Interaction of Ions in Organic and Aqueous Media with an Ion-Pair Sensor Equipped with a BODIPY Reporter: An ON1-OFF-ON2-ON3 Fluorescent Assay.

Here, we present a ditopic ion-pair sensor, B1, containing the BODIPY reporter unit in its structure, which is shown to be able-thanks to the presence of two heterogeneous binding domains-to interact with anions in an enhanced manner in the presence of cations. This enables it to interact with salts even in 99% aqueous solutions, making B1 a good candidate in terms of visual salt detection in the aquatic environment. Receptor B1's ability to extract and release salt was applied in the transport of potassium chloride through a bulk liquid membrane. Working with a concentration of B1 in the organic phase and with the presence of a specific salt in an aqueous solution, an inverted transport experiment was also demonstrated. By varying the type and the amount of the anions added to B1, we were able to develop diverse optical responses, including a unique four-step ON1-OFF-ON2-ON3 output.

Utilizing an Amino Acid Scaffold to Construct Heteroditopic Receptors Capable of Interacting with Salts under Interfacial Conditions.

A 4-nitro-L-phenylalanine scaffold was used to construct effective ion pair receptors capable of binding anions in an enhanced manner with the assistance of alkali metal cations. A benzocrown ether was linked to a receptor platform via the amide function so as to support the squaramide function in anion binding and to allow all three NHs to act simultaneously. The binding properties of the receptors were determined using UV-vis, 1H NMR, 2D NMR, and DOSY spectroscopy in MeCN and in the solid state by X-ray measurements. Ion pair receptor 2 was found to interact with the most strongly with salts, and the removal of its key structural elements was shown to hinder the receptor action. The amide proton was recognized to switch from having involvement in an intramolecular hydrogen bond to interacting with anions upon complexation. Apart from carboxylates, which promote deprotonation, and other monovalent salts creating 1:1 complexes with the receptor, more complex equilibria were established upon the complexation of 2 with sulfates. Receptor 2 was shown to be capable of the extraction of ion pairs from the aqueous to organic phase and of the cation-enhanced transport chloride and sulfate anions across a bulk chloroform membrane. These features may open the door for its use in regulating ion concertation under interfacial conditions and acting as a potential drug to treat channelopathies.

Fluorescence Recognition of Anions Using a Heteroditopic Receptor: Homogenous and Two-Phase Sensing.

In contrast to monotopic receptor 3, the anthracene functionalized squaramide dual-host receptor 1 is capable of selectively extracting sulfate salts, as was evidenced unambiguously by DOSY, mass spectrometry, fluorescent and ion chromatography measurements. The receptors were investigated in terms of anion and ion pair binding using the UV-vis and 1H NMR titrations method in acetonitrile. The reference anion receptor 3, lacking a crown ether unit, was found to lose the enhancement in anion binding induced by the presence of cations. Besides the ability to bind anions in an enhanced manner exhibited by ion pair receptors 2 and 4, changing the 1-aminoanthracene substituent resulted in their exhibiting a lower anion affinity than receptor 1. By using receptor 1 and adjusting the water content in organic phase it was possible to selectively detect sulfates both by "turn-off" and "turn-on" fluorescence, and to do so homogenously and under interfacial conditions. Such properties of receptor 1 have allowed the development of a new type of sensor capable of recognizing and extracting potassium sulfate from the aqueous medium across a phase boundary, resulting in an appropriate fluorescent response in the organic solution.

Tripodal, Squaramide-Based Ion Pair Receptor for Effective Extraction of Sulfate Salt.

Combining three features-the high affinity of squaramides toward anions, cooperation in ion pair binding and preorganization of the binding domains in the tripodal platform-led to the effective receptor 2. The lack of at least one of these key elements in the structures of reference receptors 3 and 4 caused a lower affinity towards ion pairs. Receptor 2 was found to form an intramolecular network in wet chloroform, which changed into inorganic-organic associates after contact with ions and allowed salts to be extracted from an aqueous to an organic phase. The disparity in the binding mode of 2 with sulfates and with other monovalent anions led to the selective extraction of extremely hydrated sulfate anions in the presence of more lipophilic salts, thus overcoming the Hofmeister series.

