Conjugate polymer-based membranes for gas separation applications: current status and future prospects

Conjugate polymers provide the possibility of exploiting both the chemical and physical attributes of the polymers for membrane-based gas separation. The presence of delocalized π electrons provides high chain stiffness with low packing density, thus making the membrane a rigid structure that favors facilitated transport. Historically, the polymeric membranes were constrained by the tradeoff relationship between gas permeability and gas selectivity. So, different methods were investigated to prepare the membranes that can overcome the limitation. In recent years, electroconductive polymeric membranes have gained attention with their enhanced transportation properties combining the separation behavior depending on both molecular size discrimination as well as the facilitated transport. They offer better selectivity toward polar gases such as CO₂ because of the increased solubility. This review is aimed to provide a literature survey on gas separation using conjugate polymers such as polyaniline, polypyrrole, and some derivatives of polythiophenes. It contains various methods used by different researchers to enhance the gas separation properties of the membranes with improved mechanical and thermal stability such as changing the morphology and membrane preparation methods. In addition, it provides the pros and cons of various factors affecting the conjugate polymer membrane performance. The major challenges and future work that can be done in improving the transportation properties through the membrane to achieve viable membranes are also discussed so that they can be used for commercial and practical applications in the future.     

Printing polyaniline for sensor applications

In recent years, much research has focused on the development of low-cost, printed electrochemical sensor platforms for environmental monitoring and clinical diagnostics. Much effort in this area has been based on utilising the redox properties of conducting polymers, particularly polyaniline (PANI). In tackling the inherent lack of processability exhibited by these materials, several groups have examined various mass-amenable fabrication approaches to obtain suitable thin films of PANI for sensing applications. Specifically, the approaches investigated over the years include the in situ chemical synthesis of PANI, the use of sulphonated derivatives of PANI and the synthesis of aqueousbased nano-dispersions of PANI. Nano-dispersions have shown a great deal of promise for sensing applications, given that they are inkjet-printable, facilitating the patterning of conducting polymer directly to the substrate. We have shown that inkjet-printed films of PANI can be finely controlled in terms of their two-dimensional pattern, thickness, and conductivity, highlighting the level of precision achievable by inkjet printing. Utilising these nanomaterials as inkjet-printable inks opens novel, facile, and economical possibilities for conducting polymer-printed electronic applications in areas of sensing, but also many other application areas such as energy storage, displays, organic light-emitting diodes. Given that inkjet-printing is a scalable manufacturing technique, it renders possible the large-scale production of devices such as sensors for a range of applications. Several successes have emerged from our work and from the work of others in the area of applying PANI in low-cost sensor applications, which is the focus of this review.     

Cobalt-doped silica membranes for gas separation

Cobalt-doped silica membranes were synthesized using tetraethyl orthosilicate-derived sol mixed with cobalt nitrate hexahydrate. The cobalt-doped silica structural characterization showed the formation of crystalline Co₃O₄ and silanol groups upon calcination. The metal oxide phase was sequentially reduced at high temperature in rich hydrogen atmosphere resulting in the production of high quality membranes. The cobalt concentration was almost constant throughout the film depth, though the silica to cobalt ratio changed from 33:1 at the surface to 7:1 at the interface with the alumina layer. It is possible that cobalt has more affinity to alumina, thus forming CoOAl₂O₃. The He/N₂ selectivities reached 350 and 570 at 160 °C for dry and 100 °C wet gas testing, respectively. Subsequent exposure to water vapour, the membranes was regenerated under dry gas condition and He/N₂ selectivities significantly improved to 1100. The permeation of gases generally followed a temperature dependency flux or activated transport, with best helium permeation and activation energy results of 9.5 × 10⁻⁸ mol m⁻² s⁻¹ Pa⁻¹ and 15 kJ mol⁻¹. Exposure of the membranes to water vapour led to a reduction in the permeation of nitrogen, attributed to water adsorption and structural changes of the silica matrix. However, the overall integrity of the cobalt-doped silica membrane was retained, given an indication that cobalt was able to counteract to some extent the effect of water on the silica matrix. These results show the potential for metal doping to create membranes suited for industrial gas separation.     

