Showing papers by "Xiaogang Liu published in 2011"

Tuning upconversion through energy migration in core–shell nanoparticles

Abstract: Photon upconversion is promising for applications such as biological imaging, data storage or solar cells. Here, we have investigated upconversion processes in a broad range of gadolinium-based nanoparticles of varying composition. We show that by rational design of a core-shell structure with a set of lanthanide ions incorporated into separated layers at precisely defined concentrations, efficient upconversion emission can be realized through gadolinium sublattice-mediated energy migration for a wide range of lanthanide activators without long-lived intermediary energy states. Furthermore, the use of the core-shell structure allows the elimination of deleterious cross-relaxation. This effect enables fine-tuning of upconversion emission through trapping of the migrating energy by the activators. Indeed, the findings described here suggest a general approach to constructing a new class of luminescent materials with tunable upconversion emissions by controlled manipulation of energy transfer within a nanoscopic region.

Intracellular glutathione detection using MnO2-nanosheet-modified upconversion nanoparticles

Abstract: We report a novel design, based on a combination of lanthanide-doped upconversion nanoparticles and manganese dioxide nanosheets, for rapid, selective detection of glutathione in aqueous solutions and living cells. In this approach, manganese dioxide (MnO2) nanosheets formed on the surface of nanoparticles serve as an efficient quencher for upconverted luminescence. The luminescence can be turned on by introducing glutathione that reduces MnO2 into Mn2+. The ability to monitor the glutathione concentration intracellularly may enable rational design of a convenient platform for targeted drug and gene delivery.

Single-band upconversion emission in lanthanide-doped KMnF3 nanocrystals.

Abstract: The preparation of upconversion (UC) nanocrystals that exhibit anti-Stokes emission is important for applications in fields as diverse as photonics, photovoltaics, biological imaging, and therapeutics. In particular, there has been an increasing focus on the synthesis of nanocrystals with tunable UC emission from ultraviolet to near-infrared through doping with lanthanide ions. However, the synthesis of nanocrystals featuring single-band UC with high chromatic purity remains a formidable challenge, as lanthanide ions generally have more than one metastable excited state. In principle, these lanthanide-doped nanocrystals display multipeak emission profiles. Recently, several attempts have been made to obtain high purity of single-band UC red emission attractive for anticounterfeiting and color display applications. For example, high red-to-green (R/G) emission ratio can be achieved by increasing the concentration of Yb in Yb/Er co-doped NaYF4 nanoparticles. [8] In addition, Yb/Er co-doped MnF2 and KMnF3 nanoparticles have shown substantially enhanced R/G emission ratios because of the energy transfer between the Er and Mn. However, a general method for providing single-band red emission has not been conclusively established. Herein, we report a novel oil-based procedure for the synthesis of lanthanide-doped KMnF3 nanocrystals with only single-band UC emissions from Er, Ho, and Tm dopants, respectively. Importantly, we observe that the singleband feature is independent of dopant concentration, pump power, and temperature. We also show that these nanocrystals can serve as ideal optical biolabels for deep-tissue imaging without the constraints associated with conventional multipeak UC nanocrystals. The lanthanide-doped KMnF3 nanocrystals for the present study were prepared in three steps according to a modified literature procedure. In the first step, lanthanide and manganese precursors were first dissolved in oleic acid at 150 8C to form metal–oleate complexes. A stoichiometric amount of potassium fluoride was then added to a solution of the metal–oleate complexes at room temperature to initiate the crystal growth. Subsequently, the reaction temperature was increased to 290 8C to facilitate the growth of the nanocrystals. Figure 1a shows a typical transmission electron microscopy (TEM) image of the as-synthesized KMnF3:Yb/Er (18:2 mol%) nanocrystals with cubic morphology (Figure S1 in the Supporting Information). The high-magnification TEM mage of a single KMnF3:Yb/Er nanocube shown in Figure 1b reveals lattice fringes of the {110} with a d spacing of 0.31 nm, which is typical for cubic KMnF3 (Figure 1b). Selected-area electron diffraction patterns obtained from the Fourier transform of the high-magnification TEM image confirms single-crystalline cubic phase of the nanocube (Figure 1c). It should be noted that charge balance will be disturbed when the trivalent lanthanide ions are substituted for the Mn ions in KMnF3 nanocrystals. To maintain charge balance, either manganese or potassium vacancies are formed (Figure 1d,e). X-ray powder diffraction studies (Figure S2) show peak positions and intensities that can be well indexed in accordance with cubic KMnF3 crystals (JCPDS file no. 82-1334), which is consistent with TEM analysis of the samples.

