Optical properties of metallic nanoparticles are mainly dominated by the Localized Surface Plasmon Resonance (LSPR) at optical frequencies. At this specific frequency, nano-particles exhibit intense absorption and light scattering attribute to the LSPR. Moreover, metal nanoparticles are able to transform absorbed light into heat. This photothermal conversion effect has been used for many applications. In particular, it has been used in the development of photothermal therapies (hyperthermia) for cancer treatment [1, 2], as well as laser ablation as a means of malignant cell destruction [3]. More recently, plasmonic heating has been used to promote drug release through polymer phase change [4], hybrid DNA cleavage [5], or enhanced diffusion [6].

One of the challenges is to control the temperature in the vicinity of nanoparticles. In order to address this issue, a method called fluorescence polarization anisotropy, is developed and used to measure the temperature in the near field of a metal nanoparticle. Here, we propose a new approach based on nanopolymerization thermally initiated to characterize the heat pattern in the vicinity of photoexcited gold nanoparticles (GNPs). For this we developed thermopolymerizable formulations that are characterized by a controlled threshold temperature (T_th) of polymerization (fig. 1-a). Below this threshold temperature the polymerization can not occur. As shown in fig 1-b, the T_th of polymerization could be controlled by simply adjusting the weight percentage of the initiator system. This sharp control allows us to use such material as thermal nanoprobe to measure the local temperature around the particle as well as to print the heat distribution around the nanoparticule.

Figure 1-a) Illustration of our approach of nanopolymerization induced by photothermal conversion; b) Variation of the threshold temperature of polymerization as a function of the percentage weight of the initiator system

The first experiments have been done using GNPs dispersed in a polymerizable solution characterized by a threshold temperature of 36°C. Different GNP sizes were studied in order to probe the sensitivity of our polymer and thus to validate our approach. As shown in fig. 2, the polymerization time of the solution decreases as well as the maximum of plasmon resonance approaches the wavelength of the laser used for the photothermal excitation (654 nm).

Figure 2: polymerization induced by photothermal conversion during 654 nm excitation of GNPs.

In parallel, we studied the Plasmon heating of gold nanoroads and nanospheres in solution at different wavelengths. Experiments have been done using GNPs dispersed in water and in our polymerisable solution. The measurements showed that the thermal heating spectrum is very close to the absorption spectrum of the GNPs as shown in fig 3. Experiments are underway in order to get both the absorption and the thermal signature of single gold nanoparticle at different wavelengths. New insights into the local plasmon heating in the vicinity of a single GNP are expected using our approach.

Figure 3- Absorption and plasmonic heating spectra of metallic nanoparticles dispersed in water (a) and polymerizable solution (b).

REFERENCES

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