Xiaonan Gao

Oxidation-Induced Self-Assembly of Ag Nanoshells into Transparent and Opaque Ag Hydrogels and Aerogels

The synthesis of hollow Ag nanoshells (NSs) with tunable plasmon bands in the visible spectrum and their oxidative-assembly into high-surface-area, mesoporous, transparent, and opaque Ag gel frameworks is reported. Thiolate-coated Ag NSs with varying size and shell thickness were prepared by fast chemical reduction of preformed Ag2O nanoparticles (NPs). These NSs were assembled into monolithic Ag hydrogels via oxidative removal of the surface thiolates, followed by CO2 supercritical drying to produce metallic Ag aerogels. The gelation kinetics have been controlled by tuning the oxidant/thiolate molar ratio (X) that governs the rate of NP condensation, which in turn determines the morphology, optical transparency, opacity, surface area, and porosity of the resultant gel frameworks. The monolithic Ag hydrogels prepared using high concentration of oxidant (X > 7.7) leads to oxidative etching of precursor colloids into significantly smaller NPs (3.2-7.6 nm), which appeared to eliminate the visible light scattering yielding transparent gel materials. In contrast, the opaque Ag aerogels composed entirely of hollow NSs exhibit enormously high surface areas (45-160 m(2)/g), interconnected meso-to-macro-pore network that can be tuned by varying the inner cavity of Ag colloids, and accessibility of chemical species to both inner and outer surface of the hollows, offering perspectives for a number of new technologies. An advantage of current synthesis is the ability to transform Ag NSs into monolithic hydrogels within 4-12 h, which otherwise is reported to require weeks to months for the oxidation-induced metallic gel synthesis reported to date.

Selective Recognition of Uracil and Its Derivatives Using a DNA Repair Enzyme Structural Mimic

During DNA repair, uracil DNA glycosylase (UDG) pulls unwanted uracil into its active site through hydrogen bonding and pi-pi stacking interactions. The reason why UDG binds only uracil tightly--and not its derivatives, such as thymine--remains unclear. In this study, we synthesized the stable, water-soluble receptor 1a as a structural mimic of the active site in UDG. Compound 1a contains a 2,6-bis(glycylamino)pyridine group, which mimics the amino acid residues of UDG that interact with uracil through a hydrogen-bonding network; it also possesses a pyrene moiety as a pi-pi stacking interaction element and fluorescent probe that mimics the aromatic groups (phenyl and fluorescent indolyl units) found in the active site of UDG. Receptor 1a binds selectively to uracil and derivatives (including thymine, 5-formyluracil, 5-fluorouracil, and 5-nitrouracil) and some DNA and RNA nucleosides (including thymidine and uridine) through hydrogen bonding and pi-pi stacking interactions. Interestingly, a plot of log K(b) with respect to the values of pK(a) of the N(3)H units of uracil and its derivatives was linear, with a negative slope (beta) of -0.24 +/- 0.03. Thus, compounds featuring lower values of pK(a) for their N(3)H units provided greater apparent binding constants for their complexes with receptor 1a, suggesting acidity-dependent binding of uracil and its derivatives to this receptor; notably, uracil bound more tightly than did thymine. Our study provides some insight into how uracil and its derivatives in DNA are bound by DNA repair enzymes through hydrogen bonding and pi-pi stacking interactions.

Direct Cross-Linking of Au/Ag Alloy Nanoparticles into Monolithic Aerogels for Application in Surface-Enhanced Raman Scattering.

The direct cross-linking of Au/Ag alloy nanoparticles (NPs) into high surface area, mesoporous Au/Ag aerogels via chemical oxidation of the surface ligands is reported. The precursor alloy NPs with composition-tunable morphologies were produced by galvanic replacement of the preformed Ag hollow NPs. The effect of Au:Ag molar ratio on the NP morphology and surface plasmon resonance has been thoroughly investigated and resulted in smaller Au/Ag alloy NPs (4-8 nm), larger Au/Ag alloy hollow NPs (40-45 nm), and Au/Ag alloy hollow particles decorated with smaller Au NPs (2-5 nm). The oxidative removal of surfactant ligands, followed by supercritical drying, is utilized to construct large (centimeter to millimeter) self-supported Au/Ag alloy aerogels. The resultant assemblies exhibit high surface areas (67-73 m(2)/g), extremely low densities (0.051-0.055 g/cm(3)), and interconnected mesoporous (2-50 nm) networks, making them of great interest for a number of new technologies. The influence of mesoporous gel morphology on surface-enhanced Raman scattering (SERS) has been studied using Rhodamine 101 (Rd 101) as the probe molecule. The alloy aerogels exhibit SERS signal intensities that are 10-42 times higher than those achieved from the precursor Au/Ag alloy NPs. The Au/Ag alloy aerogel III exhibits SERS sensing capability down to 1 nM level. The increased signal intensities attained for alloy aerogels are attributed to highly porous gel morphology and enhanced surface roughness that can potentially generate a large number of plasmonic hot spots, creating efficient SERS substrates for future applications.

