Cationic polyelectrolytes prevent the aggregation of l-lactate dehydrogenase under unstable conditions.

Unstructured biological macromolecules have attracted attention as protein aggregation inhibitors in living cells. Some are characterized by their free structural configuration, highly charged, and water-soluble. However, the importance of these properties in inhibiting protein aggregation remains unclear. In this study, we investigated the effect of charged poly (amino acids), which mimic these properties, on aggregation of l-lactate dehydrogenase (LDH) and compared their effects to monomeric amino acids and folded proteins. LDH was stable and active at a neutral pH (~7) but formed inactive aggregates at acidic pH (< 6). Adding cationic polyelectrolytes of poly-l-lysine and poly-l-arginine suppressed the acid-induced aggregation and inactivation of LDH under acidic pH values. Adding monomeric amino acids and cationic folded proteins also prevented LDH aggregation but with lower efficacy than cationic polyelectrolytes. These results indicate that unstructured polyelectrolytes effectively stabilize unstable enzymes because they interact flexibly and multivalently with them. Our findings provide a simple method for stabilizing enzymes under unstable conditions.

Arrayed labeling-free cultivation and growth evaluation from a single microorganism.

The development of high-throughput screening methods for microorganisms is desired because microorganisms are useful and sustainable resources with which valuable substances utilized in various industries can be produced. Micro-space-based methods are the best candidates for the efficient screening of microorganisms owing to their low reagent consumption and compact integration. In this study, we developed a picoliter-sized incubator array for quantitative and labeling-free evaluation of the growth process of Escherichia coli (E. coli) by autofluorescence. Because the array with 8464 incubators is able to compartmentalize single E. coli individually utilizing the Poisson distribution, the array can evaluate 100 single E. coli simultaneously. Our incubator array not only realized the high-throughput screening of microorganisms, but also provided an analytical tool for assessing individual differences in E. coli.

Real-Time Detection of Glyphosate by a Water-Gated Organic Field-Effect Transistor with a Microfluidic Chamber.

This paper reports the development of a real-time monitoring system utilizing the combination of a water-gated organic field-effect transistor (WG-OFET) and a microfluidic chamber for the detection of the herbicide glyphosate (GlyP). For the realization of the real-time sensing with the WG-OFET, the surface of a polymer semiconductor was utilized as a sensing unit. The aqueous solution including the target analyte, which is employed as a gate dielectric of the WG-OFET, flows into a designed microfluidic chamber on the semiconductor layer and the gate electrode. As the sensing mechanism, the WG-OFET-based sensor utilizes the competitive complexation among carboxylate-functionalized polythiophene, a copper(II) (Cu2+) ion, and GlyP. The reversible accumulation and desorption of the positively charged Cu2+ ion on the semiconductor surface induced a change in the electrical double-layer capacitance (EDLC). The optimization of the microfluidic chamber enables a uniform water flow and contributes to real-time quantitative sensing of GlyP at a micromolar level. Thus, this study would lead to practical real-time sensing in water for various fields including environmental assessment.

A Water-Gated Organic Thin-Film Transistor for Glyphosate Detection: A Comparative Study with Fluorescence Sensing.

Invited for the cover of this issue is the group of Tsuyoshi Minami at the University Tokyo. The image illustrates that despite being fabricated with the same polythiophene material, a water-gated organic thin-film transistor is a more sensitive device than a fluorescence sensor chip. Read the full text of the article at 10.1002/chem.202003529.

A Water-Gated Organic Thin-Film Transistor for Glyphosate Detection: A Comparative Study with Fluorescence Sensing.

This work reports the design of a highly sensitive solid-state sensor device based on a water-gated organic thin-film transistor (WG-OTFT) for the selective detection of herbicide glyphosate (GlyP) in water. A competitive assay among carboxylate-functionalized polythiophene, Cu2+ , and GlyP was employed as a sensing mechanism. Molecular recognition phenomena and electrical double layer (EDL) (at the polymer/water interface) originated from the field-effect worked cooperatively to amplify the sensitivity for GlyP. The limit of detection of WG-OTFT (0.26 ppm) was lower than that of a fluorescence sensor chip (0.95 ppm) which is the conventional sensing method. In contrast to the previously reported insulated molecular wires to block interchain interactions, molecular aggregates under the field-effect has shown to be effective for amplification of sensitivity through "intra"- and "inter"-molecular wire effects. The opposite strategy in this study could pave the way for fully utilizing the sensing properties of polymer-based solid-state sensor devices.

