Electrogenerated Chemiluminescence in Polyelectrolyte Multilayers: Efficiency and Mechanism

The presence of an ultrathin film of polyelectrolyte complex, formed by the multilayering method, on an electrode was shown to enhance the intensity of electrogenerated chemiluminescence (ECL) from the tris(2,2‘)bipyridylruthenium(II)/tripropylamine system. Platinum electrodes coated with up to 17 layers of poly(diallyldimethylammonium chloride) or poly(vinylmethylpyridine), alternately layered with poly(styrenesulfonate), revealed significant differences in enhancement of ECL, depending on the identity of the multilayer. ECL following deposition of each layer showed an oscillating intensity of light emission, which alludes to the importance of surface and bulk charge. This effect, along with others, such as increased output with increasing tripropylamine concentration, was used to suggest a mechanism for enhanced ECL intensity at multilayer-coated electrodes

Effect of Molecular Crowding and Ionic Strength on the Isothermal Hybridization of Oligonucleotides

The isothermal hybridization of complementary oligonucleotides, 15-mer, 25-mer, 35-mer, and a molecular beacon, was investigated under varying conditions of molecular crowding and ionic strength, using hypochromicity to follow strand pairing and polyethylene glycol as a crowding agent. Thermodynamic analysis of the results revealed the addition of counterions to the oligonucleotide backbones, ΔΨ, to be dependent on the strand GC content and the molecular crowding. A decrease in ΔΨ was observed, with both increasing GC% and solution PEG content. In contrast, the number of bound water molecules depended on the activity of Na+, where two regimes were observed. At aNa+ aNa+ ≥ 0.05, water molecules were bound to the strands, and the extent of double strand formation decreased with increasing PEG wt %

Polyelectrolyte Complex Films from Blends Versus Copolymers

The release of counterions drives the efficient blending of oppositely charged polyelectrolytes when they complex. In theory, mixing like-charged macromolecules before complexation should offer a significant scope for controlling the composition and properties of polyelectrolyte complexes/coacervates. In practice, not much is known about the relative affinities between charged components and how they might compete for limited supplies of oppositely charged partners. In this work, we contrast the use of mixtures of poly­(styrene sulfonate), PSS, and poly­(methacrylic) acid, both polyanions, with copolymers of the same repeat units competing for poly­(diallyldimethylammonium), PDADMA, a polycation. Using the layer-by-layer, or multilayering, technique ensures that the polycation is always a limiting reagent. Although the composition of the thin film of a polyelectrolyte complex faithfully mirrored the solution copolymer composition, strong deviations from solution composition were observed with blends of homopolymers. There is a degree of thermodynamic control over film composition, related to the difference in free energy of formation of PDADMA/PSS versus PDADMA/poly­(methacrylic acid) repeat unit pairs

Controlling Electroosmotic Flow in Microchannels with pH-Responsive Polyelectrolyte Multilayers

A polyelectrolyte multilayer coating comprising acrylic acid or imidazole units afforded precise control over the rate and direction of electroosmotic flow within fused silica capillaries. Morphological changes accompanying the pH-induced switching of surface charge were minimized by diluting the pH-responsive polyelectrolyte with permanently charged polymer. Electroosmotic stability over many runs was demonstrated

The Polyelectrolyte Complex/Coacervate Continuum

Stoichiometric polyelectrolyte complexes (PECs) of the strong polyelectrolytes poly­(styrenesulfonate) (PSS) and poly­(diallyldimethylammonium) (PDADMA) were dissociated and dissolved in aqueous KBr. Water was added to dilute the salt, allowing polyelectrolytes to reassociate. After appropriate equilibration, these mixtures yielded compositions spanning complexes (solid) to coacervates (elastic liquid) to dissolved solutions with increasing [KBr]. These compositions were defined by a ternary polymer/water/salt phase diagram. For coacervates, transient microphase separation could be induced by a small departure from equilibration temperature. A boundary between complex and coacervate states was defined by the crossover point between loss and storage modulus. Salt ions within the complex/coacervate were identified as either ion paired with polyelectrolytes (“doping”) or unassociated. The fraction of ion pair cross-links between polyelectrolytes as a function of KBr concentration was used to account for viscosity using a model of “sticky” reptation

Doping-Controlled Ion Diffusion in Polyelectrolyte Multilayers: Mass Transport in Reluctant Exchangers

