Proline-Based Boronic Acid Receptors for Chiral Recognition of Glucose

Dual-band terahertz chiral metasurface absorber with enhanced circular dichroism based on temperature-tunable InSb for sensing applications

Circular dichroism (CD) in terahertz (THz) regions has been widely used in biomonitoring, analytical chemistry, communication sensing, and other fields. Herein, we present a simple design for a dual-band THz chiral metasurface absorber (CMA) with a stronger CD effect based on temperature-tunable InSb for enhanced sensing applications. The proposed dual-band CMA consisted of a periodic array of the evolved C-shaped InSb adhered to a copper substrate. The designed CMA at 305 K achieved a right-handed circular polarization (RCP)-selective absorbance of 98.86% and 97.43% at 1.65 THz and 1.89 THz, respectively, and left-handed circular polarization (LCP) absorbance of 9.98% and 22.46%, respectively, and exhibited stronger CD values of 0.89 and 0.75. In addition, the CD properties of the designed CMA can be adjusted by changing the geometrical parameters of the unit-cell structure. The simulated electric field and power follow distributions indicate that this dual-band chiral-selective absorption of the designed CMA is due to the different plasma resonance mode excitations for the incident circular polarization (CP) wave. In addition, the CD properties of the designed CMA can be adjusted by changing the geometrical parameters of the unit-cell structure. Furthermore, CD spectra can be dynamically adjusted by varying the outside temperature and refraction index (RI) of the filled analytes. The designed dual-band CMA can function as a high-performance temperature sensor with sensitivities of 4.68 GHz K-1 and 5.52 GHz K-1 and also as an RI sensor with sensitivities of 1080 GHz RIU-1 and 860 GHz RIU-1, respectively. Our proposed tunable dual-band CMA with its exquisite performance has the potential to be widely applied in diverse areas such as detection, sensing, and other related optoelectronic fields.

Effective enantioselective recognition by steady-state fluorescence spectroscopy: Towards a paradigm shift to optical sensors with unusual chemical architecture

A series of amino acid-derived 1,2,3-triazoles presenting the amino acid residue and the benzazole fluorophore connected by a triazole-4-carboxylate spacer was studied for enantioselective recognition using only steady-state fluorescence spectroscopy in solution. In this investigation, the optical sensing was performed with D-(-) and L-(+)-Arabinose and (R)-(-) and (S)-(+)-Mandelic acid as chiral analytes. The optical sensors showed specific interactions with each pair of enantiomers, allowing photophysical responses, which were used for their enantioselective recognition. DFT calculations confirm the specific interaction between the fluorophores and the analytes corroborating the observed high enantioselectivity of these compounds with the studied enantiomers. Finally, this study investigated nontrivial sensors for chiral molecules by a mechanism different than turn-on fluorescence and has the potential to broad chiral compounds with fluorophoric units as optical sensors for enantioselective sensing.

Recent Progress in Diboronic-Acid-Based Glucose Sensors

Non-enzymatic sensors with the capability of long-term stability and low cost are promising in glucose monitoring applications. Boronic acid (BA) derivatives offer a reversible and covalent binding mechanism for glucose recognition, which enables continuous glucose monitoring and responsive insulin release. To improve selectivity to glucose, a diboronic acid (DBA) structure design has been explored and has become a hot research topic for real-time glucose sensing in recent decades. This paper reviews the glucose recognition mechanism of boronic acids and discusses different glucose sensing strategies based on DBA-derivatives-based sensors reported in the past 10 years. The tunable pK_a, electron-withdrawing properties, and modifiable group of phenylboronic acids were explored to develop various sensing strategies, including optical, electrochemical, and other methods. However, compared to the numerous monoboronic acid molecules and methods developed for glucose monitoring, the diversity of DBA molecules and applied sensing strategies remains limited. The challenges and opportunities are also highlighted for the future of glucose sensing strategies, which need to consider practicability, advanced medical equipment fitment, patient compliance, as well as better selectivity and tolerance to interferences.

