High-Resolution Large-Ensemble Nanoparticle Trapping with Multifunctional Thermoplasmonic Nanohole Metasurface

Hybrid plasmonic metasurface as enhanced Raman hot-spots for pesticide detection at ultralow concentrations

A surface-enhanced Raman scattering (SERS) active metasurface composed of metallic nanohole arrays and metallic nanoparticles is developed. The metasurface can operate in aqueous environments, achieves an enhancement factor of 1.83 × 10⁹ for Rhodamine 6G, and enables the detection of malachite green at a concentation of 0.46 ppb.

Opto-thermoelectric trapping of fluorescent nanodiamonds on plasmonic nanostructures

Deterministic optical manipulation of fluorescent nanodiamonds (FNDs) in fluids has emerged as an experimental challenge in multimodal biological imaging. Designing and developing nano-optical trapping strategies to serve this purpose is an important task. In this Letter, we show how chemically prepared gold nanoparticles and silver nanowires can facilitate an opto-thermoelectric force to trap individual entities of FNDs using a long working distance lens, low power-density illumination (532-nm laser, 12 µW/µm²). Our trapping configuration combines the thermoplasmonic fields generated by individual plasmonic nanoparticles and the opto-thermoelectric effect facilitated by the surfactant to realize a nano-optical trap down to a single FND that is 120 nm in diameter. We use the same trapping excitation source to capture the spectral signatures of single FNDs and track their position. By tracking the FND, we observe the differences in the dynamics of the FND around different plasmonic structures. We envisage that our drop-casting platform can be extrapolated to perform targeted, low-power trapping, manipulation, and multimodal imaging of FNDs inside biological systems such as cells.

Menezes L, Tittl A, Ren H, Maier SA. Optical Metasurfaces for Energy Conversion

Nanostructured surfaces with designed optical functionalities, such as metasurfaces, allow efficient harvesting of light at the nanoscale, enhancing light-matter interactions for a wide variety of material combinations. Exploiting light-driven matter excitations in these artificial materials opens up a new dimension in the conversion and management of energy at the nanoscale. In this review, we outline the impact, opportunities, applications, and challenges of optical metasurfaces in converting the energy of incoming photons into frequency-shifted photons, phonons, and energetic charge carriers. A myriad of opportunities await for the utilization of the converted energy. Here we cover the most pertinent aspects from a fundamental nanoscopic viewpoint all the way to applications.

Label-Free Optical Analysis of Biomolecules in Solid-State Nanopores: Toward Single-Molecule Protein Sequencing

Sequence identification of peptides and proteins is central to proteomics. Protein sequencing is mainly conducted by insensitive mass spectroscopy because proteins cannot be amplified, which hampers applications such as single-cell proteomics and precision medicine. The commercial success of portable nanopore sequencers for single DNA molecules has inspired extensive research and development of single-molecule techniques for protein sequencing. Among them, three challenges remain: (1) discrimination of the 20 amino acids as building blocks of proteins; (2) unfolding proteins; and (3) controlling the motion of proteins with nonuniformly charged sequences. In this context, the emergence of label-free optical analysis techniques for single amino acids and peptides by solid-state nanopores shows promise for addressing the first challenge. In this Perspective, we first discuss the current challenges of single-molecule fluorescence detection and nanopore resistive pulse sensing in a protein sequencing. Then, label-free optical methods are described to show how they address the single-amino-acid identification within single peptides. They include localized surface plasmon resonance detection and surface-enhanced Raman spectroscopy on plasmonic nanopores. Notably, we report new data to show the ability of plasmon-enhanced Raman scattering to record and discriminate the 20 amino acids at a single-molecule level. In addition, we discuss briefly the manipulation of molecule translocation and liquid flow in plasmonic nanopores for controlling molecule movement to allow high-resolution reading of protein sequences. We envision that a combination of Raman spectroscopy with plasmonic nanopores can succeed in single-molecule protein sequencing in a label-free way.

