Electronic Biosensing with Functionalized rGO FETs

Reduced graphene oxide electrodes meet lateral flow assays: A promising path to advanced point-of-care diagnostics

Research in electrochemical detection in lateral flow assays (LFAs) has gained significant momentum in recent years. The primary impetus for this surge in interest is the pursuit of achieving lower limits of detection, especially given that LFAs are the most widely employed point-of-care biosensors. Conventionally, the strategy for merging electrochemistry and LFAs has centered on the superposition of screen-printed electrodes onto nitrocellulose substrates during LFA fabrication. Nevertheless, this approach poses substantial limitations regarding scalability. In response, we have developed a novel method for the complete integration of reduced graphene oxide (rGO) electrodes into LFA strips. We employed a CO2 laser to concurrently reduce graphene oxide and pattern nitrocellulose, exposing its backing to create connection sites impervious to sample leakage. Subsequently, rGO and nitrocellulose were juxtaposed and introduced into a roll-to-roll system using a wax printer. The exerted pressure facilitated the transfer of rGO onto the nitrocellulose. We systematically evaluated several electrochemical strategies to harness the synergy between rGO and LFAs. While certain challenges persist, our rGO transfer technology presents compelling potential for setting a new standard in electrochemical LFA fabrication.

Enhancing Electrochemical Sensing through Molecular Engineering of Reduced Graphene Oxide-Solution Interfaces and Remote Floating-Gate FET Analysis

Two-dimensional nanomaterials such as reduced graphene oxide (rGO) have captured significant attention in the realm of field-effect transistor (FET) sensors due to their inherent high sensitivity and cost-effective manufacturing. Despite their attraction, a comprehensive understanding of rGO-solution interfaces (specifically, electrochemical interfacial properties influenced by linker molecules and surface chemistry) remains challenging, given the limited capability of analytical tools to directly measure intricate solution interface properties. In this study, we introduce an analytical tool designed to directly measure the surface charge density of the rGO-solution interface leveraging the remote floating-gate FET (RFGFET) platform. Our methodology involves characterizing the electrochemical properties of rGO, which are influenced by adhesion layers between SiO2 and rGO, such as (3-aminopropyl)trimethoxysilane (APTMS) and hexamethyldisilazane (HMDS). The hydrophilic nature of APTMS facilitates the acceptance of oxygen-rich rGO, resulting in a noteworthy pH sensitivity of 56.8 mV/pH at the rGO-solution interface. Conversely, hydrophobic HMDS significantly suppresses the pH sensitivity from the rGO-solution interface, attributed to the graphitic carbon-rich surface of rGO. Consequently, the carbon-rich surface facilitates a denser arrangement of 1-pyrenebutyric acid N-hydroxysuccinimide ester linkers for functionalizing capturing probes on rGO, resulting in an enhanced sensitivity of lead ions by 32% in our proof-of-concept test.

"Clickable" graphene nanoribbons for biosensor interfaces

We report on the synthesis of "clickable" graphene nanoribbons (GNRs) and their application as a versatile interface for electrochemical biosensors. GNRs are successfully deposited on gold-coated working electrodes and serve as a platform for the covalent anchoring of a bioreceptor (i.e., a DNA aptamer), enabling selective and sensitive detection of Interleukin 6 (IL6). Moreover, when applied as the intermediate linker on reduced graphene oxide (rGO)-based field-effect transistors (FETs), the GNRs provide improved robustness compared to conventional aromatic bi-functional linker molecules. GNRs enable an orthogonal and covalent attachment of a recognition unit with a considerably higher probe density than previously established methods. Interestingly, we demonstrate that GNRs introduce photoluminescence (PL) when applied to rGO-based FETs, paving the way toward the simultaneous optical and electronic probing of the attached biointerface.