A DNA Tetrahedron Nanoprobe with Controlled Distance of Dyes for Multiple Detection in Living Cells and in Vivo

Multicomponent quantitative detection in living samples is becoming increasingly important; however, the current detection strategy may cause fluorescence self-quenching and reduce the sensitivity of detection. To solve the problem, we develop a DNA tetrahedral nanoprobe to control the dyes distance for simultaneous detection of multiple analytes. Compared to mesoporous silica nanoparticles based nanoprobes, the DNA tetrahedral nanoprobes display enhanced fluorescence intensities due to partially avoiding the fluorescence resonance energy transfer. Confocal fluorescence images show that the nanoprobes are capable of detecting and visualizing pH and O2•- in living cells under a single wavelength excitation. In an inflammation model for mice, the nanoprobes simultaneously image the down-regulation of pH and up-regulation of O2•-. We expect that the current strategy can provide new opportunities in designing probes for multiplexed detection with reduced self-quenching and enhanced sensitivity.

Fabrication of native silica, cross-linked, and hybrid aerogel monoliths with customized geometries

In this work, a previously developed (White et al 2015 J. Mater. Chem. A 3 762) rapid synthesis approach is used to fabricate native and cross-linked aerogel monoliths with customized geometries. This technique does not require solvent exchange, therefore fabrication times do not depend on part size. To prove this, parts with a smallest dimension of approximately 3.6 cm were fabricated within the same time scale as that of small cylinders with a diameter of 7 mm. In addition, monoliths with customized geometries exhibiting physical detail on the order of 1 mm were produced to demonstrate the versatility of this technique. Furthermore, hybrid materials consisting of native silica aerogel integrated with selected regions of polymer cross-linking were produced. The cross-linked regions allow for adhesion to other surfaces or labeling while the majority of the material retains the physical characteristics of a native silica aerogel. The physical and thermal properties of all aerogel components were examined. All aerogel materials produced in this work exhibited characteristics that were within the range of aerogel materials produced using more conventional methods. For native silica materials, this includes densities in the range of 0.03–0.116 g cm−3, surface areas between 342–799 m2 g−1, mode pore sizes in the range of 30–39 nm, and thermal conductivities in the range of 0.020–0.026 W m−1 K−1. For cross-linked aerogel materials, densities ranged between 0.154–0.340 g cm−3, surface areas were between 291–388 m2 g−1, mode pore sizes were in the range of 29–41 nm, and thermal conductivities were in range of 0.038–0.066 W m−1 K−1.

Shape-Asymmetry Supramolecular Isomerism in Asymmetrical Ligand PCPs and the Expression Method of Three-Level Isomerism.

We show here the supramolecular isomerism, with respect to shape-asymmetry of ligand and the new hierarchical classification for supramolecular isomerism, the three-level isomerism, which was advanced based on a thorough investigation for the four new Ni/dpt24 polymorphs [Hdpt24 = 3-(2-pyridyl)-5-(4-pyridyl)-1,2,4-triazole)]. Compounds 1, 2, and 3 are three-dimensional twofold interpenetrated porous coordination polymers with NbO topology, while 4 with two-dimensional grid structure is termed as the primary isomer of 1/2/3 due to the difference of dimensionality. Complex 3 possessing different shape-asymmetry of single networks from 1 and 2, is called as the secondary isomer of 1 and 2. Complexes 1 and 2 possess the same topology, single shape-asymmetry networks, but different interpenetration-orientation and interpenetration-asymmetry, and are defined as the tertiary isomers. Distinct differences in H2 and CO2 adsorption capacity were observed among each level of isomers. In addition, the hierarchical classifications relationship with characteristic classifications has been discussed.