Heat-Mediated Optical Manipulation

Progress in optical manipulation has stimulated remarkable advances in a wide range of fields, including materials science, robotics, medical engineering, and nanotechnology. This Review focuses on an emerging class of optical manipulation techniques, termed heat-mediated optical manipulation. In comparison to conventional optical tweezers that rely on a tightly focused laser beam to trap objects, heat-mediated optical manipulation techniques exploit tailorable optothermo-matter interactions and rich mass transport dynamics to enable versatile control of matter of various compositions, shapes, and sizes. In addition to conventional tweezing, more distinct manipulation modes, including optothermal pulling, nudging, rotating, swimming, oscillating, and walking, have been demonstrated to enhance the functionalities using simple and low-power optics. We start with an introduction to basic physics involved in heat-mediated optical manipulation, highlighting major working mechanisms underpinning a variety of manipulation techniques. Next, we categorize the heat-mediated optical manipulation techniques based on different working mechanisms and discuss working modes, capabilities, and applications for each technique. We conclude this Review with our outlook on current challenges and future opportunities in this rapidly evolving field of heat-mediated optical manipulation.

A numerical study on the closed packed array of gold discs as an efficient dual mode plasmonic tweezers

In this report, we propose the closed pack array of gold discs on glass, as a dual mode plasmonic tweezers that benefits from two trapping modes. The first trapping mode is based on leaky surface plasmon mode (LSPM) on the gold discs with a longer penetration depth in the water and a longer spatial trapping range, so that target nanoparticles with a radius of 100 nm can be attracted toward the gold surface from a vertical distance of about 2 µm. This trapping mode can help to overcome the inherent short range trapping challenge in the plasmonic tweezers. The second trapping mode is based on the dimer surface plasmonic mode (DSPM) in the nano-slits between the neighboring gold discs, leading to isolated and strong trapping sites for nanoparticles smaller than 34 nm. The proposed plasmonic tweezers can be excited in both LSPM and DSPM modes by switching the incident wavelength, resulting in promising and complementary functionalities. In the proposed plasmonic tweezers, we can attract the target particles towards the gold surface by LSPM gradient force, and trap them within a wide half width half maximum (HWHM) that allows studying the interactions between the trapped particles, due to their spatial proximity. Then, by switching to the DSPM trapping mode, we can rearrange the particles in a periodic pattern of isolated and stiff traps. The proposed plasmonic structure and the presented study opens a new insight for realizing efficient, dual-mode tweezers with complementary characteristics, suitable for manipulation of nanoparticles. Our thermal simulations demonstrate that the thermal-induced forces does not interefe with the proposed plasmonic tweezing.

Facile open-well immunofluorescence enhancement with coplanar-electrodes-enabled optoelectrokinetics and magnetic particles

Electrokinetic manipulation has been proven powerful in enhancing the sensing capability of general-purpose biochips. However, the close-form configuration of biochips and the required use of low electric conductivity limit their practicability. In this study, an open-well microfluidic system facilitated with coplanar-electrodes-enabled optoelectrokinetic concentration and magnetic particles were therefore developed to overcome these challenges. The open side achieves optoelectrokinetic manipulation for biosignal enhancement, enabling free manual operations. Magnetic particles were employed in immunoassays to facilitate the rapid onsite separation of targets. A common cytokine biomarker found in many diseases, that is, tumor necrosis factor alpha (TNF-α), was used for assessing the immunosensing system. In addition to the benefits inherited from the immunoassays, the fluorescent signal enhanced by the optoelectrokinetic technique also featured rapid enhancement in 1 min and a limit of detection of as low as 2.9 pg/mL. The open-well architecture allowed the entire immunosensing process to be completed on site without frequent off-site washing. For a practical test, the TNF-α in human tear fluids was measured by the developed device and validated with a standard enzyme-linked immunosorbent assay (ELISA). The data show consistency in terms of trend. The developed open-well optoelectrokinetic device provides an insight into future facile clinical diagnoses. By simply modifying the surface linkers on the magnetic particles, the technique can be further extended to more other trace biomarker detections.

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.

Electrothermoplasmonic Trapping and Dynamic Manipulation of Single Colloidal Nanodiamond

Low-power trapping of nanoscale objects can be achieved by using the enhanced fields near plasmonic nanoantennas. Unfortunately, in this approach the trap site is limited to the position of the plasmonic hotspots and continuous dynamic manipulation is not feasible. Here, we report a low-frequency electrothermoplasmonic tweezer (LFET) that provides low-power, high-stability and continuous dynamic manipulation of a single nanodiamond. LFET harnesses the combined action of the laser illumination of a plasmonic nanopillar antenna array and low-frequency alternating current (ac) electric field to establish an electrohydrodynamic potential capable of the stable trapping and dynamic manipulation of single nanodiamonds. We experimentally demonstrate the fast transport, trapping, and dynamic manipulation of a single nanodiamond using a low-frequency ac field below 5 kHz and low-laser power of 1 mW. This nanotweezer platform for nanodiamond manipulation holds promise for the scalable assembly of single photon sources for quantum information processing and low noise quantum sensors.

