Influence of addition amount of protector- PVA

3.5.1 Characterization of gold colloid by UV-vis

The details of Au colloid prepared from different PVA: Au weight ratios are listed in Table 3.2. The molar ratio between reducer and Au is fixed as 5: 1. It can be seen from the inset picture of Figure 3.12 that the color of colloids from various PVA amount is generally a brown

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system. Colloid III.1-P0 displaying purple-brown is a comparison without any addition of PVA.

A smaller amount of PVA (PVA: Au ≤ 0.5 weight ratio) makes the gold colloid with dark brown.

The higher amount of PVA induces gold colloid with semitransparent light brown.

Figure 3.12 Uv-vis spectra of Au colloid prepared from different PVA amounts (left profile, 10 min after NaBH4 is added), and the comparison of fresh colloid III.1-P0 and the colloid after 1 week (right profile). a. III.1-P0, b. III.2-P0.5, c. III.3-P0.85, d, III.4-P3, e, III.5-P5, f.

III.6-P10 respectively, corresponding to different PVA: Au weight ratio. [NaBH₄] = 0.1 M, [Au]

= 10^-3 M, NaBH4: Au = 5: 1 (molar ratio).

The brown color system also depends on the selected ratio between the reducer and Au (NaBH4: Au=5: 1), which means that the color system might be red if the ratio is based on 3: 1 (as the II.4-S3 in system II). The UV-vis spectra of gold colloids by variation of PVA amount is displayed in Figure 3.12. Colloid III.1-P0 with no PVA give an intensive SPR bond at 521 nm- a characteristic peak of gold colloid as reported. The NaBH4 alone without addition of protect agent can also reduce the gold precursor into metallic gold. Without the addition of PVA, the absorption peak is much sharper than the other tested colloids. After 1 week, an additional peak around 660 nm appears in the aged colloid III.1-P0, whilst the peak around 521 nm does not shift. It is demonstrated that the sizes of the Au-NPs do not change during the aging time.

However, some large aggregations are formed due to the lack protection of Au-NPs. With the raise of PVA amount, the plasmon bond blue-shifts from 521 nm (colloid III.1-P0), 517 nm

300 400 500 600 700 800

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(colloid III.2-P0.5) to 510 nm (colloid III.3-P0.85) and then become mainly permanent. The intensity of the bonds is also reduced along with the raising of PVA amounts.. The display of plasmon bonds by UV-vis spectra reflects the basic estimation of the Au-NPs, and further information should be obtained by using the HMLD tests.

3.5.2 Home-made liquid diffusion tests of gold colloids

3.5.2.1 Colloid III.1-P0 (UV-521 nm)

The colloid III.1-P0 prepared after one day is dropped on the TLC silica gel as shown in Figure 3.13. Violet boundary formed immediately on the boundary of the initial contact layer.

The final shape of the circle is very interesting, there seems two purple diffusion layers separated by an “isolation belt” colored light purple. The drop test of colloid III.1-P0 displays unique purple color much deeper than the other colloids.

Figure 3.13 Photos of one droplet of colloid III.1-P0 onto the TLC silica gel.

The HMLD test of colloid III.1-P0 evidences the consistent result of UV spectra that this colloid varying with time possesses heterogeneous size distribution and even large aggregations.

3.5.2.2 Colloid III.2-P0.5 (UV-535 nm)

The results of HMLD test over colloid III.2-P0.5 is very similar with that of colloid III.1-P0,

Inner boundary with purple color

Formation of clear isolation belt Deep purple color resulted from

tough movement of aggregations

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expect that the essential color is light pink and the inner boundary and layer is not that obvious (Figure 3.14). A very thick pink-purple edge is observed during the drying process. If carefully watched under light, the inner diffusion layer can be observed. From the result of HMLD test, it can be seen that only very few amount of PVA (e.g. PVA: Au = 0.5 weight ratio, equal to 0.002:

1 molar ratio) is quite helpful for the protection of Au-NPs from growing.

Figure 3.14 Photos with time after one drop of colloid III.2-P0.5 onto the TLC silica gel. The third image is treated by changing the luminance and contrast to be -13% and 41% respectively for a better view of the inner layer.

3.5.2.3 Colloid III.5-P5 (UV-510 nm)

Figure 3.15 displays the photos with time after one droplet of colloid III.5-P5 onto the TLC silica gel. The satisfactory diffusion procedure is performed over this colloid as displayed below. The color of the outside circle becomes deeper pink than the inner circle at the beginning due to the fast diffusion of colloid, indicating that small particles exist in this colloid and are freely move with the liquid. No boundary is formed after the water in the initial contact layer diffuses out, suggesting that the particle sizes of Au-NPs is generally homogeneous. Besides, very thick pink boundary is displayed at the end and the silica surface is covered by uniform light pink, further evidencing that the Au-NPs in this colloid should be small with narrow size distribution.

Deeper color formed at the edge of water bubble

Deeper pink-purple centered on the edge of isolation belt

Light pink-purple color on the external surface

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Figure 3.15 Photos with time after one droplet of colloid III.5-P5 onto the TLC silica gel.

Among all the colloids prepared from the applied method, the colloid III.5-P5 (the same of II.5-S5) is applied to be the most favorable sample with both small sizes and narrow size distribution from the combination results of the colloid color, the UV-vis spectra, and the HMLD test.