Intracellular Electrochemical Sensing

Electroactive polymer tag modified nanosensors for enhanced intracellular ATP detection

ATP plays a crucial role in cell energy supply, so the quantification of intracellular ATP levels is particularly important for understanding many physio-pathological processes. The intracellular quantification of this non-electroactive molecule can be realized using aptamer-modified nanoelectrodes, but is hindered by the limited quantity of modification and electroactive tags on the nanosized electrodes. Herein, we developed a simple but effective electrochemical signal amplification strategy for intracellular ATP detection, which replaces the regular ATP aptamer-linked ferrocene monomer with a polymer, thus greatly magnifying the amounts of electrochemical reporters linked to one chain of the aptamer and enhancing the signals. This ferrocene polymer-ATP aptamer was further immobilized onto Au nanowire electrodes (SiC@C@Au NWEs) to achieve accurate quantification of intracellular ATP in single cells, presenting high electrochemical signal output and high specificity. This work not only provides a powerful tool for quantifying intracellular ATP but also offers a simple and versatile strategy for electrochemical signal amplification in the detection of broader non-electroactive molecules involved in different kinds of intracellular physiological processes.

An anti-poisoning nanosensor for in situ monitoring of intracellular endogenous hydrogen sulfide

Intracellular H₂S plays an important regulatory role in cell metabolism. The limited sensing materials and severe sensor passivation hinder its quantification. We functionalized conductive nanowires with MoS₂ and quercetin in a large-scale manner, developed single nanowire sensors with excellent electrocatalytic and anti-poisoning performance, and achieved the accurate quantification of H₂S within single cells.

Interrogation on the Cellular Nano-Interface and Biosafety of Repeated Nano-Electroporation by Nanostraw System

Cell perforation is a critical step for intracellular drug delivery and real-time biosensing of intracellular signals. In recent years, the nanostraws system has been developed to achieve intracellular drug delivery with minimal invasiveness to the cells. Repeated cell perforation via nano-system could allow delivery of multiple drugs into cells for cell editing, but the biosafety is rarely explored. In this work, a nanostraw-mediated nano-electroporation system was developed, which allowed repeated perforation of the same set of cells in a minimally invasive manner, while the biosafety aspect of this system was investigated. Highly controllable fabrication of Al2O3 nanostraw arrays based on a porous polyethylene terephthalate (PET) membrane was integrated with a microfluidic device to construct the nanostraw-electroporation system. The pulse conditions and intervals of nano-electroporation were systematically optimized to achieve efficient cells perforation and maintain the viability of the cells. The cells proliferation, the early apoptosis activities after nanostraw-electroporation and the changes of gene functions and gene pathways of cells after repeated nano-electroporation were comprehensively analyzed. These results revealed that the repeated nanostraw-electroporation did not induce obvious negative effects on the cells. This work demonstrates the feasibility of repeated nano-electroporation on cells and provides a promising strategy for future biomedical applications.

Multiplexed assessment of engineered bacterial constructs for intracellular β-galactosidase expression by redox amplification on catechol-chitosan modified nanoporous gold

Synthetic biology approaches for rewiring of bacterial constructs to express particular intracellular factors upon induction with the target analyte are emerging as sensing paradigms for applications in environmental and in vivo monitoring. To aid in the design and optimization of bacterial constructs for sensing analytes, there is a need for lysis-free intracellular detection modalities that monitor the signal level and kinetics of expressed factors within different modified bacteria in a multiplexed manner, without requiring cumbersome surface immobilization. Herein, an electrochemical detection system on nanoporous gold that is electrofabricated with a biomaterial redox capacitor is presented for quantifying β-galactosidase expressed inside modified Escherichia coli constructs upon induction with dopamine. This nanostructure-mediated redox amplification approach on a microfluidic platform allows for multiplexed assessment of the expressed intracellular factors from different bacterial constructs suspended in distinct microchannels, with no need for cell lysis or immobilization. Since redox mediators present over the entire depth of the microchannel can interact with the electrode and with the E. coli construct in each channel, the platform exhibits high sensitivity and enables multiplexing. We envision its application in assessing synthetic biology-based approaches for comparing specificity, sensitivity, and signal response time upon induction with target analytes of interest.

Nucleic Acid Tests for Clinical Translation

Nucleic acids, including deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), are natural biopolymers composed of nucleotides that store, transmit, and express genetic information. Overexpressed or underexpressed as well as mutated nucleic acids have been implicated in many diseases. Therefore, nucleic acid tests (NATs) are extremely important. Inspired by intracellular DNA replication and RNA transcription, in vitro NATs have been extensively developed to improve the detection specificity, sensitivity, and simplicity. The principles of NATs can be in general classified into three categories: nucleic acid hybridization, thermal-cycle or isothermal amplification, and signal amplification. Driven by pressing needs in clinical diagnosis and prevention of infectious diseases, NATs have evolved to be a rapidly advancing field. During the past ten years, an explosive increase of research interest in both basic research and clinical translation has been witnessed. In this review, we aim to provide comprehensive coverage of the progress to analyze nucleic acids, use nucleic acids as recognition probes, construct detection devices based on nucleic acids, and utilize nucleic acids in clinical diagnosis and other important fields. We also discuss the new frontiers in the field and the challenges to be addressed.

