum excitation and emission peaks of MIP@ZnS QDs are located at 320 nm and 596 nm, respectively. As shown in Figure 2, the prepared MIP@ZnS QDs has symmetrical emission peaks and orange fluorescence emission. The full width at half maximum (FWHM) of MIP@ZnS QDs is 40 nm, and no visible defect peaks were found. In addition, before the elution of dopamine hydrochloride molecules, the intensity of MIP@ZnS QDs was only 25.2% of that of NIP@ZnS QDs. The fluorescence quenching is due to the amount of template molecule binding on the MIP@ZnS QDs, which leads to a charge transfer interaction between the QDs and the dopamine hydrochloride molecule. After the elution, the fluorescence emission of MIP@ZnS QDs recovered significantly, and the fluorescence intensity was 97.3% of NIPs.

Characterization
Fluorescence spectra indicated that the maximum excitation and emission peaks of MIP@ZnS QDs are located at 320 nm and 596 nm, respectively. As shown in Figure 2, the prepared MIP@ZnS QDs has symmetrical emission peaks and orange fluorescence emission. The full width at half maximum (FWHM) of MIP@ZnS QDs is 40 nm, and no visible defect peaks were found. In addition, before the elution of dopamine hydrochloride molecules, the intensity of MIP@ZnS QDs was only 25.2% of that of NIP@ZnS QDs. The fluorescence quenching is due to the amount of template molecule binding on the MIP@ZnS QDs, which leads to a charge transfer interaction between the QDs and the dopamine hydrochloride molecule. After the elution, the fluorescence emission of MIP@ZnS QDs recovered significantly, and the fluorescence intensity was 97.3% of NIPs.
The structure and morphology of the MIP@ZnS QDs were characterized using JF-2100 transmission electron microscope (TEM). Figure 3 revealed that the MIP@ZnS QDs possessed spherical shape, and particle size distribution of prepared MIP@ZnS QDs is about 4.9 ± 0.53 nm. defect peaks were found. In addition, before the elution of dopamine hydrochloride molecules, the intensity of MIP@ZnS QDs was only 25.2% of that of NIP@ZnS QDs. The fluorescence quenching is due to the amount of template molecule binding on the MIP@ZnS QDs, which leads to a charge transfer interaction between the QDs and the dopamine hydrochloride molecule. After the elution, the fluorescence emission of MIP@ZnS QDs recovered significantly, and the fluorescence intensity was 97.3% of NIPs. The structure and morphology of the MIP@ZnS QDs were characterized using JF-2100 transmission electron microscope (TEM). Figure 3 revealed that the MIP@ZnS QDs possessed spherical shape, and particle size distribution of prepared MIP@ZnS QDs is about 4.9 ± 0.53 nm.

Effect of the Monomer on MIP@ZnS QDs
The amount of functional monomer (MAA) is major influencing factor in the molecular imprinting process. The changes of fluorescence quenching (ΔF) of MIP@ZnS QDs under the different amount of MAA are present in Figure 4. As the amount of MAA increases (0~400 μL), the fluorescence quenching of MIP@ZnS quantum dots gradually enhances. When the dosage of MAA monomer is 400 μL, the highest quenching effect of dopamine hydrochloride on MIP@ZnS quantum dots is achieved. Experimental results indicate that a low dosage of MAA is insufficient to form an MIP layer on the cysteinemodified ZnS QDs, resulting in inadequate binding sites for dopamine in molecular imprinting. Excessive use of monomers can increase the thickness of the MIP layer, which is detrimental to the entry of dopamine hydrochloride molecules into the binding site and affects the mass transfer efficiency. Additionally, an excess of crosslinker agents during preparation can also hinder the binding of target molecules on MIP@ZnS quantum dots.

