Matthew J Comstock, Taekjip Ha & Yann R Chemla
We present a single-molecule instrument that combines a time-shared ultrahigh-resolution dual optical trap interlaced with a confocal fluorescence microscope. In a demonstration experiment, we observed individual single fluorophore–labeled DNA oligonucleotides to bind and unbind complementary DNA suspended between two trapped beads. Simultaneous with the single-fluorophore detection, we clearly observed coincident angstrom-scale changes in tether extension. Fluorescence readout allowed us to determine the duplex melting rate as a function of force. The new instrument will enable the simultaneous measurement of angstrom-scale mechanical motion of individual DNA-binding proteins (for example, single-base-pair stepping of DNA translocases) along with the detection of properties of fluorescently labeled protein (for example, internal configuration).
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Elisa D'Este, Gabriele Baj, Paolo Beuzer, Enrico Ferrari, Giulietta Pinato, Enrico Tongiorgi and Dan Cojoc
Spatial regulation of secretory molecule release is a sophisticated mechanism used by the nervous system to control network development and finely tune the activity of each synapse. Great efforts have been made to develop techniques that mimic secretory molecule release with the aim of stimulating neurons as close as possible to physiological conditions. However, current techniques have poor spatial resolution or low flexibility. Here, we propose a novel approach to achieve focal and prolonged stimulation of neurons using optical tweezers and single microbeads functionalized with a secretory molecule, the neurotrophin brain-derived neurotrophic factor (BDNF). BDNF is a key regulator of neuronal development and plasticity. We show that single BDNF-coated microbeads can be extracted with optical tweezers from small reservoirs and positioned with submicrometric precision to specific sites on the dendrites of cultured hippocampal neurons. Localized contact of microbeads functionalized with BDNF, but not with bovine serum albumin (BSA), induced focal increase of calcium signaling in the stimulated dendrite, specific activation of the TrkB receptor pathway and influenced the development of growth cones. Remarkably, a single BDNF-coated bead localized on a dendrite was found to be enough for TrkB phosphorylation, an efficient and long-lasting activation of calcium signaling in the soma, and c-Fos signaling in the nucleus, comparable to bath stimulation conditions. These findings support the use of optical tweezer technology for long-term, localized stimulation of specific subcellular neuronal compartments.
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Ali Salehi-Reyhani, Joseph Kaplinsky, Edward Burgin, Miroslava Novakova, Andrew J. deMello, Richard H. Templer, Peter Parker, Mark A. A. Neil, Oscar Ces, Paul French, Keith R. Willison and David Klug
We have developed a generic platform to undertake the analysis of protein copy number from single cells. The approach described here is ‘all-optical’ whereby single cells are manipulated into separate analysis chambers using an optical trap; single cells are lysed by a shock wave caused by laser-induced microcavitation, and the protein released from a single cell is measured by total internal reflection microscopy as it is bound to micro-printed antibody spots within the device. The platform was tested using GFP transfected cells and the relative precision of the measurement method was determined to be 88%. Single cell measurements were also made on a breast cancer cell line to measure the relative levels of unlabelled human tumour suppressor protein p53 using a chip incorporating an antibody sandwich assay format. These results suggest that this is a viable method for measuring relative protein levels in single cells.
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Jun Chen, Jack Ng, Pei Wang, and Zhifang Lin
In a previous Letter [Opt. Lett. 35, 1674 (2010)], the mathematical notations for Cx and Sx were unclearly defined. That error is corrected here.
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Jeremy Witzens and Michael Hochberg
We theoretically investigate a novel scheme to detect target molecule induced, or suppressed, aggregation of nanoparticles. High-Q optical resonators are used to both optically trap gold nanoparticle clusters and to detect their presence via a shift in the resonance wavelength. The well depth of the optical trap is chosen to be relatively low compared to the thermal energy of the nanoparticles, so that trapping of single nanoparticles is marginal and results in a comparatively small wavelength shift. Aggregation of functionalized gold nanoparticles is mediated or suppressed via binding to a target molecule. The well depth for the resulting nanoparticle clusters scales much more favorably relative to Brownian motion, resulting in large nanoparticle concentration enhancements in the evanescent field region of the resonator. We predict a target molecule sensitivity in the tens of fM range. In order to predict the resonator response, a complete theory of time resolved nanoparticle cluster trapping dynamics is derived. In particular, the formalism of Kramers’ escape time is adapted to 2D (silicon wire) and 3D (ring resonator) optical traps.