Magnetically activated micromixers for separation membranes

Presented here is a radically novel approach to reduce concentration polarization and, potentially, also fouling by colloids present in aqueous feeds: magnetically responsive micromixing membranes. Hydrophilic polymer chains, poly(2-hydroxyethyl methacrylate) (PHEMA), were grafted via controlled surface-initiated atom transfer radical polymerization (SI-ATRP) on the surface of polyamide composite nanofiltration (NF) membranes and then end-capped with superparamagnetic iron oxide magnetite (Fe(3)O(4)) nanoparticles. The results of all functionalization steps, that is, bromide ATRP initiator immobilization, SI-ATRP, conversion of PHEMA end groups from bromide to amine, and carboxyl-functional Fe(3)O(4) nanoparticle immobilization via peptide coupling, have been confirmed by X-ray photoelectron spectroscopy (XPS) and field emission scanning electron microscopy (FESEM). These nanoparticles experience a magnetic force as well as a torque under an oscillating external magnetic field. It has been shown, using particle image velocimetry (PIV), that the resulting movement of the polymer brushes at certain magnetic field frequencies induces mixing directly above the membrane surface. Furthermore, it was demonstrated that with such membranes the NF performance could significantly be improved (increase of flux and salt rejection) by an oscillating magnetic field, which can be explained by a reduced concentration polarization in the boundary layer. However, the proof-of-concept presented here for the active alteration of macroscopic flow via surface-anchored micromixers based on polymer-nanoparticle conjugates has much broader implications.     

Biomimetic PLGA sensor: proof of principle and application

Abstract  

Spoilage of products can mainly be attributed to microorganisms which “live on the product”, i.e. which are able to utilize and/or metabolize components and/or molecules of the product. The objective of this work was to develop and optimize sensor prototypes indicating the quality of a product “in real time”, i.e. at the time the consumer is looking at the product. The design of the presented sensors relates to optical phenomena, for example anomalous absorption and remission of light. The crucial point of the sensor prototypes is a layer sensitive to the analyte, a polymer degradable by enzymes produced by the respective microorganisms. After incubation of the sensor setup with contaminated products, the lytic enzymes released from decaying cell material change the thickness of the polymer layer and generate a colour change visible by the naked eye. Production of the sensor prototypes is very simple and inexpensive and they might be successfully integrated into product packaging.     

Bioinspired membranes for multi-phase liquid and molecule separation

Water pollution is a serious problem around the world. It causes the lack of clean drinking water and brings risks to human health.Membrane technology has become a competitive candidate to treat the contaminated wastewater due to its high separation efficiency and low energy consumption. In this review, we introduce the recent development of several kinds of bioinspired separation membranes, involving the membrane design and applications. We emphasize the multi-phase liquid separation membranes inspired from nature with special wettability applied for oil/water separation, organic liquids mixture separation, and emulsion separation. After separating multi-phase liquids using these membranes, small molecule pollutants still exist in singlephase liquid. Therefore, we also expand the scope to small molecule-scale separation membranes, such as the nacre-like graphene oxide separation membrane and other nanofiltration membranes. Summary and outlook concerning the future development of separation membranes are also introduced briefly.     

Recent advances on electroactive CNT-based membranes for environmental applications: The perfect match of electrochemistry and membrane separation

Global climate change, growing population, and environmental pollution underscore the need for a greater focus on providing advanced water treatment technologies. Although electrochemical based-processes are becoming promising solutions, they still face challenges owing to mass transport and upscaling which hinder the exploitation of this technology. Electrode design and reactor configuration are key factors for achieving operational improvements. The electroactive membrane has proven to be a breakthrough technology integrating electrochemistry and membrane separation with an enhanced mass transport by convection. In this review article, we discuss recent progress in environmental applications of electroactive membranes with particular focus on those composed of carbon nanotubes (CNT) due to their intriguing physicochemical properties. Their applications in degradation of refractory contaminants, detoxification and sequestration of toxic heavy metal ions, and membrane fouling alleviations are systematically reviewed. We then discuss the existing limitations and opportunities for future research. The development of advanced electroactive systems depends on interdisciplinary collaborations in the areas of materials, electrochemistry, membrane development, and environmental sciences.     

Biomimetic material--poly(N-vinylimidazole)-zinc complex for CO2 separation

A novel material, poly(N-vinylimidazole)-zinc complex, is prepared to simulate the zinc active site of carbonic anhydrase. The obtained complex can separate CO(2) efficiently.     

