Determination of superoxide anion radical by ...

Journal of Photochemistry and Photobiology A: Chemistry 349 (2017) 1–6

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Journal of Photochemistry and Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem

Invited paper

Determination of superoxide anion radical by modified CdTe quantum dots Jiaxing Hana , Zheng Liua,* , Yajin Guoa , Guo-Cheng Hanb,* , Wei Lia , Siyang Chena , Shufen Zhanga a b

College of Chemical and Biological Engineering, Guilin University of Technology, Guilin 541004, PR China School of Life and Environmental Sciences, Guilin University of Electronic Technology, Guilin 541004, PR China

A R T I C L E I N F O

Article history: Received 22 May 2017 Received in revised form 17 August 2017 Accepted 18 August 2017 Available online 25 August 2017 Keywords: Schiff base CdTe quantum dots Superoxide anion radical Fluorescent analysis

A B S T R A C T

A novel fluorescence probe for determination of superoxide anion radical (O2) was developed with CdTe quantum dots(QDs) modified by thiophene-2-formaldehyde shrinked L-cysteine Schiff base, which was characterized by Fourier transform infrared (FT-IR), thermal gravimetric analysis (TGA), ultravioletvisible (UV–vis) and fluorescent spectrum. The particle size of CdTe QDs was 3.1 nm as confirmed by X-ray powder diffraction pattern (XRD) and Transmission Electron Microscope (TEM) in presence of b-cysteamine (CA). The fluorescent results showed that the fluorescence intensity of CdTe QDs was decreased with modified Schiff base at 305 nm, but enhanced at 610 nm with O2 and showed an apparent linear dependence in the concentration scope of 2.0 105–1.0 104 mol L1 (r = 0.9934) with a detection limit of 1.8 106 mol L1, which exhibited high selectivity and stability towards O2. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Superoxide anion (O2) is one of the important free radicals in organism and it's also the precursor of all reactive oxygen species [1], which can be converted to the other active radical, such as H2O2 [2,3], ONOO [4]. O2 is closely related to the occurrence of inflammation, aging and cancer, herein it's of all-important significance of science for its detection. Currently, the main methods for detecting O2 include electron paramagnetic resonance method [5], SOD enzyme activity assay [6–8], high performance liquid chromatography (HPLC) [9,10], chemiluminescence method [11], fluorescence analysis method [12] and electrochemical methods [13]. With these methods, fluorescenct method [14–16] is not only simple, easy operation, high sensitivity and selectivity, but also can realize in situ visualization of O2 within a living cell [17,18], so as to implement real-time observation in body. Liu and coworkers [19] established a novel fluorescent probe based on imidazole fluorescent ionic liquids with anthracene groups, which was supplied in an aqueous system, the linear range of sensitivity was 1–70 mM with detection limits of 0.1 mM. Meada and coworkers [20] found that bis(2,4-dinitrobenzenesulfonyl) fluorescein

* Corresponding authors. E-mail addresses: [email protected] (Z. Liu), [email protected] (G.-C. Han). http://dx.doi.org/10.1016/j.jphotochem.2017.08.049 1010-6030/© 2017 Elsevier B.V. All rights reserved.

compounds could be used to detect superoxide radicals, which based on a non-redox mechanism, the LOD was 1.0 pmol per well, however, it has some limitations, such as high latency, poor water solubility and high concentration of GSH interference in the biology. Quantum dots (QDs), also known as semiconductor nanocrystals, is a kind of nano-particles with a diameter of 10 nm composed of II–VI or III–V subgroup elements. When the size of nano-crystals is smaller than the exciton Bohr radius, the small size effect, surface effect, quantum effect and macroscopic quantum tunneling effect can be presented, which has attracted a wide range of research interests [21–23]. There have been many reports about Schiff bases for the analysis of the test, but less in the combination of the Schiff base and the QDs. Studies showed that the selective analysis and determination of different substances can be carried out after the cadmium based QDs were modified by different surface modification [24–28]. Schiff base not only can form stable compounds with most of transition metal elements under certain conditions, but also can modified on the surface of them to improve QDs stability and hydrophilicity. Most importantly, the group modified to QDs can improve the fluorescent recognition to some particular materials. In this paper, a Schiff base was synthesized and characterized which was derived from thiophene-2-formaldehyde and L-cysteine and was used to modify the surface of CdTe QDs stabilized by b-cysteamine (CA) as space interval material between the CdTe

