Permanent fixing or reversible trapping and release of DNA micropatterns on a gold nanostructure using continuous-wave or femtosecond-pulsed near-infrared laser light

Plasmonic dielectric antennas for hybrid optical nanotweezing and optothermoelectric manipulation of single nanosized extracellular vesicles

This paper showcases an experimental demonstration of near-field optical trapping and dynamic manipulation of an individual extracellular vesicle. This is accomplished through the utilization of a plasmonic dielectric nanoantenna designed to support an optical anapole state-a non-radiating optical state resulting from the destructive interference between electric and toroidal dipoles in the far-field, leading to robust near-field enhancement. To further enhance the field intensity associated with the optical anapole state, a plasmonic mirror is incorporated, thereby boosting trapping capabilities. In addition to demonstrating near-field optical trapping, the study achieves dynamic manipulation of extracellular vesicles by harnessing the thermoelectric effect. This effect is induced in the presence of an ionic surfactant, cetyltrimethylammonium chloride (CTAC), combined with plasmonic heating. Furthermore, the thermoelectric effect improves trapping stability by introducing a wide and deep trapping potential. In summary, our hybrid plasmonic-dielectric trapping platform offers a versatile approach for actively transporting, stably trapping, and dynamically manipulating individual extracellular vesicles.

Electron-Induced Damage by UV Photolysis of DNA Attached to Gold Nanoparticles

Photolysis of DNA attached to gold nanoparticles (AuNPs) with ultraviolet (UV) photons induces DNA damage. The release of nucleobases (Cyt, Gua, Ade, and Thy) from DNA was the major reaction (99%) with an approximately equal release of pyrimidines and purines. This reaction contributes to the formation of abasic sites in DNA. In addition, liquid chromatography-mass spectrometry/MS (LC-MS/MS) analysis revealed the formation of reduction products of pyrimidines (5,6-dihydrothymidine and 5,6-dihydro-2'-deoxyuridine) and eight 2',3'- and 2',5'-dideoxynucleosides. In contrast, there was no evidence of the formation of 5-hydroxymethyluracil and 8-oxo-7,8-dihydroguanine, which are common oxidation products of thymine and guanine, respectively. Using appropriate filters, the main photochemical reactions were found to involve photoelectrons ejected from AuNPs by UV photons. The contribution of "hot" conduction band electrons with energies below the photoemission threshold was minor. The mechanism for the release of free nucleobases by photoelectrons is proposed to take place by the initial formation of transient molecular anions of the nucleobases, followed by dissociative electron attachment at the C1'-N glycosidic bond connecting the nucleobase to the sugar-phosphate backbone. This mechanism is consistent with the reactivity of secondary electrons ejected by X-ray irradiation of AuNPs attached to DNA, as well as the reactions of various nucleic acid derivatives irradiated with monoenergetic very-low-energy electrons (∼2 eV). These studies should help us to understand the chemistry of nanoparticles that are exposed to UV light and that are used as scaffolds and catalysts in molecular biology, curative agents in photodynamic therapy, and components of sunscreens and cosmetics.

Rotational dynamics of indirect optical bound particle assembly under a single tightly focused laser

The optical binding of many particles has the potential to achieve the wide-area formation of a "crystal" of small materials. Unlike conventional optical binding, where the entire assembly of targeted particles is directly irradiated with light, if remote particles can be indirectly manipulated using a single trapped particle through optical binding, the degrees of freedom to create ordered structures can be enhanced. In this study, we theoretically investigate the dynamics of the assembly of gold nanoparticles that are manipulated using a single trapped particle by a focused laser. We demonstrate the rotational motion of particles through an indirect optical force and analyze it in terms of spin-orbit coupling and the angular momentum generation of light. The rotational direction of bound particles can be switched by the numerical aperture. These results pave the way for creating and manipulating ordered structures with a wide area and controlling local properties using scanning laser beams.

Generation of Ultralong Liposome Tubes by Membrane Fusion beneath a Laser-Induced Microbubble on Gold Surfaces

Membrane fusion (MF) is one of the most important and ubiquitous processes in living organisms. In this study, we developed a novel method for MF of liposomes. Our method is based on laser-induced bubble generation on gold surfaces (a plasmonic nanostructure or a flat film). It is a simple and quick process that takes about 1 min. Upon bubble generation, liposomes not only collect and become trapped but also fuse to form long tubes beneath the bubble. Moreover, during laser irradiation, these long tubes remain stable and move with a waving motion while continuing to grow, resulting in the creation of ultralong tubes with lengths of about 50 μm. It should be noted that the morphology of these ultralong tubes is analogous to that of a sea anemone. The behavior of the tubes was also monitored by fluorescence microscopy. The generation of these ultralong tubes is discussed on the basis of Marangoni convection and thermophoresis.

