Peroxidase-like behavior, amperometric biosensing of hydrogen peroxide and photocatalytic activity by cadmium sulfide nanoparticles

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

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Peroxidase-like behavior, amperometric biosensing of hydrogen peroxide and photocatalytic activity by cadmium sulfide nanoparticles Swarup Kumar Maji a , Amit Kumar Dutta a , Divesh N. Srivastava b , Parimal Paul b,∗ , Anup Mondal a,∗ , Bibhutosh Adhikary a,∗ a b

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

a r t i c l e

i n f o

Article history: Received 30 November 2011 Received in revised form 3 March 2012 Accepted 6 March 2012 Available online 15 March 2012 Keywords: CdS nanoparticles Single-source precursor Photocatalytic activity Peroxidase-like behavior Hydrogen peroxide sensor

a b s t r a c t A convenient solvothermal route has been developed for the synthesis of CdS nanoparticles (NPs) using a cadmium (II) complex [Cd(ACDA)2 ] of 2-aminocyclopentene-1-dithiocarboxylic acid (HACDA). Decomposition 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 with rod-like shape from EN and spherical shape from HDA or DMSO as solvents, while the optical properties suggest the quantum confinement by the nanoparticles. Superior photocatalytic activity towards the degradation of aqueous Rose Bengal (RB) solution has been achieved with the use of CdS NPs as photocatalyst under light irradiation. CdS NPs is found to possess peroxidase-like activity that can catalyze the oxidation of the peroxidase substrate 3,3 ,5,5 -tetramethylbenzidine (TMB) in the presence of H2 O2 to produce a blue color reaction. CdS NPs anchored on glassy carbon (GC) electrodes have been prepared to study the electrocatalytic reduction of H2 O2 in phosphate buffer solution. This modified electrode has also been used as amperometric biosensor for the detection of H2 O2 . © 2012 Elsevier B.V. All rights reserved.

1. Introduction Nanostructured semiconductors are drawing increasing attention for biomedical and biosensing applications due to their unique physical and chemical properties, which depend on their structures, sizes and shapes [1]. For this reason, preparation of semiconductor nanomaterials with controlled shape and size are of great importance [2]. Considerable efforts have been made to control the nanostructures in past few decades and despite remarkable progress made, it still remains challenge task to researchers to develop simple and reliable methods for fabrication of various semiconductor nanomaterials with controlled morphologies [3]. Dyes that are widely used in textile, photography, coatings etc. are therefore, common industrial pollutants in waste water [4]. Normal aerobic waste water treatment processes not very effective for removal of these toxic chemicals from the environment. In recent years, a new technology termed as advanced oxidation processes are in use for treatment of pollutants in both water and wastewater [5]. To this end, photocatalytic oxidation using semiconductor nanomaterials has turned out to be a promising

∗ Corresponding authors. Tel.: +91 3326684561; fax: +91 3326682916. E-mail addresses: [email protected] (A. Mondal), [email protected] (B. Adhikary). 1381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.molcata.2012.03.007

alternative method for environment water management [6]. The photo-degradation of several toxic compounds using TiO2 as a photocatalyst has been widely studied over the past decade [7,8]. However, the photocatalytic activity of TiO2 is limited to the UV region ( < 400 nm) and therefore is not much effective in the visible region, which is the main component solar light and indoor illuminations [9]. Thus, there is considerable interest in developing visible light sensitive photo-catalyst [10–17]. Designing of biomimetic materials exhibiting peroxidase activities 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 found to show peroxidase-like activity to catalyze oxidation of typical peroxidase substrate [20–26]. For instance, with nanostructure FeS peroxidase-like activity has been reported using 3,3 ,5,5 tetramethylbenzidine as the peroxidase substrate and detection of H2 O2 has been made by employing it as an amperometric sensor [24]. Since, H2 O2 it self is widely used for green chemical oxidation reactions, development of methodologies for detection and quantification of H2 O2 are highly important. Carbon nanotubes, noble metals, macrocyclic complexes of transition metals and protein modified electrodes have been used as amperometric sensors of H2 O2 , although each of them have some disadvantages [27,28]. Very recently semiconductor nanomaterial modified electrode has been used for this purpose [24,29].

