Orthogonal Dual‐Triggered Shape‐Memory DNA‐Based Hydrogels

DNA‐based shape‐memory hydrogels revealing switchable shape recovery in the presence of two orthogonal triggers are described. In one system, a shaped DNA/acrylamide hydrogel is stabilized by duplex nucleic acids and pH‐responsive cytosine‐rich, i‐motif, bridges. Separation of the i‐motif bridges at pH 7.4 transforms the hydrogel into a quasi‐liquid, shapeless state, that includes the duplex bridges as permanent shape‐memory elements. Subjecting the quasi‐liquid state to pH 5.0 or Ag⁺ ions recovers the hydrogel shape, due to the stabilization of the hydrogel by i‐motif or C‐Ag⁺‐C bridged i‐motif. The cysteamine‐induced transformation of the duplex/C‐Ag⁺‐C bridged i‐motif hydrogel into a quasi‐liquid shapeless state results in the recovery of the shaped hydrogel in the presence of H⁺ or Ag⁺ ions as triggers. In a second system, a shaped DNA/acrylamide hydrogel is generated by DNA duplexes and bridging Pb²⁺ or Sr²⁺ ions‐stabilized G‐quadruplex subunits. Subjecting the shaped hydrogel to the DOTA or KP ligands eliminates the Pb²⁺ or Sr²⁺ ions from the respective hydrogels, leading to shapeless, memory‐containing, quasi‐liquid states that restore the original shapes with Pb²⁺ or Sr²⁺ ions.     

Multistimuli‐Responsive,Moldable Supramolecular Hydrogels Cross‐Linked by Ultrafast Complexation of Metal Ions and Biopolymers

A new type of multistimuli‐responsive hydrogels cross‐linked by metal ions and biopolymers is reported. By mixing the biopolymer chitosan (CS) with a variety of metal ions at the appropriate pH values, we obtained a series of transparent and stable hydrogels within a few seconds through supramolecular complexation. In particular, the CS–Ag hydrogel was chosen as the model and the gelation mechanism was revealed by various measurements. It was found that the facile association of Ag⁺ ions with amino and hydroxy groups in CS chains promoted rapid gel‐network formation. Interestingly, the CS–Ag hydrogel exhibits sharp phase transitions in response to multiple external stimuli, including pH value, chemical redox reactions, cations, anions, and neutral species. Furthermore, this soft matter showed a remarkable moldability to form shape‐persistent, free‐standing objects by a fast in situ gelation procedure.     

Multistimuli‐Responsive,Moldable Supramolecular Hydrogels Cross‐Linked by Ultrafast Complexation of Metal Ions and Biopolymers

A new type of multistimuli‐responsive hydrogels cross‐linked by metal ions and biopolymers is reported. By mixing the biopolymer chitosan (CS) with a variety of metal ions at the appropriate pH values, we obtained a series of transparent and stable hydrogels within a few seconds through supramolecular complexation. In particular, the CS–Ag hydrogel was chosen as the model and the gelation mechanism was revealed by various measurements. It was found that the facile association of Ag⁺ ions with amino and hydroxy groups in CS chains promoted rapid gel‐network formation. Interestingly, the CS–Ag hydrogel exhibits sharp phase transitions in response to multiple external stimuli, including pH value, chemical redox reactions, cations, anions, and neutral species. Furthermore, this soft matter showed a remarkable moldability to form shape‐persistent, free‐standing objects by a fast in situ gelation procedure.     

pH-responsive and switchable triplex-based DNA hydrogels

New methods for the preparation of reversible pH-responsive DNA hydrogels based on Hoogsteen triplex structures are described. One system consists of a hydrogel composed of duplex DNA units that bridge acrylamide chains at pH = 7.4 and undergoes dissolution at pH = 5.0 through the reconfiguration of one of the duplex bridging units into a protonated CG·C⁺ triplex structure. The second system consists of a hydrogel consisting of acrylamide chains crosslinked in the presence of an auxiliary strand by Hoogsteen TA·T triplex interaction at pH = 7.0. The hydrogel transforms into a liquid phase at pH = 10.0 due to the separation of the triplex bridging units. The two hydrogel systems undergo reversible and cyclic hydrogel/solution transitions by subjecting the systems to appropriate pH values. The anti-cancer drug, coralyne, binds specifically to the TA·T triplex-crosslinked hydrogel thereby increasing its stiffness. The pH-controlled release of the coralyne from the hydrogel is demonstrated.     

