Mid-infrared thin-film diamond waveguides combined with tunable quantum cascade lasers for analyzing the secondary structure of proteins

Diamond has excellent optical properties including broadband transmissivity, low self-absorption, and a high refractive index, which have prompted its use for optical sensing applications. Thin-film diamond strip waveguides (DSWGs) combined with tunable quantum cascade lasers (tQCLs) providing an emission wavelength range of 5.78-6.35μm (1735-1570cm^-1) have been used to obtain mid-infrared (MIR) spectra of proteins, thereby enabling the analysis of their secondary structure via the amide I band. Three different proteins were analyzed, namely bovine serum albumin (BSA), myoglobin, and γ-globulin. The secondary structure of BSA and myoglobin has a major contribution of α-helices, whereas γ-globulins are rich in β-sheet structures, which is reflected in the amide I band. A comparison of the spectra obtained via the combination of the tQCL and DSWG with spectra obtained using conventional Fourier transform infrared (FTIR) spectroscopy and a commercial diamond attenuated total reflection (ATR) element has been performed. It is shown that the main features evident in FTIR-ATR spectra are also obtained using tQCL-DSWG sensors.