Saccharide Detection Based on the Amplified Fluorescence Quenching of a Water-Soluble Poly(phenylene ethynylene) by a Boronic Acid Functionalized Benzyl Viologen Derivative

A Perspective Looking Backward and Forward on the 25th Anniversary of Conjugated Polyelectrolytes

Conjugated polyelectrolytes are π-conjugated polymers that contain ionic charged groups such as sulfonate (R-SO3-), carboxylate (R-COO-), or ammonium (R-NR3+) combined with a π-conjugated backbone. This perspective provides a summary review of the key developments in the field, starting from the first reports of their synthesis and properties to application-focused developments. The applications include optical sensors for molecular and biomolecular targets, organic electronic applications, and specific biological applications including cellular imaging and photodynamic therapy. This perspective concludes with a discussion of where the field of conjugated polyelectrolytes is expected to lead in the coming years.

Multimode Time-Resolved Superresolution Microscopy Revealing Chain Packing and Anisotropic Single Carrier Transport in Conjugated Polymer Nanowires

Here, we developed a novel, multimode superresolution method to perform full-scale structural mapping and measure the energy landscape for single carrier transport along conjugated polymer nanowires. Through quenching of the local emission, the motion of a single photogenerated hole was tracked using blinking-assisted localization microscopy. Then, utilizing binding and unbinding dynamics of quenchers onto the nanowires, local emission spectra were collected sequentially and assembled to create a superresolution map of emission sites throughout the structure. The hole polaron trajectories were overlaid with the superresolution maps to correlate structures with charge transport properties. Using this method, we compared the efficiency of inter- and intrachain hole transport inside the nanowires and for the first time directly measured the depth of carrier traps originated from torsional disorder and chemical defects.

Molecular Boronic Acid-Based Saccharide Sensors

Boronic acids can reversibly bind diols, a molecular feature that is ubiquitous within saccharides, leading to their use in the design and implementation of sensors for numerous saccharide species. There is a growing understanding of the importance of saccharides in many biological processes and systems; while saccharide or carbohydrate sensing in medicine is most often associated with detection of glucose in diabetes patients, saccharides have proven to be relevant in a range of disease states. Herein the relevance of carbohydrate sensing for biomedical applications is explored, and this review seeks to outline how the complexity of saccharides presents a challenge for the development of selective sensors and describes efforts that have been made to understand the underpinning fluorescence and binding mechanisms of these systems, before outlining examples of how researchers have used this knowledge to develop ever more selective receptors.

Capacitive Saccharide Sensor Based on Immobilized Phenylboronic Acid with Diol Specificity

A capacitive sensor for saccharide detection is described in this study. The detection is based on selective interaction between diols and aminophenylboronic acid (APBA) immobilized on a gold electrode. Glucose, fructose, and dextran (MW: 40 kDa) were tested with the system over wide concentration ranges (1.0 x 10-8 M - 1.0 x 10-3 M for glucose, 1.0 x 10-8 M - 1.0 x 10-2 M for fructose and 1.0 x 10-10 M - 1.0 x 10-5 M for dextran). The limits of detection (LODs) were 0.8 nM for glucose, 0.6 nM for fructose, and 13 pM for dextran. These data were comparable to the others reported previously. In order to demonstrate glycoprotein detection with the same sensor, human immunoglobulin G (IgG) as well as horseradish peroxidase were used as model analytes. The sensor responded to IgG in the concentration range of 1.0 x 10-13 M - 1.0 x 10-7 M with a LOD value of 16 fM. The performance of the assay of peroxidase was compared to a spectrophotometric assay by determining the enzymatic activity of a captured analyte. The results showed that the method might be useful for label-free, fast, and sensitive detection of saccharides as well as glycoproteins over a wide concentration range.

Tuning the Potential of Electron Extraction from Microbes with Ferrocene-Containing Conjugated Oligoelectrolytes

Synthetic systems that facilitate electron transport across cellular membranes are of interest in bio-electrochemical technologies such as bio-electrosynthesis, waste water remediation, and microbial fuel cells. The design of second generation redox-active conjugated oligoelectrolytes (COEs) bearing terminal cationic groups and a π-delocalized core capped by two ferrocene units is reported. The two COEs, DVFBO and F₄ -DVFBO, have similar membrane affinity, but fluorination of the core results in a higher oxidation potential (422 ± 5 mV compared to 365 ± 4 mV vs Ag/AgCl for the neutral precursors in chloroform). Concentration-dependent aggregation is suggested by zeta potential measurements and confirmed by cryogenic transmission electron microscopy. When the working electrode potential (E_CA ) is poised below the oxidation potential of the COEs (E_CA = 200 mV) in three-electrode electrochemical cells containing Shewanella oneidensis MR-1, addition of DVFBO and F₄ -DVFBO produces negligible biocurrent enhancement over controls. At E_CA = 365 mV, DVFBO increases steady-state biocurrent by 67 ± 12% relative to controls, while the increase with F₄ -DVFBO is 30 ± 5%. Cyclic voltammetry supports that DVFBO increases catalytic biocurrent and that F₄ -DVFBO has less impact, consistent with their oxidation potentials. Overall, electron transfer from microbial species is modulated via tailoring of the COE redox properties.

