CO2-responsive polymeric materials: synthesis, self-assembly, and functional applications

Carbon Dioxide and Nitrogen-Modulated Shape Transformation of Chitosan-Based Composite Nanogels

Chitosan/poly[N-(3-(dimethylamino)propyl)methacrylamide]/poly(acrylic acid) (CS/PDMAPMA/PAA) composite nanogels (CPACNGs) were fabricated in the solution of chitosan by surfactant-free emulsion polymerization. N-(3-(Dimethylamino)propyl)methacrylamide (DMAPMA) and acrylic acid (AA) were initiated by 2,2'-azobis-2-methyl-propanimidamide to graft from the backbone of chitosan. Nanogels were formed by noncovalent forces, including of hydrogen bonds, hydrophobic, and electrostatic interaction. Nanogels were characterized by transmission electron microscopy, scanning electron microscope dynamic light scattering, X-ray photoelectron spectroscopy, Fourier transform infrared spectrometer spectra, and 1H NMR. Spherical nanoparticles were observed in the latex system. Nanogels exhibited an excellent CO2 responsivity and CO2/N2 reversible response and switchability and had a faster response rate. The morphological shape transformation of nanogels was modulated by bubbling with CO2 and N2. The responsive mechanism was explored by determining the pH and electrical conductivity. In addition, nanogels were successfully emulsified by bubbling with CO2, and then a phase transition was achieved by bubbling with N2 in the organic solvent/water mixture.

Light-, temperature-, and pH-responsive micellar assemblies of spiropyran-initiated amphiphilic block copolymers: Kinetics of photochromism, responsiveness, and smart drug delivery

Multi-responsive polymer assemblies are a significant class of smart polymers with potential applications in drug-delivery and gen-delivery systems. Poly(dimethylaminoethyl methacrylate) (PDMAEMA) is among the most applicable multi-responsive polymers that changes its physical and chemical properties in response to temperature, pH, and CO₂. Herein, different types of light-, temperature-, pH-, and CO₂-responsive polymer assemblies were developed based on multi-responsive PDMAEMA and hydrophobic poly(methyl methacrylate) blocks. In addition, spiropyran was incorporated at the chain ends by using spiropyran-initiated atom transfer radical polymerization method. Novel smart drug-delivery systems were developed by self-assembly of these amphiphilic block copolymers to micellar morphologies in aqueous media. Dynamic light scattering results showed that size of the polymer assemblies changed in response to pH variations (from 5 to 9), temperature changes (above the lower critical solution temperature (LCST) of PDMAEMA), and also UV light irradiation (wavelength of 365 nm). The LCST of PPDMAEMA showed a shift from 53 to 60 °C after isomerization of the SP to MC form, as a result of increase of polarity and water-solubility. The PDMAEMA block results in responsivity of the prepared copolymer assemblies to CO₂, which display pH variation from 8-8.6 to 5-6 after 2 min of CO₂ gas bubbling. All the multi-responsive micellar polymer assemblies showed various loading capacities and release profiles, and the DOX release can be controlled by pH, temperature, and light. The release efficiency is reached to 60-85% at pH 5.3, 80-90% at temperatures higher than the LCST of PDMAEMA (60 °C), and also 90-100% under UV light irradiation after 48 h. In summary, the multi-responsive polymer assemblies based on amphiphilic block copolymers containing spiropyran chain end groups in the current study have potential applications in smart drug-delivery systems, and offer controlling over the drug-release by different triggers, such as light irradiation, pH variation, and temperature change. A very low concentration of spiropyran molecules (one per polymer chain) showed light-controlling of drug-release from the assemblies with high efficiencies.

