High-Performance Electrochemical and Photoelectrochemical Water Splitting at Neutral pH by Ir Nanocluster-Anchored CoFe-Layered Double Hydroxide Nanosheets.

Highly efficient electrocatalysts for the oxygen evolution reaction (OER) in neutral electrolytes are indispensable for practical electrochemical and photoelectrochemical water splitting technologies. However, there is a lack of good, neutral OER electrocatalysts because of the poor stability when H+ accumulates during the OER and slow OER kinetics at neutral pH. Herein, we report Ir species nanocluster-anchored, Co/Fe-layered double hydroxide (LDH) nanostructures in which the crystalline nature of LDH-restrained corrosion associated with H+ and the Ir species dramatically enhanced the OEC kinetics at neutral pH. The optimized OER electrocatalyst demonstrated a low overpotential of 323 mV (at 10 mA cm-2) and a record low Tafel slope of 42.8 mV dec-1. When it was integrated with an organic semiconductor-based photoanode, we obtained a photocurrent density of 15.2 mA cm-2 at 1.23 V versus reversible hydrogen in neutral electrolyte, which is the highest among all reported photoanodes to our knowledge.

Zwitterion-Coated Colloidal Magnetic Nanoparticle Clusters for Reduced Nonspecific Adsorption of Biomolecules.

This paper demonstrates fabrication of silica-shell-coated magnetic nanoparticle clusters (SMNCs) and subsequent surface engineering of SMNCs to produce surface-modified SMNCs that have zwitterionic and primary amine ligands (SMNC-ZW/Am). SMNC-ZW/Am was passivated by zwitterionic ligands for improved colloidal stability and reduced nonspecific adsorption and by primary amine ligands for facilitated conjugation with biomolecules. Hydrodynamic (HD) size and zeta potential of SMNC-ZW/Am could be flexibly tuned by controlling the relative amounts of zwitterionic and primary amine ligands. SMNC-ZW/Am had higher colloidal stability in high salt concentration and broad pH range than did bare SMNC. Nonspecific adsorption with biomolecules onto SMNC-ZW/Am surface was significantly suppressed by the zwitterionic ligands. The facile bioconjugation capability of SWNC-ZW/Am enabled conjugation of biotin and antibody to the SWNC-ZW/Am surface. Biomolecule-conjugated SMNC-ZW/Am showed specific binding affinity to streptavidin and Salmonella bacteria, with reduced nonspecific adsorption; therefore, SWMC-ZW/Am has potential use as an antifouling nanosubstrate for separation and bioanalysis.

Homoepitaxial growth of ZnO nanostructures from bulk ZnO.

Material formation mechanisms and their selective realization must be well understood for the development of new materials for advanced technologies. Since nanomaterials demonstrate higher specific surface energies compared to their corresponding bulk materials, the homoepitaxial growth of nanomaterials on bulk materials is not thermodynamically favorable. We observed the homoepitaxial growth of nanowires with constant outer diameters on bulk materials in two different, solution-based growth systems. We also suggested potential mechanisms of the spontaneous and homoepitaxial growth of the ZnO nanostructures based on the characterization results. The first key factor for favorable growth was the crystal facet stabilization effect of capping agents during the early stages of growth. The second factor was the change in the dominant growth mode during the reaction in a closed system. The spontaneous, homoepitaxial growth of nanomaterials enables the realization of unprecedented, complex, hierarchical, single-crystalline structures required for future technologies.

High-performance and stable photoelectrochemical water splitting cell with organic-photoactive-layer-based photoanode.