Meta-linked cationic poly(pyridinylene vinylene) conjugated polyelectrolytes: solution photophysics and fluorescent sensing of metal ions

Two novel poly(pyridinylene vinylene) (PPV)-type conjugated polyelectrolytes 2,6-PPYPV-(2+) and 3,5-PPYPV-(2+) were synthesized via Heck coupling reaction and characterized by 1H nuclear magnetic resonance (NMR), ultraviolet-visible (UV-Vis) and photoluminescence (PL) spectroscopy. The two polymers consisting of meta-position substituted pyridinylenes as the metal recognition unit and a water-soluble divinyl-benzene derivative in the backbones. 2,6-PPYPV-(2+) and 3,5-PPYPV-(2+) both exhibit strongest fluorescence in methanol and weakest fluorescence in water among common polar solvents. In respect of ion detection, 3,5-PPYPV-(2+) has an excellent identifiability for Pd2+ in methanol with the K_SV value of 1.1×105m−1 while 2,6-PPYPV-(2+) has a good selectivity for Cu2+ and Hg2+ in aqueous solution. And all the Stern-Volmer plots of 2,6-PPYPV-(2+) and 3,5-PPYPV-(2+) in fluorescence quenching for metal ions have favorable tendencies. All the results suggest that 2,6-PPYPV-(2+) and 3,5-PPYPV-(2+) are promising materials in the applications of high performance chemosensors for some specific metal ions.

End Functionalized Nonionic Water-Dispersible Conjugated Polymers

2,7-Dibromofluorene monomers carrying two or four oligo(ethylene glycol) (OEG) side chains are synthesized. Heck coupling between the monomers and 1,4-divinylbenzene followed by end capping with [4-(4-bromophenoxy)butyl]carbamic acid tert-butyl ester leads to two nonionic water-dispersible poly(fluorene-alt-1,4-divinylenephenylene)s end-functionalized with amine groups after hydrolysis. In water, the polymer with a lower OEG density (P1) has poor water dispersibility with a quantum yield of 0.24, while the polymer with a higher OEG density (P2) possesses excellent water-dispersibility with a high quantum yield of 0.45. Both polymers show fluorescence enhancement and blue-shifted absorption and emission maxima in the presence of surfactant sodium dodecyl sulfate and dodecyltrimethylammonium bromide. The polymers are also resistant to ionic strength with minimal nonspecific interactions to bovine serum albumin. When biotin is incorporated into the end of the polymer backbones through N-hydroxysuccinimide/amine coupling reaction, the biotinylated polymers interact specifically with streptavidin on solid surface.

Oligo(p-phenylene ethynylene) with Cyanoacrylate Terminal Groups and Graphene Composite as Fluorescent Chemical Sensor for Cysteine

A chemical sensor for cysteine (Cys) was fabricated based on a fluorescent oligo(p-phenylene ethynylene)s (OPEs) and OPE-graphene oxide (GO) composite. OPE with cyanoacrylate terminal groups were synthesized by a Pd-catalyzed Sonogashira coupling reaction and Knoevenagel condensation for use as a chemical sensor for Cys. The optical properties and Cys sensing capability of the cyanoacrylate modified OPE and OPE-GO composite were investigated. In addition of Cys, the fluorescence of OPE was blue-shifted and decreased (fluorescence turn-off), while the fluorescence of the OPE-GO composite was enhanced (fluorescence turn-on). Thus, OPE with cyanoacrylate terminal groups and OPE-GO composite acts a highly sensitive fluorescent chemical sensor for Cys.

Boronlectin/Polyelectrolyte Ensembles as Artificial Tongue: Design, Construction, and Application for Discriminative Sensing of Complex Glycoconjugates from Panax ginseng

Ginsenoside is a large family of triterpenoid saponins from Panax ginseng, which possesses various important biological functions. Due to the very similar structures of these complex glycoconjugates, it is crucial to develop a powerful analytic method to identify ginsenosides qualitatively or quantitatively. We herein report an eight-channel fluorescent sensor array as artificial tongue to achieve the discriminative sensing of ginsenosides. The fluorescent cross-responsive array was constructed by four boronlectins bearing flexible boronic acid moieties (FBAs) with multiple reactive sites and two linear poly(phenylene-ethynylene) (PPEs). An "on-off-on" response pattern was afforded on the basis of superquenching of fluorescent indicator PPEs and an analyte-induced allosteric indicator displacement (AID) process. Most importantly, it was found that the canonical distribution of ginsenoside data points analyzed by linear discriminant analysis (LDA) was highly correlated with the inherent molecular structures of the analytes, and the absence of overlaps among the five point groups reflected the effectiveness of the sensor array in the discrimination process. Almost all of the unknown ginsenoside samples at different concentrations were correctly identified on the basis of the established mathematical model. Our current work provided a general and constructive method to improve the quality assessment and control of ginseng and its extracts, which are useful and helpful for further discriminating other complex glycoconjugate families.

