Indian Journal of Chemistry Vol. 51A, Jan-Feb 2012, pp. 205-225

Surface modification in electroanalysis: Past, present and future Rajendra N Goyal* & Sunita Bishnoi

Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247 667, India Email: rngcyfcy@iitr.ernet.in

Received 27 October 2011

This review discusses the electrochemical sensors and biosensors based on carbon nanotubes, fullerenes, metal nanoparticles and ionic liquid/composite modified electrodes. Subsequently, recent developments and major strategies for enhancing sensing performance, the challenges and prospects of further developments in the future are debated. Besides a brief exposition of the properties of ionic liquids and their general applications based on these properties, this review focuses on the application of ionic liquids in electroanalytical sensors. Emphasis is given to direct electron-transfer reaction, origin of electrocatalysis effect of nanomaterials and use of these sensors to determine the level of biomolecules, drugs/doping agents in human body fluids. This review aims to expand on these aspects by covering the most recent trends and advances in the utilization of nanomaterials in electroanalytical techniques during the last decade or so.

Keywords: Electrochemistry, Electroanalysis, Modified electrodes, Surface modification, Electrode modification, Sensors, Biosensors, Nanomaterials, Carbon nanotubes, Fullerenes, Ionic liquids

Exploitation of nanomaterials and nanoparticles in electroanalysis is an area of research that is continually growing. Surface modification of conventional electrodes for enhanced current response is very important in developing a stable and highly target specific interface. Sensitivity and selectivity are the crucial issues for the development of sensors for detecting biologically important molecules. The aim of this article is to provide an overview of the recent works in this field including advantages and disadvantages of surface modification by carbon nanotubes, fullerenes and metal particles. Herein we have focused on the benefits of electrode surface modification as shown by sensor performance in terms of voltammetric response to the target compounds using nanoparticles, nanomaterials, combination of nanoparticles with nanotubes and ionic liquid with carbon paste. This review does not cover all aspects of nanomaterials in electrochemistry and is restricted to electrochemical sensors based on the traditional electrochemical method, voltammetry and does not cover other methods like potentiometry, impedance spectroscopy and piezoelectricity. In addition to the origin of electrocatalytic activities of different types of nanomaterials and their fundamental electrochemical knowledge, this article also attempts to focus on pharmaceutical and biomedical applications of various nanomaterials/nanaoparticles-based electrochemical sensors.

Carbon Nanotubes and Fullerenes The subtle electronic properties of carbon

nanotubes suggest that they have the ability to promote electron transfer reactions when used as an electrode in electrochemical reactions. This provides a new application in the electrode surface modification for designing new electrochemical sensors and novel electrocatalytic materials1. As a new type of carbonaceous material, carbon nanotubes (CNTs) possess some unique properties that are very different from the conventional scaled materials. Such properties include well-defined tubular structure of nanosizes, functional surfaces, modifiable ends and sidewalls, excellent chemical stability, strong electrocatalytic activity and excellent biocompatibility2. The three-dimensional special architecture of the CNTs can lead to a high loading of electrocatalysis or biomaterial onto the solid substrate and thus can enhance the efficiency for (bio)electrocatalysis. As tubular nanomaterials, the key advantages of CNTs are their small diameter and large length-to-diameter ratio that allows them to be used as molecular wires for facilitating electron transfer between biomolecules and electrodes with ultra-sensitivity. These special properties envisage promising applications in electroanalytical chemistry and make CNTs ideal candidates for constructing sensors with high performances. Both single wall carbon nanotubes (SWNTs) and multiwall carbon

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nanotubes (MWNTs) have been widely used in biosensing3,4.

Peng et al.5 suggested that the intrinsic electronic properties of carbon nanotubes remain unaffected even when they are in direct contact with water by an ab initio study of water adsorbed on single walled carbon nanotubes which showed purely repulsive interaction without any charge transfer5. This study revealed new avenues for application of carbon nanotubes modified sensors in aqueous medium. The modification of electrodes with CNTs has been observed to apparently improve the response of substrates from small H2O2 molecules to huge redox proteins. The electron transfer and the direct electrochemistry of redox proteins at CNTs based electrochemical sensors are well reported6,7. Excellent improvement in the electrochemical behavior of biologically important compounds such as dopamine and ascorbic acid8,9, quercetin and rutine10, tryptophan11, tyrosine12, procaine13 and metformine14 has been demonstrated at CNTs modified electrodes. CNTs modified electrodes have also been utilized to determine hemoglobin in bovine blood15. Multi-walled carbon nanotubes modified carbon paste electrode (MWNT/CPE) has been used to study the electrochemical behavior of Bergenin16. The modified electrode showed excellent electrocatalytic activity in lowering the anodic overpotential and remarkable enhancement in anodic peak current of bergenin as compared to the electrochemical performance obtained at CPE. Conventional electrodes are found not suitable for the determination of catecholamines, viz., epinephrine (EP) and norepinephrine (NE) due to interference from ascorbic acid (AA) and uric acid (UA), which co-exist in a real sample at 100 times higher concentration than EP and NE. These compounds can be easily oxidized at a similar potential of EP and NE and thus always interfere with EP and NE detection. Goyal et al.17,18 have developed a MWNT modified pyrolytic graphite electrode which can be used to simultaneously monitor different biomolecules. Ascorbic acid, dopamine, norepinephrine and uric acid show oxidation peaks at −50, 80, 204 and 260 mV respectively and do not interfere with the oxidation of epinephrine, confirming thereby that this voltammetric sensor is specific for the oxidation of epinephrine at 150 mV. The proposed sensor has also been utilized for the determination of of epinephrine in human urine and plasma samples of smokers and non-smokers and it is

found that the level of epinephrine in smokers is much higher than that in non-smokers. Hydrogen peroxide (H2O2) is a product of several biological enzyme-catalyzed reactions. The detection of H2O2 plays an important role in food industry, environmental protection, and in medical diagnostics. For the sensitive detection of H2O2, Tkac and Ruzgas19 have used an electrode modified with SWNT, the sensitivity of which was highly dependent on the dispersing agent in the organic solvents and charging status of polymers (e.g. Nafion and chitosan). It is found that the dispersion of both polymers is highly stable but the SWNT in the chitosan dispersion showed higher sensitivity for H2O2 as compared to that in Nafion. The single walled carbon nanotube modified gold detector for microchip capillary electrophoresis (CE) has been constructed and successfully used for the detection of p-aminophenol, o-aminophenol, dopamine and catechol20. SWNT modified glucose biosensors exhibited a wider dynamic range and greater sensitivity in glucose determination21. Rutin is a flavonoid glycoside that has a wide range of physiological activities such as antiinflammatory, antitumor and antibacterial. CNT modified electrodes have been successfully used for the determination of rutin. A gold electrode modified with SWNT was fabricated by Zeng et al.22 to investigate the voltammetric behavior of rutin. The anodic (Epa) and cathodic peak current ratio of 1, indicates that the electrode reaction is almost reversible. The method has been applied for the determination of rutin in medicinal samples. Based on the interaction of hemoglobin with rutin, this procedure has also been used for the indirect determination of hemoglobin. A nanocomposite of poly- Nile blue with SWNT modified glassy carbon demonstrated the ability to electrocatalyze the oxidation of NADPH at a very low potential (– 80 mV versus SCE) with a substantial decrease in the overpotential by more than 700 mV as compared with the bare GCE23.

Our laboratory is also actively studying the electrochemical determination of a variety of biomolecules and drugs using CNT and fullerene modified electrodes24-38. Pyrolytic graphite electrode (PGE) has been explored as a substrate for the surface modification. The two planes of PGE, namely, edge plane (EPPGE) and basal plane (BPPGE) were modified with single-wall carbon nanotubes (SWNT) or with multiwall carbon nanotubes (MWNT) and

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used as sensors to determine a variety of steroids and drugs. A simple and highly sensitive sensor based on EPPGE coated with SWNT has been proposed24 for diclofenac determination in nanomolar concentrations. The modified electrode showed excellent catalytic activity, presenting much higher peak currents than those measured on a bare EPPGE. The purity of SWNT also affected the peak potential and peak current as shown in Table 1. Thus, it is concluded that embedded metals in MWNT play a significant role in electrocatalysis. The detection limit at peak I is almost ten times lower than that observed at bare PGE. The electrode showed good sensitivity, selectivity, reproducibility and high stability with remarkable electrocatalytic properties.

