Journal ofMaterials Chemistry C

PAPER

Publ

ishe

d on

04

June

201

4. D

ownl

oade

d by

Uni

vers

itat A

uton

oma

de B

arce

lona

on

28/1

0/20

14 1

2:43

:35.

View Article OnlineView Journal | View Issue

A single source-p

aDepartment of Chemistry, Indian Institut

721302, India. E-mail: mintu@chem.iitkgp.bDept. of Chemistry & Environment, Heritage

India. E-mail: pcam2008@gmail.com

† Electronic supplementary informa10.1039/c4tc00887a

Cite this: J. Mater. Chem. C, 2014, 2,7373

Received 30th April 2014Accepted 4th June 2014

DOI: 10.1039/c4tc00887a

www.rsc.org/MaterialsC

This journal is © The Royal Society of C

recursor route for the one-potsynthesis of highly luminescent CdS quantum dotsas ultra-sensitive and selective photoluminescencesensor for Co2+ and Ni2+ ions†

Niharendu Mahapatra,a Sudipta Panja,a Abhijit Mandal*b and Mintu Halder*a

In this study, we have demonstrated a facile, simple one-pot and low cost method for the synthesis of

3-mercaptopropionic acid (MPA)-capped, water-soluble CdS quantum dots (QDs) with highly tunable

optical properties. Initially Cd2+ coordinates with MPA at about pH 5, and the CdS QDs were then

formed at a higher pH (7–12) under refluxing conditions through the disruption of coordination

interaction with the release of sulfur. Here MPA played a dual role, as both, a source of sulfur and as a

stabilizer. The particle size and the optical properties of the as-prepared CdS QDs were found to be

dependent on the refluxing time for a given concentration ratio of the reactants and pH of the initial

mixture. The broadness and large Stokes shift of emission of MPA–CdS QDs are due to the surface-trap

state photoluminescence (PL). The PL peak around 510 nm–650 nm is due to the recombination of

shallow trapped electrons in sulfur vacancy defect states with holes in the valence band, and a �665 nm

peak (shoulder) arises from deep-trap states. The origin of the longer lifetime is presumed to be due to

the involvement of surface-trap states and their environment. Use of MPA as a capping agent eventually

enhances the water solubility as well as the stability of CdS QDs, which makes them useful for the ultra-

sensitive detection of Co2+ and Ni2+. The selective coordination interaction of Co2+ and Ni2+ with MPA–

CdS QDs through the carboxyl group of MPA provides a turn-off photoluminescence-based assay for

sensitive detection of these metal ions without any interference of other commonly coexisting metal

ions. The limit of detection (LOD) is 10 nM for Co2+ ions and 50 nM for Ni2+ ions. Co2+-induced color

(from colorless to yellow) and UV-vis spectral change of MPA–CdS QDs is the simple way to distinguish

Co2+ from Ni2+ in a higher concentration range (more than 5 mM). On the other hand the lower stability

of the Co(II)–MPA complex than the Ni(II)–MPA complex provides a disodium salt of

ethylenediaminetetraacetic acid (EDTA)-induced, time dependent turn-on photoluminescence-based

technique to distinguish Co2+ from Ni2+ in the entire range of concentrations. EDTA-induced time

dependent PL recovery of MPA–CdS QDs occurs via rapid dissociation of Co2+ ions from the surface of

QDs than that of Ni2+. Thus our synthesized MPA–CdS QDs offer a very simple, rapid, cost-effective,

turn-off–on photoluminescence-based technique for ultra-sensitive and selective detection of either

Co2+ or Ni2+ in aqueous solution without interference of other common metal ions.

1. Introduction

Colloidal semiconductor nanocrystals, known as quantum dots(QDs), have attracted utmost attention due to their outstandingoptical properties, such as the quantum effect, luminosity1–4

and fascinating applications in various elds such as elec-tronics, optoelectronics, sensing, photocatalysis and solar

e of Technology Kharagpur, Kharagpur

ernet.in

Institute of Technology, Kolkata 700107,

tion (ESI) available. See DOI:

hemistry 2014

cells.5–13 To date, various synthetic routes have been employedfor CdS QDs, such as hot injection techniques, single-sourceprecursor thermolysis, microwave-assisted methods, sol-vothermal or hydrothermal methods and the microemulsionmethod.14–20 Although all of these methods are very useful forthe synthesis of CdS QDs, most of them involve complicatedprocedures, use of expensive materials, stability related issuesand poor water solubility of the prepared QDs because of theuse of hydrophobic organic capping agents.

One of the major challenges of such synthetic approaches isto obtain water soluble QDs with a high luminescence effi-ciency. The aqueous synthetic routes have advantages overorganometallic-based synthesis because of the cost, safety, and

J. Mater. Chem. C, 2014, 2, 7373–7384 | 7373

Journal of Materials Chemistry C Paper

Publ

ishe

d on

04

June

201

4. D

ownl

oade

d by

Uni

vers

itat A

uton

oma

de B

arce

lona

on

28/1

0/20

14 1

2:43

:35.

View Article Online

ease of application in biological elds.21 Arrested precipitationin water in the presence of stabilizers (e.g., thiols) is a faster andsimpler method to synthesize water-soluble QDs and has beenapplied to several semiconductors potentially relevant to bio-logical applications (e.g., CdS, CdSe, and CdTe). Preparation ofwater-soluble CdS QDs based on either ligand exchange orhydrophilic shell growth on the presynthesized hydrophobicQDs is somewhat technically complicated requiring multipleexperimental procedures. Moreover, poor water solubility of thesynthesized CdS QDs invalidates their implementation in bio-logical studies, sensing and drug delivery.21 Again directsynthesis of water-soluble CdS QDs via arrested precipitationwith phosphates, thiols, or hydrophilic polymers as the cappingagents in aqueous medium requires additional sulfur sour-ces.14,22–27 Consequently, a simple, facile, one-pot and low-costsynthetic method for water-soluble CdS QDs with adequatehydrophilic surface capping agents to promote effectively thebiological and sensing applications of CdS QDs in aqueoussolution still remains challenging.

Photoluminescence (PL) of semiconductor nanocrystalsincludes band-edge emission and trap-state emission. Thelatter is divided into shallow-trap and deep-trap emissionaccording to the difference of electronic levels.28 The trap-statePL originates from the defects containing vacancies and inter-stitials of semiconductors. These defects can act as eithershallow or deep level states for trapping photogenerated elec-trons in the conduction band. Shallow-trap states are morespatially delocalized and lie within a few milli-electronvolts ofthe corresponding band edge, and are more likely to participatein the radiative recombination.29 On the other hand, deep-trapstates are essentially localized in space at a lattice site defectand lie in the middle of the band gap, having a tendency toundergo non-radiative recombination by emitting phonons.30

As the crystallite size decreases, shallow traps should respond tothe quantum connement process more effectively than deeptrap states and shi to higher energy due to the localization ofthe deep-trap state electronic wave-function.28,31 The emissionoccurs when a trapped electron recombines with holes in thevalence band or holes in the trap state near the valence band.30

Nowadays, functionalized QDs have generated incredibleinterest in the sensing applications owing to their high photo-luminescence (PL) quantum yield, broad absorbance spectra,and narrow size-tunable emission from visible to infraredwavelengths and high photochemical stability.32 CdS QD-basedsensing of several ions33–36 and biomolecules37–39 has beenreported to date. Among these ions, selective detection of cobalt(Co2+) and nickel (Ni2+) ions is important, as these heavy metalsrapidly diffuse in both the environment and living systems. Forexample, naturally occurring cobalt in rocks, soils, water,animals, and plants may be taken up by humans from the dietand occupational exposure in several industries such as hardmetal, diamond polishing, porcelain, chemical, and pharma-ceutical industries,40 which can cause toxic effects, includingvasodilatation, ushing, and cardiomyopathy in humans andanimals.41,42 Detection of trace amounts of Ni2+ is important forpatients suffering from acute dermatitis and skin allergies oencaused by nickel.43 Ni2+ readily forms complexes with

7374 | J. Mater. Chem. C, 2014, 2, 7373–7384

biomolecules such as amino acids, peptides, proteins, phos-phates and nucleic acids.44 Excess Ni2+ induces delayed-typehypersensitivity in the cellular response when bound toproteins. Thus, the development of a highly sensitive andselective analytical methodology for trace detection of thesemetal ions in their sources is of great importance to avoid theanticipated toxic effects.

