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Journal of Molecular Catalysis A: Chemical 358 (2012) 1– 9

Contents lists available at SciVerse ScienceDirect

Journal of Molecular Catalysis A: Chemical

j our na l ho me p age: www.elsev ier .com/ locate /molcata

ditor’s Choice paper

eroxidase-like behavior, amperometric biosensing of hydrogen peroxide andhotocatalytic activity by cadmium sulfide nanoparticles

warup Kumar Majia , Amit Kumar Duttaa , Divesh N. Srivastavab , Parimal Paulb,∗ , Anup Mondala,∗ ,ibhutosh Adhikarya,∗

Department of Chemistry, Bengal Engineering and Science University, Shibpur, Howrah 711 103, West Bengal, IndiaDepartment of Analytical Sicences, Central Salt & Marine Chemicals Research Institute, Gijubhai, Badheka Marg, Bhavnagar 364002, Gujarat, India

r t i c l e i n f o

rticle history:eceived 30 November 2011eceived in revised form 3 March 2012ccepted 6 March 2012vailable online 15 March 2012

eywords:

a b s t r a c t

A convenient solvothermal route has been developed for the synthesis of CdS nanoparticles (NPs) usinga cadmium (II) complex [Cd(ACDA)2] of 2-aminocyclopentene-1-dithiocarboxylic acid (HACDA). Decom-position of the precursor complex has been carried out by ethylenediamine (EN), hexadecylamine (HDA)or dimethyl sulfoxide (DMSO). Structural analyses reveal the formation of crystalline nanoparticles withrod-like shape from EN and spherical shape from HDA or DMSO as solvents, while the optical proper-ties suggest the quantum confinement by the nanoparticles. Superior photocatalytic activity towards the

dS nanoparticlesingle-source precursorhotocatalytic activityeroxidase-like behaviorydrogen peroxide sensor

degradation of aqueous Rose Bengal (RB) solution has been achieved with the use of CdS NPs as photo-catalyst under light irradiation. CdS NPs is found to possess peroxidase-like activity that can catalyze theoxidation of the peroxidase substrate 3,3′,5,5′-tetramethylbenzidine (TMB) in the presence of H2O2 toproduce a blue color reaction. CdS NPs anchored on glassy carbon (GC) electrodes have been preparedto study the electrocatalytic reduction of H2O2 in phosphate buffer solution. This modified electrode hasalso been used as amperometric biosensor for the detection of H2O2.

. Introduction

Nanostructured semiconductors are drawing increasing atten-ion for biomedical and biosensing applications due to theirnique physical and chemical properties, which depend on theirtructures, sizes and shapes [1]. For this reason, preparation ofemiconductor nanomaterials with controlled shape and size aref great importance [2]. Considerable efforts have been made toontrol the nanostructures in past few decades and despite remark-ble progress made, it still remains challenge task to researcherso develop simple and reliable methods for fabrication of variousemiconductor nanomaterials with controlled morphologies [3].

Dyes that are widely used in textile, photography, coatings etc.re therefore, common industrial pollutants in waste water [4].ormal aerobic waste water treatment processes not very effec-

ive for removal of these toxic chemicals from the environment.n recent years, a new technology termed as advanced oxidation

rocesses are in use for treatment of pollutants in both waternd wastewater [5]. To this end, photocatalytic oxidation usingemiconductor nanomaterials has turned out to be a promising

∗ Corresponding authors. Tel.: +91 3326684561; fax: +91 3326682916.E-mail addresses: anupmondal2000@yahoo.in (A. Mondal),

ibhutoshadhikary@yahoo.in (B. Adhikary).

381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved.oi:10.1016/j.molcata.2012.03.007

© 2012 Elsevier B.V. All rights reserved.

alternative method for environment water management [6]. Thephoto-degradation of several toxic compounds using TiO2 as aphotocatalyst has been widely studied over the past decade [7,8].However, the photocatalytic activity of TiO2 is limited to the UVregion (� < 400 nm) and therefore is not much effective in the vis-ible region, which is the main component solar light and indoorilluminations [9]. Thus, there is considerable interest in developingvisible light sensitive photo-catalyst [10–17].

Designing of biomimetic materials exhibiting peroxidase activ-ities is the focus of considerable attention [18,19]. In this regard,nanomaterials-based compounds such as Prussian blue, iron oxide,iron sulfide, cupric oxide and grapheme oxide have been foundto show peroxidase-like activity to catalyze oxidation of typicalperoxidase substrate [20–26]. For instance, with nanostructureFeS peroxidase-like activity has been reported using 3,3′,5,5′-tetramethylbenzidine as the peroxidase substrate and detection ofH2O2 has been made by employing it as an amperometric sensor[24]. Since, H2O2 it self is widely used for green chemical oxida-tion reactions, development of methodologies for detection andquantification of H2O2 are highly important. Carbon nanotubes,noble metals, macrocyclic complexes of transition metals and pro-

tein modified electrodes have been used as amperometric sensorsof H2O2, although each of them have some disadvantages [27,28].Very recently semiconductor nanomaterial modified electrode hasbeen used for this purpose [24,29].

