Three-dimensional force calibration of optical tweezers

Singer, Wolfgang; Bernet, Stefan; Hecker, N. E.; Ritsch‐Marte, Monika

doi:10.1080/09500340008232206

Cited by 51 publications

(20 citation statements)

References 12 publications

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“…͑We use trap and tweezers as synonyms.͒ They are discussed in several excellent texts. [19][20][21][22][23][24][25] The most reliable procedure interprets the power spectrum of Brownian motion of a bead in the trap. This is conventionally done with the Einstein-Ornstein-Uhlenbeck theory of Brownian motion, which predicts a Lorentzian spectrum.…”

Section: Introductionmentioning

confidence: 99%

Power spectrum analysis for optical tweezers

Berg-Sørensen¹,

Flyvbjerg

2004

Review of Scientific Instruments

847

862

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The force exerted by an optical trap on a dielectric bead in a fluid is often found by fitting a Lorentzian to the power spectrum of Brownian motion of the bead in the trap. We present explicit functions of the experimental power spectrum that give the values of the parameters fitted, including error bars and correlations, for the best such 2 fit in a given frequency range. We use these functions to determine the information content of various parts of the power spectrum, and find, at odds with lore, much information at relatively high frequencies. Applying the method to real data, we obtain perfect fits and calibrate tweezers with less than 1% error when the trapping force is not too strong. Relatively strong traps have power spectra that cannot be fitted properly with any Lorentzian, we find. This underscores the need for better understanding of the power spectrum than the Lorentzian provides. This is achieved using old and new theory for Brownian motion in an incompressible fluid, and new results for a popular photodetection system. The trap and photodetection system are then calibrated simultaneously in a manner that makes optical tweezers a tool of precision for force spectroscopy, local viscometry, and probably other applications.

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Section: Introductionmentioning

confidence: 99%

Power spectrum analysis for optical tweezers

Berg-Sørensen¹,

Flyvbjerg

2004

Review of Scientific Instruments

847

862

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“…For estimation of the trapping stiffness of our laser tweezers we used the method of trapping dynamics described in Ref. 8. We measured the bead position as a function of the time during which the bead escaped from one stationary trap and was captured by another stationary trap separated by a distance ϳd.…”

mentioning

confidence: 99%

Measurement of optical trapping forces by use of the two-photon-excited fluorescence of microspheres

et al. 2003

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A novel technique for the calibration of laser trapping systems that utilizes two-photon-excited fluorescence of commercial dye-stained microspheres has been demonstrated. The trapping forces as well as the trapping efficiency have been measured for various liquid environments and trapping depths. The trapping efficiency in water was found to decrease with an increase of trapping depths because of the enlargement of the trapping beam waist caused by aberrations of the optical system.

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“…Optical forces, which arise from transfer of the momentum the light carries itself, have been successfully applied in a variety of biological applications (Svoboda and Block, 1994;Ashkin, 1997;Ashkin et al, 1990;Quake et al, 1997;Sheetz, 2001). The force exerted on a trapped particle depends on its size, geometry, and the difference in refractive index between the particle and its environment, and scales linearly with the incident laser power (Svoboda and Block, 1994;Simmons et al, 1996;Singer et al, 2000;Wright et al, 1994). However, as the refractive index of the fused LB is not precisely known (but [n H2O ), and the size of the particle and the focus of the trapping laser are of the same order of magnitude, an experimental force calibration was required for our experiments.…”

Section: Correlation Between Laser Power and Force Exerted On A Fused Lbmentioning

confidence: 99%

“…This was performed by translational displacement of the object chamber, and therefore the surrounding bath solution, with respect to the trapped particle. Knowing the velocity v, when the particle unsnaps from the trap, the maximum trapping force, which equals the opposing viscous drag force F 5 6phrv, can be calculated according to (Svoboda and Block, 1994;Singer et al, 2000) with r denoting the particle radius and h the viscosity of the surrounding liquid.…”

Section: Correlation Between Laser Power and Force Exerted On A Fused Lbmentioning

confidence: 99%

Mechanical Forces Impeding Exocytotic Surfactant Release Revealed by Optical Tweezers

Singer

Frick

Haller

et al. 2003

Biophysical Journal

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The release of surfactant from alveolar type II cells is essential to lower the surface tension in the lung and to facilitate inspiration. However, the factors controlling dispersal and diffusion of this hydrophobic material are still poorly understood. Here we report that release of surfactant from the fused vesicle, termed lamellar body (LB), resisted mechanical forces applied by optical tweezers: At constant trapping force, the probability to expand LB contents, i.e., to "pull" surfactant into the extracellular fluid, increased with time after LB fusion with the plasma membrane, consistent with slow fusion pore expansion in these cells. Elevations of the cytoplasmic Ca(2+) concentration ([Ca(2+)](c)) had a similar effect. Inasmuch as surfactant did not disintegrate in the extracellular space, this method permitted for the first time the determination of elastic and recoil properties of the macromolecular complex, yielding a spring constant of approximately 12.5 pN/ micro m. This is the first functional evidence that release of hydrophobic material is mechanically impeded and occurs in an "all-or-none" fashion. This mode of release is most probably the result of cohesive forces of surfactant, combined with adhesive forces and/or retaining forces exerted by a constrictive fusion pore acting as a regulated mechanical barrier, withstanding forces up to 160 pN. In independent experiments equiaxial strain was exerted on cells without optical tweezers. Strain facilitated surfactant release from preexisting fused vesicles, consistent with the view of mechanical impediments during the release process, which can be overcome by cell strain.

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Three-dimensional force calibration of optical tweezers

Cited by 51 publications

References 12 publications

Power spectrum analysis for optical tweezers

Power spectrum analysis for optical tweezers

Measurement of optical trapping forces by use of the two-photon-excited fluorescence of microspheres

Mechanical Forces Impeding Exocytotic Surfactant Release Revealed by Optical Tweezers

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