Plasmonic Nanotweezers and Nanosensors for Point-of-Care Applications

The capabilities of manipulating and analyzing biological cells, bacteria, viruses, DNAs, and proteins at high resolution are significant in understanding biology and enabling early disease diagnosis. We discuss progress in developments and applications of plasmonic nanotweezers and nanosensors where the plasmon-enhanced light-matter interactions at the nanoscale improve the optical manipulation and analysis of biological objects. Selected examples are presented to illustrate their design and working principles. In the context of plasmofluidics, which merges plasmonics and fluidics, the integration of plasmonic nanotweezers and nanosensors with microfluidic systems for point-of-care (POC) applications is envisioned. We provide our perspectives on the challenges and opportunities in further developing and applying the plasmofluidic POC devices.

Optical manipulation: advances for biophotonics in the 21st century

SIGNIFICANCE

Optical trapping is a technique capable of applying minute forces that has been applied to studies spanning single molecules up to microorganisms.

AIM

The goal of this perspective is to highlight some of the main advances in the last decade in this field that are pertinent for a biomedical audience.

APPROACH

First, the direct determination of forces in optical tweezers and the combination of optical and acoustic traps, which allows studies across different length scales, are discussed. Then, a review of the progress made in the direct trapping of both single-molecules, and even single-viruses, and single cells with optical forces is outlined. Lastly, future directions for this methodology in biophotonics are discussed.

RESULTS

In the 21st century, optical manipulation has expanded its unique capabilities, enabling not only a more detailed study of single molecules and single cells but also of more complex living systems, giving us further insights into important biological activities.

CONCLUSIONS

Optical forces have played a large role in the biomedical landscape leading to exceptional new biological breakthroughs. The continuous advances in the world of optical trapping will certainly lead to further exploitation, including exciting in-vivo experiments.

Cubic-Phase Metasurface for Three-Dimensional Optical Manipulation

The optical tweezer is one of the important techniques for contactless manipulation in biological research to control the motion of tiny objects. For three-dimensional (3D) optical manipulation, shaped light beams have been widely used. Typically, spatial light modulators are used for shaping light fields. However, they suffer from bulky size, narrow operational bandwidth, and limitations of incident polarization states. Here, a cubic-phase dielectric metasurface, composed of GaN circular nanopillars, is designed and fabricated to generate a polarization-independent vertically accelerated two-dimensional (2D) Airy beam in the visible region. The distinctive propagation characteristics of a vertically accelerated 2D Airy beam, including non-diffraction, self-acceleration, and self-healing, are experimentally demonstrated. An optical manipulation system equipped with a cubic-phase metasurface is designed to perform 3D manipulation of microscale particles. Due to the high-intensity gradients and the reciprocal propagation trajectory of Airy beams, particles can be laterally shifted and guided along the axial direction. In addition, the performance of optical trapping is quantitatively evaluated by experimentally measured trapping stiffness. Our metasurface has great potential to shape light for compact systems in the field of physics and biological applications.

Optical Force-Induced Chemistry at Solution Surfaces

When an intense 1,064-nm continuous-wave laser is tightly focused at solution surfaces, it exerts an optical force on molecules, polymers, and nanoparticles (NPs). Initially, molecules and NPs are gathered into a single assembly inside the focus, and the laser is scattered and propagated through the assembly. The expanded laser further traps them at the edge of the assembly, producing a single assembly much larger than the focus along the surface. Amino acids and inorganic ionic compounds undergo crystallization and crystal growth, polystyrene NPs form periodic arrays and disklike structures with concentric circles or hexagonal packing, and Au NPs demonstrate assembling and swarming, in which the NPs fluctuate like a group of bees. These phenomena that depend on laser polarization are called optically evolved assembling at solution surfaces, and their dynamics and mechanisms are elucidated in this review. As a promising application in materials science, the optical trapping assembly of lead halide perovskites, supramolecules, and aggregation-induced emission enhancement-active molecules is demonstrated and future directions for fundamental study are discussed.

Nonlinear modulation on optical trapping in a plasmonic bowtie structure

Surface plasmon optical tweezers based on micro- and nano-structures are capable of capturing particles in a very small spatial scale and have been widely used in many front research fields. In general, distribution of optical forces and potential wells exerted on the particles can be modulated by controlling the geometric parameters of the structures. However, these fabricated structures are irreversible once processed, which greatly limits its application in dynamic manipulation. The plasmonic field in these structures can be enhanced with orders of magnitude compared to the excitation light, offering a possibility to stimulate nonlinear responses as a new degree of freedom for dynamic modulation. Here, we theoretically demonstrate that the optical force and potential well can be modulated on account of the nonlinear Kerr effect of a gold bowtie structure under a pulsed laser with high peak power. The results verify that the trapping states, including the position, width, and depth of the potential well, can be dynamically modulated by changing intensity of the incident laser. It provides an effective approach for stable trapping and dynamic controlling of particles on nanostructure-based plasmonic trapping platforms and thus has great application potential in many fields, such as enhanced Raman detection, super-resolution imaging, and optical sensing.