Effect of the Monomer on MIP@ZnS QDs
The amount of functional monomer (MAA) is major influencing factor in the molecular imprinting process. The changes of fluorescence quenching (∆F) of MIP@ZnS QDs under the different amount of MAA are present in Figure 4. As the amount of MAA increases (0~400 µL), the fluorescence quenching of MIP@ZnS quantum dots gradually enhances. When the dosage of MAA monomer is 400 µL, the highest quenching effect of dopamine hydrochloride on MIP@ZnS quantum dots is achieved. Experimental results indicate that a low dosage of MAA is insufficient to form an MIP layer on the cysteine-modified ZnS QDs, resulting in inadequate binding sites for dopamine in molecular imprinting. Excessive use of monomers can increase the thickness of the MIP layer, which is detrimental to the entry of dopamine hydrochloride molecules into the binding site and affects the mass transfer efficiency. Additionally, an excess of crosslinker agents during preparation can also hinder the binding of target molecules on MIP@ZnS quantum dots.   Figure 5 shows that the fluorescence intensity of MIP@ZnS quantum dots increases with the pH values in the range of 4.0 to 7.0. The instability of the structure of MIP@ZnS quantum dots in acidic environments leads to a decrease in their fluorescence performance. The experiment shows that dopamine hydrochloride molecules are more easily bound with MIP@ZnS quantum dots in weak alkaline environments, with significant fluorescence quenching efficiency. When the solution pH value is 7.5, the fluorescence quenching of MIP@ZnS quantum dots reaches the maximum value. Therefore, the pH value of 7.5 set in the experiment is the optimal pH value for detecting hydrochloride dopamine with MIP@ZnS quantum dots.

Response Time
As shown in Figure 6, the degree of fluorescence quenching (ΔF) of dopamine hydrochloride molecules on MIP@QDs increases with the increase of incubation time (0-12 min). Dopamine hydrochloride molecules have a significant fluorescence quenching effect on MIP@ZnS quantum dots. The fluorescence of MIP@ZnS QDs tended to stabilize after incubation for 12 min, indicating that the adsorption and binding of dopamine hydrochloride molecules on MIP@ZnS QDs reached dynamic equilibrium. As the control group, the NIP@ ZnS QDs showed slight fluorescence change in the samples reaching equilibrium at around 10 min. It is due to the lack of molecular imprinting sites for dopamine hydrochloride molecules on NIP@ZnS QDs; the binding of dopamine hydrochloride molecules is mainly through physical adsorption [45,46]. The experimental results suggest that the  The experiment shows that dopamine hydrochloride molecules are more easily bound with MIP@ZnS quantum dots in weak alkaline environments, with significant fluorescence quenching efficiency. When the solution pH value is 7.5, the fluorescence quenching of MIP@ZnS quantum dots reaches the maximum value. Therefore, the pH value of 7.5 set in the experiment is the optimal pH value for detecting hydrochloride dopamine with MIP@ZnS quantum dots.   Figure 5 shows that the fluorescence intensity of MIP@ZnS quantum dots increases with the pH values in the range of 4.0 to 7.0. The instability of the structure of MIP@ZnS quantum dots in acidic environments leads to a decrease in their fluorescence performance. The experiment shows that dopamine hydrochloride molecules are more easily bound with MIP@ZnS quantum dots in weak alkaline environments, with significant fluorescence quenching efficiency. When the solution pH value is 7.5, the fluorescence quenching of MIP@ZnS quantum dots reaches the maximum value. Therefore, the pH value of 7.5 set in the experiment is the optimal pH value for detecting hydrochloride dopamine with MIP@ZnS quantum dots.