A H+/Ag+ Dual‐Target Responsive Label‐Free Light‐Up Probe Based on a DNA Triplex

We developed a dual‐target responsive sensor for label‐free light‐up fluorescent detection of protons (H⁺) and silver ions (Ag⁺) using an “OR′′ logic gate. Berberine, a cost‐effective and non‐toxic indicator, partially intercalates the formed triplex DNA in the presence of H⁺ or Ag⁺, generating enhanced fluorescence. The designed Ag⁺ probe has high selectivity and desirable sensitivity, which is necessary for practical use. The robust ”OR“ logic gate is capable of a rapid and reversible response to the H⁺ and/or Ag⁺ inputs.     

A Shape‐Memory DNA‐Based Hydrogel Exhibiting Two Internal Memories

The synthesis of a shape‐memory acrylamide–DNA hydrogel that includes two internal memories is introduced. The hydrogel is stabilized, at pH 7.0, by two different pH‐responsive oligonucleotide crosslinking units. At pH 10.0, one of the T‐A?T triplex DNA bridging units is dissociated, resulting in the dissociation of the hydrogel into a shapeless quasi‐liquid state that includes the other oligonucleotide bridges as internal memory. Similarly, at pH 5.0, the second type of bridges is separated, through the formation of C‐G?C⁺ triplex units to yield the shapeless quasi‐liquid state that includes the other oligonucleotide bridges as internal memory. By reversible pH triggering of the hydrogel between the values 10.0?7.0?5.0, the two internal memories cycle the material across shaped hydrogel and shapeless quasi‐liquid states. The two memories enable the pH‐dictated formation of two different hydrogel structures.     

Preparation of a novel pH-responsive silver nanoparticle/poly(HEMA-PEGMA-MAA) composite hydrogel

In this paper, series of novel pH-responsive silver (Ag) nanoparticle/poly(2-hydroxyethyl methacrylate (HEMA)-poly(ethylene glycol) methyl ether methacrylate (PEGMA)-methacrylic acid (MAA)) composite hydrogel were successfully prepared by in situ reducing Ag⁺ ions anchored in the hydrogel by the deprotonized carboxyl acid groups. X-ray diffraction (XRD), UV-vis spectrophotometry, transmission electron microscopy (TEM) and electric conductivity tests were used to characterize the composite system. It was found that the size and morphology of the reduced Ag nanoparticles in the composite hydrogels could be changed by loading the Ag⁺ ions at various swelling ratios of hydrogel. Moreover, compared to the pure poly(HEMA-PEGMA-MAA) hydrogel, not only did the Ag nanoparticle/poly(HEMA-PEGMA-MAA) composite hydrogels exhibit much higher swelling ratio and faster deswelling rate, but also higher pH switchable electrical properties upon controlling the interparticle distance under pH stimulus. The pH responsive nanocomposite hydrogel reported here might be a potentially smart material in the range of applications including electronics, biosensors and drug-delivery devices.     

Stimuli‐Responsive Biomolecule‐Based Hydrogels and Their Applications

This Review presents polysaccharides, oligosaccharides, nucleic acids, peptides, and proteins as functional stimuli‐responsive polymer scaffolds that yield hydrogels with controlled stiffness. Different physical or chemical triggers can be used to structurally reconfigure the crosslinking units and control the stiffness of the hydrogels. The integration of stimuli‐responsive supramolecular complexes and stimuli‐responsive biomolecular units as crosslinkers leads to hybrid hydrogels undergoing reversible triggered transitions across different stiffness states. Different applications of stimuli‐responsive biomolecule‐based hydrogels are discussed. The assembly of stimuli‐responsive biomolecule‐based hydrogel films on surfaces and their applications are discussed. The coating of drug‐loaded nanoparticles with stimuli‐responsive hydrogels for controlled drug release is also presented.     