Optothermally Assembled Nanostructures

Nanofabrication is one of the core techniques in rapidly evolving nanoscience and nanotechnology. Conventional top-down nanofabrication approaches such as photolithography and electron beam lithography can produce high-resolution nanostructures in a robust way. However, these methods usually involve multistep processing and sophisticated instruments and have difficulty in fabricating three-dimensional complex structures of multiple materials and reconfigurability. Recently, bottom-up techniques have emerged as promising alternatives to fabricating nanostructures via the assembly of individual building blocks. In comparison to top-down lithographical methods, bottom-up assembly features the on-demand construction of superstructures with controllable configurations at single-particle resolution. The size, shape, and composition of chemically synthesized building blocks can also be precisely tailored down to the atomic scale to fabricate multimaterial architectural structures of high flexibility. Many techniques have been reported to assemble individual nanoparticles into complex structures, such as self-assembly, DNA nanotechnology, patchy colloids, and optically controlled assembly. Among them, the optically controlled assembly has the advantages of remote control, site-specific manipulation of single components, applicability to a wide range of building blocks, and arbitrary configurations of the assembled structures. In this Account, we provide a concise review of our contributions to the optical assembly of architectural materials and structures using discrete nanoparticles as the building blocks. By exploiting entropically favorable optothermal conversion and controlling optothermal-matter interactions, we have developed optothermal assembly techniques to manipulate and assemble individual nanoparticles. Our techniques can be operated both in solution and on solid substrates. First, we discuss the opto-thermoelectric assembly (OTA) of colloidal particles into superstructures by coordinating thermophoresis and interparticle depletion bonding in the solution. Localized laser heating generates a temperature gradient field, where the thermal migration of ions creates a thermoelectric field to trap charged particles. The depletion of ion species at the gap between closely positioned particles under optical heating provides strong interparticle bonding to stabilize colloidal superstructures with precisely controlled configurations and interparticle distances. Second, we discuss bubble-pen lithography (BPL) for the rapid printing of nanoparticles using an optothermal microbubble. The long-range convection flow induced by the optothermal bubble drags the colloidal particles to the substrate with a high velocity. BPL represents a general method for printing all kinds of building blocks into desired patterns in a high-resolution and high-throughput way. Third, we present the optothermally-gated photon nudging (OPN) technique, which manipulates and assembles particles on a solid substrate. Our solid-phase optical control of particles synergizes the modulation of particle-substrate interactions by optothermal effects and photon nudging of the particles by optical scattering forces. Operated on the solid surfaces without liquid media, OPN can avoid the undesired Brownian motion of nanoparticles in solutions to manipulate individual particles with high accuracy. In addition, the assembled structures can be actively reassembled into new configurations for the fabrication of tunable functional devices. Next, we discuss applications of the optothermally assembled nanostructures in surface-enhanced Raman spectroscopy, color displays, biomolecule sensing, and fundamental research. Finally, we conclude this Account with our perspectives on the challenges, opportunities, and future directions in the development and application of optothermal assembly.

Plasmonic Optical Tweezers for Particle Manipulation: Principles, Methods, and Applications

Inspired by the idea of combining conventional optical tweezers with plasmonic nanostructures, a technique named plasmonic optical tweezers (POT) has been widely explored from fundamental principles to applications. With the ability to break the diffraction barrier and enhance the localized electromagnetic field, POT techniques are especially effective for high spatial-resolution manipulation of nanoscale or even subnanoscale objects, from small bioparticles to atoms. In addition, POT can be easily integrated with other techniques such as lab-on-chip devices, which results in a very promising alternative technique for high-throughput single-bioparticle sensing or imaging. Despite its label-free, high-precision, and high-spatial-resolution nature, it also suffers from some limitations. One of the main obstacles is that the plasmonic nanostructures are located over the surfaces of a substrate, which makes the manipulation of bioparticles turn from a three-dimensional problem to a nearly two-dimensional problem. Meanwhile, the operation zone is limited to a predefined area. Therefore, the target objects must be delivered to the operation zone near the plasmonic structures. This review summarizes the state-of-the-art target delivery methods for the POT-based particle manipulating technique, along with its applications in single-bioparticle analysis/imaging, high-throughput bioparticle purifying, and single-atom manipulation. Future developmental perspectives of POT techniques are also discussed.

