Microfluidic surface-enhanced infrared spectroscopy with semiconductor plasmonics for the fingerprint region

Abstract

The combination of semiconductor plasmonics with microfluidics allows surface-enhanced infrared spectroscopy of molecules in the flow regime. Exploiting semiconductor plasmonics enables surface-enhanced mid-IR spectroscopy from 4 μm to 20 μm and accesses the so-called molecular fingerprint region from 6.7 μm to 20 μm (1500–500 cm⁻¹). Besides addressing the whole fingerprint region and allowing the identification of molecules by database comparison, the III–V semiconductor material class allows potentially an integration of semiconductor-based IR-sources, IR-detectors and IR-resonators on-chip. Miniaturized plasmonic enhanced microfluidic mid-IR spectrometry has great potential to analyse and identify minute amounts of molecules in the flow regime. This work describes technological processing to combine semiconductor plasmonics and microfluidics. Two proof-of-concept prototypes were experimentally realized and subsequently tested. Measured mid-IR spectra allow to clearly distinguish ethanol and water by their respective IR-absorption characteristics when inserted into the microfluidic flow chamber. Additionally, a semiconductor surface plasmon resonance shift can be observed according to the inserted solvent. Finally, the formation of a self-assembled monolayer under flow conditions is demonstrated by an observable mid-IR surface plasmon resonance shift of 6 ± 1 cm⁻¹ (140 ± 23 nm).