The modification of pyrolytic graphite electrode using carbon nanotubes has been found to offer a marked decrease in the peak potential, high sensitivity, low detection limit and stable voltammetric sensing for amlodipine. A detailed comparison of MWNT and SWNT modified EPPGE towards the oxidation of amlodipine indicates the enhanced performance of SWNT as a novel electrode surface modifier in comparison to MWNT25. The simultaneous determination of prednisolone and prednisone in human body fluids and pharmaceutical preparations has been proposed26 by using square wave voltammetry (SWV) in phosphate buffer medium of pH 7.2. The SWNT modified electrode exhibited good electrocatalytic properties towards prednisone and prednisolone reduction with a peak potential separation of 100 mV. The results of the quantitative estimation of prednisone and prednisolone in biological fluids were also compared with estimation by HPLC; the results were in good agreement. A sensitive voltammetric method has been described for the determination of betamethasone sodium phosphate (BSP) using EPPGE modified with SWNT-cetyltrimethyl ammonium bromide nanocomposite film27. The voltammetric response of betamethasone enhanced effectively using cationic surfactant cetyltrimethylammonium bromide (CTAB) as electrode surface modifier. The nanotubes-surfactant modified EPPGE showed great improvement in peak current and shifted the reduction potential towards less negative potential. The role of cetyltrimethylammonium bromide on electrocatalytic property is discussed. The analytical utility of the developed method is demonstrated by the direct assay of betamethasone in urine samples of pregnant

women. A fast and sensitive voltammetric method has been proposed28 for the determination of salbutamol at SWNT/EPPGE in human urine. The developed method has been successfully applied for the determination of salbutamol in commercial preparations and human body fluids. Fast analysis of salbutamol in human urine makes the proposed method of great interest for detecting doping at the site of competitive games. The electrochemistry of bisoprolol fumarate (BF,) has been investigated by differential pulse voltammetry by Goyal et al.29 at a SWNT modified glassy carbon electrode (GCE). The prepared electrode showed an excellent electrocatalytic activity towards the oxidation of BF leading to a marked improvement in sensitivity as compared to bare GCE, where electrochemical activity for the analyte cannot be observed. A novel sensing system for monitoring cases of abuse of betamethasone doping is developed31 using SWNT/EPPGE and has excellent analytical characteristics such as lower detection limit, high sensitivity, satisfactory recovery and selectivity along with good reproducibility. The method fully satisfies the requirement of World Anti Doping Agency (WADA); the limit of detection (LOD) being well below the minimum required performance limit (MRPL) of 30 ng/mL for the corticosteroid under investigation. Therefore, proposed method can be successfully applied as a wonderful analytical tool in clinical analysis and anti doping tests for the detection of illicit administration of betamethasone by athletes.

The advantage of this method is that some common metabolites like ascorbic acid, uric acid, albumin and hypoxanthine do not interfere as the determination is based on reduction. A comparison of the results observed with this method and those with HPLC clearly shows that both methods are essentially similar.

The electrochemical investigation of two corticoid isomers, viz., testosterone and epitestosterone has

Table 1⎯Effect of metallic contents of SWNT on peak potential and peak current of peak I of 50 nM diclofenac in phosphate buffer of pH 7.2

Metal content (%) Diclofenac Sample

Fe Co Ni Ep (mV) ip (μA) Untreated SWNT

0.819 0.412 0.207 439 22.1

Purified SWNT 0.373 0.023 0.101 481 18.5 Super-purified SWNT

0.318 0.013 0.078 525 16.3

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been carried out at bare and SWNT modified EPPGE32. Square wave voltammetry has been used for the simultaneous determination of isomeric steroids. This novel method has been successfully applied for the analysis of testosterone (T) and epitestosterone (E) in human urine samples of normal male as well as in patients being treated with testosterone. The studies clearly revealed that in control urine samples, the ratio of T/E was in the range 1.04–1.42, whereas in the patients treated with testosterone the ratio increased to 5.57 – 5.74. Thus, the ratio increased nearly five times with a single dose of testosterone. Hence, it is believed that the developed method can be easily used for determination of testosterone doping by athletes. A comparison of the present method with HPLC indicates that the method is sensitive and the results are comparable. The voltammetric oxidation of paracetamol on SWNT modified EPPGE has also been explored by using square wave voltammetry33. Cyclic and square wave voltammetry studies indicate the oxidation of paracetamol at the electrode surface through a two-electron reversible step and controlled by adsorption. Besides semi-infinite planar diffusion, the role of thin layer diffusion at nanotube modified electrodes is also suggested. The sensitivity at SWNT modified EPPGE is ~2 times more than that at MWNT modified EPPGE. The interfering effect of physiologically common interferents on the current response of paracetamol has also been reported. The applicability of the developed method to determine the drug in human urine samples obtained after 4 h of administration of paracetamol has also been illustrated. The simultaneous determination of adenosine and inosine has been carried out by Goyal et al.

34 at SWNT modified PGE using square wave voltammetry. The modified electrode exhibited remarkable electrocatalytic properties towards adenosine and inosine The proposed method was also used to estimate these compounds in µM range in human blood plasma and urine samples and the method was validated using HPLC. The use of such modified electrodes for the determination of purine derivatives has also been explored35. A simple and sensitive method based on SWV at SWNT modified EPPGE is proposed for the simultaneous determination of adenine and adenosine-5-monophosphate (5-AMP). The modified electrode was found to exhibit remarkable electrocatalytic properties towards adenine and 5-AMP oxidation. The

effect of pH revealed that the oxidation of adenine and 5-AMP at SWNT modified EPPGE involved an equal number of electrons and protons. The modified electrode exhibited high stability and reproducibility. Purine nucleoside, viz., 2,3-dideoxyadenosine has also been determined at PGE using voltammetric, coulometric, spectral studies and product analysis The studies indicate that the oxidation occurs in an EC reaction (electrochemical followed by chemical) involving 6e, 6H+ process at pH 7.2 to give allantoin, C–C dimer and dideoxyribose as the major products and a C–O–O–C linked dimer as a minor product. Tentative mechanisms for the formation of the products have also been suggested36. A comparison of peak potential value of 2,3-dideoxyadenosine with adenosine and 2-deoxyadensoine indicates that the difference is insignificant which has further been supported by the calculations of difference of energies of lowest unoccupied and highest occupied molecular orbitals.

Since reduction has no or very little interference from the common metabolites present in human blood or urine, an electrochemical protocol based on reduction has been developed to determine methylprednisolone using SWNT modified EPPGE. To obtain good sensitivity, instrumental variables were studied using SWV. The voltammetric results indicate that SWNT modified EPPGE remarkably enhances the reduction of methylprednisolone, which leads to considerable improvement of peak current with shift of peak potential to less negative values. The limit of detection was estimated to be 4.5×10−9 M. The developed method has been used37 for the determination of methylprednisolone in pharmaceutical dosages and human blood plasma samples of patients undergoing treatment with methylprednisolone. The major metabolites present in blood plasma did not interfere with the present investigation as they did not exhibit reduction peak in the experimental range used. A comparison of results with HPLC indicates good agreement. In another attempt, electrochemical sensor employing EPPGE and BPPGE for the sensitive detection of hydrocortisone (HC) is delineated for the first time. HC (cortisol, 11, 17, 2l-trihydroxy-4-pregnene-3,20-dione) is the main glucocorticosteroid exuded by the adrenal cortex gland and acts to limit the body's response to stress. It has been shown that EPPGE displays a better voltammetric response in comparison to BPPGE due to its low signal-to-noise ratio,

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increased sensitivity, low detection limit and large potential window38. The peculiar catalytic activity of an EPPGE relies on the crystal orientation on its surface. The high percentage of the edge orientation results in the high catalytic activity. The edge orientation on the surface of an EPPGE serves as an ‘‘active site’’ for the reduction of HC. Highly ordered pyrolytic graphite (HOPG) is used to fabricate the edge and basal plane electrodes. EPPGE is an electrode constructed from HOPG where the graphite layers are perpendicular to the disc surface and are separated with an interlayer spacing of ~ 3.35 Å. Surface defects occur in the form of steps exposing the edges of the graphite layers. Conversely, a BPPGE is fabricated such that the layers of graphite lie parallel to the surface. The difference between the edge and basal orientations may account for the fact that functional groups are easier to adsorb on the edge plane. The developed protocol has lower detection limit (88 × 10−9 M) as compared to the previously notified at carbon paste electrode modified with β-cyclodextrin (4.2 × 10−7 M). Triamcinolone, abused by athletes for doping, has been determined by Goyal et al.39 in human urine samples. The surface modification has been found to increase the effective surface area of the electrode to about two times. A comparison of the voltammetric behavior between SWNT modified PGE and fullerene–C60-modified EPPGE indicates that the SWNT modified EPPGE is more sensitive. The method was applied for the determination of triamcinolone in several commercially available

pharmaceuticals and real urine samples obtained from patients undergoing pharmacological treatment with triamcinolone. A comparison of square wave voltammograms observed for triamcinolone at different electrodes is presented in Fig. 1. The studies indicate that SWNT is a better surface modifier in comparison to fullerene for the reduction of triamcinolone, as it accelerates the rate of electron transfer faster39.

