Physical and electrochemical characterization of CdS hollow microspheres prepared by a novel template free solution phase method

Electrochimica Acta 56 (2010) 501–509

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Physical and electrochemical characterization of CdS hollow microspheres prepared by a novel template free solution phase method S. Rengaraj a,∗ , A. Ferancova a,b , S.H. Jee c , S. Venkataraj d , Y. Kim c , J. Labuda b , M. Sillanpää a,e a

University of Eastern Finland, Laboratory of Applied Environmental Chemistry (LAEC), Patteristonkatu 1, FI-50100 Mikkeli, Finland Institute of Analytical Chemistry, Slovak University of Technology in Bratislava, 81237 Bratislava, Slovakia Department of Chemical Engineering, Kwangwoon University, Wolgye, Nowon, Seoul 139-701, Republic of Korea d Crystal Growth Centre, Anna University, Chennai 600025, India e LUT Faculty of Technology, Lappeenranta University of Technology, Patteristonkatu 1, FI-50100 Mikkeli, Finland b c

a r t i c l e

i n f o

Article history: Received 29 July 2010 Received in revised form 6 September 2010 Accepted 6 September 2010 Available online 21 September 2010 Keywords: CdS microsphere Cyclic voltammetry Electrochemical impedance spectroscopy Electrochemical behavior Charge transfer resistance DNA immobilization

a b s t r a c t Novel CdS hollow microspheres have been successfully synthesized via a facile template-free solutionphase reaction from cadmium nitrate and thioacetamide precursors. The morphology of CdS hollow microspheres depends strongly on the ratio between the precursors, cadmium nitrate to thioacetamide ratio. The physical properties of the hollow microspheres have systematically been studied by different characterization methods. The stoichiometry of the hollow microspheres studied by the energy dispersive X-ray diffraction spectroscopy confirmed that the synthesized CdS hollow microspheres are nearly stoichiometric bulk like CdS. The morphology of the hollow microspheres studied by high resolution scanning electron microscopy and transmission electron microscopy observations showed that the CdS hollow microspheres of the size of 2.5 ␮m have hollow structure and are constructed by several nanoparticles of the size between 30 and 40 nm. The UV–visible diffuse reflectance spectroscopy studies showed that the band gap of the CdS hollow microspheres increased while increasing the cadmium nitrate to thioacetamide ratio. Further electrochemical characterization of CdS hollow microspheres was performed with glassy carbon electrode (GCE) after its chemical modification by CdS dispersed in dimethylformamide. The electrochemical studies showed that with decreasing the band gap energy the electron transfer resistance of CdS/GCE was also found decreased. Moreover, electrochemical impedance spectroscopic measurements showed enhanced DNA adsorption onto CdS/GCE in comparison to GCE. These experiments demonstrate that the CdS hollow microspheres act as an efficient electrode modifier that effectively decreased the charge transfer resistance and capacitance of the modified sensors, which can be used for electroanalytical purposes. © 2010 Elsevier Ltd. All rights reserved.

1. Introduction Nanomaterials have received considerable attention due to their potential applications in modern world. Recently, various nanomaterials with different morphologies such as particle, wire, and tube have been prepared [1–3]. Owing to peculiar and fascinating properties superior to their bulk counterparts, these nanomaterials are widely employed in nanoscale electronics, optoelectronic, electrochemical, and electromechanical devices [4,5]. Among various nanomaterials, II–VI binary compound semiconductors, cadmium chalcogenide materials have increasingly attracted interest because of their superior optical, photoluminescence, photosensitization and photocatalytic properties [6–13].

∗ Corresponding author. Tel.: +358 40 355 3705; fax: +358 15 33 6013. E-mail addresses: rengaraj.selvaraj@uef.fi, [email protected] (S. Rengaraj). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.09.019

It is known that semiconductor nanoparticles have a good biocompatibility that enable them to use in combination with biomolecules [14]. For example, cadmium sulfide (CdS) has found many applications in electroanalytical chemistry as label tags for DNA and oligonucleotides to detect the hybridization event [15], or as substrate for enhancement of enzyme immobilization [16] and electron transfer reactivity [17]. Excellent electrocatalytic activity of nano-CdS was observed using cyclic voltammetry [18]. However, a detailed electrochemical characterization of this material is not understood very well. The material properties greatly depend on their morphological features. Recently, nanostructured CdS with different sizes, shapes, and dimensionality has been fabricated and characterized by different groups. Especially, CdS in the form of microor nanometer size hollow sphere with considerable structure, composition, and properties has shown promising applications in many fields such as in drug delivery, photonic crystal, light filler, shape-selective adsorbent and catalyst [19–25]. A variety of meth-