Reduced Graphene Oxide Electrolyte-Gated Transistor Immunosensor with Highly Selective Multiparametric Detection of Anti-Drug Antibodies

The advent of immunotherapies with biological drugs has revolutionized the treatment of cancers and auto-immune diseases. However, in some patients, the production of anti-drug antibodies (ADAs) hampers the drug efficacy. The concentration of ADAs is typically in the range of 1-10 pm; hence their immunodetection is challenging. ADAs toward Infliximab (IFX), a drug used to treat rheumatoid arthritis and other auto-immune diseases, are focussed. An ambipolar electrolyte-gated transistor (EGT) immunosensor is reported based on a reduced graphene oxide (rGO) channel and IFX bound to the gate electrode as the specific probe. The rGO-EGTs are easy to fabricate and exhibit low voltage operations (≤ 0.3 V), a robust response within 15 min, and ultra-high sensitivity (10 am limit of detection). A multiparametric analysis of the whole rGO-EGT transfer curves based on the type-I generalized extreme value distribution is proposed. It is demonstrated that it allows to selectively quantify ADAs also in the co-presence of its antagonist tumor necrosis factor alpha (TNF-α), the natural circulating target of IFX.

Kinetics of Odorant Recognition with a Graphene-Based Olfactory Receptor Mimicry

Malaria vector mosquito species rely on a handful of specific pheromones for mating; one of them, sulcatone (6-methyl-5-hepten-2-one), is also found in human exudation. Therefore, a complete understanding of the insect’s olfaction, and rapid real-time methods for odorant detection, are required. Here, we mimic the odorant recognition of the nerve cells of an insect’s antenna with a synthetic graphene-based bio-electro-interfacial odorant receptor. By this means, we obtain the kinetics of the genuine odorant recognition reaction and compare them to electro-antennogram data that represent the more complex scenario of a living insect. The odorant-binding proteins OBP 9A and 9B only associate with their ligands weakly, showing KDs of between 2.1 mM and 3 mM, while the binding kinetics of OBP proteins depend on the structural feature of a cystine knot and are modulated by the local milieu within a protein-aided enhancement zone.

Laser-Induced Graphene-Functionalized Field-Effect Transistor-Based Biosensing: A Potent Candidate for COVID-19 Detection

Speedy and on-time detection of coronavirus disease 2019 (COVID-19) is of high importance to control the pandemic effectively and stop its disastrous consequences. A widely available, reliable, label-free, and rapid test that can recognize tiny amounts of specific biomarkers might be the solution. Nanobiosensors are one of the most attractive candidates for this purpose. Integration of graphene with biosensing devices shifts the performance of these systems to an incomparable level. Between the various arrangements using this wonder material, field-effect transistors (FETs) display a precise detection even in complex samples. The emergence of pioneering biosensors for detecting a wide range of diseases especially COVID-19 created the incentive to prepare a review of the recent graphene-FET biosensing platforms. However, the graphene fabrication and transfer to the surface of the device is an imperative factor for researchers to take into account. Therefore, we also reviewed the common methods of manufacturing graphene for biosensing applications and discuss their advantages and disadvantages. One of the most recent synthesizing techniques - laser-induced graphene (LIG) - is attracting attention owing to its extraordinary benefits which are thoroughly explained in this article. Finally, a conclusion highlighting the current challenges is presented.

Field-Effect Transistor with a Plasmonic Fiber Optic Gate Electrode as a Multivariable Biosensor Device

A novel multivariable system, combining a transistor with fiber optic-based surface plasmon resonance spectroscopy with the gate electrode simultaneously acting as the fiber optic sensor surface, is reported. The dual-mode sensor allows for discrimination of mass and charge contributions for binding assays on the same sensor surface. Furthermore, we optimize the sensor geometry by investigating the influence of the fiber area to transistor channel area ratio and distance. We show that larger fiber optic tip diameters are favorable for electronic and optical signals and demonstrate the reversibility of plasmon resonance wavelength shifts after electric field application. As a proof of principle, a layer-by-layer assembly of polyelectrolytes is performed to benchmark the system against multivariable sensing platforms with planar surface plasmon resonance configurations. Furthermore, the biosensing performance is assessed using a thrombin binding assay with surface-immobilized aptamers as receptors, allowing for the detection of medically relevant thrombin concentrations.

