Force Optical Tweezers Research Articles

Optical Tweezers Assembled Nanodiamond Quantum Sensors.

Here we show that gradient force optical tweezers can be used to mediate the self-assembly of nanodiamonds into superstructures, which can serve as optically trapped nanoscale quantum probes with superior magnetic resonance sensing capabilities. Enhanced fluorescence rates from nitrogen-vacancy NV- defect centers enable rapid acquisition of optically detected magnetic resonance (ODMR), and shape-induced forces can improve both positioning accuracy and orientation control. The use of confocal imaging can isolate the signal from individual nanodiamonds within the assembly, thereby retaining the desirable properties of a single crystal probe. The improvements afforded by the use nanodiamond assemblies has the potential to resolve dynamic changes through, for example, real-time monitoring of the ODMR contrast.

Amphibious Hybrid Laser Tweezers for Fluid and Solid Domains.

Optical trapping is a potent tool for achieving precise and noninvasive manipulation of small objects in a vacuum and liquids. However, due to the substantial disparity between optical forces and interfacial adhesion, target objects should be suspended in fluid environments, rendering solid contact surfaces a restricted area for conventional optical tweezers. In this work, by relying on a single continuous wave (CW) laser, we demonstrate an optical manipulation system applicable for both fluid and solid domains, namely, amphibious hybrid laser tweezers. The key to our system lies in modulating the intensity of the CW laser with duration shorter than the dynamic thermal equilibrium time within objects, wherein strong thermal gradient forces with ∼6 orders of magnitude higher than the forces in optical tweezers are produced, enabling moving and trapping micro/nano-objects on solid interfaces. Thereby, CW laser-based optical tweezers and pulsed laser-based photothermal shock tweezers are seamlessly fused with the advantages of cost-effectiveness and simplicity. Our concept breaks the stereotype that CW lasers are limited to generating tiny forces and instead achieve ultrawide force generation spanning from femto-newtons (10-15 N) to (10-6 N). Our work expands the horizon of optical manipulation by seamlessly bridging its applications in fluid and solid environments and holds promise for inspiring optical manipulation techniques to perform more challenging tasks, which may unearth application scenarios in diverse fields such as fundamental physical research, nanofabrication, micro/nanorobotics, biomedicine, and cytology.

Characterization of the interaction between the Sec61 translocon complex and ppαF using optical tweezers.

The Sec61 translocon allows the translocation of secretory preproteins from the cytosol to the endoplasmic reticulum lumen during polypeptide biosynthesis. These proteins possess an N-terminal signal peptide (SP) which docks at the translocon. SP mutations can abolish translocation and cause diseases, suggesting an essential role for this SP/Sec61 interaction. However, a detailed biophysical characterization of this binding is still missing. Here, optical tweezers force spectroscopy was used to characterize the kinetic parameters of the dissociation process between Sec61 and the SP of prepro-alpha-factor. The unbinding parameters including off-rate constant and distance to the transition state were obtained by fitting rupture force data to Dudko-Hummer-Szabo models. Interestingly, the translocation inhibitor mycolactone increases the off-rate and accelerates the SP/Sec61 dissociation, while also weakening the interaction. Whereas the translocation deficient mutant containing a single point mutation in the SP abolished the specificity of the SP/Sec61 binding, resulting in an unstable interaction. In conclusion, we characterize quantitatively the dissociation process between the signal peptide and the translocon, and how the unbinding parameters are modified by a translocation inhibitor.

Biphasic reinforcement of nascent adhesions by vinculin.

Vinculin is an integral component of integrin adhesions, where it functions as a molecular clutch coupling intracellular contraction to the extracellular matrix. Quantitating its contribution to the reinforcement of newly forming adhesions, however, requires ultrasensitive cell force assays covering short time and low force ranges. Here, we have combined atomic force microscopy-based single-cell force spectroscopy (SCFS) and optical tweezers force spectroscopy to investigate the role of vinculin in reinforcement of individual nascent adhesions during the first 5 min of cell contact with fibronectin or vitronectin. At minimal adhesion times (5-10 s), mouse embryonic fibroblast (MEF) wildtype (wt) and vinculin knock-out (vin(-/-) ) cells develop comparable adhesion forces on the scale of several individual integrin-ligand bonds, confirming that vinculin is dispensable for adhesion initiation. In contrast, after 60 to 120 s, adhesion strength and traction reinforce quickly in wt cells, while remaining low in vin(-/-) cells. Re-expression of full-length vinculin or a constitutively active vinculin mutant (vinT12) in MEF vin(-/-) cells restored adhesion and traction with the same efficiency, while vinculin with a mutated talin-binding head region (vinA50I) or missing the actin-binding tail-domain (vin880) was ineffective. Integrating total internal reflection fluorescence imaging into the SCFS setup furthermore enabled us to correlate vinculin-green fluorescent protein (GFP) recruitment to nascent adhesion sites with the built-up of vinculin-dependent adhesion forces directly. Vinculin recruitment and cell adhesion reinforcement followed synchronous biphasic patterns, suggesting vinculin recruitment, but not activation, as the rate-limiting step for adhesion reinforcement. Combining sensitive SCFS with fluorescence microscopy thus provides insight into the temporal sequence of vinculin-dependent mechanical reinforcement in nascent integrin adhesions.

