DETERMINATION OF BIOMOLECULES/DRUGS AT NANOMATERIAL MODIFIED ELECTRODES

Abstract

Electrochemistry has witnessed an enormous turnout in last decade on research papers reporting electroanalytical studies at nanomaterial modified electrodes. Perhaps it could be attributed to emerging novel synthetic routes for nanomaterial synthesis, and the technological advancement which makes it possible to study, and understand the principles of nanoscience and nanotechnology. Modified electrodes lead to improvement in electroanalytical determination as they show improved sensitivity and low detection limit. Modification of electrodes with nano carbon and gold nano particles is most popular due to their chemical inertness, broad potential window, and low background current. In addition carbon based electrodes also offer an additional economical advantage of low cost. Hence, in view of the advantages offered by nanomaterial modified electrodes, an attempt has been made in the dissertation to determine the utility of nano carbon modified glassy carbon electrode, and gold nanoparticles modified indium tin oxide (NGITO) electrodes for determination of physiologically important compounds, commonly used medicines, and some doping agents. The first chapter of the thesis is General Introduction, giving an up-to-date review of the recent trends in electroanalytical chemistry. Electroanalytical techniques employed for present investigations are also described briefly. Major aspects of electroanalytical chemistry like conventional electrodes, their merits and demerits are discussed along with concise outline of advantages rendered by modified electrodes over conventional electrodes. The second chapter of the thesis describes use of nanocarbon modified glassy carbon electrode (GCE) for voltammetric determination of commonly used medicines, and biomolecules of physiological relevance. Fullerene - C6o and single walled carbon in nanotubes (SWCNTs) are the two popular nanosized carbon material described in the dissertation for the modification of GCE. Fullerenes are closed convex cage molecule with only pentagonal and hexagonal faces, and truncated icosahedron shape. SWCNTs comprises of a single graphene sheet rolled up into cylindrical shape of diameter ranging between 1 - 3 nm. SWNTs are chemically inert, and characterized by strong covalent bonding. The first section of the chapter describes voltammetric determination of the commonly used drugs, like paracetamol and atenolol, and simultaneous determination of adenine and guanine at fullerene modified GCE. Paracetamol (N-acetyl-p-aminophenol) is a commonly used analgesic and antipyretic medicine which acts as painkiller by inhibiting prostaglandin's synthesis in the central nervous system, and relieves fever by sedating hypothalamic heat-regulating center. Fullerene modified GCE showed stable response for paracetamol with enhanced selectivity and sensitivity in comparison to bare GCE. Effects of common biological and chemical interferents on voltammetric quantification of paracetamol were also evaluated, and no significant interference was observed in a wide concentration range. A linear calibration plot having correlation coefficient 0.985 was obtained in the range 0.05 mM to 1.5 mM paracetamol concentration and the sensitivity of the method has been found as 13.04 uAmM"1. Method described is rapid, and has been applied for the determination of paracetamol in different tablets and urine samples with several advantages over other analytical methods. The standard deviation (SD) was 5.53% for eight determinations. Atenolol is of immense therapeutic use in the treatment of various cardiovascular disorders, such as angina pectoris, cardiac arrhythmia, and hypertension. C6o modified IV GCE has been found to exhibit excellent electrocatalytic activity towards atenolol oxidation for its voltammetric determination at physiological pH. Lowering of overpotential associated with atenolol oxidation indicates electrocatalytic nature of electrode. Determination of atenolol was carried out at pH 7.2 at modified electrode and a well-defined oxidation peak has been observed with Ep ~ 1040 mV vs. Ag/ AgCl electrode for atenolol oxidation. Calibration plot having good colinearity with a correlation coefficient 0.997 was obtained in the concentration range of 0.25 mM to 1.5 mM atenolol and the sensitivity of the method has been found to be 8.58 uA mM"1. The detection limit is found to be 0.16 mM. The method developed is applicable for atenolol determination in pharmaceutical preparations and urine samples. The modified electrode showed a good surface coverage (~ 85%) with C6o- Simultaneous determination of the two important purines; viz., adenine and guanine, at fullerene modified GCE was carried out at physiological