Characterization of gold colloid by UV-vis

For the sake of the effects of reducer amounts on the Au nanoparticles sizes during Au colloid preparation, six samples are prepared by different NaBH₄ amounts as listed in Table 3.2.

Table 3.2 The preparation condition of Au colloids with various amounts of NaBH₄ or PVA in system II.

Sample Ratio Sample Ratio

gold PVA^a NaBH4b

gold PVA^a NaBH4b

II.1-S0.5^c 1 5 0.5 III.1-P0 1 0 5

II.2-S1 1 5 1 III.2-P0.5 1 0.5 5

II.3-S2 1 5 2 III.3-P0.85 1 0.85 5

II.4-S3 1 5 3 III.4-P3 1 3 5

II.5-S5^d 1 5 5 III.5-P5^d 1 5 5

II.6-S10 1 5 10 III.6-P10 1 10 5

a The weight ratio between PVA and Au. The 1 wt% PVA is dissolved in water. ^b The molar ratio between NaBH4 and Au. The concentration of NaBH4 solution is 0.1 M. ^c For better memory and identification of the samples, every sample in Table 3.2 is labeled as II.n-Sx (x is molar ratio of sodium borohydride: Au) and III.n-Py (y is weight ratio of PVA: Au). ^d The preparation of II.5-S5 and III.5-P5 is totally the same of the I.6-GM Table 3.1, unless that the I.6-GM is diluted to be the same volume of I.1-ref in Table 3.1.

The as-prepared gold colloids are illustrated in the inset picture of top left image of Figure 3.5.

The vivid colors demonstrate that the colloidal Au-NPs have been successfully prepared. With the incremental amount of reducer, the color of the Au colloid changes from purple to red and at last dark brown. It is generally reported that the gold colloid with particles around 3-5 nm

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possesses red or wine-red color,^{[50, 51]} which is similar with our colloid II.3-S2, II.4-S3, and II.5-S5. When the molar ratio between NaBH4 and Au is lower than 2, the color of Au colloid tends to be purple. When the NaBH4: Au molar ratio is higher than 5, the color of the gold colloid changes into deep brown or even black. These variations are very interesting and are expect to be related with the size of the Au-NPs.

Figure 3.5 UV-vis spectra of Au colloids 10 min after NaBH4 addition (upper left), and the comparison of three gold colloids after prepared for 1 day (black line) and 2 weeks (red line). a.

II.1-S.0.5, b. II.2-S1, c. II.3-S2, d. II.4-S3, e. II.5-S5, and f. II.6-S10 respectively, according to different NaBH4: Au molar ratio. [NaBH4] = 0.1 M, [Au] = 10^-3 M, PVA: Au = 5: 1 (weight ratio).

As previously stated, the visualized color of gold colloid is a reasonable phenomenon that can

300 400 500 600 700 800

300 400 500 600 700 800 300 400 500 600 700 800

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be explained by the SPR effect. The incident light at the same frequency of SPR is absorbed by colloid, and the rest of the incident light is reflected as the color. The color of colloid is strictly depending on the sizes and shapes of Au-NPs in this work, which are expected to be applied for the identification of colloids. The UV-vis spectra over six Au colloids are taken 10 min after addition of NaBH₄ as shown in top left image of Figure 3.5. All the fresh Au colloids possess single absorption bond in the range of 490- 550 nm, which is a characteristic of Au colloids reported everywhere.[25, 26, 52]

