Light microscopy for real-time characterization of colloids and nanoparticles

Ivan V. Fedosov, Saratov State University, Saratov, RussiaBoris N. Khlebtsov, Institute. of Biochemistry and Physiology of Plants and Microorganisms of RAS, Saratov, RussiaValery V. Tuchin, Saratov State University, Saratov, Russia

Introduction

Brownian motion is a random motion of small particles suspended in a fluid. It results from collisions of particles with fluid molecules. Brownian motion can be described in terms of probability distribution function (PDF) of particle displacements. Root mean square displacement of a particle due to its Brownian motion characterizes fluid temperature and viscosity as well as hydraulic diameter of particle. The center of distribution characterizes the effect of external forces like gravitation, Stokes drag force or light pressure force on a particle.

One way to estimate a (PDF) of small particle displacements is based on tracking of the particle position with a microscope. Digital image processing allows for simultaneous tracking of a large number of particles over microscope field of view to provide reliable characteristic of a colloid. Earlier we reported the method for characterization of nanoparticles and colloids based on automated tracking of nanoparticles.

Goal of our current work is to perform real time measurements of PDF of nanoparticle displacements to study non stationary effects of light to heat conversion enabled with noble metal colloids.

Statistical particle tracking Frame 1 Frame 2

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Detection of particle positions through image series

Calculation of all possible displacements of each particle

Calculation histogram of displacements

Brownian particles

Mismatches

pd

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Model for particle displacement PDF is biased normal distribution: Diffusion coefficient:

particle diameter;temperature;fluid viscosity

Mean displacement:velocity of regular movement

Parallel acquisition & processing

1 – creating a memory location for image series; 2 – acquisition of image series with CMOS camera; 3 – particle detection; 4 – SPTV binning; 5 – circular buffer to average displacements over several series; 6 – PDF model fit; 7 – calculation of temperature, size etc.; 8, 9 – visualization of contour map and vector plot; 10 – updating of results time series; 11 – updating of preview monitor.

The software was developed using National Instruments LabVIEW 8.5 Professional Development System

Graphical user interface

Graphical user interface designed for widescreen digital display provides easy access to all controls and indicators for easy monitoring and adjustment of acquisition and processing.

532 nm light is not visible here because it is blocked with a filter introduced into optical path of microscope

Heating with 532 nm laser

Illumination with 473 nm

20 m

532 nm laser beam

14 m in diameter

Laser induced heating of colloids

Glass cell

Colloid

Polystyrene nanospheres

Polystyrene nanospheres of 100 nm in diameter suspended in distilled water. Concentration of 5 x 1010 particles/ml. Because of their extremely small absorption cross section polystyrene nanospheres are not impact laser induced heating. Only moderate rise of temperature caused by absorption of laser light in water is observed.

Polystyrene nanospheres & 15nm colloid gold

Mix of 100 nm polystyrene nanospheres and 15 nm colloid gold particles. Concentration of each fraction is 5 x 1010 particles/ml. Scattering cross section of 15 nm gold particles is small and camera detects only polystyrene spheres which acts as temperature sensors.

Polystyrene nanospheres & 25nm colloid gold

Mix of 100 nm polystyrene nanospheres and 25 nm colloid gold particles. Concentration of each fraction is 5 x 1010 particles/ml. Absorption cross section of 25 nm gold particles is about 3 times larger than that of 15 nm particles. Therefore 3 time less power of laser beam is needed to reach the same local temperature as in previous case.

Conclusion

We had developed software for real-time characterization of colloids by means of statistical tracking of particle images. Optimized algorithms of data processing ensure refresh rate up to 1 measurement per second.

The software makes it possible to measure temperature, particle size, concentration, fluid viscosity and flow velocity in various monodisperse colloid systems containing small particles suspended in a fluid.

Acknowledgements

•“Photonics4Life” EU Seventh Framework Program No224014

•CRDF REC-006/SA-006-00

•CRDF BRHE RUXO-006-SR-06/BP1M06

•RF №1.4.06

•RNP.2.1.1.4473

•NSh-208.2008.2

•RFBR 08-02-00399-a

This work is supported through the grants:

Thank You!

fedosov_optics@mail.ru