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ods have been developed to prepare CdS nanostructures such as solid phase reaction [26], gas phase reaction with H2 S or sulfur vapour, solvothermal route [27], sol–gel template [28], an in situ micelle–template-interface reaction route [29], ion beam synthesis [30], ultrasonic irradiation in an aqueous solution [31], one-pot synthesis [32], solution phase [33] and two-phase approach [34], etc. It has been demonstrated that solution phase method is an effective method to prepare low-dimensional nanomaterials of sulfides [32,35]. The potential applications of CdS have motivated us to prepare the CdS hollow microspheres by a simple and easily reproducible method and study their properties. In this paper, we report a new solution phase, an easily controllable template free synthesis route to prepare CdS hollow microspheres by one-step method. The physical and electroanalytical properties of the CdS hollow microspheres have been studied systematically. Since, CdS is an important candidate for sensor applications the electrochemical properties of CdS hollow microspheres were investigated using common electroanalytical methods and the surface coverage of the CdS hollow microsphere modified electrodes with dsDNA have also been reported. 2. Experimental 2.1. Synthesis of CdS microsphere The CdS hollow microspheres were synthesized by using analytical grade cadmium nitrate (Cd(NO3 )2 ·4H2 O) (Sigma–Aldrich 99%) and thioacetamide (C2 H5 NS) (Sigma–Aldrich 99%) without further purification. In a typical synthesis, 3.00 g of Cd(NO3 )2 ·4H2 O and 1.00–3.00 g of thioacetamide (TAA) were dissolved in 100 ml of deionized water and continuously stirred for 30 min to form a clear solution. Here the TAA served as the S2− source to form CdS. The solution was then transferred into a flask and then refluxed at ∼105 ◦ C for 30 min which yielded a yellow colour precipitate. The yellow precipitate was harvested by centrifugation and washed several times using deionized water and ethanol to remove the possible remaining cations and anions before being dried in oven at 70 ◦ C for 24 h. After drying, the products were calcined at 350 ◦ C for 2 h for further characterization purpose. As mentioned above, the hollow CdS microspheres were synthesized by refluxing the aqueous solution of the reagents at 105 ◦ C. According to the Ostwald ripening mechanism, the spherical morphology might have evolved by the oriented aggregation of the formed CdS primary nanocrystals [36]. From the experiments, we understood that the size and morphology of the hollow microspheres mainly depend on the precursor ratio. The probable reaction sequence for the formation of CdS hollow microspheres can be summarized as follows: CH3 CSNH2 + H2 O → CH3 CONH2 +H2 S +

−

+

H2 S → H + HS → 2H + S 2+

Cd

+S

2−

2−

→ CdS(Nanoparticle)

CdS(Nanoparticle) → CdS(Microsphere)

(1) (2)

electrode was coated by 2 ␮l of CdS dispersion and then dried at room temperature conditions. Final sensors were denoted as CdS 3:1/GCE, CdS 3:2/GCE, and CdS 3:3/GCE. The surface modification with calf thymus dsDNA (Sigma–Aldrich) was realized by casting 4 ␮l of dsDNA solution (5 mg ml−1 ) in 0.1 M phosphate buffer solution of pH 7.0 onto the surface of the sensors or GCE and dried at room temperature. 2.3. Characterization of CdS hollow microsphere The crystalline properties of the synthesized CdS hollow microspheres were studied by X-ray powder diffraction method using a Bruker (D5005) X-ray diffractometer equipped with graphite ˚ An accelerating monochromatized CuK␣ radiation ( = 1.54056 A). voltage of 40 kV and emission current of 30 mA conditions were adopted for the measurements. The elemental analysis of the hollow microspheres was performed by an energy dispersive X-ray spectrometer (EDX) attached to a Hitachi S-4800 high resolution field emission scanning electron microscope (HR-SEM). The same HR-SEM was also used to study the morphology of the hollow microspheres. The morphology of the hollow microspheres was also studied by transmission electron microscope (TEM-3010 JEOL). The absorption spectrum of the samples in the diffused reflectance spectroscopy (DRS) spectrum mode was recorded in the wavelength range between 200 and 1000 nm using a spectrophotometer (Jasco – V670), with BaSO4 as reference. The electrochemical properties of the CdS hollow microspheres were studied by cyclic voltammetry. The voltammetric measurements were performed with the Autolab potentiostat and software GPES (Eco Chemie, Netherlands). Three-electrode system was used consisting of glassy carbon electrode (GCE) with geometric surface area 7.1 mm2 , Ag/AgCl reference electrode (Ag/AgCl 3 mol L−1 KCl) and Pt counter electrode. Cyclic voltammetric measurements at prepared electrodes were performed using 1 × 10−3 M Ru(NH3 )6 Cl3 (Sigma–Aldrich) in 0.1 M KCl (Merck), 1 × 10−3 M K3 [Fe(CN)6 ] (Sigma–Aldrich) in 0.1 M KCl and 1 × 10−4 M hydroquinone (Sigma–Aldrich) in 0.5 M acetic buffer solution (ABS) of pH 5.0 in appropriate potential range using different scan rates. ABS was prepared from acetic acid (Merck) and sodium acetate (Merck). Other solutions used as supporting electrolytes in electrochemical characterization were 0.1 M NaOH (Sigma–Aldrich), 0.1 M H2 SO4 (Merck), and 0.1 M phosphate buffer solution (PBS) prepared from Na2 HPO4 (Riedel-de Haën) and NaH2 PO4 (Merck). The reduction of Cd(II) in CdS modifier layer and following oxidation was observed using square-wave voltammetry at 20 Hz, with a scan rate of 250 mV s−1 and amplitude of 25 mV. The electrochemical impedance spectroscopic measurements were carried out with the same device as voltammetric measurements using impedance module and software FRA for 0.0 V potential and over a frequency range between 10 kHz and 0.1 Hz for 10 mV amplitude. After every measurement the electrode surface was washed carefully with deionized water.

