Switch-on fluorescence sensor for ascorbic acid detection based on MoS2 quantum dots-MnO2 nanosheets system and its application in fruit samples

In this work,molybdenum disulfide quantum dots（MoS2 QDs） were firstly prepared by hydrothermal method using sodium molybdate and glutathione as precursors,and applied in ascorbic acid detection.When joining MnO2 nanosheets into MoS2 QDs solution,they produced an obvious fluorescence quenching,which should be due to inner filter effect（IFE）.Meanwhile,the fluorescent probe was formed,Interestingly,we found that this quenching phenomenon disappeared with the addition of ascorbic acid,In other words,the fluorescence gradually restored.This recovery phenomenon is mainly due to the reduction effect of ascorbic acid for MnO2 nanosheets.Under the optimum conditions,the limit of detection（LOD） of 39 nmol/L for ascorbic acid was achieved with a linear range of 0.33-5.00 μmol/L.The repeatability was better than 5.0% for ascorbic acid in both standard and fruit samples（n = 3）.Moreover,the as-fabricated fluorescent sensing system was successfully employed to detect the ascorbic acid levels in hawthorn and jujube with satisfactory results.

Simplified fabrication of high areal capacitance all-solid-state micro-supercapacitors based on graphene and MnO_2 nanosheets

All-solid-state micro-supercapacitors are acknowledged as a very promising class of microscale energy storage devices for directly integrating portable and wearable electronics. However, the improvement of electrochemical performance from materials to devices still remains tremendous challenges. Here, we demonstrate a novel and universal mask-assisted filtration technology for the simplified fabrication of all-solid-state planar micro-supercapacitors（MSCs） based on interdigital patterns of 2D pseudocapacitive MnO2 nanosheets and electrochemically exfoliated graphene film as both electrode and current collector, and polyvinyl alcohol/Li Cl gel as electrolyte. Remarkably, the resulting MSCs exhibit outstanding areal capacitance of ～355 m F/cm^2, which is among the highest values reported in the state-of-the-art MSCs. Meanwhile, MSCs possess exceptionally mechanical flexibility as high as ～92% of initial capacitance even at a highly bending angle of 180°, excellent cyclability with a capacitance retention of 95% after 3000 cycles, and impressive serial or parallel integration for modulating the voltage or capacitance. Therefore, our proposed strategy of simplified construction of MSCs will pave the ways for utilizing graphene and analogous pseudocapactive nanosheets in high-performance MSCs.

Flexible asymmetric supercapacitor based on MnO_2 honeycomb structure

A flexible asymmetric supercapacitor with high energy density was constructed by using a flexible substrate of carbonized silk-fabrics decorated with carbon nanotube, electroplating MnO2 nanosheets and dip-coating activated carbon powders as the positive and the negative electrodes, respectively. By controlling the electroplating time, the MnO2 nanosheets can be self-assembled to honeycomb structure and showed excellent electrochemical performance in 1 mol/L Na2SO4 electrolyte with SC950-EP30 performing the best. It exhibited a high specific capacitance（1110.85 F/g at a current density of 1 A/g based on the mass of MnO2） and superior rate capability（77.44% capacity retention from 1 A/g to 10 A/g）.Thus, the optimal asymmetric device assembled with this material as positive electrode can deliver a maximum energy density of 43.84 Wh/kg and a maximum power density of 6.62 kW/kg.