Electrokinetic Preconcentration and Detection of Neuropeptides at Patterned Graphene-Modified Electrodes in a Nanochannel

Voltammetric detection of Neuropeptide Y using a modified sawhorse waveform

The hormone Neuropeptide Y (NPY) plays critical roles in feeding, satiety, obesity, and weight control. However, its complex peptide structure has hindered the development of fast and biocompatible detection methods. Previous studies utilizing electrochemical techniques with carbon fiber microelectrodes (CFMEs) have targeted the oxidation of amino acid residues like tyrosine to measure peptides. Here, we employ the modified sawhorse waveform (MSW) to enable voltammetric identification of NPY through tyrosine oxidation. Use of MSW improves NPY detection sensitivity and selectivity by reducing interference from catecholamines like dopamine, serotonin, and others compared to the traditional triangle waveform. The technique utilizes a holding potential of -0.2 V and a switching potential of 1.2 V that effectively etches and renews the CFME surface to simultaneously detect NPY and other monoamines with a sensitivity of 5.8 ± 0.94 nA/µM (n = 5). Furthermore, we observed adsorption-controlled, subsecond NPY measurements with CFMEs and MSW. The effective identification of exogenously applied NPY in biological fluids demonstrates the feasibility of this methodology for in vivo and ex vivo studies. These results highlight the potential of MSW voltammetry to enable fast, biocompatible NPY quantification to further elucidate its physiological roles.

Anomalous, dielectrophoretic transport of molecules in non-electrolytes

The electric field (E-field) dielectric polarization-based separations mechanism represents a novel method for separating solutions at small length scales. An E-field gradient with a maximum strength of 0.4 MV/m applied across a 10 μm deep channel is shown to increase the concentration inside the low E-field region by ≈ 40% relative to the high E-field region. This concentration change is two orders of magnitude higher than the estimated change predicted using the classical equilibrium thermodynamics for the same E-field. The deviation between the predicted and the experimental results suggests that the change in volumetric E-field energy with solute concentration is insufficient to explain this phenomenon. The study also explores the effect of varying strength of E-field and frequency of supplied voltage on the dielectric polarization-based separation efficiency. While the increase in the former increases the separation efficiency, the increase in the latter reduces the degree of concentration change due to ineffective charging of the electrodes.

Trends and challenges in microfluidic methods for protein manipulation-A review

Proteins are important molecules involved in an immensely large number of biological processes. Being capable of manipulating proteins is critical for developing reliable and affordable techniques to analyze and/or detect them. Such techniques would enable the production of therapeutic agents for the treatment of diseases or other biotechnological applications (e.g., bioreactors or biocatalysis). Microfluidic technology represents a potential solution to protein manipulation challenges because of the diverse phenomena that can be exploited to achieve micro- and nanoparticle manipulation. In this review, we discuss recent contributions made in the field of protein manipulation in microfluidic systems using different physicochemical principles and techniques, some of which are miniaturized versions of already established macro-scale techniques.

Access and Management of Sweat for Non-Invasive Biomarker Monitoring: A Comprehensive Review

Sweat is an important biofluid presents in the body since it regulates the internal body temperature, and it is relatively easy to access on the skin unlike other biofluids and contains several biomarkers that are also present in the blood. Although sweat sensing devices have recently displayed tremendous progress, most of the emerging devices primarily focus on the sensor development, integration with electronics, wearability, and data from in vitro studies and short-term on-body trials during exercise. To further the advances in sweat sensing technology, this review aims to present a comprehensive report on the approaches to access and manage sweat from the skin toward improved sweat collection and sensing. It is begun by delineating the sweat secretion mechanism through the skin, and the historical perspective of sweat, followed by a detailed discussion on the mechanisms governing sweat generation and management on the skin. It is concluded by presenting the advanced applications of sweat sensing, supported by a discussion of robust, extended-operation epidermal wearable devices aiming to strengthen personalized healthcare monitoring systems.

Skin-Interfaced Wearable Sweat Sensors for Precision Medicine

Wearable sensors hold great potential in empowering personalized health monitoring, predictive analytics, and timely intervention toward personalized healthcare. Advances in flexible electronics, materials science, and electrochemistry have spurred the development of wearable sweat sensors that enable the continuous and noninvasive screening of analytes indicative of health status. Existing major challenges in wearable sensors include: improving the sweat extraction and sweat sensing capabilities, improving the form factor of the wearable device for minimal discomfort and reliable measurements when worn, and understanding the clinical value of sweat analytes toward biomarker discovery. This review provides a comprehensive review of wearable sweat sensors and outlines state-of-the-art technologies and research that strive to bridge these gaps. The physiology of sweat, materials, biosensing mechanisms and advances, and approaches for sweat induction and sampling are introduced. Additionally, design considerations for the system-level development of wearable sweat sensing devices, spanning from strategies for prolonged sweat extraction to efficient powering of wearables, are discussed. Furthermore, the applications, data analytics, commercialization efforts, challenges, and prospects of wearable sweat sensors for precision medicine are discussed.

