Laser-Induced Heating in Optical Traps

Abstract

In an optical tweezers experiment intense laser light is tightly focused to intensities of MW/cm(2) in order to apply forces to submicron particles or to measure mechanical properties of macromolecules. It is important to quantify potentially harmful or misleading heating effects due to the high light intensities in biophysical experiments. We present a model that incorporates the geometry of the experiment in a physically correct manner, including heat generation by light absorption in the neighborhood of the focus, balanced by outward heat flow, and heat sinking by the glass surfaces of the sample chamber. This is in contrast to the earlier simple models assuming heat generation in the trapped particle only. We find that in the most common experimental circumstances, using micron-sized polystyrene or silica beads, absorption of the laser light in the solvent around the trapped particle, not in the particle itself, is the most important contribution to heating. To validate our model we measured the spectrum of the Brownian motion of trapped beads in water and in glycerol as a function of the trapping laser intensity. Heating both increases the thermal motion of the bead and decreases the viscosity of the medium. We measured that the temperature in the focus increased by 34.2 +/- 0.1 K/W with 1064-nm laser light for 2200-nm-diameter polystyrene beads in glycerol, 43.8 +/- 2.2 K/W for 840-nm polystyrene beads in glycerol, 41.1 +/- 0.7 K/W for 502-nm polystyrene beads in glycerol, and 7.7 +/- 1.2 K/W for 500-nm silica beads and 8.1 +/- 2.1 K/W for 444-nm silica beads in water. Furthermore, we observed that in glycerol the heating effect increased when the bead was trapped further away from the cover glass/glycerol interface as predicted by the model. We show that even though the heating effect in water is rather small it can have non-negligible effects on trap calibration in typical biophysical experimental circumstances and should be taken into consideration when laser powers of more than 100 mW are used.

Analysis of laser-induced heating in optical neuronal guidance

“…This magnitude of heating is in agreement with experimentally observed laser-induced heating of vesicles by infrared optical tweezers (Liu et al, 1995), of DNA in an infrared optical trap (Braun and Libchaber, 2002), of micron-sized silica and polystyrene beads in a optical traps (Peterman et al, 2003), and temperature sensitive dyes in buffer suspension in an infrared dual-beam laser trap (Ebert et al, 2007). We find that for spot sizes with w s = 1-2 m, we have d hm = 32.7 m, which is comparable to the diameter of a neuronal growth cone.…”

“…Previous estimates of this effect in the optical neuronal guidance experiments suggested a negligible temperature increase (Albrecht-Buehler, 1991;Carnegie et al, 2008Carnegie et al, , 2009Ehrlicher et al, 2002;Graves et al, 2009;Higuchi et al, 2005Higuchi et al, , 2007Mohanty et al, 2005;Stevenson et al, 2006;Mathew et al, 2010;Koch et al, 2004), but our more detailed simulations show a temperature increase of the order 1 • C/100 mW of laser power, which is in agreement with experimental results and modelling from the field of optical trapping (Schönle and Hell, 1998;Braun and Libchaber, 2002;Peterman et al, 2003;Ebert et al, 2007). Furthermore, we find a temperature gradient of the order 1 • C/typical neuronal growth cone-radius.…”

Rapid interrogation of the physical and chemical characteristics of salbutamol sulphate aerosol from a pressurised metered-dose inhaler (pMDI)

“…The extent of the heating depends upon the characteristics of the laser beam (wavelength, power and beam profile) and the absorption characteristics of the particle and surrounding medium. 13,14 The increase in particle temperature above the ambient conditions, caused by absorption of the 514.5 and 1064 nm laser beams, is estimated using an approach similar to Knox and Reid. 15 At the typically used laser powers it is estimated that a pure salbutamol sulphate particle of 2 mm diameter would be 1.4 1C warmer than the ambient conditions (see ESI †).…”

Biophysical Tools for Cellular and Subcellular Mechanical Actuation of Cell Signaling

“…While optical tweezers can trap and manipulate individual gold nanorods as slender as 8 nm in diameter (29), heating and the creation of free radicals can have detrimental effects upon cell physiology (30,31). Endogenous particles such as lipid granules can be used as intracellular force handles (32), but the specific biological context of lipid granules and the aforementioned limitations with metallic nanostructures make it challenging to adopt optical tweezers for intracellular manipulation.…”