Goyal et al.40 recommended that the SWNT/EPPGE showed great improvement in voltammetric response of guanine and 8-hydroxy-guanine in terms of yield of large peak currents and lower peak potential as compared to bare EPPGE. Hence, a simple and reliable method based on voltammetry is proposed for the determination of oxidative DNA damage by the simultaneous determination of guanine and 8-hydroxygunine. Formic acid was used for DNA hydrolysis since it does not cause any artifacts. Other purine and pyrimidine derivatives are also likely to be present in the matrix. Thymine, adenine and cytosine oxidize at higher potentials than guanine and 8-hydroxyguanine and thus do not interfere in the determination. Figure 2 indicates that at bare EPPGE the oxidation of guanine and 8-hydroxyguanine occurs with broad peaks at 640 and 452 mV, respectively. After modification of electrode, under identical conditions the peak potential shifted negatively and oxidation occurred with well-defined peaks at 556 and 360 mV for guanine and 8-hydroxyguanine, respectively along with substantial increase in peak current.

Fig.1⎯A comparison of square-wave voltammograms of 10 nMtriamcinolone at pH 7.2 at bare BPPGE (b), bare EPPGE (c), fullerene modified EPPGE (d) and SWNT modified EPPGE (e). (a) is the background phosphate buffer solution (pH 7.2) at SWNT modified EPPGE. [Reproduced from Ref. 39 with permission from Elsevier, Amsterdam, The Netherlands].

Fig. 2⎯Cyclic voltammograms of homogeneous solution of guanine and 8-hydroxyguanine in PBS of pH 7.2 using (a) bare edge plane PGE (−·−·−·) and (b) SWNT modified EPPGE ( — ) at scan rate of 20 mV/s. [Reproduced from Ref. 40 with permission from Elsevier, Amsterdam, The Netherlands].

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Origin of electrocatalytic effect of nanotubes There has been an explosion of research on

modifying electrodes with carbon nanotubes in the search for the origin of electrocatalytic responses of nanotubes41-43. It is well documented that such improvements are due to the unique structure of the carbon nanotubes, where the presence of edge plane like-sites/defects lead to heterogeneous charge transfer44,45. In this regard, Compton’s group41,44 reported an interesting investigation related to the reason why CNTs present enhanced electrocatalytic activity and proposed that this activity is due to the presence of edge-plane like sites located at the end and the “defects” areas of the tubes. Further, ab initio calculations demonstrated that the improvement in the electron transfer is due to the curvature of the tubes that originate changes in the energy bands close to the Fermi level46. Pentagonal defects in planar and tubular structures produce regions with charge density higher than that observed in the case of hexagonal defects demonstrating the connection between topological defects and CNTs electroactivity46. Carbon nanotubes can be seen as graphene sheets rolled into tubes, with the cap region more reactive due to a much higher curve strain than the sidewall. Based on their specific structures, two distinct surface regions exist in CNT; the sidewalls and the ends. Opening of the ends of CNT by physical/chemical treatment produces a variety of oxygen-containing groups47. Moreover, the intact CNT sidewalls resemble the basal planes of pyrolytic graphite and can be regarded as electrochemically inert to electroactive species. Due to the presence of defects and oxygen-containing functional groups, the opened caps have electrochemical properties similar to those of edge planes of pyrolytic graphite. Thus, the introduction of edge-like defect sites and oxygen-containing functional groups, at both the caps and the sidewalls, by chemical or physical treatments can significantly improve the electrochemical properties of CNTs by changing the electronic structures, surface states and the wetability of the sidewalls.48 The critical roles of defect sites and oxygen-containing groups in enhancing electrochemical performances of CNT-based electrodes have been proved by several research works.49,50 The adsorbed acid moieties, during purification and acid-treatment processes, can also decrease the electrocatalytic activity of CNTs in electroanalysis. Pumera and Kolodiazhnyi51 suggested the use of dc magnetic susceptibility and electron

paramagnetic resonance for screening and quality control of CNTs before using them in electroanalysis. Recently, Dai and co-workers52 reported that vertically aligned nitrogen-doped CNTs can act as a metal-free electrode with a much better electrocatalytic activity. The electrocatalytic activity and the electroanalytical performance at CNTs modified electrodes are strongly depended on the mode of production of the CNTs, either by chemical vapour deposition (CVD) or the arc discharge (ARC) process53. CNTs produced by CVD appear to be more electrochemically reactive in their voltammetric study than those produced by the ARC methodology. The differences in the electrochemical reactivity are attributed to the smaller fraction of exposed edge planes at ARC-CNTs and higher density of edge plane defects at CVD-CNTs. The electrocatalytic activity of ARC-CNTs can be increased after pre-anodization. Wang’s group54 illustrated the effect of electrochemical pretreatment of ARC- and CVD-prepared multi-walled CNTs using nicotinamide adenine dinucleotide (NAD), ascorbic acid, hydrazine, and hydrogen peroxide model redox systems54. The fact that the ARC-CNT displays a marked improvement in electrochemical reactivity indicates that the pre-anodization effectively breaks the end caps of ARC-CNTs to expose new edge plane-like sites. The influence of metallic impurities upon the electrochemistry of CNTs has been discussed by Compton and co-workers55. They demonstrated that iron-based impurities within carbon nanotubes are responsible for the electrocatalytic oxidation of glucose55 and the reduction of hydrogen peroxide56. Further, it was also established that copper nanoparticle impurities within CNTs cause the electrocatalysis of halothane and glucose57. The residual catalyst nanoparticles that are encapsulated within the CNT graphene lattice may still be chemically accessible and could participate in the redox chemistry of biomarkers through the intercalation of molecules within the CNT lattice58, as deep as 12 nm. Differential pulse voltammetric responses of dopamine at bare PGE and non-treated as well as acid-treated SWNT modified GCE were compared by Goyal et al.59 A significant decrease in peak current of DA at acid-treated SWNT modified GCE was observed in comparison to raw-SWNT modified GCE as shown in Fig. 3. A comparison of peak current and peak potential of DA at purified and super purified SWNT confirmed that with decrease in embedded metals, the oxidation becomes difficult.

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This indicates that metallic impurities present in CNT are responsible for the enhanced peak current and it is concluded that 98 % pure SWNT are sufficient for surface modification and further purification is not necessary.

An uncontrollable amount of impurities in the CNT samples is evidently not desirable. However, from a broader perspective it is possible to direct the controllable decorating of CNTs with metallic nanoparticles and use their electrocatalytic properties in a controllable way for enhancing the performance of sensors and energy-storage devices.60, 61 Since carbon nanotubes can be seen as the graphene sheets rolled into tubes, the electrochemical properties of carbon nanotubes are comparable to those of the basal planes of pyrolytic graphite (BPPG). For intact carbon nanotubes, the defect-free structure makes the whole tubes possess almost the same properties as that of BPPG, except that the cap regions may be more reactive than the sidewall due to the much higher curve strain. The opening of the ends by physical/chemical treatments on carbon nanotubes produces a variety of oxygen-containing groups,

which possess properties similar to those of the edge planes of PGE62. Due to the simple and well-defined response at carbon materials, the Fe(CN)6

3-/Fe(CN)64-

couple has been widely used63 to characterize the surface properties of all kinds of carbon electrodes (Fig. 4). Similarly, the electrochemical properties of carbon nanotube based electrodes are generally investigated by Fe(CN)6

3-/Fe(CN)64- as the probe. The

electrochemical behaviour of aligned bundles of carbon nanotubes with other carbon electrodes of similar structures was compared by Nugent et al.64 The results indicate that Fe(CN)6

3-/Fe(CN)64- shows

an ideal redox peak separation of 59 mV at the aligned multi-walled carbon nanotubes. In comparison, they reported ΔEp of more than 100 mV and 700 mV for the basal planes of highly oriented pyrolytic graphite (HOPG) with and without electrochemical pretreatments, respectively. In contrast to the ideal response reported by Nugent et al.64, Li et al.62 observed much larger ΔEp at single-walled carbon nanotube modified electrode (ΔEp = 94 mV). At aligned MWNT with heat pretreatment to remove impurities like amorphous carbon, ΔEp was observed as 228 mV. Particularly, for aligned MWNT, the apparent electron transfer rate was found to correlate with both the area of the exposed sidewalls (with graphite basal-plane-like properties) and the density of graphite edge-plane-like defects.

