ABSTRACT
The preparation of stable hypervalent metal complexes containing Ag(III) has historically been challenging due to their propensity for reduction under ambient conditions. This work explores the preparation of a tripotassium silver bisperiodate complex as a tetrahydrate via chemical oxidation of the central silver atom and orthoperiodate chelation. The isolation of the chelate complex in high yield and purity was achieved via acidimetric titration. The comprehensive physiochemical characterization of the tribasic silver bisperiodate included single crystal X-ray diffraction, thermogravimetric and differential scanning calorimetry, and infrared and ultraviolet-visible spectroscopy. Infrared and UV-visible absorption spectra (λmax 255 and 365 nm) were in good agreement with historically prepared pentabasic diperiodatoargentate chelate complexes. The C2/c monoclinic distorted square planar structure of the bis-chelate complex affords a mutually supportive framework to both Ag(III) and I(VII), conferring stability under both thermal and long-term ambient conditions. Thermal analysis of the tribasic silver bisperiodate complex identified an endothermic mass loss, ΔH = +278.35 kJ/mol, observed at 139.0 °C corresponding to a solid-state reduction of silver from Ag(III) to Ag(I). Under ambient conditions, no significant degradation was observed over a 12 month period (P = 0.30) for the silver bisperiodate complex in a solid state, with an observed half-life of τ1/2 = 147 days in a pH-neutral aqueous solution.
ABSTRACT
Topical semi-solid formulations are ubiquitous in personal care and pharmaceutical applications. For centuries, these topical formulations have facilitated delivery of active ingredients such as botanical oils, medicinal extracts and more recently antibiotics and biologics. Numerous strategies exist for the stabilization and release of these active ingredients from semi-solid formulations, namely, inclusion of anti-oxidants and surfactants to extend shelf life and facilitate delivery respectively. However, in the instance where the active ingredient itself is an oxidizing agent, traditional strategies for formulation have limited utility. Recent evidence has highlighted the exceptional efficacy and safety of highly oxidizing silver compounds, containing Ag2+ and Ag3+. These higher oxidation states of silver provide antimicrobial and antibiofilm activity without impairing healing. However, as strong oxidizing agents, their application in medical device and pharmaceutical formulations such as semi-solid formulations are limited. The present study reports on the development of a silicone-based gel formulation of silver oxynitrate (Ag7NO11), a higher oxidation state silver complex. In this study the chemical stability of silver oxynitrate was examined through solid state characterization with X-ray diffraction, formulation stability and microstructure of the semi-solid gel evaluated through various rheological techniques, therapeutic functionality of the semi-solid formulation investigated through in-vitro planktonic and biofilm antimicrobial studies, and biocompatibility assessed though in-vitro mammalian fibroblast and in-vivo porcine wound healing models. Enhanced stability of silver oxynitrate within the semi-solid formulation was observed over a four-month X-ray diffraction study. At the end of the study, silver oxynitrate was identified as the principal diffraction pattern in the semi-solid formulation where argentic oxide diffraction peaks were observed to be dominant in silver oxynitrate powders alone. Viscoelastic or gel-like behavior of the formulation was observed under dynamic rheological study where the storage modulus (G' = 1.77 ± 0.02 × 104 Pa) significantly exceeded the loss modulus (Gâ³ = 4.89 ± 3.72 × 102 Pa) (p < 0.0001). No significant (p = 0.84) change was observed in the apparent viscous response within the last three months of the study period indicative that the formulation approached a steady rheological state. The silver oxynitrate semi-solid formulation provided sustained in-vitro antimicrobial activity (>99.99% kill) over seven days with a significant reduction in biofilm within 6 h (p < 0.001). In-vitro mammalian fibroblast studies demonstrated the formulation to be non-cytotoxic and 100% epithelialization was observed within a six-day in-vivo porcine deep partial-thickness wound. The improved chemical stability, biocompatibility and efficacy results indicate that silicone gel semi-solid formulation may be a promising medicinal configuration to facilitate expansion of the clinical use of silver oxynitrate.