Review of optical sensing and manipulation of chiral molecules and nanostructures with the focus on plasmonic enhancements [Invited]

Chiral molecules are ubiquitous in nature; many important synthetic chemicals and drugs are chiral. Detecting chiral molecules and separating the enantiomers is difficult because their physiochemical properties can be very similar. Here we review the optical approaches that are emerging for detecting and manipulating chiral molecules and chiral nanostructures. Our review focuses on the methods that have used plasmonics to enhance the chiroptical response. We also review the fabrication and assembly of (dynamic) chiral plasmonic nanosystems in this context.

Plasmonic optical trapping of nanoparticles using T-shaped copper nanoantennas

We demonstrate the optical trapping of single dielectric nanoparticles in a microfluidic chamber using a coupled T-shaped copper plasmonic nanoantenna for studying light-matter interaction. The nanoantenna is composed of two identical copper elements separated by a 50 nm gap and each element is designed with two nanoblocks. Our nanoantenna inherits three different advantages compared to previous plasmonic nanoantennas, which are usually made of gold. First, copper is a very promising plasmonic material with its very similar optical properties as gold. Second, copper is comparably cheap, which is compatible with industry-standard fabrication processes and has been widely used in microelectronics. Third, the trapping area of tweezers is expanded due to the intrinsic Fabry-Perot cavity with two parallel surfaces. We present finite element method simulations of the near-field distribution and photothermal effects. And we perform Maxwell stress tensor simulations of optical forces exerted on an individual nanoparticle in the vicinity of the nanoantenna. In addition, we examine how the existence of an oxide layer of cupric oxide and the heat sink substrate influence the optical trapping properties of copper nanoantennas. This work demonstrates that the coupled T-shaped copper nanoantennas are a promising means as optical nanotweezers to trap single nanoparticles in solution, opening up a new route for nanophotonic devices in optical information processing and on-chip biological sensing.

Plasmonic tweezers: for nanoscale optical trapping and beyond

Optical tweezers and associated manipulation tools in the far field have had a major impact on scientific and engineering research by offering precise manipulation of small objects. More recently, the possibility of performing manipulation with surface plasmons has opened opportunities not feasible with conventional far-field optical methods. The use of surface plasmon techniques enables excitation of hotspots much smaller than the free-space wavelength; with this confinement, the plasmonic field facilitates trapping of various nanostructures and materials with higher precision. The successful manipulation of small particles has fostered numerous and expanding applications. In this paper, we review the principles of and developments in plasmonic tweezers techniques, including both nanostructure-assisted platforms and structureless systems. Construction methods and evaluation criteria of the techniques are presented, aiming to provide a guide for the design and optimization of the systems. The most common novel applications of plasmonic tweezers, namely, sorting and transport, sensing and imaging, and especially those in a biological context, are critically discussed. Finally, we consider the future of the development and new potential applications of this technique and discuss prospects for its impact on science.

Label-free plasmonic assisted optical trapping of single DNA molecules

DNA molecules are hard to catch using traditional optical trapping due to the nanometer width of their chains. Here we experimentally demonstrate a label-free optical trapping of a single micrometer λ-DNA in solution by the aid of plasmonic gold nanoparticles (GNPs), where a double-laser trap induces strong optical interparticle forces for the tweezer. We examine such sub-resolved interparticle forces by tracking the GNP dynamics in solution. Moreover, surface-enhanced Raman scattering signals of trapped λ-DNA have also been measured simultaneously in the same setup. In comparison with prior works, ours benefit from the excitation in a dynamic configuration without fabrication. This technique opens a new avenue for all-optical manipulation of biomolecules, as well as ultra-sensitive bio-medical sensing applications.

Formation of Single Double-Layered Coacervate of Poly(N,N-diethylacrylamide) in Water by a Laser Tweezer

We demonstrate liquid-liquid phase separation involving both coacervation and coil-to-globule phase transition of a thermoresponsive polymer. By focusing a near-infrared laser beam into an aqueous solution of poly(N-isopropylacrylamide) (PNIPAM), a single phase-separated polymer microdroplet can be formed and stably trapped at the focal point. Such droplet formation is induced by a local elevation in temperature (induced by a photothermal effect) and an optical force. The technique allows us to selectively analyze a single polymer droplet trapped at the focal point. In this study, we applied this technique to poly(N,N-diethylacrylamide) (PDEA) in water and generated a double-layered PDEA droplet. Such an inhomogeneous and complex microstructure has not been previously observed both in steady-state heating of a PDEA solution and in the PNIPAM system. Moreover, we used micro-Raman spectroscopy to clarify that PDEA underwent dehydration due to a coil-to-globule phase transition. Despite this, the polymer concentration (C_poly) of the trapped PDEA droplet was very low and was around 30 wt %. C_poly depended on the molecular weight of PDEA and the laser power that regulates the temperature elevation. These results strongly indicate that PDEA undergoes coacervation in addition to a coil-to-globule phase transition. This study will help provide us with a fundamental understanding of the phase separation mechanisms of thermoresponsive polymers.

