Estimation of local temperature elevation induced by trapping laser irradiation

Under laser trapping condition, temperature elevation due to laser absorption cannot be

avoided. We should consider it since temperature change affects physical property and

chemical reactivity of molecules in the solution. In general, temperature elevation causes

increasing solubility of solute molecule and decreasing supersaturated value (SS). However,    

Fig. 2.5 (a) Crystal structure of L-proline and (b) hydrogen-bonded dimer structure formed between the columns in a crystal.

Fig. 2.6 Proposed model of the molecular arrangement of L-proline and water in a saturated proline solution.

temperature elevation enhanced evaporation of solvent and increasing of supersaturated

value can occur simultaneously. Thus we need to estimate temperature elevation due to the

absorption of trapping laser light at 1064 nm quantitatively.

Investigations of temperature increasing under laser trapping condition have been

reported. In the case of 1064 nm CW YAG laser, Fischerreported that T/P ~5 K/W by

thermal handing of langmuir-monolayers at the air/water interface [10]. Here T and P

represent temperature in Kelvin and irradiated laser power in Watt. Schmid and Tromberg

reported ~8 K/W and 10-14.5 K/W of temperature elevation of water, respectively [11-12].

Ito reported solvent dependent temperature elevation degree difference for ethylene,

ethanol, and water as ~62±6, 49±7, and 23±1 K/W in ethylene, ethanol, and water,

respectively, in small domain of solution base on the diffusion coefficient of fluorescent

molecular determined by FCS [13].

Here heat is generated by irradiated laser light that was absorbed by solvent and solute

molecules. First, we checked the absorption by solvent and solute molecules on the basis of

Beer-Lambert’s Law,

where I0 and I are the intensity of incident and transmitted light, respectively. α and l are

absorption coefficient and optical path length. Absorption also can be defined based on

molar absorption coefficient ε, optical length b, and concentration c.

  ₀e^l  ₀10^bc    …….………..………… (2-1)

Fig. 2.7 shows the optical path length dependent transmittance change of H2O, D2O,

EtOH, EtOD, and L-proline aqueous solution measured at 1064 nm. All the absorption and

transmittance spectra were measured by absorption spectrophotometer (JASCO, V-600). As

depicted in Fig. 2.7 optical path length dependent exponential decrease of transmittance was

observed for all samples. Deuterated solvents showed smaller diminution of transmittance

than that of H2O and EtOH. It indicates that deuterated solvents absorb less 1064 nm light

and, as a result, temperature elevation is smaller than others. Therefore we decided to use

deuterated solvent to prevent temperature elevation in this research. By fitting the

exponential curve based on the least square method, absorption coefficients of each solvent

are as follows; H2O is ~ 14.5 m^-1, D2O is ~1 m^-1, and EtOH is ~11 m^-1, and EtOD is ~4.6

m^-1.

A contribution of L-proline molecule in an absorption of L-proline solution can be

obtained from eq. 2-2, where x and y in the equation indicate solvent and proline

respectively.

Molar absorption coefficient of proline at 1064 nm is obtained as ~ 5.0x10^-3 M^-1cm^-1

from a mixture of 1.93M L-proline and 46.29 M water solution (Eq. 2-3). Compared to

water, it is about five times larger than that of water (1.1x10^-3 M^-1cm^-1).

If we assume effective optical path length is represented by focal volume size dimension and we can estimate absorption by eq. 2-4 and the horizontal and lateral size of focal volume can be derived from egs. 2-5 and 2-6,

EtOD

Fig. 2.7 Optical path length dependence of transmittance of (a) EtOD and EtOH, (b) D2O and H2O and (c) L-proline aqueous solution.



where r ,₀ Z ,₀ , NA, and n are the short and long axes, wavelength of light, numerical

aperture of the objective lens and refractive index of medium, respectively. Values of λ, NA, and n of solvents are known. Therefore r and ₀ Z are estimated to be 0.68 and 2.25 m, ₀

respectively.

Trapping laser light is tightly focused by high NA objective lens. If we consider only a

central part contribute to the absorption and heat generation, the temperature elevation in

the focal spot is represented by equation 2-7, where T, P and  are temperature, laser power

(W) and thermal conductivity, respectively [14].

Parameters in table 2-2 are substituted to eq. 2-7, and we can estimate the temperature

elevation degree during laser trapping.

Table 2-2 Absorption coefficient (1064 nm), thermal conductivity and estimation of temperature elevation by irradiation (1064 nm) in different solvent

2.5 References

1. J. Kapitan, et al., Proline Zwitterion Dynamics in Solution, Glass, and Crystalline State. Journal American Chemical Society, 2006. 128: p. 13454-13462.

2. R. Wu and T.B. McMahon, Infrared Multiple Photon Dissociation Spectra of Proline and Glycine Proton-Bound Homodimers. Evidence for Zwitterionic Structure. Journal American Chemical Society, 2007. 129: p. 4864-4865.

3. P. Zhang, et al., Neutron spectroscopic and Raman studies of interaction between water and proline. Chemical Physics, 2008. 345: p. 196-199.

4. M. Civera, M. Sironi, and S.L. Fornili, Unusual properties of aqueous solutions of L-proline:A molecular dynamics study. Chemical Physics Letters, 2005. 415: p.

274-278.

5. B. Schobert and H. Tschesche, Unusual solution properties of proline and its interaction with proteins. Biochimica et Biophysica Acta, 1978. 541: p. 270-277.

6. H.-D. Belitz, W. Grosch, and P. Schieberle, Food Chemistry 2009: Springer.

7. M. Jelifiska-Kazimierczuk and J. Szydlowski, Isotope Effect on the Solubility of Amino Acids in Water. Journal of Solution Chemistry,, 1996. 25(12): p. 1175-1184.

8. B.A. Wright and P.A. COLE, Preliminary examination of the crystal structure of l-proline. Aeta Cryst., 1949. 2: p. 129-130.

9. Y. Hayashi, et al., Large Nonlinear Effect Observed in the Enantiomeric Excess of Proline in Solution and That in the Solid State. Angew. Chem. Int. Ed., 2006. 45: p.

4593-4597.

10. S. Wurlitzer, et al., Micromanipulation of Langmuir-Monolayers with Optical Tweezers. J. Phys. Chem. B, 2001. 105: p. 182-187.

11. E.J.G. Peterman, F. Gittes, and C.F. Schmidt, Laser-Induced Heating in Optical Traps. Biophysical Journal, 2003. 84: p. 1308-1316.

12. Liu Y., et al., Physiological Monitoring of Optically Trapped Cells: Assessing the Effects of Confinement by 1064-nm Laser Tweezers Using Microfluorometry.

Biophysical Journal, 1996. 71: p. 2158-2167.

13. S. Ito, et al., Application of Fluorescence Correlation Spectroscopy to the Measurement of Local Temperature in Solutions under Optical Trapping Condition.

J. Phys. Chem. B, 2007. 111: p. 2365-2371.

14. D. Walgraef, N.M. Ghoniem, and J. Lauzeral, Deformation pattern in thin films under uniform laser irradiation. Physical Review B, 1997. 15: p. 361-376.

3. Laser Trapping Crystallization of L-proline

in Deuterated Water (D

O)

Based on the experience in glycine, laser trapping crystallization has only been achieved

by focusing near-infrared laser to the air/solution interface [1]. It suggests that utilization of

the interface increases possibility of crystallization. Thus we follow the previous example,

L-proline crystallization was examined by setting the focus at the air/solution interface.

Here I used bottom glass dish to prepare thin layered sample solution.