A new boronic acid fluorescent sensor based on fluorene for monosaccharides at physiological pH

Rational design of an acid-sensitive fluorophore from 8-hydroxy quinoline derivative exhibiting proton activated charge transfer characteristics

A pH-responsive small organic molecule 8OMeQBI has been synthesized by functionalizing 8-hydroxy quinoline and benzimidazole derivatives in an organo-aqueous medium. The pH-responsive fluorescence behavior in a semi-aqueous environment was studied using UV-visible and fluorescence spectral analysis. The newly prepared fluorophores show a protonation-activated fluorescence enhancement in an acidic environment. An increase in pH in an alkaline environment has caused deprotonation, resulting in diminished fluorescence intensity. Additionally, the protonation-induced fluorescence was confirmed through the proton-nuclear magnetic resonance titration analysis. Further, the photophysical studies reveal an enhancement in the relative quantum yield (ɸ = 0.3) and fluorescence lifetime (increases to 3 ns at pH 3) in an acidic environment. Also, the fluorophore shows fluorescence "On" with acids such as hydrochloric acid and fluorescence "Off" with bases such as triethylamine (TEA). This fluorescence "on-off" behavior is reversible and repeatable, which helps to develop a molecular-level logic gate and sequential memory unit with "Writing-Reading-Erasing-Reading" behavior. Furthermore, solid supportive experiments were carried out by preparing fluorophore-based films and paper strips. Also, the fluorophore demonstrates the practical applicability of potable water samples.

Molecular two-point recognition of fructosyl valine and fructosyl glycyl histidine in water by fluorescent Zn(II)-terpyridine complexes bearing boronic acids

Selective recognition of fructosyl amino acids in water by arylboronic acid-based receptors is a central field of modern supramolecular chemistry that impacts biological and medicinal chemistry. Fructosyl valine (FV) and fructosyl glycyl histidine (FGH) occur as N-terminal moieties of human glycated hemoglobin; therefore, the molecular design of biomimetic receptors is an attractive, but very challenging goal. Herein, we report three novel cationic Zn-terpyridine complexes bearing a fluorescent N-quinolinium nucleus covalently linked to three different isomers of strongly acidified phenylboronic acids (ortho-, 2Zn; meta-, 3Zn and para-, 4Zn) for the optical recognition of FV, FGH and comparative analytes (D-fructose, Gly, Val and His) in pure water at physiological pH. The complexes were designed to act as fluorescent receptors using a cooperative action of boric acid and a metal chelate. Complex 3Zn was found to display the most acidic -B(OH)2 group (pKa = 6.98) and exceptionally tight affinity for FV (K = 1.43 × 105 M-1) with a strong quenching analytical response in the micromolar concentration range. The addition of fructose and the other amino acids only induced moderate optical changes. On the basis of several spectroscopic tools (1H, 11B NMR, UV-Vis, and fluorescence titrations), ESI mass spectrometry, X-ray crystal structure, and DFT calculations, the interaction mode between 3Zn and FV is proposed in a 1 : 1 model through a cooperative two-point recognition involving a sp3 boronate-diol esterification with simultaneous coordination bonding of the carboxylate group of Val to the Zn atom. Fluorescence quenching is attributed to a static complexation photoinduced electron transfer mechanism as evidenced by lifetime experiments. The addition of FGH to 3Zn notably enhanced its emission intensity with micromolar affinity, but with a lower apparent binding constant than that observed for FV. FGH interacts with 3Zn through boronate-diol complexation and coordination of the imidazole ring of His. DFT-optimized structures of complexes 3Zn-FV and 3Zn-FGH show a picture of binding which shows that the Zn-complex has a suitable (B⋯Zn) distance to the two-point recognition with these analytes. Molecular recognition of fructosyl amino acids by transition-metal-based receptors has not been explored until now.

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.

