Khawla Mili

Zouhour Hsine

Yves Chevalier
email: yves.chevalier@univ-lyon1.fr

Gilles Ledoux

Rym Mlika
email: mlikarym@yahoo.fr

Application of thiol capped ZnS quantum dots

Keywords: ZnS, Quantum Dots, Fluorescence quenching, Tetracycline

1.

Introduction Since their first introduction into medicine in the 1940s, antibiotics have been widely used both in human healthcare and livestock farming throughout the world [START_REF] Zhang | Efficacy of carbonaceous nanocomposites for sorbing ionizable antibiotic sulfamethazine from aqueous solution[END_REF][START_REF] Yi | Selective prepared carbon nanomaterials for advanced photocatalytic application in environmental pollutant treatment and hydrogen production[END_REF][START_REF] Huang | Sorptive removal of ionizable antibiotic sulfamethazine from aqueous solution by graphene oxide-coated biochar nanocomposites: Influencing factors and mechanism[END_REF]. The presence of residual concentrations of these antibiotics in foods and their release in the environment is a significant danger for human health and aquatic life [START_REF] Li | Antibiotics in aquatic environments of China: A review and meta-analysis[END_REF]. Tetracycline antibiotics (TCy) are a variety of important broad-spectrum antibiotics [START_REF] Fuoco | Classification framework and chemical biology of Tetracycline-structure-based drugs[END_REF][START_REF] Wang | Development of an indirect competitive assay-based aptasensor for highly sensitive detection of tetracycline residue in honey[END_REF] that is widely used in human medicine and in animal breeding thanks to their excellent antibacterial properties and low cost. In particular, TCy allows keeping livestock in good health and thereby increases feeding efficiency toward faster animal growth [START_REF] Conzuelo | Disposable amperometric magnetoimmunosensor for direct detection of tetracyclines antibiotics residues in milk[END_REF]. Unfortunately, the overuse of TCy for growth promotion in animal breeding can result in the presence of drug residues in food of animal origin, including milk, meat, honey, fish, eggs and shrimp [START_REF] Wang | Development of a SPR aptasensor containing oriented aptamer for direct capture and detection of tetracycline in multiple honey samples[END_REF]. Long-term consumption of foods containing TCy residues results in the development of drug-resistant bacteria and generate several diseases in humans [START_REF] Chen | Near-infrared fluorescence-based multiplex lateral flow immunoassay for the simultaneous detection of four antibiotic residue families in milk[END_REF][START_REF] Chen | Instrument-free visual detection of tetracycline on an autocatalytic DNA machine using a caged G-quadruplex as the signal reporter[END_REF] such as anaphylactic reactions, gastrointestinal disorders and hepatotoxicity [START_REF] Xu | A novel visual ratiometric fluorescent sensing platform for highly-sensitive visual detection of tetracyclines by a lanthanide-functionalized palygorskite nanomaterial[END_REF][START_REF] Liu | A novel fluorescence probe for rapid and sensitive detection of tetracyclines residues based on silicon quantum dots[END_REF]. Accordingly, many countries have restricted the maximum residue limits (MRLs) of TCy in foods. MRLs are 0.1 mg kg -1 for milk, 0.2 mg kg -1 for eggs, 0.3 mg kg -1 for liver meat, and 0.6 mg kg -1 in kidney meat in EU [START_REF] Lewis | of the European parliament and the council of 18 December 2006[END_REF]. The US FDA has also established MRLs of 0.4 mg kg -1 for milk, 12 mg kg -1 for fat, 6 mg kg -1 for liver meat, and 2 mg kg -1 for muscle meat [START_REF] Liu | Recent advances in sensors for tetracycline antibiotics and their applications[END_REF]. Several analysis approaches for quantitative determination of TCy have been developed during the last few decades. Liquid chromatography-mass spectrometry (LC/MS) [START_REF] Zhou | Analysis of tetracyclines in chicken tissues and dung using LC-MS coupled with ultrasound-assisted enzymatic hydrolysis[END_REF], high performance liquid chromatography (HPLC) [START_REF] Moudgil | Validation of HPLC Multi-residue Method for Determination of Fluoroquinolones, Tetracycline, Sulphonamides and Chloramphenicol Residues in Bovine Milk, Food Anal[END_REF] and thin-layer chromatography (TLC) [START_REF] Naidong | Assay and purity control of tetracycline, chlortetracycline and oxytetracycline in animal feeds and premixes by TLC densitometry with fluorescence detection[END_REF] are the most common techniques for the detection of TCy. However, implementation of chromatographic methods is time-consuming, requires skilled operators and high-cost equipment. Other methods including capillary electrophoresis (CE) [START_REF] Ibarra | Magnetic solid phase extraction based on phenyl silica adsorbent for the determination of tetracyclines in milk samples by capillary electrophoresis[END_REF], capillary electrophoresis-mass spectrometry (CE-MS) [START_REF] Tong | Simultaneous determination of five tetracycline and macrolide antibiotics in feeds using HPCE[END_REF] and enzyme-linked immunosorbent assays (ELISA) [START_REF] Song | Multi-color quantum dot-based fluorescence immunoassay array for simultaneous visual detection of multiple antibiotic residues in milk[END_REF] have shown high sensitivity and accuracy, but there are still some limitations such as the complex sample preparation procedure and the need for a highly qualified technical staff. There is a need for alternative strategies for sensing TCy in a simple, rapid, selective, sensitive and user-friendly manner. Methods based on fluorescence are of high sensitivity and easy operation, they allow a rapid detection and possibly in situ analyses [START_REF] Sharma | Rapid sensing of lead metal ions in an aqueous medium by MoS2 quantum dots fluorescence turn-off[END_REF]. Significant attention has been paid to fluorescence sensors for the detection of TCy [START_REF] Zhang | A novel fluorescent 'turn-on' aptasensor based on nitrogen-doped graphene quantum dots and hexagonal cobalt oxyhydroxide nanoflakes to detect tetracycline[END_REF][START_REF] Sharma | Rapid optical sensor for recognition of explosive 2,4,6-TNP traces in water through fluorescent ZnSe quantum dots[END_REF] using various nanomaterials like gold nanoparticles [START_REF] Ramezani | A novel colorimetric triple-helix molecular switch aptasensor for ultrasensitive detection of tetracycline[END_REF], magnetic nanoparticles [START_REF] Wang | A colorimetric biosensor using Fe3O4 nanoparticles for highly sensitive and selective detection of tetracyclines[END_REF] and quantum dots (QDs) [START_REF] Feng | Carbon dots derived from rose flowers for tetracycline sensing[END_REF]. Fluorescence sensor based on water-dispersed nanomaterials favorably meets the analytical specifications for the detection of TCy [START_REF] Sharma | Rapid sensing of lead metal ions in an aqueous medium by MoS2 quantum dots fluorescence turn-off[END_REF][START_REF] Sharma | Synthesis of photoactivated highly fluorescent Mn 2+ -doped ZnSe quantum dots as effective lead sensor in drinking water[END_REF]. Their unique size-dependent features including easy functionalization, tunable absorption and emission spectra, high stability and large fluorescence quantum yields aiming at selective detection make them ideal fluorescent materials for being implemented into chemical and biological assays [START_REF] Brahim | Interaction of l-cysteine functionalized CdSe quantum dots with metallic cations and selective binding of cobalt in water probed by fluorescence[END_REF]. Several fluorescent sensors have been used for the detection of TCy, however their major problem is that the detection takes place in acidic or basic medias which limit their real application in biological fluids. As a few examples, Zhang et al. have developed a fluorescent sensor based on silver nanoclusters (Ag NCs) for the detection of TCy at pH = 5 [START_REF] Mili | ZnS quantum dots as fluorescence sensor for quantitative detection of tetracycline[END_REF]. Guo et al. used copper doped carbon dots (Cu-CDs) as fluorescence probes for detection of TCy at pH = 4 [START_REF] Dehghan | Improvement of the optical and photocatalytic properties of ZnSe QDs by growth of ZnS shell using a new approach[END_REF]. However, the detection of TCy in an acidic medium may limit the applicability of these sensors in real media.

