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Abstract
Quantitative applications of surface-enhanced Raman spectroscopy (SERS) often rely on surface partition layers grafted to SERS substrates to collect and trap-solvated analytes that would not otherwise adsorb onto metals. Such binding layers drastically broaden the scope of analytes that can be probed. However, excess binding sites introduced by this partition layer also trap analytes outside the plasmonic “hotspots”. We show that by eliminating these binding sites, limits of detection (LODs) can effectively be lowered by more than an order of magnitude. We highlight the effectiveness of this approach by demonstrating quantitative detection of controlled drugs down to subnanomolar concentrations in aqueous media. Such LODs are low enough to screen, for example, urine at clinically relevant levels. These findings provide unique insights into the binding behavior of analytes, which are essential when designing high-performance SERS substrates.
Highlights
T remendous efforts have been made in the development of surface-enhanced Raman spectroscopy (SERS) substrates, often utilizing colloidal self-assembly or complex patterning of metal surfaces, with many variants that showcase million-fold SERS enhancements factors (EFs).1−5 because EFs scale as |E|4, spatial inhomogeneities in field enhancement |E(x,y)| result in highly varying Raman intensities across such high-performance substrates
To demonstrate interstitial incorporation of analytes and show the benefits of preventing indiscriminate binding, plasmonic substrates consisting of selfassembled AuNPs with a range of molecular spacers of CB[n] were compared, where n is 5, 6, 7, or 8.25 Adding CB[n] to a dispersion of citrate-stabilized AuNPs induces self-assembly, forming aggregates as the particles stick together via the CB[n]
We have demonstrated an interstitial analyte incorporation mechanism in self-assembled colloidal SERS substrates and used it to show the effects of analyte “theft” by indiscriminate binding on the limit of detection (LOD)
Summary
T remendous efforts have been made in the development of surface-enhanced Raman spectroscopy (SERS) substrates, often utilizing colloidal self-assembly or complex patterning of metal surfaces, with many variants that showcase million-fold SERS enhancements factors (EFs).− because EFs scale as |E|4, spatial inhomogeneities in field enhancement |E(x,y)| result in highly varying Raman intensities across such high-performance substrates. As a consequence, the majority of the measured SERS spectra are generated by only a small fraction of the molecules, situated in highly localized optically active sites (hotspots)− (Figure S1). Local variations can be effectively mitigated by collecting signals over a large number of hotspots, averaging SERS intensities for a given analyte concentration.− Averaging, results in a large fraction of analyte molecules not contributing significantly to the collected SERS spectra This effect becomes increasingly important at low analyte concentrations when the total number of analyte molecules approaches the (large) number of binding sites available outside the hotspot, resulting in fewer analyte molecules reaching the high-performance hotspots.. Locally replacing the bounding aqueous phase with a neighboring metal nanoparticle surface renders the local chemical environment significantly different from that of a ligand-coated nanoparticle surface We show that these properties combine to allow for interstitial incorporation of analytes (i.e., outside the CB molecular cavity but within the plasmonic hotspot). Our results show that this interstitial binding principle can be employed to detect a wide range of analytes as binding does not depend on the analyte’s affinity to the metal, but rather on its preference for the amphiphilic interactions presented within the hotspot
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