Fano Resonance in Asymmetric Plasmonic Nanostructure: Separation of Sub‐10 nm Enantiomers

Tun Cao, Libang Mao, Yimei Qiu, Li Lu, Agnieszka Banas, Krzysztof Banas, Robert E. Simpson, Hsiang‐Chen Chui

Separating enantiomers is vital in chemical syntheses, life sciences, and physics. However, the usual chemical processes are inefficient. Recently, plasmonic nanostructures have drawn considerable attention for manipulating nanoparticles; however, only a few approaches are proposed to discriminate between entities that differ in terms of their handedness. This is because the chiral polarizability is much smaller than the electric polarizability, and therefore the non‐chiral gradient force dominates over the chiral gradient force. This limit means that the enantioselective sorting of chiral nanoparticles is a formidable challenge. A plasmonic nanostructure consisting of a disc‐double split ring resonator exhibiting a dipole–octupole (DO) Fano resonance (FR) is designed and fabricated. It is theoretically demonstrated that such a DO‐FR can markedly enhance the chiral gradient force on the paired enantiomers. The coaxial channel of the resonator possessing high chirality density gradients around the DO‐FR is derived. This provides an enhanced chiral gradient force that dominates over the non‐chiral gradient forces on sub‐10 nm chiral nanoparticles. Enantiomeric pairs can thus experience distinct potential wells in terms of signs. This proposed structure may advance the techniques of enantiopurification and enantioseparation, bringing a new perspective to state‐of‐the‐art all‐optical enantiopure synthesis.

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Simultaneous manipulation of microparticles in multiple planes with the Thue–Morse zone plate beam

Shubo Cheng, Tian Xia, Mengsi Liu and Shaohua Tao

We demonstrate optical manipulation with an optical beam generated by a Thue–Morse zone plate (TMZP) in this paper. The results show that the generated TMZP beam can simultaneously manipulate microparticles positioned in multiple focal planes of the beam, owing to the self-reconstruction property and the symmetry foci with the same intensity along the optical axis. The TMZP beam will be promising in the three-dimensional optical tweezers.

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Wavelength-Dependent Optical Force Aggregation of Gold Nanorods for SERS in a Microfluidic Chip

Silvie Bernatová, Maria Grazia Donato, Jan Ježek, Zdeněk Pilát, Ota Samek, Alessandro Magazzù, Onofrio M. Maragò, Pavel Zemánek, and Pietro G. Gucciardi

Optical printing of metal-nanoparticle–protein complexes in microfluidic chips is of particular interest in view of the potential applications in biomolecular sensing by surface-enhanced Raman spectroscopy (SERS). SERS-active aggregates are formed when the radiation pressure pushes the particle–protein complexes on an inert surface, enabling the ultrasensitive detection of proteins down to pM concentration in short times. However, the role of plasmonic resonances in the aggregation process is still not fully clear. Here, we study the aggregation velocity as a function of excitation wavelength and power. We use a model system consisting of complexes formed of gold nanorods featuring two distinct localized plasmon resonances bound with bovine serum albumin. We show that the aggregation speed is remarkably accelerated by 300 or 30% with respect to the off-resonant case if the nanorods are excited at the long-axis or minor-axis resonance, respectively. Power-dependent experiments evidence a threshold below which no aggregation occurs, followed by a regime with a linear increase in the aggregation speed. At powers exceeding 10 mW, we observe turbulence, bubbling, and a remarkable 1 order of magnitude increase in the aggregation speed. Results in the linear regime are interpreted in terms of a plasmon-enhanced optical force that scales as the extinction cross section and determines the sticking probability of the nanorods. Thermoplasmonic effects are invoked to describe the results at the highest power. Finally, we introduce a method for the fabrication of functional SERS substrates on demand in a microfluidic platform that can serve as the detection part in microfluidic bioassays or lab-on-a-chip devices.

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Negative optical torque on a microsphere in optical tweezers

K. Diniz, R. S. Dutra, L. B. Pires, N. B. Viana, H. M. Nussenzveig, and P. A. Maia Neto

We show that the optical force field in optical tweezers with elliptically polarized beams has the opposite handedness for a wide range of particle sizes and for the most common configurations. Our method is based on the direct observation of the particle equilibrium position under the effect of a transverse Stokes drag force, and its rotation around the optical axis by the mechanical effect of the optical torque. We find overall agreement with theory, with no fitting, provided that astigmatism, which is characterized separately, is included in the theoretical description. Our work opens the way for characterization of the trapping parameters, such as the microsphere complex refractive index and the astigmatism of the optical system, from measurements of the microsphere rotation angle.

