Electrochemical and optical sugar sensors based on phenylboronic acid and its derivatives

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.

H2O2-induced decomposition of layer-by-layer films consisting of phenylboronic acid-bearing poly(allylamine) and poly(vinyl alcohol)

Layer-by-layer (LbL) films were prepared by an alternate deposition of phenylboronic acid-bearing poly(allylamine hydrochloride) (PBA-PAH) and poly(vinyl alcohol) (PVA) on the surface of a quartz slide to develop thin films that can be decomposed in response to hydrogen peroxide (H2O2). The PBA-PAH/PVA films decomposed in the presence of H2O2; the degree and kinetics of decomposition depend on the concentration of H2O2 and on the pH of the solution. For example, the film decomposition completely occurred in 30 min in 1.0 mM H2O2 solution at pH 7.4, while it took 180 min in 0.1 mM H2O2 solution. The H2O2-induced decomposition of the film can be explained in terms of the oxidative scission of the carbon-boron bond in PBA residues in the PBA-PAH chains. A potential use of the PBA-PAH/PVA films in developing H2O2-sensitive delivery systems was suggested.

Synthesis of Water-Dispersed Ferrecene/Phenylboronic Acid-Modified Bifunctional Gold Nanoparticles and the Application in Biosensing

Phenylboronic acids can form tight covalent bonds with diol-containing biomolecules. In this work, water-dispersed bifunctional gold nanoparticles (AuNPs) modified with ferrecene (Fc)-derivatized peptides and 4-mercaptophenylboronic acids (MBA) (denoted as Fc–MBA–AuNPs) were synthesized and characterized by UV/vis spectrometry and transmission electron microscopy. To demonstrate the application and the analytical merits of the nanoparticles in biosensing, glycoprotein avidin was tested as a model analyte. Specifically, avidin was captured by the biotin-covered gold electrode via the strong biotin-avidin interaction. Then, Fc–MBA–AuNPs were attached by the captured avidin through the formation of tight covalent bonds between the boronic acid moieties of Fc–MBA–AuNPs and the oligosaccharides of avidin. As a result, a detection limit of 0.2 pM was achieved. We believe that the bifunctional nanoparticles would found many applications in amplified detection of diol-containing species by rational design of the surface chemistry of electrode.

Fluorescence enhancement of cadmium selenide quantum dots assembled on silver nanoparticles and its application to glucose detection

In this work, a new assembled glucose sensor based on the Ag nanoparticle (AgNP)-enhanced fluorescence of CdSe quantum dots (QDs) was developed. The mercaptoglycerol-modified AgNPs and aminophenylboronic acid-functionalized CdSe QDs are assembled into AgNP-CdSe QD complexes through the formation of a boronate ester bond. As compared to that of bare CdSe QDs, up to a 9-fold fluorescence enhancement and a clear blue shift of the emission peak for AgNP-CdSe QD complexes were observed, which is attributed to the surface plasmon resonance of AgNPs. In addition, the as-formed complexes are gradually disassembled in the presence of glucose molecules because they can replace the AgNPs by competitive binding with boronic acid groups, resulting in the weakening of fluorescence enhancement. The decrease in fluorescence intensity presents a linear relationship with glucose concentration in the range from 2 to 52 mM with a detection limit of 1.86 mM. Such a metal-enhanced QDs fluorescence system may have promising applications in chemical and biological sensors.

Sensing of glycoprotein via a biomimetic sensor based on molecularly imprinted polymers and graphene-Au nanoparticles

A novel strategy was proposed for preparing a highly sensitive glycoprotein sensor based on molecularly imprinted polymers (MIP), which was electropolymerized with o-phenylenediamine and 3-aminophenylboronic acid monohydrate in the presence of template molecules (bovine serum albumin (BSA)). Sensitivity improved dramatically owing to the application of a graphene-Au nanoparticles hybrid as the electrode modifier, and the immobilization of a large amount of 6-ferrocenylhexanethiol, as the electroactive species, onto nanoparticles. The quantification of BSA was realized by detecting the electrochemical oxidation signal of 6-ferrocenylhexanethiol, which was bonded onto the electrode. Under optimized conditions, a good relationship was obtained between the response current and logarithm of BSA concentration in the range of 1.0 × 10(-11) to 1.0 × 10(-5) g mL(-1) with a detection limit of 7.5 × 10(-12) g mL(-1) (S/N = 3). The resulting MIP sensor displayed good selectivity, reproducibility and stability.

