Morphologically Well-defined Gold Nanoparticles Embedded in Thermo-Responsive Hydrogel Matrices
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Morphologically Well-defined Gold Nanoparticles Embedded in Thermo-Responsive Hydrogel Matrices Chun Wang, Nolan T. Flynn, and Robert Langer* Department of Chemical Engineering, Massachusetts Institute of Technology, E25-342, Cambridge, MA 02139, U.S.A. *Corresponding author: [email protected] ABSTRACT Nanocomposite materials consisting of colloidal gold (Au) nanoparticles embedded in thermo-responsive poly(N-isopropylacrylamide) (PNIPAm) hydrogels are synthesized. Thiol groups that bind to both Au3+ ions and colloidal Au are incorporated into the side-chains of the PNIPAm hydrogels through copolymerization. This report describes formation of morphologically well-defined Au nanoparticles with varying long-term stability inside the hydrogel matrices containing adjustable concentrations of thiols. Compared with the non-Au containing PNIPAm hydrogels, the Au-PNIPAm nanocomposite hydrogels have shown higher degrees of equilibrium swelling and different temperature-triggered phase transitions. It is hypothesized that these remarkable changes in hydrogel bulk properties are related to the different morphologies and sizes, and possibly the amount of surface charges, of the Au nanoparticles. INTRODUCTION One approach to synthesizing inorganic nanoparticles with potentially well-defined size and morphology is by using polymeric templates such as porous silica [1] block copolymers [2], dendrimers [3], lyotropic liquid crystals [4], and non-aqueous polymer resins [5] and gels [6]. For biomedical applications, templates based on three-dimensional synthetic hydrogels are attractive alternatives to non-aqueous systems with the advantage of being compatible with biological molecules, cells, and tissues [7]. Controlled formation of nanoparticles has been attempted by using surfactant/polyelectrolyte gel complexes [8] or microgels with varying crosslink degrees [9]. It has not been reported, however, that defined nanoparticle structures can be achieved through tailoring the chemical functionality of the synthetic hydrogel side-chains. Hydrogels based on PNIPAm are a well-known class of thermo-responsive materials that undergo reversible phase transitions at a specific temperature defined as the lower critical solution temperature or LCST [10]. In recent years, various composite materials consisting of colloidal nanoparticles, such as palladium (Pd) or Au, embedded in PNIPAm hydrogels were synthesized and used as novel catalysts [11], biosensors [12], switchable electronics [13], and optically triggered drug delivery devices [14]. The bulk properties of these reported nanocomposite hydrogels, such as phase transition behavior, remained the same as the non-Au containing PNIPAm hydrogels, and were not affected by the incorporation of nanoparticles. In this report, Au nanoparticles are synthesized in situ PNIPAm hydrogels containing sidechains terminated with thiol groups. UV-Visible light spectroscopy and Scanning-Transmission Electron Microscopy (S-TEM) are used to characterize the morphology of the Au nanoparticles. The bulk pr
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