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

[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 cm1 with a shoulder toward lower wavenumbers [17, 18], 660B electrochemical analytical instrument (CH Instrument the upshifted graphite G band at 1590 cm1 [17, 18], and the Inc., USA). A three-electrode system, consisting of a bare broad, asymmetric peak with a maximum at 1640 cm1 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 cm1 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 experimental 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-tryptophan was from Bio Basic Inc. Nicotinamide adenine dinucleotide (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 following 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 heating 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 0.4480.0609 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 pH1) were very close to the theoretical value of second cycle with a little change in peak currents after 24 59.0 mV pH1. 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. Consequently, the separa- buffer at different pH values. It was clear that both the ca- tion of the peak potentials increased with increasing scan thodic and anodic peak potentials negatively shifted with rate. When the scan rate was higher than 3 V/s, the wave shape distorted severely, indicating that the electrode reaction became electrochemically irreversible at high scan rates. For the irreversible electrode reaction, the relationship between peak potential and the scan rate follows Laviron’s equation [21, 22] RTRTk RT E Ev ln s  ln (1) Pc nF nF nF RT RTk RT E Ev ln s  ln (2) Pa (1 )nF (1 ) nF (1 ) nF

where  is the electron-transfer coefficient, ks is the stand- ard rate constant of the surface reaction,  is the scan rate,

and E′ is the formal potential. Figure 5 shows the plot of EP vs. ln for the ND film in pH 5.42 HAc-NaAc buffer. The Figure 3 UV (325 nm) Raman spectra of ND powders before and after E′ (0.00 V) could be estimated as the midpoint between the oxidation. cathodic and the anodic peak potentials at low scan rates. At

high scan rates, the plots of EP vs. ln were linear governed by:

Figure 4 Cyclic voltammograms of the ND film in HAc-NaAc buffer (pH 5.42) after (a) 0 h, (b) 1 h, (c) 2 h, (d) 15 h, (e) 24 h at the scan rate of Figure 5 Semilogarithmic dependence of the cathodic peak potential (●) 0.1 V/s. Inset: Cyclic voltammograms of the ND film in pH 2.13 (a), 3.50 and the anodic peak potential (○) on the scan rate for the ND film in pH (b), 5.97 (c), 7.11 (d), 8.05 (e) and 10.10 (f) B-R buffer, scan rate: 0.1 V/s. 5.42 HAc-NaAc buffer. 2448 Li YS, et al. Sci China Chem November (2012) Vol.55 No.11

EPc (V) = 0.14800.05157 ln v/(V/s) (3) which should be useful for biological and chemical analyses. EPa (V) = 0.1208 + 0.04797 ln v/(V/s) (4) To test the electrocatalytic ability of the ND film modified From equations (1) and (3),  was found to be 0.48 and electrode toward biomolecules, such as tryptophan and 1 + ks to be 1.10 s . Thus, it was possible to estimate that the NAD , the following experiments were carried out. rate-determining step of the reduction process was a single Tryptophan is an essential amino acid for humans and electron-transfer reaction (n = 1,  = 0.48). From equations herbivores, which is sometimes added to dietary and feed products as food fortifier. In addition, it is the limiting ami- (2) and (4), the values of n(1) and ks were obtained as 0.52 and 258.58 s1, respectively. These results suggest that no acid in various vegetable products. On the other hand, the rate-determining step of the reduction and reoxidation tryptophan is also an important structural part of almost all processes might not be the same. proteins in the body. As shown in Figure 7(A), L-trypto- According to Refs 19, 23, 24 and our results mentioned phan exhibited an irreversible oxidation peak at 0.924 V on above, a plausible mechanism of the electrode process for the bare GC electrode in citric acid buffer (pH 4.21) at the the ND film could be described as Scheme 1. scan rate of 0.1 V/s. At the ND film-modified GC electrode, the peak potential shifted positively to 0.977 V, while the peak current increased significantly. 3.2 Electrocatalytic behavior of the carboxylated It is well known that the coenzyme couple of nicotina- nanodiamond mide adenine dinucleotide (NAD+/NADH) plays an im- Figure 6 shows the CV curves for the GC electrode with portant role in many biological redox processes, among and without modification by the ND film in 0.1 M KCl which NAD+ participates in the reactions it catalyses and is 4 + containing 5×10 M K3[Fe(CN)6] at the scan rate of 0.1 reduced simultaneously. In this study, NAD displayed an V/s . At the bare GC electrode, a pair of reversible peaks irreversible reduction peak at 1.016 V in 0.1 M phosphate was observed at 0.305 and 0.229 V associated with the oxi- buffer (pH 7.08) at the bare GC electrode at the scan rate of dation and reduction of the ferricyanide-ferrocyanide couple. 0.1 V/s (Figure 7(B), line a). At the ND film modified elec- At the ND film modified electrode, the oxidation peak of trode, the peak potential remained unchanged while the K3[Fe(CN)6] shifted positively to 0.311 V and the reduction peak current increased significantly (Figure 7(B), line b). peak potential shifted negatively to 0.199 V. The separation between the oxidation and the reduction peak was larger than that at the bare GC electrode, indicating a qua- 4 Conclusions si-reversible reaction at the ND film modified electrode. Compared with the bare GC electrode, the ND film modi- The ND films functionalized with carboxylic acid groups fied electrode exhibited a clear current enhancement for from acid oxidation showed stable cyclic voltammetric be- both the oxidation and reduction peaks. Therefore, it could havior in buffer solutions, involving the reduction of the be concluded that the performance of the GC electrode was carboxylic acid groups on the NDs. Compared with the bare significantly improved by modification with the ND film, GC electrode, the carboxylated ND film modified GC elec-

