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SCIENCE CHINA Electrochemistry of Carboxylated Nanodiamond Films

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SCIENCE CHINA Electrochemistry of Carboxylated Nanodiamond Films SCIENCE CHINA Chemistry • ARTICLES • November 2012 Vol.55 No.11: 2445–2449 doi: 10.1007/s11426-012-4619-5 Electrochemistry of carboxylated nanodiamond films LI YanShuang1, LUO HongXia1*, DAI LiMing2*, GUO Wei1, LI ShaNa1 & GUO ZhiXin1 1Department of Chemistry, Renmin University of China, Beijing 100872, China 2Department of Chemical Engineering, Case Western Reserve University, Cleveland, OH 44106, USA Received December 29, 2011; accepted February 7, 2012; published online May 30, 2012 The electrochemical behavior of nanodiamond (ND) film functionalized with carboxylic acid groups was studied systemati- cally on a glassy carbon (GC) electrode. One stable redox couple corresponding to the carboxylic acid group was observed. At the scan rate of 0.1 V/s, the cathodic and anodic peak potentials were 0.093 V and 0.088 V (vs. Ag/AgCl), respectively. The carboxylic acid groups on the ND surface were reduced to CH2OH via a four electron redox process. The ND film modified electrode showed favorable electrocatalytic behavior toward the oxidation as well as the reduction of biomolecules, such as tryptophan and nicotinamide adenine dinucleotide. carboxylated nanodiamond, film, cyclic voltammetry, catalysis, biomolecules 1 Introduction counterpart because of its weak conductivity. However, the electrochemical study of non-doped NDs could be traced back to 2004 when Novoselova et al. [9] measured redox With the recent advancement in nanoscience and nanotech- 3/4 3+/4+ nology, nanodiamonds (NDs) or nanocrystalline diamonds species, such as Fe(CN)6 and Ce , on a compacted have received considerable interest for biosensing and bio- ND powder electrode. Subsequently, Xu and his co-workers medical applications [1]. NDs are a relatively new class of [10] reported a glucose biosensor based on electrochemi- carbon nanomaterials that usually have diamond crystal cally pretreated non-doped nanocrystalline diamond, while cores at a nanometer scale with certain surface functional Wang et al. [11] fabricated a cavity packed ND powder groups [2, 3]. Particularly, ND particles generated by the electrode that showed a quasi-reversible electrode reaction 3/4 detonation technique [4] often contain many oxygen-con- in 0.1 M KCl containing Fe(CN)6 redox couple with a taining surface functional groups [5]. As a result, the deto- good electrochemical stability over a wide potential range. nated NDs possess not only the general characteristics of The cavity-packed ND powder electrode could be used to diamond but also the surface chemical properties character- detect the electrochemical oxidation of nitrite [12]. Besides, istic of those surface functional groups. The excellent elec- Foord et al. [13] compared the electrochemical properties of trochemical activity arising from its cluster structure with a nanocrystalline diamond films with those of conventional large specific surface area of numerous surface defects boron–doped diamonds for the oxidation of hydroquinone makes NDs attractive for electrode applications in dia- and ascorbic acid in aqueous solutions, and found that the mond-based electrochemical and biomedical sensors [6–8]. nanodiamonds were a class of highly active electrode mate- Although significant effort has been made to develop rials with low overpotentials and a good adhesion to metal- boron-doped diamond electrochemical sensors, there is lic electrodes. Thereafter, these authors observed an en- 3+/2+ much less discussion in the literature on its non-doped hanced faradic current for the redox couple of Ru(NH3)6 and Fe(CN)3/4 on a gold electrode modified with a drop- 6 *Corresponding authors (email: [email protected]; [email protected]) coated layer of ND with respect to the bare gold electrode © Science China Press and Springer-Verlag Berlin Heidelberg 2012 chem.scichina.com www.springerlink.com 2446 Li YS, et al. Sci China Chem November (2012) Vol.55 No.11 [14]. The ND layer was also found to promote the oxygen 3 Results and discussion reduction [15]. As is well known, carbon nanotubes functionalized with 3.1 Electrochemical behavior of the carboxylated carboxylic groups show excellent catalytic behavior toward nanodiamond biomolecules. In this manuscript, the electrochemical be- Figure 1 shows an AFM image of the ND film on a quartz havior of carboxylated nanodiamond (ND) films was inves- plate. As can be seen, NDs tend to aggregate into clusters tigated for the first time. It was demonstrated that the ND with a size of about 200 nm in diameter, though a single ND films displayed good electrocatalytic activities toward bio- particle is about 10 nm in size. molecules, such as tryptophan and nicotinamide adenine Figure 2 shows TGA results of the ND sample in com- dinucleotide. parison with the unfuntionalized