Sensing and Bio-Sensing Research 3 (2015) 46–52

Contents lists available at ScienceDirect

Sensing and Bio-Sensing Research

journal homepage: www.elsevier .com/locate /sbsr

Highly fluorescent CdTe quantum dots with reduced cytotoxicity-ARobust biomarker

http://dx.doi.org/10.1016/j.sbsr.2014.12.0012214-1804/� 2014 The Authors. Published by Elsevier B.V.This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

⇑ Corresponding author. Tel.: +82 552133436; fax: +82 552133439.E-mail address: yilee@changwon.ac.kr (Y.-I. Lee).

Jandi Kim a, Bui The Huy a, Kavitha Sakthivel a, Hye Jung Choi b, Woo Hong Joo b, Seung Kyun Shin c,Min Jae Lee c, Yong-Ill Lee a,⇑a Department of Chemistry, Changwon National University, Changwon 641-773, Republic of Koreab Department of Biology, Changwon National University, Changwon 641-773, Republic of Koreac Department of Applied Chemistry, College of Applied Sciences, Kyung Hee University, Yongin 446-701, Republic of Korea

a r t i c l e i n f o

Keywords:

CysteineCdTeQuantum dotsFluorescence reactivationMTT assay

a b s t r a c t

L-Cysteine (Cys) capped CdTe quantum dots (CdTe@Cys QDs) were successfully synthesized in an aqueousmedium. The synthesized CdTe@Cys samples were analyzed using Fourier transform infrared (FT-IR)spectroscopy, fluorescence (FL) spectroscopy, transmission electron microscopy (TEM), confocal micros-copy and subsequently subjected to the antibacterial test. Systematic investigations were carried out forthe determination of optimal conditions namely the ratios of Cd:Te, CdTe:Cys, pH value and the chemicalstability of CdTe@Cys. Moreover, the reactivation of FL intensity in the CdTe@Cys sample was done easilyby the addendum of Cys. The introduction of additional cysteine to the CdTe@Cys QDs sample showed anenhancement in terms of the FL intensity and stability along with the reduced antibacterial activity. Thiswas further confirmed through Thiazolyl blue tetrazolium bromide (MTT) assays. Both the result of thebio-stability tests namely the antibacterial test and MTT assay displayed similarities between the exter-nally added Cys and cytotoxicity of the bacteria and human HeLa cancer cell lines. Confocal microscopicimages were captured for the CdTe@Cys conjugated Escherichia coli.

� 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-NDlicense (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction

Generally, quantum dots (QDs) are referred to as the zero-dimensional colloidal crystals that possesses strong size depen-dence and multi-colored luminescence properties [1,2] along withits intrinsic features, such as the sharp and symmetric emission,photostability and high quantum yields. Owing to these inherentinimitable properties, QDs play a vital role in various avenuesnamely the identification of the chemical moieties [3], clinicaldiagnostics [4–9], optoelectronics [10,11], bio-imaging and bio-sensing [12–19].

QDs are chemically unstable and have poor solubility in waterand due to these deficits it was not preferred for biomedical appli-cations. But it associates the promising features needed for the bio-medical studies and hence, a number of attempts were put forth, tomake it water-soluble. Primary works relied on the usage of differ-ent thiol stabilizers to promote the water solubility and chemicalstability in the water phase. The thiol linkage serves as the protec-tion layer to the core and thus prevents it from surface oxidation.

Thus, the solubility of the QDs in water can be enhanced. Thiolsare often sorted as the ideal choice for the modification of QDsdue to its facile functionality [20]. A wide variety of quantum dotsare available namely cadmium selenide (CdSe), cadmium sulphide(CdS), zinc sulfide (ZnS), etc., [21,22]. The syntheses of these mate-rials require high temperature, non-aqueous medium and possesslow efficiency. These discrepancies can be rectified using cadmiumtelluride (CdTe) QDs as they are expected to show a high fluores-cence efficiency and good stability [2]. In order to obtain an opti-mum quantum efficiency the following stabilizers are usednamely mercaptoacetic acid (MAA), glutathione (GSH) and thio-glycerol (TGA) [23,24]. Few of the works reported the toxicity ofCdTe QDs in relation to the metal ions and stabilizers used for itssynthesis [25–31]. Hence the toxicity level of the stabilizers hasto be considered as a very important factor for biomedical applica-tion, where the best choice is confined to the low toxicity. Recentstudies relied on the preparation of CdTe using cysteine as a surfac-tant [32,33]. L-Cysteine (Cys) is a simple amino acid that possessesthe thiol(-SH) group and two other active functionalities (�NH2,�COOH) which is considered to be the essential component ofhuman metabolism and enzyme reactions. Specifically, the pres-ence of thiol group functions as a nucleophile and thus makes

http://crossmark.crossref.org/dialog/?doi=10.1016/j.sbsr.2014.12.001&domain=pdfhttp://creativecommons.org/licenses/by-nc-nd/4.0/http://dx.doi.org/10.1016/j.sbsr.2014.12.001http://creativecommons.org/licenses/by-nc-nd/4.0/mailto:yilee@changwon.ac.krhttp://dx.doi.org/10.1016/j.sbsr.2014.12.001http://www.sciencedirect.com/science/journal/22141804http://www.elsevier.com/locate/sbsr

J. Kim et al. / Sensing and Bio-Sensing Research 3 (2015) 46–52 47

the complex structure of proteins and is subjected to oxidationduring the alteration of the disulfide bond [34]. Yang et al.attempted the direct labeling of Escherichia coli (E. coli) using CdTeQDs by the following treatments namely chloroform–SDS, lyso-zyme–EDTA and osmotic shock and, the results obtained showedan enhancement in the sensitivity [35].

Our present work is based on the synthesis of Cys capped CdTeQDs in an aqueous medium for labeling the cell and the reactiva-tion of FL intensity using additional Cys. Li et al. also reportedthe recovery of FL intensity of CdTe to its initial level by the addi-tion of Cys [36]. But our research differs from the other reportedworks in many aspects. Herein, we carried out a systematic inves-tigation on the influence of the additional Cys to CdTe@Cys in thefacet of biomolecular protection. The results obtained proved theoptical superiority of the adopted technique. Addition of Cys tohigher concentration significantly improved both the stability ofbacterial communities and FL intensity of CdTe@Cys QDs. Further-more, the reactivation of the CdTe@Cys QDs resulted in a mitigatedactivity of CdTe in the antibacterial test. Along with the lower cyto-toxicity, reactivated CdTe@Cys QDs labeled E. coli were found to bemore adequate. MTT assay was carried out for the determination ofthe cytotoxicity of reactivated CdTe@Cys to HeLa cells. The resultsderived out of the study proved the complementary evidences forits strong implication as a novel labeling method. Thus it will havea promising future and give an unprecedented insight by the wayof an efficient labeling of cell using QDs.

