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.

Europium-doped gadolinium oxide (Gd₂O₃: Eu) nanoparticles have been synthesized, and then their surfaces have been conjugated with nucleolintargeted AS1411 aptamer to form functionalized target-specific Gd₂O₃: Eu nanoparticles (A-GdO: Eu nanoparticles). The A-GdO: Eu nanoparticles present strong fluorescence in the visible range, high magnetic susceptibility, X-ray attenuation and good biocompatibility. The A-GdO: Eu nanoparticles have been applied to test molecular expression of nucleolin highly expressed CL1-5 lung cancer cells under a confocal microscope. Fluorescence imaging clearly reveals that the nanoparticles can be applied as fluorescent tags for cancer-targeting molecular imaging. Furthermore, taking together their excellent T₁ contrast and strong computed tomography (CT) signal, the A-GdO: Eu nanoparticles demonstrate a great capability for use as a dual modality contrast agent for CT and magnetic resonance (MR) molecular imaging. Animal experiments also show that the A-GdO: Eu nanoparticles are able to contrast the tissues of BALB/c mice using CT modality. Moreover, the obvious red fluorescence of A-GdO: Eu nanoparticles can be visualized in a tumor by the naked eye. Overall, our results demonstrate that the A-GdO: Eu nanoparticles can not only serve as new medical contrast agents but also as intraoperative fluorescence imaging probes for guided surgery in the near future.