Functionalized Superwettable Fabric with Switchable Wettability for Efficient Oily Wastewater Purification, in Situ Chemical Reaction System Separation, and Photocatalysis Degradation

In view of the increasing serious water environmental and human health issues caused by oily wastewater, functional superwetting materials with controllable wettability, high durability, and scale preparation methods are highly desired for efficient oil/water separation. In this respect, a pH-responsive multifunctional fabric with switchable surface wettability, favorable mechanical durability, and self-repairing property has been developed via decorating the modified TiO₂ nanoparticles of special surface compositions onto the fabric surface. By virtue of the intelligent surface wettability, the resulted superwettable fabric can be used for controllable separation of multiple oil/water mixtures, particularly the complicated oil/water/oil ternary mixtures, showing excellent separation efficiency and high filtration flux even under extreme pH conditions, which is comparable to most of the commercial and currently reported functionalized membranes. Simultaneously, the negative pressure-driven, continuous, high-speed, and highly efficient in situ purification of large volumes of oily wastewater is successfully achieved based on the resulted superwettable fabric. More importantly, with the as-prepared superwettable fabric as the filtration membrane, the continuous in situ separation of the synthetic oily product from the corresponding chemical reaction systems is well performed without interruption of the reaction, demonstrating outstanding merits of simplifying procedures, saving operation time, and increasing product yield. In addition, it is worth noting that the alkali-treated superhydrophilic fabric presents superior photocatalysis self-cleaning performance for various water-soluble organic pollutants. These unique advantages of the functionalized smart superwettable fabric ensure that it can be competent in multifarious relevant challenging settings, indicating a broad prospect for diverse practical applications, especially the oily wastewater treatment and multiple industrial operation optimizations.

Porous, flexible, and core-shell structured carbon nanofibers hybridized by tin oxide nanoparticles for efficient carbon dioxide capture

HYPOTHESIS

Carbon based nanofibrous materials are considered to be promising sorbents for the CO₂ capture and storage. However, the precise control of porous structure with flexibility still remains a challenging task. In this research, we report a simple strategy to develop tin oxide (SnO₂) embedded, flexible and highly porous core-shell structured carbon nanofibers (CNFs) derived from polyacrylonitrile (PAN)/polyvinylidene fluoride (PVDF) core-shell nanofibers.

EXPERIMENT

PAN/PVDF core-shell solutions were electrospun using co-axial electrospinning process. The as spun PAN core, and PVDF shell, with an appropriate amount of SnO₂, fibers were stabilized followed by carbonization to develop SnO₂ embedded highly porous and flexible core-shell structured CNFs.

FINDINGS

The optimized CNFs membrane shows a prominent CO₂ capture capacity of 2.6 mmol g^-1 at room temperature, excellent CO₂ selectivity than N₂, and a remarkable cyclic stability. After 20 adsorption-desorption cycles, the CO₂ capture capacity retains >95% of the preliminary value showing the long-term stability and practical worth of the final product. The loading of SnO₂ nanoparticles in the carbon matrix not only enhanced the thermal stability of the precursor nanofibers, their surface characteristics, and porous structure to capture CO₂ molecules, but also improves the flexibility of the CNFs by serving as a plasticizer for single-fiber-crack connection. Meaningfully, the flexible SnO₂ embedded core-shell CNFs with excellent structural stability can prevail the limitations of annihilation and collapse of structures for conventional adsorbents, which makes them strongly useful and applicable. This research introduces a new route to produce highly porous and flexible materials as solid adsorbents for CO₂ capture.

Synergy of CO2 Response and Aggregation-Induced Emission in a Block Copolymer: A Facile Way To "See" Cancer Cells

Carbon dioxide (CO₂), an important gas molecule metabolite produced by the tricarboxylic acid cycle, is a direct signal for identifying cancers in cells and tissues. Herein, design and synthesis of a novel "breathable" block polymer supramolecular assembly probe consisting of a hydrophilic block, an amidine-containing CO₂-responsive block, and an aggregation-induced emission (AIE) luminescence block to detect CO₂ metabolized by cancer cells is reported. The triblock copolymer poly-(4-undecoxy tetraphenyl ethylene methacrylate)-b-poly-((N-amidino)-(2,3-dihydro-1H-1, 4-methyl-1, 2,3-triazole)-(ethenylbenzene))-b-poly(ethylene oxide) (PTPE-b-PAD-b-PEO) was successfully synthesized and characterized. This triblock copolymer could be self-assembled into "breathable" aqueous solution vesicles. In the presence of CO₂, the amidine-containing CO₂-responsive block (PAD block) of the vesicle "inhales" an amount of CO₂, which causes the volume of the vesicle to expand. The expansion of the vesicle induces the aggregation of the AIE luminescence block (PTPE block), which resulted in the fluorescence intensity enhancement. The supramolecular vesicles "exhale" CO₂, and the volume and AIE phenomenon of the vesicles decrease when N₂ is passed into the solution. On the basis of this reversible change of fluorescence intensity, HeLa cervical cancer cells, CNE1 nasopharynx cancer cells, 5-8F nasopharynx cancer cells, 16HBE human bronchial epithelial cells, and GES-1 human gastric mucosa epithelial cells have all been successfully detected and identified.