The absorption at certain wavelength is a visible phenomenon from the surface plasmon resonance, which causes strong electromagnetic radiation by the excitation of molecules.^{[17, 53]} The differences in sizes and shapes of particles result in distinctive position and shape of the absorption peak, as well as the color. With the NaBH₄: Au ratio raises from 0.5 to 5, the position of plasmon resonance bond blue-shifts from 534 nm to 510 nm, revealing the different structures/sizes of nanoparticles.^[54] It was suggested that a blue shift of plasmon bond reflected the reduction of particle sizes.^[54] It is in this case inferred that the Au-NPs sizes degrade with the addition of reducer. However, there seems to be valve value of the reducer amount for the growth of Au-NPs, since the plasmon bond slightly red-shifts to 518 nm over II.6-S10 with an excessive NaBH4 amount, and the peak becomes broader with trail until 700 nm. In addition, the intensity of plasmon bonds over colloid II.1-S0.5 and II.2-S1 are relatively weak, which means that the amounts of absorption species in colloids are quite low. An obvious peak locates at about 313 nm is observed in both the two colloids, which can be attributed to a ligand-to-metal charge transfer transition in the tetrachloroaurate ion derived from [AuCl4]⁻.^[55] It is meaning that the small amount of reducer is not sufficient for reducing all the ionic gold species into Au⁰. There exist large amounts of unreduced gold species even after the formation of Au-NPs. The intensity of the plasmon bonds raise along with the increase amount of NaBH4 until the ratio between reducer and Au becoming 2: 1. Further increase of the NaBH4 amount does not induce obvious increase of the peak intensity. It is considered that the 2:

1 ratio is generally enough for the reduction from Au³⁺ to Au⁰. When the NaBH₄: Au ratio is higher than 3, the plasmon bond become much broader. It was reported that for the gold particles possessing plasmon bond at the same location, a broader peak width was related to

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smaller Au-NPs.^[54]

After one day, the UV-vis spectra of the above six colloids generally remain the similar SPR bonds (including the half-peak width and peak position) as 1 day before, expect the colloid II.1-S0.5 and II.2-S1 (Figure 3.5). A new plasmon bond locating at 647 nm is formed over colloid II.1-S0.5 one day later. Both the absorption peaks of colloid II.1-S0.5 and II.2-S1 become sharper, and the absorption peak at 313 nm depresses gradually. After 2 weeks, deposition is formed in colloid II.1-S0.5 covering the bottom of bottle. The second absorption peak red-shifts to 731 nm and the total intensities are much weakened, proving the growing process during two weeks. The single plasmon bond of colloid II.2-S1 also becomes sharper after 2 weeks, resulting from the development of gold particles from residual precursor. When the amount of NaBH4 is lower (e.g. colloid II.1-S0.5 and II.2-S1), the Au ions cannot be totally reduced into Au⁰, and there remain Au³⁺ and even Au⁺ species in the solution. In this case, the solution possesses acid property that makes the Au-NPs unstable by the incomplete charging and lack of repulsive electrostatic interaction.^[56] The as-formed gold nuclei tend to form larger nanoparticles. No differences are observed from the UV-vis spectra or the colors of colloid II.3-S2, II.4-S3, and II.5-S5. A slight shrinking of the bond together with slight red-shift is observed in colloid II.6-S10 as shown in the lower right image of Figure 3.5, suggesting the further growth of particles within 2 weeks.

Correlating with the color changes over different colloids (the picture was taken 1 day after preparation), the insufficient reduction of Au precursor induces a purple color. The Au colloids from appropriate amount of NaBH4 display color in red. However, when the amount of reducer is too high, the gold colloid turns out to be brown and even black (for the colloid II.6-S10, we have tested several times, the result turns out to be equal). In fact, during the preparation of colloid II.6-S10, the color of solution changes from yellow to red and then brown-black very rapidly. It is inferred that when the NaBH4 amount is high, large amounts of very small germs are formed immediately. However, the intense reduction and huge number of germs provide the opportunity of small germs to be collided during string. The aggregations are more likely

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to be formed.

From the above analysis combining the color and SPR peaks of UV-vis spectra, it is still difficult to select one best colloid with small and homogeneous Au-NPs (2-4 nm). In fact, as the paper chromatography made in the organic chemistry for separating and identifying different compounds, the similar consideration also can be probational for comparing and choosing colloids with desired particle sizes and distribution. TLC silica gel is applied in the following work to understand the size distribution of Au-NPs by colloids diffusion. One drop of gold colloid is dropped onto the silica gel, the diffusion process can be complete in about 10 second and the silica gel can be totally dried after about 10 min. In order to keep an equal volume of the tested liquid, the same dropping pipette will be used for transferring different colloids, and one droplet of colloid will be utilized during the test. Before testing, the concentration of gold in each colloid is diluted to be identical. Each diffusion process is recorded by three photos along with time.