Surface enhanced raman spectroscopy using shaped gold nanoparticles

Abstract

Surface enhanced Raman scattering (SERS) spectra of 4-mercaptobenzoic acid (4-MBA) self-assembled monolayers (SAMs) on gold substrates is presented for SAMs onto which gold nanoparticles of various shapes have been electrostatically immobilized. SERS spectra of 4-MBA SAMs are enhanced in the presence of immobilized gold nanocrystals by a factor of 107-109 relative to 4-MBA in solution. Large enhancement factors are a likely result of plasmon coupling between the nanoparticles (localized surface plasmon) and the smooth gold substrate (surface plasmon polariton), creating large localized electromagnetic fields at their interface, where 4-MBA molecules reside in this sandwich architecture. Moreover, enhancement factors depend on nanoparticle shape, and vary by a factor of 102.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS[0001]This is a Divisional Application of U.S. Serial No. 11 / 721,554 filed Jul. 7, 2008, which is a U.S. National Phase of PCT / US2005 / 044963 filed Dec. 13, 2005, which claims the benefit of U.S. Application No. 60 / 635,704, filed Dec. 13, 2004, and U.S. Application No. 60 / 648,920, filed Feb. 1, 2005, all of which are hereby incorporated herein by reference in their entireties.ACKNOWLEDGEMENT[0002]This invention was made with government support under Grant CHE-0336350 awarded by the National Science Foundation. The government has certain rights in the invention.BACKGROUND[0003]Bulk solution synthetic methods often produce nanocrystals of multiple sizes and shapes, and hence there is relatively low yield of the desired size and shape. Murphy, C. J. Science 2002, 298, 2139-2141. Although colloid chemists have achieved excellent control over particle size for several metallic and semiconductor systems, there has been limited success in gaining contr...

AI Technical Summary

Benefits of technology

[0012]In one aspect, the invention relates to a method for enhancing a Raman signal comprising the steps of providing a sample comprising a metal surface, an analyte adhered to the surface, and a metallic nanoparticle coupled to the surface, wherein the nanoparticle has a plasmon resonance band; exposing the sample to incident energy of an excitation wavelength that overlaps with the metallic nanoparticle plasmon resonance band; and detecting the Raman signal of the analyte.

[0013]In a further aspect, the invention relates to a method for enhancing a Raman signal comprising the steps of providing a sample comprising a metal surface, a functionalized self-assembled monolayer adhered to the surface, wherein the self-assembled monolayer comprises an analyte, and a cetyltrimethylammonium bromide-capped metallic n...

Problems solved by technology

Bulk solution synthetic methods often produce nanocrystals of multiple sizes and shapes, and hence there is relatively low yield of the desired size and shape.

Although colloid chemists have achieved excellent control over particle size for several metallic and semiconductor systems, there has been limited success in gaining control over the shape of the nanocrystals.

However, no reports have been made for SERS on nanorods where the Raman excitation occurs at a wavelength that overlaps with nanorod plasmon resonance, a condition where the EM enhancement mechanism should be operative.

A significant challenge in SERS on colloidal nanoparticle substrates is determining the number of analyte molecules sampled during the experiment.

This is especially difficult for nanoparticles that are synthesized using strongly adsorbed capping agents including cetyltrimethylammonium bromide (CTAB), which may or may not be displaced by the analyte of interest.

In most reports, monolayer surface coverage on the nanocrystals is assumed, but if incorrect could lead to errors in calculations of EF values.

However, the tunability of the optical properties of planar SERS substrates is more difficult than solution-prepared colloids.

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