Synthesis and fluorescence study of conjugated polymers based on 2,4,6-triphenylpyridine moieties

Three novel 2,4,6-triphenylpyridine-based conjugated polymers showed strong fluorescence emission with large Stokes' shifts, tunable band gaps and high quantum yields.

Nanoparticle-enhanced fluorescence emission for non-separation assays of carbohydrates using a boronic acid–alizarin complex

Addition of crosslinked polymer nanoparticles into a solution of a 3-nitrophenylboronic acid–alizarin complex leads to significant enhancement of fluorescence emission.

A facile channel for D-glucose detection in aqueous solution

Three facile ensembles for sensing D-glucose are designed and constructed. The ensembles are comprised of fluorescent dye (NAHBDS) and boronic acid substituted viologens (BBVs) quenchers/receptors. The sensing processes of three ensembles (NAHBDS/o-BBV, NAHBDS/m-BBV and NAHBDS/p-BBV) to D-glucose were determined by fluorescence spectra at pH 7.4 buffer solution. The results show that NAHBDS/o-BBV and NAHBDS/m-BBV ensembles embody higher sensitivity for D-glucose with reversible "on-off" fluorescence response. More importantly, the recovery of relative intensity has good linear relation to low concentration of D-glucose. The action between the ensemble with D-glucose is dynamically reversible equilibrium process. The research results provide a new mode to design highly selective probe.

Positions of the glycans in molluscan hemocyanin, determined by fluorescence spectroscopy

Molluscan hemocyanins are glycoproteins with different quaternary and carbohydrate structures. It was suggested that the carbohydrate chains of some Hcs are involved in their antiviral and antitumor effect, as well in the organization of the quaternary structure of the molecules. Using a well-known complex for saccharide sensing, positions and access to the carbohydrate chains in the native hemocyanins from Rapana venosa (RvH) and Helix lucorum (HlH) and also their structural subunits (RvH1, RvH2 and βcHlH) and functional units (FUs) were analysed by fluorescence spectroscopy and circular dichroism. Almost no effect was observed in the fluorescence emission after titration of the complex with native RvH and HlH due to lack of free hydroxyl groups which are buried in the didecameric form of the molecules. Titration with the structural subunits βcHlH and RvH2, increasing of the emission indicates the presence of free hydroxyl groups compared to the native molecules. Complex titration with the structural subunit βc-HlH of H. lucorum Hcs leads to a 2.5 fold increase in fluorescence intensity. However, the highest emission was measured after titration of the complex with FU βcHlH-g. The result was explained by the structural model of βcHlH-g showing the putative position of the glycans on the surface of the molecule. The results of the fluorescent measurements are in good correlation with those of the circular dichroism data, applied to analyse the effect of titration on the secondary structure of the native molecules and functional units. The results also support our previously made suggestion that the N-linked oligosaccharide trees are involved in the quaternary organization of molluscan Hcs.

Method for analysis of different oligosacchiride structures

In this study, an improved, rapid, high yield synthesis of N,N'-4,4'-bis(benzyl-2-boronic acid)-bipyridinium dibromide (o-BBV) is described. The obtained o-BVV is applied in a two-component saccharide sensing system (complex) where it serves as a fluorescence quencher and a saccharide receptor. This system was applied to different natural oligosaccharides isolated from molluscan Rapana venosa (RvH1-a) and arthropodan Carcinus aestuarii (CaeH) hemocyanins (Hcs) and cyclodextrins (CDs). The carbohydrate contents of both Hcs were calculated in our previous work to be 1,6 % and 7 % for CaeH and RvH1-a, respectively. We propose that the difference in fluorescence increase of the native CaeH and RvH1-a when titrating them with the complex is due to the fact that the carbohydrate content of CaeH is lower and the carbohydrate chains are buried in between the structural subunits of the native molecule, while the glycans of the functional unit RvH1-a are exposed on the surface of the molecule leading to a 4-fold fluorescence's intensity change.