Technology Roadmap for Flexible Sensors

Humans rely increasingly on sensors to address grand challenges and to improve quality of life in the era of digitalization and big data. For ubiquitous sensing, flexible sensors are developed to overcome the limitations of conventional rigid counterparts. Despite rapid advancement in bench-side research over the last decade, the market adoption of flexible sensors remains limited. To ease and to expedite their deployment, here, we identify bottlenecks hindering the maturation of flexible sensors and propose promising solutions. We first analyze challenges in achieving satisfactory sensing performance for real-world applications and then summarize issues in compatible sensor-biology interfaces, followed by brief discussions on powering and connecting sensor networks. Issues en route to commercialization and for sustainable growth of the sector are also analyzed, highlighting environmental concerns and emphasizing nontechnical issues such as business, regulatory, and ethical considerations. Additionally, we look at future intelligent flexible sensors. In proposing a comprehensive roadmap, we hope to steer research efforts towards common goals and to guide coordinated development strategies from disparate communities. Through such collaborative efforts, scientific breakthroughs can be made sooner and capitalized for the betterment of humanity.

Multiplex Chroma Response Wearable Hydrogel Patch: Visual Monitoring of Urea in Body Fluids for Health Prognosis

Visual wearable devices can rapid intuitively monitor biomarkers in body fluids to indicate the human health status and provide valuable reference for further medical diagnosis. However, unavoidable interference factors such as skin color, natural light, and background luminescence can interfere with the visualization accuracy of flexible wearable devices, limiting their practical sensing application. Here, we designed a wearable sensing patch via an embedded upconversion optical probe in a 3D porous polyacrylamide hydrogel, exhibiting a multiplex chroma response to urea based on the inner filter effect, which overcomes the susceptibility to external conditions due to its near-infrared excited luminescence and improves the resolution and accuracy of visual sensing. Furthermore, a highly compatible portable sensing platform combined with a smartphone was designed to achieve in situ rapid quantitative analysis of urea. The limit of detection values of the upconversion optical probe and hydrogel sensor are as low as 1.4 and 30 μM respectively, exhibiting the practicality in different scenarios. The designed sensing patch provides a convenient and accurate sensing strategy for the detection of biomarkers in body fluids and has the potential to be developed into a point-of-care device to provide disease early warning and clinical diagnosis.

Hybridization-based DNA biosensing with a limit of detection of 4 fM in 30 s using an electrohydrodynamic concentration module fabricated by grayscale lithography

Speeding up and enhancing the performances of nucleic acid biosensing technologies have remained drivers for innovation. Here, we optimize a fluorimetry-based technology for DNA detection based on the concentration of linear targets paired with probes. The concentration module consists of a microfluidic channel with the shape of a funnel in which we monitor a viscoelastic flow and a counter-electrophoretic force. We report that the technology performs better with a target longer than 100 nucleotides (nt) and a probe shorter than 30 nt. We also prove that the control of the funnel geometry in 2.5D using grayscale lithography enhances sensitivity by 100-fold in comparison to chips obtained by conventional photolithography. With these optimized settings, we demonstrate a limit of detection of 4 fM in 30 s and a detection range of more than five decades. This technology hence provides an excellent balance between sensitivity and time to result.

Electronic sensors for alkali and alkaline earth cations based on Fullerene-C60 and silicon doped on C60 nanocages: a computational study

In this research, we have reported the electrical sensitivity of pristine C60 and silicon doped on C60 (SiC59) nanocages as sensors that can be used for detecting the presence of alkali (Li⁺, Na⁺, K⁺) and alkaline earth (Be²⁺, Mg²⁺, Ca²⁺) cations. The computations are carried out at the B3LYP level of theory with a 6-31G(d) basis set. The atoms in molecules (AIM) and natural bond orbital (NBO) analyses are performed to evaluate the intermolecular interactions between cations and nanocages. The physical properties of the selected complexes are also analyzed by the frontier molecular orbital, energy gap, electronegativity, chemical hardness, softness, and other quantities such as work function, number of transferred electron, and dipole moment. The results show that the adsorption process is exothermic and with increasing the charge of cations, the adsorption energies enhance. Our findings also reveal a decrease in the energy gap along with an increase in the electrical conductivity of the respective complexes. Finally, the density of state calculations is presented to confirm the obtained results.

Nanoparticle-Mediated Signaling for Aptamer-Based Multiplexed Detection of Cortisol and Neuropeptide Y in Serum

Multiplexed profiling of the expression of neurochemical biomarkers of stress, for periodic assessment to enable augmentation of human performance, requires wash-free detection platforms that exhibit reproducible signals from samples in biological matrices. However, alterations in aptamer conformation after binding to targets, such as cortisol, are minimal based on NMR spectra, and the methylene blue signaling is blocked by serum proteins. Hence, in this study, we explore aptamer derivatization with magnetic nanoparticles that are conjugated with multiple methylene blue moieties, to amplify signals and alter the net charge configuration for repulsing serum proteins, so that the aptamer conformation upon target recognition can lead to a signal ON assay in serum media. Based on this, a microchip platform with addressable electrodes that are immobilized with selective aptamer receptors is developed for multiplexed detection of cortisol (1–700 ng/mL) and neuropeptide Y (5–1000 pg/mL) in patient-derived serum samples, which is validated by immunoassays. We envision the application of this sensor for profiling a wider array of human performance biomarkers under stress-related events to develop stress augmentation methodologies.

