Nanostructured surfaces for surface enhanced infrared absorption spectroscopy

Surface enhanced infrared absorption (SEIRA) spectroscopy is a powerful analytical technique for enhancing vibrational signals of adsorbed analytes. This technique can be applied in many fields such as electrochemical dynamics, surface photochemistry and sensors in bio-spectroscopy. Metallic nanostructure surfaces are normally used as SEIRA active substrates. However, the controllability and reproducibility of most SEIRA active substrates are still on a poor level.\nChapter 1 provides the background of SEIRA spectroscopy. The discovery and development of surface enhanced spectroscopy is introduced, also including the development of SEIRA active substrates over past few decades. The enhancement mechanism of SEIRA is explained in terms of both electromagnetic and chemical mechanisms. Surface plasmon resonance (SPR) is also introduced here as both localized surface plasmon resonance (LSPR) and propagating SPR can be combined with SEIRA to improve its performance. The history, definitions and theory of SPR are discussed in detail. Also, the main platforms for exciting propagating SPR are illustrated.\nChapter 2 provides a novel method to prepare gradient metal films with sputter coating for applying as SEIRA active substrates. The gradient metal films have a changing morphology as a function of position along the substrate surface. With the increased film thickness, the surface morphology changed from isolated metal islands to interconnected metal clusters and became a continuous metal film finally. A maximum in SEIRA activity was observed at conditions where the metal islands begin to merge into a continuous metal film, known as the percolation threshold. This location was able to be monitored with optical transmission spectroscopy and conductivity measurements. In the UV-Vis and near-IR transmission spectrum, the tail of the LSPR band would extended into near infrared region at the percolation threshold location. And this location is the area where the film changed from conductive to insulative. This fabrication technique provides a reproducible method to fabricate metal island SEIRA active substrates and the ability to understand the role of particle size and spacing on SEIRA activity.\nChapter 3 demonstrates the synthesis and immobilization of gold nanoparticles (AuNPs) on indium tin oxide (ITO) substrates by pulsed electrodeposition (PED) technique. The prepared substrates were used as SEIRA active substrates. We demonstrated that the size, density and gaps between adjacent AuNPs were able to be well controlled by adjusting the overpotential and deposition cycles using the PED technique. The maximum SEIRA enhancement could be achieved near percolation threshold of metal nanoparticle film and is able to be monitored via optical transmission spectroscopy. This protocol provides a cost effective and controllable approach to fabricate reproducible, highly sensitive and stable SEIRA active substrates.\nChapter 4 illustrates propagating SPR response is able to be tuned in infrared region via two grating platforms. One is compressive grating platform produced by fabricating a metallic grating on a flexible, PDMS substrate. The other is a spontaneously chirped Au/PDMS grating platform, fabricated through ordered buckling structures of thin stiff film on an elastomeric substrate with mechanical deformation. By mechanically compressing the substrates, the grating pitch could be controlled on both tunable grating platforms, which resulted in shifting of the SPR peak in the infrared spectra. The results demonstrate that these two substrates provide a highly tunable and flexible platform for controlling the SPR response. They are able to combine with SEIRA spectroscopy to enhance vibrational signals of analytes adsorbed on the grating platforms.\nThis work aims to show the fabrication of novel SEIRA active substrates that can be applied in SEIRA technique to improve its performance. SEIRA activity is strongly correlated with the morphology of metal nanostructures on the substrate. The gradient metal film, AuNP substrates synthesized by PED and tunable grating platforms all show good tunability and reproducibility of their morphology. They can provide an especially controllable avenue for fabricating SEIRA active substrates and have great potential to be applied in routine SEIRA applications in the future.