Fluorescence correlation spectroscopy (FCS) was applied to the quantitative evaluation of the local heating in small domains <1 microm in solutions under the laser trapping condition in the presence of a near-infrared (NIR) laser beam at 1064 nm. On the basis of the translational diffusion coefficient of fluorescent molecules obtained by FCS, the relationship between temperature rise and the incident NIR laser power, DeltaT/DeltaP, were determined to be 62 +/- 6, 49 +/- 7, and 23 +/- 1 deg K/W in ethylene glycol, ethanol, and water, respectively, while no remarkable temperature increase was observed for deuterated water. The value of DeltaT/DeltaP linearly increased as a function of alpha/lambda (alpha is the extinction coefficient of solvent at the wavelength and lambda is the thermal conductivity of the medium). The validity and the applicability of the present method for the measurement of the local temperature increase were discussed by comparing the present results with previous ones by other various methods.

We have examined the relaxation dynamics of the dark state for water-soluble cadmium telluride (CdTe) nanocrystals by fluorescence correlation spectroscopy (FCS). Autocorrelation curves for CdTe nanocrystals were not reproduced by an analytical model including a single diffusion term and monophasic decay of dark-state contribution, but well reproduced by a model taking into account stretched-exponential type nonradiative relaxation. The present result suggests the presence of a widely distributed relaxation pathway of the dark state in the time region from μs to ms. Data analysis by the model including stretched-exponential type decay revealed that the timescale of nonradiative relaxation for CdTe nanocrystals decreased with the excitation laser power, suggesting the relaxation process of the dark state is promoted by additional absorption of a photon.

The effect of optical gradient force from a focused laser beam on the fluorescence correlation spectroscopy (FCS) was investigated by a computing method based on Brownian dynamics simulation. A series of calculations revealed that, in relatively shallow optical force potential up to 1.0kTR (TR=298.15 K), the conventional theoretical model of FCS without consideration of the optical gradient force could evaluate the increase in the average number of molecules and the diffusion time in the potential. On the other hand, large deviation between the simulated fluorescence correlation curve and the theoretical model was observed under the potential depth >1.0kTR. In addition, by integrating the optical force potential with the temperature elevation under optical trapping condition, it was deduced that the temperature rise does not seriously affect the average number of particles in the sampling area, but the average residence time is more sensitively affected by the temperature elevation. The present study using the simulation also provides a method to experimentally estimate molecular polarizabilities from FCS measurements.

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