Exploring the Potential of Nanogels: From Drug Carriers to Radiopharmaceutical Agents

Nanogels open up access to a wide range of applications and offer among others hopeful approaches for use in the field of biomedicine. This review provides a brief overview of current developments of nanogels in general, particularly in the fields of drug delivery, therapeutic applications, tissue engineering, and sensor systems. Specifically, cyclodextrin (CD)-based nanogels are important because they have exceptional complexation properties and are highly biocompatible. Nanogels as a whole and CD-based nanogels in particular can be customized in a wide range of sizes and equipped with a desired surface charge as well as containing additional molecules inside and outside, such as dyes, solubility-mediating groups or even biological vector molecules for pharmaceutical targeting. Currently, biological investigations are mainly carried out in vitro, but more and more in vivo applications are gaining importance. Modern molecular imaging methods are increasingly being used for the latter. Due to an extremely high sensitivity and the possibility of obtaining quantitative data on pharmacokinetic and pharmacodynamic properties, nuclear methods such as single photon emission computed tomography (SPECT) and positron emission tomography (PET) using radiolabeled compounds are particularly suitable here. The use of radiolabeled nanogels for imaging, but also for therapy, is being discussed.

Cyclodextrin Host-Guest Recognition in Glucose-Monitoring Sensors

Diabetes mellitus is a prevalent chronic health condition that has caused millions of deaths worldwide. Monitoring blood glucose levels is crucial in diabetes management, aiding in clinical decision making and reducing the incidence of hypoglycemic episodes, thereby decreasing morbidity and mortality rates. Despite advancements in glucose monitoring (GM), the development of noninvasive, rapid, accurate, sensitive, selective, and stable systems for continuous monitoring remains a challenge. Addressing these challenges is critical to improving the clinical utility of GM technologies in diabetes management. In this concept, cyclodextrins (CDs) can be instrumental in the development of GM systems due to their high supramolecular recognition capabilities based on the host-guest interaction. The introduction of CDs into GM systems not only impacts the sensitivity, selectivity, and detection limit of the monitoring process but also improves biocompatibility and stability. These findings motivated the current review to provide a comprehensive summary of CD-based blood glucose sensors and their chemistry of glucose detection, efficiency, and accuracy. We categorize CD-based sensors into four groups based on their modification strategies, including CD-modified boronic acid, CD-modified mediators, CD-modified nanoparticles, and CD-modified functionalized polymers. These findings shed light on the potential of CD-based sensors as a promising tool for continuous GM in diabetes mellitus management.

High-Throughput Analysis of Bacterial Toxic Lipopolysaccharide in Water by Dual-Wavelength Monitoring Using a Ratiometric Fluorescent Chemosensor

Lipopolysaccharide (LPS) is a bacterial toxin that causes fever in humans. Our small-molecule chemosensor named Zn-dpa-C2OPy shows rapid ratiometric fluorescence response to LPS in water with a detection limit of 11 pM, which is lower than that of our previously reported sensor. Spectroscopic measurements (fluorescence, absorbance, 1H NMR, and fluorescence lifetime), dynamic light scattering measurements, and transmission electron microscopy observations revealed that the fluorescence response was induced by the changes in the aggregation state via multi-point recognition of LPS through hydrophobic and electrostatic interactions, in addition to the coordination between the zinc(II)-dipicolylamine moiety of the chemosensor and the phosphate group of LPS. The proposed Zn-dpa-C2OPy chemosensor was applied to an original flow injection analysis (FIA) system with a self-developed dual-wavelength fluorophotometer, and a high throughput of 36 samples per hour was achieved. These results demonstrate the feasibility of this unique methodology combining a ratiometric fluorescent chemosensor and FIA for continuous online monitoring of LPS in water.

Supramolecular Assemblies of Fluorescent Nitric Oxide Photoreleasers with Ultrasmall Cyclodextrin Nanogels

Developing biocompatible nitric oxide (NO) photoreleasing nanoconstucts is of great interest in view of the large variety of biological roles that NO plays and the unique advantage light offers in controlling NO release in space and time. In this contribution, we report the supramolecular assemblies of two NO photodonors (NOPDs), NBF-NO and RHD-NO, as water-dispersible nanogels, ca. 10 nm in diameter, based on γ-cyclodextrins (γ-CDng). These NOPDs, containing amino-nitro-benzofurazan and rhodamine chromophores as light harvesting antennae, can be activated by visible light, are highly hydrophobic and can be effectively entrapped within the γ-CDng. Despite being confined in a very restricted environment, neither NOPD suffer self-aggregation and preserve their photochemical and photophysical properties well. The blue light excitation of the weakly fluorescent γ-CDng/NBF-NO complex results in effective NO release and the concomitant generation of the highly green, fluorescent co-product, which acts as an optical NO reporter. Moreover, the green light excitation of the persistent red fluorescent γ-CDng/RHD-NO triggers NO photorelease without significantly modifying the emission properties. The activatable and persistent fluorescence emissions of the NOPDs are useful for monitoring their interactions with the Gram-positive methicillin-resistant Staphylococcus aureus, whose growth is significantly inhibited by γ-CDng/RHD-NO upon green light irradiation.

