Viscoelasticity of Poly(ethylene glycol) Solutions on Supported Lipid Bilayers via Quartz Crystal Microbalance with Dissipation

Supported lipid bilayers of 1,2-dioleoyl-snglycero-3-phosphocholine were formed on a silicon oxide substrate, and the viscoelasticity of poly(ethylene glycol) (PEG) solutions above the bilayer was subsequently studied by quartz crystal microbalance with dissipation monitoring. No detectable adsorption of PEG molecules to the bilayer was found over a broad range of PEG molecular weight at various concentrations. The viscoelastic properties of the PEG solutions were obtained from the shifts in the resonance frequency and the energy dissipation factor of the polymer solution in contact with the resonator-supported lipid bilayer. The resulting viscoelastic properties of PEG solutions were found to be in excellent agreement with the Zimm model for linear polymer chains in a good solvent. The excluded volume scaling exponent ν for PEG in water shows an ideal-to-real crossover with increasing molecular weight. The exponent adopts a value of 0.50 for short chains and gradually increases to 0.565 for long chains. The onset of the excluded volume effect of PEG in water, a good solvent, lies in the molecular weight range between 4000 and 8000. ■ INTRODUCTION Viscoelasticity of polymer solutions has been the subject of intensive research for several decades, which provides considerable insights into the dynamics of polymer chains in both dilute and concentrated regimes. Polymer solutions usually have a wide spectrum of relaxation times which is not easy to probe experimentally. Thus, it is necessary to employ instrumentation operating at frequencies capable of probing the whole relaxation time spectrum. However, commercial rheometers employing oscillatory shear flows generated by cone-and-plate, parallel-plate, or tube geometries have a typical upper frequency limit of about 100 Hz, which is not high enough unless for high molecular weight polymers in a very viscous solvent. Such a frequency limitation can often be circumvented by employing time−temperature superposition (TTS) for polymer solutions with good solvent conditions for which the viscosity exhibits a strong dependence on temperature. However, TTS is not so effective for aqueous polymer solutions because the viscosity of water weakly depends on temperature and water represents a good solvent only over a limited temperature range depending on the polymer species. Additionally, theories of polymer dynamics in solutions and melts predict explicit scaling behaviors at high frequencies, which depend on the chain architectures, the flexibility of the chains, and the solvent−polymer interactions. Therefore, it is highly desirable to use experimental methods that can probe the polymer dynamics in the high-frequency regime. Diffusing wave spectroscopy can measure the viscoelasticity of polymer solutions up to frequencies of about 100 kHz but is not applicable, in general, to dilute polymer solutions. Instruments based on the torsional resonator, which was pioneered by Mason, have been used to study the viscoelasticity of polymer solutions up to hundreds of kilohertz. Another potential technique to probe the viscoelasticity of polymer solution at high frequency is the quartz crystal microbalance with dissipation (QCM-D). QCM was initially employed in air to measure the mass deposited on a piezoelectric resonator according to the decrease of the oscillation frequency and was later applied in liquid environments. Similar to torsional resonators which are working in the kilohertz range, QCM-D measures both resonance frequency and bandwidth (or energy dissipation factor) of the resonator in contact with the fluids, from which one can deduce the viscosity of Newtonian fluids in the megahertz frequency range. It is expected that QCM-D can also measure the viscoelasticity of polymer solutions as Received: January 16, 2015 Revised: February 23, 2015 Article pubs.acs.org/Macromolecules © XXXX American Chemical Society A DOI: 10.1021/acs.macromol.5b00095 Macromolecules XXXX, XXX, XXX−XXX torsional resonator did but at higher frequencies. However, it should be noted that the mass sensitivity constant of the adsorption on the resonator is inversely proportional to the square of the fundamental resonance frequency. Therefore, the perturbations arising from polymer adsorption on the resonator should be minimized in order to probe the viscoelasticity of polymer solutions by QCM-D. This can be achieved by coating the resonator surface with an inert layer, such as a gold film, a polymer layer, or a selfassembled monolayer or bilayer, which should be rigid and does not cause adsorption of the studied polymer molecules. Recent studies by Zhu et al. discussed the viscosity and shear modulus of poly(ethylene glycol) (PEG) solutions near a gold-coated 5 MHz resonator, but unfortunately the high-frequency character of the QCM-D results was not considered. Coating the quartz crystal resonator with a supported lipid bilayer (SLB) is a convenient and promising way to provide a nonadsorbing interface for water-soluble polymers such as PEG and dextran. PEG had been used