Three-dimensional magneto-optic trap for micro-object manipulation

Force spectroscopy with electromagnetic tweezers

Force spectroscopy using magnetic tweezers (MTs) is a powerful method to probe the physical characteristics of single polymers. Typically, molecules are functionalized for specific attachment to a glass surface at one end and a micrometer-scale paramagnetic bead at the other end. By applying an external magnetic field, multiple molecules can be stretched and twisted simultaneously without exposure to potentially damaging radiation. The majority of MTs utilize mobile, permanent magnets to produce forces on the beads (and the molecule under test). However, translating and rotating the permanent magnets may require expensive precision actuators, limit the rate at which force can be changed, and may induce vibrations that disturb tether dynamics and bead tracking. Alternatively, the magnetic field can be produced with an electromagnet, which allows fast force modulation and eliminates motor-associated vibration. Here, we describe a low-cost quadrapolar electromagnetic tweezer design capable of manipulating DNA-tethered MyOne paramagnetic beads with forces as high as 15 pN. The solid-state nature of the generated B-field modulated along two axes is convenient for accessing the range of forces and torques relevant for studying the activity of DNA motor enzymes like polymerases and helicases. Our design specifically leverages technology available at an increasing number of university maker spaces and student-run machine shops. Thus, it is an accessible tool for undergraduate education that is applicable to a wide range of biophysical research questions.

Optically controlled fiber-optic micro-gripper for sub-millimeter objects

A miniature, fully optically controlled, dielectric, opto-thermally actuated tweezer/micro-gripper that is suitable for the manipulation of small objects is presented. The tweezer/micro-gripper is formed at the tip of an optical fiber and utilizes a mid-power laser diode for its actuation. The manipulation of small objects such as short pieces of optical fibers is demonstrated. Small dimensions, fully dielectric design, non-electric actuation, remote operation through the fiber, and good harsh environment compatibility (chemical, radiation, and temperature) might provide opportunities for micromanipulation in a system and areas where current solutions are inadequate.

Rotating magnetic particles for lab-on-chip applications - a comprehensive review

Magnetic particles are widely used in lab-on-chip and biosensing applications, because they have a high surface-to-volume ratio, they can be actuated with magnetic fields and many biofunctionalization options are available. The most well-known actuation method is to apply a magnetic field gradient which generates a translational force on the particles and allows separation of the particles from a suspension. A more recently developed magnetic actuation method is to exert torque on magnetic particles by a rotating magnetic field. Rotational actuation can be achieved with a field that is uniform in space and it allows for a precise control of torque, orientation, and angular velocity of magnetic particles in lab-on-chip devices. A wide range of studies have been performed with rotating MPs, demonstrating fluid mixing, concentration determination of biological molecules in solution, and characterization of structure and function of biomolecules at the single-molecule level. In this paper we give a comprehensive review of the historical development of MP rotation studies, including configurations for field generation, physical model descriptions, and biological applications. We conclude by sketching the scientific and technological developments that can be expected in the future in the field of rotating magnetic particles for lab-on-chip applications.

Optical Trapping and Manipulation of Superparamagnetic Beads Using Annular-Shaped Beams

We propose an optical tweezers setup based on an annular-shaped laser beam that is efficient to trap 2.8 μm-diameter superparamagnetic particles. The optical trapping of such particles was fully characterized, and a direct absolute comparison with a geometrical optics model was performed. With this comparison, we were able to show that light absorption by the superparamagnetic particles is negligible for our annular beam tweezers, differing from the case of conventional Gaussian beam tweezers, in which laser absorption by the beads makes stable trapping difficult. In addition, the trap stiffness of the annular beam tweezers increases with the laser power and with the bead distance from the coverslip surface. While this first result is expected and similar to that achieved for conventional Gaussian tweezers, which use ordinary dielectric beads, the second result is quite surprising and different from the ordinary case, suggesting that spherical aberration is much less important in our annular beam geometry. The results obtained here provide new insights into the development of hybrid optomagnetic tweezers, which can apply simultaneously optical and magnetic forces on the same particles.

Mechanical determination of particle–cell interactions and the associated biomedical applications

Mechanical determination of particle–cell interactions and the associated biomedical applications.

Characterization of Individual Magnetic Nanoparticles in Solution by Double Nanohole Optical Tweezers

We study individual superparamagnetic Fe3O4 (magnetite) nanoparticles in solution using a double nanohole optical tweezer with magnetic force setup. By analysis of the trapping optical transmission signal (step size, autocorrelation, the root-mean-square signal, and the distribution with applied magnetic field), we are able to measure the refractive index, magnetic susceptibility, remanence and size of each trapped nanoparticle. The size distribution is found to agree well with scanning electron microscopy measurements, and the permeability, magnetic susceptibility and remanence values are all in agreement with published results. Our approach demonstrates the versatility of the optical tweezer with magnetic field setup to characterize nanoparticles in fluidic mixtures with potential for isolation of desired particles and pick-and-place functionality.

