VOLTAMMETRIC SENSORS FOR BIOMOLECULES AND DRUGS

Abstract

Last few years have seen a great proliferation of research interest by electrochemists at the interface of nanotechnology and electrochemistry. Advancements in electrochemistry make it possible to study and understand the principles as well as different aspects of nanoscience and nanotechnology. Electrochemical determination of variety of biomolecules, medicines, and doping agents in biological samples using electrodes modified with nanocarbon/nanoparticles/polymer film has been performed using various voltammetric techniques. Determination of physiologically important biomolecules always remains an uphill task for researchers as their concentration in biological fluids is normally very low. However, in the case of metabolic disorders and diseases, the concentration of these biological compounds gets abnormally altered. In such cases, detection of their concentration level proves out to be of diagnostic value. Voltammetric sensors have been found to show consistently good results with high sensitivity and selectivity. Quantification of common drugs is important for pharmaceutical industry, and studies on doping agents is needed to keep a check on the doping cases in competitive games. An endeavor has been made in the present investigation to develop simple, selective, and sensitive electroanalytical methods for qualitative and quantitative determination of biologically important molecules and drugs at nanomaterial/polymer film modified electrodes. An outline of the present research with a chapter-wise concise summary of the thesis is given below. The first chapter of the thesis gives a panoramic view on conventional materials, types of nanomaterials, polymeric materials and an introductory background of related materials. The chapter also presents the discussions on the application of nanomaterials and polymer films in various chemical and biosensing applications using electroanalytical techniques. This chapter also briefly adumbrates the methodology used during the investigation. The second chapter of the thesis has been divided into two parts. The first part represents the comparison of basal and edge plane pyrolytic graphite sensors towards the determination of norfloxacin (NF). The compound NF, a member of fluoroquinolones, is the first choice drug for the treatment of diseases caused by Campylobacter, E. coli, Salmonella, Shigella and V. colera. It is used for the doctoring of gonorrhoea as well as eye and urinary tract infections. The non-target toxicity of antibacterials, including NF, has also been documented in literature. The determination of NF in biological samples and medicine is considered of great importance for human health, food assurance and quality control because of its potential toxicity. When equating with the bare basal plane pyrolytic graphite sensor, the ii edge plane pyrolytic graphite sensor gave better response towards the determination of NF, both in the terms of sensitivity and detection limit. The difference in the surface morphology of the two electrodes has been studied. Also, the edge-plane pyrolytic graphite sensor delivered an analytical performance for NF with a sensitivity of 0.9064 μA μM−1 and limit of detection of 28.3×10−8 M in the concentration range 0.5-50 μM. The method is successfully utilized for the detection of NF in pharmaceuticals formulations and human urine samples. The second part of the chapter throws light on voltammetric investigation of a wellknown sulfonamide drug, sulfamethoxazole (SMZ) at poly-1,5-diaminonaphthalene modified glassy carbon electrode. The oxidation of SMZ occurred in a well-defined peak having peak potential ~850 mV at pH 7.2. The modified electrode showed an excellent catalytic response presenting much higher peak currents than those measured at a bare glassy carbon electrode. The modified sensor was characterized by Field Emission Scanning Electron Microscopy, electrochemical impedance spectroscopy and cyclic voltammetry. Under optimized conditions, SMZ showed linear response in the concentration range of 0.5-150 μM by using square wave voltammetry and the detection limit was found to be 0.05 nM with a sensitivity of 0.085 μA μM−1. In order to assess the pertinency of the proposed sensor, different commercial samples as well as human biological samples containing SMZ have been analyzed. In third chapter, electrochemical and peroxidase-catalyzed oxidation of epinephrine is documented. The hormone epinephrine (adrenaline) is commonly used as the drug of choice as a vasoconstrictor, cardiac stimulator and bronchodilator. It subsists in protonated form at physiological pH. It is synthesized in the human system from L-tyrosine and exuded by the medulla of the adrenal gland along with norepinephrine. Ratiocination of concentration of the monoamine neurotransmitters such as epinephrine is crucial for the canvassing of neurotransmission, for diagnosis of neurological disorders, such as Parkinson's, and for developing medicines to cure the diseases. In view of the importance of epinephrine in the human physiology, an attempt has been made to compare the electrochemical and peroxidase catalyzed oxidation of epinephrine in this chapter. In the electrochemical studies a single welldefined, 4e−, 4H+, pH-dependent oxidation peak was observed in square wave and cyclic sweep voltammetry at edge plane pyrolytic graphite electrode. The decay of the UV-absorbing intermediate and the first-order rate constants