Macroscopic invisibility cloak for visible light

Abstract: Invisibility cloaks, a subject that usually occurs in science fiction and myths, have attracted wide interest recently because of their possible realization. The biggest challenge to true invisibility is known to be the cloaking of a macroscopic object in the broad range of wavelengths visible to the human eye. Here we experimentally solve this problem by incorporating the principle of transformation optics into a conventional optical lens fabrication with low-cost materials and simple manufacturing techniques. A transparent cloak made of two pieces of calcite is created. This cloak is able to conceal a macroscopic object with a maximum height of 2 mm, larger than 3500 free-space-wavelength, inside a transparent liquid environment. Its working bandwidth encompassing red, green, and blue light is also demonstrated.

Copper-mediated C-H activation/C-S cross-coupling of heterocycles with thiols

Abstract: We report the synthesis of a series of aryl- or alkyl-substituted 2-mercaptobenzothiazoles by direct thiolation of benzothiazoles with aryl or alkyl thiols via copper-mediated aerobic C-H bond activation in the presence of stoichiometric CuI, 2,2'-bipyridine and Na(2)CO(3). We also show that the approach can be extended to thiazole, benzimidazole, and indole substrates. In addition, we present detailed mechanistic investigations on the Cu(I)-mediated direct thiolation reactions. Both computational studies and experimental results reveal that the copper-thiolate complex [(L)Cu(SR)] (L: nitrogen-based bidentate ligand such as 2,2'-bipyridine; R: aryl or alkyl group) is the first reactive intermediate responsible for the observed organic transformation. Furthermore, our computational studies suggest a stepwise reaction mechanism based on a hydrogen atom abstraction pathway, which is more energetically feasible than many other possible pathways including β-hydride elimination, single electron transfer, hydrogen atom transfer, oxidative addition/reductive elimination, and σ-bond metathesis.

Emerging functional nanomaterials for therapeutics

Abstract: The recent development of functional crystalline nanomaterials for therapeutics is described. In contrast to conventional therapeutic approaches, nanomaterial-based systems present novel therapeutic opportunities; for example, by allowing active agents to be site-specifically delivered and efficiently absorbed while offering fewer or reduced side effects. In addition, nanomaterials are generally amenable to surface functionalization and interior doping. These attributes provide the nanomaterials with tunable surface, optical, magnetic, thermal and mechanical properties that are important for applications ranging from controlled release of drugs to photothermal therapy. In this article, we seek to provide a conceptual framework for understanding nanomaterial-based therapeutics. We also attempt to highlight recent therapeutic applications involving some representative nanostructured materials. With improved control over the synthesis and functional characteristics of nanomaterials, the emergence of nanomaterial-based therapeutics will likely accelerate future medical research and improve patient care.

Direct Arylation of Benzothiazoles and Benzoxazoles with Aryl Boronic Acids

Abstract: Direct arylation of benzothiazole (IV) and benzoxazole (I) with a wide range of arylboronic acids is smoothly achieved in the presence of a Pd—Cu cocatalyst under aerobic conditions.

Colorimetric Detection of HIV‐1 Ribonuclease H Activity by Gold Nanoparticles

Abstract: Scheme 1 . Illustration of colorimetric detection of HIV-1 RNase H activity using unmodifi ed gold nanoparticles. Acquired immune defi ciency syndrome (AIDS) is generally characterized by infection of human immunodefi ciency virus type 1 (HIV-1). Despite enormous efforts in developing antiretroviral drugs, coping with the drug resistance of HIV-1 remains a daunting task. [ 1 ] The ribonuclease H (RNase H) activity of HIV-1 reverse transcriptase (RT) plays a crucial role in the viral replication cycle and degrades the RNA strand of an RNA–DNA hybrid. [ 2 ]

Postchemistry of inorganic-organic hybrid particles in aqueous solution: metal-cation exchange.