Double‐crosslinked reversible redox‐responsive hydrogels based on disulfide–thiol interchange

We present novel redox‐responsive hydrogels based on poly(N‐isopropylacrylamide) or poly(acrylamide), consisting of a reversible disulfide crosslinking agent N,N′‐bis(acryloyl)cystamine and a permanent crosslinking agent N,N′‐methylenebisacrylamide for microfluidic applications. The mechanism of swelling/deswelling behavior starts with the cleavage and reformation of disulfide bonds, leading to a change of crosslinking density and crosslinking points. Raman and ultraviolet‐visible spectroscopy confirm that conversion efficiency of thiol–disulfide interchange up to 99%. Rheological analysis reveals that the E modulus of hydrogel is dependent on the crosslinking density and can be repeatedly manipulated between high‐ and low‐stiffness states over at least 5 cycles without significant decrease. Kinetic studies showed that the mechanical strength of the gels changes as the redox reaction proceeds. This process is much faster than the autonomous diffusion in the hydrogel. Moreover, cooperative diffusion coefficient (D_coop) indicates that the swelling process of the hydrogel is affected by the reduction reaction. Finally, this reversibly switchable redox behavior of bulky hydrogel could be proven in microstructured hydrogel dots through short‐term photopatterning process. These hydrogel dots on glass substrates also showed the desired short response time on cyclic swelling and shrinking processes known from downsized hydrogel shapes. Such stimuli‐responsive hydrogels with redox‐sensitive crosslinkers open a new pathway in exchanging analytes for sensing and separating in microfluidics applications. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 2590–2601     

Electrochemistry and Ion Sensing Properties of Conducting Hydrogel Layers Based on Polypyrrole and Alkoxysulfonated Poly(3,4‐ethylenedioxythiophene) (PEDOT‐S)

Acidic aqueous solutions containing pyrrole and alkoxysulfonated PEDOT derivative (PEDOT‐S) were found to undergo polymerization in the absence of an external oxidizing agent. The product was a nearly black‐colored conducting hydrogel that after separation could be dispersed in water or acetone. The suspensions could be used to deposit cast films on a polycrystalline gold electrode. The polymer modified electrode showed a nearly Nernstian potentiometric response to Ag⁺ cations in the concentration range of 10^?5–10^?1 M with the slope of 54 mV/decade. The response was specific to Ag⁺ compared to a series of alkali and transition‐metal cations (pK_Ag/M>3.7).     

A Reusable DNA Single‐Walled Carbon‐Nanotube‐Based Fluorescent Sensor for Highly Sensitive and Selective Detection of Ag+ and Cysteine in Aqueous Solutions

Here we report a reusable DNA single‐walled carbon nanotube (SWNT)‐based fluorescent sensor for highly sensitive and selective detection of Ag⁺ and cysteine (Cys) in aqueous solution. SWNTs can effectively quench the fluorescence of dye‐labeled single‐stranded DNA due to their strong π–π stacking interactions. However, upon incubation with Ag⁺, Ag⁺ can induce stable duplex formation mediated by C–Ag⁺–C (C=cytosine) coordination chemistry, which has been further confirmed by DNA melting studies. This weakens the interactions between DNA and SWNTs, and thus activates the sensor fluorescence. On the other hand, because Cys is a strong Ag⁺ binder, it can remove Ag⁺ from C–Ag⁺–C base pairs and deactivates the sensor fluorescence by rewrapping the dye‐labeled oligonucleotides around the SWNT. In this way, the fluorescence signal‐on and signal‐off of a DNA/SWNT sensor can be used to detect aqueous Ag⁺ and Cys, respectively. This sensing platform exhibits high sensitivity and selectivity toward Ag⁺ and Cys versus other metal ions and the other 19 natural amino acids, with a limit of detection of 1 nM for Ag⁺ and 9.5 nM for Cys. Based on these results, we have constructed a reusable fluorescent sensor by using the covalent‐linked SWNT–DNA conjugates according to the same sensing mechanism. There is no report on the use of SWNT–DNA assays for the detection of Ag⁺ and Cys. This assay is simple, effective, and reusable, and can in principle be used to detect other metal ions by substituting C–C base pairs with other native or artificial bases that selectively bind to other metal ions.     