Pseudo-Gemini Biosurfactants with CO2 Switchability for Enhanced Oil Recovery (EOR)

Novel biosurfactants with high performance are always needed in the petroleum industry for environmental sustainability. Herein, we developed a series of biosurfactants to enhance the heavy oil recovery from Canadian oil sands. Pseudo-Gemini biosurfactants were designed to be interfacially active and CO2 switchable. The strong interfacial activity of biosurfactants promotes the liberation of heavy oil from solid substrates, which was demonstrated by the liberation visualization cell. On the other hand, the separation of heavy oil from extraction fluid was also facilitated by activating the CO2 switchability of biosurfactants. Since the efficiencies in both the heavy oil liberation and the oil-water separation were improved, the total heavy oil recovery could be significantly enhanced. Therefore, these biosurfactants are believed to be promising in the application of enhanced oil recovery from oil sands ore.

Multistimuli-Responsive Emulsifiers Based on Two-Way Amphiphilic Diblock Polymers

Diblock copolymers of poly(tert-butyl methacrylate) (PtBuMA) and poly(2-dimethylaminoethyl methacrylate) (PDMAEMA) of four different block lengths were prepared by sequential two-step reversible addition-fragmentation chain transfer radical polymerization, followed by hydrolysis of the PtBuMA blocks to obtain poly(methacrylic acid)-b-PDMAEMA (PMAA-b-PDMAEMA). The effect of the PDMAEMA block length on the multistimuli-responsive amphiphilic features of both types of diblock copolymers was investigated as CO2-switchable emulsifiers for emulsification/demulsification of n-octane (an oil) in water in response to CO2/N2 bubbling. The amphiphilicity of PtBuMA-b-PDMAEMA was switched on, and the amphiphilicity of PMAA-b-PDMAEMA was switched off by CO2 bubbling at pH 12 and 25 °C to achieve emulsification/demulsification. A longer PDMAEMA block length in PMAA-b-PDMAEMA conferred more sensitive CO2-responsive amphiphilicity but reduced the extent of recovery of emulsification ability on N2 bubbling. This newly developed diblock copolymer system could potentially serve as a "multifunctional surfactant" for CO2-switchable emulsification/demulsification of oil-in-water and water-in-oil mixtures.

Simultaneous and Reversible Triggering of the Phase Transfer and Luminescence Change of Amidine-Modified Carbon Dots by CO2

The ability to reversibly manipulate the surface nature of luminescent nanoparticles upon external stimulation enables the development of advanced optical probes for biological sensing and data encoding. Herein, we report the synthesis of a new class of smart carbon dots (CDs) via surface modification of amine-enriched CDs with CO₂-responsive groups of amidine. We present that alternative CO₂ and N₂ bubbling can not only lead to a reversible phase transfer of the CDs between an organic phase and an aqueous phase but also give rise to a corresponding reversible luminescence change between blue and cyan-green. We attribute these observations to changes in both the surface chemistry and the emission states of the CDs triggered by the alternative CO₂/N₂ introduction. We also find a similar luminescence change of the CDs upon alternative exposure to a humid vapor of CO₂ and a mixture of NH₃ and N₂ at room temperature, allowing them to be used as a new class of optical materials for optical encoding.

