Microfabricated electrochemical cell-based biosensors for analysis of living cells in vitro

Facile Synthesis and Environmental Applications of Noble Metal-Based Catalytic Membrane Reactors

Noble metal nanoparticle-loaded catalytic membrane reactors (CMRs) have emerged as a promising method for water decontamination. In this study, we proposed a convenient and green strategy to prepare gold nanoparticle (Au NPs)-loaded CMRs. First, the redox-active substrate membrane (CNT-MoS2) composed of carbon nanotube (CNT) and molybdenum disulfide (MoS2) was prepared by an impregnation method. Water-diluted Au(III) precursor (HAuCl4) was then spontaneously adsorbed on the CNT-MoS2 membrane only through filtration and reduced into Au(0) nanoparticles in situ, which involved a “adsorption–reduction” process between Au(III) and MoS2. The constructed CNT-MoS2@Au membrane demonstrated excellent catalytic activity and stability, where a complete 4-nitrophenol transformation can be obtained within a hydraulic residence time of <3.0 s. In addition, thanks to the electroactivity of CNT networks, the as-designed CMR could also be applied to the electrocatalytic reduction of bromate (>90%) at an applied voltage of −1 V. More importantly, by changing the precursors, one could further obtain the other noble metal-based CMR (e.g., CNT-MoS2@Pd) with superior (electro)catalytic activity. This study provided new insights for the rational design of high-performance CMRs toward various environmental applications.

PC-12 Cell Line as a Neuronal Cell Model for Biosensing Applications

PC-12 cells have been widely used as a neuronal line study model in many biosensing devices, mainly due to the neurogenic characteristics acquired after differentiation, such as high level of secreted neurotransmitter, neuron morphology characterized by neurite outgrowth, and expression of ion and neurotransmitter receptors. For understanding the pathophysiology processes involved in brain disorders, PC-12 cell line is extensively assessed in neuroscience research, including studies on neurotoxicity, neuroprotection, or neurosecretion. Various analytical technologies have been developed to investigate physicochemical processes and the biosensors based on optical and electrochemical techniques, among others, have been at the forefront of this development. This article summarizes the application of different biosensors in PC-12 cell cultures and presents the modern approaches employed in neuronal networks biosensing.

Microengineered platforms for characterizing the contractile function of in vitro cardiac models

Emerging heart-on-a-chip platforms are promising approaches to establish cardiac cell/tissue models in vitro for research on cardiac physiology, disease modeling and drug cardiotoxicity as well as for therapeutic discovery. Challenges still exist in obtaining the complete capability of in situ sensing to fully evaluate the complex functional properties of cardiac cell/tissue models. Changes to contractile strength (contractility) and beating regularity (rhythm) are particularly important to generate accurate, predictive models. Developing new platforms and technologies to assess the contractile functions of in vitro cardiac models is essential to provide information on cell/tissue physiologies, drug-induced inotropic responses, and the mechanisms of cardiac diseases. In this review, we discuss recent advances in biosensing platforms for the measurement of contractile functions of in vitro cardiac models, including single cardiomyocytes, 2D monolayers of cardiomyocytes, and 3D cardiac tissues. The characteristics and performance of current platforms are reviewed in terms of sensing principles, measured parameters, performance, cell sources, cell/tissue model configurations, advantages, and limitations. In addition, we highlight applications of these platforms and relevant discoveries in fundamental investigations, drug testing, and disease modeling. Furthermore, challenges and future outlooks of heart-on-a-chip platforms for in vitro measurement of cardiac functional properties are discussed.

Microfluidic-Chip-Integrated Biosensors for Lung Disease Models

Lung diseases (e.g., infection, asthma, cancer, and pulmonary fibrosis) represent serious threats to human health all over the world. Conventional two-dimensional (2D) cell models and animal models cannot mimic the human-specific properties of the lungs. In the past decade, human organ-on-a-chip (OOC) platforms-including lung-on-a-chip (LOC)-have emerged rapidly, with the ability to reproduce the in vivo features of organs or tissues based on their three-dimensional (3D) structures. Furthermore, the integration of biosensors in the chip allows researchers to monitor various parameters related to disease development and drug efficacy. In this review, we illustrate the biosensor-based LOC modeling, further discussing the future challenges as well as perspectives in integrating biosensors in OOC platforms.