2 S.K. Maji et al. / Journal of Molecular Catalysis A: Chemical 358 (2012) 1– 9

Table 1Summary of reaction conditions and experimental results of CdS NPs.

Solvent (mL) Temp (◦C) Time (min) Shape Average size (nm) Pore diameter (nm) Surface area (m2 g−1)

XRD TEM

EN (20) 120 15 Rod 4.8 D = 5aL = 70b 3.1 44.8HDA (15) + TOP (5) 120 15 Sphere 3.4 4 4.6 81.9DMSO (20) 120 15 Sphere 5.6 6 2.4 32.1

l(esvcciorinpTd

2

2

amatttSDm

2

m(oyw

3I

11[a

2

sIiC

a Diameter.b Length.

Cadmium sulfide is one of the most important visible-ight-sensitive semiconductor with narrow band gap energyEg = 2.41 eV) and has been extensively investigated because itsmission in the visible-light range can be tuned by changing theize and shape of the particles [30–32]. CdS nanomaterials witharying morphologies have been prepared recently from single pre-ursor sources by solvothermal route using different solvents andapping agents [33–39]. This method has the advantage of adopt-ng a single-pot procedure under mild condition and the productbtained in this way has fewer defects and better stoichiometry. Weeport here a simple solvothermal decomposition route for prepar-ng CdS NPs by using the complex Cd(ACDA)2 as the precursor. CdSanorods and nanoparticles thus obtained have been used for thehotocatalytic decomposition of RB and catalytic oxidation of TMB.hese NPs have also been used for evaluating their efficiencies foretection/estimation of H2O2 by electrochemical methods.

. Experimental

.1. Chemicals and materials

The chemicals used for the preparation of the ligand 2-minocyclopentene-1-dithiocarboxylic acid (HACDA) and theetal complex [Cd(ACDA)2] were of analytical grade. Ethylenedi-

mine (EN), hexadecylamine (HDA), dimethyl sulfoxide (DMSO),ri-octylphosphine (TOP), Rose Bengal (RB), commercial CdS,erephthalic acid (TA), hydrogen peroxide (H2O2) and 3,3′,5,5′-etramethyl benzidine (TMB) were purchased from Sigma–Aldrich.tandard titanium dioxide (Degussa-P25) was purchased fromegussa Company. Methanol, ethanol, acetic acid, diethyl ether andillipore water were used without any further purification.

.2. Synthesis of single-source precursor

The ligand (HACDA) was prepared according to the reportedethod [40]. To a clear methanol solution (10 mL) of HACDA

160 mg, 1 mmol) was added dropwise with stirring to an aque-us solution (10 mL) of cadmium chloride (92 mg, 0.5 mmol). Theellow compound that precipitated was filtered after 15 min andashed first with methanol and then with diethyl ether.

Yield: 89% (191 mg), CHN analyses (C12H16N2S4Cd): Calc.: C,3.59; H, 3.77; N, 6.53. Found: C, 33.53; H, 3.81; N, 6.51. Selected

R bands: �NH2: 3289 m cm−1, ıNH2: 1634s cm−1, ıCH2 + �C C:

459s cm−1, �C N + �C C S S: 1315 cm−1, �C C S S + �CN:282 m cm−1; �assymCSS: 902 cm−1, �symCSS: 623 cm−1. ESI-MS:Cd(ACDA)2 + H]+ (m/z = 430.07), [Cd(ACDA)2 + Na]+ (m/z = 451.96)nd [Cd(ACDA)2 + K]+ (m/z = 468.24).

.3. Preparation of CdS NPs

CdS NPs were prepared by solvothermal decomposition of the

ingle-source precursor using EN, HDA or DMSO as the solvents.n case of EN and DMSO, 250 mg of the precursor was dissolvedn 20 mL solvent in a round bottom flask. For the preparation ofdS NPs using HDA, 250 mg of the precursor was dissolved in 5 mL

hot TOP and then it was added to a 15 mL hot HDA solution ina round bottom flask. The resulting clear yellow solutions wereheated at 120 ◦C for 15 min and then cooled to room temperature.At this stage, 20 mL methanol was added to them. Thus obtainedyellow precipitates were centrifuged, followed by washing withethanol for several times for their purification. The pure nanoma-terials were collected after drying in oven at 50 ◦C for 30 min. Thereaction conditions and results are summarized in Table 1.

2.4. Characterization

Elemental analyses (C, H and N) were performed using Perkin-Elmer 2400II analyzer. FTIR spectra were recorded on KBr disksusing a JASCO FTIR-460-Plus spectrophotometer. Electron sprayionization mass spectroscopic measurements were carried outon a Micromass Qtof YA 263 mass spectrometer in dimethyl-formamide. X-ray diffraction (XRD) patterns were recorded ona Philips PW 1140 parallel beam X-ray diffractometer usingwith Bragg–Bretano focusing geometry and monochromatic CuK�

X-radiation (� = 1.540598 A). Transmission electron microscopy(TEM) images were collected by using JEOL JEM-2100 micro-scope working at 200 kV. EDX analyses were carried by usingHitachi S-3400 N (EDS, Horiba EMAX) instrument. N2-sorptionisotherms were obtained using a Quantachrome Instrumentsadsorption (77 K). UV–vis absorption spectra were recorded witha JASCO V-530 UV–vis spectrophotometer. Photoluminescencemeasurements were carried out using a Photon TechnologyInternational fluorometer. The catalytic oxidation of TMB was mon-itored spectrophotometrically using an Agilent 8453 diode-arrayspectrophotometer. Cyclic voltammetric and amperometric mea-surements were performed on CHI 620D electrochemical analyzer.

