High-performance bioanalysis based on ion concentration polarization of micro-/nanofluidic devices

Visual Detection of LPS at the Femtomolar Level Based on Click Chemistry-Induced Gold Nanoparticles Electrokinetic Accumulation

Lipopolysaccharide (LPS) presents a significant threat to human health. Herein, a novel method for detecting LPS was developed by coupling hybridization chain reaction (HCR), gold nanoparticles (AuNPs) agglutination (AA) triggered by a Cu(I)-catalyzed azide-alkyne cycloaddition click chemistry (CuAAC), and electrokinetic accumulation (EA) in a microfluidic chip, termed the HCR-AA-EA method. Thereinto, the LPS-binding aptamer (LBA) was coupled with the AuNP-coated Fe3O4 nanoparticle, which was connected with the polymer of H1 capped on CuO (H1-CuO) and H2-CuO. Upon LPS recognition by LBA, the polymers of H1- and H2-CuO were released into the solution, creating a "one LPS-multiple CuO" effect. Under ascorbic acid reduction, CuAAC was initiated between the alkyne and azide groups on the AuNPs' surface; then, the product was observed visually in the microchannel by EA. Finally, LPS was quantified by the integrated density of AuNP aggregates. The limit of detections were 29.9 and 127.2 fM for water samples and serum samples, respectively. The levels of LPS in the injections and serum samples by our method had a good correlation with those from the limulus amebocyte lysate test (r = 0.99), indicating high accuracy. Remarkably, to popularize our method, a low-cost, wall-power-free portable device was developed, enabling point-of-care testing.

On-Chip Electrochemical Sensing with an Enhanced Detecting Signal Due to Concentration Polarization-Based Analyte Preconcentration

Here, we integrated two key technologies within a microfluidic system, an electrokinetic preconcentration of analytes by ion Concentration Polarization (CP) and local electrochemical sensors to detect the analytes, which can synergistically act to significantly enhance the detection signal. This synergistic combination, offering both decoupled and coupled operation modes for continuous monitoring, was validated by the intensified fluorescent intensities of CP-preconcentrated analytes and the associated enhanced electrochemical response using differential pulse voltammetry and chronoamperometry. The system performance was evaluated by varying the location of the active electrochemical sensor, target analyte concentrations, and electrolyte concentration using fluorescein molecules as the model analyte and Homovanillic acid (HVA) as the target bioanalyte within both phosphate-buffered saline (PBS) and artificial sweat solution. The combination of on-chip electrochemical sensing with CP-based preconcentration renders this generic approach adaptable to various analytes. This advanced system shows remarkable promise for enhancing biosensing detection in practical applications while bridging the gap between fundamental research and practical implementation.

Cellulose-Based Ionic Conductor: An Emerging Material toward Sustainable Devices

Ionic conductors (ICs) find widespread applications across different fields, such as smart electronic, ionotronic, sensor, biomedical, and energy harvesting/storage devices, and largely determine the function and performance of these devices. In the pursuit of developing ICs required for better performing and sustainable devices, cellulose appears as an attractive and promising building block due to its high abundance, renewability, striking mechanical strength, and other functional features. In this review, we provide a comprehensive summary regarding ICs fabricated from cellulose and cellulose-derived materials in terms of fundamental structural features of cellulose, the materials design and fabrication techniques for engineering, main properties and characterization, and diverse applications. Next, the potential of cellulose-based ICs to relieve the increasing concern about electronic waste within the frame of circularity and environmental sustainability and the future directions to be explored for advancing this field are discussed. Overall, we hope this review can provide a comprehensive summary and unique perspectives on the design and application of advanced cellulose-based ICs and thereby encourage the utilization of cellulosic materials toward sustainable devices.

