A Alexander van Reenen

Accelerated particle-based target capture--the roles of volume transport and near-surface alignment.

The upcoming generations of high-sensitive and miniaturized biosensing systems need target capture methods that are as efficient and as rapid as possible, with targets ranging from molecules to cells. Capture of the targets can be achieved using particles coated with affinity molecules, but there are still fundamental questions as to the processes that limit the association rates. In this paper we quantify and compare the reaction rates of particle-based target capture with different types of actuation, namely (i) passive thermal transport, (ii) fluid agitation by vortex mixing, and (iii) actively rotating particles. In the experiments, we use fluorescent nanoparticles as targets which are biochemically captured by magnetic microparticles, and the capture efficiency is quantified using fluorescence microscopy with single target resolution. The data unravel the contributions of volume transport, near-surface alignment, and the chemical reaction to the overall rate constant of association. Vortex mixing versus passive transport gives an increase of the reaction rate constant by more than an order of magnitude, implying that the encounter frequency as well as the near-surface alignment probability are increased. The importance of near-surface alignment is underscored by the data of active particle rotation; the binding probability per encounter is 4-fold enhanced on rotating capture particles. We discuss the implications of our results for different biological systems and for the development of novel actuation methods in particle-based target capture.

Accurate quantification of magnetic particle properties by intra-pair magnetophoresis for nanobiotechnology

The application of magnetic particles in biomedical research and in-vitro diagnostics requires accurate characterization of their magnetic properties, with single-particle resolution and good statistics. Here, we report intra-pair magnetophoresis as a method to accurately quantify the field-dependent magnetic moments of magnetic particles and to rapidly generate histograms of the magnetic moments with good statistics. We demonstrate our method with particles of different sizes and from different sources, with a measurement precision of a few percent. We expect that intra-pair magnetophoresis will be a powerful tool for the characterization and improvement of particles for the upcoming field of particle-based nanobiotechnology.

How Actuated Particles Effectively Capture Biomolecular Targets

Because of their high surface-to-volume ratio and adaptable surface functionalization, particles are widely used in bioanalytical methods to capture molecular targets. In this article, a comprehensive study is reported of the effectiveness of protein capture by actuated magnetic particles. Association rate constants are quantified in experiments as well as in Brownian dynamics simulations for different particle actuation configurations. The data reveal how the association rate depends on the particle velocity, particle density, and particle assembly characteristics. Interestingly, single particles appear to exhibit target depletion zones near their surface, caused by the high density of capture molecules. The depletion effects are even more limiting in cases with high particle densities. The depletion effects are overcome and protein capture rates are enhanced by applying dynamic particle actuation, resulting in an increase in the association rate constants by up to 2 orders of magnitude.

Magnetic particle actuation in stationary microfluidics for integrated lab-on-chip biosensors

The aging population and increases in chronic diseases put high pressure on the healthcare system, which drives a need for easy-to-use and cost-effective medical technologies. In-vitro diagnostics (IVD) plays a large role in delivering healthcare and, within the IVD market, decentralized diagnostic testing, i.e. point-of-care testing (POCT), is a growing segment. POCT devices should be compact and fully integrated for maximum ease of use. A new class of POCT technologies is appearing based on actuated magnetic particles. The use of magnetic particles has important advantages: they have a large surface-to-volume ratio, are conveniently biofunctionalized, provide a large optical contrast, and can be manipulated by magnetic fields. In this chapter, we review the use of magnetic particles actuated by magnetic fields to realize integrated lab-on-chip diagnostic devices wherein several assay process steps are combined, e.g. to mix fluids, capture analytes, concentrate analytes, transfer analytes, label analytes, and perform stringency steps. We focus on realizations within the concept of stationary microfluidics and we discuss efforts to integrate different magnetically actuated assay steps, with the vision that it will become possible to realize biosensing systems in which all assay process steps are controlled and optimized by magnetic forces.

Correction to Accelerated Particle-Based Target Capture—The Roles of Volume Transport and Near-Surface Alignment

• A submitted manuscript is the version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. People interested in the research are advised to contact the author for the final version of the publication, or visit the DOI to the publishers website. • The final author version and the galley proof are versions of the publication after peer review. • The final published version features the final layout of the paper including the volume, issue and page numbers.

Translation and rotation dynamics in a magnetic-label biosensor

Magnetic particles are used in magnetic-label biosensors to accelerate molecular binding to the sensor surface as well as to apply stringency by magnetic forces [1]. The biochemical and physical interactions of the particles with the biosensor surface play a key role in the molecular association and dissociation processes. In this paper we quantify the translation and rotation dynamics of particles at a sensor surface, interacting with the surface by nucleic-acids or protein molecules. We apply magnetic fields to actuate the particles and investigate their dynamics with single-particle resolution.We will present measurements on the 3-dimensional mobility of 500 nm particles that are biologically bound to a biosensor surface, recorded using evanescent field microscopy with millisecond time resolution [2]. Our data show that the position and intensity histograms scale systematically with the length of the captured nucleic-acid analyte molecules and with the magnitude of the applied magnetic field.We also present measurements on the rotation dynamics of protein-coated particles in a rotating magnetic field [3]. We demonstrate that a controlled torque is generated by the magnetic particles, which is used to quantify the rotation behavior and torsion stiffness of proteins captured onto the sensor surface by the magnetic particles. The data show that different protein pairs have distinctly different torsion moduli.[1] D. M. Bruls et al., Lab Chip 9 (2009) 3504.[2] K. van Ommering et al., J. Phys. D: Appl. Phys. 43 (2010) 155501 & 385501.[3] X.J.A. Janssen et al., submitted to the Biophysical Journal (2010).