Cooperative Transport and Selective Extraction of Sulfates by a Squaramide-Based Ion Pair Receptor: A Case of Adaptable Selectivity.

The use of a squaramide-based ion pair receptor offers a solution to the very challenging problem of extraction and transport of extremely hydrated sulfate salt. Herein we demonstrate for the first time that a neutral receptor is able not only to selectively extract but also to transport sulfates in the form of an alkali metal salt across membranes and to do so in a cooperative manner while overcoming the Hofmeister bias. This was made possible by an enhancement in anion binding promoted by cation assistance and by diversifying the stoichiometry of receptor complexes with sulfates and other ions. The existence of a peculiar 4:1 complex of receptor 2 with sulfates in solution was confirmed by UV-vis and 1H NMR titration experiments, DOSY and DLS measurements, and supported by solid-state X-ray measurements. By varying the separation technique and experimental conditions, it was possible to switch the depletion of the aqueous layer into extremely hydrophilic or less lipophilic salts, thus obtaining the desired selectivity.

Highly Efficient, Tripodal Ion-Pair Receptors for Switching Selectivity between Acetates and Sulfates Using Solid-Liquid and Liquid-Liquid Extractions.

A tripodal, squaramide-based ion-pair receptor 1 was synthesized in a modular fashion, and 1H NMR and UV-vis studies revealed its ability to interact more efficiently with anions with the assistance of cations. The reference tripodal anion receptor 2, lacking a crown ether unit, was found to lose the enhancement in anion binding induced by presence of cations. Besides the ability to bind anions in enhanced manner by the "single armed" ion-pair receptor 3, the lack of multiple and prearranged binding sites resulted in its much lower affinity towards anions than in the case of tripodal receptors. Unlike with receptors 2 or 3, the high affinity of 1 towards salts opens up the possibility of extracting extremely hydrophilic sulfate anions from aqueous to organic phase. The disparity in receptor 1 binding modes towards monovalent anions and divalent sulfates assures its selectivity towards sulfates over other lipophilic salts upon liquid-liquid extraction (LLE) and enables the Hofmeister bias to be overcome. By changing the extraction conditions from LLE to SLE (solid-liquid extraction), a switch of selectivity from sulfates to acetates was achieved. X-ray measurements support the ability of anion binding by cooperation of the arms of receptor 1 together with simultaneous binding of cations.

The Effect of Substitution Pattern on Binding Ability in Regioisomeric Ion Pair Receptors Based on an Aminobenzoic Platform.

A series of ditopic ion pair receptors equipped with 4-nitrophenylurea and 1-aza-18-crown-6-ether linked by ortho-(1), meta-(2), and para-(3) substituted benzoic acid were readily synthesized in three steps from commercially available materials. The binding properties of these regioisomeric receptors were determined using UV-vis and 1H NMR spectroscopy in MeCN and in the solid state by single-crystal X-ray diffraction crystallography. The solution studies revealed that, apart from carboxylates, all the anions tested formed stronger complexes in the presence of sodium cations. Receptors 2 and 3 were found to interact with ion pairs with remarkably higher affinity than ortho-substituted 1. 1H NMR titration experiments showed that both urea NH protons interacted with anions with comparable strength in the case of receptors 2 and 3, but only one of the NHs was effective in anion binding in the case of receptor 1. X-ray analysis of the crystal structure of receptor 1 and 1·NaPF6 complex showed that binding was hampered due to the formation of an intramolecular hydrogen bond. Analysis of the crystal structures of 2·NaBr and 3·NaBr complexes revealed that proper mutual orientation of binding domains was responsible for the improved binding of the sodium salts.

Recognition and Extraction of Sodium Chloride by a Squaramide-Based Ion Pair Receptor.

