Wetting-Induced Polyelectrolyte Pore Bridging

Active layers of ion separation membranes often consist of charged layers that retain ions based on electrostatic repulsion. Conventional fabrication of these layers, such as polyelectrolyte deposition, can in some cases lead to excess coating to prevent defects in the active layer. This excess deposition increases the overall membrane transport resistance. The study at hand presents a manufacturing procedure for controlled polyelectrolyte complexation in and on porous supports by support wetting control. Pre-wetting of the microfiltration membrane support, or even supports with larger pore sizes, leads to ternary phase boundaries of the support, the coating solution, and the pre-wetting agent. At these phase boundaries, polyelectrolytes can be complexated to form partially freestanding selective structures bridging the pores. This polyelectrolyte complex formation control allows the production of membranes with evenly distributed polyelectrolyte layers, providing (1) fewer coating steps needed for defect-free active layers, (2) larger support diameters that can be bridged, and (3) a precise position control of the formed polyelectrolyte multilayers. We further analyze the formed structures regarding their position, composition, and diffusion dialysis performance.

Simultaneous measurement of temperature and curvature using ring-core fiber-based Mach-Zehnder interferometer

In this paper, the Mach-Zehnder interferometer (MZI) based on ring-core fiber was proposed and manufactured. Benefiting from the identical diameters of ring-core fiber, no-core fiber, and single-mode fiber, the MZI fiber sensor can be prototyped by sandwiching the ring-core fiber between the no-core fiber and the single-mode fiber (SMF). With the proposed specific structure of the ring-core fiber, the simultaneous measurement of temperature and curvature was achieved with the MZI sensor by means of monitoring the wavelength shift of interference dips. Experimental results have shown that the sensitivity of curvature sensing could reach up to -3.68 nm/m^-1 in the range from 1.3856 m^-1 to 3.6661 m^-1 with high linearity of 0.9959. Meanwhile, the maximum temperature sensitivity is measured to be 72 pm/°C with a fairly good linearity response of 0.9975. In addition, by utilizing the 2×2 matrix algorithm, the dual demodulation of temperature and curvature can be readily realized for the purpose of direct sensing. It is believed that the proposed special structure-based MZI sensor may show great potential applications in the field of fiber-optics sensing and structural health monitoring (SHM).

Dual Demodulation of Temperature and Refractive Index Using Ring Core Fiber Based Mach-Zehnder Interferometer

We report the ring core fiber spliced with single mode fiber and no core fiber which is used for temperature and refractive index (RI) sensing. The Mach-Zehnder interferometer (MZI) is formed with this kind of sandwich fiber structure and the maximum extinction ratio of the interference spectra reaches 27 dB with the free spectra range of 12 nm. The MZI fiber sensor is applied for temperature sensing with the sensitivity of 69 pm/°C and 0.051 dB/°C. The RI sensitivity reaches 182.07 dB/RIU and -31.44 nm/RIU with the RI ranging from 1.33 to 1.38. The RI value can be directly demodulated with the interference dip intensity which shows insensitivity to temperature. The demodulation of temperature can be achieved by using the linear equations between dip wavelength shift with the variation of temperature and RI.

Covalent layer-by-layer films: chemistry, design, and multidisciplinary applications

Covalent layer-by-layer (LbL) assembly is a powerful method used to construct functional ultrathin films that enables nanoscopic structural precision, componential diversity, and flexible design. Compared with conventional LbL films built using multiple noncovalent interactions, LbL films prepared using covalent crosslinking offer the following distinctive characteristics: (i) enhanced film endurance or rigidity; (ii) improved componential diversity when uncharged species or small molecules are stably built into the films by forming covalent bonds; and (iii) increased structural diversity when covalent crosslinking is employed in componential, spacial, or temporal (labile bonds) selective manners. In this review, we document the chemical methods used to build covalent LbL films as well as the film properties and applications achievable using various film design strategies. We expect to translate the achievement in the discipline of chemistry (film-building methods) into readily available techniques for materials engineers and thus provide diverse functional material design protocols to address the energy, biomedical, and environmental challenges faced by the entire scientific community.

