Tweezers Setup Research Articles

Optical twin-vortex multi-trapping by Kolakoski lenses

In this work, we design and implement a new bifocal diffractive spiral lens within an optical tweezers system. The proposed diffractive optical element coined Kolokaski Kinoform Spiral Lens (KKSL), generates twin optical vortices along the propagation direction. The axial positions, as well as the diameters of the generated vortex beams, are correlated with the Kolakoski aperiodic sequence introduced in the design of the diffractive lens. The unique properties of KKSLs make it ideal for optical trapping applications, allowing independent multiple trapping and particle displacement within each optical vortex beam. We present both the focusing properties of KKSL and the experimental results of its integration into the optical tweezer setup.

Comparative Evaluation of Interaction Force Characteristics for the Lipopolysaccharide of Yersinia pseudotuberculosis and Antibodies by Optical Trapping and Atomic Force Microscopy

Optical tweezers and atomic force microscopy were used for comparative evaluation of the interaction force between the lipopolysaccharide of Yersinia pseudotuberculosis and monoclonal antibodies. This paper discusses the peculiarities of two methods which allow determining significant differences in the values of the measured force required to rupture the interaction of probe sensitized by lipopolysaccharide (polystyrene microsphere for optical tweezers and silicon nitride cantilever for atomic force microscopy) with substrate (glass and mica, respectively) covered with monoclonal antibodies. In atomic force microscopy, the cantilever slides along the substrate for some time after the piezo stage is brought to a stop, causing changes in the spatial structure of sensitins and, therefore, redistribution of multiple bonds between the lipopolysaccharide agglomerate and antibodies. No significant displacement of the microsphere along the substrate occurs when an optical tweezers setup is used, and, unlike atomic force microscopy, the breaking of a complex bond between lipopolysaccharide and complementary antibodies is recorded in the form of a single and short-term (1–2 ms) leap of the photodetector signal employing optical tweezers. The recorded values of the force required to rupture the interaction measured by both methods are relative and vary depending on the chosen experimental conditions. It is shown that the non-specific component of the force needed to break the interaction measured by atomic force microscopy is significantly higher than that determined with optical tweezers.

In-situ aberration correction for Laguerre-Gaussian optical tweezers via optimization of orbit shape.

We introduce a concept of aberration correction under microscopy that is based on observation of circular Brownian motion of an object driven by orbital angular momentum of a Laguerre-Gaussian (LG) beam. Following the concept, we establish an aberration-correction scheme by using a holographic optical tweezers setup equipped with a spatial light modulator that produces the LG beam as well as corrects the light wavefront. The light wavefront is modified adaptively to improve circular symmetry and uniformity of the orbit of a colloidal dielectric sphere revolving in mid-water under the irradiation of the LG beam. We reveal that the proposed scheme is sensitive to tiny phase difference of less than the accuracy of a highest-grade optical flat, 0.05λ, and is applicable to aberrations of up to the first 21 terms of the Zernike series expansion. The scheme not only improves the quality of optical tweezers but also enables to distinguish individual objective lenses assigned a common product code from difference in aberration-correction patterns. The present contribution therefore provides a useful tool for microscopy and laser fabrication in addition to the immediate application to optical trapping.

Nanoscopic wetting behaviour of single oil droplets on a fibre

Multiphase fluids that contain dispersed oil droplets are fundamental to many industrial processes and formulated products. In the present work, the surface wetting and interaction between an emulsion droplet and a single fibre were quantitatively investigated as a function of surfactant chemistry, pH, and salt concentration using an optical tweezers apparatus. Silicone droplets were emulsified in water and stabilised by surfactants. An individual droplet was captured using an optical tweezers setup to allow a controlled contact with a fibre surface. During the approach process, the surface interaction was measured and spreading behaviour was quantified after contact was made between the droplet and fibre. Salt concentration was increased to change the surface interaction behaviour. The pH and ζ-potential of silicone emulsions decreased from 9.5 to 6.0 and from 39 mV to − 15 mV, respectively, when increasing salt concentration for both of the surfactants studied, SDS and CTAB. The attraction between the fibre and a silicone oil droplet was found to increase as the electrostatic repulsion was suppressed with an increased salt concentration. This was confirmed by an increased number of droplet-fibre adhesion events and in some cases droplet spreading on the fibre. Such observations for adhesion are based on electrostatic attractions exceeding the maximum force that can be exerted by the trapping laser. When adhesion was not observed, attraction could still be recorded through a thin-film hydrodynamic suction effect during the process of retraction, which becomes more pronounced on the polyester fibre. Our results provide a novel method to directly quantify the surface interaction and wetting of liquid droplets on microscopic fibres simultaneously, which could be valuable when investigating formulated products that involve emulsion droplets.