Recent Advances in Monitoring Cell Behavior Using Cell-Based Impedance Spectroscopy

Cell-based impedance spectroscopy (CBI) is a powerful tool that uses the principles of electrochemical impedance spectroscopy (EIS) by measuring changes in electrical impedance relative to a voltage applied to a cell layer. CBI provides a promising platform for the detection of several properties of cells including the adhesion, motility, proliferation, viability and metabolism of a cell culture. This review gives a brief overview of the theory, instrumentation, and detection principles of CBI. The recent applications of the technique are given in detail for research into cancer, neurodegenerative diseases, toxicology as well as its application to 2D and 3D in vitro cell cultures. CBI has been established as a biophysical marker to provide quantitative cellular information, which can readily be adapted for single-cell analysis to complement the existing biomarkers for clinical research on disease progression.

One-step phosphite removal by an electroactive CNT filter functionalized with TiO2/CeOx nanocomposites

Compared with phosphate (+5 valence), phosphite (HPO₃^2-/H₂PO₃^-, +3 valence) possesses higher solubility, and is more resistant to biotransformation. Herein, we designed a one-step electroactive filter technology for rapid and efficient phosphite removal. The filter consists of carbon nanotubes (CNT) and functionalized with nanoscale TiCe binary oxides. The phosphite removal kinetics and capacity increased with electric field (e.g., from 54.5% at 0 V to 75.6% at 2 V) and flow rate (e.g., from 63.1% at 1.5 mL/min to 81.2% at 6 mL/min). This can be attributed to synergistic effects of the filter's electrochemical reactivity, limited pore size, more exposed active sites and flow-through design. Meanwhile, phosphite can be converted to phosphate once adsorbed under electric field. The TiO₂/CeO_x-CNT filter could work effectively across a wide pH range, and the presence of various coexisting anions posed negligible impact on phosphite removal. Electrochemical characterizations verified the essential role of CeO_x and applied electric field, which synergistically accelerated electron transfer rate and increased charge capacity. The TiO₂/CeO_x-CNT filter can be regenerated effectively by chemical washing. The system efficacy was further supported by a comparable phosphite removal efficiency of 72.8% in actual lake water conditions. Therefore, this TiO₂/CeO_x-CNT filter technology is promising for mitigating the challenging issue of phosphite contamination from water bodies.

Development of Novel and Highly Specific ssDNA-Aptamer-Based Electrochemical Biosensor for Rapid Detection of Mercury (II) and Lead (II) Ions in Water

In this work, we report on the development of an electrochemical biosensor for high selectivity and rapid detection of Hg2+ and Pb2+ ions using DNA-based specific aptamer probes labeled with ferrocene (or methylene blue) and thiol groups at their 5′ and 3′ termini, respectively. Aptamers were immobilized onto the surface of screen-printed gold electrodes via the SH (thiol) groups, and then cyclic voltammetry and impedance spectra measurements were performed in buffer solutions with the addition of HgCl2 and PbCl2 salts at different concentrations. Changes in 3D conformation of aptamers, caused by binding their respective targets, e.g., Hg2+ and Pb2+ ions, were accompanied by an increase in the electron transfer between the redox label and the electrode. Accordingly, the presence of the above ions can be detected electrochemically. The detection of Hg2+ and Pb2+ ions in a wide range of concentrations as low as 0.1 ng/mL (or 0.1 ppb) was achieved. The study of the kinetics of aptamer/heavy metal ions binding gave the values of the affinity constants of approximately 9.10−7 mol, which proved the high specificity of the aptamers used.

Electrochemical inhibition bacterial sensor array for detection of water pollutants: artificial neural network (ANN) approach

This work reports on further development of an inhibition electrochemical sensor array based on immobilized bacteria for the preliminary detection of a wide range of organic and inorganic pollutants, such as heavy metal salts (HgCl₂, PbCl₂, CdCl₂), pesticides (atrazine, simazine, DDVP), and petrochemicals (hexane, octane, pentane, toluene, pyrene, and ethanol) in water. A series of DC and AC electrochemical measurements, e.g., cyclic voltammograms and impedance spectroscopy, were carried out on screen-printed gold electrodes with three types of bacteria, namely Escherichia coli, Shewanella oneidensis, and Methylococcus capsulatus, immobilized via poly l-lysine. The results obtained showed a possibility of pattern recognition of the above pollutants by their inhibition effect on the three bacteria used. The analysis of a large amount of experimental data was carried out using an artificial neural network (ANN) programme for more accurate identification of pollutants as well as the estimation of their concentration. The results are encouraging for the development of a simple and cost-effective biosensing technology for preliminary in-field analysis (screening) of water samples for the presence of environmental pollutants.