2.5. Measurements of photocatalytic activity

The experiments were carried out in a round bottom flask keptin a thermostated bath at 20 ◦C and an incandescent tungsten halo-gen lamp (200 W) was placed vertically on the reaction vessel at adistance of ca. 15 cm. The experiments were carried out with 40 mLaqueous solution of RB (3.6 × 10−5 M) and 15 mg of catalysts. Beforeirradiation, the suspension was magnetically stirred in the darkfor 30 min to reach the adsorption–desorption equilibrium. Aftera given interval of illumination, 3 mL of the aliquot was withdrawnfrom the suspension and centrifuged. The absorption spectrum ofthe filtrate solution was then measured in the range 400–650 nmand the peak at 540 nm (�max) was monitored. To test the chemicalstability of CdS NPs, it was recycled and reused for five times forthe decomposition of RB under same experimental condition. Aftereach photocatalytic test, the aqueous solution was centrifuged tocollect CdS NPs, which was then dried at 60 ◦C and used for the nextcycle.

In order to find out whether the photodegradation of RB by CdS

NPs occur through the generation of hydroxyl radical or not, thecommonly used terephthalic acid (TA) photoluminescence probingtechnique was adopted [11–13]. 40 mL aqueous solution of sodiumterephthalate (2 × 10−3 M) containing 15 mg of either TiO2 or CdS

r Catalysis A: Chemical 358 (2012) 1– 9 3

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2

aoafNaafi2oakmia

2

iswCm1sspetwmtc

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3

ps2mdws

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tncapdS

S.K. Maji et al. / Journal of Molecula

Ps was irradiated with light for a given period of time. An aliquot3 mL) of the solution was withdrawn from the suspension and cen-rifuged and its luminescence spectrum was recorded between 350nd 600 nm using 315 nm as the excitation wavelength.

.6. Peroxidase-like behavior

Peroxidase-like activity of CdS NPs was investigated by the cat-lytic oxidation of typical peroxidase substrate TMB in presencef H2O2. A series of solution mixtures were made by 3 mL sodiumcetate buffer (pH 4) with 0.1 mM of TMB and (i) 24 �g of CdS NPsrom HDA, (ii) 13 mM of H2O2, (iii) 13 mM of H2O2 and 24 �g of CdSPs from EN, (iv) 13 mM of H2O2 and 24 �g of CdS NPs from HDAnd (v) 13 mM of H2O2 and 24 �g of CdS NPs from DMSO. The kineticnalyses were carried out by using 24 �g of CdS NPs from HDA, axed amount of H2O2 (13 mM), and different amounts (0, 8.3, 16.6,9.1, 35.4, 41.6, 52, 62.5, 83.3, 104, 125 �M) of TMB solution; 24 �gf CdS NPs from HDA, a fixed amount of TMB (0.1 mM) and differentmounts (0, 6.5, 9.8, 19.5, 22.8, 26, 32, 39 mM) of H2O2 solution. Theinetic parameters were calculated using the Michaelis–Mentenodel: V0 = (Vmax × [S])/(Km

app + [S]), where V0 is the initial veloc-ty, Vmax is the maximum velocity, [S] is the substrate concentrationnd Km

app is the apparent Michaelis constant.

.7. Electrocatalytic activity

The cyclic voltammograms were obtained using a cell contain-ng 20 mL of 0.1 M phosphate buffer solution (PBS, pH 4.0) with acan rate of 0.1 V s−1 at 40 ◦C. In the typical cell setup, platinumire was used as auxiliary, an Ag/AgCl electrode as reference, withdS NPs modified GC (3 mm) as a working electrode. Before experi-ent the buffer solution was degassed by purging with pure N2 for

5 min and a N2 atmosphere was kept over the solution during mea-urements. The amperometric experiment was carried out by theuccessive addition of H2O2 to the buffer solution by applying theotential of −0.6 V (vs. Ag/AgCl). The kinetic parameter, the appar-nt Michaelis–Menten constant (Km

app) can be calculated usinghe Lineweaver–Burk equation: 1/Iss = 1/Imax + Km

app/(Imax × [S]),here Iss is the steady-state current, Imax the maximum currenteasured under conditions of enzyme saturation, [S] is the concen-

ration of substrate and Kmapp is the apparent Michaelis–Menten

onstant.