Portable fiber-based double nanohole optical tweezer for trapping small proteins

We demonstrate the trapping and analysis of individual proteins using a portable optical fiber tweezer setup with a double-nanohole in a gold film coating the fiber’s end and aligned with the fiber core. The instrument was used to trap and analyze cytochrome c, carbonic anhydrase, bovine serum albumin, and PR65 (wild-type and various point mutants). This approach was compared with a free-space optical tweezer setup that requires alignment of the laser focus to the aperture, whereas the fiber-based approach is both portable and alignment-free, which holds promise for applications in antibody discovery, small molecule drug discovery, protein interaction analysis and other applications using the standard well-plate format.

Autonomous and intelligent optical tweezers for improving the reliability and throughput of single particle analysis

Optical tweezers are an interesting tool to enable single cell analysis, especially when coupled with optical sensing and advanced computational methods. Nevertheless, such approaches are still hindered by system operation variability, and reduced amount of data, resulting in performance degradation when addressing new data sets. In this manuscript, we describe the deployment of an automatic and intelligent optical tweezers setup, capable of trapping, manipulating, and analyzing the physical properties of individual microscopic particles in an automatic and autonomous manner, at a rate of 4 particle per min, without user intervention. Reproducibility of particle identification with the help of machine learning algorithms is tested both for manual and automatic operation. The forward scattered signal of the trapped PMMA and PS particles was acquired over two days and used to train and test models based on the random forest classifier. With manual operation the system could initially distinguish between PMMA and PS with 90% accuracy. However, when using test datasets acquired on a different day it suffered a loss of accuracy around 24%. On the other hand, the automatic system could classify four types of particles with 79% accuracy maintaining performance (around 1% variation) even when tested with different datasets. Overall, the automated system shows an increased reproducibility and stability of the acquired signals allowing for the confirmation of the proportionality relationship expected between the particle size and its friction coefficient. These results demonstrate that this approach may support the development of future systems with increased throughput and reliability, for biosciences applications.

Introduction to Optical Tweezers: Background, System Designs, and Applications.

Optical tweezers are a means to manipulate objects with light. With the technique, microscopically small objects can be held and steered, allowing for accurate measurement of the forces applied to these objects. Optical tweezers can typically obtain a nanometer spatial resolution, a picoNewton force resolution, and a millisecond time resolution, which makes the technique well suited for the study of biological processes from the single-cell down to the single-molecule level. In this chapter, we aim to provide an introduction to the use of optical tweezers for single-molecule analyses. We start from the basic principles and methodology involved in optical trapping, force calibration, and force measurements. Next, we describe the components of an optical tweezers setup and their experimental relevance. Finally, we will provide an overview of the broad applications in context of biological research, with the emphasis on the measurement modes, experimental assays, and possible combinations with fluorescence microscopy techniques.

Multiplexed vortex beam-based optical tweezers generated with spiral phase mask

The design and implementation of a multiplexed spiral phase mask in an experimental optical tweezers setup are presented. This diffractive optical element allows the generation of multiple concentric vortex beams with independent topological charges and without amplitude modulation. The generalization of the phase mask for multiple concentric vortices is also shown. The design for a phase mask of two multiplexed vortices with different topological charges is developed. We experimentally show the transfer of angular momentum to the optically trapped microparticles by enabling nearly independent orbiting dynamics around the optical axis within each vortex. The angular velocity of the confined particles versus the optical power in the focal region is also discussed for different combinations of topological charges.