Long-range optofluidic control with plasmon heating

Using light to manipulate fluids has been a long-sought-after goal for lab-on-a-chip applications to address the size mismatch between bulky external fluid controllers and microfluidic devices. Yet, this goal has remained elusive due to the complexity of thermally driven fluid dynamic phenomena, and the lack of approaches that allow comprehensive multiscale and multiparameter studies. Here, we report an innovative optofluidic platform that fulfills this need by combining digital holographic microscopy with state-of-the-art thermoplasmonics, allowing us to identify the different contributions from thermophoresis, thermo-osmosis, convection, and radiation pressure. In our experiments, we demonstrate that a local thermal perturbation at the microscale can lead to mm-scale changes in both the particle and fluid dynamics, thus achieving long-range transport. Furthermore, thanks to a comprehensive parameter study involving sample geometry, temperature increase, light fluence, and size of the heat source, we showcase an integrated and reconfigurable all-optical control strategy for microfluidic devices, thereby opening new frontiers in fluid actuation technology.

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.

Quantifying the Role of the Surfactant and the Thermophoretic Force in Plasmonic Nano-optical Trapping

Plasmonic nanotweezers use intense electric field gradients to generate optical forces able to trap nano-objects in liquids. However, part of the incident light is absorbed into the metal, and a supplementary thermophoretic force acting on the nano-object arises from the resulting temperature gradient. Plasmonic nanotweezers thus face the challenge of disentangling the intricate contributions of the optical and thermophoretic forces. Here, we show that commonly added surfactants can unexpectedly impact the trap performance by acting on the thermophilic or thermophobic response of the nano-object. Using different surfactants in double nanohole plasmonic trapping experiments, we measure and compare the contributions of the thermophoretic and the optical forces, evidencing a trap stiffness 20× higher using sodium dodecyl sulfate (SDS) as compared to Triton X-100. This work uncovers an important mechanism in plasmonic nanotweezers and provides guidelines to control and optimize the trap performance for different plasmonic designs.

Stand-off trapping and manipulation of sub-10 nm objects and biomolecules using opto-thermo-electrohydrodynamic tweezers

Optical tweezers have emerged as a powerful tool for the non-invasive trapping and manipulation of colloidal particles and biological cells^1,2. However, the diffraction limit precludes the low-power trapping of nanometre-scale objects. Substantially increasing the laser power can provide enough trapping potential depth to trap nanoscale objects. Unfortunately, the substantial optical intensity required causes photo-toxicity and thermal stress in the trapped biological specimens³. Low-power near-field nano-optical tweezers comprising plasmonic nanoantennas and photonic crystal cavities have been explored for stable nanoscale object trapping^4-13. However, the demonstrated approaches still require that the object is trapped at the high-light-intensity region. We report a new kind of optically controlled nanotweezers, called opto-thermo-electrohydrodynamic tweezers, that enable the trapping and dynamic manipulation of nanometre-scale objects at locations that are several micrometres away from the high-intensity laser focus. At the trapping locations, the nanoscale objects experience both negligible photothermal heating and light intensity. Opto-thermo-electrohydrodynamic tweezers employ a finite array of plasmonic nanoholes illuminated with light and an applied a.c. electric field to create the spatially varying electrohydrodynamic potential that can rapidly trap sub-10 nm biomolecules at femtomolar concentrations on demand. This non-invasive optical nanotweezing approach is expected to open new opportunities in nanoscience and life science by offering an unprecedented level of control of nano-sized objects, including photo-sensitive biological molecules.

Opto-Thermoelectric Tweezers: Principles and Applications

Opto-thermoelectric tweezers (OTET), which exploit the thermophoretic matter migration under a light-directed temperature field, present a new platform for manipulating colloidal particles with a wide range of materials, sizes, and shapes. Taking advantage of the entropically favorable photon-phonon conversion in light-absorbing materials and spatial separation of dissolved ions in electrolytes, OTET can manipulate the particles in a low-power and high-resolution fashion. In this mini-review, we summarize the concept, working principles, and applications of OTET. Recent developments of OTET in three-dimensional manipulation and parallel trapping of particles are discussed thoroughly. We further present their initial applications in particle filtration and biological studies. With their future development, OTET are expected to find a wide range of applications in life sciences, nanomedicine, colloidal sciences, photonics, and materials sciences.

