Zhenping Guan

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.

N-Annulated Perylene Fused Porphyrins with Enhanced Near-IR Absorption and Emission

N-Annulated perylene fused porphyrins 1 and 2 were synthesized by oxidative dehydrogenation using a Sc(OTf)(3)/DDQ system. These newly synthesized hybrid molecules are highly soluble in organic solvents and exhibit remarkably intense near-IR absorption, as well as detectable photoluminescence quantum yields, all of which are comparable to or even exceed those of either meso-β doubly linked porphyrin dimer/trimer or bis/tri-N-annulated rylenes.

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 two-photon emission in coupled metal nanoparticles induced by conjugated polymers.

Interactions between noble metal (Ag and Au) nanoparticles and conjugated polymers as well as their one- and two-photon emission have been investigated. Ag and Au nanoparticles exhibited extraordinary quenching effects on the fluorescence of cationic poly(fluorinephenylene). The quenching efficiency by 37-nm Ag nanoparticles is ∼19 times more efficient than that by 13-nm Au nanoparticles, and 9-10 orders of magnitude more efficient than typical small molecule dye-quencher pairs. On the other hand, the cationic conjugated polymers induce the aggregate formation and plasmonic coupling of the metal nanoparticles, as evidenced by transmission electron microscopy images and appearance of a new longitudinal plasmon band in the near-infrared region. The two-photon emissions of Ag and Au nanoparticles were found to be significantly enhanced upon addition of conjugated polymers, by a factor of 51-times and 9-times compared to the isolated nanoparticles for Ag and Au, respectively. These studies could be further extended to the applications of two-photon imaging and sensing of the analytes that can induce formation of metal nanoparticle aggregates, which have many advantages over the conventional one-photon counterparts.

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.

Conjugated-Polymer-Based Red-Emitting Nanoparticles for Two- Photon Excitation Cell Imaging with High Contrast

Two-photon fluorescence microscopy is a widely used noninvasive bioimaging technique because of unique advantages such as a large penetration depth and 3D mapping capability. Ideal two-photon fluorophores require large two-photon absorption cross sections and red emission with high quantum yields. Here we report red-emitting-dye-doped conjugated polymer nanoparticles that display high two-photon excitation brightness. In these nanoparticles, conjugated polymer (PFV) was chosen as a two-photon light-harvesting material, and red-emitting dyes (MgPc and Nile red) were chosen as the energy acceptors and red-emitting materials. Two-photon excitation fluorescence of MgPc and Nile red was enhanced by up to ∼53 and ∼240 times, respectively. We have successfully demonstrated the application of these conjugated polymer-based nanoparticles in two-photon excitation cancer cell imaging with an excellent contrast ratio. This concept could become a general approach to the preparation of two-photon excitation red-emitting materials for deep-tissue live-cell imaging with high contrast.

Octupolar polycyclic aromatic hydrocarbons as new two-photon absorption chromophores: synthesis and application for optical power limiting.

Two-photon absorption (TPA) materials have recently attracted intensive interest due to potential applications for 3D microfabrication, multiphoton microscopy, 3D optical data storage, photodynamic therapy, bioimaging, and optical power limiting. It is important to design materials with a large TPA cross section (dmax) or a large TPA cross section per molecular weight (dmax MW ). A variety of donor–bridge–acceptor (D–p–A) dipoles, donor–bridge–donor (D–p–D) quadrupoles, paracyclophanes, and porphyrin derivatives have been developed to achieve large TPA cross sections. However, only limited works have dealt with octupolar TPA chromophores. Octupolar molecules, such as trisACHTUNGTRENNUNG(styryl)benzene derivatives (Scheme 1 a), with significant TPA cross sections was first reported by Cho et al. The three-arm octupolar structure showed significant improvement of the TPA response in comparison with the singly or doubly branched counterparts. Other octupolar chromophores, derived from a triphenylamine core, exhibited similar large TPA cross sections. The important role of a p-conjugated core on the TPA properties was investigated and it was revealed that TPA cross section increased with increased planarity and rigidity of the p-conjugated core, the length of the conjugation system, and the strength of donor–acceptor interactions. On the basis of these important studies, we intend to use a largely delocalized and rigid polycyclic aromatic hydrocarbon (PAH) molecule as a core to build up new octupolar TPA chromophores. Among various PAH molecules, hexaperi-hexabenzocoronene (HBC, Scheme 1 b) is a very attractive candidate because it possesses a large number of delocalized p electrons and rigid planar geometry. The HBC molecule also has a D6h symmetry similar to a benzene ring and can be regarded as a “superbenzene”. Thus selective attachment of certain electron-donating groups (D) and electron-withdrawing groups (A) onto the peripheries of the HBC core would result in three-fold symmetric “push–pull” type octupolar chromophores (Scheme 1 c), which are likely to show an interesting TPA response. However, up to now, there was no report on TPA materials with HBC as building block, mainly due to synthetic challenges. As shown in Scheme 2, three octupolar HBC chromophores, HBC NO2, HBC CN, and HBC CF3, with butoxy units as electron donating groups and the NO2, CN and CF3 as electron-withdrawing groups, have been synthesized. The synthesis of this type of HBC chromophores is challenging because: 1) synthesis of HBC molecules required oxidative cyclodehydrogenation from the hexaphenylbenzene (HPB) precursors by using oxidant and Lewis acid (e.g. FeCl3 or Cu ACHTUNGTRENNUNG(OTf)2–AlCl3, OTf= triflate) and such a cyclization reaction usually does not work if electron donors (e.g. arylamine, monoalkoxy) or electron acceptors (e.g. CN or COOR) are directly attached to the HPB precursors; and 2) separation of C3-symmetric HPB precursors from other isomers is not trivial. Our new strategy is to first synthesize the HBC core 8 followed by further functionalizations. The 3’,4’,5’-tris(butan-1-yloxy)-4-methyl ethy[a] Z. Zeng, Z. Guan, Prof. Q.-H. Xu, Prof. J. Wu Department of Chemistry, National University of Singapore 3 Science Drive 3, Singapore 117543 Fax: (65) 67791691 E-mail : chmwuj@nus.edu.sg chmxqh@nus.edu.sg Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201003235. Scheme 1. a) Representative octupolar TPA molecules based on the benzene core. b) Structure of HBC. c) Octupolar HBC chromophores with a “push–pull” structure.

Huge enhancement of optical nonlinearities in coupled Au and Ag nanoparticles induced by conjugated polymers

Exceptional optical limiting properties were observed in coupled Au and Ag nanoparticles that are induced by conjugated polymers. Fluence-dependent transmission measurements using 7-ns laser pulses of 532-nm wavelength showed that the optical limiting properties of Au and Ag nanoparticles were significantly enhanced upon assembly induced by addition of cationic conjugated polymers. The optical limiting performances of coupled Au and Ag nanoparticles (with optical limiting threshold as low as 2.8 J/cm2 and 2.6 J/cm2, respectively) are even better than that of the benchmark optical limiter-carbon nanotube suspensions (with threshold of 3.6 J/cm2). In addition, these coupled Au and Ag nanoparticle solutions are very stable and suitable for practical applications. Input fluence and angle-dependent scattering experiments suggested that nonlinear scattering should play an important role in the observed optical limiting effects.