Cellular Cargo Manipulation Using Magnetically Steerable Microrobots in Dense Environments.

This study presents a microrobotic system that utilizes magnetic Janus microrobots and a 3D-printed magnetic tweezers setup controlled by electromagnetic coils to transport cells in a densely crowded environment. The system was successfully demonstrated to transport cells in a densely populated sample of cells. The results indicate that this microrobotic system could address challenges such as off-target delivery, thereby realizing the full potential of medical microrobots for this and other applications. This study represents an important step towards developing a more efficient and effective method for targeted drug delivery in the field of medicine.

Exploring the binding of mitoxantrone to DNA using optical tweezers.

Mitoxantrone (MTX) is a synthetic anti-cancer drug that displays intercalation binding mode with DNA. It has been widely studied using conventional methods, but not using single molecule techniques which can provide molecular level details of the interactions. Here we explored the binding of MTX with DNA using optical tweezers. Our optical tweezers set-up uses two counter propagating lasers to trap and manipulate a single DNA molecule that is stretched to measure the force response as a function of extension. The DNA stretching was repeated in the presence of various nano-molar concentrations of MTX at different pulling rates. Stretching at much slower pulling rates exhibited closer to binding equilibrium, indicating MTX is a slow binder with the time scale of seconds compared to classical intercalators which have microsecond binding kinetics. This will provide more detail about the binding mechanism of MTX that can be used to better understand the binding of MTX and also develop similar drug designs.

Towards real-time identification of trapped particles with UMAP-based classifiers

Optical trapping provides a way to isolate, manipulate, and probe a wide range of microscopic particles. Moreover, as particle dynamics are strongly affected by their shape and composition, optical tweezers can also be used to identify and classify particles, paving the way for multiple applications such as intelligent microfluidic devices for personalized medicine purposes, or integrated sensing for bioengineering. In this work, we explore the possibility of using properties of the forward scattered radiation of the optical trapping beam to analyze properties of the trapped specimen and deploy an autonomous classification algorithm. For this purpose, we process the signal in the Fourier domain and apply a dimensionality reduction technique using UMAP algorithms, before using the reduced number of features to feed standard machine learning algorithms such as K-nearest neighbors or random forests. Using a stratified 5-fold cross-validation procedure, our results show that the implemented classification strategy allows the identification of particle material with accuracies up to 80%, demonstrating the potential of using signal processing techniques to probe properties of optical trapped particles based on the forward scattered light. Furthermore, preliminary results of an autonomous implementation in a standard experimental optical tweezers setup show similar differentiation capabilities for real-time applications, thus opening some opportunities towards technological applications such as intelligent microfluidic devices and solutions for biochemical and biophysical sensing.

Creation of rotating spiral structures using interfering Bessel beams for optical manipulation

A simple and effective optical tweezer setup which uses the interference of two counter propagating Bessel beams is proposed and investigated. The potential energy of a Rayleigh particle in the generated interference patterns is numerically evaluated by writing a MATLAB code. A rigorous analytical treatment is also performed for verification of the numerical results. The results show that the generated patterns have a spiral structure. It is also found that these spiral patterns and consequently the trapped particles can be rotated about the axis of the patterns by introducing a phase difference between the two beams. The suggested setup is hoped to provide an effective way for guiding particles along the spiral trajectories, making a laser-driven micro- and nanomotor, measuring the torsional properties of biological molecules, and separating particles from a liquid mixture according to their mass, size, or refractive index.

