Molecularly Engineered Charge-Conversion of Proteins for Sensitive Biosensing

New Insights for Biosensing: Lessons from Microbial Defense Systems

Microorganisms have gained defense systems during the lengthy process of evolution over millions of years. Such defense systems can protect them from being attacked by invading species (e.g., CRISPR-Cas for establishing adaptive immune systems and nanopore-forming toxins as virulence factors) or enable them to adapt to different conditions (e.g., gas vesicles for achieving buoyancy control). These microorganism defense systems (MDS) have inspired the development of biosensors that have received much attention in a wide range of fields including life science research, food safety, and medical diagnosis. This Review comprehensively analyzes biosensing platforms originating from MDS for sensing and imaging biological analytes. We first describe a basic overview of MDS and MDS-inspired biosensing platforms (e.g., CRISPR-Cas systems, nanopore-forming proteins, and gas vesicles), followed by a critical discussion of their functions and properties. We then discuss several transduction mechanisms (optical, acoustic, magnetic, and electrical) involved in MDS-inspired biosensing. We further detail the applications of the MDS-inspired biosensors to detect a variety of analytes (nucleic acids, peptides, proteins, pathogens, cells, small molecules, and metal ions). In the end, we propose the key challenges and future perspectives in seeking new and improved MDS tools that can potentially lead to breakthrough discoveries in developing a new generation of biosensors with a combination of low cost; high sensitivity, accuracy, and precision; and fast detection. Overall, this Review gives a historical review of MDS, elucidates the principles of emulating MDS to develop biosensors, and analyzes the recent advancements, current challenges, and future trends in this field. It provides a unique critical analysis of emulating MDS to develop robust biosensors and discusses the design of such biosensors using elements found in MDS, showing that emulating MDS is a promising approach to conceptually advancing the design of biosensors.

Label-Free Monitoring of Histone Acetylation Using Aptamer-Functionalized Field-Effect Transistor and Quartz Crystal Microbalance Sensors

Chemical and enzymatic modifications of amino acid residues in protein after translation contain rich information about physiological conditions and diseases. Histone acetylation/deacetylation is the essential post-translational modification by regulating gene transcription. Such qualitative changes of biomacromolecules need to be detected in point-of-care systems for an early and accurate diagnosis. However, there is no technique to aid this issue. Previously, we have applied an aptamer-functionalized field-effect transistor (FET) to the specific protein biosensing. Quantitative changes of target protein in a physiological solution have been determined by detecting innate charges of captured protein at the gate-solution interface. Moreover, we have succeeded in developing an integrated system of FET and quartz crystal microbalance (QCM) sensors for determining the adsorbed mass and charge, simultaneously or in parallel. Prompted by this, in this study, we developed a new label-free method for detecting histone acetylation using FET and QCM sensors. The loss of positive charge of lysine residue by chemically induced acetylation of histone subunits (H3 and H4) was successfully detected by potentiometric signals using anti-histone aptamer-functionalized FET. The adsorbed mass was determined by the same anti-histone aptamer-functionalized QCM. From these results, the degree of acetylation was correlated to the charge-to-mass ratio of histone subunits. The histone required for the detection was below 100 nM, owing to the high sensitivity of aptamer-functionalized FET and QCM sensors. These findings will guide us to a new way of measuring post-translational modification of protein in a decentralized manner for an early and accurate diagnosis.

Lactoferrin Suppresses Neutrophil Extracellular Traps Release in Inflammation

Neutrophils are central players in the innate immune system. They generate neutrophil extracellular traps (NETs), which protect against invading pathogens but are also associated with the development of autoimmune and/or inflammatory diseases and thrombosis. Here, we report that lactoferrin, one of the components of NETs, translocated from the cytoplasm to the plasma membrane and markedly suppressed NETs release. Furthermore, exogenous lactoferrin shrunk the chromatin fibers found in released NETs, without affecting the generation of oxygen radicals, but this failed after chemical removal of the positive charge of lactoferrin, suggesting that charge-charge interactions between lactoferrin and NETs were required for this function. In a model of immune complex-induced NET formation in vivo, intravenous lactoferrin injection markedly reduced the extent of NET formation. These observations suggest that lactoferrin serves as an intrinsic inhibitor of NETs release into the circulation. Thus, lactoferrin may represent a therapeutic lead for controlling NETs release in autoimmune and/or inflammatory diseases.

Current and emerging challenges of field effect transistor based bio-sensing

Field-effect-transistor (FET) based electrical signal transduction is an increasingly prevalent strategy for bio-sensing. This technique, often termed "Bio-FETs", provides an essentially label-free and real-time based bio-sensing platform effective for a variety of targets. This review highlights recent progress and challenges in the field. A special focus is on the comprehension of emerging nanotechnology-based approaches to facilitate signal-transduction and amplification. Some new targets of Bio-FETs and the future perspectives are also discussed.