Principles of odor coding in vertebrates and artificial chemosensory systems

The biological olfactory system is the sensory system responsible for the detection of the chemical composition of the environment. Several attempts to mimic biological olfactory systems have led to various artificial olfactory systems using different technical approaches. Here we provide a parallel description of biological olfactory systems and their technical counterparts. We start with a presentation of the input to the systems, the stimuli, and treat the interface between the external world and the environment where receptor neurons or artificial chemosensors reside. We then delineate the functions of receptor neurons and chemosensors as well as their overall I-O relationships. Up to this point, our account of the systems goes along similar lines. The next processing steps differ considerably: while in biology the processing step following the receptor neurons is the "integration" and "processing" of receptor neuron outputs in the olfactory bulb, this step has various realizations in electronic noses. For a long period of time, the signal processing stages beyond the olfactory bulb, i.e., the higher olfactory centers were little studied. Only recently there has been a marked growth of studies tackling the information processing in these centers. In electronic noses, a third stage of processing has virtually never been considered. In this review, we provide an up-to-date overview of the current knowledge of both fields and, for the first time, attempt to tie them together. We hope it will be a breeding ground for better information, communication, and data exchange between very related but so far little connected fields.

Market Perspectives and Future Fields of Application of Odor Detection Biosensors within the Biological Transformation-A Systematic Analysis

The technological advantages that biosensors have over conventional technical sensors for odor detection and the role they play in the biological transformation have not yet been comprehensively analyzed. However, this is necessary for assessing their suitability for specific fields of application as well as their improvement and development goals. An overview of biological basics of olfactory systems is given and different odor sensor technologies are described and classified in this paper. Specific market potentials of biosensors for odor detection are identified by applying a tailored methodology that enables the derivation and systematic comparison of both the performance profiles of biosensors as well as the requirement profiles for various application fields. Therefore, the fulfillment of defined requirements is evaluated for biosensors by means of 16 selected technical criteria in order to determine a specific performance profile. Further, a selection of application fields, namely healthcare, food industry, agriculture, cosmetics, safety applications, environmental monitoring for odor detection sensors is derived to compare the importance of the criteria for each of the fields, leading to market-specific requirement profiles. The analysis reveals that the requirement criteria considered to be the most important ones across all application fields are high specificity, high selectivity, high repeat accuracy, high resolution, high accuracy, and high sensitivity. All these criteria, except for the repeat accuracy, can potentially be better met by biosensors than by technical sensors, according to the results obtained. Therefore, biosensor technology in general has a high application potential for all the areas of application under consideration. Health and safety applications especially are considered to have high potential for biosensors due to their correspondence between requirement and performance profiles. Special attention is paid to new areas of application that require multi-sensing capability. Application scenarios for multi-sensing biosensors are therefore derived. Moreover, the role of biosensors within the biological transformation is discussed.

Current and potential biotechnological applications of odorant-binding proteins

Odorant-binding proteins (OBPs) are small soluble proteins whose biological function is believed to be facilitating olfaction by assisting the transport of volatile chemicals in both vertebrate and insect sensory organs, where they are secreted. Their capability to interact with a broad range of hydrophobic compounds combined with interesting features such as being small, stable, and easy to produce and modify, makes them suitable targets for applied research in various industrial segments, including textile, cosmetic, pesticide, and pharmaceutical, as well as for military, environmental, health, and security field applications. In addition to reviewing already established biotechnological applications of OBPs, this paper also discusses their potential use in prospecting of new technologies. The development of new products for insect population management is currently the most prevailing use for OBPs, followed by biosensor technology, an area that has recently seen a significant increase in studies evaluating their incorporation into sensing devices. Finally, less typical approaches include applications in anchorage systems and analytical tools. KEY POINTS: • Odorant-binding proteins (OBPs) present desired characteristics for applied research. • OBPs are mainly used for developing new products for insect population control. • Incorporation of OBPs into chemosensory devices is a growing area of study. • Less conventional uses for OBPs include anchorage systems and analytical purposes. Graphical Abstract.

