Reduction potentials (E(0)) of six mononuclear cobalt catalysts (1-6) for hydrogen evolution reaction and electron donating/withdrawing effect of nine X-substituents on their macrocyclic ligand are reported at solvation effect-included B3P86/6-311+G** level of density functional theory. The electrostatic potential at the Co nucleus (V(Co)) is found to be a powerful descriptor of the electronic effect experienced by Co from the ligand environment. The V(Co) values vary substantially with respect to the nature of macrocycle, type of apical ligands, nature of substituent and oxidation state of the metal center. Most importantly, V(Co) values of both the oxidized and reduced states of all the six complexes show strong linear correlation with E(0). The correlation plots between V(Co) and E(0) provide an easy-to-interpret graphical interpretation and quantification of the effect of ligand environment on the reduction potential. Further, on the basis of a correlation between the relative V(Co) and relative E(0) values of a catalyst with respect to the CF3-substituted reference system, the E(0) of any X-substituted 1-6 complexes is predicted.

A B3LYP density functional theory study on the oxidative addition of halogenobenzenes and toluene to monoligated zerovalent palladium catalysts (Pd–L) has been carried out using the “L” ligands such as phosphines, N-heterocyclic carbenes, alkynes, and alkenes. The electron deficiency of the undercoordinated Pd in Pd–L is quantified in terms of the molecular electrostatic potential at the metal center ( V Pd ), which showed significant variation with respect to the nature of the L ligand. Further, a strong linear correlation between Δ V Pd and the activation barrier ( E act ) of the reaction is established. The correlation plots between Δ V Pd and E act suggest that a priori prediction on the ability of the palladium complex to undergo oxidative addition is possible from V Pd analysis. In general, as the electron-donating nature of ligand increases, the suitability of Pd(0) catalyst to undergo oxidative addition increases. V Pd measures the electron-rich/-deficient nature of the metal center and provides a quantitative measure of the reactivity of the catalyst. By tuning the V Pd value, efficient catalysts can be designed.

pH sensing using active nanomaterials is promising in many fields ranging from chemical reactions to biochemistry, biomedicine, and environmental safety especially in the nanoscale. However, it is still challenging to achieve nanotechnology-enhanced rapid, sensitive, and quantitative pH detection with stable, biocompatible, and cost-effective materials. Here, we report a rational design of nitrogen-doped graphene quantum dot (NGQD)-based pH sensors by boosting the NGQD pH sensing properties via microplasma-enabled band-structure engineering. Effectively and economically, the emission-tunable NGQDs can be synthesized from earthabundant chitosan biomass precursor by controlling the microplasma chemistry under ambient conditions. Advanced spectroscopy measurements and density functional theory (DFT) calculations reveal that functionality-tuned NGQDs with enriched −OH functional groups and stable and large Stokes shift along the variations of pH value can achieve rapid, label-free, and ionic-stable pH sensing with a wide sensing range from pH 1.8 to 13.6. The underlying mechanism of pH sensing is related to the protonation/ deprotonation of −OH group of NGQDs, leading to the maximum pH-dependent luminescence peak shift along with the bandgap broadening or narrowing. In just 1 h, a single microplasma jet can produce a stable colloidal NGQD dispersion with 10 mg/mL concentration lasting for at least 100 pH detections, and the process is scalable. This approach is generic and opens new avenues for nanographene-based materials synthesis for applications in sensing, nanocatalysis, energy generation and conversion, quantum optoelectronics, bioimaging, and drug delivery.