(3) (4)

The reactions (3) and (4) have been clearly observed and confirmed by HR-SEM, which we will discuss in the following Section 3.3. 2.2. Preparation of glassy carbon electrode modified with CdS hollow microspheres and dsDNA In order to perform the electrochemical analysis, 7 mg of CdS 3:1, CdS 3:2, and CdS 3:3 hollow microspheres were dispersed in 1 ml of dimethylformamide (Sigma–Aldrich) and sonicated for 10 min to obtain a homogeneous mixture. The surface of a glassy carbon

3. Results and discussion 3.1. X-ray diffraction The powder X-ray diffraction patterns of CdS hollow microspheres prepared with different cadmium nitrate and thioacetamide ratio aspects are shown in Fig. 1. The experimental diffraction peaks were carefully compared to the theoretical peak positions and the miller indices were indexed. The observed peaks positions perfectly matched to the cubic phase of cadmium sulfide (JCPDS card no.: 80-0019). The sample CdS 3:1 showed diffraction peaks correspond to pure cubic crystalline phase. Where as

S. Rengaraj et al. / Electrochimica Acta 56 (2010) 501–509

100

Intensity Cps.

(222)

(311)

(220)

(200)

(111)

(c) 3:3

50 H

25

S

1000

0

Intensity Cps.

1200

CdS: Cubic phase JCPDS card No: 80-0019

50

503

Cd

C

800 600

Cd 400 200

0

(b) 3:2

50

0 0

25

1

2

3 4 Energy (eV)

5

6

7

Fig. 2. EDX spectrum of CdS microsphere.

0

(a) 3:1 50

ratio, i.e., while increasing the cadmium nitrate to thioacetamide ratio the grain size has also found increased.

25

3.2. Stoichiometry of the CdS hollow microspheres

0 20

25

30

35

40 45 2θ (°)

50

55

60

Fig. 1. XRD pattern of CdS microspheres prepared with different cadmium nitrate and thioacetamide ratios: (a) 3:1, (b) 3:2 and (c) 3:3.

for the samples CdS 3:2 and CdS 3:3, two weak diffraction peaks at 24.9◦ and 28.3◦ have been observed, indicating the existence of some hexagonal cadmium sulfide crystals in the final product. The grain sizes of the synthesized CdS hollow microspheres have been calculated from the FWHM (full width at half maximum) of the diffraction peaks using the Debye–Scherrer formula [37]. The grain size values calculated from the XRD analysis reveal that the average grain size of the CdS hollow microspheres initially possesses ∼20 nm (for sample CdS 3:1) and increased up to ∼35 nm (for sample CdS 3:3) with increasing cadmium nitrate and thioacetamide

The elemental composition of the CdS hollow microspheres is determined by EDX and is shown in Fig. 2. The EDX measurements showed three intense peaks at very closer interval between 3.0 and 3.6 keV, these three intense peaks corresponds to cadmium L␣1 (3.14 keV), L␤1 (3.35 keV) and L␤2 (3.54 keV). The strong peak observed at 2.3 keV corresponds to sulfur K␣1 (2.3 keV) peak. The EDX measurement confirmed the presence of Cd and S elements in the microsphere. The EDX measurement also showed a strong peak at 0.2 keV corresponding to carbon (C) and it can be attributed to the adsorbed C on the surface of the CdS hollow microspheres due to their exposure to the atmosphere. The quantitative analysis of these samples indicated that the atomic ratio of Cd and S in the samples is very closer to the stoichiometry of the bulk like CdS. The stoichiometry values of all three samples are listed in Table 1. From this table it is clear that within the experimental error bar limits,

Fig. 3. SEM images of CdS microspheres (sample CdS 3:2). The high magnification images of the microsphere reveal that a microsphere is constructed by several nanospheres.

