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As mentioned earlier, two methods using two different reducing agents were used in the lab experiment. To begin with, the first method utilized sodium citrate as the reducing agent for the gold nanoparticles. First, 30.21 mL of the gold (III) chloride * trihydrate (HAuCl4 * 3H2O) mixture was obtained and dissolved in 100 mL of water and placed in a round bottom flask. Then 0.11 g of the sodium citrate * dihydrate (Na3C6H5O7 * 2H2O) was weighed and dissolved in 10 mL of water and put aside. At this point, the HAuCl4 * 3H2O was being heated to a boil, with continuous stirring in the process. Once the gold chloride solution began to boil, the sodium citrate solution was rapidly added to the mixture, and the resulting solution was then set up with a condenser in order to reflux the solution for 15 minutes. While the solution was being refluxed, the solution color, which started out as a gray color after adding the sodium citrate, began to turn purple and finally, a red color. After 15 minutes, the solution was removed from the hot plate and was put aside to slowly cool at room temperature. The final color of the reaction mixture was red, as it was intended.
The second method, sodium borohydride (NaBH4) was used as the reducing agent for the gold nanoparticles. To begin with, 30 mL of distilled water was kept in an Erlenmeyer flask, in an ice bath. Next, 0.45 mL of the NaBH4 stock solution was injected into the Erlenmeyer flask containing the 30 mL of water. After this, the resulting solution mixture was stirred vigorously using the stir bar. Then, 0.3 mM potassium gold chloride solution (KAuCl4 * 3H2O) was added to the borohydride solution. Immediately after adding the KAuCl4, the solution turned grey-ish purple, and with continuous stirring for 1 minute, turned into a medium gray color. Multiple attempts were made for the trial but the results remained the same. Henceforth the rest of the procedure and results were taken by another lab group who received the correct results. In a similar experiment conducted by another group it turned from grey to rose red.
After both reduction methods were complete, the next step was aggregation of the gold nanoparticles. To do this, 1 M NaCl solution was added, dropwise, to 4 mL of the citrate stabilized gold nanoparticles, once the solution was cooled to room temperature. 12 drops of NaCl were added to the solution before the solution turned to a dark gray color. The same was repeated with another 4 mL of the citrate stabilized gold nanoparticles but this time using ethanol. This change took 120 drops, as the solution simply went from bright red, to a duller pink color. Similarly, the exact same procedure, using NaCl and ethanol, was repeated with the borohydride stabilized gold nanoparticles. After adding 60 drops of NaCl, the borohydride stabilized gold nanoparticle solution turned to a light pink that was almost a blush like color. Similar effect was seen after adding 27 drops of ethanol to the borohydride stabilized gold nanoparticles turned to a lighter red. Finally, the cuvettes were put into a UV-Vis spectrometer and the results were obtained.