2. Experimental

2.1. Reagents and materials

Tellurium powder, sodium borohydride (NaBH4), cadmiumchloride (CdCl2), L-Cysteine, ethylenediaminetetraacetic acid(EDTA), phosphate buffered saline (PBS) were obtained fromSigma-Aldrich (USA) of analytical grade. Lysozyme from chickenegg white of molecular biology grade was obtained from Sigma–Aldrich Chemical. Sucrose of ACS reagent grade for microbiologywas obtained from Fluka. Deionized distilled (D.I.) water wasobtained from a Milli-Q water purification system. All other chem-icals and solvents used for the QDs synthesis were purchased fromDeahan Co. (Korea) and are used as such without further purifica-tion. Luria–Bertani agar plates were obtained from Difco (USA).

Fig. 1. (A) The infrared spectra of L-Cysteine (a) and L-Cysteine capped-CdTe (b). (B) Effe1:0.24:x. Concentrations of Cys were (a) 2; (b) 4; (c) 6; (d) 8, respectively.

Filter paper discs were purchased from Toyo Roshi Kaisha, Ltd.(Japan). MTT assay kit was purchased from Biosesang (Korea).

2.2. Preparation of CdTe@Cys QDs

The synthesis of CdTe QDs was carried out in accordance to anearlier reported procedure [37,38] and it consisted of two steps. Inthe former step, 30 mg of CdCl2 was dissolved in 100 mL of D.I.water in a three necked flask, and to it added 363 mg of Cys care-fully under ultrasonication. The resultant solution was adjusted forthe desired pH using 1 M NaOH solution under vigorous stirring.On the other hand, Te precursor solution was prepared using Tepowder and NaBH4. Typically, 20 mg of Te powder was mixed with30 mg of NaBH4 in a vial bottle with 0.6 mL D.I. water. Then the vialbottle was sealed with a rubber stopper and heated at 50 �C for30 min. This process resulted in the generation of, NaHTe activatedsolution and it was withdrawn using a syringe. In the latter step,Cd2+ precursor solution was heated to 100 �C approximately; atthis point the prepared Te2� precursor solution was injected intothe Cd2+ precursor solution. The mixture was subjected to refluxat different reaction time interval in order to control the opticalproperties of the CdTe QDs. After completion of reaction the mix-ture was brought down to ambient temperature. Then the precip-itation of the CdTe QDs were done using iso-propanol andsubjected to centrifugation at 4000 rpm. The purification processwas repeated several times using iso-propanol/water. Finally, theproduct obtained was dried overnight at 80 �C and thus yieldingthe desired product namely, CdTe QDs.

2.3. Reactivation of CdTe@Cys QDs

Recovery of fluorescence intensity to its initial level using Cyswas previously reported by Li et al. [36]. Our experimental workwas built on this platform with slight modifications. Samples ofthe as-prepared CdTe@Cys and Cys were prepared at differentmolar ratios viz 1:1, 1:2, 1:3, and 1:4. Then, the mixtures were sub-jected to sonication at room temperature for duration of 5 min. TheFL intensity of reactivation states of CdTe QDs was analyzed.

2.4. Characterization techniques

Fluorescence spectra of QDs were recorded using a spectrofluo-rometer (FP-6500, Jasco, Japan) at an excitation wavelength of

ct of Cys concentration on the emission intensity. The molar ratio of Cd:Te:Cys was

Fig. 2. (A) Chemical stability of CdTe@Cys QDs prepared at various pH. (B) Influence of additional Cys on the FL intensity of CdTe@Cys QDs. (The molar ratio of CdTe@CysQDs:additional Cys was 1:x.) (a): original CdTe@Cys QDs without additional Cys. molar ratios of additional Cys were (b) 1; (c) 2; (d) 3; (e) 4, respectively. The inset gives animage of the adding Cys solutions under UV lamp (before and after).

48 J. Kim et al. / Sensing and Bio-Sensing Research 3 (2015) 46–52

355 nm. Fourier transform infrared (FT-IR) spectra were obtainedusing a FT-IR spectrophotometer (FT/IR-6300, Jasco, Japan). Trans-mission electron micrographs were captured using a field emissiontransmission electron microscope (JEM-2100F, JEOL, Japan) at200 kV. Fluorescence microscopic images were taken using a Laserscanning confocal microscope (FV10i, Olympus, Japan). Relativeabsorptivity of the cells were observed using UV–vis spectroscopy(Bio Tek, USA).

2.5. Evaluation of antibacterial activity

Antibacterial activities of the synthesized compounds weredetermined using disc diffusion methods [39]. Primarily, a 100 lLsuspension of E. coli IMSNU 10080 (ATCC 10798) (1.5 � 108 CFU/mL) was subjected to inoculation in a Luria-Bertani (LB) agarplates. Then the filter paper discs (6 mm) were placed over theinoculated plate, and to it added 15 lL of the compounds, andincubated at 37 �C for 24 h.

2.6. Bacterial treatment and QDs labeling

E. coli cells were washed twice with phosphate buffer solution(pH 8) and was treated using Lysozyme–EDTA [35,40]. The

Fig. 3. Schematic representation depicting the reactivation steps of CdTe@Cys on addistrengthened FL intensity.

prepared cells were resuspended in buffer A (20% sucrose, 0.03 MTris–HCl, EDTA 10 mM, pH8). After 10 min of incubation at roomtemperature, cells were resuspended by lysozyme (the sample ofa final concentration were 2 mg/mL) followed by incubation at37 �C for 20 min. Then the sample was centrifuged at 10,000 rpmfor 1 min and washed twice with buffer B (50 mM Tris–HCl,0.5 M sucrose, pH 8). Finally, E. coli cells were resuspended in buf-fer C (50 mM Tris–HCl, pH 8) and then mixed thoroughly with theprepared QDs. After shaking at room temperature for 4 h, the cellswere washed twice with buffer C.