Colorimetric Sugar Sensing Using Boronic Acid-Substituted Azobenzenes

In association with increasing diabetes prevalence, it is desirable to develop new glucose sensing systems with low cost, ease of use, high stability and good portability. Boronic acid is one of the potential candidates for a future alternative to enzyme-based glucose sensors. Boronic acid derivatives have been widely used for the sugar recognition motif, because boronic acids bind adjacent diols to form cyclic boronate esters. In order to develop colorimetric sugar sensors, boronic acid-conjugated azobenzenes have been synthesized. There are several types of boronic acid azobenzenes, and their characteristics tend to rely on the substitute position of the boronic acid moiety. For example, o-substitution of boronic acid to the azo group gives the advantage of a significant color change upon sugar addition. Nitrogen-15 Nuclear Magnetic Resonance (NMR) studies clearly show a signaling mechanism based on the formation and cleavage of the B-N dative bond between boronic acid and azo moieties in the dye. Some boronic acid-substituted azobenzenes were attached to a polymer or utilized for supramolecular chemistry to produce glucose-selective binding, in which two boronic acid moieties cooperatively bind one glucose molecule. In addition, boronic acid-substituted azobenzenes have been applied not only for glucose monitoring, but also for the sensing of glycated hemoglobin and dopamine.

Reporter-free potentiometric sensing of boronic acids and their reactions by using quaternary ammonium salt-functionalized polymeric liquid membranes

The tremendous applications of boronic acids (BAs) in chemical sensing, medical chemistry, molecular assembly, and organic synthesis lead to an urgent demand for developing effective sensing methods for BAs. This paper reports a facile and sensitive potentiometric sensor scheme for heterogeneous detection of BAs based on their unexpected potential responses on quaternary ammonium salt-doped polymeric liquid membranes. (11)B NMR data reveal that a quaternary ammonium chloride can trigger the hydrolysis of an electrically neutral BA in an aprotic solvent. Using the quaternary ammonium salt as the receptor, the BA molecules can be extracted from the sample solution into the polymeric membrane phase and undergo the concomitant hydrolysis. Such salt-triggered hydrolysis generates H(+) ions, which can be coejected into the aqueous phase with the counterions (e.g., Cl(-)) owing to their high hydrophilicities. The perturbation on the ionic partition at the sample-membrane interface changes the phase boundary potential and thus enables the potentiometric sensing of BAs. In contrast to other transduction methods for BAs, for which labeled or separate reporters are exclusively required, the present heterogeneous sensing scheme allows the direct detection of BAs without using any reporter molecules. This technique shows superior detection limits for BAs (e.g., 1.0 × 10(-6) M for phenylboronic acid) as compared to previously reported methods based on colorimetry, fluorimetry, and mass spectrometry. The proposed sensing strategy has also been successfully applied to potentiometric indication of the BA reactions with hydrogen peroxide and saccharides, which allows indirect and sensitive detection of these important species.

