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

Development of a glycated albumin sensor employing dual aptamer-based extended gate field effect transistors

Glycated albumin (GA), defined as the percentage of serum albumin glycation, is a mid-term glycemic control marker for diabetes. The concentrations of both glycated human serum albumin (GHSA) and total human serum albumin (HSA) are required to calculate GA. Here, we report the development of a GA sensor employing two albumin aptamers: anti-GHSA aptamer which is specific to GHSA and anti-HSA aptamer which recognizes both glycated and non-glycated HSA. We combine these aptamers with extended gate field effect transistors (EGFETs) to realize GA monitoring without the need to pretreat serum samples, and therefore suitable for point of care and home-testing applications. Using anti-GHSA aptamer-immobilized electrodes and EGFETs, we measured GHSA concentrations between 0.1-10 μM within 20 min. The sensor was able to measure GHSA concentration in the presence of BSA for a range of known GA levels (5-29%). With anti-HSA aptamer-immobilized electrodes and EGFETs, we measured total HSA concentrations from 1-17 μM. Furthermore, GHSA and total HSA concentrations of both healthy and diabetic-level samples were determined with GHSA and HSA sensors. The measured GHSA and total HSA concentrations in three samples were used to determine respective GA percentages, and our calculations agreed with GA levels determined by reference methods. Thus, we developed simple and rapid dual aptamer-based EGFET sensors to monitor GA through measuring GHSA and total HSA concentration, without the need for sample pretreatment, a mandatory step in the current standard of enzymatic GA monitoring.

Surface Modification of Screen-Printed Carbon Electrode through Oxygen Plasma to Enhance Biosensor Sensitivity

The screen-printed carbon electrode (SPCE) is a useful technology that has been widely used in the practical application of biosensors oriented to point-of-care testing (POCT) due to its characteristics of cost-effectiveness, disposability, miniaturization, wide potential window, and simple electrode design. Compared with gold or platinum electrodes, surface modification is difficult because the carbon surface is chemically or physically stable. Oxygen plasma (O2) can easily produce carboxyl groups on the carbon surface, which act as scaffolds for covalent bonds. However, the effect of O2-plasma treatment on electrode performance remains to be investigated from an electrochemical perspective, and sensor performance can be improved by clarifying the surface conditions of plasma-treated biosensors. In this research, we compared antibody modification by plasma treatment and physical adsorption, using our novel immunosensor based on gold nanoparticles (AuNPs). Consequently, the O2-plasma treatment produced carboxyl groups on the electrode surface that changed the electrochemical properties owing to electrostatic interactions. In this study, we compared the following four cases of SPCE modification: O2-plasma-treated electrode/covalent-bonded antibody (a); O2-plasma-treated electrode/physical adsorbed antibody (b); bare electrode/covalent-bonded antibody (c); and bare electrode/physical absorbed antibody (d). The limits of detection (LOD) were 0.50 ng/mL (a), 9.7 ng/mL (b), 0.54 ng/mL (c), and 1.2 ng/mL (d). The slopes of the linear response range were 0.039, 0.029, 0.014, and 0.022. The LOD of (a) was 2.4 times higher than the conventional condition (d), The slope of (a) showed higher sensitivity than other cases (b~d). This is because the plasma treatment generated many carboxyl groups and increased the number of antibody adsorption sites. In summary, the O2-plasma treatment was found to modify the electrode surface conditions and improve the amount of antibody modifications. In the future, O2-plasma treatment could be used as a simple method for modifying various molecular recognition elements on printed carbon electrodes.

Electrical biosensing system utilizing ion-producing enzymes conjugated with aptamers for the sensing of severe acute respiratory syndrome coronavirus 2

Viral outbreaks, which include the ongoing coronavirus disease 2019 (COVID-19) pandemic provoked by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), are a major global crisis that enormously threaten human health and social activities worldwide. Consequently, the rapid and repeated treatment and isolation of these viruses to control their spread are crucial to address the COVID-19 pandemic and future epidemics of novel emerging viruses. The application of cost-efficient, rapid, and easy-to-operate detection devices with miniaturized footprints as a substitute for the conventional optic-based polymerase chain reaction (PCR) and immunoassay tests is critical. In this context, semiconductor-based electrical biosensors are attractive sensing platforms for signal readout. Therefore, this study aimed to examine the electrical sensing of patient-derived SARS-CoV-2 samples by harnessing the activity of DNA aptamers directed against spike proteins on viral surfaces. We obtained rapid and sensitive virus detection beyond the Debye length limitation by exploiting aptamers coupled with alkaline phosphatases, which catalytically generate free hydrogen ions which can readily be measured on pH meters or ion-sensitive field-effect transistors. Furthermore, we demonstrated the detection of the viruses of approximately 100 copies/μL in 10 min, surpassing the capability of typical immunochromatographic assays. Therefore, our newly developed technology has great potential for point-of-care testing not only for SARS-CoV-2, but also for other types of pathogens and biomolecules.