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Real-time optical manipulation of particles through turbid media

Tong Peng, Runze Li, Sha An, Xianghua Yu, Meiling Zhou, Chen Bai, Yansheng Liang, Ming Lei, Chunmin Zhang, Baoli Yao, and Peng Zhang

Complex diffusive scattering media pose significant challenges for light focusing as well as optical imaging to be implemented in practice. Recently, it has been demonstrated that the wavefront shaping technique can be applied to realize focusing and imaging through scattering medium. Here we report dynamic optical manipulation of particles through turbid media by employing the interleaved segment wavefront correction method, which is an improved genetic algorithm providing faster convergence speed and higher peak to background ratio. Manipulating micro-beads behind a scattering medium along both one and two dimensional predesigned trajectories in real time has been successfully demonstrated.

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Direct observation of the fast and robust folding of a slipknotted protein by optical tweezers

Chengzhi He, Shuai Li, Xiaoqing Gao, Adam Xiao, Chunguang Hu, Xiaodong Hu, Xiaotang Hu and Hongbin Li

Understanding the folding mechanism of knotted and slipknotted proteins has attracted considerable interest. Due to their topological complexity, knotted and slipknotted proteins are predicted to fold slowly and involve large topological barriers. Molecular dynamics simulation studies suggest that a slipknotted conformation can serve as an important intermediate to help greatly reduce the topological difficulty during the folding of some knotted proteins. Here we use a single molecule optical tweezers technique to directly probe the folding of a small slipknotted protein AFV3-109. We found that stretching AFV3-109 can lead to the untying of the slipknot and complete unfolding of AFV3-109. Upon relaxation, AFV3-109 can readily refold back to its native slipknot conformation with high fidelity when the stretching force is lower than 6 pN. The refolding of AFV3-109 occurs in a sharp two-state like transition. Our results indicate that, different from knotted proteins, the folding of a slipknotted protein like AFV3-109 can be fast, and may not necessarily involve a high topological barrier.

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Opto-thermophoretic fiber tweezers

Abhay Kotnala, Yuebing Zheng

Recent advances in opto-thermophoretic tweezers open new avenues for low-power trapping and manipulation of nanoparticles with potential applications in colloidal assembly, nanomanufacturing, life sciences, and nanomedicine. However, to fully exploit the opto-thermophoretic tweezers for widespread applications, the enhancement of their versatility in nanoparticle manipulations is pivotal. For this purpose, we translate our newly developed opto-thermophoretic tweezers onto an optical fiber platform known as opto-thermophoretic fiber tweezers (OTFT). We have demonstrated the applications of OTFT as a nanoparticle concentrator, as a nanopipette for single particle delivery, and as a nanoprobe. The simple setup and functional versatility of OTFT would encourage its use in various fields such as additive manufacturing, single nanoparticle-cell interactions, and biosensing.

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Strategies for Optical Trapping in Biological Samples: Aiming at Microrobotic Surgeons

Ada‐Ioana Bunea, Jesper Glückstad

Optical trapping and manipulation of objects down to the Ångstrom level has revolutionized research at the smallest scales in all natural sciences. The flexibility of optical trapping methods facilitates real‐time monitoring of the dynamics of biological processes in model systems and even in living cells. Different optical trapping and manipulation approaches allow displacement of nanostructures with subnanometer precision and force measurements with femtonewton precision. Due to inherent constraints of optical methods, most optical trapping experiments are performed in water or simple aqueous solutions. However, in recent years, there is an ever‐growing interest of shifting from simple aqueous media towards more biologically‐relevant media. Precise optical trapping and manipulation, combined with state‐of‐the‐art microfabrication, will enable the development of microrobotic “surgeons” with tremendous potential for biomedical and microengineering applications. This review introduces the basics of optical trapping and discusses its applications for biological samples, with focus on trapping in biological media and strategies for overcoming the challenges of optical manipulation in complex environments as a stepping‐stone for microrobotic “surgeons.”