A Highly Sensitive Strategy for Fluorescence Imaging of MicroRNA in Living Cells and in Vivo Based on Graphene Oxide-Enhanced Signal Molecules Quenching of Molecular Beacon

In situ imaging of microRNA (miRNA) in living cells and in vivo is beneficial for promoting the studies on miRNA-related physiological and pathological processes. However, the current strategies usually have a low signal-to-background ratio, which greatly affects the sensitivity and imaging performance. To solve this problem, we developed a highly sensitive strategy for fluorescence imaging of miRNA in living cells and in vivo based on graphene oxide (GO)-enhanced signal molecule quenching of a molecular beacon (MB). 2Cy5-MB was designed by coupling two Cy5 molecules onto the opposite ends of MB. The fluorescence intensities of two Cy5 molecules were reduced because of the self-quenching effect. After adsorbing on the GO surface, the fluorescence quenching of the molecules was enhanced by fluorescence resonance energy transfer. This double-quenching effect significantly reduced the fluorescence background. In the presence of one miRNA molecule, the fluorescence signals of two Cy5 molecules were simultaneously recovered. Therefore, a significantly enhanced signal-to-background ratio was obtained, which greatly improved the detection sensitivity. In the presence of miRNA, the fluorescence intensity of 2Cy5-MB-GO recovered about 156 times and the detection limit was 30 pM. Compared with 1Cy5-MB-GO, the elevated fluorescence intensity was enhanced 8 times and the detection limit was reduced by an order of magnitude. Furthermore, fluorescence imaging experiments demonstrated that 2Cy5-MB-GO could visually detect microRNA-21 in various cancer cells and tumor tissues. This simple and effective strategy provides a new sensing platform for highly sensitive detection and simultaneous imaging analysis of multiple low-level biomarkers in living cells and in vivo.

Cyclic Regulation of the Sulfilimine Bond in Peptides and NC1 Hexamers via the HOBr/H2Se Conjugated System

The sulfilimine bond (-S═N-), found in the collagen IV scaffold, significantly stabilizes the architecture via the formation of sulfilimine cross-links. However, precisely governing the formation and breakup process of the sulfilimine bond in living organisms for better life functions still remains a challenge. Hence, we established a new way to regulate the breaking and formation of the sulfilimine bond through hydrogen selenide (H2Se) and hypobromous acid (HOBr), which can be easily controlled at simulated physiological conditions. This novel strategy provides a circulation regulation system to modulate the sulfilimine bond in peptides and NC1 hexamers, which can offer a substantial system for further study of the physiological function of collagen IV.

Nuclear-Targeted Photothermal Therapy Prevents Cancer Recurrence with Near-Infrared Triggered Copper Sulfide Nanoparticles

Clinical cancer treatments nowadays still face the challenge of recurrence due to the residual cancer cells and minute lesions in surgeries or chemotherapies. To effectively address the problem, we introduce a strategy for constructing cancer cell nuclear-targeted copper sulfide nanoparticles (NPs) with a significant photothermal effect to completely kill residual cancer cells and prevent local cancer recurrence. The NPs could directly target the tumor cells and further enter the nucleus by the surface modification of RGD and TAT peptides. Under the irradiation of 980 nm near-infrared laser, the NPs rapidly increase the temperature of the nucleus, destroy the genetic substances, and ultimately lead to an exhaustive apoptosis of the cancer cells. In vivo experiments show that the designed NPs could effectively treat cancer and prevent the return of cancer with a single laser irradiation for 5 min. The photothermal therapy strategy with nuclear targeting for cancer therapy and anti-recurrence will provide more possibilities to develop efficient platforms for treating cancer.

Avoiding Thiol Compound Interference: A Nanoplatform Based on High‐Fidelity Au–Se Bonds for Biological Applications

Gold nanoparticles (Au NPs) assembled through Au-S covalent bonds have been widely used in biomolecule-sensing technologies. However, during the process, detection distortions caused by high levels of thiol compounds can still significantly influence the result and this problem has not really been solved. Based on the higher stability of Au-Se bonds compared to Au-S bonds, we prepared selenol-modified Au NPs as an Au-Se nanoplatform (NPF). Compared with the Au-S NPF, the Au-Se NPF exhibits excellent anti-interference properties in the presence of millimolar levels of glutathione (GSH). Such an Au-Se NPF that can effectively avoid detection distortions caused by high levels of thiols thus offers a new perspective in future nanomaterial design, as well as a novel platform with higher stability and selectivity for the in vivo application of chemical sensing and clinical therapies.