Doctorate of Philosophy
Faculty of Science, Department of Chemistry
Lee, George Patrick, The chemical properties and characterisation of photomorphic silver nanoparticles and their application as an antimicrobial agent, Doctorate of Philosophy thesis, Faculty of Science, Department of Chemistry, University of Wollongong, 2010. http://ro.uow.edu.au/theses/3265
Spherical seed silver nanoparticles (AgNPs) have been photo-irradiated in the presence of trisodium citrate (TSC) and 4, 4’-(phenylphosphinidene) bis (benzenesulfonic acid) dipotassium salt (BSPP) to produce flat prismatic nanoparticles. Significantly, these nanoparticles were demonstrated to be capable of reversibly transforming from prisms to discs when subjected to light or dark conditions. For instance, the particles truncate from prisms to discs when placed in the dark, and switch back to prisms when returned to the light. Particles transformed to disc-shapes are referred to as dark-transformed AgNPs while the particles that are driven back to prism shapes are referred to as light-transformed AgNPs. The observed synthesis parameters of the shape-shifting AgNPs were investigated and characterised to reveal the unique mechanisms associated with the autoxidation and autoreduction process occurring in the absence and presence of light, respectively. The BSPP-Ag+ complex was observed to be responsible for the prism to disc transition driving the AgNP oxidation during dark exposure. The process was hypothesised to be a complex equilibrium between the light driven reduction and competitive reverse oxidation paths resulting in tip corner erosion, forming truncated prisms. No free silver ions (Ag+) were detectable by ion selective electrode studies below 0.1μM during dark transformation. This suggested that the BSPP remained conjugated to the Ag+ after oxidation, remaining absorbed to the surface of the AgNP as the BSPP- Ag+ complex.
During synthesis, AgNPs were observed to transform from the initial wetsynthesised spheres to prisms, over a number of hours, by photodevelopment using a 575nm fluorescent bulb light source. Characterisation of the synthesis products revealed two important factors that influenced the ability of the photomorphic AgNPs to shape-shift. First, the photomorphic effect was noted to have a limited window of opportunity in terms of photodevelopment time, in that solutions of AgNPs that were photodeveloped for more than eleven hours notably lost the ability to transform. Secondly, the optimum photodevelopment time to provide the maximum rate and magnitude for the photomorphic transformation was four hours. The photomorphic light-dark cycling of the particles was achieved up to eleven times, under the reported synthesis conditions. However, upon subsequent cycling, the magnitude of photomorphic transitions was noted to decrease with each repeat cycle. From these experiments, it was determined that the chemical species responsible for the oxidation of the tip corners was BSPP. The exposure of BSPP to light resulted in its auto-oxidation to triphenylphosphine oxide (TPPO), which was unable to complex with Ag+. Thus, when the AgNPs were being photodeveloped to prisms, concentrations of BSPP were also being oxidised to TPPO. By the end of the photodevelopment window the majority of BSPP had been consumed, thereby inhibiting further photomorphic transformations. Importantly, when fresh BSPP was added to the light passivated solutions, the photomorphic effect was restored to the AgNPs.
Research into the photomorphic shape-shifting process demonstrated that the reverse transformation of discs to prism shapes followed a distinct shape transformation evolution. The particles transformed from discs to hexagons to hexagons of increasing aspect ratios and then finally to prisms. Given that these particles demonstrated a systematic and predictable shape development, a growth mechanism was proposed. The addition of Ag ions (Ag+) was hypothesised to add to the re-entrant grooves of the AgNPs. These grooves are common amongst particles synthesised in the presence of trisodium citrate. Essentially, these grooves are defect sites on the nanoparticle faces where the crystal lattices spontaneously facet into different planes to minimise surface energies. This results in concave faces. This concave shape is capable of providing enough stabilisation by higher coordination of neighbouring molecules to add Ag+, which is enough to overcome colligative repulsive forces. This shape transformation has never been directly reported in the literature and demonstrates that the characterisation of the shape-shifting AgNPs is capable of providing further insight into fundamental nanoparticle theory.
The ability of these AgNPs to controllably release or collect Ag+ in solution was exploited in an antimicrobial application. Ag+ and Ag complexes have been known for decades to have a strong antimicrobial activity against bacteria, especially in the treatment of burns. The dark-transformed particles were demonstrated to be three times more potent against bacteria in comparison to the light-transformed prism-shaped particles. This increased antimicrobial potency between the light and dark states was attributed to the complexed BSPP- Ag+ complex, which was released by the oxidation of the prism shaped AgNPs during transformation to discs. The bacteria responded to the presence of the AgNPs, Ag+ and Ag+-BSPP complex by blistering, blebbing, fissuring and withering.