Rapid Single Step Atmospheric Pressure Plasma Jet Deposition of a SERS Active Surface
Authors: Hagger Oliver S. J. , Sener M. Emre , Khan Imran , Lockwood Estrin Francis , Agrotis Stefanos , Handoko Albertus Denny , Parkin Ivan P. , Caruana Daren J. 
Published: August 2024

Surface-enhanced Raman Spectroscopy (SERS) is a highly sensitive detection technique capable of nanomolar or lower detection limits for some analytes.1-3 The key parameter is control of the surface nanostructure to optimise this remarkable enhancement, providing non-destructive detection without the need for chemical labelling. The bottleneck for the wide applicability of SERS as a valuable analytical tool is twofold: (1) rapid fabrication with the required nanostructure and (2) the difficulty in recovering a baseline before exposure or re-exposure to the target matrix. Here we describe an atmospheric plasma jet method for rapid single-step synthesis of SERS active silver on glass and demonstrate the potential of plasma to remove the adsorbed analyte to recover a baseline.

The method described in this study employs an atmospheric pressure plasma jet (APPJ) for rapid single-step deposition of nanostructured silver materials with SERS.4,5 Non-thermal plasmas under reduced pressure offer a unique redox chemistry, partly due to the highly energised electrons and are essential for processing many materials and chemical transformations. Electrochemically, a helium-conducting gas allows various redox reactions to occur, without the need of restricting solvent mediums. An APPJ presents a medium which can be considered an electrode due to the presence of electrons and electrolytes, with the ability to drive reduction processes.6 APPJs allow fascinating chemistry to occur, balancing the thermal kinetic energy distribution between electrons and other neutral and ionised species. Consequently, plasma jets are used for etching, cleaning, and medical treatments, as well as driving highly energised electrons in redox reactions.7-9

The plasma was ignited at a stainless-steel needle electrode tip positioned in a 440 mM I.D. ceramic nozzle, with a plume extending 2 mm from the nozzle tip. Silver deposits were ‘written’ precisely in a 2D regular pattern of plasma deposited silver on borosilicate glass for Raman interrogation without any post-treatment by feeding a silver salt into the APPJ. The APPJ ceramic nozzle prints 125 ± 25 m diameter silver islands with a height of approx. 370 nm, separated by 500 mm, these islands formed the rudimentary SERS substrates.

The surface morphology and microstructure through SEM and AFM will be described– assessing how microstructure will correlate to SERS enhancement. The concentration dependence will be quantitatively evaluated using various analytes over a wide range of 1 x 10-7 M to 1 x 10-2 M. The detection limit was calculated using three times the lowest concentration detectable standard deviation, equating to 154 ppb. The particle morphology and SERS enhancement are compared to various commercial substrates. Using the same APPJ system for the deposition of silver metal, a helium plasma doped with 4 % oxygen, followed by a separate treatment of helium doped with 4 % hydrogen, was found effective in restoring the zero baseline Raman response. After the cleaning steps, all analytes could show zero baseline recovery with Raman response regeneration. After successive cycles of application of analyte and then oxygen and hydrogen treatment, there is a slight drop in the SERS signal.4,10 Silver readily oxidises to form silver oxide after exposure to helium plasma doped with oxygen, as confirmed using XRD and XPS. This plasma oxidation causes a change in the microstructure of the particles, ultimately sintering together with successive oxidation cycles.

Plasma-prepared substrates show significant gains with respect to the economical use of materials and the rapidity of the synthesis of the substrates. We showcase using the atmospheric pressure plasma jet method for synthesis and baseline recovery as an integral part of an analytical device. This approach presents the prospect of reusing the same SERS device for analysis, which may be a key advantage for academic and industrial applications. 11-13

Figure caption:

(a) Raman spectra of R6G with most prominent peak measured at 1651cm-1 after oxygen and hydrogen treatment with the black lines showing the Raman spectra of the surface before application of analyte. (b) The peak intensity at 1586cm-1 peak for 4MBA for each measurement cycle with the analyte and before analyte application (after oxygen and hydrogen treatment) (c) Depicting the cleaning and regeneration of SERS signal on a borosilicate substrate. Both 4MBA and R6G were dissolved in methanol at 1 x 10-4M and drop cast onto the substrate. (d) normal X-ray diffraction spectra of PDS between 5 – 80 o 2θ for the original plasma deposit (i), oxidised plasma clean (ii) and hydrogen plasma clean (iii). All assignment was carried out using the Bruker XRD database, using JCPDS 04-0783 and 43-0997 for Ag and Ag2O, respectively.

Figure 1