Reversible exchange of stable nitroxyl radicals on nanosilver particles
Mark A. Chappell A B , Lesley F. Miller A and Cynthia L. Price AA Environmental Laboratory, US Army Engineer Research & Development Center, 3909 Halls Ferry Road, Vicksburg, MS 39180, USA.
B Corresponding author. Email: mark.a.chappell@usace.army.mil
Environmental Chemistry 12(2) 198-203 https://doi.org/10.1071/EN14093
Submitted: 1 May 2014 Accepted: 16 September 2014 Published: 18 March 2015
Environmental context. Nanometre-sized silver particles promote unique chemical reactions on their surface. This work examines the ability of silver nanoparticles to collect and store unpaired electrons, called radicals, on their surface. This capability by silver nanoparticles could potentially serve to drive degradation reactions in the environment.
Abstract. Radicals drive important chemical reactions in the environment. These unpaired electron species can be generated by energetic inputs, such as electromagnetic radiation, or from ultrasonication processes, whereby oxygen radicals are generated in aqueous solution through a cavitation mechanism. Previous evidence has demonstrated the potential for radicals to be stored on the surface of metallic gold nanoparticles, thus suggesting a potential transference of radical species from the nanoparticle surface for catalytic reactions, particularly during preparations of nanoparticle suspensions through ultrasonication. This work investigates the potential for the nanosilver (nAg) particles to similarly scavenge radicals from solution. nAg suspensions were reacted with 0.3-mM solutions containing the stable nitroxy radicals 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), 4-oxo-2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPONE) and 4-amino-2,2,6,6-tetramethylpiperidino-1-oxyl (TEMPAMINE) analysed by quantitative electron spin resonance (ESR) spectroscopy. In ambient air, the addition of a nAg suspension to the 0.3-mM solutions reduced the integrated ESR intensity of the stable radicals by 50–93 % depending on radical species and nAg concentration, which we attributed to the sorption of the radicals onto the nAg surface. In separate experiments, the ESR intensities were further decreased under an Ar atmosphere, suggesting potential competition from ambient OH• to the sorption of the stable radicals. To verify this, we observed substantial increases in the integrated ESR intensity when the systems previously equilibrated under Ar atmosphere were exposed to ambient air. These results demonstrated that nAg scavenged the stable radicals from solution and were exchangeable from the metallic conduction band with OH•. Our work represents the first evidence for this mechanism to be demonstrated for nAg.
Additional keywords: electron spin resonance, stable radicals, ultrasonication.
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