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Conjugated polyelectrolyte dendrimers (CPDs) are monodisperse macromolecules that feature a fully π-conjugated dendrimer core surrounded on the periphery by ionic solubilizing groups. CPDs are soluble in water and polar organic solvents, and they exhibit photophysics characteristic of the π-conjugated chromophores comprising the dendrimer core. Here we describe the synthesis and photophysical characterization of series of three generations of CPDs based on a phenylene ethynylene repeat unit structure that is surrounded by an array of anionic sodium carboxylate groups. Molecular dynamics simulations indicate that the first-generation CPD is flat while the second- and third-generation CPDs adopt oblate structures. Photophysical studies, including absorption, fluorescence spectroscopy, and lifetimes, show that the ester protected precursor dendrimers exhibit highly efficient blue fluorescence in THF solution emanating from the phenylene ethynylene chromophore that is in the dendrimer core. By contrast, the water-soluble CPDs have much lower fluorescence quantum yields and the absorption and fluorescence spectra exhibit features of strong chromophore-chromophore interactions. The results are interpreted as suggesting that the CPDs exist as dimer or multimer aggregates, even in very dilute solution. Fluorescence quenching of the anionic CPDs with the dication electron acceptor N,N'-dimethylviologen (MV(2+)) is very efficient, with Stern-Volmer quenching constants (K(SV)) increasing with generation number. The third-generation CPD exhibits highly efficient amplified quenching, with K(SV) ∼ 5 × 10(6) M(-1).

We report the "light switch" effect of [Ru(bpy)2(dppz)](2+) (where bpy = 2,2'-bipyridine and dppz = dipyrido[3,2-a:2',3'-c] phenazine, Ru-dppz) in the presence of anionic conjugated polyelectrolyte dendrimers (CPDs). The metal-to-ligand charge-transfer luminescence from Ru-dppz is efficient in the presence of CPD because the complex is shielded from water by binding to the hydrophobic dendrimer core.

We report on the comparison of the electronic and photophysical properties of a series of related donor-acceptor-donor oligomers incorporating the previously known 2H-benzo[d][1,2,3]triazole (BTz) moiety as the acceptor and the recently reported BTzTD acceptor, a hybrid of BTz and 2,1,3-benzothiadiazole (BTD). Although often implied in the polymer literature that BTz has good acceptor character, we show that this moiety is best described as a weak acceptor. We present electrochemical, computational, and photophysical evidence supporting our assertion that BTzTD is a strong electron acceptor while maintaining the alkylation ability of the BTz moiety. Our results show that the identity of the central atom (N or S) in the benzo-fused heterocyclic ring plays an important role in both the electron-accepting and the electron-donating ability of acceptor moieties with sulfur imparting a greater electron-accepting ability and nitrogen affording greater electron-donating character. We report on the X-ray crystal structure of a BTzTD trimer, which exhibits greater local aromatic character in the region of the triazole ring and contains an electron-deficient sulfur that imparts strong electron-accepting ability. Additionally, we examine the transient absorption spectra of BTzTD and BTz oligomers and report that the BTz core promotes efficient intersystem crossing to the triplet state, while the presence of the thiadiazole moiety in BTzTD leads to a negligible triplet yield. Additionally, while BTz does not function as a good acceptor, oligomers containing this moiety do function as excellent sensitizers for the generation of singlet oxygen.

This protocol describes a homogeneous, convenient and sensitive DNA methylation detection method, using an optically amplifying cationic conjugated polymer (CCP, poly((1,4-phenylene)-2,7-[9,9-bis(6'-N,N,N-trimethyl ammonium)-hexyl fluorene] dibromide)). Genomic DNA from cancer cells is pretreated with a methylation-sensitive restriction endonuclease, followed by PCR amplification in the presence of fluorescein-labeled dNTP and Taq polymerase. The PCR only occurs for methylated DNA. DNA methylation of the gene sequence of interest is detected as a result of the fluorescence resonance energy transfer (FRET) between CCP and fluorescein that is incorporated into DNA. The methylated statuses of the p16, HPP1 and GALR2 promoters of five cancer cell lines (HT29, HepG2, A498, HL60 and M17) were assayed to provide an association study between the cancers and susceptibility genes, which shows great potential for early cancer diagnosis. This protocol simplifies previously available procedures by avoiding the need for primer labeling, isolation or purification steps, and sophisticated instruments. The assay takes about 20 h to obtain the methylated statuses of cancer cells.