for aggregating or fusing vesicles and cells, which is believed to be governed by attraction induced by polymer depletion from the vesicle membranes. The static and hydrodynamic forces between two substrate-supported lipid bilayers in PEG solution have been studied by surface force apparatus, and a crossover from depletion attraction to adsorption with increasing molecular weight of PEG was observed. Recent studies on giant unilamellar vesicles enclosing an aqueous solution of PEG and dextran revealed the formation of nanotubes and budding transformations, two processes that depend on the membranes’ spontaneous curvature that can be generated by depletion layers of the dissolved polymers, as first predicted in ref 28. Therefore, for PEG solutions in contact with the SLB-coated quartz crystal resonator, it is expected that PEG is depleted from the surface and that the resonator primarily probes the viscoelasticity of the solution because the thickness of the depletion layer is much smaller than the viscous penetration depth of the resonator. In the present paper, we study the formation of the SLB on silicon oxide substrates followed by an investigation of the viscoelastic properties of aqueous PEG solution in contact with the SLB using QCM-D in the high-frequency regime of 5−65 MHz. The results are compared with the frame of the Zimm model for linear polymer chains in a good solvent. ■ MATERIALS AND METHODS Materials. 1,2-Dioleoyl-sn-glycero-3-phosphocholine (DOPC) was purchased from Avanti Polar Lipids Inc. and used without further purification; the lipids were stored at −20 °C upon arrival. PEGs of different molecular weights (Mw = 200, 400, 600, 1000, 1400, 2000, 4000, 6000, 8000, 10 000, 20 000, and 35 000) were obtained from Sigma-Aldrich; they were desiccated in vacuum until no further reduction in mass was observed before use. The polydispersities Mw/ Mn of these PEG samples are less than 1.1, according to gel permeation chromatography measurement (Supporting Information Table S1). Poly(ethylene oxide)s (PEOs), synonymous to PEG, with higher molecular weight of 116 000, 278 000, and 443 000 were obtained from Polymer Laboratories Ltd. All other reagents were of analytical grade. All solutions were prepared using ultrapure water from Sartorius water purification system with a resistivity of 18.2 MΩ· cm. Preparation and Characterization of Liposomes. The liposomes were prepared by the extrusion method. About 400 μL DOPC chloroform solution (concentration 10 mg/mL) was dried under a gentle stream of nitrogen gas onto the walls of a continuously rolled vial followed by desiccating in vacuum for 3 h to completely remove the solvent. The dried lipid film was then hydrated by adding 2 mL aqueous solution containing 150 mM NaCl under gentle vortexing. Large unilamellar vesicles (LUVs) were obtained by extruding the solution 31 times through a polycarbonate membrane with a pore size of 100 nm via a mini-extruder (Avanti Polar Lipids Inc.). The obtained liposomes have a narrow size distribution with averaged diameter of 122 ± 2 nm, as determined by a Malvern Zetasizer Nano ZS90 dynamic light scattering instrument. Vesicle suspensions were stored at 4 °C and diluted to a lipid concentration of 0.1 mg/mL prior to use. QCM-D. The adsorption kinetics and the properties of the adsorbed layer were monitored on a Q-Sense E4 system (Biolin Scientific, Sweden). The AT-cut quartz crystal sensors with a fundamental resonance frequency of 4.95 MHz were cleaned with two cycles of the following procedure before each usage: soaked in 2% SDS solution for 30 min, rinsed with ultrapure water, blow-dried with nitrogen gas, and exposed to an ultraviolet (UV)/ozone cleaner for 15 min. QCM-D experiments were carried out at 24 ± 0.02 °C in an exchange mode at a flow rate of 100 μL/min. At least 1.0 mL of degassed sample solution was delivered into the chamber containing the sensor crystal (internal volume 40 μL) to ensure a complete exchange of the liquid. The shifts in the resonant frequency (Δf n) and the energy dissipation factor (ΔDn) of the sensors were acquired simultaneously at multiple harmonics (overtone number n = 1, 3, 5, 7, 9, 11, and 13). The measurements were performed after mounting the crystals in the flow module and establishing a baseline with a buffer containing 150 mM NaCl solution. DOPC vesicle solution was pumped into the chamber to form the supported lipid bilayers on the crystal surface, followed by rinsing with the above buffer again. The buffer was exchanged to water, before aqueous solution of PEG was pumped into the chamber. The adsorption behavior of PEG solution of different molecular weights at various concentrations on the SLB was checked one after another, and a sufficient amount of water was used to rinse the SLB surface between each step. For comparison, the adsorption behavior of sucrose solution of different concentrations on the SLB was also studied in the same way. For a thin and rigid film attached to the substrate, the Sauerbrey equation describes the relationship between the frequency shift Δf n and the adsorbed mass Δm. This equation has the form ρ μ Δ = − Δ = − Δ m f f n C f n ( ) 2 n n q q 1/2