Photoresistance switching of plasmonic nanopores

Nanoparticle-blockage-enabled rapid and reversible nanopore gating with tunable memory

Gated protein channels act as rapid, reversible, and fully-closeable nanoscale valves to gate chemical transport across the cell membrane. Replicating or outperforming such a high-performance gating and valving function in artificial solid-state nanopores is considered an important yet unsolved challenge. Here we report a bioinspired rapid and reversible nanopore gating strategy based on controlled nanoparticle blockage. By using rigid or soft nanoparticles, we respectively achieve a trapping blockage gating mode with volatile memory where gating is realized by electrokinetically trapped nanoparticles near the pore and contact blockage gating modes with nonvolatile memory where gating is realized by a nanoparticle physically blocking the pore. This gating strategy can respond to an external voltage stimulus (∼200 mV) or pressure stimulus (∼1 atm) with response time down to milliseconds. In particular, when 1,2-diphytanoyl-sn-glycero-3-phosphocholine liposomes are used as the nanoparticles, the gating efficiency, defined as the extent of nanopore closing compared to the opening state, can reach 100%. We investigate the mechanisms for this nanoparticle-blockage-enabled nanopore gating and use it to demonstrate repeatable controlled chemical releasing via single nanopores. Because of the exceptional spatial and temporal control offered by this nanopore gating strategy, we expect it to find applications for drug delivery, biotic-abiotic interfacing, and neuromorphic computing.

Construction of a pH-Mediated Single-Molecule Switch with a Nanopore-DNA Complex

A molecular switch is one of the simplest examples of artificial molecular machines. Even so, the development of molecular switches is still at its very early stage. Currently, building single-molecule switches mostly rely on the molecular junction technique, but many of their performance characteristics are device-dependent. Here, a pH-mediated single-molecule switch based on the combination of an α-hemolysin (αHL) nanopore and a hexacyclen-modified DNA strand is developed. The single-stranded DNA is suspended inside an αHL through biotin-streptavidin linkage and the hexacyclen-modified nucleobase interacts with amino acid residues at positions 111, 113, and 147 to cause current oscillations. Distinct current transitions are observed when pH is tuned back and forth in the range of 3.0-7.4, with a typical "up" level when pH > 6.5 and a "down" level when pH < 4.5. This nanopore-DNA complex possesses membrane-bound advantages and may find applications in single-cell studies where pH could be readily tuned to control ON-OFF functions.

The effect of laser irradiation on reducing the noise of solid-state nanopore

The performance of solid-state nanopore is affected by the noise level. This study aimed to investigate the effect of laser irradiation on the noise performance of solid-state nanoporein situ. Laser irradiation is applied to fresh and contaminated nanopores. The measurement results show that the noise of fresh and contaminated nanopores decreases with the laser power and there is a threshold of laser power in reducing the noise of contaminated nanopores. The possible reasons for reducing noise in the laser irradiation process are discussed. The laser treatment is proven to provide a convenient method for reducing the noise of solid-state nanopore.

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.

Wetting of nanopores probed with pressure

Nanopores are both a tool to study single-molecule biophysics and nanoscale ion transport, but also a promising material for desalination or osmotic power generation. Understanding the physics underlying ion transport through nano-sized pores allows better design of porous membrane materials. Material surfaces can present hydrophobicity, a property which can make them prone to formation of surface nanobubbles. Nanobubbles can influence the electrical transport properties of such devices. We demonstrate an approach which uses hydraulic pressure to probe the electrical transport properties of solid state nanopores. We show how pressure can be used to wet pores, and how it allows control over bubbles or other contaminants in the nanometer scale range normally unachievable using only an electrical driving force. Molybdenum disulfide is then used as a typical example of a 2D material on which we demonstrate wetting and bubble induced nonlinear and linear conductance in the regimes typically used with these experiments. We show that by using pressure one can identify and evade wetting artifacts.