. Results and discussion

.1. Characterization of single-source precursor

The Cd(II) complex of HACDA has been straightforwardly pre-ared in good yield (89%) by the reaction between a methanololution of HACDA and an aqueous solution of CdCl2 in the ratio:1. The precursor complex is insoluble in water and in com-on organic solvents, but fairly soluble in dimethyl sulfoxide and

imethylformamide. The composition of the precursor complexas established by elemental analyses, ESI-MS and FTIR spectro-

copic methods (supporting information Fig. S1 and 2).

.2. Structural characterizations of CdS NPs

Powder XRD pattern of CdS NPs obtained under different reac-ion conditions are shown in Fig. 1. The diffraction pattern ofanomaterials is indexed to the pure hexagonal phase of CdS withharacteristics (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (1 1 2)

nd (1 0 5) peaks (JCPDS No. 41-1049). The purity and the com-osition of the materials were also confirmed from their energyispersive X-ray (EDX) spectra, which show the signals for of Cd and

only (supporting information Fig. S3). It is of interest to note that

Fig. 1. X-ray diffraction patterns of CdS NPs obtained from (a) EN, (b) HDA and (c)DMSO.

while (1 0 1) peak is strongest for spherical nanoparticles similar tothat of the hexagonal phase, however, for nanorods the diffractionpeak (0 0 2) is the strongest one. This observation is corroboratedby the TEM results and seems to indicate that for the nanorodspreferential growth occur along the c axis. The remaining peaksare generally broad and probably indicate relatively small dimen-sions of the materials. Average crystal diameter for CdS NPs werecalculated using the Debye–Scherrer equation (D = 0.9�/( cos �)),where, D is the crystallite diameter, � is the wave length of X-rayi.e. 1.540598 A, is the value of full width at half maximum and �is the Bragg’s angle. The average crystal diameters were calculatedand are found to lie within 3.3–5.6 nm (Table 1). The dislocationdensity (ı), which is the length of dislocation lines per unit volume(amount of defects in a crystal) were estimated using the equation:ı = 1/D2. The values of ı for CdS NPs obtained with different solventsare: 0.043 (EN), 0.092 (HDA) and 0.032 nm−2 (DMSO), and the smallmagnitude of ı in all cases suggests good crystalline nature of theprepared materials. The values of microstrain (ε), as obtained byusing the relation, ε = ( cos �)/4 are 7.75 × 10−3 (EN), 9.75 × 10−3

(HDA) and 5.6 × 10−3 (DMSO), which again supports the formationof crystallites of low dimension (that is nanoparticles).

The influence of solvents on the morphologies of CdS NPswas further analyzed by TEM measurements. The TEM imagesillustrated in Fig. 2 show the rod-like morphology with averagediameter and length of 5 and 70 nm of CdS (Fig. 2a) obtained fromEN, while particles of spherical shapes with average diameter 4 and6 nm obtained from HDA (Fig. 2b) and DMSO (Fig. 2c), respectively.The solvents used have played the role of attacking nucleophile ofthe precursor complex as well as capping agents of variable effi-ciencies [41,42]. For selective growth of a face to obtain anisotropicnanoparticles, structure-directing agents are used [43–45]. In ourcase, EN seems to serve as a capping agent leaving the (0 0 2) facet togrow by following this approach. On the other hand, the formationof spherical nanoparticles in HDA and DMSO seem to be controlledmore by thermodynamic requirement of surface area minimization[46].

The average particle sizes obtained from the TEM measure-ments are in good agreement with the results obtained from XRDmeasurements (Table 1). The selected area diffraction (SAED)patterns (supporting information Fig. S4) show a set of concentricrings instead of sharps spots, as a result of the presence of smallcrystalline nanomaterials. All the diffraction pattern can be readilyindexed to (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3) and (1 1 2)

planes of the hexagonal CdS (JCPDS No. 41-1049), suggesting thepolycrystalline nature of the materials. From the high resolutionTEM (HRTEM) images (supporting information Fig. S5) latticefringes of nanocrystals can be seen, which suggests the good

4 S.K. Maji et al. / Journal of Molecular Catalysis A: Chemical 358 (2012) 1– 9

ined f

c0lteip

mai(tdmtp(a(ptfhm

3.3. Optical properties

The room temperature UV–vis absorption spectra for CdS NPs(dispersed in water) are displayed in inset of Fig. 3. The spectra

Fig. 2. TEM images of CdS NPs obta

rystalline nature of CdS NPs. The fringe spacing of 0.33, 0.31 and.32 nm correspond to the separation of the (0 0 2) and (1 0 1)

attice planes for nanorods and spheres, respectively. Furthermore,he HRTEM images reveal that the nanorods were grown prefer-ntially along the (0 0 2) direction and for the spherical particles,t was along (1 0 1) plane, which is also consistent with the XRDatterns.

