Fully integrated graphene electronic biosensor for label-free detection of lead (II) ion based on G-quadruplex structure-switching

Development of a highly sensitive CNT-metal graphene hybrid nano-IDA electrochemical biosensor for the diagnosis of Alzheimer's disease

The blood-based detection of Alzheimer's disease (AD) is becoming challenging since the blood-brain barrier (BBB) restricts the direct circulation of AD molecules in the blood, thereby precluding the detection of AD at an early-stage. Herein, we report the development of a novel CNT-metal-porous graphene hybrid (CNT-MGH) nano-interdigitated array (n-IDA) electrochemical 8-well biosensor for the successful early-stage diagnosis of AD from blood. Laser-induced graphene (LIG) technology has been used to fabricate the proposed CNT-MGH n-IDA 8-well sensor. Firstly, the electrochemical characterization (i.e., electrode gap, material composition, etc.) of the proposed sensor was demonstrated by measuring p-aminophenol (PAP) with a limit of detection (LOD) of 0.1 picomole. Subsequently, the CNT-MGH n-IDA 8-well sensor was then used to diagnose AD via novel blood biomarkers p-Tau 217 and p-Tau 181 using an electrochemical enzyme-linked immunosorbent assay (e-ELISA) enzyme by-product PAP. During e-ELISA, the alkaline phosphatase enzyme (IgG-AP) tagged to the detection antibody produced an electroactive ELISA by-product PAP by reacting with the enzyme-substrate 4-aminophenyl phosphate (PAPP). Finally, the CNT-MGH n-IDA 8-well sensor was then used to measure the current generated by the redox reaction via the e-ELISA by-product PAP. While quantified, the proposed CNT-MGH n-IDA 8-well sensor successfully detected p-Tau 217 and p-Tau 181 proteins in blood with LODs of 0.16 pg ml-1 and 0.08 pg ml-1, respectively.

Simultaneous Hg2+ and Pb2+ detection in water samples using an electrochemical aptasensor with dual signal amplification by exonuclease III and metal-organic frameworks

Heavy metal pollution in the environment has become a significant global concern due to its detrimental effects on human health and the environment. In this study, we report an electrochemical aptasensor for the simultaneous detection of Hg2+ and Pb2+. Gold nanoflower/polyethyleneimine-reduced graphene oxide (AuNFs/PEI-rGO) was introduced on the surface of a gold electrode to improve sensing performance. The aptasensor is based on the formation of a T-Hg2+-T mismatch structure and specific cleavage of the Pb2+-dependent DNAzyme, resulting in a dual signal generated by the Exo III specific digestion of methylene blue (MB) labeled at the 3' end of probe DNA-1 and the reduction of the substrate ascorbic acid (AA) catalyzed by the signal label. The decrease of MB signal and the increase of AA oxidation peak was used to indicate the content of Hg2+ and Pb2+, respectively, with detection limits of 0.11 pM (Hg2+) and 0.093 pM (Pb2+). The aptasensor was also used for detecting Hg2+ and Pb2+ in water samples with good recoveries. Overall, this electrochemical aptasensor shows promising potential for sensitive and selective detection of heavy metals in environmental samples.

Graphene-based nanotechnology in the Internet of Things: a mini review

Graphene, a 2D nanomaterial, has garnered significant attention in recent years due to its exceptional properties, offering immense potential for revolutionizing various technological applications. In the context of the Internet of Things (IoT), which demands seamless connectivity and efficient data processing, graphene's unique attributes have positioned it as a promising candidate to prevail over challenges and optimize IoT systems. This review paper aims to provide a brief sketch of the diverse applications of graphene in IoT, highlighting its contributions to sensors, communication systems, and energy storage devices. Additionally, it discusses potential challenges and prospects for the integration of graphene in the rapidly evolving IoT landscape.

DNAzyme-activated CRISPR/Cas assay for sensitive and one-pot detection of lead contamination

Hazardous lead ions (Pb2+) even at a minute level can pose side effects on human health, highlighting the need for tools for trace Pb2+ detection. Herein, we present a DNAzyme-activated CRISPR assay (termed DzCas12T) for sensitive and one-pot detection of lead contamination. Using an extension-bridged strategy eliminates the need for separation to couple the DNAzyme recognition and CRISPR reporting processes. The tandem design endowed the DzCas12T assay with high specificity and sensitivity down to the pM-level. This assay has been used to detect lead contamination in food and water samples, indicating the potential for monitoring lead-associated environmental and food safety.

Comparative Study of Field-Effect Transistors Based on Graphene Oxide and CVD Graphene in Highly Sensitive NT-proBNP Aptasensors

Graphene-based materials are actively being investigated as sensing elements for the detection of different analytes. Both graphene grown by chemical vapor deposition (CVD) and graphene oxide (GO) produced by the modified Hummers' method are actively used in the development of biosensors. The production costs of CVD graphene- and GO-based sensors are similar; however, the question remains regarding the most efficient graphene-based material for the construction of point-of-care diagnostic devices. To this end, in this work, we compare CVD graphene aptasensors with the aptasensors based on reduced GO (rGO) for their capabilities in the detection of NT-proBNP, which serves as the gold standard biomarker for heart failure. Both types of aptasensors were developed using commercial gold interdigitated electrodes (IDEs) with either CVD graphene or GO formed on top as a channel of liquid-gated field-effect transistor (FET), yielding GFET and rGO-FET sensors, respectively. The functional properties of the two types of aptasensors were compared. Both demonstrate good dynamic range from 10 fg/mL to 100 pg/mL. The limit of detection for NT-proBNP in artificial saliva was 100 fg/mL and 1 pg/mL for rGO-FET- and GFET-based aptasensors, respectively. While CVD GFET demonstrates less variations in parameters, higher sensitivity was demonstrated by the rGO-FET due to its higher roughness and larger bandgap. The demonstrated low cost and scalability of technology for both types of graphene-based aptasensors may be applicable for the development of different graphene-based biosensors for rapid, stable, on-site, and highly sensitive detection of diverse biochemical markers.