Perovskite-type oxides for high-temperature oxygen separation membranes

Oxygen permeation through dense ceramic membranes of perovskite-like SrCo_0.9−xFe_0.1Cr_xO_3−δ (x = 0.01–0.05), Sr_1−x−yLn_xCoO_3−δ(Ln = La, Nd, Sm, Gd; x = 0.30–0.35; y = 0–0.10), SrCo_1−xTi_xO_3−δ (x = 0.05–0.20) and LaM_1−xNi_xO_3−δ (M = Ga, Co, Fe; x = 0–0.6) was studied. The SrCoO_3−δ-based solid solutions with cubic perovskite structure were found to exhibit highest permeation fluxes compared to other membranes. However, high thermal expansion coefficients and interaction with gas species such as carbon dioxide may complicate the employment of SrCoO_3−δ membranes for oxygen separation membranes. Alternatively, the LaGa_1−xNi_xO_3−δ (x = 0.2–0.5) perovskites, having significant permeation fluxes as well as thermal expansion coefficients in the range of (10.8–11.6) × 10⁻⁶ K⁻¹, were demonstrated to be suitable as membrane materials at oxygen pressures from 1 × 10⁻² to 2 × 10⁴ Pa. Testing oxygen permeation at oxygen partial pressures of 1–60 atm showed that only oxides with a high oxygen deficiency such as SrCo_0.85Ti_0.15F_3−δ possess sufficient oxygen permeation fluxes. The oxygen permeability of perovskites on the basis of LaGaO₃ and LaCoO_3−δ was found to be negligible at oxygen pressures above 15 atm, caused by low oxygen vacancy concentration and ionic conductivity of such ceramic materials.     

Biomimetic and bioinspired silica: recent developments and applications

In a previous review of biological and bioinspired silica formation (S. V. Patwardhan et al., Chem. Commun., 2005, 1113 [ref. 1]), we have identified and discussed the roles that organic molecules (additives) play in silica formation in vitro. Tremendous progress has been made in this field since and this review attempts to capture, with selected examples from the literature, the key advances in synthesising and controlling properties of silica-based materials using bioinspired approaches, i.e. conditions of near-neutral pH, all aqueous environments and room temperature. One important reason to investigate biosilicifying systems is to be able to develop novel materials and/or technologies suitable for a wide range of applications. Therefore, this review will also focus on applications arising from research on biological and bioinspired silica. A range of applications such as in the areas of sensors, coatings, hybrid materials, catalysis and biocatalysis and drug delivery have started appearing. Furthermore, scale-up of this technology suitable for large-scale manufacturing has proven the potential of biologically inspired synthesis.     

Electrodialysis with ultrafiltration membranes for peptide separation

A number of bioactive peptides find their potential applications in food or pharmaceutical industry; however, there arise some limitations of their large-scale production to satisfy market demands. Although pressure-driven membrane processes are able of continuous production and separation of peptides, these technologies often demonstrate insufficient selectivity. Electrophoresis is a well-known purification process characterized by high resolution of separated species but it is limited by relatively low production capacity. On the other hand, electromembrane processes offer high production capacity but their limitation is the size of separated molecules. Electrodialysis with inserted ultrafiltration membranes is an alternative method of peptide separation into fractions, their concentration and possibly demineralization at the same time to achieve large production quantities. It is a hybrid process combining conventional electrodialysis and electrophoresis principles using ultrafiltration membranes. These membranes serve as a molecular barrier separating two types of solution while the driving force remains electric potential difference. This article offers state-of-the-art summary in the field of bioactive peptide separation and fractionation by electrodialysis with ultrafiltration membranes.     

Biomimetic photoelectric conversion systems based on artificial membranes

Biological light-driven proton pumps which could transfer light energy to electrical energy have aroused intense interest in the past years.Many related researches have been conducted to mimic this process in vitro because of its potential significant applications.This review describes the progress in biomimetic photoelectric conversion systems based on different kinds of promising artificial membranes.Both biological bacteriorhodopsin and the photosensitive chemical molecules which could be used to achieve...     

Biomimetic technique for adhesion-based collection and separation of cells in a microfluidic channel

A basic step in many biological assays is separating and isolating different types of cells from raw samples. To better meet these requirements in microfluidic devices for miniature biomedical analytical systems, an alternative method for separating cells has been devised by mimicking the physiological process of leukocyte recruitment to blood vessel walls: adhesive cell rolling and transient tethering. Reproducing these interactions for cells on surfaces of microstructured fluidic channels can serve to capture and concentrate cells and even to fractionate different cell types from a continuously flowing sample. To demonstrate this principle, two designs for microstructured fluidic channels were fabricated: an array of Square pillars and another with slender, Offset pillars. These structures were coated with E-selectin IgG chimera and the interactions of HL-60 and U-937 cells with these structures were characterized. With inflow of fluidic cell suspensions, the structures were able to efficiently capture and arrest cells directly from the rapid free stream flow. After capture, cells transit through the channel in three phases: cell rolling, cell tethering, and transient re-suspension in free stream flow before re-capture. Under these interactions, captured cells were enriched several hundred-fold from the original concentration. Additionally, among collected cells, the difference in flow-driven, adhesion-mediated cell transit in the Square design suggested that the two cell types could at least be partially fractionated.     