Brunauer–Emmett–Teller (BET) gas sorptormetry measure-ents were carried out to get more insight into the porous nature

nd surface availability sites of CdS NPs. All of the isotherms aredentified as type-IV isotherm with an H2-type hysteresis curvesupporting information Fig. S6), confirming the mesoporous struc-ures. The average porosities of three types of nanocrystals wereetermined from pore-size distribution curves (supporting infor-ation Fig. S6 inset). A sharp distribution in pore-size occurs in

he mesoporous region. The average pore diameter of all sam-les were calculated according to the Bopp–Jancso–HeinzingerBJH) method and found to be in the range of 2.4–4.6 nm, wheres the specific surface area lie in the range of 32.1–81.9 m2 g−1

Table 1). Specific surface area gradually decreases for CdS NPsrepared from HDA to EN to DMSO, suggesting that nanopar-

icles prepared from HDA would be the more active materialor catalytic applications, since larger specific surface area andigher crystallinity favor the effective catalytic activity of theaterial.

rom (a) EN, (b) HDA and (c) DMSO.

Fig. 3. Photoluminescence spectra and in the inset UV–vis absorption spectra of CdSNPs obtained from (a) EN, (b) HDA and (c) DMSO.

S.K. Maji et al. / Journal of Molecular Cata

Table 2Comparison of the kinetic parameters in terms of rate constant.

Photocatalyst Rate constant (min−1)

CdS from HDA 2.2 × 10−2

CdS from EN 1.2 × 10−2

−3

saashTtmbCsastsf

D

onX

aaertbap

Fsw(

CdS from DMSO 6.0 × 10TiO2 (Degussa-P25) 1.7 × 10−3

Commercial CdS 8.2 × 10−4

how the band edge absorption lie between 445 and 480 nm with well resolved absorption peak in the range of 420 to 460 nm. Thebsorption maxima corresponds to the first optically allowed tran-ition between the electron state in the conduction band and theole state in the valence band, i.e. the first excitonic transition [47].he excitonic features also suggest the monodispersive nature ofhe NPs [43–45]. The band gap energy (Eg) of semiconductor nano-

aterials can be evaluated from the Tauc’s plot and are found toe 2.82 (EN), 2.9 (HDA) and 2.73 (DMSO) eV. Compared to bulkdS (Eg = 2.41 eV), band gap energies of synthesized NPs are bluehifted, due the quantum effect by smaller nanocrystals [48]. Themount of blue shift increases from 0.32 to 0.5 eV as the particlesize decreases from 6 to 4 nm. The observation is consistent withhe inversely proportional relationship between Eg and the particleize [49]. The average particle size (D) can also be calculated by theollowing expression [50].

= −(6.6521 × 10-8)�3 + (1.9557 × 10-4)�2 – (9.2352 × 10-2)�

+ 13.29

Based on the absorption maxima (�) values, the values of D thusbtained are 5.2 (EN), 4.1 (HDA) and 4.6 (DMSO) nm. It should beoted that these valued are in accord with the valued obtained fromRD and TEM.

The photoluminescence properties of CdS NPs were investigatedt room temperature with the excitation wavelength 375 nm andre shown in Fig. 3. The emission spectra exhibit sharp and strongmission peak between 455 and 492 nm with an additional broaded emission at around 600 nm. The sharp emission is attributed

o the core-state radiative decay from conduction band to valenceand [49]. The band edge emission peaks are comparable to thebsorption line width, with the emission peak red-shifted com-ared to the maximum wavelength of the adsorption spectrum.

ig. 4. (a) Time dependent UV–vis spectral change of RB solution (3.6 × 10−5 M) catalyzed btudy for the photodegradation of RB under different conditions: (i) without catalyst in dith commercial CdS (15 mg) in light, (v) with commercial TiO2 (15 mg) in light, (vi) with

viii) with CdS NPs from DMSO (15 mg) in light.

lysis A: Chemical 358 (2012) 1– 9 5

This shift is the result of a combination of relaxation into trap statesand the size distribution [49]. The high intensity of the band-edgeemission indicates the high state of crystallinity with few electronicdefects and the good dispersity of nanomaterials [51]. The effec-tive passivation of surface states or defects, which are normallyassociated with semiconducting nanomaterials, is indicated by thesharp luminescence features. The additional broader low intensered emission in the emission spectra at ca. 600 nm corresponds tothe recombination of trapped electrons and holes in some surfacedefect states of CdS NPs [52].

3.4. Photocatalytic activity

To investigate the potentiality of the prepared materials as pho-tocatalyst, the catalytic performances of CdS NPs were examinedby the photodegradation of RB under the illumination of light asfollowed by spectrophotometric monitoring. We have chosen RB totest the photocatalytic activity, since it is a fluorescent dye and com-monly used in textile, photographic and photochemical industries.In recent years, a few studies have been made for the photocatalyticdecomposition of RB in the presence of semiconductor nanomate-rials [53–58], however, to the best of our knowledge no such studyhas been made using CdS NPs.

