Dual-mode mid-infrared plasmonic metasurface for real-time label-free analysis of live cells

Real-time, label-free monitoring of living cells is a central goal in biosensing, and mid-infrared spectroscopy is uniquely suited to this task because it directly probes the vibrational fingerprints of biomolecules within cells. However, its use in live-cell analysis is limited by weak absorption signals and strong water background. Here, we introduce a double-resonant plasmonic metasurface composed of gold rod-shaped nanoantennas specifically engineered to overcome these limitations. This metasurface provides two complementary spectral readouts: (i) a strong plasmonic resonance matched to the amide I-II vibrational bands of proteins, and (ii) a sharp reflectance dip around 1900 cm^-1 optimized for refractive-index mass sensing via plasmonic redshift. Initial validation with red blood cell sedimentation showed sensitivity to time-dependent refractive index variations corresponding to ∼3.5 ng of deposited material. We then applied this technology to a clinically relevant model, monitoring SAS oral squamous carcinoma cells treated with 18β-glycyrrhetinic acid, a natural compound with pro-apoptotic effects. Treatment resulted in a clear decrease in both the plasmonic redshift signal and the integrated amide absorption, consistent with biophysical and biochemical alterations associated with apoptosis. Spectral deconvolution of the amide I region revealed specific shifts in protein secondary structures, including a decrease in α-helical content, further supporting apoptosis-related molecular changes. These findings show that our plasmonic metasurface can sensitively monitor real-time cellular responses, supporting applications in drug screening, spectral biomarker discovery, and opening new opportunities in cell-based diagnostics and biosensing.