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Table 1 Chemical stoichiometry of CdS hollow microsphere calculated from EDX. Sample

CdS 3:1 CdS 3:2 CdS 3:3

Atomic % Cd

S

48.51 50.00 49.93

51.49 50.00 50.07

the stoichiometry of all samples is similar to each other and is independent to the ratio between cadmium nitrate and thioacetamide in the starting precursor solution. Even though we changed ratio of the starting materials, once after reaching the chemical equilibrium state to form CdS the increase of thioacetamide does not have any influence to change the stoichiometry. 3.3. Morphology of the CdS hollow microspheres The morphology of the CdS hollow microspheres was studied by both SEM and TEM observations. The SEM pictures recorded for the sample CdS 3:2 are shown in Fig. 3. The observed spherical morphology is same for all the samples CdS 3:1 to CdS 3:3. From the SEM images it is clear that the synthesized CdS products are composed of micro- and submicrospheres. But the sizes of these hollow microspheres are not very uniform. The high magnification image (Fig. 3b) of a microsphere reveals that the maximum diameter of the microsphere is around 2.5 ␮m. A further magnification images of these hollow microspheres reveal that the microsphere is constructed by thousands of nanoparticles (please refer Fig. 3c and d) and the size of these nanoparticles tends to be very uniform approximately 30–40 nm and this value is in very closer agreement to the grain size values calculated from XRD measurements. Similar observations have also been reported by Zhang et al. [38] for CdS hollow microspheres prepared by self-assembly method. The HR-TEM characterization has also been performed to confirm the morphology of the CdS hollow microspheres. Typical HR-TEM image recorded for the sample CdS 3:1 is shown in Fig. 4. From this figure it is clear that the CdS microsphere possesses a hollow shell structure (Fig. 4a) and the diameter of typical sphere is found to be around 2.5 ␮m. The magnified image of the shell wall reveals that the wall is constructed by several nanoparticles (Fig. 4b) and the typical wall thickness varies between 250 and 300 nm. Similar hollow structures have been observed for all samples prepared with different cadmium nitrate to thioacetamide ratios. The morphological studies clearly showed that the varia-

tion of cadmium nitrate to thioacetamide ratio did not change the hollow structure of the CdS hollow microspheres. From the SEM and TEM analysis it has been observed that the precursor ratio plays a crucial role to determine the spherical morphology, which eventually determines the size and shape of the microspheres. The formation of the CdS microspheres with hollow interior could be explained based on the Ostwald ripening process. It is well known that the ripening is generally observed in crystal growth and involves ‘growth of larger crystals from those of smaller size which have a higher solubility than the larger ones’ [39]. Recently, several hollow structures such as TiO2 [40], Cu2 O [41] and ZnS [42] have been prepared based on this well established phenomenon. The probable reaction sequences were discussed in Section 2 (refer Eqs. (1)–(4)). As soon as reaction (3) takes place, i.e., after forming the CdS nanocrystals, due to the minimization of the total energy of the system the nanocrystals would aggregate together and form solid spheres. At this stage in a newly formed sphere, compared to the crystallites in the interior the larger crystallites which are located in the exterior of the spheres in general are much loosely packed. The Ostwald ripening process will happen during this stage because smaller, less crystallized, or less dense particles in an aggregate will dissolve gradually. Simultaneously the larger, better crystallized, or denser particles in the same aggregate used to grow. Therefore the inner small crystallites of a sphere used to undergo for a mass relocation through dissolving and regrowing, whereas the outer larger ones serve as new growth sites. Due to the continuous mass transportation from the inner core to the outer surface of the same sphere, it would create a sphere with hollow interior through reaction (4), which is confirmed by SEM and TEM measurements. 3.4. UV–vis diffuse reflectance studies The optical properties of the CdS hollow microspheres were studied by UV–vis diffuse reflectance spectroscopy (DRS). The DRS spectrum and their corresponding Kubelka–Munk function of absorption curves of the samples are shown in Fig. 5. From this figure it can be seen that the absorption edge of the CdS hollow microspheres was found around 570 nm, corresponding to the absorption edge of a semiconductor material. Also it is interesting to note that the absorption edges of these samples are very close to each other. Furthermore, the steep absorption edge is an indication for a narrow size distribution and uniform crystallites of CdS hollow microspheres [43]. By extrapolating the absorption edge by linear fit method the band gap values of the samples were calculated and are listed in Table 2. From this table it is clear that the band gap values increase slightly from 2.15 to 2.16 eV.

Fig. 4. High magnification bright field TEM image of (a) CdS microsphere reveals the hollow shell like structure, and (b) magnified image of the microsphere wall reveals that the wall is constructed by several nanoparticles.

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505

current (A)

Band gap increase

(a)

0.0

(a)

CdS-3:1 CdS-3:2 CdS-3:3

1.2 Kubelka - Munk function of absorption

1.0

-5

-1.0x10

0.8

0.0

0.7

potential (V)

0.6 (b)

0.4

(b)

0.2 0.0 300

400

500 600 700 wavelength (nm)

800

900

-6

3.0x10

current (A)

Kubelka-Munk function (a.u)

Diffused Reflection

-5

1.0x10

CdS3:1 CdS3:2 CdS3:3

0.0

Fig. 5. (a) UV–vis DRS spectrum of CdS microspheres, from this figure it is clear that the absorbance edge shows a blue shift is an indication for increase of the band gap. (b) Absorbance of the samples treated by the Kubelka–Munk function.

