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Showing papers by "Xiaogang Liu published in 2010"


Journal ArticleDOI
25 Feb 2010-Nature
TL;DR: The results suggest that the doping-induced structural and size transition, demonstrated here in NaYF4 upconversion nanocrystals, could be extended to other lanthanide-doped nanocrystal systems for applications ranging from luminescent biological labels to volumetric three-dimensional displays.
Abstract: Doping is a widely applied technological process in materials science that involves incorporating atoms or ions of appropriate elements into host lattices to yield hybrid materials with desirable properties and functions. For nanocrystalline materials, doping is of fundamental importance in stabilizing a specific crystallographic phase, modifying electronic properties, modulating magnetism as well as tuning emission properties. Here we describe a material system in which doping influences the growth process to give simultaneous control over the crystallographic phase, size and optical emission properties of the resulting nanocrystals. We show that NaYF(4) nanocrystals can be rationally tuned in size (down to ten nanometres), phase (cubic or hexagonal) and upconversion emission colour (green to blue) through use of trivalent lanthanide dopant ions introduced at precisely defined concentrations. We use first-principles calculations to confirm that the influence of lanthanide doping on crystal phase and size arises from a strong dependence on the size and dipole polarizability of the substitutional dopant ion. Our results suggest that the doping-induced structural and size transition, demonstrated here in NaYF(4) upconversion nanocrystals, could be extended to other lanthanide-doped nanocrystal systems for applications ranging from luminescent biological labels to volumetric three-dimensional displays.

2,835 citations


Journal ArticleDOI
19 Jul 2010-Analyst
TL;DR: Recent developments in optical biolabeling and bio-imaging involving upconversion nanoparticles are reviewed, simultaneously bringing to the forefront the desirable characteristics, strengths and weaknesses of these luminescent nanomaterials.
Abstract: Upconversion refers to non-linear optical processes that convert two or more low-energy pump photons to a higher-energy output photon. After being recognized in the mid-1960s, upconversion has attracted significant research interest for its applications in optical devices such as infrared quantum counter detectors and compact solid-state lasers. Over the past decade, upconversion has become more prominent in biological sciences as the preparation of high-quality lanthanide-doped nanoparticles has become increasingly routine. Owing to their small physical dimensions and biocompatibility, upconversion nanoparticles can be easily coupled to proteins or other biological macromolecular systems and used in a variety of assay formats ranging from bio-detection to cancer therapy. In addition, intense visible emission from these nanoparticles under near-infrared excitation, which is less harmful to biological samples and has greater sample penetration depths than conventional ultraviolet excitation, enhances their prospects as luminescent stains in bio-imaging. In this article, we review recent developments in optical biolabeling and bio-imaging involving upconversion nanoparticles, simultaneously bringing to the forefront the desirable characteristics, strengths and weaknesses of these luminescent nanomaterials.

1,284 citations


Journal ArticleDOI
TL;DR: A comparative spectroscopic investigation of a series of Yb/Tm co-doped hexagonal-phase NaGdF4 nanoparticles with or without a thin surface protection layer shows that, through the thin layer coating, the characteristic optical features of these nanoparticles can be retained, thereby providing direct evidence to support the surface quenching effect responsible for the size-dependent UC luminescence.
Abstract: Lanthanide-doped upconversion (UC) nanoparticles have shown considerable promise in biological labeling, imaging, and therapeutics. However, although current synthetic approaches allow for preparation of ultrasmall UC nanoparticles with precise control over particle morphology and emission color, smaller nanoparticles come at the expense of weaker emissions, which is a constraint that is practically impossible to surpass. Many fundamental aspects of the UC luminescence in these nanomaterials still lack sufficient understanding. In particular, several groups have observed varied relative intensity of the multi-peak UC emissions with varying particle size. The UC luminescence primarily originates from intra-configurational 4f electron transitions within the localized lanthanide dopant ions. Due to a small Bohr radius of the exciton in UC hosts and weak interactions between 4f electrons of the lanthanide dopant ions and the host matrix, the size-dependent UC luminescence can hardly be explained by classic theories, such as quantum confinement and surface plasmon resonance related to optical properties of semiconductor and metal nanoparticles. Although phonon confinement has been used to account for the size-dependent UC luminescence, it has been a matter of much debate, owing to the constraints typically associated with solid-state sample measurements at extreme conditions (for example, low temperatures of ca. 10 K) and exclusion of vibrational energies and optical traps arising from particle surface. To this end, a surface quenching effect is proposed and correlated with size-dependent UC luminescence. However, the surface quenching effect has not been conclusively established, largely because of the lack of direct evidence on surface-quenching-induced luminescence modulation of different-sized particles. Herein, we present a comparative spectroscopic investigation of a series of Yb/Tm co-doped hexagonal-phase NaGdF4 nanoparticles (10, 15, and 25 nm) with or without a thin (ca. 2.5 nm) surface protection layer. We show that, through the thin layer coating, the characteristic optical features (such as relative emission intensities) of these nanoparticles can be retained, thereby providing direct evidence to support the surface quenching effect responsible for the size-dependent UC luminescence. Hexagonal-phase NaGdF4 was chosen as the model host system owing to its ability to render high UC efficiency and the benefits of producing relatively small (< 20 nm) and uniform nanoparticles. Furthermore, the Gd host ion that features half-filled 4f orbitals is relatively inert in the luminescence process and thus has negligible interaction with the dopant ions. To provide a direct comparison over a broad wavelength range between the relative emission intensity of the particles, the Tm ion with a ladder-like arrangement of energy levels was selected as the activator capable of generating upconverted emission peaks that span from ultraviolet (UV) to near-infrared (NIR) spectral regions (Figure 1a).