To the best of our knowledge, there have been no reports of TCy fluorescence detection in a stable biological environment (pH = 7) using thioglycolic acid functionalized Zinc based QDs. The present work addresses the development of a fluorescence probe designed for a rapid detection of low concentrations of TCy in aqueous medium with physiological pH. It is based on zinc sulfide (ZnS) QDs synthesized by a simple aqueous method using thioglycolic acid (TGA) as a stabilizer. In order to optimize the detection characteristics, pH values were also optimized. The results showed that the emission of ZnS QDs showed a strong synchronous fluorescence, a good stability and high selectivity in a neutral medium, indicating that zinc based QDs may be regarded as a suitable choice for detection of TCy in real media.

Experimental section 2.1. Reagents and Materials

Zinc acetate di-hydrate (Zn(CH3COO)2.2H2O, 98%), sodium sulfide nonahydrate (Na2S.9H2O, 98%), thioglycolic acid (TGA, C2H4O2S, 98%), sodium hydroxide (NaOH, ≥ 97%), and tetracycline (C22H24N2O8, ≥ 95%) were purchased from Sigma-Aldrich. Deionized water of resistivity 18 M cm has been used as the solvent for all steps of the ZnS QDs synthesis and for fluorescence measurements in aqueous media. All chemicals were used as they were purchased without further purification.

