Minimizing Formaldehyde Use in the Synthesis of Gold-Silver Core-Shell Nanoparticles

Through a careful analysis of the role of formaldehyde in the preparation of silver nanomaterials, a previously unnoticed function of the reagent has been discovered. Formaldehyde reacts with ammonium hydroxide to form a polymer that changes how silver attaches to and coats a substrate. In the case of a gold nanoparticle substrate, this polymer is responsible for creating a nonconcentric core shell nanoparticle with a near-infrared plasmon resonance at 700 nm. In contrast, when conditions do not favor synthesis of this polymer, concentric nanoparticles are formed that have a plasmon resonance between that of gold and silver at 498 nm. Understanding this second role of formaldehyde allowed us to decrease the amount used 100-fold compared to previous methods, providing a greener synthesis. In addition, it is shown that ascorbic acid can function as a partial substitute for formaldehyde in this synthesis. This strategy may be effective at minimizing or eliminating formaldehyde from the synthesis of other core shell nanoparticles and nanoshells, facilitating their use in medical applications.