Shengqin Wang

Facile method to prepare smooth and homogeneous polymer brush surfaces of varied brush thickness and grafting density

This Article describes a facile method to prepare smooth and homogeneous polymer brush surfaces of variable grafting density from a solid surface by combining Langmuir-Blodgett (LB) deposition with surface-initiated atom transfer radical polymerization (SI-ATRP). This method is successfully demonstrated by the preparation of thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) brush surfaces on smooth silicon and quartz substrates. With the custom-synthesized inert diluent whose chemical structure, except end-functionality, is the same as that of the reactive initiator, smooth and chemically homogeneous mixed monolayers of initiators and inert diluents are immobilized on a solid surface by LB deposition, allowing the further variation of the grafting density of PNIPAM brushes grafted from the initiator monolayers of varied initiator coverage. With the optimized molar ratio of deactivator, Cu(II) in the Cu(I)-ligand catalyst complex, the brush thickness of PNIPAM brushes at varied grafting density is controlled to grow nearly linearly with reaction time while smoothness and chemical homogeneity of PNIPAM brushes are achieved. For the demonstrated PNIPAM brush surfaces, the thermoresponsive characteristics of PNIPAM brushes are also verified. This combined LB-ATRP method can be applied to graft a variety of polymer brushes, including polyelectrolytes and block copolymers, from different solid substrates.

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

By measuring diffusion rate, the conformation change of single poly(2-vinylpyridine) chain in aqueous solution was studied by fluorescence correlation spectroscopy. The data showed a stepwise change of hydrodynamic radius when pH value was tuned, reflecting a sign of first-order conformation transition, and a continuous change was found at varying salt concentration.

Charge on a weak polyelectrolyte

Fluorescence measurements with single-molecule sensitivity are used to measure the hydrodynamic size and local pH of a weak polyelectrolyte, poly-2-vinyl pyridine end labeled with pH-sensitive dye, the polyelectrolyte having concentration so low (nanomolars) that molecular properties are resolvable only from fluorescence experiments and cannot be accessed by light scattering. We find that the local pH near the dye, inferred from its brightness, is consistently three orders of magnitude higher than the bulk pH. Upon varying the bulk pH, we measure the collapse point at which hydrophobic attraction overwhelms electrostatic repulsion between charged elements along the chain, and conclude that adding monovalent salt shifts this coil-to-globule collapse to higher pH than in the absence of salt. The influence of salt appears to shift the ionization equilibrium of this weak polyelectrolyte in the direction of the chain possessing enhanced electric charge at a given pH. Phenomenologically, this is opposite to the case for strong polyelectrolytes, although the mechanism differs.

Molecular diffusion on surface tethered polymer layers: coupling of molecular thermal fluctuation and polymer chain dynamics

The dynamics of single probe molecules on surface-tethered poly(N-isopropylacrylamide) (PNIPAM) layers is studied by fluorescence correlation spectroscopy. It is surprisingly observed that molecular surface dynamics on polymer layers is much slower than that on surfactant monolayers, despite stronger interfacial interactions for the latter case, suggesting a strong coupling of molecular thermal fluctuation and polymer chain dynamics. The effect of collective dynamics of polymer chains on the surface diffusion of adsorbed molecules is further examined with varied polymer grafting density and thickness. At low grafting density or large brush thickness, the resulting surface inhomogeneity of PNIPAM layers from the respective polymer mushroom conformation and low polymer end-segment density leads to the significant slowing down in the surface diffusion of adsorbed probe molecules. An optimal range in brush thickness to facilitate molecular surface dynamics is observed, where PNIPAM layers adopt the brush conformation with high polymer end-segment density.

Manipulating single annealed polyelectrolyte under alternating current electric fields: Collapse versus accumulation.

