Measurement of Particle Size Distribution in Turbid Solutions by Dynamic Light Scattering Microscopy

The overall goal of this experiment is to demonstrate that dynamic light scattering and confocal microscopy can be used to measure particle size distribution in turbid solutions without dilution. This is the setup for the dynamic light scattering microscope. It has a solid-state laser capable of 30 milliwatts continuous wave operation at 488 nanometers.

There is also an avalanche photodiode, an autocorrelator connected to a computer for measurements. The inverted microscope in the setup has a stage with a temperature regulator. The box contains the remaining optical elements required for the experiment.

This schematic provides a more complete view of the layout for the setup. After shaping the beam profile, the beam is introduced into the microscope. Light scattering is obtained with a back scattering geometry and guided to a detector.

A launch mirror allows some reflected light to be observed via a CCD camera. The samples for the experiment must be prepared a day ahead. Sample preparation requires 20 milliliters of degassed, deionized water as well as purified NIPA.

In addition, it requires small quantities of TMEDA and ammonium persulfate. The preparation steps also make use of an ice water bath on a stirrer, two reaction vessels, aluminum foil, and a source of argon gas flow. Begin by measuring out 9.5 milliliters of degassed and deionized water into a reaction vessel.

Add the NIPA to the water. Next, move the reaction vessel to the ice water bath on the stirrer, and add the stirring rod. Use aluminum foil to cover the bath and the vessel to shield the reaction from light.

Use a pipet tip connected to the argon source to arrange for a moderate argon flow into the solution. Turn on the stirrer to gently stir the solution for 10 minutes. Next, use a micropipet to measure 11.9 microliters of TMEDA.

Briefly, draw back the foil to add the TMEDA to the solution and then continue stirring under argon gas for another minute. While the sample is being stirred, work with the second reaction vessel. Place 4 milligrams of ammonium persulfate into the vessel.

Then, add 0.5 milliliters of degassed and deionized water. When the ammonium persulfate has dissolved, add the solution to the sample solution and continue stirring for 30 seconds under argon flow. After 30 seconds, remove the reaction vessel from the ice bath and stirrer.

In preparation for storage, cover the vessel with aluminum foil, move the solution to a refrigerator for storage at four degrees Celsius overnight before using it. After retrieving the sample solution, prepare it for use with the microscope. Have ready two cavity slides, two circular glass covers, and appropriate glue, in order to make a sample and a standard reference slide.

For the sample slide, measure 60 microliters of the sample solution. Place the solution in one of the cavity slides. Cover the solution with a circular cover, being careful not to trap air bubbles.

Remove excess solution with a micropipet and laboratory wipes. Then, apply glue to the perimeter of the cover in order to seal the solution slide. Next, create a standard reference.

For this, use a 0.1 weight percent polystyrene latex solution. In the second cavity slide, add 60 microliters of the polystyrene latex solution, cover it with a glass cover, and ensure no bubbles are trapped. After removing any excess solution, apply glue to seal the solution in the slide.

For both slides, allow the glue to dry at room temperature. At this point, take the reference slide to the inverted microscope equipped with a 10 times objective lens, and place it on the microscope stage. The cover glass should face downward.

Continue by placing a beam damper in front of the detector. Next, apply the laser beam to the sample through the objective lens. Move on to work with the objective lens.

Adjust the height of the lens to set the focal point. Viewed with the CCD, as the objective rises from its lowest position, the reflected image will first focus at the surface of the cover glass. It will then focus at the interface between the cover glass and the sample.

It will focus a third time at the interface between the sample and the slide glass. Set the focal point between the cover sample interface and the sample slide interface. Attenuate the scattered light by reducing the laser power.

Then, remove the beam block to introduce scattered light into the detector. When ready, initiate a 30 second time correlation measurement via the control computer. Next, adjust the focal point of the microscope before repeating the measurement.

Use several focal points to obtain a wide range for the initial amplitude of the time correlation function. When enough data has been collected, place a beam damper in front of the detector. At the microscope, remove the standard reference slide.

Start the sample measurement process with an empty microscope stage and set the temperature to 25 degrees Celsius. Then, place the prepared sample slide onto the stage with the cover side down. Adjust the height of the objective lens to identify cover sample interface.

Adjust it further to find the sample slide interface. For the measurement, set the focal point between these two positions. Remove the beam damper from in front of the detector.

At the computer, perform a 30 second time correlation measurement. Next, set the stage temperature to 35 degrees Celsius. Since this temperate is above the lower critical solution temperature, the solution will go from clear to turbid.

Perform another measurement of the time correlation function. Here are the time correlation functions for the polystyrene latex suspension standard at two different focal points. The red curve corresponds to a time correlation function whose initial amplitude is approximately one.

The blue curve corresponds to an initial amplitude of about 0.2. This is the size distribution obtained by inverse laplace transform of the data with an initial amplitude of 0.2. The calculation takes into account the effect of partial heterodyning.

The nominal particle radius is 50 nanometers. These time correlation functions are for poly(NIPA)below and above lower critical solution temperature. The black curve corresponds to a measurement at 25 degrees Celsius.

The red curve corresponds to a measurement at 35 degrees Celsius, and immediately after the solution became turbid. The blue curve is for a measurement at 35 degrees Celsius, 20 minutes after the solution became turbid. Here are the size distributions associated with each of the measurement conditions.

Below the lower critical solution temperature, the average hydrodynamic radius is several tenths of nanometers. Above the critical temperature, the size is about 1 micrometer. The shift in curves with temperate and time suggests the growth of aggregation.