Protein Assays on Organic Electronics: Rational Device and Material Designs for Organic Transistor-Based Sensors.

Artificial receptor-based protein assays have various attractive features such as a long-term stability, a low-cost production process, and the ease of tuning the target specificity. However, such protein sensors are still immature compared with conventional immunoassays. To enhance the application potential of synthetic sensing materials, organic field-effect transistors (OFETs) are some of the suitable platforms for protein assays because of their solution processability, durability, and compact integration. Importantly, OFETs enable the electrical readout of the protein recognition phenomena of artificial receptors on sensing electrodes. Thus, we believe that OFETs functionalized with artificial protein receptors will be a powerful tool for the on-site analyses of target proteins. In this Minireview, we summarize the recent progress of the OFET-based protein assays including the rational design strategies for devices and sensing materials.

The Power of Assemblies at Interfaces: Nanosensor Platforms Based on Synthetic Receptor Membranes.

Synthetic sensing materials (artificial receptors) are some of the most attractive components of chemical/biosensors because of their long-term stability and low cost of production. However, the strategy for the practical design of these materials toward specific molecular recognition in water is not established yet. For the construction of artificial material-based chemical/biosensors, the bottom-up assembly of these materials is one of the effective methods. This is because the driving forces of molecular recognition on the receptors could be enhanced by the integration of such kinds of materials at the 'interfaces', such as the boundary portion between the liquid and solid phases. Additionally, the molecular assembly of such self-assembled monolayers (SAMs) can easily be installed in transducer devices. Thus, we believe that nanosensor platforms that consist of synthetic receptor membranes on the transducer surfaces can be applied to powerful tools for high-throughput analyses of the required targets. In this review, we briefly summarize a comprehensive overview that includes the preparation techniques for molecular assemblies, the characterization methods of the interfaces, and a few examples of receptor assembly-based chemical/biosensing platforms on each transduction mechanism.

Systematic Investigation of Molecular Recognition Ability in FET-Based Chemical Sensors Functionalized with a Mixed Self-Assembled Monolayer System.

Exploring new strategies for simple and on-demand methods of manipulating the sensing ability of sensor devices functionalized with artificial receptors embedded in a molecular assembly is important to realizing high-throughput on-site sensing systems based on integrated and miniaturized devices such as field-effect transistors (FETs). Although FET-based chemical sensors can be used for rapid, quantitative, and simultaneous determination of various desired analytes, detectable targets in conventional FET sensors are currently restricted owing to the complicated processes used to prepare sensing materials. In this study, we investigated the relationship between the sensing features of FETs and the nanostructures of mixed self-assembled monolayers (mSAMs) for the detection of biomolecules. The FET devices were systematically functionalized using mixtures of benzenethiol derivatives (4-mercaptobenzoic acid and benzenethiol), which changed the nanostructure of the SAMs formed on gold sensing electrodes. The obtained cross-reactivity in the FETs modified with the mSAMs was derived from the multidimensional variations of the SAM characteristics. Our successful demonstration of continuous control of the molecular recognition ability in the FETs by applying the mSAM system could lead to the development of next-generation versatile analyzers, including chemical sensor arrays for the determination of multiple analytes anytime, anywhere.

Chemical Sensing Platforms Based on Organic Thin-Film Transistors Functionalized with Artificial Receptors.

