AuNP based selective colorimetric sensor for cysteine at a wide pH range: Investigation of capping molecule structure on the colorimetric sensing and catalytic properties

Gold nanoparticles (AuNPs) stabilized with different surfactants, SDS, PEG, PVA, PVP, PSS and T-80, were synthesized and explored for cysteine colorimetric sensing and catalytic properties. The sensing and catalysis studies revealed an interesting observation that AuNP surfaces covered with linear molecules, SDS and PEG, did not show any colorimetric sensing and exhibited the lowest catalytic effect. Whereas moderately improved sensing and catalysis was observed with AuNPs covered with smaller functional group-substituted PVA and PVP. Interestingly, a selective and robust colour change for cysteine (10 ^-7 M) in aqueous solution as well as the strongest catalytic effects were observed with AuNPs covered with bulky functional group-substituted PSS and highly branched T-80. These differences could be attributed to the surface accessibility of the AuNPs for the analytes. HR-TEM analysis of T-80-AuNPs with cysteine clearly showed the formation of smaller aggregates of AuNPs. Importantly, T-80-AuNPs showed selective sensing of cysteine across a wide pH range (2.0-10.0). PSS-AuNPs showed selective colorimetric sensing only in the pH range of 6.0 to 10. These studies suggest that the surface capping molecule structure plays a very significant role in both colorimetric sensing and catalysis by AuNPs.