Polyarylate gas separation membranes

The molecular design of polymers for membrane separation of gases is illustrated for the oxygen/nitrogen pair by a series of polyarylates. The use of tetrabromo ring substitutions combined with a fluorene connector group on the bisphenol with or without a t-butyl group on the isophthalic acid monomer leads to state-of-the-art productivity-selectivity combinations.     

Fluid and air-stable lipopolymer membranes for biosensor applications

The behavior of poly(ethylene glycol) (PEG) conjugated lipids was investigated in planar supported egg phosphatidylcholine bilayers as a function of lipopolymer density, chain length of the PEG moiety, and type of alkyl chains on the PEG lipid. Fluorescence recovery after photobleaching measurements verified that dye-labeled lipids in the membrane as well as the lipopolymer itself maintained a substantial degree of fluidity under most conditions that were investigated. PEG densities exceeding the onset of the mushroom-to-brush phase transition were found to confer air stability to the supported membrane. On the other hand, substantial damage or complete delamination of the lipid bilayer was observed at lower polymer densities. The presence of PEG in the membrane did not substantially hinder the binding of streptavidin to biotinylated lipids present in the bilayer. Furthermore, above the onset of the transition into the brush phase, the protein binding properties of these membranes were found to be very resilient upon removal of the system from water, rigorous drying, and rehydration. These results indicate that supported phospholipid bilayers containing lipopolymers show promise as rugged sensor platforms for ligand-receptor binding.     

Designing aromatic polyamides and polyimides for gas separation membranes

Aromatic polyamides and polyimides with improved gas permselectivity, can be designed and prepared by systematically changing structural elements that affect these properties. Indeed, a conscientious choosing of the chemical changes may still provide a promising approach to get better and better polymers for selective filtration of gases. The results of this work, in which novel monomers have been used, have confirmed that gas permeability through aromatic polyamides and polyimides much higher than that of conventional polyamides and polyimides can be achieved. It has been done by introducing bulky side groups, using non-planar monomers, and combining these elements on both monomers: diamines and dianhydrides or diamines and diacids. A theoretical study has also been made to explain the behaviour of some individual polymers, comparing experimental and calculated values of density and free volume.     

Biomimetic stimuli‐responsive nanochannels and their applications

Biomimetic smart nanochannels have been studied extensively to achieve the precise ionic transport compared to biological ion channels. Similar to ion channels in living organisms, biomimetic smart nanochannels can respond to various stimuli, which allows for promising applications in many fields. In this review, we mainly summarize the recent advances in the design of biomimetic stimuli‐responsive nanochannels and their potential applications including biosensors and drug delivery. Finally, an outlook on the challenges and opportunities for biomimetic stimuli‐responsive nanochannels is provided.     

Biomimetic piezoelectric quartz sensor for caffeine based on a molecularly imprinted polymer

A piezoelectric quartz sensor coated with molecularly imprinted polymer (MIP) for caffeine was developed. The MIP was prepared by co-polymerizing methacrylic acid (MAA) and ethylene glycol dimethacrylate (EDMA) in the presence of azobis(isobutyronitrile) as initiator, caffeine as template molecule, and chloroform as solvent. The MIP suspension in polyvinyl chloride/tetrahydrofuran (6:2:1 w/w/v) solution was spin coated onto the surface of the electrode of a 10 MHz AT-cut quartz crystal. The sensor exhibited a linear relationship between the frequency shift and caffeine concentration in the range of 1×10^–7 mg mL^–1 up to 1x10^–3 mg mL^–1 [correlation coefficient (r)=0.9935] in a stopped flow measurement mode. It has a sensitivity of about 24 Hz/ln(concentration, mg mL^–1). A steady-state response was achieved in less than 10 min. The performance characteristic of the sensor shows a promising and inexpensive alternative method of detecting caffeine. Surface studies were carried out for the reagent phase of the sensor using SEM, AFM, and XPS analysis in order to elucidate the imprinting of the caffeine molecule. The SEM micrograph, AFM image, and XPS spectra confirmed the removal of caffeine by Soxhlet extraction in the imprinting process and the rebinding of caffeine to the MIP sensing layer during measurement.