Quantification of low affinity binding interactions between natural killer cell inhibitory receptors and targeting ligands with a self-induced back-action actuated nanopore electrophoresis (SANE) sensor

A plasmonic nanopore sensor enabling detection of bimodal optical and electrical molecular signatures was fabricated and tested for its ability to characterize low affinity ligand-receptor interactions. This plasmonic nanosensor uses self-induced back-action (SIBA) for optical trapping to enable SIBA-actuated nanopore electrophoresis (SANE) through a nanopore located immediately below the optical trap volume. A natural killer (NK) cell inhibitory receptor heterodimer molecule CD94/NKG2A was synthesized to target a specific peptide-presenting Qa-1^b Qdm ligand as a simplified model of low-affinity interactions between immune cells and peptide-presenting cancer cells that occurs during cancer immunotherapy. A cancer-irrelevant Qa-1^b GroEL ligand was also targeted by the same receptor as a control experiment to test for non-specific binding. The analysis of different pairs of bimodal SANE sensor signatures enabled discrimination of ligand, receptor and their complexes and enabled differentiating between specific and non-specific ligand interactions. We were able to detect ligand-receptor complex binding at concentrations over 500 times lower than the free solution equilibrium binding constant (K _D ). Additionally, SANE sensor measurements enabled estimation of the fast dissociation rate (k _off) for this low-affinity specific ligand-receptor system, previously shown to be challenging to quantify with commercial technologies. The k _off value of targeted peptide-presenting ligands is known to correlate with the subsequent activation of immune cells in vivo, suggesting the potential utility of the SANE senor as a screening tool in cancer immunotherapy.

Applications and challenges of thermoplasmonics

Over the past two decades, there has been a growing interest in the use of plasmonic nanoparticles as sources of heat remotely controlled by light, giving rise to the field of thermoplasmonics. The ability to release heat on the nanoscale has already impacted a broad range of research activities, from biomedicine to imaging and catalysis. Thermoplasmonics is now entering an important phase: some applications have engaged in an industrial stage, while others, originally full of promise, experience some difficulty in reaching their potential. Meanwhile, innovative fundamental areas of research are being developed. In this Review, we scrutinize the current research landscape in thermoplasmonics, with a specific focus on its applications and main challenges in many different fields of science, including nanomedicine, cell biology, photothermal and hot-electron chemistry, solar light harvesting, soft matter and nanofluidics.

Plasmonic-Nanopore Biosensors for Superior Single-Molecule Detection

Plasmonic and nanopore sensors have separately received much attention for achieving single-molecule precision. A plasmonic "hotspot" confines and enhances optical excitation at the nanometer length scale sufficient to optically detect surface-analyte interactions. A nanopore biosensor actively funnels and threads analytes through a molecular-scale aperture, wherein they are interrogated by electrical or optical means. Recently, solid-state plasmonic and nanopore structures have been integrated within monolithic devices that address fundamental challenges in each of the individual sensing methods and offer complimentary improvements in overall single-molecule sensitivity, detection rates, dwell time and scalability. Here, the physical phenomena and sensing principles of plasmonic and nanopore sensing are summarized to highlight the novel complementarity in dovetailing these techniques for vastly improved single-molecule sensing. A literature review of recent plasmonic nanopore devices is then presented to delineate methods for solid-state fabrication of a range of hybrid device formats, evaluate the progress and challenges in the detection of unlabeled and labeled analyte, and assess the impact and utility of localized plasmonic heating. Finally, future directions and applications inspired by the present state of the art are discussed.

