Optical tweezers — from calibration to applications: a tutorial

Abstract

Since their invention in 1986 by Arthur Ashkin and colleagues, optical tweezers have become an essential tool in several fields of physics, spectroscopy, biology, nanotechnology, and thermodynamics. In this Tutorial, we provide a primer on how to calibrate optical tweezers and how to use them for advanced applications. After a brief general introduction on optical tweezers, we focus on describing and comparing the various available calibration techniques. Then, we discuss some cutting-edge applications of optical tweezers in a liquid medium, namely to study single-molecule and single-cell mechanics, microrheology, colloidal interactions, statistical physics, and transport phenomena. Finally, we consider optical tweezers in vacuum, where the absence of a viscous medium offers vastly different dynamics and presents new challenges. We conclude with some perspectives for the field and the future application of optical tweezers. This Tutorial provides both a step-by-step guide ideal for non-specialists entering the field and a comprehensive manual of advanced techniques useful for expert practitioners. All the examples are complemented by the sample data and software necessary to reproduce them.

Sugar-Pucker Force-Induced Transition in Single-Stranded DNA

“…The experiments are carried out using a miniaturized dual-beam setup [ 47 , 48 , 49 ]. Briefly, two tightly focused counter-propagating laser beams (P = 200 mW, = 845 nm) create a single optical trap, modelled as an harmonic potential ( Figure 1 a).…”

Work Extraction and Performance of Colloidal Heat Engines in Viscoelastic Baths

“…Extensions of Equation ( 1) to the three dimensional case, r = (x, y, z), which are relevant in many experimental situations using optical trapping techniques [52], are possible by a proper choice of the potential U(r, t) and a tensorial form of the memory kernel for particles of arbitrary shape [53]. Here, for the sake of simplicity we focus on the dynamics of a single coordinate of a spherical particle of radius a, which is confined by a harmonic potential U(x, t) = 1 2 κ(t)x(t) 2 , where κ(t) is the stiffness at time t of the corresponding restoring force.…”

“…Furthermore, without loss of generality we choose constant values of the low and high-frequencies viscosities that are typical of viscoelastic fluids prepared in aqueous solution in semidilute regimes [42,62,66,70,71]: η 0 = 0.040 Pa s and η ∞ = 0.004 Pa s, which correspond to α = 9. The diameter of the colloidal particle is set to a = 0.5 µm, while the maximum and minimum values of the trap stiffness during the Stirling cycle are chosen as κ M = 5 pN µm −1 and κ m = 1 pN µm −1 , respectively, which are easily accessible with optical tweezers [52]. The temperatures of the reservoir during the hot and cold part of the cycle are T c = 5 • C and T h = 90 • C, which are selected in such a way that they are within the temperature range in which water, which is a common solvent component of many viscoelastic fluids, remains liquid.…”

“…One further aspect that could be investigated in the future is the effect of temporal changes in the fluid parameters, as it is well-known that the rheological properties of viscoelastic fluids are dependent on their temperature, which under a thermodynamic cycle would become time-dependent. We would like to point out that, since the parameters characterizing the operation of the heat engine presented in this paper are representative of typical soft matter systems, we expect that this process can be realized in a straightforward manner by use of optical tweezers [52]. Similar ideas could be extended to Brownian particles in non-linear potentials [82], and active Brownian heat engines [32] functioning in complex fluids, which could be implemented by means of light-activated colloids in non-Newtonian liquids [69,[83][84][85][86] and hot Brownian particles [87][88][89].…”

Protocol to measure the membrane tension and bending modulus of cells using optical tweezers and scanning electron microscopy

“…There are extensive reviews in the literature describing in detail different OT calibration methods ( Gieseler et al., 2020 ; N.B.V., unpublished data), and the advantages and disadvantages of all methods. Here, we describe one of the simplest calibration procedures based on the Stokes-Faxén law for the viscous drag force acting on a microsphere immersed within a fluid ( Nicholas et al., 2014 ; N.B.V., unpublished data).…”