Response Time
As shown in Figure 6, the degree of fluorescence quenching (ΔF) of dopamine hydrochloride molecules on MIP@QDs increases with the increase of incubation time (0-12 min). Dopamine hydrochloride molecules have a significant fluorescence quenching effect on MIP@ZnS quantum dots. The fluorescence of MIP@ZnS QDs tended to stabilize after incubation for 12 min, indicating that the adsorption and binding of dopamine hydrochloride molecules on MIP@ZnS QDs reached dynamic equilibrium. As the control group, the NIP@ ZnS QDs showed slight fluorescence change in the samples reaching equilibrium at around 10 min. It is due to the lack of molecular imprinting sites for dopamine hydrochloride molecules on NIP@ZnS QDs; the binding of dopamine hydrochloride molecules is mainly through physical adsorption [45,46]. The experimental results suggest that the

Response Time
As shown in Figure 6, the degree of fluorescence quenching (∆F) of dopamine hydrochloride molecules on MIP@QDs increases with the increase of incubation time (0-12 min). Dopamine hydrochloride molecules have a significant fluorescence quenching effect on MIP@ZnS quantum dots. The fluorescence of MIP@ZnS QDs tended to stabilize after incubation for 12 min, indicating that the adsorption and binding of dopamine hydrochloride molecules on MIP@ZnS QDs reached dynamic equilibrium. As the control group, the NIP@ ZnS QDs showed slight fluorescence change in the samples reaching equilibrium at around 10 min. It is due to the lack of molecular imprinting sites for dopamine hydrochloride molecules on NIP@ZnS QDs; the binding of dopamine hydrochloride molecules is mainly through physical adsorption [42,43]. The experimental results suggest that the response time of the prepared MIP@ZnS quantum dots sensor in the sample solution is 12 min. response time of the prepared MIP@ZnS quantum dots sensor in the sample s min.

Determination of Dopamine Hydrochloride by MIP@ZnS QDs
In this experiment, the principal detection method is based on fluoresce ing of MIP@ZnS QDs by dopamine molecules. The prepared MIP@ZnS QD orange fluorescence emission was generated by photoexcitation of the host Z tal recombined by the lower-lying states of the dopped Mn 2+ ion [38]. The M could selectively capture target molecules (DA) through complementary imp on the molecularly imprinted polymers of Mn 2+ : ZnS QDs. This specific bin introduces electron transfer of Mn 2+ : ZnS QDs to the target molecule (DA), w fluorescence quenching of MIP@ZnS QDs [39]. Moreover, the quenchin MIP@ZnS QDs was positively correlated with the concentration of target mol samples. Figure 7 depicts the fluorescence emission spectra of the prepared M NIP@ZnS quantum dots under different concentrations of dopamine hydroch increased concentrations of hydrochloric acid dopamine in the sample, the spectra of MIP@ZnS and NIP@ZnS QDs presented significant changes. Com NIP@ ZnS QDs, MIP@ZnS quantum dots exhibit stronger fluorescence quenc fluorescence spectra, with a fluorescence quenching degree (F0/F − 1) much that of NIP@ ZnS quantum dots. MIP@ZnS quantum dots possess binding si plement the shape, size, and functional groups of dopamine hydrochlorid through non-covalent interactions such as hydrogen bonding and van der [21,47]. Due to the presence of the imprinting sites, more dopamine hydroch cules specifically bound to MIP@ZnS QDs and resulted in significant fluoresce ing. In contrast, dopamine hydrochloride molecules mainly bound to NIP@Q non-specific adsorption, limiting the amount of dopamine molecules adsor NIPs. Therefore, NIP@ZnS QDs displayed lower fluorescence quenching MIPs. Figure 7A