Effective manipulation and understanding of the structural and dynamic behaviors of a single polyelectrolyte (PE) under alternating current (AC) electric fields are of great scientific and technological importance because of its intimate relevance to emerging bionanotechnology. In this work, we employ fluorescence correlation spectroscopy (FCS) to study the conformational and AC-electrokinetic behaviors of a model annealed PE, poly(2-vinyl pyridine) (P2VP) under both spatially uniform and non-uniform AC fields at a single molecule level. Under spatially uniform AC-fields, we observe a gradual and continuous coil-to-globule conformational transition (CGT) of single P2VP at varied AC-frequency when a critical AC-field strength is exceeded, in contrast to the pH-induced abrupt CGT in the absence of AC-fields. On the contrary, under spatially non-uniform AC-fields, we observe field-driven net flow and accumulation of P2VP near high AC-field regions due to combined AC electro-osmosis and dielectrophoresis but surprisingly no conformational change. Thus, distinct AC-electric polarization effect on single annealed PE subject to AC-field homogeneity is suggested.

Conformation transition and electric potential of single weak polyelectrolyte: molecular weight dependence

The nature of the conformation transition of a hydrophobic weak polyelectrolyte (PE) chain is critically determined by local electrostatic environment in vicinity of the PE backbone. In this work, we examine the coil-to-globule conformation transition (CGT) and electric potential of a model weak PE, poly(2-vinyl pyridine) (P2VP) of varied molecular weight (Mn), in aqueous solutions at a single molecule level by fluorescence correlation spectroscopy (FCS) and photon counts histogram (PCH). By using FCS, the CGT of P2VP in aqueous solutions is observed upon increasing solution pH, yet showing a strong Mn dependence. The observed CGT is gradual and continuous at low Mn, in sharp contrast to an abrupt, discontinuous one at high Mn. Measured critical transition pH, pHCGT, to induce CGT is found to decrease as increasing Mn. To further elucidate the effect of Mn on CGT and local electrostatic interaction, PCH is employed with pH-sensitive fluorophore probes attached to a P2VP chain to determine the local pH, i.e. the local proton concentration immediately adjacent to an expanded P2VP coil. The electric potential, φ for a single P2VP chain is thereby estimated from the difference between local and bulk pHs, exhibiting a scaling with P2VP polymerization degree, N as φ ∼ N0.1. The increase in φ with increasing Mn suggests an enhanced electrostatic repulsion between local protons and a longer P2VP backbone, leading to the lowering of pHCGT.

Single molecule diffusion on hard, soft and fluid surfaces

Molecular diffusion on a surface is often considered as a thermal energy-activated process of molecular hopping between adjacent adsorption sites on a surface and simply determined by molecule–surface interaction. In this work, we report distinct diffusive dynamics of probe molecules on methyl-terminated self-assembled monolayer (SAM), polymer brush layer and lipid bilayer by using fluorescence correlation spectroscopy (FCS) at a single-molecule level. We have observed that despite weaker molecule–surface interaction, the surface diffusion of probe molecules on soft polymer brush surface and fluid lipid bilayer can be much slower than that on hard SAM surface, suggesting a strong impact of interfacial dynamics of the underlying coating on molecular surface diffusion. To further examine the coupling of thermal activated molecular hopping and soft surface dynamics, we have investigated the diffusion of probe molecules on polymer brushes of varied grafting density and thickness, where the molecule–surface interaction remains nearly the same, yet the adopted conformations and dynamics of surface-grafted PNIPAM chains vary considerably; it is striking to observe that the diffusion of probe molecules could be further retarded on PNIPAM brush surfaces of lower grafting density or higher brush thickness, thereby exhibiting the presence of an optimal brush thickness range to facilitate fast surface diffusion of adsorbed probe molecules. All the observations combined lead to a general model by taking the dynamics of underlying surface layers into account to elucidate the molecular diffusion mechanism on varied surfaces.

