Nanofabrication of electrode arrays by electron-beam and nanoimprint lithographies

Implantable bioelectrodes: challenges, strategies, and future directions

Implantable bioelectrodes for regulating and monitoring biological behaviors have become indispensable medical devices in modern healthcare, alleviating pathological symptoms such as epilepsy and arrhythmia, and assisting in reversing conditions such as deafness and blindness. In recent years, developments in the fields of materials science and biomedical engineering have contributed to advances in research on implantable bioelectrodes. However, the foreign body reaction (FBR) is still a major constraint for the long-term application of electrodes. In this paper, four types of commonly used implantable bioelectrodes are reviewed, concentrating on their background, development, and a series of complications caused by FBR after long-term implantation. Strategies for resisting FBRs are then devised in terms of physics, chemistry, and nanotechnology. We analyze the major trends in the future development of implantable bioelectrodes and outline some promising research to optimize the long-term operational stability of electrodes. Although current implantable bioelectrodes have been able to achieve good biocompatibility, low impedance, and low mechanical mismatch and trauma, these devices still face the challenge of FBR. Resistance to FBR is still the key for the long-term effectiveness of bioelectrodes, and a better understanding of the mechanisms of FBR, as well as miniaturization, long-term passivation, and coupling with gene therapy may be the way forward for the next generation of implantable bioelectrodes.

Spectroelectrochemical behavior of parallel arrays of single vertically oriented Pseudomonas aeruginosa cells

Pseudomonas aeruginosa is a Gram-negative opportunistic human pathogen responsible for a number of healthcare-associated infection. It is currently difficult to assess single cell behaviors of P. aeruginosa that might contribute to acquisition of antibiotic resistance, intercellular communication, biofilm development, or virulence, because mechanistic behavior is inferred from ensemble collections of cells, thus averaging effects over a population. Here, we develop and characterize a device that can capture and trap arrays of single P. aeruginosa cells in individual micropores in order to study their behaviors using spectroelectrochemistry. Focused ion beam milling is used to fabricate an array of micropores in a Au/dielectric/Au/SiO₂-containing multilayer substrate, in which individual micropores are formed with dimensions that facilitate the capture of single P. aeruginosa cells in a predominantly vertical orientation. The bottom Au ring is then used as a working electrode to explore the spectroelectrochemical behavior of parallel arrays of individual P. aeruginosa cells. Application of step-potential or swept-potential waveforms produces changes in the fluorescence emission that can be imaged and correlated with applied potential. Arrays of P. aeruginosa cells typically exhibit three characteristic fluorescence behaviors that are sensitive to nutritional stress and applied potential. The device developed here enables the study of parallel collections of single bacterial cells with well-defined orientational order and should facilitate efforts to elucidate methods of bacterial communication and multidrug resistance at the single cell level.

Micro- and nano-devices for electrochemical sensing

Electrode miniaturization has profoundly revolutionized the field of electrochemical sensing, opening up unprecedented opportunities for probing biological events with a high spatial and temporal resolution, integrating electrochemical systems with microfluidics, and designing arrays for multiplexed sensing. Several technological issues posed by the desire for downsizing have been addressed so far, leading to micrometric and nanometric sensing systems with different degrees of maturity. However, there is still an endless margin for researchers to improve current strategies and cope with demanding sensing fields, such as lab-on-a-chip devices and multi-array sensors, brain chemistry, and cell monitoring. In this review, we present current trends in the design of micro-/nano-electrochemical sensors and cutting-edge applications reported in the last 10 years. Micro- and nanosensors are divided into four categories depending on the transduction mechanism, e.g., amperometric, impedimetric, potentiometric, and transistor-based, to best guide the reader through the different detection strategies and highlight major advancements as well as still unaddressed demands in electrochemical sensing.

Graphical Abstract

Recent Advances of Nanospheres Lithography in Organic Electronics

Nanospheres lithography (NSL) is an economical technique, which makes use of highly monodispersed nanospheres such as deposition or etch masks for generating patterns with nanoscale features. Embedding nanostructures into organic electronic devices can endow them with unique capabilities and enhanced performance, which have greatly advanced the development of organic electronics. In this review, a brief summary of the methods for the preparation of monodispersed nanospheres is presented. Afterward, the authors highlight the recent advances of a wide variety of applications of nanospheres lithography in organic electronic devices. Finally, the challenges in this field are pointed out, and the future development of this field is discussed.