So far different methods have been employed to detect thesemetal ions based on different analytical strategies, such asspectrouorimetry,45,46 atomic absorption spectrometry (AAS),47

inductively coupled plasma-optical emission spectrometry (ICP-OES)42 and voltammetry.48,49 However, these techniques ingeneral require expensive equipment, and sample pretreat-ment, and also have serious inuence by the interference ofcoexisting ions. Therefore, a simple, inexpensive, and sensitivemethod that permits detection of metal ions has long been afocus of research.

Herein, we have demonstrated a novel, simple, facile, cost-effective, one-pot method for the synthesis of water soluble 3-mercaptopropionic acid (MPA) capped CdS QDs. Our syntheticmethod is advantageous in the following aspects: (1) simple,less expensive, one-pot synthesis of CdS QDs, without the needfor any additional sulfur source. Here MPA acts simultaneouslyas the sulfur source and as the capping agent. (2) Unlike theconventional organometallic [Cd(CH3)2] precursor-based hotinjection synthetic methods, nontoxic and less expensivecadmium chloride has been used here. Cd(CH3)2 is highly toxic,pyrophoric, expensive, unstable at room temperature, andexplosive at elevated temperatures.50 (3) Unlike the existingmethods for CdS QD synthesis, additional reagents, nitrogenenvironment or organic solvents are not required, and thismakes our synthetic strategy more experimentally simple. (4)Due to the presence of MPA as the capping agent, our synthe-sized CdS QDs possess good water solubility, remarkablephotochemical stability as well as longer colloidal stabilitywhich are highly effective for biological and sensing applica-tions in aqueous medium. The synthesized MPA–CdS QDs showlarge Stokes shied trap state photoluminescence (PL) withsufficiently high PL intensity, which makes them suitable forapplications such as PL probes, biomedical labeling, optoelec-tronic devices, solar energy conversion, etc. The broad yellowemission prole of all samples centered at around 510–650 nmis due to the recombination of shallow trapped electrons insulfur vacancy defect energy states with holes in the valenceband (or shallow hole trap states near the valence band). Thesecond PL shoulder at�665 nm, appearing at a relatively higherreux time, arises from the recombination of deep trappedelectrons with holes in the valence band.

In this work, we have also demonstrated a new photo-luminescence (PL) turn-off–on-based strategy for the efficientdetection and estimation of Co2+ and Ni2+ ions in aqueoussolution using MPA-capped CdS (MPA–CdS) QDs. Binding ofmetals with MPA through the carboxyl group provides aneffective PL quenching of MPA–CdS QDs. This turn-off PL ofMPA–CdS QDs has been found to occur only with Co2+ and Ni2+

and it makes their detection easy in the presence of othercommon metals e.g., Zn2+, Cr2+, Al3+, Cd2+, Ag+, Pb2+, Ca2+, Na+,

This journal is © The Royal Society of Chemistry 2014

Paper Journal of Materials Chemistry C

Publ

ishe

d on

04

June

201

4. D

ownl

oade

d by

Uni

vers

itat A

uton

oma

de B

arce

lona

on

28/1

0/20

14 1

2:43

:35.

View Article Online

K+, Hg2+, Fe2+, Fe3+, La3+, and Cu2+. These two ions are laterdistinguished by the extent of PL recovery aer a given timeinterval due to EDTA addition. The extent of EDTA-induced PLrecovery of the metal-quenched PL of MPA–CdS QDs stems fromthe difference in the stability of Co(II)–MPA and Ni(II)–MPAcomplexes. Thus we develop a MPA–CdS-based photo-luminescent sensor for ultrasensitive and selective detection ofCo2+ and Ni2+ ions.

2. Experimental section2.1. Chemicals

All the reagents were of analytical reagent grade and used asreceived without further purication. Cadmium chloride(CdCl2) and 3-mercaptopropionic acid (MPA) were purchasedfrom Sigma Aldrich. Sodium hydroxide, EDTA and all the metalsalts used for sensing studies were purchased from Merck. Fordifferent experiments the aqueous solutions were preparedusing deionized triply distilled water only. All glassware wascleaned thoroughly using piranha solution, rinsed with distilledwater and acetone, and then dried in an oven.

2.2. Equipment

Reuxing for the synthesis of CdS QD was performed in a 100ml double-neck round bottomed borosilicate-glass ask. ThepH measurement was carried out using a Eutech-510 ion pH-meter, which was pre-calibrated with standard pH buffertablets. Electronic absorption spectra were recorded using a UV-2450 (Shimadzu) absorption spectrophotometer against thesolvent reference. The steady state PL spectra were recorded ona Jobin Yvon-Spex Fluorolog-3 spectrouorimeter. The time-resolved photoluminescence lifetime decays were measured bymeans of a single photon counting apparatus (Horiba, JobinYvon, IBH Ltd, Glasgow, Scotland) equipped with an excitationsource (lexc) of 377 nm. All the decays were collected at thecorresponding emission maxima with the magic angle of 54.7�

using a Hamamatsu microchannel plate photomultiplier tube(R3809U). Data were analyzed as a sum of the exponentialcomponents with pre-exponential factors (ai) normalized tounity, using iterative reconvolution with IBH DAS-6 soware.The perfectness of t is judged in terms of the c2 value andweighted residuals. Field emission scanning electron micros-copy (FESEM) measurements were performed using a FEI NOVANANOSEM 450 with an Everhart Thornley Detector (ETD) at anoperating voltage of 5 kV. Transmission electron microscopy(TEM) images were taken using JEOL Model JEM-2010 micro-scopes at an operating voltage of 200 kV. For compositionalanalysis, energy-dispersive X-ray spectroscopy (EDX) measure-ments were performed. The X-ray diffraction (XRD) pattern ofthe sample was collected using a Bruker D8 diffractometer unitwith a nickel-ltered Cu Ka line (l ¼ 1.54 A) in the 2q range of20�–80� at a scanning rate of 3.0� min�1. The XRD data wereanalyzed using JCPDS soware. The particle size distributionwas measured by dynamic light scattering (DLS) using a Mal-vern Nano ZS instrument employing a 4 mW He–Ne laser (l ¼632.8 nm). Fourier transform infrared spectroscopic (FTIR)

This journal is © The Royal Society of Chemistry 2014

measurements were performed using a Perkin-Elmer FTIRspectrophotometer RX1.

2.3. Synthesis of MPA-capped CdS QDs

Appropriate amounts of CdCl2 and MPA were dissolved inwater, so that the concentration ratio of [cadmium chlor-ide] : MPA became 1 : 2.4, where [cadmium chloride] ¼2.35 mM. The pH of the above mixture was measured to be�2.4, and then 1 M NaOH was added dropwise to the reactionmixture to attain the desired pH. During the addition of NaOH awhite precipitate was obtained at about pH 5 which dissolvedwith further addition of alkali. The white precipitate was char-acterized by FTIR and EDX spectra. For synthesis of CdS-QDs,different pH values (7–12) of the initial reaction mixture wereused and reuxing was performed for each pH aer overnight(12 h) standing. UV-vis and photoluminescence spectra wererecorded for aliquots collected at different time intervals duringreuxing. As the PL intensity of the synthesized MPA–CdS QDswas found to be maximum at pH 9.0, the concentration varia-tion of the precursor was carried out at this pH only. Fourdifferent concentration ratios of CdCl2 andMPA were attemptedand the optimized condition for synthesis of MPA–CdS QDswith maximum PL intensity was [CdCl2] : [MPA]¼ 1 : 2.4 (where[CdCl2] ¼ 2.35 mM) at pH 9.0. The synthesized MPA–CdSQDs were characterized by conventional UV-vis spectra andphotoluminescence studies (PL), TEM measurements andXRD studies.

