Structure–reactivity relations for DC-magnetron sputtered Cu-layers during E. coli inactivation in the dark and under light

This study addresses unreported features for Cu DC-magnetron sputtering on cotton mediating inactivation of Escherichia coli K12 (from now on E. coli). In-depth profile for the different Cu-species inside the cotton fibers was determined by 5 keV Ar+ etching. Sputtering for 40-s deposited 4 × 1016 atoms Cu/cm2 (taking ∼1015 atoms/cm Cu per atomic layer) and this was the threshold amount of Cu necessary for complete bacterial inactivation. This is equivalent to a Cu-loading of 0.060% w/w or 3 nm/15 atomic layers. The inactivation of E. coli was attained within 30 min under visible light (1.2 mW/cm2) and within 120 min in the dark. XPS identified the Cu-species on the cotton as a function of the sputtering time. For a longer sputtering time of 180 s, the Cu-content was 0.294% w/w, but the bacterial inactivation kinetics under light was observed within 30 min, as was the case for the 40 s sputtered sample. This suggests that Cu-ionic species play a key role E. coli inactivation. The 40 s sputtered samples present the highest amount of Cu-sites held in exposed positions interacting on the cotton surface (or inside the cotton) with E. coli. Confocal microscopy shows the higher rugosity of Cu-cotton fibers compared to bare cotton fibers. The Cu-clusters were observed to be ∼50 nm after 40 s DC-magnetron sputtering and presented a wide size distribution. Cu DC-magnetron sputtering leads to thin metallic semi-transparent grey-brown Cu-coating presenting a moderate hydrophobic behavior as determined by contact angle measurements.