Ultrasensitive in situ label-free DNA detection using a GaN nanowire-based extended-gate field-effect-transistor sensor

Extended-Gate FET Biosensor Based on GaN Micropillar Array and Polycrystalline Layer: Application to Hg2+ Detection in Human Urine

Owing to the separation of field-effect transistor (FET) devices from sensing environments, extended-gate FET (EGFET) biosensor features high stability and low cost. Herein, a highly sensitive EGFET biosensor based on a GaN micropillar array and polycrystalline layer (GMP) was fabricated, which was prepared by using simple one-step low-temperature MOCVD growth. In order to improve the sensitivity and detection limit of EGFET biosensor, the surface area and the electrical conductivity of extended-gate electrode can be increased by the micropillar array and the polycrystalline layer, respectively. The designed GMP-EGFET biosensor was modified with l-cysteine and applied for Hg2+ detection with a low limit of detection (LOD) of 1 ng/L, a high sensitivity of -16.3 mV/lg(μg/L) and a wide linear range (1 ng/L-24.5 μg/L). In addition, the detection of Hg2+ in human urine was realized with an LOD of 10 ng/L, which was more than 30 times lower than that of reported sensors. To our knowledge, it is the first time that GMP was used as extended-gate of EGFET biosensor.

Nano-Brush Structure for Rapid Label-Free Differentiation of Alzheimer's Disease Stages and Direct Capture of Neuron-Derived Exosomes from Human Blood Plasma

The measurement of the neurofilament light chain (NFL) in human blood plasma/serum is a promising liquid biopsy for Alzheimer's disease (AD) diagnosis, offering advantages over conventional neuroimaging techniques recommended in clinical guidelines. Here, a controllable nano-brush structure comprising upstanding silicon nanowires coated with indium tin oxide was employed as the sensing substrate. This nano-brush structure was modified with an NFL antibody (NFLAb) via silane coupling and then further connected as the extended gate in a field-effect transistor (EGFET). Notable signal differences emerged within a 2 min timeframe, enabling the label-free differentiation in human blood plasmas among four distinct cohorts: healthy controls, subjective cognitive decline, mild cognitive impairment, and dementia due to AD. Our study indicates that achieving a surface roughness exceeding 400 nm on the modified nano-brush structure enables the effective electrical sensing in our EGFETs. These distinct electrical responses measured via the NFLAb-modified nano-brush EGFETs can be attributed to the combined effects of the captured NFLs and NFL-specific neuron-derived exosomes (NDEs) found in dementia patients, as confirmed by electron spectroscopy for chemical analysis, atomic force microscopy, and scanning electron microscopy. Finally, the potential of quantitatively detecting NDEs on the NFLAb-modified nano-brush structure was demonstrated using spiked solutions containing NFL-specific NDEs from IMR-32 neuroblast cells, wherein concentration-dependent changes were observed in the EGFETs output signal. Our findings show that the NFLAb-modified nano-brush EGFET enables rapid, label-free differentiation between healthy individuals and patients at varying stages of AD.

Detection of L-cysteine in urine samples based on CdS/TiO2-modified extended-gate field-effect transistor photoelectrochemical sensor

A novel extended-gate field-effect transistor (FET) photoelectrochemical (EGFET PEC) sensor was designed for highly sensitive detection of L-cysteine (L-Cys). TiO2 was initially modified on the ITO electrode by the sol-gel dip-coating method and calcined to produce TiO2/ITO. Then, CdS was synthesized on the TiO2 surface by hydrothermal method to obtain the CdS-TiO2 heterojunction material. CdS/TiO2/ITO was connected to the gate of the FET to obtain an EGFET PEC sensor. Under the irradiation of a xenon lamp simulating visible light, the CdS/TiO2 heterojunction composite absorbs light energy to produce photogenerated electron-hole pairs, which have strong photocatalytic oxidation activity and oxidize L-Cys covalently identified by Cd(II) through CdS covalent. These pairs generate a photovoltage that controls the current between the source and the drain to detect L-Cys. Under the optimized experimental conditions, the optical drain current (ID) of the sensor exhibited a good linear relationship with the logarithm of L-Cys in the range of 5.0 × 10-9-1.0 × 10-6 mol/L, and the detection limit was 1.3 × 10-9 mol/L (S/N = 3), which is lower than the values reported by other detection methods. Results showed that the CdS/TiO2/ITO EGFET PEC sensor revealed high sensitivity and good selectivity. The sensor has been used to determine L-Cys in urine samples.

A Brief Review of Graphene-Based Biosensors Developed for Rapid Detection of COVID-19 Biomarkers

The prevalence of mutated species of COVID-19 antigens has provided a strong impetus for identifying a cost-effective, rapid and facile strategy for identifying the viral loads in public places. The ever-changing genetic make-up of SARS-CoV-2 posts a significant challenfge for the research community to identify a robust mechanism to target, bind and confirm the presence of a viral load before it spreads. Synthetic DNA constructs are a novel strategy to design complementary DNA sequences specific for antigens of interest as in this review's case SARS-CoV-2 antigens. Small molecules, complementary DNA and protein-DNA complexes have been known to target analytes in minimal concentrations. This phenomenon can be exploited by nanomaterials which have unique electronic properties such as ballistic conduction. Graphene is one such candidate for designing a device with a very low LOD in the order of zeptomolar and attomolar concentrations. Surface modification will be the significant aspect of the device which needs to have a high degree of sensitivity at the same time as providing a rapid signaling mechanism.