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Three-dimensional optical trapping and orientation of microparticles for coherent X-ray diffraction imaging

Yuan Gao, Ross Harder, Stephen H. Southworth, Jeffrey R. Guest, Xiaojing Huang, Zijie Yan, Leonidas E. Ocola, Yuval Yifat, Nishant Sule, Phay J. Ho, Matthew Pelton, Norbert F. Scherer, and Linda Young
Optical trapping has been implemented in many areas of physics and biology as a noncontact sample manipulation technique to study the structure and dynamics of nano- and mesoscale objects. It provides a unique approach for manipulating microscopic objects without inducing undesired changes in structure. Combining optical trapping with hard X-ray microscopy techniques, such as coherent diffraction imaging and crystallography, provides a nonperturbing environment where electronic and structural dynamics of an individual particle in solution can be followed in situ. It was previously shown that optical trapping allows the manipulation of micrometer-sized objects for X-ray fluorescence imaging. However, questions remain over the ability of optical trapping to position objects for X-ray diffraction measurements, which have stringent requirements for angular stability. Our work demonstrates that dynamic holographic optical tweezers are capable of manipulating single micrometer-scale anisotropic particles in a microfluidic environment with the precision and stability required for X-ray Bragg diffraction experiments—thus functioning as an “optical goniometer.” The methodology can be extended to a variety of X-ray experiments and the Bragg coherent diffractive imaging of individual particles in solution, as demonstrated here, will be markedly enhanced with the advent of brighter, coherent X-ray sources.

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Inexpensive Design of a Bio‐Chip for Disease Diagnostics: Molecular Biomarker Sensing Microchip Patterned from a Soft Oxometalate‐Perylene‐Based Hybrid Composite using Thermo‐Optical Laser Tweezers

Preethi Thomas, Subhrokoli Ghosh, Apabrita Mallick, Ayan Banerjee, Soumyajit Roy

In this work, we have developed linear micro‐patterns of phosphotungstic acid soft oxometalate (SOM) and perylene (as fluorophore) hybrid composite on a glass substrate using a thermo‐optical tweezers set‐up, and used it for sensing of biologically important molecules such as glucose, uric acid and ascorbic acid. Bulk scale studies on the SOM‐fluorophore hybrid were initially performed to optimize the fabrication of the pattern. The aqueous dispersion of the hybrid composite was subjected to UV/Visible absorption and fluorescence measurements. The linear micro‐patterns were characterized using Raman spectroscopy and AFM. The ability of the patterns to sense different biomarkers was monitored by fluorescence microscopy at different time intervals and could open up possibilities for inexpensive and facile detection of biologically important molecules, and thus introduce a new paradigm in reliable, robust, and low cost disease diagnostics.

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Directionality of dynein is controlled by the angle and length of its stalk

Sinan Can, Samuel Lacey, Mert Gur, Andrew P. Carter & Ahmet Yildiz

The ability of cytoskeletal motors to move unidirectionally along filamentous tracks is central to their role in cargo transport, motility and cell division. Kinesin and myosin motor families have a subclass that moves towards the opposite end of the microtubule or actin filament with respect to the rest of the motor family1,2, whereas all dynein motors that have been studied so far exclusively move towards the minus end of the microtubule3. Guided by cryo-electron microscopy and molecular dynamics simulations, we sought to understand the mechanism that underpins the directionality of dynein by engineering a Saccharomyces cerevisiae dynein that is directed towards the plus end of the microtubule. Here, using single-molecule assays, we show that elongation or shortening of the coiled-coil stalk that connects the motor to the microtubule controls the helical directionality of dynein around microtubules. By changing the length and angle of the stalk, we successfully reversed the motility towards the plus end of the microtubule. These modifications act by altering the direction in which the dynein linker swings relative to the microtubule, rather than by reversing the asymmetric unbinding of the motor from the microtubule. Because the length and angle of the dynein stalk are fully conserved among species, our findings provide an explanation for why all dyneins move towards the minus end of the microtubule.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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Ray optics analysis of optical forces on a microsphere in a (2 + 1)D Airy beam