Recent Progress in Ferrocene-Modified Thin Films and Nanoparticles for Biosensors

This article reviews recent progress in the development of ferrocene (Fc)-modified thin films and nanoparticles in relation to their biosensor applications. Redox-active materials in enzyme biosensors commonly use Fc derivatives, which mediate electron transfer between the electrode and enzyme active site. Either voltammetric or amperometric signals originating from redox reactions of Fc are detected or modulated by the binding of analytes on the electrode. Fc-modified thin films have been prepared by a variety of protocols, including insitu polymerization, layer-by-layer (LbL) deposition, host-guest complexation and molecular recognitions. Insitu polymerization provides a facile way to form Fc thin films, because the Fc polymers are directly deposited onto the electrode surface. LbL deposition, which can modulate the film thickness and Fc content, is suitable for preparing well-organized thin films. Other techniques, such as host-guest complexation and protein-based molecular recognition, are useful for preparing Fc thin films. Fc-modified Au nanoparticles have been widely used as redox-active materials to fabricate electrochemical biosensors. Fc derivatives are often attached to Au nanoparticles through a thiol-Au linkage. Nanoparticles consisting of inorganic porous materials, such as zeolites and iron oxide, and nanoparticle-based composite materials have also been used to prepare Fc-modified nanoparticles. To construct biosensors, Fc-modified nanoparticles are immobilized on the electrode surface together with enzymes.

Electrochemical determination of glycoalkaloids using a carbon nanotubes-phenylboronic acid modified glassy carbon electrode

A versatile strategy for electrochemical determination of glycoalkaloids (GAs) was developed by using a carbon nanotubes-phenylboronic acid (CNTs-PBA) modified glassy carbon electrode. PBA reacts with α-solanine and α-chaconine to form a cyclic ester, which could be utilized to detect GAs. This method allowed GA detection from 1 μM to 28 μM and the detection limit was 0.3 μM. Affinity interaction of GAs and immobilized PBA caused an essential change of the peak current. The CNT-PBA modified electrodes were sensitive for detection of GAs, and the peak current values were in quite good agreement with those measured by the sensors.

Phenylboronic acid-functionalized polymeric micelles with a HepG2 cell targetability

Phenylboronic acid-functionalized amphiphilic block copolymer Pluronic-PMCC-BA was synthesized via ring-opening polymerization of 5-methyl-5-benzyloxycarbonyl-1,3-dioxan-2-one (MBC) with fumaric acid as a catalyst followed by the deprotection of carboxyl groups by catalyzed hydrogenation and the condensation of 3-aminophenylboronic acid with the copolymer side groups. Pluronic-PMCC-BA can form stable micelle solution by self-assembly in water. The phenylboronic acid groups are located at the shell of micelle as proved by (1)H NMR. The diameter of drug-free micelles is approximate 60 nm. Nano-spheres with narrow size distribution could be observed in the TEM image. MTT assay results show that Pluronic-PMCC-BA exhibits slight cytotoxicity when the polymer concentration is higher than 25 μg mL(-1). The toxicities of DOX@Pluronic-PMCC and DOX@Pluronic-PMCC-BA to COS7, HeLa, and HepG2 cell lines are similar with those of free DOX. Interestingly, phenylboronic acid groups located at the surface of Pluronic-PMCC-BA micelles can recognize HepG2 cells and promote the drug uptake of the cells, which are observed by confocal laser scanning microscopy (CLSM). The results imply that Pluronic-PMCC-BA would be a promising material for targeted drug delivery to the cancer cells.

Redox Response of Reduced Graphene Oxide-Modified Glassy Carbon Electrodes to Hydrogen Peroxide and Hydrazine

The surface of a glassy carbon (GC) electrode was modified with reduced graphene oxide (rGO) to evaluate the electrochemical response of the modified GC electrodes to hydrogen peroxide (H₂O₂) and hydrazine. The electrode potential of the GC electrode was repeatedly scanned from -1.5 to 0.6 V in an aqueous dispersion of graphene oxide (GO) to deposit rGO on the surface of the GC electrode. The surface morphology of the modified GC electrode was characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM). SEM and AFM observations revealed that aggregated rGO was deposited on the GC electrode, forming a rather rough surface. The rGO-modified electrodes exhibited significantly higher responses in redox reactions of H₂O₂ as compared with the response of an unmodified GC electrode. In addition, the electrocatalytic activity of the rGO-modified electrode to hydrazine oxidation was also higher than that of the unmodified GC electrode. The response of the rGO-modified electrode was rationalized based on the higher catalytic activity of rGO to the redox reactions of H₂O₂ and hydrazine. The results suggest that rGO-modified electrodes are useful for constructing electrochemical sensors.