Micro/Nano Electrode Array Sensors: Advances in Fabrication and Emerging Applications in Bioanalysis

Due to the rapid development of micro/nano manufacturing techniques and the greater understanding in electrochemical principles and methods, micro/nano electrode array sensing has received much attention in recent years, especially in bioanalysis. This review aims to explore recent progress in innovative techniques for the construction of micro/nano electrode array sensor and the unique applications of various types of micro/nano electrode array sensors in biochemical analysis. Moreover, the new area of smart sensing benefited from miniaturization of portable micro/nano electrode array sensors as well as wearable intelligent devices are further discussed.

Nanoelectrode Arrays Fabricated by Thermal Nanoimprint Lithography for Biosensing Application

Electrochemical sensors are devices capable of detecting molecules and biomolecules in solutions and determining the concentration through direct electrical measurements. These systems can be miniaturized to a size less than 1 µm through the creation of small-size arrays of nanoelectrodes (NEA), offering advantages in terms of increased sensitivity and compactness. In this work, we present the fabrication of an electrochemical platform based on an array of nanoelectrodes (NEA) and its possible use for the detection of antigens of interest. NEAs were fabricated by forming arrays of nanoholes on a thin film of polycarbonate (PC) deposited on boron-doped diamond (BDD) macroelectrodes by thermal nanoimprint lithography (TNIL), which demonstrated to be a highly reliable and reproducible process. As proof of principle, gliadin protein fragments were physisorbed on the polycarbonate surface of NEAs and detected by immuno-indirect assay using a secondary antibody labelled with horseradish peroxidase (HRP). This method allows a successful detection of gliadin, in the range of concentration of 0.5-10 μg/mL, by cyclic voltammetry taking advantage from the properties of NEAs to strongly suppress the capacitive background signal. We demonstrate that the characteristics of the TNIL technology in the fabrication of high-resolution nanostructures together with their low-cost production, may allow to scale up the production of NEAs-based electrochemical sensing platform to monitor biochemical molecules for both food and biomedical applications.

Recent advancements in microfluidics that integrate electrical sensors for whole blood analysis

Whole blood analysis reveals crucial information about various physiological and pathological conditions, including cancer metastasis, infection, and immune status, among others. Despite this rich information, the complex composition of whole blood usually required multiple sample preparation steps to purify targeted analytes. Traditionally, whole blood preparation processes, including centrifugation, lysis, dilution, or staining, are usually manually operated by well-trained technicians using bench-top instruments. This preparation can require a large blood volume and cannot be directly integrated with detection systems. Recently, various studies have integrated microfluidics with electrical sensors for whole blood analysis, with a focus on cell-based analysis, such as cell type, number, morphology, phenotype, and secreted molecules. These miniaturized systems require less sample and shorter reaction times. Besides, the sample processing and analysis can be fully integrated and automated with minimal operations. We believe these systems can transfer the current whole blood analysis from hospitals or laboratories into clinics or home settings to enable real-time and continuous health condition monitoring in point-of-care settings.

Development of a paper-based microanalysis device doped with multi-walled carbon nanotubes for in vitro evaluation of fluorene cytotoxicity

Developing a cost-effective and simple micro-analysis tool has long been an important objective in the toxicological detection of fluorene. In this study, a disposable paper-based micro-analysis device (μ-PAD) was designed using graphite doped with multi-walled carbon nanotubes (MWCNTs) to hand draw the electrode (MWCNTs/μ-PAD). We investigated the feasibility of the designed MWCNTs/μ-PAD as a cell-sensing platform using voltammetry measurements. Its application for evaluating the cytotoxicity of fluorene was studied based on the electrochemical determination of human breast cancer cells induced by fluorene. The results showed a dose-dependent toxicity effect of fluorene on cellular activity. The measurements were comparable with those obtained using a methyl-thiazolyl-tetrazolium assay. The detection limit of the MWCNTs/μ-PAD for human breast cancer cells was as low as 4.00 × 10³ cells·mL^-1 owing to the enhanced catalytic activity of the MWCNTs. Notably, the MWCNTs/μ-PAD-which had a diameter of 7.00 mm-allowed a sampling volume of 10.0 μL, which is 50.0 times less than the sampling volume required with a conventional electrode (500 μL). Advantages such as the simplicity of manufacture, low consumption, low cost, rapid detection, and disposability, suggest that the MWCNTs/μ-PAD could provide new opportunities and directions for in vitro microanalysis.