Fluorescence Sensing of Monosaccharides by Bis-boronic Acids Derived from Quinolinium Dicarboxamides: Structural and Spectroscopic Studies

Three new diboronic acid-substituted bisquinolinium salts were synthesized, structurally described by single-crystal X-ray diffraction, and studied in-depth as fluorescent receptors for six monosaccharides and two open-chain polyols in water at physiological pH. The dicationic pyridine-2,6-dicarboxamide-based receptors contain two N-quinolinium rings as the fluorescent units covalently linked to three different isomers of phenylboronic acid (ortho, 2; meta, 3; and para, 4) as chelating binding sites for polyols. Additions of glucose/fructose in the micromolar concentration range to receptors 2 and 3 induce significant fluorescence changes, but in the presence of arabinose, galactose, mannose, and xylose, only modest optical changes are observed. This optical change is attributed to a static photoinduced electron transfer mechanism. The meta-diboronic receptor 3 exhibited a high affinity/selectivity toward glucose (K = 3800 M^-1) over other monosaccharides including common interfering species such as fructose and mannitol. Based on multiple spectroscopic tools, electrospray ionization high-resolution mass spectrometry, crystal structures, and density functional theory calculations, the binding mode between 3 and glucose is proposed as a 1:1 complex with the glucofuranose form involving a cooperative chelating diboronate binding. These results demonstrate the usefulness of a new set of cationic fluorescent diboronic acid receptors with a strong ability for optical recognition of glucose in the sub-millimolar concentration range.

Recognition of d-Glucose in Water with Excellent Sensitivity, Selectivity, and Chiral Selectivity Using γ-Cyclodextrin and Fluorescent Boronic Acid Inclusion Complexes Having a Pseudo-diboronic Acid Moiety

Fluorescence recognition of d-glucose in water with excellent sensitivity, selectivity, and chiral selectivity is desired because d-glucose is an essential component in biological and pathological processes. We report an innovative approach that exploits the 1:2 stoichiometric inclusion complexes of γ-cyclodextrin (γ-CyD) with two molecules of fluorescent monoboronic acid-based receptors, which form a pseudo-diboronic acid moiety as the recognition site for d-glucose in water. Two monoboronic acids (1F and 2N) were easily synthesized without heating or column purification. The 1:2 stoichiometric inclusion complexes (1F/γ-CyD and 2N/γ-CyD) were prepared in a mixture of dimethyl sulfoxide/water (2/98 in v/v) by mixing γ-CyD and the corresponding monoboronic acids. Both 1F/γ-CyD and 2N/γ-CyD exhibited strong turn-on response to d-glucose with excellent selectivity over nine other saccharides in the water-rich solvent at pH 7.4 owing to the ditopic recognition of d-glucose by the pseudo-diboronic acid moieties. The limits of detection of 1F/γ-CyD and 2N/γ-CyD for d-glucose were 1.1 and 1.8 μM, respectively, indicating the remarkable sensitivity for the detection of d-glucose at μM levels. 1F/γ-CyD and 2N/γ-CyD also demonstrated chiral-selective recognition of d-glucose, which is apparent from the 2.0- and 6.3-fold enhancement of fluorescence by the addition of d-glucose relative to l-glucose addition, owing to the chiral pseudo-diboronic acid moieties produced by the chiral γ-CyD cavity. To the best of our knowledge, 2N/γ-CyD has the highest d/l selectivity among hitherto reported fluorescent diboronic acid-based receptors.

A Vibration-Induced-Emission-Based Fluorescent Chemosensor for the Selective and Visual Recognition of Glucose

The development of chemosensors to detect analytes in biologically relevant solutions is a challenging task. We report the synthesis of a fluorescent receptor that combines vibration-induced emission (VIE) and dynamic covalent chemistry for the detection of glucose in aqueous media. We show that the bis-2-(N-methylaminomethyl)phenylboronic acid-decorated N,N'-diphenyl-dihydrodibenzo[a,c]phenazine (DPAC) receptor 1 can detect glucose and discriminate between closely related monosaccharides including those commonly found in blood. Preliminary studies suggest monosaccharides bind to the DPAC-receptor with a 1:1 stoichiometry to produce pseudomacrocyclic complexes, which in turn leads to distinct optical changes in the fluorescent emission of the receptor for each host. Moreover, the complexation-induced change in emission can be detected visually and quantified in a ratiometric way. Our results highlight the potential of VIE-type receptors for the quantitative determination of saccharides in biological samples.