Porous Polyelectrolytes: The Interplay of Charge and Pores for New Functionalities

The past decade has witnessed rapid advances in porous polyelectrolytes and there is tremendous interest in their synthesis as well as their applications in environmental, energy, biomedicine, and catalysis technologies. Research on porous polyelectrolytes is motivated by the flexible choice of functional organic groups and processing technologies as well as the synergy of the charge and pores spanning length scales from individual polyelectrolyte backbones to their nano-/micro-superstructures. This Review surveys recent progress in porous polyelectrolytes including membranes, particles, scaffolds, and high surface area powders/resins as well as their derivatives. The focus is the interplay between surface chemistry, Columbic interaction, and pore confinement that defines new chemistry and physics in such materials for applications in energy conversion, molecular separation, water purification, sensing/actuation, catalysis, tissue engineering, and nanomedicine.

Vapor-based synthesis and micropatterning of Janus thin films with distinct surface wettability and mechanical robustness

Janus polymer thin films with distinct surface wettability and mechanical robustness were synthesized using a facile, one-step, vapor-deposition process.

A microfluidic, dual-purpose sensor for in vitro detection of Enterobacteriaceae and biotinylated antibodies

In this paper, we present a versatile, dual-purpose sensor for in vitro detection of Enterobacteriaceae (e.g. Escherichia coli) and biotinylated antibodies (e.g. IgG rabbit polyclonal antibodies), based on different detection principles for each bioanalyte. These bioanalytes are tagged individually with functionalized magnetic microparticles, suspended into a static fluid and injected into a microfluidic channel. Without the need for bulk or complicated pumping systems, the functionalized microparticles are set in motion by a magnetic force exerted on them by integrated microconductors. The fundamental detection principle is the decrease in the velocity of the microparticles that are loaded with the respective bioanalyte, due to factors inhibiting their motion. The velocity of the unloaded, bare microparticles is used as a reference. We discovered a novel mechanism on which the constrained particle motion is based; in the case of E. coli, the inhibiting factor is the enhanced Stokes' drag force due to the greater volume and altered hydrodynamic shape, whereas in the case of biotinylated antibodies, it is the increased friction force at the interface between the modified microparticle and the biosensor's surface. Friction force is for the first time employed in a scheme for resolving biomolecules. Integrated magnetic microsensors are used for the velocity measurements by detecting the microparticles' stray field. Moreover, we developed a biocompatible, easy to implement and reliable surface modification that practically diminishes the problem of bioadhesion on the sensor's surface.

Dynamics of the layer-by-layer assembly of a poly(acrylic acid)-lanthanide complex colloid and poly(diallyldimethyl ammonium)

Poly(acrylic acid) (PAA) and lanthanide (Ln) ions, such as Ce(3+), Eu(3+), and Tb(3+), were prepared as dispersed complex colloidal particles through three different protocols with rigorous control of the pH value and mixing ratio. The negatively charged PAA-Ln complex particles were layer-by-layer (LbL) assembled with positively charged poly(diallyldimethyl ammonium) (PDDA) to prepare a thin film. The film thickness growth is much quicker than PDDA/PAA film. Due to the incorporation of Ln(3+) ions, the film exhibits fluorescence. During LbL assembly, PDDA-PAA association based on electrostatic force and PAA-Ce association based on coordination are in competition, which leads to the LbL assembly of PDDA and PAA-Ln complex colloidal particles being a complicated dynamic process.

Synergistic strengthening of polyelectrolyte complex membranes by functionalized carbon nanotubes and metal ions

Hydrophilic polymers have garnered much attention due to their critical roles in various applications such as molecular separation membranes, bio-interfaces, and surface engineering. However, a long-standing problem is that their mechanical properties usually deteriorate at high relative humidity (RH). Through the simultaneous incorporation of functionalized carbon nanotubes and copper ions (Cu(2+)), this study introduces a facile method to fabricate high strength polyelectrolyte complex nanohybrid membranes resistant to high RH (90%). For example, the tensile strength of the nanohybrid membranes is 55 MPa at 90% RH (80% of the original value at 30% RH). These results are explained by copper ions depressing the swelling degree of the membrane, and functionalized carbon nanotubes promoting stress transfer between the polymer matrix and them. The nanohybrid membranes are efficient in separating water/alcohol mixtures containing relatively high water content (up to 30 wt%), whereas common hydrophilic polymer membranes usually suffer from excessive swelling under this condition.