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J. Han et al. / Journal of Photochemistry and Photobiology A: Chemistry 349 (2017) 1–6

QDs and O2, resulting in lower fluorescence intensity at 610 nm. With addition of O2, the fluorescence intensity of QDs was enhanced significantly. Therefore, a high sensitivity fluorescence analysis method for detecting concentration of O2 based on this Schiff base modified CdTe QDs fluorescent probe was developed. 2. Experimental 2.1. Materials and apparatus

Scheme 2. Synthesis route of modified CdTe QDs.

Thiophene-2-formaldehyde, L-cysteine, b-cysteamine (CA), TeO2 and dimethyl sulfoxide (DMSO) were obtained from Acros Organic Co. (Belgium), Qiangshun Chemical Co., Ltd (Shanghai, China), Jingchun Biochemical Polytron Technologies Inc. (Shanghai, China), Guangfu Fine Chemical Research Institute (Tianjin, China) and Kelong Chemical Reagent Factory (Chengdu, China), respectively. KBH4, N,N'-dicyclohexyl carbodiimide (DCC) and Nhydroxyl succinimide were obtained from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). All of the reagents used in the experiments were analytical grade. All solutions were prepared with Milli-Q water. Excitation and emission spectra were recorded on a FL-4600 fluorescence spectrofluorimeter with 5 nm bandwidths and ground state electronic absorption spectra were recorded on a Shimadzu UV–vis 2450 spectrophotometer. FT-IR spectra and thermal gravimetric analysis (TGA) were obtained on Shimadzu FTIR-8400 and STA-499, respectively. The X-ray diffraction (XRD) data analysis was carried out using Bruker SMARD CCD. 2.2. Synthesis of thiophene-2-formaldehyde shrinked L-cysteine Schiff base Thiophene-2-formaldehyde (2 mmol, 0.224 g) was dissolved in 20 mL absolute methanol and were dropped slowly into the watersoluble L-cysteine (2 mmol, 0.242 g) under magnetic stirring at 25 C for 12 h in a three-necked flask. Ocher-yellow powders were obtained, washed with water several times and dried out for further use. Yields: 68.3%, m.p. 151–154 C. The synthesis route (Scheme 1) is as follows:

Scheme 3. Synthesis route of CdTe QDs-Schiff base.

continued reacting for 24 h to make sure that CdTe QDs were combined with Schiff base steadily (Schemes 2 and 3). The CdTe QDs-Schiff base complexes were obtained at 85 C by rotary evaporation and the base solution was recycled with same volume of anhydrous ethanol for 12 h. The precipitate was washed further with anhydrous ethanol to remove unreacted CdTe QDs and Schiff base. The composite solid was obtained after another rotary evaporation, recycled base solution, centrifuged and dried. The obtained complexes were sonicated in DMF for further applications. 2.4. Method for recognition of O2

2.3. Preparation of Schiff base modified CdTe QDs CdCl2 (1 mmol, 0.2284 g) and b-cysteamine (CA) (3 mmol, 0.2315 g) were mixed into three-necked flask with 100 mL H2O as the precursor solutions of QDs. TeO2 (0.5 mmol, 0.0798 g) and KBH4 (0.5 mmol, 0.027 g) were added when the temperature heated up to 200 C in protection of N2. The solution quickly changed from colorless to orange and finally turned into wine red and quickly removed. The solution was cooled down to 3 C and sealed placed in the refrigerator and for further use. The Schiff base (1 mmol, 0.04 g) was dissolved in the N,N'dimethyl formamide (DMF) and was adjusted to pH = 9 with triethylamine in a three-necked flask. N-hydroxyl succinic diamide (NHS) (0.238 mmol, 0.049 g) were added into the above solution under stirring. After 30 min, N,N0 -dicyclohexyl carbodiimide (DDC) (0.239 mmol, 0.04 g) were added and activated for 24 h at 25 C [29]. Different amount of as-prepared CdTe QDs were added and

