Fan Xia

Smart responsive surfaces switching reversibly between super-hydrophobicity and super-hydrophilicity

Super-hydrophilicity and super-hydrophobicity are fundamentally opposite properties of special wettability, which are governed by surface chemical composition and surface roughness. Smart responsive surfaces switching reversibly between super-hydrophobicity and super-hydrophilicity can be effectively fabricated by modification of stimuli-responsive materials on rough surfaces. The externally applied stimuli include light irradiation, electrical potential, temperature, pH or selected solvents, and mechanical forces. Such surfaces with controllable wettability are of great importance to both fundamental research and practical applications.

Current Rectification in Temperature-Responsive Single Nanopores

Herein we demonstrate a fully abiotic smart single-nanopore device that rectifies ionic current in response to the temperature. The temperature-responsive nanopore ionic rectifier can be switched between a rectifying state below 34 degrees C and a non-rectifying state above 38 degrees C actuated by the phase transition of the poly(N-isopropylacrylamide) [PNIPAM] brushes. On the rectifying state, the rectifying efficiency can be enhanced by the dehydration of the attached PNIPAM brushes below the LCST. When the PNIPAM brushes have sufficiently collapsed, the nanopore switches to the non-rectifying state. The concept of the temperature-responsive current rectification in chemically-modified nanopores paves a new way for controlling the preferential direction of the ion transport in nanofluidics by modulating the temperature, which has the potential to build novel nanomachines with smart fluidic communication functions for future lab-on-chip devices.

Towards understanding the nanofluidic reverse electrodialysis system: well matched charge selectivity and ionic composition

The widespread use of tiny electrical devices, from microelectromechanical systems (MEMS) to portable personal electronics, provides a new challenge in the miniaturization and integration of power supply systems. Towards this goal, we have recently demonstrated a bio-inspired nanofluidic energy harvesting system that converts salinity gradient energy from the ambient environment into sustainable electricity with single ion-selective nanopores (Adv. Funct. Mater. 2010, 20, 1339). The nanofluidic reverse electrodialysis system (NREDS) significantly improves the performance of conventional membrane-based reverse electrodialysis systems due to a higher ionic flux and a lower fluidic resistance. However, the fundamental working mechanism of the NREDS has been largely unexplored in the literature. In this work we have systematically investigated the performance of the NREDS in relation to the electrolyte type and the charge selectivity of the nanofluidic channel using both experimental and theoretical approaches. Experimental results show that the short-circuit current, the open-circuit voltage, and the resulting electric power of the NREDS are very sensitive to the ionic composition of the electrolyte solution. Through an in-depth theoretical analysis, two dominant factors that govern the charge separation and ion selectivity of the nanochannels were identified. The results prove that, with well-matched electrolyte types and nanopore charge selectivity, the harvested electric power and energy conversion efficiency can be improved by nearly two orders of magnitude.

Wettability switching between high hydrophilicity at low pH and high hydrophobicity at high pH on surface based on pH-responsive polymer

Surfaces obtained by modifying poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) on rough silicon substrates are highly hydrophilic at low pH and highly hydrophobic at high pH; such surfaces effectively supplement the research on the wettability of solid surfaces based on the pH-responsive polymers.

Switchable wettability on cooperative dual-responsive poly-L-lysine surface.

A cooperative dual-responsive polypeptide surface switching between superhydrophilic and superhydrophobic states is presented. This macroscopic phenomenon of surface originates from the combination of the cooperative unfolding/aggregation of the poly-L-lysine (PLL) immobilized on the substrate with micro/nanocomposite structure in response to pH and temperature. At pH lower than the pK(a) of PLL (approximately 11.0), PLL mainly adopts a random coil conformation, which corresponds to the superhydrophilic state on the rough surface substrate. Raising the pH to higher than the pK(a) allows the appearance of alpha-helix conformation, which also corresponds to the hydrophilic state. However, heating up the surface at pH higher than the pK(a) destabilizes the alpha-helix conformation and induces the formation of aggregated beta-sheet structures, which represents the superhydrophobic state. Lowering the pH and temperature simultaneously switches a reversible conversion from superhydrophobic to superhydrophilic states. In the switching process, the hydrophobicity and hydrophilicity can be memorized due to the cooperative pH and temperature stimuli-induced unfolding/aggregation behaviors of PLL. This provides a new exciting prospect for understanding surface properties of polypeptides and the design of smart material surfaces with potential applications in nanodevices, bioseparation, and biosensors.

Tuning surface wettability through supramolecular interactions

A dual-stimuli-responsive surface with tunable wettability, switching between hydrophilicity and hydrophobicity, and responsivity to both temperature (T) and concentration of β-cyclodextrin (β-CD), is reported. Such surfaces are obtained by simply fabricating a poly(N-isopropyl acrylamide-co-adamantanyl acrylamide) P(NIPAAm-co-ADAAm) copolymer thin film on both a flat and an etched silicon substrate. Switching between hydrophilicity and hydrophobicity can be realized over both a temperature range of about 20 °C and over a relatively wide concentration of β-CD range from 0 M to 1 × 10−1 M.