Combining stem cell-derived hepatocytes with impedance sensing to better predict human drug toxicity

Background: The liver plays a central role in human drug metabolism. To model drug metabolism, the major cell type of the liver, the hepatocyte, is commonly used. Hepatocytes can be derived from human and animal sources, including pluripotent stem cells. Cell-based models have shown promise in modeling human drug exposure. The assays used in those studies are normally 'snap-shot' in nature, and do not provide the complete picture of human drug exposure. Research design and methods: In this study, we employ stem cell-derived hepatocytes and impedance sensing to model human drug toxicity. This impedance-based stem cell assay reports hepatotoxicity in real time after treatment with compounds provided by industry. Results: Using electric cell-substrate impedance Sensing (ECIS), we were able to accurately measure drug toxicity post-drug exposure in real time and more quickly than gold standard biochemical assays. Conclusions: ECIS is robust and non-destructive methodology capable of monitoring human drug exposure with superior performance to current gold standard 'snapshot' assays. We believe that the methodology presented within this article could prove valuable in the quest to better predict off-target effects of drugs in humans.

Human dermal fibroblast proliferation controlled by surface roughness of two-component nanostructured latex polymer coatings

In this study hierarchically-structured latex polymer coatings and self-supporting films were characterised and their suitability for cell growth studies was tested with Human Dermal Fibroblasts (HDF). Latex can be coated or printed on rigid or flexible substrates thus enabling high-throughput fabrication. Here, coverslip glass substrates were coated with blends of two different aqueous latex dispersions: hydrophobic polystyrene (PS) and hydrophilic carboxylated acrylonitrile butadiene styrene (ABS). The nanostructured morphology and topography of the latex films was controlled by varying the mixing ratio of the components in the latex blend. Thin latex-coatings retain high transparency on glass allowing optical and high resolution imaging of cell growth and morphology. Compared to coverslip glass surfaces and commercial well-plates HDF cell growth was enhanced up to 150-250 % on latex surfaces with specific nanostructure. Growth rates were correlated with selected roughness parameters such as effective surface area (S_q), RMS-roughness (S_dr) and correlation length (S_cl37). High-resolution confocal microscopy clearly indicated less actin stress-fibre development in cells on the latex surface compared to coverslip glass. The results show that surface nanotopography can, by itself, passively modulate HDF cell proliferation and cytoskeletal architecture.

LUCS (Light-Up Cell System), a universal high throughput assay for homeostasis evaluation in live cells

Observations of fluorescent cyanine dye behavior under illumination at 500 nm lead to a novel concept in cell biology allowing the development of a new live cell assay called LUCS, for Light-Up Cell System, measuring homeostasis in live cells. Optimization of the LUCS process resulted in a standardized, straightforward and high throughput assay with applications in toxicity assessment. The mechanisms of the LUCS process were investigated. Electron Paramagnetic Resonance experiments showed that the singlet oxygen and hydroxyl radical are involved downstream of the light effect, presumably leading to deleterious oxidative stress that massively opens access of the dye to its intracellular target. Reversible modulation of LUCS by both verapamil and proton availability indicated that plasma membrane proton/cation antiporters, possibly of the MATE drug efflux transport family, are involved. A mechanistic model is presented. Our data show that intracellular oxidation can be controlled by tuning light energy, opening applications in regulatory purposes, anti-oxidant research, chemotherapy efficacy and dynamic phototherapy strategies.

Quantitative assessment of specific carbonic anhydrase inhibitors effect on hypoxic cells using electrical impedance assays

Carbonic anhydrase IX (CA IX) is an important orchestrator of hypoxic tumour environment, associated with tumour progression, high incidence of metastasis and poor response to therapy. Due to its tumour specificity and involvement in associated pathological processes: tumourigenesis, angiogenesis, inhibiting CA IX enzymatic activity has become a valid therapeutic option. Dynamic cell-based biosensing platforms can complement cell-free and end-point analyses and supports the process of design and selection of potent and selective inhibitors. In this context, we assess the effectiveness of recently emerged CA IX inhibitors (sulphonamides and sulphocoumarins) and their antitumour potential using an electrical impedance spectroscopy biosensing platform. The analysis allows discriminating between the inhibitory capacities of the compounds and their inhibition mechanisms. Microscopy and biochemical assays complemented the analysis and validated impedance findings establishing a powerful biosensing tool for the evaluation of carbonic anhydrase inhibitors potency, effective for the screening and design of anticancer pharmacological agents.

