First-order conformation transition of single poly(2-vinylpyridine) molecules in aqueous solutions

Poly(ethylene oxide) Is Positively Charged in Aqueous Solutions

There have been controversies about the binding of cations to poly(ethylene oxide) (PEO) chains in aqueous solutions. In the current study, single molecular evidence of charging PEO chains by cation binding in aqueous solutions is provided. From the adoption of the photon-counting histogram method, it is discovered that the local pH value at the vicinity of the PEO chain is higher than the bulk solution, showing that the PEO chain is positively charged. Such a situation exists with and without the presence of salt (NaCl) in the solution, presumably due to the binding of cations, such as hydronium and sodium ions. Single molecular electrophoresis experiments using fluorescence correlation spectroscopy demonstrate that the PEO chains are weakly charged with a charging extent of ~5%. In comparison to the salt-free condition, the addition of external salt (NaCl) at moderate concentrations further charges the chain. The charging causes the PEO chains to expand and a further increase in the salt concentration causes the chain to shrink, exhibiting a polyelectrolyte-like behavior, demonstrated by the hydrodynamic radii of a single PEO chain. The effect of ion identity is discovered with alkali cations, with the order of the charging capacity of Li+ < Na+ < Cs+ < K+.

Studying the physics of charged macromolecules by single molecule fluorescence spectroscopy

It is well documented that conventional methods such as dynamic light scattering have encountered difficulties in characterizing charged macromolecules and, therefore, it is desirable that new methods and techniques are introduced. With the ultra-high sensitivity, single molecule fluorescence spectroscopy has successfully lowered the detection limit considerably and enabled measurement under extreme dilution conditions-around the concentration of 10^-9M-at which the effect of inter-chain electrostatic repulsion is suppressed. Furthermore, the excellent spatial and temporal resolution as well as the capacity of molecular recognition of these methods help in obtaining rich information of charged macromolecules. This paper summarizes the applications of single molecule fluorescence spectroscopy, especially fluorescence correlation spectroscopy and photon counting histogram, in the studies on charged macromolecules in aqueous solutions and plenty of new information has been revealed on the molecular conformation, counterion distribution, and a few important governing factors. The powerfulness and effectiveness of single molecule fluorescence spectroscopy make it promising in the investigations of charged macromolecules.

Molecular weight dependence of chain conformation of strong polyelectrolytes

Using sodium polystyrene sulfonate (NaPSS) and quarternized poly 4-vinylpyridine (QP4VP) as model systems, the chain conformation of polyelectrolytes under finite salt concentrations is investigated at a single molecular level. By fluorescence correlation spectroscopy (FCS), the hydrodynamic radius (R _h) of the samples with the molecular weight ranging more than one order of magnitude was measured. The variations of R _h as a function of molecular weight reveal the molecular weight dependence: under moderate salt concentrations (such as 10^-4 and 0.1M), the shorter chains of both NaPSS and QP4VP take the rod-like conformation, while the longer chains take the coiled conformation (random coil or swelled random coil conformation, respectively). At high enough salt levels, both the charged chains take the coiled conformations. Photon counting histogram (PCH) measurements of the local pH value at the vicinity of the NaPSS chain expose the higher extent of counterion adsorption for longer chains as well as higher salt concentrations, telling that the charge regularization process is the major governing factor.

Charge evolution during the unfolding of a single DNA i-motif

The effective charge and evolution of single chains of a DNA i-motif during its unfolding process are investigated at the single molecule level. Using fluorescence correlation spectroscopy and photon counting histograms, the single chain dimensions and electrical potential of cytosine-rich human telomeric oligonucleotides are monitored, during their unfolding from the i-motif to the random coil state. It is discovered that the effective charge density of the DNA chain is very sensitive to conformation changes and the results remarkably expose the existence of an intermediate state of the unfolding process. A huge difference in pH value exists in the vicinity of the DNA chain and the bulk solution, depending on the salt concentration, as reflected by a down-shift in the pH value of unfolding. The presence of an external salt in the solution helps to stabilize the i-motif structure at low pH values due to the reduction of the effective charge density. It can also destabilize the folded structure in the pH range of the conformation transition due to the elevation of the local pH value, encouraging the deprotonation of the cytosine groups. These results provide new information for understanding the structure and stability of i-motif DNA, and its biological function, as well as the building blocks for smart nanomaterials.

Huge Differences in the Kinetics of Swelling Enhancement and De-enhancement of Permanently Charged Polyelectrolyte Brushes

As demonstrated previously (X. Chu et al., Soft Matter 2014, 10, 5568), permanently charged polyelectrolyte brushes can experience an enhancement of swelling by exposure to an external monovalent salt solution in moderate concentrations. Beyond the previous static measurements, the kinetics of the swelling enhancement and de-enhancement were investigated in the current study by using a quartz crystal microbalance with dissipation (QCM-D). By developing an effective approach to quantify the response in QCM-D, a vast difference in swelling enhancement and de-enhancement of a model permanently charged polyelectrolyte brush (sodium polystyrene sulfonate, NaPSS) was discovered. The results indicate new physics of the charged brushes: the difference in the attachment and detachment of counterions to the polyelectrolyte chains.