Biosensors to Monitor Water Quality Utilizing Insect Odorant-Binding Proteins as Detector Elements

In the developing world, the identification of clean, potable water continues to pose a pervasive challenge, and waterborne diseases due to fecal contamination of water supplies significantly threaten public health. The ability to efficiently monitor local water supplies is key to water safety, yet no low-cost, reliable method exists to detect contamination quickly. We developed an in vitro assay utilizing an odorant-binding protein (OBP), AgamOBP1, from the mosquito, Anopheles gambiae, to test for the presence of a characteristic metabolite, indole, from harmful coliform bacteria. We demonstrated that recombinantly expressed AgamOBP1 binds indole with high sensitivity. Our proof-of-concept assay is fluorescence-based and demonstrates the usefulness of insect OBPs as detector elements in novel biosensors that rapidly detect the presence of bacterial metabolic markers, and thus of coliform bacteria. We further demonstrated that rAgamOBP1 is suitable for use in portable, inexpensive "dipstick" biosensors that improve upon lateral flow technology since insect OBPs are robust, easily obtainable via recombinant expression, and resist detector "fouling." Moreover, due to their wide diversity and ligand selectivity, insect chemosensory proteins have other biosensor applications for various analytes. The techniques presented here therefore represent platform technologies applicable to various future devices.

Progress in the development of olfactory-based bioelectronic chemosensors

Artificial chemosensory devices have a wide range of applications in industry, security, and medicine. The development of these devices has been inspired by the speed, sensitivity, and selectivity by which the olfactory system in animals can probe the chemical nature of the environment. In this review, we examine how molecular and cellular components of natural olfactory systems have been incorporated into artificial chemosensors, or bioelectronic sensors. We focus on the biological material that has been combined with signal transduction systems to develop artificial chemosensory devices. The strengths and limitations of different biological chemosensory material at the heart of these devices, as well as the reported overall effectiveness of the different bioelectronic sensor designs, is examined. This review also discusses future directions and challenges for continuing to advance development of bioelectronic sensors.

From radioactive ligands to biosensors: binding methods with olfactory proteins

In this paper, we critically review the binding protocols currently reported in the literature to measure the affinity of odorants and pheromones to soluble olfactory proteins, such as odorant-binding proteins (OBPs), chemosensory proteins (CSPs) and Niemann-Pick class C2 (NPC2) proteins. The first part contains a brief introduction on the principles of binding and a comparison of the techniques adopted or proposed so far, discussing advantages and problems of each technique, as well as their suitable application to soluble olfactory proteins. In the second part, we focus on the fluorescent binding assay, currently the most widely used approach. We analyse advantages and drawbacks, trying to identify the causes of anomalous behaviours that have been occasionally observed, and suggest how to interpret the experimental data when such events occur. In the last part, we describe the state of the art of biosensors for odorants, using soluble olfactory proteins immobilised on biochips, and discuss the possibility of using such approach as an alternative way to measure binding events and dissociation constants.

Label-Free and Recalibrated Multilayer MoS2 Biosensor for Point-of-Care Diagnostics

Molybdenum disulfide (MoS₂) field-effect transistor (FET)-based biosensors have attracted significant attention as promising candidates for highly sensitive, label-free biomolecule detection devices. In this paper, toward practical applications of biosensors, we demonstrate reliable and quantitative detection of a prostate cancer biomarker using the MoS₂ FET biosensor in a nonaqueous environment by reducing nonspecific molecular binding events and realizing uniform chemisorption of anti-PSA onto the MoS₂ surface. A systematic and statistical study on the capability of the proposed device is presented, and the biological binding events are directly confirmed and characterized through intensive structural and electrical analysis. Our proposed biosensor can reliably detect various PSA concentrations with a limit of 100 fg/mL. Moreover, rigorous theoretical simulations provide a comprehensive understanding of the operating mechanism of the MoS₂ FET biosensors, and further suggests the enhancement of the sensitivity through engineering device design parameters.

Ultrasensitive Ethene Detector Based on a Graphene-Copper(I) Hybrid Material

Ethene is a highly diffusive and relatively unreactive gas that induces aging responses in plants in concentrations as low as parts per billion. Monitoring concentrations of ethene is critically important for transport and storage of food crops, necessitating the development of a new generation of ultrasensitive detectors. Here we show that by functionalizing graphene with copper complexes biologically relevant concentrations of ethene and of the spoilage marker ethanol can be detected. Importantly, in addition these sensors provide us with important insights into the interactions between molecules, a key concept in chemistry. Chemically induced dipole fluctuations in molecules as they undergo a chemical reaction are harvested in an elegant way through subtle field effects in graphene. By exploiting changes in the dipole moments of molecules that occur upon a chemical reaction we are able to track the reaction and provide mechanistic insight that was, until now, out of reach.