Toward the Ultimate Limit of Analyte Detection, in Graphene-Based Field-Effect Transistors

The ultimate sensitivity of field-effect-transistor (FET)-based devices for ionic species detection is of great interest, given that such devices are capable of monitoring single-electron-level modulations. It is shown here, from both theoretical and experimental perspectives, that for such ultimate limits to be approached the thermodynamic as well as kinetic characteristics of the (FET surface)-(linker)-(ion-receptor) ensemble must be considered. The sensitivity was probed in terms of optimal packing of the ensemble, through a minimal charge state/capacitance point of view and atomic force microscopy. Through the fine-tuning of the linker and receptor interaction with the sensing surface, a record limit of detection as well as specificity in the femtomolar range, orders of magnitude better than previously obtained and in excellent accord with prediction, was observed.

Multiplexed immunosensing of cancer biomarkers on a split-float-gate graphene transistor microfluidic biochip

This work reports the development of a novel microfluidic biosensor using a graphene field-effect transistor (GFET) design for the parallel label-free analysis of multiple biomarkers. Overcoming the persistent challenge of constructing μm2-sized FET sensitive interfaces that incorporate multiple receptors, we implement a split-float-gate structure that enables the manipulation of multiplexed biochemical functionalization using microfluidic channels. Immunoaffinity biosensing experiments are conducted using the mixture samples containing three liver cancer biomarkers, carcinoembryonic antigen (CEA), α-fetoprotein (AFP), and parathyroid hormone (PTH). The results demonstrate the capability of our label-free biochip to quantitatively detect multiple target biomarkers simultaneously by observing the kinetics in 10 minutes, with the detection limit levels in the nanomolar range. This microfluidic biosensor provides a valuable analytical tool for rapid multi-target biosensing, which can be potentially utilized for domiciliary tests of cancer screening and prognosis, obviating the need for sophisticated instruments and professional operations in hospitals.

Advances in flexible graphene field-effect transistors for biomolecule sensing

With the increasing demand for biomarker detection in wearable electronic devices, flexible biosensors have garnered significant attention. Additionally, graphene field-effect transistors (GFETs) have emerged as key components for constructing biosensors, owing to their high sensitivity, multifunctionality, rapid response, and low cost. Leveraging the advantages of flexible substrates, such as biocompatibility, adaptability to complex environments, and fabrication flexibility, flexible GFET sensors exhibit promising prospects in detecting various biomarkers. This review provides a concise summary of design strategies for flexible GFET biosensors, including non-encapsulated gate without dielectric layer coverage and external gate designs. Furthermore, notable advancements in sensing applications of biomolecules, such as proteins, glucose, and ions, are highlighted. Finally, we discuss the future challenges and prospects in this field, aiming to inspire researchers to address these issues in their further investigations.

Monitoring Cd2+ in oily wastewater using an aptamer-graphene field-effect transistor with a selective wetting surface

The discharge of oily industrial wastewater containing heavy metal ions with the development of industry severely threatens the environment and human health. Therefore, it is of great significance to monitor the concentration of heavy metal ions in oily wastewater quickly and effectively. Here, an integrated Cd²⁺ monitoring system consisting of an aptamer-graphene field-effect transistor (A-GFET), oleophobic/hydrophilic surface and monitoring-alarm circuits was presented for monitoring the Cd²⁺ concentration in oily wastewater. In the system, oil and other impurities in wastewater are isolated by an oleophobic/hydrophilic membrane before detection. The concentration of Cd²⁺ is then detected by a graphene field-effect transistor with a Cd²⁺ aptamer modifying the graphene channel. Finally, the detected signal is collected and processed by signal processing circuits to judge whether the Cd²⁺ concentration exceeds the standard. Experimental results demonstrated that the separation efficiency of the oleophobic/hydrophilic membrane to an oil/water mixture was up to 99.9%, exhibiting a high oil/water separation ability. The A-GFET detecting platform could respond to changes in the Cd²⁺ concentration within 10 min with a limit of detection (LOD) of 0.125 pM. The sensitivity of this detection platform to Cd²⁺ near 1 nM was 7.643 × 10^-2 nM^-1. Compared with control ions (Cr³⁺, Pb²⁺, Mg²⁺, Fe³⁺), this detection platform exhibited a high specificity to Cd²⁺. Moreover, the system could send out a photoacoustic alarm signal when the Cd²⁺ concentration in the monitoring solution exceeds the preset value. Therefore, the system is practical for monitoring the concentration of heavy metal ions in oily wastewater.

Observing Mesoscopic Nucleic Acid Capacitance Effect and Mismatch Impact via Graphene Transistors

This work reports a molecular-scale capacitance effect of the double helical nucleic acid duplex structure for the first time. By quantitatively conducting large sample measurements of the electrostatic field effect using a type of high-accuracy graphene transistor biosensor, an unusual charge-transport behavior is observed in which the end-immobilized nucleic acid duplexes can store a part of ionization electrons like molecular capacitors, other than electric conductors. To elucidate this discovery, a cascaded capacitive network model is proposed as a novel equivalent circuit of nucleic acid duplexes, expanding the point-charge approximation model, by which the partial charge-transport observation is reasonably attributed to an electron-redistribution behavior within the capacitive network. Furthermore, it is experimentally confirmed that base-pair mismatches hinder the charge transport in double helical duplexes, and lead to directly identifiable alterations in electrostatic field effects. The bioelectronic principle of mismatch impact is also self-consistently explained by the newly proposed capacitive network model. The mesoscopic nucleic acid capacitance effect may enable a new kind of label-free nucleic acid analysis tool based on electronic transistor devices. The in situ and real-time nucleic acid detections for virus biomarkers, somatic mutations, and genome editing off-target may thus be predictable.

