Nengyue Gao

Mechanistic Investigation of Photon Upconversion in Nd3+-Sensitized Core–Shell Nanoparticles

A new type of core-shell upconversion nanoparticles which can be effectively excited at 795 nm has been designed and synthesized through spatially confined doping of neodymium (Nd(3+)) ions. The use of Nd(3+) ions as sensitizers facilitates the energy transfer and photon upconversion of a series of lanthanide activators (Er(3+), Tm(3+), and Ho(3+)) at a biocompatible excitation wavelength (795 nm) and also significantly minimizes the overheating problem associated with conventional 980 nm excitation. Importantly, the core-shell design enabled high-concentration doping of Nd(3+) (~0 mol %) in the shell layer and thus markedly enhanced the upconversion emission from the activators, providing highly attractive luminescent biomarkers for bioimaging without autofluorescence and concern of overheating.

TiO2 coated Au/Ag nanorods with enhanced photocatalytic activity under visible light irradiation

A facile method was used to prepare uniform Au NR/TiO2 and Au/Ag NR/TiO2 core-shell composite nanoparticles. Au/Ag NR/TiO2 nanoparticles were found to display significantly enhanced visible light photo-catalytic activity compared to Au NR/TiO2 and the commercially available TiO2 nanoparticles. The enhancement mechanism was ascribed to injection of hot electrons of photo-excited Au/Ag NRs to TiO2, which was confirmed by 633 nm laser induced reduction of silver ions on the surface of Au/Ag NR/TiO2 composite nanoparticles.

Plasmon-Enhanced Photocatalytic Properties of Cu2O Nanowire–Au Nanoparticle Assemblies

Cu(2)O-Au nanocomposites (NCs) with tunable coverage of Au were prepared by a facile method of mixing gold nanoparticles (Au NPs) with copper(I) oxide nanowires (Cu(2)O NWs) in various ratios. These Cu(2)O-Au NCs display tunable optical properties, and their photocatalytic properties were dependent on the coverage density of Au NPs. The photocatalytic activity of Cu(2)O-Au NCs was examined by photodegradation of methylene blue. The presence of Au NPs enhanced the photodegradation efficiency of Cu(2)O NCs. The photocatalytic efficiency of Cu(2)O-Au NCs initially increased with the increasing coverage density of Au NPs and then decreased as the surface of Cu(2)O became densely covered by Au NPs. The enhanced photocatalytic efficiency was ascribed to enhanced light absorption (by the surface plasmon resonance) and the electron sink effect of the Au NPs.

Gold nanorods as dual photo-sensitizing and imaging agents for two-photon photodynamic therapy

Gold nanorods with three different aspect ratios were prepared and their dual capabilities for two-photon imaging and two-photon photodynamic therapy have been demonstrated. These gold nanorods exhibit large two-photon absorption action cross-sections, about two orders of magnitude larger than small organic molecules, which makes them suitable for two-photon imaging. They can also effectively generate singlet oxygen under two-photon excitation, significantly higher than traditional photosensitizers such as Rose Bengal and Indocyanine Green. Such high singlet oxygen generation capability under two-photon excitation was ascribed to their large two-photon absorption cross-sections. Polyvinylpyrrolidone (PVP) coated gold nanorods displayed excellent biocompatibility and high cellular uptake efficiency. The two-photon photodynamic therapy effect and two-photon fluorescence imaging properties of PVP coated gold nanorods have been successfully demonstrated on HeLa cells in vitro using fluorescence microscopy and indirect XTT assay method. These gold nanorods thus hold great promise for imaging guided two-photon photodynamic therapy for the treatment of various malignant tumors.

Gold Nanorod Enhanced Two-Photon Excitation Fluorescence of Photosensitizers for Two-Photon Imaging and Photodynamic Therapy

Plasmon enhancement of optical properties is both fundamentally important and appealing for many biological and photonic applications. Although metal-enhanced two-photon excitation fluorescence has been demonstrated in the solid substrates, there is no report on metal enhanced overall two-photon excitation fluorescence in the colloid system. Here we systematically investigated gold nanorod enhanced one- and two-photon excitation fluorescence of a porphyrin molecule, T790. The separation distance between the metal core and T790 was varied by adjusting the silica shell thickness from 13 to 42 nm. One- and two-photon excitation fluorescence intensities of T790 were found to strongly depend on the thickness of silica shell that separates gold nanorod and T790. The optimum one- and two-photon excitation fluorescence enhancement was found to occur at shell thicknesses of 34 and 20 nm, with enhancement factors of 2.1 and 11.8, respectively. Fluorescence lifetime of T790 steadily decreased as the shell thickness decreased. The observed two-photon excitation fluorescence enhancement is ascribed to a combination effect of local electric field amplification and competition between increased radiative and non-radiative decay rates. Core-shell nanoparticles that displayed enhanced two-photon excitation fluorescence were also found to exhibit significantly improved singlet oxygen generation capability under two-photon excitation. The applications of these nanoparticles as effective agents for two-photon cell imaging and nano-photosensitizers for two-photon photodynamic therapy with improved efficiency have also been demonstrated in HepG2 cancer cells. The combined advantages of enhanced two-photon excitation fluorescence and two-photon induced singlet oxygen generation make these core-shell nanoparticles as attractive agents for two-photon imaging guided two-photon photodynamic therapy.

