- Article type
High gravimetric energy density, earth-abundance, and environmental friendliness of hydrogen sources have inspired the utilization of hydrogen fuel as a clean alternative to fossil fuels. Hydrogen evolution reaction (HER), a half reaction of water splitting, is crucial to the low-cost production of pure H2 fuels but necessitates the use of electrocatalysts to expedite reaction kinetics. Owing to the availability of low-cost oxygen evolution reaction (OER) catalysts for the counter electrode in alkaline media and the lack of low-cost OER catalysts in acidic media, researchers have focused on developing HER catalysts in alkaline media with high activity and stability. Nickel is well-known as an HER catalyst and continuous efforts have been undertaken to improve Ni-based catalysts as alkaline electrolyzers. In this review, we summarize earlier studies of HER activity and mechanism on Ni surfaces, along with recent progress in the optimization of the Ni-based catalysts using various modern techniques. Recently developed Ni-based HER catalysts are categorized according to their chemical nature, and the advantages as well as limitations of each category are discussed. Among all Ni-based catalysts, Ni-based alloys and Ni-based hetero-structure exhibit the most promising electrocatalytic activity and stability owing to the fine-tuning of their surface adsorption properties via a synergistic nearby element or domain. Finally, selected applications of the developed Ni-based HER catalysts are highlighted, such as water splitting, the chloralkali process, and microbial electrolysis cell.
Fluorescence imaging is capable of acquiring anatomical and functional information with high spatial and temporal resolution. This imaging technique has been indispensable in biological research and disease detection/diagnosis. Imaging in the visible and to a lesser degree, in the near-infrared (NIR) regions below 900 nm, suffers from autofluorescence arising from endogenous fluorescent molecules in biological tissues. This autofluorescence interferes with fluorescent molecules of interest, causing a high background and low detection sensitivity. Here, we report that fluorescence imaging in the 1, 500–1, 700-nm region (termed "NIR-Ⅱb") under 808-nm excitation results in nearly zero tissue autofluorescence, allowing for background-free imaging of fluorescent species in otherwise notoriously autofluorescent biological tissues, including liver. Imaging of the intrinsic fluorescence of individual fluorophores, such as a single carbon nanotube, can be readily achieved with high sensitivity and without autofluorescence background in mouse liver within the 1, 500–1, 700-nm wavelength region.
Oxygen evolution reaction (OER) electrolysis, as an important reaction involved in water splitting and rechargeable metal-air batteries, has attracted increasing attention for clean energy generation and efficient energy storage. Nickel/iron (NiFe)-based compounds have been known as active OER catalysts since the last century, and renewed interest has been witnessed in recent years on developing advanced NiFe-based materials for better activity and stability. In this review, we present the early discovery and recent progress on NiFe-based OER electrocatalysts in terms of chemical properties, synthetic methodologies and catalytic performances. The advantages and disadvantages of each class of NiFe-based compounds are summarized, including NiFe alloys, electrodeposited films and layered double hydroxide nanoplates. Some mechanistic studies of the active phase of NiFe-based compounds are introduced and discussed to give insight into the nature of active catalytic sites, which could facilitate further improving NiFe based OER electrocatalysts. Finally, some applications of NiFe-based compounds for OER are described, including the development of an electrolyzer operating with a single AAA battery with voltage below 1.5 V and high performance rechargeable Zn-air batteries.
Protein microarrays based on fluorescence detection have been widely utilized for high-throughput functional proteomic analysis. However, a drawback of such assays has been low sensitivity and narrow dynamic range, limiting their capabilities, especially for detecting low abundance biological molecules such as cytokines in human samples. Here, we present fluorescence-enhancing microarrays on plasmonic gold films for multiplexed cytokine detection with up to three orders of magnitude higher sensitivity than on conventional0020nitrocellulose and glass substrates. Cytokine detection on the gold plasmonic substrate is about one to two orders of magnitude more sensitive than enzyme-linked immunosorbent assay (ELISA) and can be multiplexed. A panel of six cytokines (Vascular endothelial growth factor (VEGF), Interleukin 1β (IL-1β), Interleukin 4 (IL-4), Interleukin 6 (IL-6), Interferon γ (IFN-γ), and Tumor necrosis factor (TNF)) were detected in the culture media of cancer cells. This work establishes a new method of high throughput multiplexed cytokine detection with higher sensitivity and dynamic range than ELISA.
Manganese oxides are cost-effective and green materials with rich electrochemical properties. Continuous research efforts have been undertaken to obtain MnOx materials with improved activity and stability for catalyzing the oxygen reduction reaction (ORR). Here, we have developed a novel ORR catalyst by nucleation and growth of Mn3O4 nanoparticles on graphene oxide (GO) sheets interconnected by electrically conducting multi-walled carbon nanotubes (MWCNTs). X-ray near edge absorption structure (XANES) spectroscopy revealed the partially reduced nature of GO and strong chemical coupling between the nanoparticles and the GO sheets. Incorporation of MWCNTs was found to improve the activity and stability of the hybrid by imparting higher conductivity to the hybrid material. Furthermore, surface oxidation of the manganese oxide nanoparticles through a calcination step was found to increase the density of ORR active sites. The strongly coupled and electrically interconnected Mn3O4/nanocarbon (Mn3O4/Nano-C) hybrid is one of the most active and stable manganese oxide-based ORR catalysts and shows promise for electrochemical energy conversion applications.
