Terahertz fingerprint sensing and mixture analysis of amino acid deposits on an unpatterned quartz wafer

This study presents a terahertz (THz) fingerprint sensing and mixture analysis method for amino acid deposits using an unpatterned 500 μm thick JGS1 quartz wafer. Multiple internal reflections within the wafer generate Fabry–Pérot (FP) modes, which provide frequency-selective sampling of molecular resonances over 0.2–2.1 THz without requiring patterned structures. Modeling and simulations based on the dielectric responses of quartz and the amino acids were used to clarify the sensing mechanism. The results show that substrate loss reduces the strength of higher-frequency FP modes and affects the visibility of molecular fingerprints, while the fingerprint positions remain determined by the molecular resonances. This confirms that the quartz wafer contributes actively to the spectral modulation rather than serving solely as a passive support. Wavelet reconstruction combined with model-assisted baseline correction was used to recover the molecular fingerprints from the measured spectra. Tryptophan (Trp) exhibited broad absorption bands near 1.43 and 1.84 THz, whereas tyrosine (Tyr) showed a distinct narrow marker peak near 0.96 THz. As the surface loading increased from 2.50 to 10.00 μg/mm 2 , the fingerprint signals became stronger without noticeable shifts in peak position. For binary Trp/Tyr deposits with a fixed total loading of 10.00 μg/mm 2 , the characteristic bands of both components remained distinguishable over mass ratios ranging from 4:1 to 1:4. The relative compositions retrieved from calibrated Tyr/Trp peak-area ratios showed good agreement with the designed ratios. These results demonstrate that an unpatterned quartz wafer provides a simple platform for label-free molecular identification, surface-loading analysis, and relative quantification of binary deposits.