Highly selective ensembles for D-fructose based on fluorescent method in aqueous solution

Three highly sensitive and selective switches for monosaccharides were composed by anionic polyelectrolyte PPPSO(3)Na and cationic viologen quencheres BBVs. The sensing processes of three ensembles (PPPSO(3)Na/o-BBV, PPPSO(3)Na/m-BBV and PPPSO(3)Na/p-BBV) to common seven monosaccharides have been determined by fluorescence spectra at pH 7.4 buffer solution. The results show that the three sensing ensembles all embody higher selectivity and sensitivity for d-fructose with reversible "on-off-on" fluorescence response. The research results can provide a new mode for developing highly selective probes.

A unique, two-component sensing system for fluorescence detection of glucose and other carbohydrates

In our glucose-sensing system, a boronic acid-modified viologen molecule quenches the fluorescence of a separate dye molecule. When glucose or other monosaccharides are added and bind to the boronic acid, the quenching ability of the viologen is diminished and fluorescence increases. Thus, changes in the fluorescence of the dye can be correlated with changing glucose concentration. Quenching and sugar-sensing results are explained by an electrostatic interaction between dye and quencher. This modular system can be configured in a nearly unlimited number of ways through substitution and multiplexing of the two fundamental quencher and dye components. Significantly, fluorescent quantum dots (QDs) can also be used as the reporter component. The system can also be immobilized in a hydrogel polymer to provide real-time, reversible sugar sensing.

Conjugated polymer-based real-time fluorescence caspase assays

We have developed conjugated polyelectrolyte-based fluorescence turn-on assays for caspase 3 and 8. These assays are composed of a cationic polyphenylene ethynylene polymer PPE4+ and p-nitroaniline modified caspase peptide substrate. The fluorescence of the assay is initially turned-off because of the efficient quenching of the polymer by p-nitroaniline moiety on anionic peptide substrates. A turn-on effect is observed due to the cleavage of the peptide by the enzyme and formation of the neutral p-nitroaniline unit which has no quenching on the polymer. We validated this assay design and obtained kinetic parameters of caspase 3 and caspase 8. These assays demonstrated good sensitivity as in pmol/L (0.1 units/mL) for caspase 3 and nmol/L (0.2 units/mL) for caspase 8. This method also showed high specificity by using caspase 3 assay as a model system and the results demonstrated that other proteases including caspase 8, papain, pepsin, and trypsin did not show observable fluorescence turn-on effect. The dose-response curve of a caspase inhibitor Z-VAD-FMK was evaluated by caspase 3 assay, by which the IC(50) value was determined to be 0.73 μM and was in a good agreement with the literature reported value at 0.62 μM. This design could be applied into the in vitro screening of small molecular inhibitors for drug discovery.

An anion-conjugated polyelectrolyte designed for the selective and sensitive detection of silver(I)

A novel conjugated polyelectrolyte P1, having a meta-substituted monopyridyl in the backbone, is designed and synthesized for Ag(+) detection in aqueous solution. As a chemosensor, P1 shows high sensitivity, low detection limit, and excellent selectivity for Ag(+) over other metal ions. The sensing mechanism is based on the specific interaction between Ag(+) and the pyridyl group of P1. The aggregated state of the polymer in water can amplify its quenching efficiency.

An ultra sensitive saccharides detection assay using carboxyl functionalized chitosan containing : nanoparticlesprobe

A novel saccharides detection assay based on covalent immobilization of amino phenyl boronic acid (APBA) in thin films of carboxyl functionalized chitosan (HOOC-chitosan) containing <5 nm Gd2O3 : Eu3+ nanoparticles at a platinum disc electrode was developed. The resulting HOOC-chitosan/Gd2O3 : Eu3+ nanocomposite film exhibited excellent electrochemical response to changes in the pKa values of boronate esters yielded from different vicinal diols of sugars. The covalent interaction of APBA onto the HOOC-chitosan/Gd2O3 : Eu3+ Pt-disc electrode was characterized with FT-IR, SEM, contact angle and cyclic voltammetry, whereas Gd2O3 : Eu3+ nanoparticles and HOOC-chitosan/Gd2O3 : Eu3+ nanocomposite was identified using XRD, EDX and TEM. A wide linear response was measured to boronate esters ranging from 25 nM to 13.5 μM (r2 = 0.963) with good reproducibility. The excellent electrochemical activity of the assay might be attributed to the synergistic effects of the balanced de-/protonated HOOC-chitosan, APBA and Gd2O3 : Eu3+ nanoparticles. With APBA as a model, the HOOC-chitosan/Gd2O3 : Eu3+ nanocomposite-modified Pt-electrode was constructed through a simple drop coating method. The resulting assay exhibited a good potentiometric response to different saccharides including glucose, and could be a promising application for the precise electrochemical detection of vicinal diols of specific sugars for clinical diagnostics, medicine validation, bioscience research and food analysis.