Instruments

The Fourier transform infrared spectra of ZnS QDs capped with TGA have been measured by a Nicolet Is50 FTIR spectrometer in transmittance mode at room temperature. X-ray diffraction patterns (XRD) have been recorded at room temperature by an Ital Structures APD 2000 powder diffractometer using the Cu Kα radiation and a 2θ scan rate of 2 °C min -1 . The angular resolution was 0.055 °. The morphology and size of ZnS particles have been determined by means of a JEOL JEM 1220 transmission electron microscope (TEM) operating at 120 kV. The samples were dispersed in water, deposited onto a thin Formvar film supported on a Cu grid, and dried in open air before TEM observation. The UV-vis spectra of aqueous colloidal dispersions of ZnS particles have been taken using a UV-vis spectrophotometer Shimadzu UV-310PC over a wavelength range of 200-800 nm) at room temperature. Quartz cells of 1 cm optical path were used. Photoluminescence spectra have been recorded by exciting the samples with a 10 mW CW laser model UF-F-355 (355 nm) from CNI laser focused on a spot of a 2 mm diameter. The sample has been placed on a Linkam stage THMS600 in which the temperature can be varied between 80 and 300 K. The luminescence spectra have been collected thanks to an optical fiber and fed into a Jobin-Yvon TRIAX320 monochromator coupled to a Newton EMCCD from Andor. The resolution of the system was 2 nm for all the performed measurements. The surface potential changes upon pH variation have been monitored through zeta potential determination by means of electrophoresis measurements using a Zetasizer NanoZS (Malvern, UK) instrument. Fluorescence experiments have been carried out on a Cary Eclipse fluorescence spectrometer. The spectra were collected in the range of 350-490 nm with an excitation of 325 nm.

Preparation of TGA-capped ZnS quantum dots

The ZnS QDs have been prepared in the presence of thiol capping agent (TGA) in accordance with procedures reported in the literature with minor modifications [START_REF] Li | A room-temperature phosphorescence sensor for the detection of alkaline phosphatase activity based on Mn-doped ZnS quantum dots[END_REF][START_REF] Mili | ZnS quantum dots as fluorescence sensor for quantitative detection of tetracycline[END_REF]. The Zn 2+ /S 2-/TGA molar ratio of 1/0.5/2.5 was used, corresponding to the stoichiometric requirement generating adequate coverage and full saturation of the dangling bonds on the surface of the QDs with the thiol groups. First, 0.1 M of Zn(Ac)2 and 14.3 mmol of TGA were dissolved together in 50 mL of water. The mixture was adjusted at pH 11 by adding 1 M NaOH, and magnetically stirred for 45 min at room temperature under N 2 atmosphere. Thereafter, 25 mL of Na2S solution (0.1 M) was added for operating the chemical reaction. The resulting QDs solution has been agitated for about 2 h at 100 °C under N2 atmosphere. The obtained ZnS nanoparticles were precipitated with ethanol, separated by centrifugation at 4000 rpm, and then dried in a desiccator under vacuum.

Preparation of the TCy solutions and of TGA-ZnS QDs solution

In a typical test, one millimole of TCy has been dissolved in 10 mL deionized water to make a 10 -1 M stock solution, which was then diluted to the appropriate concentrations and the pH adjusted as needed. All spectroscopic measurements of synthesized QDs were carried out in an aqueous solution containing 20 mg of dry QDs dispersed in 100 mL of water (0.2 mg mL -1 ) [START_REF] Ouni | High impact of thiol capped ZnS nanocrystals on the degradation of single and binary aqueous solutions of industrial azo dyes under sunlight[END_REF]. To investigate the influence of the quenching by TCy on the fluorescence intensity of TGA-ZnS QDs, various concentrations of TCy were added to a 2 mL calibrated test tube, diluted to the mark with deionized water, and well shaken. After equilibration for 5 min at room temperature, the fluorescence spectra were recorded in the wavelength range of 350-480 nm with an excitation wavelength of 325 nm.

Results and discussion

Chemical and structural characterization of synthesized MPA capped ZnS QDs 3.1.1. Fourier transform infrared (FTIR) studies

The IR spectrum of ZnS stabilized by TGA (Fig. 1) allowed identification of the coating of the particles by TGA. The peaks for the S-H vibration (2550-2670 cm -1 ) were absent in TGA capped ZnS QDs. The origin of the disappearance of the S-H group vibration on the surface of ZnS QDs is due to the strong interaction between the electron doublet of sulfur and the zinc atoms on the surface of the QDs. [START_REF] Dehghan | Improvement of the optical and photocatalytic properties of ZnSe QDs by growth of ZnS shell using a new approach[END_REF][START_REF] Pourghobadi | Highly sensitive fluorescent probe for detection of paraquat based on nanocrystals[END_REF]. The absorption spectrum of TGA-ZnS given in Fig. 1 shows an absorption band at 562 cm -1 corresponding to Zn-S stretching vibrations [START_REF] Khafajeh | Synthesis of ZnSe and ZnSe:Cu quantum dots by a room temperature photochemical (UV-assisted) approach using Na2SeO3 as Se source and investigating optical properties[END_REF]. Besides, this spectrum shows the stretching bending corresponding to C-S, C-H and C-O at 784 cm -1 , 899 cm -1 and 1226 cm -1 , respectively [START_REF] Li | Application of thioglycolic acid capped nano-ZnS as a fluorescence probe for the determination of nevirapine[END_REF]. The intense bands at 1375 and 1560 cm -1 are assigned to C=O symmetric and asymmetric stretching modes resulting from the absorption of atmospheric CO2 on the surface of the ZnS QDs [START_REF] Khafajeh | Synthesis of ZnSe and ZnSe:Cu quantum dots by a room temperature photochemical (UV-assisted) approach using Na2SeO3 as Se source and investigating optical properties[END_REF][START_REF] Amirian | A new and simple UV-assisted approach for synthesis of water soluble ZnS and transition metals doped ZnS nanoparticles (NPs) and investigating optical and photocatalyst properties[END_REF]. The broad band around 3000-3600 cm -1 is due to the O-H stretching of water absorbed on the surface of QDs. These results clearly confirm the binding of TGA on the surface of ZnS QDs.

