Toward the Design of Highly Stable Small Colloidal SERS Substrates with Supramolecular Host–Guest Interactions for Ultrasensitive Detection

Abstract

In this work, we report a simple strategy to obtain ultrasensitive SERS nanostructures by self-assembly of Au nanospheres (NSs). This novel protocol allow us to obtain in a reproducible manner Au NS dimers using cucurbit[6]uril (CB[6]) molecules as linkers. The resulting dimers are stable in colloidal dispersion over several days, generating in this way nanostructures with highly reproducible hot spots. This feature is due to the precise subnanometric control of the molecules that generates interparticle distance and, at the same time, of the capability of placing analyte molecules just within these junctions of ultrahigh field enhancement due to the host−guest properties of the CB[6] molecule. The dimer formation is based on the modification of the metal surface with cysteamine molecules (Cys) previous incubation with CB [6] molecules. The cysteamine-functionalized NSs (Cys/ NSs) are positive charged due to the protonation of the amine groups of the Cys molecule at the working pH. These positive groups interact through H-bonds with the carbonyls groups at the portal of the CB[6] molecules. The dimer formation is based on the stoichiometric control between Cys/NPs and CB[6] (ratio 2:1), generating a gap of 1.8 nm and giving rise to SERS enhancements of around 10 8 . The host−guest properties of the CB [6] molecule are used to detect the average SERS enhancement produced by adding methyl viologen (MV) as plasmonic probe. It was found that the average analytical enhancement factor (AEF) for MV is as good as that obtained for the CB[6] itself (around 10 8 ).

Tunable Coffee Ring Formation on Polycarbonate Nanofiber Film for Sensitive SERS Detection of Phenylalanine in Urine

“…This low density of metal nanostructures in the colloidal solution results in low sensitivity of SERS measurements, with the observed sensitivity usually 1 or 2 orders of magnitude lower than that observed for other substrate types. 18,19…”

Quantitative SERS by hot spot normalization – surface enhanced Rayleigh band intensity as an alternative evaluation parameter for SERS substrate performance

“…$$\text{italicAEF}=\frac{I_{\mathit{\text{SERS}}}{c}_{\mathit{\text{normal}}}}{I_{\mathit{\text{normal}}}{c}_{\mathit{\text{SERS}}}}$$In contrast to SSEF, AEF is particularly useful for colloidal SERS substrates. 15 For solid substrates, large variations in AEF are expected due to variations in the way analyte molecules are added to the substrates, as such variations affect analyte adsorption to the substrate surface. Analyte adsorption to a SERS substrate is generally described using the Langmuir isotherm, 16, 17 such that the coverage of the analyte on the metal surface is only linearly correlated with the analyte solution concentration over a low concentration range.…”

“…For example, in a recent study, Fraire et al reported two AEF values that varied over five orders of magnitudes (1×10 8 vs. 3×10 3 ) corresponding to two different c SERS (i.e., 10 −11 vs. 10 −7 M). 15 Accordingly, AEF reflects the combined properties of both the SERS substrate and the number of analyte molecules on the surface rather than the intrinsic enhancing capability of the SERS substrate. As Le Ru stated, “It is not a good characterization of the SERS substrate itself, and it cannot be used to easily compare the performances of different substrates.”…”

Cucurbiturils‐Mediated Noble Metal Nanoparticles for Applications in Sensing, SERS, Theranostics, and Catalysis

“…Reproduced with permission. [ 36 ] Copyright 2015, American Chemical Society. c) 1D assembly of negatively charged NPs with CB[ n ]s and salts through electrostatic interactions.…”