Huge Enhancement in Two-Photon Photoluminescence of Au Nanoparticle Clusters Revealed by Single-Particle Spectroscopy

Aggregated metal nanoparticles have been known to display significantly enhanced two-photon photoluminescence (TPPL) compared to nonaggregated nanoparticles, which could be utilized to develop platforms for two-photon sensing and imaging applications. Here we have conducted single-particle spectroscopic studies on gold (Au) nanoparticle clusters of different sizes to understand the enhancement mechanisms and explore the limit of maximum achievable enhancement. Our studies show that the TPPL intensity of Au nanoparticle clusters significantly increases from monomer to trimer. The averaged intensity of the Au nanosphere dimers and linear trimers is ~7.8 × 10(3) and ~7.0 × 10(4) times that of Au nanosphere monomers, respectively. A highest enhancement of 1.2 × 10(5) folds was obtained for the linear trimer. The TPPL spectra of these single Au nanosphere clusters closely resemble their corresponding scattering spectra, suggesting strong correlation between their TPPL with plasmon resonance. The scattering spectra of dimers and linear trimers displayed cos(2) dependence on the detection polarization, while their TPPL displayed cos(4) dependence on the excitation polarization, which are very similar to Au nanorods. These results suggest that two-photon excitation of dimer and linear trimer is strongly coupled to their longitudinal plasmon resonance modes. These studies help to provide insight on fundamental understanding of the enhancement mechanisms as well as development of biomedical and photonic applications.

Enhanced Optical Properties of Graphene Oxide–Au Nanocrystal Composites

A simple strategy based on electrostatic interactions was utilized to assemble Au nanocrystals of various morphologies onto graphene oxide (GO). This method allows deposition of metal nanocrystals of different shapes onto GO. The linear and nonlinear optical properties of GO-Au nanocrystal composites have been examined. The extinction spectra of Au nanocrystals became broadened and red-shifted from the visible to the near IR upon formation of GO-Au nanocrystal composites. A more than 4-fold increase in two-photon excitation emission intensity was observed from the GO-Au nanocrystal composites compared to pure Au nanocrystals. The SERS signals of the composites were found to be strongly dependent on the morphology of Au nanocrystals, with SERS enhancement factors ranging from 9 to 20.

Band-Selective Coupling-Induced Enhancement of Two-Photon Photoluminescence in Gold Nanocubes and Its Application as Turn-on Fluorescent Probes for Cysteine and Glutathione

We have demonstrated that cysteine and glutathione induced edge-to-edge coupling of gold nanocubes (Au NCs) caused a band-selective enhancement of two-photon photoluminescence (TPPL). The photoluminescence intensity of the X-band of Au NCs was found to be enhanced up to 60-fold and 46-fold upon addition of cysteine and glutathione, respectively, while the intensity of L-band remained almost unchanged. This band-selective enhancement behavior is totally different from the previously observed aggregation enhanced TPPL of spherical metal nanoparticles (NPs). The band-selective enhancement was ascribed to preferential enhancement of the X-band emission through resonant coupling with longitudinal surface plasmon resonance (SPR) band of the Au NCs assembly. This phenomenon was utilized to develop a new two-photon fluorescence turn-on sensing platform for detection of cysteine and glutathione. This method displayed high sensitivity and excellent selectivity over the other 19 amino acids. Together with the advantage of deep tissue penetration and localized excitation of two-photon near-infrared excitation, this strategy has promising applications in in vivo biosensing and imaging.

Two-Photon Induced Photoluminescence and Singlet Oxygen Generation from Aggregated Gold Nanoparticles

Metal nanoparticles have potential applications as bioimaging and photosensitizing agents. Aggregation effects are generally believed to be adverse to their biomedical applications. Here we have studied the aggregation effects on two-photon induced photoluminescence and singlet oxygen generation of Au nanospheres and Au nanorods of two different aspect ratios. Aggregated Au nanospheres and short Au nanorods were found to display enhanced two-photon induced photoluminescence and singlet oxygen generation capabilities compared to the unaggregated ones. The two-photon photoluminescence of Au nanospheres and short Au nanorods were enhanced by up to 15.0- and 2.0-fold upon aggregation, and the corresponding two-photon induced singlet oxygen generation capabilities were enhanced by 8.3 and 1.8-fold, respectively. The two-photon induced photoluminescence and singlet oxygen generation of the aggregated long Au nanorods were found to be lower than the unaggregated ones. These results support that the change in their two-photon induced photoluminescence and singlet oxygen generation originate from aggregation modulated two-photon excitation efficiency. This finding is expected to foster more biomedical applications of metal nanoparticles as Au nanoparticles normally exist in an aggregated form in the biological environments. Considering their excellent biocompatibility, high inertness, ready conjugation, and easy preparation, Au nanoparticles are expected to find more applications in two-photon imaging and two-photon photodynamic therapy.

Red-Emitting DPSB-Based Conjugated Polymer Nanoparticles with High Two-Photon Brightness for Cell Membrane Imaging

New red-emitting conjugated polymers have been successfully synthesized by incorporating classical two-photon absorption (TPA) units, electron-rich units, and a small amount of electron-deficient units along the polymer backbones. Water-dispersible nanoparticles (NPs) based on these polymers were also fabricated for applications in two-photon excitation fluorescence imaging of cell membrane. Through optimization of the polymer/matrix mass ratio and the initial feed concentration of the polymer solution, a high quantum yield (QY) of 24% was achieved for the red-emitting NPs in water. TPA cross section and two-photon action cross section values of these polymers at 750 nm reached up to 1000 GM and 190 GM per repeat unit in aqueous media, 2.5 × 10(5) GM and 4.7 × 10(4) GM per NP, respectively. Furthermore, these NPs displayed excellent photostability and biocompatibility. Their applications as two-photon excitation fluorescence probes for cell membrane imaging have been demonstrated in three different cell lines with excellent imaging contrast. These results demonstrated that these polymer NPs hold great potentials as excellent two-photon excitation fluorescence probes in various biological applications.