Conducting hydrogels based on semi‐interpenetrating networks of polyaniline in poly(acrylamide‐co‐itaconic acid) matrix: synthesis and characterization

In this work, acrylamide/itaconic acid copolymeric hydrogels are prepared by free radical polymerization initiated by redox initiators of potassium persulfate and N ,N ,N ′,N ′‐tetramethyl ethylene diamine; N ,N ′methylene bisacrylamide was employed as a crosslinking agent. Aniline monomer was absorbed in the network of poly(acrylamide‐co‐itaconic acid) P(AAm‐co‐IA) hydrogel and followed by gamma radiation induced polymerization at room temperature. The novel semi‐interpenetrating network was comprised of linear polyaniline immersed in P(AAm‐co‐IA) matrix. Electrical conductivity of the hydrogels was measured using four‐probe technique. The conductivities for the prepared hydrogels are found to increase from 5.5 × 10^?7 S cm^?1 for P(AAm‐co‐IA) alone to 4.4 × 10^?3 S cm^?1 for semi‐interpenetrating polymer network P(AAm‐co‐IA)/polyaniline. Thus, a new composite hydrogel with good conductive properties also displaying enhanced mechanical strength and pH sensitivity was prepared. Copyright © 2017 John Wiley & Sons, Ltd.     

Bifunctional Ion‐Selective Electrode Based on C60‐Cryptand 22 for Silver and Iodine Ions

Fullerence C60‐cryptand 22 was prepared and successfully applied as the electric carrier in the PVC electrode membrane of a bifunctional ion‐selective electrode for cations, e.g., Ag⁺ ions as well as anions, e.g., I^? ions. The bifunctional ion‐selective electrode based on C60‐cryptand 22 can be applied as a Silver (Ag⁺) ion selective electrode with an internal electrode solution of 10^?3 M AgNO₃ in water (pH = 6.3), or as an Iodide (I^?) ion selective electrode with an acidic internal electrode solution of 10^?4 M KI_(aq) (pH = 2) in which the cryptand 22 is protonated, and the C60‐cryptand 22 is changed to C60‐Cryptand22–H⁺ and becomes an anionic electro‐carrier to absorb the I^? ion. The Ag⁺ ion selective electrode based on C60‐cryptand 22 gave a linear response with a near‐Nernstian slope (59.5 mV decade^?1) within the concentration range 10^?1‐10^?3 M Ag⁺_(aq). The Ag⁺ ion electrode exhibited comparatively good selectivity for silver ions, over other transition‐metal ions, alkali and alkaline earth metal ions. The Ag⁺ ion selective electrode with good stability and reproducibility was successfully used for the titration of Ag⁺_(aq) with Cl^? ions. The Iodide (I^?) Ion selective electrode based on protonated C60–cryptand22‐H⁺ also showed a linear response with a nearly Nernstian slope (58.5 mV decade^?1) within 10^?1 ‐ 10^?3 M I^? _(aq) and exhibited good selectivity for I^? ions and had small selectivity coefficients (10^?2–10^?3) for most of other anions, e.g., F^? , OH^?, CH₃COO^?, SO₄^2?, CO₃^2?, CrO₄^2?, Cr₂O₇^2? and PO₄^3? ions.     

Stimuli‐Responsive Polypeptide‐Based Supramolecular Hydrogels Mediated by Ca2+ Ion Cross‐Linking

Hydrogels cross‐linked with metal ions (e.g., Ca²⁺) represent a promising class of bioinspired materials for a wide range of biomedical applications. Herein, we report a facile approach to obtain cross‐linked stimuli‐responsive supramolecular polypeptide hydrogels. The hydrogel is prepared by statistical/block copoly(L‐glutamate)s based copolymers cross‐linked with calcium ions. The incorporation of both oligo(ethylene glycol) (OEG) and glutamic acid residues in the polymer offers thermal‐responsive property and cooperative binding sites with Ca²⁺ ions simultaneously. We present a systematic study of the influence of calcium ions on the gelation behaviors of these copolymers. It is observed that the addition of calcium ions induces the formation of hydrogels. Increasing the concentration of Ca²⁺ ions can significantly enhance the gelation ability of the samples as indicated by increased storage modulus and decreased sol‐to‐gel transition temperature (T_sol‐gel). We further demonstrate that the influence of monomer distribution on the gelation behavior is trivial, which is possibly due to similar morphology of the self‐assemblies. The obtained hydrogels exhibit thermal‐responsive gelation behavior mediated by ion cross‐linking, which enables them to be ideal smart hydrogel system for many applications.     

