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Author Notes:

semory@chem.wwu.edu; snie@emory.edu

Subjects:

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

Tools:

Journal Title:

Journal of the American Chemical Society

Volume:

Volume 134, Number 4

Publisher:

, Pages 2000-2003

Type of Work:

Article | Post-print: After Peer Review

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.
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