Anchoring Molecular Chromophores to Colloidal Gold Nanocrystals: Surface-Enhanced Raman Evidence for Strong Electronic Coupling and Irreversible Structural Locking

Abstract

High-affinity anchoring groups such as isothiocyanate (-N=C=S) are often used to attach organic chromophores (reporter molecules) to colloidal gold nanocrystals for surface-enhanced Raman scattering (SERS), to atomically smooth gold surfaces for tip-enhanced Raman scattering (TERS), and also to scanning tunneling microscopy (STM) probes (nanosized electrodes) for single-molecule conductance measurements. However, it is still unclear how the attached molecules interact electronically with the underlying surface, and how the anchoring group might affect the electronic and optical properties of such nanoscale systems. Here we report systematic surface-enhanced Raman studies of two organic chromophores (malachite green and its isothiocyanate derivative) that have very different functional groups for surface binding, but nearly identical spectroscopic properties. A surprise finding is that under the same experimental conditions, the SERS signal intensities for the isothiocyanate derivative (MGITC) are nearly 500-fold higher than that of the parent dye (MG). Correcting for the intrinsic difference in scattering cross sections of these two dyes, we estimate that the MGITC enhancement factors are approximately 200-fold higher than that for MG. Furthermore, pH-dependent studies reveal that the surface structure of the isothiocyanate derivative is irreversibly stabilized or “locked” in its π-conjugated form and is no longer responsive to pH changes. In contrast, the electronic structure of adsorbed malachite green is still sensitive to pH and can be switched between its localized and delocalized electronic forms. These results indicate that isothiocyanate is indeed an unusual anchoring group that enables strong electronic coupling between gold and the adsorbed dye, leading to more efficient chemical enhancement and higher overall enhancement factors.

Self-Assembly of Gold Nanocrystals into Discrete Coupled Plasmonic Structures

“…Isothiocyanide (-N=C=S) groups are also known to possess high affinity versus gold. So far this affinity has mostly been exploited to attach organic chromophores to colloidal gold nanocrystals or to nanoscale electrodes for surface-enhanced Raman scattering (SERS) applications [23], formation of metal junctions for molecular electronic applications [24], bioimmunoassays [25] and cancer therapy applications [26]. However, recent theoretical work by Sikora et al showed that bis-isothiocyanide molecules can also be used as alternative to dithiols as conjugation molecules for bridging two or more nanocrystals together [27], whereby the lowest reactivity of isothiocyanide groups compared to thiols for citrate stabilized gold nanoparticles was shown useful to achieve good control in the growth rate of nanoparticle clusters.…”

Highly Robust, Recyclable Displacement Assay for Mercuric Ions in Aqueous Solutions and Living Cells

“…11 RBITC molecules can tightly adsorb and anchor around AuNP surfaces by means of the strong affinity between the ITC group and Au. 24 In general, the most commonly used 13 nm AuNPs are prepared using a citrate reduction method and are stabilized by the negatively-charged citrate. Upon addition of RBITC, the citrate, weakly adsorbed on Au surfaces, is replaced by RBITC molecules through Au–ITC bonds.…”

Hybrid Au–SiO2Core–Satellite Colloids as Switchable SERS Tags

“…We thus expect that RhB adsorbs on both the small Au 15 spheres and the exposed Au surface of the Janus particles (1 molecule per 2 nm 2 of total Au surface available). 52 Therefore, two Raman-active species are incorporated within the final nanostructures: RhB in the hot spots generated at the gaps between assembled Au particles and 4-MBA at the gold− silica interface of the Janus nanoparticles. Importantly, addition of the dye did not trigger aggregation of the particles, as indicated by UV−vis spectroscopy (Figure 2).…”