Organic thin-film transistors (OTFTs) have attracted intense attention as promising electronic devices owing to their various applications such as rollable active-matrix displays, flexible nonvolatile memories, and radiofrequency identification (RFID) tags. To further broaden the scope of the application of OTFTs, we focus on the host-guest chemistry combined with the electronic devices. Extended-gate types of OTFTs functionalized with artificial receptors were fabricated to achieve chemical sensing of targets in complete aqueous media. Organic and inorganic ions (cations and anions), neutral molecules, and proteins, which are regarded as target analytes in the field of host-guest chemistry, were electrically detected by artificial receptors. Molecular recognition phenomena on the extended-gate electrode were evaluated by several analytical methods such as photoemission yield spectroscopy in the air, contact angle goniometry, and X-ray photoelectron spectroscopy. Interestingly, the electrical responses of the OTFTs were highly sensitive to the chemical structures of the guests. Thus, the OTFTs will facilitate the selective sensing of target analytes and the understanding of chemical conversions in biological and environmental systems. Furthermore, such cross-reactive responses observed in our studies will provide some important insights into next-generation sensing systems such as OTFT arrays. We strongly believe that our approach will enable the development of new intriguing sensor platforms in the field of host-guest chemistry, analytical chemistry, and organic electronics.

Fabrication of a Flexible Biosensor Based on an Organic Field-effect Transistor for Lactate Detection.

A novel flexible lactate sensor based on organic field-effect transistors (OFETs) is demonstrated. Because lactate is known as a biomarker for assessing our physical performance, wearable lactate sensors could contribute to the monitoring of human health conditions. The flexible and low-voltage operatable OFET possesses an extended-gate modified with enzymes and an osmium-redox polymer for the lactate detection, meaning that the continuous measurement of lactate levels (0 - 10 mM) has been successfully achieved. We believe that insight obtained will open up opportunities for applying OFETs in wearable biosensors.

An electrolyte-gated polythiophene transistor for the detection of biogenic amines in water.

We herein propose a novel sensing strategy for biogenic amines using an electrolyte-gated polythiophene transistor. This ultra-low voltage operatable device responds to amines on the basis of the molecular recognition ability of a polythiophene pendant with a carboxy side-chain, which indicates that the sensing platform can contribute to the realisation of easy-to-fabricate and integrated chemical sensors.

Label-Free Direct Electrical Detection of a Histidine-Rich Protein with Sub-Femtomolar Sensitivity using an Organic Field-Effect Transistor.

The front cover artwork is provided by the group of Dr. Tsuyoshi Minami at the Institute of Industrial Science, the University of Tokyo (Japan). Easy-to-use sensing systems for on-site biomarker testing have been researched numerously, because conventional approaches for biomarker detection (e.g. enzyme linked-immunosorbent assays, etc.) are too complicated. In that regard, organic filed-effect transistors (OFETs) are some of the most promising platforms for construction of on-site testing systems. As OFETs can be easily fabricated on flexible substrates using wet processes, these are not only valuable transducers for chemo-/biosensors, but also the prospective device for rollable displays and low-cost radio frequency identification tags. Thus, the components of the sensing system could be integrated into a single chip by using OFET-based circuits. For more details, see the full text of the Communication at 10.1002/open.201700070.

Label-Free Direct Electrical Detection of a Histidine-Rich Protein with Sub-Femtomolar Sensitivity using an Organic Field-Effect Transistor.

There is a growing interest in achieving sensor systems to enable on-site testing of biomarkers. Herein, a new strategy for highly sensitive protein detection at sub-femtomolar levels without any labelling has been demonstrated by using an organic field-effect transistor (OFET). An artificial histidine-rich protein receptor (NiII-nitrilotriacetic acid complex, NiII-nta) functionalizes a detection portion (i.e. an extended-gate electrode) of the fabricated OFET device. The OFET responds electrically and selectively to a target analyte (bovine serum albumin), meaning that the binding processes at the NiII-nta on the extended-gate electrode for the analyte affect the field-effect properties of the device. Our results demonstrate that the combination of the OFET with the artificial receptor is an ideal approach for label-free and immune-free protein detection.

A molecular self-assembled colourimetric chemosensor array for simultaneous detection of metal ions in water.

We propose a novel strategy for a high-throughput sensing of metal ions using a molecular self-assembled colourimetric chemosensor array. The proposed colourimetric assay has been achieved by only using the combination of commercially available materials. Importantly, the easy-to-prepare assay can be utilised to quantitatively detect metal ions under competitive conditions.