X-ray diffraction (XRD) analysis

XRD gives information about many parameters such as d-spacing, crystal structure, crystal size and even the composition of the material [START_REF] Tilakraj | Simple one pot synthesis and characterization of biocompatible Lactic acid capped fluorescent ZnS QDs[END_REF]. Fig. 2 shows the XRD pattern of the synthesized X-ZnS QDs. The three diffraction peaks at 2θ values of 28.6 °, 47.90 ° and 55.27 ° correspond to the (111), ( 220) and (311) diffraction planes of the crystalline structure of ZnS. The broad diffraction peaks in the XRD pattern show the production of TGA-capped ZnS quantum dots having small crystallites. In addition, no impurity-related reflections were found in the pattern, confirming the high purity of the final product. The comparison with the standard card (JCPDS No. 80-0020) proves the formation of a cubic Zinc Blende structure. The average crystallite size (D) has been calculated from the full width at half maximum (FWHM) measured by fitting the Gaussian profile to the Bragg peaks of the (111), ( 220) and (311) planes using the Debye-Scherrer equation:

D = K/ cos (1)
where D is the average crystallite size, K is the geometric factor (0.9), λ is the X-ray wavelength (1.54 Å for Cu Kα), β is the FWHM of the diffraction peak (in radian) and θ is the Bragg diffraction angle. The average crystallite size of TGA-ZnS was estimated to be 10.6 nm. The dislocation density (δ) which represents the amount of defects in the sample is determined from the average crystallite size D by using the relationship (δ = 1/D 2 ) [START_REF] Dhanam | An investigation on silar Cu(In1-xAlx)Se2 thin films[END_REF]. The d-spacing is calculated using the relation (dhkl = nλ / 2sinθ) [START_REF] Dhanam | Analysis of ZnS nanoparticles prepared by surfactant micelletemplate inducing reaction[END_REF]. The lattice parameter of TGA-ZnS QDs is a = 5.4 Å. All these structural parameters are listed in Table 1.

TEM study

The TEM image in Fig. 3(a) indicates the formation of nano-sized crystals with a quasispherical shape, which is similar to the results published by several researchers [START_REF] Wang | Simple and greener synthesis of highly photoluminescence Mn 2+ -doped ZnS quantum dots and its surface passivation mechanism[END_REF][START_REF] Subha | Efficient photoluminescence of Mn 2+ -doped ZnS quantum dots excited by two-photon absorption in near-infrared window II[END_REF][START_REF] Li | Highly photoluminescent and stable aqueous ZnS quantum dots[END_REF]. Fig. 3(b) shows the particle size distribution resulting from 100 measurements, and the best fit of a Gaussian size distribution to it. The estimated average particle size of TGA-ZnS is 10.63 ± 0.6 nm with a standard deviation of 5.6%. This result is consistent with DRX measurements. has shown an absorption band at 297 nm which was related to the first electronic transition 1S e -1Sh [START_REF] Ouni | High impact of thiol capped ZnS nanocrystals on the degradation of single and binary aqueous solutions of industrial azo dyes under sunlight[END_REF]. This is much lower than that of the bulk ZnS crystal (344 nm), that indicated the formation of a smaller crystal with a significantly reduced diameter that is smaller than the excitonic radius of the material. Hence, the information from the absorption spectrometer confirms the formation of QDs.

In order to know the actual value of the band gap, the Tauc diagram is one of the most efficient and famous methods [START_REF] Tauc | Optical properties and electronic structure of amorphous Ge and Si[END_REF]. The values of absorption intensity and energy parameters are allowed to determine the band gap energy (Eg) of ZnS QDs by using the Tauc relation:

𝜶𝒉𝝂 = 𝑨(𝒉𝝂 -𝑬 𝒈 ) 𝒏 (2)
where, α is the absorption coefficient, hυ is the incident photon energy, A is a constant, Eg is the band gap energy of the material and the exponent n is a constant (n = 1/2 for direct semiconductors).

The band gap of TGA-ZnS QDs was determined by a linear extrapolation of (αhν) 2 to the xaxis (Fig. 4b). The band gap was found to be 3.99 eV. There was a blue shift compared to the bang gap of bulk ZnS (Eg = 3.6 eV), which is caused by quantum confinement of charge carriers [START_REF] Brahim | Synthesis, characterization and spectral temperature-dependenceof thioglycerol-CdSe nanocrystals[END_REF][START_REF] Juine | Surfactant-Free Green Synthesis of ZnS QDs with Active Surface Defects for Selective Nanomolar Oxalic Acid Colorimetric Sensors at Room Temperature[END_REF][START_REF] Prasanth | Nonlinear optical property and fluorescence quenching behavior of PVP capped ZnS nanoparticles co-doped with Mn 2+ and Sm 3+[END_REF].