Dynamic electrical measurement of biomolecule behavior via plasmonically-excited nanogap fabricated by electromigration

The dynamic motion of DNA oligomers at the nanoscale gap between nanoelectrodes is measured under plasmonic excitation using laser irradiation. The use of a nanogap enables highly sensitive detection of individual molecules using an electrical readout or an optical readout such as Raman spectroscopy. However, the target molecule must reach the nanogap in order to be detected. This study focuses on the use of plasmonic excitation to trap molecules at the nanogap surface. The nanogap electrode is fabricated by electromigration and is, therefore, a much smaller nanogap than the top-down fabrication in the conventional plasmonic trapping studies. To demonstrate the individual molecule detection and to investigate the molecular behavior, the molecules are monitored using an electrical readout under a bias voltage instead of an optical readout used in the conventional studies. The conductance change due to DNA oligomer penetration to the nanogap is observed with the irradiated light intensity of over 1.23 mW. The single-molecule detection is confirmed irradiating the laser to the nanogap. The results suggest that DNA oligomers are spontaneously attracted and concentrated to the nanogap corresponding to the detection point, resulting in high detection probability and sensitivity.

Particle Manipulation with External Field; From Recent Advancement to Perspectives

Physical forces, such as dielectric, magnetic, electric, optical, and acoustic force, provide useful principles for the manipulation of particles, which are impossible or difficult with other approaches. Microparticles, including polymer particles, liquid droplets, and biological cells, can be trapped at a particular position and are also transported to arbitrary locations in an appropriate external physical field. Since the force can be externally controlled by the field strength, we can evaluate physicochemical properties of particles from the shift of the particle location. Most of the manipulation studies are conducted for particles of sub-micrometer or larger dimensions, because the force exerted on nanomaterials or molecules is so weak that their direct manipulation is generally difficult. However, the behavior, interactions, and reactions of such small substances can be indirectly evaluated by observing microparticles, on which the targets are tethered, in a physical field. We review the recent advancements in the manipulation of particles using a physical force and discuss its potentials, advantages, and limitations from fundamental and practical perspectives.

Optical Tweezers Exploring Neuroscience

Over the past decade, optical tweezers (OT) have been increasingly used in neuroscience for studies of molecules and neuronal dynamics, as well as for the study of model organisms as a whole. Compared to other areas of biology, it has taken much longer for OT to become an established tool in neuroscience. This is, in part, due to the complexity of the brain and the inherent difficulties in trapping individual molecules or manipulating cells located deep within biological tissue. Recent advances in OT, as well as parallel developments in imaging and adaptive optics, have significantly extended the capabilities of OT. In this review, we describe how OT became an established tool in neuroscience and we elaborate on possible future directions for the field. Rather than covering all applications of OT to neurons or related proteins and molecules, we focus our discussions on studies that provide crucial information to neuroscience, such as neuron dynamics, growth, and communication, as these studies have revealed meaningful information and provide direction for the field into the future.

Next-Generation Optical Nanotweezers for Dynamic Manipulation: From Surface to Bulk

Optical traps based on strongly confined electromagnetic fields at metal-dielectric interfaces are far more efficient than conventional optical tweezers. Specifically, these near-field nanotweezers allow the trapping of smaller particles at lower optical intensities, which can impact diverse research fields ranging from soft condensed matter physics to materials science and biology. A major thrust in the past decade has been focused on extending the capabilities of plasmonically enhanced nanotweezers beyond diffusion-limited trapping on surfaces such as to achieve dynamic control in the bulk of fluidic environments. Here, we review the recent efforts in optical nanotweezers, especially those involving hybrid forcing schemes, covering both surface and bulk-based techniques. We summarize the important capabilities demonstrated with this promising approach, with niche applications in reconfigurable nanopatterning and on-chip assembly as well as in sorting and separating colloidal nanoparticles.

Nanoscale rotational optical manipulation

Light has momentum, and hence, it can move small particles. The optical tweezer, invented by Ashkin et al. [Opt. Lett. 11, 288 (1986)] is a representative application. It traps and manipulates microparticles and has led to great successes in the biosciences. Currently, optical manipulation of "nano-objects" is attracting growing attention, and new techniques have been proposed and realized. For flexible manipulation, push-pull switching [Phys. Rev. Lett. 109, 087402 (2012)] and super-resolution trapping by using the electronic resonance of nano-objects have been proposed [ACS Photonics 5, 318 (2017)]. However, regarding the "rotational operation" of nano-objects, the full potential of optical manipulation remains unknown. This study proposes mechanisms to realize rotation and direction switching of nano-objects in macroscopic and nanoscopic areas. By controlling the balance between the dissipative force and the gradient force by using optical nonlinearity, the direction of the macroscopic rotational motion of nano-objects is switched. Further, conversion between the spin angular momentum and orbital angular momentum by light scattering through localized surface plasmon resonance in metallic nano-complexes induces optical force for rotational motion in the nanoscale area. This study pieces out fundamental operations of the nanoscale optical manipulation of nanoparticles.

