Hybrid,
organic–inorganic, biocidal films exhibiting polishing
properties were developed as effective long-lasting antimicrobial
surface coatings. The films were prepared using cationically modified
chitosan, synthesized by the reaction with 3-bromo-N,N,N-trimethylpropan-1-aminium
bromide, to introduce permanent biocidal quaternary ammonium salt
(QAS) groups along the polymer backbone and were cross-linked by a
novel, pH-cleavable acetal cross-linker, which allowed polishing the
hybrid coatings with the solution pH. TiO2 nanoparticles,
modified with reduced graphene oxide (rGO) sheets, to narrow their
band gap energy value and shift their photocatalytic activity in the
visible light regime, were introduced within the polymer film to enhance
its antibacterial activity. The hybrid coatings exhibited an effective
biocidal activity in the dark (∼2 Log and ∼3 Log reduction
for Gram-negative and Gram-positive bacteria, respectively), when
only the QAS sites interacted with the bacteria membrane, and an excellent
biocidal action upon visible-light irradiation (∼5 Log and
∼6 Log reduction for Gram-negative and Gram-positive bacteria,
respectively) due to the synergistic antimicrobial effect of the QAS
moieties and the rGO-modified TiO2 nanoparticles. The gradual
decrease in the film thickness, upon immersion of the coatings in
mildly basic (pH 8), neutral (pH 7), and acidic (pH 6) media, reaching
10, 20, and 70% reduction, respectively, after 60 days of immersion
time, confirmed the polishing behavior of the films, whereas their
effective antimicrobial action was retained. The biocompatibility
of the hybrid films was verified in human cell culture studies. The
proposed approach enables the facile development of highly functional
coatings, combining biocompatibility and bactericidal action with
a “kill and self-clean” mechanism that allows the regeneration
of the outer surface of the coating leading to a strong and prolonged
antimicrobial action.