818 citations


Journal ArticleDOI
TL;DR: The results suggest that the photocatalysis efficiency of nanocrystals can be significantly improved by tailoring the shape and the surface structure of nanocystals, which provides a new concept for rational design and development of high-performance photocatalysts.
Abstract: Particle size is generally considered to be the primary factor in the design of nanocrystal photocatalysts, because the reduction of particle size increases the number of active sites. However, the benefit from the size reduction can be canceled by a higher electron-hole recombination rate due to the confined space in sphere-shaped nanoparticles. Here we report a mechanistic study on a novel nanobelt structure that overcomes the drawback of sphere-shaped nanoparticles. Single-crystalline anatase TiO(2) nanobelts with two dominant surfaces of (101) facet exhibit enhanced photocatalytic activity over the nanosphere counterparts with an identical crystal phase and similar specific surface area. The ab initio density functional theory (DFT) calculations show that the exposed (101) facet of the nanobelts yields an enhanced reactivity with molecular O(2), facilitating the generation of superoxide radical. Moreover, the nanobelts exhibit a lower electron-hole recombination rate than the nanospheres due to the following three reasons: (i) greater charge mobility in the nanobelts, which is enabled along the longitudinal dimension of the crystals; (ii) fewer localized states near the band edges and in the bandgap due to fewer unpassivated surface states in the nanobelts; and (iii) enhanced charge separation due to trapping of photogenerated electrons by chemisorbed molecular O(2) on the (101) facet. Our results suggest that the photocatalysis efficiency of nanocrystals can be significantly improved by tailoring the shape and the surface structure of nanocrystals, which provides a new concept for rational design and development of high-performance photocatalysts.

668 citations


Journal ArticleDOI
TL;DR: A novel method for the synthesis of vinyl sulfides by the decarboxylative cross-coupling of arylpropiolic acids with thiols using copper(I) salts as catalysts has been developed.

163 citations


Journal ArticleDOI
TL;DR: A complete nanoreaction system whereby colloidal NPs are rationally assembled and purified, and the integration of hetero-assembly, stoichiometry control, protection scheme and separation method may provide a scalable way to fabricate sophisticated nanostructures.
Abstract: A central theme in nanotechnology is to advance the fundamental understanding of nanoscale component assembly, thereby allowing rational structural design that may lead to materials with novel properties and functions. Nanoparticles (NPs) are often regarded as 'artificial atoms', but their 'reactions' are not readily controllable. Here, we demonstrate a complete nanoreaction system whereby colloidal NPs are rationally assembled and purified. Two types of functionalized gold NPs (A and B) are bonded to give specific products AB, AB(2), AB(3) and AB(4). The stoichiometry control is realized by fine-tuning the charge repulsion among the B-NPs. The products are protected by a polymer, which allows their isolation in high purity. The integration of hetero-assembly, stoichiometry control, protection scheme and separation method may provide a scalable way to fabricate sophisticated nanostructures.

151 citations


Journal ArticleDOI
TL;DR: In this paper, the synthesis of tetragonal-phase LiYF 4 nanoparticles doped with upconverting lanthanide ions was reported, where the size of the nanoparticles can be tuned by varying the precursor ratio of F to L ions.

114 citations


Journal ArticleDOI
TL;DR: The cross-coupling reactions of nitro- and aldehyde-substituted aryl halides with benzyl thiols under the same reaction conditions were demonstrated to afford benzothiazole and phenylbenzo[b]thiophene derivatives.