Time-Evolved SERS Signatures of DEP-Trapped Aβ and Zn2+Aβ Peptides Revealed by a Sub-10 nm Electrode Nanogap

Alzheimer's disease (AD) has become highly relevant in aging societies, yet the fundamental molecular basis for AD is still poorly understood. New tools to study the undergoing structural conformation changes of amyloid beta (Aβ) peptides, the pathogenic hallmark of AD, could play a crucial role in the understanding of the underlying mechanisms of misfolding and cytotoxicity of this peptide. It has been recently reported that Zn²⁺ interacts with Aβ and changes its aggregation pathway away from less harmful fibrillar forms to more toxic species. Here, we present a versatile platform based on a set of sub-10 nm nanogap electrodes for the manipulation and sensing of biomolecules in the physiological condition at a low copy number, where molecules are trapped via dielectrophoresis (DEP) across the nanogap, which also serves as a surface-enhanced Raman spectroscopy hotspot. In this study, we demonstrate that our electrode nanogap platform can be used to study the structural difference between Aβ40 and ZnAβ40 peptides at different aggregation stages in the physiologically relevant concentration and in solution phase. The Raman spectroscopic signatures of the DEP-captured neuropeptides prove the device to be attractive as a label-free bioanalytical tool.

Solvent-mediated forces in protein dielectrophoresis

DEP is an established method to manipulate micrometer-sized particles, but standard continuum theories predict only negligible effects for nanometer-sized proteins despite contrary experimental evidence. A theoretical description of protein DEP needs to consider details on the molecular scale. Previous work toward this goal addressed the role of orientational polarization of static protein dipole moments for dielectrophoretic effects, which successfully predicts the general magnitude of dielectrophoretic forces on proteins but does not readily explain negative DEP forces observed for proteins in some experiments. However, contributions to the protein chemical potential due to protein-water interactions have not yet been considered in this context. Here, we utilize atomistic molecular dynamics simulations to evaluate polarization-induced changes in the protein solvation free energy, which result in a solvent-mediated contribution to dielectrophoretic forces. We quantify solvent-mediated dielectrophoretic forces for two proteins and a small peptide in water, which follow expectations for protein-water dipole-dipole interactions. The magnitude of solvent-mediated dielectrophoretic forces exceeds predictions of nonmolecular continuum theories, but plays a minor role for the total dielectrophoretic force for the simulated proteins due to dominant contributions from the orientational polarization of their static protein dipoles. However, we extrapolate that solvent-mediated contributions to negative protein DEP forces will become increasingly relevant for multidomain proteins, complexes and aggregates with large protein-water interfaces, as well as for high electric field frequencies, which provides a potential mechanism for corresponding experimental observations of negative protein DEP.

Dielectrophoresis: Developments and applications from 2010 to 2020

The 20th century has seen tremendous innovation of dielectrophoresis (DEP) technologies, with applications being developed in areas ranging from industrial processing to micro- and nanoscale biotechnology. From 2010 to present day, there have been 981 publications about DEP. Of over 2600 DEP patents held by the United States Patent and Trademark Office, 106 were filed in 2019 alone. This review focuses on DEP-based technologies and application developments between 2010 and 2020, with an aim to highlight the progress and to identify potential areas for future research. A major trend over the last 10 years has been the use of DEP techniques for biological and clinical applications. It has been used in various forms on a diverse array of biologically derived molecules and particles to manipulate and study them including proteins, exosomes, bacteria, yeast, stem cells, cancer cells, and blood cells. DEP has also been used to manipulate nano- and micron-sized particles in order to fabricate different structures. The next 10 years are likely to see the increase in DEP-related patent applications begin to result in a greater level of technology commercialization. Also during this time, innovations in DEP technology will likely be leveraged to continue the existing trend to further biological and medical-focused applications as well as applications in microfabrication. As a tool leveraged by engineering and imaginative scientific design, DEP offers unique capabilities to manipulate small particles in precise ways that can help solve problems and enable scientific inquiry that cannot be addressed using conventional methods.

Measurement of Neuropeptide Y Using Aptamer-Modified Microelectrodes by Electrochemical Impedance Spectroscopy

Aptamer-modified microelectrodes for Neuropeptide Y measurement by electrochemical impedance spectroscopy was described here. The advantages of using carbon fiber or platinum microelectrodes are because they are promising materials with high electrical conductivity, chemical stability, and high surface area that can be easily modified on their surface. The immobilization and biofouling were studied and compared using EIS. Moreover, the adsorption of NPY to the aptamer-modified microelectrodes was also demonstrated by EIS. Changes of -ω*Zimag, an impedance factor that gives information of the capacitance, is directly correlated with concentrations. A widely linear range was obtained from 10 to 1000 ng/mL of NPY. This method was able to detect NPY without performing a redox reaction by adsorption at the surface of the microelectrodes, with the specificity provided by aptamer functionalization of the microelectrode surface.

