Fig. 4

Plasmonic core and shell selection. Electric field enhancement maps of different plasmonic cores: a 60-nm AuNP, b 40-nm length–10-nm diameter AuNR, and c AuNR with 2 nm gap and 2 nm Ag shell. The average electric field enhancement factor is increased by 2 orders of magnitude predicting a higher SERS enhancement with a AuNR core and a Ag shell. Scale bars are 10 nm. d Synthesis of the optimized FRNPs. Different thicknesses of Ag shell are formed by reducing different concentrations of AgNO₃ with ascorbic acid in the presence of DNA cloaked AuNR cores. e–g TEM images (lower panels are magnified views) of fluorophore cloaked AuNR cores, AuNR core with a 2 nm Ag shell and fully optimized FRNPs with PEG-2000 coated silica shell. All the scale bars are 100 nm. h UV–vis spectra of the AuNRs with increasing thickness of silver shell showing a hypsochromic shift of localized surface plasmon resonance (LSPR) peaks. i Fluorescence background subtracted Raman spectra of 10 fM of the spherical AuNP-based FRNPs and 1 fM of AuNR-based FRNPs with increasing thickness of Ag. The FRNP with longitudinal LSPR maximum of 585 nm exhibits the maximum Raman enhancement