Surface chemistry for point-of-care Raman spectroscopy

Abstract

This work explores the fundamentals required for developing waveguide enhanced Raman spectroscopy (WERS) as a suitable technique for point-of-use biological detection. WERS has the capability to use low-cost and miniaturised or integrated optical components, with the potential for highly sensitive and low-burden detection. Currently, WERS devices being developed globally are immature and are not yet suitable for use outside of controlled laboratory conditions. This research aims to provide information on the surface chemistry required for waveguide sensors to drive the future development of highly sensitive WERS.
In this research, there has been a focus on the chemical functionalisation of tantalum pentoxide waveguides to tune the surface properties, enabling the future immobilisation of biorecognition elements. Self-assembled monolayers of octadecylphosphonic acid were used to show the suitability of phosphonic acid derivatives for waveguide surface modification. These were characterised using contact angle goniometry, measuring the hydrophobicity imparted by the monolayer. A probe molecule, m-nitrophenylphosphonic acid, was adsorbed to the waveguide surface, and WERS of a monolayer was shown.
Typical Raman reference materials cannot be used with WERS; thus, benzyl alcohol has been identified as a suitable standard to assess the function and calibration of the WER spectrometer. The initial characterisation of benzyl alcohol formed a study into the scattering cross-sections of several solvents, which were determined using Raman microscopy and density functional theory. WERS was then performed, producing spectra of benzyl alcohol, benzyl-d7 alcohol, and m-diethynylbenzene.
Substrates for surface enhanced Raman spectroscopy (SERS) were also investigated. Comparisons were drawn between the two methods, with SERS able to strongly enhance the signal, but with difficulties in reproducibility and stability. Biofunctionalized silver nanostars were produced, with the aim of assessing a combined WERS-SERS sandwich assay, although the research did not reach this stage.
A wax printer has been used to produce a paper fluidic device capable of incubating the glucose oxidase reaction with glucose and delivering the product to a SERS active surface.
Combining the above elements together with signal enhancement and more targeted efforts towards biological assays should demonstrate the benefits of WERS for accessible and rapid biosensing.

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Identifiers

ORCID for Bethany May Bowden: orcid.org/0000-0002-2312-2727

ORCID for Philip Bartlett: orcid.org/0000-0002-7300-6900

ORCID for Sumeet Mahajan: orcid.org/0000-0001-8923-6666

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