2.4. Selective detection of Co2+ and Ni2+

Turn-off PL-based detection of Co2+ and Ni2+ in aqueous solu-tion at room temperature was performed by using the synthe-sized MPA–CdS QDs at pH 9.0 aer 10 h reux which shows themaximum PL intensity. The desired concentrations of theaqueous solution for all metal ions were prepared by dissolvingtheir respective salts in deionized triply distilled water andstored at room temperature. The required amount of a partic-ular metal ion solution was mixed together with MPA–CdS QDsolution to get the desired concentration of the metal ion. Theresulting solution was used to monitor the change of PLintensity. To distinguish between these two ions, the PLrecovery kinetics of MPA–CdS QDs was performed for 30minutes aer addition of EDTA to Co2+/Ni2+ containing QDsolution. The molar ratio of [Co2+/Ni2+] : [EDTA] was 1 : 1.2 forany concentration of metal ions.

3. Results and discussion3.1. Characterization of synthesized MPA–CdS QDs

Fig. 1A shows the FTIR spectra of MPA (black line) and theprecipitate formed during addition of NaOH to the CdCl2 andMPA reactionmixture at pHz 5.0 (blue line). For MPA, peaks at2667, 2573, 1711 and 1411 cm�1 are assigned to the O–H (nO–H),S–H (nS–H), C]O (nC]O) and C–O (nC–O) stretching bands,respectively.51 In the FTIR spectra (red line) of the sodium salt ofMPA (Na–MPA) the characteristic peak of the O–H stretchingdisappears, the peak of nC]O shis from 1711 to 1547 cm�1

J. Mater. Chem. C, 2014, 2, 7373–7384 | 7375

Fig. 1 (A) FTIR spectra of MPA (black), Na–MPA (red) and Cd–MPA(blue). (B) EDX analysis of Cd–MPA. Cd–MPA is formed by the additionof NaOH to the CdCl2 and MPA mixture at pH 5.

Scheme 1 Schematic representation of the formation of MPA cappedCdS QDs through the coordination interaction between Cd2+ andMPA.

Fig. 2 Absorbance (a) and photoluminescence (b) spectra of MPA–CdS QDs formed by using a concentration ratio of [CdCl2] : [MPA] ¼1 : 2.4 (where [CdCl2] ¼ 2.35 mM) at pH 9.0 after 10 h of reflux time.Excitation wavelength for the PL spectra is 393 nm.

Journal of Materials Chemistry C Paper

Publ

ishe

d on

04

June

201

4. D

ownl

oade

d by

Uni

vers

itat A

uton

oma

de B

arce

lona

on

28/1

0/20

14 1

2:43

:35.

View Article Online

(�164 cm�1), the peak of nS–H and nC–O remains almost thesame. The disappearance of the O–H peak is due to deproto-nation of the carboxyl group of MPA (pKa of the carboxyl groupis 4.352). The shi of the nC]O peak is due to partial single bondcharacteristics of the C]O bond aer deprotonation of thecarboxyl group. Again in FTIR spectra of the white precipitate,formed aer addition of NaOH to the CdCl2 and MPA mixture,the characteristic peak of the O–H and S–H stretching disap-pears, the peak of nC]O shis from 1711 to 1547 cm�1

(�164 cm�1) and the peak of nC–O remains almost unchanged.The disappearance of the O–H peak and shiing of the C]Opeak occur due to the same reasons as in the case of Na–MPA.On the other hand the disappearance of the S–H peak is not dueto the deprotonation of the sulydryl group of MPA (as the pKa

of the thiol moiety is quite high, 10.3,53 and the working pH is 5)which suggests that there must be a coordination interactionbetween Cd2+ andMPA, in which Cd2+ is bound to the sulydrylgroup and the hydroxyl oxygen of carboxyl (as reported else-where54,55), as shown in Scheme 1. The energy-dispersive X-ray(EDX) spectrum of the white precipitate (Fig. 1B) further

7376 | J. Mater. Chem. C, 2014, 2, 7373–7384

conrms the presence of C, O, S, and Cd in the Cd–MPA coor-dination complex, in which C, O, and S elements are attributedto the presence of MPA-salt. The Au peak arises from the goldcoating. Under the experimental conditions, Cd(OH)2 is notformed56 because of the stronger coordination interactionbetween Cd2+ and MPA than that of Cd2+ and OH�.

As mentioned in the Experimental section, the whiteprecipitate dissolves by the addition of excess NaOH, when thepH of the reaction mixture becomesz7.0, and so we performedthe synthesis of our MPA-capped CdS QDs by varying the pH ofthe initial reaction mixture from 7.0 to 12.0. At high pH andreuxing conditions the coordination interaction between Cd2+

and MPA breaks down with the release of sulfur (as reported inthe case of thioglycolic acid, glutathione and thio Schiff-base18,57,58), which acts as a sulfur precursor during formation ofCdS QDs. The absorbance and photoluminescence (PL) spectraof the synthesized MPA–CdS QDs at different pH and withincrease in reux time are shown in the ESI S1 (Fig. S1A–F†).

In order to study the effect of the [Cd] : [MPA] ratio on PLefficiency, several batches of MPA capped CdS QDs are synthe-sized at different [Cd] : [MPA] ratios at pH 9.0. The UV-vis andPL spectra at different reuxing times are shown in the ESI S2(Fig. S2A–D).

The optimized condition for the synthesis of MPA–CdS QDswith maximum PL intensity is found to be [CdCl2] : [MPA] ¼1 : 2.4 (where [CdCl2] ¼ 2.35 mM) at pH 9.0. The synthesizedMPA–CdS QDs with maximum PL intensity have been used forfurther studies. Fig. 2 represents the UV-vis and PL spectra ofthe synthesized MPA–CdS QDs with the absorption peak at

This journal is © The Royal Society of Chemistry 2014

Paper Journal of Materials Chemistry C

Publ

ishe

d on

04

June

201

4. D

ownl

oade

d by

Uni

vers

itat A

uton

oma

de B

arce

lona

on

28/1

0/20

14 1

2:43

:35.

View Article Online

393 nm and the emission peak at 594 nm (excitation at 393 nm).The TEM image (Fig. 3B) of MPA–CdS QDs shows mostlyspherical particles with an average size of 3.77 � 1.17 nm, asevident from the statistical histogram of the size of CdS QDs(Fig. 3B, inset). The high-resolution TEM (HRTEM) image of theCdS QDs (Fig. 4A) shows the lattice fringes with an interplanarspacing of 3.38 A, which corresponds to the (111) plane. Theselected-area electron diffraction (SAED) pattern (Fig. 4B, inset)also indicates the presence of (111), (220) and (311) planes ofthe cubic zinc blende structure of CdS. The XRD pattern(Fig. 4B) further conrms the existence of such planes and is ingood agreement with the standard values (JCPDS le no. 10-454). The relatively broad diffraction peaks are attributed to thesmall sizes of our synthesized CdS nanocrystals,58 which isconsistent with the size distribution histogram shown in theinset of Fig. 3B. The EDX spectrum (Fig. 4C) further conrmsthe formation of MPA–CdS QDs with the atomic ratio of Cd to Sbeing 1.00 : 2.46. All of these analyses demonstrate thatreuxing of alkaline Cd2+ and MPA solution can be exploited for

Fig. 3 TEM images of MPA–CdSQDs formed by using a concentrationratio of [CdCl2] : [MPA] ¼ 1 : 2.4 (where [CdCl2] ¼ 2.35 mM) at pH 9.0after (A) 1 h, (B) 10 h and (C) 20 h of reflux time. The insets show theparticle size distribution and magnified high resolution TEM images ofMPA–CdS QDs under corresponding conditions.