Enzyme assays are receiving more and more research and application interest because of the rapidly increasing demands of clinical diagnosis, environmental analysis, drug discovery, and molecular biology. Water-soluble light-harvesting conjugated polymers (CPs) coordinate the action of a large number of absorbing units to afford an amplified fluorescence signal, which makes them useful as optical platforms in highly sensitive chemical and biological sensors. This Feature Article highlights recent developments of water-soluble CPs for fluorescent assays of enzymes. Different signal transduction mechanisms, such as electron transfer, fluorescence resonance energy transfer (FRET), and aggregation or conformation changes of CPs, are employed in these assays according to the dissimilar nature of enzymes. Potential challenges and future research directions in these approaches based on CPs are also discussed.

Nuclear magnetic resonance imaging (MRI) has become a powerful technique in clinical diagnostics. In this work, a new MRI contrast agent by covalently linking Gd(III) chelates to the side chain of conjugated polymer (PF–Gd) is synthesized by Suzuki cross-coupling reaction. The PF–Gd exhibits a higher relaxivity and a pronounced enhancement in contrast than that of (NMG) 2 –Gd–DTPA widely used for clinical diagnosis. This work should be feasible to potentially lead to a new class of imaging contrast agents.

A simple, rapid, continuous and homogeneous fluorescent assay for β-galactosidase was developed, combining an enzyme-coupled reaction and signal amplification property of conjugated polyelectrolytes (CPs). The procedure is based on a sequence of two coupled biocatalytic steps in which the β-galactosidase hydrolyzes its substrate to a phenol derivative followed by conversion to quinone (secondary product) with fluorescence quenching ability by the tyrosinase. The fluorescence of PFP-SO 3 - is efficiently quenched by the quinone via an electron transfer process. The limit of detection (LOD) of this assay is less than 0.0005 U mL -1 , which is better than that of electrochemical method and is comparable to that of most sensitive chemiluminescent techniques. In principle, this sensor mechanism will extend the application window of CPs for wide-spectrum enzyme detections. This "mix-and-detect" approach could be expanded to a high-throughput manner.

An energy-transfer cascade is generated from a cationic conjugated polymer (PFP) and negatively charged, Y-shaped DNA labeled with three dyes at its termini (fluorescein (Fl), Tex Red, and Cy5). Multistep fluorescence resonance energy transfer regulates the fluorescence intensities of PFP and the dyes. Different types of logic gates can be operated by observing the emission wavelengths of different dyes with multiplex nucleases as inputs.

A new platform has been developed for DNA lesion detection using a cationic conjugated polymer (CCP). DNA that contains two adjacent thymine bases is irradiated with ultraviolet light to allow for the formation of cyclobutane pyrimidine dimers and pyrimidine-pyrimidone dimers. The DNA lesions block the primer extension, and the base labeled with fluorescein cannot be incorporated into the DNA strand. Addition of the CCP leads to inefficient fluorescence resonance energy transfer (FRET) from CCP to fluorescein. For the case without DNA lesions, successful primer extension allows for efficient FRET between them. In view of the FRET signal changes, the DNA lesions can be detected. This new protocol offers a convenient detection for DNA lesions in aqueous solution without any isolation and washing steps.

A new water-soluble oligo(phenylenevinylene) containing a tyrosine unit (OPV-Tyr) was synthesized as a fluorescent probe to optically detect tyrosinase activity. Upon the addition of tyrosinase, the tyrosine moiety was oxidized to quinone, which quenched the fluorescence of OPV-Tyr via intramolecular electron transfer from the phenylenevinylene unit to the quinone site. OPV-Tyr was elaborated to detect tyrosinase activity both in aqueous buffer solution and in agarose gel.