Subnanomolar detection of promethazine abuse using a gold nanoparticle-graphene nanoplatelet-modified electrode

A simple, sensitive, and effective adsorptive stripping voltammetric sensor for the detection of trace-level promethazine was created based on a gold nanoparticle-graphene nanoplatelet-modified glassy carbon electrode (AuNP-GrNP/GCE). AuNP-GrNP nanocomposites were synthesized using an electroless deposition process, and the morphology was characterized using UV-vis spectroscopy, Fourier transform infrared spectroscopy, field emission scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The electrochemical behavior and detection of promethazine at the AuNP-GrNP/GCE were investigated utilizing cyclic voltammetry and adsorptive stripping voltammetry. The AuNP-GrNP/GCE showed outstanding synergistic electrochemical activity for promethazine oxidation, a highly active surface area, great adsorptivity, and outstanding catalytic properties. The electrolyte pH, amount of AuNP-GrNP nanocomposite, preconcentration potential (vs. Ag/AgCl), and time were optimized to obtain a high performance electrochemical sensor. Under optimal conditions, the proposed sensor displayed two linear concentration ranges from 1.0 nmol L^-1 to 1.0 μmol L^-1 and from 1.0 to 10 μmol L^-1. The limits of detection and quantitation were 0.40 and 1.4 nmol L^-1, respectively. This sensor displayed high sensitivity, a capability for rapid analysis, and excellent repeatability and reproducibility. The developed sensor was effective and practical for promethazine detection in biological fluids and forensic samples, and the obtained results exhibited excellent agreement with the results obtained using the method described in the British Pharmacopoeia. Graphical abstract.

NiO/SWCNTs coupled with an ionic liquid composite for amplified carbon paste electrode; A feasible approach for improving sensing ability of adrenalone and folic acid in dosage form

Electrochemical sensors have shown great appeal for the simultaneous analysis of pharmaceutical compounds. In this way, the presence study described first electroanalytical sensor for simultaneous determination of adrenalone and folic acid. The two-amplified voltammetric sensor was developed by modifying carbon paste electrode (CPE) with NiO/SWCNTs composite and 1-butyl-3-methylimidazolium methanesulfonate (1B3MIMS) and used for simultaneous determination of adrenalone and folic acid. The NiO/SWCNTs was synthesised by a fast and low-cost precipitation strategy and then characterised by EDS, FESEM and XRD methods. The results confirmed a particle size range of ⁓ 26.93-33.87 nm for NiO nanoparticle decorated at SWCNTs. The cyclic voltammetric investigation showed that oxidation potentials of adrenalone and folic acid depend on changing the pH value. The maximum oxidation current for the simultaneous analysis of two compounds occurred at pH = 7.0. In this condition, the sensor showed linear dynamic range 0.01-400 μM and 0.3-350 μM for determination of adrenalone and folic acid, respectively. The NiO/SWCNTs/1B3MIMS/CPE was then used as an ultrasensitive electroanalytical sensor for determination of adrenalone and folic acid in injection samples with recovery ratio between 98.2-103.66 %.

Metal-Organic-Framework FeBDC-Derived Fe3O4 for Non-Enzymatic Electrochemical Detection of Glucose

Present-day science indicates that developing sensors with excellent sensitivity and selectivity for detecting early signs of diseases is highly desirable. Electrochemical sensors offer a method for detecting diseases that are simpler, faster, and more accurate than conventional laboratory analysis methods. Primarily, exploiting non-noble-metal nanomaterials with excellent conductivity and large surface area is still an area of active research due to its highly sensitive and selective catalysts for electrochemical detection in enzyme-free sensors. In this research, we successfully fabricate Metal-Organic Framework (MOF) FeBDC-derived Fe₃O₄ for non-enzymatic electrochemical detection of glucose. FeBDC synthesis was carried out using the solvothermal method. FeCl₂.4H₂O and Benzene-1,4-dicarboxylic acid (H₂BDC) are used as precursors to form FeBDC. The materials were further characterized utilizing X-ray Powder Diffraction (XRD), Scanning Electron Microscopy (SEM), and Fourier-Transform Infrared Spectroscopy (FTIR). The resulting MOF yields good crystallinity and micro-rod like morphology. Electrochemical properties were tested using Cyclic Voltammetry (CV) and Differential Pulse Voltammetry (DPV) with a 0.1 M of Phosphate Buffer Saline (PBS pH 7.4) solution as the supporting electrolyte. The measurement results show the reduction and oxidation peaks in the CV curve of FeBDC, as well as Fe₃O₄. Pyrolysis of FeBDC to Fe₃O₄ increases the peak of oxidation and reduction currents. The Fe₃O₄ sample obtained has a sensitivity of 4.67 µA mM⁻¹.cm⁻², a linear range between 0.0 to 9.0 mM, and a glucose detection limit of 15.70 µM.