Fig. 4a shows the time dependent UV–vis spectral changes ofRB solution in the presence of CdS NPs (HDA) under the irra-diation of light for 200 min. The characteristic peak at 540 nmgradually decreases with irradiation time and disappears com-pletely after 200 min. During the reaction, no new peaks aregenerated, whereas, the absorption intensities at 350, 305, 255and 212 nm also decreases, suggesting the complete photodegra-dation of RB rather than decolorization or bleaching. In order toestablish relative performances of the three different CdS NPs asphotocatalysts among themselves as well as commercial CdS andTiO2 (Degussa P25) the following comparative studies were madeusing 3.6 × 10−5 M RB solutions: (i) without catalyst in dark, (ii)without catalyst in light, (iii) with CdS NPs from HDA (15 mg) indark, (iv) with commercial CdS (15 mg) in light, (v) with commer-cial TiO2 (15 mg) in light, (vi) with CdS NPs from EN (15 mg) in

light, (vii) with CdS NCs from HDA (15 mg) in light and (viii) withCdS NPs from DMSO (15 mg) in light. As shown in Fig. 4b, the photo-catalytic activity decrease in the order CdS (HDA) > CdS (EN) > CdS(DMSO) > TiO2 > CdS (commercial). It may also me noted that the

y 15 mg CdS NPs (from HDA) under light irradiation for 200 min, and (b) comparativeark, (ii) without catalyst in light, (iii) with CdS NPs from HDA (15 mg) in dark, (iv)

CdS NPs from EN (15 mg) in light, (vii) with CdS NCs from HDA (15 mg) in light and

6 S.K. Maji et al. / Journal of Molecular Catalysis A: Chemical 358 (2012) 1– 9

Fw

cltars

ttmogoaorohetp

bm(fdgppwtagrcasidTpii

of H2O2 [26].

ig. 5. Photoluminescence spectral changes of TA solution under light irradiationith TiO2 and in the inset for CdS NPs (from HDA).

hange in relative concentration of RB with time with added cata-yst is nominal in absence of light. The rate constant values for allhe systems are given in Table 2. The catalyst (CdS NPs) recoveredfter complete degradation of RB when reused showed no deterio-ation of its activity. Indeed, even after five successive recycling noignificant change of photocatalytic activity was made.

Catalytic conversation reactions are generally correlated withhe adsorption and desorption of molecules on the surface ofhe catalyst. A larger specific surface area allows more reactive

olecules to be adsorbed onto the surface of the catalyst [59]. Asbtained from BET measurements, CdS NPs from HDA providesreater specific surface area than that of EN to DMSO, which isbviously beneficial for the enhancement of photocatalytic activitynd matches well with the result of photocatalytic decompositionf RB. In addition, semiconductor photocatalysis reactions are alsoelated to the crystallinity of the photocatalyst. Higher crystallinityf materials reduces the formation of trap states in the crystals andence, the number of recombination centers for photogeneratedlectrons and holes. This allows for higher photocatalytic degrada-ion of RB. Therefore, it can be concluded that the photocatalyticroperty of CdS NPs is directly related to their structural features.

The photocatalytic degradation of RB in aqueous solution haseen previously reported and indicates the possibility of involve-ent of several steps like, (i) photo-absorption of catalyst and dye,

ii) generation of photo-induced electrons and holes, (iii) trans-er of charge carriers to the surface and (iv) photodegradation ofye [53–55]. The degradation of the dye may takes place by theenerated hydroxyl radicals (•OH) or by the direct participation ofhotogenerated holes. Therefore, to establish the mechanism of thehotodegradation process, the detection of hydroxyl radicals (•OH)ere made by the well known photoluminescence technique using

erephthalic acid (TA) as a probe molecule [9,11–13]. In this process, strong fluorescent molecule 2-hydroxylterephthalic acid (HTA) isenerated by the capture of •OH by terephthalic acid. We have car-ied out the detection process using CdS NPs (HDA) and TiO2 asatalyst with the solution sodium terephthalate under light irradi-tion. Fig. 5 is the photoluminescence spectral changes of TiO2/TAystem after certain time interval of irradiation. A gradual increasen emission intensity at 425 nm is observed with increasing the irra-iation time, which indicates the generation of hydroxyl radicals.hese hydroxyl radicals are the main active species for the decom-

osition of dye molecule in presence of TiO2 as catalyst. However,

n the case of CdS/TA system, no emission peak is observed (Fig. 5nset), which suggests that hydroxyl radicals were not produced in

Fig. 6. UV–vis absorption–time course curve of TMB using three types of CdS NPsunder different conditions.

the reaction system and hence are not the main active species forthe photodecomposition of RB.

It appears that the position of the valence band of CdS NPs isless positive than that of OH−/•OH couple (2.7 V vs. SCE) [60], asa result the photogenerated holes on the surface cannot interactwith OH− to generate •OH radical. Thus, unlike TiO2 where the dyeis decomposed by •OH, in the case of CdS NPs photodegradation ofRB seems to be actuated by the photogenerated holes.

3.5. Peroxidase-like activity

Peroxidase-like behavior of CdS NPs was examined by the oxida-tion of TMB as peroxidase substrate in presence of H2O2. It is wellestablished that peroxidase can catalyze the oxidation of peroxi-dase substrate by producing a color change and the color reactioncan generally be quenched by adding H2SO4 (supporting informa-tion Fig. S7) [22]. The color of TMB in presence of H2O2 and CdS NPsturns from colorless to blue, suggesting the catalytic oxidation ofTMB.