Fig. 4 (A) HRTEM image, (B) XRD pattern and (C) EDX spectrum ofMPA–CdS QDs formed by using a concentration ratio of[CdCl2] : [MPA]¼ 1 : 2.4 (where [CdCl2]¼ 2.35mM) at pH 9.0 after 10 hrefluxing. The inset of (B) shows the corresponding SAED pattern ofMPA–CdS QDs.

This journal is © The Royal Society of Chemistry 2014

the one-pot facile synthesis of CdS QDs, which is more simpleand cost-effective than the traditional synthetic approaches(Table S1 of the ESI S3†). The synthesized CdS QDs could beuseful for the enhancement of solar cell efficiency, which mayreduce the cost of renewable energy resources.

3.2. Tunable optical properties of MPA–CdS QDs

The optical properties of our synthesized MPA–CdS QDsstrongly depend on pH, concentration ratios of the reactantsand also on reuxing time (see ESI S1 and S2†). Thermolysis ofeach sample was performed; absorption and PL spectra wererecorded. It should be noted that the initial reaction mixture ofCdCl2 and MPA (i.e. at 0 min reux) has no absorbance and alsoPL, for any starting pH and concentration ratio of reactants, i.e.MPA–CdS-QD formation takes place only aer reuxing,because of better crystallinity and surface passivation of QDs.For all the samples the absorption peak position shiedtowards longer wavelengths with increase in the reuxing time,indicating the increase of the size of the QDs. The shi in theabsorbance peak position from 30 min to 10 h reuxing for aparticular concentration ratio of reactants ([CdCl2] : [MPA] ¼1 : 2.4) at different pH is as follows: 350 nm/ 398 nm (Fig. S1-A;† pH 7), 357 nm / 399 nm (Fig. S1-B;† pH 8), 349 nm / 392nm (Fig. S1-C;† pH 9), 340 nm / 391 nm (Fig. S1-D;† pH 10),345 nm / 404 nm (Fig. S1-E;† pH 11), and 357 nm / 441 nm(Fig. S1-F;† pH 12). Again, at pH 9 for different concentrationratios of reactants the shi in the absorption peak positionfrom 30 min to 30 h reuxing is as follows: 353 nm / 454 nm(Fig. S2-A;† [CdCl2] : [MPA] ¼ 1 : 1.5), 349 nm / 442 nm(Fig. S2-B;† [CdCl2] : [MPA] ¼ 1 : 2.4), 358 nm / 409 nm(Fig. S2-C;† [CdCl2] : [MPA] ¼ 1 : 4.0), and 370 nm / 403 nm

J. Mater. Chem. C, 2014, 2, 7373–7384 | 7377

Scheme 2 Energy-level diagram (not in absolute scale) explaining theprocess of (a) shallow and (b) deep trap-state photoluminescencefrom MPA–CdS QDs.

Fig. 5 (A) Absorbance (dotted line) and (B) photoluminescence (solidline) spectra of MPA–CdS QDs formed by using a concentration ratioof [CdCl2] : [MPA] ¼ 1 : 2.4 (where [CdCl2] ¼ 2.35 mM) at pH 9.0 atdifferent reflux times. The spectra are recorded at room temperature.The insets are photographs of an aqueous solution of MPA cappedCdSQDs taken under daylight (left side) and UV light (right side) at differentreflux times.

Journal of Materials Chemistry C Paper

Publ

ishe

d on

04

June

201

4. D

ownl

oade

d by

Uni

vers

itat A

uton

oma

de B

arce

lona

on

28/1

0/20

14 1

2:43

:35.

View Article Online

(Fig. S2-D;† [CdCl2] : [MPA] ¼ 1 : 8.0). Despite the extensive redshi observed during thermolysis, the absorbance peak posi-tion still remains blue shied compared to the absorbance peakposition of bulk CdS (515 nm). This blue shi is ascribed to bedue to the quantum connement effect i.e. restriction of theelectron and hole in a small volume. This phenomenon ariseswhen the particle size becomes comparable to the de Brogliewavelength of a charge carrier. The de Broglie wavelength of anelectron in a crystal lattice is longer than that of an electron infree space and thence the effects can be seen in particles of a fewnanometers diameter.59 Again, for all samples we observed alarge Stoke shied PL spectrum of MPA–CdS QDs, whichbecomes red shied with increasing reuxing time. The shi inthe PL peak position from 30 min to 10 h reuxing for aparticular concentration ratio of reactants ([CdCl2] : [MPA] ¼1 : 2.4) at different pH is as follows: 536 nm/ 597 nm (Fig. S1-A;† pH 7), 537 nm / 598 nm (Fig. S1-B;† pH 8), 528 nm /

594 nm (Fig. S1-C;† pH 9), 513 nm / 597 nm (Fig. S1-D;† pH10), 523 nm / 608 nm (Fig. S1-E;† pH 11), and 540 nm /

644 nm (Fig. S1-F;† pH 12). Again, at pH 9 for differentconcentration ratios of reactants the shi in the PL peak posi-tion from 30 min to 30 h reuxing is as follows: 543 nm /

644 nm (Fig. S2-A;† [CdCl2] : [MPA]¼ 1 : 1.5), 528 nm/ 642 nm(Fig. S2-B;† [CdCl2] : [MPA] ¼ 1 : 2.4), 546 nm / 611 nm(Fig. S2–C;† [CdCl2] : [MPA] ¼ 1 : 4.0), and 613 nm / 645 nm(Fig. S2-D;† [CdCl2] : [MPA] ¼ 1 : 8.0). The emission maximumof the aqueous MPA–CdS QDs is about 1 eV lower than theabsorption edge, indicating that the photoluminescence of theQDs is not band-edge emission. The relatively broad and Stokes-shied emission indicates that trap-states (lattice defects) areinvolved in the photoluminescence.

Sulfur vacancies are formed more easily in the CdS mate-rials58 because of the smaller radius of S2� than that of Cd2+.These sulfur vacancies can act as both, a shallow or deep trapfor photogenerated electrons in the conduction band, resultingin the yellow59,60 or red luminescence,60,61 respectively, of CdSthrough recombination with holes in the valence band (orshallow hole trap states near the valence band). The density ofthe trap states is relative to the content of surface Cd2+ danglingbonds in CdS QDs. The higher content of surface Cd2+ danglingbonds results in more trapped electrons and trapped holes, andhence more effective surface trap state photoluminescence62,63

occurs. Scheme 2 provides a detailed understanding of thesetwo types of trap state photoluminescence. Deep traps areessentially localized in space at a lattice site defect and lie in themiddle of the band gap, whereas shallow traps lie within a fewmilli-electronvolts of the corresponding band edge. As thecrystallite size decreases, shallow traps should respond to thesmall size more than deep traps and shi to higher energy.28,31

Under our present experimental conditions we haveobserved the wide yellow emission prole of all samplescentered at around 510–650 nm and can be considered assurface trapped induced emission,59,60 i.e. recombination ofshallow trapped electrons in sulfur vacancy defect energy stateswith holes in the valence band (or shallow hole trap states nearthe valence band). With increase in reuxing time the particlesize of QDs increases (decrease in the band gap) and the trap

7378 | J. Mater. Chem. C, 2014, 2, 7373–7384

states are modied accordingly, i.e., the energy gap between thetrap states also decreases, which in turns causes a gradual redshi of the PL peak position. Again, the considerably large redshi of the PL peak position with increase in the size of QDsimplies that the emission occurs from shallow electron trapstates, as the energies of these shallow trap states are easilymodied by the nanocrystal size.28

Now let us take a closer look at absorbance (Fig. 5A) and PL(Fig. 5B) spectra of MPA–CdS QDs for a particular starting pH(e.g. pH 9) and concentration ratio ([CdCl2] : [MPA] ¼ 1 : 2.4).Here the absorbance peak position of QDs gets red shied andalso the absorption intensity increases with increase in

This journal is © The Royal Society of Chemistry 2014

Fig. 6 Decrease in photoluminescence (PL) intensity of MPA–CdSQDs in the presence of (A) Co2+ and (B) Ni2+. The inset shows thedecrease in PL color intensity of MPA–CdS QDs in the presence of (A)Co2+ and (B) Ni2+ under UV light. Plot of relative PL intensity (F0/F) vs.[Co2+]/[Ni2+] for (C) the total range of concentration and (D) the lowerrange of concentration. The QDs having maximum PL intensity,synthesized via optimized conditions, were used for this study.