Tailoring Sensors and Solvents for Optimal Analysis of Complex Mixtures Via Discriminative 19F NMR Chemosensing

Separation-free analytic techniques capable of providing precise and real-time component information are in high demand. ¹⁹F NMR-based chemosensing, where the reversible binding between analytes and a ¹⁹F-labeled sensor produces chromatogram-like output, has emerged as a valuable tool for the rapid analysis of complex mixtures. However, the potential overlap of the ¹⁹F NMR signals still limits the number of analytes that can be effectively differentiated. In this study, we systematically investigated the influence of the sensor structure and NMR solvents on the resolution of structurally similar analytes. The substituents adjacent and distal to the ¹⁹F labels are both important to the resolving ability of the ¹⁹F-labeled sensors. More pronounced separation between ¹⁹F NMR peaks was observed in nonpolar and aromatic solvents. By using a proper sensor and solvent combination, more than 20 biologically relevant analytes can be simultaneously identified.

Recent advances of BINOL-based sensors for enantioselective fluorescence recognition

Enantioselective fluorescent sensors show large potential for fast, real-time, and highly sensitive measurement of the concentration and enantiomeric composition of chiral molecules. Among all of the sensors, BINOL-based sensors have been actively investigated and extensively used to carry out highly enantioselective, sensitive recognition of chiral α-hydroxycarboxylic acids, amino acids, amino acid derivatives, amino alcohols and amines. In this manuscript, the recent progress of chiral BINOL-based sensors for enantioselective fluorescence recognition of different substrates is reviewed and discussed. The structure of BINOL is tuned by introducing various groups or molecules which systematically changed its fluorescence properties and offered potential for rapid assays of chiral organic molecules. From the development of this area, we gain fresh insight into the challenges and chances of BINOL-based sensors.

Balancing interactions in proline-based receptors for chiral recognition of l-/d-DOPA

Proline based receptors (1-14) attached with phenylboronic acid and benzaldehyde binding groups at the N-/C- or C-/N-termini of the proline residue were created for chiral recognition of l-/d-DOPA, in an attempt to examine if balancing the two binding events would influence the recognition. By changing the positions of boronic acid and aldehyde groups substituted on the phenyl rings (1-4, 5-8) and the site at which phenylboronic acid and benzaldehyde moieties attached respectively to the N- and C-termini or C- and N-termini of the proline residue (1-4vs.5-8), and by introducing an electron-withdrawing fluorine atom in the phenyl ring of the weaker binder the benzaldehyde moiety (11vs.1, 14vs.5), we were able to show that a better balance of the two binding events does improve the chiral recognition. This finding can only be made with the current version of receptors that were equipped with two different binding groups. Together with the finding that the chiral recognition performance in mixed organic-aqueous solutions is tunable by varying the solvent composition, we have now arrived at a protocol for designing proline based receptors for extended applications in chiral recognition.

Accurate chiral pattern recognition for amines from just a single chemosensor

The current work proposes a novel method for accurate pattern recognition of (mono- and di-) amines and determination of enantiomeric excess (ee) using molecular self-assembly.

The challenges of glycan recognition with natural and artificial receptors

Glycans - simple or complex carbohydrates - play key roles as recognition determinants and modulators of numerous physiological and pathological processes. Thus, many biotechnological, diagnostic and therapeutic opportunities abound for molecular recognition entities that can bind glycans with high selectivity and affinity. This review begins with an overview of the current biologically and synthetically derived glycan-binding scaffolds that include antibodies, lectins, aptamers and boronic acid-based entities. It is followed by a more detailed discussion on various aspects of their generation, structure and recognition properties. It serves as the basis for highlighting recent key developments and technical challenges that must be overcome in order to fully deal with the specific recognition of a highly diverse and complex range of glycan structures.

Reaction-based chiroptical sensing of ClO− using circularly polarized luminescence via self-assembly organogel

A chiral amino acid functionalized probe, PTZ-D, could self-assemble into a chiral organogel displaying unprecedented chiroptical monitoring of ClO⁻ with switchable CPL signals.

Spiro[pyrrol-benzopyran]-based probe with high asymmetry for chiroptical sensing via circular dichroism

We proposed an efficient approach to construct a novel spiro[pyrrol-benzopyran] scaffold with high asymmetry for reaction-based chiroptical sensing via circular dichroism.