Synthesis and Nonlinear Optical Behavior of Thermally Stable Chromophores Based on 9,9-Dimethyl-9H-fluoren-2-amine: Improving Intrinsic Hyperpolarizability through Modulation of "Push-Pull"

Improvement in the first hyperpolarizability (βHRS) as well as intrinsic hyperpolarizability (βint) of chromophores based on 9,9-dimethyl-9H-fluoren-2-amine through modulation of the conjugation pathway is described. A series of six novel chromophores with "linear" conjugation showed significant enhancement of βHRS as well as βint compared to the counterparts lacking a "linear" conjugation but having an identical combination of donor, acceptor, and the intervening π-conjugated linker. The hyperpolarizability (βHRS as well as βint) values of the new series measured using hyper-Rayleigh scattering exceeded the apparent limit set by the latter set of fluorene-based chromophores. The experimental results are analyzed and interpreted in the context of linear optical properties, single-crystal X-ray analysis, electrochemistry, etc. and corroborated by theoretical studies. We find that modulation of the "push-pull" of the conjugation pathway in these donor-acceptor chromophores compares favourably with the corresponding changes in the optical gaps, transition dipole moments, and dipole moment difference between the ground and excited states.

A new fluorescent boronic acid sensor based on carbazole for glucose sensing via aggregation-induced emission

A water-soluble fluorescent sensor based on carbazole pyridinium boronic acid (CPBA) was designed and synthesized. Its structure has been confirmed by CHN and 1H and 13C NMR, FT-IR, and MS spectral data. Fluorescence studies of the synthesized chemosensor CPBA showed a selective ratiometric fluorescent response for glucose among different monosaccharides. The results specified that CPBA is a pH-sensitive sensor that behaves differently in the absence and presence of glucose in the pH range 4-10. The pH, DLS, Job's plot, UV-visible, and fluorescence titration studies showed that the selectivity of CPBA towards glucose is through the aggregation-induced emission (AIE) phenomenon. The fluorescence emission intensity of CPBA changes by more than 2100 fold by adding glucose, whereas it is 2 fold for fructose. The calculated binding constant value of CPBA for glucose (K = 2.3 × 106 M-1) is 85 times greater than for fructose, indicating the high affinity of the sensor for glucose.

Colorimetric and fluorimetric chemosensor based on upper rim-functionalized calix[4]arene for selective detection of fluoride ion

A new colorimetric and fluorescent chemosensor for fluoride anion based on calix [4]arene bearing four sulfonamide-fluorenone subunits on the upper rim was conveniently synthesized. It showed a remarkable color change as well as the fluorescence quenching upon addition of F^- even in the presence of a wide range of anions in DMSO. The binding property of L with F^- was studied by a combination of various spectroscopic techniques, such as absorption and emission titration, Job's plot and ¹H NMR titration. It is anticipated that this design with functional group attached to upper rim of calix[4]arene platform can provide a new approach for the development of F^- chemosensor.

4-Iodophenylboronic Acid Stabilized Gold Cluster as a New Fluorescent Chemosensor for Saccharides Based on Excimer Emission Quenching

4-iodophenylboronic acid (IPBA) ligated luminescent gold cluster was synthesized by mixing an aqueous solution of IBPA and polyvinylpyrrolidone stabilized gold cluster (Au:PVP) in water at room temperature through chemisorption of iodine on gold nano surface. Transmission Electron microscopy (TEM) and matrix assisted laser desorption ionization (MALDI) analysis revealed that the size of these Au-clusters (1.4±0.2 nm) remain unchanged without any noticeable aggregation during synthesis. Owing to the formation of excimer between aryl moieties grafted over Au surface, the cluster exhibit strong emission peak at 335 nm. This luminescent gold cluster is used for sensing different saccharides in water at physiological pH through quenching of excimer emission peak. This strong excimer emission is significantly quenched in presence of saccharides through interaction with boronic acid moieties. The selectivity for different saccharides follows the order: fructose > galactose > maltose > glucose ~ ribose > sorbitol with hight affinity for fructose (K_SV = 1.54 × 10⁴ M^-1) with Limit of Detection (LOD) of 100 μM.