Fabrication, Characterization, and Analytical Application of Silica Nanopore Array-Modified Platinum Electrode

In this work, we report a new approach to fabricate the nanopore array electrode (NAE) by transferring silica nanochannel membrane (SNM) to the surface of Pt electrode (0.5 mm in diameter) sealed by glass capillary (designated as Pt-NAE for simplicity). The SNM is supported via the irreversible covalent-bond formation with the surrounding glass capillary treated by plasma, thus providing long-term stability to Pt-NAE. Meanwhile, this fabrication process does not require pregrafting or premodification of Pt electrode surface, providing well-defined active surface domains. Thanks to the small pore diameter (∼2.3 nm) and negatively charged channel walls, the SNM is permselective and thus the electrochemical behavior of Pt-NAE is dependent on both electrolyte concentration and charge state of redox molecules. The permeability of SNM was determined by the scanning electrochemical microscopy (SECM) approach curve measurements coupled with finite-element simulations from a quantitative viewpoint. The permeability of anionic Ru(CN)₆^4- was varied from 150 to 10.3 μm s^-1 as the electrolyte concentration decreased from 1.0 to 0.01 M, while there is no obvious change for cationic Ru(NH₃)₆³⁺. Finally, the as-prepared Pt-NAE is able to continuously monitor dissolved oxygen for up to 2 h in a solution containing biofouling reagents, exhibiting an enhanced antifouling ability and therefore excellent current stability. We believe the NAE with unique mass transport properties can be extended further for other analytical applications.

A Comprehensive Review: Development of Electrochemical Biosensors for Detection of Cyanotoxins in Freshwater

Cyanobacteria harmful algal blooms are increasing in frequency and cyanotoxins have become an environmental and public concern in the U.S. and worldwide. In this Review, the majority of reported studies and developments of electrochemical affinity biosensors for cyanotoxins are critically reviewed and discussed. Essential background information about cyanobacterial toxins and electrochemical biosensors is combined with the rapidly moving development of electrochemical biosensors for these toxins. Current issues and future challenges for the development of useful electrochemical biosensors for cyanotoxin detection that meet the demands for applications in field freshwater samples are discussed. The major aspects of the entire review article in a prescribed sequence include (i) the state-of-the-art knowledge of the toxicity of cyanotoxins, (ii) important harmful algal bloom events, (iii) advisories, guidelines, and regulations, (iv) conventional analytical methods for determination of cyanotoxins, (v) electrochemical transduction, (vi) recognition receptors, (vii) reported electrochemical biosensors for cyanotoxins, (viii) summary of analytical performance, and (ix) recent advances and future trends. Discussion includes electrochemical techniques and devices, biomolecules with high affinity, numerous array designs, various detection approaches, and research strategies in tailoring the properties of the transducer-biomolecule interface. Scientific and engineering aspects are presented in depth. This review aims to serve as a valuable source to scientists and engineers entering the interdisciplinary field of electrochemical biosensors for detection of cyanotoxins in freshwaters.

A gold coated polystyrene ring microarray formed by two-step patterning: construction of an advanced microelectrode for voltammetric sensing

