Silver Nanocrescents with Infrared Plasmonic Properties As Tunable Substrates for Surface Enhanced Infrared Absorption Spectroscopy

Abstract

We exploit the unique infrared plasmonic properties of silver nanocrescents (AgNCs) in preparing tunable substrates for surface enhanced infrared absorption (SEIRA) spectroscopy. Fabrication provides good control over the crescents' structural properties which enables tuning of the localized surface plasmon resonances (LSPRs) from the visible through the infrared (IR) regions of the spectrum. Using AgNCs as uniquely tunable IR LSPR substrates, we demonstrate the impact of spectral tuning on maximizing SEIRA signal enhancements measured for adsorbed alkylthiolates. The AgNCs demonstrate the largest reported area-normalized SEIRA signal enhancements which increase from 7,700 to 46,000 depending on the relative positions of the AgNC's LSPR frequency and the molecular vibration frequency. The SEIRA enhancement increases and the absorption band line shape becomes more asymmetric as the AgNCs' LSPR frequency overlaps more extensively with the frequency of the probed molecular vibration. The tunability of the LSPR properties will enable fundamental SEIRA studies and the development of optimized SEIRA substrates for detection and identification of molecular adsorbates.

Ultra-sensitive vibrational spectroscopy of protein monolayers with plasmonic nanoantenna arrays

“…The enhanced signal mainly comes from a small quantity of molecules at the close vicinity of the N 2 nanorod tips (n ϭ 63 is the number of rows and columns). Second, in contrast to the studies conducted with self-assembled monolayers (SAMs) (7)(8)(9)(10)(11)(12)(13), here the protein molecules are physisorbed (see SI Text), which results in the following differences. (i) Unlike the SAMs, the transition dipole moments of the physisorbed proteins have no fixed orientation with respect to the metal surface normal.…”

“…The plasmonic enhancement of optical near fields can also be extended to the infrared frequencies enabling dramatic signal enhancement in infrared (IR) spectroscopy. In analogy to SERS, this method is called surface enhanced infrared absorption (SEIRA) spectroscopy (7)(8)(9)(10)(11)(12)(13)(14). Until recently, the bulk of SEIRA studies have revolved around enhancements achieved via chemically prepared or roughened metal surfaces (7)(8)(9)(10)14).…”

“…Furthermore, plasmonic resonances in these structures can be tuned to the spectral regions of interest with high spatial reproducibility. Recently, enhanced absorption signals have been observed from individual nanoantennas (8,(11)(12)(13). However, surprising new phenomena arise as a result of the collective resonant excitation of the nanoantenna ensembles.…”

Periodic array-based substrates for surface-enhanced infrared spectroscopy

“…As the spheres cover only a few percent of the surface in a random fashion, the distance between individual spheres is usually comparably large, and interaction between the individual particles is not expected to occur. Bukasov and Shumaker-Parry used this technique to prepare nanocrescents, shortly after linear nanoantennas were introduced for SEIRS [100]. They measured transmittance and converted it to what they call "normalized extinction" (normalized extinction is based on 1 − T instead of −lgT).…”

Plasmon resonances of Ag capped Si nanopillars fabricated using mask-less lithography

“…Plasmonic nanostructures [1,2] are the key components for extensive applications such as surface-enhanced Raman Scattering (SERS) [3][4][5], refractive index sensing [6][7][8], surfaceenhanced infrared absorption (SEIRA) [9,10], plasmonic optical devices [11,12] and thermal sensing [13], due to their ability to controllably confine light energy at the nanoscale. For sensing applications based on localized surface plasmon resonance (LSPR), emphasis has been drawn on the development of nanostructures exhibiting high and uniform field enhancement factors (EF), wide range and controllable plasmonic tunability, suitableness for mass-production, and reproducibility.…”