Fig. 3⎯Differential pulse voltammograms of 25 μM DA recorded at pH 7.2 at (i) basal plane pyrolytic graphite electrode, (ii) edge plane pyrolytic graphite electrode, (iii) super-purified SWNT modified GCE and (iv) untreated SWNT modified GCE.[Reproduced from Ref. 59 with permission from Elsevier, Amsterdam, The Netherlands].

Fig. 4⎯Cyclic voltammograms for the reduction of 1 mMferricyanide for different carbon electrodes at a scan rate of 100 mV s-1. [Reproduced from Ref. 64 with permission from The Royal Society of Chemistry, London, UK].

Fullerenes

An important application of fullerenes is their use as mediators in electrochemistry for the chemical modification of electrodes in electrocatalysis.

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Fullerene (C60 or C70) modified electrodes catalyze the redox reaction of a variety of compounds due to the formation of more conducting C60

n- species during partial reduction of C60, which helps electron transfer at the interface65,66. Fullerene anions (reduced fullerene) can abstract protons from biomolecules. A very interesting feature of the partial reduction of fullerene films in aqueous solutions is the proposed ‘sandwich like’ reduction. Partially reduced fullerene films have a structure with a polar inner and outer surface, while the inside is non-polar. The structure cloaely resembles a biological membrane, raising the possibility of using fullerene as solid state modifiers to study the electrochemistry of biomolecules67. Therefore, fullerene modified electrodes meet the conditions essential for the analysis of biomolecules, i.e., excellent reproducibility, high sensitivity, wide potential range and high stability in biological samples68. The performance of fullerene –C60–modified electrodes has been reported to produce electrocatalytic responses compared to the underlying electrode for certain target analytes. Jehoul et al.69 demonstrated the formation of C60 film on an electrode surface by evaporation of fullerene solutions and suggested the requirement of further studies of its electrochemistry69. Szucs and co-workers70 explored gold surfaces for the electrochemical determination of cytochrome c. Further, the electrochemical response of cytochrome c was also determined by Csiszar et al.71

using fullerene C60 modified electrodes with optimum results. Tan, Bond and co-workers72 reported the electrochemical oxidation of L-cysteine in aqueous solution using C60-modified glassy carbon electrode. Goyal et al.73 reported the electrochemical oxidation of uric acid mediated by C60 supported on glassy carbon electrodes. The introduction of C60 on the glassy carbon surface facilitated resolution of the overlapping voltammetric response of uric acid and ascorbic acid. Two well-defined voltammetric peaks with a potential difference of ~150 mV were observed. Simultaneous voltammetric determination of adenine and guanine at C60 modified glassy carbon electrode at physiological pH has been described by Goyal et al.74. Analytical application of the developed protocol for determination of (G+C)/(A+T) ratio in DNA samples has been suggested. Nandrolone (also known as nortestosterone or 17-hydroxy-19-nor-4-androsten-3-one) is one of the most abused androgenic anabolic steroid which occurs naturally in tiny quantities in the human body. The

electrochemical behaviour of nandrolone has been investigated at fullerene-C60-modified electrode75. The modified electrode shows an excellent electrocatalytic activity towards the oxidation of nandrolone, resulting in a marked lowering in the peak potential and considerable improvement of the peak current as compared to the electrochemical activity at the bare glassy carbon electrode. This method was successfully employed for the determination of nandrolone in human blood serum and urine samples. Simlarly, adenosine and guanosine have also been studied by the same group76. This method was not only easy to perform, but also required less time, financial input and sample amount than other reported approaches, and hence, is an attractive alternative to HPLC analysis in both routine and research laboratories. Origin of electrocatalytic activity of fullerenes

The unique structure of C60 has a distinct lack of edge plane like-sites/defects78,79 and consequently the origin of the reported electrocatalysis is quite interesting. The work by Compton and co-workers80

has clearly indicated that the origin of the electrocatalytic response observed at C60 modified carbon electrodes as reported by Tan, Bond and co-workers for the electrocatalytic detection of cysteine72 is unambiguously due to graphite impurities in C60. More recently, determination of salbutamol81 and dopamine in the presence of ascorbic acid82 have been reported using C60–modified electrodes. The authors of these reports suggest that the observed electrocatalytic activity is due to the partially reduced conductive C60 film in addition to graphite impurities. The reduction of C60 films in aqueous media is in fact the electrochemically reversible reduction of adventitious C60On, with subsequent rapid loss of “O2

−” in an irreversible chemical step. There is no evidence that C60 itself is reduced within the potential window of aqueous electrolytes. In another attempt, in the case of the target analyte nandrolone, Goyal and co-workers83 have carefully examined the effect of metallic impurities in the C60 and found that just like with CNT, the removal of embedded metals from fullerene shifts the peak potential of nandrolone to more positive potentials with decrease in peak current as shown in Table 2. Thus, the untreated fullerene modified electrode exhibits enhanced catalytic effect as compared to the acid purified and super-purified C60 modified electrodes.

Although, the origin of the electrocatalysis has always been attributed due to partially reduced

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conductive C60 film, recent reports have indicated that this is not the only reason. The presence of graphite impurities84, metal impurities85 and the pretreatment employed86 may all be the origin or contribute significantly to the observed electrocatalysis at C60–modified electrodes depending on the experimental parameters, chosen not only during the electrochemical measurements, but also during the synthesis of fullerenes. It is likely that other overlooked parameters may also contribute to the electrocatalytic effect of C60 modified electrodes and, thus it is recommended that control experiments should be performed before electrocatalysis of C60 modified electrodes is claimed. It must also be realized that it is practically impossible to get 100 % pure fullerenes without breaking the structure as embedded metals in the cavities cannot be completely removed. Metal Nanoparticles

Metal nanoparticles (NP) have wide applications in different kinds of electroanalytical methods and can be used to construct novel and improved sensing devices, particularly electrochemical sensors and biosensors. Owing to their small size (in order of 1-100 nm) metal nanoparticles exhibit unique chemical, physical and electronic properties. They can absorb biomolecules strongly and play an important role in the modification of electrodes to improve their electrocatalytic activities. Metal nanoparticles increase electrochemical activities as they exhibit higher catalytic efficiency per gram than the bulk materials, good performance, enhancement of mass transport and good biocompatibility. Bioactivity of biomolecules is retained on the surface of nanoparticles due to their biocompatibility. Metal nanomaterials enhance the performance of the biosensors by enlarging the effective surface area87. Large surface area of deposited metal nanoparticles permits improvement of analytical performance in terms of low detection limit and short deposition time. Transition metal nanomaterials possess high catalytic activity and facilitate electron transfer for many

electrochemical reactions. A wide variety of metallic nanoparticles have been studied to assess the applications of these materials in electroanalysis. Biosensors incorporating metal nanomaterials, including platinum black88, copper89, silver90 palladium91 and gold92 have exhibited good biocompatibility and enhanced performance. Nanoparticles of bismuth and iridium have also been synthesized recently.

Campbell et al.93 studied H2O2 reduction at a silver nanoparticles (AgNPs) on BPPGE, and have shown that the voltammetric trace for H2O2 reduction varies with both nanoparticles size and the extent of surface coverage. A decrease in nanoparticles size causes a negative shift in the peak potential, whereas increasing coverage causes a positive shift. Additionally, nanoparticles size effects have been simulated by Ward-Jones et al.94 for the anodic stripping voltammetry of various sizes of AgNPs. Theory has been presented for modeling the voltammetry produced on stripping the nanoparticles from the surface of an electrode. Radial diffusion and the proximity of the particles to each other were considered in the model. Electrochemical detection of hydrogen peroxide using an edge-plane pyrolytic-graphite electrode, a glassy carbon electrode, and a silver nanoparticle-modified glassy carbon electrode has been reported by Welch et al.95. The hydrogen peroxide which could not be detected directly on either the EPPGE or GCE electrodes in phosphate buffers was facilitated by modification of the glassy-carbon surface with nanosized silver assemblies.