Plasmonic Manipulation of DNA using a Combination of Optical and Thermophoretic Forces: Separation of Different-Sized DNA from Mixture Solution

We demonstrate the size-dependent separation and permanent immobilization of DNA on plasmonic substrates by means of plasmonic optical tweezers. We found that a gold nanopyramidal dimer array enhanced the optical force exerted on the DNA, leading to permanent immobilization of the DNA on the plasmonic substrate. The immobilization was realized by a combination of the plasmon-enhanced optical force and the thermophoretic force induced by a photothermal effect of the plasmons. In this study, we applied this phenomenon to the separation and fixation of size-different DNA. During plasmon excitation, DNA strands of different sizes became permanently immobilized on the plasmonic substrate forming micro-rings of DNA. The diameter of the ring was larger for longer DNA (in base pairs). When we used plasmonic optical tweezers to trap DNA of two different lengths dissolved in solution (φx DNA (5.4 kbp) and λ-DNA (48.5 kbp), or φx DNA and T4 DNA (166 kbp)), the DNA were immobilized, creating a double micro-ring pattern. The DNA were optically separated and immobilized in the double ring, with the shorter sized DNA and the larger one forming the smaller and larger rings, respectively. This phenomenon can be quantitatively explained as being due to a combination of the plasmon-enhanced optical force and the thermophoretic force. Our plasmonic optical tweezers open up a new avenue for the separation and immobilization of DNA, foreshadowing the emergence of optical separation and fixation of biomolecules such as proteins and other ncuelic acids.

Plasmonic tweezers for optical manipulation and biomedical applications

This comprehensive minireview highlights the recent research on the subtypes, optical manipulation, and biomedical applications of plasmonic tweezers.

Towards biological applications of nanophotonic tweezers

Optical trapping (synonymous with optical tweezers) has become a core biophysical technique widely used for interrogating fundamental biological processes on size scales ranging from the single-molecule to the cellular level. Recent advances in nanotechnology have led to the development of 'nanophotonic tweezers,' an exciting new class of 'on-chip' optical traps. Here, we describe how nanophotonic tweezers are making optical trap technology more broadly accessible and bringing unique biosensing and manipulation capabilities to biological applications of optical trapping.

Plasmonic Tweezers towards Biomolecular and Biomedical Applications

With the capability of confining light into subwavelength scale, plasmonic tweezers have been used to trap and manipulate nanoscale particles. It has huge potential to be utilized in biomolecular research and practical biomedical applications. In this short review, plasmonic tweezers based on nano-aperture designs are discussed. A few challenges should be overcome for these plasmonic tweezers to reach a similar level of significance as the conventional optical tweezers.

Bio-Molecular Applications of Recent Developments in Optical Tweezers

In the past three decades, the ability to optically manipulate biomolecules has spurred a new era of medical and biophysical research. Optical tweezers (OT) have enabled experimenters to trap, sort, and probe cells, as well as discern the structural dynamics of proteins and nucleic acids at single molecule level. The steady improvement in OT's resolving power has progressively pushed the envelope of their applications; there are, however, some inherent limitations that are prompting researchers to look for alternatives to the conventional techniques. To begin with, OT are restricted by their one-dimensional approach, which makes it difficult to conjure an exhaustive three-dimensional picture of biological systems. The high-intensity trapping laser can damage biological samples, a fact that restricts the feasibility of in vivo applications. Finally, direct manipulation of biological matter at nanometer scale remains a significant challenge for conventional OT. A significant amount of literature has been dedicated in the last 10 years to address the aforementioned shortcomings. Innovations in laser technology and advances in various other spheres of applied physics have been capitalized upon to evolve the next generation OT systems. In this review, we elucidate a few of these developments, with particular focus on their biological applications. The manipulation of nanoscopic objects has been achieved by means of plasmonic optical tweezers (POT), which utilize localized surface plasmons to generate optical traps with enhanced trapping potential, and photonic crystal optical tweezers (PhC OT), which attain the same goal by employing different photonic crystal geometries. Femtosecond optical tweezers (fs OT), constructed by replacing the continuous wave (cw) laser source with a femtosecond laser, promise to greatly reduce the damage to living samples. Finally, one way to transcend the one-dimensional nature of the data gained by OT is to couple them to the other large family of single molecule tools, i.e., fluorescence-based imaging techniques. We discuss the distinct advantages of the aforementioned techniques as well as the alternative experimental perspective they provide in comparison to conventional OT.

Trapping and Deposition of Dye-Molecule Nanoparticles in the Nanogap of a Plasmonic Antenna

Plasmonic nanostructures, which allow light focusing at the deep subwavelength scale, and colloidal nanoparticles with unique optoelectronic properties are nowadays fabricated with nanometer precision. However, to fully control and exploit nanoscale light-matter interactions in hybrid plasmonic-nanophotonic devices, both materials must be assembled in heterostructures with similar precision. Near-field optical forces have recently attracted much attention, as they can precisely trap and position nanoparticles at plasmonic hotspots. However, long-range attraction and the surface bonding of nanoparticles usually require other specific techniques, such as electrothermal heating and surface chemical treatments. This Letter reports on the optical trapping and deposition of dye-molecule nanoparticles in the nanogap of a gold antenna. The nanoparticles are captured by focusing a near-infrared laser beam on a targeted plasmonic antenna. This single-step deposition process requires only a few seconds under 1.4-1.8 MW·cm^-2 continuous-wave illumination and shows a polarization dependence smaller than expected. Fluorescence and electronic microscopy observations suggest that nanoparticle deposition arises from a trade-off between optical and thermal effects.