O2 was generated in alkaline DMSO according to literature methods [30,31]. Briefly, 100 mL 0.5 mol L1 NaOH was dropped into 10 mL DMSO. The reaction was lasted 30 min under magnetic stirring. The concentration of O2 could be calculated according to Beer’s law with its molar absorptivity (20.061 L/(mol cm)1 at 271 nm) in DMSO [32,33]. The DMSO was reacted in alkaline as follows: 2 DMSO + 2 OH + 3 O2 ! 2 O2 + 2 CH3SOOH + CH3OOCH3

(1)

The CdTe QDs-Schiff base complexes dissolved in DMF were added to various concentration of O2 which were diluted by DMF and ultrasonicated for 15 min. Hence, the mixture was allowed to equilibrate for 30 min before recording the fluorescence signal for each concentration of O2. All measurements were recorded at room temperature. 3. Results and discussion 3.1. Characterization of modified CdTe QDs

Scheme 1. Synthesis route of thiophene-2-formaldehyde shrinked Schiff-base.

L-cysteine

3.1.1. Spectra characteristic of CdTe QDs Fig. 1 shows the X-ray powder diffraction (XRD) pattern which was employed to elucidate the crystal nature and size of the QDs with the Scheler formula. The 2u values for CA-CdTe QDs were


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Fig. 3. FT-IR spectrum of Schiff base (a), CdTe QDs-Schiff base (b) and CdTe QDs (c). Fig. 1. XRD pattern of CdTe QDs.

24.0 , 39.7 and 47.0 corresponds to the diffraction pattern with planes at [110], [220] and [311], respectively. They were coincided with the position of diffraction peak, compared with the standard spectra (PDF No. 01-075-2083) and the diffraction peaks were narrow as shown in the XRD pattern of CdTe QDs. It can be seen from Fig. 2 that quantum dots are uniform in particle size and have good dispersibility. The diameter of CdTe QDs showed in TEM (Fig. 2) is 3.87 nm, a little higher than the calculated value of 3.1 nm according to Scherrer equation [34], which may due to the fact that the surface atoms are chemically more active and caused precipitation [35]. At the same time, the diffraction peak slightly shifts towards CdS cubic, the S-H is hydrolyzed in the formation of QDs because little sulfur is coordinated with Cd2+ and developed CdS. 3.1.2. FT-IR analysis Fig. 3 shows the IR spectra of thiophene-2-formaldehyde shrinked L-cysteine Schiff base, CdTe QDs-Schiff base and CdTe QDs in the range of 4000–400 cm1. For the Schiff base (Fig. 3a), the broad band around 3100–2400 cm1 may be assigned to free COOH groups and the band at 3432 cm1 may be ascribed to O H stretching vibration. For CdTe QDs (Fig. 3c), the band between 3437 and 3135 cm1 may be assigned to NH2 stretching which may due to the stabilizer of b-cysteamine (CA) coated on the surface of CdTe QDs. Fig. 3b shows the IR spectra of CdTe QDs-Schiff base. It was observed that the broad band around 3100–2400 cm1 was diminished and a new band assigned to the CO NH

Fig. 2. TEM image of CdTe QDs.

stretch was observed at 1619 cm1. Such band could also be proved by the group of NH- at 3438 cm1 and was not observed in pure Schiff base and CdTe QDs alone (Fig. 3a and c). Therefore, the IR spectra demonstrate clearly that the COOH of the Schiff base had been successfully shrinked with NH2 of CA-CdTe QDs. 3.1.3. Thermal properties Fig. 4 shows the Schiff base and CdTe QDs-Schiff base thermal curves measured under a nitrogen atmosphere. It can be seen from Fig. 4 that when the temperature is increased, the thermal decomposition of the Schiff base and CdTe QDs-Schiff base occurs at different decomposition steps [36]. For them, the weight loss steps were in the range of 135–390 C (curve a) and 263–496 C (curve b), respectively. It concluded that the CdTe QDs-Schiff base had better thermal stability than that of Schiff base alone. 3.1.4. Spectrophotometric properties Fig. 5 shows the UV–vis absorption of the Schiff base (1.0 105 mol L1) in DMF. The maximum absorption wavelength value of the Schiff base is at 305 nm, which can be assigned to conjugated C¼N p–p* transition. Fig. 6 shows the fluorescence spectra of the Schiff base (1.0 105 mol L1) and the CdTe QDs (1.678 105 mol L1). It can be seen from Fig. 6 that the strongest emission (Fig. 6b) wavelength is at 337 nm (lex = 305 nm, Fig. 6a), due to the closed-loop in the formation of Schiff base and the closed-ring structure which showed excellent fluorescence intensity. And CdTe QDs have significant fluorescence properties, the

Fig. 4. TG curves of thiophene-2-formaldehyde shrinked L-cysteine Schiff base (a) and CdTe QDs-Schiff base (b).

