In situ biosynthesis of bacterial cellulose hydrogel spheroids with tunable dimensions

Bacterial cellulose(BC)hydrogel spheroid plays a significant role in diverse fields due to its spatial 3D structure and properties.In the present work,a series of BC spheroids with controllable size and shape was obtained via an in situ biosynthesis.Crucial factors for fabricating BC spheroid in-cluding inoculum concentration of 1.35×10^(3)CFU/mL,shaking speeds at 100 r/min,and 48-96 h incubation time during the biosynthetic process,were comprehensively established.An operable mechanism model for tuning the size of BC spheroids from 0.4 to 5.0 mm was proposed with a fresh feeding medium strategy of dynamic culture.The resulting BC spheroids exhibit an inter-active 3D network of nanofibers,a crystallinity index of 72.3%,a specific surface area of 91.2 m^(2)/g,and good cytocompatibility.This study reinforces the understanding of BC spheroid forma-tion and explores new horizons for the design of BC spheroids-derived functional matrix materials for medical care.

The modulating effect of N coordination on single-atom catalysts researched by Pt-N_(x)-C model through both experimental study and DFT simulation

N-doped carbon-based single-atom catalysts(NC-SACs) are widely researched in various electrochemical reactions due to high metal atom utilization and catalytic activity.The catalytic activity of NC-SACs originates from the coordinating structure between single metal site(M) and the doped nitrogen(N) in carbon matrix by forming M-N_(x)-C structure(1≤x≤4).The M-N4-C structure is widely considered to be the most stable and effective catalytic site.However,there is no in-depth research for the "x" modulation in Pt-Nx-C structure and the corresponding catalytic properties.Herein,atomically dispersed Pt on N-doped carbon(Pt-NC) with Pt-Nx-C structure(1≤x≤4),as a research model,is fabricated by a ZIF-8 template and applied to catalytic oxygen reduction.Different carbonization temperatures are used to control N loss,and then modulate the N coordination of Pt-Nx-C structure.The Pt-NC has the predictable low half-wave potential(E_(1/2)) of 0.72 V vs RHE compared to the Pt/C 20% of 0.81 V due to low Pt content.Remarkably,the Pt-NC shows a high onset potential(1.10 V vs RHE,determined for j=-0.1 mA cm^(2)) and a high current density of 5.2 mA cm^(-2),more positive and higher than that of Pt/C 20%(0.96 V) and 4.9 mA cm^(-2),respectively.As the structural characterization and DFT simulation confirmed,the reducing PtN coordination number induces low valence of Pt atoms and low free energy of oxygen reduction,which is responsible for the improved catalytic activity.Furthermore,the Pt-NC shows high mass activity(172 times higher than that of Pt/C 20%),better stability and methanol crossover resistance.