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Table 1 Summary of reaction conditions and experimental results of CdS NPs. Solvent (mL)

Temp (◦ C)

Time (min)

Shape

EN (20) HDA (15) + TOP (5) DMSO (20)

120 120 120

15 15 15

Rod Sphere Sphere

a b

Average size (nm) XRD

TEM

4.8 3.4 5.6

D = 5a L = 70b 4 6

Pore diameter (nm)

Surface area (m2 g−1 )

3.1 4.6 2.4

44.8 81.9 32.1

Diameter. Length.

Cadmium sulfide is one of the most important visiblelight-sensitive semiconductor with narrow band gap energy (Eg = 2.41 eV) and has been extensively investigated because its emission in the visible-light range can be tuned by changing the size and shape of the particles [30–32]. CdS nanomaterials with varying morphologies have been prepared recently from single precursor sources by solvothermal route using different solvents and capping agents [33–39]. This method has the advantage of adopting a single-pot procedure under mild condition and the product obtained in this way has fewer defects and better stoichiometry. We report here a simple solvothermal decomposition route for preparing CdS NPs by using the complex Cd(ACDA)2 as the precursor. CdS nanorods and nanoparticles thus obtained have been used for the photocatalytic decomposition of RB and catalytic oxidation of TMB. These NPs have also been used for evaluating their efficiencies for detection/estimation of H2 O2 by electrochemical methods.

hot TOP and then it was added to a 15 mL hot HDA solution in a round bottom flask. The resulting clear yellow solutions were heated at 120 ◦ C for 15 min and then cooled to room temperature. At this stage, 20 mL methanol was added to them. Thus obtained yellow precipitates were centrifuged, followed by washing with ethanol for several times for their purification. The pure nanomaterials were collected after drying in oven at 50 ◦ C for 30 min. The reaction conditions and results are summarized in Table 1. 2.4. Characterization

The chemicals used for the preparation of the ligand 2aminocyclopentene-1-dithiocarboxylic acid (HACDA) and the metal complex [Cd(ACDA)2 ] were of analytical grade. Ethylenediamine (EN), hexadecylamine (HDA), dimethyl sulfoxide (DMSO), tri-octylphosphine (TOP), Rose Bengal (RB), commercial CdS, terephthalic acid (TA), hydrogen peroxide (H2 O2 ) and 3,3 ,5,5 tetramethyl benzidine (TMB) were purchased from Sigma–Aldrich. Standard titanium dioxide (Degussa-P25) was purchased from Degussa Company. Methanol, ethanol, acetic acid, diethyl ether and millipore water were used without any further purification.

Elemental analyses (C, H and N) were performed using PerkinElmer 2400II analyzer. FTIR spectra were recorded on KBr disks using a JASCO FTIR-460-Plus spectrophotometer. Electron spray ionization mass spectroscopic measurements were carried out on a Micromass Qtof YA 263 mass spectrometer in dimethylformamide. X-ray diffraction (XRD) patterns were recorded on a Philips PW 1140 parallel beam X-ray diffractometer using with Bragg–Bretano focusing geometry and monochromatic CuK␣ ˚ Transmission electron microscopy X-radiation ( = 1.540598 A). (TEM) images were collected by using JEOL JEM-2100 microscope working at 200 kV. EDX analyses were carried by using Hitachi S-3400 N (EDS, Horiba EMAX) instrument. N2 -sorption isotherms were obtained using a Quantachrome Instruments adsorption (77 K). UV–vis absorption spectra were recorded with a JASCO V-530 UV–vis spectrophotometer. Photoluminescence measurements were carried out using a Photon Technology International fluorometer. The catalytic oxidation of TMB was monitored spectrophotometrically using an Agilent 8453 diode-array spectrophotometer. Cyclic voltammetric and amperometric measurements were performed on CHI 620D electrochemical analyzer.

2.2. Synthesis of single-source precursor

2.5. Measurements of photocatalytic activity

The ligand (HACDA) was prepared according to the reported method [40]. To a clear methanol solution (10 mL) of HACDA (160 mg, 1 mmol) was added dropwise with stirring to an aqueous solution (10 mL) of cadmium chloride (92 mg, 0.5 mmol). The yellow compound that precipitated was filtered after 15 min and washed first with methanol and then with diethyl ether. Yield: 89% (191 mg), CHN analyses (C12 H16 N2 S4 Cd): Calc.: C, 33.59; H, 3.77; N, 6.53. Found: C, 33.53; H, 3.81; N, 6.51. Selected IR bands: NH2 : 3289 m cm−1 , ıNH2 : 1634s cm−1 , ıCH2 + C C:

