The reason why I don’t want to use NaOH to adjust its pH value is because the gold solution (HAuCl4) starts to precipitate when adjusted to pH>7.
Dear Chen-Wei Lai many thanks for asking this very interesting technical question. To my knowledge, adjusting the pH value of tetrachloroauric acid (HAuCl4) with NaOH to ca. 9-10 in aqueous solution should present no problem. This has been described for example in the attached Suplementary Information of the article
Plasmonic Giant Quantum Dots: Hybrid Semiconductor-Metal
Nanostructures for Truly Simultaneous Optical Imaging, Photothermal Effect
and Thermometry
(see first paragraph on page 2)
An obvious alternative would be to use an aqueous solution of ammonia (NH3) to adjust the pH value. However, I would strongly suggest to stay away from using ammonia in this case. Addition of ammonia to a solution of HAuCl4 could lead to the formation (precipitation) of so-called "fulminating gold", which is highly explosive. For more information about fulminating gold please have a look at the respective Wikipedia entry.
Good luck with your research and best wishes!
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