Dielectrophoresis of proteins: experimental data and evolving theory

The ability to selectively move and trap proteins is core to their effective use as building blocks and for their characterization. Analytical and preparative strategies for proteins have been pursued and modeled for nearly a hundred years, with great advances and success. Core to all of these studies is the separation, isolation, purification, and concentration of pure homogeneous fractions of a specific protein in solution. Processes to accomplish this useful solution include biphasic equilibrium (chromatographies, extractions), mechanical, bulk property, chemical equilibria, and molecular recognition. Ultimately, the goal of all of these is to physically remove all non-like protein molecules-to the finest detail: all atoms in the full three-dimensional structure being identical down the chemical bond and bulk structure chirality. One strategy which has not been effectively pursued is exploiting the higher order subtle electrical properties of the protein-solvent system. The advent of microfluidic systems has enabled the use of very high electric fields and well-defined gradients such that extremely high resolution separations of protein mixtures are possible. These advances and recognition of these capabilities have caused a re-evaluation of the underlying theoretical models and they were found to be inadequate. New theoretical descriptions are being considered which align more closely to the total forces present and the subtlety of differences between similar proteins. These are focused on the interfacial area between the protein and hydrating solvent molecules, as opposed to the macroscale assumptions of homogeneous solutions and particles. This critical review examines all data which has been published that place proteins in electric field gradients which induce collection of those proteins, demonstrating a force greater than dispersive effects or countering forces. Evolving theoretical constructs are presented and discussed, and a general estimate of future capabilities using the higher order effects and the high fields and precise gradients of microfluidic systems is discussed. Graphical abstract.

Protein Dielectrophoresis: I. Status of Experiments and an Empirical Theory

The dielectrophoresis (DEP) data reported in the literature since 1994 for 22 different globular proteins is examined in detail. Apart from three cases, all of the reported protein DEP experiments employed a gradient field factor ∇Em2 that is much smaller (in some instances by many orders of magnitude) than the ~4 1021 V2/m3 required, according to current DEP theory, to overcome the dispersive forces associated with Brownian motion. This failing results from the macroscopic Clausius-Mossotti (CM) factor being restricted to the range 1.0 > CM > -0.5. Current DEP theory precludes the protein's permanent dipole moment (rather than the induced moment) from contributing to the DEP force. Based on the magnitude of the β-dispersion exhibited by globular proteins in the frequency range 1 kHz-50 MHz, an empirically derived molecular version of CM is obtained. This factor varies greatly in magnitude from protein to protein (e.g., ~37,000 for carboxypeptidase; ~190 for phospholipase) and when incorporated into the basic expression for the DEP force brings most of the reported protein DEP above the minimum required to overcome dispersive Brownian thermal effects. We believe this empirically-derived finding validates the theories currently being advanced by Matyushov and co-workers.

Self-aligned microfluidic contactless dielectrophoresis device fabricated by single-layer imprinting on cyclic olefin copolymer

The trapping and deflection of biological cells by dielectrophoresis (DEP) at field non-uniformities in a microfluidic device is often conducted in a contactless dielectrophoresis (cDEP) mode, wherein the electrode channel is in a different layer than the sample channel, so that field penetration through the interceding barrier causes DEP above critical cut-off frequencies. In this manner, through physical separation of the electrode and sample channels, it is possible to spatially modulate electric fields with no electrode-induced damage to biological cells in the sample channel. However, since this device requires interlayer alignment of the electrode to sample channel and needs to maintain a thin interceding barrier (~ 15 μm) over the entire length over which DEP is needed (~ 1 cm), variations in alignment and microstructure fidelity cause wide variations in cDEP trapping level and frequency response across devices. We present a strategy to eliminate interlayer alignment by fabricating self-aligned electrode and sample channels, simultaneously with the interceding barrier layer (14-μm width and 50-μm depth), using a single-layer imprint and bond process on cyclic olefin copolymer. Specifically, by designing support structures, we preserve fidelity of the high aspect ratio insulating posts in the sample channel and the interceding barrier between the sample and electrode channels over the entire device footprint (~ 1 cm). The device operation is validated based on impedance measurements to quantify field penetration through the interceding barrier and by DEP trapping measurements. The presented fabrication strategy can eventually improve cDEP device manufacturing protocols to enable more reproducible DEP performance. Graphical abstract.

Impact of Self-Assembled Monolayer Design and Electrochemical Factors on Impedance-Based Biosensing

Real-time sensing of proteins, especially in wearable devices, remains a substantial challenge due to the need to convert a binding event into a measurable signal that is compatible with the chosen analytical instrumentation. Impedance spectroscopy enables real-time detection via either measuring electrostatic interactions or electron transfer reactions while simultaneously being amenable to miniaturization for integration into wearable form-factors. To create a more robust methodology for optimizing impedance-based sensors, additional fundamental studies exploring components influencing the design and implementation of these sensors are needed. This investigation addresses a sub-set of these issues by combining optical and electrochemical characterization to validate impedance-based sensor performance as a function of (1) biorecognition element density, (2) self-assembled monolayer chain length, (3) self-assembled monolayer charge density, (4) the electrochemical sensing mechanism and (5) the redox reporter selection. Using a pre-existing lysozyme aptamer and lysozyme analyte combination, we demonstrate a number of design criteria to advance the state-of-the-art in protein sensing. For this model system we demonstrated the following: First, denser self-assembled monolayers yielded substantially improved sensing results. Second, self-assembled monolayer composition, including both thickness and charge density, changed the observed peak position and peak current. Third, single frequency measurements, while less informative, can be optimized to replace multi-frequency measurements and in some cases (such as that with zwitterionic self-assembled monolayers) are preferred. Finally, various redox reporters traditionally not used in impedance sensing should be further explored. Collectively, these results can help limit bottlenecks associated with device development, enabling realization of next-generation impedance-based biosensing with customize sensor design for the specific application.

