Nitrite-triggered surface plasmon-assisted catalytic conversion of p -aminothiophenol to p, p ′-dimercaptoazobenzene on gold nanoparticle: Surface-enhanced raman scattering investigation and potential for nitrite detection

The stunning large enhancement factor (∼10⁸) of the surface-enhanced Raman scattering (SERS) effect leads people to wonder about the underlying enhancement mechanisms of the effect. But, a strong evidence of the existence of one commonly accepted mechanism (chemical enhancement), the origin of the symbolic "b₂" bands (ca. 1140,1390, 1432 cm^-1) of p-aminothiophenol (PATP), was recently found to be a false explanation, which were actually arisen from the product of a surface plasmon-assisted coupling reaction of PATP, p,p′-dimercaptoazobenzene (DMAB). However, the debate is far from over, especially because the mechanism of the above reaction has not been fully understood yet. In this paper, we for the first time report a new surface plasmon-assisted catalytic conversion of PATP to DMAB that NO₂^- ions can trigger the formation of DMAB on gold nanoparticles (GNPs) suspension under light illumination. The mechanism of the conversion is also discussed. All relevant data suggest the nitrite-triggered conversion of PATP to DMAB on GNPs is a surface plasmon-assisted oxidation reaction, involving transfer of multiple electrons from PATP to NO₂^- (electron acceptors) and protons, leading to the formation of DMAB. The proposed mechanisms may also help to understand the unclear surface plasmon-assisted catalytic coupling of PATP on the SERS substrates. Furthermore, inspired by the high selectivity of the above nitrite-triggered catalysis reaction, a simple and fast nitrite screening method was also developed, exhibiting good sensitivity. Considering other advantages of the assay, such as rapidness, simplicity of the detection procedures, and requirement of no sample pretreatment, it is a promising method for on-site fast screening or point-of-care application.