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

Abstract

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 5.78–6.35 μm (1735–1570 cm−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.

Towards label-free mid-infrared protein assays: in-situ formation of bare gold nanoparticles for surface enhanced infrared absorption spectroscopy of bovine serum albumin

“…Last but not least, quantitative analysis of BSA protein was demonstrated in both water and deuterium oxide with the latter medium minimizing the background absorption of water within the spectral regime of interest (i.e., the amide I band). Hence, the present study evidences the potential of IR-ATR spectroscopy and sensing schemes as label-free mid-infrared protein conformation assay technology with enhanced sensitivity, which is of particular interest in combination with recently emerging miniaturized mid-infrared sensing schemes based on thin-film waveguide technologies combined with quantum cascades lasers [18,[34][35][36][37].…”

Quantum cascade lasers (QCLs) in biomedical spectroscopy

“…Particularly the work on breath analysis benefits from the long-lasting experience of developing advanced QCL-based detection schemes for trace gas analysis. In contrast, current implementations of QCLs for biomedical analysis of liquids seem rather limited to the quantification of glucose in blood, but recent advancements in protein analysis 65,74 promise near-term extension to further applications, e.g. milk analysis.…”

“…Pulsed and standard CW operation allow largest spectral tuning ranges and are frequently used for condensed phase spectroscopy and have found their applications in far-field 59 as well as near-field 60,61 infrared microscopy, stand-off measurements of solid residues 62 as well as in analysis of liquids. [63][64][65] However, these configurations might cause problems in high resolution gas-phase spectroscopy due to mode-hops that can occur during tuning. 54 Mode-hops originate from competition of different optical modes for the available net gain in the laser medium.…”

Infrared spectroscopy based on broadly tunable quantum cascade lasers and polycrystalline diamond waveguides

“…Thin-film diamond waveguide technology in combination with laser sources has already been shown for the detection of organic compounds and proteins in the MIR. [9][10][11] In these contributions, free-standing core-only (i.e., air-clad) diamond thinfilm waveguides supported by a silicon frame have demonstrated their utility for analytical applications. However, processing limitations including high strain within the deposited diamond layers, thermal expansion, and demanding processing parameters at elevated temperatures in a hydrogen atmosphere compete with matching the required optical parameters such as the refractive index.…”