Nano particles, medicine and heat conduction
23 Oct, 2007 11:10 am
Metal nano particles have a particularly large optical cross-section that can be used in high resolution imaging and treating cancer. Heat transport, which depends on the propagation of electrons and phonons, is strongly modified by both the size of the nano particle and the distance between nano particles. As a result the control of the nano particle size and organization are expected to lead to significant advancement in heat management and effective cancer therapies. Strongly light absorbing nano particles are extremely localized heat sources. They can be used not only to destroy biological cells but can also be used to induce chemical reactions on specific locations by infrared or optical illumination.
Nano particle optics and imaging: Nanoscale metal particles interact strongly with light. Their optical cross-section is four orders of magnitude greater than dye molecules. The scattering cross section of a noble metal nano particles show a broad resonance in the visible spectral range (Au: 520nm) that depends on their size and shape. The strong response is due to collective charge oscillations (surface plasmons) in metal particles. In other words, the particle at the size smaller then the wavelength of light, acts as an antenna for visible light. The surface plasmon resonance (SPR) is sensitive to the environment of the particles and shifts to higher wavelength with increasing local index of refraction. This effect can be used to monitor the binding of nano particles to specific cells and they can be used as contrast agents. Nano particles have the advantage of not being influenced by illumination (photo bleaching) compared to traditional contrast agents such as dyes.
Local heat source and photo thermal cancer therapy: Cancer is one of the most often found causes of death. Cancer is caused by uncontrolled cell growth. In what way are nanoparticles useful for cancer therapies? The resonant optical response has the effect that the plasmon-decay leads to substantial heating of the nano particle and its environment. The illuminated nano particle is therefore a local heat source which can destroy biological cells. Biological molecules are fragile and disintegrate rapidly even with a 5-10 degree increase in temperature. By marking cancer cells with metal nano particles one can destroy cancer cells through illumination. Photothermal tumor therapy has been found to be highly effective compared to chemotherapy or radiation therapy. Only the part of the body which contains the nano particles is exposed and only the cells targeted by the functionalized metal nano particles are heated. To access deeper tissues near infrared (650-900nm) light is used. Visible light can not penetrate far due to strong scattering. By changing the shape of nano particle into rods or shells it is possible to shift the optical resonance into the infrared spectral region. Photo thermal cancer therapy is still at an early stage and is currently being clinically tested.
Heat transport: Heat is conducted in solids by electrons and collective vibrations. Collective vibrations are quantized at the atomic scale into phonons. Phonons have a characteristic wavelengths and have a mean free path of typically 10-100nm. Scattering on defects, impurities or interfaces reduce heat conduction. Using nano particles one can influence heat conduction or phonon transport. The interface of the nano particles or their organization can be used to increase or to reduce heat transport to values which have not been possible with bulk materials so far. Thermal management of integrated circuits is the biggest challenge in microelectronics today. There are a number of applications where thermal barrier coatings or heat sinks are needed and where nano particles will be able to solve heat transport issues. Carbon nanotubes can also be considered to be nano particles. Nanotubes have phonon velocities greater than in diamond. This is due to the different chemical bonding in nanotubes and diamond. The highest thermal conductivity ever, was measured in single wall carbon nanotubes (3500W/mK) which is 10% higher than in diamond. Thermal transport is believed to be ballistic in some carbon nanotubes as compared to the normally observed diffusive transport. This means carbon nanotubes are waveguides for phonons.
We conclude that strongly light absorbing nano particles are extremely localized heat sources. They can be used not only to destroy biological cells but can also be used to induce chemical reactions on specific locations by infrared or optical illumination.