700
800 900
Viscosity (cP)
30°C 35°C 44°C
Figure 6.11: Viscosity as function of the transmembrane voltage on the GUV at 30◦C, 35◦C and 44◦C calculated from the fluorescence lifetime (Figure6.10) using the calibration (Figure5.4) and eq. 6.9.
0 1e-22 2e-22 3e-22 4e-22 Electrical energy (J.lipid-1)
600 700 800 900 1000
Viscosity (cP)
30°C 35°C 44°C
Figure 6.12: Viscosity as function of the electrical energy on the bilayer. In the gel phase (T<41◦C), the viscosity depends linearly on the electrical energy. For compar-ison, the energy difference between the gel and liquid phase of the bilayer is about 5 10−20 J per lipid.
6.6 Conclusion
To conclusion, the voltage-induced phase transition has been investigated on Giant Unilamellar Vesicles by FLIM measurements. The method is validated by the recovering of a phase transition temperature compatible with the 41◦C tabulated in the literature. Fluorescence anisotropy and photoselection exper-iments exploiting the spherical geometry of the vesicles demonstrated that the BODIPY embedded in the bilayer lie oriented horizontally with regard to the bilayer. The fluorescence lifetime of the embedded fluorophore in the bilayer has been measured with transmembrane voltage from 0 to 140 mV, those mea-surements reveal the voltage can decrease the viscosity of the bilayer by a 23
% while heating the system from 35◦C to 44◦C decreases its viscosity by only 15 %. The structure of the membrane is thus altered but to understand the change of structure, further investigations are required. Molecular Dynamics have proven to be able to reproduce structural changes of bilayer in the case of electroporation [127,155,151] and could be used to investigate the effect of a smaller electric field as the one applied here and even compute the viscosity variations.
Conclusion
The introduction of nanotechnologies in biology promises more localized and effective medical applications with fewer side effects and that might cure lethal diseases. A large part of these applications take root in the thermal transfer at the nanoscale. However, despite the extensive studies performed worldwide, most of the heat relaxation properties of nanoscale biological materials remain incompletely addressed. There are two types of nanoscale systems of biological interests:
• nanoscale objects designed to interact with biological materials.
• biological materials themselves.
In this thesis, the case of a nanoparticle designed for local plasmonic hyperther-mia has been addressed from a numerical point of view. This object is among the best candidates for local hyperthermia applications. A microscopic analysis of the heat transfer at the solid/liquid interface has been proposed in this work with a vibrational analysis and transmission calculations concluding to that polymer grafting reduces the overall solid/liquid resistance by creating addi-tional relaxation channels. The largest part of the work presented here concerns lipid bilayers that are the main constituent of cell membranes. Such material is thus widely studied but heat relaxation problems remain un-addressed such as the voltage-induced phase transition that has been investigated here. A nu-merical study of the electrical capacity and infrared absorption of this system has been proposed here showing that the absorption spectra do not depends on the bilayer structure but the localisation of the absorption is strongly impacted by the phase: it occurs mostly in the heads of the lipids in the a gel phase, while in a liquid phase, the absorption is more homogeneous. To show a voltage-induced phase transition, fluorescence lifetime imaging microscopy experiments have been conducted on two lipid systems: suspended lipid bilayers and giant unilamellar vesicles. In the first case, the experimental results show that the suspended bilayer’s viscosity increases with the temperature, and specially at the phase transition temperature at about 310 K. This unexpected result can be explained by solvent molecules localised in the bilayer being shooed due to the increase of the interdigitation with the temperature. This hypothesis was assessed using Molecular Dynamics simulations. The numerical model al-lows to compute the variation of the concentration of the solvent in the bilayer and the results are in agreement with the experimental measurements. The
suited systems found for the investigation of the voltage-induced phase transi-tion are giant unilamellar vesicles. The spherical geometry of this system allows performing fluorescence anisotropy measurements that demonstrated that the fluorophore embedded in the membrane lie horizontally oriented with regard to the bilayer. The measure of the viscosity as function of the temperature and voltage were performed using fluorescence lifetime imaging microscopy. A vari-ation of the viscosity by 15 % is found at the tabulated transition temperature of the bilayer and an even larger variation of 23 % is found with 140 mV of transmembrane voltage. However a sharp variation of the viscosity with the voltage is not found thus the measures reveal a structure change in the bilayer but not a first order phase transition and further calculations and experiments are required to conclude.
The work that have been presented in this thesis reveals the complexity of thermal relaxation in biological nanosystems that involves heat conduction, convection, radiation and phase transition at the same time. The complexity of the system studied leads to very unexpected results and the numerical tool was required to understand the measurements. This thesis illustrates the need to make experimental measurements interact with numerical simulations and model in order to address the nanoscale energy transfer problems in biological environments.