A Supported Lipid Bilayer-Based Lab-on-a-Chip Biosensor for the Rapid Electrical Screening of Coronavirus Drugs

With the rapid spread and multigeneration variation of coronavirus, rapid drug development has become imperative. A major obstacle to addressing this issue is adequately constructing the cell membrane at the molecular level, which enables in vitro observation of the cell response to virus and drug molecules quantitatively, shortening the drug experiment cycle. Herein, we propose a rapid and label-free supported lipid bilayer-based lab-on-a-chip biosensor for the screening of effective inhibition drugs. An extended gate electrode was prepared and functionalized by an angiotensin-converting enzyme II (ACE2) receptor-incorporated supported lipid bilayer (SLB). Such an integrated system can convert the interactions of targets and membrane receptors into real-time charge signals. The platform can simulate the cell membrane microenvironment in vitro and accurately capture the interaction signal between the target and the cell membrane with minimized interference, thus observing the drug action pathway quantitatively and realizing drug screening effectively. Due to these label-free, low-cost, convenient, and integrated advantages, it is a suitable candidate method for the rapid drug screening for the early treatment and prevention of worldwide spread of coronavirus.

Rapid and low-cost, and disposable electrical sensor using an extended gate field-effect transistor for cardiac troponin I detection

Field effect transistor (FET) biosensor is based on metal oxide field effect transistor that is gated by changes in the surface charges induced the reaction of biomolecules. In most cases of FET biosensor, FET biosensor is not being reused after the reaction; therefore, it is an important concept of investigate the biosensor with simplicity, cheap and reusability. However, the conventional cardiac troponin I (cTnI) sensing technique is inadequate owing to its low sensitivity and high operational time and cost. In this study, we developed a rapid and low-cost, and disposable electrical sensor using an extended gate field-effect transistor (EGFET) to detect cTnI, as a key biomarker for myocardiac infarction. We first investigated pH sensing characteristics according to the pH level, which provided a logarithmically linear sensitivity in the pH sensing buffer solution of approximately 57.9 mV/pH. Subsequently, we prepared a cTnI sample and monitored the reaction between cTnI and cTnI antibodies through the changes in the drain current and transfer curves. Our results showed that the EGFET biosensor could successfully detect the cTnI levels as well as the pH with low-cost and rapid detection.

Nanowire-based sensor electronics for chemical and biological applications

Detection and recognition of chemical and biological species via sensor electronics are important not only for various sensing applications but also for fundamental scientific understanding. In the past two decades, sensor devices using one-dimensional (1D) nanowires have emerged as promising and powerful platforms for electrical detection of chemical species and biologically relevant molecules due to their superior sensing performance, long-term stability, and ultra-low power consumption. This paper presents a comprehensive overview of the recent progress and achievements in 1D nanowire synthesis, working principles of nanowire-based sensors, and the applications of nanowire-based sensor electronics in chemical and biological analytes detection and recognition. In addition, some critical issues that hinder the practical applications of 1D nanowire-based sensor electronics, including device reproducibility and selectivity, stability, and power consumption, will be highlighted. Finally, challenges, perspectives, and opportunities for developing advanced and innovative nanowire-based sensor electronics in chemical and biological applications are featured.

A Bio-inspired Extended-Gate Metal-Oxide-Semiconductor Field-Effect-Transistor for Highly Sensitive Amino Acid Enantiodiscrimination

As the most important small molecules revealing the origins of life, amino acids (AAs) play essential roles in living organisms and their facile enantiodiscrimination has long been a great challenge for analytical chemists. Inspired by the specific stereomatching effect between biomolecules and AA enantiomers, herein, we first developed a bio-inspired highly sensitive platform based on an extended-gate metal-oxide-semiconductor field-effect-transistor (EG-MOSFET) for highly sensitive AA enantiodiscrimination. Bovine serum albumin (BSA) was self-assembled on deposited Au surfaces to afford the extended gate (EG) sensing unit, and its enantiorecognition ability was initially verified using common electrochemical techniques. The EG was thereafter installed to a MOSFET to build the desired BSA-EG-MOSFET highly sensitive chiral sensing platform, which realized the efficient enantiodiscrimination of essential AAs with high sensitivity, where effective chiral resolution was achieved at the femtomole level to phenylalanine (Phe). Combining molecular docking and circular dichroism spectroscopy, the weak intermolecular interactions between BSA and AAs enantiomers were investigated and the mechanism for signal amplification was proposed. Our results demonstrate that the as-fabricated biosensor has great potential in highly sensitive chiral sensing fields and can also afford a potential tool for biomolecular interaction investigations.

Rapid and Sensitive Detection of Mycobacterium tuberculosis by an Enhanced Nanobiosensor

Tuberculosis (TB) mostly spreads from person to person through Mycobacterium tuberculosis (MTB). However, the majority of conventional detection methods for MTB cannot satisfy the requirements for actual TB detection. As one of the most promising powerful platforms, a silicon nanowire field-effect transistor (SiNW-FET) biosensor shows good prospect in TB detection. In this study, an enhanced SiNW-FET biosensor was developed for the rapid and sensitive detection of MTB. The surface functional parameters of the biosensor were explored and optimized. The SiNW-FET biosensor has good sensitivity with a detection limit of 0.01 fg/mL toward protein. The current change value shows a linear upward trend with the increase in protein concentration in the range of 1 fg/mL to 100 μg/mL. One whole test cycle can be accomplished within only 30 s. More importantly, a good distinction was realized in the sputum without pretreatment between normal people and TB patients, which greatly shortened the TB detection time (only 2-5 min, considering the dilution of sputum). Compared with other methods, the SiNW-FET biosensor can detect MTB with a remarkably broad dynamic linear range in a shorter time.

