In addition to the single stimuli,2D plasmonic assemblies are responsive to multiple stimuli. Two mixed polymers wereutilized to assemble amphiphilic 2D plasmonic arrays at oil-water interface.37 By choosing different grafting ligands, the assembly processes canbe switched reversibly under different solvent quality or pH. Specifically, binary ligands containing poly (ethyleneglycol) (PEG) and poly (methyl methacrylate) (PMMA) induced a solvent quality response, while mixed ligands of PEG and poly (2-(diethylamino)ethylmethacrylate) (PDEA) brought pH response to the assemblies (Figure 2.7c).
In awater and chloroform system, the nanoparticles suspension stayed in chloroform,adding hexane will trigger the assembly of Au nanoparticle, forming golden colored films. When replacing hexane-chloroformwith pure chloroformwill dissolve the assemblies back to suspension, leading to a reversibleassembly (Figure 2.7d). For the [email protected]/PDEA system, both adding hexane andadjusting the pH can lead to reversible transition between oil and aqueousphases, due to the hydrophilic-to hydrophobic switching of PDEA under differentsolvent quality/pH. Since the reversible process is controlled by the surface properties, not the building blocks, suchmethod can be extended to gold nanorod system as well.The above examples are all based onthe response that comes from ligands, inducing specific substrates can alsoachieve structure transformation of the 2D assemblies as well.
For example, thermalsensitive polymer- Poly (N-isopropylacrylamide)(PNIPAM) can act as a temperature-responsive polymer substrate to controlnanoparticle spacing of an Ag film transferred from air-water interface.100 Since PNIPAM can shrink above the transition temperature whileswell blew the transition temperature, the nanoparticles were pushed closer toeach other under the shrinkage of PNIPAM film. Also, the strong interactionbetween hydrophobic nanoparticles and thepolymer substrate enable the reorganization of nanoparticles on the substrate,resulting in a uniform and densely packed structure. Reduction of Au or Agnanoparticle in situ on a hyperbranched poly (ether amine) (hPEA)substrate provided another way to obtain transformable assemblies.101 Such kind of hybrid nanosheet is amphiphilic since hPEA compriseda hydrophilic outer layer and a hydrophobic inner layer. Thus, this hybridnanosheet is responsive to multiple stimuli such as temperature, ionic strengthand pH, and can reversibly transfer between water and oil phase. For instance,increasing temperature will destroy the bonding between water and thehydrophilic chains of hPEA, resulting in the aggregation (Figure 2.7e).
Decreasing the temperature will reversibly disperse the hybrid nanosheets inwater. Increasing the ionic strength will transfer the hybrid nanosheets to theoil phase. Although the hydrodynamicvolumes of hybrid nanosheets are over 500nm, the films can be reversibly transferred between oil and waterphase by controlling of pH or temperature. Such hydrophilic/hydrophobictransformation of substrates provide new models in the design of environment-sensitiveassemblies.