Biological and technical challenges for implementation of yeast-based biosensors

Biosensors are low-cost and low-maintenance alternatives to conventional analytical techniques for biomedical, industrial and environmental applications. Biosensors based on whole microorganisms can be genetically engineered to attain high sensitivity and specificity for the detection of selected analytes. While bacteria-based biosensors have been extensively reported, there is a recent interest in yeast-based biosensors, combining the microbial with the eukaryotic advantages, including possession of specific receptors, stability and high robustness. Here, we describe recently reported yeast-based biosensors highlighting their biological and technical features together with their status of development, that is, laboratory or prototype. Notably, most yeast-based biosensors are still in the early developmental stage, with only a few prototypes tested for real applications. Open challenges, including systematic use of advanced molecular and biotechnological tools, bioprospecting, and implementation of yeast-based biosensors in electrochemical setup, are discussed to find possible solutions for overcoming bottlenecks and promote real-world application of yeast-based biosensors.

A Bioinspired Artificial Gustatory Neuron for a Neuromorphic Based Electronic Tongue

A novel biomimicked neuromorphic sensor for an energy efficient and highly scalable electronic tongue (E-tongue) is demonstrated with a metal-oxide-semiconductor field-effect transistor (MOSFET). By mimicking a biological gustatory neuron, the proposed E-tongue can simultaneously detect ion concentrations of chemicals on an extended gate and encode spike signals on the MOSFET, which acts as an input neuron in a spiking neural network (SNN). Such in-sensor neuromorphic functioning can reduce the energy and area consumption of the conventional E-tongue hardware. pH-sensitive and sodium-sensitive artificial gustatory neurons are implemented by using two different sensing materials: Al₂O₃ for pH sensing and sodium ionophore X for sodium ion sensing. In addition, a sensitivity control function inspired by the biological sensory neuron is demonstrated. After the unit device characterization of the artificial gustatory neuron, a fully hardware-based E-tongue that can classify two distinct liquids is demonstrated to show a practical application of the artificial gustatory neurons.

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.

Modeling of Quasi-Static Floating-Gate Transistor Biosensors

Floating-gate transistors (FGTs) are a promising class of electronic sensing architectures that separate the transduction elements from molecular sensing components, but the factors leading to optimum device design are unknown. We developed a model, generalizable to many different semiconductor/dielectric materials and channel dimensions, to predict the sensor response to changes in capacitance and/or charge at the sensing surface upon target binding or other changes in surface chemistry. The model predictions were compared to experimental data obtained using a floating-gate (extended gate) electrochemical transistor, a variant of the generic FGT architecture that facilitates low-voltage operation and rapid, simple fabrication using printing. Self-assembled monolayer (SAM) chemistry and quasi-statically measured resistor-loaded inverters were utilized to obtain experimentally either the capacitance signals (with alkylthiol SAMs) or charge signals (with acid-terminated SAMs) of the FGT. Experiments reveal that the model captures the inverter gain and charge signals over 3 orders of magnitude variation in the size of the sensing area and the capacitance signals over 2 orders of magnitude but deviates from experiments at lower capacitances of the sensing surface (<1 nF). To guide future device design, model predictions for a large range of sensing area capacitances and characteristic voltages are provided, enabling the calculation of the optimum sensing area size for maximum charge and capacitance sensitivity.

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

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

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.

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.

Nanoscale FET-Based Transduction toward Sensitive Extended-Gate Biosensors

Owing to their simple and low-cost architecture, extended-gate biosensors based on the combination of a disposable sensing part and a reusable transducer have been widely utilized for the label-free electrical detection of chemical and biological species. Previous studies have demonstrated that sensitive and selective detection of ions and biomolecules can be achieved by controlled modification of the sensing part with an ion-selective membrane and receptors of interest. However, no systematic studies have been performed on the impact of the transducer on sensing performance. In this paper, we introduce the concept of a nanoscale field-effect transistor (FET) as a reusable and sensitive transducer for extended-gate biosensors. The capacitive effect from the external sensing part can degrade the sensing performance, but the nanoscale FET can reduce this effect. The nanoscale FET with a gate-all-around (GAA) structure exhibits a higher pH sensitivity than a commercially available FET, which is widely used in conventional extended-gate biosensors. A sensitivity reduction is observed for the commercial FET, whereas the pH sensitivity is insensitive to the area of the sensing region in the nanoscale FET, thus allowing the scaling of the detection area. Our analysis based on a capacitive model suggests that the high pH sensitivity in the compact sensing area originates from the small input capacitance of the nanoscale FET transducer. Moreover, a decrease in the nanowire width of the GAA FET leads to an improvement in the pH sensitivity. The extended-gate approach with the nanoscale FET-based transduction can pave the way for a highly sensitive analysis of chemical and biological species with a small sample volume.