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.

Fano-Resonant, Asymmetric, Metamaterial-Assisted Tweezers for Single Nanoparticle Trapping

Plasmonic nanostructures overcome Abbe's diffraction limit to create strong gradient electric fields, enabling efficient optical trapping of nanoparticles. However, it remains challenging to achieve stable trapping with low incident laser intensity. Here, we demonstrate Fano resonance-assisted plasmonic optical tweezers for single nanoparticle trapping in an array of asymmetrical split nanoapertures on a 50 nm gold thin film. A large normalized trap stiffness of 8.65 fN/nm/mW for 20 nm polystyrene particles at a near-resonance trapping wavelength of 930 nm was achieved. The trap stiffness on-resonance is enhanced by a factor of 63 compared to that of off-resonance due to the ultrasmall mode volume, enabling large near-field strengths and a cavity effect contribution. These results facilitate trapping with low incident laser intensity, thereby providing new options for studying transition paths of single molecules such as proteins.

Two-state switchable plasmonic tweezers for dynamic manipulation of nano-objects

In this work, we present a plasmonic platform capable of trapping nano-objects in two different spatial configurations. The switch between the two trapping states, localized on the tip and on the outer wall of a vertical gold nanochannel, can be activated by varying the focusing position of the excitation laser along the main axis of the nanotube. We show that the switching of the trapping site is induced by changes in the distribution of the electromagnetic field and of the trapping force. The "inner" and "outer" trapping states are characterized by a static and a dynamic behavior respectively, and their stiffness is measured by analyzing the positions of the trapped specimens as a function of time. In addition, we demonstrate that the stiffness of the static state is high enough to trap particles with diameter as small as 40 nm. These results show a simple, controllable way to generate a switchable two-state trapping regime, which could be used as a model for the study of dynamic trapping or as a mechanism for the development of nanofluidic devices.

Opto-thermoelectric speckle tweezers

Opto-thermoelectric tweezers present a new paradigm for optical trapping and manipulation of particles using low-power and simple optics. New real-life applications of opto-thermoelectric tweezers in areas such as biophysics, microfluidics, and nanomanufacturing will require them to have large-scale and high-throughput manipulation capabilities in complex environments. Here, we present opto-thermoelectric speckle tweezers, which use speckle field consisting of many randomly distributed thermal hotspots that arise from an optical speckle pattern to trap multiple particles over large areas. By further integrating the speckle tweezers with a microfluidic system, we experimentally demonstrate their application for size-based nanoparticle filtration. With their low-power operation, simplicity, and versatility, opto-thermoelectric speckle tweezers will broaden the applications of optical manipulation techniques.

Overcoming Diffusion-Limited Trapping in Nanoaperture Tweezers Using Opto-Thermal-Induced Flow

Nanoaperture-based plasmonic tweezers have shown tremendous potential in trapping, sensing, and spectroscopic analysis of nano-objects with single-molecule sensitivity. However, the trapping process is often diffusion-limited and therefore suffers from low-throughput. Here, we present bubble- and convection-assisted trapping techniques, which use opto-thermally generated Marangoni and Rayleigh-Bénard convection flow to rapidly deliver particles from large distances to the nanoaperture instead of relying on normal diffusion, enabling a reduction of 1-2 orders of magnitude in particle-trapping time (i.e., time before a particle is trapped). At a concentration of 2 × 107 particles/mL, average particle-trapping times in bubble- and convection-assisted trapping were 7 and 18 s, respectively, compared with more than 300 s in the diffusion-limited trapping. Trapping of a single particle at an ultralow concentration of 2 × 106 particles/mL was achieved within 2-3 min, which would otherwise take several hours in the diffusion-limited trapping. With their quick delivery and local concentrating of analytes at the functional surfaces, our convection- and bubble-assisted trapping could lead to enhanced sensitivity and throughput of nanoaperture-based plasmonic sensors.

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.