A new fluorescence method has been developed for DNA detection at room temperature in a sensitive, selective, economical, and real-time manner that interfaces the superiority of a molecular beacon in mismatch discrimination with the light-harvesting property of water-soluble conjugated polyelectrolytes. The probe solution contains a cationic conjugated polyelectrolyte (PFP-NMe3+), a molecular beacon with a five base pairs double-stranded stem labeled at the 5'-terminus with fluorescein (DNA P-Fl), and ethidium bromide (EB, a specific intercalator of dsDNA). The electrostatic interactions between DNA P-Fl and PFP-NMe3+ keep them in close proximity, facilitating the fluorescence resonance energy transfer (FRET) from PFP-NMe3+ to fluorescein. Upon adding a complementary strand to the probe solution, the conformation of DNA P-Fl transits into dsDNA followed by the intercalation of EB into the grooves. Two-step FRET, from PFP-NMe3+ to DNA P-Fl (FRET-1), followed by FRET from DNA P-Fl to EB (FRET-2) takes place. In view of the observed fluorescein or EB emission changes, DNA can be detected in aqueous solution. Because the base mismatch in target DNA inhibits the transition of DNA P-Fl from the stem-loop to duplex structure, single nucleotide mismatch can be clearly detected.

Water-soluble conjugated polymers have attracted much attention in the biosensor community because of recent exciting experimental results. The transfer of excitation energy along the whole backbone of the conjugated polymer to the reporter results in an amplified fluorescence signal, which makes them useful as optical platforms in highly sensitive chemical and biological sensors. In this Research News Article, we provide a brief review of recent developments in this field, focusing especially on the application of water-soluble optical conjugated polymers in biomacromolecule (DNA and protein) detection.

An assay method has been developed to detect oxalic acid from other structurally similar dicarboxylic acids using a cationic polythiophene derivative (PFT). Upon adding oxalic acid, the PFT can be cross-linked through the bifunctional diacids of oxalic acid to form tense interpolymer π-stacking aggregation, which results in the fluorescence self-quenching of the PFT. Upon adding other dicarboxylic acids with a longer tether, loose interpolymer π-stacking aggregation forms and the PFT fluorescence is less quenched. The fluorescence analysis was carried out in solution for the aggregates prior to precipitation. As a result of the short tether of oxalic acid, the unique fluorescent response from the PFT/oxalic acid assembly can be used to detect oxalic acid.

Fluorescence correlation spectroscopy (FCS) is applied to demonstrate avidin-induced cross-linking in a system consisting of a helical anionic conjugated polyelectrolyte (P1) and a biotin-tetramethylrhodamine (TMR) conjugate (2). In a previous study, we used fluorescence spectroscopy to demonstrate that 2 binds to P1 via intercalation of the TMR chromophore into the P1 helix. Addition of avidin to the P1/2 complex induces little change in the fluorescence of the system; however, FCS reveals a remarkable increase in the diffusion time of the P1/2 complex in the presence of avidin. This change is attributed to supramolecular polymer aggregates produced by cross-link formation between the biotin unit of intercalated 2 and avidin. Atomic force microscopy imaging provides evidence supporting the existence of these aggregates. The highly sensitive FCS method is used to develop a novel sensor for the biotin-avidin interaction, with a detection limit of

Summary: A reversible and highly selective assay methodhas been developed to detect mercury( II )(Hg 2þ ) ionsusing aconjugated polymer (CP). The transduction mechanism isbased on Hg 2þ -mediated interpolymer p-stacking aggrega-tion, which results in the fluorescence self-quenching of theCP. CPs that contain thymine moieties, poly[3-(N-thymin-1-ylacetyl)ethylamine-thiophene] (PTT), have been synthe-sizedandcharacterized.IntheabsenceofHg 2þ ions,thePTTchains remain separated from each other and the CP exhibitsstrong fluorescence emission. Upon adding Hg 2þ ions, theformationofinterpolymerp-stackingaggregationinducedbyspecific thymine–Hg–thymine interactions results in thefluorescencequenchingofPTT.Distinguishingaspectsofthisassayinclude thesignalamplificationofCPsandthespecificbinding of Hg 2þ ions to thymine-thymine (T–T) base pairs.The binding of Hg 2þ ions causes the separate conductingpolymer chains to aggregate with subsequent fluorescenceself-quenching. Macromol. Rapid Commun. 2006, 27, 389–392 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