Chemosensing of neurotransmitters with selectivity and naked eye detection of l-DOPA based on fluorescent Zn(ii)-terpyridine bearing boronic acid complexes

A new range of fluorescent Zn(ii)-terpyridine complexes are described and studied in-depth as chemosensors for catecholamine-based neurotransmitters in pure water.

A water-soluble boronic acid sensor for caffeic acid based on double sites recognition

Due to reversibly and covalently binding with Lewis bases and polyols, boronic acid compounds as fluorescent sensors have been widely reported to recognize carbohydrates, ions, hydrogen peroxide, and so on. However, boronic acid sensors for highly selective recognition of caffeic acid rather than catechol or catechol derivatives have not been reported yet. Herein a novel water-soluble sensor 5c with double recognition sites based on a boronic acid was reported. When 2.3 × 10-4 M of caffeic acid was added, the fluorescence intensity of sensor 5c decreased by 99.6% via inner filter effect (IFE) because its excitation spectrum well overlaps with the absorption spectrum of caffeic acid under neutral condition, while the fluorescence increased or did not change obviously after binding with other analytes including carbohydrates and other catechol derivatives. In addition, the response time to caffeic acid is fast at room temperature, and a high binding constant (9245.7 ± 348.3 M-1) and low LOD (1.81 × 10-6 M) was calculated. Moreover, determination of caffeic acid content in caffeic acid tablets was studied, and the recovery rate is sufficient. Therefore, sensor 5c can be used as a potential tool for detecting biologically significant caffeic acid in real samples.

Efficient Conversion of Glucose into Fructose via Extraction-Assisted Isomerization Catalyzed by Endogenous Polyamine Spermine in the Aqueous Phase

In the present study, natural polyamine spermine is demonstrated as a potential basic catalyst for glucose-to-fructose isomerization. For instance, spermine achieves a decent fructose yield (30% wt) and selectivity (74%) during the single-step aqueous phase isomerization under the modest operating conditions (100 °C for 15 min). In addition to the expected reaction byproduct monosugar mannose, spermine also assists in the synthesis of rare and important monosugar, that is, psicose up to 4% wt. Psicose is a zero calorie rare sugar, exhibits a low caloric value, and possesses anti-adipogenic property. A comparative study involving other polyamines concluded that the presence of 20 amines tends to exhibit the most significant impact in improving the target product yield by releasing a higher number of OH- ions, which are responsible for isomerization through the formation of an enediol anion. An attempt was made to further improve the fructose yield through the addition of neutral salts, but it promoted a meager achievement. In an alternate study, a selective extraction strategy was followed for the isolation of fructose from the reaction mixture. The employed aryl monoboronic acid remarkably improved the net fructose concentration, that is, fructose productivity up to 75% wt (cumulative) and 70% selectivity within three consecutive extractions and isomerization cycles, which is comparatively three times shorter than that reported in the literature. Notably, spermine itself provided the essential and necessary basic environment for selective fructose extraction and glucose isomerization, ruling out the use of any external reagents and thus establishing itself as a versatile material suitable for a typical isomerization reaction in an upscaled reactor.

A novel boronic acid-based fluorescent sensor for selectively recognizing Fe3+ ion in real time

Boronic acid provides faster fluorescence response to Fe3+ compared to other reported sensors, which is critical for continuous dynamic detection. Herein, we reported a novel boronic acid-based sensor 4 that could recognize Fe3+ ion in real time. After 10 equiv. of Fe3+ ion (1 mM) was added, the fluorescence of sensor 4 was immediately quenched by 96%. While other ions, including Ba2+, Ca2+, Cr2+, Cd2+, Co2+, Cs2+, Cu2+, Fe2+, K+, Li+, Mg2+, Mn2+, Na+, Ni2+ or Zn2+, respectively, did not change the fluorescence significantly. Further tests indicated that the high selectively sensing Fe3+ ion benefits from the two boronic acid functionalities in the structure. Moreover, interference experiments showed this sensor has an excellent anti-interference ability. In addition, we performed binding activity test in rabbit plasma and other real samples for practical applications, obtaining similar results. And the thin layer loading sensor 4 was also successfully applied to recognize Fe3+ ion among various ions. Therefore, 4 may serve as a potential sensor for continuous monitoring and detecting Fe3+ ion in real time.

