Page 55 - Annual Report_21-22

Development of antimicrobial polycationic, ultra-small silver nanoclusters

The evolving resistance of bacteria to common antibiotics nanoclusters (pAgNCs) and characterized them by various
has prompted urgent demand for alternative antibacterial spectroscopic and microscopic techniques (below Figure).
agents that not only control infections but are also able
to prevent the development of resistance. Nanoparticles,
such as silver, have attracted tremendous attention and
have been applied in many medical and consumer products
to protect from infections. However, recent studies
reveal that bacteria may be able to develop resistance
to silver nanoparticles (Ag NPs) after repeated exposure.
Moreover, most of the current silver nanomaterials may
not have sufficient potency against anaerobic bacteria,
such as those causing dental infections, due to the lack
of oxygen to dissolve Ago to Ag+. The next generation of
silver nanoantibiotics will also need to have the capacity
to eliminate biofilms which are notoriously difficult to
eradicate. To develop the next generation of highly potent
silver nanoantibiotics, that are needed in numerous
medical and everyday applications, a precise approach
should be developed that rationally nanoengineer the Figure: (a) UV-vis absorption spectra of pAgNCs with
most important features of Ag NPs such as size, structure, inset showing solution colour under ambient light.
and surface functionality. (b) Representative TEM micrographs of pAgNCs with
corresponding size distribution analysis (inset). (c) High
resolution XPS analysis of purified pAgNCs. (d) TGA
analysis of purified pAgNCs showing high percentage of
cationic ligands (chitosan) on the surface.

Importantly, these pAgNCs were designed to carry a
positively charged inner surface layer containing a high
percentage (>50%) of silver ions (Figure above (c)), which
we call a Ag - nanoreservoir. The presence of such kind of
+
Ag - nanoreservoir in the pAgNCs is a key feature allowing
+
very high potency (MIC down to 3 µg/ml) against various
Gram-negative and Gram-positive medically relevant
pathogens.
The unique design features of our pAgNCs made them
effective in destroying mature multispecies biofilms. The
Scheme: Schematic illustration of the ideal design of availability of plentiful Ag+ in the nanoreservoir reduces
antibacterial silver nanoclusters, followed by their the dependence on oxygen-rich environment (to reduce
mechanism of action. Ago to Ag+) for the activity of our pAgNCs. When tested in
anaerobic conditions against F. nucleatum and S. sanguinis,
our pAgNCs showed higher efficiency compared to similar-
An ideal cationic Ag NPs should have the following features: sized negatively charged AgNPs or antibiotics (Figure 2).
(1) polycationic surface layer for better interaction with Importantly, the unique features of pAgNCs were also
the bacterial membrane leading to disruption and/or critical in preventing the development of resistance by
higher intracellular uptake as well as to induce a strong bacteria even after 21 cycles of continuous culture.
interaction with DNA, (2) ultra-small size (~3 nm) to
achieve optimum antimicrobial performance, (3) high Ag - The outstanding antibacterial efficacy of the pAgNCs
+
content for rapid bacterial eradication, (4) high stability against both aerobic and anaerobic bacteria, capacity to
in solution and (5) low toxicity to the mammalian cells eradicate established biofilms with minimal toxicity to
(Scheme Above). mammalian cells, as well as difficulty for microorganism
to develop resistance over time, presents a valuable
Considering all the above mentioned factors, we set of properties that could be attractive for a range of
have developed such kind of polycationic ideal silver applications in the medical field and beyond.

ANNUAL REPORT 2021-22 53