3.5.1. Investigation by cyclic voltammetry Electrochemical behavior of three different redox active systems, [Ru(NH3 )6 ]3+ /[Ru(NH3 )6 ]2+ , [Fe(CN)6 ]3− /[Fe(CN)6 ]4− , and hydroquinone/quinone (H2 Q/Q) at CdS modified GCE was investigated using cyclic voltammetry and Fig. 6 shows respective voltammograms. Cyclic voltammograms of the redox system [Fe(CN)6 ]3− /[Fe(CN)6 ]4− show well developed redox peaks at all electrodes under investigation (Fig. 6a). The peak potential separations (Ep ) (Table 3) decrease in the following order: CdS 3:3/GCE > CdS 3:2/GCE > CdS 3:1/GCE. The increasing of the peak potential separation with the scan rate was also observed for all the studied electrodes. Dependence of peak current on scan rate showed that cathodic peak current depends linearly on the square root of the scan rate with correlation coefficients very close to 1.0 for all electrodes under study. These results indicate the diffusion limited process. Hydroquinone/quinone is reported as redox system exchanging two electrons and depending strongly on the material of the electrode [44]. Fig. 6b shows that the highest CV peak current and the most reversible behavior was observed for the CdS 3:2/GCE. For the all electrodes well developed peaks have been observed. In the case of hydroquinone/quinone redox system, we observed the Ep (Table 3) decreases in the order of CdS 3:1/GCE > CdS 3:2/GCE > CdS 3:3/GCE. Similarly to previous redox system ([Fe(CN)6 ]3− /[Fe(CN)6 ]4− ) the increasing of the peak potential separation with the scan rate was also observed for all the studied electrodes. Investigations of dependence of anodic peak Table 2 Band gap values of CdS microspheres calculated from DRS data. Sample

Wavelength (nm)

Bang gap (eV)

CdS 3:1 CdS 3:2 CdS 3:3

575.114 574.202 573.199

2.156 2.159 2.163

-6

-3.0x10

0.0

potential (V)

0.6

-5

1.0x10

(c)

current (A)

3.5. Electrochemical characterization of CdS hollow microspheres immobilized at the GCE surface

CdS-3:1 CdS-3:2 CdS-3:3

0.0

-5

CdS 3:1/GCE CdS 3:2/GCE CdS 3:3/GCE

-1.0x10

-0.5

0.0

potential (V) Fig. 6. Cyclic voltammograms of 1 × 10−3 M [Fe(CN)6 ]3− in 0.1 M KCl (a), 1 × 10−4 M H2 Q in 0.5 M ABS (b), and 1 × 10−3 M [Ru(NH3 )6 ]3+ in 0.1 M KCl (c) measured at CdS modified GCE at scan rate of 50 mV s−1 .

current on square root of scan rate showed linear dependence with correlation coefficients very close to 1.0 for all electrodes under study indicating diffusion limited process. In the case of [Ru(NH3 )6 ]3+ /[Ru(NH3 )6 ]2+ system, well developed CV peaks were also observed, however, in this case no significant difference in electron transfer was observed for modified electrodes (Fig. 6c). All electrodes show the most reversible behavior towards the ruthenium redox system in comparison to previous redox systems. For all the modified electrodes used for this study the peak potential separation was independent on scan rate with value of 73 mV, which is close to a value of Ep = 57/n mV (n is the number of electrons transferred per molecule reacting at the electrode surface) typical for reversible process under planar diffusion conditions [45]. Value |Ep − Ep/2 | = 56.6/n mV is considered

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Table 3 ˇ cik Peak potential separations (Ep ) obtained from CV scans at scan rate of 50 mV s−1 , effective surface area (Aeff ) of studied electrodes calculated according to Randles–Sevˇ equation as well as the surface density of CdS at electrode surface ( ). Modifier

CdS 3:1 CdS 3:2 CdS 3:3

K3 [Fe(CN)6 ]

[Ru(NH3 )6 ]Cl3

Ep (mV)

Aeff (mm2 )

Ep (mV)

Aeff (mm2 )

Ep (mV)

Aeff (mm2 )

252 310 430

3.0 3.2 3.3

73 73 73

6.5 7.4 7.3

356 264 191

3.6 3.5 3.7

as another criterion of the reversibility for linear sweep and cyclic voltammograms [46]. This condition is fulfilled in the case of ruthenium complex for all electrodes (53 mV) in contrast to ferricyanide (145 mV) or quinone (80 mV) redox systems. The linear dependence of the peak current on square root of the scan rate indicating the diffusion limited process was also found. From these results we can conclude that CdS hollow microspheres modified electrodes show higher affinity towards the positively charged ruthenium complex at given experimental conditions. For [Fe(CN)6 ]3− /[Fe(CN)6 ]4− and H2 Q/Q redox systems we observed shifting of the peak potential indicating the kinetic limitation of the electrochemical reactions. For the evaluation of effective electrode area (Aeff ), the depenˇ cik equation [46] (5) were dence Ip vs. 1/2 and the Randles–Sevˇ used. Ip = 2.687 × 105 n3/2 D1/2 1/2 Aeff c