Paper Journal of Materials Chemistry C

Publ

ishe

d on

04

June

201

4. D

ownl

oade

d by

Uni

vers

itat A

uton

oma

de B

arce

lona

on

28/1

0/20

14 1

2:43

:35.

View Article Online

reuxing time. The red shi of the absorbance peak positionindicates a faster growth rate during the process of QD forma-tion and the increase of the size of the QDs (as stated earlier)with increase in reuxing time up to 10 h is also evident fromthe TEM images in Fig. 3A and B. Beyond 10 h of reux furtherred shi in the absorption peak position of QDs occurs becauseof the increase in particle size and also the higher degree ofclustering makes close proximity between the QDs, as evidentfrom the TEM image (Fig. 3C). The increase in absorptionintensity is due to the higher amount of CdS QD generation,which in turn increases the PL intensity up to 10 h of reux.Beyond 10 h of reux the PL intensity decreases althoughabsorbance increases i.e. a greater amount of CdS QD formationoccurs. The decrease in PL intensity of the QDs aer 10 h reuxmay be due to their higher degree of clustering as shown in theTEM image (Fig. 3C). Because of various size distributionsduring different cluster formation, the excitation energy of oneQD can be transferred to another in a non-radiative way. Theclose proximity between the QDs in a cluster provided moredissipation paths, which could also suppress the PL. Again, withincrease in reuxing time the population of sulfur vacanciesmay decrease, which could be a possible reason for the decreasein intensities of trap-state emission. Note that initially the PLspectrum of CdS QDs shows a single peak, but aer a certaintime of reux the PL spectrum splits into two peaks, one ofwhich (initial peak) gets red shied from 528 nm to 642 nmwithincrease in reux time and the other peak, appearing as ashoulder, shows very little shiing from 659 nm to 670 nm. Nowif we have a closer look at PL spectra, one can see that thesecond peak prominently appears aer 15 h reuxing. So, aer15 h to 30 h reuxing, the shi in the rst peak is �24 nm (618nm to 642 nm) and that of the second peak is �11 nm (659 nmto 670 nm). This implies that the second peak also get redshied during reuxing, but to a lesser extent than the rst one.As we stated earlier, the rst PL peak is due to the recombina-tion of shallow electrons trapped in sulfur vacancy defect energystates with holes in the valence band (or shallow hole trap statesnear the valence band). Along with the band edges, the energiesof the shallow trap states could also change with particle sizeand degree of clustering, resulting in particle size-dependentand proximity-dependent shallow trap state emission.23,26 Thuswith the increase in particle size and degree of clustering, theshallow trap-state emission peaks of MPA–CdS QDs get redshied. On the other hand the second PL peak at lower energy(red luminescence) arises from deep trapped electrons in sulfurvacancy defect energy states.61 Little amount of shiing of thesecond PL peak also supports the above fact, i.e. emissionoccurs from deep trap-states, as the photoluminescence bandowing to deep level recombination is only slightly affected bythe connement process and degree of clustering,31 due to thelocalization of the electronic wave-function.64 During reuxing,the deeply trapped sulfur vacancy states may be generated fromshallow trapped states,65 which in turn gives rise to the secondPL peak at lower energy. The intensity of the second PL peak issubstantially low, which could be due to the little amount ofdeep trap sulfur vacancies, or deep level traps that tend toundergo non-radiative recombination by emitting phonons.30

This journal is © The Royal Society of Chemistry 2014

Finally, because of clustering of QDs, quantum connement islost and densely packed CdS particles would behave as bulkCdS, whichmay be a possible cause for considerable decrease inPL intensity of both peaks with reuxing time.66

3.3. Sensing of Co2+ and Ni2+ ions by MPA–CdS QDs

On the basis of metal–ligand interaction between the carboxylacid group of MPA and metal ions,67 our synthesized MPA–CdSQDs can be used for metal sensing and it has been observed thatthese QDs have excellent sensitivity towards Co2+ and Ni2+. Thesensing ability of MPA–CdS QDs for Co2+ and Ni2+ was exploredby photoluminescence (PL) spectroscopy. The MPA-capped CdSQDs having maximum PL intensity were used for our sensingstudies. The PL of QDs dramatically decreased in the presenceof Co2+ and Ni2+ ions (Fig. 6A and B, respectively), which isevidenced by the decrease in PL color intensity (Fig. 6A insetand B inset, respectively). Different physiologically importantmetal ions are used along with some transition metal ions toexplore the sensing properties of MPA coated CdS QDs towardsthem. We have found that other common metal ions (as statedearlier in the Introduction section) have almost no inuence onPL properties of CdS QDs studied here. Hence turn-off photo-luminescence (PL) of MPA–CdS QDs is the basic mechanism forthe detection of Co2+ and Ni2+ in the presence of other commonmetal ions.

The UV-vis spectra of MPA–CdS QDs do not change duringthe addition of Ni2+ ions (data not shown). The same trend isalso observed for the UV-vis spectra of MPA–CdS QDs in thelower range of concentration (up to 5 mM) due to addition of

J. Mater. Chem. C, 2014, 2, 7373–7384 | 7379

Scheme 3 Schematic representation of the coordination interactionbetween metal ions and MPA–CdS QDs through the carboxyl groupof MPA.

Journal of Materials Chemistry C Paper

Publ

ishe

d on

04

June

201

4. D

ownl

oade

d by

Uni

vers

itat A

uton

oma

de B

arce

lona

on

28/1

0/20

14 1

2:43

:35.

View Article Online

Co2+. But beyond 5 mM concentration of Co2+ a gradual increasein the absorbance of MPA–CdS QDs occurs at 393 nm with anoticeable blue shi in absorbance maxima (�5 nm), and at thesame time an additional broad absorption band is observed atabout 467 nm (Fig. 7) with a visual color change from colorlessto yellow (Fig. 7 inset).

In our synthesized MPA-capped CdS QDs, the sulfur atom ofthe thiol group remains attached to the surface of the QDs andthe negatively charged carboxyl group (at high pH) is accessiblefor selective coordination with Co2+ and Ni2+. Here Ni2+ cancoordinate with a single MPA-capped CdS QD (as shown inScheme 3), resulting in a decrease in PL intensity, without anychange in absorbance of MPA–CdS QDs. On the other hand Co2+

can coordinate with two or more MPA-capped CdS QDs67 (asshown in Scheme 3), resulting in a decrease in PL intensity, witha noticeable change in absorbance and color of MPA–CdS QDsas stated earlier. The change in absorbance and color of MPA–CdS QDs with the Co2+ ion is primarily due to close proximity ofQDs following an aggregation, as evident from dynamic lightscattering (DLS) studies and also the TEM image (see Fig. S3Aand B of the ESI S4). At lower concentration of Co2+ ions aconsiderably less amount of aggregation of QDs occurs, whichis not sufficient for bringing a change in absorbance and alsocolor of MPA–CdS-QDs. With increase in the concentration ofCo2+ ions the amount of aggregation increases and aer acertain concentration (5 mM) a sufficient amount of aggregationoccurs which is shown by the change in absorbance and color ofMPA–CdS-QDs. The same phenomenon was also observed byGore et al.67 at high concentrations of Co2+ but they did notprovide any idea about the lower range of concentration of Co2+,which may be due to the low sensitivity of thioglycolic acid(TGA)-capped CdS QDs.