Specific and label-free immunosensing of protein-protein interactions with silicon-based immunoFETs

The importance of specific and label-free detection of proteins via antigen-antibody interactions for the development of point-of-care testing devices has greatly influenced the search for a more accessible, sensitive, low cost and robust sensors. The vision of silicon field-effect transistor (FET)-based sensors has been an attractive venue for addressing the challenge as it potentially offers a natural path to incorporate sensors with the existing mature Complementary Metal Oxide Semiconductor (CMOS) industry; this provides a stable and reliable technology, low cost for potential disposable devices, the potential for extreme minituarization, low electronic noise levels, etc. In the current review we focus on silicon-based immunological FET (ImmunoFET) for specific and label-free sensing of proteins through antigen-antibody interactions that can potentially be incorporated into the CMOS industry; hence, immunoFETs based on nano devices (nanowire, nanobelts, carbon nanotube, etc.) are not treated here. The first part of the review provides an overview of immunoFET principles of operation and challenges involved with the realization of such devices (i.e. e.g. Debye length, surface functionalization, noise, etc.). In the second part we provide an overview of the state-of-the-art silicon-based immunoFET structures and novelty, principles of operation and sensing performance reported to date.

Label-free and reagentless capacitive aptasensor for thrombin

This investigation develops a label-free and reagentless aptasensor, based on a capacitive transducer with simple face-to-face electrode pairs. The electrode pairs of the transducer are composed of a gold electrode and an indium tin oxide film with micrometer separation with a double-side polyethylene terephthalate tape. Aptamers and 1-dodecanethiol are modified to form a self-assembled monolayer (SAM) on the gold electrode surfaces, and function as bio-recognition elements and preventers of non-specific protein binding, respectively. Electrochemical characterization results indicate that the SAM also forms an effective insulating layer, which is sufficient for capacitive sensing. The feasibility of the capacitive biosensor is validated using thrombin as a model analyte. The ultra-small value changes of capacitance originating from thrombin binding with the aptamers modified on the biosensor were measured with a home-made capacitance measuring circuit based on switched capacitor (SC) technology. The developed biosensor has detection limits of 1 pM and 10 pM of thrombin in phosphate buffered saline and mimic serum solution, respectively. The linear range for thrombin detection in human serum solution is from 10 pM to 1 μM, with a regression coefficient of 0.98. Additionally, the proposed aptasensor does not have significant levels of non-specific binding of bovine serum albumin and human serum albumin. Accordingly, the combination of SC and SAM bringing capacitive transduction at the forefront of ultrasensitive label-free and reagentless biosensing devices, particularly for point-of-care clinical analysis, which adopts small numbers of biological samples with low analyte concentrations.

Development of an aptamer-based field effect transistor biosensor for quantitative detection of Plasmodium falciparum glutamate dehydrogenase in serum samples

There has been a continuous strive to develop portable, stable, sensitive and low cost detection system for malaria to meet the demand of effective screening actions in developing countries where the disease is most endemic. Herein, we report an aptamer-based field effect transistor (aptaFET) biosensor, developed by using an extended gate field effect transistor with inter-digitated gold microelectrodes (IDµE) for the detection of the malaria biomarker Plasmodium falciparum glutamate dehydrogenase (PfGDH) in serum samples. A 90 mer long ssDNA aptamer (NG3) selective to PfGDH was used in the aptaFET to capture the target protein. The intrinsic surface net charge of the captured protein led to change in gate potential of the aptaFET device, which could be correlated to the concentration of the protein. This biosensor exhibited a sensitive response in broad dynamic range of 100 fM -10 nM with limits of detection of 16.7 pM and 48.6 pM in spiked buffer and serum samples, respectively. The high selectivity of the biosensor for PfGDH was verified by testing relevant analogous human and parasitic proteins on the device. Overall, the results validated the application potential of the developed aptaFET for diagnosis of both symptomatic and asymptomatic malaria.

An ''off-on'' phosphorescent aptasensor switch for the detection of ATP

An "off-on" phosphorescent aptasensor based on the 3-mercaptopropionic acid (MPA) capped Mn-doped ZnS quantum dots (MPA-Mn:ZnS QDs)/aptamer hybrid system was developed to detect adenosine triphosphate (ATP) in biological fluids. The phosphorescence of MPA-Mn:ZnS QDs was obviously quenched when ATP aptamer was added due to the aggregation induced effect. ATP aptamer, adsorbed on the surface of the phosphorescent MPA-Mn:ZnS QDs, has a high affinity for ATP. And then, with the addition of ATP, phosphorescence was gradually recovered because of the stronger special binding interaction between ATP and ATP aptamer than that between QDs and ATP aptamer. In this case, a high sensitivity and selectivity of phosphorescent aptasensor for the detection of ATP has constructed with a low detection limit of 0.9 nM and a wide linear range from 2 nM to 9 µM. What's more, the phosphorescent aptasensor does not require complex pretreatments and can effectively eliminate the interference from auto fluorescence and scattering light.