We synthesized an ion pair receptor 1 consisting of a crown ether cation binding site and a squaramide anion binding domain and compared its binding properties to those of its analogous urea counterpart 2. We studied their salt binding properties using spectrophotometric and spectroscopic measurements in an acetonitrile solution and in acetonitrile/water mixtures. Apart from carboxylate anions, all of the anions tested were found to associate with receptor 1 and 2 more strongly in the presence of sodium cations. A homotopic anion receptor 3, lacking a crown ether unit, was unable to bind sodium salt more strongly than tetrabutylammonium salts. Solution and solid-state X-ray measurements revealed strong sodium chloride coordination to receptor 1, which is able to bind this salt even in the presence of 10% water. In contrast to the urea-based ion pair receptor 2 or anion receptor 3, ditopic receptor 1 is capable of extracting sodium chloride from aqueous media to the organic phase, as was evidenced unambiguously by 1H nuclear magnetic resonance, mass spectrometry, and atomic absorption spectroscopy analyses.

Dual Sensing by Simple Heteroditopic Salt Receptors Containing an Anthraquinone Unit.

We synthesized simple ion pair receptors consisting of a crown ether cation binding site and an anthraquinone-supported thiourea anion binding domain and studied their anion-, cation-, and salt-binding properties using spectroscopic, spectrophotometric, and electrochemical measurements in acetonitrile solution. Apart from carboxylate anions, which cause deprotonation, all the anions tested were found to associate with receptor 1 more strongly in the presence of sodium cations, whereas in the presence of potassium or ammonium cation the anion binding strength was greatly diminished. A homotopic anion receptor 3, lacking a crown ether unit, was unable to bind sodium salt more strongly than tetrabutylammonium salts. Solution and solid-state X-ray measurements revealed that strong sodium coordination with the cation-binding domain is responsible for the salt-binding enhancement. Electrochemical measurements showed that the addition of anions to the receptor 1 pretreated with sodium cations resulted in greater changes in reduction potentials compared to the addition of anions to receptor 1 in the absence of Na(+).

Smart Hydrogel Based on Derivatives of Natural α-Amino Acids for Efficient Removal of Metal Ions from Wastewater.

A novel class of hydrogels, rich in a variety of functional groups capable of interacting/complexing with metal ions was successfully synthesized. This was achieved by using acryloyl derivatives of natural α-amino acids, specifically ornithine and cystine. The δ-amino group of ornithine was modified with an acryloyl group to facilitate its attachment to the polymer chain. Additionally, N,N'-bisacryloylcystine, derived from cystine, was employed as the cross-linker. The hydrogel was obtained through a process of free radical polymerization. This hydrogel, composed only from derivatives of natural amino acids, has proven to be a competitive sorbent and has been effectively used to remove heavy metal pollutants, mainly lead, copper, and silver ions, from aqueous media. The maximum sorption capacities were ca. 155 mg·g-1, 90 mg·g-1, and 215 mg·g-1, respectively for Pb(II), Cu(II), and Ag(I). The material was characterized by effective regeneration, maintaining the sorption capacity at around 80%, 85%, and 90% for Cu(II), Ag(I), and Pb(II), respectively, even after five cycles. The properties of sorption materials, such as sorption kinetics and the effect of pH on sorption, as well as the influence of the concentration of the examined metal ions on the swelling ratio and morphology of the gel, were investigated. The EDS technique was employed to investigate the composition and element distribution in the dry gel samples. Additionally, IR spectroscopy was used to identify the functional groups responsible for binding the studied metal ions, providing insights into their specific interactions with the hydrogel.

Selective ammonium nitrate recognition by a heteroditopic macrotricyclic ion-pair receptor.

The heteroditopic macrotricyclic molecular receptor 1, which bears a tripodal anion binding domain and 4,10,16-triaza-18-crown-6 cation recognition domain, proves to be an effective ion-pair receptor. In the absence of the cobound cation (TBA(+) salts) receptor 1 preferably binds nitrate and nitrite over other anions, including basic anions such as acetate or dihydrogenphosphate. Ammonium cation binding by the 4,10,16-triaza-18-crown-6 subunit significantly enhances the strength of the nitrate and nitrite complexation at the triamide recognition site of the receptor. In the presence of ammonium cations, the association constants of nitrate binding reach an impressive value of 1050 M(-1) in highly polar DMSO-d6. Interestingly, the binding of other anions such as chloride and bromide is not enhanced in the presence of a cobound NH4(+) cation. The increased affinity of [1·NH4(+)]PF6(-) for anionic species is attributed to a strong cooperative effect that arises from the properly positioned binding sites in the receptor 1 cavity, thus allowing for the formation of the ion pair. Under liquid/liquid conditions, receptor 1 is able to extract NH4NO3 from an aqueous to an organic phase, as inferred from (1)H NMR spectroscopic and nitrite/nitrate colorimetric analyses.