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Marcus Jahnel, Martin Behrndt, Anita Jannasch, Erik Schäffer, and Stephan W. Grill
The use of optical traps to measure or apply forces on the molecular level requires a precise knowledge of the trapping force field. Close to the trap center, this field is typically approximated as linear in the displacement of the trapped microsphere. However, applications demanding high forces at low laser intensities can probe the light-microsphere interaction beyond the linear regime. Here, we measured the full nonlinear force and displacement response of an optical trap in two dimensions using a dual-beam optical trap setup with back-focal-plane photodetection. We observed a substantial stiffening of the trap beyond the linear regime that depends on microsphere size, in agreement with Mie theory calculations. Surprisingly, we found that the linear detection range for forces exceeds the one for displacement by far. Our approach allows for a complete calibration of an optical trap.
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Raktim Dasgupta, Ravi Shanker Verma, Sunita Ahlawat, Deepa Chaturvedi, and Pradeep Kumar Gupta
We show that the axial spread of the focal volume of a tightly focused beam propagating through a glass–water interface is much reduced for Laguerre–Gaussian (LG) modes as compared to the TEM00mode. Therefore, use of the LG beam helps in achieving a significant improvement of the axial trapping range in optical tweezers. We demonstrate the use of LG modes to manipulate biological cells from the bottom layer of the medium to the top surface layer. Exposure of the cells to a higher oxygen concentration at the surface layer is used for estimation of the intramembrane oxygen diffusion rate.
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Josep Mas, Michelle S. Roth, Estela Martín-Badosa, and Mario Montes-Usategui
In this study, we present a method designed to generate dynamic holograms in holographic optical tweezers. The approach combines our random mask encoding method with iterative high-efficiency algorithms. This hybrid method can be used to dynamically modify precalculated holograms, giving them new functionalities—temporarily or permanently—with a low computational cost. This allows the easy addition or removal of a single trap or the independent control of groups of traps for manipulating a variety of rigid structures in real time.
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I Naydenova, E Leite, Tz Babeva, N Pandey, T Baron, T Yovcheva, S Sainov, S Martin, S Mintova and V Toal
Acrylamide-based photopolymerizable nanocomposites containing three types of nanosized crystals with controlled microporosity, Silicalite-1 (MFI-structure), AlPO-18 (AEI-structure) and Beta (BEA-structure) are studied. The influence of the porous nanoparticles on the average refractive index, optical scattering and holographic recording properties of the nanocomposite are characterized. The redistribution of nanoparticles as a result of the holographic recording in the layers is investigated by Raman spectroscopy. It is observed that in all three nanocomposites the nanoparticles are redistributed according to the illuminating light pattern. This redistribution improves the refractive index modulation only in the case of the MFI nanoparticles, while no improvement is observed in AEI and BEA doped layers. The results can be explained by the hydrophobic/hydrophilic nature of the nanoparticles and their interactions, or absence of interactions, with the host photopolymer.
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N K Metzger, M Mazilu, L Kelemen, P Ormos and K Dholakia
In the realm of low Reynolds number flow there is a need to find methods to pump, move and mix minute amounts of analyte. Interestingly, micro-devices performing such actuation can be initiated by means of the light–matter interaction. Light induced forces and torques are exerted on such micro-objects, which are then driven by the optical gradient or scattering force. Here, different driving geometries can be realized to harness the light induced force. For example, the scattering force enables micro-gears to be operated in a tangential setup where the micromotor rotors are in line with an optical waveguide. The operational geometry we investigate has the advantage that it reduces the complexity of the driving of such a device in a microfluidic environment by delivering the actuating light by means of a waveguide or fiber optic. In this paper we explore the case of a micromotor being driven by a fiber optically delivered light beam. We experimentally investigate how the driving light interacts with and diffracts from the motor, utilizing two-photon imaging. The micromotor rotation rate dependence on the light field parameters is explored. Additionally, a theoretical model based on the paraxial approximation is used to simulate the torque and predict the rotation rate of such a device and compare it with experiment. The results presented show that our model can be used to optimize the micromotor performance and some example motor designs are evaluated.
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