Simultaneous Three-Dimensional Velocity And Temperature Measurement In Microfluidics Using An LED And A Double-frame Camera

With astigmatism particle tracking velocimetry combined with luminescence lifetime imaging, the temperature and all velocity components of spherical trace particles in a volume of fluid can be simultaneously measured with one optical access. Generally, a pulsed laser is required for this measurement technique. However, it may cause additional heating of a small volume of stagnant fluid, as often present in microfluidics. Here we show that this problem can be avoided by replacing the pulsed laser with an LED. To compensate for the lower power provided by the LED, the illumination time and the exposure time for the image acquisition were vastly extended. In addition, we replaced the typically used high-speed camera with an ordinary double-frame camera. With these modifications, the temperature of the fluid is unaffected, while low uncertainties on all the measured quantities can be achieved. Temperature can be measured with an uncertainty of ∼1°C with a varying location. With a varying temperature covering the range of 20-50°C, measurement uncertainties were found to be lower than 0.2 μm for x- and y-coordinates and lower than 1.1 μm for the z-coordinates for the current setup. The system was applied to measure the location and temperature of particles trapped in an acoustic tweezer setup. No heating on the fluid can be observed using the new technique. Therefore, the heating of the particles could be solely related to the acoustic field. The measurement technique may serve as a powerful tool to investigate the thermodynamic and fluid dynamic phenomena in microfluidic environments. The lower cost on the hardware and the comparable or even lower measurement uncertainties on the measured quantities are further advantages.

Single Molecular Chelation Dynamics Reveals That DNA Has a Stronger Affinity toward Lead(II) than Cadmium(II)

Lead ions can bind to DNA via nonelectrostatic interactions and hence alter its structure, which may be related to their adverse effects. The dynamics of Pb2+-DNA interaction has not been well understood. In this study, we report the monomolecular dynamics of the Pb2+-DNA interaction using a magnetic tweezers (MT) setup. We found that lead cations could induce DNA compaction at ionic strengths above 1 μM, which was also confirmed by morphology characterization. The chelation behavior of the Pb2+-DNA and the Cd2+-DNA complex solutions after adding EDTA were compared. The results showed that EDTA chelated with the bound metal ions on DNA and consequently led to restoring the DNA to its original length but with different restoration speeds for the two solutions. The fast binding dynamics and the slower chelation dynamics of the Pb2+ scenario compared to that of Cd2+ suggested that Pb2+ was more capable to induce DNA conformational change and that the Pb2+-DNA complex was more stable than the Cd2+-DNA complex. The stronger affinities for DNA bases and the inner binding of lead cations were two possible causes of the dynamics differences. Three agents, including EDTA, sodium gluconate, and SDBS, were used to remove the bound lead ions on DNA. It was shown that EDTA was the most efficient, and sodium gluconate could not fully restore DNA from its compact state. We concluded that both EDTA and SDBS were good candidates to restore the Pb2+-bound DNA to its original state.

Upconversion, MRI imaging and optical trapping studies of silver nanoparticle decorated multifunctional NaGdF4:Yb,Er nanocomposite

The multifunctional upconversion nanoparticles (UCNPs) are fascinating tool for biological applications. In the present work, photon upconverting NaGdF4:Yb,Er and Ag nanoparticles decorated NaGdF4:Yb,Er (NaGdF4:Yb,Er@Ag) nanoparticles were prepared using a simple polyol process. Rietveld refinement was performed for detailed crystal structural and phase fraction analysis. The morphology of the NaGdF4:Yb,Er@Ag was examined using high-resolution transmission electron microscope, which reveals silver nanoparticles of 8 nm in size were decorated over spherical shaped NaGdF4:Yb,Er nanoparticles with a mean particle size of 90 nm. The chemical compositions were confirmed by EDAX and inductively coupled plasma-optical emission spectrometry analyses. The upconversion luminescence (UCL) of NaGdF4:Yb,Er at 980 nm excitation showed an intense red emission. After incorporating the silver nanoparticles, the UCL intensity decreased due to weak scattering and surface plasmon resonance effect. The VSM magnetic measurement indicates both the UCNPs possess paramagnetic behaviour. The NaGdF4:Yb,Er@Ag showed computed tomography imaging. Magnetic resonance imaging study exhibited better T1 weighted relaxivity in the NaGdF4:Yb,Er than the commercial Gd-DOTA. For the first time, the optical trapping was successfully demonstrated for the upconversion NaGdF4:Yb,Er nanoparticle at near-infrared 980 nm light using an optical tweezer setup. The optically trapped UCNP possessing paramagnetic property exhibited a good optical trapping stiffness. The UCL of trapped single UCNP is recorded to explore the effect of the silver nanoparticles. The multifunctional properties for the NaGdF4:Yb,Er@Ag nanoparticle are demonstrated.