Interfacial Charge Contributions to Chemical Sensing by Electrolyte-Gated Transistors with Floating Gates

The floating gate, electrolyte-gated transistor (FGT) is a chemical sensing device utilizing a floating gate electrode to physically separate and electronically couple the active sensing area with the transistor. The FGT platform has yielded promising results for the detection of DNA and proteins, but questions remain regarding its fundamental operating mechanism. Using carboxylic acid-terminated self-assembled monolayers (SAMs) exposed to solutions of different pH, we create a charged surface and hence characterize the role that interfacial charge concentration plays relative to capacitance changes. The results agree with theoretical predictions from conventional double-layer theory, rationalizing nonlinear responses obtained at high analyte concentrations in previous work using the FGT architecture. Our study elucidates an important effect in the sensing mechanism of FGTs, expanding opportunities for the rational optimization of these devices for chemical and biochemical detection.

Multiplexing determination of cancer-associated biomarkers by surface-enhanced Raman scattering using ordered gold nanohoneycomb arrays

AIM

Here, a multiplex surface-enhanced Raman scattering (SERS) based assay for simultaneous quantitation of carcinoembryonic antigen (CEA) and α-fetoprotein (AFP) was developed.

METHODS

SERS tags of nanostars and SERS substrates of nanobowl arrays were functionalized with labeling and capturing antibodies, respectively. In presence of antigens, SERS tags, antigens and SERS substrates formed sandwich structure.

RESULTS

The SERS-based technique showed a wide linear range from 0.5 to 100 ng/ml and detection limits were 0.41 and 0.35 ng/ml for CEA and AFP in phosphate-buffered saline buffer, respectively. Analysis results of clinical serum samples using this technique were similar to that shown in phosphate-buffered saline buffer. The LODs were 0.44 and 0.40 ng/ml for CEA and AFP, respectively. Conclusion: The precision and stability of this analysis technique were satisfactory, meanwhile, no obvious cross-reactivity could be found. What's more, it also suggested that this novel multiplex SERS-based technique could be a simple, specific, reliable, sensitive and multiplexed tool for important diagnostic and prognostic applications.

Direct and label-free influenza virus detection based on multisite binding to sialic acid receptors

A system to discriminate human or avian influenza A remains a highly sought-after tool for prevention of influenza pandemics in humans. Selective binding of the influenza A viral hemagglutinin (HA) to specific sialic acid (SA) receptors (Neu5Acα(2-6)Gal in humans, Neu5Acα(2-3)Gal in birds) is determined by the genotype of the HA and neuraminidase (NA) segments, making it one of the key characteristics that distinguishes human or avian influenza A virus. Here we demonstrate the direct detection of whole H1N1 influenza A virus using 6′-sialyllactose (Neu5Acα(2-6)Galβ(1-4)Glc, 6SL)-immobilized gold electrodes as biosensing surfaces. The sensitivity was higher than that of conventional immunochromatographic technique (ICT) for influenza virus and not restricted by genetic drift. The label-free detection technology via direct attachment of a whole virus using a chemically modified electrode is a promising means to provide a simple and rapid diagnostic system for viral infections.

On-Line Monitoring the Growth of E. coli or HeLa Cells Using an Annular Microelectrode Piezoelectric Biosensor

Biological information is obtained from the interaction between the series detection electrode and the organism or the physical field of biological cultures in the non-mass responsive piezoelectric biosensor. Therefore, electric parameter of the electrode will affect the biosensor signal. The electric field distribution of the microelectrode used in this study was simulated using the COMSOL Multiphysics analytical tool. This process showed that the electric field spatial distribution is affected by the width of the electrode finger or the space between the electrodes. In addition, the characteristic response of the piezoelectric sensor constructed serially with an annular microelectrode was tested and applied for the continuous detection of Escherichia coli culture or HeLa cell culture. Results indicated that the piezoelectric biosensor with an annular microelectrode meets the requirements for the real-time detection of E. coli or HeLa cells in culture. Moreover, this kind of piezoelectric biosensor is more sensitive than the sensor with an interdigital microelectrode. Thus, the piezoelectric biosensor acts as an effective analysis tool for acquiring online cell or microbial culture information.