Preparation of ultrasmall cyclodextrin nanogels by an inverse emulsion method using a cationic surfactant

Stable water-in-oil emulsion membranes can be prepared using [dilauryl(dimethyl)ammonium] bromide (DDAB), a cationic surfactant. We prepared ultrasmall cyclodextrin (γ-CyD) nanogels (γ-CyDngs) by forming ionic pairs between the secondary hydroxyl groups of γ-CyDs and DDAB. Fluorescence and NMR characterisation of the obtained γ-CyDngs revealed superior inclusion affinities compared with native γ-CyDs, beneficial for the solubilisation of hydrophobic compounds in water.

Electrochemical detection of boric acid using gallacetophenonato-(β-diketonato) ruthenium complex in water

A simple and practical method for boron detection in water is desired in various fields such as seawater desalination, water conservation, and plant production. To develop a method for detecting boron as boric acid in water, we synthesized [Ru(acac)₂(H₂thap)] (acac = acetylacetonat ion, thap = 2',3',4'-trihydroxyacetophenonate (gallacetophenonate) ion) possessing a cis-diol moiety that interacts with boric acid. A comparison of UV-visible (UV-vis) absorption spectra measured in the presence and absence of boric acid at various pH values revealed that [Ru(acac)₂(H₂thap)] shows the highest response to boric acid at pH 8.5. Cyclic voltammograms (CVs) and differential pulse voltammograms (DPVs) of [Ru(acac)₂(H₂thap)] aqueous solution at pH 8.5 with varying boric acid concentrations showed a decrease in the peak current value at 0.032 V (vs. Ag|AgClaq.) and an increase in the peak current value at 0.444 V with increasing boric acid concentration. On the basis of the relationship between the ratio of current values (at 0.032 V and 0.444 V) and boric acid concentrations, the binding constant (assuming a 1:1 binding model) for the interaction between [Ru(acac)₂(H₂thap)] and boric acid was estimated to be 135.1 ± 9.1 mol^-1 dm³, and the Limit of Detection (LOD) was calculated to be 1.03 mg B L^-1.

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.

Selective ATP recognition by boronic acid-appended cyclodextrin and fluorescent probe supramolecular complex in water

Various organic compound-based chemosensors for adenosine triphosphate (ATP) have been reported; however, the conventional designs have problems with their water solubility, selectivity, and sensitivity. Herein, a supramolecular complex of boronic...

Boronic Acid-Based Dendrimers with Various Surface Properties for Bacterial Recognition with Adjustable Selectivity

The need for a selective bacterial recognition method is evident to overcome the global problem of antibiotic resistance. Even though researchers have focused on boronic acid-based nanoprobes that immediately form boronate esters with saccharides at room temperature, the mechanism has not been well studied. We have developed boronic acid-modified poly(amidoamine) (PAMAM) dendrimers with various surface properties to investigate the mechanism of bacterial recognition. The boronic acid-based nanoprobes showed selectivity toward strains, species, or a certain group of bacteria by controlling their surface properties. Our nanoprobes showed selectivity toward Gram-positive bacteria or Escherichia coli K12W3110 without having to modify the boronic acid recognition sites. The results were obtained in 20 min and visible to the naked eye. Selectivity toward Gram-positive bacteria was realized through electrostatic interaction between the bacterial surface and the positively charged nanoprobes. In this case, the recognition target was lipoteichoic acid on the bacterial surface. On the other hand, pseudo-zwitterionic nanoprobes showed selectivity for E. coli K12W3110, indicating that phenylboronic acid did not recognize the outermost O-antigen on the lipopolysaccharide layer. Boronic acid-based nanoprobes with optimized surface properties are expected to be a powerful clinical tool to recognize multidrug-resistant strains or highly pathogenic bacteria.