Influence of environmental conditions and surface treatments on the photoluminescence properties of GaN nanowires and nanofins

Due to their intrinsically large surface-to-volume ratio, nanowires and nanofins interact strongly with their environment. We investigate the role of the main air constituents nitrogen, oxygen and water on the efficiency of radiative recombination in GaN nanostructures as a function of different surface treatments and at temperatures up to 200 °C. Oxygen and water exposures exhibit a complex behavior as they can both act quenching and enhancing on the photoluminescence intensity dependent on the temperature. For oxygen, these characteristics are already observed for low concentrations of below 0.5% in nitrogen. While the photoluminescence intensity changes induced by oxygen occur independently of illumination, the influence of water is light-induced: it evolves within tens of seconds under ultraviolet light exposure and is heavily influenced by the nanostructure pre-treatment. In contrast to observations in dry atmospheres, water prevents a recovery of the photoluminescence intensity in the dark. Combined measurements of the electrical current through GaN nanofins and their photoluminescence intensity reveal the environmental influence on the interaction of non-radiative recombination processes and changes in the surface band bending of the nanostructures. Several investigated solvents show an enhancing effect on the PL intensity increase, peaking in c-hexane with a 26-fold increase after 6 min of light exposure. Stabilization of the PL intensity was achieved by a passivation of the GaN surface with Ga_xO_y, and ZnO shells. Surprisingly, Al₂O₃coatings resulted in a highly instable PL intensity during the first minutes of illumination. Our findings reveal the high importance of controlled environmental conditions for the investigation of nanostructures, especially when aimed at their applications in the fields of environmental sensing, photo-catalysis and light-emitting diodes.

Nucleic acid amplification-free detection of DNA and RNA at ultralow concentration

The broad spectrum of approaches for nucleic acid amplification-free detection of DNA and RNA at single-digit attomolar (10^-18 M) concentration and lower is reviewed. These low concentrations correspond roughly to the most clinically desirable detection range for pathogen-specific nucleic acid as well as the detection limits of commercially available, nucleic acid amplification tests based primarily on polymerase chain reaction (PCR). The need for more rapid and inexpensive, yet still highly accurate tests, has become evident during the pandemic. It is expected that publication of reports describing improved tests will accelerate soon, and this review covers the wide variety of detection methods based on both optical and electrical measurements that have been conceived over recent years, enabled generally by the advent of nanotechnology.

Fabrication and Characterization of In0.9Ga0.1O EGFET pH Sensors

In this study, the In0.9Ga0.1O sensing membrane were deposited by using the RF magnetron sputtering at room temperature and combined with commercial MOSFETs as the extended gate field effect transistor (EGFET) pH sensors. The sensing performance of the In0.9Ga0.1O EGFET pH sensors were measured and analyzed in the pH value of range between 2 to 12. In the saturation region, the pH current sensitivity calculated from the linear relationship between the IDS and pH value was approximately 56.64 μA/pH corresponding to the linearity of 97.8%. In the linear region, the pH voltage sensitivity exhibited high sensitivity and linearity of 43.7 mV/pH and 96.3%, respectively. The In0.9Ga0.1O EGFET pH sensors were successfully fabricated and exhibited great linearity. The analyzed results indicated that the In0.9Ga0.1O was a robust material as a promising sensing membrane and effectively used for pH sensing detection application.

Single-molecule, hybridization-based strategies for short nucleic acids detection and recognition with nanopores

DNA nanotechnology has seen large developments over the last 30 years through the combination of detection and discovery of DNAs, and solid phase synthesis to increase the chemical functionalities on nucleic acids, leading to the emergence of novel and sophisticated in features, nucleic acids-based biopolymers. Arguably, nanopores developed for fast and direct detection of a large variety of molecules, are part of a revolutionary technological evolution which led to cheaper, smaller and considerably easier to use devices enabling DNA detection and sequencing at the single-molecule level. Through their versatility, the nanopore-based tools proved useful biomedicine, nanoscale chemistry, biology and physics, as well as other disciplines spanning materials science to ecology and anthropology. This mini-review discusses the progress of nanopore- and hybridization-based DNA detection, and explores a range of state-of-the-art applications afforded through the combination of certain synthetically-derived polymers mimicking nucleic acids and nanopores, for the single-molecule biophysics on short DNA structures.

A photoelectrochemical sensor based on an acetylcholinesterase-CdS/ZnO-modified extended-gate field-effect transistor for glyphosate detection

A new photoelectrochemical enzyme biosensor based on an extended-gate field-effect transistor (EGFET) was constructed for the highly sensitive detection of glyphosate based on the inhibition of acetylcholinesterase (AChE) activity by glyphosate. First, a two-step hydrothermal method was used to introduce ZnO and CdS onto an activated indium tin oxide (ITO) electrode to prepare a CdS/ZnO/ITO electrode. Then, AChE was immobilized on CdS/ZnO/ITO with chitosan to obtain an AChE/CdS/ZnO EGFET sensor. Under optimal experimental conditions, the logarithmic value of glyphosate in the range of 1.0 × 10-15-1.0 × 10-11 mol L-1 exhibited a good linear relationship with the photo-drain current response. The detection limit was 3.8 × 10-16 mol L-1 (signal-to-noise ratio = 3). The results show that the AChE/CdS/ZnO EGFET sensor has extremely high sensitivity and good selectivity. Moreover, the sensor was used for the determination of glyphosate in vegetables, demonstrating its application for the real-time detection of samples.