Sensitive fluorescence detection of ATP based on host-guest recognition between near-infrared β-Cyclodextrin-CuInS2 QDs and aptamer

We have developed a near-infrared (NIR) fluorescent aptamer-based sensor for sensitive detection of adenosine-5'-triphosphate (ATP) by using a ATP-binding aptamer and β-Cyclodextrin-CuInS₂ quantum dots (β-CD-CuInS₂ QDs). The fluorescence of β-CD-CuInS₂ QDs has a slight enhancement with the addition of ATP-binding aptamer due to the host-guest recognition between aptamer and β-CD. When ATP is added, it will bind to aptamer to form G-quadruplexes. Aptamer-ATP complexes can enter into the hydrophobic cavities of β-CD and result in great enhancement of the fluorescence intensity. Under the optimum conditions, the fluorescence intensity of β-CD-CuInS₂ QDs is proportional to the concentration of ATP, which shows a good linear response toward ATP concentration range of 6-1200μmolL^-1, the detection limit for ATP is 3μmolL^-1. The present assay shows a good selectivity for ATP over other biologically important proteins, and it is applied to the determination of ATP in human serum sample with satisfactory results.

Horizontally Aligned Carbon Nanotube Based Biosensors for Protein Detection

A novel horizontally aligned single-walled carbon nanotube (CNT) Field Effect Transistor (FET)-based biosensing platform for real-time and sensitive protein detections is proposed. Aligned nanotubes were synthesized on quartz substrate using catalyst contact stamping, surface-guided morphological growth and chemical vapor deposition gas-guided growth methods. Real-time detection of prostate-specific antigen (PSA) using as-prepared FET biosensors was demonstrated. The kinetic measurements of the biosensor revealed that the drain current (I_d) decreased exponentially as the concentration of PSA increased, indicating that the proposed FET sensor is capable of quantitative protein detection within a detection window of up to 1 µM. The limit of detection (LOD) achieved by the proposed platform was demonstrated to be 84 pM, which is lower than the clinically relevant level (133 pM) of PSA in blood. Additionally, the reported aligned CNT biosensor is a uniform sensing platform that could be extended to real-time detections of various biomarkers.

Immobilized rolling circle amplification on extended-gate field-effect transistors with integrated readout circuits for early detection of platelet-derived growth factor

Detection of tumor-related proteins with high specificity and sensitivity is important for early diagnosis and prognosis of cancers. While protein sensors based on antibodies are not easy to keep for a long time, aptamers (single-stranded DNA) are found to be a good alternative for recognizing tumor-related protein specifically. This study investigates the feasibility of employing aptamers to recognize the platelet-derived growth factor (PDGF) specifically and subsequently triggering rolling circle amplification (RCA) of DNAs on extended-gate field-effect transistors (EGFETs) to enhance the sensitivity. The EGFETs are fabricated by the standard CMOS technology and integrated with readout circuits monolithically. The monolithic integration not only avoids the wiring complexity for a large sensor array but also enhances the sensor reliability and facilitates massive production for commercialization. With the RCA primers immobilized on the sensory surface, the protein signal is amplified as the elongation of DNA, allowing the EGFET to achieve a sensitivity of 8.8 pM, more than three orders better than that achieved by conventional EGFETs. Moreover, the responses of EGFETs are able to indicate quantitatively the reaction rates of RCA, facilitating the estimation on the protein concentration. Our experimental results demonstrate that immobilized RCA on EGFETs is a useful, label-free method for early diagnosis of diseases related to low-concentrated tumor makers (e.g., PDGF) for serum sample, as well as for monitoring the synthesis of various DNA nanostructures in real time. Graphical Abstract The tumor-related protein, PDGF, is detected by immobilizing rolling circle amplification on an EGFET with integrated readout circuit.

A review on electronic bio-sensing approaches based on non-antibody recognition elements

In this review, recent advances in the development of electronic detection methodologies based on non-antibody recognition elements such as functional liposomes, aptamers and synthetic peptides are discussed. Particularly, we highlight the progress of field effect transistor (FET) sensing platforms where possible as the number of publications on FET-based platforms has increased rapidly. Biosensors involving antibody-antigen interactions have been widely applied in diagnostics and healthcare in virtue of their superior selectivity and sensitivity, which can be attributed to their high binding affinity and extraordinary specificity, respectively. However, antibodies typically suffer from fragile and complicated functional structures, large molecular size and sophisticated preparation approaches (resource-intensive and time-consuming), resulting in limitations such as short shelf-life, insufficient stability and poor reproducibility. Recently, bio-sensing approaches based on synthetic elements have been intensively explored. In contrast to existing reports, this review provides a comprehensive overview of recent advances in the development of biosensors utilizing synthetic recognition elements and a detailed comparison of their assay performances. Therefore, this review would serve as a good summary of the efforts for the development of electronic bio-sensing approaches involving synthetic recognition elements.

