I noticed that most literature doing photothermal therapy were using a laser power intensity of 10W/cm2 to 20W/cm2 and they reported a dramatic temperature increase as high as to 40oC which is enough to kill cancer cells. However, on the other hand, for research on solar vapor generation of plasmonic nanoparticles, the laser intensity is at least 10^4W/cm2 or even higher, which induce a temperature change of 10s oC. Also, theoretical calculation showed that for a 20 nm AuNP, the temperature increase is only 4oC when a illumination of 520 nm and 10^5 W/cm2. Why the temperature increase is so dramatic in photothermal therapy at such low light intensity? Thanks a lot!
It is a question of absorption by, and the emissivity and conduction away from the heated object. Your Au nanoparticle will absorb very little energy and radiate it away quite easily so it requires a very high intensity beam to heat it. The loss from skin is far, far less, being almost all conduction and convection, so you need much much less power input. Consider how hot a (say) 25 watt incandescent type electric globe gets in order to lose the 23 watts of heat it generates.
It is a question of absorption by, and the emissivity and conduction away from the heated object. Your Au nanoparticle will absorb very little energy and radiate it away quite easily so it requires a very high intensity beam to heat it. The loss from skin is far, far less, being almost all conduction and convection, so you need much much less power input. Consider how hot a (say) 25 watt incandescent type electric globe gets in order to lose the 23 watts of heat it generates.
If all the power gets in and the heat capacity of wet tissue is about 4.2 J/K/cc, then a 1 cc cube will go up in temperature by over 2 K/s (ignoring escape of heat by conduction and blood flow). If the laser doesn't even penetrate 1 cm, or the absorption length is less than 1 cm, the temperature rise over the absorption distance will be even faster (and the escape of heat less significant).
Not sure if this thread is still active, but it's interesting for my present research. This question has been very much in my mind. As pointed out by others, the thermophysical properties of the surrounding tissue cannot be the reason for the high temperature. Firstly, water has pretty low thermal conductivity and high heat capacity. These values are comparable to tissue. In fact there are no biological materials with properties better for this purpose.
Now, one of the reason can be the assembly. Due to clustering, the heat may be concentrated as pointed out by Zhaoxia. I am not sure how much it will still matter.
In general laser heats up tissues/substrates. This heating by absorption is also in play I suppose.. if the heating of tissues without nanoparticles can be quantified, I think it can be made clear. I am still in search for an answer.
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