Nengyue Gao

National University of Singapore, Singapore, Singapore

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Publications (15)81.07 Total impact

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    ABSTRACT: Plasmon coupling between noble metal nanoparticles has been demonstrated to dramatically enhance linear and nonlinear optical properties of nearby chromophores and metal nanoparticles themselves. The interparticle distance is expected to have significant influence on the coupling strength. Here we have prepared DNA tuned Au nanoparticle assemblies with well controlled separation distance from 2.0 to 12.2 nm to investigate plasmon coupling strength and particle size effects on TPPL enhancement. TPPL intensities of these DNA coupled nano-assemblies were found to increase rapidly as the separation distance decreases. The largest TPPL enhancement factors of 115 and 265 were achieved at the shortest available separation distance of 2.0 nm for 21 and 41 nm Au NPs-dsDNA assemblies respectively. We have further utilized DNA induced coupling of Au NPs and TPPL enhancement to develop a two-photon sensing scheme for detection of DNA sequences. This TPPL based method displayed high sensitivity with LOD of 2.9 pM and excellent selectivity against ssDNA with mismatched base. Single mismatch can be easily differentiated at room temperature. Taking the unique advantages of two-photon excitation, this method could be potentially further extended to DNA detection inside cells or even in vivo. These findings can provide important insight for fundamental understanding of plasmon-coupling enhanced TPPL and development of various two-photon excitation based applications.
    ACS Applied Materials & Interfaces 07/2014; · 5.90 Impact Factor
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    ABSTRACT: 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.
    The Journal of Physical Chemistry C. 06/2014; 118(25):13904–13911.
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    ABSTRACT: 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.
    ACS Applied Materials & Interfaces 02/2014; · 5.90 Impact Factor
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    ABSTRACT: Thrombin plays an important role in pathological conditions. It is important however challenging to detect thrombin in complex biological media for clinical practice and diagnostic applications. Here we demonstrate a label free, fast, highly sensitive and selective two-photon sensing scheme for detection of thrombin on the picomolar level. The assay is based on interactions between thrombin and a DNA aptamer, which induce aggregation of silver nanoparticles to display significantly enhanced two-photon photoluminescence. The limit of detection (LOD) of this two-photon sensing assay is as low as 3.1 pM in the buffer solution, more than 400 times lower than that of the extinction method (1.3nM). The dynamic range of this method covers more than 4 orders of magnitude. Furthermore, this two-photon sensing assay can be applied to detection of thrombin in 100% fetal bovine serum with LOD of 1.8 nM. In addition to the unique advantages of two-photon sensing such as deep penetration and localized detection, this method could be potentially integrated with two-photon microscopy to offer additional advantages of 3D detection and mapping for potential in-vivo applications.
    ACS Applied Materials & Interfaces 09/2013; · 5.90 Impact Factor
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    ABSTRACT: PEG-SH capped gold nanorods (Au NRs) well dispersed in various organic solvents have been prepared to study the solvent effects on the two-photon photoluminescence (TPPL) properties and ultrafast excitation dynamics of Au NRs. The TPPL intensities of Au NRs in different organic solvents including DMF, DEG, CH3OH, C2H5OH, CH3CN, DMSO and THF were found to be significantly quenched than that of Au NRs in H2O. Ultrafast time-resolved transient absorption and pump-probe measurements have been performed on Au NRs in H2O, DMF and DEG to understand the corresponding quenching mechanisms. Different from the excitation decay behaviors of Au NRs in H2O, an additional transient species was observed in Au NRs in DMF or DEG upon photo-excitation. This intermediate state is ascribed to the charge separated species, which serves as a direct evidence to support that the observed TPPL quenching in various organic solvents is due to electron transfer from electron-donating solvents to the excited Au NRs. The charge transfer mechanism was further supported by the observation that the TPPL intensity of Au NRs in H2O was found to be quenched by nearly 5.0-fold in the presence of S2-. These studies provide useful information on fundamental understanding the TPPL properties as well as development of various potential biological applications of gold nanoparticles.
    The Journal of Physical Chemistry B 08/2013; · 3.61 Impact Factor
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    ABSTRACT: A new type of core-shell upconversion nanoparticles which could be effectively excited at 795 nm has been designed and synthesized through spatially confined doping of neodymium (Nd3+) ions. The use of Nd3+ ions as sensitizers facilitates the energy transfer and photon upconversion of a series of lanthanide activators (Er3+, Tm3+, and Ho3+) under a biocompatible excitation wavelength (795 nm) and also significantly minimizes the overheating problem associated with conventional 980-nm excitation. Importantly, the core-shell design could enable high concentration doping (~ 20 mol%) of Nd3+ at the shell layer and thus markedly enhance upconversion emission from the activators, providing highly attractive luminescent biomarkers for bioimaging without autofluorescence and concern of overheating.
    Journal of the American Chemical Society 08/2013; · 10.68 Impact Factor
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    ABSTRACT: Metal nanoparticles have potential applications as bio-imaging 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 un-aggregated 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 un-aggregated 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.
    ACS Applied Materials & Interfaces 05/2013; · 5.90 Impact Factor
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    ABSTRACT: Aggregated metal nanoparticles have been known to display significantly enhanced two-photon photoluminescence (TPPL) compared to non-aggregated 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 ~7.8×103 and 7.0×104 times that of Au nanosphere monomers, respectively. A highest enhancement of 1.2×105 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 cos2 dependence on the detection polarization while their TPPL displayed cos4 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.
    Journal of the American Chemical Society 04/2013; · 10.68 Impact Factor
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    ABSTRACT: 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.
    Nanoscale 04/2013; · 6.73 Impact Factor
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    ABSTRACT: Conventional photosensitizers generally suffer from low efficiency in novel non-invasive two-photon photodynamic cancer therapy due to their small two-photon absorption cross section and they lack an imaging capability for therapy guiding due to their low fluorescence yield. Demonstrated here is the first water-soluble conjugated polymers as direct two-photon photosensitizers with dual capability of two-photon cell imaging and two-photon photodynamic therapy. By introducing a strong electron-withdrawing cyano group into the phenyl ring of the backbone, the cyano-substituted poly(fluorene-2,7-ylenevinylene-co-phenylene) (PFVCN) displays a 2.4 times higher maximum two-photon absorption cross section per repeat unit and significantly higher fluorescence quantum yield in water than the unsubstituted PFV. The large two-photon absorption cross section of PFVCN allows it to efficiently generate singlet oxygen under two-photon excitation, which is critical for two-photon photodynamic therapy. Two-photon excitation cell imaging and efficient two-photon-induced photodynamic therapy effect on cancer cells of PFVCN are successfully demonstrated. These studies provide insight in designing novel photosensitizing agents for simultaneous two-photon imaging and two-photon photodynamic therapy, which allows two-photon imaging guided therapy to fully take the unique advantages of two-photon excitation such as deep penetration and 3D selectivity.
    ADVANCED OPTICAL MATERIALS. 01/2013; 1(1):92-99.
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    ABSTRACT: 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.
    Nanoscale 11/2012; · 6.73 Impact Factor
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    ABSTRACT: 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.
    ACS Applied Materials & Interfaces 10/2012; 4(10):5711-6. · 5.90 Impact Factor
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    ABSTRACT: 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.
    Langmuir 07/2012; 28(33):12304-10. · 4.38 Impact Factor
  • Langmuir 01/2012; · 4.38 Impact Factor
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    ABSTRACT: 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.
    Langmuir 11/2011; 28(1):321-6. · 4.38 Impact Factor