We report a visual chronometric assay for Cr based on the CuO nanocube-mediated clock reaction of methylene blue (MB) and NH. MB with a blue color can be easily reduced by excess NH to form colorless leuco-MB in the presence of CuO nanocubes as a catalyst in a short time. However, the addition of Cr decreases the catalytic activity of CuO nanocubes owing to the coordination interactions between them, leading to an increase in the reaction time. The reaction time change was employed for selective detection of Cr. This assay has a wide dynamic range from 0.03 to 600 μM with a limit of detection of 2.7 nM. Remarkably, different concentrations of Cr can be directly detected with the naked eye by observing the color-fading speed. Additionally, this assay was successfully used to determine Cr in real water samples.

The laser-driven acceleration of proton beams from a double-layer cone target, comprised of a cone shaped high-Z material target with a low density proton layer, is investigated via two-dimensional fully relativistic electro-magnetic particle-in-cell simulations. The dependence of the inside diameter (ID) of the tip size of a double-layer cone target on proton beam characteristics is demonstrated. Our results show that the peak energy of proton beams significantly increases and the divergence angle decreases with decreasing ID size. This can be explained by the combined effects of a stronger laser field that is focused inside the cone target and a larger laser interaction area by reducing the ID size.

We use two-dimensional particle-in-cell simulations to investigate how the plasma density n0 of the channel target affects energetic-electron generation and transportation. The simulations show that the optimum plasma-density regime is 10 ≤ n0 ≤ 25 for the present simulation parameters, which results in a peak energy flux and coupling efficiency from laser to electrons. In this case, the laser beam propagates stably in the channel, which has the advantage of increasing the acceleration length and more effectively generating high-energy electrons. Furthermore, the high-current electron beam and the density modulation induce strong azimuthal magnetic fields and double-layer radial electric fields around the inner surface of the channel, which consistently guide and collimate the hot-electron bunch so that it propagates over rather long times and distances. Upon further increasing the plasma density n0, the hot electrons are scattered out of the channel by the damped laser pulse and the reduced quasistatic interface electromagnetic fields, reducing the long-time transport. The use of a proper plasma-density channel stably guides the relativistically intense laser pulse and greatly improves the properties of the electron beam.