AC electrokinetic immobilization of organic dye molecules

The application of inhomogeneous AC electric fields for molecular immobilization is a very fast and simple method that does not require any adaptions to the molecule’s functional groups or charges. Here, the method is applied to a completely new category of molecules: small organic fluorescence dyes, whose dimensions amount to only 1 nm or even less. The presented setup and the electric field parameters used allow immobilization of dye molecules on the whole electrode surface as opposed to pure dielectrophoretic applications, where molecules are attracted only to regions of high electric field gradients, i.e., to the electrode tips and edges. In addition to dielectrophoresis and AC electrokinetic flow, molecular scale interactions and electrophoresis at short time scales are discussed as further mechanisms leading to migration and immobilization of the molecules.

Dimension-reconfigurable bubble film nanochannel for wetting based sensing

Dimensions and surface properties are the predominant factors for the applications of nanofluidic devices. Here we use a thin liquid film as a nanochannel by inserting a gas bubble in a glass capillary, a technique we name bubble-based film nanofluidics. The height of the film nanochannel can be regulated by the Debye length and wettability, while the length independently changed by applied pressure. The film nanochannel behaves functionally identically to classical solid state nanochannels, as ion concentration polarizations. Furthermore, the film nanochannels can be used for label-free immunosensing, by principle of wettability change at the solid interface. The optimal sensitivity for the biotin-streptavidin reaction is two orders of magnitude higher than for the solid state nanochannel, suitable for a full range of electrolyte concentrations. We believe that the film nanochannel represents a class of nanofluidic devices that is of interest for fundamental studies and also can be widely applied, due to its reconfigurable dimensions, low cost, ease of fabrication and multiphase interfaces.

Development of a flexible, sweat-based neuropeptide Y detection platform

Neuropeptide Y (NPY) biomarker levels have a close association with the diagnosis of Major Depression Disorder (MDD) and anxiety disorders. Quantifying NPY in correlation to self-reported symptoms will be an important measure to ensure a relatively uniform diagnosis and help with disease prognosis of these disorders. The work presented is a novel, passive eccrine sweat based, electrochemical detection platform for quantification of NPY biomarker levels. The paper offers a comparison between non-porous and porous sensor platforms using various electrochemical detection techniques. This work uses a novel strategy towards designing an optimal nanobioelectronic interface to measure NPY. The portability aspect of this detection platform is discussed by the demonstration a novel, portable EmStat Pico based electronic platform. The detection limit of the sensor is 10 pg mL⁻¹ and its range is 20–500 pg mL⁻¹. The NPY detection platform is envisioned to be a wearable point-of need monitoring system for management of chronic anxiety disorders and MDD.

Novel wearable NPY biomarker tracking system, envisioned to be a chronic anxiety and MDD management platform via self-monitoring.

Ion concentration polarization (ICP) of proteins at silicon micropillar nanogaps

Fast detection of low-abundance protein remains a challenge because detection speed is limited by analyte transport to the detection site of a biosensor. In this paper, we demonstrate a scalable fabrication process for producing vertical nanogaps between micropillars which enable ion concentration polarization (ICP) enrichment for fast analyte detection. Compared to horizontal nanochannels, massively paralleled vertical nanogaps not only provide comparable electrokinetics, but also significantly reduce fluid resistance, enabling microbead-based assays. The channels on the device are straightforward to fabricate and scalable using conventional lithography tools. The device is capable of enriching protein molecules by >1000 fold in 10 min. We demonstrate fast detection of IL6 down to 7.4 pg/ml with only a 10 min enrichment period followed by a 5 min incubation. This is a 162-fold enhancement in sensitivity compared to that without enrichment. Our results demonstrate the possibility of using silicon/silica based vertical nanogaps to mimic the function of polymer membranes for the purpose of protein enrichment.

Trapping of sub-100 nm nanoparticles using gigahertz acoustofluidic tweezers for biosensing applications

In this study, we present a nanoscale acoustofluidic trap (AFT) that manipulates nanoparticles in a microfluidic system actuated by a gigahertz acoustic resonator. The AFT generates independent standing closed vortices with high-speed rotation. Via careful design and optimization of geometric confinements, the AFT was able to effectively capture and enrich sub-100 nm nanoparticles with a low power consumption (0.25-5 μW μm-2) and rapid trapping (within 30 s), showing significantly enhanced particle-operating ability as compared to its acoustic and optical counterparts; using specifically functionalized nanoparticles (SFNPs) to selectively capture target molecules from the sample, the AFT led to the molecular concentration enhancement of ∼200 times. We investigated the feasibility of the SFNP-assisted AFT preconcentration method for biosensing applications and successfully demonstrated the capability of this method for the detection of serum prostate-specific antigen (PSA). The AFT was prepared via a fully CMOS-compatible process and thus could be conveniently integrated on a single chip, with potential for "lab-on-a-chip" or point-of-care (POC) nanoparticle-based biosensing applications.