We demonstrate the fabrication of high-density aligned graphene nanoribbon (GNR) arrays by plasma etching of graphene sheets through a nanomask derived from self-assembled poly (styrene-block-dimethylsiloxane) (PS–PDMS) diblock copolymer films. This approach produces parallel GNR (~12 nm wide) arrays at ~35 nm pitch. Microscopy and polarized Raman spectroscopy are used to reveal the high-degree of alignment of GNRs. Electrical measurements show that parallel GNRs in a 1 μm wide region can deliver ~0.38 mA current at a source–drain bias of 1 V. This novel patterning approach allows for the fabrication of densely aligned GNR arrays on various substrates and could provide a route to large scale integration of GNRs into nanoelectronics, optoelectronics and biosensors.
Supercapacitors operating in aqueous solutions are low cost energy storage devices with high cycling stability and fast charging and discharging capabilities, but generally suffer from low energy densities. Here, we grow Ni(OH)2 nanoplates and RuO2 nanoparticles on high quality graphene sheets in order to maximize the specific capacitances of these materials. We then pair up a Ni(OH)2/graphene electrode with a RuO2/graphene electrode to afford a high performance asymmetrical supercapacitor with high energy and power density operating in aqueous solutions at a voltage of ~1.5 V. The asymmetrical supercapacitor exhibits significantly higher energy densities than symmetrical RuO2–RuO2 supercapacitors or asymmetrical supercapacitors based on either RuO2–carbon or Ni(OH)2–carbon electrode pairs. A high energy density of ~48 W·h/kg at a power density of ~0.23 kW/kg, and a high power density of ~21 kW/kg at an energy density of ~14 W·h/kg have been achieved with our Ni(OH)2/graphene and RuO2/graphene asymmetrical supercapacitor. Thus, pairing up metal-oxide/graphene and metal-hydroxide/graphene hybrid materials for asymmetrical supercapacitors represents a new approach to high performance energy storage.
Short single-walled carbon nanotubes (SWNTs) functionalized by PEGylated phospholipids are biologically non-toxic and long-circulating nanomaterials with intrinsic near infrared photoluminescence (NIR PL), characteristic Raman spectra, and strong optical absorbance in the near infrared (NIR). This work demonstrates the first dual application of intravenously injected SWNTs as photoluminescent agents for in vivo tumor imaging in the 1.0-1.4 μm emission region and as NIR absorbers and heaters at 808 nm for photothermal tumor elimination at the lowest injected dose (70 μg of SWNT/mouse, equivalent to 3.6 mg/kg) and laser irradiation power (0.6 W/cm2) reported to date. Ex vivo resonance Raman imaging revealed the SWNT distribution within tumors at a high spatial resolution. Complete tumor elimination was achieved for large numbers of photothermally treated mice without any toxic side effects after more than six months post-treatment. Further, side-by-side experiments were carried out to compare the performance of SWNTs and gold nanorods (AuNRs) at an injected dose of 700 μg of AuNR/mouse (equivalent to 35 mg/kg) in NIR photothermal ablation of tumors in vivo. Highly effective tumor elimination with SWNTs was achieved at 10 times lower injected doses and lower irradiation powers than for AuNRs. These results suggest there are significant benefits of utilizing the intrinsic properties of biocompatible SWNTs for combined cancer imaging and therapy.
A graphene/TiO2 nanocrystals hybrid has been successfully prepared by directly growing TiO2 nanocrystals on graphene oxide (GO) sheets. The direct growth of the nanocrystals on GO sheets was achieved by a two-step method, in which TiO2 was first coated on GO sheets by hydrolysis and crystallized into anatase nanocrystals by hydrothermal treatment in the second step. Slow hydrolysis induced by the use of EtOH/H2O mixed solvent and addition of H2SO4 facilitates the selective growth of TiO2 on GO and suppresses growth of free TiO2 in solution. The method offers easy access to the GO/TiO2 nanocrystals hybrid with a uniform coating and strong interactions between TiO2 and the underlying GO sheets. The strong coupling gives advanced hybrid materials with various applications including photocatalysis. The prepared graphene/TiO2 nanocrystals hybrid has superior photocatalytic activity to other TiO2 materials in the degradation of rhodamine B, showing an impressive three-fold photocatalytic enhancement over P25. It is expected that the hybrid material could also be promising for various other applications including lithium ion batteries, where strong electrical coupling to TiO2 nanoparticles is essential.