Extending the limits of direct force measurements: colloidal probes from sub-micron particles

Hydrodynamic function and spring constant calibration of FluidFM micropipette cantilevers

Fluidic force microscopy (FluidFM) fuses the force sensitivity of atomic force microscopy with the manipulation capabilities of microfluidics by using microfabricated cantilevers with embedded fluidic channels. This innovation initiated new research and development directions in biology, biophysics, and material science. To acquire reliable and reproducible data, the calibration of the force sensor is crucial. Importantly, the hollow FluidFM cantilevers contain a row of parallel pillars inside a rectangular beam. The precise spring constant calibration of the internally structured cantilever is far from trivial, and existing methods generally assume simplifications that are not applicable to these special types of cantilevers. In addition, the Sader method, which is currently implemented by the FluidFM community, relies on the precise measurement of the quality factor, which renders the calibration of the spring constant sensitive to noise. In this study, the hydrodynamic function of these special types of hollow cantilevers was experimentally determined with different instruments. Based on the hydrodynamic function, a novel spring constant calibration method was adapted, which relied only on the two resonance frequencies of the cantilever, measured in air and in a liquid. Based on these results, our proposed method can be successfully used for the reliable, noise-free calibration of hollow FluidFM cantilevers.

Electrochemical grippers based on the tuning of surface forces for applications in micro- and nanorobotics

Existing approaches to robotic manipulation often rely on external mechanical devices, such as hydraulic and pneumatic devices or grippers. Both types of devices can be adapted to microrobots only with difficulties and for nanorobots not all. Here, we present a fundamentally different approach that is based on tuning the acting surface forces themselves rather than applying external forces by grippers. Tuning of forces is achieved by the electrochemical control of an electrode's diffuse layer. Such electrochemical grippers can be integrated directly into an atomic force microscope, allowing for 'pick and place' procedures typically used in macroscopic robotics. Due to the low potentials involved, small autonomous robots could as well be equipped with these electrochemical grippers that will be particularly useful in soft robotics as well as nanorobotics. Moreover, these grippers have no moving parts and can be incorporated in new concepts for actuators. The concept can easily be scaled down and applied to a wide range of objects, such as colloids, proteins, and macromolecules.

Cell-substratum and cell-cell adhesion forces and single-cell mechanical properties in mono- and multilayer assemblies from robotic fluidic force microscopy

The epithelium covers, protects, and actively regulates various formations and cavities of the human body. During embryonic development the assembly of the epithelium is crucial to the organoid formation, and the invasion of the epithelium is an essential step in cancer metastasis. Live cell mechanical properties and associated forces presumably play an important role in these biological processes. However, the direct measurement of cellular forces in a precise and high-throughput manner is still challenging. We studied the cellular adhesion maturation of epithelial Vero monolayers by measuring single-cell force-spectra with high-throughput fluidic force microscopy (robotic FluidFM). Vero cells were grown on gelatin-covered plates in different seeding concentrations, and cell detachment forces were recorded from the single-cell state, through clustered island formation, to their complete assembly into a sparse and then into a tight monolayer. A methodology was proposed to separate cell-substratum and cell-cell adhesion force and energy (work of adhesion) contributions based on the recorded force-distance curves. For comparison, cancerous HeLa cells were also measured in the same settings. During Vero monolayer formation, a significantly strengthening adhesive tendency was found, showing the development of cell-cell contacts. Interestingly, this type of step-by-step maturation was absent in HeLa cells. The attachment of cancerous HeLa cells to the assembled epithelial monolayers was also measured, proposing a new high-throughput method to investigate the biomechanics of cancer cell invasion. We found that HeLa cells adhere significantly stronger to the tight Vero monolayer than cells of the same origin. Moreover, the mechanical characteristics of Vero monolayers upon cancerous HeLa cell influence were recorded and analyzed. All these results provide insight into the qualitative assessment of cell-substratum and cell-cell mechanical contacts in mono- and multilayered assemblies and demonstrate the robustness and speed of the robotic FluidFM technology to reveal biomechanical properties of live cell assemblies with statistical significances.