Capture of Single Silver Nanoparticles in Nanopore Arrays Detected by Simultaneous Amperometry and Surface-Enhanced Raman Scattering

The attoliter volumes and confinement abilities of zero-dimensional nanopore-electrode arrays (NEAs) hold considerable promise for examining the behavior of single nanoparticles. In this work, we use surface-enhanced Raman scattering (SERS) in tandem with amperometry in order to monitor single Ag Raman-sentinel nanoparticles transported to and captured in single nanopores. To that end, highly ordered solid-state NEAs were fabricated to contain periodic arrays of nanopores, each housing a single recessed Au-ring electrode. These were used to electrostatically capture and trap single silver nanoparticles (AgNPs) functionalized with the electrochemically stable Raman reporter, 1,4-bis(2-methylstyryl)benzene (bis-MSB). Transport and capture of the bis-MSB-tagged AgNPs in the nanopores was followed by simultaneous amperometry and SERS signals characteristic of AgNP oxidation and enhanced Raman scattering by bis-MSB at silver-gold hot spots, respectively. The frequency and magnitude of oxidation-current spikes increased with stepwise increases in DC voltage, and characteristic bis-MSB SERS spectra were observed. Under AC excitation, on the other hand, two distinctly different types of SERS signals were observed, independent of frequency and amplitude: (1) strong, transient (<10 s) spectra and (2) slow (>100 s) monotonically diminishing spectra. We hypothesize that the former behavior results from AgNP aggregates, whereas the latter occurs as a result of multiple incomplete AgNP-oxidation events in succession. These results show that attoliter-volume NEAs are competent for acquiring concurrent SERS spectra and for amperometry of single nanoparticles and that together these measurements can illuminate the collision dynamics of nanoparticles in confined environments.

Self-induced back action actuated nanopore electrophoresis (SANE)

We present a novel method to trap nanoparticles in double nanohole (DNH) nanoapertures integrated on top of solid-state nanopores (ssNP). The nanoparticles were propelled by an electrophoretic force from the cis towards the trans side of the nanopore but were trapped in the process when they reached the vicinity of the DNH-ssNP interface. The self-induced back action (SIBA) plasmonic force existing between the tips of the DNH opposed the electrophoretic force and enabled simultaneous optical and electrical sensing of a single nanoparticle for seconds. The novel SIBA actuated nanopore electrophoresis (SANE) sensor was fabricated using two-beam GFIS FIB. Firstly, Ne FIB milling was used to create the DNH features and was combined with end pointing to stop milling at the metal-dielectric interface. Subsequently, He FIB was used to drill a 25 nm nanopore through the center of the DNH. Proof of principle experiments to demonstrate the potential utility of the SANE sensor were performed with 20 nm silica and Au nanoparticles. The addition of optical trapping to electrical sensing extended translocation times by four orders of magnitude. The extended electrical measurement times revealed newly observed high frequency charge transients that were attributed to bobbing of the nanoparticle driven by the competing optical and electrical forces. Frequency analysis of this bobbing behavior hinted at the possibility of distinguishing single from multi-particle trapping events. We also discuss how SANE sensor measurement characteristics differ between silica and Au nanoparticles due to differences in their physical properties and how to estimate the charge around a nanoparticle. These measurements show promise for the SANE sensor as an enabling tool for selective detection of biomolecules and quantification of their interactions.

Advanced Nanoscale Approaches to Single-(Bio)entity Sensing and Imaging

Individual (bio)chemical entities could show a very heterogeneous behaviour under the same conditions that could be relevant in many biological processes of significance in the life sciences. Conventional detection approaches are only able to detect the average response of an ensemble of entities and assume that all entities are identical. From this perspective, important information about the heterogeneities or rare (stochastic) events happening in individual entities would remain unseen. Some nanoscale tools present interesting physicochemical properties that enable the possibility to detect systems at the single-entity level, acquiring richer information than conventional methods. In this review, we introduce the foundations and the latest advances of several nanoscale approaches to sensing and imaging individual (bio)entities using nanoprobes, nanopores, nanoimpacts, nanoplasmonics and nanomachines. Several (bio)entities such as cells, proteins, nucleic acids, vesicles and viruses are specifically considered. These nanoscale approaches provide a wide and complete toolbox for the study of many biological systems at the single-entity level.