The experiments were carried out in a round bottom flask kept in a thermostated bath at 20 ◦ C and an incandescent tungsten halogen lamp (200 W) was placed vertically on the reaction vessel at a distance of ca. 15 cm. The experiments were carried out with 40 mL aqueous solution of RB (3.6 × 10−5 M) and 15 mg of catalysts. Before irradiation, the suspension was magnetically stirred in the dark for 30 min to reach the adsorption–desorption equilibrium. After a given interval of illumination, 3 mL of the aliquot was withdrawn from the suspension and centrifuged. The absorption spectrum of the filtrate solution was then measured in the range 400–650 nm and the peak at 540 nm (max ) was monitored. To test the chemical stability of CdS NPs, it was recycled and reused for five times for the decomposition of RB under same experimental condition. After each photocatalytic test, the aqueous solution was centrifuged to collect CdS NPs, which was then dried at 60 ◦ C and used for the next cycle. In order to find out whether the photodegradation of RB by CdS NPs occur through the generation of hydroxyl radical or not, the commonly used terephthalic acid (TA) photoluminescence probing technique was adopted [11–13]. 40 mL aqueous solution of sodium terephthalate (2 × 10−3 M) containing 15 mg of either TiO2 or CdS

2. Experimental 2.1. Chemicals and materials

1459s cm−1 , C N + C C S S: 1315 cm−1 , C C S S + CN: 1282 m cm−1 ; assym CSS: 902 cm−1 , sym CSS: 623 cm−1 . ESI-MS: [Cd(ACDA)2 + H]+ (m/z = 430.07), [Cd(ACDA)2 + Na]+ (m/z = 451.96) and [Cd(ACDA)2 + K]+ (m/z = 468.24). 2.3. Preparation of CdS NPs

CdS NPs were prepared by solvothermal decomposition of the single-source precursor using EN, HDA or DMSO as the solvents. In case of EN and DMSO, 250 mg of the precursor was dissolved in 20 mL solvent in a round bottom flask. For the preparation of CdS NPs using HDA, 250 mg of the precursor was dissolved in 5 mL