Photoluminescence studies

The photoluminescence of ZnS quantum dots is complex, as it is influenced by several synthetic conditions like refluxing time, capping agent and its concentration, reaction temperature, agitation etc. [START_REF] Bahmani | Synthesis of zinc sulfide semiconductor nanoparticles by coprecipitation method for biological diagnostics[END_REF][START_REF] Sana | Luminescence and Morphological Kinetics of Functionalized ZnS Colloidal Nanocrystals[END_REF] and crystal sizes and shapes [START_REF] Reetz | Size-Selective Synthesis of Nanostructured Transition Metal Clusters[END_REF][START_REF] Cizeron | Solid solution of CdyZn1-yS nanosize particles made in reverse micelles[END_REF]. It is reported in literature that the emission spectrum of ZnS can be adjusted in a very broad range from 300 nm to 550 nm [START_REF] Bansal | Effect of Mn 2+ and Cu 2+ co-doping on structural and luminescent properties of ZnS nanoparticles[END_REF]. Typically, two emission peaks can be observed from semiconductor nanoparticles: exciton luminescence and luminescence from trapped electronic states. In most cases of ZnS, a broad blue emission (centered around 400-500 nm) has been reported, which

3.76

was ascribed originating from surface defect states [START_REF] Zhou | Understanding the mechanoluminescent mechanisms of manganese doped zinc sulfide based on load effects[END_REF][START_REF] Ebrahimi | Enhanced optoelectrical properties of Mn-doped ZnS films deposited by spray pyrolysis for ultraviolet detection applications[END_REF]. However, in this work, as shown in Fig. 5(a), for the TGA capped ZnS QDs, there appears an asymmetrical and broadened emission with two distinct maxima. A major peak centered at ~405 nm and a second minor peak at 484 nm are observed, respectively. It is interesting to mention that the broad PL emission of the ZnS QDs is related to the transitions originating from sulfur vacancies [START_REF] Sakthivel | Structural, band gap and photoluminescence behaviour of Mn-doped ZnS quantum dots annealed under Ar atmosphere[END_REF]. The emission peak observed at 405 nm is assigned to the defect trapping states present in the ZnS QDs caused by sulfur vacancies [START_REF] Bansal | Effect of Mn 2+ and Cu 2+ co-doping on structural and luminescent properties of ZnS nanoparticles[END_REF][START_REF] Chalana | Tailoring the visible emissions in ZnS:Mn films for white light generation[END_REF][START_REF] Panda | Chelating ligand-mediated synthesis of hollow ZnS microspheres and its optical properties[END_REF][START_REF] Pathak | Blue light emission from barium doped zinc sulfide nanoparticles[END_REF]. During the synthesis, the passivating ligands' thiol group (-SH) is separated from the organic part, which is subsequently attached to Zn ions. These organic groups are responsible for suppressing Zn dangling orbitals. However, the unsaturated sp 3 hybridized orbitals of the surface dangle out of the crystals surface provoking deep hole trap states. This leads to red shifted emission at longer wavelengths, as observed at 484 nm in our case, and also has been reported by others [START_REF] Bahmani | Synthesis of zinc sulfide semiconductor nanoparticles by coprecipitation method for biological diagnostics[END_REF][START_REF] Bi | Synthesis and characterization of Co-doped wurtzite ZnS nanocrystals[END_REF]. The photoluminescence excitation spectrum (PLE) is a very effective method to study electronic transitions in materials with a high sensitivity compared to absorption measurements [START_REF] Hien | Structural, optical properties, energy transfer mechanism and quantum cutting of Tb 3+ doped ZnS quantum dots[END_REF]. Fig. 5(b) also shows a representative PLE spectra of the sample corresponding to the emission at 405 nm and 484 nm, which is centered at ~326 and ~324 nm, respectively. The PLE spectra red-shifted compared to the corresponding absorption spectrum centered at 297 nm. These red-shifted PLE spectra suggest that the lowest electronic levels of the excited states contribute dominantly to the PL emissions.

Temperature dependent PL spectroscopy

Thermal evolution of the photoluminescence (within the range of 80 K ≤ T ≤ 300 K) has been studied, which revealed in details the role of optically active defect states in carrier storage and escape. The PL spectra obtained at various temperatures from 80 K to 300 K of TGA-ZnS powder are plotted in Fig. 6. The PL spectrum of these QDs is different at low temperature from the one observed at room temperature. Cooling more clearly reveals the emission from defects in the materials [START_REF] Hafeez | Critical role of defect states on visible luminescence from ZnS nanostructures doped with Au, Mn and Ga[END_REF]. In addition, a new peak appears at 442 nm arising from the interstitial zinc lattice defects when the temperature decreases [START_REF] Ebrahimi | Enhanced optoelectrical properties of Mn-doped ZnS films deposited by spray pyrolysis for ultraviolet detection applications[END_REF]. The PL peak position and intensity as a function of measurement temperature are shown in Fig. 7(a,b). At low temperature (80 K), the emission spectra of ZnS QDs contain three large bands located at 405 nm (Peak I), 442 nm (Peak II) and 483 nm (Peak III). The PL bands clearly turned narrower as their intensities have increased (Fig. 7b). In fact, the width at half maximum (FWHM) of the defects emission increases with temperature together with a slight change of the position of their maximum (Fig. 7a). This temperature dependence of these bands is indicative of a thermal process, which is probably due to the fact that the impurity levels (trap state or surface defects) in the band gap are tightly attached to the ZnS-TGA lattice [START_REF] Wang | Visible emission characteristics from different defects of ZnS nanocrystals[END_REF].