Mitochondrial isolation: when size matters

Mitochondrial vitality is critical to cellular function, with mitochondrial dysfunction linked to a growing number of human diseases. Tissue and cellular heterogeneity, in terms of genetics, dynamics and function means that increasingly mitochondrial research is conducted at the single cell level. Whilst there are several technologies that are currently available for single-cell analysis, each with their advantages, they cannot be easily adapted to study mitochondria with subcellular resolution. Here we review the current techniques and strategies for mitochondrial isolation, critically discussing each technology's limitations for future mitochondrial research. Finally, we highlight and discuss the recent breakthroughs in sub-cellular isolation techniques, with a particular focus on nanotechnologies that enable the isolation of mitochondria from subcellular compartments. This allows isolation of mitochondria with unprecedented spatial precision with minimal disruption to mitochondria and their immediate cellular environment.

Recent Advances in Electrochemical Sensors for the Detection of Biomolecules and Whole Cells

Electrochemical sensors are considered an auspicious tool to detect biomolecules (e.g., DNA, proteins, and lipids), which are valuable sources for the early diagnosis of diseases and disorders. Advances in electrochemical sensing platforms have enabled the development of a new type of biosensor, enabling label-free, non-destructive detection of viability, function, and the genetic signature of whole cells. Numerous studies have attempted to enhance both the sensitivity and selectivity of electrochemical sensors, which are the most critical parameters for assessing sensor performance. Various nanomaterials, including metal nanoparticles, carbon nanotubes, graphene and its derivatives, and metal oxide nanoparticles, have been used to improve the electrical conductivity and electrocatalytic properties of working electrodes, increasing sensor sensitivity. Further modifications have been implemented to advance sensor platform selectivity and biocompatibility using biomaterials such as antibodies, aptamers, extracellular matrix (ECM) proteins, and peptide composites. This paper summarizes recent electrochemical sensors designed to detect target biomolecules and animal cells (cancer cells and stem cells). We hope that this review will inspire researchers to increase their efforts to accelerate biosensor progress-enabling a prosperous future in regenerative medicine and the biomedical industry.

Mitochondrial isolation: when size matters

Mitochondrial vitality is critical to cellular function, with mitochondrial dysfunction linked to a growing number of human diseases. Tissue and cellular heterogeneity, in terms of genetics, dynamics and function means that increasingly mitochondrial research is conducted at the single cell level. Whilst there are several technologies that are currently available for single-cell analysis, each with their advantages, they cannot be easily adapted to study mitochondria with subcellular resolution. Here we review the current techniques and strategies for mitochondrial isolation, critically discussing each technology's limitations for future mitochondrial research. Finally, we highlight and discuss the recent breakthroughs in sub-cellular isolation techniques, with a particular focus on nanotechnologies that enable the isolation of mitochondria from subcellular compartments. This allows isolation of mitochondria with unprecedented spatial precision with minimal disruption to mitochondria and their immediate cellular environment.

Recent Advances in Electrochemiluminescence-Based Systems for Mammalian Cell Analysis

Mammalian cell analysis is essential in the context of both fundamental studies and clinical applications. Among the various techniques available for cell analysis, electrochemiluminescence (ECL) has attracted significant attention due to its integration of both electrochemical and spectroscopic methods. In this review, we summarize recent advances in the ECL-based systems developed for mammalian cell analysis. The review begins with a summary of the developments in luminophores that opened the door to ECL applications for biological samples. Secondly, ECL-based imaging systems are introduced as an emerging technique to visualize single-cell morphologies and intracellular molecules. In the subsequent section, the ECL sensors developed in the past decade are summarized, the use of which made the highly sensitive detection of cell-derived molecules possible. Although ECL immunoassays are well developed in terms of commercial use, the sensing of biomolecules at a single-cell level remains a challenge. Emphasis is therefore placed on ECL sensors that directly detect cellular molecules from small portions of cells or even single cells. Finally, the development of bipolar electrode devices for ECL cell assays is introduced. To conclude, the direction of research in this field and its application prospects are described.

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

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

Review-Recent Advances in Nanosensors Built with Pre-Pulled Glass Nanopipettes and Their Applications in Chemical and Biological Sensing

Nanosensors built with pre-pulled glass nanopipettes, including bare or chemically modified nanopipettes and fully or partially filled solid nanoelectrodes, have found applications in chemical and biological sensing via resistive-pulse, current rectification, and electrochemical sensing. These nanosensors are easily fabricated and provide advantages through their needle-like geometry with nanometer-sized tips, making them highly sensitive and suitable for local measurements in extremely small samples. The variety in the geometry and layout have extended sensing capabilities. In this review, we will outline the fundamentals in fabrication, modification, and characterization of those pre-pulled glass nanopipette based nanosensors and highlight the most recent progress in their development and applications in real-time monitoring of biological processes, chemical ion sensing, and single entity analysis.

