Peterman Erwin J G, Gittes Frederick, Schmidt Christoph F
Division of Physics and Astronomy, Vrije Universiteit, Amsterdam, The Netherlands.
Biophys J. 2003 Feb;84(2 Pt 1):1308-16. doi: 10.1016/S0006-3495(03)74946-7.
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.
在光镊实验中，强激光被紧密聚焦至兆瓦每平方厘米的强度，以便对亚微米粒子施加力或测量大分子的力学性质。在生物物理实验中，由于高光强会产生潜在有害或误导性的热效应，因此对其进行量化很重要。我们提出了一个模型，该模型以物理上正确的方式纳入了实验的几何结构，包括焦点附近光吸收产生的热量，由向外的热流平衡，以及样品腔玻璃表面的散热。这与早期仅假设被困粒子产生热量的简单模型形成对比。我们发现，在最常见的实验情况下，使用微米级的聚苯乙烯或二氧化硅珠子时，被困粒子周围溶剂中激光的吸收，而非粒子本身，是加热的最重要贡献。为了验证我们的模型，我们测量了被困在水中和甘油中的珠子的布朗运动光谱作为俘获激光强度的函数。加热既增加了珠子的热运动，又降低了介质的粘度。我们测量到，对于甘油中直径为2200纳米的聚苯乙烯珠子，1064纳米激光下焦点处的温度以每瓦34.2±0.1开尔文的速率升高；对于甘油中840纳米的聚苯乙烯珠子，为43.8±2.2开尔文每瓦；对于甘油中502纳米的聚苯乙烯珠子，为41.1±0.7开尔文每瓦；对于水中500纳米的二氧化硅珠子，为7.7±1.2开尔文每瓦，对于水中444纳米的二氧化硅珠子，为8.1±2.1开尔文每瓦。此外，正如模型所预测的，我们观察到在甘油中，当珠子被困在离盖玻片/甘油界面更远的地方时，加热效应会增加。我们表明，尽管水中的加热效应相当小，但在典型的生物物理实验情况下，它对陷阱校准可能有不可忽略的影响，当使用超过100毫瓦的激光功率时应予以考虑。