High spatial resolution nanoslit SERS for single-molecule nucleobase sensing

Solid-state nanopores promise a scalable platform for single-molecule DNA analysis. Direct, real-time identification of nucleobases in DNA strands is still limited by the sensitivity and the spatial resolution of established ionic sensing strategies. Here, we study a different but promising strategy based on optical spectroscopy. We use an optically engineered elongated nanopore structure, a plasmonic nanoslit, to locally enable single-molecule surface enhanced Raman spectroscopy (SERS). Combining SERS with nanopore fluidics facilitates both the electrokinetic capture of DNA analytes and their local identification through direct Raman spectroscopic fingerprinting of four nucleobases. By studying the stochastic fluctuation process of DNA analytes that are temporarily adsorbed inside the pores, we have observed asynchronous spectroscopic behavior of different nucleobases, both individual and incorporated in DNA strands. These results provide evidences for the single-molecule sensitivity and the sub-nanometer spatial resolution of plasmonic nanoslit SERS.

Direct and real-time identification of nucleobases in DNA strands is still limited by the sensitivity and spatial resolution of the established solid-state nanopore devices. Here, the authors use CMOS compatible, plasmonic nanoslits to locally enable SERS for identifying nucleobases, both individual and incorporated in DNA strands.

Label-Free Single-Molecule Thermoscopy Using a Laser-Heated Nanopore

When light is used to excite electronic transitions in a material, nonradiative energy during relaxation is often released in the form of heat. In this work, we show that photoexcitation of a silicon nitride nanopore using a focused visible laser results in efficient localized photothermal heating, which reduces the nearby electrolyte viscosity and increases the ionic conductance. In addition, a strong localized thermal gradient in the pore vicinity is produced, evidenced by finite-element simulations and experimental observation of both ion and DNA thermophoresis. After correcting for thermophoresis, the nanopore current can be used as a nanoscale thermometer, enabling rapid force thermoscopy. We utilize this to probe thermal melting transitions in synthetic and native biomolecules that are heated at the nanopore. Our results on single molecules are validated by correspondence to bulk measurements, which paves the way to various biophysical experiments that require rapid temperature and force control on individual molecules.

An optothermally generated surface bubble and its applications

Under laser illumination, a solid-state surface or nanostructure can turn into a micro/nano heating source with the so-called optothermal effect. This effect allows for non-invasive control of heat at the micro/nanoscale. In the presence of a liquid, a surface bubble can be generated on top of the solid surface or nanostructure at a temperature much higher than the boiling point of the liquid. The high temperature and the fluid flow associated with the optothermally generated surface bubble enable many intriguing applications, ranging from the micro/nano-manipulation of fluids, particles, cells, and light to the synthesis of micro/nano-structures under ambient conditions. In this review article, we present the fundamentals, recent developments, and future perspectives in this emerging field.

Light-Enhancing Plasmonic-Nanopore Biosensor for Superior Single-Molecule Detection

A stacked plasmonic nanowell-nanopore biosensor strongly suppresses the background fluorescence from the bulk and yields net more than tenfold enhancement of the fluorescence intensity. The device offers extremely high signal-to-background (S/B) ratio for single-molecule detection at ultralow excitation laser intensities, while maintaining extremely high temporal bandwidth for single-DNA sensing.

Asymmetric plasmonic induced ionic noise in metallic nanopores

We present distinct asymmetric plasmon-induced noise properties of ionic transport observed through gold coated nanopores. We thoroughly investigated the effects of bias voltage and laser illumination. We show that the potential drop across top-coated silicon nanocavity pores can give rise to a large noise asymmetry (∼2-3 orders of magnitude). Varying the bias voltage has an appreciable effect on the noise density spectra, typically in the Lorentzian components. The laser power is found to strongly affect the ionic noise level as well as the voltage threshold for light-induced noise generation. The asymmetric noise phenomenon is attributed to plasmon-induced interfacial reactions which promote light-induced charge fluctuation in the ion flow and allow voltage modulation of photo-induced carriers surmounting over such Schottky junctions. We further compare the ionic noise performances of gold nanocavities containing different material stacks, among which thermal oxide passivation of the silicon successfully mitigates the light-induced noise and is also fully CMOS-compatible. The understanding of the described noise characteristics will help to foster multiple applications using related structures including plasmonic-based sensing or plasmon-induced catalysis such as water splitting or solar energy conversion devices.