Determination of Dopamine Hydrochloride by MIP@ZnS QDs
In this experiment, the principal detection method is based on fluorescence quenching of MIP@ZnS QDs by dopamine molecules. The prepared MIP@ZnS QDs possessed orange fluorescence emission was generated by photoexcitation of the host ZnS nanocrystal recombined by the lower-lying states of the dopped Mn 2+ ion [38]. The MIP@ZnS QDs could selectively capture target molecules (DA) through complementary imprinting sites on the molecularly imprinted polymers of Mn 2+ : ZnS QDs. This specific binding process introduces electron transfer of Mn 2+ : ZnS QDs to the target molecule (DA), which leads to fluorescence quenching of MIP@ZnS QDs [39]. Moreover, the quenching degree of MIP@ZnS QDs was positively correlated with the concentration of target molecules in the samples. Figure 7 depicts the fluorescence emission spectra of the prepared MIP@ZnS and NIP@ZnS quantum dots under different concentrations of dopamine hydrochloride. With increased concentrations of hydrochloric acid dopamine in the sample, the fluorescence spectra of MIP@ZnS and NIP@ZnS QDs presented significant changes. Compared with NIP@ ZnS QDs, MIP@ZnS quantum dots exhibit stronger fluorescence quenching in their fluorescence spectra, with a fluorescence quenching degree (F 0 /F − 1) much higher than that of NIP@ ZnS quantum dots. MIP@ZnS quantum dots possess binding sites that complement the shape, size, and functional groups of dopamine hydrochloride molecules through non-covalent interactions such as hydrogen bonding and van der Waals forces [21,44]. Due to the presence of the imprinting sites, more dopamine hydrochloride molecules specifically bound to MIP@ZnS QDs and resulted in significant fluorescence quenching. In contrast, dopamine hydrochloride molecules mainly bound to NIP@QDs through nonspecific adsorption, limiting the amount of dopamine molecules adsorption on the NIPs. Therefore, NIP@ZnS QDs displayed lower fluorescence quenching than that of MIPs. Figure 7A showed that the fluorescence quenching of MIP@ZnS QDs is proportion to the concentration of dopamine hydrochloride in the samples. Curve fitting ( Figure 7C) indicates that there is a good linear relationship (R 2 = 0.9936) between F 0 /F − 1 and concentration of dopamine hydrochloride (Q) in the range of 0.01~1.0 µM. The Stern-Volmer equation is F 0 /F = 2.8445 Q -0.00373. The limit of detection is calculated by 3δ/S (IUPAC criteria), δ is the standard deviation of the blank signal (n = 20), and S is the slope of the linear calibration plot. The corresponding detection limit of MIP@ZnS QDs for dopamine is 3.6 nM.

Selective Experimental Methods
In selective experiments, dopamine hydrochloride significantly exhibited fluorescence quenching constants on the MIP@ZnS QDs compared with other molecules ( Figure  8). During the molecular imprinting process, a series of binding sites were formed on the cysteine-modified Mn 2+ : ZnS QDs, which complement the DA molecule in shape, structure, size and functional groups. Due to the presence of these molecular imprinting binding sites, MIP@ZnS QDs is more likely to bind with DA molecules. Therefore, a large amount of dopamine hydrochloride molecules were specifically bound to MIP@ZnS QDs, which results in significant fluorescence quenching. However, as reference compounds, p-aminophenol, pyrocatechol and carbamide molecules are different in shape, size, and functional groups from dopamine hydrochloride molecules. These structural analog molecules are only bound to MIP through non-specific effects such as physical adsorption and cannot match the molecularly imprinted cavies on MIP@ZnS QDs. Thus, their adsorption capacity and fluorescence quenching degree were lower on MIP@ZnS QDs. As a control,