NDs, indicating the pres- ence of about 5%–7% surface COOH groups on ND. 2 Experimental The UV Raman spectra of ND powders are shown in Figure 3. NDs exhibited the characteristic Raman features: the asymmetrically broadened sharp diamond peak at 1324 Electrochemical measurements were carried out on a CHI cm1 with a shoulder toward lower wavenumbers [17, 18], 660B electrochemical analytical instrument (CH Instrument the upshifted graphite G band at 1590 cm1 [17, 18], and the Inc., USA). A three-electrode system, consisting of a bare broad, asymmetric peak with a maximum at 1640 cm1 GC electrode ( = 3.0 mm) or ND modified GC electrode which might be assigned to a superposition of sp2 carbon as working electrode, an Ag/AgCl electrode as reference band at 1590 cm1 with a peak of O–H bending vibration at electrode, and a platinum wire as counter electrode, was 1 used. Atomic force microscopy (AFM) was performed on a 1640 cm [18]. It was obvious that the oxidation led to a Veeco D3100 scanning probe microscope. Thermogravi- significant increase in the relative intensity of the diamond metric analysis (TGA) was carried out using a Model TGA Q500 thermogravimetric analyzer (TA Instruments) at 20 º C/min under N2 flow. Raman analysis of the initial and oxidized powders was conducted using a LabRAM HR800 spectrometer (HORIBA Jobin Yvon) with an excitation wave length of 325 nm (He-Cd laser). The pH values of solutions were measured with a Sartorius PB-10 pH meter (Germany). Deionized water (Milli-Q water purification system; Millipore, USA) was used through all the experi- mental procedures. All experiments were carried out at room temperature. Nanodiamond powders (~95% in purity) used in this study were prepared by the detonation method. L-trypto- phan was from Bio Basic Inc. Nicotinamide adenine dinu- cleotide (NAD+) was from Sigma. All the other chemicals were analytical grade reagents. The nanodiamond powders Figure 1 An AFM image of the carboxylated nanodiamond (ND). were carboxylated and oxidized through acid oxidation fol- lowing the reported procedure [16]. Briefly, the ND sample (0.5 g) was first heated in a 9:1 (v/v) mixture of concentrat- ed H2SO4 and HNO3 at 75 °C for 3 days, followed by heat- ing in 0.1 M NaOH aqueous solution at 90 °C for 2 h, and then in 0.1 M HCl aqueous solution at 90 °C for 2 h. The resulting carboxylated/oxidized nanodiamonds were exten- sively rinsed with deionized water. To prepare the ND film, 1 mg of ND was dispersed with the aid of ultrasonic agitation in 1 mL of water to give a 1 mg/mL aqueous suspension. The GC electrode was first abraded with emery paper (No.1500) and polished with 0.3 m alumina slurry, then washed ultrasonically in deionized water and ethanol, respectively. The GC electrode was then coated by gradually casting 25 L of the ND suspension Figure 2 TGA thermograms of nanodiamonds before (a) and after (b) oxidation recorded at a heating rate of 20 °C/min under the nitrogen at- prepared above and then dried under an infrared lamp. mosphere. Li YS, et al. Sci China Chem November (2012) Vol.55 No.11 2447 peak. increasing pH according to the linear equations: EPa = In pH 5.42 HAc-NaAc buffer solution, the ND film 0.4480.0609 pH and EPc = 0.291 0.0601 pH, respectively. modified GC electrode exhibited a pair of reduction/reoxi- The slopes of the plots of EPc and EPa vs. pH (60.9 and dation peak. The peak potentials remained stable after the 60.1 mV pH1) were very close to the theoretical value of second cycle with a little change in peak currents after 24 59.0 mV pH1. These results suggest that four protons are hours (Figure 4). At a scan rate of 0.1 V/s, the cathodic and involved in the four-electron redox reaction associated with anodic peak potentials were 0.093 V and 0.088 V (vs. the ND film during the electrode reaction. Ag/AgCl), respectively. In view of the electrochemical be- The cyclic voltammetric behavior of the ND film was havior of an acid-oxidized single-wall carbon nanotube film also found to be affected by the potential scan rate. As ex- [19], it could be deduced that the carboxylic acid groups on pected for a surface wave [20], a higher scan rate led to a the ND surface were the electroactive species, which were higher current flow. The reduction peak current i was pro- reduced to –CH2OH via a four electron redox process. portional to the scan rate over the range of 0.005–3 V/s. The cyclic voltammetric behavior of the ND film in dif- When the scan rate was lower than 0.2 V/s, both the ca- ferent media, including KH2PO4-NaH2PO4, HAc-NaAc and thodic and anodic peak potentials (EPc and EPa) remained B-R buffer (pH 5.42) were tested, a similar electrochemical almost unchanged with an increase in the scan rate. At behavior was obtained in all cases. The insert of Figure 4 higher scan rates, EPc shifted negatively and EPa shifted pos- shows the electrochemical behavior of the ND film in B-R itively as the scan rate increased.
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