Comparison of Boron-Doped Diamond and Glassy Carbon Electrodes for Determination of Procaine Hydrochloride

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Comparison of Boron-Doped Diamond and Glassy CarbonElectrodes for Determination of Procaine Hydrochloride

Min Wei,a Yanli Zhou,b Jinfang Zhi,b Degang Fu,a Yasuaki Einaga,c Akira Fujishima,d Xuemei Wang,a* Zhongze Gua*a State Key Laboratory of Bioelectronics, Southeast University, Nanjing 210096, P. R. China*e-mail: [email protected] Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100080, P. R. Chinac Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Yokohama 223-8522, Japand Kanagawa Academy of Science and Technology, KSP, 3-2-1 Sakado, Kawasaki 213-0012, Japan

Received: July 4, 2007Accepted: September 6, 2007

AbstractThe electrochemical oxidation of procaine hydrochloride (PC ·HCL, 2-diethylaminoethyl 4-aminobenzoate hydro-chloride) was investigated at as-deposited boron-doped diamond (ad-BDD) electrode, anodically oxidized BDD (ao-BDD) electrode and glassy carbon (GC) electrode using cyclic voltammetry (CV). Well-defined cyclic voltammo-grams were obtained for PC ·HCL oxidation with high signal-to-background (S/B) ratio, low tendency for adsorption,good reproducibility and long-term stability at ad-BDD electrode, demonstrating its superior electrochemicalbehavior and significant advantages in contrast to ao-BDD and GC electrode. At 100 mM PC ·HCL, the voltammetricS/B ratio was nearly one order of magnitude higher at an ad-BDD electrode than that at a GC electrode. In a separateset of experiments for oxidation of 100 mM PC ·HCL, 96%, 92% and 84% of the initial oxidation peak current wasretained at the ad-BDD, ao-BDD and GC electrode, respectively, by stirring the solution after the tenth cycle. Thecurrent response was linearly proportional to the square root of the scan rate within the range 10 – 1000 mV s�1 in10 mM PC ·HCL solutions, indicating that the oxidation process was diffusion-controlled with negligible adsorption atan ad-BDD surface. The good linearity was observed for a concentration range from 5 to 200 mM with a linearequation of y¼ 0.03517xþ 0.65346 (r¼ 0.999), and the detection limit was 0.5 mM for oxidation of PC ·HCL at the ad-BDD electrode. The ad-BDD electrode could maintain 100% of its original activity after intermittent use for3 months.

Keywords: Boron-doped diamond electrode, Glassy carbon electrode, Procaine hydrochloride, Cyclic voltammetry

DOI: 10.1002/elan.200704024

1. Introduction

Theboron-doped diamond (BDD) thin-films have attracteda great deal of attention as unique electrodematerials due totheir superior electrochemical properties such as widepotential window, low background current, high sensitivityand long-term stability, negligible adsorption of neutral andpolar species, controllable surface termination and relativeease of functionalization [1]. BDD thin-films have thusemerged in several applications including electroanalysis,electrocatalysis, electrosynthesis, etc. As-deposited boron-doped diamond (ad-BDD) electrodes are initially hydro-gen-terminated. The ad-BDD electrode has been reportedto be much superior to the glassy carbon (GC) electrode fordetection of many molecules in terms of high signal-to-background (S/B) ratio, long-term stability, high sensitivityand good reproducibility [2 – 10]. Some appropriate oxida-tion treatment such as anodic treatment and oxygen-plasmatreatment can convert hydrogen termination to oxygentermination. In contrast, oxygen termination can also bealtered to hydrogen termination by means of hydrogen-flame or hydrogen-plasma treatment. The advantages of

oxygen-terminated BDD electrodes in comparison withhydrogen-terminated ones have beendemonstrated in someways including much wider potential window and highersurface stability from fouling [11 – 14]. Several biologicalsubstances [15 – 17] and chemical compounds [18, 19] havebeen detected at both ad-BDD electrodes and anodicallyoxidized BDD (ao-BDD) electrodes, and the results dem-onstrated that the ao-BDD electrode possessed betterresponse and higher stability and was superior to the ad-BDD electrode. However, it is known that the surfacecharge of a BDD electrode could be converted to benegative by anodic treatment due to the formation ofcarbon-oxygen functionalities [20]. Consequently, for thedirect oxidation of negatively charged substances such asoxalic acid [21], DNA [22], and AQDS [23], an ao-BDDelectrode is inferior to an ad-BDD electrode owing to theelectrostatic repulsion between these compounds and thenegatively charged electrode surface. At the same time,manymodifiedBDDelectrodes have been applied to detectglucose [24 – 26], phenolic compounds [27, 28], etc.Procaine hydrochloride (PC ·HCL) is the first synthetic