Distinct Effects of Multivalent Macroion and Simple Ion on the Structure and Local Electric Environment of a Weak Polyelectrolyte in Aqueous Solution

Adding ionic species can critically affect the structure of weak polyelectrolyte (PE) chains, whose charge density in aqueous solution can be greatly regulated by bathing solution conditions such as pH and added ions. Distinct from simple ions that can be treated as point charges, multivalent macroions of finite size, including many charged nanoparticles and biopolymers, could show strong electrostatic coupling with PEs and effectively modify the conformation and assembly of PEs in aqueous solution. In this work, we have compared the effects of hydrophilic multivalent macroion of finite size and simple divalent ion on the conformational transition of a model weak polybase, poly(2-vinylpyridine) (P2VP), in dilute aqueous solution. By using fluorescence correlation spectroscopy combined with photon counting histogram analysis, we have examined the swollen-to-collapsed conformational transition and local electric potential of a P2VP chain in ionic aqueous solution at a single-molecule level. Adding inorganic polytungstate ([W12]) macroion bearing eight negative charges per [W12] of ∼0.8 nm in diameter at increased concentration from 10-9 to 10-5 mM can lead to a shift of the critical conformational transition pH, pHcr, of P2VP to lower pH values, in an opposite trend to the previously reported effect of adding simple monovalent anion. Conversely, adding simple divalent sulfate anion can lead to a nonmonotonic change of pHcr when increasing its concentration from 0.03 to 15 mM. Additionally, at pH < pHcr where P2VP is highly protonated and adopts a swollen conformation, a monotonic decrease of P2VP size is observed with increased sulfate ionic concentration, exhibiting the typical ionic screening effect. In contrast, the size of the P2VP chain shows little change with increasing [W12] concentration before the precipitation of P2VP from water. To investigate the distinct effects of multivalent ion and macroion on the conformational transition of P2VP in aqueous solution, we have also measured the local proton concentration in the vicinity to a P2VP chain by an attached pH-sensitive fluorescence probe. In both cases, we have observed the monotonic reduction of the local electric potential of a swollen P2VP chain with increased ionic concentration, despite the increased protonation degree of P2VP. The results suggest that counterion condensation of multivalent ion and macroion can modify the effective net charge density of P2VP chains in dilute aqueous solution. However, possibly due to its high multivalency and finite size, multivalent [W12] macroion is much more effective in modifying the local electric environment and structure of P2VP chains at 3-7 orders of magnitude lower concentrations than simple sulfate counterion.

AC-electrokinetic manipulation and controlled encapsulate release of surfactant based micelles

Effective manipulation and understanding of the dynamic responses of complex micelle nanocolloids under ac-electric fields is of great scientific and technological importance due to the emerging interest in using rich electrokinetic phenomena for medical diagnostics and drug delivery. In this work, we employ fluorescence correlation spectroscopy (FCS) to study the ac-electrokinetic behaviors of surfactant-based micelles encapsulated with varied fluorescence and model drug probes at a single micelle level. Under applied non-uniform ac-fields of constant voltage across two adjacent microelectrodes in a quadrupole configuration, probe-encapsulated micelles exhibit positive dielectrophoresis (DEP) characteristics at the ac-frequency range of 10–500 kHz, in contrast to negative or zero DEP at higher frequency of approximately greater than 500 kHz. The transition from positive to negative or zero DEP is indicated by the field-induced decrease of micelle concentration in the FCS focal volume located in the high field regions between two adjacent microelectrodes. The estimated DEP crossover frequency from ω-dependent micelle concentration profile agrees with advanced DEP theory, which predicts the scaling of crossover frequency with Stern layer conductance. At a lower frequency range of ω = 5–10 kHz, unique and surprising response in the measured fluorescence autocorrelation functions of charged probe-encapsulated micelles is observed, but not so with neutral encapsulated probes, suggesting ac-field induced micelle instability with the resulting release of interior charged probe encapsulates. Intriguingly, the release of encapsulates from micelles by non-uniform ac-electric fields shows strong dependence on ac-frequency and encapsulate chemistry.