Considering their superior charge-transfer characteristics, easy tenability of energy levels, and low production cost, organic semiconductors are ideal for photoelectrochemical (PEC) hydrogen production. However, organic-semiconductor-based photoelectrodes have not been extensively explored for PEC water-splitting because of their low stability in water. Herein, we report high-performance and stable organic-semiconductors photoanodes consisting of p-type polymers and n-type non-fullerene materials, which is passivated using nickel foils, GaIn eutectic, and layered double hydroxides as model materials. We achieve a photocurrent density of 15.1 mA cm-2 at 1.23 V vs. reversible hydrogen electrode (RHE) with an onset potential of 0.55 V vs. RHE and a record high half-cell solar-to-hydrogen conversion efficiency of 4.33% under AM 1.5 G solar simulated light. After conducting the stability test at 1.3 V vs. RHE for 10 h, 90% of the initial photocurrent density are retained, whereas the photoactive layer without passivation lost its activity within a few minutes.

Multiplexed In Vivo Imaging Using Size-Controlled Quantum Dots in the Second Near-Infrared Window.

PbS/CdS core/shell quantum dots (QDs) that emit at the second near-infrared (NIR-II, 1000-1700 nm) window are synthesized. The PbS seed size and CdS shell thicknesses are carefully controlled to produce bright and narrow fluorescence that are suitable for multiplexing. A polymer encapsulation yields polymer-encapsulated NIR-II QDs (PQDs), which provides the QDs with long-term fluorescence stability over a week in biological media. Exploiting the simple bioconjugation capability of PQDs, folic acids are conjugated to PQDs that can efficiently label folate receptor overexpressing cell lines. The PQDs afford multiplexed and nearly real-time longitudinal whole-body in vivo imaging. Two NIR-II QD probes are prepared: folic acid-conjugated PQDs (FA-PQDs) emitting at 1280 nm and unconjugated PQDs emitting at 1080 nm. The two PQDs are engineered to have compact and similar hydrodynamic sizes. A mixture of the folic acid-conjugated PQD and unconjugated PQDs is injected intravenously into a tumor-xenografted mouse, and the signals from them are monitored. This NIR-II whole-body imaging with the two PQDs provides precise evaluation of the active ligand-assisted tumor-targeting capability of the FA-PQD probe because the hydrodynamic size control of the two PQDs effectively eliminates effects from the size-dependent accumulations by permeations and retentions in tumors.

Nanoporous Films and Nanostructure Arrays Created by Selective Dissolution of Water-Soluble Materials.

Highly porous thin films and nanostructure arrays are created by a simple process of selective dissolution of a water-soluble material, Sr3Al2O6. Heteroepitaxial nanocomposite films with self-separated phases of a target material and Sr3Al2O6 are first prepared by physical vapor deposition. NiO, ZnO, and Ni1- x Mg x O are used as the target materials. Only the Sr3Al2O6 phase in each nanocomposite film is selectively dissolved by dipping the film in water for 30 s at room temperature. This gentle and fast method minimizes damage to the remaining target materials and side reactions that can generate impurity phases. The morphologies and dimensions of the pores and nanostructures are controlled by the relative wettability of the separated phases on the growth substrates. The supercapacitor properties of the porous NiO films are enhanced compared to plain NiO films. The method can also be used to prepare porous films or nanostructure arrays of other oxides, metals, chalcogenides, and nitrides, as well as films or nanostructures with single-crystalline, polycrystalline, or amorphous nature.

A RNA producing DNA hydrogel as a platform for a high performance RNA interference system.

RNA interference (RNAi) is a mechanism in which small interfering RNA (siRNA) silences a target gene. Herein, we describe a DNA hydrogel capable of producing siRNA and interfering with protein expression. This RNAi-exhibiting gel (termed I-gel for interfering gel) consists of a plasmid carrying the gene transcribing siRNA against the target mRNA as part of the gel scaffold. The RNAi efficiency of the I-gel has been confirmed by green fluorescent protein (GFP) expression assay and RNA production quantification. The plasmid stability in the I-gel results in an 8-times higher transcription efficiency than that of the free plasmid. We further applied the I-gel to live cells and confirmed its effect in interfering with the GFP expression. The I-gel shows higher RNAi effect than plasmids in free form or complexed with Lipofectamine. This nanoscale hydrogel, which is able to produce RNA in a cell, provides a platform technology for efficient RNAi system.

