Conjugated polyelectrolyte based real-time fluorescence assay for adenylate kinase

Facile Synthesis of Fluorescent Conjugated Polyelectrolytes Using Polydentate Sulfonate as Highly Selective and Sensitive Copper(II) Sensors

Fluorescent conjugated polyelectrolytes represent an exciting area of research into new chemosensors. By virtue of their rapid electron and energy transfer paths, these highly correlated, one-dimensional systems have been depicted as "molecular wires" and show "million-fold" sensitivity compared to monomolecular sensor analogs. In this paper, a novel polyelectrolyte sensor, the ttp-PPESO₃, has been designed by incorporating terpyridine and sulfonate functional groups into the polyelectrolyte. This specifically tailored sensor has displayed remarkable quenching response toward copper(II) with a detection limit of 14.7 nM (0.93 ppb). It is capable of selectively screening copper without interference from 12 common cations. Molecular modeling suggests that binding occurs through a coordination interaction of the terpyridine and sulfonate. The additional multidentate nature from the sulfonate offers extraordinary chelating ability to the analyte. We anticipate that this unique binding mode will provide insight for the design of future more sensitive and selective systems.

Detection of bisphenol A in food packaging based on fluorescent conjugated polymer PPESO3 and enzyme system

Bisphenol A (BPA) is a kind of carcinogen, which can interfere with the body's endocrine system. In this paper, a new kind of fluorescent sensor for BPA detection was established based on the fluorescent conjugated polymer PPESO3. The oxidative product of BPA is able to quench PPESO3 in the presence of HRP and H2O2, and the quenched PL intensity of PPESO3 was proportionally to the concentration of BPA in the range of 1-100 μmol/L with a detection limit of 4 × 10(-7) mol/L. The proposed method has been applied to detect BPA in eight food packaging samples with satisfactory results. The proposed method has the potential for the assay of BPA in food or food packaging samples.

Fluorescence array-based sensing of metal ions using conjugated polyelectrolytes

Array-based sensing offers several advantages for detecting a series of analytes with common structures or properties. In this study, four anionic conjugated polyelectrolytes (CPEs) with a common poly(p-pheynylene ethynylene) (PPE) backbone and varying pendant ionic side chains were designed. The conjugation length, repeat unit pattern, and ionic side chain composition were the main factors affecting the fluorescence patterns of CPE polymers in response to the addition of different metal ions. Eight metal ions, including Pb(2+), Hg(2+), Fe(3+), Cr(3+), Cu(2+), Mn(2+), Ni(2+), and Co(2+), categorized as water contaminants by the Environmental Protection Agency, were selected as analytes in this study. Fluorescence intensity response patterns of the four-PPE sensor array toward each of the metal ions were recorded, analyzed, and transformed into canonical scores using linear discrimination analysis (LDA), which permitted clear differentiation between metal ions using both two-dimensional and three-dimensional graphs. In particular, the array could readily differentiate between eight toxic metal ions in separate aqueous solutions at 100 nM. Our four-PPE sensor array also provides a practical application to quantify Pb(2+) and Hg(2+) concentrations in blind samples within a specific concentration range.

Near-infrared fluorescence probe for the determination of acid phosphatase and imaging of prostate cancer cells

In this paper, we developed a near-infrared mercaptopropionic acid (MPA)-capped CuInS2 quantum dot (QD) fluorescence probe for the detection of acid phosphatases (ACP), which is an important biomarker and indicator of prostate cancer. The fluorescence of CuInS2 QDs could be quenched by Cu(2+), and then the addition of adenosine-5'-triphosphate (ATP) could effectively turn on the quenched fluorescence due to the strong interaction between Cu(2+) and ATP. The ACP could catalyze the hydrolysis of ATP, which would disassemble the complex of Cu(2+)-ATP. Therefore, the recovered fluorescence could be quenched again by the addition of ACP. In our method, the limit of detection (LOD) is considerably low for ACP detection in solution. Using the CuInS2 QDs fluorescence probe, we successfully performed in vitro imaging of human prostate cancer cells.

Highly sensitive and selective detection of the pyrophosphate anion biomarker under physiological conditions

Self-assembled monolayers on gold of a multidentate adsorbate bearing a bis(carbazolyl)urea unit are prepared and used as a surface resonance plasmon sensor for the detection of hydrogen pyrophosphate anions under physiological conditions.

Photochemistry of "end-only" oligo-p-phenylene ethynylenes: complexation with sodium dodecyl sulfate reduces solvent accessibility

Cationic oligo-p-phenylene ethynylenes are very effective light-activated biocides and biosensors but degrade upon exposure to light. In this study, we explore the photochemistry of a class of "end-only" compounds from this series, which have cationic moieties on the ends of the backbone. Product characterization by mass spectrometry reveals that the photoreactivity of these molecules is higher than that of a previously studied oligomer and that the primary products of photolysis result from the addition of water or oxygen across the triple bond. In addition, a product suggesting the addition of peroxide or other reactive oxygen species across the triple bond was observed. To explore avenues by which the photodegradation of these compounds can be mitigated, the effects of complexation with sodium dodecyl sulfate micelles on their photochemistry was explored. Classical molecular dynamics simulations revealed that compounds that were protected from photolysis by SDS buried their phenylene ethynylene backbones into the interior of the micelle, protecting it from contact with water. This work has revealed a molecular basis for the protection of a novel class of light-activated biocides from irradiation that is consistent with the proposed photochemistry of these compounds. This information can be useful for developing photodegradation-resistant biocidal materials and applications for current compounds and leads to new molecular design.

