Atomic Force Microscopy (AFM) is a promising tool to measure dissociation rates and binding distances in the single-molecule regime. However, interpretation of the obtained rupture data can be challenging. The unbinding process between two complex molecules is often characterized by a spectrum of barrier heights that cannot be parametrized by a single barrier height. In standard analysis, different barriers can be discerned only by measuring over at least 4-5 orders of magnitude of the pulling force rate, which can be difficult to achieve with AFM. Here, we used fits of the full rupture force histograms to address this complexity. We found that multiple unbinding paths can be discerned from measurements at just a few force rates using this approach. We also show how multiple rupture events - situations where multiple bonds breaking simultaneously appear as single rupture events - can be addressed in the analysis.

Microbial fouling involves the physicochemical interactions between microorganisms and solid surfaces. An electromagnetic field (EMF) may change the diffusion rates of microbial cells and the electrical double layer around the cells and contacting surfaces. In the current study, polycardanol exhibiting antibiofouling activity was modified with ferromagnetic iron oxide (IO) to investigate the EMF effects on bacterial adhesion. When there was a flow of electrolyte that contained bacterial cells, flow-induced EMF was generated according to Faraday's principle. It was observed that the IO-ionic solution (IS)-modified surfaces, with an induced current of 44, 53, 66 nA, showed decreases in the adhesion of bacteria cells more than the unmodified (polycardanol) and IO-nanoparticles-modified ones. In addition to the EMF effects, the nano-scale uniform roughness of the modified surfaces appeared to play an important role in the reduction of cell adhesion. The results demonstrated that the IOIS-modified surface (3.2 x 10-6 mM IO) had the highest antibiofouling activity.

We present methods for making and testing the membrane biophysics of model lipid droplets (LDs). Methods are described for imaging LDs ranging in size from 0.1 to 40 μm in diameter with high-resolution microscopy and spectroscopy. With known LD compositions, membrane binding, sorting, diffusion, and tension were measured via fluorescence correlation spectroscopy (FCS), fluorescence recovery after photobleaching (FRAP), fluorescence lifetime imaging microscopy (FLIM), atomic force microscopy (AFM), and imaging flow cytometry. Additionally, a custom, small-volume pendant droplet tensiometer is described and used to measure the association of phospholipids to the LD surface. These complementary, cross-validating methods of measuring LD membrane behavior reveal the interplay of biophysical processes on lipid droplet monolayers.

We utilized the momentum transfer (Q)-dependence of quasi-elastic neutron scattering (QENS) to measure the dynamics of water and ethanol confined in graphene oxide (GO) powder or membranes at different temperatures and in different orientations. We found reduced diffusivities (up to 30% in the case of water) and a depression of dynamic transition temperatures. While water showed near Arrhenius behavior with an almost bulk-like activation barrier in a temperature range of 280-310 K, the diffusivity of ethanol showed little temperature dependence. For both water and ethanol, we found evidence for immobile and mobile fractions of the confined liquid. The mobile fraction exhibited jump diffusion, with a jump length consistent with the expected average spacing of hydroxide groups in the GO surfaces. From anisotropy measurements, we found weak anisotropy in the diffusivity of the mobile species and in the fraction and geometry of immobile species.

Aiming to develop a new class of metallosurfactants with unidirectional electron transfer properties, a (terpyridine) ruthenium complex containing a semiquinone derivative L2, namely [RuIII(Lterpy)(L2)Cl]PF6 (1), was synthesized and structurally characterized as a solid and in solution. The electronic and redox behaviour of 1 was studied experimentally as well as by means of DFT methods, and is indicative of significant orbital mixing and overlap between metal and ligands. The complex forms stable Pockels-Langmuir films at the air-water interface and allows for the formation of thin films onto gold electrodes to prepare nanoscale Au|LB 1|Au junctions for current-voltage (I/V) analysis. Complex 1 shows asymmetric electron transfer with a maximum rectification ratio of 32 based on tunnelling through MOs of the aminocatechol derivative.

Background: Discoidin Domain Receptors (DDRs) are membrane-tethered proteins of the receptor tyrosine kinase family, which signal in response to collagen. DDR expression is associated with human diseases, including fibrosis and cancer. The role of DDRs in human pathogenesis is mediated by dysregulated receptor function in response to the collagenous milieu. Thus, understanding DDR-collagen interactions is important for developing novel therapeutic strategies against DDRs. Methods: We developed a biophysical method to isolate and measure specific interactions between DDR1 and collagen in live cells at the single molecule level using atomic force microscopy. This new method is capable of providing density and kinetics of membrane receptors in live cells. Results: We isolated DDR1-collagen interactions and quantified the association and dissociation rates of the DDR1-collagen I complex. We estimated separate binding probabilities of collagen I to DDR and integrin, and by combining kinetic and binding probability data, we were able to estimate the density of receptors in two cancer cell types. We also tested the viability of a DDR1 blocking antibody and determined its efficacy in suppressing DDR1-collagen binding. Conclusions: The new method shows promise in quantifying receptor-ligand kinetics and receptor density on live cells. General significance: The new approach is applicable to other receptor-ligand systems and allows the determination of critical parameters at the single cell/single molecule level - in particular, the direct determination of kinetic and density differences of receptors in different cell types. This capability should prove to be useful in cancer research and drug design.