Water-soluble diboronic acid-based fluorescent sensors recognizing d-sorbitol

Water-soluble diboronic acid sensor 15c exhibited higher selectivity for d-sorbitol than monoboronic acid sensor 1 and hydroxy derivative 2. And it could be applied for d-sorbitol sensing and analysis in real samples.

A simple ratiometric fluorescent sensor for fructose based on complexation of 10-hydroxybenzo[h]quinoline with boronic acid

A simple ratiometric fluorescent sensor for fructose was presented. It consisted of 10-hydroxybenzo[h]quinoline (HBQ) which showed emission at 572nm and 3-pyridylboronic acid (PDBA) whose complex with HBQ gave emission at 500nm. The reaction of fructose with PDBA inhibited the complexation of HBQ with PDBA, resulting in the change of dual-emission intensity ratio. The sensor well quantified fructose in the range of 0.015-2.5mM with detection limit of 0.005mM. Besides, this sensor exhibited excellent selectivity and was successfully applied to fructose detection in food. This work provides a simple ratiometric sensing platform for sensitive and selective detection of fructose.

Amidofluorene-appended lower rim 1,3-diconjugate of calix[4]arene: synthesis, characterization and highly selective sensor for Cu(2.)

Functionalization of calix[4]arene with amidofluorene moieties at the lower rim led to formation of the 1,3-diconjugate of calix[4]arene L as a novel fluorescent chemosensor for Cu²⁺. The receptor molecule L exhibited a pronounced selectivity towards Cu²⁺ over other mono and divalent ions. The formation of the complex between L and Cu²⁺ was evaluated by absorption, fluorescence and ¹H NMR spectroscopy. The sensor L showed a remarkable color change from colorless to purple and a fluorescence quenching only upon interaction with Cu²⁺. The 1:1 stoichiometry of the obtained complex has been determined by Job’s plot. The association constant determined by fluorescence titration was found to be 1.8 × 10⁶ M⁻¹. The sensor showed a linear response toward Cu²⁺ in the concentration range from 1 to 10 µM with a detection limit of 9.6 × 10⁻⁸ M.

A new selective fluorene-based fluorescent internal charge transfer (ICT) sensor for sugar alcohols in aqueous solution

Sugar alcohols, such as sorbitol, are commonly used as a replacement for sucrose in the food industry, applied as starting material for vitamin C synthesis, and involved as one of the causative factors in diabetic complications. Therefore, their detection and quantification in aqueous solution are necessary. The reversible covalent interactions between boronic acids and diols are the basis of efficient methods for the detection of saccharides. Herein, we report a new internal charge transfer (ICT) fluorene-based fluorescent boronic acid sensor (1) 2-[(9,9-dimethyl-9H-fluoren-2-yl-amino)methyl] phenyl boronic acid that shows significant fluorescence changes upon addition of saccharides. The boronic acid has high affinity (K a = 1107.9 M(-1)) and selectivity for sorbitol at pH = 8.31. It showed a linear response toward sorbitol in the concentration range from 1.0 × 10(-5) to 6.0 × 10(-4) mol L(-1) with the detection limit of 7.04 × 10(-6) mol L(-1). Sensor 1 was used to detect sorbitol in real samples with good recovery.

Fluorescent chemosensors of carbohydrate triols exhibiting TICT emissions

4-4′-Disubstituted biphenyl boronic acids (BBAs) are push–pull fluorophores with “turn-on” fluorescence properties.