Detection of Brownian Torque in a Magnetically-Driven Rotating Microsystem

Thermal fluctuations significantly affect the behavior of microscale systems rotating in shear flow, such as microvortexes, microbubbles, rotating micromotors, microactuators and other elements of lab-on-a-chip devices. The influence of Brownian torque on the motion of individual magnetic microparticles in a rotating magnetic field is experimentally determined using optical tweezers. Rotational Brownian motion induces the flattening of the breakdown transition between the synchronous and asynchronous modes of microparticle rotation. The experimental findings regarding microparticle rotation in the presence of Brownian torque are compared with the results of numerical Brownian dynamics simulations.

Optical trapping performance of dielectric-metallic patchy particles

We demonstrate a series of simulation experiments examining the optical trapping behavior of composite micro-particles consisting of a small metallic patch on a spherical dielectric bead. A full parameter space of patch shapes, based on current state of the art manufacturing techniques, and optical properties of the metallic film stack is examined. Stable trapping locations and optical trap stiffness of these particles are determined based on the particle design and potential particle design optimizations are discussed. A final test is performed examining the ability to incorporate these composite particles with standard optical trap metrology technologies.

Optomagnetically Controlled Microparticles Manufactured with Glancing Angle Deposition

Optical trapping and magnetic trapping are common micro-manipulation techniques for controlling colloids including micro- and nano-particles. Combining these two manipulation strategies allows for a larger range of applied forces and decoupled control of rotation and translation; each of which are beneficial properties for many applications including force spectroscopy and advanced manufacturing. However, optical trapping and magnetic trapping have conflicting material requirements inhibiting the combination of these methodologies. In this paper, anisotropic micron scaled particles capable of being simultaneously controlled by optical and magnetic trapping are synthesized using a glancing angle deposition (GLAD) technique. The anisotropic alignment of dielectric and ferromagnetic materials limits the optical scattering from the metallic components which typically prevents stable optical trapping in three dimensions. Compared to the current state of the art, the benefits of this approach are two-fold. First, the composite structure allows for larger volumes of ferromagnetic material so that larger magnetic moments may be applied without inhibiting the stability of optical trapping. Secondly, the robustness of the synthesis process is greatly improved. The dual optical and magnetic functionality of the synthesized colloids is demonstrated by simultaneously optically translating and magnetically rotating a magnetic GLAD particle using a custom designed opto-magnetic trapping system.

Nanomechanical force transducers for biomolecular and intracellular measurements: is there room to shrink and why do it?

Over the past couple of decades there has been a tremendous amount of progress on the development of ultrasensitive nanomechanical instruments, which has enabled scientists to peer for the first time into the mechanical world of biomolecular systems. Currently, work-horse instruments such as the atomic force microscope and optical/magnetic tweezers have provided the resolution necessary to extract quantitative force data from various molecular systems down to the femtonewton range, but it remains difficult to access the intracellular environment with these analytical tools as they have fairly large sizes and complicated feedback systems. This review is focused on highlighting some of the major milestones and discoveries in the field of biomolecular mechanics that have been made possible by the development of advanced atomic force microscope and tweezer techniques as well as on introducing emerging state-of-the-art nanomechanical force transducers that are addressing the size limitations presented by these standard tools. We will first briefly cover the basic setup and operation of these instruments, and then focus heavily on summarizing advances in in vitro force studies at both the molecular and cellular level. The last part of this review will include strategies for shrinking down the size of force transducers and provide insight into why this may be important for gaining a more complete understanding of cellular activity and function.

Nickel foil microcantilevers for magnetic manipulation and localized heating

Cellular manipulation has been investigated by a number of techniques. In this manuscript nickel foil microcantilevers were used for magnetophoresis and manipulation of microparticles and magnetically labeled HeLa cells. The cantilevers were also used for localized heating in liquid, reaching biologically relevant temperatures. This work aims to develop cantilevers for sample enrichment, manipulation, and thermal applications, offering an inexpensive and versatile solution compatible with standard tools in research and clinical diagnostic testing, such as microwell plates.

Fabrication of nanoassemblies using flow control

Synthetic nanostructures, such as nanoparticles and nanowires, can serve as modular building blocks for integrated nanoscale systems. We demonstrate a microfluidic approach for positioning, orienting, and assembling such nanostructures into nanoassemblies. We use flow control combined with a cross-linking photoresist to position and immobilize nanostructures in desired positions and orientations. Immobilized nanostructures can serve as pivots, barriers, and guides for precise placement of subsequent nanostructures.