Construction and Application of Graphene Oxide-Bovine Serum Albumin Modified Extended Gate Field Effect Transistor Chiral Sensor

Chirality is an essential natural attribute of organisms. Chiral molecules exhibit differences in biochemical processes, pharmacodynamics, and toxicological properties, and their enantioselective recognition plays an important role in explaining life science processes and guiding drug design. Herein, we developed an ultra-sensitive enantiomer recognition platform based on an extended-gate metal-oxide semiconductor field-effect-transistor (Nafion–GO@BSA–EG-MOSFET) that achieved effective chiral resolution of ultra-sensitive Lysine (Lys) and α-Methylbenzylamine (α-Met) enantiodiscrimination at the femtomole level. Bovine serum albumin (BSA) was immobilized on the surface of graphene oxide (GO) through amide bond coupling to prepare the GO@BSA complex. GO@BSA was drop-cast on deposited Au surfaces with a Nafion solution to afford the extended-gate sensing unit. Effective recognition of chiral enantiomers of mandelic acid (MA), tartaric acid (TA), tryptophan (Trp), Lys and α-Met was realized. Moreover, the introduction of GO reduced non-specific adsorption, and the chiral resolution concentration of α-Met reached the level of picomole in a 5-fold diluted fetal bovine serum (FBS). Finally, the chiral recognition mechanism of the as-fabricated sensor was proposed.

Piezocatalytic Effect Induced Hydrogen Production from Water over Non-noble Metal Ni Deposited Ultralong GaN Nanowires

Piezoelectric material-based catalysis that relies on an external stress-induced piezopotential has been demonstrated to be an effective strategy toward various chemical reactions. In this work, non-noble metal Ni-decorated ultralong monocrystal GaN nanowires (NWs) were prepared through a chemical vapor deposition (CVD) technique, followed by a photodeposition method. The piezocatalytic activity of the GaN NWs was enhanced by ∼9 times after depositing the Ni cocatalyst, generating hydrogen gas of ∼88.3 μmol·g^-1·h^-1 under ultrasonic vibration (110 W and 40 kHz), which is comparable to that of Pt-loaded GaN NWs. Moreover, Ni/GaN NWs with smaller diameters (∼100 nm) demonstrated superior piezocatalytic efficiency, which can be attributed to the large piezoelectric potential evidenced by both finite-element analysis and piezoresponse force microscopy measurements. These results demonstrate the promising application potential of non-noble metal loaded GaN nanostructures in hydrogen generation driven by weak mechanical energy from the surrounding environment.

Synthesis of highly crystalline black phosphorus thin films on GaN

Black phosphorus (BP) has recently garnered significant attention due to its specific physical properties. At present, high-quality few-layer and thin-film BP is obtained principally by mechanical exfoliation, restricting its device applications in the future. Here, a facile, direct synthesis of highly crystalline thin-film BP on GaN(001) substrates is achieved by conversion of red phosphorus to BP under atmospheric pressure. The synthesized ≈100-500 nm thick BP thin films with a length ranging from 4 to 15 μm can maintain long-term stability with no sign of oxidation after 5 months of exposure to ambient conditions, as indicated by energy dispersive spectroscopy (EDS). Cross-sectional spherical aberration correction transmission electron microscopy (STEM) analysis of the entire thin-film BP sample did not show any aggregation nucleation through the selected sample. The interface of the BP/GaN heterostructure is atomically sharp, which is very critical for high-performance device fabrication using a direct step in the future. And it is worth noting that there are fluctuations of a few atoms on the surface of GaN. Moreover, using first-principles approaches, here we establish a novel kinetic pathway for fabricating thin-film BP via epitaxial growth. The step of fluctuations with a few atoms on the GaN surface are first preferentially covered by P adatoms, then P adatoms cover the remaining part. Once formed, such a structure of thin-film BP is stable, as tested using EDS and STEM. Combining the results of the experiment and simulation, it can be revealed that the P adatom on undulatory GaN is sufficiently mobile and the undulating surface of GaN plays a major role in forming high-quality thin-films of BP. The preferentially covered nearby step growth mechanism discovered here may enable the mass production of high-quality thin-film BP, and could also be instrumental in achieving the epitaxial growth of thin-film BP on GaN and other 2D materials.

Detecting DNA and RNA and Differentiating Single-Nucleotide Variations via Field-Effect Transistors

We detect short oligonucleotides and distinguish between sequences that differ by a single base, using label-free, electronic field-effect transistors (FETs). Our sensing platform utilizes ultrathin-film indium oxide FETs chemically functionalized with single-stranded DNA (ssDNA). The ssDNA-functionalized semiconducting channels in FETs detect fully complementary DNA sequences and differentiate these sequences from those having different types and locations of single base-pair mismatches. Changes in charge associated with surface-bound ssDNA vs double-stranded DNA (dsDNA) alter FET channel conductance to enable detection due to differences in DNA duplex stability. We illustrate the capability of ssDNA-FETs to detect complementary RNA sequences and to distinguish from RNA sequences with single nucleotide variations. The development and implementation of electronic biosensors that rapidly and sensitively detect and differentiate oligonucleotides present new opportunities in the fields of disease diagnostics and precision medicine.