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Fig. 7. Effect of addition of different concentrations of Schiff base on the fluorescence of CdTe QDs. Ratios of QDs and the Schiff base: (a) 0, (b) 1:1667, (c) 1:2000, (d) 1:2500, (e) 1:3333, (f) 1:5000, (g) 1:10,000; Insert: Relationship between concentration of schiff base and fluorescence intensity.

Fig. 5. UV–vis spectrum of the Schiff base.

represent different ratio of QDs to Schiff base. It is noteworthy that the fluorescence intensity decreased significantly with addition of Schiff base. It can be seen from illustration that when the concentration of Schiff base is 3333 mmol L1, the curve is almost parallel to the X axis. It indicated that the fluorescence of the quantum dot no longer changes with the addition of Schiff base. So 3333 mmol L1 is the most suitable concentration of Schiff base, in which case, the modified quantum dots yield reaches maximum. The fluorescence quantum yields of the CdTe QDs modified before and after were calculated according to literature methods [39]. The fluorescence yield of the modified CA-CdTe quantum dots is calculated by Eq. (3): y fFmodif ðQDÞ ¼ fF ðQDÞ

y F modif QD

F QD

ð3Þ

In the formula: fF ðQDÞ is the fluorescence quantum yield of CAFig. 6. Fluorescence excitation (a) and emission spectra (b) of Schiff base and fluorescence excitation (c) and emission spectra (d) of CA-CdTe QDs.

strongest emission located at 610 nm (Fig. 6d) (lex = 305 nm, Fig. 6c), the emission spectrum is narrow and symmetrical, and the excitation spectrum is broad. Using Rhodamine B as the standard sample, the quantum yield of CA-CdTe quantum dots before the modification was calculated by the Eq. (2):

fF ¼ fF ðStdÞ

F AStd n2 F Std An2Std

ð2Þ

In the formula: A and AStd are the absorbance of the sample to be tested and the standard sample at each excitation wavelength respectively (A = 0.017, AStd = 0.007). F and FStd are the integral areas of the fluorescence curve of the quantum dots and the standard samples. n and nStd are the refractive indices (n = 1.33, nStd = 1.36) of the solvent used for the sample to be tested and the standard sample. The calculated value of fF of CA-CdTe quantum dots is 0.15. 3.2. Effect of Schiff base yields on the fluorescence of CdTe QDs Effect of Schiff base yields on the fluorescence of CdTe QDs was studied. The fluorescence spectra of the CdTe QDs complexed with different yields of Schiff base (Fig. 7) were measured at room temperature. It indicates that CdTe QDs showed broad emission spectra as expected [37,38]. The Fig. 7 curve (a) represents the fluorescence spectra of CdTe QDs alone and Fig. 7 curve b-g

CdTe quantum dots before modification, and is used as a standard sample. F ðQDÞ is the fluorescence intensity of the CA-CdTe modif y is the fluorescence quantum dots before modification, and F QD

intensity after quantum dot modification. The results show that the fluorescence quantum yields of QDs complexes are much lower than the values for the QDs individual. The fluorescence quantum yields of CA-CdTe quantum dots and modified thiophene-2-formaldehyde Schiff base are shown in Table 1. 3.3. Fluorescence recognition of O2 Fluorescence response of the CdTe QDs-Schiff base in the presence of varying concentration of O2 was examined. The original concentration of Schiff base is 2000 mol L1 and the fluorescence intensity is 4458 a.u. with addition of different concentrations of O2. As shown in Fig. 8, with an increase of O2 concentration, the fluorescence intensity was markedly enhanced and thus gives a linear relationship in the range of 2.0 105– 1.0 104 mol L1. The linear equation is y = 56.27c (104 mol L1) + 4362.810 with r = 0.9934 (RSD = 0.156%) [40]. The corresponding limit of detection (LOD) was calculated following the 3 d/S criteria where d is the standard deviation of blank measurement and S is the slope of the calibration graph (inserts in Fig. 8). The LOD value for CdTe QDs-Schiff base was 1.8 106 mol L1. The covalent bond of the Schiff base may has an effect on the QDs to set up a new trap state on the surface of them, which leads to the attenuation of the fluorescence emission. After the addition