Selective Experimental Methods
In selective experiments, dopamine hydrochloride significantly exhibited fluorescence quenching constants on the MIP@ZnS QDs compared with other molecules (Figure 8). During the molecular imprinting process, a series of binding sites were formed on the cysteine-modified Mn 2+ : ZnS QDs, which complement the DA molecule in shape, structure, size and functional groups. Due to the presence of these molecular imprinting binding sites, MIP@ZnS QDs is more likely to bind with DA molecules. Therefore, a large amount of dopamine hydrochloride molecules were specifically bound to MIP@ZnS QDs, which results in significant fluorescence quenching. However, as reference compounds, p-aminophenol, pyrocatechol and carbamide molecules are different in shape, size, and functional groups from dopamine hydrochloride molecules. These structural analog molecules are only bound to MIP through non-specific effects such as physical adsorption and cannot match the molecularly imprinted cavies on MIP@ZnS QDs. Thus, their adsorption capacity and fluorescence quenching degree were lower on MIP@ZnS QDs. As a control, NIP@QDs showed similar fluorescence quenching degree because there are no molecular imprinting sites formed during the preparation process. The imprinting factor of MIP@ZnS QDs for dopamine hydrochloride is 29.5, which is much higher than that of p-aminophenol and pyrocatechol and carbamide (3.6, 5.9, and 2.7, respectively). The experimental results indicated that MIP@ZnS QDs have good selectivity for dopamine hydrochloride. The selectivity of MIP@ZnS QDs to DA molecules is mainly due to the non-covalent interactions (hydrogen bonding, van der Waals forces, etc.) formed between functional monomers, crosslinkers and template molecules during the preparation process. The selectivity of MIP@ZnS quantum dots to DA molecules is mainly due to the non-covalent interaction (hydrogen bond, van der Waals force, etc.) between functional monomers and template molecules and the formation of complementary molecular imprinting cavities in the molecularly imprinting process. The method was compared with other reported dopamine hydrochloride detection techniques (Table 1). Compared with the reported detection methods based on QDs and other novel nanomaterials (COF, GQD, graphene), MIP@ZnS QDs based fluorescence sensors exhibit satisfactory linear range, detection limit, and good recovery rates for the detection of dopamine hydrochloride. In addition, the developed methods have the advantages of simple and convenient operation, low equipment requirement without any complex sample treatment process, and have better selectivity. NIP@QDs showed similar fluorescence quenching degree because there are no molecular imprinting sites formed during the preparation process. The imprinting factor of MIP@ZnS QDs for dopamine hydrochloride is 29.5, which is much higher than that of paminophenol and pyrocatechol and carbamide (3.6, 5.9, and 2.7, respectively). The experimental results indicated that MIP@ZnS QDs have good selectivity for dopamine hydrochloride. The selectivity of MIP@ZnS QDs to DA molecules is mainly due to the non-covalent interactions (hydrogen bonding, van der Waals forces, etc.) formed between functional monomers, crosslinkers and template molecules during the preparation process. The selectivity of MIP@ZnS quantum dots to DA molecules is mainly due to the non-covalent interaction (hydrogen bond, van der Waals force, etc.) between functional monomers and template molecules and the formation of complementary molecular imprinting cavities in the molecularly imprinting process. The method was compared with other reported dopamine hydrochloride detection techniques (Table 1). Compared with the reported detection methods based on QDs and other novel nanomaterials (COF, GQD, graphene), MIP@ZnS QDs based fluorescence sensors exhibit satisfactory linear range, detection limit, and good recovery rates for the detection of dopamine hydrochloride. In addition, the developed methods have the advantages of simple and convenient operation, low equipment requirement without any complex sample treatment process, and have better selectivity.

Application
In this study, real samples of bovine urine, sheep urine and human urine were used to evaluate the practical application performance of the MIP@ZnS QDs based sensor. As shown in Table 2, dopamine hydrochloride was not detected in any of the three urine samples. In order to further verify the accuracy of MIP@ZnS quantum dot sensor in detecting dopamine hydrochloride, a standard addition method was used to conduct recovery experiments on dopamine hydrochloride in the samples. The recoveries of dopamine Molecules 2023, 28, 3646 9 of 14 hydrochloride in different samples ranged from 95.2% to 103.8% with RSD of 3.75% to 5.73%. The results show that the developed molecularly imprinted quantum dots sensor exhibited satisfactory reliability and accuracy in the detection of dopamine hydrochloride in real samples.