Simple and Rapid Endotoxin Recognition Using a Dipicolylamine-Modified Fluorescent Probe with Picomolar-Order Sensitivity

Endotoxin is a lipopolysaccharide (LPS) that is found in the outer membrane of the cell wall of Gram-negative bacteria. Due to its high toxicity, the allowable endotoxin limit for water for injection is set at a very low value. Conventional methods for endotoxin detection are time-consuming and expensive and have low reproducibility. A previous study has shown that dipicolylamine (dpa)-modified pyrene-based probes exhibit fluorescence enhancement in response to LPS; however, the application of such probes to the sensing of LPS is not discussed. Against this backdrop, we have developed a simple and rapid endotoxin detection method using a dpa-modified pyrenyl probe having a zinc(II) center (Zn-dpa-C4Py). When LPS was added into Zn-dpa-C4Py solution, excimer emission of the pyrene moiety emerged at 470 nm. This probe can detect picomolar concentrations of LPS (limit of detection = 41 pM). The high sensitivity of the probe is ascribed to the electrostatic and hydrophobic interactions between the probe and LPS, which result in the dimer formation of the pyrene moieties. We also found that Zn-dpa-C4Py has the highest selectivity for LPS compared with other phosphate derivatives, which is probably caused by the co-aggregation of the probe with LPS. We propose that Zn-dpa-C4Py is a promising chemical sensor for the detection of endotoxin in medical and pharmaceutical applications.

NMR Investigation of the Supramolecular Complex Formed by a Phenylboronic Acid-Ferrocene Electroactive Probe and Native or Derivatized β-Cyclodextrin

Specifically designed electrochemical sensors are standing out as alternatives to enzyme-based biosensors for the sensing of metabolites. In our previous works, we developed a new electrochemical assay based on cyclodextrin supramolecular complexes. A ferrocene moiety (Fc) was chemically modified by phenylboronic acid (4-Fc-PB) and combined with two different kinds of cyclodextrins (CDs): β-CD and β-CD modified by a dipicolylamine group (dpa-p-HB-β-CDs) for the sensing of fructose and adenosine-triphosphate (ATP), respectively. The aim of the present work is to better comprehend the features underlining the aforementioned complex formation. For the first time, a study about inclusion phenomena between the 4-Fc-PB electroactive probe with β-CD and with dpa-p-HB-β-CD was performed by using nuclear magnetic resonance (NMR) analysis. In particular, we focused on providing insights on the interaction involved and on the calculation of the binding constant of 4-Fc-PB/β-CD supramolecular complex, and elucidation about a drift in the time observed during the control experiments of the electrochemical measurements for the 4-Fc-PB/dpa-p-HB-β-CD supramolecular complex. In this sense, this paper represents a step further in the explanation of the electrochemical results obtained, pointing out the nature of the interactions present both in the formation of the inclusions and in the sensing with the analytes.

Synthesis and Antioxidant Activity of Silver Nanoparticles Using the Odontonema strictum Leaf Extract

The aqueous extract of the leaves of Odontonema strictum (OSM) is used in folk medicine for its antihypertensive properties, and it contains a wide range of secondary metabolites, mostly polyphenols such as verbascoside and isoverbascoside, which could play a major role in the preparation of silver nanoparticles. In this study, we aimed to prepare AgNPs for the first time using the OSM leaf extract (OSM-AgNPs) to investigate their free radical-scavenging potency against 1,1-diphenyl-2-picrylhydrazyl (DPPH) and hydrogen peroxide (H₂O₂). Dynamic light scattering (DLS), UV/Vis, Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), and X-ray photoelectron spectroscopy (XPS) were used to characterize the OSM-AgNPs. With a size around 100 nm and a ζ-potential of −41.1 mV, OSM-AgNPs showed a good stability and a better colloidal property due to electrostatic repulsion and the dispersity. The strong absorption peak at 3 keV in the EDX spectra indicated that silver was the major constituent. Additionally, the existence of silver atoms was confirmed by the Ag 3d_5/2 peak around 367 eV in the XPS spectra. IC₅₀ values of 116 μg/mL and 4.4 μg/mL were obtained for the scavenging activities of DPPH and H₂O₂, respectively. The synthetic OSM-AgNPs can be further exploited as potential antioxidant agents.

Ratiometric fluorescence sensing of d-allulose using an inclusion complex of γ-cyclodextrin with a benzoxaborole-based probe

Because d-allulose has been attracting attention as a zero-calorie sugar, the selective sensing of d-allulose is desired to investigate its health benefits. We report herein a novel fluorescence chemosensor that is based on an inclusion complex of γ-cyclodextrin (γ-CyD) with a benzoxaborole-based probe. Two inclusion complexes, 1/γCyD and 2/γCyD, were prepared by mixing γ-CyD with their corresponding probes in a water-rich solvent, where γ-CyD encapsulates two molecules of the probes inside its cavity to form a pyrene dimer. Both 1/γCyD and 2/γCyD exhibit monomeric and dimeric fluorescence from the pyrene moieties. By the reaction of 1/γCyD with saccharides, the intensities of monomeric and dimeric fluorescence remained unchanged and decreased, respectively. We have demonstrated that 1/γCyD has much higher affinity for d-allulose than for the other saccharides (d-fructose, d-glucose, and d-galactose). The conditional equilibrium constants for the reaction systems were determined to be 498 ± 35 M^-1 for d-fructose, 48.4 ± 25.3 M^-1 for d-glucose, 15.0 ± 3.3 M^-1 for d-galactose, and (8.05 ± 0.59) × 10³ M^-1 for d-allulose. These features of 1/γCyD enable ratiometric fluorescence sensing with high sensitivity and selectivity for d-allulose. The limits of detection and quantification of 1/γCyD for d-allulose at pH 8.0 were determined to be 6.9 and 21 μM, respectively. Induced circular dichroism spectral study has shown that the reaction of 1/γCyD with d-allulose causes the monomerisation of the dimer of probe 1 that is encapsulated by γ-CyD, which leads to the diminishment of the dimeric fluorescence.

