Microbial Biosensors for Some Heavy Metal Ions

Abstract

Whole cell microbes having phosphatase activity were identified for the preparation of biosensor using Pikovskaya media for bacterial culture and Beneck s media for algal culture from the rhizospheric soil. Identified microbial colonies were tested for their phosphatase solubilizing capacity. Microbes having maximum solubilizing capacity were cultured and re-cultured using BG-11 inorganic media and were starved with respect to phosphate by suspending the media in phosphate free BG-11 media, before being used for immobilization on to the electrode surface. Starved microbes were immobilized on different electrode surfaces using bovine serum albumin (BSA) and were cross-linked with glutaraldehyde vapours for 30 minutes. The immobilized electrode was characterized for maximum output current by changing its cell density and optimum working environment by variation in pH of the solution. Immobilized electrode was also characterized for its composition, permeability and viability of the microbes in the membrane matrix with various techniques like scanning electron micrograph (SEM), cyclic voltammetry (CV) and spectrofluoremetry. Whole cell based amperometric biosensor was fabricated using an identified phosphatase solubilizing unicellular microalgae Chlorella sp. as the biocatalyst. The microbial culture was entrapped in a polymeric membrane of BSA directly interfaced to the surface of a platinum electrode for the detection of bioavailable heavy metal ions like zinc, copper, cadmium, cobalt and nickel. . The fabricated biosensor had a life time of 7 days and was found sensitive to a lower concentration level of 10and#8722;9 M of nickel ions, 10and#8722;10 M of cadmium and cobalt ions, 10and#8722;11 M of copper and zinc ions. The electrode system was workable in a detection range of 10-12 M to 10-6 M. The amperometric biosensor responds to heavy metal ions with a relative selectivity in the order: Zn2+ gt Cu2+ gt Cd2+ gt Co2+ gt Ni2+.