Surface-Enhanced Dual-Frequency Two-Dimensional Vibrational Spectroscopy of Thin Layers at an Interface

Abstract

The development of spectroscopic approaches to study molecules at interfaces is important as the molecular properties often differ from those in the bulk. Implementation of surface-enhanced two-dimensional infrared (SE 2DIR) spectroscopy using lithographically fabricated plasmonic nanoarrays is demonstrated for nanometer thick films. The sample, 4-azidobutyrate-N-hydroxysuccinimide ester (azNHS), dispersed in polystyrene was deposited onto the nanoarray. Raw enhancements in the SE 2DIR spectra exceeding 5 × 104 and 1.3 × 103 fold were achieved for the CO and NN peaks, respectively. The field enhancement provided by the nanoarray was sufficient to record cross-peaks in 1 nm thick samples under dilute conditions for azNHS (∼0.1 M). Note that the cross-peaks were recorded for vibrational modes frequency separated by ∼350 cm–1 with the enhancement factor of 4.1 × 104. The effective electric field enhancement factors, measured for NN and CO modes via linear and two nonlinear IR techniques, have similar sample-thickness dependences, which permit using linear spectroscopy for enhancement evaluation. High-quality cross-peak waiting-time dependences were recorded for samples as thin as 1 nm involving several IR reporters demonstrating the applicability of an arsenal of 2DIR approaches, including spectral diffusion, chemical exchange, and relaxation-assisted 2DIR, to interrogate samples in nanometer thick films. The study opens new opportunities in analyzing structures and dynamics of molecules at interfaces.

Enhancing the signal strength of surface sensitive 2D IR spectroscopy

“…SEAR 2D IR spectroscopy builds from previous work that allows for surface sensitive 2D IR spectroscopy. [24][25][26][27][28][29]32,33 Here, the combination of a reflection geometry and localized surface plasmons effectively decreases the local oscillator intensity while also increasing the third-order signal, providing enhancements of the measured ∆OD of the amide I mode of a peptide monolayer. Table II outlines the signal enhancement observed from the localized surface plasmon, the inverted reflection geometry, and the total enhancement of SEAR 2D IR signals for cys-ovispirin-1 on 3 nm Au/CaF 2 .…”

“…[24][25][26] Others have used gold nano-antennas to obtain even larger signals through on-resonant enhancement, albeit not in an ATR geometry. [27][28][29] In Hamm's surface enhanced 2D ATR IR spectroscopy, a nanostructured layer of gold coats the surface of a prism that acts as one face of the sample cell. These gold nanostructures have a maximum plasmon absorption in the visible region that extends into the mid-IR.…”

“…Nishida et al reported signal enhancement of 50-times. 32 While this enhancement is not as large as plasmon enhanced 2D IR, [24][25][26][27][28][29] it is applicable to solution surfaces where interesting phenomena using metal carbonyl molecules has been observed. 33 In this article, we report a variation of 2D IR spectroscopy that combines the approaches of Fayer and Hamm (Fig.…”

“…As the plasmon frequency moves closer to resonance with the vibrational modes of interest, the enhancement increases drastically. 28,29 Other plasmonic structures, such as nanoarrays, nano-antennas, or nano-stars may have different plasmon frequencies and polarization dependencies that can be considered to provide more enhancement. Here, the rough nanometer thin Au on CaF 2 substrates used have a plasmon frequency in the visible (Fig.…”

Optical Spectroscopy of Surfaces, Interfaces, and Thin Films: A Status Report

“…30 A recent paper by Mackin et al demonstrated the utilization of SEIRA principles in two-dimensional IR (2DIR) (surface enhanced 2DIR spectroscopy, SE-2DIR) to study thin layers of materials down to 1 nm. 31 A key advantage of 2DIR is the ability to probe cross peaks or changes that occur in the IR spectrum not in the region actively being pumped that can be linked to molecular structures and their dynamics. However, in SE-2DIR, cross-peak analysis can be problematic, as the cross peaks are typically several orders of magnitude weaker than the diagonal peaks.…”