Optical manipulation: advances for biophotonics in the 21st century.

.Significance: Optical trapping is a technique capable of applying minute forces that has been applied to studies spanning single molecules up to microorganisms.Aim: The goal of this perspective is to highlight some of the main advances in the last decade in this field that are pertinent for a biomedical audience.Approach: First, the direct determination of forces in optical tweezers and the combination of optical and acoustic traps, which allows studies across different length scales, are discussed. Then, a review of the progress made in the direct trapping of both single-molecules, and even single-viruses, and single cells with optical forces is outlined. Lastly, future directions for this methodology in biophotonics are discussed.Results: In the 21st century, optical manipulation has expanded its unique capabilities, enabling not only a more detailed study of single molecules and single cells but also of more complex living systems, giving us further insights into important biological activities.Conclusions: Optical forces have played a large role in the biomedical landscape leading to exceptional new biological breakthroughs. The continuous advances in the world of optical trapping will certainly lead to further exploitation, including exciting in-vivo experiments.

Optimizing Optical Tweezers Experiments for Magnetic Resonance Sensing with Nanodiamonds

In this Article we explore the requirements for enabling high quality optically detected magnetic resonance (ODMR) spectroscopy in a conventional gradient force optical tweezers using nanodiamonds containing nitrogen-vacancy (NV-) centers. We find that modulation of the infrared (1064 nm) trapping laser during spectroscopic measurements substantially improves the ODMR contrast compared with continuous wave trapping. The work is significant as it allows trapping and quantum sensing protocols to be performed in conditions where signal contrast is substantially reduced. We demonstrate the utility of the technique by resolving NV- spin projections within the ODMR spectrum. Manipulating the orientation of the nanodiamond via the trapping laser polarization, we observe changes in spectral features. Theoretical modeling then allows us to infer the crystallographic orientation of the NV-. This is an essential capability for future magnetic sensing applications of optically trapped nanodiamonds.

Molecular origins of reduced activity and binding commitment of processive cellulases and associated carbohydrate-binding proteins to cellulose III

Efficient enzymatic saccharification of cellulosic biomass into fermentable sugars can enable production of bioproducts like ethanol. Native crystalline cellulose, or cellulose I, is inefficiently processed via enzymatic hydrolysis but can be converted into the structurally distinct cellulose III allomorph that is processed via cellulase cocktails derived from Trichoderma reesei up to 20-fold faster. However, characterization of individual cellulases from T. reesei, like the processive exocellulase Cel7A, shows reduced binding and activity at low enzyme loadings toward cellulose III. To clarify this discrepancy, we monitored the single-molecule initial binding commitment and subsequent processive motility of Cel7A enzymes and associated carbohydrate-binding modules (CBMs) on cellulose using optical tweezers force spectroscopy. We confirmed a 48% lower initial binding commitment and 32% slower processive motility of Cel7A on cellulose III, which we hypothesized derives from reduced binding affinity of the Cel7A binding domain CBM1. Classical CBM–cellulose pull-down assays, depending on the adsorption model fitted, predicted between 1.2- and 7-fold reduction in CBM1 binding affinity for cellulose III. Force spectroscopy measurements of CBM1–cellulose interactions, along with molecular dynamics simulations, indicated that previous interpretations of classical binding assay results using multisite adsorption models may have complicated analysis, and instead suggest simpler single-site models should be used. These findings were corroborated by binding analysis of other type-A CBMs (CBM2a, CBM3a, CBM5, CBM10, and CBM64) on both cellulose allomorphs. Finally, we discuss how complementary analytical tools are critical to gain insight into the complex mechanisms of insoluble polysaccharides hydrolysis by cellulolytic enzymes and associated carbohydrate-binding proteins.

