Electrochemical Surface Plasmon Resonance Measurement Based on Gold Nanohole Array Fabricated by Nanoimprinting Technique

ZnO Metal Oxide Semiconductor in Surface Acoustic Wave Sensors: A Review

Surface acoustic wave (SAW) gas sensors are of continuous development interest to researchers due to their sensitivity, short detection time, and reliability. Among the most used materials to achieve the sensitive film of SAW sensors are metal oxide semiconductors, which are highlighted by thermal and chemical stability, by the presence on their surface of free electrons and also by the possibility of being used in different morphologies. For different types of gases, certain metal oxide semiconductors are used, and ZnO is an important representative for this category of materials in the field of sensors. Having a great potential for the development of SAW sensors, the discussion related to the development of the sensitivity of metal oxide semiconductors, especially ZnO, by the synthesis method or by obtaining new materials, is suitable and necessary to have an overview of the latest results in this domain.

Electrochemical Long Period Fiber Grating Sensing for Electroactive Species

We present an electrochemical long period fiber grating (LPFG) sensor for electroactive species with an optically transparent electrode. The sensor was fabricated by coating indium tin oxide onto the surface of LPFG using a polygonal barrel-sputtering method. LPFG was produced by an electric arc-induced technique. The sensing is based on change in the detection of electron density on the electrode surface during potential application and its reduction by electrochemical redox of analytes. Four typical electroactive species of methylene blue, hexaammineruthenium(III), ferrocyanide, and ferrocenedimethanol were used to investigate the sensor performance. The concentrations of analytes were determined by the modulation of the potential as the change in transmittance around the resonance band of LPFG. The sensitivity of the sensor, particularly to methylene blue, was high, and the sensor responded to a wide concentration range of 0.001 mM to 1 mM.

Coulomb Blockade Effect in Well-Arranged 2D Arrays of Palladium Nano-Islands for Hydrogen Detection at Room Temperature: A Modeling Study

The fast growth of hydrogen usage as a clean fuel in civil applications such as transportation, space technology, etc. highlights the importance of the reliable detection of its leakage and accumulation under explosion limit by sensors with a low power consumption at times when there is no accumulation of hydrogen in the environment. In this research, a new and efficient mechanism is presented for hydrogen detection-using the Coulomb blockade effect in a well-arranged 2D array of palladium nano-islands-which can operate at room temperature. We demonstrated that under certain conditions of size distribution and the regularity of palladium nano-islands, with selected sizes of 1.7, 3 and 6.1 nm, the blockade threshold will appear in current-voltage (IV) characteristics. In reality, it will be achieved by the inherent uncertainty in the size of the islands in nano-scale fabrication or by controlling the size of nanoparticles from 1.7 to 6.1 nm, considering a regular arrangement of nanoparticles that satisfies single-electron tunneling requirements. Based on the simulation results, the threshold voltage is shifted towards lower ones due to the expansion of Pd nanoparticles exposed to the environment with hydrogen concentrations lower than 2.6%. Also, exploring the features of the presented structure as a gas sensor, provides robustness against the Gaussian variation in nano-islands sizes and temperature variations. Remarkably, the existence of the threshold voltage in the IV curve and adjusting the bias voltage below this threshold leads to a drastic reduction in power consumption. There is also an improvement in the minimum detectable hydrogen concentration as well as the sensor response.

Ultrathin layer solid transformation-enabled-surface enhanced Raman spectroscopy for trace harmful small gaseous molecule detection

Detection of trace harmful small gaseous molecules (h-SGMs), based on surface enhanced Raman spectroscopy (SERS), has been expected to be a useful strategy but is challenging due to the extremely small Raman cross section (RCS) and weak metal affinity of the h-SGMs. Here, a new strategy, ultrathin layer solid transformation-enabled (ULSTE)-SERS, is proposed. It uses the chemical reaction between the target h-SGM and an ultrathin layer of solid sensing matter coated on a plasmonic metal SERS substrate. This reaction in situ produces a new solid matter with large RCS, which ensures the detection of trace h-SGMs via SERS. The validity of this strategy has been demonstrated by detecting trace H₂S gas with an ultrathin CuO layer wrapped around Au nanoparticles. Furthermore, this strategy allows fast and ultrasensitive detection. The detection limit can be down to ppb (even ppt) levels with 10 min preprocessing. Importantly, this strategy has good universality for various other h-SGMs, such as SO₂, CS₂, CH₃SH, and HCl, etc., using appropriate sensing matter. Additionally, the ULSTE-SERS is also suitable for unstable molecules and fast portable detection due to the stable solid layer. This work provides highly efficient SERS-based detection of trace h-SGMs, which is easily applied in practical situations.