2.7. In vitro cell viability

Cell viability was checked by the standard MTT assay withslight modifications to the earlier reported procedure [41]. Briefly,the cells were grown in 96 well plates at 37 �C in a humidifiedatmosphere of 95% air/5% CO2. After the treatment of the extraCys at various concentrations, thiazolyl blue tetrazolium bromidewas added to each well containing the cells (final conc. 0.5 mg/mL) and incubated for 2.5 h. Dimethyl sulfoxide was added to sol-ubilize the blue MTT-formazan product and the sample was incu-bated for further 30 min at room temperature. Absorbance of thesolution was recorded at a wavelength of 550 nm.

tion of Cys. (A) Bare Cd2+ ions and (B) Bare Cd2+ ions/additional Cys showing the

J. Kim et al. / Sensing and Bio-Sensing Research 3 (2015) 46–52 49

3. Results and discussion

3.1. Characterization of CdTe@Cys

3.1.1. Structural propertiesFig. 1A depicts the FT-IR spectra of free Cys (a) and CdTe@Cys

QDs (b). The peaks observed for the CdTe@Cys QD sample at anintensity of 1549 cm�1 (asymmetric stretch) and 1304 cm�1

(symmetric stretch) corresponds to the presence of COO� group.Another band located at 2900 cm�1 corresponds to the C–Hstretching vibration (asymmetric, symmetric) of Cys in CdTe@CysQDs. The band located at approximately 3286 cm�1 and3330 cm�1 can be assigned to the N–H stretching. Also, the peakaround 1598 cm�1 can be correlated to the N–H bend since Cyshas an amine group [36,42]. The absence of the peak at2550 cm�1 confirmed the formation of Cys capped-CdTe QDs. Itindicates S–H stretches (symmetric) and thus confirming theformation of covalent bonds between thiols group and Cd2+ ionson the surface of CdTe QDs.

3.1.2. Influence of molar radio and acidity on the optical propertiesThe Emission spectra were recorded for determining the opti-

mized molar ratios of Cd:Te:Cys and the pH value. Fig. S1 depictsthe emission spectra of CdTe@Cys QDs sample which show astrong dependence of FL intensity against the change in Te2+ molarprecursor solution while the concentrations of Cys and Cd2+ werefixed to be 4 and 1, respectively.

The higher degree of QDs luminescence can be correlated to thepresence of excessive cadmium monomers in the initial stage ofnanocrystals growth [43]. The intensity of PL showed a steepincrease with increasing concentration of Te2+ and the further raisein Te concentration leads to a decrease in intensity. When themolar concentration of Te2+ was 0.24, the FL intensity reachedmaxima. The presence of excess Te2+ paved the way for oxidationand in-turn attached itself to the surface of QDs. This resulted inthe appearance of defects on the QDs surface which in-turn dimin-ished the FL intensity of CdTe.

The surfactant plays a pivotal role in the synthesis of CdTe QDs,and henceforth the molar concentration of the surfactant was opti-mized by maintaining the ratio of precursors (Cd:Te) as constant.The maximum FL intensity of CdTe was observed, when the molarratio of Cd:Te:Cys was maintained to be 1:0.24:4, at a fixed Cd:Te(1:0.24), as shown in Fig. 1B. A further increase in Cys concentra-tions to 6 and 8 give rise to the red shift in the emission peak. Thismay be attributed to the presence of excessive Cys molecules,which impede the formation of CdTe QDs and finally affix itself

Fig. 4. Antibacterial activity of additional Cys introduced CdTe@Cys QDs on E. colicells (a–g: 0.09, 0.18, 1.8, 18, 54, 90, 144 ng/mL of additional Cys and h: controlgroup).

to the Cys and/or Cd2+on the CdTe QD surface. A recent report byOmoho’s group portrays the QDs of Te2+-rich surface, where alower quantum yield was observed than that of the Cd2+-rich sur-face. This may be ascribed to the density of uncoordinated Te2+

ions that gives rise to the process of hole-trapping [44].Another important parameter to be considered in the aqueous

media synthesis of quantum dots is the pH value as it controls boththe emission wavelength and PL quantum yield. Three different pHvalues namely pH = 4, 8 and 11 were chosen for the synthesis ofCdTe@Cys QDs and their PL spectra, shape and size were studiedin detail. The pH value of precursor solution of as-prepared QDssamples was controlled by 1 N NaOH solution. This experimentgave a strong evidence for the influence of pH in determinationof the maximum emission wavelength of CdTe@Cys QDs. A com-plete depletion in the PL emission intensity was observed at a pHvalue as lower as 4 or at a higher pH value of 11. But, at the pHvalue of 8, emission intensity was considerably strong (resultsnot shown). Transmission electron micrograph (TEM) reveals thecomplete profile of the particle size and the state of aggregationof Cys capped CdTe QDs, and is shown in Fig. S2. There are translu-cent gray areas for the surfactant of organic Cys on the edge ofparticles. The average particle size of CdTe@Cys QDs was about5 nm with a good particle size distribution.

3.1.3. Fluorescence stability of CdTe@Cys QDsIn order to extend the application of CdTe@Cys QDs to bio-

labeling studies, a systematic investigation of FL intensity at vari-ous times for the CdTe@Cys QDs prepared at various pH [45] wascarried out in a phosphate buffer saline as medium (Fig. 2A).

The intensity of PL was decreased to a formal arc shape for pris-tine CdTe@Cys QDs in the absence of PBS. At a pH value of 6, the FLintensity of PBS that contains CdTe@Cys QDs was almost quench-ing, and a gradual aggregation of nanoparticles was visualized.Few of the researchers have reported, the absence of aggregationat a pH value of 8 [46]. The brightest FL intensity was found withinthis range. The quenching patterns of CdTe@Cys QDs in PBS (pH 7)were in analogue to that of pristine QDs; however a weaker FLintensity was noted. At a pH value of 8, the decrease in FL intensitywas noted after an hours’ duration. But after an interval of 2 h, anenhancement in the intensity was observed when, compared tothat of the original intensity of the unmingled QDs. After 24 h,the FL intensity of CdTe@Cys in PBS at pH 6–7 and pristineCdTe@Cys were almost quenched. However at a pH value of 8, FLintensity of CdTe@Cys in PBS was found to be higher than that ofpristine CdTe@Cys. Therefore, further experiments were precededusing PBS dispersed CdTe@Cys prepared at a pH value of 8.

3.2. Reactivation of fluorescence in CdTe@Cys QDs

3.2.1. Influence of additional Cys on FL intensity of CdTe@Cys QDsThe surfactants play a significant role in protection of the sur-

face and core from oxidation and subsequent degeneration. Fewof the notable surfactants are TGA and mercaptopropionic acid[20]. Recent reports revealed the possible recovery of FL intensityof CdTe QDs to its initial level by the addition of Cys [36]. There-fore, we investigated the efficacy of supplemental Cys onCdTe@Cys by the addition of Cys solutions at different molar ratiosand the FL intensity was recorded. However, we inferred that theaddition of Cys to the CdTe@Cys QDs resulted in a steep increaseof FL intensity when compared to that of the as-synthesizedCdTe@Cys QDs (Fig. 2B and Fig. 3).