A two-step patterning process was developed based on nanosphere lithography and plasma etching to fabricate an array of electrodes with two different gold ring structures: the arrays of Au micro-ring electrode (Au-MRE) and Au covered with polystyrene micro-ring electrode (Au-PS-MRE). The Au-MRE structure was fabricated by etching a monolayer of polystyrene (PS) spheres on indium tin oxide (ITO) surface to generate PS rings on ITO glass. PS rings served as a mask in secondary etching for blocking an interaction of oxygen plasma and ITO surface to create a ring-patterned ITO surface. Then, the PS residue was removed and gold was deposited. The site-selective electrodeposition of gold was carried out and an array of a gold ring structure was formed on the ITO glass. The Au-PS-MRE structure was fabricated by keeping the PS residue from second etching before deposition of gold. The Au-PS-MRE microelectrode was studied by using hexacyanoferrate as an electrochemical probe where it displayed steady state current in cyclic voltammetry. The respective calibration plots were acquired at a working potential of 0.31 V and 0.12 V (vs. Ag/AgCl) for oxidation and reduction reaction, respectively. The sensitivity is as high as 163.4-220.7 μA·mM^-1·mm^-2 which is larger by a factor of 95-132 compared to a conventional gold film macroelectrode. The detection limit (at a signal-to-noise ratio of 3) is 2.2 μM. This approach thus yields relatively effective and low-cost fabrication without resorting to high resolution instruments. Conceivably, the technique may be used to produce microelectrode arrays on a large scale. Graphical abstract Schematic presentation of a novel fabrication process of micro-ring electrode arrays. Two-step patterning based on nanosphere lithography leads to electrodes with great electrochemical performance. Direct deposition metal in the presence of polystyrene (PS) mask induces the formation of a new structure with arrays of gold covered with PS microring on the indium tin oxide (ITO) coated glass. The microelectrode-like behavior has been achieved using this fabrication process.

Enhanced Thermal Stability of Thermoplastic Polymer Nanostructures for Nanoimprint Lithography

Thermoplastic polymer micro- and nanostructures suffer pattern decay when heated to a temperature close to or above the polymer’s glass transition temperature. In this work, we report enhanced thermal stability of polycarbonate nanostructures at temperatures well above their glass transition temperatures. Based on this observation, we develop a unique technique for high-resolution polymer patterning by polymer reflows. This technique is characterized as the precise control of polymer reflows regardless of the annealing time, which avoids the time-domain nonlinear reflow of the polymer melt. We also implement thermal nanoimprinting in a step-and-repeat fashion, which dramatically increases the throughput of the thermal nanoimprint. The enhanced pattern stability against thermal reflow also allows for multiple imprinting at the same location to generate complex resist patterns from a simple mold structure. Since modern lithography often uses thin resist films (sub-100 nm) due to the restraint from the pattern aspect ratio, the unusual annealing behavior of thin polymer films is highly relevant in sub-100 nm lithographic processing.

Electrochemistry at single molecule occupancy in nanopore-confined recessed ring-disk electrode arrays

Electrochemical reactions at nanoscale structures possess unique characteristics, e.g. fast mass transport, high signal-to-noise ratio at low concentration, and insignificant ohmic losses even at low electrolyte concentrations. These properties motivate the fabrication of high density, laterally ordered arrays of nanopores, embedding vertically stacked metal-insulator-metal electrode structures and exhibiting precisely controlled pore size and interpore spacing for use in redox cycling. These nanoscale recessed ring-disk electrode (RRDE) arrays exhibit current amplification factors, AF_RC, as large as 55-fold with Ru(NH₃)₆^2/3+, indicative of capture efficiencies at the top and bottom electrodes, Φ_t,b, exceeding 99%. Finite element simulations performed to investigate the concentration distribution of redox species and to assess operating characteristics are in excellent agreement with experiment. AF_RC increases as the pore diameter, at constant pore spacing, increases in the range 200-500 nm and as the pore spacing, at constant pore diameter, decreases in the range 1000-460 nm. Optimized nanoscale RRDE arrays exhibit a linear current response with concentration ranging from 0.1 μM to 10 mM and a small capacitive current with scan rate up to 100 V s^-1. At the lowest concentrations, the average pore occupancy is 〈n〉 ∼ 0.13 molecule establishing productive electrochemical signals at occupancies at and below the single molecule level in these nanoscale RRDE arrays.

Integrated dual-modality microfluidic sensor for biomarker detection using lithographic plasmonic crystal