Off-Axis Dipole Forces in Optical Tweezers by an Optical Analog of the Magnus Effect.

It is shown that a circular dipole can deflect the focused laser beam that induces it and will experience a corresponding transverse force. Quantitative expressions are derived for Gaussian and angular top hat beams, while the effects vanish in the plane wave limit. The phenomena are analogous to the Magnus effect, pushing a spinning ball onto a curved trajectory. The optical case originates in the coupling of spin and orbital angular momentum of the dipole and the light. In optical tweezers the force causes off-axis displacement of the trapping position of an atom by a spin-dependent amount up to λ/2π, set by the direction of a magnetic field. This suggests direct methods to demonstrate and explore these effects, for instance, to induce spin-dependent motion.

Characterization of blood coagulation dynamics and oxygenation in ex-vivo retinal vessels by fluorescence hyperspectral imaging.

Blood coagulation mechanisms forming a blood clot and preventing hemorrhage have been extensively studied in the last decades. Knowing the mechanisms behind becomes very important particularly in the case of blood vessel diseases. Real-time and accurate diagnostics accompanied by the therapy are particularly needed, for example, in diseases related to retinal vasculature. In our study, we employ for the first time fluorescence hyperspectral imaging (fHSI) combined with the spectral analysis algorithm concept to assess physical as well as functional information of blood coagulation in real-time. By laser-induced local disruption of retinal vessels to mimic blood leaking and subsequent coagulation and a proper fitting algorithm, we were able to reveal and quantify the extent of local blood coagulation through direct identification of the change of oxyhemoglobin concentration within few minutes. We confirmed and illuminated the spatio-temporal evolution of the essential role of erythrocytes in the coagulation cascade as the suppliers of oxygenated hemoglobin. By additional optical tweezers force manipulation, we showed immediate aggregation of erythrocytes at the coagulation site. The presented fluorescence-based imaging concept could become a valuable tool in various blood coagulation diagnostics as well as theranostic systems if coupled with the laser therapy.

Lateral forces on particles induced by magnetic spin-orbit coupling.

Optical forces in optical tweezers enable non-contact, non-destructive trapping and manipulation of particles. One such force has been found to originate from the spin-orbit coupling of light, which produces a counter-intuitive lateral optical force on metal nanoparticles due to the spin of the electric-field components of light. Here we reveal that the spin-orbit coupling of the magnetic-field components of light also produces a lateral optical force on particles. To study this lateral force, we designed a gapped structure composed of a dielectric particle near photonic crystal surface, and found that the lateral force originates from the spin-dependent excitation of a Bloch surface wave. We further demonstrate that the lateral force can be modified by tuning the structural parameters of the gapped structure and by exploiting the magnetic resonance modes of the particle. This work should contribute to a deeper understanding of the magnetic spin-orbit coupling between light and matter and promote the development of particle manipulation on dielectric platforms.

Analytical calculation of optical forces on spherical particles in optical tweezers: tutorial

Arthur Ashkin was awarded the 2018 Nobel prize in physics for the invention of optical tweezers. Since the first publication in 1986 Optical Tweezers have been used as a tool to measure forces and rheological properties of microscopic systems. For the calibration of these measurements, the knowledge of the forces is fundamental. However, it is still common to deduce the optical forces from assumptions based on the particle size with respect to the trapping laser wavelength. This show the necessity to develop a complete and accurate electromagnetic model that does not depend on early approximations of the force model. Furthermore, the model we developed has several advantages, such as morphology--dependent resonances, size dependence for large spheres, and multipole effects from smaller particles, just to name a few. In this tutorial, we review and discuss the physical modeling of optical forces in optical tweezers, which are the resultant forces exerted by a trapping beam on a sphere of any size and composition.

