Synthesis and characterisation of some osmium and ruthenium complexes and the application of Os(bpy)2-(pyridine-4-COOH)-CI in an electrochemical DNA biosensor

Abstract

This thesis details the synthesis and characterisation of seven osmium and ruthenium bipyridyl complexes, reports on their electrochemical characteristics and outlines an application for one of the synthesised complexes in an electrochemical DNA biosensor. Complexes synthesised included cis-bis(2,2-bipyridine-N,N)dichlororuthenium, cis-bis(2,2- bipyridine-N,N)dichloroosmium, [Os(bpy)2-(4 -tetrazine)-Cl](PF6), [Ru(bpy)2(4 -tetrazine)- C1](PF6), [Os(bpy)2-(pyridine-4-COOH)-Cl](PF6), [Ru(bpy)2-(pyridine-4-COOH)-Cl](PF6), Ru(bpy)2(4-tetrazine)2(PF6)2- Diffusion coefficients determined using a glassy carbon electrode ranged from 1.51 x 10' to 3.28 x 10' cm s' for the osmium complexes and from c c ^ 1 2.39 x 10' to 4.76 x 10“ cm s" for the ruthenium complexes. Using the Laviron approach, solution phase electron transfer rates ranged from 0.29 cm s' 1 for [Os(bpy)2-(4-tet)-Cl](PF6) to 1.61 cm s' 1 for Ru(bpy)2(4-tet)2(PF6)2* Self assembled monolayers of [Os(bpy)2-(4-tet)- C1](PF6)> [Ru(bpy)2-(4-tet)-Cl](PF6) and Ru(bpy)2(4-tet)2(PF6)2 formed spontaneously on platinum electrodes and electron transfer rate constants were determined as 9.0 x 103, 5.7 x 103 and 1.4 x 104 s_1 respectively. Monolayers of [Os(bpy)2-(pyridine-4-COOH)-Cl](PF6) and [Ru(bpy)2-(pyridine-4“COOH)-Cl](PF6) were formed on gold electrodes in a two-step assembly and the former complex shown to spontaneously adsorb on indium tin oxide. Complexes adsorbed by the two-step assembly exhibited much reduced electron transfer rate constants for example 33.86 s' 1 for [Os(bpy)2-(pyridine-4-COOH)-Cl](PF6). Rates of adsorption on platinum were determined in-situ and closely followed Langmurian adsorption and first order adsorption kinetics. Adsorption rate constants determined for complexes where adsorption occurred on a single site ranged from 14.30 ± 4.35 M 1 s' 1 for [Os(bpy)2-(4- tet)-Cl](PF6) to 30.93 ± 9.15 M' 1 s' 1 for Ru(bpy)2(4-tet)2(PF6)2. Rates of desorption followed first order kinetics for the complexes investigated and desorption rate constants ranged from 2.33 x lO 'V to 5.66 x 10'5s \ In a further two-step assembly procedure, a 5’ thiolated hairpin ssDNA probe specific to the bacterial genus Salmonella spp.: 5’ SH-(CH2)6- GCA GTA AC A AGA ATA AAA CGC ACT GC- (CH2)6- NH2 3’ was immobilised on a gold electrode surface and then [Os(bpy)2- (pyridine-4-COOH)-Cl](PF6) coupled to the amine group at the 3* end using carbodiimide coupling. This DNA biosensor was capable of hybridisation detection in the presence of a complementary sequence due to the conformational change in hairpin structure. This is the first report of the use of a covalently attached osmium bipyridyl complex as a redox label for the purpose of hybridisation detection.