a-D-Glucose is the chief energy source in organisms and plays an important role in the metabolism of most mammalian tissues and cells. Glycolysis, the major route of glucose metabolism, is initiated by the catalytic phosphorylation to D-glucose-6-phosphate (G-6-P) by hexokinase (HK) in the presence of adenosine triphosphate (ATP) and Mg. Phosphorylation of glucose is important for both energy metabolism and biosynthesis in the cell. It accelerates glucose metabolism and prevents glucose from effusing from cells. Furthermore, the rates of ATP-dependent glucose phosphorylation can be used to identify new members of the hexokinases. Glucose phosphorylation is currently measured by coupling to the reduction of NADP+ using glucose-6-phosphate dehydrogenase as an auxiliary enzyme. The generated NADPH is then monitored by absorption spectroscopy and/or HPLC techniques. These methods share the drawback of including a coupled additional enzymatic reaction and are timeconsuming discontinuous assays. As a way to circumvent these limitations, a synthetic multifunctional pore, octakis(Gla-LeuArg-Trp-His-Val-NH2)-p-octiphenylene (SMP), has been used for developing fluorometric assays. Such methods require intricate design and the synthesis of SMPs. These considerations provide motivation for developing simpler and more convenient methods to detect and monitor glucose phosphorylation. Conjugated polymers have been used as the optical platform for chemical and biological detection. Water-soluble polythiophene derivatives display changes in their conformation and aggregation states upon forming interpolyelectrolyte complexes with DNA. Recently, a cationic conjugated polythiophene was used to successfully respond to the presence of ATP, via a combination of electrostatic and hydrophobic interactions, and to detect DNA cleavage. As demonstrated here, it is possible to use these properties for designing a simple, homogeneous, continuous and real-time protocol to assay glucose phosphorylation. The overall strategy is illustrated in Scheme 1. Cationic poly[3-(3′-N,N,N-trimethylamino-1′-propyloxy)-4-methyl-2,5thiophene hydrochloride] (CPT) was chosen as the optical probe. Upon mixing, electrostatic interactions bring together

In this letter, we report on the direct measurement of the intercalation interactions between acridine and double-stranded DNA (dsDNA) using single molecule force spectroscopy. The interaction between acridine and dsDNA is broken by force of 36 pN at a loading rate of 5.0 nN/s. The most probable rupture force between acridine and dsDNA is dependent on the loading rate, indicating that the binding of acridine and dsDNA is a dynamic process. The combination of SMFS experimental data with the theoretical model clearly suggests the presence of two energy barriers along with an unbinding trajectory of acridine-dsDNA.

Disulfide has been the only widely used functionality to serve as a reduction responsive trigger in drug delivery. We introduce thioester as a novel thiol responsive chemistry for drug delivery, whose reactivity can be conveniently modulated by choosing the appropriate steric environment around the thioester. Compared with disulfides, thioesters are facile to synthesize and have an order of magnitude broader kinetic tunability. A novel traceless reversible protein PEGylation reagent is developed based on thioester chemistry.

Enzyme assays are receiving more and more research and application interest because of the rapidly increasing demands of clinical diagnosis, environmental analysis, drug discovery, and molecular biology. Water-soluble light-harvesting conjugated polymers (CPs) coordinate the action of a large number of absorbing units to afford an amplified fluorescence signal, which makes them useful as optical platforms in highly sensitive chemical and biological sensors. This Feature Article highlights recent developments of water-soluble CPs for fluorescent assays of enzymes. Different signal transduction mechanisms, such as electron transfer, fluorescence resonance energy transfer (FRET), and aggregation or conformation changes of CPs, are employed in these assays according to the dissimilar nature of enzymes. Potential challenges and future research directions in these approaches based on CPs are also discussed.

