Catching Single Molecules with Plasmonic InGaN Quantum Dots

Single molecules are elusive and often produce misleading signals. Surface-enhanced Raman spectroscopy (SERS) is one of the few techniques capable of verifying the presence of single molecules. To achieve the goal, the bianalyte proof is favored by researchers as it relies on the statistical analysis of thousands of spectra, rather than the fluctuating signals observed at limited spots. Since the hotspot of SERS is extremely small (<10 nm), less than 1% of the adsorbed molecules can deliver boosted Raman intensities, making the capture of single molecules a rare event. Here, the proof with single-molecule signals covering 89.6% of the scanned spots is presented. This is achieved by plasmonically coupling subsurface InGaN quantum dots (QDs) to every Au nanoparticle on the SERS substrate. The QD-Au complexes extend the plasmonic fields far away from the metallic nanojunction, redefining the bianalyte rule for single-molecule detection.