Line optical tweezers instrument for measuring nanoscale interactions and kinetics

Abstract

We describe an optical tweezers instrument for measuring short-ranged colloidal interactions, based on a combination of a continuous wave line optical tweezers, high speed video microscopy, and laser illumination. Our implementation can measure the separation of two nearly contacting microspheres to better than 4 nm at rates in excess of 10 kHz. A simple image analysis algorithm allows us to sensibly remove effects from diffraction blurring and microsphere image overlap for separations ranging from contact to at least 100 nm. The result is a versatile instrument for measuring steric, chemical and single-molecular interactions and dynamics, with a force resolution significantly better than achievable with current atomic force microscopy. We demonstrate the effectiveness of the instrument with measurements of the pair interactions and dynamics of microspheres in the presence of transient molecular bridges of DNA or surfactant micelles. We describe an optical tweezers instrument for measuring short-ranged colloidal interactions, based on a combination of a continuous wave line optical tweezers, high speed video microscopy, and laser illumination. Our implementation can measure the separation of two nearly contacting microspheres to better than 4 nm at rates in excess of 10 kHz. A simple image analysis algorithm allows us to sensibly remove effects from diffraction blurring and microsphere image overlap for separations ranging from contact to at least 100 nm. The result is a versatile instrument for measuring steric, chemical and single-molecular interactions and dynamics, with a force resolution significantly better than achievable with current atomic force microscopy. We demonstrate the effectiveness of the instrument with measurements of the pair interactions and dynamics of microspheres in the presence of transient molecular bridges of DNA or surfactant micelles.

Direct measurements of DNA-mediated colloidal interactions and their quantitative modeling

“…The formation of transient bridges pulls the particles together and leads to the development of a sharp peak in the histogram of relative separation, PðhÞ, near h ≈ L. The equilibrium pair free energy, FðhÞ, can be computed from the experimental PðhÞ, up to an additive constant, by inverting the Boltzmann relation, PðhÞ ∝ exp½−FðhÞ∕k B T , where k B T is the thermal energy (22). Fig.…”

“…We measure DNA-mediated colloidal interactions using a scanning-line optical tweezers instrument and video microscopy (see SI Text for details of the line optical tweezers). The imaging setup is described elsewhere (22). We achieve an elongated optical potential by rapidly scanning an ordinary optical trap back-and-forth in the focal plane of an inverted light microscope (Leica DM IRB, Leica Microsystems) with a 16-kHz resonant scanning mirror (Electro-Optical Products Corporation).…”

“…All pair-interaction measurements were performed in aqueous buffer containing 125 mM NaCl/10 mM Tris/1 mM EDTA∕pH ¼ 8.0. We fit the measured pair-interaction potentials to numerically blurred solutions of our model using procedures detailed elsewhere (22,32) and in SI Text. Briefly, we convert our model solution of ΔFðhÞ into a probability distribution, PðhÞ, using the Boltzmann relation.…”

On bias of kinetic temperature measurements in complex plasmas

“…The results demonstrated in this paper are applicable not only to complex plasmas but also to colloids [26][27][28][29] and granular materials. [30][31][32][33][34] In Sec.…”

Imaging of a linear diode bar for an optical cell stretcher

“…In this, opposing antipodal stretching forces are generated along the linear stretcher when the beam width and length are shorter and longer than the cell size, respectively [19]. Whereas linear optical traps can be generated by other methods [21,22], the inherent geometric anisotropy characteristic to such linear diode bar sources, typically 100:1 or greater, allows for direct application as an optical stretcher. For example, a diode bar may have emitter dimensions of 1 µm by 100 to 300 µm.…”