Abstract: The inorganic–organic hybrid micro/nanoparticles have emerged as an exciting new research area owing to their highly tailorable properties, as well as their potential applications in drug delivery, catalysis, biosensing, biomedical imaging, gas storage, spin-crossover, and optics. Unlike inorganic particles, the inorganic–organic hybrid micro/nanostructures can be easily tuned by switching the different metal centers and linkers. So far, work in this area has predominatly focused on size/shape control, property-tailoring, and nano-technological applications. The inherent chemistry of inorganic–organic hybrid particles remains largely unexplored. The conversion of inorganic-organic hybrid particles into novel micro/nanostructures through the exchange of metal cations or organic ligands, could be technologically useful for creating novel inorganic– organic hybrid materials and achieving new physical phenomena. Recently, our group has already demonstrated the possibility of chemical reactions among organic particles. As part of our ongoing efforts to explore the chemical reactivity of organic and organic–inorganic hybrid micro/nanomaterials, we are increasingly interested in developing inorganic– organic hybrid micro/nanostructures with more-tunable properties through a judicious choice of coordination force and environment. Such procedures would allow for an additional level of control over their morphology as well as the optical, magnetic, and electrical properties. Herein, we report that the discovery of the sub-microspherical particles of polymerized ZnACHTUNGTRENNUNG(SPh)2 networks, formed by a reprecipitation method, can perform a metal– cation exchange in aqueous solution to produce crystalline nanoparticles and nanowires. In a typical experiment, the starting inorganic–organic hybrid material {Zn ACHTUNGTRENNUNG(SPh)2}n (1) was synthesized according to a previously reported procedure. The spherical particles of 1 formed spontaneously on slow addition of a solution of 1 in N,N-dimethylformamide into water in the presence of P123 (poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol)) nonionic surfactant. The Zn cations in these spherical particles can be exchanged in aqueous solution by Pb , Cd , or Hg ions to form different shapes, such as wires or pebble-like particles. The successful exchange of metal cations in the framework is due to the different bond energy of the M S bonds and the solubility of the products, that is, the Ksp value. To the best of our knowledge, the metal–cation exchange in the metal–chalcogenide framework together with the change of shape is unprecedented, although the exchange of free-metal cations (not in a framework) in crystalline metal– chalcogenide materials has been demonstrated to enlarge the pore volume or remove hazardous metals from aqueous solution. In most cases, this kind of exchange does not affect the spatial arrange of frameworks. Scanning electron microscopy (SEM) and optical microscopy images of the as-prepared hybrid particles are shown in Figure 1. The SEM image (Figure 1 a) of Zn ACHTUNGTRENNUNG(SPh)2 compositions shows spherical particles with an average diameter of 300 ( 200) nm, which is consistent with that measured by dynamic light scattering (DLS). After exchanging Zn ions with Pb , Cd , or Hg ions in aqueous solution, the spherical particles became wire-shaped structures for PbACHTUNGTRENNUNG(SPh)2 and Hg ACHTUNGTRENNUNG(SPh)2, and pebble-like particles for CdACHTUNGTRENNUNG(SPh)2. SEM images revealed that the diameter and length of Pb ACHTUNGTRENNUNG(SPh)2 nanowires were in the range of 500 nm–1 mm and above 5 mm, respectively (Figure 1 b), whilst HgACHTUNGTRENNUNG(SPh)2 has a relatively narrow size distribution (200–500 nm) with [a] Dr. Y. Liu, Prof. F. Boey, Dr. L. L. Lao, Prof. H. Zhang, Prof. Q. Zhang School of Materials Science and Engineering Nanyang Technological University 50 Nanyang Avenue, Singapore 639798 (Singapore) Fax: (+65) 67909081 E-mail : qczhang@ntu.edu.sg [b] Prof. X. Liu Department of Chemistry National University of Singapore 21 Lower Kent Ridge Road, Singapore 119077 (Singapore) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201000865.

EcoRI-modified gold nanoparticles for dual-mode colorimetric detection of magnesium and pyrophosphate ions.

Abstract: Magnesium in its ionic form (Mg 2 + ) is essential for many physiological processes, including much of metabolism, enzyme activation and catalysis, photosynthesis development, signal transduction, and protection against hypertension and blood vessel spasm. [ 1 ] Therefore, it has been long recognized that for suitable diagnosis of various ailments, the accurate and rapid measurement of Mg 2 + is important. Additionally, the ability to detect Mg 2 + is also important in the area of environmental monitoring for effective pollution control. [ 2 ]

Rare-Earth Doped Upconversion Nanophosphors

Abstract: Rare-earth-doped upconversion nanophosphors possess a unique ability to convert long-wavelength near-infrared excitations to shorter wavelength emissions under moderate excitation intensities (1–103 W cm−2), thus opening up vast opportunities for creating novel biological labels and three-dimensional (3D) display systems. This chapter reports most recent work in upconversion nanophosphor research, which includes developments of various synthetic approaches and chemical tuning of upconversion properties, as well as technological applications of these luminescent nanomaterials.