Modulus characterization of cells with submicron colloidal probes by atomic force microscope

Colloidal probes have been increasingly demanded for the characterization of cellular modulus in atomic force microscope because of their well-defined geometry and large contact area with cell. In this work, submicron colloidal probes are prepared by scanning electron microscope/focused ion beam and compared with sharp tip and micron colloidal probe, in conjunction with loading velocity and indentation depth on the apparent elastic modulus. NIM and cartilage cells are used as specimens. The results show that modulus value measured by sharp tip changes significantly with loading velocity while remains almost stable by colloidal probes. Also, submicron colloidal probe is superior in characterizing the modulus with increasing indentation depth, which could help reveal the mechanical details of cellular membrane and the modulus of the whole cell. To test the submicron colloidal probe further, the modulus distribution map of cell is scanned with submicron colloidal probe of 50 nm radius during small and large indentation depths with high spatial resolution. The outcome of this work will provide the effective submicron colloidal probe according to the effect of loading velocity and indentation depth, characterizing the mechanical properties of the cells.

Nanonewton scale adhesion force measurements on biotinylated microbeads with a robotic micropipette

Binding force between biomolecules has a crucial role in most biological processes. Receptor-ligand interactions transmit physical forces and signals simultaneously. Previously, we employed a robotic micropipette both in live cell and microbead adhesion studies to explore the adhesion force of biomolecules such as cell surface receptors including specific integrins on immune cells. Here we apply standard computational fluid dynamics simulations to reveal the detailed physical background of the flow generated by the micropipette when probing microbead adhesion on functionalized surfaces. Measuring the aspiration pressure needed to pick up the biotinylated 10 μm beads on avidin coated surfaces and converting it to a hydrodynamic lifting force on the basis of simulations, we found an unbinding force of 12 ± 2 nN, when targeting the beads manually; robotic targeting resulted in 9 ± 4 nN (mean ± SD). We measured and simulated the effect of the targeting offset, when the microbead was out of the axis (off-axis)of the micropipette. According to the simulations, the higher offset resulted in a higher lifting force acting on the bead. Considering this effect, we could readily correct the impact of the targeting offset to renormalize the experimental data. Horizontal force and torque also appeared in simulations in case of a targeting offset. Surprisingly, simulations show that the lifting force acting on the bead reaches a maximum at a flow rate of ~ 5 μl/s if the targeting offset is not very high (<5 μm). Further increasing the flow rate decreases the lifting force. We attribute this effect to the spherical geometry of the bead. We predict that higher flow rates cannot increase the hydrodynamic lifting force acting on the precisely targeted microbead, setting a fundamental force limit (16 nN in our setup) for manipulating microbeads with a micropipette perpendicular to the supporting surface. In order to extend the force range, we propose the offset targeting of microbeads.

Inter-particle biomolecular reactivity tuned by surface crowders

The rate at which colloidal particles can form biomolecular bonds controls the kinetics of applications such as particle-based biosensing, targeted drug delivery and directed colloidal assembly. Here we study how the reactivity of the particle surface depends on its molecular composition, quantified by the inter-particle rate of aggregation in an optomagnetic cluster experiment. Particles were functionalized with DNA or with proteins for specific binding, and with polyethylene glycol as a passive surface crowder. The data show that the inter-particle binding kinetics are dominated by specific interactions, which surprisingly can be tuned by the passive crowder molecules for both the DNA and the protein system. The experimental results are interpreted using model simulations, which show that the crowder-induced decrease of the particle surface reactivity can be described as a reduced reactivity of the specific binder molecules on the particle surface.