Designing with Iontronic Logic Gates─From a Single Polyelectrolyte Diode to an Integrated Ionic Circuit

This article presents the implementation of on-chip iontronic circuits via small-scale integration of multiple ionic logic gates made of bipolar polyelectrolyte diodes. These ionic circuits are analogous to solid-state electronic circuits, with ions as the charge carriers instead of electrons/holes. We experimentally characterize the responses of a single fluidic diode made of a junction of oppositely charged polyelectrolytes (i.e., anion and cation exchange membranes), with a similar underlying mechanism as a solid-state p- and n-type junction. This served to carry out predesigned logical computations in various architectures by integrating multiple diode-based logic gates, where the electrical signal between the integrated gates was transmitted entirely through ions. The findings shed light on the limitations affecting the number of logic gates that can be integrated, the degradation of the electrical signal, their transient response, and the design rules that can improve the performance of iontronic circuits.

Sensitive detection of microRNAs using polyadenine-mediated fluorescence spherical nucleic acids and a microfluidic electrokinetic signal amplification chip

The identification of tumor-related microRNAs (miRNAs) exhibits excellent promise for the early diagnosis of cancer and other bioanalytical applications. Therefore, we developed a sensitive and efficient biosensor using polyadenine (polyA)-mediated fluorescent spherical nucleic acid (FSNA) for miRNA analysis based on strand displacement reactions on gold nanoparticle (AuNP) surfaces and electrokinetic signal amplification (ESA) on a microfluidic chip. In this FSNA, polyA-DNA biosensor was anchored on AuNP surfaces via intrinsic affinity between adenine and Au. The upright conformational polyA-DNA recognition block hybridized with 6-carboxyfluorescein-labeled reporter-DNA, resulting in fluorescence quenching of FSNA probes induced by AuNP-based resonance energy transfer. Reporter DNA was replaced in the presence of target miRNA, leading to the recovery of reporter-DNA fluorescence. Subsequently, reporter-DNAs were accumulated and detected in the front of with Nafion membrane in the microchannel by ESA. Our method showed high selectivity and sensitivity with a limit of detection of 1.3 pM. This method could also be used to detect miRNA-21 in human serum and urine samples, with recoveries of 104.0%–113.3% and 104.9%–108.0%, respectively. Furthermore, we constructed a chip with three parallel channels for the simultaneous detection of multiple tumor-related miRNAs (miRNA-21, miRNA-141, and miRNA-375), which increased the detection efficiency. Our universal method can be applied to other DNA/RNA analyses by altering recognition sequences.

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FSNA assisted microfluidic chip was developed for miRNAs detection.

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Three different miRNAs were detected simultaneously.

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The excellent sensitivity and specificity were displayed toward miRNAs.

Engineering the Redox-Driven Channel for Precisely Regulating Nanoconfined Glutathione Identification and Transport

Bioinspired artificial nanochannels for molecular and ionic transport have extensive applications. However, it is still a huge challenge to achieve an intelligent transport system with high selectivity/efficiency and controllability. Inspired by glutathione transport across the plasma membrane via redox regulation, we herein designed and fabricated a redox-reactive artificial nanochannel based on the host-guest chemical strategy. The nanochannel platform achieved high selectivity/efficiency for the identification and transmission of glutathione in the confined space. In addition, this nanochannel can switch between the ON and OFF states through the redox reaction. This redox-regulated system can provide a potential application for detection/binding of biological analytes and redox-controlled drug release.

Manipulation and Mixing of 200 Femtoliter Droplets in Nanofluidic Channels Using MHz-Order Surface Acoustic Waves

Controllable manipulation and effective mixing of fluids and colloids at the nanoscale is made exceptionally difficult by the dominance of surface and viscous forces. The use of megahertz (MHz)-order vibration has dramatically expanded in microfluidics, enabling fluid manipulation, atomization, and microscale particle and cell separation. Even more powerful results are found at the nanoscale, with the key discovery of new regimes of acoustic wave interaction with 200 fL droplets of deionized water. It is shown that 40 MHz-order surface acoustic waves can manipulate such droplets within fully transparent, high-aspect ratio, 100 nm tall, 20-130 micron wide, 5-mm long nanoslit channels. By forming traps as locally widened regions along such a channel, individual fluid droplets may be propelled from one trap to the next, split between them, mixed, and merged. A simple theory is provided to describe the mechanisms of droplet transport and splitting.