MEA Recordings and Cell-Substrate Investigations with Plasmonic and Transparent, Tunable Holey Gold

Microelectrode arrays are widely used in different fields such as neurobiology or biomedicine to read out electrical signals from cells or biomolecules. One way to improve microelectrode applications is the development of novel electrode materials with enhanced or additional functionality. In this study, we fabricated macroelectrodes and microelectrode arrays containing gold penetrated by nanohole arrays as a conductive layer. We used this holey gold to optically excite surface plasmon polaritons, which lead to a strong increase in transparency, an effect that is further enhanced by the plasmon's interaction with cell culture medium. By varying the nanohole diameter in finite-difference time domain simulations, we demonstrate that the transmission can be increased to above 70% with its peak at a wavelength depending on the holey gold's lattice constant. Further, we demonstrate that the novel transparent microelectrode arrays are as suitable for recording cellular electrical activity as standard devices. Moreover, we prove using spectral measurements and finite-difference time domain simulations that plasmonically induced transmission peaks of holey gold red-shift upon sensing medium or cells in close vicinity (<30 nm) to the substrate. Thus, we establish plasmonic and transparent holey gold as a tunable material suitable for cellular electrical recordings and biosensing applications.

Large Area Nanohole Arrays for Sensing Fabricated by Interference Lithography

Several fabrication techniques are recently used to produce a nanopattern for sensing, as focused ion beam milling (FIB), e-beam lithography (EBL), nanoimprinting, and soft lithography. Here, interference lithography is explored for the fabrication of large area nanohole arrays in metal films as an efficient, flexible, and scalable production method. The transmission spectra in air of the 1 cm2 substrate were evaluated to study the substrate behavior when hole-size, periodicity, and film thickness are varied, in order to elucidate the best sample for the most effective sensing performance. The efficiency of the nanohole array was tested for bulk sensing and compared with other platforms found in the literature. The sensitivity of ~1000 nm/RIU, achieved with an array periodicity in the visible range, exceeds near infrared (NIR) performances previously reported, and demonstrates that interference lithography is one of the best alternative to other expensive and time-consuming nanofabrication methods.

Nanohole array plasmonic biosensors: Emerging point-of-care applications

Point-of-care (POC) applications have expanded hugely in recent years and is likely to continue, with an aim to deliver cheap, portable, and reliable devices to meet the demands of healthcare industry. POC devices are designed, prototyped, and assembled using numerous strategies but the key essential features that biosensing devices require are: (1) sensitivity, (2) selectivity, (3) specificity, (4) repeatability, and (5) good limit of detection. Overall the fabrication and commercialization of the nanohole array (NHA) setup to the outside world still remains a challenge. Here, we review the various methods of NHA fabrication, the design criteria, the geometrical features, the effects of surface plasmon resonance (SPR) on sensing as well as current state-of-the-art of existing NHA sensors. This review also provides easy-to-understand examples of NHA-based POC biosensing applications, its current status, challenges, and future prospects.

Nanoplasmonic sensors for biointerfacial science

In recent years, nanoplasmonic sensors have become widely used for the label-free detection of biomolecules across medical, biotechnology, and environmental science applications. To date, many nanoplasmonic sensing strategies have been developed with outstanding measurement capabilities, enabling detection down to the single-molecule level. One of the most promising directions has been surface-based nanoplasmonic sensors, and the potential of such technologies is still emerging. Going beyond detection, surface-based nanoplasmonic sensors open the door to enhanced, quantitative measurement capabilities across the biointerfacial sciences by taking advantage of high surface sensitivity that pairs well with the size of medically important biomacromolecules and biological particulates such as viruses and exosomes. The goal of this review is to introduce the latest advances in nanoplasmonic sensors for the biointerfacial sciences, including ongoing development of nanoparticle and nanohole arrays for exploring different classes of biomacromolecules interacting at solid-liquid interfaces. The measurement principles for nanoplasmonic sensors based on utilizing the localized surface plasmon resonance (LSPR) and extraordinary optical transmission (EOT) phenomena are first introduced. The following sections are then categorized around different themes within the biointerfacial sciences, specifically protein binding and conformational changes, lipid membrane fabrication, membrane-protein interactions, exosome and virus detection and analysis, and probing nucleic acid conformations and binding interactions. Across these themes, we discuss the growing trend to utilize nanoplasmonic sensors for advanced measurement capabilities, including positional sensing, biomacromolecular conformation analysis, and real-time kinetic monitoring of complex biological interactions. Altogether, these advances highlight the rich potential of nanoplasmonic sensors and the future growth prospects of the community as a whole. With ongoing development of commercial nanoplasmonic sensors and analytical models to interpret corresponding measurement data in the context of biologically relevant interactions, there is significant opportunity to utilize nanoplasmonic sensing strategies for not only fundamental biointerfacial science, but also translational science applications related to clinical medicine and pharmaceutical drug development among countless possibilities.