A Sub-Micron Spherical Atomic Force Microscopic Tip for Surface Measurements

We report a novel methodology for fabricating a sub-micron spherical atomic force microscope (AFM) tip controllably-a silicon sub-micron sphere atop microcantilevers, which is desired for precise nanoscale tribology measurements, biological studies, and colloid science. Silicon sub-micron spheres are fabricated through swelling of single-crystal silicon with proper high-energy helium ion dosing, a traditionally undesired phenomenon known in helium ion microscopy. Silicon sub-micron spheres with diameters from 100 nm to 1 μm are demonstrated, and the placement of silicon sub-micron spheres can be as accurate as 10 nm or even below. This AFM tip demonstrates robust measurements during friction tests on graphene/silicon oxide substrates for more than 10 000 cycles. This AFM tip overcomes a critical challenge of reducing the size of spherical AFM tips from the micrometer scale to the sub-micron scale and is promising in cross-scale mechanics studies, nanotribology, colloid science, and biology.

Forces between solid surfaces in aqueous electrolyte solutions

This review addresses experimental findings obtained with direct force measurements between two similar or dissimilar solid surfaces in aqueous electrolyte solutions. Interpretation of these measurements is mainly put forward in terms of the classical theory of Derjaguin, Landau, Verwey, and Overbeek (DLVO). This theory invokes a superposition of attractive van der Waals forces and repulsive double layer forces. DLVO theory is shown to be extremely reliable, even in the case of multivalent ions. However, such a description is only successful, when appropriate surface charge densities, charge regulation characteristics, and ion pairing or complexation equilibria in solution are considered. Deviations from DLVO theory only manifest themselves at distances of typically below few nm. More long-ranged non-DLVO forces can be observed in some situations, particularly, in concentrated electrolyte solutions, in the presence of strongly adsorbed layers, or for hydrophobic surfaces. The latter forces probably originate from patch-charge surface heterogeneities, which can be induced by ion-ion correlation effects, charge fluctuations, or other types of surface heterogeneities.

Electrokinetics in Micro-channeled Cantilevers: Extending the Toolbox for Reversible Colloidal Probes and AFM-Based Nanofluidics

The combination of atomic force microscopy (AFM) with nanofluidics, also referred to as FluidFM, has facilitated new applications in scanning ion conductance microscopy, direct force measurements, lithography, or controlled nanoparticle deposition. An essential element of this new type of AFMs is its cantilever, which bears an internal micro-channel with a defined aperture at the end. Here, we present a new approach for in-situ characterization of the internal micro-channels, which is non-destructive and based on electrochemical methods. It allows for probing the internal environment of a micro-channeled cantilever and the corresponding aperture, respectively. Acquiring the streaming current in the micro-channel allows to determine not only the state of the aperture over a wide range of ionic strengths but also the surface chemistry of the cantilever’s internal channel. The high practical applicability of this method is demonstrated by detecting the aspiration of polymeric, inorganic and hydrogel particles with diameters ranging from several µm down to 300 nm. By verifying in-situ the state of the aperture, i.e. open versus closed, electrophysiological or nano-deposition experiments will be significantly facilitated. Moreover, our approach is of high significance for direct force measurements by the FluidFM-technique and sub-micron colloidal probes.

Single-Dimer Formation Rate Reveals Heterogeneous Particle Surface Reactivity

Biofunctionalized micro- and nanoparticles are important for a wide range of applications, but methodologies to measure, modulate, and model interactions between individual particles are scarce. Here, we describe a technique to measure the aggregation rate of two particles to a single dimer, by recording the trajectory that a particle follows on the surface of another particle as a function of time. The trajectory and the interparticle potential are controlled by a magnetic field. Particles were studied with and without conjugated antibodies in a wide range of pH conditions. The data shows that the aggregation process strongly depends on the particle surface charge density and hardly on the antibody surface coverage. Furthermore, microscopy videos of single particle dimers reveal the presence of reactive patches and thus heterogeneity in the particle surface reactivity. The aggregation rates measured with the single-dimer experiment are compared to data from an ensemble aggregation experiment. Quantitative agreement is obtained using a model that includes the influence of surface heterogeneity on particle aggregation. This single-dimer experiment clarifies how heterogeneities in particle reactivity play a role in colloidal stability.