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NPs was irradiated with light for a given period of time. An aliquot (3 mL) of the solution was withdrawn from the suspension and centrifuged and its luminescence spectrum was recorded between 350 and 600 nm using 315 nm as the excitation wavelength. 2.6. Peroxidase-like behavior Peroxidase-like activity of CdS NPs was investigated by the catalytic oxidation of typical peroxidase substrate TMB in presence of H2 O2 . A series of solution mixtures were made by 3 mL sodium acetate buffer (pH 4) with 0.1 mM of TMB and (i) 24 ␮g of CdS NPs from HDA, (ii) 13 mM of H2 O2 , (iii) 13 mM of H2 O2 and 24 ␮g of CdS NPs from EN, (iv) 13 mM of H2 O2 and 24 ␮g of CdS NPs from HDA and (v) 13 mM of H2 O2 and 24 ␮g of CdS NPs from DMSO. The kinetic analyses were carried out by using 24 ␮g of CdS NPs from HDA, a fixed amount of H2 O2 (13 mM), and different amounts (0, 8.3, 16.6, 29.1, 35.4, 41.6, 52, 62.5, 83.3, 104, 125 ␮M) of TMB solution; 24 ␮g of CdS NPs from HDA, a fixed amount of TMB (0.1 mM) and different amounts (0, 6.5, 9.8, 19.5, 22.8, 26, 32, 39 mM) of H2 O2 solution. The kinetic parameters were calculated using the Michaelis–Menten model: V0 = (Vmax × [S])/(Km app + [S]), where V0 is the initial velocity, Vmax is the maximum velocity, [S] is the substrate concentration and Km app is the apparent Michaelis constant. 2.7. Electrocatalytic activity The cyclic voltammograms were obtained using a cell containing 20 mL of 0.1 M phosphate buffer solution (PBS, pH 4.0) with a scan rate of 0.1 V s−1 at 40 ◦ C. In the typical cell setup, platinum wire was used as auxiliary, an Ag/AgCl electrode as reference, with CdS NPs modified GC (3 mm) as a working electrode. Before experiment the buffer solution was degassed by purging with pure N2 for 15 min and a N2 atmosphere was kept over the solution during measurements. The amperometric experiment was carried out by the successive addition of H2 O2 to the buffer solution by applying the potential of −0.6 V (vs. Ag/AgCl). The kinetic parameter, the apparent Michaelis–Menten constant (Km app ) can be calculated using the Lineweaver–Burk equation: 1/Iss = 1/Imax + Km app /(Imax × [S]), where Iss is the steady-state current, Imax the maximum current measured under conditions of enzyme saturation, [S] is the concentration of substrate and Km app is the apparent Michaelis–Menten constant. 3. Results and discussion 3.1. Characterization of single-source precursor The Cd(II) complex of HACDA has been straightforwardly prepared in good yield (89%) by the reaction between a methanol solution of HACDA and an aqueous solution of CdCl2 in the ratio 2:1. The precursor complex is insoluble in water and in common organic solvents, but fairly soluble in dimethyl sulfoxide and dimethylformamide. The composition of the precursor complex was established by elemental analyses, ESI-MS and FTIR spectroscopic methods (supporting information Fig. S1 and 2). 3.2. Structural characterizations of CdS NPs Powder XRD pattern of CdS NPs obtained under different reaction conditions are shown in Fig. 1. The diffraction pattern of nanomaterials is indexed to the pure hexagonal phase of CdS with characteristics (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (1 1 2) and (1 0 5) peaks (JCPDS No. 41-1049). The purity and the composition of the materials were also confirmed from their energy dispersive X-ray (EDX) spectra, which show the signals for of Cd and S 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 to that of the hexagonal phase, however, for nanorods the diffraction peak (0 0 2) is the strongest one. This observation is corroborated by the TEM results and seems to indicate that for the nanorods preferential growth occur along the c axis. The remaining peaks are generally broad and probably indicate relatively small dimensions of the materials. Average crystal diameter for CdS NPs were calculated using the Debye–Scherrer equation (D = 0.9/(ˇ cos )), where, D is the crystallite diameter,  is the wave length of X-ray ˚ ˇ is the value of full width at half maximum and  i.e. 1.540598 A, is the Bragg’s angle. The average crystal diameters were calculated and are found to lie within 3.3–5.6 nm (Table 1). The dislocation density (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 solvents are: 0.043 (EN), 0.092 (HDA) and 0.032 nm−2 (DMSO), and the small magnitude of ı in all cases suggests good crystalline nature of the prepared materials. The values of microstrain (ε), as obtained by using the relation, ε = (ˇ cos )/4 are 7.75 × 10−3 (EN), 9.75 × 10−3 (HDA) and 5.6 × 10−3 (DMSO), which again supports the formation of crystallites of low dimension (that is nanoparticles). The influence of solvents on the morphologies of CdS NPs was further analyzed by TEM measurements. The TEM images illustrated in Fig. 2 show the rod-like morphology with average diameter and length of 5 and 70 nm of CdS (Fig. 2a) obtained from EN, while particles of spherical shapes with average diameter 4 and 6 nm obtained from HDA (Fig. 2b) and DMSO (Fig. 2c), respectively. The solvents used have played the role of attacking nucleophile of the precursor complex as well as capping agents of variable efficiencies [41,42]. For selective growth of a face to obtain anisotropic nanoparticles, structure-directing agents are used [43–45]. In our case, EN seems to serve as a capping agent leaving the (0 0 2) facet to grow by following this approach. On the other hand, the formation of spherical nanoparticles in HDA and DMSO seem to be controlled more by thermodynamic requirement of surface area minimization [46]. The average particle sizes obtained from the TEM measurements are in good agreement with the results obtained from XRD measurements (Table 1). The selected area diffraction (SAED) patterns (supporting information Fig. S4) show a set of concentric rings instead of sharps spots, as a result of the presence of small crystalline nanomaterials. All the diffraction pattern can be readily indexed 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 the polycrystalline nature of the materials. From the high resolution TEM (HRTEM) images (supporting information Fig. S5) lattice fringes of nanocrystals can be seen, which suggests the good

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Fig. 2. TEM images of CdS NPs obtained from (a) EN, (b) HDA and (c) DMSO.

