Article pubs.acs.org/JPCC
Dispersion Stability, Ligand Structure and Conformation, and SERS Activities of 1‑Alkanethiol Functionalized Gold and Silver Nanoparticles Siyam M. Ansar,† Manuel Gadogbe,† Kumudu Siriwardana,† Jane Y. Howe,⊥ Stas Dogel,⊥ Hooman Hosseinkhannazer,∥ Willard E. Collier,§ Jose Rodriguez,‡ Shengli Zou,# and Dongmao Zhang*,† †
Department of Chemistry and ‡Mississippi State Chemical Laboratory, Mississippi State University, Mississippi State, Mississippi 39762, United States § Department of Chemistry, Tuskegee University, Tuskegee, Alabama 36088, United States ∥ Norcada Inc., Edmonton, AB T6E 5B6, Canada ⊥ Hitachi High-Technologies Canada, Toronto, M9W 6A4, Canada # Department of Chemistry, University of Central Florida, Orlando, Florida 32816, United States S Supporting Information *
ABSTRACT: Dispersion stability, ligand structure and conformation, and SERS activities of 1-alkanethiol (CnH2n+1SH, n = 2−14) functionalized gold and silver nanoparticles (AuNPs and AgNPs) were studied as a function of alkanethiol carbon chain length and nanoparticle (NP) type and size. The dispersion stability of alkanethiol functionalized NPs in water increases with increasing alkanethiol chain length and NP size, and the stabilities of the alkanethiol-containing AuNPs are higher than their AgNP counterparts. C3H7SH and longer alkanethiols are highly ordered on AgNPs but disordered on AuNPs. The SERS intensity of the C−S stretch band for the model alkanethiols on AgNPs and AuNPs decays exponentially (I = I0 exp(−Nc/N0)) with increasing number of carbon atoms (Nc). The empirical decay length N0, in terms of the number of the carbon atoms, is 1.29, 0.53, and 0.10 for AgNPs with diameters of 50, 30, and 10 nm, respectively. This decay length is less than 1 for AuNPs of difference sizes. These results show that changing the NP gap size by a distance equivalent to a single chemical bond can have a significant impact on the NP integrated SERS activities.
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INTRODUCTION Ligand interactions with nanoparticles (NPs) and NP/NP interactions are implicated in essentially all NP applications including bottom-up device fabrications,1 biosensing,2 cancer therapy,3 and catalysis.4 Collective oscillations of free electrons in plasmonic NPs under resonance excitations generate strong local electrical fields, which lead to surface-enhanced optical spectroscopic phenomena including surface-enhanced Raman spectroscopy (SERS). Theory predicts that plasmonic NPs with nanoscale separation undergo effective plasmon coupling, producing exceedingly large electrical fields in the small NP junctions. SERS measurements confirm that the aggregated nanoparticles have drastically higher SERS activities than isolated NPs,5−7 and spatially averaged SERS spectra are dominated by a small fraction of molecules located in the junctions or crevices between neighboring particles. While classic electromagnetic theory predicts that the SERS enhancement increases with reducing NP gap size, quantum mechanics calculations show that when the gap size is smaller than 0.5 nm, electrons can tunnel through the gap between plasmonic NPs.8 This quantum plasmonic resonance reduces the SERS enhancement of the nanoparticle junction molecules.8 Recent © 2014 American Chemical Society
electron energy loss spectroscopic measurements and theoretical modeling suggest that the quantum plasmonic resonance effect is important between NPs with gap sizes smaller than 1.3 nm.9 Although there has been extensive theoretical and experimental work on the correlation between SERS enhancement and NP gap size, experimental evaluation of the effect of ultrasmall gap sizes (