Scheme 1 Possible mechanism proposed for the reduction/reoxidation processes of ND film. Li YS, et al. Sci China Chem November (2012) Vol.55 No.11 2449

1 Xing Y, Dai L. Nanodiamonds for nanomedicine. Nanomedicine, 2009, 4(4): 207–218 2 Aleksenskii AE, Baidakova MV, Vul AY, Siklitskii V. The structure of diamond nanoclusters. Phys Solid State, 1999, 41(4): 668–671 3 Kulakova II. Surface chemistry of nanodiamonds. Phys Solid State, 2004, 46(4): 636–643 4 Greiner NR, Phillips DS, Johnson JD, Volk F. Diamonds in detonation soot. Nature, 1988, 333: 440–442 5 Chen P, Huang F, Yun S. Characterization of the condensed carbon in detonation soot. Carbon, 2003, 41(11): 2093–2099 6 Gruen DM. Nanocrystalline diamond films. Annu Rev Mater Sci, 1999, 29: 211–259 7 Huang TS, Tzeng Y, Liu YK, Chen YC, Walker KR, Guntupalli R, Liu C. Immobilization of antibodies and bacterial binding on nanodiamond and carbon nanotubes for biosensor applications. Dia- mond Relat Mater, 2004, 13(4-8): 1098–1102 8 Lasseter TL, Clare BH, Abbott NL, Hamers RJ. Covalently modified 4 Figure 6 Cyclic voltammograms of 5×10 M K3[Fe(CN)6] in 0.1 M KCl silicon and diamond surfaces: Resistance to nonspecific protein ad- at the bare GC electrode (a) and the ND film modified GC electrode (b) at sorption and optimization for biosensing. J Am Chem Soc, 2004, the scan rate of 0.1 V/s. 126(33): 10220–10221 9 Novoselova IA, Fedoryshena EN, Panov ÉV, Bochechka AA, Romanko LA. Electrochemical properties of compacts of nano- and microdisperse diamond powders in aqueous electrolytes. Phys Solid State, 2004, 46(4): 748–750 10 Zhao W, Xu JJ, Qiu QQ, Chen HY. Nanocrystalline diamond modified gold electrode for glucose biosensing. Biosens Bioelectron, 2006, 22(5): 649–655 11 Zang JB, Wang YH, Zhao SZ, Bian LY, Lu J. Electrochemical properties of nanodiamond powder electrodes. Diamond Relat Mater, 2007, 16(1): 16–20 12 Chem LH, Zang JB, Wang YH, Bian LY. Electrochemical oxidation of nitrite on nanodiamond powder electrode. Electrochem Acta, 2008, 53(8): 3442–3445 13 Hian LC, Grehan KJ, Compton RG, Foord JS, Marken F. Influence of thin film properties on the electrochemical performance of diamond electrodes. Diamond Relat Mater, 2003, 12: 590–595 14 Holt KB, Ziegler C, Caruana DJ, Zang J, Millán-Barrios EJ, Hu J, Foord JS. Redox properties of undoped 5 nm diamond nanoparticles. Phys Chem Chem Phys, 2008, 10(2): 303–310 15 Holt KB, Ziegler C, Zang J, Hu J, Foord JS. Scanning electrochemical microscopy studies of redox processes at undoped nanodiamond surfaces. J Phys Chem C, 2009, 113(7): 2761–2770 16 Ushizawa K, Sato Y, Mitsumori T, Machinami T, Ueda T, Ando T. Covalent immobilization of DNA on diamond and its verification by diffuse reflectance infrared spectroscopy. Chem Phys Lett, 2002, 351(1–2): 105–108 17 Osswald S, Yushin G, Mochalin V, Kucheyev SO, Gogotsi Y. Con- trol of sp2/sp3 carbon ratio and surface chemistry of nanodiamond powders by selective oxidation in air. J Am Chem Soc, 2006, 128(35): 11635–11642 Figure 7 (A) Cyclic voltammograms of 1.1×104 M L-tryptophan in 0.1 18 Mochalin V, Osswald S, Gototsi Y. Contribution of functional groups M citric acid buffer (pH 4.21) at the bare GC electrode (a) and the ND film to the raman spectrum of nanodiamond powders. Chem Mater, 2009, modified GC electrode (b) at the scan rate of 0.1 V/s; (B) cyclic voltam- 21(2): 273–279 mograms of 1.5×104 M NAD+ in 0.1 M phosphate buffer (pH 7.08) at the 19 Luo H, Shi Z, Li N, Gu Z, Zhuang Q. Investigation of the electro- bare GC electrode (a) and the ND film modified GC electrode (b) at the chemical and electrocatalytic behavior of single-wall carbon nanotube film on a glassy carbon electrode. Anal Chem, 2001, 73(5): 915– scan rate of 0.1 V/s. 920 20 Bard AJ, Faulkner LR. Electrochemical Methods. New York: Wiley, 1980, 522 trode significantly improved the faradic responses of bio- 21 Laviron E. Adsorption, autoinhibition and autocatalysis in polarog- molecules, such as tryptophan and nicotinamide adenine raphy and in linear potential sweep voltammetry. J Electroanal Chem, dinucleotide. Our results indicate the carboxylated nanodi- 1974, 52: 355–393 amond is a promising electrocatalyst for both the anodic and 22 Laviron E. General expression of the linear potential sweep voltam- mogram in the case of diffusionless electrochemical systems. J Elec- cathodic processes. troanal Chem, 1979, 101: 19–28 23 Baizer MM, Lund H. Organic Electrochemistry. New York: Marcel Dekker, 1983, 379 This research was sponsored by the National Natural Science Foundation 24 Baizer MM, Lund H. Organic Electrochemistry. New York: Marcel of China (21075136). Dekker, 1983, 507