Seedless, surfactantless, high-yield synthesis of branched gold nanocrystals in HEPES buffer solution

In this work, three-dimensional branched gold nanocrystals were produced at high yield by reacting an aqueous solution of chloroauric acid with a Good's buffer, HEPES, at room temperature. This particular method of preparation was scalable to gram-quantity. The branched nanocrystals containing one to eight tips were stable at room temperature and could be stored as a powder after freeze-drying. They were, however, unstable at higher temperatures and transformed into spherical particles upon boiling. The formation of the branched gold nanocrystals was kinetically controlled, as shown by the dependence of shapes on temperature and precursor salt concentration. The growth of branched gold nanocrystals in the HEPES buffer was monitored by microscopic and spectroscopic techniques, allowing the detection of several key intermediates in the growth process. Piperazine in HEPES molecule was identified as the principal moiety responsible for forming highly branched Au nanocrystals in the HEPES buffer.