An immunoassay readout method based on surface-enhanced Raman scattering (SERS) is described. The method exploits the SERS-derived signal from reporter molecules that are coimmobilized with biospecific species on gold colloids. This concept is demonstrated in a dualanalyte sandwich assay, in which two different antibodies covalently bound to a solid substrate specifically capture two different antigens from an aqueous sample. The captured antigens in turn bind selectively to their corresponding detection antibodies. The detection antibodies are conjugated with gold colloids that are labeled with different Raman reporter molecules, which serve as extrinsic labels for each type of antibody. The presence of a specific antigen is established by the characteristic SERS spectrum of the reporter molecule. A near-infrared diode laser was used to excite efficiently the SERS signal while minimizing fluorescence interference. We show that, by using different labels with little spectral overlap, two different antigenic species can be detected simultaneously. The potential of this concept to function as a readout strategy for multiple analytes is briefly discussed.

Bimetallic monolayer-protected nanoparticles have been synthesized by the core metal galvanic exchange reaction of dodecylthiolate monolayer-protected metal (Ag, Pd, Cu) clusters with the more noble metal metal thiolate complexes Au I [SCH2(C6H4)C(CH3)3] and Pd II [S(CH2)11CH3)2]. The bimetal nanoparticles produced are stable and can be isolated without core aggregation or decomposition. These new materials have been examined by UV-vis spectroscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and elemental analysis. Their optical properties reflect bimetal cluster formation by timedependent shifts in the surface plasmon resonance absorbance. Transmission electron microscopy results suggest that the core metal replacement can also effect a change in nanoparticle core size. Formation of bimetallic nanoparticles appears to stabilized the less stable member of the metal pair.

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