Porphyrin Dyes for Nonlinear Optical Imaging of Live Cells

Second harmonic generation-based (SHG) biological probes are useful for nonlinear optical imaging of biological structures such as the plasma membrane. Several amphiphilic porphyrin-based dyes with high SHG coefficients have been synthesized with different hydrophilic head-groups and their cellular targeting has been studied. The probes with cationic head-groups localize better at the plasma membrane than the neutral probes with zwitterionic or non-charged ethylene glycol-based head-groups. Porphyrin dyes with only dications as hydrophilic head-groups localize inside HEK 293T cells to give SHG, while tricationic dyes localize robustly at the plasma membrane of cells including neurons in vitro and ex vivo. The copper(II) complex of the tricationic dye with negligible fluorescence quantum yield works as a SHG-only dye. The free-base tricationic dye has been demonstrated for two-photon fluorescence and SHG-based multimodal imaging. This study demonstrates the importance of a balance between the hydrophobicity and hydrophilicity of amphiphilic dyes for effective plasma membrane localization