Enhancement of cortisol measurement sensitivity by laser illumination for AlGaN/GaN transistor biosensor

In this study, high electron mobility transistor (HEMT) device was used as an immuno biosensor to measure concentration of a stress hormone, cortisol, by using selective binding on cortisol monoclonal antibody (c-Mab). Also, the HEMT sensor was enhanced in its sensitivity through light illumination to generate photocurrent. The optical pumping could assist the biosensor to discriminate more detailed change, which could result in an increment of limit of detection (LOD) to 1.0 pM cortisol level. It was the lowest level of detection with semiconductor device-based cortisol biosensors and the enhancement of surface potential sensitivity was induced by laser light (532 nm). Output current amplificated by photocurrent was higher than dark original current at about 3.39% when gate voltage is applied with -3 V. Since the device could be applied to not only standard cortisol solution but also real human salivary sample, it is expected to apply for in vitro direct diagnosis of point-of-care test (POCT).

Aptamer-Based Field-Effect Transistor for Detection of Avian Influenza Virus in Chicken Serum

Early diagnosis of the highly pathogenic H5N1 avian influenza virus (AIV) is significant for preventing and controlling a global pandemic. However, there is no existing electrical biosensor for detecting biomarkers for AIV in clinically relevant samples such as chicken serum. Herein, we report the first use of an aptamer-functionalized field-effect transistor (FET) as a label-free sensor for AIV detection in chicken serum. A DNA aptamer is employed as a sensitive and selective receptor for hemagglutinin (HA) protein, which is a biomarker for AIVs. This aptamer is immobilized on a gold microelectrode that is connected to the gate of a reusable FET transducer. The specific binding of the target protein results in a change in the surface potential, which generates a signal response of the FET transducer. We hypothesize that a conformational change in the DNA aptamer upon specific binding of HA protein may alter the surface potential. The signal of the aptamer-based FET biosensor increased linearly with the increase in the logarithm of HA protein concentration in a dynamic range of 10 pM to 10 nM with a detection limit of 5.9 pM. The selectivity of the biosensor for HA protein was confirmed by employing relevant interfering proteins. The proposed biosensor was successfully applied to the selective detection of HA protein in a chicken serum sample. Owing to its simple and low-cost architecture, portability, and sensitivity, the aptamer-based FET biosensor has potential as a point-of-care diagnosis of H5N1 AIVs in clinical samples.

An electronic enzyme-linked immunosorbent assay platform for protein analysis based on magnetic beads and AlGaN/GaN high electron mobility transistors

The AlGaN/GaN high electron mobility transistor (HEMT) biosensors have the characteristics of high sensitivity, stability and fast response in the detection of biomolecules.

Microfluidic opportunities in printed electrolyte-gated transistor biosensors

Printed electrolyte-gated transistors (EGTs) are an emerging biosensor platform that leverage the facile fabrication engendered by printed electronics with the low voltage operation enabled by ion gel dielectrics. The resulting label-free, nonoptical sensors have high gain and provide sensing operations that can be challenging for conventional chemical field effect transistor architectures. After providing an overview of EGT device fabrication and operation, we highlight opportunities for microfluidic enhancement of EGT sensor performance via multiplexing, sample preconcentration, and improved transport to the sensor surface.

Establishing a Field-Effect Transistor Sensor for the Detection of Mutations in the Tumour Protein 53 Gene (TP53)-An Electrochemical Optimisation Approach

We present a low-cost, sensitive and specific DNA field-effect transistor sensor for the rapid detection of a common mutation to the tumour protein 53 gene (TP53). The sensor consists of a commercially available, low-cost, field-effect transistor attached in series to a gold electrode sensing pad for DNA hybridisation. The sensor has been predominantly optimised electrochemically, particularly with respect to open-circuit potentiometry as a route towards understanding potential (voltage) changes upon DNA hybridisation using a transistor. The developed sensor responds sensitively to TP53 mutant DNA as low as 100 nM concentration. The sensor responds linearly as a function of DNA target concentration and is able to differentiate between complementary and noncomplementary DNA target sequences.

New Disposable Nitric Oxide Sensor Fabrication Using GaN Nanowires

Gallium nitride (GaN) nanowires anchored on the surface of cost-effective pencil graphite electrodes (PGEs) have been developed as a new disposable nitric oxide (NO) sensor through a hydrothermal method followed by annealing treatment. The as-obtained nanomaterials were examined by field emission scanning electron microscopy, high-resolution transmission electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and EIS. Concurrently, the electrocatalytic performance has been analyzed using cyclic voltammetry and amperometric measurements. The experimental results exhibit good electrochemical sensing performance toward the generated NO in NO2 - with a wide linear detection range of 1.0 μM to 1.0 mM with a correlation coefficient of 0.999 and a detection limit of 0.180 μM. In addition, the GaN nanowire-modified PGE surface showed high selectivity for the detection of NO as compared to other relevant biomolecules. This confirms that the PGE/GaN nanowire is a new promising electrochemical sensor for the sensitive detection of NO.

Rapid, label-free genetic detection of enteropathogens in stool without genetic isolation or amplification

Current genetic detection methods require gene isolation, gene amplification and detection with a fluorescent-tagged probe. They typically require sophisticated equipment and expensive fluorescent probes, rendering them not widely available for rapid acute infection diagnoses at the point of care to ensure timely treatment of the diseases. Here we report a rapid genetic detection method that can detect the bacterial gene directly from patient stools using a piezoelectric plate sensor (PEPS) in conjunction with a continuous flow system with two temperature zones. With stools spiked with sodium dodecyl sulfate (SDS) in situ bacteria lysing and DNA denaturation occurred in the high-temperature zone whereas in situ specific detection of the denatured DNA by the PEPS occurred in the lower-temperature zone. The outcome was a rapid genetic detection method that directly detected bacterial genes from stool in < 40 min without the need of gene isolation, gene amplification, or expensive fluorescent tag but with polymerase chain reaction (PCR) sensitivity. In 40 blinded patient stools, it detected the toxin B gene of Clostridium difficile with 95% sensitivity and 95% specificity. The all-electrical, label-free nature of the detection further supports its potential as a low-cost genetic test that can be used at the point of care.

