Single-beam Gradient Optical Trap Research Articles

Optical Trap Loading of Dielectric Microparticles In Air.

We demonstrate a method to trap a selected dielectric microparticle in air using radiation pressure from a single-beam gradient optical trap. Randomly scattered dielectric microparticles adhered to a glass substrate are momentarily detached using ultrasonic vibrations generated by a piezoelectric transducer (PZT). Then, the optical beam focused on a selected particle lifts it up to the optical trap while the vibrationally excited microparticles fall back to the substrate. A particle may be trapped at the nominal focus of the trapping beam or at a position above the focus (referred to here as the levitation position) where gravity provides the restoring force. After the measurement, the trapped particle can be placed at a desired position on the substrate in a controlled manner. In this protocol, an experimental procedure for selective optical trap loading in air is outlined. First, the experimental setup is briefly introduced. Second, the design and fabrication of a PZT holder and a sample enclosure are illustrated in detail. The optical trap loading of a selected microparticle is then demonstrated with step-by-step instructions including sample preparation, launching into the trap, and use of electrostatic force to excite particle motion in the trap and measure charge. Finally, we present recorded particle trajectories of Brownian and ballistic motions of a trapped microparticle in air. These trajectories can be used to measure stiffness or to verify optical alignment through time domain and frequency domain analysis. Selective trap loading enables optical tweezers to track a particle and its changes over repeated trap loadings in a reversible manner, thereby enabling studies of particle-surface interaction.

Multibuilding Block Janus Synthesized by Seed-Mediated Self-Assembly for Enhanced Photothermal Effects and Colored Brownian Motion in an Optical Trap.

The asymmetrical features and unique properties of multibuilding block Janus nanostructures (JNSs) provide superior functions for biomedical applications. However, their production process is very challenging. This problem has hampered the progress of JNS research and the exploration of their applications. In this study, an asymmetrical multibuilding block gold/iron oxide JNS has been generated to enhance photothermal effects and display colored Brownian motion in an optical trap. JNS is formed by seed-mediated self-assembly of nanoparticle-loaded thermocleavable micelles, where the hydrophobic backbones of the polymer are disrupted at high temperatures, resulting in secondary self-assembly and structural rearrangement. The JNS significantly enhances photothermal effects compared to their homogeneous counterpart after near-infrared (NIR) light irradiation. The asymmetrical distribution of gold and iron oxide within JNS also generates uneven thermophoretic force to display active colored Brownian rotational motion in a single-beam gradient optical trap. These properties indicate that the asymmetrical JNS could be employed as a strong photothermal therapy mediator and a fuel-free nanoscale Janus motor under NIR light.

Improved calibration of the nonlinear regime of a single-beam gradient optical trap.

We report an improved method for calibrating the nonlinear region of a single-beam gradient optical trap. Through analysis of the position fluctuations of a trapped object that is displaced from the trap center by controlled flow we measure the local trap stiffness in both the linear and nonlinear regimes without knowledge of the magnitude of the applied external forces. This approach requires only knowledge of the system temperature, and is especially useful for measurements involving trapped objects of unknown size, or objects in a fluid of unknown viscosity.

The initial single yeast cell adhesion on glass via optical trapping and Derjaguin–Landau–Verwey–Overbeek predictions

We used an optical tweezer to investigate the adhesion of yeast Saccharomyces cerevisiae onto a glass substrate at the initial contact. Micromanipulation of free-living objects with single-beam gradient optical trap enabled to highlight mechanisms involved in this initial contact. As a function of the ionic strength and with a displacement parallel to the glass surface, the yeast adheres following different successive ways: (i) Slipping and rolling at 1.5 mM NaCl, (ii) slipping, rolling, and sticking at 15 mM NaCl, and (iii) only sticking at 150 mM. These observations were numerous and reproducible. A kinetic evolution of these adhesion phenomena during yeast movement was clearly established. The nature, range, and relative intensity of forces involved in these different adhesion mechanisms have been worked out as a quantitative analysis from Derjaguin-Landau-Verwey-Overbeek (DLVO) and extended DLVO theories. Calculations show that the adhesion mechanisms observed and their affinity with ionic strength were mainly governed by the Lifshitz-van der Waals interaction forces and the electrical double-layer repulsion to which are added specific contact forces linked to "sticky" glycoprotein secretion, considered to be the main forces capable of overcoming the short-range Lewis acid-base repulsions.