New 2-in-1 polyelectrolyte step-by-step film buildup without solution alternation: from PEDOT-PSS to polyelectrolyte complexes

Although never emphasized and increasingly used in organic electronics, PEDOT-PSS (poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate)) layer-by-layer (lbl) film construction violates the alternation of polyanion and polycation rule stated as a prerequisit for a step-by-step film buildup. To demonstrate that this alternation is not always necessary, we studied the step-by-step construction of films using a single solution containing polycation/polyanion complexes. We investigated four different systems: PEDOT-PSS, bPEI-PSS (branched poly(ethylene imine)-poly(sodium 4-styrene sulfonate)), PDADMA-PSS (poly(diallyl dimethyl ammonium)-PSS), and PAH-PSS (poly(allylamine hydrochloride)-PSS). The film buildup obtained by spin-coating or dipping-and-drying process was monitored by ellipsometry, UV-vis-NIR spectrophotometry, and quartz-crystal microbalance. The surface morphology of the films was characterized by atomic force microscopy in tapping mode. After an initial transient regime, the different films have a linear buildup with the number of deposition steps. It appears that, when the particles composed of polyanion-polycation complex and complex aggregates in solution are more or less liquid (case of PEDOT-PSS and bPEI-PSS), our method leads to smooth films (roughness on the order of 1-2 nm). On the other hand, when these complexes are more or less solid particles (case of PDADMA-PSS and PAH-PSS), the resulting films are much rougher (typically 10 nm). Polycation/polyanion molar ratios in monomer unit of the liquid, rinsing, and drying steps are key parameters governing the film buildup process with an optimal polycation/polyanion molar ratio leading to the fastest film growth. This new and general lbl method, designated as 2-in-1 method, allows obtaining regular and controlled film buildup with a single liquid containing polyelectrolyte complexes and opens a new route for surface functionalization with polyelectrolytes.

Layer-by-layer assembly of salt-containing polyelectrolyte complexes for the fabrication of dewetting-induced porous coatings

The layer-by-layer (LbL) assembly of salt-containing nonstoichiometric polyelectrolyte complexes (PECs) with oppositely charged uncomplexed polyelectrolyte for the fabrication of dewetting-induced porous polymeric films has been systematically investigated. Salt-containing poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) complexes (noted as PAH-PAA) with a molar excess of PAH were LbL assembled with polyanion poly(sodium 4-styrenesulfonate) (PSS) to produce PSS/PAH-PAA films. The structure of the PAH-PAA complexes is dependent on the concentration of NaCl added to their aqueous dispersions, which can be used to tailor the structure of the LbL-assembled PSS/PAH-PAA films. Porous PSS/PAH-PAA films are fabricated when salt-containing PAH-PAA complexes with a large amount of added NaCl are used for LbL assembly with PSS. In-situ and ex-situ atomic force microscopy measurements disclose that the dewetting process composed of pore nucleation and pore growth steps leads to the formation of pores in the LbL-assembled PSS/PAH-PAA films. The present study provides a facile way to fabricate porous polymeric films by dewetting LbL-assembled polymeric films comprising salt-containing PECs.

Highly sensitive and fast responsive fiber-optic modal interferometric pH sensor based on polyelectrolyte complex and polyelectrolyte self-assembled nanocoating

A new fiber-optic pH sensor is demonstrated by coating negatively charged polyelectrolyte complex (PEC(-)) nanoparticles, made of sodium carboxymethyl cellulose and poly(diallyldimethylammonium chloride) (PDDA), and positively charged PDDA on the surface of a thin-core fiber modal interferometer (TCFMI) with a layer-by-layer (LbL) electrostatic self-assembly method. The fabricated TCFMI pH sensor has different transmission dip wavelengths under different pH values and shows high sensitivities of 0.6 nm/pH unit and -0.85 nm/pH unit for acidic and alkaline solutions, respectively, and short response time of 30-50 s. The LbL electrostatic self-assembly process of a PEC(-)/PDDA multilayer is traced by quartz crystal microbalance and shows a fast thickness growth. Atomic force microscopy shows the root mean square (RMS) surface roughness of electrostatic self-assembly nanocoating of polyelectrolyte complex/polyelectrolyte is much higher than that of polyelectrolyte/polyelectrolyte due to the larger size of PEC(-) colloidal nanoparticles. The enhanced RMS surface roughness and thickness of the nanocoating can shorten the response time and raise the sensitivity of the TCFMI pH sensor, respectively. In addition, the TCFMI pH sensor has highly reversible performance and good durability.