Xiangjun Gong

Direct measurement of the nanobubble-induced weak depletion attraction between a spherical particle and a flat surface in an aqueous solution

Previously, it has been shown that stable nanobubbles (∼150 nm) free in an alkaline α-cyclodextrin (α-CD) solution can be removed by repeated filtration and regenerated by air injection. Recently, we found that the existence of nanobubbles can induce attraction between a spherical latex particle (radius ∼3.2 μm) and a flat hydrophilic surface. Using total internal reflection microscopy (TIRM), we are able to directly measure such a weak interaction (down to ∼0.1 μN m-1 within a range up to 200 nm). The weak attraction disappears as soon as the nanobubbles are removed. Our results suggest that the origin of such weak attraction comes from the exclusion or depletion of the charged nanobubbles from the gap between the particle and surface; namely, an imbalance between the osmotic pressures inside and outside the gap.

Investigation of cell behaviors on thermo-responsive PNIPAM microgel films

The use of poly(N-isopropylacrylamide) (PNIPAM) as building blocks for engineering responsive coatings and their potential use as switchable substrates such as biosensors have attracted great attention in recent years. However, few studies have been conducted regarding the cell behaviors and the related mechanism on thermos-responsive surfaces consisting of PNIPAM microgel particles. In this work, monodisperse PNIPAM microgels were synthesized and used to prepare PINPAM microgel films. Uniform microgel surfaces can be fabricated by drop-coating with the precoating of a polyethylenimine (PEI) layer. Cell experiments indicate that unlike PNIPAM polymer brushes reported with controllable detachment ability, HepG2, which is a human liver carcinoma cell line, remains adherent on the microgel films upon cooling. Surface plasmon resonance (SPR) experiments show an irreversible adsorption of serum proteins on the microgel surface upon cooling, whose adsorption is a prerequisite of cell adhesion during cell culture. This fact may account for the irreversible adhesion of HepG2 cells.

Interactions between solid surfaces mediated by polyethylene oxide polymers: effect of polymer concentration.

Using total internal reflection microscopy (TIRM), we have systematically measured the interactions between a microsphere and a flat hydrophilic surface in the presence of polyethylene oxide (PEO) polymer solution. Our results reveal that PEO significantly mediates the interaction forces between the two surfaces. At low polymer concentration, the interactions between two surfaces in the presence of PEO are mainly dominated by repulsive forces, originating from diffuse layer overlap. At intermediate polymer concentration, a long-range and weak attraction sets in. This force is likely attributed to the depletion attraction due to the presence of free PEO chains in bulk solution; however, a simple hard-sphere AO model fails to precisely describe the attraction. At high polymer concentration where PEO chains overlap, the attraction disappears, and levitation of the microsphere probe is detected. We argue that at this overlapping region, the correlation length of PEO chains is much smaller than the size of single PEO molecule, leading to weakening and disappearing of the depletion attraction. Finally, at very high concentration, oscillatory structural force is obviously found, indicating the significant structural ordering of the PEO chains under confinement.

Investigating interactions between cationic particles and polyelectrolyte brushes with Total Internal Reflection Microscopy (TIRM)

The manufacturing of switchable surfaces can be achieved when polymer chains are adsorbed or grafted densely on solid surfaces. These so-called “smart” surfaces have been often used to control the adsorption of various colloidal particles and biomolecules. To have an insight into the adsorption process, knowledge of the interaction forces between the surface and colloidal particle or biomolecule is critical. In this work, we used Total Internal Reflection Microscopy (TIRM) to directly measure the interaction potentials between poly(2-(dimethylaminoethyl methacrylate)) (PDMAEMA) brushes with two different lengths grafted on a glass slide and a positively charged polystyrene (PS) particle with pre-adsorbed layers of poly(ethyleneimine) (PEI, Mw = 2000 g mol−1), in aqueous solutions. As can be shown by direct interaction measurements, the interactions were strongly affected by the conformation of the polyelectrolyte brushes, pH values and salt concentrations. For short polymer brushes (∼30 nm), at pH 4.2 and 3.5 the interaction between the partially protonated and swollen PDMAEMA brush and the positively charged PS particle was dominated by repulsive forces at low salt concentrations, originating from diffuse layer overlap. However, when the pH is decreased to 3.0, a long-range attraction sets in. For longer polymer brushes (∼75 nm), the influences of the pH and salinity were more complex. Our results showed that the interaction between the longer polymer brushes and the particle could be switched reversibly between pure repulsion at pH 4.0, medium attraction at pH 3.6 and strong attraction at pH 3.0. The interaction mechanisms that act at these pH values and salt concentrations were discussed.

