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.

The amphiphilic, pH-responsive amine derivative, 3-[(2-carboxy-ethyl)-hexadecyl-amino]-propionic acid (C16CA) was used for the functionalization of gold nanorods (Au NRs) prepared with cetyltrimethylammonium bromide (CTAB). The Au NRs could also be stabilized with C16CA owing to the selective adsorption of the amino moiety, and the Au NRs were well dispersed following one-step ligand exchange using C16CA. Based on a change in the nature of C16CA, self-assemblies of spherical micelles (pH > 5) or lamellar precipitates (pH 2–5) are formed in dispersion. Au NRs were incorporated into the precipitates at pH 2–5, but could be redispersed by redissolution of C16CA at pH > 5. The pH-induced recovery–redispersion of Au NRs was successfully accomplished without affecting the morphology of the Au NRs, the amount of Au in the dispersion, or the catalytic activity of the Au NRs for the reduction of p-nitrophenol.

The pH-responsive self-assembly of zwitterionic amphiphile C16CA was expanded to the recovery of gold (Au) nanoparticles for environmentally friendly chemistry applications. Multilayered lamellae at pH ∼ 4 were successfully incorporated into nanoparticles by dispersion. Redispersion of nanoparticles was achieved under basic conditions by the transition of self-assembly.

Micro-facet formation processes were observed, with atomic force microscopy (AFM), at various crystal faces of two aragonite-type minerals, cerussite (PbCO3) and strontianite (SrCO3), in dilute aqueous acetic acid. Among the prismatic faces, parallel to the c-axis, electrically neutral (1 0 0) and (1 3 0) faces were stabilized in the acidic environment, as in the case of aragonite (CaCO3). On the other hand, polar (1 1 0) and (0 1 0) faces of PbCO3 remained to be stable, in contrast with the case of aragonite. Even atom-resolved AFM images were observed with the (1 1 0) face, corresponding to the arrangement of carbonate ions, most probably protonated. Since Pb2+ is a more polarizable soft acid, Pb–O bonds bear covalent nature in addition to Coulombic nature. Due to diminished polarity with the bonds, the (1 1 0) face remained stable even in the acidic condition. Among the crystal faces intersecting with the c-axis, stabilization, in the acidic condition, of (1 1 1) faces of SrCO3 and (0 2 1) faces of PbCO3 was observed in contrast to the stabilization of (1 1 2) faces of aragonite, suggesting flexible nature of the mineral faces having larger cations. Relationship between natural crystal forms and growth environment was discussed.

Micro-facet formation processes were observed, with atomic force microscopy (AFM), at presumably less stable (0 1 1) and (1 1 1) faces of anhydrite (CaSO4) crystal in aqueous solutions containing either Ca2+ or SO42− ions. The order of relative stabilities of the crystal faces was (0 1 0)∼(1 0 0)>(1 1 1)>(0 1 1)≥(0 0 1). While (0 1 1), (0 1 0) and (1 1 1) faces were stabilized by either ions, (0 0 1) face was stabilized mainly by SO42− ions. Atom-resolved AFM images were observed in air for (0 1 1) and (1 1 1) faces after soaking in CaCl2 aq. (0.01 mol dm−3). At these faces, positive charges of Ca2+ at the outermost layers are partly cancelled by the negative charges of O atoms of the SO42− ions centered at lower levels. At (1 0 0) face, {1 1 1} ledges with bilayer height make growth or dissolution fronts around a screw dislocation. The relationship between natural crystal forms and chemical growth environment is discussed. Growth in sulfate-rich and Ca2+-rich conditions gives different crystal shapes.

The pH-responsive self-assembly of zwitterionic amphiphile C16CA was expanded to the recovery of gold (Au) nanoparticles for environmentally friendly chemistry applications. Multilayered lamellae at pH ∼ 4 were successfully incorporated into nanoparticles by dispersion. Redispersion of nanoparticles was achieved under basic conditions by the transition of self-assembly.