Screen-printed electrodes are planar devices with plastic substrates that are coated with layers of electroconductive and insulating inks at controlled thickness. Such an electrode modified with silver nanoparticles by using electrochemical deposition has been utilized successfully by Calvo et al96. Carbon screen-printed electrodes (CSPE) modified with silver nanoparticles present an interesting alternative in the determination of lamotrigine (LTG) using differential pulse adsorptive stripping voltammetry. The electrodes developed in this work present an

Table 2⎯Effect of metallic contents of fullerene on peak potential and peak current of nandrolone in phosphate buffer of pH 7.2

Metal content (%) Nandrolone Sample Fe Cu Co Ni Ep (mV) ip (μA)

Untreated fullerenes 0.416 0.191 0.007 0.392 456 11.7 Purified fullerenes 0.162 0.164 0.001 0.263 492 7.25 Super-purified fullerenes 0.099 0.071 0.001 0.086 548 5.43

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environmental-friendly method for the analysis of LTG. The most important advantage is high sensitivity in the determination of LTG in real samples. In fact, the detection limit obtained was less than the values found when LTG was measured with carbon and mercury-film carbon screen-printed electrodes.

Electrochemical study of sodium hypochlorite was carried out on dendrimer stabilized gold nano particles modified glassy carbon electrode97. Deposition of gold nanoparticles on the surface of glassy carbon electrode was observed by transmission electron microscopy and X-ray photoelectron spectroscopy. Both anodic and cathodic peak currents were found to increase after the deposition of these gold nanoparticles. Gold nanoparticles modified glassy carbon electrode has also been used to detect Hg(II) in drinking water, sediments and pharmaceutical preparations by anodic stripping voltammetry. Mercury concentrations in the low range (ng/ml) were easily quantified with high accuracy and precision98. Further, gold nano particles were deposited onto the surface of glassy carbon electrode and potential utility of electrode constructed was demonstrated by applying it for the analytical determination of As(III) with low detection limit99 of 2.5 ppb. Gold being very sensitive to Cr(VI), gold nanoparticles based nanostructured electrode was developed for detection of ultra trace amount of the carcinogenic Cr(VI) and evaluated for detection of Cr(VI) in ground water100. Arsenic is highly toxic and a novel method for the detection of arsenic(III) in 1 M HCl at gold nanoparticles modified glassy carbon electrode has been developed101. After optimization, a limit of detection (LOD) of 0.0096 ppb was obtained with linear sweep voltammetry (LSV). These results point towards the applicability of the modified sensor for detection of arsenic in natural water samples.

Simultaneous determinations of various biomolecules and drugs have vital importance in biomedical research. A gold nanoparticle-modified carbon paste electrode has been used successfully for the simultaneous determination of acetaminophen and atenolol102 using differential pulse voltammetry (DPV). The modified electrode exhibited electrocatalytic properties towards acetaminophen and atenolol oxidation with a peak potential of 20.0 and 50.0 mV lower than that at the bare carbon paste electrode, respectively. Also, the enhanced peak current response is clear evidence of the catalytic activity of gold nanoparticles modified carbon paste electrode towards

oxidation of acetaminophen and atenolol. The large peak separations obtained using this electrode allows simultaneous detection of these drugs. Goyal et al.103-107 prepared nanogold modified indiumtin oxide (NGITO) electrode and deposition of nanogold (~40 nm) was confirmed by scanning electron microscopy (SEM).105 The electrode was used for the individual and simultaneous electrochemical determination of various drugs and biomolecules including nandrolone, methylprednisolone, atenolol, adenosine and adenosine-5’-triphosphate, 5-hydroxy-tryptamine and 5-hydroxyindoleacetic acid. As such the electrode is not suitable for reduction and the determinations are based on the oxidation of target molecules. Application of the electrode has been successfully demonstrated by determining these molecules in human urine and blood plasma. The common metabolites present in urine and blood did not interfere. The method using NGITO is simple, fast and accurate and opens new avenues for quick estimation of physiologically important compounds. Further, as gold nanoparticles provide larger conductive area, it leads to improved electron transfer kinetics and enhancement in the oxidation current. Also, the amount of nano gold on the electrode surface affects the electrocatalytic activity of the modified electrode. The interesting characteristics of gold nanoparticles diminish if they gather to form large clusters at the surface of the electrode.

Electrocatalytic oxidation of aspirin and acetaminophen at cobalt hydroxide nanoparticles modified glassy carbon electrode was performed by Houshmand et al.108 using cyclic voltammetry, chronoamperometry and steady-state polarization measurements. Voltammetric studies show that in the presence of drugs, the anodic peak current of low valence cobalt species increases, followed by a decrease in the corresponding cathodic current. This indicates that drugs are oxidized on the redox mediator which is immobilized on the electrode surface via an electrocatalytic mechanism. Chronoamperometric studies showed a large anodic current at the oxidation potential of low-valence cobalt hydroxide in further support of the mediated electrooxidation. An amperometric procedure was successfully applied for quantification of these drugs in the bulk form and to the assay of aspirin and acetaminophen in human urine samples. Nanofibers

Recently, due to its low toxicity and insensitivity toward the dissolved oxygen in a solution,

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Bi electrodes have attracted much attention as a new alternative of common mercury electrode due to their highly reproducible response109. Platinum and palladium nanoparticles based graphite nano fibers were used as cathodic electrocatalysts for proton exchange membrane water electrolysis for hydrogen evolution reactions110. Sensitive analytical methodology was presented at nano TiO2-Au-KI modified glassy carbon and indium tin oxide electrode for the investigation of hydrogen peroxide. The practical utility of the modified electrodes was examined111 by analyzing the real samples as antiseptic and contact lens cleaner solution containing H2O2.

Platinum metal is widely used as industrial catalyst and is found in catalytic converters of vehicles to remove pollutants from car exhaust fumes. Platinum wires are often used in electrochemistry as electrodes owing to their stability and conductivity. Chang et al.112 presented a method for the attachment and structural growth of platinum nano particles (PtNPs) on indium tin oxide (ITO). The PtNPs attached ITO (PtNP/ITO) has been applied for the study of electrochemical oxidation of methanol and the results are presented in Fig. 5. It can be seen that the peak current in the case of PtNP/ITO is much more than other two electrodes.

Mixture of Metal Nanoparticles and Nanotubes Electrodeposition of metal nanoparticles on carbon

nanotubes has been found very beneficial in electroanalysis of biologically important molecules. It has been observed that carbon nanotubes work as catalyst supporting materials and significantly enhance the electro catalytic activity of metal nanoparticles for electrooxidation. These supporting carbon nanomaterials change the morphology and electronic structure of metal particles due to which metal nanoparticles-carbon nanotubes catalysts exhibit high current density and low overpotential for electooxidation and can be used in direct methanol fuel cells113. Incorporation of nanoparticles in CNTs for modification of electrodes enhances the electrocatalytic activity in many electrochemical processes and therefore is suitable for sensing applications. In particular, the combination of metal nanomaterials and CNTs for surface modification of biosensors has proved to be more effective than using either nanomaterial alone. Hrapovic et al.114 focused on metal nanoparticles/CNTs nanocomposites for electrochemical detection of trinitrotoluene (TNT) and other nitroaromatics. They found that Cu nanoparticles and SWNT solubilized in Nafion

provided the highest sensitivity for TNT with the detection limit of 1 ppb in tap water, river water and contaminated soil.

Fig. 5⎯Cyclic voltammograms obtained for 0.1 M methanol oxidation in the N2-saturated 0.5 M sulfuric solution recorded with (a) the PtNP/ITO electrode prepared via 24 h of growth, (b) a Pt bulk electrode and (c) a bare ITO electrode. [Reproduced from Ref. 112 with permission from American Chemical Society, Washington DC, USA].

