Microbe-engineered nanocatalysts for thermal, photo, and electrocatalytic green transformations: a critical review of structure-activity relationships and performance metrics

Microbe-engineered nanocatalysts offer a sustainable and adaptable platform for green chemical transformations in thermal, photocatalytic, and electrocatalytic systems. Biological synthesis techniques using bacteria, fungi, yeast, and algae enable the controlled formation of nanoparticles via enzymatic reduction, respiratory metal transformation, and photosynthetic redox pathways. However, previous reviews have mostly emphasised biological synthesis pathways, with limited quantitative integration of structure-activity connections and cross-platform catalytic performance measurements. These biogenic nanocatalysts typically have particle sizes ranging from 2 to 100 nm, high defect densities, variable surface functionalisation, and organic capping layers to improve colloidal stability and catalytic selectivity. Quantitative study demonstrates activation energy reductions of 20-60%, turnover frequencies of 0.1-10 s^-1, and overpotentials as low as 200-500 mV in electrocatalytic systems. Environmental applications exhibit removal efficiency of over 90% for dyes, heavy metals, and pharmaceutical micropollutants. Energy applications show hydrogen evolution rates of up to 10 mmol g^-1 h^-1 and CO₂ conversion selectivity of up to 90%. Despite these performance benchmarks, the field faces significant hurdles in batch-to-batch reproducibility and standardized reporting of metal-normalized activity. Structure-activity relationships suggest that nanoscale defect engineering, active site density, and surface biomolecule interactions all have a significant impact on catalytic performance. This review combines quantitative measures from synthesis, structural characterisation, catalytic mechanisms, and environmental deployment to provide a cohesive framework for rationally designing next-generation bioinspired nanocatalysts for sustainable chemical processes.