Characterization and manipulation of single nanoparticles using a nanopore-based electrokinetic tweezer

Manipulation and characterization of nanoscale objects through electrokinetic techniques offer numerous advantages compared to the existing optical methods and hold great potential for both fundamental research and practical applications. Here we present a novel electrokinetic tweezer for single nanoparticle manipulation and characterization based on electrokinetic trapping near a low-aspect-ratio nanopore. We find that this nanopore-based electrokinetic tweezer share lots of similarity with optical tweezers and can be modeled as an overdamped harmonic oscillator, with the spring constant of the system being the trap stiffness. We show that different values of ionic currents through the nanopore and trap stiffnesses are achieved when trapping nanoparticles with different sizes (down to 100 nm) and/or zeta potentials. We also demonstrate that the trap stiffness and nanoparticle position can be easily tuned by changing the applied voltage and buffer concentration. We envision that further development of this electrokinetic tweezer will enable various advanced tools for nanophotonics, drug delivery, and biosensing.

SERS discrimination of single DNA bases in single oligonucleotides by electro-plasmonic trapping

Surface-enhanced Raman spectroscopy (SERS) sensing of DNA bases by plasmonic nanopores could pave a way to novel methods for DNA analyses and new generation single-molecule sequencing platforms. The SERS discrimination of single DNA bases depends critically on the time that a DNA strand resides within the plasmonic hot spot. In fact, DNA molecules flow through the nanopores so rapidly that the SERS signals collected are not sufficient for single-molecule analysis. Here, we report an approach to control the residence time of molecules in the hot spot by an electro-plasmonic trapping effect. By directly adsorbing molecules onto a gold nanoparticle and then trapping the single nanoparticle in a plasmonic nanohole up to several minutes, we demonstrate single-molecule SERS detection of all four DNA bases as well as discrimination of single nucleobases in a single oligonucleotide. Our method can be extended easily to label-free sensing of single-molecule amino acids and proteins.

Nanophotonic Array-Induced Dynamic Behavior for Label-Free Shape-Selective Bacteria Sieving

Current particle sorting methods such as microfluidics, acoustics, and optics focus on exploiting the differences in the mass, size, refractive index, or fluorescence staining. However, there exist formidable challenges for them to sort label-free submicron particles with similar volume and refractive index yet distinct shapes. In this work, we report an optofluidic nanophotonic sawtooth array (ONSA) that generates sawtooth-like light fields through light coupling, paving the physical foundation for shape-selective sieving. Submicron particles interact with the coupled hotspots which impose different optical torques on the particles according to their shapes. Unstained S. aureus and E. coli are used as a model system to demonstrate this shape-selective sorting mechanism based on the torque-induced body dynamics, which was previously unattainable by other particle sorting technologies. More than 95% of S. aureus is retained within ONSA, while more than 97% of E. coli is removed. This nanophotonic chip offers a paradigm shift in shape-selective sorting of submicron particles and expands the boundary of optofluidics-based particle manipulation.

All optical dynamic nanomanipulation with active colloidal tweezers

Manipulation of colloidal objects with light is important in diverse fields. While performance of traditional optical tweezers is restricted by the diffraction-limit, recent approaches based on plasmonic tweezers allow higher trapping efficiency at lower optical powers but suffer from the disadvantage that plasmonic nanostructures are fixed in space, which limits the speed and versatility of the trapping process. As we show here, plasmonic nanodisks fabricated over dielectric microrods provide a promising approach toward optical nanomanipulation: these hybrid structures can be maneuvered by conventional optical tweezers and simultaneously generate strongly confined optical near-fields in their vicinity, functioning as near-field traps themselves for colloids as small as 40 nm. The colloidal tweezers can be used to transport nanoscale cargo even in ionic solutions at optical intensities lower than the damage threshold of living micro-organisms, and in addition, allow parallel and independently controlled manipulation of different types of colloids, including fluorescent nanodiamonds and magnetic nanoparticles.

Tunable size selectivity and nanoparticle immobilization on a photonic crystal optical trap

We harness residual thermal effects in a low-absorptivity system to manipulate parallel optical trapping of particles on the nanoscale. A photonic crystal is used to generate a 2D array of optical traps. We show that the size selectivity of the trap can be tuned by adding a non-ionic surfactant to the solution, altering the thermophoretic effect that delivers nanoparticles to trapping sites. We further show that particles can be permanently immobilized on the photonic crystal via photopolymerization of the trapping medium.