Effect of iron and iron oxide combined with nanotubes as electrode surface modifier has been studied by Abolanle et al.115 Electrochemical sensors using EPPGE modified with single-wall carbon nanotubes-iron(III) oxide (SWNT/Fe2O3) nanoparticles have been examined for the sensitive detection of DA. When compared with the bare electrode and electrode modified with only nanotubes, the EPPGE-SWNT-Fe2O3 gave the best response towards the detection of DA. This electrode was reliably used to assay DA in its real drug composition. As can be easily seen from the voltammograms in Fig 6, the electrodes containing SWNT are associated with large background (capacitive) current responses, characteristic of acid-treated SWNT.

The electrochemical behavior of rutin on a gold nanoparticle/ethylenediamine/multi-wall carbon nanotubes modified glassy carbon electrode (AuNPs/en/MWNTs/GCE) was investigated and the electrochemical parameters of rutin were calculated116. Rutin effectively accumulated on the AuNPs/en/MWNTs/GCE. The coexisting substance, AA, caused no interference to the determination of rutin. The proposed method was further applied to the determination of rutin in rutin tablet samples with satisfactory results. Figure 7 shows the cyclic voltammograms of rutin at different modified electrodes. Rutin does not display any redox peak at the bare GCE (a), which demonstrates the weaker

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adsorption and slower electrochemical reaction of rutin on the GCE surface. However, well-defined redox peaks are seen at the en/MWNTs/GCE (b), MWNTs/GCE (c) and AuNPs/en/MWNTs/GCE (d) in 0.1 M phosphate buffer solution (pH 3.5). The heights of the redox peaks were obviously higher at the AuNPs/en/MWNTs/GCE than at the en/MWNTs/GC or MWNTs/GCE. The ratio of ipa/ipc was approximately 1.2, which shows that the electrode reaction was almost reversible. Nano-gold and MWNTs are expected to enhance the electron-transfer rate and increase the participation of rutin in the electrochemical reaction due to accumulation and catalytic ability.

Fig. 6⎯Typical cyclic voltammograms of (i) bare EPPGE, (ii) EPPGE-SWNT, (iii) EPPGE-SWNT–Fe and (iv) EPPGE-SWNT–Fe2O3 in 0.1 M PBS (pH 7.0) containing 2 × 10−4 M dopamine at scan rate = 25 mV s−1. [(a) original; (b) background-subtracted]. [Reproduced from Ref. 115 with permission from Elsevier, Amsterdam, The Netherlands].

Tashkhourian et al.117 reported that silver nanoparticle-carbon nanotube modified carbon paste electrode (Ag/CNT–CPE) exhibits excellent electrochemical catalytic activities towards the oxidation of DA and AA by significantly decreasing their oxidation overpotentials and enhancing the peak currents. The peak separation between DA and AA was found to be 67 mV, indicating that the Ag/CNT–CPE facilitated their simultaneous determination. Further, multi-walled carbon nanotubes were functionalized with acid treatment and thereafter gold-copper nanoparticles were electrodeposited on the MWNT by applying several repetitive scans, forming a Cu-Au-MWNT/GCE interface. The electrochemical reduction of oxygen was studied on

this modified electrode in 0.1 M NaOH solution.118

The peak potential of O2 reduction at the Cu-Au-MWNT/GCE shifted from ca. 70 mV to higher positive potentials as compared to that of a polished bare glassy carbon electrode. Significant current enhancement was obtained at Cu-Au-MWNT/GCE as compared to that of bare GCE, MWNT/GCE, Cu-MWNT/GCE and Au-MWNT/GCE.

Fig. 7⎯Cyclic voltammograms of 8.0 × 10−5 M rutin in 0.1 M phosphate buffer (pH 3.5) on different electrodes. [(a) bare GCE, (b) en/MWNT /GCE, (c) MWNT/GCE and (d) AuNPs/en/MWNT/GCE; scan rate 100 mV s−1].[Reproduced from Ref. 116 with permission from Elsevier, Amsterdam, The Netherlands].

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Electrochemically deposited Pt nano-clusters on MWNT modified GCE showed a strong electrocatalytic activity towards the oxidation of estrogens involving estradiol, estrone, and estriol.119 The electrode showed linear response from 0.5-15 μM, 2.0-50 μM and 1.0-75 μM for estradiol, estrone, and estriol, respectively using square-wave voltammetry In comparison with the MWNT/GCE or Pt nanoparticles modified GCE prepared in a similar manner, this composite modified electrode exhibited much higher current sensitivity and catalytic activity. Wei and co-workers120 used a composite of nano-silver coated MWNT to determine traces of thiocyanate in urine and saliva samples from smokers and non-smokers by cyclic voltammetry with LOD in nM range. The characterization of nano-silver coated multi-walled carbon nanotubes was carried out with SEM and XRD methods. Liu et al.121 designed a sensor by electropolymerization of thionine at the GCE modified with gold nanoparticles-multiwalled carbon nanotubes (GNPs/MWNT) composites for simultaneous determination of adenine and guanine in DNA. The modified electrode exhibited enhanced electrocatalytic behavior and good stability for the detection of guanine and adenine.

An important catecholamine neurotransmitter is epinephrine, which is involved in the message transfer of the mammalian central nervous system. In biological fluids such as blood and urine, EP coexists with AA, and UA, which interferes during the electrochemical detection of EP at conventional electrodes. Chen’s group122 developed a method for the simultaneous determination of AA, EP, and UA at physiologically relevant conditions by using the film composed of functionalized-MWNT and Nafion incorporating platinum and gold nanoparticles modified glassy carbon electrode. A new glucose electrochemical biosensor has also been designed by Norouzi et al.123 in which glucose oxidase, the important sensing material which catalyzes the oxidation of glucose was immobilized on to gold nano particles and MWNT modified glassy carbon electrode. The electrode was found to exhibit an outstanding and reproducible sensitivity for glucose oxidation. Electrochemical sensing of adenine was studied at SnO2 nanoparticles modified carbon paste electrode. The coupling of these nanoparticles with carbon paste brought in some advantages such as an improved voltammetric response of adenine with high sensitivity and selectivity. The modified electrode was

successfully utilized to detect specific nucleic acid hybridization and for monitoring of some proteins124.

Preparation and characterization of nano-copper coated multi-walled carbon nanotubes modified glassy carbon electrode (Cunano/CNTs–Nf/GCE) for the determination of nitrite was demonstrated by Yang et al.125 In this work, Cu nanoparticles were electrodeposited onto the film of Nafion-solubilized multi-walled carbon nanotubes deposited on GCE. The resulting Cunano/CNTs–Nf/GCE exhibited excellent electrocatalytic activity towards the reduction of nitrite. The simplicity, low detection limit, low applied potential, high sensitivity, fast response time and wide linear range is attributed to the synergistic effect of CNTs and Cu nanoparticles.

The electrochemical decoration of EPPGE with cobalt and cobalt oxide nanoparticles integrated with single-walled carbon nanotubes (EPPGE–SWNT–Co) was presented by Adekunle et al.126 The current response of the EPPGE–SWNT–Co towards nitrite was approximately 3 folds higher than that of bare electrode (Fig. 8a) with slightly lower onset potentials (0.78 V). At pH 3.0, the EPPGE-SWNT–Co exhibits faster catalysis as compared to EPPGE-Co (less positive potential, 100 mV lower than Co electrodes without SWNT), although the current response was almost the same. It is evident from Fig. 8(a) that the oxidation potential was lower with high current response on the EPPGE–SWNT–Co nanoparticles modified electrode. The better electrooxidation reaction of the EPPGE–SWNT–Co can be attributed to the SWNT itself acting as an electrical conducting nanowire which enhances electron transport between the base electrode and analyte.

The integration of nano ZnO and MWNT has been explored by Zhang et al.127 to enhance the electrochemical signal of DNA and increase the detection sensitivity. The remarkable synergistic effect of the zinc oxide nanoparticles and multi-walled carbon nanotubes for the ssDNA probe immobilization was observed. The simple modification procedure, high selectivity, low cost, fast response and broad linear range are the main features of the proposed DNA biosensor.

