The photothermal effect of metal nanoparticles embedded in polymeric materials can be used to effi ciently generate local heat for in situ thermally processing within an existing material. Fluorescent probes are employed as thermal sensors to allow dynamical measurement of the amplitude and rate of temperature change within the polymer matrix. The effi cacy of this technique is demonstrated in polymer nanocomposite samples with different morphological characteristics, namely nanofi brous mats and thin fi lm samples. For similarly thick materials and both types of sample morphology, average temperature increases on the order of ≈ 100s ° C are readily obtained with dilute nanoparticle concentrations under relatively low irradiation intensity. Thus, the in situ photothermal heating approach has great potential for controllably driving a multitude of thermal processes, such as triggering phase transitions, generating site-specifi c cross-linking, or initiating chemical reactions from within a material.

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In this work, the photothermal effect of gold nanorods (GNRs) is used to selectively process a collection of polymeric nanofi bers, for example, completely melting those fi bers lying along a chosen direction while leaving the remaining material largely unheated and unaffected. While melting is utilized as a proofof-principle to demonstrate this effect, such capability could be usefully utilized for any level of thermal processing. This ability to select only a subset of the sample, heat it dramatically and thus transform it in situ, is due to the innate specifi city of the plasmon-mediated photothermal effect of metal nanoparticles embedded within the nanofi bers. In particular, heating occurs only where 1) GNRs are present and 2) the wavelength and polarization of the light irradiation match the GNRs' surface plasmon. Although most studies of the photothermal properties of metal nanoparticles have been performed in solution, this specifi city is highlighted in solid environments where the particle placement can be non-uniform and particle orientation can be controlled, the latter point being the focus of this work. Thus the specifi city of the photothermal effect provides the ability to create and modify nanostructured materials by design, for instance driving phase-transitions, [ 1,2 ] cross-linking, or chemical reactions, or manipulating crystallinity in situ solely within specifi c subsets of a material, even if such subsets spatially overlap. One example of the unique potential power of the photothermal processing scheme, where the heat is generated from within the material, is uniformly irradiating an entire homogeneous sample, but using the polarization of the light to selectively "break the symmetry" and only couple heat into the sample along a single direction, even though GNRs are equally present in all fi bers. Here, electron microscopy is used to monitor changes in the morphology of nanofi brous materials under photothermal heating; in addition, a fl uorescence-based thermometer measures the average temperature in the sample interior, and fl uorescence polarization anisotropy is used to confi rm melting. By selective placement of the temperature-and viscosity-sensing fl uorophores, the absence of heating in sample regions with GNRs not aligned with the light fi eld linear polarization direction is confi rmed.In the photothermal process, visible light is absorbed by metal nanoparticles due to a resonance with the collective electron oscillation referred to as the surface plasmon. [ 3 ] Because of the absence or weakness of radiative relaxation mechanisms, the conversion of optical energy into heat via non-radiative electron relaxation [ 4,5 ] is the dominant process, resulting in strong photothermal properties. As previously demonstrated, [ 1,2 ] polymer nanofi bers and fi lms containing even low concentrations of spherical gold nanoparticles can be heated to a few hundred °C by irradiation with relatively low-intensity resonant light. Amongst the different types of metal nanostructures, anisotropic g...