Mesoporous Silica Nanospheres Decorated by Ag–Nanoparticle Arrays with 5 nm Interparticle Gap Exhibit Insignificant Hot-Spot Raman Enhancing Effect

Arrays of 20 nm silver nanoparticles (AgNPs) with tunable interparticle gaps have been grown on mesoporous silica (MPS) nanospheres to form AgNPs@MPS nanohybrids for surface-enhanced Raman scattering (SERS). Adenine was employed as the probing molecule to study the hot-spot SERS effect as a function of interparticle gap. The SERS signal intensity increased by 8–10-fold when the gaps were reduced to 5 nm, appearing to suggest that the hot-spot Raman enhancing effect was derived from adjacent AgNP pairs. However, after rescaling the signal intensity by the number of AgNPs on each MPS corresponding to different interparticle gaps, the renormalized signal exhibits an insignificant dependence on the gap size, invalidating the existence of the hot-spot effect on AgNPs@MPS even for a gap as small as 5 nm. The observation is in sharp contrast to the large hot-spot effect clearly demonstrated by narrowing the gaps in arrays of AgNPs grown on a flat anodic alumina with vertical nanochannels. The unexpected lack of a hot-spot effect from AgNPs@MPS could be understood in terms of the misalignment between the directions of the field for the impinging laser with those of the axes between most adjacent AgNP pairs due to the spherical shape of MPS.