Quantitatively Revealing the Anomalous Enhancement in Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy Using Single-Nanoparticle Spectroscopy.

Surface-enhanced Raman spectroscopy (SERS) is an ultrasensitive spectroscopic technique that has been extensively applied in the studies of catalysis, electrochemistry, material science, etc.; however, it is substrate and material limited. The development of shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) effectively offsets this limitation that attracts enormous attention due to its potential to be applied to any surface. As the core of the SHINERS technique, the inert shell prevents the exposure of the active metal surface, however, also significantly enlarges the metallic gap where the light is trapped. Consequently, the shell is widely considered a side issue to debilitate the coupling efficiency and hinder the sensitivity of SHINERS without systematic studies. Herein, we investigate the shell and structural effect of SHINERS by performing the quantitative optical and structural characterization of single nanostructures. By a statistic of over two hundred nanostructures, we observe that the field enhancement loss due to the shell could be overcome by optimizing the coupling geometry of the shell-isolated nanoparticles (SHINs). An example of SHIN dimers shows even higher field enhancement than their bare Au nanoparticle counterparts as confirmed and explained by FDTD simulations. We demonstrate the signal enhancement of SHINERS saturates with the increasing number of hot spots but could be further optimized by altering the aggregation geometries of the nanoparticles. The sensitivity improvement of the SHINERS technique will boost its broader applications in material science.