InP Nanowire Biosensor with Tailored Biofunctionalization: Ultrasensitive and Highly Selective Disease Biomarker Detection

Electrically active field-effect transistors (FET) based biosensors are of paramount importance in life science applications, as they offer direct, fast, and highly sensitive label-free detection capabilities of several biomolecules of specific interest. In this work, we report a detailed investigation on surface functionalization and covalent immobilization of biomarkers using biocompatible ethanolamine and poly(ethylene glycol) derivate coatings, as compared to the conventional approaches using silica monoliths, in order to substantially increase both the sensitivity and molecular selectivity of nanowire-based FET biosensor platforms. Quantitative fluorescence, atomic and Kelvin probe force microscopy allowed detailed investigation of the homogeneity and density of immobilized biomarkers on different biofunctionalized surfaces. Significantly enhanced binding specificity, biomarker density, and target biomolecule capture efficiency were thus achieved for DNA as well as for proteins from pathogens. This optimized functionalization methodology was applied to InP nanowires that due to their low surface recombination rates were used as new active transducers for biosensors. The developed devices provide ultrahigh label-free detection sensitivities ∼1 fM for specific DNA sequences, measured via the net change in device electrical resistance. Similar levels of ultrasensitive detection of ∼6 fM were achieved for a Chagas Disease protein marker (IBMP8-1). The developed InP nanowire biosensor provides thus a qualified tool for detection of the chronic infection stage of this disease, leading to improved diagnosis and control of spread. These methodological developments are expected to substantially enhance the chemical robustness, diagnostic reliability, detection sensitivity, and biomarker selectivity for current and future biosensing devices.

Successive Release of Tissue Inhibitors of Metalloproteinase-1 Through Graphene Oxide-Based Delivery System Can Promote Skin Regeneration

The purpose of this study was to testify the hypothesis that graphene oxide (GO) could act as an appropriate vehicle for the release of tissue inhibitors of metalloproteinase-1 (TIMP-1) protein in the context of skin repair. GO characteristics were observed by scanning electron microscopy, atomic force microscopy, and thermal gravimetric analysis. After TIMP-1 absorbing GO, the release profiles of various concentrations of TIMP-1 from GO were compared. GO biocompatibility with fibroblast viability was assessed by measuring cell cycle and apoptosis. In vivo wound healing assays were used to determine the effect of TIMP-1-GO on skin regeneration. The greatest intensity of GO was 1140 nm, and the most intensity volume was 10,674.1 nm (nanometer). TIMP-1 was shown to be continuously released for at least 40 days from GO. The proliferation and viability of rat fibroblasts cultured with TIMP-1-GO were not significantly different as compared with the cells grown in GO or TIMP-1 alone (p > 0.05). Skin defect of rats treated with TIMP-1 and TIMP-1-GO showed significant differences in histological and immunohistochemical scores (p < 0.05). GO can be controlled to release carrier materials. The combination of TIMP-1 and GO promoted the progression of skin tissue regeneration in skin defect.

Sensing at the Surface of Graphene Field-Effect Transistors

Recent research trends now offer new opportunities for developing the next generations of label-free biochemical sensors using graphene and other two-dimensional materials. While the physics of graphene transistors operated in electrolyte is well grounded, important chemical challenges still remain to be addressed, namely the impact of the chemical functionalizations of graphene on the key electrical parameters and the sensing performances. In fact, graphene - at least ideal graphene - is highly chemically inert. The functionalizations and chemical alterations of the graphene surface - both covalently and non-covalently - are crucial steps that define the sensitivity of graphene. The presence, reactivity, adsorption of gas and ions, proteins, DNA, cells and tissues on graphene have been successfully monitored with graphene. This review aims to unify most of the work done so far on biochemical sensing at the surface of a (chemically functionalized) graphene field-effect transistor and the challenges that lie ahead. The authors are convinced that graphene biochemical sensors hold great promise to meet the ever-increasing demand for sensitivity, especially looking at the recent progresses suggesting that the obstacle of Debye screening can be overcome.