A simple supramolecular complex of boronic acid-appended β-cyclodextrin and a fluorescent boronic acid-based probe with excellent selectivity for d-glucose in water

A simple supramolecular complex of modified cyclodextrin with a boronic acid probe enabled the highly selective detection of d-glucose in water.

Rapid Bacterial Recognition over a Wide pH Range by Boronic Acid-Based Ditopic Dendrimer Probes for Gram-Positive Bacteria

We have developed a convenient and selective method for the detection of Gram-positive bacteria using a ditopic poly(amidoamine) (PAMAM) dendrimer probe. The dendrimer that was modified with dipicolylamine (dpa) and phenylboronic acid groups showed selectivity toward Staphylococcus aureus. The ditopic dendrimer system had higher sensitivity and better pH tolerance than the monotopic PAMAM dendrimer probe. We also investigated the mechanisms of various ditopic PAMAM dendrimer probes and found that the selectivity toward Gram-positive bacteria was dependent on a variety of interactions. Supramolecular interactions, such as electrostatic interaction and hydrophobic interaction, per se, did not contribute to the bacterial recognition ability, nor did they improve the selectivity of the ditopic dendrimer system. In contrast, the ditopic PAMAM dendrimer probe that had a phosphate-sensing dpa group and formed a chelate with metal ions showed improved selectivity toward S. aureus. The results suggested that the targeted ditopic PAMAM dendrimer probe showed selectivity toward Gram-positive bacteria. This study is expected to contribute to the elucidation of the interaction between synthetic molecules and bacterial surface. Moreover, our novel method showed potential for the rapid and species-specific recognition of various bacteria.

Effect of Spacer Length in Pyrene-Modified-Phenylboronic Acid Probe/CyD Complexes on Fluorescence-based Recognition of Monosaccharides in Aqueous Solution

The chemical sensing of saccharides is of importance for the diagnosis of diabetes. Various enzymatic sensors have been developed, but their heat and pH instability issues need to be resolved. In this regard, the development of artificial saccharide sensors with high stability is attracting attention. We have designed a heat- and pH-stable supramolecular inclusion complex system composed of cyclodextrin (CyD) as a host and a phenylboronic acid (PB) probe possessing pyrene as a fluorescent guest. Several probes possessing alkyl spacers having various lengths between the PB and the pyrene moiety, Cn-APB (n = 1 - 4), were newly synthesized and evaluated with respect to their monosaccharide recognition ability on the basis of the fluorescence response through the cyclic esterification of monosaccharide and PB. These Cn-APB/CyD supramolecular inclusion complexes have exhibited a selective fluorescence response towards fructose in aqueous solution based on the photo-induced electron transfer mechanism. The spacer length of the alkyl group in Cn-APB significantly affects the affinity for saccharides. With respect to the complex between C4-APB and PB-modified CyD (3-PB-γ-CyD), it was found that the supramolecular inclusion complexes had high selectivity for glucose with significant fluorescence enhancement. These results indicate that the lengths of the alkyl spacers in the probe molecules are important to control the recognition of saccharides in aqueous solution.

Supramolecular Zn(II)-Dipicolylamine-Azobenzene-Aminocyclodextrin-ATP Complex: Design and ATP Recognition in Water

