SERS-active nanoparticles for sensitive and selective detection of cadmium ion (Cd²⁺)

We report a new class of turn-on surface enhanced Raman scattering (SERS) sensors for the sensitive and selective detection of cadmium ion (Cd ²⁺) by taking advantage of the interparticle plasmonic coupling generated in the process of Cd²⁺-selective nanoparticle self-aggregation. The SERS-active nanoparticles consist of 41-nm gold nanoparticles, encoded with a Raman-active dye through a disulfide anchoring group, and a layer of Cd²⁺-chelating polymer brush coating grafted on the nanoparticle via surface-initiated atom transfer radical polymerization. These SERS nanoparticles are optimized to remain spectrally silent when staying as single particles. Addition of Cd²⁺ leads to interparticle self-aggregation and immediately turns on the SERS fingerprint signal with up to 90-fold of signal enhancement. The selectivity of the SERS nanoparticle for Cd²⁺ was also examined, showing that various common metal ions cannot induce interparticle self-aggregation and the turn-on of SERS signal. In contrast to nanoparticle-based colorimetric assays, the SERS probe is also capable of detecting Cd²⁺ in heavily colored samples.