Self-Propelled and Electrobraking Synergetic Liquid Manipulator toward Microsampling and Bioanalysis

Droplet manipulation is of paramount significance for microfluidics-based biochips, especially for bioanalytical chips. Despite great progresses made on droplet manipulation, the existing bioanalytical methods face challenges in terms of capturing minute doses toward hard-to-obtain samples and analyzing biological samples at low temperatures immediately. To circumvent these limitations, a self-propelled and electric stimuli synergetic droplet manipulator (SES-SDM) was developed by a femtosecond laser microfabrication strategy followed by post-treatment. Combining the inspiration from cactus and Nepenthes pitcher plants, the wedge structure with the microbowl array and silicone oil infusion was endowed cooperatively with the SES-SDM. With the synergy of the ultralow voltage (4.0 V) stimuli, these bioinspired features enable the SES-SDM to transport the droplet spontaneously and controllably, showing the maximum fast motion (15.7 mm/s) and long distance (96.2 mm). Remarkably, the SES-SDM can function at -5 °C without the freezing of the droplets, where the self-propelled motion and electric-responsive pinning can realize the accurate capture and real-time analysis of the microdroplets of the tested samples. More importantly, the SES-SDM can realize real-time diagnosis of excessive heavy metal in water by the cooperation of self-propulsion and electro-brake. This work opens an avenue to design a microsampling (5-20 μL) manipulator toward producing the minute samples for efficient bioanalysis and offers a strategy for microanalysis using the synergistic droplet manipulation.

Tutorial review: Enrichment and separation of neutral and charged species by ion concentration polarization focusing

Ion concentration polarization focusing (ICPF) is an electrokinetic technique, in which analytes are enriched and separated along a localized electric field gradient in the presence of a counter flow. This field gradient is generated by depletion of ions of the background electrolyte at an ion permselective junction. In this tutorial review, we summarize the fundamental principles and experimental parameters that govern selective ion transport and the stability of the enriched analyte plug. We also examine faradaic ICP (fICP), in which local ion concentration is modulated via electrochemical reactions as an attractive alternative to ICP that achieves similar performance with a decrease in both power consumption and Joule heating. The tutorial covers important challenges to the broad application of ICPF including undesired pH gradients, low volumetric throughput, samples that induce biofouling or are highly conductive, and limited approaches to on- or off-chip analysis. Recent developments in the field that seek to address these challenges are reviewed along with new approaches to maximize enrichment, focus uncharged analytes, and achieve enrichment and separation in water-in-oil droplets. For new practitioners, we discuss practical aspects of ICPF, such as strategies for device design and fabrication and the relative advantages of several types of ion selective junctions and electrodes. Lastly, we summarize tips and tricks for tackling common experimental challenges in ICPF.

Electropreconcentration diagrams to optimize molecular enrichment with low counter pressure in a nanofluidic device

Concentration polarization (CP)-based focusing electrokinetics nanofluidic devices have been developed in order to simultaneously detect and enrich highly diluted analytes on-a-chip. However, stabilization of focal points over long time under the application of the electric field remains as a technical bottleneck. If pressure-assisted preconcentration methods have been proposed to stabilize propagating modes at low inverse Dukhin number ( 1 / D u ≪ 1 ) , these recent protocols remain laborious for optimizing experimental parameters. In this paper, "electric field E/counter-pressure P" diagrams have been established during pressure-assisted electro-preconcentration of fluorescein as a model molecule. Such E/P diagram allows direct observation of the region for which the optimal counter-pressure P leads to a stable focusing regime. This region of stable focusing is shown to vary depending of the nanoslit length (100 μm < L_nanoslit < 500 μm) and the nature of the background electrolyte (KCl and NaCl). Longer nanoslits (500 μm) produce stabilization at low counter-pressure P, whereas NaCl offers a narrower region of stable focusing in the E/P diagram compared to KCl. Finally, the ability of such pressure-assisted protocol to concentrate negatively charged proteins has been tested with a more applicative protein, i.e., ovalbumin. The corresponding E/P diagram confirms the existence of the stable focusing regime at both low electric field E (≤20 V) and counter-pressure P (≤0.4 bar). With an enrichment factor as high as 70 after 2 min for ovalbumin at a concentration of 10 μM, such pressure-assisted nanofluidic electro-preconcentration protocol appears very promising to concentrate and detect biomolecules.

