Aluminum-Coated Nanoridge Arrays with Dual Evanescent Wavelengths for Real-Time and Label-Free Cellular Analysis

Label-free sensing techniques, designed for the observation of dynamic cell activities, have the potential to advance studies of cell biology, immunotherapy, and drug discovery. In this context, we introduce the concept of Fano resonances featuring dual evanescent wavelengths in aluminum-coated nanoridge arrays for real-time and label-free analysis of cellular adhesion. The distinctive optical characteristics of dual evanescent lengths were confirmed through finite-difference time-domain (FDTD) calculations and experimental assessments, including tests of the refractive index and surface (thickness) sensitivity. The findings reveal the evanescent lengths measured at 874 and 316 nm for the peak and dip of the Fano resonance at the air/aluminum interface, aligning with FDTD simulations (891 and 338 nm). Wavelength shifts and intensity variations of the Fano resonance correspond to changes in the effective refractive index and absorption/scattering of surface plasmon waves, respectively. Employing four-parameter plots─time constants of Δλpeak, Δλdip, ΔImax, and ΔImin─we exemplify the investigation of long-range and short-range adhesion behaviors of lung cancer cells. In contrast to conventional gold-based SPR sensors with evanescent lengths ranging from 100 to 300 nm, the aluminum-coated nanoridge array exhibits an extended sensing depth and dual evanescent lengths. This dual capability enables the simultaneous exploration of cell behaviors in the proximity of and at a distance from the metal surface.