The PL of MPA-capped CdS QDs is quenched upon additionof either Co2+ or Ni2+ due to the coordination interactionbetween QDs and metal through the carboxyl group of MPA. PLquenching of QDs by transition metal ions is a complicatedprocess and several mechanisms have been proposed to explainhow metal ions quench the PL intensity of QDs.68 UV-vis and PLspectra reveal that energy transfer from MPA–CdS QDs to eitherCo2+ or Ni2+ is ruled out because of no spectral overlap betweenthe absorption spectra of these metal ions and the emission

Fig. 7 UV-vis absorption spectra of MPA–CdS QD solution in theabsence and presence of various concentrations of aqueous Co2+ ions(as indicated). The inset shows the change of color of MPA–CdS QDsolution with increasing concentrations of Co2+.

7380 | J. Mater. Chem. C, 2014, 2, 7373–7384

spectra of QDs (data not shown). Time-resolved PL experiments(Fig. 8) show that the lifetime of MPA–CdS QDs (the averagelifetime (sav) of our synthesized MPA–CdS QDs is �34.38 ns)does not change upon addition of either Co2+ or Ni2+ (Table 1),which rules out the possibility of excited state PL quenchingand electron transfer processes. The possibility for either Co2+

or Ni2+ to compete with Cd2+ for thiol ligands, resulting inchemical etching of the surface of MPA–CdS QDs to form newnon-radiative surface channels,69 is minimal, because of themuch bigger Ksp value of either CoS or NiS than that of CdS (thepKsp values of CoS, NiS and CdS are 20.4, 18.5 and 26.1,respectively).68 Moreover, the unchanged PL peak position(lemmax) of MPA–CdS QDs in the presence of either Co2+ or Ni2+

(Fig. 6A and B) excludes the possibility of incorporation of thesemetal ions into the crystal lattice of CdS. Hence the coordina-tion interaction between QDs and metal ions through thecarboxyl group of MPAmay lead to some non-radiative channelsfor energy dissipation through which the PL intensity of MPA–CdS QDs is quenched in the presence of either Co2+ or Ni2+.

3.4. Selectivity and sensitivity of Co2+ and Ni2+ ions by MPA–CdS QDs

To evaluate the selectivity and sensitivity of our PL-quenchingtechnique, we have plotted the relative PL intensity (F0/F), forthe MPA–CdS QD sensor in the presence of different metals atsome xed concentration of 50 mM as represented in Fig. 9 (barplot), where F0 and F represent the PL intensities of MPA–CdSQDs at 594 nm in the absence and presence of metal ions,

Fig. 8 Photoluminescence lifetime decays of MPA–CdS QDs in theabsence and presence of varying concentrations of (A) Co2+ and EDTAand (B) Ni2+ and EDTA. The samples were excited using a 377 nm lasersource and all the decays were monitored at 594 nm.

This journal is © The Royal Society of Chemistry 2014

Fig. 9 Plots of relative photoluminescence intensity of MPA–CdSQDs(F0/F) for different metal ions (50 mM): (A) the orange bars indicate theresponses of individual metal ions while the gray bars denote theresponses of Co2+ ions (50 mM) in the presence of other coexistingmetal ions (50 mM); (B) the orange bars show the responses of indi-vidual metal ions while the purple bars indicate the responses of Ni2+

ions (50 mM) in the presence of other coexisting metal ions (50 mM).

Table 2 Linear equation and linearity coefficient for Co2+ and Ni2+

ions at different concentration ranges

Metal ion (differentconcentration ranges) Linear equation

Linearitycoefficient

Co2+ (10 nM–1 mM) F0/F ¼ 0.270[Co2+] + 1.009 0.9794Co2+ (1 mM–50 mM) F0/F ¼ 0.020[Co2+] + 1.286 0.9866Ni2+ (50 nM–1 mM) F0/F ¼ 0.144[Ni2+] + 1.002 0.9934Ni2+ (1 mM–50 mM) F0/F ¼ 0.016[Ni2+] + 1.142 0.9956

Table 1 Average PL life time (sav) of MPA–CdSQDs in the absence andpresence of different concentrations of Co2+ ions and EDTA or Ni2+

ions and EDTA

SamplesAverage lifetime, sav (ns) c2

CdS QDs 34.38 1.13CdS QDs + 200 nM Co2+ 32.72 1.11CdS QDs + 20 mM Co2+ 32.82 1.10CdS QDs + 20 mM Co2+ + 24 mM EDTA 31.94 1.10CdS QDs + 200 nM Ni2+ 32.14 1.13CdS QDs + 20 mM Ni2+ 31.76 1.11CdS QDs + 20 mM Ni2+ + 24 mM EDTA 30.51 1.13

Paper Journal of Materials Chemistry C

Publ

ishe

d on

04

June

201

4. D

ownl

oade

d by

Uni

vers

itat A

uton

oma

de B

arce

lona

on

28/1

0/20

14 1

2:43

:35.

View Article Online

respectively. Fig. 9 (bar graph) shows the selectivity of the MPA–CdS QDs as optical sensors. As shown in Fig. 6A and B, only theaddition of either Co2+ or Ni2+ leads to the decrease in PL

This journal is © The Royal Society of Chemistry 2014

intensity of the MPA–CdS QDs, whereas no substantial changesare observed upon addition of other metal cations such as Zn2+,Cr2+, Al3+, Cd2+, Ag+, Pb2+, Ca2+, Na+, K+, Hg2+, Fe2+, Fe3+, La3+

and Cu2+. This shows that exclusively Co2+ and Ni2+ result in PLquenching (Fig. 9), presumably via coordination interactionwith the carboxyl group of MPA in the CdS QDs.67 Moreover,competitive studies are also carried out in the presence of othermetal ions by adding either Co2+ or Ni2+ to solutions of MPA–CdS QDs. As in Fig. 9, marked changes in the PL intensity ofMPA–CdS QDs are observed upon addition of Co2+ or Ni2+,either in the absence or presence of other metal ions. Theresults indicate that the sensing of these two metal ions (Co2+

and Ni2+) is merely affected by the presence of these commonlycoexisting ions. Hence the observed turn-off photo-luminescence of MPA–CdS QDs is exclusively due to either Co2+

or Ni2+.The analysis of the limit of detection is performed by

titrating the MPA–CdS QDs with either Co2+ or Ni2+ and moni-toring the change in PL behavior. Fig. 6A and B show the typicalPL spectroscopic response aer addition of various concentra-tions of either Co2+ or Ni2+ to the MPA–CdS QD solution. The PLintensity of the MPA–CdS QDs at 594 nm gradually decreaseswith increasing concentrations of either Co2+ or Ni2+ (Fig. 6Aand B). From these PL quenching processes, we have plotted therelative PL intensity (F0/F) against the added concentration ofeither Co2+ or Ni2+ over the range from 10 nM to 50 mM (Fig. 6C).From the plot of F0/F vs. [Co2+] or [Ni2+], one can obtain that twolinear portions exist over the whole range of concentrations: (1)the rst linear correlation remains over the range from 10 nM to1 mM for Co2+ and 50 nM to 1 mM for Ni2+ (Fig. 6D), (2) thesecond linear correlation remains over the range from 1 mM to50 mM for both Co2+ and Ni2+ (Fig. 6C). The linear equationswith linearity coefficients at different concentration ranges forthese two metals (Co2+ and Ni2+) are summarized in Table 2.From these linear equations one can estimate the unknownconcentration of either Co2+ or Ni2+ without any interference ofother commonly coexisting metals as mentioned previously.The limit of detection (LOD) is calculated by the equation LOD¼ (3s/k), where s is the standard deviation of the y-intercept ofregression lines and k is the slope of the calibration graph.67 TheLOD is found to be �10 nM for Co2+ and �50 nM for Ni2+.

Although, our synthesized MPA–CdS QDs can easily, rapidlyand selectively detect both Co2+ and Ni2+ ions through the PLturn-off technique, but still there is an issue involved in dis-tinguishing between these two metal ions. In a higher concen-tration range (more than 5 mM), one can easily differentiate Co2+

J. Mater. Chem. C, 2014, 2, 7373–7384 | 7381

Fig. 10 Plot of F � F0 vs. time of MPA–CdS QDs with EDTA (1.2 mM) inthe presence of either Co2+ or Ni2+ ions (1.0 mM) over 30 min.

