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

Department of Biomedical Engineering, Emory University and Georgia Institute of Technology, 101 Woodruff Circle, Suite 2001, Atlanta, Georgia 30322, USA.
Journal of the American Chemical Society (Impact Factor: 12.11). 02/2012; 134(4):2000-3. DOI: 10.1021/ja210992b
Source: PubMed


High-affinity anchoring groups such as isothiocyanate (ITC, -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, and to scanning tunneling microscopy 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 (MG) and its ITC derivative (MGITC), 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 MGITC are nearly 500-fold higher than those of MG. Correcting for the intrinsic difference in scattering cross sections of these two dyes, we estimate that the MGITC enhancement factors are ~200-fold higher than for MG. Furthermore, pH-dependent studies reveal that the surface structure of MGITC 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 MG is still sensitive to pH and can be switched between its localized and delocalized electronic forms. These results indicate that ITC 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.

Download full-text


Available from: Shuming Nie, Feb 17, 2014
  • [Show abstract] [Hide abstract]
    ABSTRACT: We designed a recyclable Hg2+ probe based on Rhodamine B isothiocyanate (RBITC) - poly (ethylene glycol) (PEG)-co-modified gold nanoparticles (AuNPs) with excellent robustness, selectivity and sensitivity. Based on reasonable design, only Hg2+ can displace RBITC from the AuNP surfaces, resulting in a remarkable enhancement of RBITC fluorescence initially quenched by AuNPs. To maintain stability and monodispersity of AuNPs in real samples, thiol-terminated PEG was employed to bind with the remaining active sites of AuNPs. Besides, this displacement assay can be regenerated by adding fresh RBITC into the AuNPs solutions that were already used for detecting Hg2+. Importantly, the detection limit of this assay for Hg2+ (2.3 nM) was lower than the maximum limits guided by the United States Environmental Protection Agency as well as that permitted by the World Health Organization. The efficiency of this probe was demonstrated in monitoring Hg2+ in complex samples such as river water and living cells.
    No preview · Article · Nov 2012 · ACS Nano
  • [Show abstract] [Hide abstract]
    ABSTRACT: Probing and controlling the configurations and mechanical motions of the azobenzenes adsorbed on the metal substrates are preliminary for their applicability in various functional devices. In this work, we presented a detailed investigation of Raman properties of the dimercaptoazobenzene (DMAB) bound to gold nanoclusters using density functional calculations. It is demonstrated that the spectral features of the trans conformation of DMAB are quite different from the cis conformation, and the Raman intensities of the trans-DMAB are much larger. Magnitude of chemical enhancement for the adsorbed trans-DMAB is found to be close to or less than that for the adsorbed cis-DMAB for the molecule–cluster complexes. This change trend can be, to a large extent, governed by the energy difference between the highest occupied energy level of the molecule and the lowest unoccupied energy level of the gold. Moreover, it is further demonstrated that differences in Raman intensities of the two conformations can be amplified for the cluster–molecular–cluster junctions, and thus chemical enhancement is much larger for the trans conformation than the cis conformation, possibly facilitating the experimental identification of the trans/cis DMAB. Copyright © 2012 John Wiley & Sons, Ltd.
    No preview · Article · Mar 2013 · Journal of Raman Spectroscopy
  • [Show abstract] [Hide abstract]
    ABSTRACT: Nanoscale mapping of adsorption sites for molecules or ions at solid–liquid interfaces has not been explored in detail because of the difficulty in probing both stochastic adsorption/desorption events and heterogeneous surface structures. We report here the application of single-molecule-based super-resolution fluorescence microscopy using a catechol-modified boron–dipyrromethene dye (CA-BODIPY), which serves as a fluorescent reporter, to identify the locations of effective adsorption sites on metal oxide surfaces. Upon adsorption on a TiO2 nanoparticle, individual CA-BODIPY molecules exhibited detectable fluorescence because of the formation of chelating complexes between the catechol moiety and the surface Ti sites. Interestingly, a significant effect of the crystal face on the adsorption preference for CA-BODIPY was found in the case of anatase TiO2 microcrystals in neutral water: {101} > {001} ≈ {100}. In an aprotic solvent such as acetonitrile, however, the opposite crystal face effect was observed; this implies a significant contribution of solvent molecules to the adsorption of organic compounds on specific surfaces. From the quantitative analysis of the formation rate of fluorescent complexes per unit area, it was found that nanometer-sized TiO2 crystals have superior adsorptivity over micrometer-sized TiO2 crystals and an atomically flat TiO2 surface. This observation is consistent with the higher density of surface defects on the nanoparticles. Furthermore, it was revealed that CA-BODIPY molecules are preferentially adsorbed on the top branches of α-Fe2O3 micropines, where a high density of exposed Fe cations is expected. Our methodology and findings yield new insights into the mechanisms underlying the synthesis and (photo)catalytic activity of metal oxide particles with different sizes and shapes.
    No preview · Article · May 2013 · The Journal of Physical Chemistry C
Show more