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Table 1 Fluorescence quantum yield of CA-CdTe quantum dots and modified thiophene-2-formaldehyde Schiff base. Number

The proportion of modification (QDs: schiff base)

fluorescence intensity (F)

Fluorescence quantum yield (FF)

a b c d e f g

CA-QDS 1:1667 1:2000 1:2500 1:3333 1:5000 1:10000

8525 6313 5082 2797 396.1 321.3 282.5

0.150a 8.998 102 7.244 102 3.987 102 5.646 103 4.580 103 4.027 103

a

is the fluorescence quantum yield of CA-CdTe quantum dots before modification, calculated by Eq. (2), used as the standard value of Eq. (3).

3.3 106 mol L1. Blank is a control group with no interfering substance. It can be seen from Fig. 9 that there were almost no change in fluorescence intensities in the presence of the above interferences, which means it is possible to discriminate between O2 and other species. We can conclude that the CdTe QDs-Schiff base system is highly selective for O2 4. Conclusion

Fig. 8. Effect of addition of different concentrations of O2 on the fluorescence of CdTe QDs-Schiff base. Concentration of O2-: (a) 0, (b) 2.0 105, (c) 4.0 105, (d) 6.0 105, (e) 8.0 105, (f) 1.0 104 (mol L1); Insert: Linear relationship plot.

of O2, the space and distance between the Schiff base and QDs increase when the Schiff base is served as spacer molecule, and enhance the fluorescence intensity. 3.4. Interference study on the recognition of O2 The interference of the CdTe QDs-Schiff base of various possible co-existing species (8-hydroxyquinoline, lithium perchlorate, sodium nitrite, potassium nitrate, L-cysteine and b-thiol) was carried out to investigate the selectivity of the proposed QDs. Before adding the interfering substance, the fluorescence intensity of each group is the same. Fig. 9 shows the fluorescence intensity of QDs-Schiff-O2 when the interference ion concentration is 1 103 mol L1 and the superoxide anion concentration is

Fig. 9. Effect of different interfering species on CdTe QDs-Schiff base system.

