We demonstrate advanced fluid manipulations using magnetic polymeric artificial cilia on the walls of a microfluidic channel. In nature, cilia are little hairs covering the surface of micro-organisms which enable them to manipulate a fluid on the micro-scale. The asymmetric movement of natural cilia is crucial to obtain a net fluid flow. We have developed a ferromagnetic polymer made from iron nanoparticles and polydimethylsiloxane, and describe a process that can structure the material into high aspect ratio lying artificial cilia with a length of 300 microm. These artificial cilia were actuated with a homogeneous rotating magnetic field (micro(0)H < 50 mT) generated with a compact external electromagnet. An asymmetric movement involving torsion could be created when the cilia were provided with a remanent magnetisation perpendicular to the plane of rotation of the magnetic field vector. The artificial cilia could be actuated in fluid up to a frequency of approximately 50 Hz. In an aqueous solution in a microfluidic chamber we were able to generate rotational as well as translational fluid movements with fluid velocities up to approximately 0.5 mm s(-1).
We describe an optomagnetic bionanotechnology for rapid and sensitive solution-based affinity assays. Nanoactuators made from bioactive magnetic nanoparticles undergo rotational motion in the volume of a fluid under frequency-controlled magnetic actuation. The nanoactuators show a time-dependent scattering cross-section to an incoming light beam. We demonstrate that the temporal behavior of the scattered light intensity relates to the number, the magnetic properties and the size distribution of the nanoactuators, independently revealing the average value and variation in the magnetic properties of the nanoparticles as well as the concentration of nanoactuators. The method is applied to detect biomolecules in fluid by interparticle binding. In a total assay time of less than 3 min, we demonstrate a limit of detection lower than 400 fM in buffer and 5 pM in human plasma.
Biofunctionalized colloidal particles are widely used as labels in bioanalytical assays, lab-on-chip devices, biophysical research, and in studies on live biological systems. With detection resolution going down to the level of single particles and single molecules, understanding the nature of the interaction of the particles with surfaces and substrates becomes of paramount importance. Here, we present a comprehensive study of motion patterns of colloidal particles maintained in close proximity to a substrate by short molecular tethers (40 nm). The motion of the particles (500-1000 nm) was optically tracked with a very high localization accuracy (below 3 nm). A surprisingly large variation in motion patterns was observed, which can be attributed to properties of the particle-molecule-substrate system, namely the bond number, the nature of the bond, particle protrusions, and substrate nonuniformities. Experimentally observed motion patterns were compared to numerical Monte Carlo simulations, revealing a close correspondence between the observed motion patterns and properties of the molecular system. Particles bound via single tethers show distinct disc-, ring-, and bell-shaped motion patterns, where the ring- and bell-shaped patterns are caused by protrusions on the particle in the direct vicinity of the molecular attachment point. Double and triple tethered particles exhibit stripe-shaped and triangular-shaped motion patterns, respectively. The developed motion pattern analysis allows for discrimination between particles bound by different bond types, which opens the possibility to improve the limit of detection and the dynamic range of bioanalytical assays, with a projected increase of dynamic range by nearly 2 orders of magnitude.
In this article, we report on the development of a flow cell optimized for the heat‐transfer method, a versatile biosensing technique. The design of the flow cell ensures that the heat flow is focused with minimal heat loss through the surroundings of the cell. This results in a more stable measuring signal and an improved sensitivity of the measuring technique. The sensor was characterized by performing background measurements in air, water, and phosphate buffered saline (PBS) solution. Heat flow through the setup was simulated using COMSOL in order to provide insight in the contribution of convection to the heat flow and recommendations for possible future improvements to the cell. Additionally, a two‐step algorithm for calculating thermal resistance was defined, allowing the user to accurately derive thermal conductivity from experimental data. Finally, the potential of the flow cell for bacteria (Escherichia coli) detection was assessed and compared with the results obtained in the original HTM setup in a similar experiment. This experiment demonstrates that we were able to improve the limit‐of‐detection (LoD) to 2.10 × 104 colony forming units (CFU) mL−1 by changing the geometry of the measuring cell. Sensor setup for thermal biodetection experiments a directed heat flow.
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