Simple, rapid, and sensitive technologies to detect nucleic acid modifications have important applications in genetic analysis, clinical diagnosis, and molecular biology. Because genetic modifications such as single nucleotide polymorphisms (SNP), DNA methylation, and other lesions can serve as hallmarks of human disease, interest in such methods has increased in recent years. This Account describes a new strategy for the optical detection of these DNA targets using cationic conjugated polymers (CCPs). Because of their unique signal amplification properties, researchers have extensively investigated conjugated polymers as optical transducers in highly sensitive biosensors. Recently, we have shown that cationic polyfluorene can detect SNPs within the DNA of clinical samples. When we incorporated deoxyguanosine triphosphate (dGTP-Fl) into the DNA chain at an SNP site where the target/probe pair is complementary, we observed higher fluorescence resonance energy transfer (FRET) efficiency between cationic polyfluorene and fluorescein label on the dGTP. By monitoring the change in emission intensity of cationic polyfluorene or fluorescein, we identified the homozygous or heterozygous SNP. The high sensitivity of this assay results from the 10-fold enhancement of fluorescein emission intensity by the FRET from polyfluorene. This method can detect allele frequencies (the proportion of all copies of a gene that is made up of a particular gene variant) as low as 2%. Using this novel method, we clearly discriminated among the SNP genotypes of 76 individuals of Chinese ancestry. Improving on this initial system, we designed a method for multicolor and one-tube SNP genotyping assays based on cationic polyfluorene using fluorescein-labeled deoxyuridine triphosphate (dUTP-Fl) and Cy3-labeled deoxycytidine triphosphate (dCTP-Cy3) in extension reactions. We also developed a one-step method for direct detection of SNP genotypes from genomic DNA by combining allele-specific PCR with CCPs. In 2008, we developed a new method for DNA methylation detection based on single base extension reaction and CCPs. Treatment of DNA with bisulfite followed by PCR amplification converts unmethylated DNA into a C/T polymorphism, which allows us to characterize the methylation status of the target DNA. Furthermore, we used CCPs to detect DNA lesions caused by ultraviolet light irradiation for the first time. By monitoring the color change of cationic polythiophene before and after DNA cleavage, we also detected oxidative damage to DNA by hydroxyl radical. These CCP-based new assays avoid primer labeling, cumbersome workups, and sophisticated instruments, leading to simpler procedures and improved sensitivity. We expect that these features could lead to major advances in human disease diagnostics and genomic study in the near future.

A new methodology has been developed for DNA detection that interfaces optical amplification properties of cationic conjugated polyelectrolytes with highly selective target-induced DNA strand displacement. The probe solution contains a cationic conjugated polyelectrolyte (CCP-1), partly hybridized duplex DNA labeled with a fluorescein at the 5'-terminus, and endonuclease Hae III. Excitation of the CCP-1 leads to efficient energy transfer from CCP-1 to fluorescein. In the presence of a complementary DNA strand to one strand of the probe duplex, a hairpin DNA with the recognition site of endonuclease Hae at the double-stranded stem is released following its cleavage by Hae III to generate short DNA fragment carrying fluorescein. The relatively weak electrostatic interactions between the DNA fragment and CCP-1 lead fluorescein far away from CCP-1 and inefficient energy transfer between them is present. Thus, the DNA can be detected by fluorescence spectra in view of the observed CCP-1 or fluorescein emission changes in aqueous solutions. To avoid utilizing unstable Hae III endonuclease, a new system based on RNA-cleaving DNAzyme was further developed. The protocol offers a convenient approach for homogeneous, selective, and sensitive DNA assay in aqueous solution without using any denaturation steps. Compared with previously reported DNA sensors based on conjugated polyelectrolytes, our new method is highly sequence specific and a single-nucleotide mismatch can be clearly detected in target DNA.

