Photophysical and photochemical characterisation of bacterial semiconductor cadmium sulfide particles

Klebsiella pneumoniae forms electron-dense cadmium sulfide particles (ca. 5-200 nm in diameter) on the cell surface in response to the presence of cadmium ions in the growth medium. In the current study, these 'bio-semiconductor' particles have been spectroscopically characterised using UV-VIS absorption and luminescence analysis. The spectroscopic properties observed suggest that they are similar in size and possess photoactive traits analogous to CdS systems prepared by conventional chemical methods. The optical nature of the bacterial semiconductor particles means that, in principle, they are capable of performing a variety of photoredox reactions. The reactions involving photoelectrochemical indicators such as methyl viologen (MV²⁺) and methyl orange (MO^-) are considered and, by comparing initial rates of reaction and altering reaction variables, a general mechanism of photoactivity for the cadmium sulfide 'bio-semiconductor' is proposed.