On-chip colorimetric detection of Cu²⁺ ions via density-controlled plasmonic core-satellites nanoassembly

We report on an on-chip colorimetric method for the detection and analysis of Cu²⁺ ions via the targeted assembly of plasmonic silver nanoparticles (2.6 nm satellites) on density-controlled plasmonic gold nanoparticles (50 nm cores) on a glass substrate. Without any ligand modification of the nanoparticles, by directly using an intrinsic moiety (carboxylate ion, COO^-) surrounded with nanoparticles, the method showed a high selectivity for Cu²⁺, resulting in a nearly 2 times greater optical response compared to those of other metal ions via the targeted core-satellites assembly. By modulating the surface chemistry, it was possible to control the density of core gold nanoparticles on the surface, thus permitting easy tuning of the optical responses induced by plasmon coupling generated between each core-satellites nanostructure. Using chips with a controlled optimal core density, we observed the remarkable scattering color changes of the chips from green to yellow and finally to orange with the increase of Cu²⁺ concentration. The detection limits of the fabricated chips with controlled core densities (ca. 1821 and 3636 particles/100 μm²) are 10 nM and 10 pM, respectively, which are quite tunable and below the level of 20 μM (or 1.3 ppm) defined by the United States Environmental Protection Agency. The findings suggest that the method is a potentially promising protocol for detecting small molecules with target selectivity and the tunability of the detection limits by replacing with ligands and adjusting core densities.