In this work, CdTe QDs were successfully conjugated to the thiophene-2-formaldehyde shrinked L-cysteine Schiff base to form a CdTe QDs-Schiff base complex. The complex was characterized. The synthesized complex was successfully recognized O2 in aqueous solution. Interference study illustrates that the complex has a high selectivity towards O2. Hence, the proposed method presents great promise in the recognition of O2 at 610 nm in the concentration scope of 2.0 105–1.0 104 mol L1 (r = 0.9934) with a detection limit of 1.8 106 mol L1. Acknowledgments This work was supported by the National Nature Science Foundation of China (No. 21266006, 61661014, 61301038), the Nature Science Foundation of Guangxi Province (No. 2016 GXNSFAA3 80109, 2015GXNSFBA139041, 2013GXNSFAA019224) and Guangxi Key Laboratory of Electro chemical and Magnetochemical Functional Materials, Collaborative Innovation Center for Exploration of Hidden Nonferrous Metal Deposits and Development of New Materials in Guangxi. References [1] O. Adegoke, P.B.C. Forbes, Challenges and advances in quantum dot fluorescent probes to detect reactive oxygen and nitrogen species: a review, Anal. Chim. Acta 862 (2015) 1–13, doi:http://dx.doi.org/10.1016/j.aca.2014.08.036. [2] I.A. Abreua, D.E. Cabellib, Superoxide dismutases—a review of the metalassociated mechanistic variations, Biochim. Biophys. Acta 1804 (2010) 263– 274, doi:http://dx.doi.org/10.1016/j.bbapap.2009.11.005. [3] S. Singh, V.K. Dubey, Multiwalled, carbon nanotube-superoxide dismutase conjugate towards alleviating induced oxidative stress, Int. J. Pept. Res. Ther. 22 (2016) 171–177, doi:http://dx.doi.org/10.1007/s10989-015-9495-3. [4] R. Radi, Peroxynitrite, a stealthy biological oxidant, J. Biol. Chem. 288 (2013) 26464–26472, doi:http://dx.doi.org/10.1074/jbc.R113.472936. [5] U.A. Nilsson, G. Haraldsson, S. Bratell, ESR-measurement of oxygen radicals in vivo after renal ischaemia in the rabbit, effects of pre-treatment with superoxide dismutase and heparin, Acta Physiol. 147 (1993) 263–270, doi: http://dx.doi.org/10.1111/j.1748-1716.1993.tb09498.x. [6] W.M. Armstead, R. Mirro, C.W. Leffler, D.W. Busija, Cerebral superoxide anion generation during seizures in newborn pigs, J. Cereb. Blood Folw Met. 9 (1989) 175–179, doi:http://dx.doi.org/10.1038/jcbfm.1989.26. [7] D.U. Jin feng, H.Y. Hao, Q. Zhang, W.B. Yuan, H.Y. Wang, Formation of Ca(II) and Zn(II) with chitosan and the study on GFC spectra of oligosaccharides by oxidizing degradation and their reactivity of scavenging active oxygen, Int. J. Mol. Sci. 22 (2006) 38–376, doi:http://dx.doi.org/10.3969/j.issn.10009035.2006.06.004. [8] A.J. Ležai c, I. Pašti, M. Vukomanovi c, G. Ciri c-Marjanovi c, Polyaniline tannatesynthesis, characterization and electrochemical assessment of superoxide anion radical scavenging activity, Electrochim. Acta 142 (2014) 92–100, doi: http://dx.doi.org/10.1016/j.electacta.2014.07.073.

