Shape-Dependent Two-Photon Photoluminescence of Single Gold Nanoparticles

Gold (Au) nanoparticles that display strong two-photon photoluminescence (TPPL) are attractive contrast agents for noninvasive live cell/tissue imaging with deep penetration because of their excellent biocompatibility and low cytotoxicity. The TPPL properties of Au nanoparticles are strongly dependent on the particle shape. As chemically prepared nanoparticles are generally inhomogeneous, conventional ensemble-based TPPL measurements can only give averaged results of particles of different morphologies. Single-particle spectroscopy can avoid the complication induced by the sample inhomogeneity in ensemble measurements and help to establish the morphology–property relationship. Here we have investigated the scattering spectra and TPPL properties of Au nanoparticles of different shapes on the single particle level and explored their potential applications in cancer cell imaging. Au nanoparticles of five different shapes (nanospheres, nanocubes, nanotriangles, nanorods, and nanobranches) with similar dimensions have been chosen for the study. The TPPL spectra of these Au nanoparticles were found to be strongly modulated by plasmon resonance. TPPL intensity increases in the order of nanospheres, nanocubes, nanotriangles, nanorods, and nanobranches. The averaged TPPL intensity of a single Au nanobranch is 47750 times that of a single Au nanosphere. Two-photon action cross sections of single Au NSs, Au NCs, Au NTs, Au NRs, and Au NBs were estimated to be ∼83, ∼500, ∼1.5 × 10³, ∼4.2 × 10⁴, and ∼4.0 × 10⁶ GM, respectively. Laser-induced melting experiments on single Au nanobranches demonstrate that the tips played an important role in the observed strong TPPL. Application of these Au nanobranches as excellent two-photon imaging contrast agents has been demonstrated on HepG2 cancer cells.