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We eventually succeded in producing electroless (auto catalytic) gold coating on fused silica glass pieces. There are two known methods (there may be more) of depositing electroless gold coating: cyanide based and non-cyanide based. Cyanide based electroless gold coating is done using potassium gold cyanide KAu(CN)2 [chemical name: potassium dicyanoaurate(I)], whereas non-cyanide based electroless gold coating is done using gold(III) chloride trihydrate (HAuCl4.3H2O) [other names: hydrogen tetrachloroaurate(III), tetrachloroauric(III) acid].
Our first attempt was cyanide bath, which didn't work well. So, we switched to the non-cyanide bath. We followed the method described by Jiandong Hu et al with some modifications. Here are the five steps:
- Surface hydroxylation
- Surface functionalization (-NH2 addition) using APTMS (3-aminopropyltrimethoxysilane)
- Preparation of self-assembled monolayers (SAMs) of gold nanoparticles (AuNPs)
- Electroless gold deposition using HAuCl4 solution
- 30 minutes sonication in acetone (CH3COCH3).
- 30 minutes sonication in methanol (CH3OH).
- 30 minutes sonication in 1:1 :: methanol:HCl.
- 30 minutes in concentrated sulfuric acid (H2SO4).
- Rinse with DI water, isopropyl alcohol, and dry with N2 gas.
2. Surface hydroxilation (addition of -OH groups on the surface)
- 30 minutes immersion in Piranha solution (7:3 :: H2SO4:H2O2) maintained at 70oC.
- Rinse with DI water and dry with N2 gas.
3. Surface functionalization (-NH2 addition)
[ using APTMS (3-aminopropyltrimethoxysilane) ]
- 16 hours in 9.1% APTMS solution in methanol (i.e., 10 ml APTMS in 100 ml ultra pure methanol).
- Rinse with methanol, DI water, and dry with N2 gas.
[Note: All the glassware were cleaned with aqua regia before steps 4 and 5. It is a very crucial step.]
4. Preparation of self-assembled monolayers (SAMs) of gold nanoparticles (AuNPs).
- 6 hours immersion in colloidal gold nanoparticles (AuNPs) solution. Click here to read in detail the preparation of gold nanoparticles.
- Rinse with DI water (please note that the electroless gold deposition should be done immediately after this step).
5. Electroless gold deposition using HAuCl4 solution
- 10 minutes immersion and vigorous agitation in 5 ml 0.01% HAuCl4 solution + 0.25 ml H2O2 (within 2 minutes the color change was observed).
- Rinse with DI water, dry with N2 gas, and store it in a clean container until further excercise.
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