Portable immunosensor directly and rapidly detects Mycobacterium tuberculosis in sputum

Tuberculosis (TB) remains a public health problem that cannot be ignored. The portable and efficient detection of Mycobacterium tuberculosis (MTB) is important for the effective control of this disease. However, current detection techniques do not meet the requirements for MTB detection in the actual environment and often require cumbersome detection steps that are time consuming and inflexible. In this study, a portable immunosensor to detect MTB in sputum was prepared and then subjected to interface characterizations, such as scanning electron microscopy, hydrophilic angle test, and fluorescence characterization. The source and gate voltage of the device were optimized and tested using a non-contact photoresponse. The results showed that the sensitivity of the sensor to luminance increases with the decrease in source voltage. The gate voltage can substantially improve the response of the immunosensor to the normalized current of protein and amplify the signal at least 1.6 times. The optimal voltage detection conditions of source voltage (0.3 V) and gate voltage (0.1 V) were also determined. Several common proteins present in simulated saliva were used for anti-interference tests, and the sensor exhibited good specificity. Finally, the dilution gradient of an actual TB sputum sample was optimized. In the absence of preconditioning, a double-blind experiment was used to distinguish between the sputum from patients with TB and healthy individuals to shorten the TB detection time to a few minutes. Compared with the hospital's conventional detection method using cultures, the proposed method can complete the detection in a shorter time. This study provides a new strategy for the portable diagnosis of TB.

Molecular Engineering of 2D Nanomaterial Field-Effect Transistor Sensors: Fundamentals and Translation across the Innovation Spectrum

Over the last decade, 2D layered nanomaterials have attracted significant attention across the scientific community due to their rich and exotic properties. Various nanoelectronic devices based on these 2D nanomaterials have been explored and demonstrated, including those for environmental applications. Here, the fundamental attributes of 2D layered nanomaterials for field-effect transistor (FET) sensors and tunneling FET (TFET) sensors, which provide versatile detection of water contaminants such as heavy-metal ions, bacteria, nutrients, and organic pollutants, are discussed. The major challenges and opportunities are also outlined for designing and fabricating 2D nanomaterial FET/TFET sensors with superior performance. Translation of these FET/TFET sensors from fundamental research to applied technology is illustrated through a case study on graphene-based real-time FET water sensors. A second case study centers on large-scale sensor networks for water-quality monitoring to enable intelligent drinking water and river-water systems. Overall, 2D nanomaterial FET sensors have significant potential for enabling a human-centered intelligent water system that can likely be applied to other precarious water supplies around the globe.

Ten Years Progress of Electrical Detection of Heavy Metal Ions (HMIs) Using Various Field-Effect Transistor (FET) Nanosensors: A Review

Heavy metal pollution remains a major concern for the public today, in line with the growing population and global industrialization. Heavy metal ion (HMI) is a threat to human and environmental safety, even at low concentrations, thus rapid and continuous HMI monitoring is essential. Among the sensors available for HMI detection, the field-effect transistor (FET) sensor demonstrates promising potential for fast and real-time detection. The aim of this review is to provide a condensed overview of the contribution of certain semiconductor substrates in the development of chemical and biosensor FETs for HMI detection in the past decade. A brief introduction of the FET sensor along with its construction and configuration is presented in the first part of this review. Subsequently, the FET sensor deployment issue and FET intrinsic limitation screening effect are also discussed, and the solutions to overcome these shortcomings are summarized. Later, we summarize the strategies for HMIs' electrical detection, mechanisms, and sensing performance on nanomaterial semiconductor FET transducers, including silicon, carbon nanotubes, graphene, AlGaN/GaN, transition metal dichalcogenides (TMD), black phosphorus, organic and inorganic semiconductor. Finally, concerns and suggestions regarding detection in the real samples using FET sensors are highlighted in the conclusion.

Multiplexed detection of heavy metal ions by single plasmonic nanosensors

Detection of multiple analytes simultaneously in small liquid samples with high efficiency and precision is highly important to the fields like water quality monitoring. In this letter, we present a multiplexed nanosensors with position-encoded aptamer functionalized gold nanorods for heavy metal ions detection. The individual gold nanorods respond specifically to two different heavy metal ions (Pb²⁺ and Hg²⁺) with a spectral shift in the scattering spectrum. We used a home-built spectral imaging dark-field microscope to measure the response of thousands of single plasmonic nanosensors with relatively high time resolution and precision. To explore the performance and limit of detection (LOD) of our nanosensor and setup, we recorded the concentration-dependent response of our position-encoded nanosensors with a series of mixture solutions that contain different concentrations of Hg²⁺ and Pb²⁺ ions. The LOD levels of our system are around 5 nM for Pb²⁺ ions and 1 nM for Hg²⁺ ions. Our method and results demostrate the nanomolar sensitivity and the potential to detect more different heavy metal ions.

Ratiometric G-Quadruplex Assay for Robust Lead Detection in Food Samples

Lead (Pb²⁺) pollution is a serious food safety issue, rapid detection of Pb²⁺ residual in food is vital to guarantee food quality and safety. Here we proposed ratiometric aptamer probes, allowing robust Pb²⁺ supervision in food samples. Pb²⁺ specific aptamer can bolster a transition of G-quadruplex structural response to Pb²⁺; this process can be monitored by N-methyl mesoporphyrin IX (NMM), which is highly specific to G-quadruplex. Particularly, the utilization of G-quadruplex specific dye and terminal-labeled fluorophore allowed to endue ratiometric signal outputs towards Pb²⁺, dramatically increase the robustness for lead detection. The ratiometric G-quadruplex assay allowed a facile and one-pot Pb²⁺ detection at room temperature using a single-stranded DNA aptamer. We demonstrated its feasibility for detecting lead pollution in fresh eggs and tap water samples. The ratiometric G-quadruplex design is expected to be used for on-site Pb²⁺ testing associated with food safety.