The Next Generation of Colloidal Probes: A Universal Approach for Soft and Ultra-Small Particles

The colloidal probe technique, which is based on the atomic force microscope, revolutionizes direct force measurements in many fields, such as interface science or biomechanics. It allows for the first time to determine interaction forces on the single particle or cell level. However, for many applications, important "blind spots" remain, namely, the possibility to probe interaction potentials for nanoparticles or complex colloids with a soft outer shell. Definitely, these are colloidal systems that are currently of major industrial importance and interest from theory. The here-presented novel approach allows for overcome the aforementioned limitations. Its applicability has been demonstrated for 300 nm sized carboxylate-modified latex particles as well as sub-micron core-shell particles with a soft poly-N-isopropylacrylamide hydrogel shell and a rigid silica core. For the latter, which until now cannot be studied by the colloidal probe technique, determined is the temperature dependency of electrosteric and adhesion forces has been determined on the single particle level.

Spring constant and sensitivity calibration of FluidFM micropipette cantilevers for force spectroscopy measurements

The fluidic force microscope (FluidFM) can be considered as the nanofluidic extension of the atomic force microscope (AFM). This novel instrument facilitates the experimental procedure and data acquisition of force spectroscopy (FS) and is also used for the determination of single-cell adhesion forces (SCFS) and elasticity. FluidFM uses special probes with an integrated nanochannel inside the cantilevers supported by parallel rows of pillars. However, little is known about how the properties of these hollow cantilevers affect the most important parameters which directly scale the obtained spectroscopic data: the inverse optical lever sensitivity (InvOLS) and the spring constant (k). The precise determination of these parameters during calibration is essential in order to gain reliable, comparable and consistent results with SCFS. Demonstrated by our literature survey, the standard error of previously published SCFS results obtained with FluidFM ranges from 11.8% to 50%. The question arises whether this can be accounted for biological diversity or may be the consequence of improper calibration. Thus the aim of our work was to investigate the calibration accuracy of these parameters and their dependence on: (1) the aperture size (2, 4 and 8 µm) of the hollow micropipette type cantilever; (2) the position of the laser spot on the back of the cantilever; (3) the substrate used for calibration (silicon or polystyrene). It was found that both the obtained InvOLS and spring constant values depend significantly on the position of the laser spot. Apart from the theoretically expectable monotonous increase in InvOLS (from the tip to the base of the cantilever, as functions of the laser spot's position), we discerned a well-defined and reproducible fluctuation, which can be as high as ±30%, regardless of the used aperture size or substrate. The calibration of spring constant also showed an error in the range of -13/+20%, measured at the first 40 µm of the cantilever. Based on our results a calibration strategy is proposed and the optimal laser position which yields the most reliable spring constant values was determined and found to be on the first pair of pillars. Our proposed method helps in reducing the error introduced via improper calibration and thus increases the reliability of subsequent cell adhesion force or elasticity measurements with FluidFM.

Measuring Inner Layer Capacitance with the Colloidal Probe Technique

The colloidal probe technique was used to measure the inner layer capacitance of an electrical double layer. In particular, the forces were measured between silica surfaces and sulfate latex surfaces in solutions of monovalent salts of different alkali metals. The force profiles were interpreted with Poisson-Boltzmann theory with charge regulation, whereby the diffuse layer potential and the regulation properties of the interface were obtained. While the diffuse layer potential was measured in this fashion in the past, we are able to extract the regulation properties of the inner layer, in particular, its capacitance. We find systematic trends with the type of alkali metal ion and the salt concentration. The observed trends could be caused by difference in ion hydration, variation of the binding capacitance, and changes of the effective dielectric constant within the Stern layer. Our results are in agreement with recent experiments involving the water-silica interface based on a completely independent method using X-ray photoelectron spectroscopy in a liquid microjet. This agreement confirms the validity of our approach, which further provides a means to probe other types of interfaces than silica.