Friday, February 22, 2019

All-fiber impurity collector based on laser-induced microbubble

Zhihai Liu, Jiaojie Lei, Yu Zhang, Keqiang Liu, Wei Liu, Ruiwei Zhang, Yaxun Zhang, Xinghua Yang, Jianzhong Zhang, Jun Yang, Libo Yuan

Microbubbles have attracted interests in polluted water treatment industry. We proposed and constructed an all-fiber impurity collector based on the laser-induced microbubbles characteristics for polluted water treatment. Without any special pretreatment, such as metallically coating or carbon materials doping, the proposed impurity collector performed microbubbles generation and impurities extraction and collection. We employed the graphite oxide nanoplatelets in an aqueous solution to construct the polluted water environment to show the performance of the all-fiber impurity collector. With the optimal incident laser power of 11 mW, we performed the collection efficiency as high as 16.3104 /s. The optimal microbubbles diameter was in the range of 80–120 . The proposed all-fiber impurity collector extends the potential applications of fiber-based optical manipulation for polluted water treatment.


Optical tweezers as an effective tool for spermatozoa isolation from mixed forensic samples

Nicole Auka, Michael Valle, Bobby D. Cox, Peter D. Wilkerson, Tracey Dawson Cruz, Joseph E. Reiner, Sarah J. Seashols-Williams

A single focus optical tweezer is formed when a laser beam is launched through a high numerical aperture immersion objective. This objective focuses the beam down to a diffraction-limited spot, which creates an optical trap where cells suspended in aqueous solutions can be held fixed. Spermatozoa, an often probative cell type in forensic investigations, can be captured inside this optical trap and dragged one by one across millimeter-length distances in order to create a cluster of cells which can be subsequently drawn up into a capillary for collection. Sperm cells are then ejected onto a sterile cover slip, counted, and transferred to a tube for DNA analysis workflow. The objective of this research was to optimize sperm cell collection for maximum DNA yield, and to determine the number of trapped sperm cells necessary to produce a full STR profile. A varying number of sperm cells from both a single-source semen sample and a mock sexual assault sample were isolated utilizing optical tweezers and processed using conventional STR analysis methods. Results demonstrated that approximately 50 trapped spermatozoa were required to obtain a consistently full DNA profile. A complete, single-source DNA profile was also achieved by isolating sperm cells via optical trapping from a mixture of sperm and vaginal epithelial cells. Based on these results, optical tweezers are a viable option for forensic applications such as separation of mixed populations of cells in forensic evidence.


Numerical study of optical trapping properties of nanoparticle on metallic film with periodic structure

Cheng-Xian Ge, Zhen-Sen Wu, Jing Bai, Lei Gong

Based on the three-dimensional dispersive finite difference time domain method and Maxwell stress tensor equation, the optical trapping properties of nanoparticle placed on the gold film with periodic circular holes are investigated numerically. Surface plasmon polaritons are excited on the metal-dielectric interface, with particular emphasis on the crucial role in tailoring the optical force acting on a nearby nanoparticle. Utilizing a first order corrected electromagnetic field components for a fundamental Gaussian beam, the incident beam is added into the calculation model of the proposed method. To obtain the detailed trapping properties of nanoparticle, the selected calculations on the effects of beam waist radius, sizes of nanoparticle and circular holes, distance between incident Gaussian beam and gold film, material of nanoparticle and polarization angles of incident wave are analyzed in detail to demonstrate that the optical-trapping force can be explained as a virtual spring which has a restoring force to perform positive and negative forces as a nanoparticle moves closer to or away from the centers of circular holes. The results of optical trapping properties of nanoparticle in the vicinity of the gold film could provide guidelines for further research on the optical system design and manipulation of arbitrary composite nanoparticles.


Reconfigurable optical forces induced by tunable mode interference in gold core-silicon shell nanoparticles

Zheng-Xun Xiang, Xiang-Shi Kong, Xu-Bo Hu, Hai-Tao Xu, Yong-Bing Long, and Hai-Dong Deng

The effects of resonant mode interference on optical forces acting on gold core-silicon shell nanoparticles are theoretically investigated with the multipolar expansion method based on the Mie scattering theory. It is found that the total optical radiation force and its two components, the incident force and the recoil force, can be tuned flexibly by engineering the interference interaction among electric, magnetic, and anapole modes. The recoil force acting on the core-shell nanoparticles can be enhanced up to 17 pN compared with the pure silicon nanoparticles with the same size as that of the core-shell nanoparticles when the magnetic dipole resonant mode totally interferes with the electric dipole resonant mode. In addition, the incident force can also be improved to 25 pN by suppressing the interference between the electric dipole and the magnetic dipole resonances. More importantly, the maximum optical radiation force is not dominated by the strongest resonant scattering mode of the hybrid nanostructure due to the modes’ interference induced giant negative recoil forces. We hope our results not only improve the optical trapping and manipulation of core-shell nanoparticles but also help to understand the underlying physical mechanism regarding the tunable optical radiation forces induced by the tunable interference among different resonant modes in core-shell nanoparticles.