Electrochemically Controlled Release from a Thin Hydrogel Layer.

In this study, we present a thermoresponsive thin hydrogel layer based on poly(N-isopropylacrylamide), functionalized with ß-cyclodextrin groups (p(NIPA-ßCD)), as a novel electrochemically controlled release system. This thin hydrogel layer was synthesized and simultaneously attached to the surface of a Au quartz crystal microbalance (QCM) electrode using electrochemically induced free radical polymerization. The process was induced and monitored using cyclic voltammetry and a quartz crystal microbalance with dissipation monitoring (QCM-D), respectively. The properties of the thin layer were investigated by using QCM-D and scanning electron microscopy (SEM). The incorporation of ß-cyclodextrin moieties within the polymer network allowed rhodamine B dye modified with ferrocene (RdFc), serving as a model metallodrug, to accumulate in the p(NIPA-ßCD) layer through host-guest inclusion complex formation. The redox properties of the electroactive p(NIPA-ßCD/RdFc) layer and the dissociation of the host-guest complex triggered by changes in the oxidation state of the ferrocene groups were investigated. It was found that oxidation of the ferrocene moieties led to the release of RdFc. It was crucial to achieve precise control over the release of RdFc by applying the appropriate electrochemical signal, specifically, by applying the appropriate potential to the electrode. Importantly, the electrochemically controlled RdFc release process was performed at a temperature similar to that of the human body and monitored using a spectrofluorimetric technique. The presented system appears to be particularly suitable for transdermal delivery and delivery from intrabody implants.

A novel self-healing hydrogel based on derivatives of natural α-amino acids with potential applications as a strain sensor.

We successfully synthesized a novel hydrogel based on derivatives of natural α-amino acids: ornithine and cystine. To make ornithine attachable to the polymer chain, the δ-amino group was modified with an acryloyl group and the main monomer was obtained. From cystine, the cross-linker N,N'-bisacryloylcystine was obtained. Then, by free radical polymerization of the monomers in the presence of Fe3+, the hydrogel was obtained. The presence of iron ions in the pre-gel solution accelerated the decomposition of a free radical initiator (ammonium persulfate) and allowed uniform distribution of complexed Fe3+ in the hydrogel to be obtained. The presence of free α-amino acid groups in the main monomer and then in the polymeric network of the gels enables this complexation. As a result, the obtained hydrogel benefits from the chemical and physical cross-links, disulfide bonds, and metal-ligand complex, respectively. The composition of the hydrogel was optimized to obtain improved mechanical properties and self-healing ability. Thereby we identified a hydrogel exhibiting fast and conclusive self-healing, which recovered approximately 99% (efficiency of self-healing based on fracture strain) of its original properties after 15 min. The conductivity and electrical response of the hydrogel were investigated. The results revealed a rapid electrical response to minor stretching of the hydrogel, allowing it to be used as a strain sensor. In addition, the presence of the disulfide bonds in the hydrogel structure enabled the hydrogel to degrade in a redox environment.

Macrocyclic squaramides as ion pair receptors and fluorescent sensors selective towards sulfates.

Through the high dilution technique, we obtained macrocyclic ion pair receptors R1 and R2, an anion receptor R3, and a fluorescent sensor R4 using a combination of particular members of simple libraries consisting of synthesized diamines and methyl squarates, respectively. The receptors were investigated in terms of anion and ion pair binding using the 1H NMR titration method in DMSO-d6. We found that the major contribution to the anion binding comes from the interaction with the squaramide protons rather than with the amide functions of the receptors. The receptors demonstrated the highest affinity towards benzoates and sulfates over the anions tested, and in the case of sulfate binding more complex equilibria in solution were observed. Unlike the anion receptor R3, the ion pair receptor R1 was found to recognize anions in an enhanced manner with the assistance of sodium or potassium cations. Tethering of a simple fluorophore in close proximity to the amide function of receptor R4 resulted in an optical ion pair sensor selective towards sulfates. DFT calculations carried out for the 1 : 1 complexes of R3 with the anions helped clarify this selectivity, showing more effective participation of tetrahedral sulfate anions in binding with the amide function than in the case of benzoates or chlorides.

