Surprisingly High Selectivity and High Affinity in Mercury Recognition by H-Bonded Cavity-Containing Aromatic Foldarands

Shaping Macromolecules for Sensing Applications—From Polymer Hydrogels to Foldamers

Sensors are tools for detecting, recognizing, and recording signals from the surrounding environment. They provide measurable information on chemical or physical changes, and thus are widely used in diagnosis, environment monitoring, food quality checks, or process control. Polymers are versatile materials that find a broad range of applications in sensory devices for the biomedical sector and beyond. Sensory materials are expected to exhibit a measurable change of properties in the presence of an analyte or a stimulus, characterized by high sensitivity and selectivity of the signal. Signal parameters can be tuned by material features connected with the restriction of macromolecule shape by crosslinking or folding. Gels are crosslinked, three-dimensional networks that can form cavities of different sizes and forms, which can be adapted to trap particular analytes. A higher level of structural control can be achieved by foldamers, which are macromolecules that can attain well-defined conformation in solution. By increasing control over the three-dimensional structure, we can improve the selectivity of polymer materials, which is one of the crucial requirements for sensors. Here, we discuss various examples of polymer gels and foldamer-based sensor systems. We have classified and described applied polymer materials and used sensing techniques. Finally, we deliberated the necessity and potential of further exploration of the field towards the increased selectivity of sensory devices.

Tweezer-type binding cavity formed by the helical folding of a carbazole-pyridine oligomer

We have synthesised a new aromatic foldamer based on the carbazole-pyridine oligomers that adopt helical conformations via dipole-dipole interactions and π-stacking between two ethynyl bond-linked monomers. This foldamer scaffold has been further modified into a synthetic receptor with a tweezer-type binding cavity outside the helical backbone upon folding, in contrast to most aromatic foldamers with internal binding cavities. The tweezer-type cavity is composed of two parallel pyrenyl planes, allowing for the intercalation of a naphthalenediimide guest via π-stacking and CH⋯O interactions, as demonstrated using its ¹H NMR spectra and X-ray crystal structure.

Subtle Modification of Imine-linked Helical Receptors to Significantly Alter their Binding Affinities and Selectivities for Chiral Guests

Aromatic helical receptors P-1 and P-2 were slightly modified by aerobic oxidation to afford new receptors P-7 and P-8 with right-handed helical cavities. This subtle modification induced significant changes in the binding properties for chiral guests. Specifically, P-1 was reported to bind d-tartaric acid (K_a =35500 M^-1 ), used as a template, much strongly than l-tartaric acid (326 M^-1 ). In contrast, its modified receptor P-7 exhibited significantly reduced affinities for d-tartaric acid (3600 M^-1 ) and l-tartaric acid (125 M^-1 ). More dramatic changes in the affinities and selectivities were observed for P-2 and P-8 upon binding of polyol guests. P-2 was determined to selectively bind d-sorbitol (52000 M^-1 ) over analogous guests, but P-8 showed no binding selectivity: d-sorbitol (1890 M^-1 ), l-sorbitol (3330 M^-1 ), d-arabitol (959 M^-1 ), l-arabitol (4970 M^-1 ) and xylitol (4960 M^-1 ) in 5% (v/v) DMSO/CH₂ Cl₂ at 25±1 °C. These results clearly demonstrate that even subtle post-modifications of synthetic receptors may significantly alter their binding affinities and selectivities, in particular for guests of long and flexible chains.

Sensing a binding event through charge transport variations using an aromatic oligoamide capsule

The selective binding properties of a 13-mer oligoamide foldamer capsule composed of 4 different aromatic subunits are reported. The capsule was designed to recognize dicarboxylic acids through multiple-point interactions owing to a combination of protonation/deprotonation events, H-bonding, and geometrical constraints imparted by the rigidity of the foldamer backbone. Compared to tartaric acid, binding of 2,2-difluorosuccinic acid or 2,2,3,3-tetrafluorosuccinic acid resulted in symmetry breaking due to deprotonation of only one of the two carboxylic acid groups of the encapsulated species as shown by NMR studies in solution and by single-crystal X-ray diffraction in the solid state. An analogous 14-mer foldamer capsule terminated with a thiol anchoring group was used to probe the complexation event in self-assembled monolayers on Au substrates. Ellipsometry and polarization-modulation infrared absorption-reflection spectroscopy studies were consistent with the formation of a single molecule layer of the foldamer capsule oriented vertically with respect to the surface. The latter underwent smooth complexation of 2,2-difluorosuccinic acid with deprotonation of one of the two carboxylic acid groups. A significant (80-fold) difference in the charge transport properties of the monolayer upon encapsulation of the dicarboxylic acid was evidenced from conducting-AFM measurements (S = 1.1 × 10-9 vs. 1.4 × 10-11 ohm-1 for the empty and complexed capsule, respectively). The modulation in conductivity was assigned to protonation of the aromatic foldamer backbone.

