Li-Min Fu

Luminescence properties of transparent hybrid thin film covalently linked with lanthanide complexes

Novel hybrid thin films covalently doped with Eu3+ (Tb3+) have been prepared via direct routes involving co-condensation of tetraethoxysilane and phen-Si in the presence of Eu3+ (Tb3+) by spin-casting and their luminescence properties have been investigated in detail. Lanthanide ions can be sensitized by anchored phenanthroline in hybrid thin films. Excitation at the ligand absorption wavelength (272 nm) resulted in the strong emission of the lanthanide ions i.e. Eu3+ D-5(0)-F-7(J) (J=0, 1, 2, 3, 4) emission lines and Tb3+ D-5(4)-F-7(J) (J = 6, 5, 4, 3) due to the energy transfer from the ligands to the lanthanide ions.

PEDOT nanocomposites mediated dual-modal photodynamic and photothermal targeted sterilization in both NIR I and II window

PEDOT nanoparticles with a suitable nanosize of 17.2 nm, broad adsorption from 700 to 1250 nm, and photothermal conversion efficiency (η) of 71.1%, were synthesized using an environmentally friendly hydrothermal method. Due to the electrostatic attraction between indocyanine green (ICG) and PEDOT, the stability of ICG in aqueous solution was effectively improved. The PEDOT nanoparticles modified with glutaraldehyde (GTA) targeted bacteria directly, and MTT experiments demonstrated the low toxicity of PEDOT:ICG@PEG-GTA in different bacteria and cells. Pathogenic bacteria were effectively killed by photodynamic therapy (PDT) and photothermal therapy (PTT) with PEDOT:ICG@PEG-GTA in the presence of near-infrared (NIR) irradiation (808 nm for PDT, and 1064 nm for PTT). The combination of the two different bacteriostatic methods was significantly more effective than PTT or PDT alone. The obtained PEDOT:ICG@PEG-GTA may be used as a novel synergistic agent in combination photodynamic and photothermal therapy to inactivate pathogenic bacteria in both the NIR I and II window.

Colloidal Nanoparticles of a Europium Complex with Enhanced Luminescent Properties

We report an alternative approach, that is, forming Eu(tta)3dpbt (dpbt = 2-( N, N-diethylanilin-4-yl)-4,6-bis(3,5-dimethylpyrazol-1-yl)-1,3,5-triazine, tta = thenoyltrifluoroacetonato) nanoparticles in water/methanol mixtures, to satisfy the combined requirements of good dispersibility in water solutions and efficient long-wavelength sensitization for Eu (III) complexes to be used in biological applications. The size of Eu(tta)3dpbt colloidal particles with very high luminescent capabilities can be modulated to some extent by changing the preparation conditions. The optical excitation window for the Eu (III) luminescence of Eu(tta)3dpbt nanoparticles, extending up to 475 nm, is wider than that of Eu(tta)3dpbt molecules dissolved in toluene. This is the first example for obviously extending the sensitization window of luminescent lanthanide materials to the long-wavelength region by forming nanoparticles of a lanthanide complex. Quantum yields of Eu (III) luminescence of the prepared Eu(tta)3dpbt colloidal particles, with an average diameter of 33.1 nm, are 0.27, 0.27, 0.24, 0.19, 0.14, and 0.01 upon excitation at 402, 420, 430, 440, 450, and 475 nm, respectively. The Eu(tta)3dpbt nanoparticles exhibited excellent two-photon sensitization performance with a highest delta Phi value of 3.2 x 10(5) GM (1 GM = 10(-50) cm4 s photo(-1) particle(-1)) at the excitation wavelength of 832 nm, which is about 7 times higher than the highest value reported for the CdSe/ZnS core-shell quantum dots. The favorable luminescent properties and the good dispersibility in water solutions of the Eu(tta)3dpbt nanoparticles are very promising for the development of new luminescent nanoprobes for bioanalysis.

