Pyruvate in Aqueous Media Probed by Mid-Infrared Quantum Spectroscopy Based on Induced Coherence

As a key intermediate in energy metabolism, pyruvate concentration can reflect cellular metabolic status. Conventional methods such as enzymatic colorimetric assays offer high sensitivity, but rely on fresh reagents and can modify or deplete the target analyte. Mid-infrared (MIR) spectroscopy simplifies the measurement. However, conventional MIR detection is constrained by limited MIR detector sensitivity and high background noise. Here we present an induced-coherence MIR quantum spectroscopy system that requires neither MIR sources nor detectors. Using a nonlinear Michelson interferometer with an AgGaSe₂ crystal (type-I nondegenerate SPDC), we achieve the MIR spectral characterization of pyruvate at 8.5 μm (≈1176 cm⁻¹), corresponding to the C–C vibrational band, while only the near-infrared signal photons are detected. A 10-μm-path liquid cell containing 5 μL of pyruvate solution in ultrapure water and simulated body fluid (SBF) is inserted in the MIR interferometer arm. The absorbance spectra retrieved from quantum interferograms via fast Fourier transform agree with those obtained by conventional MIR spectroscopy using the same sample. Using only near-infrared detection, we perform rapid scans near the zero-path-difference position, enabling nondestructive, label-free quantitative analysis of microliter-scale samples and establishing a linear response to pyruvate concentration. These results highlight the potential of quantum spectroscopy for biomedical sensing and provide a foundation for high-sensitivity MIR spectral analysis in complex physiological environments.