Deep Submicron EGFET Based on Transistor Association Technique for Chemical Sensing

Extended-gate field-effect transistor (EGFET) is an electronic interface originally developed as a substitute for an ion-sensitive field-effect transistor (ISFET). Although the literature shows that commercial off-the-shelf components are widely used for biosensor fabrication, studies on electronic interfaces are still scarce (e.g., noise processes, scaling). Therefore, the incorporation of a custom EGFET can lead to biosensors with optimized performance. In this paper, the design and characterization of a transistor association (TA)-based EGFET was investigated. Prototypes were manufactured using a 130 nm standard complementary metal-oxide semiconductor (CMOS) process and compared with devices presented in recent literature. A DC equivalence with the counterpart involving a single equivalent transistor was observed. Experimental results showed a power consumption of 24.99 mW at 1.2 V supply voltage with a minimum die area of 0.685 × 1.2 mm². The higher aspect ratio devices required a proportionally increased die area and power consumption. Conversely, the input-referred noise showed an opposite trend with a minimum of 176.4 nV_rms over the 0.1 to 10 Hz frequency band for a higher aspect ratio. EGFET as a pH sensor presented further validation of the design with an average voltage sensitivity of 50.3 mV/pH, a maximum current sensitivity of 15.71 mA^1/2/pH, a linearity higher than 99.9%, and the possibility of operating at a lower noise level with a compact design and a low complexity.

A highly sensitive label-free electrochemical immunosensor based on an aligned GaN nanowires array/polydopamine heterointerface modified with Au nanoparticles

Aligned GaN nanowire arrays show great potential not only in optoelectronic devices, but also in sensitive biosensor applications, owing to their excellent chemical stability and biocompatibility, as well as high electron mobility and surface-to-volume ratio. However, to construct electrochemical immunosensors, proper surface modification of GaN nanowires, which can enable efficient charge transfer and provide large densities of immobilization sites for antibodies to anchor, is still challenging. Herein we demonstrate a highly sensitive label-free electrochemical immunosensing platform based on the integration of polydopamine (PDA) on a GaN nanowire surface. The PDA polymer was self-assembled on GaN nanowire surfaces via organic polymerization. The interface dipole layer generated at the GaN nanowire array/PDA polymer heterointerface enabled efficient charge transfer. The aligned GaN nanowire array/PDA hybrids were further modified with gold nanoparticles for subsequent covalent binding of antibodies. The fabricated immunosensor yielded a wide linear range between 0.01 and 100 ng ml^-1 and a detection limit as low as 0.003 ng ml^-1 for the detection of alpha-fetoprotein (AFP). The immunosensor showed good selectivity, reproducibility, and stability and was utilized in human serum samples for AFP detection. This work demonstrates the superiority of taking advantage of a nanowire array configuration and a semiconductor/polymer heterointerface in an immunosensing platform for sensitivity enhancement.

EGFET-Based Sensors for Bioanalytical Applications: A Review

Since the 1970s, a great deal of attention has been paid to the development of semiconductor-based biosensors because of the numerous advantages they offer, including high sensitivity, faster response time, miniaturization, and low-cost manufacturing for quick biospecific analysis with reusable features. Commercial biosensors have become highly desirable in the fields of medicine, food, and environmental monitoring as well as military applications, whereas increasing concerns about food safety and health issues have resulted in the introduction of novel legislative standards for these sensors. Numerous devices have been developed for monitoring biological processes such as nucleic acid hybridization, protein–protein interaction, antigen–antibody bonds, and substrate–enzyme reactions, just to name a few. Since the 1980s, scientific interest moved to the development of semiconductor-based devices, which also include integrated front-end electronics, such as the extended-gate field-effect transistor (EGFET) biosensor, one of the first miniaturized chemical sensors. This work is intended to be a review of the state of the art focused on the development of biosensors and chemosensors based on extended-gate field-effect transistor within the field of bioanalytical applications, which will highlight the most recent research reported in the literature. Moreover, a comparison among the diverse EGFET devices will be presented, giving particular attention to the materials and technologies.

In situ, amplification-free double-stranded mutation detection at 60 copies/ml with thousand-fold wild type in urine

We have investigated amplification-free in situ double-stranded mutation detection in urine in the concentration range 10^-19 M - 10^-16 M using piezoelectric plate sensors (PEPs). The detection was carried out in a close-loop flow with two temperature zones. The 95 °C high-temperature zone served as the reservoir where the sample was loaded and DNA de-hybridized. The heated urine was cooled flowing through a 1 m long tubing immersed in room-temperature water bath at a flow rate of 4 ml/min to reach the detection cell at the desired temperature for the detection to take place. With hepatitis B virus double mutation (HBVDM) and KRAS G12V point mutation as model double mutations, it is shown that PEPS was able to detect double-stranded HBVDM and KRAS with 70% detection efficiency or better at concentration as low as 10^-19 M against single-stranded mutation detection at the same concentrations, which was validated by the following in situ fluorescent reporter microspheres (FRMs) detection as well as microscopic visualization of the FRMs bound to the captured mutant on the PEPS surface. Furthermore, the same double-stranded mutation detection efficacy was demonstrated at 10^-19 M - 10^-16 M in a background of 250-fold wildtype for HBVDM and 1000-fold wildtype for KRAS. Also demonstrated was detection of KRAS mutation at 10^-19 M - 10^-16 M of SW480 DNA fragments in urine.

