A. Preparation of Gold Seed Solution.

1. 2.94 mL 10 -2 M TX-100 and 0.06 mL 4.975 × 10 -3 M Au-(III) (HAuCl4. xH2O) solutions are taken in a 1 cmquartz cuvette.
2. The cuvette is kept about 3 cm away from a germicidal (UV) lamp and is irradiated with the UV light for 30 min. The yellow colored solution turns purple indicating the formation of fine colloidal gold nanoparticles.
3. Turn on the spectrophotometer instrument by clicking on the power button and wait for 30 min for initialization of the instrument.
4. Click on the spectrophotometer lid to open it.
5. Click on the cuvette to place it in the sample cell holder of the instrument. One has to use aqueous TX-100 solution as the sample blank or reference in this measurement. Here a double beam spectrophotometer is shown.
6. Run the wavelength scan by clicking on the Computer monitor and then on the ‘Scan’ tab and observe the scan. In the real spectrophotometer, an appropriate wavelength range of incident light for the sample can be chosen and the wavelength scan are run via the accompanied computer software. One can run the scan in absorbance or transmittance mode. The scan data is stored in the computer.
7. Click on the spectrophotometer lid again to open it.
8. Click on the cuvette to take it out to the instrument table.
9. Collect data by clicking on the Data tab.
10. Plot the absorbance (and transmittance) of the sample at various wavelengths and determine the wavelength of maximum absorption i.e., spectral peak-position. Colloidal gold particles absorb maximum light near wavelength 520 nm.

B. Preparation of Various-Sized Colloidal Gold Particles and Kinetic Studies of Their Evolution.

The seed particles are left generally for 3-4 h before employing them for the generation of larger gold particles. Various sized colloidal gold particles can be prepared by a two-step seed mediated (non-iterative) growth method. The conditions for preparing gold particles of three different sizes are given in the following table.

1. Turn on the spectrophotometer instrument by clicking on the power button and wait for 30 min for initialization of the instrument.
2. For the preparation of Sample 1 gold particle proceed as follows.
3. Take the micropipette by clicking on it.
4. Click on the micropipette again to collect the required quantity of seed solution.
5. Add the seed solution from micropipette to the required quantity of Au(III) ion solution in the cuvette by clicking on the pipette once again.
6. Add 0.04 mL 10-2 M ascorbic acid at a time (not drop by drop) by clicking on the ascorbic acid solution.
7. Immediately after addition of ascorbic acid, the kinetics of the particle development from the seed should be followed by following the increase in absorbance value at λ=520 nm with time (at 331°C).
8. Click on the spectrophotometer lid to open it.
9. Click on the cuvette to place it in the sample cell holder of the instrument.One has to use water as the sample blank or reference in this measurement. Here a double beam spectrophotometer is shown. In this case, one can place the sample in the sample holder and the sample bank or reference in the reference holder simultaneously.
10. Close the chamber lid by clicking on it.
11. Click on the computer monitor to open the scan page.
12. Run the scan in the kinetics mode by clicking on the ‘Scan’ button and observe the scan. In the real spectrophotometer, an appropriate wavelength of light is selected (here λ=520 nm) at which increase in absorbance vs. time scan is performed by using the accompanied computer software.
13. When the scan is complete click on the ‘Next’ tab for beginning the kinetic measurements of Sample 2 and Sample 3.
14. Repeat the above steps and study the developments of Sample 2 and Sample 3 gold particles. Thus one prepares various-sized gold particles depending on the seed particle to Au(III) ion concentration ratio. All the samples left for ~12 hrs before using them as catalysts for eosin reduction. The rate of catalysis was found to be very slow within 3-4 hrs of their preparations.
15. Collect data by clicking on the Data tab.
16. Plot the absorbance vs. time for the sample. What is the nature of the plot: sigmoidal?
17. Attempt an autocatalytic fit of the data.

C. Catalytic Eosin Reduction Kinetics Study.

1. Turn on the spectrophotometer instrument by clicking on the power button and wait for 30 min for initialization of the instrument.
2. Take 2 mL of a sample containing particles of a given average size in a cuvette.
3. Click on the micropipette to add 0.04 mL 10 -2 M eosin solution and then 0.40 mL 0.50 M sodium borohydride (NaBH4) solution to the particle solution present in the cuvette.
4. Immediately after the addition of NaBH4, kinetics of particle catalyzed eosin reduction is followed by following the decrease in absorbance value at λ=535 nm with time (at 30 1 °C).
5. Click on the spectrophotometer lid to open it.
6. Click on the cuvette to place it in the sample cell holder of the instrument.One has to use water as the sample blank or reference in this measurement. Here a double beam spectrophotometer is shown. In this case, one can place the sample in the sample holder (often the front one) and the sample bank or reference in the reference holder (often the back one) simultaneously.
7. Close the spectrophotometer lid by clicking on it.
8. Run the scan in the kinetics mode by clicking on the Computer monitor and then on the ‘Scan’ tab and observe the scan. In the real spectrophotometer, an appropriate wavelength of light is selected (here λ=535 nm) at which decrease in absorbance vs. time scan is performed by using the accompanied computer software.
9. When the scan is complete click on the ‘Next’ tab for beginning the kinetic measurements with Sample 2 and Sample 3.
10. Repeat the above steps and study the kinetics of catalytic reduction of eosin by Sample 2 and Sample 3 gold particles.
11. Collect data by clicking on the ‘Data’ tab.
12. Plot the absorbance vs. time for every sample. What is the nature of the plot?
13. Attempt a first order fit of the data.