Xiaoyong Zhao

University of Florida, Gainesville, FL, United States

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Publications (12)31.81 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Here we report spectroscopic studies on the interaction of negatively charged, amphiphilic polyphenylene ethynylene (PPE) polymers with liposomes prepared either from negative, positive or zwitterionic lipids. Emission spectra of PPEs of 7 and 49 average repeat units bearing carboxylate terminated side chains showed that the polymer embeds within positively charged lipids where it exists as free chains. No interaction was observed between PPEs and negatively charged lipids. Here the polymer remained aggregated giving rise to broad emission spectra characteristic of the aggregate species. In zwitterionic lipids, we observed that the majority of the polymer remained aggregated yet a small fraction readily embedded within the membrane. Titration experiments revealed that saturation of zwitterionic lipids with polymer typically occurred at a polymer repeat unit to lipid mole ratio close to 0.05. No further membrane embedding was observed above that point. For liposomes prepared from positively charged lipids, saturation was observed at a PPE repeat unit to lipid mole ratio of ~0.1 and liposome precipitation was observed above this point. FRET studies showed that precipitation arose as PPEs induced lipid mixing and liposome fusion. This behavior was prominent for the longer polymer and negligible for the shorter one at a repeat unit to lipid mole ratio of 0.05. We postulate that fusion is the consequence of membrane destabilization whereby the longer polymer gives rise to more extensive membrane deformation than the shorter polymer.
    Langmuir : the ACS journal of surfaces and colloids. 08/2014;
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    ABSTRACT: A new series of conjugated polyelectrolytes (CPE) consisting of an arylene–ethynylene backbone featuring phenyl (Ph), 2,1,3-benzothiadiazole (BTD), or 4,7-bis(2′-thienyl)-2,1,3-benzothiadiazole (TBT) units have been synthesized and characterized. On each polymer repeat unit the CPEs contain two branched ionic side groups each featuring a “triad” of carboxylate (R-CO2–Na+) or ammonium (R-NH3+Cl–) units, giving the polymers six ionic charges per repeat. The photophysical properties of the series of CPEs were investigated in CH3OH and H2O solution by absorption, steady-state fluorescence, and fluorescence lifetime spectroscopy. The different arylene units in the backbone lead to the variation of the HOMO–LUMO gap across the series. The branched, polyionic side chains suppress aggregation of the polymer chains, even in aqueous solution, leading to higher fluorescence quantum yields relative to similar CPEs with linear side chains. UV–vis absorption spectra show that CPEs with anionic branched side chains (R-bCO2–Na) aggregate at low pH, while retaining the photophysical properties of their organic-soluble precursors at high pH. CPEs having branched cationic side chains (R-bNH3+Cl–) exhibit the opposite response to pH change.
    Macromolecules. 06/2011;
  • Xiaoyong Zhao, Kirk S Schanze
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    ABSTRACT: Direct detection of pyrophosphate (PPi) in aqueous solution is demonstrated using a cationic poly(phenylene-ethynylene) with polyamine side chains. Pyrophosphate-induced polymer aggregation causes a significant spectroscopic change, which in turn allows quantification of dissolved PPi using ratiometric signals.
    Chemical Communications 09/2010; 46(33):6075-7. · 6.38 Impact Factor
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    ABSTRACT: A series of poly(arylene ethynylene) conjugated polyelectrolytes (CPEs) substituted with carboxylic acid side groups have been synthesized and characterized. The polymers feature a backbone consisting of a carboxylated dialkoxyphenylene-1,4-ethynylene unit alternating with a second arylene ethynylene moiety of variable electron demand. The HOMO-LUMO gap is varied across the series, giving rise to a set of four polymers that have absorption maxima ranging from 404 to 495 nm. The CPEs adsorb effectively from solution onto nanostructured TiO(2) films, giving rise to TiO(2)/CPE films that absorb approximately 90% of the incident light at the absorption band maximum. The photocurrent generation efficiency of the TiO(2)/CPE films was examined in a solar cell configuration using an I(3)(-)/I(-) propylene carbonate electrolyte and a Pt/fluorine-doped tin oxide counter electrode. Most of the films exhibit good photocurrent generation efficiency with a peak quantum efficiency of approximately 50% at wavelengths corresponding to the polymers' absorption band maximum. Interestingly, the photocurrent generation efficiency for the lowest-band-gap polymer is substantially lower compared to the other three systems. This effect is attributed to efficient nonradiative decay of excitons at trap sites arising from interchain contacts distal from the TiO(2)/CPE interface.
