Reduced cytotoxicity of CTAB-templated silica layer on gold nanorod using fluorescence dyes and its application in cancer theranostics

Cytotoxicity of cetyltrimethylammonium bromide (CTAB)-templated silica-coated gold nanorod (CGNR@SiO2) was confirmed to be severe even after the washing process because of the residual CTAB surfactant. Calcination to eliminate the CTAB template induced dissolution of the gold nanorod (GNR) core and caused a blue-shift in the longitudinal surface plasmon resonance (LSPR) mode of the GNR from the near-infrared to visible wavelength. In contrast, the CTAB template was effectively removed from CGNR@SiO2 without interference of the plasmonic property of the core GNR by amine-functionalized fluorescence dyes such as methylene blue (MB) and rhodamine B (RB). The RB was more efficient in substituting the residual CTAB through electrostatic interactions than the MB. Moreover, the resulting products exhibited excellent surface-enhanced Raman scattering performance to detect a few cancer cells. The MB and RB molecules as visible light photosensitizer (PS) were activated by near-infrared (NIR) excitation light through transfer of the NIR-driven plasmonic hot electrons from the core GNR to the PS molecules. It is noteworthy that the RB-loaded CGNR@SiO2 exhibited excellent cancer cell killing efficiency via generation of abundant reactive oxygen species (ROS) under NIR laser irradiation, although the concentration of the intracellular GNR was too low to induce the photothermal effect.