Two-dimensional arbitrary nano-manipulation on a plasmonic metasurface

In this Letter, we report on a plasmonic nano-ellipse metasurface with the purpose of trapping and two-dimensional (2D) arbitrary transport of nanoparticles by means of rotating the polarization of an excitation beam. The locations of hot spots within a metasurface are polarization dependent, thus making it possible to turn on/off the adjacent hot spots and then convey the trapped target by rotating the incident polarization state. For the case of a metasurface with a unit cell of perpendicularly orientated nano-ellipses, the hot spots with higher intensities are located at both apexes of the nano-ellipse whose major axis is parallel to the direction of polarization. When the polarization gradually rotates to its counterpart direction, the trapped particle may move around the ellipse and transfer to the most adjacent ellipse, due to the unbalanced trap potentials around the nano-ellipse. Clockwise and counterclockwise rotation would guide the particle in a different direction, which makes it possible to convey the particle arbitrarily within the plasmonic metasurface by setting a time sequence of polarization rotation. As confirmed by the three-dimensional finite-difference time-domain analysis, our design offers a novel scheme of 2D arbitrary transport with nanometer accuracy, which could be used in many on-chip optofluidic applications.

Plasmofluidics for Biosensing and Medical Diagnostics

Plasmofluidics, an extension of optofluidics into the nanoscale regime, merges plasmonics and micro-/nanofluidics for highly integrated and multifunctional lab on a chip. In this chapter, we focus on the applications of plasmofluidics in the versatile manipulation and sensing of biological cell, organelles, molecules, and nanoparticles, which underpin advanced biomedical diagnostics.

Optical tweezing and binding at high irradiation powers on black-Si

Nowadays, optical tweezers have undergone explosive developments in accordance with a great progress of lasers. In the last decade, a breakthrough brought optical tweezers into the nano-world, overcoming the diffraction limit. This is called plasmonic optical tweezers (POT). POT are powerful tools used to manipulate nanomaterials. However, POT has several practical issues that need to be overcome. First, it is rather difficult to fabricate plasmonic nanogap structures regularly and rapidly at low cost. Second, in many cases, POT suffers from thermal effects (Marangoni convection and thermophoresis). Here, we propose an alternative approach using a nano-structured material that can enhance the optical force and be applied to optical tweezers. This material is metal-free black silicon (MFBS), the plasma etched nano-textured Si. We demonstrate that MFBS-based optical tweezers can efficiently manipulate small particles by trapping and binding. The advantages of MFBS-based optical tweezers are: (1) simple fabrication with high uniformity over wafer-sized areas, (2) free from thermal effects detrimental for trapping, (3) switchable trapping between one and two - dimensions, (4) tight trapping because of no detrimental thermal forces. This is the NON-PLASMONIC optical tweezers.

Plasmonic optical trapping of nanometer-sized J- /H- dye aggregates as explored by fluorescence microspectroscopy

In the present study, we explored plasmonic optical trapping (POT) of nanometer-sized organic crystals, carbocyanine dye aggregates (JC-1). JC-1 dye forms both J- and H- aggregates in aqueous solution. POT behavior was analyzed using fluorescence microspectroscopy. POT of JC-1 aggregates was realized in an increase in their fluorescence intensity from the focus area upon plasmon excitation. Repeating on-and-off plasmonic excitation resulted in POT of JC-1 aggregates in a trap-and-release mode. Such POT of nanometer-sized dye aggregates lying in a Rayleigh scattering regime (< 100 nm) is important toward molecular manipulation. Furthermore, interestingly, we found that the J-aggregates were preferentially trapped than H-aggregates. It possibly indicates semi-selective optical trapping of nanoparticles on the basis of molecular alignments.

Plasmonic Trapping and Release of Nanoparticles in a Monitoring Environment

Plasmonic tweezers use surface plasmon polaritons to confine polarizable nanoscale objects. Among the various designs of plasmonic tweezers, only a few can observe immobilized particles. Moreover, a limited number of studies have experimentally measured the exertable forces on the particles. The designs can be classified as the protruding nanodisk type or the suppressed nanohole type. For the latter, microscopic observation is extremely challenging. In this paper, a new plasmonic tweezer system is introduced to monitor particles, both in directions parallel and orthogonal to the symmetric axis of a plasmonic nanohole structure. This feature enables us to observe the movement of each particle near the rim of the nanohole. Furthermore, we can quantitatively estimate the maximal trapping forces using a new fluidic channel.