    ACS Applied Materials & Interfaces 02/2009; 1(2):381-7. · 5.90 Impact Factor
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    ABSTRACT: A time-resolved and steady state photophysical study of a family of conjugated polyelectrolytes (CPEs) with variable chain lengths (ranging from 8 to 108 polymer repeat units per chain) is reported. The CPEs investigated are poly(phenylene ethynelene)s substituted with two carboxylate groups per polymer repeat unit to provide water and methanol soluble conjugated polyelectrolytes. Steady state and ultrafast time-resolved fluorescence and anisotropy measurements were performed to explore the role of chain lengths on the energy transfer processes. We find that the CPEs aggregate under almost all conditions, with the degree of aggregation depending on the length of the conjugated polyelectrolyte chains. These CPEs are highly rigid and planar and present a very small loss of anisotropy during their emission lifetime.
    Journal of Physical Chemistry C - J PHYS CHEM C. 09/2008; 112(41).
  • Xiaoyong Zhao, Hui Jiang, Kirk S. Schanze
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    ABSTRACT: This paper reports the synthesis and photophysical study of a series of anionic, carboxylate-substituted poly(phenylene ethynylene)-based conjugated polyelectrolytes (CPEs) with variable chain lengths. These CPEs are of interest as they allow the study of the effect of chain length on amplified fluorescence quenching. The CPEs were synthesized via organic soluble ester precursor polymers. The degree of polymerization of the precursor polymers was controlled by addition of a monofunctional “end-cap” to the polymerization reaction. The CPEs were obtained postpolymerization by base-promoted hydrolysis of the ester protecting groups. Stern−Volmer fluorescence quenching of the CPEs in methanol with monovalent electron-transfer quenchers (MV+ and HV+) show that the Stern−Volmer quenching constant (KSV) increases with polymer chain length reaching a maximum of ca. 2 × 105 M−1 at a degree of polymerization of 49. The results indicate that a maximum quenching amplification factor of 53 is attained under conditions where monovalent quencher ions interact with nonaggregated (single) polymer chains.
    Macromolecules. 04/2008; 41(10).
  • Hui Jiang, Xiaoyong Zhao, Kirk S Schanze
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    ABSTRACT: The fluorescence of conjugated polyelectrolytes (CPEs) is efficiently quenched by low concentrations of quenchers with opposite charges. We have reported the close correlation between this amplified quenching phenomenon and CPE chain aggregation. In this paper, we further demonstrate the profound correlation between the fluorescence quenching efficiency, CPE chain aggregation, and quencher molecular size. Aggregation of a poly(phenylene ethynylene)-type CPE (PPE-CO2-) is induced by the addition of either water or Ca2+ to methanol solution, as indicated by absorption, fluorescence, dynamic light scattering, and fluorescence microscope measurements. For quencher ions with a small molecular size, such as methyl viologen (MV2+), either the loose (induced by the addition of Ca2+) or the compact (induced by the addition of water) CPE chain aggregates are beneficial to the fluorescence quenching. For quencher ions with large molecular size, such as tris(4,7-diphenyl-1,10-phenanthroline)ruthenium (Ru(dpp)32+), however, the loose chain aggregates are found to be favorable for quenching, while the quenching efficiency is lower for the compact polymer aggregates present in aqueous solution.
    Langmuir 09/2007; 23(18):9481-6. · 4.38 Impact Factor
  • Xiaoyong Zhao, Yan Liu, Kirk S Schanze
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    ABSTRACT: A new fluorescence turn-on sensor consisting of PPE-CO(2)(-)/Cu(2+) shows high selectivity for pyrophosphate over other anions and is used to develop a real-time assay for alkaline phosphatase.
    Chemical Communications 08/2007; · 6.38 Impact Factor
  • Chemistry of Materials - CHEM MATER. 12/2006; 18(26).
