A vivid Au-porous anodic alumina composite film with the inverted taper structure for label-free detection

Localized surface plasmon resonance (LSPR) has been widely used in medical detection because of its time effectiveness, non-invasiveness, high sensitivity, and relatively simple fabrication process. Porous anodic alumina (PAA) can be regarded as a plasma substrate for label-free detection due to its unique two-dimensional structure. In this work, a vivid Au-PAA composite film with the inverted taper structure was developed by multi-step anodic oxidation and pore-widening processes followed by magnetron sputtering with Au nanoparticles (AuNPs). The highly saturated and bright structural color was generated by the synergistic effect of photonic and plasmonic modes. Interestingly, various Au-PAA composite films with structural colors altering from purple to red were obtained via adjusting the height/diameter ratio of PAA. Benefiting from the inverted taper structure, light trap characteristics were effectively enhanced by increasing the incident light and reducing the diffuse light. In addition, a finite difference time domain (FDTD) model was proposed to predict the relationship between the reflectance peak and the height of the composite film, and the simulated data were in good agreement with the experimental results. As a proof of concept, label-free detections of various reagents (water, ethanol, glycol, glycerol, and glucose), the concentration of glucose (refractive index sensitivity of 376 nm/RIU, RIU: refractive index unit), and thrombin (detection limit of 0.1 × 10⁻⁷ mol/L) were realized by the Au-PAA composite film. This vivid Au-PAA composite film provides a very powerful tool for in-situ label-free bio-detection.