This paper reports an integrated dual-modality microfluidic sensor chip, consisting of a patterned periodic array of nanoposts coated with gold (Au) and graphene oxide (GO), to detect target biomarker molecules in a limited sample volume. The device generates both electrochemical and surface plasmon resonance (SPR) signals from a single sensing area of Au-GO nanoposts. The Au-GO nanoposts are functionalized with specific receptor molecules, serving as a spatially well-defined nanostructured working electrode for electrochemical sensing, as well as a nanostructured plasmonic crystal for SPR-based sensing via the excitation of surface plasmon polaritons. High sensitivity of the electrochemical measurement originates from the presence of the nanoposts on the surface of the working electrode where radial diffusion of redox species occurs. Complementarily, the SPR detection allows convenient tracking of dynamic antigen-antibody interactions, to describe the association and dissociation phases occurring at the sensor surface. The soft-lithographically formed nanoposts provide high reproducibility of the sensor response to epidermal growth factor receptor (ErbB2) molecules even at a femtomolar level. Sensitivities of the electrochemical measurements to ErbB2 are found to be 20.47 μA μM^-1 cm^-2 in a range from 1 fM to 0.1 μM, and those of the SPR measurements to be 1.35 nm μM^-1 in a range from 10 pM to 1 nM, and 0.80 nm μM^-1 in a range from 1 nM to 0.1 μM. The integrated dual-modality sensor offers higher sensitivity (through higher surface area and diffusions from nanoposts for electrochemical measurements), as well as the dynamic measurements of antigen-antibody bindings (through the SPR measurement), while operating simultaneously in a same sensing area using the same sample volume.

Mesoporous Silica Thin Films for Improved Electrochemical Detection of Paraquat

An electrochemical method was developed for rapid and sensitive detection of the herbicide paraquat in aqueous samples using mesoporous silica thin film modified glassy carbon electrodes (GCE). Vertically aligned mesoporous silica thin films were deposited onto GCE by electrochemically assisted self-assembly (EASA). Cyclic voltammetry revealed effective response to the cationic analyte (while rejecting anions) thanks to the charge selectivity exhibited by the negatively charged mesoporous channels. Square wave voltametry (SWV) was then used to detect paraquat via its one electron reduction process. Influence of various experimental parameters (i.e., pH, electrolyte concentration, and nature of electrolyte anions) on sensitivity was investigated and discussed with respect to the mesopore characteristics and accumulation efficiency, pointing out the key role of charge distribution in such confined spaces on these processes. Calibration plots for paraquat concentration ranging from 10 nM to 10 μM were constructed at mesoporous silica modified GCE which were linear with increasing paraquat concentration, showing dramatically enhanced sensitivity (almost 30 times) as compared to nonmodified electrodes. Finally, real samples from Meuse River (France) spiked with paraquat, without any pretreatment (except filtration), were analyzed by SWV, revealing the possible detection of paraquat at very low concentration (10-50 nM). Limit of detection (LOD) calculated from real sample analysis was found to be 12 nM, which is well below the permissible limits of paraquat in drinking water (40-400 nM) in various countries.

Nanochannel Arrays for Molecular Sieving and Electrochemical Analysis by Nanosphere Lithography Templated Graphoepitaxy of Block Copolymers

The ability to design, fabricate, and manipulate materials at the nanoscale is fundamental to the quest to develop technologies to assemble nanometer-scale pieces into larger-scale components and materials, thereby transferring unique nanometer-scale properties to macroscopic objects. In this work, we describe a new approach to the fabrication of highly ordered, ultrahigh density nanochannel arrays that employs nanosphere lithography to template the graphoepitaxy of polystyrene-polydimethylsiloxane, diblock copolymers. By optimizing the well-controlled solvent vapor annealing, overcoating conditions, and the subsequent reactive ion etching processes, silica nanochannel (SNC) arrays with areal densities, ρ_A, approaching 1000 elements μm^-2, are obtained over macroscopic scales. The integrity and functionality of the SNC arrays was tested by using them as permselective ion barriers to nanopore-confined disk electrodes. The nanochannels allow cations to pass to the disk electrode but reject anions, as demonstrated by cyclic voltammetry. This ion gating behavior can be reversed from cation-permselective to anion-permselective by chemically inverting the surface charge from negative to positive. Furthermore, the conformal SNC array structures obtained could easily be lifted, detached, and transferred to another substrate, preserving the hierarchical organization while transferring the nanostructure-derived properties to a different substrate. These results demonstrate how nanoscale behavior can be replicated over macroscale distances, using electrochemical analysis as a model.