Recyclability and Stability
As shown in Figure 9, the regeneration cycle (adsorption/elution/re-adsorption) was carried out to investigate the recyclability of MIP @ZnS QDs. After four cycles, the fluorescence of MIP@ZnS QDs significantly decreased. Compared with the initial value of the sensor, the fluorescence intensity has decreased by about 20%. Furthermore, the fluorescence stability of the MIP@ZnS QDs was evaluated and depicted in Figure 10. After being stored in the dark for 31 days; the fluorescence intensity of the MIP@ZnS QDs has no significant changes compared to the initial, indicating good stability of the MIP@ZnS QDs. These experimental results indicate that the designed MIP@ZnS QDs-based sensor has good recyclability and stability and has high practical application value as a fluorescence sensor.
In this study, real samples of bovine urine, sheep urine and human urine were used to evaluate the practical application performance of the MIP@ZnS QDs based sensor. As shown in Table 2, dopamine hydrochloride was not detected in any of the three urine samples. In order to further verify the accuracy of MIP@ZnS quantum dot sensor in detecting dopamine hydrochloride, a standard addition method was used to conduct recovery experiments on dopamine hydrochloride in the samples. The recoveries of dopamine hydrochloride in different samples ranged from 95.2% to 103.8% with RSD of 3.75% to 5.73%. The results show that the developed molecularly imprinted quantum dots sensor exhibited satisfactory reliability and accuracy in the detection of dopamine hydrochloride in real samples.

Recyclability and Stability
As shown in Figure 9, the regeneration cycle (adsorption/elution/re-adsorption) was carried out to investigate the recyclability of MIP @ZnS QDs. After four cycles, the fluorescence of MIP@ZnS QDs significantly decreased. Compared with the initial value of the sensor, the fluorescence intensity has decreased by about 20%. Furthermore, the fluorescence stability of the MIP@ZnS QDs was evaluated and depicted in Figure 10. After being stored in the dark for 31 days; the fluorescence intensity of the MIP@ZnS QDs has no significant changes compared to the initial, indicating good stability of the MIP@ZnS QDs. These experimental results indicate that the designed MIP@ZnS QDs-based sensor has good recyclability and stability and has high practical application value as a fluorescence sensor.

Conclusions
In this study, a molecular imprinted fluorescence sensor for dopamine hydrochloride was successfully designed by combining water-soluble ZnS QDs with surface molecular imprinting technology. After binding with dopamine hydrochloride molecules, MIP@ZnS QDs undergo significant quenching and the quenching is proportional to the concentration of the dopamine hydrochloride molecules. The constructed MIP@ZnS QDs sensor

Preparation of Cysteine Modified Mn 2+ : ZnS QDs
Cysteine modified Mn 2+ : ZnS QDs were prepared by chemical precipitation method with some modifications [49,50]. In brief, 1.80 g ZnSO 4 ·7H 2 O, 0.10 g MnCl 2 4H 2 O, were ultrasonically dispersed into 20 mL ultrapure water, and stirred for 1.0 h under nitrogen protection at 25 • C. Then, 5.0 mL of Na 2 S·4H 2 O solution was dropwise added. After stirring another 10 h, 10 mg of cysteine was added and continued to be stirred for 12 h in the dark. Finally, the product was washed and separated by centrifuging, and dried under vacuum.

Preparation of MIP@ZnS QDs
The dopamine molecularly imprinted layer was constructed using surface imprinting technology onto the surface of cysteine modified Mn 2+ : ZnS QDs. First, 2.0 mg dopamine hydrochloride is dissolved in 10 mL of methanol, 400 µL of MAA is added to the flask and stirred for 10 min. Second, 100 µL of EGDMA and 100 mg of cysteine modified Mn 2+ : ZnS QDs were added. Third, AIBN (15 mg) was added, and UV (360 nm) initiated polymerization at 50 • C in the dark for 20 h. Finally, the prepared product was collected by centrifugation and washed with methyl alcohol. The non-imprinted ZnS QDs (NIP@ZnS QDs) were synthesized by the same procedure without dopamine hydrochloride molecules. The obtained product was eluted by methanol and acetic acid (9:1, v/v) solution to remove dopamine hydrochloride molecules until no dopamine hydrochloride molecules were detected in the eluent. The eluted product is washed to be neutral by methanol and is freeze-dried in vacuum for further use.