injectable local anesthetic. The IUPAC name for this

137

Electroanalysis 20, 2008, No. 2, 137 – 143 K 2008 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim

compound is 2-diethylaminoethyl 4-aminobenzoate hydro-chloride and the chemical structure is as follows:

PC ·HCL can produce a reversible loss of sensation bydiminishing the conduction of sensory nerve impulses nearto the site of application [29], and be used to alleviate thepain or treat depletion of acute renal function. However,PC ·HCLoften shows a short duration of action and adverseside effects, such as cardiac and neurological toxicity, and, insome cases, accompanied by allergic reactions [30]. In ordertominimize the risk of side effects, it is important to analyzePC ·HCL for quality assurance in pharmaceutical prepara-tions and for obtaining optimum therapeutic concentra-tions. Simultaneously, the analysis of PC ·HCL can offeruseful information to study themechanism for thebindingofPC ·HCL to DNA and other biomolecules.Many methods such as spectrophotometry and chroma-

tography have been employed to determine PC ·HCL.Recently, electrochemical methods have been used todetermine PC ·HCL owing to their high sensitivity andrapid response time. For instance, Wu et al. [31] and Wanget al. [32] respectively utilized multi-wall carbon nanotubefilm-coated GC electrode and pumice-modified carbonpaste electrode to determine PC ·HCL and obtainedfavorable results with the detection limit of 2.0� 10�7 molL�1 and 5.0� 10�8 mol L�1, which were one or two orders ofmagnitude lower than those obtained by spectrophotometryand chemiluminescence method [33, 34].Although the above electrochemical methods have been

successful in detecting PC ·HCL, in most cases, the process-es of modifying bare electrodes were complex, time-consuming and inconvenient. Also, strong adsorption atthe bare GC electrode surface induced unsatisfactoryresults. In general, BDD electrodes are less prone todrawbacks described above and may thus be suitable forsensitive determination of the PC ·HCL. In this paper,comparison of BDD electrodes and GC electrodes for thedetermination of PC ·HCL was performed. We demon-strated the advantages of ad-BDD electrodes over ao-BDDelectrodes and GC electrodes for PC ·HCL oxidation interms of high S/B, low tendency for adsorption, goodreproducibility and long-term stability.

2. Experimental

2.1. Materials and Methods

2.1.1. Preparation of the BDD Film

BDD films were prepared on silicon (100) wafers bymicrowave assisted plasma chemical vapor deposition

(CVD) technique using a commercial microwave plasmareactor (ASTeX Corp., Woburn, MA) at 5 kW with highpurity hydrogen as the carrier gas. Prior to deposition, thesilicon substrates were hand-polishedwith diamond powder(0.5 mm) for nucleation, after which they were rinsed with 2-propanol. Amixture of acetone andmethanol (9 :1 v/v) wasused as the carbon source. B2O3 as the boron source wasdissolved in the above-mentioned mixture at B/C molarratio of 1 :100. A film thickness of ca. 40 mm was achievedafter 10 hour deposition.

2.1.2. Chemicals

Si wafer was fromFURUYAMetal Co., Ltd. B2O3 was fromWako. PC ·HCL was purchased from Nanjing pharmacyfactory (Nanjing, China). All chemicals were of analyticalreagent grade and usedwithout further purification.Milli-Qwater was used throughout the experiments. A stocksolution of PC ·HCL (1.0 mM) was prepared and kept inthe dark under refrigeration (low 4 8C).A 0.07 Mphosphatebuffer saline (PBS; pH 7.0) was used as a supportingelectrolyte.