Lead-Free Piezoelectric Diaphragm Biosensors Based on Micro-Machining Technology and Chemical Solution Deposition

In this paper, we present a new approach to the fabrication of integrated silicon-based piezoelectric diaphragm-type biosensors by using sodium potassium niobate-silver niobate (0.82KNN-0.18AN) composite lead-free thin film as the piezoelectric layer. The piezoelectric diaphragms were designed and fabricated by micro-machining technology and chemical solution deposition. The fabricated device was very sensitive to the mass changes caused by various targets attached on the surface of diaphragm. The measured mass sensitivity value was about 931 Hz/μg. Its good performance shows that the piezoelectric diaphragm biosensor can be used as a cost-effective platform for nucleic acid testing.

Dual aptamer-immobilized surfaces for improved affinity through multiple target binding in potentiometric thrombin biosensing

We developed a label-free and reagent-less potentiometric biosensor with improved affinity for thrombin. Two different oligomeric DNA aptamers that can recognize different epitopes in thrombin were introduced in parallel or serial manners on the sensing surface to capture the target via multiple contacts as found in many biological systems. The spacer and linker in the aptamer probes were optimized for exerting the best performance in molecular recognition. To gain the specificity of the sensor to the target, an antifouling molecule, sulfobeaine-3-undecanethiol (SB), was introduced on the sensor to form a self-assembled monolayer (SAM). Surface characterization revealed that the aptamer probe density was comparable to the distance of the two epitopes in thrombin, while the backfilling SB SAM was tightly aligned on the surface to resist nonspecific adsorption. The apparent binding parameters were obtained by thrombin sensing in potentiometry using the 1:1 Langmuir adsorption model, showing the improved dissociation constants (Kd) with the limit of detection of 5.5 nM on the dual aptamer-immobilized surfaces compared with single aptamer-immobilized ones. A fine control of spacer and linker length in the aptamer ligand was essential to realize the multivalent binding of thrombin on the sensor surface. The findings reported herein are effective for improving the sensitivity of potentiometric biosensor in an affordable way towards detection of tiny amount of biomolecules.

Label-free Protein Detection Based on the Heat-Transfer Method--A Case Study with the Peanut Allergen Ara h 1 and Aptamer-Based Synthetic Receptors

Aptamers are an emerging class of molecules that, because of the development of the systematic evolution of ligands by exponential enrichment (SELEX) process, can recognize virtually every target ranging from ions, to proteins, and even whole cells. Although there are many techniques capable of detecting template molecules with aptamer-based systems with high specificity and selectivity, they lack the possibility of integrating them into a compact and portable biosensor setup. Therefore, we will present the heat-transfer method (HTM) as an interesting alternative because this offers detection in a fast and low-cost manner and has the possibility of performing experiments with a fully integrated device. This concept has been demonstrated for a variety of applications including DNA mutation analysis and screening of cancer cells. To the best our knowledge, this is the first report on HTM-based detection of proteins, in this case specifically with aptamer-type receptors. For proof-of-principle purposes, measurements will be performed with the peanut allergen Ara h 1 and results indicate detection limits in the lower nanomolar regime in buffer liquid. As a first proof-of-application, spiked Ara h 1 solutions will be studied in a food matrix of dissolved peanut butter. Reference experiments with the quartz-crystal microbalance will allow for an estimate of the areal density of aptamer molecules on the sensor-chip surface.

Label and Label-Free Detection Techniques for Protein Microarrays

Protein microarray technology has gone through numerous innovative developments in recent decades. In this review, we focus on the development of protein detection methods embedded in the technology. Early microarrays utilized useful chromophores and versatile biochemical techniques dominated by high-throughput illumination. Recently, the realization of label-free techniques has been greatly advanced by the combination of knowledge in material sciences, computational design and nanofabrication. These rapidly advancing techniques aim to provide data without the intervention of label molecules. Here, we present a brief overview of this remarkable innovation from the perspectives of label and label-free techniques in transducing nano-biological events.

Nanopore-based DNA-probe sequence-evolution method unveiling characteristics of protein-DNA binding phenomena in a nanoscale confined space

Almost all of the important functions of DNA are realized by proteins which interact with specific DNA, which actually happens in a limited space. However, most of the studies about the protein-DNA binding are in an unconfined space. Here, we propose a new method, nanopore-based DNA-probe sequence-evolution (NDPSE), which includes up to 6 different DNA-probe systems successively designed in a nanoscale confined space which unveil the more realistic characteristics of protein-DNA binding phenomena. There are several features; for example, first, the edge-hindrance and core-hindrance contribute differently for the binding events, and second, there is an equilibrium between protein-DNA binding and DNA-DNA hybridization.