Hyper‐Crosslinkers Lead to Temperature‐ and pH‐Responsive Polymeric Nanogels with Unusual Volume Change

Hydrogels consisting of carboxylic acid groups and N‐isopropylacrylamide as pendants on their polymeric network usually exhibit volume expansion upon deprotonation or volume contraction when being heated. Demonstrated here is an anti‐intuitive case of a hydrogel containing multiple carboxylic acid groups at each crosslinking point in the polymeric network, which shrinks upon increasing pH from 1 to 7 at 37 °C or expands upon heating from 25 to 37 °C at pH 1. The unexpected volume change originates from the high percentage of the crosslinker in the polymers, as detected by solid‐state ¹³C NMR spectroscopy. In addition, the volume changes are thermally reversible. As the first example of the use of functional hyper‐crosslinkers to control the pH and thermal responses of nanogels, this work illustrates a new way to design soft materials with unusual behaviors.     

Poly[μ2‐(N‐hydroxypyridine‐2‐carboxamidine)‐μ2‐nitrato‐silver(I)]

In the title complex, [Ag(NO₃)(C₆H₇N₃O)]_n or [Ag(NO₃)(pyaoxH₂)] (pyaoxH₂ is N‐hydroxypyridine‐2‐carboxamidine), the Ag⁺ ion is bridged by the pyaoxH₂ ligands and nitrate anions, giving rise to a two‐dimensional molecular structure. Each pyaoxH₂ ligand coordinates to two Ag⁺ ions using its pyridyl and carboxamidine N atoms, and the OH and the NH₂ groups are uncoordinated. Each nitrate anion uses two O atoms to coordinate to two Ag⁺ ions. The Ag...Ag separation via the pyaoxH₂ bridge is 2.869 (1) Å, markedly shorter than that of 6.452 (1) Åvia the nitrate bridge. The two‐dimensional structure is fishscale‐like, and can be described as pyaoxH₂‐bridged Ag₂ nodes that are further linked by nitrate anions. Hydrogen bonding between the amidine groups and the nitrate O atoms connects adjacent layers into a three‐dimensional network.     

Sensitive and Selective Fluorescent and Colorimetric Sensor for Ag+ Based on the Supramolecular Self‐Assembly in Semi‐Water

Specific recognition of ultratrace levels of ions in semi‐water using super‐quicker methods is still a challenge for environmental monitoring. Herein we report a fluorescent and colormetric sensor ( ZH ) based on supramolecular self‐assembly, whose structure was destroyed by the addition of ultratrace of silver ions. The process promoted either naked eye visible color changes or fluorescence intensity quenched in conjunction with a wide pH range. Systematic studies revealed very high selectivity (0.07 µmol/L) for silver ions, and other common cations, e.g ., Hg²⁺, Cu²⁺, Cd²⁺, Pb²⁺ had nearly no influence on the sensing behavior. This sensor also served as a multiple use of component in sensing materials by addition of I⁻ into the mixture of ZH and Ag⁺ (about 5 times). What's more, ZH containing filter paper emerged distinct color and fluorescence changes upon exposure to silver (Ag⁺), which could be used as a portable method to undertake field testing for Ag ⁺ .     

