Qing-Hua Xu

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.

A Graphene Oxide–Organic Dye Ionic Complex with DNA‐Sensing and Optical‐Limiting Properties

Graphene oxide (GO), a nonstoichiometric, two-dimensionalcarbonsheetresultingfromacidexfoliationofgraphite,offersa new class of solution-dispersible polyaromatic platform forperforming chemistry. Graphene oxide bears covalentlybound epoxide (1,2-ether) and hydroxyl functional groupson either side of its basal plane, while carboxyl groups arelocated at the edge sites.

Separation distance dependent fluorescence enhancement of fluorescein isothiocyanate by silver nanoparticles

Nanocomposites consisting of a metal core, a silica-spacer shell with controlled thickness, and a dye-labelled shell were synthesized and separation distance dependent fluorescence enhancement of fluorescein isothiocyanate by silver nanoparticles was studied; the results indicated an optimum enhancement of 4.8 times with a spacer shell thickness of 21 nm.

One- and Two-Photon Turn-on Fluorescent Probe for Cysteine and Homocysteine with Large Emission Shift

A novel dendritic chromophore with efficient intramolecular charge transfer (ICT) and strong two-photon absorption (TPA) was designed as a turn-on fluorescent probe for cysteine (Cys) and homocysteine (Hcy). The probe exhibited greatly enhanced fluorescence intensity as well as a very large emission peak shift (165 nm) upon addition of Cys/Hcy due to ICT switch off. The sensing process was also monitored by two-photon excited fluorescence (TPEF).

Optical sensing of biological, chemical and ionic species through aggregation of plasmonic nanoparticles

Plasmonic nanoparticles made of gold and silver have attracted a great deal of research attention in various fields, such as biosensors, imaging, therapy, nanophotonics, catalysis and light harvesting due to their unique optical and electronic properties. Plasmonic nanoparticle colloids may exhibit strong colours in the visible region due to localized surface plasmon resonances, whereas their aggregates exhibit different linear and nonlinear optical properties. Therefore, a smart design of chemical interactions between analytes and the nanoparticles surface may lead to gradual optical changes, which can be probed by various sensing methods, allowing quantitative analyte detection. A significant amount of research has been carried out toward the development of plasmonic sensors based on analyte-induced aggregation of Au or Ag nanoparticles, and the sensitivity and selectivity of such plasmonic biosensors have been greatly improved over the years. In this feature article, we summarize different design strategies that have been employed to induce the aggregation of plasmonic nanoparticles upon the addition of various analytes such as DNA, proteins, organic molecules and inorganic ions. We introduce various optical assays, such as colorimetry, surface-enhanced Raman scattering, two-photon photoluminescence, dynamic light scattering, hyper-Rayleigh scattering and chiroptical activity. From the discussion, it can be concluded that plasmonic sensors based on nanoparticle aggregation offer simple, highly sensitive and selective detection of various analytes. Finally, we discuss some of the future directions of plasmonic nanosensors toward device integration for practical applications.

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.

Water-soluble conjugated polymer-induced self-assembly of gold nanoparticles and its application to SERS.

Gold nanochains were prepared by the assembly of citrate-stabilized gold nanospheres induced by cationic conjugated polymers. This assembly method was rapid, and the assembled product was very stable. A longitudinal plasmon resonance band was formed as a result of the coupling of gold nanoparticles and can be tuned from visible to near-infrared by adjusting the polymer/Au molar ratio. The gold nanochains were used as a SERS substrate and gave an enhancement factor of 8.4 x 10 (9), which is approximately 400 times larger than that on the isolated gold nanosphere substrate. The giant SERS enhancement is ascribed to the large electromagnetic fields of coupled gold nanoparticles.

Graphene Oxides as Tunable Broadband Nonlinear Optical Materials for Femtosecond Laser Pulses

Graphene oxide (GO) thin films on glass and plastic substrates were found to display interesting broadband nonlinear optical properties. We have investigated their optical limiting activity for femtosecond laser pulses at 800 and 400 nm, which could be tuned by controlling the extent of reduction. The as-prepared GO films were found to exhibit excellent broadband optical limiting behaviors, which were significantly enhanced upon partial reduction by using laser irradiation or chemical reduction methods. The laser-induced reduction of GO resulted in enhancement of effective two-photon absorption coefficient at 400 nm by up to ∼19 times and enhancement of effective two- and three-photon absorption coefficients at 800 nm by ∼12 and ∼14.5 times, respectively. The optical limiting thresholds of partially reduced GO films are much lower than those of various previously reported materials. Highly reduced GO films prepared by using the chemical method displayed strong saturable absorption behavior.

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.

Stable and Functionable Mesoporous Silica-Coated Gold Nanorods as Sensitive Localized Surface Plasmon Resonance (LSPR) Nanosensors

Core-shell structured Au NRs with a surface-exposed gold core and a mesoporous silica shell (MS Au NRs) were demonstrated as a promising platform for localized surface plasmon resonance (LSPR) based molecular sensing. Mesoporous silica shell not only allows the Au NRs core to be directly exposed to their surrounding environment but also stabilizes Au NRs dispersion in various water-organic mixtures and pure organic solvents. The LSPR band of MS Au NRS displays a stable and linear response in spectral shift to the changes in their surrounding refractive index with a sensitivity of 325 nm/RIU. To demonstrate the application of MS Au NRs as LSPR nanosensors in molecular sensing, the plasmon response to molecular adsorbates (GSH) was demonstrated. MS Au NRs provide a more stable and sensitive response than CTAB-capped Au NRs in GSH sensing. In addition, we have also demonstrated that the LSPR response of Au NRs is highly sensitive to changes of local refractive index in mesoporous silica shell, which renders the feasibility of using MS Au NRs as effective molecule-sensing platforms when mesoporous silica shells were functionalized with various chemical and biological ligands.