Classical strategies for gene microarrays require labeling of probes or target nucleic acids with signaling molecules, a process that is expensive, time consuming and not always reliable. Bazan and colleagues showed that a nucleic acid-binding cationic conjugated polyelectrolyte can be used in label-free DNA microarrays based on surfaces modified with neutral peptide nucleic acid (PNA) probes. This technique provides a simple and sensitive method for DNA detection without the need for covalent labeling of target DNA.

G-quartet DNA converts to duplex form in the presence of its complementary strand. This conformational change can be detected in real time by a homogeneous assay method based on the signal amplification of conjugated polyelectrolytes and the specific interaction of intercalating dyes with double-stranded DNA (dsDNA). The probe solution contains a cationic, conjugated polymer (CCP), G-quadruplex labeled with a fluorescein at the 5'-terminus (G-quadruplex-Fl), and ethidium bromide (EB). The addition of a complementary target results in the transition from G-quadruplex to duplex (dsDNA-Fl) and EB intercalation within the duplex structure. Excitation of the CCP leads to energy transfer from CCP to dsDNA-Fl (FRET-1) and then energy transfer from dsDNA-Fl to EB (FRET-2). Increasing the number of mismatched bases discourages dsDNA formation, which is detected in the assay.

A new method has been developed for continuous and real-time assays for enzymes using complexes of conjugated polymers/enzymatic substrates by electrostatic interactions, where the products are the substrate fragments and the free chains of conjugated polymers. The charged substrates and enzymes as a proof of concept are anionic adenosine triphosphate (ATP) and alkaline phosphatase (ALP), as well as cationic polyarginine peptide (Arg6) and trypsin. The charged water-soluble polyfluorenes containing 2,1,3-benzothiadiazole (BT) units demonstrate intramolecular energy transfer from the fluorene units to the BT sites when oppositely charged substrates (ATP or Arg6) are added, following by a shift in emission color from blue to green. When the substrates are cleaved into fragments, the relatively weak electrostatic interactions of substrate fragments with conjugated polymers keep their main chains separated and fluorescence resonance energy transfer (FRET) from the fluorene units to the BT sites is inefficient, and the conjugated polymers emit blue fluorescence. By triggering the shift in emission color and the change of emission intensity of conjugated polymers, it is possible to assay the enzyme activity and screen drugs based on the inhibition of enzymes. This protocol combines the fluorescence resonance energy transfer and light harvesting properties of conjugated polymers. The assay technique has two significant characteristics offered by the conjugated polymers: (i) the use of water-soluble conjugated polymers imparts the sensor high sensitivity; (ii) this method does not require fluorescent labels on the substrates, which should significantly reduce the cost.

A new water-soluble cationic polyfluorene with boronate-protected fluorescein (peroxyfluor-1) covalently linking to the polymer backbone (PF-FB) was synthesized as a fluorescence probe to optically detect hydrogen peroxide (H2O2) and glucose in serum. The peroxyfluor-1 exists as a lactone form which is colorless and non-fluorescent. Without addition of H2O2, the fluorescence resonance energy transfer (FRET) from fluorene units (donor) to peroxyfluor-1 (acceptor) is absent and only blue donor emission is observed upon excitation of the fluorene units. In the presence of H2O2, the peroxyfluor-1 can specifically react with H2O2 to deprotect the boronate protecting groups and to generate green fluorescent fluorescein. The absorption of fluorescein overlaps the emission of polyfluorene, which encourages efficient FRET from the fluorene units to the fluorescein. By triggering the shift in emission color and the ratio change of blue to green emission intensities, it is possible to assay H2O2 and its concentration change in buffer solution and in serum. The PF-FB probe can detect H2O2 in the range from 4.4 to 530 µM. Since glucose oxidases (GOx) can specifically catalyze the oxidation of β-D-(+)-glucose to generate H2O2, glucose detection is also realized with the PF-FB probe as the signal transducer. Thus, an optical assay for hydrogen peroxide (H2O2) and glucose in serum was created which combines a fluorescent ratiometric technique based on FRET with the light-harvesting properties of conjugated polymers.