Additionally, we observed the change in color for the additionalCys introduced CdTe@Cys QDs sample even with the naked eyeunder visible light. A further increase in the amount of Cys led tothe red shift which is notable by the peak value of the emissionspectra of additional Cys-introduced CdTe@Cys QDs. The FL

Fig.5. Fluorescence confocal micrographs of untreated and more Cys treated E. coli. Images of untreated E. coli (A) fluorescence images excited at 525 nm. (B) Bright fieldimages and (C) Overlaid micrograph of A and B. Images of additional Cys treated E. coli (D) fluorescence images excited at 525 nm; (E) bright field images and (F) overlaidmicrograph of D and F.

50 J. Kim et al. / Sensing and Bio-Sensing Research 3 (2015) 46–52

intensity of additional Cys-introduced CdTe@Cys QDs reachedoptimization after 5 min when the ratio of CdTe@Cys:Cys was fixedto be 1:1.

3.2.2. Photoluminescence stability of additional Cys introducedCdTe@Cys QDs

Systematic photoluminescence stability was studied for theadditional Cys introduced CdTe@Cys samples. Typically, five sam-ples were prepared with various molar ratios of CdTe@Cys QDsto supplemental Cys, namely 1:1, 1:2, 1:3, and 1:4 along withthe pristine sample as reference. The stability of pristine and addi-tional Cys introduced CdTe@Cys QDs were evaluated by analyzingits FL intensity at various time intervals from 5 s to a week at pH 8in PBS medium. It can be clearly observed that FL intensity ofCdTe@Cys QDs rose steeply with increasing concentration of addi-tional Cys. But, in the case of pristine CdTe@Cys QDs sample the FLintensity was substantially quenched after 1 h. Fig S3 depicts the FLintensity of additional Cys introduced CdTe@Cys samples in whichit showed an instant recovery and thus contributes to the increasedintensity after a time period of 2 h. But, it dwindled after a days’time. In the case of samples with the ratios of 1:3 and 1:4, the FLintensity was observed to be stronger even after a week.

3.3. Application

3.3.1. Effects of the additional Cys to CdTe@Cys QDs on antibacterialactivity and in vitro cell viability

The antibacterial effects were evaluated by the addition of Cysand the role of excessive Cys was predicted in the case of Esche-richia coli (E. coli) cells. Fig. 4, depicts the efficiency of theCdTe@Cys QDs towards the execution of bacteria. However, thereis a decrement in the antibacterial activity with an incrementalconcentration of additional Cys (sample a–h in Fig 4). Cytotoxic

level of E. coli was completely eliminated at a Cys concentrationof 18 ng/mL. Hence, the survival of E. coli follows a concentra-tion-dependent inclination towards the added Cys [47]. Therefore,it can be well established that, the Cys addendum is the key factorin the reduction of antibacterial capacity; and the concentration ofadded Cys should reach a particular level for an efficient interfer-ence in the antibacterial efficacy of CdTe@Cys QDs.

Further investigations were carried out with respect to the colorvariation of CdTe@Cys QDs solution to that of the externally intro-duced additional Cys. Primarily, the as-synthesized CdTe@Cys QDssolution was colorless but after the addition of Cys it turned intored. This phenomenon may be ascribed to the formation of anew complex by the introduction of additional Cys to CdTe@CysQDs. A relatively lower oxidative ability was discernible in the caseof Cys reactivated CdTe@Cys QDs and thus cause only a meageramount of damage to the bacteria, when compared to that of thebare CdTe QDs and CdTe@Cys without introduction of additionalCys. Also, in vitro efficacy of extra Cys was examined by cell viabil-ity assay using a human cervical cancer HeLa cell line (Fig. S4).

In this work, we substantiated the enhanced proliferation sta-bility in the living cells as a function of additional Cys concentra-tions. As the concentration of additional Cys increased, anincrease in absorbance was noted. It gives a clear picture of theincreased levels of living cells than at a Cys concentration of0 mg/mL. When the concentration of Cys was 5 mg/mL, anenhanced stability was observed. The results obtained from boththe bio-stability studies namely the antibacterial test and MTTassay are in good agreement with each other in terms of the viabil-ity of the bacteria and living cells. The Cytotoxicity factor related tothe cadmium metal ion has been eliminated. The presence ofexcessive Cys gives rise to the formation of complex via thiolatecoordination between the cysteine residues and cadmium ions.The presence of additional Cys are extremely competent in the

J. Kim et al. / Sensing and Bio-Sensing Research 3 (2015) 46–52 51

process of cadmium ions capture, whereby the ability to block cad-mium toxicity was increased in vivo [48]. All these facets namelythe disappearance of antibacterial effect and the sharp improve-ment in cell viability reveals the reduction in toxicity.

3.3.2. CdTe@Cys QDs labeled E. coli cellsConfocal microscopic images also demonstrated an enhance-

ment in the fluorescence intensity by the addition of Cys to thatof the labeled bacterial cells. Initially, the pretreated E. coli cellswere labeled with both the samples namely the pristine CdTe@CysQDs and additional Cys introduced CdTe@Cys QDs based on an ear-lier reported procedure [34]. Lysozyme plays a significant role inthe membrane fluidity and permeability of the cell membraneand thus promotes the QDs to enter into cells for labeling[35,40]. When E. coli cells were treated with the pristine CdTe@CysQDs, an absence of fluorescence was observed in the bacterial cells.On the other hand, when the E. coli cells were subjected to treat-ment by the additional Cys introduced CdTe@Cys QDs an enhance-ment in the fluorescence was observed and this may be attributedto the introduction of additional Cys molecules (Fig. 5).

4. Conclusions

In summary, we have successfully demonstrated a novelmethod for the reactivation of degenerated FL intensity along withthe reduction in antibacterial activity of CdTe@Cys QDs using addi-tional Cys supplementation. The influence of precursor’s ratio, pHvalue and concentrations of additional Cys were studied systemat-ically and optimized. Along with an enhanced chemical stability astrong FL intensity was recorded at a pH value of 8. It was inferredthat an additional Cys is required for CdTe@Cys QDs samples tobring about the requisite enhancement in FL intensity when com-pared to the pristine CdTe@Cys QDs. Additionally, an impededcytotoxicity of CdTe@Cys QDs was discernible due to the introduc-tion of additional Cys. Addition of the higher concentration of Cysplays a vital role in the enhancement of the bacterial activity ofCdTe@Cys QDs and cell viability. Antibacterial studies, confirmedthat the addition of Cys at a concentration greater than 18 ng/mLhave restricted the antibacterial effect. Cell viability studies alsoconfirm the reduction in cytotoxicity at a concentration greaterthan 5 mg/mL of additional Cys. Confocal microscopic images alsoproved its efficiency. All these results provided significant evi-dences for it to be utilized as an efficient labeling agent for the tar-geted cells.