Journal of Materials Chemistry C Paper

Publ

ishe

d on

04

June

201

4. D

ownl

oade

d by

Uni

vers

itat A

uton

oma

de B

arce

lona

on

28/1

0/20

14 1

2:43

:35.

View Article Online

by following the change in color (from colorless to yellow) andUV-vis spectra of MPA–CdS QDs during addition of Co2+ (asstated earlier). However, one apparent problem still remains ina lower concentration range (from 10 nM to 5 mM). In order todistinguish between Co2+ and Ni2+ ions we used a disodium saltof ethylenediaminetetraacetic acid (EDTA), a good chelator. Thelower stability of the Co(II)–MPA complex than the Ni(II)–MPAcomplex70 provides an EDTA-induced, time dependent turn-onphotoluminescence-based technique to distinguish Co2+ fromNi2+ in the entire range of concentrations. EDTA-induced rapiddissociation of Co2+ ions from the surface of QDs through breakdown of the Co(II)–MPA complex provides a time dependent PLrecovery method for selective detection of Co2+ in the presenceof Ni2+. We have performed PL recovery kinetics of MPA–CdSQDs for both Co2+ and Ni2+ by using the concentration of EDTA1.2 times of the concentration of metal ions (Fig. 10). The metalion induced quenched PL of CdS QDs is recovered aer theaddition of EDTA for Co2+ ions, whereas the quenched PLalmost remains unchanged during 30 minutes aer the addi-tion of EDTA for Ni2+ ions. Thus the Co2+-induced PL quenchingand subsequent EDTA-induced PL recovery for MPA–CdS QDsform a solid base for the present QD-based turn-on photo-luminescent sensor for selectively detecting Co2+.

4. Conclusions

In summary, we have demonstrated an environmentallyfriendly, easy, cost-effective and facile synthetic route for thesynthesis of highly luminescent, remarkably stable, water-soluble MPA-capped CdS QDs through reuxing the alkaline,cadmium chloride and MPA reaction mixture at 80–90 �C.Coordination of Cd2+ with MPA at around pH 5 and subsequentdisruption of the coordination interaction at higher pH (7–12),under reuxing conditions, result in the release of the sulfurprecursor for the formation of MPA–CdS QDs. MPA simulta-neously acts as the sulfur precursor and stabilizer. Dependingon reuxing time for a particular concentration ratio of thereactants and pH the particle size and the clustering tendency ofthe formed MPA–CdS QDs can be easily tuned, resulting intunable absorption and emission wavelengths of QDs. The largeStokes shied photoluminescence (PL) peak with sufficiently

7382 | J. Mater. Chem. C, 2014, 2, 7373–7384

high PL intensity is due to the recombination of trapped elec-trons in sulfur vacancy defect energy states with holes in thevalence band. The broad yellow emission prole at around 510–650 nm arises from shallow trap states, whereas the PL shoulderat �665 nm arises from deep level trap states.

The synthesized MPA–CdS QDs provide a simple, rapidturn-off photoluminescence-based assay for ultra-sensitiveand selective detection of either Co2+ or Ni2+ over other metalsin aqueous solution. The coordination interaction betweenQDs and metal ions through the carboxyl group of MPA couldlead to some non-radiative channels for energy dissipationthrough which the PL intensity of MPA–CdS QDs is quenchedin the presence of either Co2+ or Ni2+ ions. The LOD is 10 nMfor Co2+ and 50 nM for Ni2+. In a higher concentration range(more than 5 mM), Co2+ can be distinguished from Ni2+, byfollowing the change of color (from colorless to yellow) andUV-vis spectra of MPA–CdS QDs during addition of Co2+. Againfor distinctive detection of Co2+ ions over the whole concen-tration range, we have used EDTA-induced time dependent PLrecovery kinetics of MPA–CdS-QDs. The present turn-off-onphotoluminescence technique offers a new cost-effective,simple, rapid, MPA CdS QD-based assay for the ultra-sensitiveand selective detection of either Co2+ or Ni2+ in aqueoussolution without any interference of other commonly coexist-ing metal ions.

Abbreviations

QD

Quantum dot MPA 3-Mercaptopropionic acid PL Photoluminescence EDTA Disodium salt of ethylenediaminetetraacetic acid

Acknowledgements

MH thanks DST SERB-India (Fund no. SB/S1/PC-041/2013) fornancial support. NM thanks CSIR-India and SP thanks UGCfor their individual fellowships. We thank Prof. N. Sarkar forhelp in measuring DLS in his lab-setup. Sincere thanks are alsodue to Prof. S. Basu, Head of the Department, ChemicalSciences Division, Saha Institute of Nuclear Physics, Kolkata forallowing us to measure time-resolved uorescence data usingher departmental TCSPC setup. Also we would like to thank theanonymous reviewers for their critical comments andsuggestions.

References

1 Y. Wang and N. Herron, J. Phys. Chem., 1991, 95, 525–532.2 D. A. Wheeler and J. Z. Zhang, Adv. Mater., 2013, 25, 2878–2896.

3 G. Sagarzazu, Y. Kobayashi, N. Murase, P. Yang andN. Tamai, Phys. Chem. Chem. Phys., 2011, 13, 3227–3230.

4 Y. Kobayashi, T. Nishimura, H. Yamaguchi and N. Tamai,J. Phys. Chem. Lett., 2011, 2, 1051–1055.

This journal is © The Royal Society of Chemistry 2014

Paper Journal of Materials Chemistry C

Publ

ishe

d on

04

June

201

4. D

ownl

oade

d by

Uni

vers

itat A

uton

oma

de B

arce

lona

on

28/1

0/20

14 1

2:43

:35.

View Article Online

5 S. V. Kershaw, A. S. Susha and A. L. Rogach, Chem. Soc. Rev.,2013, 42, 3033–3087.

6 T. Nann and W. M. Skinner, ACS Nano, 2011, 5, 5291–5295.7 T. Y. Zhai, X. S. Fang, L. Li, Y. Bando and D. Golberg,Nanoscale, 2010, 2, 168–187.

8 X. Q. Fu, J. Y. Liu, Y. T. Wan, X. M. Zhang, F. L. Meng andJ. H. Liu, J. Mater. Chem., 2012, 22, 17782–17791.

9 R. Freeman and I. Willner, Chem. Soc. Rev., 2012, 41, 4067–4085.

10 X. D. Wang, O. S. Woleis and R. J. Meier, Chem. Soc. Rev.,2013, 42, 7834–7869.

11 V. Rajendran, M. Lehnig and C. M. Niemeyer, J. Mater.Chem., 2009, 19, 6348–6353.

12 K. Zhang and L. J. Guo, Catal. Sci. Technol., 2013, 3, 1672–1690.

13 R. S. Selinsky, Q. Ding, M. S. Faber, J. C. Wright and S. Jin,Chem. Soc. Rev., 2013, 42, 2963–2985.

14 Z. B. Zhuang, X. T. Lu, Q. Peng and Y. D. Li, J. Am. Chem. Soc.,2010, 132, 1819–1821.

15 J. C. Bruce, N. Revaprasadu and K. R. Koch, New J. Chem.,2007, 31, 1647–1653.

16 P. S. Nair and G. D. Scholes, J. Mater. Chem., 2006, 16, 467–473.

17 T. T. Xuan, X. J. Wang, J. Q. Liu, H. L. Li, L. K. Pan and Z. Sun,J. Mater. Chem. C, 2013, 1, 4550–4555.

18 M. Shakouri-Arani and M. Salavati-Niasari, New J. Chem.,2014, 38, 1179–1185.

19 Y. X. Li, L. F. Tang, S. Q. Peng, Z. C. Li and G. X. Lu,CrystEngComm, 2012, 14, 6974–6982.

20 R. B. Khomane, A. Manna, A. B. Mandale and B. D. Kulkarni,Langmuir, 2002, 18, 8237–8240.

21 D. R. Larson, W. R. Zipfel, R. M. Williams, S. W. Clark,M. P. Bruchez, F. W. Wise and W. W. Webb, Science, 2003,300, 1434–1436.

