The interest of encapsulating noble metal nanoparticles stems from applications related to their interesting optical properties, which are based on coherent oscillations of conduction electrons when irradiated with a suitable electromagnetic radiation. Such electron oscillations in nanoparticles are known as Localized Surface Plasmon Resonances or LSPRs. The corresponding resonance frequency can be tuned through the composition, size and shape of the nanoparticles, typically occurring (for gold, silver and copper) at the visible or near-IR spectral range. This gives rise to sharp and intense extinction bands at the LSPR frequency, but additionally originates high electric fields at the nanoparticles surface, which can notably affect the chemistry and spectroscopy of molecules located next to it.
One of the most widely studied effects is surface-enhanced Raman scattering (SERS), in which large enhancements of the Raman scattering signal are recorded when the molecules are adsorbed onto metal nanostructures. The requirement that the molecules are in close proximity to the metallic surface has restricted the applications of SERS as a general ultrasensitive technique, and therefore there is a need for the development of coating materials that can actively trap the analyte molecules and bring them close to the metallic nanostructure.
In this context, nanocomposite particle colloids comprising a metal nanoparticle within a polymer hydrogel shell can be seen as a suitable candidate for solving this problem, since they combine the photonic properties of the nanoparticle cores and the trapping ability of the smart microgel coating. Obviously, efficient fabrication of such hybrid colloids requires a precise control over the size and shape of the core particles, as a means to properly modulate the optical response of the system.
This can be achieved through advanced colloid chemistry methods, which have mostly been developed during the last couple of decades. Regarding the polymer shells, stimuli-responsive materials are particularly interesting because of their potential for external switching and manipulation. A common example is poly(N-isopropylacrylamide) (pNIPAM), a thermoresponsive polymer that undergoes a phase transition from a hydrophilic, water-swollen state to a hydrophobic, globular state when heated above its lower critical solution temperature (LCST), at around 32 ºC in water. Addition of co-monomers has been proposed to add responsiveness toward different stimuli such as temperature, pH, ionic strength or light.
We have recently developed a novel and efficient method to coat Cetyltrimethylammonium Bromide (CTAB)-capped gold nanoparticles with pNIPAM, involving initial coating with a thin polystyrene shell and subsequent emulsion polymerization of NIPAM monomers on the polystyrene-primed nanoparticles. The resulting core-shell structure was conclusively characterized through detailed TEM, AFM and UV-vis spectroscopy analysis. A temperature-driven, reversible swelling-deswelling transition was identified in the core-shell system, with a transition temperature similar to that of the pure microgel system, which can be easily monitored through (reversible) surface plasmon shifts.
Further growth of the metallic cores within the microgel leads to different morphologies as a function of CTAB concentration, which allows tuning the optical response and environmental sensitivity. All these results demonstrate the accessibility of the metal cores, which is crucial for applications such as catalysis or biosensing.
For example, the thermoresponsive behavior of the pNIPAM shell has been exploited to capture organic contaminants, which could be readily detected through surface enhanced Raman scattering (SERS) assisted by the plasmon resonance of the gold core. The operation of this sensor is illustrated in Figure 1 for the identification of naphthol in solution. Naphthol contains no functional groups that can chemically bind to metallic surfaces, but it can be trapped within the microgel network when collapsed above the LCST, thus reaching the central core and allowing us to record meaningful SERS spectra. Interestingly, the naphthol molecules get released when temperature is lowered and the microgel is swollen, so that we can say that the sensing element works in a reversible fashion.
Figure 1. SERS spectra recorded from an Au@pNIPAM colloid in contact with 10 µM 1-naphthol, at low (left), high (middle) and low temperature again (right), corresponding to swollen (left and right) and collapsed (middle) microgel, as shown in the cartoons. A high quality SERS spectrum can only be recorded in the collapsed state because the naphthol molecules are trapped next to the gold cores.
Additional advances in the design of these smart plasmonic sensors include:
- The encapsulation of gold nanorods and their in situ coating with silver , or
- The incorporation of magnetic functionality, through reduction of nickel on the surface of the gold cores  or
- Incorporation of small iron oxide nanoparticles within the same microgels .
All these strategies open new avenues toward the fabrication of miniaturized sensing devices for ultrasensitive identification of a wide variety of analytes.
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