Oral ENCAPSULATION GOLD NANOPARTICLES IN THERMORESPONSIVE MICROGELS. MOLECULAR TRAPS FOR SERS

Nanocomposite materials consisting of a colloidal metal nanoparticle within a synthetic polymer hydrogel shell have attracted great attention due to potential applications in several fields such as catalysis, photonics, electronics, optics and biomedicine. Within the polymeric nanoparticles the field concerning stimuli-responsive nanomaterials has been investigated intensively in the past years.[1] Among then one of the most commonly studied is the poly(N-isopropylacrylamide) (pNIPAM), that is, a thermoresponsive polymer that undergoes a phase transition from a hydrophilic, water-swollen state to hydrophobic, globular state when heated above its lower critical solution temperature (LCST) which is about 31-32oC in water. In this work we are proposing an easy two-step procedure to pNIPAM encapsulate cetyltrimethyl ammonium bromide (CTAB) stabilized metal nanoparticlesn;[2] the first step consisting of a CTAB promoted polystyrene coating of the metal nanoparticles in order to avoid aggregation and make them fully compatible with the precipitation polymerization of NIPAM in the second step. Figure 1 shows a representative TEM image of the Au-pNIPAM core-shell system. Further confirmation of the core-shell structure of the obtained nanocomposite is given by the UV-Vis analysis of the influence of the temperature on the system (see Figure 1). A closer look at the position of the surface plasmon band of the gold nanoparticles reveals that as the microgel collapses the band red shifts ca. 10nm. This effect should be interpreted as a consequence of a core-shell structure since an increase in the local refractive index of the gold nanoparticles. Control over the NIPAM shell thickness can be easily achieved by using appropriate amounts of NIPAM monomer. This allows us to control the thickness of the NIPAM shell, in the collapse state, between 20nm and 250 nm. Figure 2 shows representative TEM images of the gold nanoparticles with different thicknesses. The thermoresponsive properties of the systems is expected to allow us to control the catalytic properties of metal nanoparticles, as well as to control the interparticle distance in order to obtained two dimensional arrays. Recently, we have shown its applicability as molecular traps for surfaceenhanced, spectroscopic, ultra-sensitive analysis.[3]