Citrate-stabilized
silver nanoparticles (AgNPs) were functionalized with a pH-responsive
amphiphile, 3-[(2-carboxy-ethyl)-hexadecyl-amino]-propionic acid (C16CA).
At pH ∼ 4, the zwitterionic C16CA assembled into lamellar structures
due to the protonation of the amine groups of the amphiphile that
neutralized the anionic charge of the carboxylate groups. The lamellar
supramolecules incorporated the AgNPs into their 3D network and extracted
them from water. C16CA
supramolecules dissolved into water (at pH > 6) and organic solvents;
consequently, the recovered C16CA-AgNPs were redispersed not only
to water but also to chloroform and tetrahydrofuran without any additional
functionalization. C16CA acted as a pH-responsive stabilizer of AgNPs
and formed a solvent-switchable molecular layer such as a bilayered
structure in water and densely packed monolayer in chloroform and
tetrahydrofuran. Redispersion of the AgNPs was achieved in different
solvents by changing the solvent affinity of the adsorbed C16CA molecular
layer based on the protonation of the amine groups of the pH-responsive
amphiphile. The morphology of redispersed AgNPs did not change during
the recovery and redispersion procedure, due to the high steric effect
of the network structure of C16CA supramolecules. These observations
can lead to a novel solvent-exchange method for nanocrystals without
aggregation and loss of nanocrystals, and they enable effective preparations
of stimuli-responsive plasmonic nanomaterials.