DNA interfaces with dimensional materials for biomedical applications

DNA interfaces with nano, micro, and macro materials have gained widespread attention for various applications. Such interfaces exhibit distinct functions and properties not only due to the unique properties of interfacing materials but also sequence- and conformation-dependent characteristics of the DNA. Therefore, DNA interfaces with diverse dimensional materials have advanced our understanding of the interaction mechanisms and the properties of such interfaces. The unique interfacial properties of such novel materials have applications in nanotechnology, biophysics, cell biology, biosensing, and bioelectronics. The field is growing rapidly with the frequent emergence of new interfaces carrying remarkable interfacial character. In this review article, we have classified the DNA interfaces into 0D, 1D, 2D, and 3D categories based on the types of dimensional materials. We review the key efforts made in the last five years and focus on types of interfaces, interfacing mechanisms, and their state-of-the-art applications. This review will draw a general interest because of the diversity in the DNA materials science but also the unique applications that will play a cutting-edge role in biomedical and biosensing research.

An Intelligent Graphene-Based Biosensing Device for Cytokine Storm Syndrome Biomarkers Detection in Human Biofluids

Abnormal elevated levels of cytokines such as interferon (IFN), interleukin (IL), and tumor necrosis factor (TNF), are considered as one of the prognosis biomarkers for indicating the progression to severe or critical COVID-19. Hence, it is of great significance to develop devices for monitoring their levels in COVID-19 patients, and thus enabling detecting COVID-19 patients that are worsening and to treat them before they become critically ill. Here, an intelligent aptameric dual channel graphene-TWEEN 80 field effect transistor (DGTFET) biosensing device for on-site detection of IFN-γ, TNF-α, and IL-6 within 7 min with limits of detection (LODs) of 476 × 10^-15 , 608 × 10^-15 , or 611 × 10^-15 m respectively in biofluids is presented. Using the customized Android App together with this intelligent device, asymptomatic or mild COVID-19 patients can have a preliminary self-detection of cytokines and get a warning reminder while the condition starts to deteriorate. Also, the device can be fabricated on flexible substrates toward wearable applications for moderate or even critical COVID-19 cases for consistently monitoring cytokines under different deformations. Hence, the intelligent aptameric DGTFET biosensing device is promising to be used for point-of-care applications for monitoring conditions of COVID-19 patients who are in different situations.

A colorimetric sensing platform for azodicarbonamide detection in flour based on MnO2 nanosheets oxidative system

Azodicarbonamide (ADA), as a dough conditioner food additive in flour, can be turned into toxic biurea and semicarbazide after high temperature processing. Hence, the using of ADA in food material should be strictly controlled, and the detection of ADA is very important for consumers' safety and health. Herein, a simple and fast colorimetric strategy has been developed for ADA detection based on the MnO₂ nanosheets-3,3',5,5'-tetramethylbenzidine (TMB)-glutathione (GSH) as oxidative sensing system (MnO₂-TMB-GSH). Since the ADA can selectively react with GSH via oxidizing the sulfydryl (-SH) group of GSH to disulfide bond (S-S), which makes GSH unable to reduce MnO₂ nanosheets and restore its oxidase-like activity. The absorbance changes of the TMB solution depended on ADA content. The MnO₂-TMB-GSH colorimetric platform can detect the ADA with a linear range of 10 μmol L^-1 (11.6 ppm) to 400 μmol L^-1 (464 ppm), and the limit of detection (LOD) is 3.3 μmol L^-1 (3.51 ppm). Some potential interferences in real sample were tested and did not affect the MnO₂-TMB-GSH colorimetric platform for ADA detection. Furthermore, the sensing platform was applied for detecting ADA in real flour sample with a recovery of 96%-105% (RSD < 5%). This colorimetric method can effectively and rapidly detect ADA additives in flour less than the prescribed standard (45 mg kg^-1), which shows a great potential for visualization analysis and on-site detection of ADA in flour. A simple and fast colorimetric strategy has been developed for azodicarbonamide (ADA) detection based on the MnO₂ nanosheets-3,3',5,5'-tetramethylbenzidine (TMB)-glutathione (GSH) as oxidative sensing system (MnO₂-TMB-GSH).

Recent Progress in DNA Motor-Based Functional Systems

The designability, functionalization, and diverse secondary structures of DNA enable the construction of DNA motors with stimuli-responsiveness. Therefore, it has been widely used to fabricate functional systems or generate mechanical power under external stimuli, such as pH, light, heat, electrical, and chemical molecular signals. Furthermore, the DNA motor has also been demonstrated to promote the applications of smart devices and materials, particularly in controllable drug delivery and reversible molecular switching. In this review, we have summarized and discussed recent progress of the construction and applications of DNA motor-based functional systems, such as responsive nanodevices, modified surfaces, and hydrogels.