Degradable nanohydrogel with high doxorubicin loadings exhibiting controlled drug release and decreased toxicity against healthy cells.

A new nanogel/drug carrier of 100-150 nm size, based on poly(N-isopropylacrylamide-co-sodium acrylate) and degradable crosslinker (cystine derivative), was synthesized. Using the electrostatic interactions between the carboxylic groups in the polymer network and the protonated amine groups of doxorubicin it was possible to load the drug into the carrier to a very high level of 28-30% relative to the dry mass of the polymer. The presence of the -S-S- groups made the polymer network susceptible to degradation by glutathione. The size of the nanoparticles was small enough to enable them to easily penetrate the cells. The MTT assay indicated that compared to free doxorubicin the nanogel particles loaded with doxorubicin were more cytotoxic against the MCF-7 and A2780 cancer cells, while they were 150 times less toxic against the MCF-10A healthy cells. The new carrier nanoparticles appeared also to be useful for prolonged drug delivery.

Ion-pair induced supramolecular assembly formation for selective extraction and sensing of potassium sulfate.

Selective extraction of sulfates in the form of alkali metal salts using charge-neutral molecular receptors is one of the holy grails of supramolecular chemistry. Herein we describe, for the first time, a squaramide-based ion pair receptor equipped with a crown ether site that is able to extract potassium sulfate from the aqueous to the organic phase (an analogous monotopic anion receptor lacking the crown ether unit lacks this ability). 1H NMR, UV-vis, DOSY-NMR, DLS, and MS experiments and the solid-state single crystal structure provided evidence of the formation of a supramolecular core-shell like assembly upon interaction of the receptor with potassium sulfate. The presence of monovalent potassium salts, in contrast, promoted the formation of simple 1 : 1 complexes. Unlike the 4 : 1 assembly, the 1 : 1 complexes are poorly soluble in organic media. This feature was utilized to overcome the Hofmeister bias and allow for selective extraction of extremely hydrophilic sulfates over lipophilic nitrate anions, which was unambiguously proved by quantitative AES and ion chromatography measurements. A simple modification of the receptor structure led to a "naked eye" optical sensor able to selectively detect sulfates under both SLE and LLE conditions.

A Novel Strategy for the Synthesis of Amphiphilic and Thermoresponsive Poly(N-isopropylacrylamide)-b-Polystyrene Block Copolymers via ATRP.

A new synthetic approach is presented for the preparation of Poly(N-isopropylacrylamide-block-styrene) PNIPAM-b-PS via an Atom Transfer Radical Polymerization (ATRP) technique. The proposed method is based on application of 2-chloro-N-(2-hydroxyethyl)propanamide (NCPAE) as a bifunctional initiator, which enables ATRP of two monomers, differing in activity and polarity, into two stages. The synthesized copolymer molecules contain two well-defined polymer chains connected by a linker, which is a derivative of the proposed initiator. Using NCPAE led to PNIPAMs with well-planned molecular weight, low polydispersities (PDI=1.1÷1.3) and hydroxyl functionality. Activation of such blocks for initiation of styrene polymerization was performed using α-bromoisobutyryl bromide. After such a modification, the synthesized homopolymers acted as macroinitiators in ARGET ATRP and a well-defined polystyrene block, as the next one in the polymer chain was successfully formed. Both of the synthesized macromolecules, PNIPAM and PNIPAM-b-PS, exhibit a thermoresponsive behavior with explicit lower critical solution temperatures (LCST) in their aqueous solutions. The synthesized homopolymers and subsequently derived block copolymers were characterized using Size-Exclusion Chromatography, Differential Scanning Calorimetry, Dynamic Light Scattering, and NMR spectroscopy.

Degradable, thermo-, pH- and redox-sensitive hydrogel microcapsules for burst and sustained release of drugs.