Cyclodextrins (CyDs) are water-soluble host molecules possessing a nanosized hydrophobic cavity. In the realm of molecular recognition, this cavity is used not only as a recognition site but also as a reaction medium, where a hydrophobic sensor recognizes a guest molecule. Based on the latter concept, we have designed a novel supramolecular sensing system composed of Zn(II)-dipicolylamine metal complex-based azobenzene (1-Zn) and 3A-amino-3A-deoxy-(2AS,3AS)-γ-cyclodextrin (3-NH₂-γ-CyD) for sensing adenosine-5′-triphosphate (ATP). 1-Zn showed redshifts in the UV-Vis spectra and induced circular dichroism (ICD) only when both ATP and 3-NH₂-γ-CyD were present. Calculations of equilibrium constants indicated that the amino group of 3-NH₂-γ-CyD was involved in the formation of supramolecular 1-Zn/3-NH₂-γ-CyD/ATP. The Job plot of the ICD spectral response revealed that the stoichiometry of 1-Zn/3-NH₂-γ-CyD/ATP was 2:1:1. The pH effect was examined and 1-Zn/3-NH₂-γ-CyD/ATP was most stable in the neutral condition. The NOESY spectrum suggested the localization of 1-Zn in the 3-NH₂-γ-CyD cavity. Based on the obtained results, the metal coordination interaction of 1-Zn and the electrostatic interaction of 3-NH₂-γ-CyD were found to take place for ATP recognition. The “reaction medium approach” enabled us to develop a supramolecular sensing system that undergoes multi-point interactions in water. This study is the first step in the design of a selective sensing system based on a good understanding of supramolecular structures.

Electrochemical Sensing of Adenosin Triphosphate by Specific Binding to Dipicolylamine Group in Cyclodextrin Supramolecular Complex

Electrochemical detection based on cyclodextrin supramolecular complexes is founded on the competitive binding between electroactive probes and target molecules. This limits their versatility to be used for sensing a broad range of metabolites. In this work, we demonstrate the significant role of zinc ions as well as of β-cyclodextrins modified with dipicolylamine and of a phenylboronic acid-modified ferrocene probe to address a selective electrochemical detection of adenosin triphosphate (ATP). Our findings will definitively have an impact in oncological point-of-care systems, since a high level of extracellular ATP reveals the inflammatory response due to chemotherapeutic treatments.

Electrochemical Assay for Extremely Selective Recognition of Fructose Based on 4-Ferrocene-Phenylboronic Acid Probe and β-Cyclodextrins Supramolecular Complex

The aim of the present paper is to highlight a novel electrochemical assay for an extremely-selective detection of fructose thanks to the use of a supramolecular complex between β-cyclodextrins (β-CDs) and a chemically modified ferrocene with boronic acid named 4-Fc-PB/natural-β-CDs. Another kind of β-CDs, the 4-Fc-PB/3-phenylboronic-β-CDs, is proposed for the detection of glucose. The novel electrochemical probe is fully characterized by ¹ H nuclear magnetic resonance, mass spectroscopy, and elemental analysis, while the superior electrochemical performance is assessed in terms of sensitivity and detection limit. The novelty of the present work consists in the role of CDs that for the first time are employed in electrochemistry with a unique detection mechanism based on specific chemical interactions with the target molecule by the introduction of proper binding groups. A highly selective detection of fructose is obtained and it is believed that the proposed mechanism of detection represents a new way to electrochemically sense other molecules by varying the combination of specific groups of the supramolecular complex. The findings are of impactful importance since a quick, easy, cheap, and extremely selective detection of fructose is not yet available in the market, here achieved by using electrochemical methods which are a very growing field.

Phosphate-sensing with (di-(2-picolyl)amino)quinazolines based on a fluorescence on–off system

Detection and visualization of phosphates such as ATP in living organisms can facilitate the elucidation of various biological events. Although substantial efforts had been made in this area, present methods have disadvantages such as the need for specialized equipment and poor sensitivities. To address these limitations, novel fluorescent probes, (di-(2-picolyl)amino)quinazolines, were developed for application in ATP detection. They selectively recognized copper ions by fluorescence quenching, and their copper complexes displayed fluorescence enhancement in the presence of phosphoric acid derivatives. This fluorescence on–off system enabled highly sensitive fluorescence detection of ATP when combined with a phenyl boronic acid-modified γ-cyclodextrin through a plausible multipoint recognition system.

Supramolecular probe Cu-dpa-QZ2/FPB-γ-CyD recognized ATP with high sensitivity.

Role of alkan-1-ol solvents in the synthesis of yellow luminescent carbon quantum dots (CQDs): van der Waals force-caused aggregation and agglomeration

Carbon quantum dots (CQDs; luminescent carbon nanoparticles, size < 10 nm) have attracted much attention with respect to their eco-friendliness and multi-functionality. The solvent-dependent photoluminescence of CQDs has been well investigated to optimize the synthesis process and homogeneous dispersion. Although some alkan-1-ol solvents, such as ethanol, have been well utilized empirically as good solvents when synthesizing highly photoluminescent CQDs, the role of alkan-1-ol solvents, particularly long-chain alkan-1-ols (e.g., 1-nonanol, 1-decanol), has not yet been clarified. Herein, we demonstrate a method for the synthesis of strongly yellow emitting CQDs using solvothermal treatment and elucidate the role of alkan-1-ol solvents in the photoluminescence of CQDs. These CQDs have been characterized using theoretical calculations, ex situ morphological observations using transmission electron microscopy (TEM) and dynamic light scattering (DLS), and 500 MHz ¹H nuclear magnetic resonance (NMR) and ¹³C NMR spectroscopy. A comparative study of alkan-1-ol solvents suggests a mechanism for the agglomeration and aggregation of carbon precursors, intermediates, and CQDs, which is expected to lead to further synthesis studies on highly luminescent CQDs.