Nanobiosensing with Arrays and Ensembles of Nanoelectrodes

Since the first reports dating back to the mid-1990s, ensembles and arrays of nanoelectrodes (NEEs and NEAs, respectively) have gained an important role as advanced electroanalytical tools thank to their unique characteristics which include, among others, dramatically improved signal/noise ratios, enhanced mass transport and suitability for extreme miniaturization. From the year 2000 onward, these properties have been exploited to develop electrochemical biosensors in which the surfaces of NEEs/NEAs have been functionalized with biorecognition layers using immobilization modes able to take the maximum advantage from the special morphology and composite nature of their surface. This paper presents an updated overview of this field. It consists of two parts. In the first, we discuss nanofabrication methods and the principles of functioning of NEEs/NEAs, focusing, in particular, on those features which are important for the development of highly sensitive and miniaturized biosensors. In the second part, we review literature references dealing the bioanalytical and biosensing applications of sensors based on biofunctionalized arrays/ensembles of nanoelectrodes, focusing our attention on the most recent advances, published in the last five years. The goal of this review is both to furnish fundamental knowledge to researchers starting their activity in this field and provide critical information on recent achievements which can stimulate new ideas for future developments to experienced scientists.

Lift-Off Free Fabrication Approach for Periodic Structures with Tunable Nano Gaps for Interdigitated Electrode Arrays

We report a simple, low-cost and lift-off free fabrication approach for periodic structures with adjustable nanometer gaps for interdigitated electrode arrays (IDAs). It combines an initial structure and two deposition process steps; first a dielectric layer is deposited, followed by a metal evaporation. The initial structure can be realized by lithography or any other structuring technique (e.g., nano imprint, hot embossing or injection molding). This method allows the fabrication of nanometer sized gaps and completely eliminates the need for a lift-off process. Different substrate materials like silicon, Pyrex or polymers can be used. The electrode gap is controlled primarily by sputter deposition of the initial structure, and thus, adjustable gaps in the nanometer range can be realized independently of the mask or stamp pattern. Electrochemical characterizations using redox cycling in ferrocenemethanol (FcMeOH) demonstrate signal amplification factors of more than 110 together with collection factors higher than 99%. Furthermore, the correlation between the gap width and the amplification factor was studied to obtain an electrochemical performance assessment of the nano gap electrodes. The results demonstrate an exponential relationship between amplification factor and gap width.

A miniaturized electrochemical device integrating a biconical microchannel and carbon fiber disk ultramicroelectrode

A simple, cheap and practicable miniaturized electrochemical device was developed based on a biconical microchannel.

Cyclic voltammogram on ridge/pore array architectured electrode inspired by butterfly-wings

Porous architectured electrodes are intensely investigated for promoting electrochemical performance. Besides the high surface area, mass transport plays an irreplaceable role in the architecture assisting effect, which is, however, far beyond expression due to the complexity and irregularity of various electrode materials. Here, we took advantage of elaborate architectures from butterfly wings and obtained carbon electrode with ridge/pore array hierarchical architecture (ridge/pore-C) using a carbonizing-graphite coating method. A basic one-electron transfer process using the redox couple ferri/ferrocyanide as a benchmark under cyclic voltammetric conditions was conducted. The peak potential separation for ridge/pore-C was decreased by 117 mV compared to its non-architectured counterpart, with obvious enhancement of peak current density, indicating prominent beneficial impact on electrochemical responses. Further finite element simulation demonstrated the additional lateral diffusion within the ridge domain and partial thin layer diffusion within the pore array domain of ridge/pore-C, and simultaneously verified the experimental results. By constructing and investigating the well-organized porous architecture for affecting cyclic voltammogram, this work provides a prototype and cost-effective method for structural design of efficient electrodes by drawing inspiration from nature.

Double layer lift-off nanofabrication controlled gaps of nanoelectrodes with sub-100 nm by nanoimprint lithography

Basic research on nanoelectronics is often limited by the high cost and large-scale methods to fabricate electrodes with controlled gap size in nanometer scales. Here nanoelectrodes with a controlled gap size of sub-100 nm were fabricated by modified nanoimprint lithography (NIL) via a double-layer lift-off process utilizing polymethylmethacrylate (PMMA) and polydimethylglutarimide (PMGI) as the lift-off resist. Firstly the patterns of the electrode mold were transferred onto the upper PMMA layer by NIL techniques and then through controlling the developing time and concentration of developer of the PMGI under layer, regulating the exact gap size of the transferred metal nanoelectrode. The result indicated that the 'undercut' phenomenon was observed of the PMGI transfer layer during the developing process; through controlling the feature size of the undercut length, the gap size of the transferred metal nanoelectrode was precisely controlled, which showed shrinkage behavior. The nanoelectrodes with gap sizes of 800, 400, 200, and 100 nm can be reduced to about 440, 120, 80, and 70 nm. Our result provides a low-cost and large-scale route to prepare nanoelectrodes with controlled gap size, which can be valuable for current efforts in nanoelectronics.