Optical Methods to Study Protein-DNA Interactions in Vitro and in Living Cells at the Single-Molecule Level

The maintenance of intact genetic information, as well as the deployment of transcription for specific sets of genes, critically rely on a family of proteins interacting with DNA and recognizing specific sequences or features. The mechanisms by which these proteins search for target DNA are the subject of intense investigations employing a variety of methods in biology. A large interest in these processes stems from the faster-than-diffusion association rates, explained in current models by a combination of 3D and 1D diffusion. Here, we present a review of the single-molecule approaches at the forefront of the study of protein-DNA interaction dynamics and target search in vitro and in vivo. Flow stretch, optical and magnetic manipulation, single fluorophore detection and localization as well as combinations of different methods are described and the results obtained with these techniques are discussed in the framework of the current facilitated diffusion model.

Trapping and rotating microparticles and bacteria with moiré-based optical propelling beams

We propose and demonstrate trapping and rotation of microparticles and biological samples with a moiré-based rotating optical tweezers. We show that polystyrene beads, as well as Escherichia coli cells, can be rotated with ease, while the speed and direction of rotation are fully controllable by a computer, obviating mechanical movement or phase-sensitive interference. Furthermore, we demonstrate experimentally the generation of white-light propelling beams and arrays, and discuss the possibility of optical tweezing and particle micro-manipulation based on incoherent white-light rotating patterns.

Cellular strategies for regulating DNA supercoiling: a single-molecule perspective

Entangling and twisting of cellular DNA (i.e., supercoiling) are problems inherent to the helical structure of double-stranded DNA. Supercoiling affects transcription, DNA replication, and chromosomal segregation. Consequently the cell must fine-tune supercoiling to optimize these key processes. Here, we summarize how supercoiling is generated and review experimental and theoretical insights into supercoil relaxation. We distinguish between the passive dissipation of supercoils by diffusion and the active removal of supercoils by topoisomerase enzymes. We also review single-molecule studies that elucidate the timescales and mechanisms of supercoil removal.

Single-molecule manipulation reveals supercoiling-dependent modulation of lac repressor-mediated DNA looping

Gene expression regulation is a fundamental biological process which deploys specific sets of genomic information depending on physiological or environmental conditions. Several transcription factors (including lac repressor, LacI) are present in the cell at very low copy number and increase their local concentration by binding to multiple sites on DNA and looping the intervening sequence. In this work, we employ single-molecule manipulation to experimentally address the role of DNA supercoiling in the dynamics and stability of LacI-mediated DNA looping. We performed measurements over a range of degrees of supercoiling between -0.026 and +0.026, in the absence of axial stretching forces. A supercoiling-dependent modulation of the lifetimes of both the looped and unlooped states was observed. Our experiments also provide evidence for multiple structural conformations of the LacI-DNA complex, depending on torsional constraints. The supercoiling-dependent modulation demonstrated here adds an important element to the model of the lac operon. In fact, the complex network of proteins acting on the DNA in a living cell constantly modifies its topological and mechanical properties: our observations demonstrate the possibility of establishing a signaling pathway from factors affecting DNA supercoiling to transcription factors responsible for the regulation of specific sets of genes.

Fast dynamics of supercoiled DNA revealed by single-molecule experiments

The dynamics of supercoiled DNA play an important role in various cellular processes such as transcription and replication that involve DNA supercoiling. We present experiments that enhance our understanding of these dynamics by measuring the intrinsic response of single DNA molecules to sudden changes in tension or torsion. The observed dynamics can be accurately described by quasistatic models, independent of the degree of supercoiling initially present in the molecules. In particular, the dynamics are not affected by the continuous removal of the plectonemes. These results set an upper bound on the hydrodynamic drag opposing plectoneme removal, and thus provide a quantitative baseline for the dynamics of bare DNA.

Magneto-optical tweezers built around an inverted microscope

We present a simple experimental setup of magneto-optical tweezers built around an inverted microscope. Two pairs of coils placed around the focal point of the objective generate a planar-rotating magnetic field that is perpendicular to the stretching direction. This configuration allows us to control the rotary movement of a paramagnetic bead trapped in the optical tweezers. The mechanical design is universal and can be simply adapted to any inverted microscope and objective. The mechanical configuration permits the use of a rather large experimental cell and the simple assembly and disassembly of the magnetic attachment.

Exploring molecular motors and switches at the single-molecule level

Single-molecule techniques have propelled an impressive number of biophysical studies during the last decade. From relatively simple video-microscopy techniques, to sophisticated manipulation and detection apparata, single-molecule techniques are capable of tracking the movements and the reaction trajectories of single enzymatic units. By observing microspheres attached to biomolecules it is possible to follow the motion of molecular motors, or to detect conformational "switching" induced by regulatory proteins. Micromanipulation tools like optical tweezers have been widely applied to understand the mechanisms of linear molecular motors, and have allowed the measurement of the elementary steps and the forces produced by several motor proteins, including myosin, kinesin, and dynein. New experimental assays based on magnetic or optical "wrenches," which are able to apply and detect torques on rotary motors and biopolymers, are opening new possibilities in this field. Here, established and emerging magneto-optical manipulation and video-tracking techniques are reviewed, in the perspective of single molecular motors and regulatory proteins studies.

