Current Analytical Chemistry - Volume 18, Issue 6, 2022

Background: Bipolar electrode (BPE), as an immersed electrical conductor in the electrolyte, can be polarized into cathodic and anodic poles under a sufficient electric field without direct contact, which affords a unique way to promote asymmetrical reactions at two poles. Up to date, bipolar electrochemistry has been widely used in the preparation of Janus materials, the fabrication of sensing/screening platform, target focusing and microswimmers. However, the wireless feature of BPE makes monitoring the Faradaic current difficult. Electrochemiluminescence (ECL), the light emission via an electrochemical reaction, matches the feature of bipolar electrochemistry well and is widely adopted to achieve the record of the Faradaic current flowing through the BPE. The objective of the present review aims to demonstrate the most recent advances in analytical applications (2016-2020) in combination with the high sensitive ECL as the output. Methods: Due to the difficulty of the recording of the Faradaic current flowing the BPE, the ECL, as a simple, sensitive and detectable signal-out, has become a popular method for analytical application based on the BPE. This review mainly summarizes the recent research of BPE/ECL according to the configuration and sensing principle of BPE designed in the ECL analysis. Results: Various sensors based on the BPE/ECL have been proposed for the electroactive targets and the bio-relevant molecules without the electroactivity by different ingenious designs. Besides, the microelectrode array and ultra-microelectrode (UME) array have also been applied in the BPE/ECL field to achieve the high temporal-spatial resolution imaging of the sample molecules based on the BPE microelectrode array. Conclusion: The combination of BPE and ECL provides a simple, portable and versatile sensor strategy for various targets due to the unique advantages of BPE and ECL, and can be applied for the fast, accurate and point-of-care diagnostics of numerous diseases. Though the BPE/ECL analysis has many merits such as high-throughput, excellent sensitivity, and high spatial-temporal resolution, the sensitive and commercial ECL analysis based on the BPE is still difficult to realize and the analysis research of BPE/ECL is still in the early stage.

Cells, regarded as the structural and functional units of organisms, have become one of the most important objects in many research areas. Specific recognition and detection of malignant cells are critical for disease diagnosis, therapy and prognosis. Aptamers are short; single-stranded oligonucleotides screened from a random library by an in vitro technology termed “Systematic Evolution of Ligands by Exponential Enrichment” (SELEX) on the basis of their specific binding to target cargos. With the advantages of small size, easy synthesis, convenient modification, high chemical stability and low immunogenicity, aptamers have attracted broad attention in bioanalysis. Using intact living cells as the selection target, the cell-SELEX technology enables the generation of many aptamers that can specifically recognize molecular signatures of target cells. These aptamers have been extensively utilized in various cell-based research. In this mini-review, we focus on recent advances in aptamer-based recognition and detection of cells, particularly circulating tumor cells (CTCs).

Background: Imaging of RNA in vivo is of great significance for elucidating their biological functions, revealing mechanisms behind the disease, and for further diagnosis and treatment. Over the past decade, a variety of DNA-based molecular imaging techniques have been developed for RNA imaging in living cells. Nevertheless, non-invasive imaging of RNA in animals is still limited. Methods: An overview of the literature involving RNA imaging in vivo based on the integration of DNA probes with nanotechnology has been reviewed. Results: Attributed to DNA’s designability of sequences and specificity of recognition, molecular beacon, strand displacement and hybridization chain reaction would confer quick, efficient and specific response to target RNA. Multifunctional nanomaterials provide powerful support for the intracellular delivery of such DNA probes with spatiotemporal control over their sensing function, thereby achieving RNA imaging in vivo. Conclusion: Merging DNA probes with nanotechnology has gained substantial prospects for RNA imaging in vivo, which not only helps us to better elucidate biological functions of RNA, but also provides valuable information for further disease diagnosis and treatment.