2.1.3. Electrochemical Measurement

Electrochemical measurements were carried out usingIM6ex instrument (ZAHNER Elektrik, Germany) and athree-electrode electrochemical cell consisting of either aBDD electrode or a GC electrode as the working electrode,a saturated calomel reference electrode (SCE) and a Pt wireas the counter electrode. BDD electrodes were sonicatedsuccessively in 2-propanol andMilli-Q water (>18 MW cm)before use. GC electrodes were polished with 0.1 and0.05 mm alumina powder, respectively, rinsed thoroughlywith Milli-Q water between each polishing step, and thensonicated in acetone and Milli-Q water successively. Thegeometric areas of the working electrodes in the cell wereestimated to be 0.07 cm2. The electrical contact for theBDDelectrode was made through the backside of the scratchedsilicon substrate by contacting the brass current-collectingback plate. Ad-BDD films were altered to ao-BDD films byapplying a potential ofþ2.6 V (vs. SCE) for 75 min in 0.1 MNaOH solutions. Prior to experiments, all solutions werepurged with nitrogen for 15 min to remove oxygen. All themeasurements were performed at room temperature.

3. Results and Discussion

3.1. Comparison of Signal to Background Current

The electrochemical oxidation of PC ·HCL in aqueoussolution has been studied onother carbon electrodes [31, 36]which corresponds to a two-electron, two-proton process.The proposed reaction mechanism is shown in Scheme 2 byconsidering the earlier studies [32].Figure 1 and Table 1 present comparison of the cyclic

voltammetric data and S/B ratios at a GC electrode, an ad-

Scheme 1. The chemical structure of PC ·HCl.

138 M. Wei et al.

Electroanalysis 20, 2008, No. 2, 137 – 143 www.electroanalysis.wiley-vch.de K 2008 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim

www.electroanalysis.wiley-vch.de

BDD electrode and an ao-BDD electrode for oxidation ofPC ·HCL.From our experimental results, the background current

for ad-BDD electrode was one order of magnitude lowerthan that for GC electrode, which was ascribed to thehydrogen termination and the low double layer capacitanceof the BDD surface. It is known that a high quality ad-BDDelectrode can exhibit a capacitance as low as 3 mF cm�2,

which is about one order of magnitude lower than thatusually observed at a cleanGCelectrode [1]. It was seen thatthe potential window of the ao-BDD electrodes becamesomewhat wider than that of the ad-BDD electrodes. Thebackground current for the ao-BDD electrode was 2 – 3times larger than that for the ad-BDD electrode. Asdiscussed in the previous reports [37, 38], it was found thatan additional capacitance element was generated and theacceptor densities in the near-surface region of the anodi-cally treated surfaces were found to be extremely low afterthe anodic treatment. The passivation layers generated onthe BDD surfaces arose as a result of the removal ofhydrogen acting as an acceptor in the near-surface region ofhydrogen-terminated surface by the anodic treatment,which resulted in a change in the surface conductivity.It can be seen that the peak currents were increased by

about 16% at ao-BDD electrode in comparison with thoseat ad-BDD electrode. The difference of peak currentsobtained at ad-BDD electrodes and ao-BDD electrodescould be considered as the result of elimination of the non-diamond carbon impurities from the surface during anodicoxidation by applying high potentials (þ2.6 V vs. SCE) [17,39]. The non-diamond carbon compounds may be intro-duced at BDD surface during its deposition in CVDmachine. The peak potentials at ao-BDD electrode wereabout 0.03 V more negative than those at ad-BDD elec-trode, which could be regarded as a negligible change.According to the literature [13, 21, 22, 40], compounds withpositive charge were easily oxidized at ao-BDD electrodeand the anodic peaks were shifted to the negative directioncompared to those obtained at ad-BDDelectrode due to theelectrostatic attraction force between these positivelycharged compounds and negatively charged surface of ao-BDD electrode. As mentioned in previous report [12],oxygen functional groups such as carbonyl or hydroxylgroups, formed on the facets of the diamond microcrystal,would form a negative dipolar field during anodic oxidation,which electrostatically attracts the positively charged mol-ecules. On the contrary, anodic peaks for negative chargecompounds were more clearly observed at an ad-BDDelectrode than those at an ao-BDD electrode due toexistence of the electrostatic repulsion [18, 23]. In thepresent work, there was negligible effect of the surfacetermination on the peak potential, as previously noted forseveral purines, pyrimidines and dopamine [17, 41]. Asmentioned in the literature [41], the main reason for theabove results is that PC ·HCL is protonated and in thecationic form (pKa 8.9) in pH7.0 PBS buffer, and theinteraction of the positively charged quaternary ammoniumgroup with the surface is relatively strong and nearly equalfor both hydrogen- and oxygen-terminated surfaces, so thepeak potential for PC ·HCLoxidation is not affected greatly

Scheme 2. Electrochemical oxidation reaction of PC ·HCL.