Molecular Dynamics Simulation of DNA Capture and Transport in Heated Nanopores

The integration of local heat sources with solid-state nanopores offers new means for controlling the transmembrane transport of charged biomacromolecules. In the case of electrophoretic transport of DNA, recent experimental studies revealed unexpected temperature dependences of the DNA capture rate, the DNA translocation velocity, and the ionic current blockades produced by the presence of DNA in the nanopore. Here, we report the results of all-atom molecular dynamics simulations that elucidated the effect of temperature on the key microscopic processes governing electric field-driven transport of DNA through nanopores. Mimicking the experimental setup, we simulated the capture and subsequent translocation of short DNA duplexes through a locally heated nanopore at several temperatures and electrolyte conditions. The temperature dependence of ion mobility at the DNA surface was found to cause the dependence of the relative conductance blockades on temperature. To the first order, the effective force on DNA in the nanopore was found to be independent of temperature, despite a considerable reduction of solution viscosity. The temperature dependence of the solution viscosity was found to make DNA translocations faster for a uniformly heated system but not in the case of local heating that does not affect viscosity of solution surrounding the untranslocated part of the molecule. Increasing solution temperature was also found to reduce the lifetime of bonds formed between cations and DNA. Using a flow suppression algorithm, we were able to separate the effects of electro-osmotic flow and direct ion binding, finding the reduced durations of DNA-ion bonds to increase, albeit weakly, the effective force experienced by DNA in an electric field. Unexpectedly, our simulations revealed a considerable temperature dependence of solvent velocity at the DNA surface-slip velocity, an effect that can alter hydrodynamic coupling between the motion of DNA and the surrounding fluid.

Tuning Fano resonances with a nano-chamber of air

By designing a polymer-film-coated asymmetric metallic slit structure that only contains one nanocavity side-coupled with a subwavelength plasmonic waveguide, the Fano resonance is realized in the experiment. The Fano resonance originates from the interference between the narrow resonant spectra of the radiative light from the nanocavity and the broad nonresonant spectra of the directly transmitted light from the slit. The lateral dimension of the asymmetric slit is only 825 nm. Due to the presence of the soft polymer film, a nano-chamber of air is constructed. Based on the opto-thermal effect, the air volume in the nano-chamber is expanded by a laser beam, which blueshifts the Fano resonance. This tunable Fano resonance in such a submicron slit structure with a nano-chamber is of importance in the highly integrated plasmonic circuits.

Can Nanofluidic Chemical Release Enable Fast, High Resolution Neurotransmitter-Based Neurostimulation?

Artificial chemical stimulation could provide improvements over electrical neurostimulation. Physiological neurotransmission between neurons relies on the nanoscale release and propagation of specific chemical signals to spatially-localized receptors. Current knowledge of nanoscale fluid dynamics and nanofluidic technology allows us to envision artificial mechanisms to achieve fast, high resolution neurotransmitter release. Substantial technological development is required to reach this goal. Nanofluidic technology—rather than microfluidic—will be necessary; this should come as no surprise given the nanofluidic nature of neurotransmission. This perspective reviews the state of the art of high resolution electrical neuroprostheses and their anticipated limitations. Chemical release rates from nanopores are compared to rates achieved at synapses and with iontophoresis. A review of microfluidic technology justifies the analysis that microfluidic control of chemical release would be insufficient. Novel nanofluidic mechanisms are discussed, and we propose that hydrophobic gating may allow control of chemical release suitable for mimicking neurotransmission. The limited understanding of hydrophobic gating in artificial nanopores and the challenges of fabrication and large-scale integration of nanofluidic components are emphasized. Development of suitable nanofluidic technology will require dedicated, long-term efforts over many years.