Ghalkhani et al.128 utilized MWNT decorated with silver nanoparticles (AgNPs-MWNT) as an effective strategy for modification of the surface of pyrolytic graphite electrode by colloidal dispersion of decorated MWNT in water to obtain uniform and stable thin films for altering the surface properties of

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Fig. 8⎯Comparative current response (after background current subtraction) of the EPPGE, EPPGE–SWNT,EPPGE–SWNT–Co and EPPGE–SWNT–CoO in (a) 1 mM nitrite solution in pH 7.4 PBS and (b) 1 mM nitrite solution in pH 3.0 PBS. [Scan rate = 25 mVs−1].[Reproduced from Ref. 126 with permission from Elsevier, Amsterdam, The Netherlands].

the working electrode. Comparative study of electrochemical behavior of sumatriptan (Sum) at bare PGE and AgNPs-MWNT modified electrode indicates that the AgNPs-MWNT modified PGE significantly enhanced the oxidation peak current of Sum. A remarkable enhancement in microscopic area of the electrode together with strong adsorption of Sum at the surface of the modified electrode resulted in a considerable increase in the peak current of Sum. A carbon nanotube modified electrode is made by Streeter et al.129 in which a glassy carbon electrode is partially covered with a layer of gold nanoparticle-modified CNTs as presented in Fig. 9. The carbon surfaces are passivated by the physisorption of anthraquinone-2, 6-disulfonic acid so that only the gold nanotubes can be electroactive. Linear sweep voltammetry of ferrocyanide has been studied using this modified electrode, and the results interpreted by numerical simulation. It is shown that the voltammetric determinations can measure the total length of the CNTs present on the surface. The effect of surface modification of indium tin oxide (ITO) by MWNT and gold nanoparticles attached MWNT has been studied recently to determine tryptophan, an important and essential amino acid for humans and herbivores130. A comparison of SEM images as shown in Fig. 10 clearly indicates the deposition of MWNT and Au nano particles on the surface of ITO. A

detailed comparison has been made among the voltammetric response of bare ITO, MWNT/ITO and AuNP-MWNT/ITO in respects of several essential analytical parameters viz. sensitivity, detection limit, peak current and peak potential of tryptophan.

Fig. 9⎯Schematic diagram of gold-modified MWNT randomly distributed on a glassy carbon macroelectrode. [Reproduced from Ref. 129 from permission of American Chemical Society, Washington DC, USA]

Ionic Liquid/Carbon Paste

Ionic liquids (ILs) present a high degree of asymmetry that inhibits crystallisation at room temperatures. ILs posses some fascinating properties including nonvolatile nature, low melt point, strong

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electrostatic field, high polarity, favourable viscosity and density as solvents, high thermal stability and the ability to solvate a wide range of species including organic, inorganic, and organometallic compounds. Due to the unique features such as wide potential windows (a voltage range between which the electrolyte is not oxidized or reduced) and high electrical conductivity, hydrophobicity, insolubility in water and plasticizing ability, ILs are used in construction of electrochemical sensors and biosensors. ILs have shown good compatibility with biomolecules and enzymes, and even whole cells. Thus, ILs can be used in electrochemical biosensors typically as both binder and conductor. The difference between the potentials of their anodic (Ea) and cathodic (Ec) decomposition131 is usually greater than 3 V, while for aqueous electrolytes it is about 1.2 V. Because of this remarkable property of ionic liquids, they find wide use in electrochemical biosensors. Electrochemical properties of ionic liquids

The unique properties of ILs can be tailored by combining different cations with suitable anions. The fact that ILs are intrinsically good ion conductors and generally of very low vapor pressure makes them attractive as device electrolytes. The viscosity of an IL is an important consideration in electrochemical studies due to its strong effect on the rate of mass transport within solution. ILs generally are more viscous than most molecular solvents due to the large internal friction of fluids. Viscosity of the ILs depends on the temperature and impurities present in the ILs. The addition of co-solvent such as, water, acetonitrile, acetone, alcohol, dichloromethane, benzene and toluene can result in a dramatic decrease in viscosity of ILs. Typically, ILs have a potential window of

more than 2.0 V. However impurities in the ILs have a profound impact on the anodic or cathodic potential limits and the corresponding electrochemical potential window. The large potential window of ILs makes them significant as electrolytes for electrodeposition of metals. This property also permits stable electrochemical cycling of intrinsically conductive polymer electrodes that have been widely used in applications including electrochemical sensors.

Various electrochemical sensors using ILs as a functional media have been developed. Maleki and co-workers132 developed a sensor with electrochemical properties that would be well suited for use in sensor and biosensor applications. N-octylpyridinium hexafluorophosphate, [Opyr] [PF6], was used as a binder of graphite powder in the construction of a carbon paste electrode. The authors claim several advantages by using this composite electrode including increment in electron-transfer rate and marked decrement in overvoltage for biomolecules such as NADH, dopamine and ascorbic acid. The composite electrode circumvents NADH surface fouling effects. Also, higher current densities for a wide range of compounds are observed. All these properties indicate it to be a sensitive, simple, and stable composite electrode for the detection of biomolecules and other electroactive compounds. Zhu et al.133 described the potential application of MWNT/AuNP/IL composite electrode for the fabrication of a non-enzymatic glucose sensor. The AuNPs embedded in MWNT/IL gel show strong and sensitive voltammetric response to glucose, owing to a possible combined synergistic effect of AuNPs, MWNT and IL. The simultaneous determination of dopamine, ascorbic acid and uric acid has been developed by Safavi et al.134 It was observed that

Fig. 10⎯Typical FE-SEM images observed for (A) MWNT/ITO, (B) AuNP-MWNT/ITO and (C) bare ITO surfaces. [Reproduced from Ref. 130 with permission from Elsevier, Amsterdam, The Netherlands].

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the apparent reversibility and kinetics of the electrochemical reaction at the carbon ionic liquid electrode (CILE) was significantly enhanced as compared to those obtained using a conventional carbon paste electrode. The same group has fabricated a non-enzymatic composite electrode by mixing nano- Ni(OH)2 with graphite powder and [Opyr][PF6], which showed excellent electocatalytic activity towards oxidation of glucose in an alkaline solution135 .

A composite electrode based on mixing nano copper(II) hydroxide with graphite powder and [Opyr][PF6] was successfully utilized by Safavi et al.136

for the simultaneous electrochemical detection of glutathione and glutathione disulfide. Application of copper(II) hydroxide in the composite electrode results in complexation of Cu(II) with the thiol group of glutathione and leads to a significant decrease in glutathione oxidation overpotential, while an anodic peak corresponding to the direct oxidation of glutathione disulfide as the product of glutathione oxidation is observed at higher overvoltages. An ethanol biosensor has been developed137 based on the lithium methylsulfonyl group-containing IL prepared from the precursors, poly(propylene glycol)-block-(ethylene glycol)-block-(propylene glycol)-bis(2-aminopropyl ether) with varying molecular weight (600, 900, and 2000 g/mol, commercially denoted as Jeffamine ED-600, ED-900, and ED-2000, respectively). To determine the sensitivity of the sensor, these liquids are analyzed with nickel electrodes, where a redox reaction of the Ni(OH)2/NiOOH couple was observed. Further, Tu et al.138 used a composite material based on SWNT, a water-insoluble porphyrin (hydroxyl-ferriprotoporphyrin, hematin) and 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim] [PF6]) to modify a GCE to study the direct electrochemistry and electrochemical properties of porphyrin. The porphyrin dissolved in [bmim][PF6] was self-assembled on SWNT by π–π non-covalent interaction, which leads to good dispersion of the SWNTs in the [bmim][PF6] and a direct electrochemical response corresponding to the Fe3+/2+ redox couple. This study provides a facile way for preparing biofunctional materials, accelerating electron-transfer, and extending the application of porphyrins/IL-based composite materials in sensor applications. Wang et al.139 fabricated an electrode by mixing a room temperature ionic liquid, 1-butyl-3-methyl-imidazolium tetrafluoroborate, and MWNT along

with chitosan. The use of ionic liquid and MWNT at the electrode surface increases ionic and electrical conductivities, thus enhancing the sensitivity of the sensor for NADH sensing139.

Recently, a colloidal gold-modified carbon ionic liquid electrode was constructed by mixing colloidal gold-modified graphite powder with a solid RTIL, n-octyl-pyridinium hexafluorophosphate (OPPF6)140. The electrode showed good bioactivity and excellent stability. The sensor was capable of distinguishing the complementary target DNA at low concentration from the three-base mismatched DNA at higher concentration. A room-temperature ionic liquid, N-butylpyridinium hexafluorophosphate, was used as a binder to construct an ionic liquid modified carbon paste electrode, which was characterized by scanning electron microscopy and electrochemical impedance spectroscopy141. The ionic liquid carbon paste electrode (IL-CPE) showed enhanced electrochemical response and strong analytical activity towards the electrochemical oxidation of dopamine. The IL-CPE was sensitive, selective and showed good ability to distinguish the coexisting ascorbic acid and uric acid. Figure 11 clearly indicates that the oxidation process of dopamine using the IL-CPE is greatly improved.