Fig. 1. Cyclic voltammograms for 10 mM PC ·HCL in 0.07 MPBS (pH 7) at a) GC electrode (0.07 cm2); b) ad-BDD electrode(0.07 cm2); c) ao-BDD electrode (0.07 cm2). Scan rate: 100 mV s�1;dashed lines: background current.

139Determination of Procaine Hydrochloride



by surface termination when the pH was below the pK, i.e.,when it was protonated.As shown in the Table 1, though the oxidation of PC ·

HCL occurred at a higher potential at a BDD electrodethan that at aGC electrode, and the oxidation peak currentsat aGC electrode were larger than those obtained at a BDDelectrode, the S/B ratios were about 4 times and nearly oneorder ofmagnitude higher for ad-BDDelectrode than thosefor the GC electrode at 10 mM and 100 mM of PC ·HCLconcentration, respectively.

3.2. Comparison of Reproducibility and Stability

Figure 2 shows comparison of a ten-successive cyclicvoltammograms of PC ·HCL. As shown in Figure 2, theoxidation peak currents dropped with each scan until itreached a minimum value after the tenth cycle due to theadsorption at the three electrodes. However, at ad-BDDelectrode, a relatively well-defined peak was still obtainedafter the tenth cycle, while a voltammogram with nearlypeak-shaped feature was observed in the last cycle at ao-BDD electrode. In contrast, the tenth voltammogramobtained at the GC electrode was featureless due to a largerising background current and severe block of oxidativeproducts. The results demonstrated that ad-BDD electrodehad favorable quality about low tendency for adsorptionandwas superior to ao-BDD and GC electrode.As mentioned above, fouling of the electrodes occurred

due to the continuous adsorption of the oxidative products,which resulted in the formation of a passive film on theelectrode. We removed the passive film on the electrodesurface by stirring the solution. Figure 3 and Table 2 showcomparison of cyclic voltammograms recorded for 100 mMPC ·HCL and those obtained by stirring the solution afterone hour. Herein, the purpose of stirring the solution was toremove the fouling on the electrode surface and help thediffusion of PC ·HCL to the electrode surface. It can be seenthat there was inconspicuous drop before and after stirringthe solution after the tenth cycle at ad-BDD and ao-BDDelectrode, and the oxidation peak currents retained 95.59%and 91.99% of there initial signals, respectively. Forcomparison, at GC electrode, the peak current decreasedby 16.26%,which is about 4 times higher than that happenedat ad-BDD electrode (4.41%), and retained 83.74% of its

initial signal under the same conditions, demonstrating thatoxidation of PC ·HCL resulted in obvious deactivation atthe GC electrode and was suppressed after the tenth cycle.The differences in stability at the three electrodes could beascribed to the more severe blocks of reaction products attheGCsurface than those at theBDDsurface, and indicated

Table 1. Comparison of the cyclic voltammetric data and S/B ratios at an ad-BDD electrode, an ao-BDD electrode and a GC electrode.The data were obtained from the cyclic voltammetric data for the oxidation of PC ·HCL in 0.07 M PBS (pH 7) with scan rate of 100 mVs�1.

Electrode PC ·HCL (mM) Eoxp (V vs. SCE) Ioxp (mA) Background current (mA) S/B ([itotal – ibkd]/ibkd) [a]

ad-BDD 100 1.037 3.859 0.119 31.4310 0.952 0.733 0.114 7.35

ao-BDD 100 1.024 3.876 0.381 8.6110 0.926 0.856 0.230 2.72

GC 100 1.011 12.61 2.5 4.0410 0.894 2.088 0.731 1.86

[a] Calculated according to the criteria in [35]

Fig. 2. A series of cyclic voltammograms for 10 mM PC ·HCL in0.07 M PBS (pH 7) at a) GC electrode (0.07 cm2); b) ad-BDDelectrode (0.07 cm2 ); c) ao-BDD electrode (0.07 cm2). Scan rate:100 mV s�1; cycles, ten consecutive scans.

