Fig. 4: Study of cleave-bind-and-trap detection mechanism.

a Schematic illustration of the cleavage-binding-and-trap detection mechanism involving MMP peptide cleavage, biotin-avidin recognition and SERS nanoparticles trapped in nanogel. b A series of SEM experiments demonstrating nanogel swelling associated with the concentration of MMP-2. c Trapping process of nanotags in nanogel as evidenced by SEM images. d CO intensity enhanced by nanotags, aggregated nanotags and clustered nanotags on nanopillars. A representative image of three individual experiments is shown (scale bar: 100 nm).