Cancer-Microenvironment-Sensitive Activatable Quantum Dot Probe in the Second Near-Infrared Window.

Recent technological advances have expanded fluorescence (FL) imaging into the second near-infrared region (NIR-II; wavelength = 1000-1700 nm), providing high spatial resolution through deep tissues. However, bright and compact fluorophores are rare in this region, and sophisticated control over NIR-II probes has not been fully achieved yet. Herein, we report an enzyme-activatable NIR-II probe that exhibits FL upon matrix metalloprotease activity in tumor microenvironment. Bright and stable PbS/CdS/ZnS core/shell/shell quantum dots (QDs) were synthesized as a model NIR-II fluorophore, and activatable modulators were attached to exploit photoexcited electron transfer (PET) quenching. The quasi type-II QD band alignment allowed rapid and effective FL modulations with the compact surface ligand modulator that contains methylene blue PET quencher. The modulator was optimized to afford full enzyme accessibility and high activation signal surge upon the enzyme activity. Using a colon cancer mouse model, the probe demonstrated selective FL activation at tumor sites with 3-fold signal enhancement in 10 min. Optical phantom experiments confirmed the advantages of the NIR-II probe over conventional dyes in the first near-infrared region.

Super-resolution visible photoactivated atomic force microscopy.

Imaging the intrinsic optical absorption properties of nanomaterials with optical microscopy (OM) is hindered by the optical diffraction limit and intrinsically poor sensitivity. Thus, expensive and destructive electron microscopy (EM) has been commonly used to examine the morphologies of nanostructures. Further, while nanoscale fluorescence OM has become crucial for investigating the morphologies and functions of intracellular specimens, this modality is not suitable for imaging optical absorption and requires the use of possibly undesirable exogenous fluorescent molecules for biological samples. Here we demonstrate super-resolution visible photoactivated atomic force microscopy (pAFM), which can sense intrinsic optical absorption with ~8 nm resolution. Thus, the resolution can be improved down to ~8 nm. This system can detect not only the first harmonic response, but also the higher harmonic response using the nonlinear effect. The thermoelastic effects induced by pulsed laser irradiation allow us to obtain visible pAFM images of single gold nanospheres, various nanowires, and biological cells, all with nanoscale resolution. Unlike expensive EM, the visible pAFM system can be simply implemented by adding an optical excitation sub-system to a commercial atomic force microscope.

"Smart" gold nanoparticles for photoacoustic imaging: an imaging contrast agent responsive to the cancer microenvironment and signal amplification via pH-induced aggregation.

'Smart' gold nanoparticles can respond to mild acidic environments, rapidly form aggregates, and shift the absorption to red and near-infrared. They were used as a photoacoustic imaging agent responsive to the cancer microenvironment, and have demonstrated the cancer-specific accumulation at the cellular level and an amplified signal which is twice higher than the control in vivo.

Mesenchymal Stem Cells Aggregate and Deliver Gold Nanoparticles to Tumors for Photothermal Therapy.

Gold nanoparticles (AuNPs) have been extensively studied for photothermal cancer therapy because AuNPs can generate heat upon near-infrared irradiation. However, improving their tumor-targeting efficiency and optimizing the nanoparticle size for maximizing the photothermal effect remain challenging. We demonstrate that mesenchymal stem cells (MSCs) can aggregate pH-sensitive gold nanoparticles (PSAuNPs) in mildly acidic endosomes, target tumors, and be used for photothermal therapy. These aggregated structures had a higher cellular retention in comparison to pH-insensitive, control AuNPs (cAuNPs), which is important for the cell-based delivery process. PSAuNP-laden MSCs (MSC-PSAuNPs) injected intravenously to tumor-bearing mice show a 37-fold higher tumor-targeting efficiency (5.6% of the injected dose) and 8.3 °C higher heat generation compared to injections of cAuNPs after irradiation, which results in a significantly enhanced anticancer effect.

