Application of Voltage in Dynamic Light Scattering Particle Size Analysis

Our protocol investigates changes in stimuli-responsive polymers which are used in electrochemical sensors under an applied voltage in solution and allows influences in stimuli-responsive polymers to be observed. The advantages of this technology are that it's simple and can be used to observe the dynamic of poly-NIPAM with applied voltage. The analysis of polymers and particles under an applied voltage has applications in sensors, soft robotics and energy storage.

New trainees should take care to prepare the sample carefully and to avoid air bubble with cuvette for optimal data acquisition. It is easy to be careless with straightforward analytic techniques so take care as small changes in the protocol could lead to variable data. To prepare samples for DLS analysis, dissolve 10 milligrams of polymer powder in 10 milliliters of filtered deionized water and store the mixture at four degrees Celsius overnight.

To prepare the DLS cuvette, cut two 6.3 millimeter by 7 centimeter pieces from single-sided copper tape and use tweezers to stick each piece of tape to opposite sides of the inside of a DLS sample cuvette perpendicular to the light path with the bottom of the tape near the bottom of the cuvette. Fold the edges of the copper tape over the top of the cuvette making sure that the copper tape is near the top of the cuvette to ensure good electrical contact. Then wash the cuvette three times with deionized water dabbing the excess water off with a lab wipe after the last wash.

To set up the DLS instrument controls, the next morning, add 1.5 milliliters of deionized water to the prepared cuvette and add two drops of a standard solution to the cuvette. Insert the cuvette to the cuvette holder taking care that the small arrow on top of the cuvette is aligned with the cuvette holder and close the lid. Select measure in the instrument software and set the temperature to the experimental starting point.

After the measurement, rinse the cuvette and filter the prepared polymer test solution into the cuvette. Then load and measure the cuvette as just demonstrated. A clear measurement of the initial test solution should be observed.

To set up the DLS measurement protocol, in the instrument software, select file and new to set up a new standard operating procedure and click measurement type to select the trend, temperature and size. Under material, select the appropriate material and refractive index. Under dispersant, select the appropriate solvent.

Under sequence, set the start temperature and the end temperature for both the heating and cooling experiments. Then uncheck the return to starting temperature box. Select an interval for each temperature step change and under size measurement set the equilibrium time.

Select three measurements in automatic for the measurement duration. Then save the protocol and close the file. If applied voltage is to be used, select two wires that are thin enough to fit through the small crevice on the upper right edge of the DLS cuvette holder area.

Strip off the insulation from one end of one wire to facilitate a connection to the potentiostat. On the opposite end of the same wire, solder a short aligator clamp to the wire and attach the clamp to the cuvette. Clamp the white reference potentiostat lead and the red counter-potentiostat lead to one of the prepared wires and clamp the green working potentiostat lead and the blue working sense potentiostat lead to the other prepared wire.

Leave the orange counter sense and black ground potentiostat leads floating without touching any other equipment or materials. Within the Gamry software toolbar, click experiment and E physical electrochemistry and select chronoamperometry. Set pre-step, step one and step two voltage versus reference to the applied voltage across the entire field of the cuvette.

Set voltage to one volt versus the reference for all three steps. Set both the step one time and step two time to control how long the voltage will be applied and set the sample period to select how frequently the graph will read and record the current and voltage values. Click OK.An active sign will be displayed indicating that voltage is being applied.

In the Malvern DLS software, click measure and click start SOP. When the text at the bottom of the standard operating protocol window reads insert cell and press start when ready, click the start button to start the experiment. Each real-time file output of each run in the temperature ramp can be selected independently to view the volume size and the correlation coefficient.

Correlation graphs that have a generally smooth curve are considered good quality whereas non-smooth graphs or low-quality data should be considered for exclusion from the analysis. As observed, poly-NIPAM exhibits an LCST at 30 degrees Celsius, a temperature close to the literature described values. Without voltage, poly-NIPAM is able to aggregate and disaggregate within the tested temperature range returning to its original size and demonstrating the expected reversibility.

With voltage, poly-NIPAM changes from being soluble to aggregating to a size of 2, 000 nanometers then reduces to a size of around 1, 000 nanometers during cooling, never returning to the original soluble state. Here, the current data from poly-NIPAM with the applied voltage and heating and cooling experiments corresponding to previous data is shown. For this experiment, 26 degrees Celsius was a key transition point of poly-NIPAM at which a phase change was observed with DLS.

At 40 degrees Celsius, the maximum temperature in the measurement was achieved before the cooling cycle. If the current is not carefully monitored, the data can be misconstrued and potentially misunderstood. For example, in this analysis, the voltage was only applied randomly and sporadically resulting in a trend more similar to the no voltage condition.

The standard is a useful indicator of the setup and quality of the data. Clean standard results show the experiment can be completed with a higher chance of success. Researchers can use this procedure to test aggregation behavior of polymers or other electrochemically responsive polymers with applied voltage.

We are currently investigating why the LCST shift and why the irreversible aggregation behavior occurs. We expect this scientific question to provide greater insight into LCST behavior.