Conflict of interest

The authors declared that we have no conflicts of interest to thiswork.

Acknowledgments

This work was supported by the Regional University ResearchProgram (NRF 2014-008793) and the Priority Research CentersProgram (NRF 2010-0029634), through the National ResearchFoundation of Korea (NRF) funded by the Ministry of Education,Science and Technology.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.sbsr.2014.12.001.

References

[1] M.G. Bawendi, M.L. Steigerwald, L.E. Brus, The quantum mechanics of largersemiconductor clusters (‘‘quantum dots’’), Annu. Rev. Phys. Chem. 41 (1990)477–496.

[2] S.F. Wuister, I. Swart, F. van Driel, S.G. Hickey, C. de Mello Donegá, Highlyluminescent water-soluble CdTe quantum dots, Nano Lett. 3 (2003) 503–507.

[3] Y. Li, Y. Zhou, H.-Y. Wang, S. Perrett, Y. Zhao, Z. Tang, G. Nie, Chirality ofglutathione surface coating affects the cytotoxicity of quantum dots, Angew.Chem. Int. Ed. 50 (2011) 5860–5864.

[4] M. Geszke-Moritz, M. Moritz, Quantum dots as versatile probes in medicalsciences: synthesis, Mater. Sci. Eng. C 33 (2013) 1008–1021.

[5] V. Bagalkot, L. Zhang, E. Levy-Nissenbaum, S. Jon, P.W. Kantoff, R. Langer, O.C.Farokhzad, Quantum dot-aptamer conjugates for synchronous cancer imaging,therapy, and sensing of drug delivery based on bi-fluorescence resonanceenergy transfer, Nano Lett. 7 (2007) 3065–3070.

[6] M. Zhang, H. Ping, X. Cao, H. Li, F. Guan, C. Sun, J. Liu, Rapid determination ofmelamine in milk using water-soluble CdTe quantum dots as fluorescenceprobes, Food Addit. Contam. Part A: Chem. Anal. Control. 29 (2012) 333–344.

[7] E. Mohamed Ali, Y. Zheng, H.-H. Yu, J.Y. Ying, Ultrasensitive Pb2+ detection byglutathione-capped quantum dots, Anal. Chem. 79 (2007) 9452–9458.

[8] Y. Chen, Z. Chen, Y. He, H. Lin, P. Sheng, C. Liu, S. Luo, Q. Cai, L-Cysteine-cappedCdTe QD-based sensor for simple and selective detection of trinitrotoluene,Nanotechnology 21 (2010) 125502–125506.

[9] E.J. Llorent-Martínez, L. Molina-García, R. Kwiatkowski, A. Ruiz-Medina,Application of quantum dots in clinical and alimentary fields usingmulticommutated flow injection analysis, Talanta 109 (2013) 203–208.

[10] X. Geng, L. Niu, Z. Xing, R. Song, G. Liu, M. Sun, G. Cheng, H. Zhong, Z. Liu, Z.Zhang, L. Sun, H. Xu, L. Lu, L. Liu, Aqueous-processable noncovalent chemicallyconverted graphene-quantum dot composites for flexible and transparentoptoelectronic films, Adv. Mater. 22 (2010) 638–642.

[11] S.M. Ng, D.S.N. Wong, J.H.C. Phung, H.S. Chua, Integrated miniature fluorescentprobe to leverage the sensing potential of ZnO quantum dots for the detectionof copper (II) ions, Talanta 116 (2013) 514–519.

[12] R.S. Chouhan, J.H. Niazi, A. Qureshi, E. coli-quantum dot bioconjugates aswhole-cell fluorescent reporters for probing cellular damage, J. Mater. Chem. B1 (2013) 2724–2730.

[13] A. Bhirde, J. Xie, M. Swierczewska, X. Chen, Nanoparticles for cell labeling,Nanoscale 3 (2011) 142–153.

[14] B. Chang, X. Yang, F. Wang, Y. Wang, R. Yang, N. Zhang, B. Wang, Water solublefluorescence quantum dot probe labeling liver cancer cells, J. Mater. Sci. Mater.Med. 24 (2013) 2505–2508.

[15] I.L. Medintz, H.T. Uyeda, E.R. Goldman, H. Mattoussi, Quantum dotbioconjugates for imaging, labelling and sensing, Nat. Mater. 4 (2005) 435–446.

[16] M.-K. So, C. Xu, A.M. Loening, S.S. Gambhir, J. Rao, Self-illuminating quantumdot conjugates for in vivo imaging, Nat. Biotechnol. 24 (2006) 339–343.

[17] X. Michalet, F. Pinaud, L. Bentolila, J. Tsay, S. Doose, J. Li, G. Sundaresan, A. Wu,S. Gambhir, S. Weiss, Quantum dots for live cells, in vivo imaging, anddiagnostics, Science 307 (2005) 538–544.

[18] Y. Zhu, Z. Li, M. Chen, H.M. Cooper, Z.P. Xu, Tuning core-shell SiO2@CdTe@SiO2fluorescent nanoparticles for cell labeling, J. Mater. Chem. B 1 (2013) 2315–2323.

[19] X. Ai, L. Niu, Y. Li, F. Yang, X. Su, A novel b-Cyclodextrin-QDs optical biosensorfor the determination of amantadine and its application in cell imaging,Talanta 99 (2012) 409–414.

[20] S.A. Owens, M.C. Carpenter, J.W. Sonne, C.A. Miller, J.R. Renehan, C.A. Odonkor,E.M. Henry, D.T. Miles, Reversed-phase HPLC separation of water-soluble,monolayer-protected quantum dots, J. Phys. Chem. C 115 (2011) 18952–18957.

[21] M.A. Walling, J.A. Novak, J.R. Shepard, Quantum dots for live cell and in vivoimaging, Int. J. Mol. Sci. 10 (2009) 441–491.

[22] B.A. Kairdolf, A.M. Smith, T.H. Stokes, M.D. Wang, A.N. Young, S. Nie,Semiconductor quantum dots for bioimaging and biodiagnostic applications,Annu. Rev. Anal. Chem. 6 (2013) 143–162.

