Carboxy-terminated immuno-SERS tags overcome non-specific aggregation for the robust detection and localization of organic media in artworks.

Methods combining immunology and surface-enhanced Raman scattering (SERS) have been developed for the simultaneous detection, identification, and localization of proteinaceous binding media found in artworks. However, complex surface topographies and heterogeneous compositions of art samples represent significant challenges for the general optimization of this technique. In particular, aggregation of immuno-SERS nanoparticles can lead to non-specific SERS response across the sample surface, resulting in inaccurate identification of binding media or dubious localization maps. This aggregation also diminishes the sample area available for analysis, as excitation of visible nanoparticle aggregates by the Raman laser must be avoided during data collection. In the present work, we synthesize several types of immuno-SERS nanoparticles and investigate their applicability for the detection and localization of ovalbumin-rich (egg-based) binding media in art samples. Dimers of gold nanoparticles (Au NPs) connected by a Raman-active dithiolated linker are conjugated to secondary antibodies through either an amino or a carboxyl functional group (SERS tags). The SERS tags display localized surface plasmon resonance (LSPR) at 532 nm. SERS spectra are acquired at 633 nm (SERS-633) in order to maximize tuning between laser excitation and LSPR, while avoiding sample burning. In an indirect immunoassay applied to replica art samples, carboxy-terminated SERS-633 tags show strong Raman reporter signal, specificity for the target protein, robust response in the presence of various inorganic pigments, and reduced aggregation on sample surfaces compared to amino-terminated or commercial SERS tags. Scanning electron microscopy (SEM) is used to visualize Au NPs bound to egg media in situ, demonstrating that carboxy-terminated SERS-633 tags remain as discrete dimer units throughout the assay.

Gold nanostar substrates for SERS-based chemical sensing in the femtomolar regime.

We report a novel approach for fabricating gold nanostar-functionalized substrates for highly sensitive surface enhanced Raman spectroscopy (SERS)-based chemical sensing. Gold nanostars immobilized on a gold substrate via a Raman silent organic tether serve as the SERS substrate, and facilitate the chemical sensing of analytes that can either be chemisorbed or physisorbed on the nanostars. Our SERS substrates are capable of detecting chemisorbed 4-mercaptobenzoic acid at a concentration as low as 10 fM with a reproducible SERS enhancement factor of 10(9), and enable the semi-quantitative multiplexed identification of analytes from mixtures in which they have been dissolved in variable stoichiometry. Most importantly, they afford the detection of physisorbed analytes, such as crystal violet, with an excellent signal-to-noise ratio, hence serving as a versatile platform for the chemical identification of in principle any molecular analyte. These characteristics make a strong case for the use of our nanostar-based SERS substrate in practical chemical sensing applications.