(5)

where Ip is peak current (A), n is number of electrons exchanged, D is diffusion coefficient (7.63 × 10−6 cm2 s−1 for [Fe(CN)6 ]3− in 0.1 M KCl, 8.06 × 10−6 cm2 s−1 for [Ru(NH3 )6 ]3+ in 0.1 M KCl, and 7.64 × 10−6 cm2 s−1 for H2 Q in 0.5 M ABS) [47,48],  is scan rate (V s−1 ), Aeff is effective electrode area (cm2 ) and c is concentration of electroactive species (mol cm−3 ) and the calculated values of Aeff are given in Table 3. Effective surface areas obtained from CV of [Fe(CN)6 ]3− and H2 Q are very similar, however, effective surface areas obtained from CV of [Ru(NH3 )6 ]3+ are approximately two times higher. From these results we can conclude that the CdS microsphere modified electrodes have higher affinity to a positive charged ruthenium complex. The basic electrochemical behavior of the GCE modified with CdS 3:1, CdS 3:2, and CdS 3:3 hollow microspheres was studied by using cyclic voltammetry in different supporting electrolytes. A cathodic signal, which can correspond to CdS/Cd0 couple [18,49] was observed at potentials between −0.9 V and −1.0 V depending on supporting electrolyte. Similarly to this, small anodic stripping peak at the reverse scan corresponding to the stripping of Cd0 was observed at potential between −0.75 V and −0.95 V. Both, cathodic peaks and anodic counter peaks were identified after addition of CdCl2 into supporting electrolyte. Similar cathodic and anodic peaks were also reported for KH2 PO4 [18] and HCl [50] used as supporting electrolyte. The dependence of CV cathodic peak current on scan rate was used to estimate the surface density of the CdS modifiers by using the following Eq. (6) [51]: Ipc =

n2 F 2 A 4RT

 (mol cm−2 )

H2 Q

(6)

where Ipc is cathodic peak current (A), n is number of electrons exchanged, R is the gas constant, T is the absolute temperature, F denotes the Faraday constant, A is geometric electrode area (cm2 ),  is surface density (mol cm−2 ) and  is scan rate (V s−1 ). Calculated values of surface density are given in Table 3 and the highest value of  has been obtained for CdS 3:1/GCE. Increased surface density of CdS enhanced the electroactivity of the modified electrode towards [Fe(CN)6 ]3− and found decreased for hydroquinone. Cyclic voltammetric response of [Fe(CN)6 ]3− was used to evaluate the stability of electrodes modification and repeatability of their

6.431 × 10−9 4.129 × 10−9 2.325 × 10−9

preparation. The CV signal was measured within 10 days with the same sensor. Significant decrease of the signal was observed after three days. In order to avoid the experimental errors that could happen due to the damaged modifier layer, new electrodes were prepared freshly for experiments. The working stability characterized by RSD value and determined from CV response of [Fe(CN)6 ]3− with the same sensor was 7.2% (n = 10). The repeatability of the electrode preparation was determined as RSD value for CV response of [Fe(CN)6 ]3− obtained with 10 newly prepared modified electrodes and varied from 12% to 15% for CdS 3:1/GCE, CdS 3:2/GCE, and CdS 3:3/GCE, respectively. 3.5.2. Investigation by square-wave voltammetry The behavior of the CdS hollow microsphere modified electrodes in different electrolytes (ABS, NaOH, KCl, H2 SO4 , and PBS) was characterized by using square-wave voltammetry (SWV). Optimum scan rate with respect to a peak shape was found to be 250 mV s−1 for both the cathodic and anodic scans. In the case of KCl, no cathodic and anodic peaks were observed. However, in the case of ABS small cathodic peak at −1.05 V for CdS 3:3/GCE and anodic peaks at the potential of −0.8 V for CdS 3:2/GCE and CdS 3:3/GCE were found. For NaOH electrolyte, the cathodic peak (−1.1 V) was found only for CdS 3:1/GCE while anodic peak (−0.9 V) was found for CdS 3:3/GCE. These results are not presented here due to the fact that they cannot be used for comparison of behavior of all the modified electrodes. Fig. 7 shows cathodic as well as anodic SWV scans obtained for H2 SO4 and PBS electrolytes. In the case of H2 SO4 no cathodic peak related to CdS/Cd0 couple was observed, probably because of rapid increasing of current at more negative potentials due to a hydrogen evolution which can overlap reduction signals. However, at positive scans multiple anodic peaks related to stripping of Cd0 at around −0.7 V are remarkable at all studied electrodes. Similar peaks for Cd0 oxidation in KH2 PO4 as well as HCl solution were reported [18,50]. For PBS solution, cathodic as well as anodic peaks were observed for all electrodes and corresponding peak potentials are listed in Table 4. It has been observed that the cathodic peak potentials obtained for PBS supporting electrolyte were found increasing (i.e., peak potential of CdS 3:1 < CdS 3:2 < CdS 3:3), whereas the anodic peak potentials were decreased while increasing the ratio between cadmium nitrate to thioacetamide in the starting precursor to form the CdS hollow microspheres. For PBS we calculated the difference between peak potentials observed on cathodic and anodic scans and the values are listed in Table 4 and the same trend corresponding to a trend in band gap energy values calculated from UV–vis diffuse reflectance studies (Section 3.4) was observed. The changes in both the cathodic and anodic peak potentials can be attributed to the increase of band gap (refer