[23] X. Wang, Y. Lv, X. Hou, A potential visual fluorescence probe for ultratracearsenic (III) detection by using glutathione-capped CdTe quantum dots,Talanta 84 (2011) 382–386.

[24] G.-L. Wang, H.-J. Jiao, X.-Y. Zhu, Y.-M. Dong, Z.-J. Li, Enhanced fluorescencesensing of melamine based on thioglycolic acid-capped CdS quantum dots,Talanta 93 (2012) 398–403.

[25] Y. Su, M. Hu, C. Fan, Y. He, Q. Li, W. Li, L.-H. Wang, P. Shen, Q. Huang, Thecytotoxicity of CdTe quantum dots and the relative contributions fromreleased cadmium ions and nanoparticle properties, Biomaterials 31 (2010)4829–4834.

[26] A. Hoshino, K. Fujioka, T. Oku, M. Suga, Y.F. Sasaki, T. Ohta, M. Yasuhara, K.Suzuki, K. Yamamoto, Physicochemical properties and cellular toxicity ofnanocrystal quantum dots depend on their surface modification, Nano Lett. 4(2004) 2163–2169.

[27] A. Hoshino, S. Hanada, K. Yamamoto, Toxicity of nanocrystal quantum dots:the relevance of surface modifications, Arch. Toxicol. 85 (2011) 707–720.

[28] S.J. Tan, N.R. Jana, S. Gao, P.K. Patra, J.Y. Ying, Surface-ligand-dependent cellularinteraction, subcellular localization, and cytotoxicity of polymer-coatedquantum dots, Chem. Mater. 22 (2010) 2239–2247.

http://dx.doi.org/10.1016/j.sbsr.2014.12.001http://refhub.elsevier.com/S2214-1804(14)00042-7/h0005http://refhub.elsevier.com/S2214-1804(14)00042-7/h0005http://refhub.elsevier.com/S2214-1804(14)00042-7/h0005http://refhub.elsevier.com/S2214-1804(14)00042-7/h0010http://refhub.elsevier.com/S2214-1804(14)00042-7/h0010http://refhub.elsevier.com/S2214-1804(14)00042-7/h0015http://refhub.elsevier.com/S2214-1804(14)00042-7/h0015http://refhub.elsevier.com/S2214-1804(14)00042-7/h0015http://refhub.elsevier.com/S2214-1804(14)00042-7/h0020http://refhub.elsevier.com/S2214-1804(14)00042-7/h0020http://refhub.elsevier.com/S2214-1804(14)00042-7/h0025http://refhub.elsevier.com/S2214-1804(14)00042-7/h0025http://refhub.elsevier.com/S2214-1804(14)00042-7/h0025http://refhub.elsevier.com/S2214-1804(14)00042-7/h0025http://refhub.elsevier.com/S2214-1804(14)00042-7/h0030http://refhub.elsevier.com/S2214-1804(14)00042-7/h0030http://refhub.elsevier.com/S2214-1804(14)00042-7/h0030http://refhub.elsevier.com/S2214-1804(14)00042-7/h0035http://refhub.elsevier.com/S2214-1804(14)00042-7/h0035http://refhub.elsevier.com/S2214-1804(14)00042-7/h0035http://refhub.elsevier.com/S2214-1804(14)00042-7/h0040http://refhub.elsevier.com/S2214-1804(14)00042-7/h0040http://refhub.elsevier.com/S2214-1804(14)00042-7/h0040http://refhub.elsevier.com/S2214-1804(14)00042-7/h0040http://refhub.elsevier.com/S2214-1804(14)00042-7/h0040http://refhub.elsevier.com/S2214-1804(14)00042-7/h0045http://refhub.elsevier.com/S2214-1804(14)00042-7/h0045http://refhub.elsevier.com/S2214-1804(14)00042-7/h0045http://refhub.elsevier.com/S2214-1804(14)00042-7/h0050http://refhub.elsevier.com/S2214-1804(14)00042-7/h0050http://refhub.elsevier.com/S2214-1804(14)00042-7/h0050http://refhub.elsevier.com/S2214-1804(14)00042-7/h0050http://refhub.elsevier.com/S2214-1804(14)00042-7/h0055http://refhub.elsevier.com/S2214-1804(14)00042-7/h0055http://refhub.elsevier.com/S2214-1804(14)00042-7/h0055http://refhub.elsevier.com/S2214-1804(14)00042-7/h0060http://refhub.elsevier.com/S2214-1804(14)00042-7/h0060http://refhub.elsevier.com/S2214-1804(14)00042-7/h0060http://refhub.elsevier.com/S2214-1804(14)00042-7/h0065http://refhub.elsevier.com/S2214-1804(14)00042-7/h0065http://refhub.elsevier.com/S2214-1804(14)00042-7/h0070http://refhub.elsevier.com/S2214-1804(14)00042-7/h0070http://refhub.elsevier.com/S2214-1804(14)00042-7/h0070http://refhub.elsevier.com/S2214-1804(14)00042-7/h0075http://refhub.elsevier.com/S2214-1804(14)00042-7/h0075http://refhub.elsevier.com/S2214-1804(14)00042-7/h0075http://refhub.elsevier.com/S2214-1804(14)00042-7/h0080http://refhub.elsevier.com/S2214-1804(14)00042-7/h0080http://refhub.elsevier.com/S2214-1804(14)00042-7/h0085http://refhub.elsevier.com/S2214-1804(14)00042-7/h0085http://refhub.elsevier.com/S2214-1804(14)00042-7/h0085http://refhub.elsevier.com/S2214-1804(14)00042-7/h0090http://refhub.elsevier.com/S2214-1804(14)00042-7/h0090http://refhub.elsevier.com/S2214-1804(14)00042-7/h0090http://refhub.elsevier.com/S2214-1804(14)00042-7/h0090http://refhub.elsevier.com/S2214-1804(14)00042-7/h0095http://refhub.elsevier.com/S2214-1804(14)00042-7/h0095http://refhub.elsevier.com/S2214-1804(14)00042-7/h0095http://refhub.elsevier.com/S2214-1804(14)00042-7/h0100http://refhub.elsevier.com/S2214-1804(14)00042-7/h0100http://refhub.elsevier.com/S2214-1804(14)00042-7/h0100http://refhub.elsevier.com/S2214-1804(14)00042-7/h0100http://refhub.elsevier.com/S2214-1804(14)00042-7/h0105http://refhub.elsevier.com/S2214-1804(14)00042-7/h0105http://refhub.elsevier.com/S2214-1804(14)00042-7/h0110http://refhub.elsevier.com/S2214-1804(14)00042-7/h0110http://refhub.elsevier.com/S2214-1804(14)00042-7/h0110http://refhub.elsevier.com/S2214-1804(14)00042-7/h0115http://refhub.elsevier.com/S2214-1804(14)00042-7/h0115http://refhub.elsevier.com/S2214-1804(14)00042-7/h0115http://refhub.elsevier.com/S2214-1804(14)00042-7/h0120http://refhub.elsevier.com/S2214-1804(14)00042-7/h0120http://refhub.elsevier.com/S2214-1804(14)00042-7/h0120http://refhub.elsevier.com/S2214-1804(14)00042-7/h0125http://refhub.elsevier.com/S2214-1804(14)00042-7/h0125http://refhub.elsevier.com/S2214-1804(14)00042-7/h0125http://refhub.elsevier.com/S2214-1804(14)00042-7/h0125http://refhub.elsevier.com/S2214-1804(14)00042-7/h0130http://refhub.elsevier.com/S2214-1804(14)00042-7/h0130http://refhub.elsevier.com/S2214-1804(14)00042-7/h0130http://refhub.elsevier.com/S2214-1804(14)00042-7/h0130http://refhub.elsevier.com/S2214-1804(14)00042-7/h0135http://refhub.elsevier.com/S2214-1804(14)00042-7/h0135http://refhub.elsevier.com/S2214-1804(14)00042-7/h0140http://refhub.elsevier.com/S2214-1804(14)00042-7/h0140http://refhub.elsevier.com/S2214-1804(14)00042-7/h0140