were calculated at different pH. At pH 7.2, the electrooxidation product was characterized using NMR and DEPT studies as leucoadrenochrome. The peroxidase catalyzed oxidation was carried out using horseradish peroxidase and initiated by adding H2O2. The identical spectral changes, rate constants for the decay of the UV-absorbing intermediate and product formed during electrochemical and iii enzymatic oxidation suggest that the same intermediate species is generated during both the oxidations. It is concluded that the electrochemical pathway and peroxidase-catalyzed oxidation of epinephrine proceed by an identical mechanism. The fourth chapter throws light on simultaneous voltammetric investigation of adenine and adenosine monophosphate at single-walled carbon nanotubes (SWNT) modified edge plane pyrolytic graphite electrode (EPPGE). Under optimized conditions, well-defined oxidation peaks are observed at SWNT modified EPPGE where the peak shifted negatively and the peak current increased remarkably in comparison to bare edge plane pyrolytic graphite electrode. The significant enhancement in current response followed by a decrease in peak potential indicates that SWNT modified EPPGE acts as a promoter to enhance the electrochemical reaction, considerably accelerating the rate of electron transfer. The electrocatalytic activity of SWNT has been assigned due to the metallic impurities present within it. The electrode process is adsorption-controlled and irreversible in nature. The effect of pH revealed that the oxidation of adenine and 5′-AMP at SWNT modified EPPGE involved equal number of electrons and protons. Linear calibration curves are obtained over the concentration range of 5-100 nM for adenine and 10-100 nM for 5′-AMP with sensitivity of 677 and 476 nA nM−1 for adenine and 5’-AMP, respectively. The limit of detection for adenine and 5′-AMP was found to be 37×10−10 M and 76×10−10 M, respectively. The modified electrode exhibited high stability and reproducibility. In order to assess the pertinency of the proposed method, different plasma samples have been analyzed using standard addition method. Epinephrine Tryptophan The fifth chapter of the thesis elaborates the effect of surface modification of indium tin oxide (ITO) by the use of multi walled carbon nanotubes (MWNT/ITO) and gold nanoparticles attached carboxylated multi walled carbon nanotubes (AuNP-MWNT/ITO) for the electrochemical oxidation of tryptophan. Tryptophan (2-amino-3-(1H-indol-3-yl)-propionic acid) is an important and essential amino acid for humans and herbivores, and is also a potent precursor of several metabolites such as serotonin, melatonin and niacin. A detailed comparison has been made among the voltammetric response of bare ITO, MWNT/ITO and AuNPiv MWNT/ITO in respects of several essential analytical parameters viz. sensitivity, detection limit, peak current and peak potential of tryptophan. It is observed that the oxidation peak current of tryptophan was found to increase significantly along with a substantial shift in peak potential towards less positive potentials by using AuNP-MWNT/ITO in comparison to bare ITO and MWNT/ITO electrodes. Under optimum conditions linear calibration curve was obtained over tryptophan concentration range 0.5-90.0 μM in phosphate buffer solution of pH 7.2 with detection limit and sensitivity of 0.025 μM and 0.12 μA μM−1, respectively. The oxidation of tryptophan occurred in a pH dependent, 2e− and 2H+ process and the electrode reaction followed adsorption controlled pathway. The origin of electrocatalytic properties of nanotubes has been assigned to the embedded metal impurities in CNT samples and edgeplane- like defects which are present at the open ends of nanotubes. The AuNP-MWNT/ITO has also been utilized for the electrochemical determination of tryptophan in human blood plasma and urine samples with reproducible results. Paracetamol Aspirin 1,5-diaminonapthalene Caffeine Sixth, the last chapter of the thesis is devoted to the simultaneous monitoring of aspirin, paracetamol and caffeine in human urine at poly-1,5-diaminonapthalene modified pyrolytic graphite sensor. Recently, conductive polymers have acquired much attention due to their potential applications to battery electrodes, electrochromic devices, electroluminescent devices, and biological sensors. Of these, aromatic compound possessing two amine groups has been studied for polymer film-coated electrodes in this chapter. Poly-1,5-diaminonapthalene has shown incredible interest due to its fascinating properties. The main advantage of the present method is that poly-1,5-diaminonapthalene layer contributes to its efficiency leading to v the lowering of oxidation peak potential and marked enhancement in the peak current. The electrooxidation of aspirin, paracetamol and caffeine occurred in well-defined peaks at pH 7.2. After optimization of analytical conditions exploiting this sensor, the peak currents for the three compounds were found to increase linearly with increase in their concentration in the range of 0.1-120 nM and detection limits of 0.93×10−10, 0.57×10−10 and 0.64×10−10 M were observed for aspirin, paracetamol and caffeine respectively. The proposed sensor exhibited good stability and reproducibility towards the determination of aspirin, caffeine and paracetamol in urine samples and tablets. The sensor can also be recommended for detecting doping cases of caffeine at the site of competitive games due to its rapid response and accuracy.