Label-Free Detection of Human Glycoprotein (CgA) Using an Extended-Gated Organic Transistor-Based Immunosensor.

Herein, we report on the fabrication of an extended-gated organic field-effect transistor (OFET)-based immunosensor and its application in the detection of human chromogranin A (hCgA). The fabricated OFET device possesses an extended-gate electrode immobilized with an anti-CgA antibody. The titration results of hCgA showed that the electrical changes in the OFET characteristics corresponded to the glycoprotein recognition ability of the monoclonal antibody (anti-CgA). The observed sensitivity (detection limit: 0.11 µg/mL) and selectivity indicate that the OFET-based immunosensor can be potentially applied to the rapid detection of the glycoprotein concentration without any labeling.

Selective nitrate detection by an enzymatic sensor based on an extended-gate type organic field-effect transistor.

First selective nitrate biosensor device based on an extended-gate type organic field-effect transistor (OFET) is reported. The fabricated sensor device consists of the extended-gate electrode functionalized by a nitrate reductase with a mediator (=a bipyridinium derivative) and an OFET-based transducer. The mechanism of the nitrate detection can be explained by an electron-relay on the extended-gate electrode, resulting in changes of the electric properties of the OFET. The detection limit of nitrate in water is estimated to be 45 ppb, which suggests that the sensitivity of our fabricated sensor is comparable to those of some conventional detection methods. As a practical application of the OFET sensor, the nitrate detection in diluted human saliva has been successfully demonstrated; the results agreed well with those by conventional colorimetric measurement. The advantages of OFETs are printability, mechanical flexibility, stretchability and disposability, meaning that the fabricated OFET could open up a new approach for low-cost electronic devices toward on-site detection of nitrate in aqueous media.

Antibody- and Label-Free Phosphoprotein Sensor Device Based on an Organic Transistor.

An antibody- and label-free detection of a phosphoprotein (α-casein) has been achieved using an organic field-effect transistor (OFET)-based sensor. The fabricated OFET device possesses an extended-gate electrode functionalized with an artificial phosphoprotein receptor (Zn(II)-dipicolylamine complex, Zn(II)-DPA). It is shown that the OFET responds to the molecular recognition processes involving the Zn(II)-DPA at the extended-gate electrode. The binding of α-casein to the receptor in a HEPES solution results in an analyte-specific changes of the drain current of the OFET. The successful demonstration of the antibody- and label-free detection using the OFET could pave the way to the development of low-cost, disposable, and portable electronic sensor devices.

A mercury(II) ion sensor device based on an organic field effect transistor with an extended-gate modified by dipicolylamine.

Herein, we report an organic field effect transistor (OFET) with an extended-gate modified by an artificial receptor for the detection of mercury(II) ions (Hg(2+)) in water. The sensor device is easy to fabricate, reusable, disposable, and portable. Thus OFET sensors could be applied for low-cost on-site detection of Hg(2+).

An Extended-gate Type Organic FET Based Biosensor for Detecting Biogenic Amines in Aqueous Solution.

This study is the first to report on the detection of biogenic amines in an aqueous solution using an organic field-effect transistor (OFET) device with an extended gate electrode modified with a layer of diamine oxidase and a horseradish peroxidase osmium-redox polymer. The limit of detection (LOD) for histamine was estimated to be 1.2 µM. These results reveal that extended-gate type OFET devices are highly suitable enzyme-based biosensors for the detection of biogenic amine levels.

An Organic Field-effect Transistor with an Extended-gate Electrode Capable of Detecting Human Immunoglobulin A.

We herein report on the development of an extended-gate type organic field-effect transistor (OFET)-based immunosensor for the detection of human immunoglobulin A (IgA). The titration results of IgA exhibited shifts in the transfer characteristics of the OFET sensor device with increasing IgA concentration. A linear detection range from 0 to 10 µg/mL was realized with a detection limit of 2.1 µg/mL, indicating that the OFET-based immunosensor can be potentially applied to the monitoring of infectious diseases and psychological stress in daily life.