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    ABSTRACT: A series of poly(arylene ethynylene) (PAE) conjugated polyelectrolytes (CPEs) have been prepared using palladium-mediated (Sonogashira) coupling chemistry. The series consists of five pairs of polymers that share the same poly(arylene ethynylene) backbone. One member of each pair contains anionic sulfonate (R−SO3-) side groups, whereas the other member contains cationic bis-alkylammonium (R-N+−R-N+−R) side groups. The repeat unit structure of the poly(arylene ethynylene) backbone consists of a bis(alkoxy)phenylene-1,4-ethynylene unit alternating with a second arylene ethynylene moiety, and five different arylenes were used, Ar = 1,4-phenyl, 2,5-pyridyl (Py), 2,5-thienyl (Th), 2,5-(3,4-ethylenedioxy)thienyl (EDOT), and 1,4-benzo[2,1,3]thiodiazole (BDT). The different arylene units induce variation in the HOMO−LUMO band gap across the series of polymers, resulting in a series of materials that display absorption maxima at wavelengths ranging from 400 to 550 nm and fluorescence maxima ranging from 440 to 600 nm. The absorption and fluorescence properties of the CPEs were investigated in methanol, water, and in methanol/water mixtures. The photophysical data suggest that the CPE chains aggregate in water, but in methanol, the polymers are well solvated such that the optical properties are characteristic of the “molecularly dissolved” chains. Stern−Volmer (SV) fluorescence quenching studies were carried out using ionic naphthalene diimides as electron acceptors. The results show that the fluorescence from the CPEs was quenched with very high efficiency (amplified quenching) when the ionic diimide was charged opposite to the charge on the CPE chain. The sensitivity of the Stern−Volmer quenching response varies strongly across the series of CPEs, with the most efficient quenching seen for polymers that display efficient fluorescence when they are aggregated. The relationship between CPE side chain structure, band gap, fluorescence quantum yield, extent of chain aggregation, and fluorescence quenching efficiency is discussed.
    Macromolecules. 08/2006; 39(19).
  • Hui Jiang, Xiaoyong Zhao, Kirk S Schanze
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    ABSTRACT: The fluorescence of conjugated polyelectrolytes (CPEs) is quenched with very high efficiency by small molecule quenchers. This effect has been referred to as amplified quenching. In the present communication, we demonstrate that aggregation of a poly(phenylene ethynylene)-type CPE (PPE-CO2-) induced by Ca2+ has a pronounced effect on the amplified quenching of the polymer by the dication methyl viologen (MV2+). In particular, absorption and fluorescence spectroscopy of PPE-CO2- in methanol solution indicate that addition of a low concentration of Ca2+ induces aggregation of the polymer chains. The range of MV2+ concentrations within which linear Stern-Volmer quenching behavior is observed systematically decreases with increasing Ca2+ concentration to a point where superlinear quenching is observed immediately upon addition of MV2+. This finding is unequivocal evidence that the superlinear Stern-Volmer quenching behavior typically observed in CPE-quencher systems arises due to quencher-induced aggregation of the CPE chains.
    Langmuir 07/2006; 22(13):5541-3. · 4.38 Impact Factor
  • Xiaoyong Zhao, Kirk S Schanze
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    ABSTRACT: A water soluble, meta-linked poly(phenylene ethynylene) featuring chiral and optically active side groups based on L-alanine (mPPE-Ala) has been studied by using absorption, fluorescence, and circular dichroism spectroscopy. Studies of mPPE-Ala in methanol/water solvent mixtures show that the polymer folds into a helical conformation, and the extent of helical folding increases with the volume % water in the solvent. The presence of the helical conformation is signaled by the appearance of a broad, excimer-like visible fluorescence band, combined with a strong bisignate circular dichroism signal in the region of the pi,pi absorption of the polymer backbone. The circular dichroism signal exhibits negative chirality, suggesting that the left-handed (M-form) of the helix is in enantiomeric excess. Binding of the metallointercalator [Ru(bpy)2(dppz)]2+ (where bpy = 2,2-bipyridine and dppz = dipyrido[3,2-a:2',3'-c]phenazine) with the helical polymer is accompanied by the appearance of the orange-red photoluminescence from the metal complex. This effect is directly analogous to that observed when [Ru(bpy)2(dppz)]2+ binds to DNA via intercalation, suggesting that the metal complex binds to mPPE-Ala by intercalating between the pi-stacked phenylene ethynylene residues. Cationic cyanine dyes also bind to the periphery of the helical polymer in a manner that is interpreted as "groove binding". A circular dichroism signal is observed that is believed to arise from exciton coupling within the chiral cyanine dye chromophore aggregate that is formed as the dye molecules are oriented by the helical mPPE-Ala "template".
    Langmuir 06/2006; 22(10):4856-62. · 4.38 Impact Factor