Nanoscaled aptasensors for multi-analyte sensing

Introduction: Nanoscaled aptamers (Aps), as short single-stranded DNA or RNA oligonucleotides, are able to bind to their specific targets with high affinity, upon which they are considered as powerful diagnostic and analytical sensing tools (the so-called "aptasensors"). Aptamers are selected from a random pool of oligonucleotides through a procedure known as "systematic evolution of ligands by exponential enrichment".

Methods: In this work, the most recent studies in the field of aptasensors are reviewed and discussed with a main focus on the potential of aptasensors for the multianalyte detection(s).

Results: Due to the specific folding capability of aptamers in the presence of analyte, aptasensors have substantially successfully been exploited for the detection of a wide range of small and large molecules (e.g., drugs and their metabolites, toxins, and associated biomarkers in various diseases) at very low concentrations in the biological fluids/samples even in presence of interfering species.

Conclusion: Biological samples are generally considered as complexes in the real biological media. Hence, the development of aptasensors with capability to determine various targets simultaneously within a biological matrix seems to be our main challenge. To this end, integration of various key scientific dominions such as bioengineering and systems biology with biomedical researches are inevitable.

Rapid and simple G-quadruplex DNA aptasensor with guanine chemiluminescence detection

Cost-effective and sensitive aptasensor with guanine chemiluminescence detection capable of simply quantifying thrombin in human serum was developed using thrombin aptamer (TBA), one of the G-quadruplex DNA aptamers, without expensive nanoparticles and complicated procedures. Guanines of G-quadruplex TBA-conjugated carboxyfluorescein (6-FAM) bound with thrombin do not react with 3,4,5-trimethoxylphenylglyoxal (TMPG) in the presence of tetra-n-propylammonium hydroxide (TPA), whereas guanines of free TBA- and TBA-conjugated 6-FAM immobilized on the surface of graphene oxide rapidly react with TMPG to emit light. Thus, guanine chemiluminescence in 5% human serum with thrombin was lower than that without thrombin when TBA-conjugated 6-FAM was added in two samples and incubated for 20 min. In other words, the brightness of guanine chemiluminescence was quenched due to the formation of G-quadruplex TBA-conjugated 6-FAM bound with thrombin in a sample. High-energy intermediate, capable of emitting dim light by itself, formed from the reaction between guanines of TBA and TMPG in the presence of TPA, transfers energy to 6-FAM to emit bright light based on the principle of chemiluminescence energy transfer (CRET). G-quadruplex TBA aptasensor devised using the rapid interaction between TBA-conjugated 6-FAM and thrombin quantified trace levels of thrombin without complicated procedures. The limit of detection (LOD = background + 3 × standard deviation) of G-quadruplex TBA aptasensor with good linear calibration curve, accuracy, precision, and recovery was as low as 12.3 nM in 5% human serum. Using the technology reported in this research, we expect that various types of G-quadruplex DNA aptasensors capable of specifically sensing a target molecule such as ATP, HIV, ochratoxin, potassium ions, and thrombin can be developed.

Electrochemiluminescence aptasensor for adenosine triphosphate detection using host-guest recognition between metallocyclodextrin complex and aptamer

A sensitive and label-free electrochemiluminescence (ECL) aptasensor for the detection of adenosine triphosphate (ATP) was successfully designed using host-guest recognition between a metallocyclodextrin complex, i.e., tris(bipyridine)ruthenium(II)-β-cyclodextrin [tris(bpyRu)-β-CD], and an ATP-binding aptamer. In the protocol, the NH2-terminated aptamer was immobilized on a glassy carbon electrode (GCE) by a coupling interaction. After host-guest recognition between tris(bpyRu)-β-CD and aptamer, the tris(bpyRu)-β-CD/aptamer/GCE produced a strong ECL signal as a result of the photoactive properties of tris(bpyRu)-β-CD. However, in the presence of ATP, the ATP/aptamer complex was formed preferentially, which restricted host-guest recognition, and therefore less tris(bpyRu)-β-CD was attached to the GCE surface, resulting in an obvious decrease in the ECL intensity. Under optimal determination conditions, an excellent logarithmic linear relationship between the ECL decrease and ATP concentration was obtained in the range 10.0-0.05 nM, with a detection limit of 0.01 nM at the S/N ratio of 3. The proposed ECL-based ATP aptasensor exhibited high sensitivity and selectivity, without time-consuming signal-labeling procedures, and is considered to be a promising model for detection of aptamer-specific targets.