52 J. Kim et al. / Sensing and Bio-Sensing Research 3 (2015) 46–52

[29] B.A. Rzigalinski, J.S. Strobl, Cadmium-containing nanoparticles: perspectiveson pharmacology and toxicology of quantum dots, Toxicol. Appl. Pharmacol.238 (2009) 280–288.

[30] E.Q. Contreras, M. Cho, H. Zhu, H.L. Puppala, G. Escalera, W. Zhong, V.L. Colvin,Toxicity of quantum dots and cadmium salt to Caenorhabditis elegans aftermultigenerational exposure, Environ. Sci. Technol. 47 (2012) 1148–1154.

[31] L. Wang, H. Zheng, Y. Long, M. Gao, J. Hao, J. Du, X. Mao, D. Zhou, Rapiddetermination of the toxicity of quantum dots with luminous bacteria, J.Hazard. Mater. 177 (2010) 1134–1137.

[32] L. Sáez, J. Molina, D.I. Florea, E.M. Planells, M.C. Cabeza, B. Quintero,Characterization of L-cysteine capped CdTe quantum dots and application totest Cu (II) deficiency in biological samples from critically ill patients, Anal.Chim. Acta 785 (2013) 111–118.

[33] Y. Yu, K. Zhang, S. Sun, One-pot aqueous synthesis of near infrared emittingPbS quantum dots, Appl. Surf. Sci. 258 (2012) 7181–7187.

[34] Z. Wei, L. Sun, J. Liu, J.Z. Zhang, H. Yang, Y. Yang, L. Shi, Cysteine modified rare-earth up-converting nanoparticles for in vitro and in vivo bioimaging,Biomaterials 35 (2014) 387–392.

[35] C. Yang, H. Xie, Y. Li, J.-K. Zhang, B.-L. Su, Direct and rapid quantum dotslabelling of Escherichia coli cells, J. Colloid Interface Sci. 393 (2013) 438–444.

[36] M. Li, H. Zhou, H. Zhang, P. Sun, K. Yi, M. Wang, Z. Dong, S. Xu, Preparation andpurification of L-Cysteine capped CdTe quantum dots and its self-recovery ofdegenerate fluorescence, J. Lumin. 130 (2010) 1935–1940.

[37] H.-B. Bu, H. Kikunaga, K. Shimura, K. Takahasi, T. Taniguchi, D. Kim,Hydrothermal synthesis of thiol-capped CdTe nanoparticles and their opticalproperties, Phys. Chem. Chem. Phys. 15 (2013) 2903–2911.

[38] S. Chen, J. Tian, Y. Jiang, Y. Zhao, J. Zhang, S. Zhao, A one-step selectivefluorescence turn-on detection of cysteine and homocysteine based on a facilecdte/cds quantum dots-phenanthroline system, Anal. Chim. Acta 787 (2013)181–188.

[39] P.R. Murray, E.J. Baron, M.A. Pfaller, F.C. Tenover, R.H. Yolke, Manual of ClinicalMicrobiology, 7th ed., ASM, Washington, DC, 1999, pp. 1527–1539.

[40] P. Liu, Q. Wang, X. Li, Studies on CdSe/L-cysteine quantum dots synthesized inaqueous solution for biological labeling, J. Phys. Chem. C 113 (2009) 7670–7676.

[41] S. Lee, K. Saito, H.-R. Lee, M.J. Lee, Y. Shibasaki, Y. Oishi, B.-S. Kim,Hyperbranched double hydrophilic block copolymer micelles of poly(ethylene oxide) and polyglycerol for pH-responsive drug delivery,Biomacromolecules 13 (2012) 1190–1196.

[42] B. The Huy, M.-H. Seo, X. Zhang, Y.-I. Lee, Selective optosensing of clenbuteroland melamine using molecularly imprinted polymer-capped CdTe quantumdots, Biosens. Bioelectron. 57 (2014) 310–316.

[43] L. Li, H. Qian, N. Fang, J. Ren, Significant enhancement of the quantum yield ofCdTe nanocrystals synthesized in aqueous phase by controlling the pH andconcentrations of precursor solutions, J. Lumin. 116 (2006) 59–66.

[44] B. Omogo, J.F. Aldana, C.D. Heyes, Radiative and nonradiative lifetimeengineering of quantum dots in multiple solvents by surface atomstoichiometry and ligands, J. Phys. Chem. C 117 (2013) 2317–2327.

[45] M. Gui, L. Bao, Y. Xia, C. Wei, S. Zhang, C. Zhu, Indication of intracellularphysiological pH changes by L-Cysteine-coated CdTe quantum dots with anacute alteration in emission color, Biosens. Bioelectron. 30 (2011) 324–327.

[46] J. Tian, R. Liu, Y. Zhao, Q. Xu, S. Zhao, Controllable synthesis and cell-imagingstudies on CdTe quantum dots together capped by glutathione and thioglycolicacid, J. Colloid Interface Sci. 336 (2009) 504–509.