Copy Number Variations at the Prader‐Willi Syndrome Region on Chromosome 15 and associations with Obesity in Whites

Abstract: Obesity is a serious health problem with strong genetic determination. Copy number variation (CNV) is a common type of genomic variant associated with some complex human diseases. However, it is not clear how CNVs contribute to the etiology of obesity. In this study, we examined 1,000 unrelated US whites to search for CNVs that may predispose to obesity. We focused our analyses on the Prader-Willi syndrome (PWS) critical region (chromosome 15q11–q13), because the PWS region is a hotspot for CNV generation and obesity is one of the major clinical manifestations for chromosome abnormalities at this region. We constructed a map containing 39 CNVs at the PWS critical region with CNV occurrence rates higher than 1%. Among them, three CNVs were significantly associated with body fat mass (P < 0.05), with a higher copy number (CN) associated with an increase of 5.08–9.77 kg in body fat mass. These three CNVs are close to two known PWS genes, NDN (necdin homolog) and C15orf2 (chromosome 15 open reading frame 2), and partially overlap with another obesity gene PWRN1 (Prader-Willi region nonprotein-coding RNA 1). Interestingly, our recently published whole genome association scan study using the same sample by examining single-nucleotide polymorphisms (SNPs) did not find any significant associations at these CNV regions, suggesting the importance of examining both CNVs and SNPs for better understanding of genetic basis of obesity. Further studies are warranted to validate these CNVs and their importance to obesity.

Genome-wide Association Study of Exercise Behavior in Dutch

Abstract: Introduction—The objective of this study was to identify genetic variants that are associated with adult leisure-time exercise behavior using genome-wide association in two independent samples. Methods—Exercise behavior was measured in 1,772 unrelated Dutch and 978 unrelated American adults with detailed questions about type, frequency and duration of exercise. Individuals were classified into regular exercisers or non-exercisers using a threshold of 4 METhours (metabolic equivalents*hours per week). Regular exercisers were further divided into 5 categories of METhours, ranging from moderate (>=4 METhours) to highly vigorous (>=40 METhours) exercisers. Genome-wide association analyses with a total of 470,719 SNPs were conducted in both samples independently using regression-based techniques in SNPtest, including sex, age and BMI as covariates. Results—SNPs located in SGIP1, DNASE2B, PRSS16, ERCC2 and PAPSS2 were associated with exercise participation (combined p-value between 0.0004 and 4.5*10 -6 with the same

Vertically standing, highly ordered, and dimension and morphology controllable TiO2 nanotube arrays via template-assisted atomic layer deposition

Abstract: We demonstrate that TiO2 nanotube arrays (TNTAs) prepared by template-assisted atomic layer deposition (ALD) has successfully yielded vertically standing, highly ordered, and dimension and morphology controllable TNTAs on substrates. The prepared TNTAs are ideal photoanode structures in solid-state dye sensitized solar cells (DSSCs) for enhanced solar energy conversion efficiency.

Pathway-based association analyses identified TRAIL pathway for osteoporotic fractures.

Abstract: Introduction Hip OF carries the highest morbidity and mortality. Previous studies revealed that individual genes/loci in the Tumor Necrosis Factor (TNF) -Related Apoptosis-Inducing Ligand (TRAIL) pathway were associated with bone metabolism. This study aims to verify the potential association between hip OF and TRAIL pathway.

Molecular origins of commercial laser dye functionality in azacoumarins and 2-quinolones: LD 425, LD 489 and LD 473

Abstract: The molecular structures of three compounds, LD 425 (C13H14N2O3) (1), LD 489 (C15H15F3N2O2) (2) and LD 473 (C17H19F3N2O) (3), are determined by single-crystal X-ray diffraction (XRD) at 180 K. Azacoumarins (1) and (2) possess para-quinoidal bond-length patterns in their benzene rings due to intramolecular charge transfer (ICT) from these rings to the adjoining rings. In contrast, substitution of O with N within the coumarin heterocycle, to form a 2-quinolone, results in the suppression of this ICT effect. Instead, charge transfer within the heterocycle is shown to become more pronounced. Resonance theory is employed to discuss these bond pattern differences and characteristic spectral blue shifts in relation to their coumarin analogues. The application of this theory offers an intuitive understanding of the structure–property relationships in azacoumarins and 2-quinolones which is further supported by quantum chemical calculations. Such an understanding is important for recognizing ICT mechanisms in these compounds which can then be used to facilitate the molecular design of new laser dyes with the desired spectral shifts.