Polymer microcapsules offer a possibility of storing increased amounts of drugs. Appropriate design and composition of the microcapsules allow tuning of the drug-release process. In this paper, we report on synthesis of hydrogel microcapsules sensitive to temperature and pH and degradable by glutathione and hydrogen peroxide. Microcapsules were based on thermo-responsive poly(N-isopropylacrylamide) and degradable cystine crosslinker, and were synthesized by applying precipitation polymerization. Such way of polymerization was appropriately modified to limit the crosslinking in the microcapsule center. This led to a possibility of washing out the pNIPA core at room temperature and the formation of a capsule. Microcapsules revealed rather high drug-loading capacity of ca. 17%. The degradation of the microcapsules by the reducing agent (GSH) and the oxidizing agent (H2O2) was confirmed by using the DLS, UV-Vis, SEM and TEM techniques. Depending on pH and concentration of the reducing/oxidizing agents a fast or slow degradation of the microcapsules and a burst or long-term release of doxorubicin (DOX) were observed. The DOX loaded microcapsules appeared to be cytotoxic against A2780 cancer cells similarly to DOX alone, while unloaded microcapsules did not inhibit proliferation of the cells.

Nanohydrogel with N,N'-bis(acryloyl)cystine crosslinker for high drug loading.

Substantially improved hydrogel particles based on poly(N-isopropylacrylamide) (pNIPA) have been obtained. First, as a result of replacing commercially available N,N'-bis(acryloyl)cystamine (BAC), the crosslinker, with acryloyl derivative of cystine containing a carboxylic group (BISS), the hydrogel particles acquired improved stability vs. ionic strength and allowed further chemical modification of the chains, including the attachment of drug molecules. Next, a redox-initiated aqueous precipitation polymerization via the semi-batch method was used. This led to substantially increased BISS content and diminished size of the nanoparticles that made them suitable to an endocytic process. In addition, the obtained nanogels revealed high loading capacity of anticancer drug vs. dry gel (circa 16%) and they exhibited much better stability and enhanced drug release under the typical conditions existing in cancer cells. Size of obtained nanogels was investigated by dynamic light scattering (DLS). It appeared that nanoparticle size was in the range from ca. 40 to 200nm. In 0.01M solution of glutathione (GSH) the -S-S- bonds were reduced and the nanogel particles were degraded. This could be seen in obtained SEM and TEM micrographs. The cytotoxicity investigation against the HeLa cells showed that DOX loaded nanogels were more cytotoxic (IC50=0.51µM) than free DOX (IC50=0.83µM), while unloaded nanogels did not inhibit proliferation of the cells. It was also found that the nanogels loaded with DOX reached a high intracellular concentration in HeLa cells just after 2h while free DOX needed 6h for that.

Stable and degradable microgels linked with cystine for storing and environmentally triggered release of drugs.

Environmentally sensitive, degradable microgels based on poly(N-isopropylacrylamide) (pNIPA) cross-linked with the diacryloyl derivative of cystine (BISS) were synthesized by applying surfactant-free emulsion polymerization. pNIPA contributed the sensitivity to temperature to the microgels and the cross-linker made them degradable and sensitive to pH. The morphology of the microgels was investigated by using scanning and transmission electron microscopies (SEM and TEM). The gels formed spherical particles with a narrow size distribution. The influence of temperature, pH and ionic strength on the swelling behavior and the stability of new microgels with various contents of BISS (0, 1 and 3%) were investigated by dynamic light scattering (DLS). It was found that microgels with 3% content of amino acid were highly stable over a wide range of investigated temperatures, pH values and ionic strengths, including the physiological conditions (pH = 7.4, IS = 0.15 M, and 37 °C). The reduction-induced degradation of these microgels by 0.01 M solution of dithiothreitol (DTT) or glutathione (GSH) was studied by means of SEM and TEM; the obtained micrographs showed the destruction of spherical microgel particles. The microgels containing 3% of BISS could be loaded with doxorubicin (DOX) by employing the electrostatic interactions between the DOX amine group and the ionized carboxyl group from BISS. A significant increase in the cumulative release of DOX was observed after changing pH from that characteristic to blood (∼7.4) to that existing in affected cells (∼5.0) and in the presence of GSH (CGSH∼ 10 mM). The cytotoxicity tests proved that the obtained microgels are interesting as useful carriers in directed drug delivery systems.