The intensities of three trappings, on the other hand, rose at lower temperatures, which was attributable to the elimination of phonon-coupled thermal quenching and differing temperature sensitivity between trapping and exciton states of QDs [START_REF] Wuister | Luminescence temperature antiquenching of water-soluble CdTe quantum dots: Role of the solvent[END_REF][START_REF] Nonoguchi | Size-and temperature-dependent emission properties of zinc-blende CdTe nanocrystals in ionic liquid[END_REF]. The schematic diagram which describes the emission mechanism of the TGA-ZnS QDs is presented in Fig. 8, which shows the three-trap state emission in ZnS QDs. In the ZnS QDs, these excited electrons decompose successively in certain defect states such as sulfur vacancy (VS), interstitial sulfur (IS) atoms and interstitial zinc (IZn) atoms by normal recombination.

According to a previous work on defects in ZnS QDs, Sulfur atom vacancies (VS) and zinc atom interstitials (IZn) are donor states, while interstitials of sulfur atoms (IS) are acceptor states [START_REF] Chalana | Tailoring the visible emissions in ZnS:Mn films for white light generation[END_REF][START_REF] Wang | Visible emission characteristics from different defects of ZnS nanocrystals[END_REF].

The introduction of additional zinc and sulfur atoms into the crystal sites can cause the deformation of the host crystal lattice. The atomic size of the incorporated atoms has a significant impact on the amount of deformation. Because sulfur ions are bigger than zinc ones, they will cause more strain in the lattice than zinc ones [START_REF] Denzler | Luminescence studies of localized gap states in colloidal ZnS nanocrystals[END_REF]. It results in a low binding energy value for sulfur interstitial states compared with that for zinc interstitials. The modest binding energy value of interstitial sulfur states minimizes the separation between its energy level and the valence band when compared to the energy difference between zinc interstitial and conduction band [START_REF] Denzler | Luminescence studies of localized gap states in colloidal ZnS nanocrystals[END_REF]. Similarly, the separation of energy between the sulfur vacancy and the conduction band edge is smaller than that between the zinc vacancy and the valence band edge [START_REF] Denzler | Luminescence studies of localized gap states in colloidal ZnS nanocrystals[END_REF]. Additionally, vacancy states are situated at deep energy levels than the interstitial states [START_REF] Wang | Visible emission characteristics from different defects of ZnS nanocrystals[END_REF][START_REF] Denzler | Luminescence studies of localized gap states in colloidal ZnS nanocrystals[END_REF]. Taking all these facts into consideration, when comparing the emission of interstitial sulfur states to that of zinc interstitial states, it can be deduced that the emission of interstitial sulfur states arises at a shorter wavelength [START_REF] Denzler | Luminescence studies of localized gap states in colloidal ZnS nanocrystals[END_REF]. Following a molar ratio of Zn/S ≥ 2, the synthesized QDs contains an excess of zinc or a sulfur deficiency. Similarly, defect-related emissions were assigned to a transition to the valence band from either an interstitial sulfur atom (IS), an interstitial zinc atom (IZn), and a sulfur vacancy (VS) [START_REF] Bel Haj Mohamed | Use of MPAcapped CdS quantum dots for sensitive detection and quantification of Co 2+ ions in aqueous solution[END_REF].

Fluorescence detection of TCy 3.3.1. Effect of pH

The acidity and basicity of the solution plays an important role in the fluorescent properties of TGA-ZnSe QDs. The zeta potential of the TGA-ZnS QDs have been measured in pH range between 1 and 13 to find the most suitable pH condition (Fig. 9(a)). The TGA-capped ZnS QDs have negative charges in all the selected pH range. This is explained by the ionization of the carboxylic acid moiety of the TGA, which causes a negative charge on its surface [START_REF] Ramírez-Herrera | NIR-emitting alloyed CdTeSe QDs and organic dye assemblies: A nontoxic, stable, and efficient FRET system[END_REF]. Fig. 9(b) shows the charge of TCy at different pH. Moreover, the higher and lower pH provokes an electrostatic repulsive interaction with the ZnS QDs as recently reported [START_REF] Mili | ZnS quantum dots as fluorescence sensor for quantitative detection of tetracycline[END_REF]. highly acidic media are not suggested as the TGA-ZnS QDs dissociate via protonation of the surface-bound TGA [START_REF] Chen | CdSe quantum dots decorated by mercaptosuccinic acid as fluorescence probe for Cu 2+[END_REF][START_REF] Mahmoud | Functionalized ME-capped CdSe quantum dots based luminescence probe for detection of Ba 2+ ions[END_REF] and the interaction between TGA ligand and ZnS QDs can be destroyed, resulting in decreased stability of the QDs [START_REF] Cao | Facile synthesis of chitosan-capped ZnS quantum dots as an eco-friendly fluorescence sensor for rapid determination of bisphenol A in water and plastic samples[END_REF][START_REF] Duan | Facile synthesis of N-acetyl-l-cysteine capped ZnS quantum dots as an eco-friendly fluorescence sensor for Hg 2+[END_REF][START_REF] Mandani | Development of a new simple spectroscopic determination coupled acid-motivated delivery system based on fluorescence turn-off MSNs@MPA-ZnS QDs for infection[END_REF]. In addition, at higher and lower pH, TC may be slightly denatured [START_REF] Zhang | Fluorescence Probe Based on Hybrid Mesoporous Silica/Quantum Dot/Molecularly Imprinted Polymer for Detection of Tetracycline[END_REF], pH = 7 was selected in this study for the formation of the TGA-ZnS QDs/TCy complex through hydrogen bonding with the neutral zwitterionic form of TCy and the fluorescence quenching detection of TCy. Apart from the good formation of the TGA-ZnS QDs/TCy complex provided by the pH = 7, this pH value is compatible with the pH value of the biological fluids which make our TGA-ZnS sensor easily applied in real media without needing to change its pH.