CO2 -Activated Reversible Transition between Polymersomes and Micelles with AIE Fluorescence

Fluorescent polymersomes with both aggregation-induced emission (AIE) and CO₂ -responsive properties were developed from amphiphilic block copolymer PEG-b-P(DEAEMA-co-TPEMA) in which the hydrophobic block was a copolymer made of tetraphenylethene functionalized methacrylate (TPEMA) and 2-(diethylamino)ethyl methacrylate (DEAEMA) with unspecified sequence arrangement. Four block copolymers with different DEAEMA/TPEMA and hydrophilic/hydrophobic ratios were synthesized, and bright AIE polymersomes were prepared by nanoprecipitation in THF/water and dioxane/water systems. Polymersomes of PEG₄₅ -b-P(DEAEMA₃₆ -co-TPEMA₆ ) were chosen to study the CO₂ -responsive property. Upon CO₂ bubbling vesicles transformed to small spherical micelles, and upon Ar bubbling micelles returned to vesicles with the presence of a few intermediate morphologies. These polymersomes might have promising applications as sensors, nanoreactors, or controlled release systems.

Tunable microfluidic standing air bubbles and its application in acoustic microstreaming

Microbubbles are often used in chemistry, biophysics, and medicine. Properly controlled microbubbles have been proved beneficial for various applications by previous scientific endeavors. However, there is still a plenty of room for further development of efficient microbubble handling methods. Here, this paper introduces a tunable, stable, and robust microbubble interface handling mechanism, named as microfluidic standing air bubbles (μSABs), by studying the multiphysical phenomena behind the gas-liquid interface formation and variation. A basic μSAB system consists specially structured fluidic channels, pneumatic channels, and selectively permeable porous barriers between them. The μSABs originate inside the crevice structures on the fluidic channel walls in a repeatable and robust manner. The volumetric variation of the μSAB is a multiphysical phenomenon that dominated by the air diffusion between the pneumatic channel and the bubble. Theoretical analysis and experimental data illustrate the coupling processes of the repeatable and linear μSAB volumetric variation when operated under common handling conditions (control pneumatic pressure: -90 kPa to 200 kPa). Furthermore, an adjustable acoustic microstreaming is demonstrated as an application using the alterable μSAB gas-liquid interface. Derived equations and microscopic observations elucidate the mechanism of the continuous and linear regulation of the acoustic microstreaming using varying μSAB gas-liquid interfaces. The μSAB system provides a new tool to handle the flexible and controllable gas-liquid interfaces in a repeatable and robust manner, which makes it a promising candidate for innovative biochemical, biophysical, and medical applications.

Switchable Oil-in-Water Emulsions Stabilized by Like-Charged Surfactants and Particles at Very Low Concentrations

A novel CO₂/N₂ switchable n-decane-in-water emulsion was prepared, which is stabilized by a CO₂/N₂ switchable surfactant [ N'-dodecyl- N, N-dimethylacetamidine (DDMA)] in cationic form in combination with positively charged alumina nanoparticles at concentrations as low as 0.01 mM and 0.001 wt %, respectively. The particles do not adsorb at the oil-water interface but remain dispersed in the aqueous phase between surfactant-coated droplets. A critical zeta potential of the particles of ca. +18 mV is necessary for the stabilization of the novel emulsions, suggesting that the electrical double-layer repulsions between particles and between particles and oil droplets are responsible for their stability. By bubbling N₂ into the emulsions, demulsification occurs following transformation of DDMA molecules from the surface-active cationic form to the surface-inactive neutral form and desorption from the oil-water interface. Bubbling CO₂ into the demulsified mixtures, cationic DDMA molecules are re-formed, which adsorb to the droplet interfaces, ensuring stable emulsions after homogenization. Compared with Pickering emulsions and traditional emulsions, the amount of switchable surfactant and number of like-charged particles required for stabilization are significantly reduced, which is economically and environmentally benign for practical applications.

Hydrogenation of Carbon Dioxide to Value-Added Chemicals by Heterogeneous Catalysis and Plasma Catalysis

Due to the increasing emission of carbon dioxide (CO2), greenhouse effects are becoming more and more severe, causing global climate change. The conversion and utilization of CO2 is one of the possible solutions to reduce CO2 concentrations. This can be accomplished, among other methods, by direct hydrogenation of CO2, producing value-added products. In this review, the progress of mainly the last five years in direct hydrogenation of CO2 to value-added chemicals (e.g., CO, CH4, CH3OH, DME, olefins, and higher hydrocarbons) by heterogeneous catalysis and plasma catalysis is summarized, and research priorities for CO2 hydrogenation are proposed.

