Ferrous ion as a reducing agent in the generation of antibiofilm nitric oxide from a copper-based catalytic system

The work found that the electron-donating properties of ferrous ions (Fe²⁺) can be used for the conversion of nitrite (NO₂^-) into the biofilm-dispersing signal nitric oxide (NO) by a copper(II) complex (CuDTTCT) catalyst, a potentially applicable biofilm control technology for the water industries. The availability of Fe²⁺ varied depending on the characteristics of the aqueous systems (phosphate- and carbonate-containing nitrifying bacteria growth medium, NBGM and phosphate buffered saline, PBS at pH 6 to 8, to simulate conditions typically present in the water industries) and was found to affect the production of NO from nitrite by CuDTTCT (casted into PVC). Greater amounts of NO were generated from the CuDTTCT-nitrite-Fe²⁺ systems in PBS compared to those in NBGM, which was associated with the reduced extent of Fe²⁺-to-Fe³⁺ autoxidation by the iron-precipitating moieties phosphates and carbonate in the former system. Further, acidic conditions at pH 6.0 were found to favor NO production from the catalytic system in both PBS and NBGM compared to neutral or basic pH (pH 7.0 or 8.0). Lower pH was shown to stabilize Fe²⁺ and reduce its autoxidation to Fe³⁺. These findings will be beneficial for the potential implementation of the NO-generating catalytic technology and indeed, a 'non-killing' biofilm dispersal activity of CuDTTCT-nitrite-Fe²⁺ was observed on nitrifying bacteria biofilms in PBS at pH 6.