Hagger, Oliver S. J.; (2025) Single-step Atmospheric Pressure Plasma Jet deposition of metals for SERS-active substrates. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Plasma technology is at the forefront of advancements in the scientific sector, with its applications expanding daily and an increasing number of publications in the field. A key advantage of plasma is its ability to deliver high energy at cold temperatures, making it adaptable for modifying substrate surface characteristics. This study utilises an RF-powered atmospheric pressure plasma jet (APPJ) for zerovalent metallic deposition to produce surface-enhanced Raman scattering (SERS) substrates. The substrates primarily consist of silver particles with distinct sizes and shapes, deposited from a silver nitrate (AgNO3) precursor. SERS characterisation is performed using two organic analytes that bind strongly (chemisorption) to the surface and exhibit well-defined peaks. Substrate SERS enhancement factors are calculated. Substrates are also characterised by X-ray Photoelectron Spectroscopy (XPS), X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and Transmission Electron Microscopy (TEM). The SERS substrates are produced through a simple, one-step method requiring minimal plasma power and short incubation time. These substrates exhibit significantly higher enhancement factors than those of commercially available alternatives. This versatile deposition technique has been extensively explored, including the incorporation of five axes of movement to extend its capabilities. It enables deposition not only on 2D surfaces but also on topologically complex and temperature-sensitive materials, all of which produce effective SERS spectra. An additional advantage of the plasma-deposited SERS substrates is their reusability. By introducing a small concentration of hydrogen (4%) and oxygen (4%) into the plasma, the substrates can be effectively ‘cleaned’ to remove organic contaminants, including carbonaceous residues. This cleaning step is essential for eliminating background signals that can obscure SERS spectra. The APPJ system used in this work was carefully controlled to prevent unwanted mixing of gases, and a sheath gas was employed to isolate the jet in a negatively pressurised environment, minimising interference from atmospheric oxygen. The combined hydrogen-oxygen plasma not only enables effective SERS by reducing background noise but also facilitates the removal of the analyte after analysis, restoring the substrate surface to a clean, pre-analyte state. This approach significantly reduces production costs by allowing substrates to be reused multiple times. Although the plasma cleaning process alters the silver surface slightly—particularly during oxygen exposure—experimental results indicate that the substrates can be reused at least five times without any measurable degradation in SERS signal intensity. This deposition technique and the resulting SERS substrate production represent a significant advancement in analytical science. The low-cost, one-step production method is simple, efficient, and adaptable for deposition on various surfaces and in diverse patterns. Furthermore, the ability to reuse these surfaces repeatedly provides a significant cost-saving benefit, making this approach a notable step forward in SERS technology.

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