Low-cost photolithographic fabrication of nanowires and microfilters for advanced bioassay devices

Integrated microfluidic devices with nanosized array electrodes and microfiltration capabilities can greatly increase sensitivity and enhance automation in immunoassay devices. In this contribution, we utilize the edge-patterning method of thin aluminum (Al) films in order to form nano- to micron-sized gaps. Evaporation of high work-function metals (i.e., Au, Ag, etc.) on these gaps, followed by Al lift-off, enables the formation of electrical uniform nanowires from low-cost, plastic-based, photomasks. By replacing Al with chromium (Cr), the formation of high resolution, custom-made photomasks that are ideal for low-cost fabrication of a plurality of array devices were realized. To demonstrate the feasibility of such Cr photomasks, SU-8 micro-pillar masters were formed and replicated into PDMS to produce micron-sized filters with 3–4 µm gaps and an aspect ratio of 3. These microfilters were capable of retaining 6 µm beads within a localized site, while allowing solvent flow. The combination of nanowire arrays and micro-pillar filtration opens new perspectives for rapid R&D screening of various microfluidic-based immunoassay geometries, where analyte pre-concentration and highly sensitive, electrochemical detection can be readily co-localized.

Microchannel conductivity measurements in microchip for on line monitoring of dephosphorylation rates of organic phosphates using paramagnetic-beads linked alkaline phosphatase

This paper presents the use of polymer coated microelectrodes for the realtime conductivity monitoring in a microchannel photoablated through the polymer without contact. Based on this strategy, a small conductometry sensor has been developed to record in time conductivity variation when an enzymatic reaction occurs through the channel. The rate constant determination, k2, for the dephosphorylation of organic phosphate-alkaline phosphatase-superparamagnetic beads complex using chemically different substrates such as adenosine monoesterphosphate, adenosine diphosphate and adenosine triphosphate was taken as an example to demonstrate selectivity and sensivity of the detection scheme. The k2 value measured for each adenosine phosphate decreases from 39 to 30 s(-1) in proportion with the number (3, 2 and 1) of attached phosphate moiety, thus emphasizing the steric hindrance effect on kinetics.

Nanostructures: a platform for brain repair and augmentation

Nanoscale structures have been at the core of research efforts dealing with integration of nanotechnology into novel electronic devices for the last decade. Because the size of nanomaterials is of the same order of magnitude as biomolecules, these materials are valuable tools for nanoscale manipulation in a broad range of neurobiological systems. For instance, the unique electrical and optical properties of nanowires, nanotubes, and nanocables with vertical orientation, assembled in nanoscale arrays, have been used in many device applications such as sensors that hold the potential to augment brain functions. However, the challenge in creating nanowires/nanotubes or nanocables array-based sensors lies in making individual electrical connections fitting both the features of the brain and of the nanostructures. This review discusses two of the most important applications of nanostructures in neuroscience. First, the current approaches to create nanowires and nanocable structures are reviewed to critically evaluate their potential for developing unique nanostructure based sensors to improve recording and device performance to reduce noise and the detrimental effect of the interface on the tissue. Second, the implementation of nanomaterials in neurobiological and medical applications will be considered from the brain augmentation perspective. Novel applications for diagnosis and treatment of brain diseases such as multiple sclerosis, meningitis, stroke, epilepsy, Alzheimer's disease, schizophrenia, and autism will be considered. Because the blood brain barrier (BBB) has a defensive mechanism in preventing nanomaterials arrival to the brain, various strategies to help them to pass through the BBB will be discussed. Finally, the implementation of nanomaterials in neurobiological applications is addressed from the brain repair/augmentation perspective. These nanostructures at the interface between nanotechnology and neuroscience will play a pivotal role not only in addressing the multitude of brain disorders but also to repair or augment brain functions.

