Significantly improved stability of silver nanodots via nanoparticles encapsulation

Luminescent silver nanodots are bright, silver cluster-based emitters with tunable emission windows from the blue to near-IR. Their stability and photophysical properties depend highly on the protection group that forms coordinate bonds with the cluster core. The coordinate nature of such protections suggests that silver nanodots are vulnerable to any materials that competitively bind to the silver-cluster core, resulting in deterioration of nanodots. Given the excellent photophysical properties of silver nanodots, it is necessary to investigate methods to stabilize silver nanodots. While nanoparticles offer diverse platforms to protect silver nanodots and adequate room to build smart, robust, and multi-functional silver nanodot-nanoparticle hybrids, we examined the construction of nanoparticle-encapsulated silver nanodots in reverse micelles, liposomes, and silica nanoparticles. Charges of surfactants in organic nanoparticles strongly influence the stability of silver nanodots. Both reverse micelles and liposomes built of charged surfactants destabilize silver nanodots, but silver nanodots are stable in non-ionic reverse micelles. However, it is difficult to encapsulate a silica layer on top of silver nanodots due to electrostatic repulsions between the DNA molecules and hydrolyzed tetraethyl orthosilicate. Such repulsions are overcome by introducing an amino silane to cross-link silver species and orthosilicate and to initiate the growth of silica surrounding silver nanodots. This optimized protocol can be applied to any silver nanodot, yielding multi-color, chemically and photophysically stable silica nanoparticle-encapsulated silver nanodots in PBS.