Effective energy transfer and luminescence of LB films based on europium-substituted heteropolytungstate

The europium-substituted heteropolytungstate K13Eu(SiW11O39)(2) was successfully assembled into two lipids by LB technique for the first time. X-ray diffraction has shown a well defined lamellar for the LB films. The LB films have been characterized by fluorescence spectra and the characteristic luminescence behaviors were discussed. The ligand-metal charge transfer band could be observed in the spectra of the LB films, which could not be found in that of heteropolytungstate solid. The results of fluorescence spectra indicate the energy could be effectively transferred from ligands to the Eu3+ ions in the LB films and the luminescence efficiency was increased greatly. The influences of various lipids on the luminescence of polyoxometalates were investigated. The various interactions between monolayer and polyanions have different effect on the luminescence properties of europium-substituted heteropolytungstate.

Role of ligand-to-metal charge transfer state in nontriplet photosensitization of luminescent europium complex.

We have investigated, by means of steady-state and time-resolved optical spectroscopies, the excited-state dynamics of the luminescent europium complex Eu(III)(tta)(3)dpbt (tta = henoyltrifluoroacetonate; dbpt = 2-(N,N-diethylanilin-4-yl)-4,6-bis(3,5-dimethylpyrazol-1-yl)-1,3,5-triazine) with Gd(III)(tta)(3)dpbt and Tb(III)(tta)(3)dpbt as the reference complexes that cannot be photosensitized. In the Eu(III)(tta)(3)dpbt complex, the ligand dpbt exhibited biphasic fluorescence decay kinetics; the faster component (decay time constant, 8.5 ps) is ascribed to the rapid conversion of the lowest-lying singlet excited state of dpbt (S(1) or (1)dpbt*) to a ligand-to-metal charge transfer singlet state of the complex ((1)LMCT*), whereas the slower one (1.8 ns) is shown by temperature-dependent luminescence spectroscopy to be delayed fluorescence due to the LMCT-to-dpbt backward energy transfer and represents the time scale of efficient excitation energy flow from the (1)LMCT* state to the (5)D(1) state of Eu(III). On the basis of the spectroscopic results of the Ln(III)(tta)(3)dpbt complexes (Ln = Eu, Gd, and Tb), the crucial role of the (1)LMCT* state in photosensitization of the Eu(III)(tta)(3)dpbt complex is established, and a LMCT-mediated nontriplet sensitization mechanism is proposed, which is advantageous in high efficiency and low excitation photon energy as well as in low susceptibility against oxygen quenching.

Excitation dynamics of two spectral forms of the core complexes from photosynthetic bacterium Thermochromatium tepidum.

The intact core antenna-reaction center (LH1-RC) core complex of thermophilic photosynthetic bacterium Thermochromatium (Tch.) tepidum is peculiar in its long-wavelength LH1-Q(y) absorption (915 nm). We have attempted comparative studies on the excitation dynamics of bacteriochlorophyll (BChl) and carotenoid (Car) between the intact core complex and the EDTA-treated one with the Q(y) absorption at 889 nm. For both spectral forms, the overall Car-to-BChl excitation energy transfer efficiency is determined to be approximately 20%, which is considerably lower than the reported values, e.g., approximately 35%, for other photosynthetic purple bacteria containing the same kind of Car (spirilloxanthin). The RC trapping time constants are found to be 50 approximately 60 ps (170 approximately 200 ps) for RC in open (closed) state irrespective to the spectral forms and the wavelengths of Q(y) excitation. Despite the low-energy LH1-Q(y) absorption, the RC trapping time are comparable to those reported for other photosynthetic bacteria with normal LH1-Q(y) absorption at 880 nm. Selective excitation to Car results in distinct differences in the Q(y)-bleaching dynamics between the two different spectral forms. This, together with the Car band-shift signals in response to Q(y) excitation, reveals the presence of two major groups of BChls in the LH1 of Tch. tepidum with a spectral heterogeneity of approximately 240 cm(-1), as well as an alteration in BChl-Car geometry in the 889-nm preparation with respect to the native one.

Baicalin in Radical Scavenging and Its Synergistic Effect with β-Carotene in Antilipoxidation

The lipophilic flavonoid glycoside baicalin from the traditional oriental herb Scutellaria baicalensis Georgi (logP = 1.27, pK(a1) = 7.6, pK(a2) = 10.1 as determined at 25 degrees C in 0.1 M NaCl) is found to be as reducing (0.39 V vs NHE, reversible two-electron oxidation by CV at pH 7.4) as other catechol flavonoids but a poor radical scavenger (TEAC = 1.12, pH 7.4) and a poor antioxidant against free radical initiated lipid oxidation in liposomes. However, this compound is able to regenerate beta-carotene (beta-Car) from beta-Car(*+) with a second-order rate constant of (5.6 +/- 0.5) x 10(9) L mol(-1) s(-1) in the methanol/chloroform binary solvent (1:9, v/v) and, more importantly, to exhibit a prominent synergistic effect with beta-Car against the lipoxidation induced by AMVN-derived peroxyl radical in liposomal membrane. Thus, baicalin by itself is not an effective antioxidant, but it becomes one via interaction with beta-Car. The radical scavenging and antilipoxidation properties of baicalin are discussed in terms of its physicochemical properties and molecular structures.