Investigating RNA Hairpin Stability and Protein Binding Using Optical Tweezers

More than 1.2 million people in the US are living with HIV. This RNA virus must be reverse‐transcribed into DNA before it can be integrated into the host genome and subsequently replicated. Retroviral DNA synthesis is generally understood, but questions remain about many identified critical elements. The transactivation response (TAR) RNA hairpin, specifically, inhibits the minus‐strand transfer step in reverse transcription and thereby regulates the life cycle of the HIV‐1 retrovirus. Before proceeding with reverse transcription, the TAR hairpin must be destabilized: chaperone assistance from the nucleocapsid (NC) protein can stimulate this rate‐limiting step 3000‐fold. Recent interest in the folding landscape of the TAR hairpin has revealed in‐vitro thermodynamic properties in the presence and absence of NC. These discoveries were enabled by optical tweezers force spectroscopy, the nature of which requires a complex TAR hairpin construct. Unfortunately, design and synthesis of the necessary TAR hairpin construct has limited the quantity and diversity of thermodynamic analyses regarding this essential facet of HIV‐1. We aim to develop efficient protocols to produce TAR RNA hairpin constructs optimized for optical tweezer manipulations.In order to manipulate individual hairpin molecules, the optical tweezers require that the RNA hairpin be ligated to two long double‐stranded DNA “handles.” One handle of the final construct specifically binds a bead in the optical tweezer dual‐laser trap, while the other handle binds a second bead that is manipulated via a micropipette. Tiny displacements of the second bead in the laser trap are used to calculate the force required to unzip the hairpin, and the thermodynamic parameters for force‐induced unfolding. The two ~3,000 base pair handles were constructed by amplification of bacterial plasmid DNA by PCR, restricted to create specific sticky ends, and purified by gel electrophoresis. RNA hairpins were transcribed from short DNA templates that were themselves synthesized by automated solid‐phase synthesis with specific sequences for RNA pol binding and the hairpin structure of interest. The handles were then individually ligated via several short DNA and RNA linker sequences to the RNA hairpin. Gel purification between ligation steps removed alternative ligation products, allowing size‐based verification and isolation of intermediates and the complete hairpin construct. Initial unzipping data support the correct length and composition of the final construct. We anticipate that subsequent data will contribute to a broader understanding of RNA hairpin stability as well as provide useful insight about a potential HIV‐1 drug target.Support or Funding InformationNIH‐NIGMS (Grant 2R01‐GM072462‐10), NSF‐MCB (Grant 1213883), Camille and Henry Dreyfus FoundationThis abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Effects of non-Gaussian Brownian motion on direct force optical tweezers measurements of the electrostatic forces between pairs of colloidal particles.

Measurements of the electrostatic force with separation between a fixed and an optically trapped colloidal particle are examined with experiment, simulation and analytical calculation. Non-Gaussian Brownian motion is observed in the position of the optically trapped particle when particles are close and traps weak. As a consequence of this motion, a simple least squares parameterization of direct force measurements, in which force is inferred from the displacement of an optically trapped particle as separation is gradually decreased, contains forces generated by the rectification of thermal fluctuations in addition to those originating directly from the electrostatic interaction between the particles. Thus, when particles are close and traps weak, simply fitting the measured direct force measurement to DLVO theory extracts parameters with modified meanings when compared to the original formulation. In such cases, however, physically meaningful DLVO parameters can be recovered by comparing the measured non-Gaussian statistics to those predicted by solutions to Smoluchowski's equation for diffusion in a potential.

The Nanomechanical Properties of Lactococcus lactis Pili Are Conditioned by the Polymerized Backbone Pilin.

Pili produced by Lactococcus lactis subsp. lactis are putative linear structures consisting of repetitive subunits of the major pilin PilB that forms the backbone, pilin PilA situated at the distal end of the pilus, and an anchoring pilin PilC that tethers the pilus to the peptidoglycan. We determined the nanomechanical properties of pili using optical-tweezers force spectroscopy. Single pili were exposed to optical forces that yielded force-versus-extension spectra fitted using the Worm-Like Chain model. Native pili subjected to a force of 0–200 pN exhibit an inextensible, but highly flexible ultrastructure, reflected by their short persistence length. We tested a panel of derived strains to understand the functional role of the different pilins. First, we found that both the major pilin PilB and sortase C organize the backbone into a full-length organelle and dictate the nanomechanical properties of the pili. Second, we found that both PilA tip pilin and PilC anchoring pilin were not essential for the nanomechanical properties of pili. However, PilC maintains the pilus on the bacterial surface and may play a crucial role in the adhesion- and biofilm-forming properties of L. lactis.

Nonconservative dynamics of optically trapped high-aspect-ratio nanowires.

We investigate the dynamics of high-aspect-ratio nanowires trapped axially in a single gradient force optical tweezers. A power spectrum analysis of the dynamics reveals a broad spectral resonance of the order of kHz with peak properties that are strongly dependent on the input trapping power. A dynamical model incorporating linear restoring optical forces, a nonconservative asymmetric coupling between translational and rotational degrees of freedom, viscous drag, and white noise provides an excellent fit to experimental observations. A persistent low-frequency cyclical motion around the equilibrium trapping position, with a frequency distinct from the spectral resonance, is observed from the time series data.

