||The work described in this thesis has been focused on the use of cobalt(II)-tetra (3-methoxy-4-hydroxy-phenyl) porphyrin [Co(II) TMHPP] modified electrodes, which have demonstrated to be effective electrocatalysts for dioxygen reduction in the application of cyanide determination. To prepare chemically modified electrodes, we have employed the sol-gel technique because sol-gel films are chemically inert, photo-stable and thermally stable supporting materials. The Co(II) TMHPP doped sol-gel based Pt electrode has been characterized electrochemically by rotating disk voltammetry, cyclic voltammetry and chronoamperometry, respectively. The results show that the charge transfer diffusion coefficient Dct within the modified layer is comparable to other chemically modified electrodes. The charge transfer rate constants ks, for the electron transfer between the electrode and the localized redox sites, and ko2, for the oxygenated adduct formation have been calculated to study the catalytic power and mechanism of metalloporphyrins. Furthermore, we have worked out that the binding equilibrium constant of cyanide is much larger than oxygen and the mode of inhibition is non-competitive. The immobilization of metalloporphyrins with silica gel can avoid unwanted dimerization to μ-oxo-bridged dimers via peroxo intermediates and therefore the electrode has a storage stability of more than 10 days at room temperature. This electrode shows an extremely high stability towards dioxygen reduction after more than 1700 cyclic potential scans with a deviation of less than 9%. For electrocatalytic processes involving an inner-sphere mechanism, strongly binding species such as CN poison the reaction by coordinating to the active electrocatalyst sites. This poisoning effect has been employed here for the first time in literature to analyze trace quantities of cyanide in aqueous solution. A linear relationship between the catalytic current decrease and the cyanide ion concentration is observed in the range of 10 nM to 100 nM, which is two orders of magnitude more sensitive than all other described methods, and correlates well to the standard method. Cyanide is determined amperometrically with a detection limit of 7-10x10[to the power of negative 9]M at 0 mV vs. Ag/AgCl and a short response time of 10 minutes. Finally, this measurement method was tested on real samples, such as reef fish blood and wastewater samples, without the use of any toxic reagent. It is well-known that cyanide may gain access to freshwater and seawater mainly from artificial sources including slow continuous discharge of industrial waste containing cyanide or cyanogens. Less is known about the cyanide fishing in the South Asia region. The impact of cyanide, as a broad-spectrum poison, on marine life is irreversible and chronic. In order to combat cyanide fishing and to investigate the cyanide level in discharge, a simple and sensitive analytical tool would be ideal for the purpose of monitoring in the field. Chemically modified electrodes can be used as a fast, sensitive and reliable analytical tool to detect cyanide in our environment.