Modification to axial tracking for mobile magnetic microspheres

Three-dimensional particle tracking is a routine experimental procedure for various biophysical applications including magnetic tweezers. A common method for tracking the axial position of particles involves the analysis of diffraction rings whose pattern depends sensitively on the axial position of the bead relative to the focal plane. To infer the axial position, the observed rings are compared with reference images of a bead at known axial positions. Often the precision or accuracy of these algorithms is measured on immobilized beads over a limited axial range, whereas many experiments are performed using freely mobile beads. This inconsistency raises the possibility of incorrect estimates of experimental uncertainty. By manipulating magnetic beads in a bidirectional magnetic tweezer setup, we evaluated the error associated with tracking mobile magnetic beads and found that the error of tracking a moving magnetic bead increases by almost an order of magnitude compared with the error of tracking a stationary bead. We found that this additional error can be ameliorated by excluding the center-most region of the diffraction ring pattern from tracking analysis. Evaluation of the limitations of a tracking algorithm is essential for understanding the error associated with a measurement. These findings promise to bring increased resolution to three-dimensional bead tracking of magnetic microspheres.

Upcoming flow promotes the bundle formation of bacterial flagella

Flagellated bacteria swim by rotating a bundle of helical flagella and commonly explore the surrounding environment in a “run-and-tumble” motility mode. Here, we show that the upcoming flow could impact the bacterial run-and-tumble behavior by affecting the formation and dispersal of the flagellar bundle. Using a dual optical tweezers setup to trap individual bacteria, we characterized the effects of the imposed fluid flow and cell body rotation on the run-and-tumble behavior. We found that the two factors affect the behavior differently, with the imposed fluid flow increasing the running time and decreasing the tumbling time and the cell body rotation decreasing the tumbling time only. Using numerical simulations, we computed the flagellar bundling time as a function of flow velocity, which agrees well with our experimental observations. The mechanical effects we characterized here provide novel, to our knowledge, ingredients for further studies of bacterial chemotaxis in complex environments such as dynamic fluid environments.

Microsphere-coupled optical tweezers.

In this Letter, we study the optical trapping of particles in a focal spot engineered by a combination of a dielectric microsphere and the conventional optical tweezers setup. The dielectric microsphere is placed in the laser path before the focal spot, and its impact on the trapping stiffness is theoretically and experimentally studied in detail. The suggested method for considering the geometric phase shift, due to the presence of the microsphere, into the Debye diffraction integral shows a good agreement with the experiment. We stably trap particles as small as 350 nm in the microsphere-coupled optical trap using a low numerical aperture objective (NA=0.7), while in a conventional optical tweezers setup, it requires at least an NA=1.1. Moreover, the results show that choosing an appropriate microsphere and depth of trapping can enhance the trapping efficiency in the axial direction by a factor of ∼3.8.

Profiling selective binding to promoter CpG islands by a single-DNA mechanical footprinting assay

Profiling selective binding to promoter CpG islands by a single-DNA mechanical footprinting assay

Bessel beam optical tweezers for manipulating superparamagnetic beads.

We propose a Bessel beam optical tweezers setup that can stably trap superparamagnetic beads. The trap stiffness measured is practically independent of the radius of the Bessel beam and of the bead height (distance from the coverlip of the sample chamber), indicating that the beads can be trapped with high accuracy within a wide range of such parameters. On the other hand, the trap stiffness exhibits the expected linear increase with the laser power, despite the non-negligible absorption coefficient of the superparamagnetic beads. A geometrical optics model that considers spherical aberration and light absorption by the beads was used to predict the optical forces and trap stiffness, showing excellent agreement with the experimental data. We believe the results presented here advance the field of optical trapping manipulation of absorbing magnetic particles, and future applications will involve, for example, the design of new hybrid optomagnetic tweezers.