Carbon quantum dots (CQDs; luminescent carbon nanoparticles, size < 10 nm) have attracted much attention with respect to their eco-friendliness and multi-functionality.

Selective Sugar Recognition by Anthracene-Type Boronic Acid Fluorophore/Cyclodextrin Supramolecular Complex Under Physiological pH Condition

We synthesized novel PET (photoinduced electron transfer)-type fluorescence glucose probe 1 [(4-(anthracen-2-yl-carbamoyl)-3-fluorophenyl)boronic acid], which has a phenylboronic acid (PBA) moiety as the recognition site and anthracene as the fluorescent part. Although the PBA derivatives dissociate and bind with sugar in the basic condition, our new fluorescent probe can recognize sugars in the physiological pH by introducing an electron-withdrawing fluorine group into the PBA moiety. As a result, the pK _a value of this fluorescent probe was lowered and the probe was able to recognize sugars at the physiological pH of 7.4. The sensor was found to produce two types of fluorescent signals, monomer fluorescence and dimer fluorescence, by forming a supramolecular 2:1 complex of 1 with glucose inside a γ-cyclodextrin (γ-CyD) cavity. Selective ratiometric sensing of glucose by the 1/γ-CyD complex was achieved in water at physiological pH.

Rapid and Selective Discrimination of Gram-Positive and Gram-Negative Bacteria by Boronic Acid-Modified Poly(amidoamine) Dendrimer

There is an urgent need to develop a rapid and selective method for the detection of bacteria because delayed diagnosis and the overuse of antibiotics have triggered drug resistance in bacteria. To this end, we prepared boronic acid-modified poly(amidoamine) generation 4 (B-PAMAM(G4)) dendrimer as cross-linking molecules that form aggregates with bacteria. Within 5 min of adding B-PAMAM(G4) dendrimer solution to a bacterial suspension, large aggregates were observed. Interestingly, the aggregate formation with various bacteria was pH-dependent. In basic pH, both Gram-positive and Gram-negative bacteria formed aggregates, but in neutral pH, only Gram-positive bacteria formed aggregates. We revealed that this bacteria-selective aggregation involved the bacterial surface recognition of the phenylboronic acid moiety of B-PAMAM(G4) dendrimer. In addition, we demonstrated that the spherical structure of B-PAMAM(G4) was one of the important factors for the formation of large aggregates. The aggregation was also observed in the presence of ≤10 mM fructose. B-PAMAM(G4) dendrimer is expected to be a powerful tool for the rapid and selective discrimination between Gram-positive and Gram-negative bacteria.

Metal and Phosphate Ion Recognition Using Dipicolylamine-modified Fluorescent Silica Nanoparticles

Dipicolylamine-modified fluorescent silica nanoparticles were prepared by introducing dipicolylamine to the surface of silica nanoparticles possessing terminal amines. We examined the selectivities of dipicolylamine-hydroxycoumarin carbonate (dpa-HCC) and dpa-HCC/fluorescent silica nanoparticles (FSiNP) for metal ions and phosphate anions. The dipicolylamine-modified silica nanoparticles responded to PPi, Tri and Pb²⁺, indicating novel selectivity derived from the assembly effect of dpa-HCC on the silica nanoparticle surface. Surface-modified fluorescent silica nanoparticles are expected to be used as a sensor for environmental and biological applications.

Structural effects of ditopic azoprobe–cyclodextrin complexes on the selectivity of guest-induced supramolecular chirality

Guest-induced supramolecular chirality: the guest ion selectivity was dramatically altered by a slight change in the spacer length of (15C5-Azo-n-dpa)₂–γ-CyD complexes in water.

Development of Supramolecular Saccharide Sensors Based on Cyclodextrin Complexes and Self-assembling Systems

Cyclodextrins (CDs) are water-soluble host compounds having nano-size hydrophobic cavities that enable them to incorporate organic molecules in water. Optically inert CDs can be efficiently combined with various types of chromoionophores and fluoroionophores. In this study, using diverse combinations of phenylboronic acid fluorescent sensors and azoprobes with CDs, the unique saccharide recognition functions of CD, chemically modified CD, and CD gel complexes based on their synergistic function are clarified, thereby confirming their use as supramolecular saccharide sensors. To realize novel supramolecular chirality, the twisted structure of two ditopic azoprobes inside the γ-CD chiral cavity is controlled by multi-point recognition of guest ions in water. As different types of supramolecular saccharide sensors, phenylboronic acid-based self-assembling systems are also reviewed.