Shuhe Zhang, Jinhua Zhou, and Yu-Xuan Ren

The optical forces of a (2 + 1)D Airy beam on a microsphere are studied in the ray optics regime. The ray model of a (2 + 1)D Airy beam is derived from its Fourier angular spectrum using a stable aggregate of the flexible elements theory. Numerical results demonstrate that the microsphere can be trapped by the transverse optical force and pulled towards the beam major lobe. Longitudinal optical forces further push the microsphere towards the positive z-direction. The trend for the movement of a microsphere in an Airy beam is clearly demonstrated as the stream line of optical forces, which is consistent with the observed phenomena in optical trapping experiments. In the meantime, both the transverse and the longitudinal optical forces increase when the relative refractive index of the trapped microsphere increases. Calculation of optical forces on microspheres with larger size reveals that the optical forces contributed by rays on each hemisphere are actually different due to the asymmetry of the Airy beam. The force difference could cause natural torque on the trapped objects if they are ovals or other asymmetric shapes.

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Optomechanical properties of optically self-arranged colloidal waveguides

Oto Brzobohatý, Lukáš Chvátal, and Pavel Zemánek

When a suspension of wavelength-sized polystyrene spheres is illuminated with non-interfering counter-propagating Gaussian beams, the particles self-arrange into a colloidal waveguide (CWG). Mutual force interaction among particles is mediated by scattered light, referred to as the optical binding. We analyzed the longitudinal and lateral motion of particles in such CWGs made of an increasing number of particles with diameters of either 520 or 657 nm. We observed the enhancement of the binding stiffness of neighboring particles by more than an order of magnitude. This enhancement is done by optical means, mainly due to a local increase of optical intensity due to multiple light scattering in an optically bound structure.

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Optical Forces in the T-matrix formalism

Paolo Polimeno, Rosalba Saija, Cristian Degli Esposti Boschi, Onofrio M. Maragò, Maria Antonia Iatì
Optical tweezers are a crucial tool for the manipulation and characterisation, without mechanical contact, of micro- and nanoparticles, ranging from biological components, such as biomolecules, viruses, bacteria, and cells, to nanotubes, nanowires, layered materials, plasmonic nanoparticles, and their composites. Despite the many interdisciplinary applications, only recently it has been possible to develop an accurate theoretical modelling for the mesoscale size range. This goes beyond the strong approximations typically used for the calculation of optical forces on particles much smaller (dipole approximation) or much larger (ray optics) than the wavelength of the trapping light. Among the different methods used to calculate optical forces on model particles, the ones based on the transition matrix (T-matrix) are currently among the most accurate and efficient, particularly when applied to non-spherical particles, both isolated and interacting, or in composite structures. Here, we first give an overview of the theoretical background on optical forces, optomechanics, and T-matrix methods. Then, we focus on calculations of optical trapping on model polystyrene nanowires with the aim to investigate their scaling with nanowire length at the mesoscale. We compare the force constant dependence with approximations at small or large length with respect to the trapping wavelength and with calculations on spheres, pointing out the role of shape.

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Microfluidic platforms for cell cultures and investigations

Maria Laura Coluccio, Gerardo Perozziello, Natalia Malara, Elvira Parrotta, Peng Zhang, Francesco Gentile, Tania Limongi, Pushparani Michael Raj, Gianni Cuda,Patrizio Candeloro, Enzo Di Fabrizio

This review covers several aspects of microfluidic devices used for culturing and monitoring of both adherent and non-adherent cells, including a multitude of applications. A comparison of available platforms with high throughput analysis, automation capability, interface to sensors and integration, is reported. Aspects, such as operational versatility of the devices, are scrutinized in terms of their analytical efficacy. It is found that due to multi-functionality capability of modern microfluidics, there is big amount of experimental data obtainable from a single device, allowing complex experimental control and efficient data correlation, particularly important when biomedical studies are considered. Hence several examples on cell culture and monitoring are given in this review, including details on design of microfluidic devices with their distinctive technological peculiarities.