Recent Advances in Nanotechnology-Based Biosensors Development for Detection of Arsenic, Lead, Mercury, and Cadmium

Heavy metals cause considerable environmental pollution due to their extent and non-degradability in the environment. Analysis and trace levels of arsenic, lead, mercury, and cadmium as the most toxic heavy metals show that they can cause various hazards in humans' health. To achieve rapid, high-sensitivity methods for analyzing ultra-trace amounts of heavy metals in different environmental and biological samples, novel biosensors have been designed with the participation of strategies applied in nanotechnology. This review attempted to investigate the novel, sensitive, efficient, cost-benefit, point of care, and user-friendly biosensors designed to detect these heavy metals based on functional mechanisms. The study's search strategies included examining the primary databases from 2015 onwards and various keywords focusing on heavy metal biosensors' performance and toxicity mechanisms. The use of aptamers and whole cells as two important bio-functional nanomaterials is remarkable in heavy metal diagnostic biosensors' bioreceptor design. The application of hybridized nanomaterials containing a specific physicochemical function in the presence of a suitable transducer can improve the sensing performance to achieve an integrated detection system. Our study showed that in addition to both labeled and label-free detection strategies, a wide range of nanoparticles and nanocomposites were used to modify the biosensor surface platform in the detection of heavy metals. The detection limit and linear dynamic range as an essential characteristic of superior biosensors for the primary toxic metals are studied. Furthermore, the perspectives and challenges facing the design of heavy metal biosensors are outlined. The development of novel biosensors and the application of nanotechnology, especially in real samples, face challenges such as the capability to simultaneously detect multiple heavy metals, the interference process in complex matrices, the efficiency and stability of nanomaterials implemented in various laboratory conditions.

Formation of Novel Octuplex DNA Molecules from Guanine Quadruplexes

Guanine quadruplex (G4)-DNA structures have sparked the interest of many scientists due to their important biological roles and their potential use in molecular nanoelectronics and nanotechnology. The high guanine content in G4-DNA endows it with mechanical stability, robustness, and improved charge transport properties-attractive attributes for a molecular nanowire. The self-driven formation of a novel G4-DNA-based nanostructure, coined guanine octuplex (G8)-DNA, is reported herein. Atomic force microscopy and scanning tunneling microscopy characterization of this molecule reveal its organized coiled-coil structure, which is found to be stable under different temperatures and surrounding conditions. G8-DNA exhibits enhanced stiffness, mechanical and thermodynamic stability when compared to its parent G4-DNA. These, along with its high guanine content, make G8-DNA a compelling new molecule, and a highly prospective candidate for molecular nanoelectronics.

Review of recent progress on DNA-based biosensors for Pb2+ detection

Lead (Pb) is a highly toxic heavy metal of great environmental and health concerns, and interestingly Pb²⁺ has played important roles in nucleic acids chemistry. Since 2000, using DNA for selective detection of Pb²⁺ has become a rapidly growing topic in the analytical community. Pb²⁺ can serve as the most active cofactor for RNA-cleaving DNAzymes including the GR5, 17E and 8-17 DNAzymes. Recently, Pb²⁺ was found to promote a porphyrin metalation DNAzyme named T30695. In addition, Pb²⁺ can tightly bind to various G-quadruplex sequences inducing their unique folding and binding to other molecules such as dyes and hemin. The peroxidase-like activity of G-quadruplex/hemin complexes was also used for Pb²⁺ sensing. In this article, these Pb²⁺ recognition mechanisms are reviewed from fundamental chemistry to the design of fluorescent, colorimetric, and electrochemical biosensors. In addition, various signal amplification mechanisms such as rolling circle amplification, hairpin hybridization chain reaction and nuclease-assisted methods are coupled to these sensing methods to drive up sensitivity. We mainly cover recent examples published since 2015. In the end, some practical aspects of these sensors and future research opportunities are discussed.

Ultrasensitive ratiometric detection of Pb2+ using DNA tetrahedron-mediated hyperbranched hybridization chain reaction

A fluorescent sensing strategy was developed for rapid, highly sensitive and specific detection of lead (II) ion (Pb²⁺) on the basis of Pb²⁺ DNAzyme-controlled tetrahedral DNA nanostructure (TDN)-mediated hyper-branched hybridization chain reaction (hHCR). In this strategy, DNA hairpins used for HCR amplification are modified on the four vertexes of TDN, which are then used to perform rapid TDN-hHCR in the presence of an initiator strand, producing large-sized cross-linked reaction products and thus giving greatly improved fluorescence resonance energy transfer (FRET) signal output. Pb²⁺ DNAzyme catalyzes the cleavage of the initiator strand, inhibiting the initiation of TDN-hHCR and giving decreased FRET signal. Synergetic signal amplification of Pb²⁺ DNAzyme-catalyzed cleavage reaction and subsequent TDN-hHCR confers the sensing platform with ultrahigh sensitivity. As low as 0.25 pM Pb²⁺ can be detected by using either signal "turn-on" or "turn-off" mode. The whole detection process can be finished within 20 min. Strong anti-interference capacity of FRET-based ratiometric detection and high specificity of Pb²⁺ DNAzyme endow the sensing platform with great practical application potential, which was demonstrated by the accurate detection of Pb²⁺ in real river water, fruit, vegetable and grain samples.

Graphene field-effect transistors as bioanalytical sensors: design, operation and performance

Graphene field-effect transistors (GFETs) are emerging as bioanalytical sensors, in which their responsive electrical conductance is used to perform quantitative analyses of biologically-relevant molecules such as DNA, proteins, ions and small molecules. This review provides a detailed evaluation of reported approaches in the design, operation and performance assessment of GFET biosensors. We first dissect key design elements of these devices, along with most common approaches for their fabrication. We compare possible modes of operation of GFETs as sensors, including transfer curves, output curves and time series as well as their integration in real-time or a posteriori protocols. Finally, we review performance metrics reported for the detection and quantification of bioanalytes, and discuss limitations and best practices to optimize the use of GFETs as bioanalytical sensors.