Exploiting the reversible covalent bonding of boronic acids: recognition, sensing, and assembly

Boronic acids can interact with Lewis bases to generate boronate anions, and they can also bind with diol units to form cyclic boronate esters. Boronic acid based receptor designs originated when Lorand and Edwards used the pH drop observed upon the addition of saccharides to boronic acids to determine their association constants. The inherent acidity of the boronic acid is enhanced when 1,2-, 1,3-, or 1,4-diols react with boronic acids to form cyclic boronic esters (5, 6, or 7 membered rings) in aqueous media, and these interactions form the cornerstone of diol-based receptors used in the construction of sensors and separation systems. In addition, the recognition of saccharides through boronic acid complex (or boronic ester) formation often relies on an interaction between a Lewis acidic boronic acid and a Lewis base (proximal tertiary amine or anion). These properties of boronic acids have led to them being exploited in sensing and separation systems for anions (Lewis bases) and saccharides (diols). The fast and stable bond formation between boronic acids and diols to form boronate esters can serve as the basis for forming reversible molecular assemblies. In spite of the stability of the boronate esters' covalent B-O bonds, their formation is reversible under certain conditions or under the action of certain external stimuli. The reversibility of boronate ester formation and Lewis acid-base interactions has also resulted in the development and use of boronic acids within multicomponent systems. The dynamic covalent functionality of boronic acids with structure-directing potential has led researchers to develop a variety of self-organizing systems including macrocycles, cages, capsules, and polymers. This Account gives an overview of research published about boronic acids over the last 5 years. We hope that this Account will inspire others to continue the work on boronic acids and reversible covalent chemistry.

Applications of reversible covalent chemistry in analytical sample preparation

Reversible covalent chemistry (RCC) adds another dimension to commonly used sample preparation techniques like solid-phase extraction (SPE), solid-phase microextraction (SPME), molecular imprinted polymers (MIPs) or immuno-affinity cleanup (IAC): chemical selectivity. By selecting analytes according to their covalent reactivity, sample complexity can be reduced significantly, resulting in enhanced analytical performance for low-abundance target analytes. This review gives a comprehensive overview of the applications of RCC in analytical sample preparation. The major reactions covered include reversible boronic ester formation, thiol-disulfide exchange and reversible hydrazone formation, targeting analyte groups like diols (sugars, glycoproteins and glycopeptides, catechols), thiols (cysteinyl-proteins and cysteinyl-peptides) and carbonyls (carbonylated proteins, mycotoxins). Their applications range from low abundance proteomics to reversible protein/peptide labelling to antibody chromatography to quantitative and qualitative food analysis. In discussing the potential of RCC, a special focus is on the conditions and restrictions of the utilized reaction chemistry.

The development of boronic acids as sensors and separation tools

Synthetic receptors for diols that incorporate boronic acid motifs have been developed as new sensors and separation tools. Utilizing the reversible interactions of diols with boronic acids to form boronic esters under new binding regimes has provided new hydrogel constructs that have found use as dye-displacement sensors and electrophoretic separation tools; similarly, molecular boronic-acid-containing chemosensors were constructed that offer applications in the sensing of diols. This review provides a somewhat-personal perspective of developments in boronic-acid-mediated sensing and separation, placed in the context of the seminal works of others in the area, as well as offering a concise summary of the contributions of the co-authors in the area.

pH gated glucose responsive biomimetic single nanochannels

The development of pH gated glucose (Glu) biosensor is of great significance to human health. Herein, we have designed a pH gated Glu responsive biomimetic nanochannel, modified with 3-aminobenzeneboronic acid. The Glu responsive property can be regulated by pH which can switch nanochannels from the "on" to "off" state.