The oxidation of TMB by H2O2 in presence of CdS NPs as cata-lyst were carried out at optimized pH 4 and temperature at 40 ◦C(supporting information Fig. S8). The absorption spectral changeof TMB-H2O2 system catalyzed by CdS NPs (HDA) shows the grad-ual raise of characteristic peaks at 370 and 652 nm, which are thecharacteristic peaks for the oxidation product of TMB [22]. There-fore, similar to horseradish peoxidase (HRP), CdS NPs from differentsources can quickly catalyze the oxidation of typical HRP substrateslike TMB, suggesting the peroxidase-like activity. The comparativeexperiment for the oxidation of TMB using three types of CdS NPsand H2O2 are shown in Fig. 6. As shown in Fig. 6, it is observed thatneither H2O2 nor CdS alone can effectively oxidize TMB, which indi-cates that the interaction between CdS, H2O2 and TMB are neededfor the catalytic reaction process. Among three types of CdS NPs,the one which was obtained from HDA shows the highest activ-ity and then decreases to EN to DMSO, which matches well withthe results of photocatalytic decomposition of RB. Similar interpre-tations in their catalytic activities can be placed here as has beenmade for RB degradation. The mechanism of the reaction may beexplained by the electron transfer from the amino group of TMBto CdS NPs, followed by the electron transfer from the conductionband of CdS NPs to the lowest unoccupied molecular orbital (LUMO)

The apparent steady-state reaction kinetic parameters by ini-tial rate method were determined to investigate the mechanismof the peroxidase-like activity of CdS NPs. In this case we have

S.K. Maji et al. / Journal of Molecular Catalysis A: Chemical 358 (2012) 1– 9 7

Fig. 7. Steady-state kinetic analyses using Michaelis–Menten model and Lineweaver–Burk model (insets) for CdS NPs (HDA) by (a) varying the concentration of TMB withfi of TM

ofAtabotMatgi

oivmNh3nTa

3

torsie

TK

xed amount of H2O2 and (b) varying the concentration of H2O2 with fixed amount

nly determined the kinetic parameters of the catalyst preparedrom HDA, since it has the highest catalytic activity among them.pparent steady-state reaction parameters at different concentra-

ions of substrate were obtained by calculating the slopes of initialbsorption changes with time. The curves shown in Fig. 7a and

indicate the typical Michaelis–Menten kinetic for the oxidationf TMB by varying the concentration of TMB and H2O2, respec-ively. To obtain the catalytic parameters, the data was fitted to the

ichaelis–Menten equation and the resulted kinetic parametersre shown in Table 3. All the parameters were again calculated fromhe Lineweaver–Burk double-reciprocal plot, which gives analo-ous result. The double reciprocal plots (1/V0 vs. 1/S0) are presentedn insets of Fig. 7a and b.

The apparent Michaelis constant (Kmapp), which is the measure

f enzyme affinity for its substrate of CdS NPs with TMB as substrates 0.0095, while for HRP [22] the value is about 0.434. The Km

app

alue is thus significantly lower than HRP and peroxides nano-imetics reported recently [20–26]. The lowest Km

app value of CdSPs suggests that it has the highest affinity to TMB. On the otherand, the Km

app value of CdS NPs with H2O2 as substrate is about.62, which is also slightly lower than HRP and reported peroxidesano-mimetics, suggesting the highest affinity to H2O2 [20–26].herefore, our results indicate that CdS NPs possess peroxide-likectivity and shows good affinity to both TMB and H2O2.

.6. Electrocatalytic activity and amperometric biosensor

The electrocatalytic activity of CdS NPs (HDA) modified GC elec-rode (CdSNPs/GC) was examined by the electrochemical reductionf H2O2. The cyclic voltammogram of modified electrode was car-

◦

ied out in phosphate buffer solution (0.1 M, pH 4) at 40 C. Amall background current is observed for the bare electrode (GC)n buffer medium, while a dramatic increase of current signal isvident for CdSNPs/GC electrode (Fig. 8a), suggesting an excellent

able 3inetic parameters for the peroxidase-like activity of CdS NPs.

Catalyst Substrate Kmapp [mM] Vmax [Ms−1]

CdSTMB 0.0095 3.57 × 10−8

H2O2 3.62 5.6 × 10−8

HRP[22]

TMB 0.434 10.0 × 10−8

H2O2 3.7 8.71 × 10−8

B.

electrochemical property by it. Upon successive addition of H2O2,the cathodic peak current at −0.6 V increases significantly (Fig. 8a),indicating an obvious electrocatalytic reduction of H2O2 and there-fore suggesting the novel sensing application of CdS NPs in sensorsin aqueous solution. The excellent electrochemical behavior of CdSNPs is may be due to the smaller size, larger surface area and supe-rior electron transfer.

