The overall goal of this analysis technique is to gain an understanding of nanoparticle incorporation into polymer matrices. The direct imaging of particles allows for an estimation of local concentrations and overall assimilation rates. This method can help answer key questions in materials chemistry, including nanochemistry, concept materials design, and catalysis.
The main advantage of this technique is that the incorporation of nanoparticular materials are imaged directly, such that the information gained cannot be achieved by any other straightforward method. Demonstrating the procedure will be Dr.Sacha Noimark, Dr.Joe Bear, and Dr.William Peveler. Post-doc from Professor Parkin's laboratory at UCL.
Begin this procedure with preparation of the cadmium selenide cores, as detailed in the text protocol. To a 100 milliliter browned-bottomed flask, add zinc diethyldithiocarbamate, 1-Octadecene, Oleylamine, Trioctylphosphine, and the cores in hexane. Add a stir bar, and exchange the reaction atmosphere to nitrogen.
Heat the reaction on a hot plate stirrer at 3.3 degrees Celsius per minute under partial vacuum until 70 degrees Celsius while removing the hexane using the Schlenk line. Switch the atmosphere to nitrogen, and continue heating at this rate to 120 degrees Celsius. Stir at 120 degrees Celsius for two hours.
Allow the reaction to cool, and split the mixture between two 15 milliliter centrifuge tubes. Inject chloroform into the sample before pouring the solution into the tubes. Ensure the tubes are topped up to 50 milliliters with ethanol to precipitate the particles and centrifuge at 3600 times g for 10 minutes.
Discard the supernatant and redisperse the pellets in a total of 10 milliliters of n-Hexane. Centrifuge the solution once more to remove any insoluble impurities before decanting into a sample tube. Store at four degrees Celsius under a nitrogen atmosphere for up to three months.
For the swelling solution preparation, prepare a stock solution of cadmium selenide quantum dots by mixing 36 milliliters of n-Hexane with four milliliters of the synthesized cadmium selenide quantum dot dispersion. Stir the solution magnetically. Set aside two vials, each containing nine milliliters of the stock solution as designated swelling solutions.
Use the remainder of the stock solution for the preparation of further swelling solutions of varying quantum dot concentrations. Prepare three swelling solutions of decreasing quantum dot concentration by diluting the stock solution to give a 66%solution, a 50%solution, and a 33%solution, as detailed in the text protocol. Store all of the quantum dot solutions under dark conditions at room temperature.
Next, cut out four medical grade silicone squares using a fresh scalpel blade. Immerse a medical grade silicone square in each of the four swelling solutions of varying percent quantum dot concentration:stock solution, 66%50%and 33%After the polymer samples swell at room temperature for 24 hours under dark conditions, remove the swollen polymer samples from the respective swelling solutions. Air dry the samples under dark conditions for 48 hours, during which time the residual solvent evaporates and the polymers shrink back to their initial dimensions.
Next, wash the quantum dot incorporated samples thoroughly with deionized water to remove any surface bound materials. Then prepare four more medical grade silicone squares. Immerse them in the stock swelling solution for varying time periods:one hour, three hours, six hours, and 24 hours.
After removal from the swelling solution, air dry the swollen polymer samples under dark conditions for 48 hours, such that the sample shrinks back to its previous dimensions. Wash the quantum dot incorporated samples thoroughly with deionized water to remove any surface bound materials or residual solvent. Cut out two swell shrink silicone squares with a fresh scalpel blade.
Ensure that this exposes the internal surface of the silicone samples. Place the silicone samples on a microscope slide for imaging. Ensure that the freshly cut side of the polymer makes full contact with the glass slide.
Press the silicone portion down lightly to ensure a smooth contact with the microscope slide before completing the lifetime fluorescence measurements, as described in the text protocol. Then place the sample on the stage of the microscope. To complete lifetime fluorescence measurements, directly couple the laser output to an acousto-optic tunable filter system to generate the 488 nanometer laser line.
Focus the laser beam using a custom-built laser scanning unit which is reflected by a dichroic mirror into the back aperture of a 10X objective and then onto the sample. Refer to the text protocol for instructions on performing measurements and processing data. Chilled quantum dots are characterized using photoluminescence spectroscopy to measure emission, excitation spectra, and the overall quantum yield.
The swell-encapsulated quantum dots are highly photoluminescent, therefore brightness is indicative of successful swell encapsulation. Cross sections of the polymer can be mounted in the fluorescence microscope to measure the degree of swell encapsulation. Once mastered, this technique can be done in around 15 minutes with swell encapsulated samples, if it's performed properly.
While attempting the procedure, it is important to remember that the blade used for cutting the polymer samples is uncontaminated and relatively sharp, as inconsistencies at this stage may introduce artifacts in the fluorescence images. Following this procedure, other methods like energy dispersive X-ray spectroscopy in addition to functional testing, can be performed to further gauge encapsulation amount or surface concentration. After its development, this technique paved the way for researchers in the field of material science to explore nanoparticle composite design using a range of host matrices.
Don't forget that working with cadmium selenide can be extremely hazardous, and precautions, such as wearing suitable personal protective equipment and working in a fume cupboard, should always be taken while performing this procedure.