Probing the Inhomogeneous Charge Distribution on Annealed Polyelectrolyte Star Polymers in Dilute Aqueous Solutions

The conformational structure of a polyelectrolyte chain in dilute aqueous solution is strongly coupled with its surrounding electrostatic environment. With the introduction of branched topology, the distribution of counterions in the vicinity of a polyelectrolyte chain becomes highly inhomogeneous, giving rise to complex structures of branched polyelectrolytes in dilute aqueous solution. To directly probe the local electrostatic conditions near a branched polyelectrolyte in aqueous solutions, star-shaped poly(2-vinylpyridine) (P2VP) polymers with precise labeling of one single fluorophore at different locations, for example, the star center or the terminal group of one arm, were synthesized using reversible addition-fragmentation chain transfer (RAFT) polymerization of vinyl-terminated P2VP macromonomers. Using fluorescence correlation spectroscopy (FCS) combined with photon counting histogram (PCH) analysis, the conformational structures and local electric potential of P2VP star polyelectrolytes were investigated in dilute aqueous solutions of varied pH at a single molecule level. Despite the same hydrodynamic radius of P2VP stars, pH-sensitive fluorophores labeled at different locations sensitively differentiated the higher electric potential at the star center from the lower electric potential at the periphery in dilute aqueous solutions.

Tuning the adhesion of silica microparticles to a poly(2-vinyl pyridine) brush: an AFM force measurement study

AFM force measurements have been performed to study the influence of the pH value and salt concentration on the interactions between poly(2-vinyl pyridine) brushes and microsized silica spheres, focusing on attractive and adhesion forces. It was found that the interaction was composed of a repulsive component reflecting the conformation of the brush and an additional attractive force. It can therefore be switched reversibly between purely repulsive at pH 2.5 to strong and medium adhesion by changing the pH value to pH 4 and 6, respectively. Addition of KCl showed different effects: at pH 2.5 high salt concentrations induced an attractive force; at pH 4 the interaction changed from strong attraction in the osmotic brush regime to repulsion and weaker adhesion in the salted brush regime; at pH 6 increase of the KCl concentration weakened the attractive force. These effects could partly be explained by the theory of polyelectrolyte brushes; under some conditions the mechanism of the attractive force is still unclear.

Diffusion of ionic fluorescent probes atop polyelectrolyte brushes

The lateral diffusion of ionic fluorescent molecules atop polyelectrolyte brushes was adopted to probe the distribution of counterions of the polyelectrolyte brushes. With a combination of single molecule fluorescence techniques, fluorescence correlation spectroscopy and single molecule fluorescence imaging, the lateral diffusion of the ionic probes (sulforhodamine B, rhodamine 6G) at the top of the model polyelectrolyte brushes with the opposite charges, poly([2-(methylacryloyloxyl)ethyl] trimethylammonium chloride) (PMETAC) and polystyrene sulfonate (PSS), was studied with different external salt concentrations. A huge decrease of the diffusion rate of the probes was observed at salt concentrations 2-3 orders of magnitude lower than that for any detectable change of brushes thickness could be observed. The results reflect the early collapse of the top portion of the polyelectrolyte brushes and also the penetration of the probes into the brushes due to the increase of osmotic pressure by the salt level in the solution. The diffusion of the fluorescent counterion can serve as a very sensitive probe of the structure atop the polyelectrolyte brushes.

Hofmeister effect on the interfacial dynamics of single polymer molecules

The Hofmeister effect on interfacial dynamics has been discovered for single charged polymer molecules (sodium polystyrene sulfonate) adsorbed on a hydrophobic surface from an aqueous solution. The presence of ions in the aqueous solution affects the surface diffusivity, and its amplitudes and the surface friction follow the Hofmeister series-the kosmotropic ions slowed down the surface diffusivity and the chaotropic ions speeded it up. The amplitude of the surface friction exhibits a good correlation with the surface tension increment, indicating the interfacial feature of the Hofmeister effect.

Single chain diffusion of poly(ethylene oxide) in its monolayers before and after crystallization

Lateral diffusion of single chain related to the crystallization process of poly(ethylene oxide) (PEO) in its monolayers on silica surfaces is studied by single molecule fluorescence microscopy and single molecule tracking techniques. Diffusion of PEO chains is observed in the supercooled state before crystallization as well as in the noncrystalline regions after crystallization. In the postcrystallization monolayers, the diffusion coefficient of PEO chains is an order of magnitude lower than that in the supercooled state before crystallization. The origin is attributed to the change of polymer surface concentration due to the consumption of polymer molecules in the crystal formation. This is supported by the results showing a monotonous decrease of diffusion coefficient with the thickness decrease of the monolayer in its supercooled state. The PEO chains take a more flattened conformation under lower surface concentration and stick stronger to the surface. As a consequence, the diffusion rate is reduced. The results clearly demonstrate a strong mutual effect between the crystallization process and the mass transportation for the polymer crystallization under surface confinement.

Effective charge and coil-globule transition of a polyelectrolyte chain

Considering the adsorption of counterions on an isolated polyelectrolyte (PE) chain and using a variational theory, phase boundaries and the critical point for the first-order coil-globule transition are calculated. The transition is induced cooperatively by counterion adsorption and chain conformations and the calculation is done self-consistently. The size of the PE chain is a single-valued function of charge. The discontinuous transition of the coil size is accompanied by a discontinuous transition of the charge. Phase boundaries for the coil-globule transitions induced by both Coulomb strength (inverse temperature or dielectric constant) and ionic strength (salt) show that the PE chain collapses at a substantially lower Coulomb strength in the presence of salt. In the expanded state of the coil, an analytical formula is derived for the effective charge of the chain for conditions where the coupling between chain conformations and counterion adsorption is weak. In general, the dielectric heterogeneity of the solvent close to the polymer backbone is found to play a crucial role in the charge regularization and the chain collapse.