Characterization of the near-field and convectional transport behavior of micro and nanoparticles in nanoscale plasmonic optical lattices

Here, we report the characterization of the transport of micro- and nanospheres in a simple two-dimensional square nanoscale plasmonic optical lattice. The optical potential was created by exciting plasmon resonance by way of illuminating an array of gold nanodiscs with a loosely focused Gaussian beam. This optical potential produced both in-lattice particle transport behavior, which was due to near-field optical gradient forces, and high-velocity (∼μm/s) out-of-lattice particle transport. As a comparison, the natural convection velocity field from a delocalized temperature profile produced by the photothermal heating of the nanoplasmonic array was computed in numerical simulations. This work elucidates the role of photothermal effects on micro- and nanoparticle transport in plasmonic optical lattices.

Fano resonant Ge2Sb2Te5 nanoparticles realize switchable lateral optical force

Sophisticated optical micromanipulation of small biomolecules usually relies on complex light, e.g., structured light, highly non-paraxial light, or chiral light. One emerging technique is to employ chiral light to drive the chiral nanoparticle along the direction perpendicular to the propagation of the light, i.e., the lateral optical force. Here, we theoretically study the lateral optical force exerted by a entirely Gaussian beam. For the very first time we demonstrate that the Fano resonances (FRs) of the Ge2Sb2Te5 (GST) phase-change nanoparticles encapsulated with Au shells could enable a conventional Gaussian laser to exert a lateral force on such a dielectric GST nanoparticle, attributed to the strongly asymmetric energy flow around the sphere in the dipole-quadrupole FRs. More interestingly, the direction of this lateral force could be reversible during the state transition (i.e., from amorphous to crystalline). By bonding small biomolecules to the outer surface of the phase-change nanoparticle, the particle behaves as a direction-selective vehicle to transport biomolecules along opposite directions, at pre-assessed states of the Ge2Sb2Te5 core correspondingly. Importantly, the origin of the reversal of the lateral optical force is further unveiled by the optical singularity of the Poynting vector. Our mechanism of tailoring the FRs of phase-change nanoparticles, not just limited to GST, may bring a new twist to optical micromanipulation and biomedical applications.

Shell formation in short like-charged polyelectrolytes in a harmonic trap

Inspired by recent experiments and simulations on pattern formation in biomolecules by optical tweezers, a theoretical description based on the reference interaction site model (RISM) is developed to calculate the equilibrium density profiles of small polyelectrolytes in an external potential. The formalism is applied to the specific case of a finite number of Gaussian and rodlike polyelectrolytes trapped in a harmonic potential. The density profiles of the polyelectrolytes are studied over a range of lengths and numbers of polyelectrolytes in the trap, and the strength of the trap potential. For smaller polymers we recover the results for point charges. In the mean field limit the longer polymers, unlike point charges, form a shell at the boundary layer. When the interpolymer correlations are included, the density profiles of the polymers show sharp shells even at weaker trap strengths. The implications of these results are discussed.

Dynamic motions of DNA molecules in an array of plasmonic traps

The dynamic motion of a DNA near a plasmonic nanohole.

A measurement of the maximal forces in plasmonic tweezers

Plasmonic tweezers that are designed to trap nanoscale objects create many new possibilities for single-molecule targeted studies. Numerous novel designs of plasmonic nanostructures are proposed in order to attain stronger forces and weaker laser intensity. Most experiments have consisted only of immobilization observations--that is, particles stick when the laser is turned on and fall away when the laser is turned off. Studies of the exertable forces were only theoretical. A few studies have experimentally measured trap stiffness. However, as far as we know, no studies have addressed maximal forces. In this paper, we present a new experimental design in which the motion of the trapped particle can be monitored in either parallel or orthogonal directions to the plasmonic structure's symmetric axis. We measured maximal trapping force through such monitoring. Although stiffness would be useful for force-calibration or immobilization purposes, for which most plasmonic tweezers are used, we believe that the maximal endurable force is significant and thus, this paper presents this aspect.

Plasmofluidics: Merging Light and Fluids at the Micro-/Nanoscale

Plasmofluidics is the synergistic integration of plasmonics and micro/nanofluidics in devices and applications in order to enhance performance. There has been significant progress in the emerging field of plasmofluidics in recent years. By utilizing the capability of plasmonics to manipulate light at the nanoscale, combined with the unique optical properties of fluids and precise manipulation via micro/nanofluidics, plasmofluidic technologies enable innovations in lab-on-a-chip systems, reconfigurable photonic devices, optical sensing, imaging, and spectroscopy. In this review article, the most recent advances in plasmofluidics are examined and categorized into plasmon-enhanced functionalities in microfluidics and microfluidics-enhanced plasmonic devices. The former focuses on plasmonic manipulations of fluids, bubbles, particles, biological cells, and molecules at the micro/nanoscale. The latter includes technological advances that apply microfluidic principles to enable reconfigurable plasmonic devices and performance-enhanced plasmonic sensors. The article is concluded with perspectives on the upcoming challenges, opportunities, and possible future directions of the emerging field of plasmofluidics.