Design and Function of Supramolecular Recognition Systems Based on Guest-Targeting Probe-Modified Cyclodextrin Receptors for ATP

In this study, we have developed a rational design strategy to obtain highly selective supramolecular recognition systems of cyclodextrins (CyDs) on the basis of the lock and key principle. We designed and synthesized dipicolylamine (dpa)-modified γ-CyD-Cu²⁺ complexes possessing an azobenzene unit (Cu·1-γ-CyD) and examined how they recognized phosphoric acid derivatives in water. The results revealed that Cu·1-γ-CyD recognized ATP with high selectivity over other phosphoric acid derivatives. The significant blue shift in the UV-vis spectra and ¹H NMR analysis suggested that the selective ATP recognition was based on the multipoint interactions between the adenine moiety of ATP and both the CyD cavity and the azobenzene unit in addition to the recognition of phosphoric moieties by the Cu-dpa complex site. Our unique receptor made it capable of distinguishing ATP from AMP and ADP, revealing the discrimination of even a length of one phosphoric group. This study demonstrates that, compared to conventional recognition systems of CyDs, this multipoint recognition system confers a higher degree of selectivity for certain organic molecules, such as ATP, over their similar derivatives.

Saccharide Recognition Based on Self-Assembly of Amphiphilic Phenylboronic Acid Azoprobes

We designed amphiphilic phenylboronic acid azoprobes (B-Azo-Cn) and evaluated their saccharide recognition function in relation to the micelle formation changes of the self-assembled B-Azo-Cn. First, we evaluated B-Azo-C8 in a 1% methanol-99% water solution under basic conditions. The wavelength of maximum absorption in the ultraviolet-visible (UV-vis) spectra of B-Azo-C8 was shifted, and the solution showed a color change with the addition of saccharides. The morphology of B-Azo-C8 was evaluated using dynamic light scattering (DLS) measurements and transmission electron microscopy (TEM) observations. B-Azo-C8 formed aggregates in the absence of saccharides and in the presence of glucose. In the presence of fructose, micelle-formed B-Azo-C8 was dispersed, indicating that B-Azo-C8 changed its dispersion state by recognizing fructose. The effect of alkyl chain length on the saccharide recognition ability was examined as well. B-Azo-C4 and B-Azo-C12 did not recognize saccharides in a 1% methanol-99% water solution under basic conditions, indicating that an appropriate alkyl chain length was required for recognizing saccharides. The control of the hydrophilic-lipophilic balance (HLB) was a key factor for saccharide recognition.

Solvent effect on the fluorescence response of hydroxycoumarin bearing a dipicolylamine binding site to metal ions

The fluorescence behavior of a probe dpa-HC, which has a coumarin derivative that acts as a fluorophore and a dipicolylamine (DPA) unit that functions as a metal ion-recognition site, was investigated with various metal ions in aqueous and several non-aqueous solvents. In aqueous solution, the fluorescence of dpa-HC was enhanced by Zn(2+) and Cd(2+), but was quenched by other metal ions. On the other hand, in an acetonitrile solution, only Mg(2+) enhanced the fluorescence, and the addition of a small amount of water quenched this fluorescence. This dramatic selectivity change is explained by stabilization of a metal-dpa-HC complex due to acetonitrile coordination and ON-OFF switching of the intramolecular photoinduced electron transfer (PET) from the nitrogen lone pair of DPA to the coumarin derivative.

Guest-induced supramolecular chirality in a ditopic azoprobe-cyclodextrin complex in water

We report a novel supramolecular chirality induced by the twisted structural change of two ditopic azoprobes (15C5-Azo-dpa) inside the chiral cavity of γ-cyclodextrin (γ-CyD) due to multi-point recognition of guest ions by 15C5-Azo-dpa molecules in water.

Effects of cyclodextrins on intramolecular photoinduced electron transfer in a boronic acid fluorophore

An inclusion complex consisting of a boronic acid fluorophore (C1-APB) and β-cyclodextrin (β-CyD) acts as a supramolecular sugar sensor whose response mechanism is based on photoinduced electron transfer (PET) from the excited pyrene to the boronic acid. We have investigated the PET process in C1-APB/CyD complexes by using time-resolved photoluminescence (TRPL) measurements at room temperature, and have succeeded in estimating the electron-transfer time to be about 1 ns. We have also studied the effects of CyDs on the PET process by comparing two kinds of CyDs (α-CyD, β-CyD) under different water-dimethyisulfoxide (DMSO) concentration conditions. We found that the CyDs interacting with the boronic acid moiety completely inhibits PET quenching and increases the monomer fluorescence intensity.