Extracellular matrix mechanical cues regulate lipid metabolism through Lipin-1 and SREBP

Patrizia Romani, Irene Brian, Giulia Santinon, Arianna Pocaterra, Matteo Audano, Silvia Pedretti, Samuel Mathieu, Mattia Forcato, Silvio Bicciato, Jean-Baptiste Manneville, Nico Mitro & Sirio Dupont

Extracellular matrix (ECM) mechanical cues have powerful effects on cell proliferation, differentiation and death. Here, starting from an unbiased metabolomics approach, we identify synthesis of neutral lipids as a general response to mechanical signals delivered by cell–matrix adhesions. Extracellular physical cues reverberate on the mechanical properties of the Golgi apparatus and regulate the Lipin-1 phosphatidate phosphatase. Conditions of reduced actomyosin contractility lead to inhibition of Lipin-1, accumulation of SCAP/SREBP to the Golgi apparatus and activation of SREBP transcription factors, in turn driving lipid synthesis and accumulation. This occurs independently of YAP/TAZ, mTOR and AMPK, and in parallel to feedback control by sterols. Regulation of SREBP can be observed in a stiffened diseased tissue, and contributes to the pro-survival activity of ROCK inhibitors in pluripotent stem cells. We thus identify a general mechanism centered on Lipin-1 and SREBP that links the physical cell microenvironment to a key metabolic pathway.


Coarse-grained particle dynamics along helical orbit by an optical vortex irradiated in photocurable resins

Ryo Nagura, Tempei Tsujimura, Tetsuro Tsuji, Kentaro Doi, and Satoyuki Kawano

Optical vortices, which carry orbital angular momentum, have attracted much attention in various research fields, such as materials processing, chirality control, and particle manipulation. A recent study experimentally confirmed that twisted fibers of polymerized photocurable resins with a constant period can be formed via irradiation by an optical vortex. It is suspected that this phenomenon is caused by the projection of the angular momentum of an optical vortex to the photocurable resin. The detailed mechanism of the growth of such peculiar fibers has not yet been clarified. In this study, which focuses on one aspect of polymerized structure formation, we develop a coarse-grained particle model in which the particle dynamics in the framework of the Rayleigh scattering theory involving light absorption is theoretically simulated. The period of the twisted fibers expressed using the coarse-grained particles is found to be in reasonable agreement with experimental values and independent of the input power of the laser. In addition, the shape of the polymerized fibers can be controlled by modulating the degree of light absorption.


Wednesday, February 20, 2019

All-dielectric nanotweezers for trapping and observation of a single quantum dot

Zhe Xu and Kenneth B. Crozier

We report the optical trapping of a single streptavidin-coated CdSe/ZnS quantum dot whose overall diameter is around 15–20 nm, in a microfluidic chamber by an all-dielectric (silicon) nanotweezer with negligible local heating. The use of fluorescence microscopy allows us to readily observe trapping events, tracking the fluorescence emission from, and the position of, each individual trapped quantum dot as a function of time. The blinking behavior of the quantum dots is observed during the trapping process, that is, in the near field region of the silicon nanoantenna. We furthermore show that the continuous wave infrared laser employed to trap the quantum dots can also excite photoluminescence from them via two-photon absorption. We present Maxwell stress tensor simulations of optical forces applied to a single quantum dot in the nanoantenna’s vicinity. This work demonstrates that all-dielectric nanotweezers are a promising means to handle quantum dots in solution, enabling them to be localized for observations over extended periods of time.


Functional significance of HCM mutants of tropomyosin, V95A and D175N, studied with in vitro motility assays

Shuya Ishii, Madoka Suzuki, Shin’ichi Ishiwata, Masataka Kawai

The majority of hypertrophic cardiomyopathy (HCM) is caused by mutations in sarcomere proteins. We examined tropomyosin (Tpm)’s HCM mutants in humans, V95A and D175N, with in vitro motility assay using optical tweezers to evaluate the effects of the Tpm mutations on the actomyosin interaction at the single molecular level. Thin filaments were reconstituted using these Tpm mutants, and their sliding velocity and force were measured at varying Ca2+ concentrations. Our results indicate that the sliding velocity at pCa ≥8.0 was significantly increased in mutants, which is expected to cause a diastolic problem. The velocity that can be activated by Ca2+ decreased significantly in mutants causing a systolic problem. With sliding force, Ca2+ activatable force decreased in V95A and increased in D175N, which may cause a systolic problem. Our results further demonstrate that the duty ratio determined at the steady state of force generation in saturating [Ca2+] decreased in V95A and increased in D175N. The Ca2+ sensitivity and cooperativity were not significantly affected by the mutations. These results suggest that the two mutants modulate molecular processes of the actomyosin interaction differently, but to result in the same pathology known as HCM.


Stochastic heating and self-induced cooling in optically bound pairs of atoms

Angel T. Gisbert, Nicola Piovella, and Romain Bachelard

The light scattered by cold atoms induces mutual optical forces between them, which can lead to bound states. In addition to the trapping potential, this light-induced interaction generates a velocity-dependent force which damps or amplifies the stretching vibrational mode of the two-atom “molecule.” This velocity-dependent force acts on time scales much longer than the mode period or the dipole dynamics, determining the true stability of the bound state. We show that, for two atoms, the stochastic heating due to spontaneous emission always exceeds the bounding effect, so pairs of cold atoms cannot be truly stable without an extra cooling mechanism.


Optimal work in a harmonic trap with bounded stiffness

Carlos A. Plata, David Guéry-Odelin, E. Trizac, and A. Prados

We apply Pontryagin's principle to drive rapidly a trapped overdamped Brownian particle in contact with a thermal bath between two equilibrium states corresponding to different trap stiffness  κ. We work out the optimal time dependence κ(t) by minimizing the work performed on the particle under the nonholonomic constraint  0≤κ≤κmax, an experimentally relevant situation. Several important differences arise, as compared with the case of unbounded stiffness that has been analyzed in the literature. First, two arbitrary equilibrium states may not always be connected. Second, depending on the operating time tf and the desired compression ratio κf/κi, different types of solutions emerge. Finally, the differences in the minimum value of the work brought about by the bounds may become quite large, which may have a relevant impact on the optimization of heat engines.