Abstract
This work demonstrates the dynamic potential for tailoring the surface plasmon resonance (SPR), size, and shapes of gold nanoparticles (AuNPs) starting from an Au(I) precursor, chloro(dimethyl sulfide)gold (I) (Au(Me2S)Cl), in lieu of the conventional Au(III) precursor hydrogen tetrachloroaurate (III) hydrate (HAuCl4). Our approach presents a one-step method that permits regulation of an Au(I) precursor to form either visible-absorbing gold nanospheres or near-infrared-window (NIRW)-absorbing anisotropic AuNPs. A collection of shapes is obtained for the NIR-absorbing AuNPs herein, giving rise to spontaneously formed nanomosaic (NIR-absorbing anisotropic gold nanomosaic, NIRAuNM) without a dominant geometry for the tesserae elements that comprise the mosaic. Nonetheless, NIRAuNM exhibited high stability; one test sample remains stable with the same SPR absorption profile 7 years post-synthesis thus far. These NIRAuNM are generated within thermoresponsive poly(N-isopropylacrylamide) (PNIPAm) microgels, without the addition of any growth-assisting surfactants or reducing agents. Our directed-selection methodology is based on the photochemical reduction of a light-, heat-, and water-sensitive Au(I) precursor via a disproportionation mechanism. The NIRAuNM stabilized within the thermoresponsive microgels demonstrates a light-activated size decrease of the microgels. On irradiation with a NIR lamp source, the percent decrease in the size of the microgels loaded with NIRAuNM is at least five times greater compared to the control microgels. The concept of photothermal shrinkage of hybrid microgels is further demonstrated by the release of a model luminescent dye, as a drug release model. The absorbance and emission of the model dye released from the hybrid microgels are over an order of magnitude higher compared to the absorbance and emission of the dye released from the unloaded-control microgels.
Highlights
Stimuli-responsive polymers, hydrogels, and microgels are appealing to biomedical researchers as potential drug delivery candidates, based upon their ability to undergo phase transitions in response to various environmental factors
The schematics representing the in situ formation of both visible absorbing spherical AuNPs and NIR-absorbing anisotropic AuNPs within PNIPAm microgels are shown in Scheme 1
We deduced this hypothesis based on both the literature as well as our earlier AuNP work, which revealed that dimethyl sulfide (Me2S) is an ideal ligand for making nontoxic [55] gold nanoparticles, because of its inherent low toxicity and high volatility
Summary
Stimuli-responsive polymers, hydrogels, and microgels are appealing to biomedical researchers as potential drug delivery candidates, based upon their ability to undergo phase transitions in response to various environmental factors (e.g., temperature, light, pH, and/or ionic strength). Hydrophilicity, softness, flexibility, biocompatibility, and properties similar to biological tissues render such polymer hydrogel materials as suitable platforms for various biomedical applications [1,2,3,4,5,6,7]. To extend their functionality, numerous investigations have demonstrated the loading and/or stabilization of different nanomaterials such as metallic nanoparticles and quantum dots within these microgels [8]. AuNPs with anisotropic shapes (e.g., rods, triangles, cubes, truncated octahedra, shells, stars, etc.) with surface plasmon resonances (SPR) that extend beyond the visible region are desirable for sensing and plasmonic photothermal therapy (PPTT) applications [10,11,12]
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