140 M. Wei et al.



that BDD surfaces clearly exhibit lower tendency foradsorption and higher stability than GC surfaces.

3.3. Effect of Scan Rate

The effect of scan rate on the peak current of PC ·HCL wasalso shown in Figure 4. From the cyclic voltammograms, itwas found that in 10 mMPC ·HCL solutions, a positive shiftin the peak potential existed with increasing scan rate andthe current response was linearly proportional to the squareroot of the scan rate within the range 10 – 1000 mV s�1. Thelinear regression statistical analysis yields r¼ 0.999. Thelinearity suggested that the current was limited by semi-infinite linear diffusion of PC ·HCL in the interfacialreaction zone and that rate-limiting adsorption steps andspecific surface interaction could be neglected [2].

3.4. Calibration Curve

Figure 5 shows a series of CVs for the PC ·HCLoxidation atvarious concentrations from 5 – 200 mM at ad-BDD elec-trode. Inset shows the dependence of the current peak onthe additive amount of PC ·HCL, and the good linearity wasobserved with a linear equation of y¼ 0.03517xþ 0.65346(r¼ 0.999). And the detection limit was 0.5 mMaccording tothe calculated formula 3sb/m criteria, where sb is thestandard deviation of the background current and m theslope of the calibration graph [42]. This detection limitdecreased one order of magnitude in comparison with thatobtained using the ion-selective electrodes [43, 44], and, itwas lower than that obtained at a carbon nanotube film-

Fig. 3. Cyclic voltammograms for 100 mM PC ·HCL (solid lines)and those obtained by stirring the solution after the tenth cycle(dashed lines) in 0.07 M PBS (pH 7) at a) GC electrode(0.07 cm2); b) ad-BDD electrode (0.07 cm2); c) ao-BDD electrode(0.07 cm2). Dotted lines: background currents; scan rate: 100 mVs�1.

Table 2. Comparison of stability for diamond and GC electrodes. Eoxp : oxidation peak potential: Ioxp : oxidation peak current

Electrode Eoxp (V vs. SCE) Ioxp1 (mA) [a] Ioxp2 (mA) [b] Decreased (%)

ad-BDD 1.037 3.859 3.689 4.41ao-BDD 1.024 3.876 3.565 8.01GC 1.011 12.61 10.56 16.26

[a], [b] are oxidation peak currents recorded for the oxidation of 100 mM PC ·HCL(Ioxp1Þ and those obtained by stirring the solution after the tenth cycle(Ioxp2Þ in 0.07 M PBS (pH 7) with scan rate of 100 mV s�1.

Fig. 4. Cyclic voltammograms for 10 mM PC ·HCL in 0.07 MPBS (pH 7) at the ad-BDD electrode (0.07 cm2) at different scanrates. Inset: calibration plot for the dependence of peak current onthe square root of scan rate.




modified GC electrode [31], demonstrating the superiorityof ad-BDD electrode containing lower interference, rapidand simple operation and high accuracy. There were nocathodic peaks observed during the reverse scan within theworking potential range of 0 – 1.3 V (vs. SCE), indicatingthat PC ·HCL oxidation was an irreversible reaction. Inaddition, 100% and 93% of the initial current response wasretained after 3 months at the ad-BDD and ao-BDDelectrode, respectively, demonstrating that the inactivationof the electrode with time was avoided due to the long-termstability of the ad-BDD electrode.

4. Conclusion

In this work, we compared the electrochemical oxidation ofPC ·HCL at ad-BDD electrode, ao-BDD electrode andGCelectrode. In contrast to GC electrode, well-defined cyclicvoltammograms were obtained for PC ·HCLoxidation withhigh S/B, surface inertness for adsorption, good reproduci-bility and long-term stability at ad-BDD electrode, demon-strating its superior electrochemical behavior and signifi-cant advantages. A linear range from 5 to 200 mM wasachieved, and PC ·HCL could be detected with the detec-tion limit of 0.5 mM at the ad-BDD electrode.

5. Acknowledgements

This work was supported by the Ministry of Education ofChina (Grant No. 20040286024) and the National NaturalScience Foundation of China (Grant No. 60121101).

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Comparison of Boron-Doped Diamond and Glassy Carbon Electrodes for Determination of Procaine Hydrochloride