Radiofrequency heating of nanoclusters and nanoparticles for enzyme activation
Radiofrequency (RF) fields heat magnetic nanoparticles. Applications include remote drug delivery and noninvasive cancer therapy. The underlying objective is to increase temperatures precisely where desired without heating the surroundings. Before our work, the replicable limit of this precision has been in the micrometer range. To investigate the lower limit of this concept of local heat, we explored the RF remote activation of a thermophilic enzyme, thermolysin, covalently attached to gold coated iron oxide nanoparticles (6.1 nm in core diameter). Thermolysin activity was a function of applied radiofrequency power, independent of bulk solution temperature. Besides iron oxides, many other inorganic nanomaterials have been investigated for RF heating. One of the more interesting and controversial material choices is gold nanoparticles, primarily investigated for cancer treatment. Early reports of gold nanoparticle heating conflated the heating of supporting electrolytes with the heating of nanoparticles. Multiple mechanisms were proposed to account for the observed heat of gold nanoparticle suspensions. There was no consensus among specialists on how gold nanoparticles heat, if they heat at all. To investigate this, a critical review of the RF heating of gold nanoparticles field was performed with new experimental results presented to elucidate the confounding results. Following this, Au25(SR)18 gold nanoclusters were used as a model system to investigate the controversial heating rates reported for gold nanoparticles. For the first time the observed rates were explained entirely with a single mechanism: electrophoretic heating. The success of this reconciliation of theory and experiment considered the screened charge of the particle in addition to the contribution of the counterions. To enable deconvolution of possible contributions to gold nanoparticle heating by cluster decomposition products, an investigation of the practical stability of gold nanoclusters was conducted. It was found that organo-soluble particles with large ligands possessed over one month long lifetime in air-free non-polar solvents.
enzyme, radiofrequency heating, nanoparticles, conjugate
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