Developing a Self‐Healing Supramolecular Nucleoside Hydrogel Based on Guanosine and Isoguanosine

Recently, supramolecular hydrogels have attracted increasing interest owing to their tunable stability and inherent biocompatibility. However, only few studies have been reported in the literature on self‐healing supramolecular nucleoside hydrogels, compared to self‐healing polymer hydrogels. In this work, we successfully developed a self‐healing supramolecular nucleoside hydrogel obtained by simply mixing equimolar amounts of guanosine (G) and isoguanosine (isoG) in the presence of K⁺. The gelation properties have been studied systematically by comparing different alkali metal ions as well as mixtures with different ratios of G and isoG. To this end, rheological and phase diagram experiments demonstrated that the co‐gel not only possessed good self‐healing properties and short recovery time (only 20 seconds) but also could be formed at very low concentrations of K⁺. Furthermore, nuclear magnetic resonance (NMR), powder X‐ray diffraction (PXRD), and circular dichroism (CD) spectroscopy suggested that possible G₂isoG₂‐quartet structures occurred in this self‐healing supramolecular nucleoside hydrogel. This co‐gel, to some extent, addressed the problem of isoguanosine gels for the applications in vivo, which showed the potential to be a new type of drug delivery system for biomedical applications in the future.     

A Highly K+‐Selective Phenylaza‐[18]crown‐6‐Lariat‐Ether‐Based Fluoroionophore and Its Application in the Sensing of K+ Ions with an Optical Sensor Film and in Cells

Herein, we report the synthesis of two phenylaza‐[18]crown‐6 lariat ethers with a coumarin fluorophore ( 1 and 2 ) and we reveal that compound 1 is an excellent probe for K⁺ ions under simulated physiological conditions. The presence of a 2‐methoxyethoxy lariat group at the ortho position of the anilino moiety is crucial to the substantially increased stability of compounds 1 and 2 over their lariat‐free phenylaza‐[18]crown‐6 ether analogues. Probe 1 shows a high K⁺/Na⁺ selectivity and a 2.5‐fold fluorescence enhancement was observed in the presence of 100 mM K⁺ ions. A fluorescent membrane sensor, which was prepared by incorporating probe 1 into a hydrogel, showed a fully reversible response, a response time of 150 s, and a signal change of 7.8 % per 1 mM K⁺ within the range 1–10 mM K⁺. The membrane was easily fabricated (only a single sensing layer on a solid polyester support), yet no leaching was observed. Moreover, compound 1 rapidly permeated into cells, was cytocompatible, and was suitable for the fluorescent imaging of K⁺ ions on both the extracellular and intracellular levels.     

From poly(N‐isopropylacrylamide)‐block‐poly(ethylene oxide)‐block‐poly(N‐isopropylacrylamide) triblock copolymer to poly(N‐isopropylacrylamide)‐block‐poly(ethylene oxide) hydrogels: Synthesis and rapid deswelling and reswelling behavior of hydrogels

Poly(N‐isopropylacrylamide)‐block‐poly(ethylene oxide)‐block‐poly(N‐isopropylacrylamide) (PNIPAAm‐b‐PEO‐b‐PNIPAAm) triblock copolymer was synthesized via the reversible addition‐fragmentation chain transfer/macromolecular design via the interchange of xanthate (RAFT/MADIX) process with xanthate‐terminated poly(ethylene oxide) (PEO) as the macromolecular chain transfer agent. The successful synthesis of the ABA triblock copolymer inspired the preparation of poly(N‐isopropylacrylamide)‐block‐poly(ethylene oxide) (PNIPAAm‐b‐PEO) copolymer networks with N,N′‐methylenebisacrylamide as the crosslinking agent with the similar approach. With the RAFT/MADIX process, PEO chains were successfully blocked into poly(N‐isopropylacrylamide) (PNIPAAm) networks. The unique architecture of PNIPAAm‐b‐PEO networks allows investigating the effect of the blocked PEO chains on the deswelling and reswelling behavior of PNIPAAm hydrogels. It was found that with the inclusion of PEO chains into the PNIPAAm networks as midblocks, the swelling ratios of the hydrogels were significantly enhanced. Furthermore, the PNIPAAm‐b‐PEO hydrogels displayed faster response to the external temperature changes than the control PNIPAAm hydrogel. The accelerated deswelling and reswelling behaviors have been interpreted based on the formation of PEO microdomains in the PNIPAAm networks, which could act as the hydrophilic tunnels to facilitate the diffusion of water molecules in the PNIPAAm networks. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012