Towards label-free mid-infrared protein assays : in-situ formation of bare gold nanoparticles for surface enhanced infrared absorption spectroscopy of bovine serum albumin

Abstract

Surface enhanced infrared absorption spectroscopy (SEIRAS) with attenuated total reflection (ATR) configuration has been used to determine bovine serum albumin (BSA) protein adsorbed onto bare gold nanoparticles (AuNPs). The AuNPs forming the SEIRA-active substrate were directly synthesized inside the ATR liquid cell taking advantage of stainless steel assisted synthesis via the walls composing the ATR compartment. The formation of the AuNPs can be directly monitored via the enhancement of the corresponding water absorption features. The absorbance of BSA at the AuNPs deposited onto the Si ATR waveguide is significantly enhanced when compared to bare Si, thereby improving the sensitivity of detection. Apparently, the presence of the AuNPs layer at the ATR waveguide surface does not affect the secondary structure of the protein. Measurements were performed in water and confirmed in deuterium oxide, which significantly reduces the strongly absorbing mid- infrared background of H2O in the spectral regime of interest.