Amplification-free, sequence-specific 16S rRNA detection at 1 aM

A nucleic acid amplification-free, optics-free platform has been demonstrated for sequence-specific detection of Escherichia coli (E. coli) 16S rRNA at 1 aM (10-18 M) against a 106-fold (1 pM) background of Pseudomonas putida (P. putida) RNA. This work was driven by the need for simple, rapid, and low cost means for species-specific bacterial detection at low concentration. Our simple, conductometric sensing device functioned by detecting blockage of a nanopore fabricated in a sub-micron-thick glass membrane. Upon sequence-specific binding of target 16S rRNA, otherwise charge-neutral, PNA oligonucleotide probe-polystyrene bead conjugates become electrophoretically mobile and are driven to the glass nanopore of lesser diameter, which is blocked, thereby generating a large, sustained and readily observable step decrease in ionic current. No false positive signals were observed with P. putida RNA when this device was configured to detect E. coli 16S rRNA. Also, when a universal PNA probe complementary to the 16S rRNA of both E. coli and P. putida was conjugated to beads, a positive response to rRNA of both bacterial species was observed. Finally, the device readily detected E. coli at 10 CFU mL-1 in a 1 mL sample, also against a million-fold background of viable P. putida. These results suggest that this new device may serve as the basis for small, portable, low power, and low-cost systems for rapid detection of specific bacterial species in clinical samples, food, and water.

Insufficiency of the Young's modulus for illustrating the mechanical behavior of GaN nanowires

We use a non-classical modified couple stress theory including the acceleration gradients (MCST-AG), to precisely demonstrate the size dependency of the mechanical properties of gallium nitride (GaN) nanowires (NWs). The fundamental elastic constants, Young's modulus and length scales of the GaN NWs were estimated both experimentally, using a novel experimental technique applied to atomic force microscopy, and theoretically, using atomic simulations. The Young's modulus, static and the dynamic length scales, calculated with the MCST-AG, were found to be 323 GPa, 13 and 14.5 nm, respectively, for GaN NWs from a few nanometers radii to bulk radii. Analyzing the experimental data using the classical continuum theory shows an improvement in the experimental results by introducing smaller error. Using the length scales determined in MCST-AG, we explain the inconsistency of the Young's moduli reported in recent literature, and we prove the insufficiency of the Young's modulus for predicting the mechanical behavior of GaN NWs.

Molecular recognition by synthetic receptors: Application in field-effect transistor based chemosensing

Molecular recognition, i.e., ability of one molecule to recognize another through weak bonding interactions, is one of the bases of life. It is often implemented to sensing systems of high merits. Preferential recognition of the analyte (guest) by the receptor (host) induces changes in physicochemical properties of the sensing system. These changes are measured by using suitable signal transducers. Because of possibility of miniaturization, fast response, and high sensitivity, field-effect transistors (FETs) are more frequently being used for that purpose. A FET combined with a biological material offers the potential to overcome many challenges approached in sensing. However, low stability of biological materials under measurement conditions is a serious problem. To circumvent this problem, synthetic receptors were integrated with the gate surface of FETs to provide robust performance. In the present critical review, the approach utilized to devise chemosensors integrating synthetic receptors and FET transduction is discussed in detail. The progress in this field was summarized and important outcome was provided.

Recent advances in nanowires-based field-effect transistors for biological sensor applications

Nanowires (NWs)-based field-effect transistors (FETs) have attracted considerable interest to develop innovative biosensors using NWs of different materials (i.e. semiconductors, polymers, etc.). NWs-based FETs provide significant advantages over the other bulk or non-NWs nanomaterials-based FETs. As the building blocks for FET-based biosensors, one-dimensional NWs offer excellent surface-to-volume ratio and are more suitable and sensitive for sensing applications. During the past decade, FET-based biosensors are smartly designed and used due to their great specificity, sensitivity, and high selectivity. Additionally, they have the advantage of low weight, low cost of mass production, small size and compatible with commercial planar processes for large-scale circuitry. In this respect, we summarize the recent advances of NWs-based FET biosensors for different biomolecule detection i.e. glucose, cholesterol, uric acid, urea, hormone, proteins, nucleotide, biomarkers, etc. A comparative sensing performance, present challenges, and future prospects of NWs-based FET biosensors are discussed in detail.

Crystal engineering by tuning the growth kinetics of GaN 3-D microstructures in SAG-HVPE

The growth of GaN 3-D microstructures is investigated by SAG-HVPE.

Beyond the Debye length in high ionic strength solution: direct protein detection with field-effect transistors (FETs) in human serum

In this study, a new type of field-effect transistor (FET)-based biosensor is demonstrated to be able to overcome the problem of severe charge-screening effect caused by high ionic strength in solution and detect proteins in physiological environment. Antibody or aptamer-immobilized AlGaN/GaN high electron mobility transistors (HEMTs) are used to directly detect proteins, including HIV-1 RT, CEA, NT-proBNP and CRP, in 1X PBS (with 1%BSA) or human sera. The samples do not need any dilution or washing process to reduce the ionic strength. The sensor shows high sensitivity and the detection takes only 5 minutes. The designs of the sensor, the methodology of the measurement, and the working mechanism of the sensor are discussed and investigated. A theoretical model is proposed based on the finding of the experiments. This sensor is promising for point-of-care, home healthcare, and mobile diagnostic device.