Modulating the Linker Immobilization Density on Aptameric Graphene Field Effect Transistors Using an Electric Field

Aptameric graphene-based field-effect transistors (A-GFETs) always employ linkers, which could immobilize on graphene through π-π stacking between contained pyrenyl groups and graphene, to anchor aptamers. Aptamer density is closely associated with the A-GFET sensitivity and determined by the linker density. Using known linker immobilization methods, the linker density is random, uncontrollable, and limited. In this work, we propose a novel linker immobilization method which can be used to effectively modulate the linker density using an electric field and further bridge the relationship between the linker density and the A-GFET sensitivity. Here, polar molecule 1-pyrenebutanoic acid succinimidyl ester (PASE) is used as a linker representative. In the electric field, PASE is arranged regularly with the electron-rich pyrenyl group forced toward graphene in the solution due to electrostatic repulsion, thereby making it possible to modulate the quantity of PASE molecules that could interact with graphene by tuning the electric field application and then realizing the regulation of the A-GFET sensitivity. Experimental results indicate that the limits of detection (LODs) of A-GFETs for detecting interleukin-6 (IL-6) and insulin can be significantly improved to be 618 and 766 fM, respectively, by applying an electric field at -0.3 V for 3 h during PASE immobilization.

Recent developments in carbon-based two-dimensional materials: synthesis and modification aspects for electrochemical sensors

This review (162 references) focuses on two-dimensional carbon materials, which include graphene as well as its allotropes varying in size, number of layers, and defects, for their application in electrochemical sensors. Many preparation methods are known to yield two-dimensional carbon materials which are often simply addressed as graphene, but which show huge variations in their physical and chemical properties and therefore on their sensing performance. The first section briefly reviews the most promising as well as the latest achievements in graphene synthesis based on growth and delamination techniques, such as chemical vapor deposition, liquid phase exfoliation via sonication or mechanical forces, as well as oxidative procedures ranging from chemical to electrochemical exfoliation. Two-dimensional carbon materials are highly attractive to be integrated in a wide field of sensing applications. Here, graphene is examined as recognition layer in electrochemical sensors like field-effect transistors, chemiresistors, impedance-based devices as well as voltammetric and amperometric sensors. The sensor performance is evaluated from the material's perspective of view and revealed the impact of structure and defects of the 2D carbon materials in different transducing technologies. It is concluded that the performance of 2D carbon-based sensors is strongly related to the preparation method in combination with the electrical transduction technique. Future perspectives address challenges to transfer 2D carbon-based sensors from the lab to the market. Graphical abstract Schematic overview from synthesis and modification of two-dimensional carbon materials to sensor application.

Graphene-Based Biosensor for Early Detection of Iron Deficiency

Iron deficiency (ID) is the most prevalent and severe nutritional disorder globally and is the leading cause of iron deficiency anemia (IDA). IDA often progresses subtly symptomatic in children, whereas prolonged deficiency may permanently impair development. Early detection and frequent screening are, therefore, essential to avoid the consequences of IDA. In order to reduce the production cost and complexities involved in building advanced ID sensors, the devices were fabricated using a home-built patterning procedure that was developed and used for this work instead of lithography, which allows for fast prototyping of dimensions. In this article, we report the development of graphene-based field-effect transistors (GFETs) functionalized with anti-ferritin antibodies through a linker molecule (1-pyrenebutanoic acid, succinimidyl ester), to facilitate specific conjugation with ferritin antigen. The resulting biosensors feature an unprecedented ferritin detection limit of 10 fM, indicating a tremendous potential for non-invasive (e.g., saliva) ferritin detection.

On-chip estimation of hematocrit level for diagnosing anemic conditions by Impedimetric techniques

An on-chip device has been fabricated on Si platform for precise estimation of the hematocrit (Hct) level of human blood with rapid turn out. Impedance/capacitance spectroscopy and current-voltage (I-V) measurements have been employed to observe the variation of electrical parameters of erythrocyte suspensions with varying Hct level. Experimentally obtained values of capacitance, impedance and conductance with Hct level suggests a linear variation. Current-time measurement has also been performed to ensure the susceptibility of red blood cells under a fixed external electric field for certain duration of time. The online real time sample analyzes have also been performed by the device connected with an embedded electronic circuit interfaced with a laptop through appropriate software. This has enabled the development of a novel Si-chip based compact point-of-care (POC) diagnosis system for Hct level estimation by measuring the dielectric/capacitive variation of an erythrocyte cell suspension. The relevant performance parameters of such a compact system including range, resolution, limit of detection and throughput have also been evaluated.

Fully Solid-State Graphene Transistors with Striking Homogeneity and Sensitivity for the Practicalization of Single-Device Electronic Bioassays

To break through a critical barrier in the practical application of graphene biosensors, namely, device-to-device performance inhomogeneity, this work presents a novel scenario employing a fully solid-state (FSS) transistor configuration. Herein, the graphene sensing unit is completely encapsulated by a high-κ solid dielectric material, which isolates the sensing unit from solution contaminants and thus homogeneously maintains the extraordinary carrier mobility of pristine graphene in batch-made devices. To create an interface sensitive to biomolecular interactions based on the FSS configuration, a metallic floating gate functionalized by conductive mercapto-phenyl molecular linkers is defined on the top-layer solid dielectric. As the solid dielectric layer beneath the metal floating gate enables a higher capacitive gating efficiency than the regular graphene-solution electrical double layer (EDL) interface, the overall transistor amplification gain is further enhanced. As a proof of principle, a label-free DNAzymatic bioassay of Pb²⁺ is conducted. Without the traditional one-by-one device normalization, an excellent concentration detection limit of 929.8 fM is achieved, which is almost 2 orders of magnitude lower than that in existing works. The FSS configuration allows enhanced sensitivity and homogeneity, thereby providing new developmental guidelines for graphene biosensors beyond the laboratory investigation stage. Additionally, it has the potential to be universally applicable for cost-efficient single-device bioassays.