Stability of the TGA-ZnS QDs Fluorescence Emission Measurement

The excellent stability of QDs is the prerequisite for their application as fluorescent probes. In order to investigate the stability of fluorescence intensity, fluorescence signals of the TGA-ZnS QDs were recorded every 5 min .As shown in the results in Fig. 10(a), the emission of ZnS QDs showed good stability at pH = 7 with a relative standard deviation (RSD) of 0.27%.

Influence of incubation time

The influence of incubation time on the fluorescence intensity was investigated and the result is shown in Fig. 10(b). It could be seen that the fluorescent intensity decreases rapidly upon adding TCy. The reaction between TGA-ZnS QDS and TCy was completed within 5 min. After that, the fluorescent intensity remains stable for at least 1 h. Therefore, the ZnS fluorescence spectra in the presence of different concentrations of TCy were recorded after 5 min.

where F0 and F are the fluorescence intensities of the QDs in the absence and the presence of TCy, respectively, KSV is the Stern-Volmer constant, and [TCy] refers to the corresponding concentrations of TCy.

As can be seen, Fig. 11(b) shows two linear parts at low and high concentrations of TCy which prove the affinity of the TGA-ZnS towards TCy analytes. Furthermore, as recently reported [START_REF] Mili | ZnS quantum dots as fluorescence sensor for quantitative detection of tetracycline[END_REF], the downward curvature plot at low concentration (Fig. 11(b)) may indicate that dynamic quenching is predominant at low concentration, with a static quenching mechanism at higher concentrations.

The calculated Stern-Volmer quenching constants KSV of TGA-ZnS QDs (which represents the quenching efficiency) from the linearized equation are found to be KSV = 8.87×10 5 L mol -1 . This high value of the Stern-Volmer quenching constant demonstrates the excellent quenching efficiency of TCy as well as good affinity of TCy for adsorption to TGA-ZnS QDs.

Additionally, the limit of detection (LOD) has been calculated as 3σ/K = 0.03 nM (σ is the standard deviation of the blank, and K is the slope of the calibration curve). Owing to the very low LOD and the wide linear range of response, a high sensitivity is disclosed. The response of TGA-ZnS QDs might be due to two factors. Firstly, the carboxylate group has a high basicity. The consequence is a strong hydrogen bonding with the phenol and hydroxyl groups of TCy. Secondly, the TGA ligand has a short hydrocarbon chain between the thiol and the carboxylate groups, which can result in a highly ordered distribution on the surface of ZnS QDs, potentially increasing the hydrogen bonding interaction between TGA and TCy (Fig. 12).

Recently, there have been many reports on the detection methods of TCy listed in Table 2.

Compared with these methods, our material is environment-friendly and the method is timesaving and an easy of operation and its works over a wide linear range. The afore mentioned findings show that the TGA-ZnS QDs can be used as suitable candidate for the detection of TCy, based on the lower detection limit and a wider detection range than previously reported methods.

Analysis of tetracycline in biological samples

To investigate the applicability of the proposed method, our fluorescent probe was adapted to detect TCy in real samples (raw and pasteurized milk). Five spiked levels of TCy were added to the raw and pasteurized milk samples. As shown in Fig. 13, there is a linear relationship between the fluorescent intensity of TGA-ZnS QDs and the concentration of TCy in the range of 50 nM-3000 nM. The lowest detectable concentration of TCy in raw and pasteurized milk is 4.3 nM and 5.1 nM, respectively. As shown in Table 3, the excellent results (recoveries ranging from 98.4% to 100.8% and RSD values less than 0.6) confirm that the application of the proposed fluorescent probe in biological sample analysis was anticipated to be promising.

Conclusion

A simple, low cost, efficient and eco-friendly fluorescence sensor for the detection of trace TCy has been reported. The simple colloidal chemical approach employed for the synthesis of TGA-ZnS QDs provides a colloidal material with an excellent purity, low cost, easy implementation, and low energy consumption. Detection of TCy is based on the fluorescence quenching of TGA-ZnS QDs, is due to the electrostatic and hydrogen bonding interactions between the surface groups of ZnS QDs and TCy. Meanwhile, the TGA-ZnS QDs probe can be used for the detection of tetracycline in the range of 25-6000 nM with the LODs of 0.03 nM. In comparison to earlier published fluorescent sensors, the simple and low-cost TGA-ZnS QD fluorescence sensor reaches a very low limit of detection at the picomolar level. Besides, the sensor was successfully applied to TCy analysis in biological samples. It is believed that this research work has opened another avenue for the point of care detection of antibiotics in the biological fluids.