Group III nitride nanomaterials for biosensing

Biosensing has found wide applications in biological and medical research, and in clinical diagnosis, environmental monitoring and other analytical tasks. Recognized as novel and outstanding transducing materials because of their superior and unique physical/chemical properties, group III nitride (III-nitride) nanomaterials have been introduced into biosensor development with remarkable advancements achieved in the past few decades. This paper presents the first comprehensive review on biosensor development with III-nitride nanomaterials. The review starts with the introduction of the material properties and biocompatibility of III-nitrides that are useful for biosensing. The focus is then placed on surface treatments of III-nitrides, which lay the foundation for biosensing, and on biosensing mechanisms where the exceptional properties of III-nitride nanomaterials lead to superior biosensing performance. From a practical point of view, techniques for biosensor fabrication are then summarized. Finally, existing biosensing applications and future directions are discussed.

Highly sensitive extended gate-AlGaN/GaN high electron mobility transistor for bioassay applications

An extended gate-AlGaN/GaN high electron mobility transistor (EG-AlGaN/GaN HEMT) with a high sensitivity for bioassay has been developed.

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.

Electrochemical DNA biosensors based on long-range electron transfer: investigating the efficiency of a fluidic channel microelectrode compared to an ultramicroelectrode in a two-electrode setup

Here, we describe the transposition of an ultramicroelectrode (UME) setup into a microfluidic chip configuration for DNA biosensors. The hydrodynamic properties of the fluidic channel microelectrode were screened with an [Fe(iii)(CN)₆]^3-/[Fe(ii)(CN)₆]^4- redox couple by cyclic voltammetry to provide a basis for further biological processes. A 23-base DNA probe was self-assembled into a monolayer on gold microelectrodes both in classical configuration and integrated in a microfluidic setup. Special interest was focused on the DNA target mimicking the liver-specific micro-ribonucleic acid 122 (miRNA122). Long-range electron transfer was chosen for transducing the hybridization. This direct transduction was indeed significantly enhanced after hybridization due to DNA-duplex π-stacking and the use of redox methylene blue as a DNA intercalator. Quantification of the target was deduced from the resulting electrical signal characterized by cyclic voltammetry. The limit of detection for DNA hybridization was 0.1 fM in stopped flow experiments, where it can reach 1 aM over a 0.5 μL s^-1 flow rate, a value 10⁴-fold lower than the one measured with a conventional UME dipped into an electrolyte droplet under the same analytical conditions. An explanation was that forced convection drives more biomolecules to the area of detection even if a balance between the speed of collection and the number of biomolecules collected has been found. The latter point is discussed here along with an attempt to explain why the sensor has reached such an unexpected value for the limit of detection.

A silicon carbide nanowire field effect transistor for DNA detection

This work reports on the label-free electrical detection of DNA molecules for the first time, using silicon carbide (SiC) as a novel material for the realization of nanowire field effect transistors (NWFETs). SiC is a promising semiconductor for this application due to its specific characteristics such as chemical inertness and biocompatibility. Non-intentionally n-doped SiC NWs are first grown using a bottom-up vapor-liquid-solid (VLS) mechanism, leading to the NWs exhibiting needle-shaped morphology, with a length of approximately 2 μm and a diameter ranging from 25 to 60 nm. Then, the SiC NWFETs are fabricated and functionalized with DNA molecule probes via covalent coupling using an amino-terminated organosilane. The drain current versus drain voltage (I d-V d) characteristics obtained after the DNA grafting and hybridization are reported from the comparative and simultaneous measurements carried out on the SiC NWFETs, used either as sensors or references. As a representative result, the current of the sensor is lowered by 22% after probe DNA grafting and by 7% after target DNA hybridization, while the current of the reference does not vary by more than ±0.6%. The current decrease confirms the field effect induced by the negative charges of the DNA molecules. Moreover, the selectivity, reproducibility, reversibility and stability of the studied devices are emphasized by de-hybridization, non-complementary hybridization and re-hybridization experiments. This first proof of concept opens the way for future developments using SiC-NW-based sensors.

Development of an electrochemical surface-enhanced Raman spectroscopy (EC-SERS) aptasensor for direct detection of DNA hybridization

Rapid detection of disease biomarkers at the patient point-of-care is essential to timely and effective treatment. The research described herein focuses on the development of an electrochemical surface-enhanced Raman spectroscopy (EC-SERS) DNA aptasensor capable of direct detection of tuberculosis (TB) DNA. Specifically, a plausible DNA biomarker present in TB patient urine was chosen as the model target for detection. Cost-effective screen printed electrodes (SPEs) modified with silver nanoparticles (AgNP) were used as the aptasensor platform, onto which the aptamer specific for the target DNA was immobilized. Direct detection of the target DNA was demonstrated through the appearance of SERS peaks characteristic for adenine, present only in the target strand. Modulation of the applied potential allowed for a sizeable increase in the observed SERS response and the use of thiol back-filling prevented non-specific adsorption of non-target DNA. To our knowledge, this work represents the first EC-SERS study of an aptasensor for the direct, label-free detection of DNA hybridization. Such a technology paves the way for rapid detection of disease biomarkers at the patient point-of-care.