80 citations


Journal ArticleDOI
TL;DR: Thermoelectric investigation of these nanocrystals shows that the Bi(3+) doping increases electrical conductivity from 350 to 650 K and changes the Seebeck coefficient sign from positive to negative.
Abstract: We report the synthesis of a series of monodispersed Bi-doped PbTe nanocrystals with tunable morphologies by using a doping precursor of bismuth(III) 2-ethylhexanoate. The as-synthesized Pb1−xBixTe (x = 0.005, 0.010, 0.015, 0.020) nanocrystals are characterized by X-ray diffraction, X-ray photoelectron spectroscopy and Hall measurements. The nanocrystals with controlled spherical, cuboctahedral, and cubic shapes were readily prepared by varying the Bi doping concentration. Thermoelectric investigation of these nanocrystals shows that the Bi3+ doping increases electrical conductivity from 350 to 650 K and changes the Seebeck coefficient sign from positive to negative.

49 citations


Journal ArticleDOI
TL;DR: In this paper, upconversion multicolor tuning based on uniform β-NaYF4:Yb/Tm/Er nanoparticles is reported, and the color output can be precisely modulated by adjusting activator ratios of Tm3+ to Er3+.
Abstract: We report upconversion multicolor tuning based on uniform β-NaYF4:Yb/Tm/Er nanoparticles. The as-synthesized nanoparticles with an average diameter of 25 nm are well dispersed in a wide range of nonpolar solvents including hexane, cyclohexane, dichloromethane, and toluene. These nanoparticles show intense upconversion emissions and the color output can be precisely modulated by adjusting activator ratios of Tm3+ to Er3+. Dopant-concentration dependent emission properties of the triply doped particle system are also investigated. In addition, we demonstrate that these nanoparticles can be readily transferred to polar solvents such as ethanol and water by growing a thin silica layer (10 nm) around the particles, providing potential applications in biological labeling and imaging.

42 citations


Journal ArticleDOI
TL;DR: This replication study confirmed 38 single-nucleotide polymorphisms (SNPs) out of 139 SNPs previously reported in three recent genome-wide association studies (GWASs) in an independent US white sample to show promise for replication of some initial genome- wide association signals.
Abstract: Summary Osteoporosis is a major public health problem characterized by low bone mineral density (BMD). This replication study confirmed 38 single-nucleotide polymorphisms (SNPs) out of 139 SNPs previously reported in three recent genome-wide association studies (GWASs) in an independent US white sample. Ten SNPs achieved combined p < 3.6 × 10−4.

Journal ArticleDOI
TL;DR: This study used a developed pathway-based approach to further test biological pathways for potential association with stature, by examining ∼370 000 single-nucleotide polymorphisms across the human genome in 618 unrelated elder Han Chinese.
Abstract: Recent success of genome-wide association studies (GWASs) on human height variation emphasized the effects of individual loci or genes. In this study, we used a developed pathway-based approach to further test biological pathways for potential association with stature, by examining ∼370 000 single-nucleotide polymorphisms (SNPs) across the human genome in 618 unrelated elder Han Chinese. A total of 626 biological pathways annotated by any of the three major public pathway databases (KEGG, BioCarta and Ambion GeneAssist Pathway Atlas) were tested. The regulation-of-autophagy (ROA) (nominal P=0.012) pathway was marginally significantly associated with human stature after our family wise error rate multiple-testing correction. We also used 1000 random recruited US whites for further replication. Interestingly, the ROA pathway presented the strongest signals in whites for height variation (nominal P=0.002). The results correspond to biological roles of the ROA pathway in human long bone development and growth. Our findings also implied that multiple-genetic factors may work jointly as a functional unit (pathway), and the traditional GWASs could have missed important genetic information imbedded in those less significant markers.

Journal ArticleDOI
TL;DR: The findings suggest that the DBP gene might be one of the genetic factors influencing CSI phenotype in Caucasians, especially in males.
Abstract: Summary This study was conducted to test whether there exists an association between vitamin D-binding protein (DBP) gene and compression strength index (CSI) phenotype. Candidate gene association analyses were conducted in total sample, male subgroup, and female subgroup, respectively. Two single-nucleotide polymorphisms (SNPs) with significant association results were found in males, suggesting the importance of DBP gene polymorphisms on the variation in CSI especially in Caucasian males.

Journal ArticleDOI
TL;DR: The findings indicate that EFNB2 gene may be a candidate susceptibility gene for schizophrenia in the Han Chinese population, and also provide further support for the potential importance of the NMDA receptor pathway in the etiology of schizophrenia.
Abstract: Recently, evidence of linkage of schizophrenia to chromosome 13q22-q34 has been demonstrated in multiple studies. Based on structure and function, EFNB2 may be considered as a compelling candidate gene for schizophrenia on chromosome 13q33. We genotyped three single-nucleotide polymorphisms (SNPs: rs9520087, rs11069646, and rs8000078) in this region in 846 Han Chinese subjects (477 cases and 369 controls). Significant association between an allele of marker rs9520087 and schizophrenia was found. Furthermore, since no LD was observed in the three SNPs linkage disequilibrium estimation, all three SNPs were used in multiple SNPs haplotype analysis, and a strongly significant difference was found for the common haplotype TTC. Overall our findings indicate that EFNB2 gene may be a candidate susceptibility gene for schizophrenia in the Han Chinese population, and also provide further support for the potential importance of the NMDA receptor pathway in the etiology of schizophrenia.

