Capping agent-free gold nanostars show greatly increased versatility and sensitivity for biosensing.

We report the first assessment of the plasmonic biosensing capabilities of capping agent-free gold nanostars. Capping agent removal was carried out using aqueous solutions of sodium borohydride, which yielded a refractive index sensitivity of 474 nm/RIU for the polyvinylpyrrolidone (PVP)-free nanostars compared with 98 nm/RIU for PVP-coated gold nanostars. Following PVP removal, biotinylated thiol and streptavidin protein were added to the nanostars, which resulted in red shifts as large as 51 nm and a limit of detection as low as 0.1 pM. Refractive index-based sensing of prostate specific antigen (PSA) both in buffer and serum was then carried out and was shown to yield shifts as large as 127 nm and have a limit of detection of 100 pM in serum. Last, a sandwich assay involving PSA was developed to aggregate the nanostars together for greater sensitivity. The sandwich assay did, indeed, give shifts close to 200 nm and was capable of detecting 10(-17) M PSA in serum. The greatly increased sensitivity and amenability to functionalization of PVP-free gold nanostars should prove useful in applications ranging from catalysis to drug delivery.