A fluorescence turn on trypsin assay based on aqueous polyfluorene

A new method based on the electrostatic interaction of a novel anionic water soluble polymer P1 with a positively charged polypeptide Arg₆ was developed for a continuous and real time turn on assay for the enzymatic activity of trypsin under alkaline conditions with a limit of detection of 0.17 nM. This method was also able to screen the inhibitors of trypsin. P1 fluorescence intensity was significantly decreased by the positively charged Arg₆ due to the electrostatic interaction, whereas the enzymatic action recovered P1 fluorescence due to the fragmentation of Arg₆ into small positively charged fragments and these were unable to quench the P1 fluorescence. Therefore, by triggering the fluorescence intensity change, it was possible to assay the enzymatic activity. Use of water soluble conjugated polymer P1 and no labeling on the substrate enhances the utility of this method significantly.

A real-time fluorescence turn-on assay for trypsin based on a conjugated polyelectrolyte

We report a continuous and sensitive fluorescence turn-on assay for trypsin by using an anionic conjugated polyelectrolyte (PPE-CO2H) and a cationic peptide substrate labelled with p-nitroaniline (R_xG-pNA). Applications of the assay in trypsin activity study and high-throughput screening of protease inhibitors were demonstrated.

Photochemistry of a Model Cationic p-Phenylene Ethynylene in Water

One important aspect of p-phenylene-ethynylenes that has not yet been explored is the possible photochemical reactions that can take place in different chemical environments. This is especially important when considering the possible breakdown of these compounds in applications that involve exposure to light, air, and water. In this Letter, a study of the photochemical reaction processes of a cationic oligomer based on the p-phenylene-ethynylene repeat unit is performed in aqueous solution in both the presence and absence of oxygen. Clearly different reaction pathways were observed in the presence and absence of oxygen in aqueous solution. The results of this study revealed the photoaddition of water across the triple bond of the ethynyl group in the absence of oxygen, the addition of singlet-oxygen across the triple-bond in the presence of oxygen, and the cleavage of the alkoxy side chains leaving phenols in both cases.

Synthesis, self-assembly, and photophysical properties of cationic oligo(p-phenyleneethynylene)s

Three series of cationic oligo p-phenyleneethynylenes (OPEs) have been synthesized to study their structure-property relationships and gain insights into the transition from molecular to macromolecular properties. The absorbance maxima and molar extinction coefficients in all three sets increase with increasing number of repeat units; however, the increase in λ(max) between the oligomers having 2 and 3 repeat units is very small, and the oligomer having 3 repeat units shows virtually the same spectra as a p-phenyleneethynylene polymer having 49 repeat units. A computational study of the oligomers using density functional theory calculations indicates that while the simplest oligomers (OPE-1) are fully conjugated, the larger oligomers are nonplanar and the limiting "segment chromophore" may be confined to a near-planar segment extending over three or four phenyl rings. Several of the OPEs self-assemble on anionic "scaffolds", with pronounced changes in absorption and fluorescence. Both experimental and computational results suggest that the planarization of discrete conjugated segments along the phenylene-ethynylene backbone is predominantly responsible for the photophysical characteristics of the assemblies formed from the larger oligomers. The striking differences in fluorescence between methanol and water are attributed to reversible nucleophilic attack of structured interfacial water on the excited singlet state.

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.

Colorimetric assays for acetylcholinesterase activity and inhibitor screening based on the disassembly-assembly of a water-soluble polythiophene derivative

A complex between an anionic polythiophene derivative (PT-COO(-)) and a cationic surfactant, myristoylcholine, has been prepared and applied to be colorimetric probe for acetylcholinesterase (AChE) assays. The complex formation process, AChE activity assay and inhibitor screening has been studied by absorption spectroscopy. It was confirmed that the introduction of myristoylcholine into PT-COO(-) phosphate buffer solution resulted in the disassembly of PT-COO(-) aggregates, and further addition of AChE into the above solution led to the reassembly of PT-COO(-) due to the catalyzed hydrolysis of myristoylcholine and the collapse of the complex. The colorimetric assay for AChE can be readily realized with the concentration of AChE as low as 0.2 U/mL. The results also demonstrate that the colorimetric approach can be applied for screening inhibitors of AChE.