Graphene-based fully integrated portable nanosensing system for on-line detection of cytokine biomarkers in saliva

Saliva has been reported to contain various cytokine biomarkers which are associated with some severe diseases such as cancers. Non-invasive saliva diagnosis using wearable or portable devices may pave a new avenue for monitoring conditions of the high risk population. Here, a graphene-based fully integrated portable nanosensing system, the entire size of which is smaller than a smart-phone and can be handheld, is presented for on-line detection of cytokine biomarkers in saliva. This miniaturized system employs an aptameric graphene-based field effect transistor (GFET) using a buried-gate geometry with HfO₂ as the dielectric layer and on-line signal processing circuits to realize the transduction and processing of signals which reflect cytokine concentrations. The signal can be wirelessly transmitted to a smart-phone or cloud sever through the Wi-Fi connection for visualizing the trend of the cytokine concentration change. Interleukin-6 (IL-6) is used as a representative to examine the sensing capability of the system. Experimental results demonstrate that the nanosensing system responds to the change of IL-6 concentration within 400s in saliva with a detection limit down to 12 pM. Therefore, this portable system offers the practicality to be potentially used for non-invasive saliva diagnosis of diseases at early stage.

Exploiting electrostatic shielding-effect of metal nanoparticles to recognize uncharged small molecule affinity with label-free graphene electronic biosensor

Label-free electronic biosensors as the non-electrochemical analytical tools without requirement of sophisticated instrumentation have become attractive, although their application in competitive affinity sensing of uncharged small molecules is still hindered by a difficulty in the development of competing analogues. To break through this bottleneck, we report a novel analogue made by epitope-modified metal nanoparticles to enable the electronic signaling of small-molecule analyte recognition via competitive affinity. While the electronic signaling capability of metal nanoparticle analogues is demonstrated by a graphene field-effect transistor bioassay of small-molecule glucose as a proof-of-principle, interestingly, we discover a new electronic signaling mechanism in the metal nanoparticle affinity, different to the intuitive charge accumulation expectation. On the basis of Kelvin-probe force microscopic potential characterization and theoretical discussion, we fundamentally elucidated the signaling mechanism as a seldom used electrostatic shielding-effect, that is, in the analogue-receptor affinity, metal nanoparticles with the charge density lower than receptor biomolecules can reduce the collective electrical potential via charge dispersion. Further consider the convenient epitope-modifiability of metal nanoparticles, the easy-to-develop analogues for diverse target analyte might potentially be predictable in the future. And the application of label-free electronic biosensors for the competitive affinity bioassay of range-extended small molecules may thus be promoted based on the electrostatic shielding-effect.

Measurement of cytokine biomarkers using an aptamer-based affinity graphene nanosensor on a flexible substrate toward wearable applications

We present an approach for the label-free detection of cytokine biomarkers using an aptamer-functionalized, graphene-based field effect transistor (GFET) nanosensor on a flexible, SiO2-coated substrate of the polymer polyethylene naphthalate (PEN). The nanosensor conforms to the underlying nonplanar surface and performs GFET-based rapid transduction of the aptamer-biomarker binding, thereby potentially allowing the detection of cytokine biomarkers that are sampled reliably from human bodily fluids (e.g., sweat) in wearable sensing applications. In characterizing the suitability of the nanosensor for wearable applications, we investigate the effects of substrate bending on the equilibrium dissociation constant between the aptamer and the biomarker as well as the graphene transconductance. The utility of the nanosensor is demonstrated by the detection of tumor necrosis factor-α (TNF-α), an inflammatory cytokine biomarker. Experimental results show that the flexible nanosensor can specifically respond to changes in the TNF-α concentration within 5 minutes with a limit of detection as low as 26 pM in a repeatable manner.

Selective detection of water pollutants using a differential aptamer-based graphene biosensor

Graphene field-effect transistor (GFET) sensors are an attractive analytical tool for the detection of water pollutants. Unfortunately, this application has been hindered by the sensitivity of such sensors to nonspecific disturbances caused by variations of environmental conditions. Incorporation of differential designs is a logical choice to address this issue, but this has been difficult for GFET sensors due to the impact of fabrication processes and material properties. This paper presents a differential GFET affinity sensor for the selective detection of water pollutants in the presence of nonspecific disturbances. This differential design allows for minimization of the effects of variations of environmental conditions on the measurement accuracy. In addition, to mitigate the impact of the fabrication process and material property variations, we introduce a compensation scheme for the individual sensing units of the sensor, so that such variations are accounted for in the compensation-based differential sensing method. We test the use of this differential sensor for the selective detection of the water pollutant 17β-estradiol in buffer and tap water. Consistent detection results can be obtained with and without interferences of pH variations, and in tap water where unknown interferences are present. These results demonstrate that the differential graphene affinity sensor is capable of effectively mitigating the effects of nonspecific interferences to enable selective water pollutant detection for water quality monitoring.

Graphene FET Array Biosensor Based on ssDNA Aptamer for Ultrasensitive Hg2+ Detection in Environmental Pollutants

Invisible mercury ion is an extremely poisonous environmental pollutant, therefore, a fast and highly sensitive detection method is of significant importance. In this study, a liquid-gated graphene field-effect transistor (GFET) array biosensor (6 × 6 GFETs on the chip) was fabricated and applied for Hg²⁺ quantitate detection based on single-stranded DNA (ssDNA) aptamer. The biosensor showed outstanding selectivity to Hg²⁺ in mixed solutions containing various metal ions. Moreover, the sensing capability of the biosensor was demonstrated by real-time responses and showed a fairly low detection limit of 40 pM, a wide detection ranged from 100 pM to 100 nM and rapid response time below one second. These results suggest that the GFET array biosensor based on ssDNA aptamer offers a simple fabrication procedure and quite fast method for mercury ion contaminant detection and are promising for various analytical applications.