Biomechanical and Structural Features of CS2 Fimbriae of Enterotoxigenic Escherichia coli

Enterotoxigenic Escherichia coli (ETEC) are a major cause of diarrhea worldwide, and infection of children in under-developed countries often leads to high mortality rates. Isolated ETEC expresses a plethora of colonization factors (fimbriae/pili), of which CFA/I and CFA/II, which are assembled via the alternate chaperone pathway (ACP), are among the most common. Fimbriae are filamentous structures whose shafts are primarily composed of helically arranged single pilin-protein subunits, with a unique biomechanical ability to unwind and rewind. A sustained ETEC infection, under adverse conditions of dynamic shear forces, is primarily attributed to this biomechanical feature of ETEC fimbriae. Recent understanding about the role of fimbriae as virulence factors points to an evolutionary adaptation of their structural and biomechanical features. In this work, we investigated the biophysical properties of CS2 fimbriae from the CFA/II group. Homology modeling of its major structural subunit, CotA, reveals structural clues related to the niche in which they are expressed. Using optical-tweezers force spectroscopy, we found that CS2 fimbriae unwind at a constant force of 10 pN and have a corner velocity (i.e., the velocity at which the force required for unwinding rises exponentially with increased speed) of 1300 nm/s. The biophysical properties of CS2 fimbriae assessed in this work classify them into a low-force unwinding group of fimbriae together with the CFA/I and CS20 fimbriae expressed by ETEC strains. The three fimbriae are expressed by ETEC, colonize in similar gut environments, and exhibit similar biophysical features, but differ in their biogenesis. Our observation suggests that the environment has a strong impact on the biophysical characteristics of fimbriae expressed by ETEC.

Cancellation of non-conservative scattering forces in optical traps by counter-propagating beams.

Non-conservative forces in optical tweezers generate undesirable behavior, such as particle loss due to radiation pressure and the preclusion of the thermodynamic equilibrium. Here, we rigorously derive criteria for the elimination of non-conservative forces, and describe how these criteria can be met by a large class of counter-propagating, focused optical beams.

Toward optical-tweezers-based force microscopy for airborne microparticles.

Optical tweezers have found widespread application in biological and colloidal physics for the measurement of pN forces over nanometer to micrometer length scales. Similar aerosol-phase measurements of interparticle force have not been reported in spite of the potential to better resolve particle coagulation kinetics. Various refractive index mismatches in the beam path as well as the need to explicitly account for gravity and inertial particle motion provide a number of challenges that must be overcome to make such measurements tractable. In this regard, we demonstrate schemes by which the particle position and trap stiffness may be unambiguously measured using bright-field microscopy with resolution comparable with analogous condensed-phase measurements. Moreover, some of the challenges of working with highly dynamic aqueous particles are introduced and exploited to observe size-dependent phenomena in aerosol optical tweezers. Notably, when combined with cavity-enhanced Raman spectroscopy, this provides a unique opportunity to explore trapping forces over a continuum of particle size and refractive index. It is expected that the methods developed will provide a basis for the measurement of pairwise interaction forces in aerosol optical tweezers while providing a probe of fundamental airborne particle trapping dynamics.

Optically Driven Gear-Based Mechanical Microtransducer for a Lab-on-a-Chip

In this study, we propose an optically driven gear-based micro-transducer composed of a mechanical arm, a sphere, and two gears, which are fabricated by two-photon polymerization technique. Optical tweezers is employed to grasp the sphere as a force exerted point to manipulate the mechanical arm. The two gears with different diameters to form a required gear ratio are meshed for the tranducing demonstration. Thanks to the classic lever function, the mechanical arm can multiply the optical tweezers force as the request of tranducing performance. The experimental result indicates that the transducer enables precise applied forces and the directions of the micro-transducer by optical tweezers at micron scale. The ratio of rotational angles has good agreement with the gear ratio as the classic gear function. Such gear-based optically driven mechanical transducer provides a possibility for driving micron-sized complex mechanisms, which is expected to perform as a mechanical operator in micro-channels for the applications of the “Lab-on-a-chip.”

Computational models for new fiber optic tweezers

This paper discusses the calculation of the trapping forces in optical tweezers using a combination of the finite differences time domain (FDTD) method and the Lorentz force on electric dipoles. The results of 2D simulations of the trapping of a circular particle by a waveguide with a circular tip are presented and discussed.