Inverse sensitivity of plasmonic nanosensors at the single-molecule limit

Recent work using plasmonic nanosensors in a clinically relevant detection assay reports extreme sensitivity based upon a mechanism termed inverse sensitivity, whereby reduction of substrate concentration increases reaction rate, even at the single-molecule limit. This near-homœopathic mechanism contradicts the law of mass action. The assay involves deposition of silver atoms upon gold nanostars, changing their absorption spectrum. Multiple additional aspects of the assay appear to be incompatible with settled chemical knowledge, in particular the detection of tiny numbers of silver atoms on a background of the classic ‘silver mirror reaction’. Finally, it is estimated here that the reported spectral changes require some 2.5× 10 times more silver atoms than are likely to be produced. It is suggested that alternative explanations must be sought for the original observations.