22 S. Sapra, J. Nanda, D. D. Sarma, F. A. El-Al and G. Hodes,Chem. Commun., 2001, 37, 2188–2189.

23 H. Li, W. Y. Shih and W. H. Shih, Ind. Eng. Chem. Res., 2007,46, 2013–2019.

24 L. Zou, Z. Fang, Z. Y. Gu and X. H. Zhong, J. Lumin., 2009,129, 536–540.

25 E. Gabellieri, P. Cioni, E. Balestreri and E. Morelli, Eur.Biophys. J., 2011, 40, 1237–1245.

26 L. M. Qi, H. Colfen andM. Antonietti, Nano Lett., 2001, 1, 61–65.

27 V. Maheshwari and R. F. Saraf, Langmuir, 2006, 22, 8623–8626.

28 L. Brus, J. Phys. Chem., 1986, 90, 2555–2560.29 X. S. Zhao, J. Schroeder, P. D. Persans and T. G. Bilodeau,

Phys. Rev. B: Condens. Matter Mater. Phys., 1991, 43, 12580–12589.

30 P. Hu, Y. Cao, Y. Lou, B. Lu, M. Shao, J. Ni and M. Cao, Adv.Mater. Phys. Chem., 2013, 3, 10–18.

31 D. Nesheva, C. Raptis, Z. Levi, Z. Popovic and I. Hinic, J.Lumin., 1999, 82, 233–240.

32 X. Michalet, F. F. Pinaud, L. A. Bentolila, J. M. Tsay, S. Doose,J. J. Li, G. Sundaresan, A. M. Wu, S. S. Gambhir and S. Weiss,Science, 2005, 307, 538–544.

This journal is © The Royal Society of Chemistry 2014

33 B. B. Campos, M. Algarra, B. Alonso, C. M. Casado andJ. C. G. E. da Silva, Analyst, 2009, 134, 2447–2452.

34 Y. B. Lou, Y. X. Zhao, J. X. Chen and J. J. Zhu, J. Mater. Chem.C, 2014, 2, 595–613.

35 G. L. Wang, H. J. Jiao, X. Y. Zhu, Y. M. Dong and Z. J. Li,Analyst, 2013, 138, 2000–2006.

36 P. Wu, T. Zhao, S. L. Wang and X. D. Hou, Nanoscale, 2014, 6,43–64.

37 C. M. Tyrakowski and P. T. Snee, Phys. Chem. Chem. Phys.,2014, 16, 837–855.

38 L. Lin, Y. Q. Wen, Y. X. Liang, N. Zhang and D. Xiao, Anal.Methods, 2013, 5, 457–464.

39 M. S. Steiner, A. Duerkop and O. S. Woleis, Chem. Soc. Rev.,2011, 40, 4805–4839.

40 S. A. El-Say, Adsorption, 2009, 15, 227–239.41 M. Gharehbaghi, F. Shemirani and M. D. Farahani, J.

Hazard. Mater., 2009, 165, 1049–1055.42 A. R. Khorrami, T. Hashempur, A. Mahmoudi and

A. R. Karimi, Microchem. J., 2006, 84, 75–79.43 M. E. Rothenberg, Nat. Immunol., 2010, 11, 781–782.44 A. Sigel, H. Sigel and R. K. O. Sigel, in Nickel and Its Surprising

Impact in Nature: Metal Ions in Life Sciences, John Wiley &Sons, Ltd., West Sussex, UK, 2007, vol. 2.

45 Z. T. Zeng and R. A. Jewsbury, Analyst, 1998, 123, 2845–2850.46 E. J. Song, J. Kang, G. R. You, G. J. Park, Y. Kim, S. J. Kim,

C. Kim and R. G. Harrison, Dalton Trans., 2013, 42, 15514–15520.

47 S. Carter, A. S. Fisher, M. W. Hinds and S. Lancaster, J. Anal.At. Spectrom., 2012, 27, 2003–2053.

48 H. Zhang, R. Wollast, J. C. Vire and G. J. Patriarche, Analyst,1989, 114, 1597–1602.

49 E. N. Iliadou, S. T. Girousi, U. Dietze, M. Otto,A. N. Voulgaropoulos and C. G. Papadopoulos, Analyst,1997, 122, 597–600.

50 Z. A. Peng and X. G. Peng, J. Am. Chem. Soc., 2001, 123, 183–184.

51 A. Ihs and B. Liedberg, J. Colloid Interface Sci., 1991, 144,282–292.

52 S. C. Burris, Y. Zhou, W. A. Maupin, A. J. Ebelhar andM. W. Daugherty, J. Phys. Chem. C, 2008, 112, 6811–6815.

53 U. Srinivasan, P. A. Mieyal and J. J. Mieyal, Biochemistry,1997, 36, 3199–3206.

54 M. Belcastro, T. Marino, N. Russo and E. Sicilia, J. Phys.Chem. A, 2004, 108, 8407–8410.

55 M. B. Mishra and H. L. Nigam, Indian J. Chem., 1971, 9, 601–602.

56 T. Noyhouzer and D. Mandler, Anal. Chim. Acta, 2011,684, 1–7.

57 M. Salavati-Niasari, M. R. Loghman-Estarki and F. Davar,Chem. Eng. J., 2008, 145, 346–350.

58 P. C. Huang, Q. Jiang, P. Yu, L. F. Yang and L. Q. Mao, ACSAppl. Mater. Interfaces, 2013, 5, 5239–5246.

59 S. Karan, M. Majumder and B. Mallik, Photochem. Photobiol.Sci., 2012, 11, 1220–1232.

60 L. Huang, J. H. Yang, X. L. Wang, J. F. Han, H. X. Han andC. Li, Phys. Chem. Chem. Phys., 2013, 15, 553–560.

J. Mater. Chem. C, 2014, 2, 7373–7384 | 7383

Journal of Materials Chemistry C Paper

Publ

ishe

d on

04

June

201

4. D

ownl

oade

d by

Uni

vers

itat A

uton

oma

de B

arce

lona

on

28/1

0/20

14 1

2:43

:35.

View Article Online

61 K. J. van Schooten, J. Huang, D. V. Talapin, C. Boehme andJ. M. Lupton, Phys. Rev. B: Condens. Matter Mater. Phys.,2013, 87, 125412.

62 X. C. Wu, A. M. Bittner and K. Kern, J. Phys. Chem. B, 2005,109, 230–239.

63 B. A. Harruff and C. E. Bunker, Langmuir, 2003, 19, 893–897.64 N. Chestnoy, T. D. Harris, R. Hull and L. E. Brus, J. Phys.

Chem., 1986, 90, 3393–3399.65 E. Lifshitz, I. Dag, I. Litvin, G. Hodes, S. Gorer, R. Reisfeld,

M. Zelner and H. Minti, Chem. Phys. Lett., 1998, 288, 188–196.

7384 | J. Mater. Chem. C, 2014, 2, 7373–7384

66 H. L. Lee, A. M. Issam, M. Belmahi, M. B. Assouar, H. Rinnertand M. Alnot, J. Nanomater., 2009, 2009, 1–9.

67 A. H. Gore, D. B. Gunjal, M. R. Kokate, V. Sudarsan,P. V. Anbhule, S. R. Patil and G. B. Kolekar, ACS Appl.Mater. Interfaces, 2012, 4, 5217–5226.

68 P. Wu and X. P. Yan, Biosens. Bioelectron., 2010, 26, 485–490.69 E. M. Ali, Y. G. Zheng, H. H. Yu and J. Y. Ying, Anal. Chem.,

2007, 79, 2583.70 I. Filipovic, M. Tkalcec and B. S. Grabaric, Inorg. Chem., 1990,

29, 1092–1097.

This journal is © The Royal Society of Chemistry 2014