Localized surface plasmon resonance sensing structure based on gold nanohole array on beveled fiber edge

This paper proposes a simple, stable, sensitive, and angle-dependent localized surface plasmon resonance (LSPR) sensing structure based on multi-mode optical fiber. We adopted the template transfer method to integrate a nanohole array onto a fiber tip with beveled angle. Experimental results indicated that beveled angle structured probe sensor outperform the flat optical fiber tip structured LSPR sensor in our experiment. We tested the sensitivity and the figure of merit (FOM) of the probe beveled angle from 5°-22°, with refractive index ranging from 1.333-1.385, to find that sensitivity and FOM were optimal at fiber tip bevel angle of 7°, reaching 487 nm/RIU and 29 respectively.

Imaging Electrogenerated Chemiluminescence at Single Gold Nanowire Electrodes

We report electrogenerated chemiluminescence (ECL) generated at single gold nanowire electrodes supported on tin-doped indium oxide. Unlike other single nanoparticle electrochemical characterization techniques, ECL provides a massively parallel direct readout of electrochemical activity on individual nanoparticle electrodes without the need for extrinsic illumination or a scanning electrochemical probe. While ECL is not observed from as-purchased nanowires due to the surfactant layer, by removing the layer and coating the nanowires with a polymer blend, ECL from single nanowire electrodes is readily measured. With an increase in polymer thickness, an increase in ECL image quality and reproducibility over multiple redox cycles is observed. The polymer coating also provides a strategy for stabilizing gold nanoparticle electrodes against complete surface oxidation in aqueous environments.

Electrochemical plasmonic sensing system for highly selective multiplexed detection of biomolecules based on redox nanoswitches

In this paper, we present the development of a nanoswitch-based electrochemical surface plasmon resonance (eSPR) transducer for the multiplexed and selective detection of DNA and other biomolecules directly in complex media. To do so, we designed an experimental set-up for the synchronized measurements of electrochemical and electro-plasmonic responses to the activation of multiple electrochemically labeled structure-switching biosensors. As a proof of principle, we adapted this strategy for the detection of DNA sequences that are diagnostic of two pathogens (drug-resistant tuberculosis and Escherichia coli) by using methylene blue-labeled structure-switching DNA stem-loop. The experimental sensitivity of the switch-based eSPR sensor is estimated at 5 nM and target detection is achieved within minutes. Each sensor is reusable several times with a simple 8M urea washing procedure. We then demonstrated the selectivity and multiplexed ability of these switch-based eSPR by simultaneously detecting two different DNA sequences. We discuss the advantages of the proposed eSPR approach for the development of highly selective sensor devices for the rapid and reliable detection of multiple molecular markers in complex samples.

Quantification of a cardiac biomarker in human serum using extraordinary optical transmission (EOT)

Nanoimprinting lithography (NIL) is a manufacturing process that can produce macroscale surface areas with nanoscale features. In this paper, this technique is used to solve three fundamental issues for the application of localized surface plasmonic resonance (LSPR) in practical clinical measurements: assay sensitivity, chip-to-chip variance, and the ability to perform assays in human serum. Using NIL, arrays of 140 nm square features were fabricated on a sensing area of 1.5 mm x 1.5 mm with low cost. The high reproducibility of NIL allowed for the use of a one-chip, one-measurement approach with 12 individually manufactured surfaces with minimal chip-to-chip variations. To better approximate a real world setting, all chips were modified with a biocompatible, multi-component monolayer and inter-chip variability was assessed by measuring a bioanalyte standard (2.5-75 ng/ml) in the presence of a complex biofluid, human serum. In this setting, nanoimprinted LSPR chips were able to provide sufficient characteristics for a 'low-tech' approach to laboratory-based bioanalyte measurement, including: 1) sufficient size to interface with a common laboratory light source and detector without the need for a microscope, 2) high sensitivity in serum with a cardiac troponin limit of detection of 0.55 ng/ml, and 3) very low variability in chip manufacturing to produce a figure of merit (FOM) of 10.5. These findings drive LSPR closer to technical comparability with ELISA-based assays while preserving the unique particularities of a LSPR based sensor, suitability for multiplexing and miniaturization, and point-of-care detections.

Tuning the 3D plasmon field of nanohole arrays

Modern photonics is being revolutionized through the use of nanostructured plasmonic materials, which confine light to sub-diffraction limit resolution providing universal, sensitive, and simple transducers for molecular sensors. Understanding the mechanisms by which light interacts with plasmonic crystals is essential for developing application-focussed devices. The strong influence of grating coupling on electromagnetic field distribution, frequency and degeneracy of plasmon bands has now been characterized using hexagonal nanohole arrays. An equation for nanohole arrays was derived to demonstrate the strong influence of incidence and rotation angle on optical properties of 2D plasmonic crystals such as nanohole arrays. Consequently, we report experimental data that are in strong agreement with finite difference time-domain (FDTD) simulations that clearly demonstrate the influence of the grating coupling conditions on the optical properties (such as plasmon degeneracy and bandwidth), and on the distribution of the plasmon field around nanohole arrays (including tuneable penetration depths and highly localized fields). The tuneable 3D plasmon field allowed for controlled sensing properties and by increasing the angle of incidence to 30 degrees, the resonance wavelength was tuned from 1000 to 600 nm, and the sensitivity was enhanced by nearly 300% for a protein assay using surface plasmon resonance (SPR) and by 40% with surface-enhanced Raman scattering (SERS) sensors.