crystalline nature of CdS NPs. The fringe spacing of 0.33, 0.31 and 0.32 nm correspond to the separation of the (0 0 2) and (1 0 1) lattice planes for nanorods and spheres, respectively. Furthermore, the HRTEM images reveal that the nanorods were grown preferentially along the (0 0 2) direction and for the spherical particles, it was along (1 0 1) plane, which is also consistent with the XRD patterns. Brunauer–Emmett–Teller (BET) gas sorptormetry measurements were carried out to get more insight into the porous nature and surface availability sites of CdS NPs. All of the isotherms are identified as type-IV isotherm with an H2-type hysteresis curve (supporting information Fig. S6), confirming the mesoporous structures. The average porosities of three types of nanocrystals were determined from pore-size distribution curves (supporting information Fig. S6 inset). A sharp distribution in pore-size occurs in the mesoporous region. The average pore diameter of all samples were calculated according to the Bopp–Jancso–Heinzinger (BJH) method and found to be in the range of 2.4–4.6 nm, where as 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 NPs prepared from HDA to EN to DMSO, suggesting that nanoparticles prepared from HDA would be the more active material for catalytic applications, since larger specific surface area and higher crystallinity favor the effective catalytic activity of the material.

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

S.K. Maji et al. / Journal of Molecular Catalysis A: Chemical 358 (2012) 1–9 Table 2 Comparison of the kinetic parameters in terms of rate constant. Photocatalyst

Rate constant (min−1 )

CdS from HDA CdS from EN CdS from DMSO TiO2 (Degussa-P25) Commercial CdS

2.2 × 10−2 1.2 × 10−2 6.0 × 10−3 1.7 × 10−3 8.2 × 10−4

show the band edge absorption lie between 445 and 480 nm with a well resolved absorption peak in the range of 420 to 460 nm. The absorption maxima corresponds to the first optically allowed transition between the electron state in the conduction band and the hole state in the valence band, i.e. the first excitonic transition [47]. The excitonic features also suggest the monodispersive nature of the NPs [43–45]. The band gap energy (Eg ) of semiconductor nanomaterials can be evaluated from the Tauc’s plot and are found to be 2.82 (EN), 2.9 (HDA) and 2.73 (DMSO) eV. Compared to bulk CdS (Eg = 2.41 eV), band gap energies of synthesized NPs are blue shifted, due the quantum effect by smaller nanocrystals [48]. The amount of blue shift increases from 0.32 to 0.5 eV as the particles size decreases from 6 to 4 nm. The observation is consistent with the inversely proportional relationship between Eg and the particle size [49]. The average particle size (D) can also be calculated by the following expression [50]. D = −(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 thus obtained are 5.2 (EN), 4.1 (HDA) and 4.6 (DMSO) nm. It should be noted that these valued are in accord with the valued obtained from XRD and TEM. The photoluminescence properties of CdS NPs were investigated at room temperature with the excitation wavelength 375 nm and are shown in Fig. 3. The emission spectra exhibit sharp and strong emission peak between 455 and 492 nm with an additional broad red emission at around 600 nm. The sharp emission is attributed to the core-state radiative decay from conduction band to valence band [49]. The band edge emission peaks are comparable to the absorption line width, with the emission peak red-shifted compared to the maximum wavelength of the adsorption spectrum.

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This shift is the result of a combination of relaxation into trap states and the size distribution [49]. The high intensity of the band-edge emission indicates the high state of crystallinity with few electronic defects and the good dispersity of nanomaterials [51]. The effective passivation of surface states or defects, which are normally associated with semiconducting nanomaterials, is indicated by the sharp luminescence features. The additional broader low intense red emission in the emission spectra at ca. 600 nm corresponds to the recombination of trapped electrons and holes in some surface defect states of CdS NPs [52]. 3.4. Photocatalytic activity To investigate the potentiality of the prepared materials as photocatalyst, the catalytic performances of CdS NPs were examined by the photodegradation of RB under the illumination of light as followed by spectrophotometric monitoring. We have chosen RB to test the photocatalytic activity, since it is a fluorescent dye and commonly used in textile, photographic and photochemical industries. In recent years, a few studies have been made for the photocatalytic decomposition of RB in the presence of semiconductor nanomaterials [53–58], however, to the best of our knowledge no such study has been made using CdS NPs. Fig. 4a shows the time dependent UV–vis spectral changes of RB solution in the presence of CdS NPs (HDA) under the irradiation of light for 200 min. The characteristic peak at 540 nm gradually decreases with irradiation time and disappears completely after 200 min. During the reaction, no new peaks are generated, whereas, the absorption intensities at 350, 305, 255 and 212 nm also decreases, suggesting the complete photodegradation of RB rather than decolorization or bleaching. In order to establish relative performances of the three different CdS NPs as photocatalysts among themselves as well as commercial CdS and TiO2 (Degussa P25) the following comparative studies were made using 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) in dark, (iv) with commercial CdS (15 mg) in light, (v) with commercial 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) with CdS NPs from DMSO (15 mg) in light. As shown in Fig. 4b, the photocatalytic activity decrease in the order CdS (HDA) > CdS (EN) > CdS (DMSO) > TiO2 > CdS (commercial). It may also me noted that the