Micro magnetic tweezers for nanomanipulation inside live cells

This study reports the design, realization, and characterization of a multi-pole magnetic tweezers that enables us to maneuver small magnetic probes inside living cells. So far, magnetic tweezers can be divided into two categories: I), tweezers that allow the exertion of high forces but consist of only one or two poles and therefore are capable of only exerting forces in one direction; and II), tweezers that consist of multiple poles and allow exertion of forces in multiple directions but at very low forces. The magnetic tweezers described here combines both aspects in a single apparatus: high forces in a controllable direction. To this end, micron scale magnetic structures are fabricated using cleanroom technologies. With these tweezers, magnetic flux gradients of nablaB = 8 x 10(3) T m(-1) can be achieved over the dimensions of a single cell. This allows exertion of forces up to 12 pN on paramagnetic probes with a diameter of 350 nm, enabling us to maneuver them through the cytoplasm of a living cell. It is expected that with the current tweezers, picoNewton forces can be exerted on beads as small as 100 nm.

High-precision measurements of light-induced torque on absorbing microspheres

Laser beams have been demonstrated to be capable of exerting torque as well as forces on microparticles. Using a custom magneto-optic manipulator, we directly measured the torque exerted by laser light on absorbing microspheres as a result of the transfer of spin angular momentum. A general method for measuring torque has been developed, and the experimental apparatus has shown a sensitivity of approximately 1 pN/nm.

Optical torque wrench: angular trapping, rotation, and torque detection of quartz microparticles

We describe an apparatus that can measure the instantaneous angular displacement and torque applied to a quartz particle which is angularly trapped. Torque is measured by detecting the change in angular momentum of the transmitted trap beam. The rotational Brownian motion of the trapped particle and its power spectral density are used to determine the angular trap stiffness. The apparatus features a feedback control that clamps torque or other rotational quantities. The torque sensitivity demonstrated is ideal for the study of known biological molecular motors.

Near-field photonic forces

A review of recent advancements in photonic forces is presented. We discuss in detail the interaction of light and sub-wavelength particles on a substrate illuminated by total internal reflection, and we study the optical forces experienced by the particles. The effects of plasmon-mode excitations on the resulting photonic forces on metallic particles are also addressed. Moreover, we explore the possibility of using the metallic tip of a classical apertureless microscope to create optical tweezers, and thus to achieve a selective manipulation of nanoparticles.

Direct Manipulation and Observation of the Rotational Motion of Single Optically Trapped Microparticles and Biological Cells in Microvortices

This paper describes a method for manipulating and monitoring the rotational motion of single, optically trapped microparticles and living cells in a microvortex. To induce rotation, we placed the microparticle at the center of rotation of the vortex and used the recirculating fluid flow to drive rotation. We have monitored the rotation of single beads (which ranged in diameter from a few micrometers to tens of micrometers) and living cells in a microvortex. To follow the rotation of a smooth and symmetrically shaped bead, we first ablated a small region ( approximately 1 microm) on the bead. An Ar(+) laser was then tightly focused ( approximately 0.5-microm spot size) onto the bead, and rotation was tracked by recording changes in the level of backscattered laser light as the ablated region repeatedly transited the laser focus. Using this method, we have followed bead rotation that varied in frequency from 0.15 to 100 Hz and have studied the effect of bead diameter on the rate of rotation at a given fluid flow rate. To monitor the rotation of single living cells, we selectively stained portions of B-lymphocytes with the fluorescent dye DiOC(6). We observed rotation by following changes in the fluorescence signal as the dye-stained region transited the laser focal volume. This technique provides a simple and sensitive method for controlling and monitoring the rotational motion of microparticles in a microfluidic environment.

Coupled electrorotation of polymer microspheres for microfluidic sensing and mixing

We show that coupled electrorotation (CER) of microscopic particles using microfabricated electrodes can be used for localized sensing and mixing. The effective use of microelectromechanical systems and micro total analysis systems requires many types of control. These include the abilityto (1) manipulate objects within microchannels by noncontact means, (2) mix fluids, and (3) sense local chemical parameters. Coupled electrorotation, in which the interactions between induced electric dipoles of adjacent particles lead to particle rotation, addresses aspects of all three challenges simultaneously. CER is a simple means of controlling the rotation of dielectric objects using homogeneous external radio frequency electric fields. CER is sensitive to several chemical and physical parameters such as the solution conductivity, pH, and viscosity. As a step toward integrating CER devices into microfluidic systems, a simple chip was designed to induce local mixing and to detect local changes in salt concentration, pH, and viscosity.