Effect of pH on MIP@ZnS QDs
20 mg of MIP@ZnS QDs were ultrasonically dispersed in dopamine hydrochloride solutions with different pH values. The dopamine hydrochloride stock solution was added and diluted with buffer solution. The concentration of dopamine hydrochloride in the sample was 0.5 µM. Using the solution without dopamine hydrochloride as a control, the change of the fluorescence intensity of the samples were analyzed by fluorescence spectrofluorometer.

Determination of Dopamine Hydrochloride Using MIP@ZnS QDs
To carry out the rebinding experiments, 19 mg of dopamine hydrochloride standard solution is dissolved in 100 mL of methanol as the stock solution. 20 mg of MIP@ZnS QDs or NIP@ZnS QDs are dissolved in 100 mL of pH 7.5 buffer solution, and 5 mL of the polymer solution is taken into a 10-mL calibrated test tube after ultrasonic dispersion. Then, a certain amount of dopamine hydrochloride standard solution is sequentially added and diluted to 10 mL. The concentration of dopamine hydrochloride ranging from 1.0 × 10 −8 to 1.0 × 10 −6 mol/L. After incubating for 12 min, the fluorescence emission spectra with different dopamine hydrochloride concentrations was recorded. The fluorescence quenching of the MIP@ZnS QDs in the system is calculated by the Stern-Volmer equation [51]: F 0 is the initial fluorescence intensity in the absence of the dopamine hydrochloride molecule; F is the fluorescence intensity with dopamine hydrochloride; K sv is the quenching constant of the equation; Q is the concentration of the dopamine hydrochloride. The degree of quenching of the prepared MIP@QDs by dopamine hydrochloride molecules is represented by ∆F (∆F = F 0 /F − 1).

Selective Experiment
p-Aminophenol, pyrocatechol, and carbamide were used as structural analogues to conduct the selectivity experiments. Dopamine hydrochloride, p-aminophenol, pyrocatechol and carbamide stock solution were prepared. After ultrasonic dispersion, 5 mL of the MIP@ZnS QDs and NIP@ZnS QDs dispersed solution is taken into a 10-mL calibrated test tube. Then, a certain amount of the structural analogues standard solution was added and diluted to 1.0 × 10 −8~1 .0 × 10 −6 mol/L. After incubation that lasted 12 min, the changes in fluorescence intensity of samples were measured. The imprinting factor (IF) of MIP@ZnS QDs for each compound was calculated as follows: K SV,MIP and K SV,NIP are the quenching constants of MIP@ZnS QDs and NIP@ZnS QDs in different substance solutions, respectively.

Real Sample Analysis
Practical application is one of the important indicators to evaluate the prepared sensor. The samples were collected from the urine of cattle, sheep and healthy volunteers. All samples were filtered with 0.45 µm filter and stored in a 4 • C refrigerator. The prepared MIP@ZnS QDs were dispersed in 10 mL sample solution. After incubation, the changes of fluorescence intensity in the sample solutions were measured and the concentration of dopamine hydrochloride in the samples was calculated. To further verify the reliability of the method, a recovery test was conducted using standard addition method.

Conclusions
In this study, a molecular imprinted fluorescence sensor for dopamine hydrochloride was successfully designed by combining water-soluble ZnS QDs with surface molecular imprinting technology. After binding with dopamine hydrochloride molecules, MIP@ZnS QDs undergo significant quenching and the quenching is proportional to the concentration of the dopamine hydrochloride molecules. The constructed MIP@ZnS QDs sensor presents high selectivity towards dopamine hydrochloride and has been successfully used for real sample determination. The developed MIP@ZnS QDs based fluorescence sensor has advantages such as simple operation, fast response, low cost, and high stability and selectivity. It has great potential in food safety, biosensing, environmental protection, and other fields.

Informed Consent Statement: Not applicable.
Data Availability Statement: All the data generated by this research is included in the article.

Conflicts of Interest:
The authors declare no conflict of interest.