Fig. 4. (a) Time dependent UV–vis spectral change of RB solution (3.6 × 10−5 M) catalyzed by 15 mg CdS NPs (from HDA) under light irradiation for 200 min, and (b) comparative study for the photodegradation of RB under different conditions: (i) without catalyst in dark, (ii) without catalyst in light, (iii) with CdS NPs from HDA (15 mg) in dark, (iv) with commercial CdS (15 mg) in light, (v) with commercial 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) with CdS NPs from DMSO (15 mg) in light.

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

change in relative concentration of RB with time with added catalyst is nominal in absence of light. The rate constant values for all the systems are given in Table 2. The catalyst (CdS NPs) recovered after complete degradation of RB when reused showed no deterioration of its activity. Indeed, even after five successive recycling no significant change of photocatalytic activity was made. Catalytic conversation reactions are generally correlated with the adsorption and desorption of molecules on the surface of the catalyst. A larger specific surface area allows more reactive molecules to be adsorbed onto the surface of the catalyst [59]. As obtained from BET measurements, CdS NPs from HDA provides greater specific surface area than that of EN to DMSO, which is obviously beneficial for the enhancement of photocatalytic activity and matches well with the result of photocatalytic decomposition of RB. In addition, semiconductor photocatalysis reactions are also related to the crystallinity of the photocatalyst. Higher crystallinity of materials reduces the formation of trap states in the crystals and hence, the number of recombination centers for photogenerated electrons and holes. This allows for higher photocatalytic degradation of RB. Therefore, it can be concluded that the photocatalytic property of CdS NPs is directly related to their structural features. The photocatalytic degradation of RB in aqueous solution has been previously reported and indicates the possibility of involvement of several steps like, (i) photo-absorption of catalyst and dye, (ii) generation of photo-induced electrons and holes, (iii) transfer of charge carriers to the surface and (iv) photodegradation of dye [53–55]. The degradation of the dye may takes place by the generated hydroxyl radicals (• OH) or by the direct participation of photogenerated holes. Therefore, to establish the mechanism of the photodegradation process, the detection of hydroxyl radicals (• OH) were made by the well known photoluminescence technique using terephthalic acid (TA) as a probe molecule [9,11–13]. In this process, a strong fluorescent molecule 2-hydroxylterephthalic acid (HTA) is generated by the capture of • OH by terephthalic acid. We have carried out the detection process using CdS NPs (HDA) and TiO2 as catalyst with the solution sodium terephthalate under light irradiation. Fig. 5 is the photoluminescence spectral changes of TiO2 /TA system after certain time interval of irradiation. A gradual increase in emission intensity at 425 nm is observed with increasing the irradiation time, which indicates the generation of hydroxyl radicals. These hydroxyl radicals are the main active species for the decomposition of dye molecule in presence of TiO2 as catalyst. However, in the case of CdS/TA system, no emission peak is observed (Fig. 5 inset), which suggests that hydroxyl radicals were not produced in

