Vibrational spectroscopic techniques, such as infra-red (IR), normal Raman Spectroscopy and Surface Enhanced Raman Spectroscopy (SERS), have been considered for analyte detection. Of these, SERS has evolved as one of the most sensitive techniques for analyte detection due to enhancement of the Raman spectral intensity by interaction of the adsorbed SERS active analyte molecules with the surface of a metal substrate.
In SERS, the intensity of the vibrational spectra of a molecule is enhanced by several orders of magnitude when the molecule is in close proximity to metallic nanoparticles such as gold and silver. SERS has been successfully applied for labeling biological systems even in cells and tissues to sense multiplexed biomarkers. Nanoparticle tags that use SERS, herein referred as SERS-nanotags, to generate detectable Raman signals have been shown to be a successful alternative to fluorescence labeling, which has the drawbacks of photobleaching, peaks overlapping in multiplexed experiments, and inability to function in some extreme environments in biological systems.
Current state of the art SERS nanotags include immobilizing a Raman active dye (Raman reporter) on a metal colloid followed by bioconjugation to target specific locations. Such a nanoparticle—Raman reporter assembly is called a Raman tag in analogy with quantum dots and can provide a platform for multiplexing, targeting and tracking in bioimaging and sensing applications.
The types of reporter molecules and metal nanoparticles are major determinants of the sensitivity of a Raman tag. Among the different reporter molecules, triphenylmethine (TM) compounds exhibit absorption at visible ranges that allows the compounds to be a useful Raman reporter in visible-near infrared (visible-NIR) excitation. Although a few TM compounds, such as malachite green isothiocyanate (MGITC) and crystal violet (CV), have been used as reporters, there remains a need for reporter molecules which generate higher SERS intensities and are easily identifiable within a multiplexed analysis platform.
In view of the above, there is a need for an improved compound that can be used for detecting an analyte using SERS, as well as improved methods for forming it.