6


[9] L. Diez, M.H. Livertoux, et al., High-performance liquid chromatographic assay of hydroxyl free radical using salicylic acid hydroxylation during in vitro experiments involving thiols, J. Chromatogr. B: Biomed. Sci. Appl. 763 (2001) 185–193, doi:http://dx.doi.org/10.1016/S0378-4347(01). [10] B. Kalyanaraman, B.P. Dranka, M. Hardy, R. Michalski, J. Zielonka, HPLC-based monitoring of products formed from hydroethidine-based fluorogenic probesthe ultimate approach for intra- and extracellular superoxide detection, Biochim. Biophys. Acta 1840 (2014) 739–744, doi:http://dx.doi.org/10.1016/j. bbagen.2013.05.008. [11] U. Dirnagl, U. Lindauer, A. Them, S. Schreiber, H.W. Pfister, U. Koedel, Regine Reszka, Dorette Freyer, A. Villringer, Global cerebral ischemia in the rat: online monitoring of oxygen free radical production using chemiluminescence in vivo, J. Cereb. Blood Flow Met. 15 (1995) 929–940, doi:http://dx.doi.org/ 10.1038/jc bfm.1995.118. [12] T. Münzel, I.B. Afanas’Ev, A.L. Kleschyov, D.G. Harrison, Detection of superoxide in vascular tissue, Arterioscler. Thromb. Vasc. Biol. 22 (2002) 1761–1768, doi: http://dx.doi.org/10.1161/01.ATV.0000034 022.11764.EC. [13] Y.Z. Xian, W. Zhang, J. Xue, X. Ying, L. Jin, Measurement of nitric oxide released in the rat heart with an amperometric microsensor, Analyst 125 (2000) 1435– 1439, doi:http://dx.doi.org/10.1039/B0 00354I. [14] J.J. Hu, N.K. Wong, S. Ye, Fluorescent probe HKSOX for imaging and detection of endogenous superoxide in live cells and in vivo, J. Am. Chem. Soc. 137 (2015) 6837–6843, doi:http://dx.doi.org/10.1021/jacs.5b01881. [15] J.C. Qin, L. Fan, Z.Y. Yang, A small-molecule and resumable two-photon fluorescent probe for Zn2+ based on a coumarin Schiff-base, Sens. Actuators B: Chem. 228 (2016) 156–161, doi:http://dx.doi.org/10.1016/j.snb.2016.01.031. [16] A. Degirmenci, D. Iskenderkaptanoglu, F. Algi, A novel turn-off fluorescent Pb (II) probe based on 2,5-di(thien-2-yl)pyrrole with a pendant crown ether, Tetrahedron Lett. 56 (2015) 602–607, doi:http://dx.doi.org/10.1016/j. tetlet.2014.12.039. [17] S. Fang, L. Yang, Y. Kelu, Z. Wenwan, A mitochondrion targeting fluorescent probe for imaging of intracellular superoxide radicals, Chem. Commun. 51 (2015) 7931–7934, doi:http://dx.doi.org/10.1039/c5cc01075f. [18] J. Karolin, C.D. Geddes, Metal-enhanced fluorescence based excitation volumetric effect of plasmon enhanced singlet oxygen and super oxide generation, Phys. Chem. Chem. Phys. 15 (2013) 15740–15745, doi:http://dx.doi. org/10.1039/c3cp50950h. [19] H. Liu, L. Zhang, J.M. Chen, Y.Y. Zhai, Y.B. Zeng, L. Li, A novel functional imidazole fluorescent ionic liquid: simple and efficient fluorescent probes for superoxide anion radicals, Anal. Bioanal. Chem. 405 (2013) 9563–9570, doi: http://dx.doi.org/10.1007/s00216-013-7357-4. [20] H. Maeda, K. Yamamoto, Y. Nomura, I. Kohno, L. Hafsi, et al., A design of fluorescent probes for superoxide based on a nonredox mechanism, J. Am. Chem. Soc. 127 (2005) 68–69, doi:http://dx.doi.org/10.1021/ja047018k. [21] A.S. Karakoti, R. Shukla, R. Shanker, S. Singh, Surface functionalization of quantum dots for biological applications, Adv. Colloid Interface Sci. 215 (2015) 28–45, doi:http://dx.doi.org/10.1016/j.cis.2014.11.004. [22] D. Kumar, K. Singh, V. Verma, H.S. Bhatti, Synthesis and characterization of carbon quantum dots from orange juice, J. Bionanosci. 8 (2014) 274–279, doi: http://dx.doi.org/10.1166/jbns.2014.1236. [23] D. Yue, J.W. Zhang, J.B. Zhang, Y. Lin, Preparation of PbS quantum dots using inorganic sulfide as precursor and their characterization, Acta Phys. Chim. Sin. 27 (5) (2011) 1239–1243, doi:http://dx.doi.org/10.3866/PKU.WHXB20110513. [24] Q.F. Chen, W.X. Wang, Y.X. Ge, M.Y. Li, S.K. Xu, X.J. Zhang, Direct aqueous synthesis of cysteamine-stabilized CdTe quantum dots and its deoxyribonucleic acid bioconjugates, Chin. J. Anal. Chem. 35 (2007) 135–138, doi:http://dx.doi.org/10.1016/S1872-2040(07)60030-9. [25] C.H. Wu, L.X. Shi, C.Y. Wu, D.D. Guo, M. Selke, X.M. Wang, Enhanced in vitro anticancer activity of quercetin mediated by functionalized CdTe QDs, Sci. China Chem. 57 (2014) 1579–1588, doi:http://dx.doi.org/10.1007/s11426-014-5165-0.