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

the reaction system and hence are not the main active species for the photodecomposition of RB. It appears that the position of the valence band of CdS NPs is less positive than that of OH− /• OH couple (2.7 V vs. SCE) [60], as a result the photogenerated holes on the surface cannot interact with OH− to generate • OH radical. Thus, unlike TiO2 where the dye is decomposed by • OH, in the case of CdS NPs photodegradation of RB seems to be actuated by the photogenerated holes. 3.5. Peroxidase-like activity Peroxidase-like behavior of CdS NPs was examined by the oxidation of TMB as peroxidase substrate in presence of H2 O2 . It is well established that peroxidase can catalyze the oxidation of peroxidase substrate by producing a color change and the color reaction can generally be quenched by adding H2 SO4 (supporting information Fig. S7) [22]. The color of TMB in presence of H2 O2 and CdS NPs turns from colorless to blue, suggesting the catalytic oxidation of TMB. The oxidation of TMB by H2 O2 in presence of CdS NPs as catalyst were carried out at optimized pH 4 and temperature at 40 ◦ C (supporting information Fig. S8). The absorption spectral change of TMB-H2 O2 system catalyzed by CdS NPs (HDA) shows the gradual raise of characteristic peaks at 370 and 652 nm, which are the characteristic peaks for the oxidation product of TMB [22]. Therefore, similar to horseradish peoxidase (HRP), CdS NPs from different sources can quickly catalyze the oxidation of typical HRP substrates like TMB, suggesting the peroxidase-like activity. The comparative experiment for the oxidation of TMB using three types of CdS NPs and H2 O2 are shown in Fig. 6. As shown in Fig. 6, it is observed that neither H2 O2 nor CdS alone can effectively oxidize TMB, which indicates that the interaction between CdS, H2 O2 and TMB are needed for the catalytic reaction process. Among three types of CdS NPs, the one which was obtained from HDA shows the highest activity and then decreases to EN to DMSO, which matches well with the results of photocatalytic decomposition of RB. Similar interpretations in their catalytic activities can be placed here as has been made for RB degradation. The mechanism of the reaction may be explained by the electron transfer from the amino group of TMB to CdS NPs, followed by the electron transfer from the conduction band of CdS NPs to the lowest unoccupied molecular orbital (LUMO) of H2 O2 [26]. The apparent steady-state reaction kinetic parameters by initial rate method were determined to investigate the mechanism of the peroxidase-like activity of CdS NPs. In this case we have

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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 with fixed amount of H2 O2 and (b) varying the concentration of H2 O2 with fixed amount of TMB.

only determined the kinetic parameters of the catalyst prepared from HDA, since it has the highest catalytic activity among them. Apparent steady-state reaction parameters at different concentrations of substrate were obtained by calculating the slopes of initial absorption changes with time. The curves shown in Fig. 7a and b indicate the typical Michaelis–Menten kinetic for the oxidation of TMB by varying the concentration of TMB and H2 O2 , respectively. To obtain the catalytic parameters, the data was fitted to the Michaelis–Menten equation and the resulted kinetic parameters are shown in Table 3. All the parameters were again calculated from the Lineweaver–Burk double-reciprocal plot, which gives analogous result. The double reciprocal plots (1/V0 vs. 1/S0 ) are presented in insets of Fig. 7a and b. The apparent Michaelis constant (Km app ), which is the measure of enzyme affinity for its substrate of CdS NPs with TMB as substrate is 0.0095, while for HRP [22] the value is about 0.434. The Km app value is thus significantly lower than HRP and peroxides nanomimetics reported recently [20–26]. The lowest Km app value of CdS NPs suggests that it has the highest affinity to TMB. On the other hand, the Km app value of CdS NPs with H2 O2 as substrate is about 3.62, which is also slightly lower than HRP and reported peroxides nano-mimetics, suggesting the highest affinity to H2 O2 [20–26]. Therefore, our results indicate that CdS NPs possess peroxide-like activity and shows good affinity to both TMB and H2 O2 . 3.6. Electrocatalytic activity and amperometric biosensor The electrocatalytic activity of CdS NPs (HDA) modified GC electrode (CdSNPs/GC) was examined by the electrochemical reduction of H2 O2 . The cyclic voltammogram of modified electrode was carried out in phosphate buffer solution (0.1 M, pH 4) at 40 ◦ C. A small background current is observed for the bare electrode (GC) in buffer medium, while a dramatic increase of current signal is evident for CdSNPs/GC electrode (Fig. 8a), suggesting an excellent Table 3 Kinetic parameters for the peroxidase-like activity of CdS NPs. Catalyst CdS HRP [22]

Substrate

Km app [mM]

Vmax [Ms−1 ]