One attractive approach to using ionic liquids with carbon nanotubes in biosensors involves the use of multiwall carbon nanotubes ionic liquid modified glassy carbon electrode (MWCN-ILs/GCE). In this approach, MWNT were thoroughly mixed with the ILs by grinding in a mortar to create a gel-like paste

Fig. 11⎯Cyclic voltammograms of 1.0×10−4 mol/L dopamine (DA)on the CPE (a) and the IL-CPE (b) with a scan rate of 50 mV/s. [Reproduced from Ref. 140 with permission fromSpringer, Germany]

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which was then applied to the surface of a cleaned glassy carbon electrode. Cyclic voltammograms of dopamine solutions were recorded using a platinum wire and a saturated calomel electrode as auxiliary and reference electrodes, respectively. An immediate advantage of the MWCN-ILs was found to be the larger peak current with smaller peak separations, an indication of faster electron transport to the electrode surface142. Figure 12 clearly indicates that peak current of uric acid increased significantly by using MWNT-IL-gel/GCE as compared to bare GCE. The modified electrode has been successfully applied for the assay of DA in human blood serum.

A nanocomposite material consisting of amine functionalized multi-walled carbon nanotubes and a room temperature ionic-liquid, 1-butyl-3-methyl-imidazolium tetrafluoroborate, was reported for use in construction of a novel catalase based biosensor for the measurement of hydrogen peroxide143. The modified electrode exhibited a quasi-reversible cyclic voltammogram corresponding to the Fe(II)/Fe(III) redox couple in the heme prosthetic group of catalase. The nanocomposite film showed an obvious promotion of the direct electron transfer between catalase and the underlying electrode. Recently, Zhao et al. 144 proposed a novel strategy for investigating the electrical-ionic properties of RTILs and carbon composite materials formed by mixing a water-insoluble IL [bmim] [PF6] and carbon materials of two types, one of which was MWNT with a tube shape and the other meso-carbon micro-beads (MCMB). The hybrid MWNT/RTIL and MCMB/RTIL materials showed a different

conductivity mechanism as determined by ac impedance. The ILs and carbon composite materials can also be used as modifiers in the direct electrochemistry of protein. [bmim][PF6]-single-walled carbon nanotubes gel modified glassy carbon electrode (bmimPF6-SWNT/GCE) was fabricated to determine p-nitroaniline (PNA). The feasibility to determine other nitroaromatic compounds (NACs) with the modified electrode was also tested. Under the optimized conditions, the peak current was linear to NACs concentration and the detection limits down to nanomolar level, which are better than the values obtained by using carbon materials based electrodes without RTILs 145.

Fig. 12⎯Cyclic voltammograms of (a) 1.0 mM UA at bare GC electrode and (b) 1.0 mM UA at MWNT-IL-gel/GC electrode.[Reproduced from Ref. 142 with permission from Elsevier, Amsterdam, The Netherlands].

A voltammetric method was developed for the determination of tetracycline (TC) using an ionic liquid (1-octyl-3-methylimidazolium-hexafluorophosphate)– multiwalled carbon nanotubes film coated glassy carbon electrode146. The results indicate that both IL and MWNT can facilitate the TC oxidation with good reproducibility. Zhang et al.147 developed a RTIL supported three-dimensional network SWNT electrode. The glucose oxidase was directly covalently anchored on the SWNT-poly-N-succinimidyl acrylate assembly. The application of the RTIL-supported three-dimensional network SWNT electrode overcame the difficulties of homogeneous electrochemical functionalization of SWNTs in large quantities. Salimi et al.148 have recently reported a new carbon nanotubes-ionic liquid and chloropromazine modified electrode for the determination of NADH. The nanocomposite modified electrode displays excellent electrocatalytic activity towards the oxidation of NADH. The proposed electrode has potential application in the third generation reagentless biosensor. The composites of SWNT, gold nanoparticles and ionic liquid (1-octyl-3-methylimidazolium hexafluoro-phosphate) were used to fabricate a modified GCE for the sensitive voltammetric detection of chloramphenicol.149 The composition of the film affected the detection limit and was found to be 5.0 nM under optimum conditions.

Concluding Remarks Various approaches for the surface modification of

electrodes are available these days with their advantages and limitations. It is reasonable to use current density (current per surface area) to evaluate the performance of sensors. The ideal case is that in which the layer is permeable to target compounds,

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while resistant to the interfering compounds. For surface modification approaches aimed at enhancing the performance of sensors, four factors should be considered: (1) enlarged surface area. Both carbon nanotubes and metal nanomaterials increase the effective surface area of the electrodes; (2) enhanced electron transfer rate due to the catalytic ability of nanomaterials. The electron transfer and adsorption reactivity of graphitic carbon electrodes depends strongly on the surface coverage of edge relative to basal plane graphite, with the edge sites more reactive to electron transfer, adsorption and chemical modification; (3) Redox systems differ dramatically in their sensitivity to the state of the carbon electrode surface, due to differences in their redox reaction mechanisms, ionic charge, etc. Consequently, certain standards should be set up to evaluate the performance of the sensors. It is recommended that for a particular target a specific sensor exhibits optimal response. It is unwise and often incorrect to discuss “activation” of a carbon electrode without stating the redox reaction used as the indicator of electrode reactivity; (4) Carbon materials vary more in both bulk and surface structures as compared to metals, hence procedure for electrode preparation is particularly important in the case of carbon materials for achieving reproducible electrochemical behavior. Even the most common preparation procedures, such as polishing, significantly modify the surface structure and chemistry and can have dramatic effects on reactivity and adsorption.

In the future, more developments in applications of ionic liquids in electrochemistry are expected. The simple dissolution of a lithium salt always results in electrolytes in which only a fraction of the current is actually carried by Li+ ions. It would be interesting to see whether the incorporation of lithium salts with a polyanionic chain, through specific interaction with the fixed negative charge of polarizing Li+ ions, can change the flux balance. All of these fundamental aspects of the physical chemistry and electrochemistry of ionic liquids remain to be thoroughly explained, and hold the promise to further improve their potential in various electrochemical applications.

New strategies for the synthesis of nanostructered metal oxide-based sensors are likely to result in new bioelectronic sensing applications. Functionalizing nanostructered metal oxide-based sensors with the desired groups to bind target biomolecules, and doping these with electronically active materials to obtain enhanced charge transfer, may lead to

innovative methods in sensor development. The synthesis of nanoparticles with various morphologies at the nano domain level may offer a suitable environment for oriented immobilization of the desired biomolecules and thereby amplified signals. These nanostructered metal based sensors can be fabricated and tested in desired patterns for the development of functional integrated devices. The interface of nanomaterial based devices could also be used for parallel real-time monitoring of multiple analytes. It will be interesting to focus on new methods for the fabrication of innovative sensors with desired properties for health care and confining different biomolecules by using metal nanomaterials and their nano-structured oxides.

The implication of nanomaterials in electroanalysis is due to very specific properties of nanomaterials, exhibited only at the nanoscale. These include enhanced diffusion based on convergent rather than linear diffusion, high active surface area, improved selectivity, catalytic activity, higher signal-to-noise ratio and unique optical properties. The use of nanomaterials also grants control over the local microenvironment. This can be vastly profitable while incorporating sensitive or biological material into a system. These unique properties make nanomaterials extremely suited for electroanalytical applications. Improved convergent mass transport to nanoelectrodes assists the study of faster electrochemical processes. At the nanoscale, crystal planes can be exposed which are not accessible at the macroscale, in turn giving rise to enhanced current responses and catalysis. In terms of construction of nanomaterial based electrodes, costs can be reduced compared with the costs for manufacture of conventional macroelectrodes as only a fraction of the nanomaterial is needed. Therefore, the electroanalytical application of such nanomaterials has been found to be quite extensive which clearly indicates that there is a large scope for further study in this area of electrochemistry. Acknowledgement

One of the authors (RNG) is thankful to CSIR, New Delhi, DST, New Delhi and DBT New Delhi for their constant research support through funding of different projects. References

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