Butterfly-scale architecture directed electrodeposition of Ag microband arrays for electrochemical detection

The ridge array architecture of Troides aeacus butterfly scales was used as guidance to gather electric field compactly around the ridge tips to obtain an Ag microband array by electrodeposition.

Gold nanotip array for ultrasensitive electrochemical sensing and spectroscopic monitoring

A gold nanotip array platform with a combination of ultrasensitive electrochemical sensing and spectroscopic monitoring capability is reported. Adenosine triphosphate is detected down to 1 pM according to the impedance changes in response to aptamer-specific binding. Furthermore, the local molecular information can be monitored at the individual plasmonic nanotips, and hence provide the capability for a better understanding of complex biological processes.

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.

Redox cycling in nanoscale-recessed ring-disk electrode arrays for enhanced electrochemical sensitivity

An array of nanoscale-recessed ring-disk electrodes was fabricated using layer-by-layer deposition, nanosphere lithography, and a multistep reactive ion etching process. The resulting device was operated in generator-collector mode by holding the ring electrodes at a constant potential and performing cyclic voltammetry by sweeping the disk potential in Fe(CN)6(3-/4-) solutions. Steady-state response and enhanced (~10×) limiting current were achieved by cycling the redox couple between ring and disk electrodes with high transfer/collection efficiency. The collector (ring) electrode, which is held at a constant potential, exhibits a much smaller charging current than the generator (disk), and it is relatively insensitive to scan rate. A characteristic feature of the nanoscale ring-disk geometry is that the electrochemical reaction occurring at the disk electrodes can be tuned by modulating the potential at the ring electrodes. Measured shifts in Fe(CN)6(3-/4-) concentration profiles were found to be in excellent agreement with finite element method simulations. The main performance metric, the amplification factor, was optimized for arrays containing small diameter pores (r < 250 nm) with minimum electrode spacing and high pore density. Finally, integration of the fabricated array within a nanochannel produced up to 50-fold current amplification as well as enhanced selectivity, demonstrating the compatibility of the device with lab-on-a-chip architectures.

Nanopatterning using a simple bi-layer lift-off process for the fabrication of a photonic crystal nanostructure

A simple and versatile method for fabricating nanopatterns by a lift-off procedure is demonstrated. The technique involves the use of molecular transfer lithography based on water-soluble templates to form a nanopatterned UV-curable material on a PMGI layer, which serves as an underlying resin suitable for lift-off processes. This bi-layer procedure is used for the fabrication of nickel patterns, which are subsequently used as a hard mask for plasma etch processing. Using this procedure, a two-dimensional TiO(2) photonic crystal layer with a 450 nm lattice constant is fabricated on Y(3)Al(5)O(12):Ce(3+) (YAG:Ce) yellow ceramic plate phosphor to enhance its forward emission. The yellow emission in the forward direction is improved by a factor of 3.5 compared to that of a conventional non-scattering YAG:Ce phosphor plate excited by a blue LED.

Label-free electrochemical impedance detection of kinase and phosphatase activities using carbon nanofiber nanoelectrode arrays

We demonstrate the feasibility of a label-free electrochemical method to detect the kinetics of phosphorylation and dephosphorylation of surface-attached peptides catalyzed by kinase and phosphatase, respectively. The peptides with a sequence specific to c-Src tyrosine kinase and protein tyrosine phosphatase 1B (PTP1B) were first validated with ELISA-based protein tyrosine kinase assay and then functionalized on vertically aligned carbon nanofiber (VACNF) nanoelectrode arrays (NEAs). Real-time electrochemical impedance spectroscopy (REIS) measurements showed reversible impedance changes upon the addition of c-Src kinase and PTP1B phosphatase. Only a small and unreliable impedance variation was observed during the peptide phosphorylation, but a large and fast impedance decrease was observed during the peptide dephosphorylation at different PTP1B concentrations. The REIS data of dephosphorylation displayed a well-defined exponential decay following the Michaelis-Menten heterogeneous enzymatic model with a specific constant, k(cat)/K(m), of (2.1±0.1)×10(7) M(-1)s(-1). Consistent values of the specific constant was measured at PTP1B concentration varying from 1.2 to 2.4 nM with the corresponding electrochemical signal decay constant varying from 38.5 to 19.1s. This electrochemical method can be potentially used as a label-free method for profiling enzyme activities in fast reactions.