Single molecule biosensing using color coded plasmon resonant metal nanoparticles.

Plasmonic metal nanoparticles (NPs) exhibit size, shape, and composition-dependent optical properties and have been studied extensively in the field of biosensing, but wide applications so far are limited by a lack of both highly sensitive and quantitative yet cost-effective optical detection schemes. We present a single molecule biosensing method based on color differentiation of scattered light between single plasmonic NPs and DNA hybridization-induced NP aggregates. With a seed-mediated NP growth method and a fast DNA modification method, highly stable, spectrally uniform and monodisperse Au NP (40 nm) and Au/Ag/Au composite NP (33 nm) probes were successfully prepared. Through theoretical calculations, single NP spectral measurements, and real time single-NP tracking experiments, we show that binding of a single target molecule between two NP probes can be recognized without separation from the unbound NPs by simply using a darkfield microscope equipped with a conventional light source and a color charge coupled device (CCD) camera. The detection limit of this homogeneous assay reached 0.02 pM. As an initial demonstration of multiplexed sensing at the single NP level, we used Au NPs and Au/Ag/Au composite NPs as different color probes. This scheme could be potentially applied to other areas such as multiplexed immunoassay, single cell analysis, and real time biomolecule interaction studies.