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Research: Conjugated Polyelectrolytes


 

 

Amplified Quenching

The fluorescence of conjugated polyelectrolytes (CPEs) is quenched with very high efficiency by small molecule quenchers with opposite charges. This effect has been referred to as amplified quenching or superquenching. We study the profound correlation between the fluorescence quenching efficiency, CPE chain aggregation (induced by the addition of either Ca2+ or H2O to methanol solution), and quencher molecular size. We propose that the superlinear Stern-Volmer quenching behavior typically observed in CPE-quencher systems arises due to quencher-induced aggregation of the CPE chains.


 

Capsules

We are interested in the development of conjugated polyelectrolyte capsules from inorganic colloids towards use in sensing applications from these new materials. Synthesis of highly monodisperse manganese carbonate particles has been used to serve as template particles for layer-by-layer (LBL) fabrication of polyelectrolyte capsules. Using the LBL technique conjugated polyelectrolytes are deposited on the inorganic colloid to form bilayers of oppositely charged polyelectrolytes, followed by dissolution of the inorganic core leaving behind the PE capsule. This has been demonstrated using poly(diallyldimethylammonium chloride) (PDDA) and poly(phenylene ethynylene) (PPE) with a carboxlate side group (PPE-CO2-) to form PE capsules. Capsules have been synthesized in a well dispersed fashion ranging in size from 4-6 μm. Characterization under fluorescent and confocal microscopes displays the stability of the capsules structure, along with the fluorescent emission from the conjugated polyelectrolyte capsule. Future work with these materials will be in the development of sensing applications utilizing the fluorescent properties of the CPE.


 

 

CPE Surface Grafted Colloids

This is an on-going project in collaboration with Dr. Whitten's group at the University of New Mexico (UNM). In this project, PPE-type conjugated polymers are grafted on a surface of silica particles. Two grafting methods are available: "graft-from" and "graft-on." In the "graft-from" approach, the surface of the particles are modified with grafting points (iodobenzene) and polymer brushes are grown from there via in situ polymerization. In "graft-on" approach, the surface of the particles are modified with amino-functional groups.

Polymers are synthesized separately and the terminal of the polymer chains are functionalized with carboxyl functional groups. Then, polymer brushes are attached onto the surface through amide bonds via DCC/DMAP chemistry. These colloids with surface grafted polyelectrolytes retain the fluorescence ability of the polymer as shown in the confocal microscopy image. The presence of the polymers can also be observed in the SEM image.

Quenching experiments using a cyanine dye shows very efficient amplified quenching effect that is characteristic of conjugated polyelectrolyte. Possible applications of such particles include quantum dots and fluorescent sensors. At UNM, these particle are currently investigated for possible applications in light-activated bactericides.

Reference:
Ogawa, K; Chemburu, S.; Lopez, G.P.; Whitten, D.G.; Schanze, K.S. "Conjugated Polyelectrolyte-Grafted Silica Microspheres." Langmuir 2007, 23, 4541-4548.


 

Dye-Sensitized Solar Cells

A series of poly(arylene ethynylene) conjugated polyelectrolytes (CPEs) have been prepared, featuring a backbone consisting of a carboxylated bis(alkoxy)phenylene-1,4-ethynylene unit alternating with a second arylene ethynylene moiety (1,4-phenyl (PPE), 2,5-thienyl (TH), 2,5-(3,4-ethylenedioxy)thienyl (EDOT), and 1,4-benzo[2,1,3]-thiodiazole (BTD)). Using these CPEs as sensitizer, nanocrystalline TiO2 solar cells are fabricated and characterized. The IPCE and power conversion efficiency indicate close correlation with the band gap energy. In order to gain some insights into the kinetics for the involved charge transfer process, ns transient absorption spectroscopy are applied.

References:

  1. Zhao, X.; Pinto, M.R.; Hardison, L.M.; Mwaura, J.; Mueller, J.; Jiang, H.; Witker, D.; Kleiman, V.D.; Reynolds, J.R.; Schanze, K.S. "Variable Band Gap Poly(arylene ethynylene) Conjugated Polyelectrolytes." Macromolecules 2006, 39, 6355-6366.
  2. Mwaura, J.K.; Zhao, X.; Jiang, H.; Schanze, K.S.; Reynolds, J.R. "Spectral Broadening in Nanocrystalline TiO2 Solar Cells Based on Poly(p-phenylene ethynylene) and Polythiophene Sensitizers." Chem. Mater. 2006, 18, 6109-6111.
  3. Taranekar, P.; Qiao, Q.; Jiang, H.; Ghiviriga, I.; Schanze, K.S.; Reynolds, J.R. "Hyperbranched Conjugated Polyelectrolyte Bilayers for Solar-Cell Applications." J. Am. Chem. Soc. 2007, 129, 8958-8959.

 

Sensors

Based on the amplified quenching or superquenching effect, conjugated polyelectrolytes can be used to sense a variety of analytes, such as metal ion, inorganic ions, organic ions, and bio-molecules (DNA, proteins, etc.). For example, the fluorescence of PPE-CO2- can be efficiently quenched by Cu2+. This system was used to sense pyrophosphate, an important anion involved in many biological processes. Because of the stronger binding of Cu2+ to PPi, the PPE-CO2- fluorescence is recovered as shown in the following Figure. We have developed a real-time assay for alkaline phosphatase (ALP) based on this system, and efforts to use such a system to sense other important biological analytes, such as ATP, are under the way.

A fluorescence turn-off assay for phospholipase C (PLC) is developed based on the reversible interaction between the natural substrate, phosphatidylcholine and a fluorescent, CPE. The fluorescence intensity of the CPE in water is increased substantially by the addition of the phospholipid due to the formation of a CPE-lipid complex. Incubation of the CPE-lipid complex with the enzyme PLC causes the fluorescence intensity to decrease (turn-off sensor); the response arises due to PLC-catalyzed hydrolysis of the phosphatidylcholine, which effectively disrupts the CPE-lipid complex. The PLC assay operates with phospholipid substrate concentrations in the mM range and the analytical detection limit for PLC is < 1 nM. It demonstrated a good selectivity by testing the response of PLC and other enzymes. The optimized assay provides convenient, rapid and real-time sensor for PLC activity. The real-time fluorescence intensity from the CPE can be converted to substrate concentration by using an ex-situ calibration curve, allowing PLC catalyzed reaction rates and kinetic parameters to be determined.

As another example, meta-inked PPE-type CPEs can self-assemble into a helical conformation in aqueous solution. This allows mPPESO3 to be used as a platform for sensing biological targets with high sensitivity. A biotin-linked Re complex can quench the fluorescence of the polymer. In the presence of avidin, it is expected that due to stronger interaction between biotin and avidin, the intercalation of Re complex into polymer would be disrupted, and the polymer fluorescence would recover.

References:

  1. Zhao, X.; Liu, Y.; Schanze, K. S. "A Conjugated Polyelectrolyte-based Fluorescence Sensor for Pyrophosphate" Chem. Commun., 2007, 2914-2916.
  2. Liu, Y.; Ogawa, K.; Schanze, K. S. "Conjugated Polyelectrolyte Based Real-Time Fluorescence Assay for Phospholipase C" Anal. Chem., 2008, 80, 150-158.