pH. Well-defined oxidation peaks were observed with Ep at ~ 990 mV and 692 mV for adenine and guanine respectively. Good co-linearity was obtained in the concentration range 0.5 to 100.0 uM for adenine and guanine with sensitivity of -0.06 uA uM"1 and -0.03 uA uM" respectively. Recovery studies for adenine and guanine in biological samples were also carried out. Interfering effect of some common metabolites including ascorbic acid has been evaluated. (G+C)/ (A+T) ratio is a significant indicator of probable defects in purine metabolism and the analytical application of the developed protocol for the determination of the ratio in DNA sample has also been described. Second section of the chapter describes the role of metallic impurities in nanotubes in enhancing the electrochemical current. The simultaneous voltammetric determination of adenosine (ADS) and dopamine (DA) using a single wall carbon nanotube (SWCNT) modified glassy carbon electrode (GCE) is reported, which is of emerging significance in controlling Parkinson's disease. In phosphate buffer medium of pH 7.2, the concentration versus peak current plots were linear in the range 1-100 uM for ADS and DA. The detection limit (3a) and sensitivity observed for ADS and DA were 34.7 uM, 7 uM and 9.5 nA uM"1, 77.9 nA uM"1 respectively. The third chapter of the thesis deals with voltammetric determination of physiologically important biomolecules like uric acid and simultaneous determination of adenosine and adenosine-5'-triphosphate using NGITO electrodes. Voltammetric quantification of an important anti-asthmatic drug salbutamol, which is generally abused by athletes for doping, has been also described in this chapter. The determination of uric acid has been carried out at pH 7.2 at nanogold modified indium tin oxide electrode by differential pulse voltammetry in absence and presence of ascorbic acid and other common interferents. A well-defined oxidation peak has been observed at around 840 mV vs. Ag/AgCl electrode for uric acid at pH 7.2. The plot of peak current vs. concentration plot is linear in the range 0.5-100 uM with a correlation coefficient of 0.995 and a sensitivity of 0.412 nA uM"1. The detection limit (3a) is found to be 0.5 uM. Ascorbic acid in concentrations lower than 10 uM does not interfere, whereas, at higher concentrations it interferes. Xanthine, hypoxanthine and adenine do not interfere even at eight-fold excess. The method is simple and convenient with the only limitation that the electrode cannot be used more than eight times. A balanced ADS/ATP level is critically important in neural transmission because of its antagonistic effect on nerve excitability and impulse progression. Simultaneous VI determination of adenosine (ADS) and adenosine-5'-triphosphate (ATP) has been described using NGITO electrode. Gold nanoparticles catalyze ADS oxidation which results in increasing separation of oxidation peaks of ADS and ATP, making it possible to determine ADS and ATP simultaneously. The detection limits for ADS and ATP were found as 0.07 uM and 0.10 uM respectively with sensitivity 22.9 nA uM"1 and 20.9 nA uM"1. The proposed method was also used for sensing the compounds in biological samples. Influence of various square wave parameters and different pH conditions on peak current has also been reported. The determination of salbutamol, abused for doping, has been carried out at NGITO electrode using Osteryoung square wave voltammetry. The NGITO electrode exhibited an effective catalytic response towards salbutamol oxidation as compared to bare ITO and bulk gold electrodes. The peak current was also found to be significantly increased. The determination was carried out in phosphate containing electrolyte in the pH range 4.0 to 10.0 and two well-defined peaks were noticed. The first peak was found to be linearly dependent on concentration of salbutamol in the concentration range 50 to 2000 ng/mL with a detection limit of 75 ng/mL, whereas the second peak was due to adsorption of product. The proposed method has been applied to determine the concentration of salbutamol in pharmaceutical formulations and humanblood plasma and urine samples. A comparison of salbutamol concentration determined by proposed method and GC/MS indicated a good agreement. It is believed that the method is simple, rapid and can be easily used in determining this drug in case of its abuse in doping. Fourth, the final chapter of the thesis compares the electrochemical response of prednisolone (I) at fullerene - C60 - modified gold electrode and NGITO electrode in vu phosphate buffer solution (PBS) of pH 7.2. During oxidation of prednisolone, an anodic peak with peak potential (Ep) at 570 mV and 400 mV appeared at NGITO and fullerene modified gold electrode, respectively. ..................