Ion Current Rectification in Extra-Long Nanofunnels

Nanofluidic systems offer new functionalities for the development of high sensitivity biosensors, but many of the interesting electrokinetic phenomena taking place inside or in the proximity of nanostructures are still not fully characterized. Here, to better understand the accumulation phenomena observed in fluidic systems with asymmetric nanostructures, we study the distribution of the ion concentration inside a long (more than 90 µm) micrometric funnel terminating with a nanochannel. We show numerical simulations, based on the finite element method, and analyze how the ion distribution changes depending on the average concentration of the working solutions. We also report on the effect of surface charge on the ion distribution inside a long funnel and analyze how the phenomena of ion current rectification depend on the applied voltage and on the working solution concentration. Our results can be used in the design and implementation of high-performance concentrators, which, if combined with high sensitivity detectors, could drive the development of a new class of miniaturized biosensors characterized by an improved sensitivity.

Nanofluidic-Based Accumulation of Antigens for Miniaturized Immunoassay

The continuous advances of Nanofluidics have been stimulating the development of novel nanostructures and strategies to accumulate very diluted analytes, for implementing a new class of high sensitivity miniaturized polymeric sensors. We take advantage of the electrokinetic properties of these structures, which allow accumulating analytes inside asymmetric microfluidic structures to implement miniaturized sensors able to detect diluted solutions down to nearly 1.2 pg/mL. In particular, exploiting polydimethylsiloxane devices, fabricated by using the junction gap breakdown technique, we concentrate antigens inside a thin microfunnel functionalized with specific antibodies to favor the interaction and, if it is the case, the recognition between antigens in solution and antibodies anchored to the surface. The transduction mechanism consists in detecting the fluorescence signal of labeled avidin when it binds to biotinylated antigens. Here, we demonstrate that exploiting these electrokinetic phenomena, typical of nanofluidic structures, we succeeded in concentrating biomolecules in correspondence of a 1 pL sensing region, a strategy that grants to the device performance comparable to standard immunoassays.

Development of microfluidic concentrator using ion concentration polarization mechanism to assist trapping magnetic nanoparticle-bound miRNA to detect with Raman tags

MicroRNAs (miRNAs) are small noncoding single-stranded ribonucleic acid molecules. This type of endogenous oligonucleotide could be secreted into the circulation and exist stably. The detection of specific miRNAs released by cancer cells potentially provides a noninvasive means to achieve early diagnosis and prognosis of cancers. However, the typical concentration of miRNAs in blood is below the ultratrace level. This study uses a simple thermoplastic microfluidic concentration device based on an ion concentration polarization mechanism to perform enrichment and cleanup and Raman sensing beads to determine miRNA quantitatively. One sample solution containing target miRNA molecules having been hybridized with two nucleotide probes, where one probe is on a Raman tag of a nanoaggregate embedded bead (NAEB) and the other probe is on a magnetic nanoparticle (MNP), is first filled into the device. When an external field is applied across a cation exchange membrane stationed in the middle conduit of the device, the MNP-miRNA-NAEB complexed particles are enriched near the membrane edge of the cathode side. The concentrated complexed particles are further trapped using an external magnet to perform washing steps to remove excess noncomplexed NAEBs. When cleanup steps are accomplished, the remaining complexed particles are loaded into one detection capillary to acquire Raman signals from the sensing beads. Compared with that using a conventional magnetic trapping device, the cleanup time is shortened from nearly an hour to less than 10 min. Sample loss during the washing steps becomes more controllable, resulting in adequate standard curve linearity (R > 0.99) ranging from 1 to 100 pM.