Using an ultra-sensitive home-built atomic force microscope, we have studied the dynamic mechanical responses of pure water, 1 M NaCl, and 1 M CsCl aqueous solutions to understand the effects of ions on the viscoelastic properties of nanoconfined ( ≤ 1 nm) water films. In 1 M NaCl, we observed peaks in the Maxwell relaxation time, indicating a solid-like, elastic response due to jamming (dynamic solidification) during squeeze-out. NaCl also extended the range of ordering of water molecules further away from the mica surface up to 4–5 molecular layers ( ∼ 1 nm). By contrast, in 1 M CsCl, the relaxation time peaks were suppressed, even at high compression speeds, indicating a more liquid-like, viscous response. The addition of NaCl significantly increases the probability of the nanoconfined water film to react elastically in response to compression, while 1 M CsCl decreases the probability of water layers to show an elastic response. Our measurements support the notion that Na + acts as a kosmotrope (order enhancer) and Cs + as a chaotrope (order destroyer), directly influencing the hydration structure of water, and altering the mechanical response of the nanoconfined liquid layers to compression and squeeze-out.

Nanoconfined water plays an important role in many areas of research such as oil recovery or molecular biology. However, the mechanics and dynamics of nanoconfined water are not well understood. Using an ultra-sensitive homebuilt atomic force microscope, we have shown that pure water film (ρ = 18 MΩ-cm) exhibits squeeze rate dependent viscoelastic properties, and a solid-like behavior at a squeeze rate ≥ 0.8 nm/s, as manifested by an increased Maxwell’s relaxation time (Phys. Rev. Lett. 2010). The introduction of NaCl to the film enhances this tendency by showing the solid-like behavior even at a lower squeeze rate of 0.2 nm/s, as well as extending the range of ordering of water molecules along a mica surface (Langmuir 2016). Here, we will discuss these results as well as the changes introduced by another alkali salt (CsCl) to form a collective picture of the role played by alkali salts in shaping the dynamic mechanical response of nanoconfined water film.

Charged surfaces and electrolytes play an important role in many scientific and technological applications such as molecular friction, transport through bio-membranes, and generation of electrochemical energy. Using a home-built ultra-sensitive atomic force microscope, we have measured the stiffness, damping coefficient, and Maxwell’s relaxation time of a few molecular layers thin film of electrolytic water against atomically smooth mica surface. Water containing NaCl and CsCl have been compared with ultrapure water. The viscoelastic properties of the nanoconfined film exhibit a strong dependence on the compression rate of the film as well as the type of electrolyte present. The increase in Maxwell’s relaxation time as a function of NaCl concentration gives rise to a solidlike dynamic mechanical response under quasi-static conditions (compression rate = 0.2 nm/s). In contrast, the CsCl containing electrolytic water seems to suppress the solidlike dynamic mechanical behavior, which results from an apparent jamming of the molecules during a squeeze-out, as suggested elsewhere (Khan et al., Phys. Rev. Lett. 2010). These results suggest that the size and hydration number of ions play an important role in nanoconfined water.

Polymeric nanoparticles have been studied for gene and drug delivery. These nanoparticles can be modified to utilize a targeted delivery approach to selectively deliver their payload to specific cells, while avoiding unwanted delivery to healthy cells. One commonly over-expressed receptor which can be targeted by ligand-conjugated nanoparticles is the folate receptor alpha (FRα). The ability to target FRα remains a promising concept, and therefore, understanding the binding dynamics of the receptor with the ligand of the nanoparticle therapeutic can provide valuable insight. This manuscript focuses on the interaction between self-assembled nanoparticles decorated with a folic acid (FA) ligand and FRα. The nanoparticles consist of micelles formed with a FA conjugated triblock copolymer (PEI-g-PCL-b-PEG-FA) which condensed siRNA to form micelleplexes. By combining biological and biophysical approaches, this manuscript explores the binding kinetics and force of the targeted siRNA containing nanoparticles to FRα in comparison with free FA. We demonstrate via flow cytometry and atomic force microscopy that multivalent micelleplexes bind to FRα with a higher binding probability and binding force than monovalent FA. Furthermore, we revisited why competitive inhibition studies of binding of multivalent nanoparticles to their respective receptor are often reported in literature to be inconclusive evidence of effective receptor targeting. In conclusion, the results presented in this paper suggest that multivalent targeted nanoparticles display strong receptor binding that a monovalent ligand may not be able to compete with under in vitro conditions and that high concentrations of competing monovalent ligands can lead to measurement artifacts.

