Antibacterial activity of copper pyrazolate coordination polymers

Novel copper(ii) coordination polymers (CPs) [Cu(mu-4-NO2pz)2]n and [Cu(mu-4-NO2pz)2 center dot DMF H2O]n (4-NO2pz- = 4-nitropyrazolate; DMF = N,N '-dimethylformamide) were synthesized and characterized in the solid state and their thermal behaviour and structural features were investigated. Together with the known CPs [Cu(mu-pz)(mu-OH)]n, [Cu(mu-pz)2(H2O)]n and [Cu(mu-4-Xpz)2(DMF)]n [pz- = pyrazolate; 4-Xpz- = 4-X-pyrazolate, with X = H, Cl, Br, I], they were isolated in the form of very small particles whose dimensions range within 20-300 nm and, as such, they were investigated as potential antibacterial agents. Their antibacterial activity was assessed against two Gram-negative bacteria, namely Escherichia coli (E. coli) and Pseudomonas aeruginosa (P. aeruginosa), and the Gram-positive bacterium Staphylococcus aureus (S. aureus), by monitoring the bacterial growth curve and the antibacterial rate as a function of time. The bactericidal mechanism seems to be related to the ROS production, directly activated by the contact of the CP tiny particles with the bacterial membrane, without a significant release of Cu(ii) ions.Cu(ii) pyrazolate coordination polymers display antibacterial activity through a "contact kill" mechanism.