Background: Integrating metal-organic frameworks (MOFs) with other functional materials to form MOF-composites has attracted great attention. Their diverse and synergetic performance (e.g., mechanical stability, conductivity, optical signal and catalytic activity) facilitates their applications in sensing of various target molecules. Methods: Up to now, a wide range of MOF-composites have been designed and synthesized. The choice of appropriate parent materials, as well as their combination strategy, is of great importance for their performance. Prior to the design of MOF-composites for certain sensing applications, it is necessary to evaluate the advantages of the composites compared to the pristine MOFs and other functional materials. Results: In this review, an overview of the significant advances in the development of diverse MOF-composites is presented, with special emphasis on the synergistic effects in sensing (e.g., optical, electrochemical and biological) applications of the composites. Additionally, the challenges and future prospects of MOF-composites as an innovative sensing platform have been discussed to seek further development in this emerging research area. Conclusion: MOF-composites show great potential in sensing applications. Further efforts are still urgently required to advance their applications in point-of-care (POC) detection and in vitro diagnosis (IVD).

Background: In recent years, electrochemical sensors are widely preferred because of their high sensitivity, rapid response, low cost and easy miniaturization. Covalent organic frameworks (COFs), a porous crystalline polymer formed by organic units connected by covalent bonds, have been widely used in gas adsorption and separation, drug transportation, energy storage, photoelectric catalysis, electrochemistry and other aspects due to their large specific surface, excellent stability, high inherent porosity, good crystallinity as well as structural and functional controllability. The topological structure of COFs can be designed in advance, the structural units and linkage are diversified, and the structure is easy to be functionalized, which are all beneficial to their application in electrochemical sensors. Methods: The types, synthesis methods, properties of covalent organic frameworks and some examples of using covalent organic frameworks in electrochemical sensors are reviewed. Results: Due to their characteristics of a large specific surface, high porosity, orderly channel and periodically arranged π electron cloud, COFs are often used to immobilize metal nanoparticles, aptamers or other materials to achieve the purpose of building electrochemical sensors with high sensitivity and good stability. Since the structure of COFs can be predicted, different organic units can build COFs with different structures and properties. Therefore, organic units with certain functional groups can be selected to build COFs with certain properties and used directly for electrochemical sensors. Conclusion: COFs have a good application prospect in electrochemical sensors.

Background: Cell membrane is a physical barrier for cells, as well as an important structure with complex functions in cell activities. The cell membrane can not only receive external mechanical signal stimulation and make response (e.g. cell migration, differentiation, tumorigenesis, growth), but it can also spontaneously exert force on the environment to regulate cell activities (such as tissue repair, tumor metastasis, extracellular matrix regulation, etc.). Methods: This review provides the introduction of single-molecule force methods, as atomic force microscopy, optical tweezers, magnetic tweezers, micropipette adhesion assay, tension gauge tethers and traction force microscopy. Results: This review summarizes the principles, advantages and disadvantages of single-molecule force methods developed in recent years as well as their application in terms of force received and generated by cells. The study of cell mechanics enables us to understand the nature of mechanical signal transduction and the manifestation of the cell's own movement. Conclusion: The study of the mechanical properties of cell microenvironment leads to a gradual understanding of the important role of cell mechanics in development, physiology and pathology. Recently developed combined methods are beneficial to further study of cell mechanics. The optimization of these methods and the invention of new methods enable the continuing research on cell mechanics.

Adenosine 5'-triphosphate (ATP) plays a significant role in biological processes and the ATP level is closely associated with many diseases. In order to detect ATP in live cells, tissues, and body fluids with high sensitivity and selectivity, researchers have developed various sensing strategies. Particularly, owing to the distinct physicochemical properties of nanomaterials and the high sensitivity of fluorescence, a lot of efforts have been devoted to developing nanomaterials-based approaches for fluorescent ATP sensing in recent years. In this review, we focus on the current development of nanomaterial-based fluorescent ATP sensors and discuss the sensing mechanisms in detail. The advantages and disadvantages of ATP sensing using different kinds of nanomaterials, including carbon nanomaterials, metal nanoparticles, semiconductor quantum dots, metal-organic frameworks, and up-conversion nanoparticles have been thoroughly compared and discussed. Finally, current challenges and future prospects in this field are examined.

