Malachite green isothiocyanate (MGITC)
is frequently used as a
surface bound Raman reporter for metal nanoparticle-enabled surface
enhanced Raman scattering (SERS). To date, however, no study has focused
on the application of MGITC for the formation of stable “hot-spot”
aggregates for Raman imaging applications. Herein we report a method
to produce a series of suspensions of MGITC functionalized gold nanoparticles
(MGITC-AuNPs) that at one extreme consist primarily of monomers and
at the other extreme as mixtures of multimers and monomers. Monomer
and multimer morphologies were characterized by scanning electron
microscopy and atomic force microscopy using a reliable spin-coating
deposition sampling method. The multimers generally include 2, 3,
or 4 individual AuNPs with an average number of 3 ± 1. The number
of multimers produced in a given suspension was found to be dependent
on the volume and concentration of MGITC initially applied. The surface
binding of MGITC to both monomeric and multimeric MGITC-AuNPs was
investigated by Raman and SERS, and the degree of aggregation in the
multimer suspension was evaluated based upon the measured variation
of the MGITC SERS intensity of the AuNPs. Using an estimated extinction
coefficient of 1.22 ± 0.41 × 10<sup>11</sup> M<sup>–1</sup> cm<sup>–1</sup> at ≈850 nm for the localized surface
plasmon resonance (LSPR) band of the MGITC-AuNP multimers, the multimer
concentrations were calculated by Beer’s Law