Cobalt oxide nanoparticle synthesis by cell surface-engineered recombinant Escherichia coli and the potential application on photocatalytic degradation of norfloxacin

• Cobalt peptides were displayed using YiaT. • The CF4 peptide fraction exhibited superior cobalt adsorption efficiency. • Adsorbed cobalt was calcinated to synthesize Co3O4 nanoparticles. • Co3O4 demonstrated efficient photocatalytic degradation of norfloxacin. Peptide-mediated biosorption offers a sustainable and selective approach for cobalt recovery, enabling the synthesis of functional Co 3 O 4 nanoparticles for environmental remediation. This research investigated the cobalt-binding affinity of the peptide CP2 and its derivative fractions, CF3 and CF4, displayed on the Escherichia coli cell surface via the outer membrane protein YiaT. Adsorption experiments showed that cells displaying CF4 exhibited the highest cobalt adsorption of 1865 ± 90 µmol/g dry cell weight (DCW) at 2 mM cobalt concentration, outperforming CP2 and CF3. Selectivity analysis at 1 mM revealed that both CF3 and CF4 preferentially adsorbed cobalt over nickel and manganese. Calcination of cobalt-adsorbed cells resulted in the formation of cobalt oxide (Co 3 O 4 ) nanoparticles, as confirmed by energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and UV-diffuse reflectance spectroscopy (UV–DRS). Further particle characterization using field-emission scanning electron microscopy (FE-SEM) and zeta particle size analysis revealed an average particle size of 104.5 nm and a zeta potential of –10.2 mV. The photocatalytic performance of Co 3 O 4 nanoparticles was evaluated for norfloxacin degradation, achieving 90% degradation at a catalyst dosage of 1 g/L, while pH-dependent studies at 0.8g/L identified pH 7 as the optimum condition, yielding a degradation efficiency of 86% within 90 min. These findings highlight the effectiveness of YiaT-mediated peptide display for selective cobalt biosorption and the potential of peptide-mediated Co 3 O 4 nanoparticles for photocatalytic environmental remediation.