Crystallization of high aspect ratio HKUST-1 thin films in nanoconfined channels for selective small molecule uptake

We present the ability to create unique morphologies of a prototypical metal organic framework (MOF), HKUST-1, by carrying out its crystallization within a set of nano-confined fluidic channels. These channels are fabricated on cyclic olefin copolymer by the high-fidelity hot embossing imprinting method. The picoliter volume synthesis in the nanochannels is hypothesized to bias the balance between nucleation and growth rates to obtain high aspect ratio large-crystalline domains of HKUST-1, which are grown in defined morphologies due to the patterned nanochannels. Confined crystal growth is achieved in nanofluidic channels as shallow as 50 nm. HKUST-1 crystalline domains with aspect ratios greater than 2500, and lengths up to 144 μm are obtained using the nanochannels, exceeding values obtained using chemical modulation and other confinement methods. HKUST-1 crystals are characterized using optical microscopy and scanning electron microscopy with energy dispersive spectroscopy. Porosity of the MOF and selective molecular uptake is demonstrated through inclusion of anthracene and methylene blue within the HKUST-1 framework, and with exclusion of rhodamine B and riboflavin, characterized using a confocal fluorescence microscope. We attribute this selectivity to the analyte size and electrostatic characteristics. Nanoconfined crystallization of MOFs can thus yield control over crystalline morphology to create ideal MOF crystals for enabling selective molecular enrinchment and sensing.

Hydroxyl phenyl imino modified phenanthro[9,10-d] imidazole: An AIEE-active sensor for determination of Cu2+ in water samples and subsequent "turn-on" recognition of Cr3+ with logic gates

A 2-hydroxyl phenyl imino modified phenanthro[9,10-d]imidazole probe (PPIP) was synthesized and its aggregation-induced emission enhancement (AIEE) phenomenon in aqueous medium was characterized by the Tyndall phenomena, morphologies of scanning electron microphotograph (SEM) as well as the enhanced fluorescence intensities. The AIEE-active PPIP in aggregated states exhibited a sensitive detection for Cu²⁺ along with the elimination of fluorescence intensity; subsequently, in situPPIP-Cu²⁺ complex showed selective fluorescence "turn-on" for Cr³⁺, providing a potential platform for tandem recognition of Cu²⁺ and Cr³⁺. The 1:1 M ratio between PPIP and Cu²⁺ was confirmed by Job's plot, ¹H NMR spectra, mass spectra and density functional theory (DFT) calculations. The detection limits for Cu²⁺ and Cr³⁺ were 4.127 × 10^-2 μM and 9.404 × 10^-1 μM, respectively. Moreover, this successive recognition could be utilized to construct the molecular logic gates by the fluorescence changes. In addition, PPIP was successfully employed for quantitative detecting Cu²⁺ in real water samples with satisfactory results.

Separation of Nano- and Microparticle Flows Using Thermophoresis in Branched Microfluidic Channels

Particle flow separation is a useful technique in lab-on-a-chip applications to selectively transport dispersed phases to a desired branch in microfluidic devices. The present study aims to demonstrate both nano- and microparticle flow separation using microscale thermophoresis at a Y-shaped branch in microfluidic channels. Microscale thermophoresis is the transport of tiny particles induced by a temperature gradient in fluids where the temperature variation is localized in the region of micrometer order. Localized temperature increases near the branch are achieved using the Joule heat from a thin-film micro electrode embedded in the bottom wall of the microfluidic channel. The inlet flow of the particle dispersion is divided into two outlet flows which are controlled to possess the same flow rates at the symmetric branches. The particle flow into one of the outlets is blocked by microscale thermophoresis since the particles are repelled from the hot region in the experimental conditions used here. As a result, only the solvent at one of outlets and the residual particle dispersion at the other outlet are obtained, i.e., the separation of particles flows is achieved. A simple model to explain the dynamic behavior of the nanoparticle distribution near the electrode is proposed, and a qualitative agreement with the experimental results is obtained. The proposed method can be easily combined with standard microfluidic devices and is expected to facilitate the development of novel particle separation and filtration technologies.

A novel diarylethene-based fluorescent "turn-on" sensor for the selective detection of Mg2

A new photochromic diarylethene derivative with a 4-methylphenol unit has been designed and synthesized. It displayed distinct photochromism and fluorescent ''turn on'' features to Mg2+ in acetonitrile solution. With the addition of Mg2+, there was an obvious increase of fluorescent emission intensity at 552 nm, accompanied by a clear change of fluorescent color from dark purple to green. Meantime, the 1 : 1 stoichiometry between the derivative and Mg2+ was verified by Job's plot and HRMS. Furthermore, the sensor was successfully applied in the detection of Mg2+ in practical samples. Moreover, based on the multiple-responsive fluorescence switching behaviors, it also could be used to construct a molecular logic circuit with UV/vis lights and Mg2+/EDTA as input signals and the emission at 552 nm as the output signal.