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Flow-Induced Transport via Optical Heating of a Single Gold Nanoparticle

Jun-ichi Chikazawa, Takayuki Uwada, Akihiro Furube, and Shuichi Hashimoto

Optothermal trapping has gained increasing popularity in manipulation such as selecting, guiding, and positioning submicron objects because of a few mW laser power much lower than that required for optical trapping. Optothermal trapping uses thermal-gradient-induced phoretic motions, but the underlying physics of driving force has not been fully understood. In this study, we performed optothermal trapping of 500 nm-diameter colloidal silica via a continuous laser illumination of a single gold nanoparticle from the bottom in a closed chamber. Under illumination, the tracer particles were attracted to the gold nanoparticle and trapped. Notably, the direction of migrating particles was always to hot gold nanoparticles regardless of the configuration of gold nanoparticles placed at two opposite sides of the chamber, on the bottom surface of an upper substrate (ceiling) or on the top surface of a lower substrate (floor). The previous interpretation based on thermal convective flow from the bottom to the top and circulating inside the chamber was only applicable to floor configuration and failed to explain our observation for the ceiling. Instead, temperature-induced Marangoni effect at the water/superheated water interface is likely to play a role. This study promoted a better understanding of the driving mechanism in optothermal trapping. Moreover, as an application of the single-particle platform, we showed the photothermal phase separation-induced microdroplet formation of thermoresponsive polymers and the coating of non-thermoresponsive polymers on nanoparticles.

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Selective Localization of Hierarchically Assembled Particles to Plasma Membranes of Living Cells

Asish C. Misra,  Tae‐Hong Park,  Randy P. Carney,  Giulia Rusciano,  Francesco Stellacci,  Joerg Lahann

Particles that preferentially partition to a specific cellular subunit, such as the nucleus, mitochondria, or the cytoskeleton, are of relevance to a number of applications, including drug delivery, genetic manipulation, or self‐assembly. Here, hierarchical assemblies of fully synthetic particles that selectively localize to the plasma membrane of mammalian cells are presented. A multimodal approach is used to create assemblies of polymer‐based carrier particles with amphiphilic gold nanoparticles immobilized on one hemisphere. These assemblies persist in the plasma membrane of cells for several days and undergo rearrangements and clustering, typically considered to be hallmarks of membrane‐bound receptors.

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Magnetic hot-spot generation at optical frequencies: from plasmonic metamolecules to all-dielectric nanoclusters

Eugenio Calandrini, Andrea Cerea, Francesco De Angelis, Remo Proietti Zaccaria, Andrea Toma

The weakness of magnetic effects at optical frequencies is directly related to the lack of symmetry between electric and magnetic charges. Natural materials cease to exhibit appreciable magnetic phenomena at rather low frequencies and become unemployable for practical applications in optics. For this reason, historically important efforts were spent in the development of artificial materials. The first evidence in this direction was provided by split-ring resonators in the microwave range. However, the efficient scaling of these devices towards the optical frequencies has been prevented by the strong ohmic losses suffered by circulating currents. With all of these considerations, artificial optical magnetism has become an active topic of research, and particular attention has been devoted to tailor plasmonic metamolecules generating magnetic hot spots. Several routes have been proposed in these directions, leading, for example, to plasmon hybridization in 3D complex structures or Fano-like magnetic resonances. Concurrently, with the aim of electromagnetic manipulation at the nanoscale and in order to overcome the critical issue of heat dissipation, alternative strategies have been introduced and investigated. All-dielectric nanoparticles made of high-index semiconducting materials have been proposed, as they can support both magnetic and electric Mie resonances. Aside from their important role in fundamental physics, magnetic resonances also provide a new degree of freedom for nanostructured systems, which can trigger unconventional nanophotonic processes, such as nonlinear effects or electromagnetic field localization for enhanced spectroscopy and optical trapping.

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Optofluidic control of the dispersion of nanoscale dumbbells

M. Meléndez, N. Alcázar-Cano, R. P. Peláez, J. J. Sáenz, and R. Delgado-Buscalioni

Previous research has shown that gold nanoparticles immersed in water in an optical vortex lattice formed by the perpendicular intersection of two standing light waves with a
π/2 rad phase difference will experience enhanced dispersion that scales with the intensity of the incident laser. We show that flexible nanoscale dumbbells (created by attaching two such gold particles by means of a polymer chain) in the same field display different types of motion depending on the chain length and field intensity. We have not disregarded the secondary optical forces due to light scattering. The dumbbells may disperse, rotate, or remain trapped. For some values of the parameters, the (enhanced) dispersion possesses a displacement distribution with exponential tails, making the motion anomalous, though Brownian.

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