TMB H2 O2 TMB H2 O2

0.0095 3.62 0.434 3.7

3.57 × 10−8 5.6 × 10−8 10.0 × 10−8 8.71 × 10−8

electrochemical property by it. Upon successive addition of H2 O2 , the cathodic peak current at −0.6 V increases significantly (Fig. 8a), indicating an obvious electrocatalytic reduction of H2 O2 and therefore suggesting the novel sensing application of CdS NPs in sensors in aqueous solution. The excellent electrochemical behavior of CdS NPs is may be due to the smaller size, larger surface area and superior electron transfer. For the fabrication of amperometric biosensor, the current response with the concentration of H2 O2 at a given applied potential (−0.6 V vs. Ag/AgCl) was studied by the chronoamperometric response of CdSNPs/GC electrode in phosphate buffer solution with a scan rate of 0.1 V s−1 at 40 ◦ C. The current response of the modified electrode by the successive addition of H2 O2 to phosphate buffer solution is shown in Fig. 8b. The reduction current increases steeply upon the successive addition of H2 O2 and finally reaches a saturation position. The electrode achieves 95% of the steady-statecurrent within 7 s. This result indicates very fast electrocatalytic response of the CdSNPs/GC electrode. The calibration curve (upper left inset of Fig. 8b) shows that it has a linear relationship in the range 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 sheetlike FeS/GC electrode [24] and 0–1.4 mM for Co3 O4 /GC electrode [29]. From the calibration curve, the sensitivity of the sensor is estimated to be 0.989 mA/mM, and larger than that of the above mention electrodes fabricated by different researchers. The detection limit of the sensor was also determined and shows 0.28 ␮M (signal to noise of 3), which is lower than HRP/gold nanoparticles/chitosan modified electrode [63], gold nanotube ensembles [64], NCNT/GC electrode [62] and Co3 O4 /GC electrode [29]. The above mentioned result demonstrates an attractive performance of the proposed H2 O2 sensor by CdS NPs modified GC electrode. The calibration curve for CdSNPs/GC electrode shows that the electrocatalytic current (icat ) increases linearly with successive addition of H2 O2 and then reaches a steady state position after a certain period. The calibration curve follows typical Michaelis–Menten mechanism, 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 that of 2.3 mM for HRP immobilized on a collide/cysteamine modified gold electrode [65], free HRP (11 mM), immobilized HRP by sol–gel (4.8 mM) [66] and cytochrome c immobilized on colloidal gold

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

Fig. 8. (a) Cyclic voltammogram of bare GC and CdSNPs/GC electrode upon successive addition of H2 O2 to the buffer solution. (b) Amperometric response of CdSNPs/GC electrode with successive addition of H2 O2 to the phosphate buffer solution. Inset: corresponding calibration plot and Lineweaver–Burk plot.

modified carbon past electrode (2.11 mM) [67], indicating higher affinity of CdSNPs/GC electrode to H2 O2 than others. Therefore, we have successfully designed amperometric biosensors for H2 O2 which exhibit a comparable high affinity for H2 O2 . The stability of the sensor is an important factor for its practical application, which is measured by the effects temperature and pH on the electrocatalytic activity of CdSNPs/GC electrode. The temperature dependence curves indicate that the CdSNPs/GC electrode has the highest activity at 40 ◦ C, while the pH effects show the maxima at pH 4 (supporting information, Fig. S9).

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 to the Department of Chemistry, BESU, India. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.molcata.2012.03.007. References

4. Conclusions In summary, we have successfully made CdS NPs with different shapes (rod and sphere) and sizes by a simple chemical method from a single-source precursor complex. They posses a specific surface area in the range 32.1–81.9 m2 g−1 , which leads to substantially more effective photocatalytic performance compare to that of Degussa P-25 TiO2 . The as prepared CdS NPs show excellent photocatalytic decomposition of RB solution (99%) under the light irradiation and they are stable enough to be recycled multiple times. The photodecomposition process follows the pseudo-first-order reaction kinetic with maximum rate constant of 2.2 × 10−2 min−1 . Furthermore, our synthesized CdS NPs show peroxidase-like activity and the results are similar to that of HRP. In addition, we have also fabricated amperometric biosensors for H2 O2 . Both the catalytic activities are strongly dependent on pH and temperature. The kinetic analyses for both the peroxidaselike activity and amperometric response of CdS NPs indicate the typical Michaelis–Menten kinetics. More importantly, CdS NPs are highly effective as a catalyst with a higher binding affinity to the TMB substrate than HRP and also other peroxidase nano-mimetics. Considering all the experimental results, the nanosized CdS may be potentially effective as photocatalyst for the waste water treatment, biocatalyst, biosensors and artificial peroxidase. It should be noted that cytotoxicity of CdS be taken in consideration for practical application in waste water treatment and as nano-mimetics.

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Acknowledgements [29]

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

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