An Os^II-bisbipyridine-4-picolinic acid complex mediates the biocatalytic growth of Au nanoparticles: Optical detection of glucose and acetylcholine esterase inhibition

The complex Os^II-bisbipyridine-4-picolinic acid, [Os(bpy) ₂PyCO₂H]²⁺ (1), mediates the biocatalyzed growth of Au nanoparticles, Au NPs, and enables the spectroscopic assay of biocatalyzed transformations and enzyme inhibition by following the Au NP plasmon absorbance. In one system, [Os(bpy)₂PyCO₂H] ²⁺ mediates the biocatalyzed oxidation of glucose and the growth of Au NPs in the presence of glucose oxidase, GOx, AuCl₄^-, citrate and Au NP seeds. The mechanism of the Au NPs growth involves the oxidation of the [Os(bpy)₂-PyCO₂H]²⁺ complex by AuCl₄^- to form [Os(bpy)₂PyCO ₂H]³ and Au^I. The [Os(bpy)₂PyCO ₂H]³⁺ complex mediates the GOx biocatalyzed oxidation of glucose and the regeneration of the mediator 1. Citrate reduces Au^I and enlarges the Au seeds by the catalytic deposition of gold on the Au NP seeds. In the second system, the enzyme acetylcholine esterase, AChE, is assayed by the catalytic growth of the Au NPs. The hydrolysis of acetylcholine (2) by AChE to choline is followed by the [Os(bpy)₂PyCO₂H] ³⁺ mediated oxidation of choline to betaine and the concomitant growth of the Au NPs. The mediated growth of the Au NPs is inhibited by 1,5-bis(4-allyldimethylammonium-phenyl)pentane-3-one dibromide (3). A competitive inhibition process was demonstrated (K_M = 0.13 IBM, K_I = 2.6 μM) by following the growth of the Au NPs.