The overall goal of this procedure is to efficiently prepare gold nanorods with an aspect ratio between two and three. This method can help prepare gold nanorod in a reproducible and convenient way compared to other synthetic protocol in the scientific literature. The main advantages in the use of hydroquinone as the reducing agent are the dramatic increase in the gold ion reduction rate, and the lower use of cetyltrimethylammonium bromide, with substantially lower cost.
This protocol consists of two steps. First is most spherical gold seeds are prepared using sodium borohydride. Secondly, gold nanorods are prepared from a seed using a seed-grow solution.
To begin preparation of the gold seeds, first dissolve 364.4 milligrams of CTAB in five milliliters of water at 40 degrees Celsius using ultrasonication. Remove the sample from the sonicator once the solution becomes clear, and allow it to cool to room temperature before proceeding. Next, prepare five milliliters of a 0.5 millimolar solution of tetrochlorauric acid in water and add it to the vigorously stirring CTAB solution at a constant temperature of 27 degrees Celsius.
Subsequently, make 600 microliters of a sodium borohydride solution in water at four degrees Celsius, and add this to the rapidly stirred CTAB mixture at 27 degrees Celsius. Check that the color immediately changes from yellow to brownish. Stir the resulting suspension for an additional 20 minutes.
To determine the size of the seeds, first pipette 100 microliters of the seed suspension into 400 microliters of water in a plastic microcuvette. Next, transfer the microcuvette to a UV-visible spectrometer and analyze it at a wavelength range of 400 to 840 nanometers. For the seeds to be small enough to use, there should be no pronounced plasmonic peak in the range between 505 to 520 nanometers.
Start preparation of the growth solution by dissolving CTAB and hydroquinone in five milliliters of water at 40 degrees Celsius using ultrasonication. Allow the solution to cool to 27 degrees Celsius before proceeding. Separately, prepare 200 microliters of a four millimolar aqueous silver nitrate solution and five milliliters of a one millimolar aqueous tetrachloroauric acid solution.
Next, add the silver nitrate solution, followed by the tetrachloroauric acid solution, to the magnetically stirred CTAB solution. Once combined, immediately add 12 microliters of the previously prepared gold seed suspension. After 30 minutes, check to see that the color of the suspension has changed to brown.
Sample the suspension every five minutes by pipetting 100 microliters of it into 400 microliters of water in a plastic microcuvette, and analyze the sample by UV-visible spectroscopy. In the UV-visible spectrum, a shift to the lower wavelength of the far red shifted plasmonic peak stabilizes after about 30 minutes indicating the reaction is complete. Divide the suspension into one milliliter aliquots in centrifuge tubes and centrifuge them.
The gold nanorods form a dark precipitate at the bottom of the tubes. Subsequently re-suspend each of the precipitates by pipetting and sonicating them in one milliliter of water. Combine all the suspensions into one container and store at room temperature for later use.
Analyze the gold nanorod suspension by TEM at 200 kilovolts, according to the manufacturer's protocol, and by UV-visible spectroscopy, at a wavelength range of 400 to 840 nanometers. To vary the nanorods'aspect ratio using silver ions, first prepare a four millimolar solution of silver nitrate in water. Next, set up three vials, each containing five milliliters of the CTAB and hydroquinone solution.
Add 100 microliters, 150 microliters, or 200 microliters of the silver nitrate solution to the appropriate vial. Continue by adding five milliliters of the one millimolar tetrachloroauric acid solution to each vial. Complete the aspect ratio tuning process by adding 12 microliters of the gold seeds, sampling the reaction, and isolating the gold nanorods by centrifugation as previously described.
To start, make CTAB solutions of different concentrations by adding the appropriate amount of CTAB according to Table One, to vials containing 22 milligrams of hydroquinone in five milliliters of water. Subsequently, add 200 microliters of the four millimolar silver nitrate solution and five milliliters of the one millimolar tetrachloroauric acid solution to each magnetically stirred vial of CTAB solution. Add 12 microliters of the gold seeds suspension to the vials and observe the change in color from blue to brown.
Analyze the suspension by UV-visible spectroscopy. Discontinue stirring when the suspension's color and the UV-visible spectrum are stabilized. Prepare the sample for centrifugation.
Centrifuge the particles and then re-suspend the precipitate of gold nanorods in one milliliter of water for final analysis by TEM and UV-visible spectroscopy. The lengths of the gold nanorods formed by different amounts of silver ions in the growth solution were measured using TEM. The images show that the length of the nanorods increases proportionally to the amount of silver ions used.
Analysis of TEM images and UV-visible spectra, showed that when CTAB concentrations increased, the nanorods'length also increased. The nanorods'aspect ratios peaked at a CTAB concentration of 50 millimoles, because the rods'width grew more rapidly than their length at CTAB concentrations greater than 50 millimoles. Once mastered, this technique can be done in half a day if it is performed properly.
While attempting this procedure, it's important to remember to always use clean glassware and to control the temperature of the growth solution in order to obtain reproducible results. Beginners may struggle with this method, as temperature, stirring speed, and the reagent reaction order, all need to be controlled in order to obtain gold nanorods of a desired size and aspect ratio. After watching this video you should have a good understanding of how to prepare a gold nanorod using hydroquinone as a reducing agent.
Don't forget, working with chemicals can be extremely hazardous, and precautions such as wearing gloves, lab coat, and goggles, should always be taken while performing these procedures.