Time-resolved spectroscopic behavior of Fe2O3 and ZnFe2O4 nanocrystals

Using nanosecond (ns) and femtosecond (fs) time-resolved absorption spectroscopies (pump-probe technique), the carrier dynamics in transition metal oxide nanocrystals of alpha-Fe2O3 and ZnFe2O4 was studied during the photolysis process. For Fe2O3 and ZnFe2O4 nanocrystals, the fs measurements detect similar profiles of a positive nonlinear absorption in their capped nanocrystals, whereas much weak signals in the naked particles. In the nanosecond measurements Fe2O3 and ZnFe2O4 nanocrystals show obvious excitation-power dependent absorption properties and at the low pump power they show weak photobleaching, but at high pump power they produce positive nonlinear absorptions. For Fe2O3 nanocrystals, the threshold power of negative absorption (bleach) to positive absorption increases with reducing size, whereas for the ZnFe2O4 samples, the threshold powers reach minimum at a critical size of 11 nm, grow for both the bigger and the smaller nanocrystals. These results reflect the influences of their microscopic magnetic couplings and carrier correlation on biexciton absorption in Fe2O3 and ZnFe2O4 nanocrystals. All the results indicate that the time resolved photoabsorption techniques are useful to study the microscopic spin interactions and carrier correlations in transition metal oxide nanocrystals.

Correlation between Energy and Spatial Distribution of Intragap Trap States in the TiO2 Photoanode of Dye-Sensitized Solar Cells

The energy and spatial distribution of intragap trap states of the TiO2 photoanode of dye-sensitized solar cells and their impact on charge recombination were investigated by means of time-resolved charge extraction (TRCE) and transient photovoltage (TPV). The photoanodes were built from TiO2 nanospheroids with different aspect ratios, and the TRCE results allowed differentiation of two different types of trap states, that is, deep and shallow ones at the surface and in the bulk of the TiO2 particles, respectively. These trap states exhibit distinctly different characteristic energy with only a slight variation in the particle size, as derived from the results of the density of states. Analyses of the size-dependent TPV kinetics revealed that in a moderate photovoltage regime of about 375-625 mV, the dynamics of electron recombination are dominated by shallow trap states in the bulk, which can be well accounted for by the mechanism of multiple-trap-limited charge transport.

Syntheses of novel 1,3-diazaazulene derivatives and their nonlinear optical characterization

We have synthesized three new 1,3-diazaazulene derivatives, namely, 2-(4′-N,N-dimethylaminophenyl)-6-nitro-1,3-diazaazulene (18, DMAPNA), 2-(4′-aminophenyl)-6-nitro-1,3-diazaazulene (19, APNA) and 2-[4′-N-(2-hydroxyethyl)aminophenyl]-6-nitro-1,3-diazaazulene (20, HEAPNA), each of them contains an amino substitute as the electron donor (D), 2-phenyl-1,3-diazaazulene as the π-conjugated bridge and a nitro as the electron acceptor (A). Our theoretical results have predicted that these D–π–A type chromophores possess low ground-state dipole moment (μg) and large first-order hyperpolarizability (β), which may facilitate a low degree of aggregation for the chromophores dispersed in a polymeric matrix as well as a large nonlinear optical (NLO) response. The expected NLO performance has been confirmed by the experimental β and μg values, e.g., for 18, 407.8 × 10−30 esu and 4.7 D, respectively. The origins of large β and low μg are explained in terms of a two-state quantum model. The DMAPNA (18)-doped and poled polymethylmethacrylate film exhibits a large second harmonic generation (SHG) coefficient of d33 = 10.9 pm V−1 with excellent thermal stability (above 70% of the maximal SHG coefficient remains at ∼100 °C).