Photonic crystals: emerging biosensors and their promise for point-of-care applications

Biosensors are extensively employed for diagnosing a broad array of diseases and disorders in clinical settings worldwide. The implementation of biosensors at the point-of-care (POC), such as at primary clinics or the bedside, faces impediments because they may require highly trained personnel, have long assay times, large sizes, and high instrumental cost. Thus, there exists a need to develop inexpensive, reliable, user-friendly, and compact biosensing systems at the POC. Biosensors incorporated with photonic crystal (PC) structures hold promise to address many of the aforementioned challenges facing the development of new POC diagnostics. Currently, PC-based biosensors have been employed for detecting a variety of biotargets, such as cells, pathogens, proteins, antibodies, and nucleic acids, with high efficiency and selectivity. In this review, we provide a broad overview of PCs by explaining their structures, fabrication techniques, and sensing principles. Furthermore, we discuss recent applications of PC-based biosensors incorporated with emerging technologies, including telemedicine, flexible and wearable sensing, smart materials and metamaterials. Finally, we discuss current challenges associated with existing biosensors, and provide an outlook for PC-based biosensors and their promise at the POC.

Electronic control of gene expression and cell behaviour in Escherichia coli through redox signalling

The ability to interconvert information between electronic and ionic modalities has transformed our ability to record and actuate biological function. Synthetic biology offers the potential to expand communication ‘bandwidth' by using biomolecules and providing electrochemical access to redox-based cell signals and behaviours. While engineered cells have transmitted molecular information to electronic devices, the potential for bidirectional communication stands largely untapped. Here we present a simple electrogenetic device that uses redox biomolecules to carry electronic information to engineered bacterial cells in order to control transcription from a simple synthetic gene circuit. Electronic actuation of the native transcriptional regulator SoxR and transcription from the PsoxS promoter allows cell response that is quick, reversible and dependent on the amplitude and frequency of the imposed electronic signals. Further, induction of bacterial motility and population based cell-to-cell communication demonstrates the versatility of our approach and potential to drive intricate biological behaviours.

Synthetic biology offers the ability to explore new ways of manipulating gene expression and function. Here the authors demonstrate an electrogenetic device that allows control of transcription by an exogenous electrical signal.

Microfluidic-Based Multi-Organ Platforms for Drug Discovery

Development of predictive multi-organ models before implementing costly clinical trials is central for screening the toxicity, efficacy, and side effects of new therapeutic agents. Despite significant efforts that have been recently made to develop biomimetic in vitro tissue models, the clinical application of such platforms is still far from reality. Recent advances in physiologically-based pharmacokinetic and pharmacodynamic (PBPK-PD) modeling, micro- and nanotechnology, and in silico modeling have enabled single- and multi-organ platforms for investigation of new chemical agents and tissue-tissue interactions. This review provides an overview of the principles of designing microfluidic-based organ-on-chip models for drug testing and highlights current state-of-the-art in developing predictive multi-organ models for studying the cross-talk of interconnected organs. We further discuss the challenges associated with establishing a predictive body-on-chip (BOC) model such as the scaling, cell types, the common medium, and principles of the study design for characterizing the interaction of drugs with multiple targets.

Exploiting plug-and-play electrochemistry for drug discovery

Electrochemistry has emerged as a powerful analytical technique for chemical analysis of living cells, biologically active molecules and metabolites. Electrochemical biosensor, microfluidics and mass spectrometry are the most frequently used methods for electrochemical detection and monitory, which comprise a collection of extremely useful measurement tools for various fields of biology and medicine. Most recently, electrochemistry has been shown to be coupled with nanotechnology and genetic engineering to generate new enabling technologies, providing rapid, selective, and sensitive detection and diagnosis platforms. The primary focus of this review is to highlight the utility of electrochemical strategies and their conjunction with other approaches for drug metabolism and discovery. Current challenges and possible future developments and applications of electrochemistry in drug studies are also discussed.