Recent Progress in Lab-On-a-Chip Systems for the Monitoring of Metabolites for Mammalian and Microbial Cell Research

Lab-on-a-chip sensing technologies have changed how cell biology research is conducted. This review summarises the progress in the lab-on-a-chip devices implemented for the detection of cellular metabolites. The review is divided into two subsections according to the methods used for the metabolite detection. Each section includes a table which summarises the relevant literature and also elaborates the advantages of, and the challenges faced with that particular method. The review continues with a section discussing the achievements attained due to using lab-on-a-chip devices within the specific context. Finally, a concluding section summarises what is to be resolved and discusses the future perspectives.

Site-Specific Cytosol Sampling from a Single Cell in an Intact Tumor Spheroid Using an Electrochemical Syringe

A multicellular tumor aggregate, known as a spheroid, is an indispensable tool to study cancer biology. Owing to its three-dimensional organization, a spheroid exhibits an inherent gradient of nutrients, oxygen, and metabolites within itself. The spheroid provides culture conditions that resemble the microenvironment of certain cancer cells and causes these cells to acquire characteristics relevant to tumors in our body. However, site-specific gene expression analysis in an intact spheroid with single-cell resolution has not been explored. Recently, some types of electrochemical syringes were developed to extract cellular materials from living single cells for transcriptomic analysis. Here, we investigated whether an electrochemical syringe could be used to evaluate site-specific gene expression in a spheroid. A small amount of cytosol (roughly 540-1480 fL, less than the volume of a single cell) was successfully collected from the first, second, and third layers of the spheroid using an electrochemical syringe without causing damage to the spheroid architecture. We found that the CCNB1 and CCNA2 expression levels were different between the surface and the average of the entire spheroid, indicating that there are heterogeneous cellular functions across different regions of the spheroid. This method provides opportunities to improve our understanding of spatial gene expression of single cells in a three-dimensional environment.

Electric and Electrochemical Microfluidic Devices for Cell Analysis

Microfluidic devices are widely used for cell analysis, including applications for single-cell analysis, healthcare, environmental monitoring, and organs-on-a-chip that mimic organs in microfluidics. Moreover, to enable high-throughput cell analysis, real-time monitoring, and non-invasive cell assays, electric and electrochemical systems have been incorporated into microfluidic devices. In this mini-review, we summarize recent advances in these systems, with applications from single cells to three-dimensional cultured cells and organs-on-a-chip. First, we summarize microfluidic devices combined with dielectrophoresis, electrophoresis, and electrowetting-on-a-dielectric for cell manipulation. Next, we review electric and electrochemical assays of cells to determine chemical section activity, and oxygen and glucose consumption activity, among other applications. In addition, we discuss recent devices designed for the electric and electrochemical collection of cell components from cells. Finally, we highlight the future directions of research in this field and their application prospects.

Electrochemical Biosensing System for Single Cells, Cellular Aggregates and Microenvironments

Applications of electrochemical biosensing for surveying intact cells and tissues have been focus of attention. Two experimental approaches have been used when performing amperometric measurements on biological cells, the stylus-type microelectrode probes and the electrode-integrated microdevices based on lithographic technologies. For the probe scanning approach, various types of microsensors were developed to monitor localized physical or chemical natures at a variety of surfaces in situ under wet conditions. Scanning electrochemical microscopy (SECM) has been applied for monitoring local oxygen, enzyme activity, and collection of transcripts. For the non-scanning type of approach, electrode array devices allow very rapid response, parallel monitoring, and multi-analyte assay. Sveral topics of on-chip-culture system were introduced especially concerning on gene expression monitoring by reporter system and reconstruction of in vivo-like nature by controlling microenvironments. Electrochemical reporter assay has been demonstrated to monitor the gene expression process of the gene-modified cultured cells. Long-term monitoring of cellular function of spheroids and three dimensionally-cultured cells were carried out by controlling microenvironments on the cellular chip.

Electrochemical Imaging of Cell Activity in Hydrogels Embedded in Grid-shaped Polycaprolactone Scaffolds Using a Large-scale Integration-based Amperometric Device

Tissue engineering requires analytical methods to monitor cell activity in hydrogels. Here, we present a method for the electrochemical imaging of cell activity in hydrogels embedded in printed polycaprolactone (PCL) scaffolds. Because a structure made of only hydrogel is fragile, PCL frameworks are used as a support material. A grid-shaped PCL was fabricated using an excluder printer. Photocured hydrogels containing cells were set at each grid hole, and cell activity was monitored using a large-scale integration-based amperometric device. The electrochemical device contains 400 microelectrodes for biomolecule detection, such as dissolved oxygen and enzymatic products. As proof of the concept, alkaline phosphatase and respiration activities of embryonic stem cells in the hydrogels were electrochemically monitored. The results indicate that the electrochemical imaging is useful for evaluating cells in printed scaffolds.