Journal ArticleDOI
TL;DR: A novel approach to the fabrication of self-aligned nanoscale trench structures in a thin polymer layer covering on conductive materials by passing AC current through a polymer-coated nanowire in the presence of an appropriate solvent is reported.
Abstract: We report a novel approach to the fabrication of self-aligned nanoscale trench structures in a thin polymer layer covering on conductive materials. By passing AC current through a polymer-coated nanowire in the presence of an appropriate solvent, a self-aligned nanotrench is formed in the polymer overlayer as a result of accelerated dissolution while the rest of the device remains covered. Similar results have been achieved for polymer-coated graphene ribbons. Such polymer-protected devices in which only the active component is exposed should find important applications as electrical sensors in aqueous solutions, particularly in cases where parasitic ionic currents often obscure sensing signals.

Journal ArticleDOI
TL;DR: In situ trapping of a thiolated DNA duplex with eight base pairs into a polymer-protected gold nanogap device under near-physiological conditions and the control of DNA dehybridization by heating the device to temperatures above the melting point of the DNA are reported.
Abstract: We report in situ trapping of a thiolated DNA duplex with eight base pairs into a polymer-protected gold nanogap device under near-physiological conditions. The double-stranded DNA was captured by electrophoresis and covalently attached to the nanogap electrodes through sulfur-gold bonding interaction. The immobilization of the DNA duplex was confirmed by direct electrical measurements under near-physiological conditions. The conductance of the DNA duplex was estimated to be 0.09 μS. We also demonstrate the control of DNA dehybridization by heating the device to temperatures above the melting point of the DNA.

Journal ArticleDOI
TL;DR: In this article, a cross coupling of nitroaryl chlorides with various aromatic and aliphatic thiols proceeds smoothly under metal-free conditions to give the corresponding sulfides, e.g. (III), (V) or (VII), with concomitant reduction of the nitro group.
Abstract: Cross coupling of nitroaryl chlorides with various aromatic and aliphatic thiols proceeds smoothly under metal-free conditions to give the corresponding sulfides, e.g. (III), (V) or (VII), with concomitant reduction of the nitro group.



Journal ArticleDOI
05 Nov 2010-Small
TL;DR: The standard procedures to create large-area conjugatedpolymer-array patterns generally involve sequential polymer deposition, photolithography, and a subsequent etching step to remove dissolved polymers by appropriate solvents, but the ultraviolet light, typically used in the process of photolithographic patterning, may cause degradation of the conjugate polymers.
Abstract: Conjugated polymers have enormous potential as novel electronic materials that combine the electrical properties of semiconductors with the good mechanical strength and processing advantages of plastics. [ 1 , 2 ] Perhaps nowhere is this more evident than in the synthesis of conjugated heterocyclic polymers based on thiophene units or monomers. [ 3 ] Owing to their facile preparation, relatively high conductivity, and good long-term stability under ambient conditions, these polymers have been the focus of intense study for use in devices such as plastic photovoltaics, light-emitting diodes, transistors, and fl exible displays. [ 4–6 ] The ability to change color, due to twisting of the polymer backbone in response to changes in solvent, temperature, applied potential, and binding to other molecules, also makes the polythiophenes attractive as optical sensors. [ 7 ] Despite the phenomenal growth in diverse synthetic methodologies that allow the fabrication of the polythiophenes with desired chain length and structure, however, the deposition and patterning of these molecules on a substrate has remained challenging. Conventional techniques for depositing fi lms of conjugated polymers include solution casting, dip coating, and spin coating of the polymer from solutions. However, for many conductive polymers, these approaches are often limited by the insolubility of the polymers, which form aggregates without extended molecular ordering upon deposition. Another common route to deposit conjugated polymers is through an electrochemical polymerization process on an electrode substrate that involves monomer oxidation, dimerization, and the growth of oligomers and polymers of the oxidized monomer. [ 8 ] Because of localized deposition of polymers at the electrode surface, this approach can be used to prepare patterned arrays of conjugated polymers on prepatterned electrodes. The standard procedures to create large-area conjugatedpolymer-array patterns generally involve sequential polymer deposition, photolithography, and a subsequent etching step to remove dissolved polymers by appropriate solvents. However, the ultraviolet light, typically used in the process of photolithographic patterning, may cause degradation of the conjugated polymers. In addition, many types of etching solvents can also dissolve and swell the conjugated polymer