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The aggregation of histidine-functionalized Au nanoparticles induced by Fe 3+ was identified to show pHdependent character. At low pH value, the Fe 3+ ion was effective to induce aggregation of the Au nanoparticles via its coordination with imidazole group of the histidine ligand. At pH value higher than 9, the histidine ligand provided an additional coordination site for the Fe 3+ ion due to deprotonization of its R-NH 2 group. Therefore, the Fe 3+ ion-induced aggregation of the Au nanoparticles was suppressed as a result of the transformation of the histidine from monodentate to bidentate ligand upon increased pH value.

There is an increasing interest in using liquid crystalline media as mobile phases in two-dimensional nanofluidic systems. Their small-scale, reduced dimensionality, and plentiful opportunities for functionalisation render such phases advantageous. However, flow control has been difficult to achieve, as the wetting processes which drive area expansion are not dynamically controllable. Here, we report on temperature-controlled monolayer spreading of 1,2-dielaidoyl-sn-glycero-3-phosphoethanolamine (DEPE) on the hydrophobic substrates SU-8, and Teflon AF (amorphous fluoropolymer). The gel/ liquid phase transition of DEPE at T c $ 38 C is exploited to toggle spreading of a molecular lipid film on SU-8. We observed that on Teflon AF, DEPE monolayer spreading occurs even below T c , and exhibits strongly accelerated spreading above the phase transition temperature. Our results demonstrate that switching DEPE monolayer spreading on and off, or, alternatively, switching between fast and slow area expansion, is a feasible approach towards establishing control over lipid film flow in two-dimensional fluidic systems. We also present a chip-based device integrating a patterned surface for 2D-microfluidics and on-chip heating.

Nanoparticles that are responsive to multiple stimuli allow more precise control over the aggregation process and thus the structures and properties of the resulting aggregates. Au nanoparticles functionalized by poly-L-histidine (PLH), a simple polypeptide with a pK(a) value (approximately 6.2) around the physiological pH, are sensitive to pH and temperature simultaneously. The dual stimuli, pH and temperature, must act in a cooperative way to switch the hydrophobic beta-sheet structure of PLH and thus trigger the assembly/disassembly of the Au nanoparticles. The aggregation process and thus the conformation change of PLH can be well recognized by the color change of the Au nanoparticles by naked eyes.

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