Microparticle templating as a route to nanoscale polymer vesicles with controlled size distribution for anticancer drug delivery

Polymer vesicles are self-assembled shells of amphiphilic block copolymers (BCPs) that have attracted tremendous interest due to their encapsulation ability and intracellular delivery of therapeutic agents. However, typical processes for the formation of polymer vesicles lead to ensembles of structures with a broad size distribution (from nanometer to micrometer scale) which result in a limitation for efficient cellular uptake. In this study, we present a simple and efficient approach for the fabrication of polymer vesicles with uniform nanoscale dimensions from template formation of electrosprayed particles in a high throughput manner. First, electrospraying was applied to produce micrometer-sized templates of a block copolymer before polymer vesicles were formed from the pre-prepared microparticles via rehydration. Four different biocompatible diblock and triblock copolymers were used to successfully fabricate polymer vesicles with uniform size around 150nm using this approach. Furthermore, we encapsulate anticancer drug doxorubicin (DOX) within the polymer vesicles via this method. The kinetics of cellular uptake (HeLa cell) and intracellular distribution of DOX-loaded polymer vesicles have been quntified and monitored by flow cytometry and confocal microscopy, respectively. The results show that our new method provides a promising way to fabricate drug-loaded polymer vesicles with controllable nanoscale size for intracellular anticancer drug delivery.

Measurements of Long-Range Interactions between Protein-Functionalized Surfaces by Total Internal Reflection Microscopy

Understanding the interaction between protein-functionalized surfaces is an important subject in a variety of protein-related processes, ranging from coatings for biomedical implants to targeted drug carriers and biosensors. In this work, utilizing a total internal reflection microscope (TIRM), we have directly measured the interactions between micron-sized particles decorated with three types of common proteins concanavalin A (ConA), bovine serum albumin (BSA), lysozyme (LYZ), and glass surface coated with soy proteins (SP). Our results show that the protein adsorption greatly affects the charge property of the surfaces, and the interactions between those protein-functionalized surfaces depend on solution pH values. At pH 7.5-10.0, all these three protein-functionalized particles are highly negatively charged, and they move freely above the negatively charged SP-functionalized surface. The net interaction between protein-functionalized surfaces captured by TIRM was found as a long-range, nonspecific double-layer repulsion. When pH was decreased to 5.0, both protein-functionalized surfaces became neutral and double-layer repulsion was greatly reduced, resulting in adhesion of all three protein-functionalized particles to the SP-functionalized surface due to the hydrophobic attraction. The situation is very different at pH = 4.0: BSA-decorated particles, which are highly charged, can move freely above the SP-functionalized surfaces, while ConA- and LYZ-decorated particles can only move restrictively in a limited range. Our results quantify these nonspecific kT-scale interactions between protein-functionalized surfaces, which will enable the design of surfaces for use in biomedical applications and study of biomolecular interactions.

Tuning the particle-surface interactions in aqueous solutions by soft microgel particles.

Due to the softness and deformability, interaction between colloidal surfaces induced by soft particles varies in a more complex way than for solid particles and thus has attracted much attention in recent years. In the present study, we use total internal reflection microscopy (TIRM) to directly measure the interaction between polystyrene (PS) microparticles and a flat glass surface in a poly(N-isopropylacrylamide) (PNIPAM) microgel dispersion with concentration varying from dilute (0.1 wt %) to highly concentrated regime (7.5 wt %). Our result shows that the PS particle-surface interactions mediated by the soft microgels are greatly affected by the particle concentration, the configuration of those microgels adsorbed on the surfaces, and the structure and packing of microgels in bulk solution. With increasing the microgel concentration (Cmicrogel), the interaction between the PS particle and surface turned from bridging attraction to steric repulsion, and then depletion attraction, which were mainly governed by the adsorption amount and configuration of microgels on the two surfaces. By further increasing Cmicrogel to condensed situation, structural force with oscillated energy wells was detected. The variation of interactions induced by the soft microgels was further confirmed by optical imaging. Crystallization of the PS microparticles appeared at moderate Cmicrogel; however, crystallization was hindered at higher Cmicrogel where the microgels are highly packed in the bulk solution. Furthermore, using TIRM, microgel packing with local energy well (0.1-1.0 kBT) at the highly condensed state (7.5 wt %) was resolved from the interaction profiles. Therefore, the shear force and modulus generated by such microgel packing can be determined as ∼0.2 pN and tens of mPa, respectively, which are much weaker than data given by conventional active methods.