DNA hydrogel delivery vehicle for light-triggered and synergistic cancer therapy.

A DNA hydrogel is reported as a delivery vehicle for gold nanorods and doxorubicin. The two photothermal and chemo cancer agents were co-loaded using electrostatic and DNA binding interactions, respectively. Light-triggered and highly synergistic combination cancer therapy was demonstrated in cellular and animal models.

Light-responsible DNA hydrogel-gold nanoparticle assembly for synergistic cancer therapy.

Assembled AuNPs in a DNA hydrogel (Dgel) showed strongly coupled plasmon modes, and the Dgel vehicle can co-load anticancer drugs such as doxorubicin (Dox) as a light-controlled releasing cargo by DNA intercalations. Upon laser excitation, local heat shock generation was accompanied by the release of Dox. A highly synergistic combination of thermo- and chemotherapy was demonstrated in cellular and animal models. Our Dgel vehicle can be fragmented after the excitation-induced heat generations, which subsequently causes the dispersion of the AuNPs. Our system may be less toxic because it uses small sizes of AuNPs, and the inherently biocompatible scaffold may reduce the long-term toxicity by rapid clearance.

Gold nanoparticle-mediated photothermal therapy: current status and future perspective.

Gold nanoparticles (AuNPs) are attractive photothermal agents for cancer therapy because they show efficient local heating upon excitation of surface plasmon oscillations. The strong absorption, efficient heat conversion, high photostability, inherent low toxicity and well-defined surface chemistry of AuNPs contribute to the growing interest in their photothermal therapy (PTT) applications. The facile tunability of gold nanostructures enables engineering of AuNPs for superior near-infrared photothermal efficacy and target selectivity, which guarantee efficient and deep tissue-penetrating PTT with mitigated concerns regarding side effects by nonspecific distributions. This article discusses the current research findings with representative near-infrared-active AuNPs, which include nanoshell, nanorod, nanocage, nanostar, nanopopcorn and nanoparticle assembly systems. AuNPs successfully demonstrate potential for use in PTT, but several hurdles to clinical applications remain, including long-term toxicity and a need for sophisticated control over biodistribution and clearance. Future research directions are discussed, especially regarding the clinical translation of AuNP photosensitizers.

Detection of pH-induced aggregation of "smart" gold nanoparticles with photothermal optical coherence tomography.

We report the feasibility of a novel contrast agent, namely "smart" gold nanoparticles (AuNPs), in the detection of cancer cells with photothermal optical coherence tomography (PT-OCT). "Smart" AuNPs form aggregation in low pH condition, which is typical for cancer cells, and this aggregation results in a shift of their absorption spectrum. A PT-OCT system was developed to detect this pH-induced aggregation by combining an OCT light source and a laser with 660 nm in wavelength for photothermal excitation. Optical detection of pH-induced aggregation was tested with solution samples at two different pH conditions. An increase in optical path length (OPL) variation was measured at mild acidic condition, while there was not much change at neutral condition. Detection of cancer cells was tested with cultured cell samples. HeLa and fibroblast cells, as cancer and normal cells respectively, were incubated with "smart" gold nanoparticles and measured with PT-OCT. An elevated OPL variation signal was detected with the HeLa cells while not much of a signal was detected with the fibroblast cells. With the novel optical property of "smart" AuNPs and high sensitivity of PT-OCT, this technique is promising for cancer cell detection.

pH-responsive gold nanoparticles-in-liposome hybrid nanostructures for enhanced systemic tumor delivery.

We report a pH-responsive gold nanoparticles-in-liposome hybrid nanostructure, which effectively combines the pH-responsive assembly and surface plasmon property changes of 'smart' gold nanoparticles and enhanced systemic circulation and tumor accumulation of the PEG-grafted liposomes.