Carbon Dot-Mediated Synthesis of Manganese Oxide Decorated Graphene Nanosheets for Supercapacitor Application

In this work, we demonstrate that carbon dots (CDs) can be used as a dispersing agent for graphene as well as a reducing agent for KMnO₄ for the synthesis of manganese oxide (MnO_<i>x</i>)–graphene hybrid nanocomposites for supercapacitor applications. CDs obtained from the pyrolysis of ammonium citrate under dry heating possess excellent solubility in water due to their oxygen- and nitrogen-containing functional groups. In addition, the sp²-carbon-rich CDs exhibited strong interaction with graphene through π–π stacking for self-immobilizing on graphene in the preparation of water-soluble CD/graphene nanocomposites (CDGs). Interestingly, MnO_<i>x</i> could be grown in situ on CDGs after reaction with KMnO₄ in aqueous solution under a mild reaction temperature (75 °C). Under the mild reaction conditions, CDs undergo sacrificial oxidation for the formation of MnO_<i>x</i> nanoparticles on graphene, whereas the graphene’s graphitic carbons are protected. The as-formed nanostructured MnO_<i>x</i> on CDGs (MnO_<i>x</i>–CDGs) was employed to fabricate flexible solid-state supercapacitor which exhibited good capacitance properties (specific capacitance ∼280 F g^–1) with very high charge–discharge cyclic stability (>10 000 cycles) and good capacitance retention at 90° bending angle. Compared to other graphene-based nanocomposites, our one-pot synthesis route for MnO_<i>x</i>–CDGs is relatively green, simple, rapid, and cost-effective and has a great potential for the synthesis of different metal oxide-decorated graphene nanocomposites for energy conversion and storage application

Self-Assembled Chiral Gold Supramolecules with Efficient Laser Absorption for Enantiospecific Recognition of Carnitine

Stereospecific recognition of chiral molecules is ubiquitous in chemical and biological systems, thus leading to strong demand for the development of enantiomeric drugs, enantioselective sensors, and asymmetric catalysts. In this study, we demonstrate the ratio of d-Cys and l-Cys playing an important role in determining the optical properties and the structures of self-assembled Cys–Au­(I) supramolecules prepared through a simple reaction of tetrachloroaurate­(III) with chiral cysteine (Cys). The irregularly shaped −[d-Cys–Au­(I)]_<i>n</i>– or – [l-Cys–Au­(I)]_<i>n</i>– supramolecules with a size larger than 500 nm possessing strong absorption in the near-UV region and chiroptical characteristics were only obtained from the reaction of Au­(III) with d-Cys or l-Cys. On the other hand, spindle-shaped −[d/l-Cys–Au­(I)]_<i>n</i>– supramolecules were formed when using Au­(III) with mixtures of d/l-Cys. Our results have suggested that Au­(I)···Au­(I) aurophilic interactions, and stacked hydrogen bonding and zwitterionic interactions between d/l-Cys ligands are important in determining their structures. The NaBH₄-mediated reduction induces the formation of photoluminescent gold nanoclusters (Au NCs) embedded in the chiral −[d-Cys–Au­(I)]_<i>n</i>– or −[l-Cys–Au­(I)]_<i>n</i>– supramolecules with a quantum yield of ca. 10%. The as-formed Au NCs/–[d-Cys–Au­(I)]_<i>n</i>– and Au NCs/–[l-Cys–Au­(I)]_<i>n</i>– are an enantiospecific substrate that can trap l-carnitine and d-carnitine, respectively, and function as a nanomatrix for surface-assisted laser desorption/ionization mass spectrometry (LDI-MS). The high absorption efficiency of laser energy, analyte-binding capacity, and homogeneity of the Au NCs/–[Cys–Au­(I)]_<i>n</i>– allow for quantitation of enantiomeric carnitine down to the micromolar regime with high reproducibility. The superior efficiency of the Au NCs/–[d-Cys–Au­(I)]_<i>n</i>– substrate has been further validated by quantification of l-carnitine in dietary supplements with accuracy and precision. Our study has opened a new avenue for chiral quantitation of various analytes through LDI-MS using metal nanocomposites consisting of NCs and metal–ligand complexes