Mid-IR refractive index sensor for detecting proteins employing an external cavity quantum cascade laser-based Mach-Zehnder interferometer

Abstract

Novel laser light sources in the mid-infrared region enable new spectroscopy schemes beyond classical absorption spectroscopy. Herein, we introduce a refractive index sensor based on a Mach-Zehnder interferometer and an external-cavity quantum cascade laser that allows rapid acquisition of high-resolution spectra of liquid-phase samples, sensitive to relative refractive index changes down to 10−7. Dispersion spectra of three model proteins in deuterated solution were recorded at concentrations as low as 0.25 mg mL−1. Comparison with Kramers-Kronig-transformed Fourier transform infrared absorbance spectra revealed high conformance, and obtained figures of merit compare well with conventional high-end FTIR spectroscopy. Finally, we performed partial least squares-based multivariate analysis of a complex ternary protein mixture to showcase the potential of dispersion spectroscopy utilizing the developed sensor to tackle complex analytical problems. The results indicate that laser-based dispersion sensing can be successfully used for qualitative and quantitative analysis of proteins.

A mid-infrared lab-on-a-chip for dynamic reaction monitoring

“…In addition, we extract important performance values of the sensor such as the LOD and the effective path length within the probed analyte d e f f . This is in contrast to previous studies, mainly performing a qualitative analysis 34 , 35 , 40 , 68 .…”

“…2 d, ref. 40 ). This is a remarkable result, because the refractive index of air ( n a i r ≈ 1) is significantly lower than that for D 2 O of = 1.3 40 .…”

“… 40 ). This is a remarkable result, because the refractive index of air ( n a i r ≈ 1) is significantly lower than that for D 2 O of = 1.3 40 . Still, the mode remains very well confined and guided from laser to detector, showing the excellent suitability of the DLSPP waveguide for liquid spectroscopy.…”

“…Still, the mode remains very well confined and guided from laser to detector, showing the excellent suitability of the DLSPP waveguide for liquid spectroscopy. Adding the BSA to the D 2 O has only a negligible effect on the refractive index (e.g., Δ n ~ 10 −4 for 0.25–2% m v −1 40 ) as compared to pure deuterium oxide.…”

Dual-Beam Photothermal Spectroscopy Employing a Mach–Zehnder Interferometer and an External Cavity Quantum Cascade Laser for Detection of Water Traces in Organic Solvents

“…In recent years, room temperature-operated, small but powerful mid-IR lasers operated in pulsed as well as CW mode have become commercially available from different providers, and as such they have been used in mid-IR sensing applications . QCLs equipped with an external diffraction grating, called external cavity (EC), allow for a broad tunability across the mid-IR range (a few hundred wavenumbers) that can be particularly exploited for liquid-phase analytes. − The favorable properties of these sources are high spectral power densities up to several hundreds of milliwatts in terms of peak power as well as several tens of milliwatts in CW operation. Furthermore, coherence, inherent polarization, and the possibility for fast albeit accurate amplitude and frequency modulation offer opportunities to develop new sensing schemes.…”