[47] Z. Lu, C.M. Li, H. Bao, Y. Qiao, Y. Toh, X. Yang, Mechanism of antimicrobialactivity of CdTe quantum dots, Langmuir 24 (2008) 5445–5452.

[48] F. Jalilehvand, B.O. Leung, V. Mah, Cadmium(II) complex formation withcysteine and penicillamine, Inorg. Chem. 48 (2009) 5758–5771.

http://refhub.elsevier.com/S2214-1804(14)00042-7/h0145http://refhub.elsevier.com/S2214-1804(14)00042-7/h0145http://refhub.elsevier.com/S2214-1804(14)00042-7/h0145http://refhub.elsevier.com/S2214-1804(14)00042-7/h0150http://refhub.elsevier.com/S2214-1804(14)00042-7/h0150http://refhub.elsevier.com/S2214-1804(14)00042-7/h0150http://refhub.elsevier.com/S2214-1804(14)00042-7/h0155http://refhub.elsevier.com/S2214-1804(14)00042-7/h0155http://refhub.elsevier.com/S2214-1804(14)00042-7/h0155http://refhub.elsevier.com/S2214-1804(14)00042-7/h0160http://refhub.elsevier.com/S2214-1804(14)00042-7/h0160http://refhub.elsevier.com/S2214-1804(14)00042-7/h0160http://refhub.elsevier.com/S2214-1804(14)00042-7/h0160http://refhub.elsevier.com/S2214-1804(14)00042-7/h0160http://refhub.elsevier.com/S2214-1804(14)00042-7/h0160http://refhub.elsevier.com/S2214-1804(14)00042-7/h0165http://refhub.elsevier.com/S2214-1804(14)00042-7/h0165http://refhub.elsevier.com/S2214-1804(14)00042-7/h0170http://refhub.elsevier.com/S2214-1804(14)00042-7/h0170http://refhub.elsevier.com/S2214-1804(14)00042-7/h0170http://refhub.elsevier.com/S2214-1804(14)00042-7/h0175http://refhub.elsevier.com/S2214-1804(14)00042-7/h0175http://refhub.elsevier.com/S2214-1804(14)00042-7/h0180http://refhub.elsevier.com/S2214-1804(14)00042-7/h0180http://refhub.elsevier.com/S2214-1804(14)00042-7/h0180http://refhub.elsevier.com/S2214-1804(14)00042-7/h0180http://refhub.elsevier.com/S2214-1804(14)00042-7/h0180http://refhub.elsevier.com/S2214-1804(14)00042-7/h0185http://refhub.elsevier.com/S2214-1804(14)00042-7/h0185http://refhub.elsevier.com/S2214-1804(14)00042-7/h0185http://refhub.elsevier.com/S2214-1804(14)00042-7/h0190http://refhub.elsevier.com/S2214-1804(14)00042-7/h0190http://refhub.elsevier.com/S2214-1804(14)00042-7/h0190http://refhub.elsevier.com/S2214-1804(14)00042-7/h0190http://refhub.elsevier.com/S2214-1804(14)00042-7/h0195http://refhub.elsevier.com/S2214-1804(14)00042-7/h0195http://refhub.elsevier.com/S2214-1804(14)00042-7/h0195http://refhub.elsevier.com/S2214-1804(14)00042-7/h0200http://refhub.elsevier.com/S2214-1804(14)00042-7/h0200http://refhub.elsevier.com/S2214-1804(14)00042-7/h0200http://refhub.elsevier.com/S2214-1804(14)00042-7/h0200http://refhub.elsevier.com/S2214-1804(14)00042-7/h0200http://refhub.elsevier.com/S2214-1804(14)00042-7/h0205http://refhub.elsevier.com/S2214-1804(14)00042-7/h0205http://refhub.elsevier.com/S2214-1804(14)00042-7/h0205http://refhub.elsevier.com/S2214-1804(14)00042-7/h0205http://refhub.elsevier.com/S2214-1804(14)00042-7/h0210http://refhub.elsevier.com/S2214-1804(14)00042-7/h0210http://refhub.elsevier.com/S2214-1804(14)00042-7/h0210http://refhub.elsevier.com/S2214-1804(14)00042-7/h0215http://refhub.elsevier.com/S2214-1804(14)00042-7/h0215http://refhub.elsevier.com/S2214-1804(14)00042-7/h0215http://refhub.elsevier.com/S2214-1804(14)00042-7/h0220http://refhub.elsevier.com/S2214-1804(14)00042-7/h0220http://refhub.elsevier.com/S2214-1804(14)00042-7/h0220http://refhub.elsevier.com/S2214-1804(14)00042-7/h0225http://refhub.elsevier.com/S2214-1804(14)00042-7/h0225http://refhub.elsevier.com/S2214-1804(14)00042-7/h0225http://refhub.elsevier.com/S2214-1804(14)00042-7/h0225http://refhub.elsevier.com/S2214-1804(14)00042-7/h0225http://refhub.elsevier.com/S2214-1804(14)00042-7/h0230http://refhub.elsevier.com/S2214-1804(14)00042-7/h0230http://refhub.elsevier.com/S2214-1804(14)00042-7/h0230http://refhub.elsevier.com/S2214-1804(14)00042-7/h0235http://refhub.elsevier.com/S2214-1804(14)00042-7/h0235http://refhub.elsevier.com/S2214-1804(14)00042-7/h0240http://refhub.elsevier.com/S2214-1804(14)00042-7/h0240

Highly fluorescent CdTe quantum dots with reduced cytotoxicity-A Robust biomarker1 Introduction2 Experimental2.1 Reagents and materials2.2 Preparation of CdTe@Cys QDs2.3 Reactivation of CdTe@Cys QDs2.4 Characterization techniques2.5 Evaluation of antibacterial activity2.6 Bacterial treatment and QDs labeling2.7 In vitro cell viability

3 Results and discussion3.1 Characterization of CdTe@Cys3.1.1 Structural properties3.1.2 Influence of molar radio and acidity on the optical properties3.1.3 Fluorescence stability of CdTe@Cys QDs

3.2 Reactivation of fluorescence in CdTe@Cys QDs3.2.1 Influence of additional Cys on FL intensity of CdTe@Cys QDs3.2.2 Photoluminescence stability of additional Cys introduced CdTe@Cys QDs

3.3 Application3.3.1 Effects of the additional Cys to CdTe@Cys QDs on antibacterial activity and in vitro cell viability3.3.2 CdTe@Cys QDs labeled E. coli cells

4 ConclusionsConflict of interestAcknowledgmentsAppendix A Supplementary dataReferences