Fig. 1 .

 1 Fig. 1. IR spectrum of TGA-ZnS nanoparticles.

Fig. 2 .

 2 Fig. 2. X-ray diffraction (XRD) pattern of TGA-ZnS nanoparticles.

Fig. 3 .

 3 Fig. 3. (a) TEM image of TGA-ZnS QDs. (b) Particle size distribution measured from 100 QDs.

2 .

 2 Photophysical characteristics of the TGA capped ZnS QDs 3.2.1. Band gap determination from optical studies UV-Visible absorption spectra of the dispersed solution of TGA-capped ZnS QDs (Fig. 4(a))

Fig. 4 .

 4 Fig. 4. (a) Absorption spectrum of TGA-ZnS nanoparticles and (b) graphical method for the calculation of their optical gap energy.

Fig. 5 .

 5 Fig. 5. (a) Gaussian band deconvolution for photoluminescence spectra of TGA-ZnS nanoparticles recorded for λexc = 325 nm and (b) PL excitation spectra recorded for the 405 nm (blue line) and 484 nm (red line) emission bands at room temperature.

Fig. 6 .

 6 Fig. 6. The temperature-dependent PL spectra of ZnS-TGA nanocrystals.

Fig. 7 .

 7 Fig. 7. (a) PL peak position and (b) PL intensity as a function of temperature for Peak I, Peak II and Peak III defects.

Fig. 8 .

 8 Fig. 8. Transitions leading to PL emission in TGA-ZnS QDs at 80 K; CB-Conduction Band, VB-Valence Band, IS-Interstitial Sulfur, IZn-Interstitial Zinc, VS-Sulfur Vacancy.

Fig. 9 .

 9 Fig. 9. (a) Zeta potential of the TGA-ZnS QDs at different pH values, (b) pH-dependent speciation of the tetracycline molecule.

Fig. 10 .

 10 Fig. 10. The fluorescent intensity of TGA-ZnS QDs versus its different concentrations.

Fig. 11 .

 11 Fig. 11. Fluorescence spectra (λexc = 325 nm) of (a) TGA-ZnS QDs (0.2 mg mL -1 ) in the presence of various concentrations of TCy (from top to bottom, 0, 25, 50, 100, 200, 400, 1000, 2000, 3000 and 6000 nM). (b) Linear fitted result using Stern-Volmer quenching plot of TCy concentration dependence on fluorescence intensity of TGA-ZnS QDs. Insert: zoom of Stern-Volmer plot at low concentration of TCy.

Fig. 12 .

 12 Fig. 12. Schematic representation of the electrostatic interaction between the positive charges of TCy and the negative charges of TGA-ZnS QDs.

Fig. 13 .

 13 Fig. 13. Fluorescence spectra of TGA-ZnS QDs in (a) raw and (b) pasteurized milk samples containing 0, 50, 100, 200, 1000 and 3000 nM TCy. The inset is the linear relationship between the fluorescent intensity of TGA-ZnS and the concentrations of TCy in milk samples.

Table 1 .

 1 Structural parameters of TGA-ZnS QDs.

	2θ (°)	Plane (hkl)	Interplanar spacing d (Å)	Lattice constant a (Å)	Average crystallite size D (nm)	Dislocation density δ (lines/m 2 )
	28.6	(111)	3.117
	47.9	(220)	1.896	5.4	10.6	8.95×10 15
	55.27	(311)	1.660

Table 2 .

 2 The

	Adsorptive voltammetry carbon nanotube		100-2000	40	[51]
	Ratiometric molecular imprinting		0-50	1.19	[52]
	PEC detection ZrO2/g-C3N4/FTO electrode		50-150	8.7	[53]
	(TAP)/γ-cyclodextrin probe/cyclodextrin fluorescent sensing triple-helix	aptamer	5-100	1.6	[54]
	Carbon-doped WO3 electrochemical aptasensor		0.1-100	0.048	[55]
	Thioglycolic acid-ZnS QDs fluorescence probe	25-6000	0.03	This work

linear range and detection limit of different assay methods for TCy Method Linear range (nM) LOD (nM) Reference

Table 3 .

 3 Results for the determination of TCy in raw and pasteurized milk samples.

	Samples	Added (nM)	Found a (nM) ± RSD (n = 3)	Recovery (%)
	Raw milk	50	49.9  0.4	99.8
		100	100.3  0.3	100.3
		200	201.4  0.4	100.7
		1000	992.7  0.5	99.2
		3000	3003  0.4	100.1
	Pasteurized milk	50	50.2  0.5	100.4
		100	98.4  0.6	98.4
		200	201.7  0.5	100.8
		1000	1001.6  0.5	100.1
		3000	2998.1  0.3	99.3

a Average of three replicate measurements ± relative standard deviation.