Disposable chip based platforms for waveguide and metasurface enhanced Raman and mid-infrared spectroscopy via evanescent waves

Abstract

Rapid point-of-care medical diagnostics are vital for improving patient outcomes. Neonatal and acute respiratory distress syndromes are lung conditions that both require development of rapid point-of-care tests to enable faster treatment and more in-depth understanding of their pathologies. Fourier transform infrared (FTIR) and Raman spectroscopies provide complementary analytical techniques for analysis of a wide range of samples. Molecular bonds that can undergo a change in dipole moment tend to absorb strongly in the mid-infrared at characteristic wavelengths, enabling identification of molecules from their ‘fingerprint’ spectra. Similarly, molecular bonds that undergo a change in polarisability upon inelastic scattering of monochromatic light produce a Raman spectrum. However, the standard attenuated total reflection technique to measure solutions in FTIR spectroscopy via the evanescent field requires expensive crystals and is highly susceptible to inclusion of absorptions by atmospheric interferents, whilst Raman scattering is an inherently weak effect. Thus, this project aimed to design and fabricate low-cost, single-use chip based
Raman and attenuated total reflection (ATR)-FTIR spectroscopic platforms using evanescent waves to provide enhanced spectra and replace ATR crystals in ATR-FTIR.
Two approaches were utilised to address this aim using lower cost materials and fabrication methodologies than the current gold-standards. A recipe for low loss amorphous germanium films was developed and used to produce ATR chips microstructured on both sides for enhanced absorption spectroscopic measurements using tunable resonances. To remove spectral features of water vapour and CO2 absorptions in the infrared, a simple technique was developed using just two sequentially measured spectra and minimisation of Euclidean spectral length. Furthermore, given that the evanescent field is not diffraction limited, altering the evanescent field penetration depth achieved using microstructured single reflection elements (mSREs) was investigated as a potential technique for depth profiling of samples without causing structural damage. A novel method to fabricate pedestal waveguides for waveguide enhanced Raman spectroscopy (WERS) was also developed via hot embossing of plastics, followed by deposition of a light-guiding thin film of tantalum pentoxide.

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Identifiers

ORCID for Senthil Murugan Ganapathy: orcid.org/0000-0002-2733-3273

ORCID for Aneesh Vincent Veluthandath: orcid.org/0000-0003-4306-6723

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