Escape trajectories of single-beam optically trapped micro-particles in a transverse fluid flow

We have studied the transverse and axial equilibrium positions of dielectric micro-spheres trapped in a single-beam gradient optical trap and exposed to an increasing fluid flow transverse to the trapping beam axis. It is demonstrated that the axial equilibrium position of a trapped micro-sphere is a function of its transverse position in the trapping beam. Moreover, although the applied drag-force acts perpendicularly to the beam axis, reaching a certain distance r(0) from the beam axis (r(0)/a approximately 0.6, a being the sphere radius) the particle escapes the trap due to a breaking axial equilibrium before the actual maximum transverse trapping force is reached. The comparison between a theoretical model and the measurements shows that neglecting these axial equilibrium considerations leads to a theoretical overestimation in the maximal optical transverse trapping forces of up to 50%.

Direct measurement of interaction forces between a single bacterium and a flat plate

A technique for precisely measuring the equilibrium and viscous interaction forces between a single bacterium and a flat surface as functions of separation distance is described. A single-beam gradient optical trap was used to micromanipulate the bacterium against a flat surface while evanescent wave light scattering was used to measure separation distances. Calibrating the optical trap far from the surface allowed the trapped bacterium to be used as a force probe. Equilibrium force–distance profiles were determined by measuring the deflection of the cell from the center of the optical trap at various trap positions. Simultaneously, viscous forces were determined by measuring the relaxation time for the fluctuating bacterium. Absolute distances were determined using a best-fit approximation to the theoretical prediction for the hindered mobility of a diffusing sphere near a wall. Using this approach, forces in the range from 0.01 to 4 pN were measured at near-nanometer resolution between Staphylococcus aureus and glass that was bare or coated with adsorbed protein.

Direct Measurement of Static and Dynamic Forces between a Colloidal Particle and a Flat Surface Using a Single-Beam Gradient Optical Trap and Evanescent Wave Light Scattering

A new measurement technique is described for the simultaneous measurement of static and dynamic interactions between a micron-sized colloidal particle and a flat surface. The technique uses a single-beam gradient optical trap as a sensitive force transducer and evanescent wave light scattering to precisely measure the particle position within the trap. The static force is determined from the deflection of the particle position from the trap center, and the viscous force is measured from the relaxation time of the particle fluctuations near the equilibrium position. Each force contribution is measured as a function of the particle−surface separation distance by scanning the particle toward the surface. Absolute separation distances are determined by curve fitting the viscous force data to hydrodynamic theory in regions where the static force is negligible. The static force data were found to agree well with Derjaguin−Landau−Verwey−Overbeek theory over the entire range of separation distances using 1.0 and 1.5 μm silica spheres in solutions of NaCl. The viscous force data obeyed hydrodynamic theory well until there was an appreciable overlap of the double layers at close separations. This departure from theory is likely due to electroviscous phenomena that enhance the effective drag coefficient of the particle as it moves normal to the flat plate. We also observed light interference effects as the trap focus was placed near the solid−liquid interface. A simple method was found to reduce this effect sufficiently to yield accurate force−distance profiles.

Quantitative measurements of force and displacement using an optical trap

We combined a single-beam gradient optical trap with a high-resolution photodiode position detector to show that an optical trap can be used to make quantitative measurements of nanometer displacements and piconewton forces with millisecond resolution. When an external force is applied to a micron-sized bead held by an optical trap, the bead is displaced from the center of the trap by an amount proportional to the applied force. When the applied force is changed rapidly, the rise time of the displacement is on the millisecond time scale, and thus a trapped bead can be used as a force transducer. The performance can be enhanced by a feedback circuit so that the position of the trap moves by means of acousto-optic modulators to exert a force equal and opposite to the external force applied to the bead. In this case the position of the trap can be used to measure the applied force. We consider parameters of the trapped bead such as stiffness and response time as a function of bead diameter and laser beam power and compare the results with recent ray-optic calculations.

Measurement of the Optical Force and Trapping Range of a Single-beam Gradient Optical Trap for Micron-sized Latex Spheres

Abstract A single-beam gradient optical trap was constructed using a 20 mW 632·8 nm He–Ne laser coupled to an optical microscope. Latex spheres were trapped in water at the focal point of a tightly-focused laser beam, which was generated using a 100 × objective. The efficiency of the trap was evaluated by determining the maximum speeds at which the trapped particles could be manipulated. Typical maximum speeds of tens of microns per second were recorded, at the maximum trapping power of 6·7 mW. The effective transverse trapping range for 1–7 μm diameter latex spheres was measured to be 1–3 μm, and the maximum transverse optical force on 1–12 μm diameter latex spheres varied in the range 0·4–4·5 pN.