Immobilization of Gold Nanoparticles in Localized Surface Plasmon Polariton-Coupled Hot Spots via Photolytic Dimerization of Aromatic Amine Groups for SERS Detection in a Microfluidic Regime

Abstract

Plasmon-assisted chemistry is an effective tool for triggering various chemical transformations that can be performed with high spatial precision. Plasmon-triggering efficiency is ensured by a high concentration of light energy near the plasmonic surface and the resulting enhancement of the local electric field (EF). The coupling of different plasmonic nanostructures by multimodal plasmonic hot spots can significantly enhance the EF, increasing plasmon-triggering efficiency. In this work, we demonstrate that the coupling between the traveled surface plasmon polariton (SPP) wave and the localized surface plasmon (LSP) resonance can be used to effectively realize plasmon-induced organic reactions, even in hot spots between SPP-and LSP-supported Au nanostructures. The periodically modulated surface of the gold grating was used as an SPP-active plasmonic support, with spherical Au nanoparticles (AuNPs) ensuring LSP excitation. To model the chemical transformation, we utilized the dimerization of amino groups (previously grafted to Au nanostructures) with azo bridges between the AuNPs and Au grating. The reaction was performed in a microfluidic regime and resulted in AuNP immobilization on the grating surface. Experiments and theoretical studies indicated that the azo-bridge formation and AuNP immobilization occurred only under illumination with the SPP wavelength and proceeded more effectively on the grating walls (particularly the inflection points), where the EF enhancement is greatest due to LSP−SPP coupling. Created structures were subsequently demonstrated as a promising substrate for the SERS-based detection in the microfluidic regime.

Perspective on Tailored Nanostructure‐Dominated SPP Effects for SERS

“…To achieve more effective coupling between SPPs and LSPR; and thus, stronger SERS effects, plasmonic films with fishbone‐like periodic slit structures, [ 50 ] sinusoidal, [ 47–49,108–110 ] nano‐bowl, [ 111,112 ] and paraboloid‐like [ 51 ] surfaces instead of flat surfaces were adopted, as revealed in Figure . These special structures are normally designed to manipulate SPPs wave, somewhat like the manipulation of wavefront by metasurfaces.…”

“…The penetration depth in a dielectric layer could be even one order of magnitude larger relative to only dozens of nanometers in a plasmonic layer, suggesting a typical long-range characteristic, in sharp contrast with a few nanometers for LSPR only as well. [47][48][49][50][51] Despite the presence of some distinct differences, SPPs wave can couple with not only LSPR but also with themselves, other SPPs waves, and even excitons to create new localized/non-localized electromagnetic modes. [47,52,53] For example, for a typical system with plasmonic nanoparticles on a metallic film spaced by a dielectric layer, the LSPR produced by nanoparticles with an incident light can interact with the SPPs wave generated at the interface between plasmonic film and dielectric layer, or the SPPs wave is reflected into a free space via the LSPR.…”

Ag Nanoparticles@Au Nanograting Array as a 3D Flexible and Effective Surface-Enhanced Raman Scattering Substrate

“…To improve the SERS sensitivity, some attempts have been achieved to decorate the plasmonic NPs onto uniform nanostructured patterns via self-assembly and chemical linking to fabricate three-dimensional (3D) hybrid substrates. − The bottom 3D nanostructured patterns can provide a larger surface area as the support matrix to assemble more plasmonic NPs to obtain high-density hot spots, demonstrating a significantly enhanced SERS activity. − However, the support nanostructured patterns were usually fabricated by the above expensive and complicated electron-beam lithography and other micro/nanomanufacturing techniques, which seriously limits the large-scale fabrication of SERS substrates for industrial applications. Meanwhile, to anchor the plasmonic NPs on the bottom nanostructured patterns, coupling agents such as biphenyl-4,4′-dithiol (BFDT) or DNA chains had to be used. , Nevertheless, the background Raman signals of the coupling agents would seriously interfere with those of the targeted analytes.…”

Investigation of SERS Frequency Fluctuations Relevant to Sensing and Catalysis

“…The dimerization of para- aminothiophenol (PATP) to 4,4′-dimercaptoazobenzene (DMAB) is a commonly studied example. Originally, excitation wavelength and potential dependent bands were attributed to b 2 modes of PATP until it was shown those bands were the result of the dimerization of PATP to DMAB. , Additional azo-bridge formations have also been reported as the result of hot-carrier generation in SERS . The effects of hot carriers have also been seen in tip-enhanced Raman spectroscopy (TERS), where the plasmonic properties of nanoparticles on the coated apex of an atomic force microscopy tip allows for spatial monitoring of hot carrier catalysis …”

“…18,19 Additional azo-bridge formations have also been reported as the result of hot-carrier generation in SERS. 20 The effects of hot carriers have also been seen in tip-enhanced Raman spectroscopy (TERS), where the plasmonic properties of nanoparticles on the coated apex of an atomic force microscopy tip allows for spatial monitoring of hot carrier catalysis. 21 The ability to monitor catalytic and chemical transformations through hot carrier generation with SERS is advantageous.…”