Soft Matter-Regulated Active Nanovalves Locally Self-Assembled in Femtoliter Nanofluidic Channels

Well-tailored thermoresponsive polymer brushes locally self-assembled in tiny nanofluidic channels enable the active regulation of femtoliter-scale fluids. Such soft-matter-regulated active nanovalves within nanofluidic channels can be extended to build well-controlled functional nanofluidic systems, allowing complex fluidic processes to be performed at the nanometer scales.

Raman fingerprinting of single dielectric nanoparticles in plasmonic nanopores

Plasmonic nano-apertures are commonly used for the detection of small particles such as nanoparticles and proteins by exploiting electrical and optical techniques. Plasmonic nanopores are metallic nano-apertures sitting on a thin membrane with a tiny hole. It has been shown that plasmonic nanopores with a given geometry identify internal molecules using Surface Enhanced Raman Spectroscopy (SERS). However, label-free identification of a single dielectric nanoparticle requires a highly localized field comparable to the size of the particle. Additionally, the particle's Brownian motion can jeopardize the amount of photons collected from a single particle. Here, we demonstrate that the combination of optical trapping and SERS can be used for the detection and identification of 20 nm polystyrene nanoparticles in plasmonic nanopores. This work is anticipated to contribute to the detection of small bioparticles, optical trapping and nanotribology studies.

Plasmonic Nanopores for Trapping, Controlling Displacement, and Sequencing of DNA

With the aim of developing a DNA sequencing methodology, we theoretically examine the feasibility of using nanoplasmonics to control the translocation of a DNA molecule through a solid-state nanopore and to read off sequence information using surface-enhanced Raman spectroscopy. Using molecular dynamics simulations, we show that high-intensity optical hot spots produced by a metallic nanostructure can arrest DNA translocation through a solid-state nanopore, thus providing a physical knob for controlling the DNA speed. Switching the plasmonic field on and off can displace the DNA molecule in discrete steps, sequentially exposing neighboring fragments of a DNA molecule to the pore as well as to the plasmonic hot spot. Surface-enhanced Raman scattering from the exposed DNA fragments contains information about their nucleotide composition, possibly allowing the identification of the nucleotide sequence of a DNA molecule transported through the hot spot. The principles of plasmonic nanopore sequencing can be extended to detection of DNA modifications and RNA characterization.

Full wetting of plasmonic nanopores through two-component droplets

Benefiting from the prospect of extreme light localization, plasmonic metallic nanostructures are bringing advantages in many applications. However, for use in liquids, the hydrophobic nature of the metallic surface inhibits full wetting, which is related to contact line pinning in the nanostructures. In this work, we use a two-component droplet to overcome this problem. Due to a strong internal flow generated from the solutal Marangoni effect, these droplets can easily prime metallic nanostructures including sub-10 nm nanopores. We subsequently evaluate the local wetting performance of the plasmonic structures using surface enhanced Raman spectroscopy (SERS). Compared with other commonly used surface cleaning based wetting methods such as the oxygen plasma treatment, our two-component drop method is an efficient method in resolving the pinning of contact lines and is also non-destructive to samples. Thus the method described here primes plasmonic devices with guaranteed performances in liquid applications.

Self-Aligned Plasmonic Nanopores by Optically Controlled Dielectric Breakdown

We present a novel cost-efficient method for the fabrication of high-quality self-aligned plasmonic nanopores by means of an optically controlled dielectric breakdown. Excitation of a plasmonic bowtie nanoantenna on a dielectric membrane localizes the high-voltage-driven breakdown of the membrane to the hotspot of the enhanced optical field, creating a nanopore that is automatically self-aligned to the plasmonic hotspot of the bowtie. We show that the approach provides precise control over the nanopore size and that these plasmonic nanopores can be used as single molecule DNA sensors with a performance matching that of TEM-drilled nanopores. The principle of optically controlled breakdown can also be used to fabricate nonplasmonic nanopores at a controlled position. Our novel fabrication process guarantees alignment of the nanopore with the optical hotspot of the nanoantenna, thus ensuring that pore-translocating biomolecules interact with the concentrated optical field that can be used for detection and manipulation of analytes.