The development of smart nanomaterials has attracted great attention in several fields like nanoscience, materials science, engineering and nanotechnology due to their unique response to external stimuli. Many of them are based on polymers that can exhibit great shape-changes when submitted to environmental modifications. Poly(N-isopropylacrylamide), PNIPAM, is such a thermo-responsive polymer. It is water soluble at room temperature, forming gels by cross-linking but undergoes a reversible coil-to-globule volume phase transition (VPT) at a lower critical solution temperature (LCST) close to 32 °C due to the dehydration and subsequent collapse of its chains into compact globules.
Hybrid nanocomposite microgels associating PNIPAM and gold nanoparticles (GNP) have been designed in order to take advantage of the outstanding plasmonic and photo-thermal properties of GNP to promote the volume phase transition of the microgels through an efficient photo-thermal conversion. With their strong diameter-dependent optical absorption in the near infrared (NIR) and their large surface area favoring photo-thermal transfer, semiconducting SWNT (s-SWNT) are good candidates for photo-thermal conversion in the NIR and may therefore be used to prepare multi-responsive hybrid microgels (Figure 1). However, to the best of our knowledge, no thorough studies of such nanomaterials have been reported so far.
Here we report the preparation of smart SWNT/PNIPAM nanocomposites through non-covalent functionalization techniques. These SWNT/PNIPAM hybrid microgels are stable in water and show a VPT, which can be promoted either by direct heating or by excitation of the resonant absorption of s-SWNT in the near infrared. Furthermore, the photoluminescence (PL) signal of s- SWNT is modulated at the phase transition and therefore, the PL signal can be used to monitor the VPT. This is illustrated in Figure 2, showing coupled Raman/PL measurements below and above the LCST, where a redshift of the PL bands is observed when crossing the LCST while the Raman signatures remain essentially the same.