A polyelectrolyte-containing copolymer with a gas-switchable lower critical solution temperature-type phase transition

A polyelectrolyte-containing copolymer with a CO₂/N₂-switchable cloud point, resulting from the gas-induced alternation of hydrophilicity, was prepared.

Dual CO2/temperature-responsive diblock copolymers confer controlled reversible emulsion behavior

Dual-stimuli responsive diblock copolymers possessing unique temperature-sensitive and CO₂/N₂-switching ability were successfully developed to promote efficient manipulation of reversible emulsification processes.

CO2-Driven reversible wettability in a reactive hierarchically patterned bio-inspired honeycomb film

A triple structured honeycomb film is fabricated through block copolymer directed self-assembly in “Breath Figure” templating as a clickable patterned platform to enhance its reversible surface wettability between hydrophobicity and hydrophilicity upon a biological CO₂ trigger.

Intelligent CO2- and photo-dual-responsive polymer vesicles with tunable wall thickness

CO₂- and photo-dual-responsive polymer vesicles with tunable wall thickness were explored and used as a potential “smart” platform for drug release.

Open-air preparation of cross-linked CO2-responsive polymer vesicles by enzyme-assisted photoinitiated polymerization-induced self-assembly

An open-air strategy via enzyme-assisted photoinitiated polymerization-induced self-assembly (photo-PISA) in water is developed for preparing cross-linked CO₂-responsive vesicles at high solids contents.

Bimodal detection of carbon dioxide using fluorescent molecular aggregates

Fluorescent aggregates of a cyano-substituted phenylenevinylene derivative (R-1) have been used as a bimodal probe for the easy and fast detection of CO₂.

The light-controlling of temperature-responsivity in stimuli-responsive polymers

Light-controlling of phase separation in temperature-responsive polymer solutions by using light-responsive materials for reversible controlling physical and chemical properties of the media with an out-of-system stimulus with tunable intensity.

Reversible Stability of Emulsion and Polymer Latex Controlled by Oligochitosan and CO₂

The addition of salt to a colloid solution ensures that emulsions can be easily separated into two phases and that polymer latexes can be coagulated. The switchable stability of emulsions and polymer latexes would improve the properties for their current applications. A switchable process of salt addition can be achieved using CO₂ and switchable water, and it is a novel, benign approach to achieving a switchable ionic strength in an aqueous solution. However, the problem associated with switchable water is that its additives are all synthetic tertiary amines, most of which are harmful to human beings and the environment. Oligochitosan, as a natural product, can also be used as a switchable water additive. In this paper, a new switchable water system using oligochitosan to change the ionic strength was explored for use in several potential industrial applications. The conductivity of the aqueous solution of oligochitosan (0.2 wt.%) was switched from 0.2 to 331 μS/cm through the addition and removal of CO₂. Oligochitosan and CO₂ were successfully utilized to reversibly break a crude oil emulsion. Polystyrene (PS) latexes could also be reversibly destabilized; the zeta potential of the PS latex changed between -5.8 and -45.2 mV in the absence and presence of CO₂ after oligochitosan was dissolved in the PS latex. The use of oligochitosan is a more environmentally friendly means for reversibly separating colloid solutions.

Polymersomes: Breaking the Glass Ceiling?

Polymer vesicles, also known as polymersomes, have garnered a lot of interest even before the first report of their fabrication in the mid-1990s. These capsules have found applications in areas such as drug delivery, diagnostics and cellular models, and are made via the self-assembly of amphiphilic block copolymers, predominantly with soft, rubbery hydrophobic segments. Comparatively, and despite their remarkable impermeability, glassy polymersomes (GPs) have been less pervasive due to their rigidity, lack of biodegradability and more restricted fabrication strategies. GPs are now becoming more prominent, thanks to their ability to undergo stable shape-change (e.g., into non-spherical morphologies) as a response to a predetermined trigger (e.g., light, solvent). The basics of block copolymer self-assembly with an emphasis on polymersomes and GPs in particular are reviewed here. The principles and advantages of shape transformation of GPs as well as their general usefulness are also discussed, together with some of the challenges and opportunities currently facing this area.