The design of phenylboronic acid azoprobe–polyamidoamine dendrimer complexes as supramolecular sensors for saccharide recognition in water

Phenylboronic acid azoprobe–PAMAM dendrimer complex responded to saccharides and exhibited selective aggregation particularly with glucose at neutral pH.

Glucose recognition by a supramolecular complex of boronic acid fluorophore with boronic acid-modified cyclodextrin in water

A boronic acid fluorophore (C1-APB)/boronic acid-modified γ-cyclodextrin (3-PB-γ-CyD) complex as a supramolecular sensor has been designed for selective glucose recognition in water. The fluorescent response behavior of the C1-APB/3-PB-γ-CyD complex under various pH conditions revealed that a C1-APB/3-PB-γ-CyD complex solution containing glucose showed a large increase in the fluorescence intensity under alkaline pH conditions. In contrast, only small increases in the fluorescence intensity were noted for fructose and without sugar solutions. The observed response selectivity for the C1-APB/3-PB-γ-CyD complex was on the order of glucose >> galactose, mannose > fructose. The evidence on a large value of the inclusion constant (K(L·CyD) = 6.5 × 10(3) M(-1)), a marked broadening of the (1)H NMR spectra, and an enhancement of induced circular dichloism (ICD) intensity for the C1-APB/3-PB-γ-CyD complex by glucose binding supported the multi-point interaction of the C1-APB/3-PB-γ-CyD complex with glucose. These results demonstrated that the C1-APB/3-PB-γ-CyD complex functioned as an efficient supramolecular sensor for selective glucose recognition in water.

Selective glucose recognition by boronic acid azoprobe/gamma-cyclodextrin complexes in water

The phenylboronic acid azoprobe (BA-Azo)/gamma-cyclodextrin (gamma-CD) complex exhibits a selective response for D-glucose by forming a supramolecular 2:1 inclusion complex of the azoprobes with D-glucose inside the gamma-CD cavity.

Fluorescence response mechanism of D-glucose selectivity for supramolecular probes composed of phenylboronic-acid-modified beta-cyclodextrin and styrylpyridinium dyes

Supramolecular complex formation of phenylboronic-acid-modified beta-cyclodextrin (1) with 1-methyl-4-(4-dimethylaminostyryl)pyridinium (C1SP) in aqueous solutions containing saccharides was fully clarified to gain an insight into the observed D-glucose (D-glc) selectivity of a supramolecular fluorescent probe composed of 1 and the 1-heptyl analogue of C1SP (Chem. Commun., 2006, 4319). At pH 9.6, where 1 was in its anionic form, both the stability and the fluorescence of the 1/C1SP complex were reduced by the formation of boronate esters of 1 with saccharides. Among the saccharides, D-glc had the smallest effect on destabilization of the 1/C1SP complex, almost completely retaining the fluorescence of the 1/C1SP complex that was reduced by other saccharides by approximately 2/3. Under neutral conditions, D-glc enhanced the fluorescence of the 1/C1SP complex by increasing the fraction of anionic 1 while minimally decreasing the stability and fluorescence of the 1/C1SP complex. Although other saccharides also increased the fraction of the anionic 1, their relatively large effects on the destabilization and reduction of fluorescence of the 1/C1SP complex limited the enhancement of the fluorescence of the 1-C1SP system under neutral conditions.

Test Strips for Lead(II) Based on a Unique Color Change of PVC-film Containing O-Donor Macrocycles and an Anionic Dye

Glassy test strips partially coated with PVC-film including O-donor macrocyclic receptors (L), tetrabromophenolphthalein ethyl ester (TBPE(-)), and a plasticizer sensed Pb(2+) in aqueous solutions by a unique color change. Yellow films successively changed color to green, dark-blue and purple with increases of the Pb(2+) concentration. In contrast with the ordinary "optode", a characteristic absorption band at 525 nm was newly appeared independently of the protonation and deprotonation of HTBPE (yellow to blue). The unique color change occurred only when asymmetric receptors with respect to the basal plane were coupled with Pb(2+). This optical-structural correlation is likely to be induced by the H aggregate of two sets of TBPE(-) in the 1:2 ion-pair, [Pb-L](2+).(TBPE(-))(2). The color change, based on metachromasy, was exclusive for Pb(2+) among common metal cations (Ca(2+), Al(3+), Cd(2+), Zn(2+), Fe(3+), Co(2+), Hg(2+)) and anions (Cl(-), SO(4)(2-), PO(4)(3-), S(2)O(3)(2-)).

Pseudorotaxane-type fluorescent receptor exhibiting unique response to saccharides

Pseudorotaxane formed by reacting beta-cyclodextrin bearing a phenylboronic acid residue with 1-heptyl-4-(4'-dimethylaminostyryl)pyridinium functioned as a novel fluorescent saccharide receptor having unique responses.