From oil recovery to molecular biology, nanoconfined water plays important role in many areas of research. However, the mechanics and dynamics of nanoconfined water are not well understood. Over the last ten years, a number of groups have measured the mechanics of confined water using atomic force microscopy (AFM) or surface forces apparatus (SFA) – often with contradictory results. We use a high resolution AFM to measure the mechanics and dynamics of nanoconfined liquids. We have shown that water shows a dynamic “solidification” when confined to a few layers (Phys. Rev. Lett. 2010), a finding that explains the contradictory findings in earlier measurements and points to surprisingly complex behavior in this seemingly simple system. We also discovered an unusual relationship of viscous damping of nanoconfined liquids on the lateral size of the confinement region (App. Phys. Lett. 2014). Here we will review these findings, as well as present new findings such as Young’s modulus of nanoconfined liquids. Also, we will show the profound effect of ion concentration on the dynamics of water at the interface.

Molecular motors are the workhorses of living cells. Seemingly by ‘magic’, these molecules are able to complete purposeful tasks while being immersed in a sea of thermal chaos. Here, we review the current understanding of how these machines work, present simple models based on thermal ratchets, discuss implications for statistical physics, and provide an overview of ongoing research in this important and fascinating field of study.

Purpose:Discoidin domain receptors (DDR) have recently been recognized as important players in cancer progression. DDRs are cell receptors that interact with collagen, an extracellular matrix (ECM) protein. However the detailed mechanism of their interaction is unclear. Here we attempted to examine their interaction in terms of structural (surface topography), mechanical (rupture force), and kinetic (binding probability) information on the single molecular scale with the use of atomic force microscopy (AFM).Methods:The Quantitative Nano‐mechanical property Mapping (QNM) mode of AFM allowed to assess the cells in liquid growth media at their optimal physiological while being viable. Human benign prostate hyperplasia (BPH‐1) cell line was genetically regulated to suppress DDR expression (DDR‐ cells) and was compared with naturally DDR expressing cells (DDR+).Results:Binding force measurements (n = 1000) were obtained before and after the two groups were treated with fibronectin (FN), an integrin‐inhibiting antibody to block the binding of integrin. The quantification indicates that cells containing DDR bind with collagen at a most probable force of 80.3–83.0 ±7.6 pN. The probability of them binding is 0.167 when other interactions (mainly due to integrin‐collagen binding) are minimized.Conclusion:Together with further force measurements at different pulling speeds will determine dissociation rate, binding distance and activation barrier. These parameters in benign cells provides some groundwork in understanding DDR's behavior in various cell microenvironments such as in malignant tumor cells.Funding supported by Richard Barber Interdisciplinary Research Program of Wayne State University

In material science, bioengineering, and biology, thin liquid films and soft matter membranes play an important role in micro-lubrication, ion transport, and fundamental biological processes. Various attempts have been made to characterize the elastic properties, such as Young's modulus, of such films using Hertz theory by incorporating convoluted mathematical corrections. We propose a simple way to extract tip-size independent elastic properties based on stiffness and force measurement through a spherical tip on a flat surface. Using our model, the Young's modulus of nanoconfined, molecularly-thin, layers of a model liquid TEHOS (tetrakis 2-ethylhexoxy silane) and water were determined using a small-amplitude AFM. This AFM can simultaneously measure the stiffness and forces of nanoscale films. While the stiffness scales linearly with the tip radius, the measured Young's modulus essentially remains constant over an order of magnitude variation in the tip radius. The values obtained for the elastic modulus of TEHOS and water films on the basis of our method are significantly lower than the confining surfaces' elastic moduli, in contrast with the uncorrected Hertz model, suggesting that our method can serve as a simple way to compare elastic properties of nanoscale thin films as well as to characterize a variety of soft films. In addition, our results show that the elastic properties (elastic modulus) of nanoconfined liquid films remain fairly independent of increasing confinement.

Understanding the dynamics of water under nanoscale confinement is important for biology, geology, tribology, and nanotechnology. In many naturally occurring situations, ions are present in water at various concentrations. Here we report on how the addition of sodium ions alters the squeeze-out behavior of water nanoconfined between a mica surface and silicon oxide tip. We find that Na+ ions enhance molecular ordering and lead to longer mechanical relaxation times. We also observed a critical ion concentration, above which the confined water switches from a viscous to an elastic (solid-like) response at very slow, quasistatic compression speeds.