Aromatic Helical Foldamers as Nucleophilic Catalysts for the Regioselective Acetylation of Octyl β-d-Glucopyranoside

Two indolocarbazole-naphthyridine foldamers 2 and 3 that fold into helical conformations were prepared. The 4-(N,N-dimethylamino)pyridine (DMAP) moiety was introduced at one end of the foldamer strands to develop foldamer-based catalysts for the site-selective acylation of polyols. These foldamers adopt helical conformations containing internal cavities capable of binding octyl β-d-glucopyranoside. The association constants were determined to be 1.9 (±0.1)×10⁵  M^-1 for 2 and 2.1 (±0.1)×10⁵  M^-1 for 3 in CH₂ Cl₂ at 25 °C. In the presence of DMAP, 2 or 3 as the catalysts, octyl β-d-glucopyranoside was subjected to acetylation under identical reaction conditions. The DMAP-catalysed reaction afforded the random distribution of the monoacetylates (6-OAc : 4-OAc : 3-OAc : 2-OAc=33 : 24 : 41 : 2). In contrast, foldamers 2 and 3 led to the predominant formation of 6-OAc. The relative distributions were estimated to be 6-OAc : 4-OAc : 3-OAc=88 : 4 : 6 : ∼0 with 2 and 6-OAc : 4-OAc : 3-OAc : 2-OAc=90 : 3 : 6 : 1 with 3.

Macrocyclic Pyridone Pentamers for Highly Selective High-Capacity Removal of Caesium Ions from Radioactive High-Salinity Waste

We report here that macrocyclic H-bonded pyridone pentamers, containing five properly and convergently spaced electron-rich O-atoms that decorate a rigid cavity of 2.85 Å across, exhibit an extraordinarily high yet pH-independent capacity and selectivity in Cs⁺ removal. In particular, with [host]=240 μM and [Cs⁺ ]=15 μM, a single extraction efficiently removes more than 91% of Cs⁺ ions from artificial sea water, containing various competitive metal ions at a total concentration of 0.68 M ([total Mⁿ⁺ ]/[Cs⁺ ]=4.5×10⁴ ]). To our best knowledge, these pyridone pentamers represent the best small organic molecule-based extractants that target Cs⁺ ions.

Highly efficient synthesis of hydrogen-bonded aromatic tetramers as macrocyclic receptors for selective recognition of lithium ions

Lithium ion receptor based on novel hydrogen-bonded aromatic tetramer biphenyl-cyclo[4]aramide has been developed.

Biomimetic folding of small organic molecules driven by multiple non-covalent interactions

The supramolecular self-folding of UPy-based monomers with low molecular weight driven by multiple non-covalent interactions has been developed.

Carbohydrate binding through first- and second-sphere coordination within aromatic oligoamide metallofoldamers

Aromatic oligoamide capsules that fold upon metal binding recognize carbohydrate guests in solution as evidenced by CD and NMR titrations. Crystallographic data reveal that, besides their structural role, metal ions also contribute to guest recognition through either first- or second-sphere coordination.

Alkali and alkaline earth metal ion binding by a foldamer capsule: selective recognition of magnesium hydrate

Alkali and alkaline earth metal ion binding by an aromatic oligoamide foldamer was shown to induce its folding into a helical capsule. CD and NMR titrations revealed tight and selective binding of Mg²⁺. Crystallographic studies demonstrated that, depending on the metal, binding may involve the first or second coordination spheres of the metal hydrates.

Stronger host–guest binding does not necessarily give brighter particles: a case study on polymeric AIEE-tunable and size-tunable supraspheres

Compared with the tritopic guest without chains, the tritopic guest with flexible alkyl chains bound to the polymeric host more strongly and induced the formation of larger but duller supraspheres.

Multiplexed ratiometric photoluminescent detection of pyrophosphate using anisotropic boron-doped nitrogen-rich carbon rugby ball-like nanodots

B-doped N-rich carbon rugby ball-like nanodots have been synthesized by a heterophase polymerization route and can act not only as a ratiometric sensor for Hg²⁺ but also as a dual-mode PPi-selective sensor.