Optically selective and electrochemically active chemosensors for Cu (II) ions based on a skeleton of 2-(benzylideneamino)-4,5,6,7-tetrachloro-3',6'-dihydroxyspiro-[isoindoline-1, 9'-xanthen]-3-one

A new series of fluorescent chemosensors (7a-7f) based on a core skeleton of 2-(benzylideneamino)-4,5,6,7-tetrachloro-3',6'-dihydroxyspiro-[isoindoline-1,9'-xanthen]-3-one was synthesized and characterized by FT-IR, NMR and mass spectrometric techniques. The sensitivity and selectivity of probes (7a-7f) for Cu²⁺ ions were investigated by colorimetric, UV-vis absorption, fluorescence emission spectral studies and electrochemical analysis. These optical probes exhibited higher sensitivity and selectivity towards Cu²⁺ ions in DMSO solution over various other metal cations under consideration. Induced changes were observable by naked-eye. Based on fluorescence titration spectra and Job's-plot, it was found that the complexes formed between probes (7a-7f) and Cu²⁺ ions were in 1:1 stoichiometric ratio. The detection limit for chemosensors (7a-7f) were calculated to be 3.0834 × 10^-7-3.6425 × 10^-7 M.

Cost-effective approach to detect Cu(ii) and Hg(ii) by integrating a smartphone with the colorimetric response from a NBD-benzimidazole based dyad

A new optical chemosensor N1 was designed and synthesized by condensing 4-chloro-7-nitrobenzofurazan with 2-aminophenylbenzimidazole. In CH₃OH : H₂O (1 : 1, v/v) medium, sensor N1 exhibited high selectivity and sensitivity towards Cu²⁺ and Hg²⁺ ions by showing a distinct colour change from pale yellow to pink due to the internal charge transfer occurring between the sensor N1 and the Cu²⁺/Hg²⁺ ions upon complexation in 1 : 1 stoichiometry.

Stress Biomarkers in Biological Fluids and Their Point-of-Use Detection

Hormones produced by glands in the endocrine system and neurotransmitters produced by the nervous system control many bodily functions. The concentrations of these molecules in the body are an indication of its state, hence the use of the term biomarker. Excess concentrations of biomarkers, such as cortisol, serotonin, epinephrine, and dopamine, are released by the body in response to a variety of conditions, for example, emotional state (euphoria, stress) and disease. The development of simple, low-cost modalities for point-of-use (PoU) measurements of biomarkers levels in various bodily fluids (blood, urine, sweat, saliva) as opposed to conventional hospital or lab settings is receiving increasing attention. This paper starts with a review of the basic properties of 12 primary stress-induced biomarkers: origin in the body (i.e., if they are produced as hormones, neurotransmitters, or both), chemical composition, molecular weight (small/medium size molecules and polymers, ranging from ∼100 Da to ∼100 kDa), and hydro- or lipophilic nature. Next is presented a detailed review of the published literature regarding the concentration of these biomarkers found in several bodily fluids that can serve as the medium for determination of the condition of the subject: blood, urine, saliva, sweat, and, to a lesser degree, interstitial tissue fluid. The concentration of various biomarkers in most fluids covers a range of 5-6 orders of magnitude, from hundreds of nanograms per milliliter (∼1 μM) down to a few picograms per milliliter (sub-1 pM). Mechanisms and materials for point-of-use biomarker sensors are summarized, and key properties are reviewed. Next, selected methods for detecting these biomarkers are reviewed, including antibody- and aptamer-based colorimetric assays and electrochemical and optical detection. Illustrative examples from the literature are discussed for each key sensor approach. Finally, the review outlines key challenges of the field and provides a look ahead to future prospects.

Ultrasound-electrospinning-assisted fabrication and sensing evaluation of a novel membrane as ultrasensitive sensor for copper (II) ions detection in aqueous environment

The present study has reported an optimized fabrication and application of a novel PVA/TEOS/Schiff base nanofibers membrane as a highly sensitive copper (II) ions in aqueous environment. Here in, for first time, an ultrasound-assisted synthesized symmetric Schiff base has been immobilized on a hybrid polyvinyl alcohol (PVA) and TEOS using electrospinning technique for detection and filtration of copper ions. For this purpose, various working parameters were evaluated and finally the optimized nano fibers membrane was synthesized with 72 nm thickness and PVA/TEOS/Schiff base ratio of (wt%) 8:6:1. The optimized sample named PTLNFM has been employed successfully as an ultra sensitive chemosensor for Cu (II) detection in real samples. The immobilized Schiff base used as a chelating agent could detect copper (II) in the range from 9.34 × 10^-8 to 1.15 × 10^-5 mol L^-1 with the following correlation equation: Absorbance = 0.066 [Cu²⁺] × 10^-6 + 0.095 and R² = 0.992 and LOD of 1.27 × 10^-8 mol L^-1 which was lower than most of the reported detection limits in the previous literatures. Validity of this method has been carried out by analysis of Cu²⁺ in real samples with satisfying recoveries of over 96.11-99.24%.The developed membrane could be offered for diverse use such as medical or industrial applications.