Equilibrium electrostatics of responsive polyelectrolyte monolayers

Enabling High-Capacitance Supercapacitors by Polyelectrolyte Brushes

Polyelectrolyte brushes (PEBs) hold excellent potential for designing high-capacitance electrical double-layer capacitors (EDLCs), a crucial component of supercapacitors. Both experiments and computational simulations have shown their energy-storage advantage. However, the effect of PEBs on the energy storage of EDLCs is not yet fully understood. Herein, we systematically study the energy-storage effects of polyanionic (PA) and polycationic (PC) brushes using polymer density functional theory (DFT). First, the application of polymer DFT in polyelectrolyte-grafted EDLCs is successfully validated using molecular dynamics simulations. With the help of polymer DFT, an interfacial adhesion microstructure of the PA/PC brushes is observed. Most importantly, the results show that polyelectrolyte-grafted EDLCs achieve a significant increase in capacitance at low salt concentrations and surface voltages, offering an excellent energy-storage advantage over traditional EDLCs. However, this advantage is considerably diminished at high salt concentrations or surface voltages, showing unusual salt- and voltage-dependent behaviors of energy-storage capacity. Nonetheless, the PC-grafted EDLCs maintain their outstanding energy-storage performance, even at relatively high salt concentrations and surface voltages. These findings deepen our comprehension of PEBs at the molecular level and provide insights for the molecular design of high-capacitance supercapacitors.

Cation-size-dependent DNA adsorption kinetics and packing density on gold nanoparticles: an opposite trend

The property of DNA is strongly influenced by counterions. Packing a dense layer of DNA onto a gold nanoparticle (AuNP) generates an interesting colloidal system with many novel physical properties such as a sharp melting transition, protection of DNA against nucleases, and enhanced complementary DNA binding affinity. In this work, the effect of monovalent cation size is studied. First, for free AuNPs without DNA, larger group 1A cations are more efficient in inducing their aggregation. The same trend is observed with group 2A metals using AuNPs capped by various self-assembled monolayers. After establishing the salt range to maintain AuNP stability, the DNA adsorption kinetics is also found to be faster with the larger Cs(+) compared to the smaller Li(+). This is attributed to the easier dehydration of Cs(+), and dehydrated Cs(+) might condense on the AuNP surface to reduce the electrostatic repulsion effectively. However, after a long incubation time with a high salt concentration, Li(+) allows ∼30% more DNA packing compared to Cs(+). Therefore, Li(+) is more effective in reducing the charge repulsion among DNA, and Cs(+) is more effective in screening the AuNP surface charge. This work suggests that physicochemical information at the bio/nanointerface can be obtained by using counterions as probes.

Morphological transformations in polymer brushes in binary mixtures: DPD study

Morphological transformations in polymer brushes in a binary mixture of good and bad solvents are studied using dissipative particle dynamics simulations drawing on a characteristic example of polyisoprene natural rubber in an acetone-benzene mixture. A coarse-grained DPD model of this system is built based on the experimental data in the literature. We focus on the transformation of dense, collapsed brush in bad solvent (acetone) to expanded brush solvated in good solvent (benzene) as the concentration of benzene increases. Compared to a sharp globule-to-coil transition observed in individual tethered chains, the collapsed-to-expanded transformation in brushes is found to be gradual without a prominent transition point. The transformation becomes more leveled as the brush density increases. At low densities, the collapsed brush is highly inhomogeneous and patterned into bunches composed of neighboring chains due to favorable polymer-polymer interaction. At high densities, the brush is expanded even in bad solvent due to steric restrictions. In addition, we considered a model system similar to the PINR-acetone-benzene system, but with the interactions between the solvent components worsened to the limit of miscibility. Enhanced contrast between good and bad solvents facilitates absorption of the good solvent by the brush, shifting the collapsed-to-expanded transformation to lower concentrations of good solvent. This effect is especially pronounced for higher brush densities.