Table 4 SWV cathodic and anodic peak potentials obtained at the electrodes under study in 0.1 M PBS obtained at scan rate of 250 mV s−1 . Modifier

Epc (V)

Epa (V)

|Epc − Epa | (V)

CdS 3:1 CdS 3:2 CdS 3:3

−1.07 −1.15 −1.22

−0.87 −0.86 −0.83

0.20 0.29 0.39

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CdS 3:1/GCE CdS 3:2/GCE CdS 3:3/GCE GCE

(a)

H 2SO 4 1200

-4

Z’’ (Ω)

current (A)

2.0x10

507

600 C

(b) 0.0

R1

0

W Rct

0.0

- 1 .0

potential (E)

-0 .5

0

2000

4000

current (A)

Z’ (Ω) Fig. 8. (a) Nyquist plots obtained at bare GCE as well as CdS modified GCE in 1 × 10−3 M [Fe(CN)6 ]3−/4− in 0.1 M KCl. (b) Scheme of equivalent circuit (R1 = solution resistance, Rct = charge transfer resistance, C = capacitance, W = Warburg impedance).

-5.0x10

-4

CdS 3:1/GCE CdS 3:2/GCE CdS 3:3/GCE

PBS

-6

current (A)

8.0x10

-1.0

potential (V)

-0.5

current (A)

0.0

-5.0x10

-5

CdS 3:1/GCE CdS 3:2/GCE CdS 3:3/GCE

Fig. 7. SWV scans at CdS modified GCE measured in 0.5 M H2 SO4 and 0.1 M PBS as supporting electrolytes. Conditions: scan rate of 250 mV s−1 , frequency of 20 Hz, amplitude of 25 mV.

Table 2) calculated from UV-DRS, which is due to the increase of thioacetamide ratio in the starting precursor. These results are in good agreement with those obtained by CV, and have also been reported by Haram et al., for CdS nanoparticles prepared by templating method [52]. 3.5.3. Investigation by electrochemical impedance spectroscopy Electrochemical impedance spectroscopy is effective tool for the investigation of surface-modified electrodes. Therefore we also used this method for the evaluation of the properties of CdS hollow microspheres modified electrodes. Fig. 8a shows impedance spectra, represented by Nyquist plots, obtained for CdS 3:1/GCE, CdS

3:2/GCE, and CdS 3:3/GCE. For comparison, Nyquist plot of unmodified GCE is also shown in Fig. 8a. All measurements were performed in the presence of redox system [Fe(CN)6 ]3−/4− , i.e., under Faradaic conditions. Impedance spectra of CdS 3:1/GCE, CdS 3:2/GCE, and CdS 3:3/GCE show complex behavior consistent with the Randles equivalent circuit. The observed semicircle shaped curve at high frequencies is typical for the interfacial electron transfer, and the linear part observed at low frequencies is typical for a process controlled by the diffusion of the redox probe [53]. The diameter of the semicircle shaped curve is found decreased in the following order CdS 3:3/GCE > CdS 3:2/GCE > CdS 3:1/GCE, indicating lowering of the charge transfer resistance in the same order. In the case of GCE without any CdS modification, the diameter of the semicircle shaped curve is found higher than the value observed for the CdS modified electrodes and linear part of the spectrum was not observed. This behavior is an indication for a process which is mainly controlled by the interfacial electron transfer within the entire range of the applied frequencies [53]. The impedance spectra have been analyzed by using an equivalent circuit, which is depicted in Fig. 8b. The charge transfer resistance (Rct ) and capacitance (C) values were obtained by fitting of the experimental data and are listed in Table 5. From this table, it can be seen that both parameters, i.e., the electron transfer resistance and capacitance were increased while increasing the thioacetamide ratio in the precursor and follow the trend CdS 3:1 < CdS 3:2 < CdS 3:3 in a sequence. These results are in good agreement with the band gap values listed in Table 2, i.e., with increasing of the band gap of CdS hollow microspheres both resistance and capacitance of the modifier layer are increased. While comparing the resistance values (refer Table 5), it is clear that the resistance value obtained for bare GCE is higher than that of the value obtained for the CdS hollow microsphere modified electrodes. This clearly demonstrates that CdS hollow microspheres would be a successful candidate to decrease the charge transfer resistance of an electrode surface.