Simple and robust strategy for potentiometric detection of glucose using fluorinated phenylboronic acid self-assembled monolayer

BACKGROUND

Field effect transistor (FET) based signal-transduction (Bio-FET) is an emerging technique for label-free and real-time basis biosensors for a wide range of targets. Glucose has constantly been of interest due to its clinical relevance. Use of glucose oxidase (GOD) and a lectin protein Concanavalin A are two common strategies to generate glucose-dependent electrochemical events. However, these protein-based materials are intolerant of long-term usage and storage due to their inevitable denaturing.

METHODS

A phenylboronic acid (PBA) modified self-assembled monolayer (SAM) on a gold electrode with an optimized disassociation constant of PBA, that is, 3-fluoro-4-carbamoyl-PBA possessing its pKa of 7.1, was prepared and utilized as an extended gate electrode for Bio-FET.

RESULTS

The prepared electrode showed a glucose-dependent change in the surface potential under physiological conditions, thus providing a remarkably simple rationale for the glyco-sensitive Bio-FET. Importantly, the PBA modified electrode showed tolerance to relatively severe heat and drying treatments; conditions under which protein based materials would surely be denatured.

CONCLUSIONS

A PBA modified SAM with optimized disassociation constant (pKa) can exhibit a glucose-dependent change in the surface potential under physiological conditions, providing a remarkably simple but robust method for the glyco-sensing.

GENERAL SIGNIFICANCE

This protein-free, totally synthetic glyco-sensing strategy may offer cheap, robust and easily accessible platform that may be useful in developing countries. This article is part of a Special Issue entitled Organic Bioelectronics-Novel Applications in Biomedicine.

Label-free and reagent-less protein biosensing using aptamer-modified extended-gate field-effect transistors

We have developed biosensors based on an aptamer-modified field-effect transistor (FET) for the detection of lysozyme and thrombin. An oligonucleotide aptamer as a sensitive and specific ligand for these model proteins was covalently immobilized on a gold electrode extended to the gate of FET together with thiol molecules to make a densely packed self-assembled monolayer (SAM). The aptamer-based potentiometry was achieved in a multi-parallel way using a microelectrodes array format of the gate electrode. A change in the gate potential was monitored in real-time after introduction of a target protein at various concentrations to the functionalized electrodes in a buffer solution. Specific protein binding altered the charge density at the gate/solution interface, i.e., interface potential, because of the intrinsic local net-charges of the captured protein. The potentiometry successfully determined the lysozyme and thrombin on the solid phase with their dynamic ranges 15.2-1040 nM and 13.4-1300 nM and the limit of detection of 12.0 nM and 6.7 nM, respectively. Importantly, robust signals were obtained by the specific protein recognition even in the spiked 10% fetal bovine serum (FBS) conditions. The technique herein described is all within a complementary metal oxide semiconductor (CMOS) compatible format, and is thus promising for highly efficient and low cost manufacturing with the readiness of downsizing and integration by virtue of advanced semiconductor processing technologies.

Interpretation of protein adsorption through its intrinsic electric charges: a comparative study using a field-effect transistor, surface plasmon resonance, and quartz crystal microbalance

We describe the highly sensitive detection of the nonspecific adsorption of proteins onto a 1-undecanethiol self-assembled monolayer (SAM)-formed gold electrode by parallel analysis using field effect transistor (FET), surface plasmon resonance (SPR), and quartz crystal microbalance (QCM) sensors. The FET sensor detects the innate electric charges of the adsorbed protein at the electrode/solution interface, transforming the change in charge density into a potentiometric signal in real time, without the requirement for labels. In particular, using the Debye-Huckel model, the degree of potential shift was proportional to the dry mass of adsorbed albumin and β-casein. A comparison of the FET signal with SPR and QCM data provided information on the conformation and orientation of the surface-bound protein by observing characteristic break points in the correlation slopes between the signals. These slope transitions reflect a multistage process that occurs upon protein adsorption as a function of protein concentration, including interim coverage, film dehydration, and monolayer condensation. The FET biosensor, in combination with SPR and QCM, represents a new technology for interrogating protein-material interactions both quantitatively and qualitatively.

A hairpin DNA aptamer coupled with groove binders as a smart switch for a field-effect transistor biosensor

We report here that a hairpin-structured DNA that possesses an anti-ATP aptamer sequence successfully detected target ATP or adenosine in a temperature-dependent manner by nanoscale intramolecular displacement on the surface of a gold electrode as an extended gate of a field-effect transistor (FET). The structural switching of the hairpin aptamer from closed loop to open-loop conformations was accompanied by the release of the preloaded DNA binder (DAPI) from the stem part of the hairpin aptamer into the solution phase. The loss of intrinsic positive charges of DAPI (2+) from the diffusion layer at the gate/solution nano-interface as a result of target capturing was responsible for generating a specific signal by the field-effect. We emphasize a new aspect of the structured DNA aptamer in combination with FET: the DAPI-loaded hairpin aptamer successfully detected even uncharged adenosine, which remains a major challenge for FET-based biosensors. Given the simplicity in design of the primary and secondary structures of oligonucleotide aptamers, it is easy to apply this technology to a wide variety of bio-analytes, irrespective of their electric charges. In view of these advantages, our findings may offer a new trend in the design of stimuli-responsive "smart" biomolecular switches for semiconductor-based biosensors.