For the fabrication of amperometric biosensor, the currentresponse with the concentration of H2O2 at a given applied poten-tial (−0.6 V vs. Ag/AgCl) was studied by the chronoamperometricresponse of CdSNPs/GC electrode in phosphate buffer solution witha scan rate of 0.1 V s−1 at 40 ◦C. The current response of the mod-ified electrode by the successive addition of H2O2 to phosphatebuffer solution is shown in Fig. 8b. The reduction current increasessteeply upon the successive addition of H2O2 and finally reaches asaturation position. The electrode achieves 95% of the steady-state-current within 7 s. This result indicates very fast electrocatalyticresponse of the CdSNPs/GC electrode. The calibration curve (upperleft inset of Fig. 8b) shows that it has a linear relationship in therange of 1.0 �M–1.9 mM, with a correlation coefficient of 0.9998,which is much wider than 1–73 �M for a HRP based electrode [61],1.76–139 �M for NCNT/GC electrode [62], 0.5–150 �M for sheet-like FeS/GC electrode [24] and 0–1.4 mM for Co3O4/GC electrode[29]. From the calibration curve, the sensitivity of the sensor isestimated to be 0.989 mA/mM, and larger than that of the abovemention electrodes fabricated by different researchers. The detec-tion limit of the sensor was also determined and shows 0.28 �M(signal to noise of 3), which is lower than HRP/gold nanoparti-cles/chitosan modified electrode [63], gold nanotube ensembles[64], NCNT/GC electrode [62] and Co3O4/GC electrode [29]. Theabove mentioned result demonstrates an attractive performanceof the proposed H2O2 sensor by CdS NPs modified GC electrode.

The calibration curve for CdSNPs/GC electrode shows that theelectrocatalytic current (icat) increases linearly with successiveaddition of H2O2 and then reaches a steady state position after a cer-tain period. The calibration curve follows typical Michaelis–Mentenmechanism, from which apparent Michaelis–Menten constant(Km

app) can be obtained by using the Lineweaver–Burk model(lower right insets of Fig. 8b). The Km

app value is estimated to

be 1.6 mM. The Km

app for CdSNPs/GC electrode is lower than thatof 2.3 mM for HRP immobilized on a collide/cysteamine modifiedgold electrode [65], free HRP (11 mM), immobilized HRP by sol–gel(4.8 mM) [66] and cytochrome c immobilized on colloidal gold

8 S.K. Maji et al. / Journal of Molecular Catalysis A: Chemical 358 (2012) 1– 9

F ssive ae corres

maww

tattt

4

fmstpsucpopIHalthTCbmna

A

C

[[

[[[

[[[[[[[[

[

[[

[[

[

ig. 8. (a) Cyclic voltammogram of bare GC and CdSNPs/GC electrode upon succelectrode with successive addition of H2O2 to the phosphate buffer solution. Inset:

odified carbon past electrode (2.11 mM) [67], indicating higherffinity of CdSNPs/GC electrode to H2O2 than others. Therefore,e have successfully designed amperometric biosensors for H2O2hich exhibit a comparable high affinity for H2O2.

The stability of the sensor is an important factor for its prac-ical application, which is measured by the effects temperaturend pH on the electrocatalytic activity of CdSNPs/GC electrode. Theemperature dependence curves indicate that the CdSNPs/GC elec-rode has the highest activity at 40 ◦C, while the pH effects showhe maxima at pH 4 (supporting information, Fig. S9).

. Conclusions

In summary, we have successfully made CdS NPs with dif-erent shapes (rod and sphere) and sizes by a simple chemical

ethod from a single-source precursor complex. They posses apecific surface area in the range 32.1–81.9 m2 g−1, which leadso substantially more effective photocatalytic performance com-are to that of Degussa P-25 TiO2. The as prepared CdS NPshow excellent photocatalytic decomposition of RB solution (99%)nder the light irradiation and they are stable enough to be recy-led multiple times. The photodecomposition process follows theseudo-first-order reaction kinetic with maximum rate constantf 2.2 × 10−2 min−1. Furthermore, our synthesized CdS NPs showeroxidase-like activity and the results are similar to that of HRP.

n addition, we have also fabricated amperometric biosensors for2O2. Both the catalytic activities are strongly dependent on pHnd temperature. The kinetic analyses for both the peroxidase-ike activity and amperometric response of CdS NPs indicate theypical Michaelis–Menten kinetics. More importantly, CdS NPs areighly effective as a catalyst with a higher binding affinity to theMB substrate than HRP and also other peroxidase nano-mimetics.onsidering all the experimental results, the nanosized CdS maye potentially effective as photocatalyst for the waste water treat-ent, biocatalyst, biosensors and artificial peroxidase. It should be

oted that cytotoxicity of CdS be taken in consideration for practicalpplication in waste water treatment and as nano-mimetics.

cknowledgements

Authors are thankful to Prof. K. Nag, Department of Inorganichemistry, IACS, Kolkata, India, for helpful discussion. S. K. Maji is

[

[

ddition of H2O2 to the buffer solution. (b) Amperometric response of CdSNPs/GCponding calibration plot and Lineweaver–Burk plot.

indebted to CSIR, India for his SRF fellowship and A. K. Dutta to UGC,India, for his SRF fellowship. We are also acknowledging MHRD(India) and UGC-SAP (India) for providing instrumental facilities tothe Department of Chemistry, BESU, India.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.molcata.2012.03.007.

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