Highly sensitive solution-gated graphene transistor based sensor for continuous and real-time detection of free chlorine

The concentration of free chlorine used for sterilizing drinking water, recreational water, and food processing water is critical for monitoring potential environmental and human health risks, and should be strictly controlled. Here, we report a highly efficient solution-gated graphene transistor (SGGT) device, for the detection of free chlorine in a real-time and convenient manner with excellent selectivity and high sensitivity. The detection mechanism of the SGGT with Au gate electrode is attributed to two combined effects: the reduction of the free chlorine on Au gate electrode; and the direct oxidization of graphene by the free chlorine in solution. The SGGT device shows a linear response range of free chlorine from 1 μM to 100 μM, with detection limit as low as 100 nM, far beyond the sensitivity required for practical applications. Finally, we also demonstrate the performance of the SGGT for determination of free chlorine in local tap water samples. The results presented herein have important implications in the development of portable and disposable devices based on SGGT sensing platform for the simple, real-time, and selective determination of free chlorine.

Simultaneous detection and determination of mercury (II) and lead (II) ions through the achievement of novel functional nucleic acid-based biosensors

The serious threats of mercury (Hg²⁺) and lead (Pb²⁺) ions for the public health makes it important to achieve the detection methods of the ions with high affinity and specificity. Metal ions usually coexist in some environment and foodstuff or clinical samples. Therefore, it is very necessary to develop a fast and simple method for simultaneous monitoring the amount of metal ions, especially when Hg²⁺ and Pb²⁺ coexist. DNAzyme-based biosensors and aptasensors have been highly regarded for this purpose as two main groups of the functional nucleic acid (FNA)-based biosensors. In this review, we summarize the recent achievements of functional nucleic acid-based biosensors for the simultaneous detection of Hg²⁺ and Pb²⁺ ions in two main optical and electrochemical groups. The tremendous interest in utilizing the various nanomaterials is also highlighted in the fabrication of the FNA-based biosensors. Finally, some results are presented based on the advantages and disadvantages of the studied FNA-based biosensors to compare their validation.

A simple lateral flow biosensor for rapid detection of lead(ii) ions based on G-quadruplex structure-switching

A novel strip biosensor equipped with a colorimetric card shows great promise for in-field Pb²⁺ detection.

Pulse-Driven Capacitive Lead Ion Detection with Reduced Graphene Oxide Field-Effect Transistor Integrated with an Analyzing Device for Rapid Water Quality Monitoring

Rapid and real-time detection of heavy metals in water with a portable microsystem is a growing demand in the field of environmental monitoring, food safety, and future cyber-physical infrastructure. Here, we report a novel ultrasensitive pulse-driven capacitance-based lead ion sensor using self-assembled graphene oxide (GO) monolayer deposition strategy to recognize the heavy metal ions in water. The overall field-effect transistor (FET) structure consists of a thermally reduced graphene oxide (rGO) channel with a thin layer of Al₂O₃ passivation as a top gate combined with sputtered gold nanoparticles that link with the glutathione (GSH) probe to attract Pb²⁺ ions in water. Using a preprogrammed microcontroller, chemo-capacitance based detection of lead ions has been demonstrated with this FET sensor. With a rapid response (∼1-2 s) and negligible signal drift, a limit of detection (LOD) < 1 ppb and excellent selectivity (with a sensitivity to lead ions 1 order of magnitude higher than that of interfering ions) can be achieved for Pb²⁺ measurements. The overall assay time (∼10 s) for background water stabilization followed by lead ion testing and calculation is much shorter than common FET resistance/current measurements (∼minutes) and other conventional methods, such as optical and inductively coupled plasma methods (∼hours). An approximate linear operational range (5-20 ppb) around 15 ppb (the maximum contaminant limit by US Environmental Protection Agency (EPA) for lead in drinking water) makes it especially suitable for drinking water quality monitoring. The validity of the pulse method is confirmed by quantifying Pb²⁺ in various real water samples such as tap, lake, and river water with an accuracy ∼75%. This capacitance measurement strategy is promising and can be readily extended to various FET-based sensor devices for other targets.

Graphene-Based Materials for Biosensors: A Review

The advantages conferred by the physical, optical and electrochemical properties of graphene-based nanomaterials have contributed to the current variety of ultrasensitive and selective biosensor devices. In this review, we present the points of view on the intrinsic properties of graphene and its surface engineering concerned with the transduction mechanisms in biosensing applications. We explain practical synthesis techniques along with prospective properties of the graphene-based materials, which include the pristine graphene and functionalized graphene (i.e., graphene oxide (GO), reduced graphene oxide (RGO) and graphene quantum dot (GQD). The biosensing mechanisms based on the utilization of the charge interactions with biomolecules and/or nanoparticle interactions and sensing platforms are also discussed, and the importance of surface functionalization in recent up-to-date biosensors for biological and medical applications.

Real-Time Monitoring of Insulin Using a Graphene Field-Effect Transistor Aptameric Nanosensor

This paper presents an approach to the real-time, label-free, specific, and sensitive monitoring of insulin using a graphene aptameric nanosensor. The nanosensor is configured as a field-effect transistor, whose graphene-based conducting channel is functionalized with a guanine-rich IGA3 aptamer. The negatively charged aptamer folds into a compact and stable antiparallel or parallel G-quadruplex conformation upon binding with insulin, resulting in a change in the carrier density, and hence the electrical conductance, of the graphene. The change in the electrical conductance is then measured to enable the real-time monitoring of insulin levels. Testing has shown that the nanosensor offers an estimated limit of detection down to 35 pM and is functional in Krebs-Ringer bicarbonate buffer, a standard pancreatic islet perfusion medium. These results demonstrate the potential utility of this approach in label-free monitoring of insulin and in timely prediction of accurate insulin dosage in clinical diagnostics.