Modified electrodes used for electrochemical detection of metal ions in environmental analysis

Heavy metal pollution is one of the most serious environmental problems, and regulations are becoming stricter. Many efforts have been made to develop sensors for monitoring heavy metals in the environment. This review aims at presenting the different label-free strategies used to develop electrochemical sensors for the detection of heavy metals such as lead, cadmium, mercury, arsenic etc. The first part of this review will be dedicated to stripping voltammetry techniques, on unmodified electrodes (mercury, bismuth or noble metals in the bulk form), or electrodes modified at their surface by nanoparticles, nanostructures (CNT, graphene) or other innovative materials such as boron-doped diamond. The second part will be dedicated to chemically modified electrodes especially those with conducting polymers. The last part of this review will focus on bio-modified electrodes. Special attention will be paid to strategies using biomolecules (DNA, peptide or proteins), enzymes or whole cells.

Three-dimensional cell culture systems and their applications in drug discovery and cell-based biosensors

Three-dimensional (3D) cell culture systems have gained increasing interest in drug discovery and tissue engineering due to their evident advantages in providing more physiologically relevant information and more predictive data for in vivo tests. In this review, we discuss the characteristics of 3D cell culture systems in comparison to the two-dimensional (2D) monolayer culture, focusing on cell growth conditions, cell proliferation, population, and gene and protein expression profiles. The innovations and development in 3D culture systems for drug discovery over the past 5 years are also reviewed in the article, emphasizing the cellular response to different classes of anticancer drugs, focusing particularly on similarities and differences between 3D and 2D models across the field. The progression and advancement in the application of 3D cell cultures in cell-based biosensors is another focal point of this review.

A generic microfluidic biosensor of G protein-coupled receptor activation – impedance measurements of reversible morphological changes of reverse transfected HEK293 cells on microelectrodes

Flowcell with micro-IDEs (250–500 μm) covered with both stable and reverse transfected cells overexpressing membrane receptors to demonstrate impedance responses to serial injections of analyte.

An iridium oxide microelectrode for monitoring acute local pH changes of endothelial cells

A microelectrode on a chip was modified to detect the local pH changes of the attached endothelial cells under the stimulation of thrombin.

A novel electrochemical method to evaluate the cytotoxicity of heavy metals

There is an ongoing search to develop techniques for detection of heavy metals which are highly toxic and can cause damaging effects even at very low concentrations. In this present study, we report a label-free electrochemical method based on the direct voltammetric response of human cervical carcinoma (HeLa) cells on a highly sensitive graphene modified electrode. Five heavy metals were tested with the method and the results were validated by the traditional methyl tetrazolium (MTT) assay. The results revealed that the most toxic metal was Cr, followed by Cd, Cu, Pb and Zn. A good correlation between the two methods was observed. This work will be beneficial in providing a novel monitoring method to detect hazardous pollutants in the field of environmental toxicology.

Detection of Micrococcus luteus biofilm formation in microfluidic environments by pH measurement using an ion-sensitive field-effect transistor

Biofilm formation in microfluidic channels is difficult to detect because sampling volumes are too small for conventional turbidity measurements. To detect biofilm formation, we used an ion-sensitive field-effect transistor (ISFET) measurement system to measure pH changes in small volumes of bacterial suspension. Cells of Micrococcus luteus (M. luteus) were cultured in polystyrene (PS) microtubes and polymethylmethacrylate (PMMA)-based microfluidic channels laminated with polyvinylidene chloride. In microtubes, concentrations of bacteria and pH in the suspension were analyzed by measuring turbidity and using an ISFET sensor, respectively. In microfluidic channels containing 20 μL of bacterial suspension, we measured pH changes using the ISFET sensor and monitored biofilm formation using a microscope. We detected acidification and alkalinization phases of M. luteus from the ISFET sensor signals in both microtubes and microfluidic channels. In the alkalinization phase, after 2 day culture, dense biofilm formation was observed at the bottom of the microfluidic channels. In this study, we used an ISFET sensor to detect biofilm formation in clinical and industrial microfluidic environments by detecting alkalinization of the culture medium. 

Electrochemical based biosensors

This editorial summarizes the general approaches of the electrochemical based biosensors described in the manuscripts published in this Special Issue. Electrochemical based biosensors are scientifically and economically important for the detection and early diagnosis of many diseases, and they will be increasingusedand developed in the coming years. The importance of the selection of recognition processes, fabrication techniques and biosensor materials will be introduced.