Donnan potential caused by polyelectrolyte monolayers

The Donnan potential is successfully isolated from ion pair potential on a ferrocene-labeled polyelectrolyte (DNA) monolayer. The isolated Donnan potential shifts negatively upon the increase in NaClO4 concentration with a slope of -58.8 mV/decade. With the salt concentration grown up to 1 M, the stretched DNA chains in low salt concentration are found to experience a gradual conformation relaxing process. At salt concentrations higher than 2 M, Donnan breakdown occurs where only the ion pair effect modulates the apparent potential. The apparent formal potential also shows strong dependence on solution pH, which reveals that the charge density in the polyelectrolyte monolayer plays an important role in the establishment of Donnan equilibrium.

Multimodal electrochemical sensing of transcription factor–operator complexes

Changes in diffusive movements, surface potential, and interfacial impedance of DNA monolayers are combined to analyze binding of unlabeled transcription factors.

DNA surface hybridization: comparison of theory and experiment

The design and interpretation of surface hybridization assays is complicated by poorly understood aspects of the interfacial environment that cause both kinetic and thermodynamic behaviors to deviate from those in solution. The origins of these differences lie in the additional interactions experienced by hybridizing strands at the surface. In this report, an analysis of surface hybridization equilibria is provided for end-tethered, single-stranded oligonucleotide "probes" hybridizing with similarly sized, single-stranded solution "target" molecules. Theoretical models by Vainrub and Pettitt (Phys. Rev. E 2002, 66, 041905) and by Halperin, Buhot, and Zhulina (Biophys. J. 2004, 86, 718), and an "extended" model that in addition includes a solution-like salt dependence of probe-target dimerization, are compared to experiments as a function of salt concentration and probe coverage. Good agreement with experiment is observed when the DNA volume fraction at the surface remains below approximately 0.25. None of the models, however, can account for strong suppression of hybridization when the volume fraction of DNA approaches 0.3, realizable in the limit of high buffer strength and densely tethered films. Under these conditions, hybridization yields become insensitive to increases in analyte concentration even though many probes remain available to bind targets. These observations are attributed to the onset of packing constraints which, interestingly, become limiting significantly below maximum DNA coverages estimated from ideally efficient hexagonal packing. By delineating conditions under which specific hybridization behaviors are observed, the results advance fundamental knowledge in support of DNA microarray and biosensor applications.

Molecular mechanisms in morpholino-DNA surface hybridization

Synthetic nucleic acid mimics provide opportunity for redesigning the specificity and affinity of hybridization with natural DNA or RNA. Such redesign is of great interest for diagnostic applications where it can enhance the desired signal against a background of competing interactions. This report compares hybridization of DNA analyte strands with morpholinos (MOs), which are uncharged nucleic acid mimics, to the corresponding DNA-DNA case in solution and on surfaces. In solution, MO-DNA hybridization is found to be independent of counterion concentration, in contrast to DNA-DNA hybridization. On surfaces, when immobilized MO or DNA "probe" strands hybridize with complementary DNA "targets" from solution, both the MO-DNA and DNA-DNA processes depend on ionic strength but exhibit qualitatively different behaviors. At lower ionic strengths, MO-DNA surface hybridization exhibits hallmarks of kinetic limitations when separation between hybridized probe sites becomes comparable to target dimensions, whereas extents of DNA-DNA surface hybridization are instead consistent with limits imposed by buildup of surface (Donnan) potential. The two processes also fundamentally differ at high ionic strength, under conditions when electrostatic effects are weak. Here, variations in probe coverage have a much diminished impact on MO-DNA than on DNA-DNA hybridization for similarly crowded surface conditions. These various observations agree with a structural model of MO monolayers in which MO-DNA duplexes segregate to the buffer interface while unhybridized probes localize near the solid support. A general perspective is presented on using uncharged DNA analogues, which also include compounds such as peptide nucleic acids (PNA), in surface hybridization applications.