A new paradigm for nonlinear doping-controlled ion transport in soft condensed matter is presented, where the mobility of a minority “probe” ion is controlled by majority “salt” ion. The class of materials to which this paradigm applies is represented by ultrathin films of polyelectrolyte complexes, or multilayers. Intersite hopping of probe ions of charge ν occurs only when the charge of the destination site, produced by clustering of monovalent salt ions, is at least −ν, conserving electroneutrality. Salt ions are reversibly “doped” into the multilayer under the influence of external salt concentration. In situ ATR-FTIR reveals that the doping level, y, is proportional to salt concentration. Because hopping requires coincidence, or clustering, of salt, a strongly nonlinear dependence of flux, J, on salt concentration is observed:  J ∼ [NaCl]ν ∼ yν. This scaling was reproduced both by Monte Carlo simulations of ion hopping and by continuum probability expressions. The theory also predicts the observed scaling, though it underestimates the magnitude, of the strong selectivity of multilayers for ions of different charge

Mechanical Properties of Reversibly Cross-Linked Ultrathin Polyelectrolyte Complexes

Tensile properties of microcoupons of polyelectrolyte complex, formed by the multilayering method, were determined using a micromechanical analysis system. The degree of internal ion-pair (“electrostatic”) cross-linking was reversibly controlled by exposure to salt solution of varying concentration, which “doped” counterions into the films, breaking polymer/polymer ion pairs in the process. Linear stress−strain behavior was observed for a poly(styrene sulfonate)/poly(diallyldimethylammonium) multilayer up to 2% deformation. The dependence of modulus on cross-link density could be rationalized well by classical theories of rubber elasticity, including some insight on the topology of polyelectrolyte complexes

Driving Forces for Oppositely Charged Polyion Association in Aqueous Solutions: Enthalpic, Entropic, but Not Electrostatic

Driving forces for association between oppositely charged biological or synthetic polymers in aqueous solution have long been identified as electrostatic in origin. This attraction is broken down into an entropic component, due to loss of counterions, and an enthalpic component, stemming from Coulombic attraction between opposite charges. While the balance between entropic and enthalpic contributions shifts according to the conditions, the presence of exotherms or endotherms on mixing, though small, are viewed as signatures of Coulombic interactions which support theories of polyelectrolyte association rooted in continuum electrostatics. Here, a head-to-head comparison is made between mechanisms based on electrostatics and those based on specific ion pairing, or ion exchange. Using a Hofmeister series of counterions for a common polycation, poly­(diallyldimethylammonium), enthalpy changes on association with poly­(styrenesulfonate) are shown to derive from changes in water perturbation, revealed by Raman scattering studies of water O–H vibrations. The free energy for complexation is almost completely entropic over all salt concentrations

The Polyelectrolyte Complex/Coacervate Continuum

Stoichiometric polyelectrolyte complexes (PECs) of the strong polyelectrolytes poly­(styrenesulfonate) (PSS) and poly­(diallyldimethylammonium) (PDADMA) were dissociated and dissolved in aqueous KBr. Water was added to dilute the salt, allowing polyelectrolytes to reassociate. After appropriate equilibration, these mixtures yielded compositions spanning complexes (solid) to coacervates (elastic liquid) to dissolved solutions with increasing [KBr]. These compositions were defined by a ternary polymer/water/salt phase diagram. For coacervates, transient microphase separation could be induced by a small departure from equilibration temperature. A boundary between complex and coacervate states was defined by the crossover point between loss and storage modulus. Salt ions within the complex/coacervate were identified as either ion paired with polyelectrolytes (“doping”) or unassociated. The fraction of ion pair cross-links between polyelectrolytes as a function of KBr concentration was used to account for viscosity using a model of “sticky” reptation

Polyelectrolyte Multilayers with Reversible Thermal Responsivity

Thermally responsive polyelectrolyte multilayers were made from charged poly(N-isopropylacrylamide) (PNIPAM) copolymers. The temperature-dependent water content of the thin film, studied in situ using attenuated total reflectance Fourier transform infrared (ATR−FTIR) spectroscopy, revealed microscopic and macroscopic transitions at 33 and 45 °C, respectively. About seven water molecules per NIPAM repeat unit were found to be reversibly lost from, or recovered by, the film upon cycling over a temperature range of 10−55 °C. Assuming each ion pair represents a cross-link, swelling theory was used to translate these results into polymer−solvent interaction parameters and enthalpies of mixing for the various polymer components. The flux of a charged probe molecule, ferricyanide, through the NIPAM-rich multilayer was assessed with rotating disk electrode voltammetry. Thermally reversible modulation of ion transport was demonstrated