Background: Soft wearable electrochemical biosensors are attracting increasing attention over the past several years due to their potential for non-invasive personalized health monitoring in real-time and in-situ. Objective: Herein, we cover the design, fabrication, and applications of soft electrochemical sensing systems. Firstly, we describe key design requirements for fabricating the mechanically compliant electrochemical biosensors. This is followed by a narration of typical sensor configurations and the detecting methodologies. Next, on-body soft electrochemical biosensing and cell/tissue-based “wearable” sensing applications are summarized. Detection of key biochemical markers, including metabolites (glucose, lactate, uric acid and ethanol), electrolytes (Na+ and K+), nutrients (vitamin C), hormones (cortisol) and proteins (TNF-α), as well as cellular signalling molecules (nitric oxide, hydrogen peroxide and serotonin), is the focus of the discussion in this review. Conclusion: We conclude the review with discussions on future opportunities and challenges of the soft and wearable electrochemical biosensors.

Background: Personal Glucose Meter (PGM) has become the most successful biosensor in past decades due to its advantages of small size, convenient operation, and low cost. To take advantage of many years of research and development of PGMs, new signal transduction methods have been developed to expand the PGM from simple monitoring of blood glucose to the detection of numerous non-glucose targets. Objectives: This review summarizes recent advances of PGM-based biosensors for non-glucose targets, including signal transduction, signal amplification, and target molecule recognition and analysis. Current challenges and future directions are also discussed. Conclusion: PGM can be used as a biosensor readout to detect various non-glucose targets from metal ion, small molecules to protein and even living organisms such as bacteria and other pathogens by using different signal transduction elements such as invertase and amylase, and different signal amplification methods such as nanomaterials, nucleic acid reaction, liposome encapsulation, hydrogel trapping, DNAzyme amplification and biotin-streptavidin reaction.

Background: The comprehensive understanding of nanomaterials’ behavior in biological systems is essential in accurately modeling and predicting nanomaterials’ fate and toxicity. Synchrotron radiation (SR) X-ray techniques, based on their ability to study electronic configuration, coordination geometry, or oxidative state of nanomaterials with high sensitivity and spatial resolution, have been introduced to analyze the transformation behavior of nanomaterials in biological systems. Methods: Previous researches in this field are classified and summarized. Results: To start with, a brief introduction of a few widely used SR-based analytical techniques including X-ray absorption spectroscopy, X-ray fluorescence microprobe, scanning transmission Xray microscopy and circular dichroism spectroscopy is provided. Then, the recent advances of their applications in the analysis of nanomaterial behaviors are elaborated based on different nanomaterial transformation forms such as biodistribution, biomolecule interaction, decomposition, redox reaction, and recrystallization/agglomeration. Finally, a few challenges faced in this field are proposed. Conclusion: This review summarizes the application of SR X-ray techniques in analyzing the fate of inorganic nanomaterials in biological systems. We hope it can help the readers to have a general understanding of the applications of SR-based techniques in studying nanomaterial biotransformation and to stimulate more insightful research in relevant fields.

Background: Nanozymes are a kind of emerging nanomaterials that can mimic the catalytic activity of natural enzymes with good stability. Objective: Benefited by the unique coordination structure and constitution, metal-organic frameworks (MOFs) have been widely exploited as novel nanozymes. Importantly, various MOFs engineered with fascinating functions provide great opportunities to enhance their enzyme-like activity and improve their applied performance, achieving the goal of vividly mimicking natural enzymes. Conclusion: This review summarized recent advances in the fabrication of the MOFs-based nanozymes and their applications in biosensing. First, MOFs-based nanomaterials containing pristine MOFs, functionalized MOFs, MOFs-based composites and MOFs derivatives are introduced, where the design strategy, enzyme-like activity and the catalytic mechanisms are highlighted systematically. Then, their applications in various target assays are summarized. Finally, the challenges and possible research directions for the development and application of MOFs-based nanozymes are provided.

Single-atom (SA) catalysts, as a rising star in the catalytic field, have many advantages over traditional nanocatalysts. SA catalysts have improved catalytic activity, a simple and tunable structure, and obvious active sites, which might provide a good opportunity for biosensing technique innovation. This paper will review the latest research progress of SA catalysts in the biosensing field. In particular, we will emphasize on the biosensing strategies for the determination of disease-related biological matrices (H2O2, biological enzyme, NO) and environmental pollutants (organophosphorus pesticides, heavy metal ions, and volatile organic compounds). Finally, we will provide some perspective and discuss the challenges that SA catalysts continue to face.