An active one-particle microrheometer: Incorporating magnetic tweezers to total internal reflection microscopy

We present a novel microrheometer by incorporating magnetic tweezers in the total internal reflection microscopy (TIRM) that enables measuring of viscoelastic properties of materials near solid surface. An evanescent wave generated by a solid∕liquid interface in the TIRM is used as the incident light source in the microrheometer. When a probe particle (of a few micrometers diameter) moves near the interface, it can interact with the evanescent field and reflect its position with respect to the interface by the scattered light intensity. The exponential distance dependence of the evanescent field, on the one hand, makes this technique extremely sensitive to small changes from z-fluctuations of the probe (with a resolution of several nanometers), and on the other, it does not require imaging of the probe with high lateral resolution. Another distinct advantage is the high sensitivity in determining the z position of the probe in the absence of any labeling. The incorporated magnetic tweezers enable us to effectively manipulate the distance of the embedded particle from the interface either by a constant or an oscillatory force. The force ramp is easy to implement through a coil current ramp. In this way, the local viscous and elastic properties of a given system under different confinements can therefore be measured by resolving the near-surface particle motion. To test the feasibility of applying this microrheology to soft materials, we measured the viscoelastic properties of sucrose and poly(ethylene glycol) solutions and compared the results to bulk rheometry. In addition, we applied this technique in monitoring the structure and properties of deformable microgel particles near the flat surface.

Long-range interactions between protein-coated particles and POEGMA brush layers in a serum environment

Hydrophilic poly[oligo(ethylene glycol) methyl methacrylate] (POEGMA) brush layers with different thickness and graft densities were prepared by surface-initiated atom transfer radical polymerization (SI-ATRP) to construct a model surface to examine protein-surface interactions in a serum environment. The thickness of the POEGMA brush layers could be well controlled by the polymerization time and density of the immobilized initiators. The interactions between these brush-modified surfaces and the protein-coated polystyrene (PS) particles in newborn calf serum (NBCS) environment were then measured by total internal reflection microscopy (TIRM). In addition, protein adsorption properties onto the polymer brush surface layers were examined by atomic force microscopy (AFM). Relatively large amounts of protein adsorbed to short (4nm and 9nm-thick) POEGMA-coated surfaces or surfaces grafted with a low density of polymer chains. It was considered that shorter polymer chains or chains with low grafted density cannot fully cover the surfaces, proteins in serum could directly interact with the material surface and then deposited to form an adsorbed layer. The TIRM measurements showed that such adsorbed protein layer could mediate the interactions between the two surfaces by generating steric or bridging forces, resulting in different interaction potentials. Some particles were freely diffusing, some experienced intermittent diffusion and more than 50% of particles were irreversibly deposited to the surfaces covered by short polymer brushes. However, for longer (17 and 30nm-thick) POEGMA brush layer surfaces, material surface would be sufficiently covered by the dense coating and the first step of protein adsorption on surface was avoided. TIRM measurements showed that around 95% of the protein-coated particles could freely move in the serum and no attractive force between two surfaces was detected. The steric repulsion generated from the long POEGMA brush layer in the swollen state was long-range and strong so that the protein adsorption is very unlikely. These results concluded that the adsorbed protein layer on POEGMA surfaces plays an important role in regulating the interaction between protein-coated particles and POEGMA surfaces which are highly repellent toward protein adsorption.

Near-Surface Microrheology Reveals Dynamics and Viscoelasticity of Soft Matters

The development of experimental techniques able to probe the microscale viscoelastic properties of soft matter over a broad time scale is essential to uncover the physics that govern their behavior. Herein, we report the development of a microrheology technique that can determine the near-surface dynamics and viscoelastic behaviors of soft matter like polymer solution/gels and colloidal dispersions. Our approach combines a magnetic-field-induced stimulator with total internal reflection microscopy (TIRM) to apply mechanical loading (∼pN) to a micro-sized probe particle and capture its axial displacement near the surface with nano-scaled sensitivity. We demonstrate the use of this technique to measure the detachment of a colloid to a solid substrate and identify three quantitatively different regimes of mechanical coupling that differ in colloid-surface separation and interaction: exclusion, aging, and non-exclusion. We also apply it to study a physical gelation process of a volume-phase transition in thermosensitive microgels and a chemically cross-linked sol-gel transition of 4-arm star polymers by monitoring the evolution of complex modulus near solid surface with frequency, time, and separation distance. In contrast to passive microrheology techniques that rely on particle tracking, we can probe the viscoelastic behavior over five orders of magnitude in stiffness, from 10-3 to 102 Pa, providing excellent coverage for dynamics and heterogeneous samples. We expect this technique will stimulate the development of new experimental methods to explore the complex microscale rheology of macromolecular networks, soft materials, and living cytoplasm.