Origin and Future of Plasmonic Optical Tweezers

Plasmonic optical tweezers can overcome the diffraction limits of conventional optical tweezers and enable the trapping of nanoscale objects. Extension of the trapping and manipulation of nanoscale objects with nanometer position precision opens up unprecedented opportunities for applications in the fields of biology, chemistry and statistical and atomic physics. Potential applications include direct molecular manipulation, lab-on-a-chip applications for viruses and vesicles and the study of nanoscale transport. This paper reviews the recent research progress and development bottlenecks and provides an overview of possible future directions in this field.

Trapping and assembling of particles and live cells on large-scale random gold nano-island substrates

We experimentally demonstrated the use of random plasmonic nano-islands for optical trapping and assembling of particles and live cells into highly organized pattern with low power density. The observed trapping effect is attributed to the net contribution due to near-field optical trapping force and long-range thermophoretic force, which overcomes the axial convective drag force, while the lateral convection pushes the target objects into the trapping zone. Our work provides a simple platform for on-chip optical manipulation of nano- and micro-sized objects, and may find applications in physical and life sciences.

Large-area one-step assembly of three-dimensional porous metal micro/nanocages by ethanol-assisted femtosecond laser irradiation for enhanced antireflection and hydrophobicity

The capability to realize 2D-3D controllable metallic micro/nanostructures is of key importance for various fields such as plasmonics, electronics, bioscience, and chemistry due to unique properties such as electromagnetic field enhancement, catalysis, photoemission, and conductivity. However, most of the present techniques are limited to low-dimension (1D-2D), small area, or single function. Here we report the assembly of self-organized three-dimensional (3D) porous metal micro/nanocages arrays on nickel surface by ethanol-assisted femtosecond laser irradiation. The underlying formation mechanism was investigated by a series of femtosecond laser irradiation under exposure time from 5 to 30 ms. We also demonstrate the ability to control the size of micro/nanocage arrays from 0.8 to 2 μm by different laser pulse energy. This method features rapidness (∼10 min), simplicity (one-step process), and ease of large-area (4 cm(2) or more) fabrication. The 3D cagelike micro/nanostructures exhibit not only improved antireflection from 80% to 7% but also enhanced hydrophobicity from 98.5° to 142° without surface modification. This simple technique for 3D large-area controllable metal microstructures will find great potential applications in optoelectronics, physics, and chemistry.

Plasmonic Optical Tweezers toward Molecular Manipulation: Tailoring Plasmonic Nanostructure, Light Source, and Resonant Trapping

This Perspective describes recent progress in optical trappings of nanoparticles based on localized surface plasmon. This plasmonic optical trapping has great advantages over the conventional optical tweezers, being potentially applicable for a molecular manipulation technique. We review this novel trapping technique from the viewpoints of (i) plasmonic nanostructure, (ii) the light source for plasmon excitation, and (iii) the polarizability of the trapping target. These findings give us future outlook for plasmonic optical trapping. In addition to a brief review, recent developments on plasmonic optical trapping of soft nanomaterials such as proteins, polymer chains, and DNA will be discussed to point out the important issue for further development on this trapping method. Finally, we explore new directions of plasmonic optical trapping.

Ammonia-assisted semicarbonization: a simple method to introduce micropores without damaging a 3D mesoporous carbon nanonetwork structure

A simple and effective way to introduce micropores into skeleton of carbon aerogel (CA) without damaging its unique 3D mesoporous nanonetwork has been successfully developed by NH3-assisted semicarbonization. During the NH3-assisted semicarbonization process, nitrogen functional groups with high thermo-decomposable ability like pyrrolic/pyridine and pyridinic can be introduced into the semicarbonized aerogel framework by substituting oxygen functional groups with low thermo-decomposable ability like C═O quinone-type groups and then escape from the resultant CA framework during the subsequent carbonization, thus forming abundant micropores inside carbon framework under the circumstance of keeping wonderful stability of mesoporous nanonetwork structure. Compared with the traditional CA without NH3 assistance during semicarbonization, the as-prepared novel CA represents a much higher surface area (1100 vs 620 m(2) g(-1)) and a compatible mesopore structure. Meanwhile, such a NH3 treatment confers many useful nitrogen functional groups on the nanonetwork framework. The novel CA is then used as electrode material of supercapacitors and shows a much higher capacitance and comparable high capacitance retention as compared with the traditional CA.

Trapping of a single DNA molecule using nanoplasmonic structures for biosensor applications

Conventional optical trapping using a tightly focused beam is not suitable for trapping particles that are smaller than the diffraction limit because of the increasing need of the incident laser power that could produce permanent thermal damages. One of the current solutions to this problem is to intensify the local field enhancement by using nanoplasmonic structures without increasing the laser power. Nanoplasmonic tweezers have been used for various small molecules but there is no known report of trapping a single DNA molecule. In this paper, we present the trapping of a single DNA molecule using a nanohole created on a gold substrate. Furthermore, we show that the DNA of different lengths can be differentiated through the measurement of scattering signals leading to possible new DNA sensor applications.