On-chip nanohole array based sensing: a review

The integration of nanohole array based plasmonic sensors into microfluidic systems has enabled the emergence of platforms with unique capabilities and a diversified palette of applications. Recent advances in fabrication techniques together with novel implementation schemes have influenced the progress of these optofluidic platforms. Here, we review the advances that nanohole array based sensors have experienced since they were first merged with microfluidics. We examine established and new fabrication methodologies that have enabled both the fabrication of nanohole arrays with improved optical attributes and a reduction in manufacturing costs. The achievements of several platforms developed to date and the significant benefits obtained from operating the nanoholes as nanochannels are also reviewed herein. Finally, we discuss future opportunities for on-chip nanohole array sensors by outlining potential applications and the use of the abilities of the nanostructures beyond the optical context.

Promises and Challenges of Nanoplasmonic Devices for Refractometric Biosensing

Optical biosensors based on surface plasmon resonance (SPR) in metallic thin films are currently standard tools for measuring molecular binding kinetics and affinities - an important task for biophysical studies and pharmaceutical development. Motivated by recent progress in the design and fabrication of metallic nanostructures, such as nanoparticles or nanoholes of various shapes, researchers have been pursuing a new generation of biosensors harnessing tailored plasmonic effects in these engineered nanostructures. Nanoplasmonic devices, while demanding nanofabrication, offer tunability with respect to sensor dimension and physical properties, thereby enabling novel biological interfacing opportunities and extreme miniaturization. Here we provide an integrated overview of refractometric biosensing with nanoplasmonic devices and highlight some recent examples of nanoplasmonic sensors capable of unique functions that are difficult to accomplish with conventional SPR. For example, since the local field strength and spatial distribution can be readily tuned by varying the shape and arrangement of nanostructures, biomolecular interactions can be controlled to occur in regions of high field strength. This may improve signal-to-noise and also enable sensing a small number of molecules. Furthermore, the nanoscale plasmonic sensor elements may, in combination with nanofabrication and materials-selective surface-modifications, make it possible to merge affinity biosensing with nanofluidic liquid handling.

Ultrasmooth metallic films with buried nanostructures for backside reflection-mode plasmonic biosensing

We present a new plasmonic device architecture based on ultrasmooth metallic surfaces with buried plasmonic nanostructures. Using template-stripping techniques, ultrathin gold films with less than 5 Å surface roughness are optically coupled to an arbitrary arrangement of buried metallic gratings, rings, and nanodots. As a prototypical example, we present linear plasmonic gratings buried under an ultrasmooth 20 nm thick gold surface for biosensing. The optical illumination and collection are completely decoupled from the microfluidic delivery of liquid samples due to the backside, reflection-mode geometry. This allows for sensing with opaque or highly scattering liquids. With the buried nanostructure design, we maintain high sensitivity and decoupled backside (reflective) optical access as with traditional prism-based surface plasmon resonance (SPR) sensors. In addition, we also gain the benefits offered by nanoplasmonic sensors such as spectral tunability and high-resolution, wide-field SPR imaging with normal-incidence epi-illumination that is simple to construct and align. Beyond sensing, our buried plasmonic nanostructures with ultrasmooth metallic surfaces can benefit nanophotonic waveguides, surface-enhanced spectroscopy, nanolithography, and optical trapping.

Nanochannels preparation and application in biosensing

Selective transport in nanochannels (protein-based ion channels) is already used in living systems for electrical signaling in nerves and muscles, and this natural behavior is being approached for the application of biomimetic nanochannels in biosensors. On the basis of this principle, single nanochannels and nanochannel arrays seem to bring new advantages for biosensor development and applications. The purpose of this review is to provide a general comprehensive and critical overview on the latest trends in the development of nanochannel-based biosensing systems. A detailed description and discussion of representative and recent works covering the main nanochannel fabrication techniques, nanoporous material characterizations, and especially their application in both electrochemical and optical sensing systems is given. The state-of-the-art of the developed technology may open the way to new advances in the integration of nanochannels with (bio)molecules and synthetic receptors for the development of novel biodetection systems that can be extended to many other applications with interest for clinical analysis, safety, and security as well as environmental and other industrial studies and applications.