[26] F. Zhao, J. Kim, Synthesis and spectral analysis of head-to-tail shaped Ag2S/ZnS near-infrared quantum dots with manganese dopant, J. Nanosci. Nanotechnol. 15 (2015) 7871–7875, doi:http://dx.doi.org/10.1166/jnn.2015.10426. [27] K. Zhang, Q.S. Mei, G.J. Guan, B. Liu, S.H. Wang, Z.P. Zhang, Ligand replacementinduced fluorescence switch of quantum dots for ultrasensitive detection of organophosphorothioate pesticides, Anal. Chem. 82 (2010) 9579–9586, doi: http://dx.doi.org/10.1021/ac102531z. [28] Y. Wei, J. Yang, J.Y. Ying, Reversible phase transfer of quantum dots and metal nanoparticles, Chem. Commun. 46 (2010) 3179–3181, doi:http://dx.doi.org/ 10.1039/b926194j. [29] K.E. Sekhosana, E. Antunes, S. Khen, S. D’Souza, T. Nyokong, Fluorescence behavior of glutathione capped CdTe@ZnS quantum dots chemically coordinated to zinc octacarboxy phthalocyanines, J. Lumin. 136 (2013) 255– 264, doi:http://dx.doi.org/10.1016/j.jlumin.2012.11.044. [30] J. Di, S. Bi, M. Zhang, Third-generation superoxide anion sensor based on superoxide dismutase directly immobilized by sol-gel thin film on gold electrode, Biosens. Bioelectron. 19 (2004) 1479–1486, doi:http://dx.doi.org/ 10.1016/j.bios.2003.12.006. [31] Y.J. Wang, J. Xu, J.J. Li, F. Wu, Natural montmorillonite induced photooxidation of As (III) in aqueous suspensions: roles and sources of hydroxyl and hydroperoxyl/superoxide radicals, J. Hazard. Mater. 260 (2013) 255–262, doi: http://dx.doi.org/10.1016/j.jhazmat.2013.05.028. [32] K. Hyland, C. Auclair, The formation of superoxide radical anions by a reaction between O2, OH and dimethyl sulfoxide, Biochem. Biophys. Res. Commun. 102 (1981) 531–537, doi:http://dx.doi.org/10.1016/0006-291X(81)91552-7. [33] J. Di, S. Bi, M. Zhang, Third-generation superoxide anion sensor based on superoxide dismutase directly immobilized by sol-gel thin film on gold electrode, Biosens. Bioelectron. 19 (2004) 1479–1486, doi:http://dx.doi.org/ 10.1016/j.bios.2003.12.006. [34] Z. Hao, Z. Zhen, Y. Bai, The influence of carboxyl groups on the photoluminescence of mercaptocarboxylic acid-stabilized CdTe nanoparticle, J. Phys. Chem. B 107 (2003) 8–13, doi:http://dx.doi.org/10.1021/ jp025910c. [35] C. Cai, Y.H. Cheng, Y.C. Wang, H.F. Bao, Mercaptosuccinic acid modified CdTe quantum dots as a selective fluorescence sensor for Ag+ determination in aqueous solutions, RSC Adv. 4 (2014) 59157–59163, doi:http://dx.doi.org/ 10.1039/c4ra07891h. [36] S.S. Yang, C.L. Ren, Z.Y. Zhang, J.J. Hao, Q. Hu, X.G. Chen, Aqueous synthesis of CdTe/CdSe core/shell quantum dots as pH-sensitive fluorescence probe for the determination of ascorbic acid, J. Fluoresc. 21 (2011) 1123–1129, doi:http://dx. doi.org/10.1007/s10895-010-0788-9. [37] R. Ran, D.D. Miao, Y.M. Liang, L.B. Qu, J.J. Li, P.D. Harrington, Ultrasensitive electrochemical sensor based on CdTe quantum dots-decorated poly (diallyldimethylammonium chloride)- functionalized graphene nanocomposite modified glassy carbon electrode for the determination of puerarin in biological samples, Electrochim. Acta 173 (2015) 839–846, doi: http://dx.doi.org/10.1016/j.electacta.2015.05.139. [38] M. Hou, X.Y. Yan, L. Xiong, Determination of sparfloxac in with CdSe/ CdSquantum dots as fluorescent probes, J. Lumin. 157 (2015) 58–62, doi:http:// dx.doi.org/10.1016/j.jlumin.2014.08.006. [39] S. Chandra, J. Doran, S.J. McCormack, M. Kennedya, A.J. Chattenc, Enhanced quantum dot emission for luminescent solar concentrators using plasmonic interaction, Sol. Energy Mater. Sol. Cells 98 (2012) 385–390, doi:http://dx.doi. org/10.1016/j.solmat.2011.11.030. [40] W. Liu, A.X. Zhang, G.H. Xu, F.D. Wei, J. Yang, Q. Hu, Manganese modified CdTe/ CdS quantum dots as an immunoassay biosensor for the detection of Golgi protein-73, J. Pharm. Biomed. 117 (2016) 18–25, doi:http://dx.doi.org/10.1016/ j.jpba.2015.08.020.