A label-free electrochemiluminescence aptasensor for thrombin detection based on host-guest recognition between tris(bipyridine)ruthenium(II)-β-cyclodextrin and aptamer

An ultrasensitive label-free electrochemiluminescence (ECL) aptasensor for the detection of thrombin was developed based on the specific recognition between tris(bipyridine)ruthenium(II)-β-cyclodextrin (tris(bpyRu)-β-CD) and the anti-thrombin aptamer (aptamer). The NH2-aptamer was first immobilized on the activated glassy carbon electrode (GCE) by coupling interaction. By use of the specific recognition between tris(bpyRu)-β-CD and aptamer, tris(bpyRu)-β-CD was then attached on the surface of GCE. Resulting from the outstanding photoactive properties of tris(bpyRu)-β-CD, the fabricated GCE performed strong ECL signal with the coreactant of 2-(dibutylamino)ethanol (DBAE). However, in the presence of thrombin, aptamer-thrombin bioaffinity complexes were formed, which restricted the recognition activities between aptamer and tris(bpyRu)-β-CD. Thus, fewer tris(bpyRu)-β-CD could be attached on the surface of GCE and led to an obvious decrease of ECL signal. Fortunately, the difference of ECL intensity before and after combination with thrombin was logarithmically linear with the concentration of thrombin in a wide range of 10 nM-1 pM. Meantime, a detection limit of 0.1 pM without any other signal labeling or amplifying procedures indicated that the biosensor performed excellent sensitivity, operability and simplicity.

Signal amplification for thrombin impedimetric aptasensor: sandwich protocol and use of gold-streptavidin nanoparticles

In this work, we report a highly specific amplification strategy demonstrated for the ultrasensitive biosensing of thrombin with the use of gold-streptavidin nanoparticles (strep-AuNPs) and silver reduction enhancement. The biotinylated aptamer of thrombin was immobilized onto an avidin-graphite epoxy composite (AvGEC) electrode surface by affinity interaction between biotin and avidin; electrochemical impedance measurements were performed in a solution containing the redox marker ferrocyanide/ferricyanide. The change in interfacial charge transfer resistance (Rct) experimented by the redox marker, was recorded to confirm aptamer complex formation with target protein, thrombin (Thr), in a label-free first stage. A biotinylated second thrombin aptamer, with complementary recognition properties was then used in a sandwich approach. The addition of strep-AuNPs and silver enhancement treatment led to a further increment of Rct thus obtaining significant signal amplification. The AptThrBio1-Thr-AptThrBio2 sandwich formation was inspected by confocal microcopy after incubation with streptavidin quantum dots. In order to visualize the presence of gold nanoparticles, the same silver enhancement treatment was applied to electrodes already modified with the nanoparticle-sandwich conjugate, allowing direct observation by scanning electron microscopy (SEM). Results showed high sensitivity and selectivity for thrombin detection, with an improvement from ca. 4.7 pM in a simple assay to 0.3 pM in the amplified reported scheme.

Resonance light scattering determination of trace bisphenol A with signal amplification by aptamer-nanogold catalysis

HAuCl4 was reduced by sodium citrate to prepare 10 nm gold nanoparticles (AuNPs) that were modified by the bisphenol A aptamer (Apt) to obtain an aptamer-nanogold probe (Apt-AuNP) for bisphenol A (BPA). The probes were aggregated nonspecifically to form large clusters, which showed a strong resonance light scattering (RLS) peak at 520 nm, under preparation conditions (pH 7.6 Na2HPO4-NaH2PO4 buffer and ultrasonication). Upon addition of BPA, the probe reacted specifically to form dispersed BPA-Apt-AuNP conjugates that exhibited strong catalysis of the two particle reactions of glucose-Cu(II) and hydrazine hydrochloride-Cu(II) with a strong RLS peak at 360 nm and 510 nm respectively. When the BPA concentration increased, the RLS intensity at 360 nm and 510 nm increased respectively. Accordingly, two new and highly-sensitive RLS methods were established for the detection of BPA, using the Apt-AuNP catalytic amplification.

Thrombin detection using a piezoelectric aptamer-linked immunosorbent assay

The development of diagnostic assays using highly targeted specific aptamers with existing detection platforms has been an endeavor with few opportunities until now. Many current commercially available diagnostic platforms make use of detection systems employing capture agents composed of modified antigen-specific antibodies coupled with a variety of detection modalities, including radioimmunoassays, fluorescence-based detection assays, electro/chemiluminescence assays, and immunoradiometric assays. In the studies presented here, a novel frequency-modulating technology from BioScale called Acoustic Membrane MicroParticle (AMMP) detection was used to demonstrate a sensitive and reproducible method of incorporating aptamers as capture and detection agents. The method provides a robust and rapid detection of thrombin in human serum while also eliminating the labor-intensive efforts of Western blot analysis and is not affected by the interfering substances found in serum that often affect optical-based detection systems. In addition, we have demonstrated, for the first time, the adaptation of the AMMP platform to exploit aptamers against a clinically relevant target. The AMMP platform is an ideal medium for using aptamers in commercial assay development for application in a clinical setting.