Solvent-assisted optical modulation of FRET-induced fluorescence for efficient conjugated polymer-based DNA detection

The solvent effects were studied in fluorescence resonance energy transfer (FRET) from a cationic polyfluorene copolymer (FHQ, FPQ) to a fluorescein (Fl)-labelled oligonucleotide (ssDNA-Fl). Upon addition of dimethyl sulfoxide (DMSO), the optical properties of polymers and the probe dye were substantially modified and the FRET-induced PL signal was enhanced 3.8-37 times, relative to that in phosphate buffer solution (PBS). The hydrophobic interaction between polymers and ssDNA-Fl is expected to decrease in the presence of DMSO, which induces the weaker polymer/ssDNA-Fl complexation with longer intermolecular donor-acceptor separation and perturbs the competition between the FRET and PL quenching processes such as photo-induced charge transfer. The gradual decrease in Fl PL quenching with increasing the DMSO content was investigated by measuring the Stern-Volmer quenching constants (3.3-4.2 × 10(6) M(-1) in PBS, 0.56-1.1 × 10(6) M(-1) in 80 vol% DMSO) and PL lifetime of the excited Fl* in polymer/ssDNA-Fl (600 ps in PBS and 2120 ps in 80 vol% DMSO for FHQ/ssDNA-Fl) in PBS/DMSO mixtures. The substantially reduced PL quenching would amplify the resulting FRET Fl signal. The signal amplification in real DNA detection was also demonstrated with fluorescein-labelled PNA (probe PNA) in the presence of a complementary target DNA and noncomplementary DNA in aqueous DMSO solutions. This approach suggests a simple way of modifying the fine-structure of polymer/ssDNA-Fl and improving the detection sensitivity in conjugated polymer-based FRET bioassays.

Universal platform for sensitive and label-free nuclease assay based on conjugated polymer and DNA/intercalating dye complex

By taking advantage of the large signal amplification through efficient fluorescence resonance energy transfer (FRET) from the conjugated polymer to the intercalating dye mediated by DNA, a new strategy for nuclease assay has been developed using conjugated polymer and DNA/intercalating dye complex. The discrimination of DNA before or after digestion by nuclease denotes the universal application of the approach, in which either dsDNA or ssDNA substrates could be used for detecting nuclease activity. This method can be extended to most nucleases by simply changing the substrate DNA. The present method is label-free, rapid, and highly sensitive, with a detection limit much better or at least comparable to previous reports. In addition, this assay is easy to implement for visual detection with the assistance of a UV transilluminator. We reason that a similar strategy can be adopted for the detection of other analytes.

Hyper-efficient quenching of a conjugated polyelectrolyte by dye-doped silica nanoparticles: better quenching in the nonaggregated state

We investigated the quenching properties of poly(phenylene ethynylene) with anionic alkoxyl sulfonate side groups, PPE-SO3, by tris(2,2-bipyridyl)dichlororuthenium(II) (Rubpy)-doped silica nanoparticles (SiNPs) in water, methanol, and the surfactant Triton X-100 aqueous solution. SiNPs demonstrated hyper-efficient quenching characterized by the Stern-Volmer constants in the range of 10(9)-10(10) M(-1), which is about 100-10,000 times more efficient compared to the Rubpy dye. More importantly, quenching by SiNPs was found to be more efficient when the polymer existed as a nonaggregated state, such as in methanol solution and in the surfactant solution. Investigations on confocal fluorescence images and time-resolved fluorescence decays were carried out to study the quenching mechanism.

A new light-harvesting conjugated polyelectrolyte microgel for DNA and enzyme detections

A new fluorescent microgel containing the CP moiety (PFP-NIPAm) was prepared with the size within 100 nm. Covalent linking of the conjugated polyelectrolyte moiety into the microgel prevents leakage of the fluorophore while keeping its fluorescence property. The new fluorescent microgel can be used as an optical platform to detect DNA and enzyme. More importantly, because the electrostatic attraction dominates the interactions between cationic PFP-NIPAm microgel and negatively charged target, washing with high ionic strength aqueous solution can block their interactions and displace the target from the PFP-NIPAm microgel. Thus, PFP-NIPAm can be readily reusable for detection. Another unique feature is that the PFP-NIPAm microgel extends the detection media from homogeneous solution to solid phase, which shows great potential for biodetection in the real world using conjugated polymers. Furthermore, the detection can be carried out under UV light, and no expensive detection instrumentation is needed. In principle, such an assay system could be expanded to a high-throughput assay.

Conjugated polyelectrolytes: synthesis, photophysics, and applications

Organic optoelectronic polymers have evolved to the point where fine structural control of the conjugated main chain, coupled with solubilizing and property-modifying pendant substituents, provides an entirely new class of materials. Conjugated polyelectrolytes (CPEs) provide a unique set of properties, including water solubility and processability, main-chain-controlled exciton and charge transport, variable band gap light absorption and fluorescence, ionic interactions, and aggregation phenomena. These characteristics allow these materials to be considered for use in applications ranging from light-emitting diodes and electrochromic color-changing displays, to photovoltaic devices and photodetectors, along with chemical and biological sensors. This Review describes the evolution of CPE structures from simple polymers to complex materials, describes numerous photophysical aspects, including amplified quenching in macromolecules and aggregates, and illustrates how the physical and electronic properties lead to useful applications in devices.