The Plasmonic Ruler Goes 3D

Abstract: Noble metal nanoparticles have distinct optical properties arising from localized surface plasmon resonance (LSPR). It is wellestablished that a LSPR spectrum depends strongly on the size and shape of the nanoparticles as well as on the distance between the nanoparticles. The precise measurement of the interparticle distance-dependent LSPR spectrum has been the fundamental principle of many analytical methods in biology. These types of bioassays can be done with a large ensemble of particles either in homogenous solutions (i.e. colorimetric assays) or on substrates. Alternatively, the assay can be carried out through individual particles (i.e. single-particle-based LSPR). The single-particle-based LSPR technique has the advantages of high detection sensitivity, a good S/N ratio, low sample consumption, and multiplexing potential. Under a dark-field microscope, individual metal nanoparticles can easily be observed as they scatter light intensely and do not blink or photobleach. When two nanoparticles are in close proximity, their plasmon resonances couple with each other and generate a light-scattering spectrum depending strongly on the interparticle distance. This effect has been used to create one-dimensional plasmon rulers, that is, two nanoparticles linked with (bio)chemical linkers. Under high-resolution dark-field imaging spectroscopy, distinct spectral shifts triggered by biomolecular binding events and/or biological processes can be measured based on their modulation of the linkers and in turn the distances of two individual plasmonic particles. This one-dimensional plasmonic ruler has been successfully used to detect DNA conformational change, DNA bending and RNA cleavage. Recently, writing in Science, Na Liu, Mario Hentschel, Thomas Weiss, A. Paul Alivisatos, and Harald Giessen have reported a more powerful plasmonic ruler, a 3D plasmonic ruler, where multiple plasmonic nanoparticles are put in a spatial arrangement. This 3D plasmonic ruler in combination with high-resolution plasmon spectroscopy and plasmon-induced transparency as well as high-order resonances offers a blueprint for optically determining the structural dynamics of single 3D entities. The emergence of this pioneering work is a result of the advances in nanofabrication techniques, theoretical simulation, and high-resolution spectroscopy techniques. Using high-precision electron-beam-based top-down fabrication techniques and layer-by-layer stacking nanotechnologies, the researchers fabricated a 3D plasmonic structure containing five gold nanorods (Figure 1). In their design, a nanorod is stacked between two pairs of parallel nanorods. The two pairs of parallel rods

Copy Number Variation in CNP267 Region May Be Associated with Hip Bone Size

Abstract: Osteoporotic hip fracture (HF) is a serious global public health problem associated with high morbidity and mortality. Hip bone size (BS) has been identified as one of key measurable risk factors for HF, independent of bone mineral density (BMD). Hip BS is highly genetically determined, but genetic factors underlying BS variation are still poorly defined. Here, we performed an initial genome-wide copy number variation (CNV) association analysis for hip BS in 1,627 Chinese Han subjects using Affymetrix GeneChip Human Mapping SNP 6.0 Array and a follow-up replicate study in 2,286 unrelated US Caucasians sample. We found that a copy number polymorphism (CNP267) located at chromosome 2q12.2 was significantly associated with hip BS in both initial Chinese and replicate Caucasian samples with p values of 4.73E-03 and 5.66E-03, respectively. An important candidate gene, four and a half LIM domains 2 (FHL2), was detected at the downstream of CNP267, which plays important roles in bone metabolism by binding to several bone formation regulator, such as insulin-like growth factor-binding protein 5 (IGFBP-5) and androgen receptor (AR). Our findings suggest that CNP267 region may be associated with hip BS which might influence the FHL2 gene downstream.

Synthesis of Vinyl Sulfides by Copper-Catalyzed Decarboxylative C—S Cross-Coupling.

Abstract: The reaction of the arylpropiolic acids with a wide range of thiols proceeds mostly with high or complete (Z)-selectivity tolerating functionalized aromatic, benzylic, heteroaromatic and aliphatic thiols.

Imaging past obstructions

Abstract: The optical detection of a disk geometrically obscured by a second disk and illuminated by a plane wave is considered. Auto and cross-correlation comparisons are found to yield good detection capability.

Direct Arylation of Benzothiazoles and Benzoxazoles with Aryl Boronic Acids.

Abstract: Direct arylation of benzothiazole (IV) and benzoxazole (I) with a wide range of arylboronic acids is smoothly achieved in the presence of a Pd—Cu cocatalyst under aerobic conditions.