Table 5 Charge transfer resistance (Rct ) and capacitance (C) obtained from fitting of measured impedance spectra as well as electrode surface coverage () with dsDNA. Electrode

Rct (k)

C (␮F)

 (%)

CdS 3:1/GCE CdS 3:2/GCE CdS 3:3/GCE GCE

1.415 1.519 1.807 3.542

12.4 13.5 17.6 24.1

80 72 67 60

508

S. Rengaraj et al. / Electrochimica Acta 56 (2010) 501–509

15000 CdS 3:2/GCE 6000

10000

Z'' (Ω)

Z'' (Ω)

CdS 3:1/GCE

5000

3000

0

0 0

6000 Z' (Ω)

12000

0

4000

Z', (Ω)

8000

4000 10000

Z'' (Ω)

Z'' (Ω)

CdS 3:3/GCE

2000

0

GCE

5000

0 0

2000

4000

6000

0

3000

Z' (Ω)

6000 Z' (Ω)

9000

Fig. 9. Nyquist plots obtained at CdS modified GCE as well as GCE in 1 × 10−3 M [Fe(CN)6 ]3−/4− in 0.1 M KCl before (black squares) and after (red dots) modification with dsDNA. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

In order to demonstrate the practical applicability of the modified CdS/GCE, the surface of the sensors has been coated by a very thin layer of dsDNA and the electrochemical impedance spectra were measured before and after modification with dsDNA (Fig. 9). Here the layer of dsDNA at the surface of electrode acts as an insulator layer and therefore a significant increase in the diameter of the semicircle shaped curve was observed in all cases. This is an indication for the increase of the charge transfer resistance value observed for the dsDNA modified electrodes. From the charge transfer resistance (Rct ) values, one can easily understand the resistive properties of the modifier (here in our case dsDNA). From this Rct value, one can easily calculate the surface coverage () of the electrode under investigation by using the following Eq. (7) [54]:



=

1−

Rct



DNA Rct

× 100%

(7)

where Rct is charge transfer resistance of electrode obtained before DNA is charge transfer resistance modification with dsDNA and Rct of electrode obtained after modification with dsDNA. As can be seen from Table 5, around 80% of CdS 3:1/GCE surface was covered with dsDNA while in the case of GCE the surface coverage reached only 60%. These results clearly demonstrate that the CdS hollow microspheres synthesized by this template free method effectively modified the surface of GCE and enhanced further dsDNA immobilization. Moreover, the difference in impedance spectra obtained for CdS modified electrodes before and after modification with dsDNA seems to be large enough to utilize it for the detection of amount/state of dsDNA layer and dsDNA damage. 4. Conclusions In summary, high quality novel CdS hollow microspheres composed of nanocrystals were synthesized by a one-step solution growth approach. We concluded that the ratio of cadmium nitrate to thioacetamide ratio in the precursor plays a major role to determine the properties of the CdS hollow microspheres. The formation mechanism of the hollow microsphere has been explained by the Ostwald ripening process. The powder X-ray diffraction confirmed the cubic structure of the CdS hollow microspheres and

the EDX revealed the stoichiometric phase of CdS. The CdS microspheres possess hollow shell like structure of diameter ∼2.5 ␮m and the thickness of the microsphere wall has been found varied between 250 and 300 nm. It has also been observed that the microsphere wall is formed by the aggregation of several thousands of nanoparticles of size between 30 and 40 nm and these observations were confirmed by HR-SEM and TEM measurements. The DRS confirmed the increase in the band gap of CdS hollow microspheres while increasing cadmium nitrate to thioacetamide ratio. Further the electrochemical characterization of CdS hollow microspheres was performed with glassy carbon electrode after its modification by CdS dispersed in dimethylformamide. Welldefined voltammetric responses for different redox systems were obtained for all the CdS/GCE modified electrodes. In the case of [Fe(CN)6 ]3− /[Fe(CN)6 ]4− and H2 Q/Q redox systems the observed changes in electroactivity are similar to the variations observed for the band gap energy values. The best reversibility has been observed for the [Ru(NH3 )]3+ /[Ru(NH3 )]2+ redox system, however, insignificant changes in its cyclic voltammetric responses did not allow to discriminate the differences of electron transfer resistance of the individual electrodes. The values of charge transfer resistance and capacitance were obtained by fitting the electrochemical impedance spectra. We have clearly demonstrated that while increasing the thioacetamide ratio in the precursor, which slightly increased the band gap, the charge transfer resistance of the CdS/GCE modified electrode can be decreased. The surface of the bare GCE and CdS modified electrodes was successfully covered by dsDNA and the electrochemical impedance measurements were performed to estimate the surface coverage. CdS/GCE effectively enhanced the dsDNA immobilization and it was confirmed by electrochemical impedance spectra. From these experiments we conclude that the CdS hollow microsphere modified GCE can be used to detect the amount, state of dsDNA and dsDNA damage. Acknowledgements The authors gratefully acknowledge the research funding support from the European Commission (Marie Curie Transfer of Knowledge Fellowship of the Sixth Framework Program under con-

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