We study the properties and applications of a class of conjugated polymers called conjugated polyelectrolytes (CPEs). These polymers are substituted with ionic (charged) groups such as sulfonate (-SO3–), carboxylate (-COO–) and ammonium (-NR3+), and the strong interactions between the charged groups and water molecules makes the polymers water soluble. CPEs have a number of remarkable properties, which present interesting systems for basic science investigations, as well as for developing materials and devices for useful applications such as fluorescence sensors, biosensors and dye-sensitized solar cells. Group members apply synthetic organic and polymer chemistry to prepare new and known conjugated polyelectrolytes and then we utilize them in a studies including fluorescence spectroscopy, applications in biology, and fabrication and testing of solar cells.
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.
For an example, two of the poly(phenyleneethynylene)-based CCPEs bearing cationic imidazolium groups synthesized in our lab showed excellent biocidal activity. These two polymers having same backbone differ only by the frequency of the imidazolium groups present on the chain, which in turn changes their solubility property in water. Addition of an anionic electron-acceptor like anthraquinone disulphate, even in a relatively low concentration to these CCPEs solution quenches their fluorescence via the amplified quenching effect. These CCPEs also display aggregation-induced fluorescence quenching property when exposed to pyrophosphates. Our investigation on their biocidal activity against Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus bacteria in the dark and under blue-light-illumination revealed that these CCPEs are effective biocides, exhibiting greater than 3 log kill with 30-60 min of light exposure at concentrations of <10 mg mL-1.
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.
1. S. Li; T. Chen; Y. Wang; L. Liu; F. Lv; Z. Li; Y. Huang, K. S. Schanze; S. Wang “Conjugated Polymer with Intrinsic Alkyne Units for Synergistically Enhanced Raman Imaging in Living Cells”, Angew. Chem. Int. Ed. 2017, 56, 13455–13458. DOI: 10.1002/anie.201707042
2. Y. Huang; H. C. Pappas; L. Zhang; S. Wang; R. Cai; W. Tan; S. Wang; D. G. Whitten; K. S. Schanze “Selective Imaging and Inactivation of Bacteria over Mammalian Cells by Imidazolium-Substituted Polythiophene”, Chem. Mater. 2017, 29, 6389–6395. DOI: 10.1021/acs.chemmater.7b01796
3. S. Wang; C. J. Zeman IV; J. Jiang; Z. Pan; K. S. Schanze “Intercalation of Alkynylplatinum(II) Terpyridine Complexes into a Helical Poly(phenylene ethynylene) Sulfonate: Application to Protein Sensing”, ACS Appl. Mater. Interfaces 2017, 9, 33461–33469. DOI: 10.1021/acsami.7b01587