Cationic colloidal gold assisting delivery of macromolecular fluoresceins into target CHO-K1 cells by focused femtosecond laser

We describe a new method for delivering macromolecules into the target cells based on light-absorbing cationic colloidal gold nanoparticles that are irradiated by focused femtosecond laser pulses. Cationic colloidal 15nm gold particles which were made by conjugation with poly-L-Lysine, were attached on the anionic sites, especially on the membrane, of CHO-K1 cells because of their strong positive charge at physiological pH. Target cells labeled with cationic gold nanoparticles were imaged under two-photon fluorescence microscopy, and lifetime images of the same targets were taken by TCSPC technique in order to verify the fluorescence of the marker and the luminescence of the gold particles. A macromolecular 10k Dalton fluorescein isothiocyanate dextran (FITC-D), was added into the sample and the focused femtosecond laser of two-photon fluorescence microscopy was employed to scan the target cells layer by layer. Typical laser power level used in biological imaging is about 3-5 mW. Here the laser power of scanning was below 5 mW in order to prevent photochemical damage of the fs-pulses alone and to localize effects to the nanoparticles on a nano-scale. After scanning the target cells under stack mode, macromolecular fluoresceins surrounding the cells was observed to cross the membrane and to diffuse in the cytoplasma. Comparing with the images before scanning, the two-photon fluorescence and fluorescence lifetime images revealed the delivery of FITC-D into target cells.