Label-free biosensors based on aptamer-modified graphene field-effect transistors

A label-free immunosensor based on an aptamer-modified graphene field-effect transistor (G-FET) is demonstrated. Immunoglobulin E (IgE) aptamers with an approximate height of 3 nm were successfully immobilized on a graphene surface, as confirmed by atomic force microscopy. The aptamer-modified G-FET showed selective electrical detection of IgE protein. From the dependence of the drain current variation on the IgE concentration, the dissociation constant was estimated to be 47 nM, indicating good affinity and the potential for G-FETs to be used in biological sensors.     

Highly responsive biosensors based on organic field-effect transistors under light irradiation

High responsivity and sensitivity play essential roles in the development of organic field-effect transistors (OFETs)-based biosensors with regard to biological detections, particularly for disease diagnosis. Nonetheless, how to design a biosensor which improves these two outstanding properties while achieving low cost, easy processing, and time saving is a daunting challenge. Herein, a novel biosensor based on OFET with copolymer thin film, whose surface is illuminated with a suitable light beam is reported. This film can be used as both an organic semiconductor material and as a photoelectric active material. Due to amplification of signals as a result of the film’s strong response to light, the biosensor possesses higher responsivity and sensitivity compared to dark condition and even realizes a maximum responsivity of up to 10³ for alpha-fetoprotein (AFP) detection. The simple combination of light and transistor builds a bridge between photoelectric effect and biological system. In addition, the emergence of more excellent photoelectric active materials is expected to pave a way for ultrasensitive bio-chemical diagnostic tools.     

Recent advances on the development of graphene-based field-effect transistors,electrochemical biosensors and electrochemiluminescence biosensors

Graphene, a honeycomb lattice of carbon material with single-atom-layer structure, demonstrates extraordinary mechanical, thermal, chemical and electronic properties. Thus, it has sparked tremendous interests in various fields, such as energy storage and conversion devices, field-effect transistors (FET), chemical sensors and biosensors. In this review, we will first focus on the synthesis method of graphene and the fabrication strategy of graphene-based materials. Subsequently, the construction of graphene-based biosensors are introduced, in which three kinds of biosensors are discussed in details, including the FET, electrochemical biosensors and electrochemiluminescence (ECL) biosensors. The performances of the state-of-the-art biosensors on the detection of biomolecules are also displayed. Finally, we also highlight some critical challenges remain to be solved and the development in this field for further research.     

Advances in organic transistor-based biosensors: from organic electrochemical transistors to electrolyte-gated organic field-effect transistors

Organic electronics have, over the past two decades, developed into an exciting area of research and technology to replace classic inorganic semiconductors. Organic photovoltaics, light-emitting diodes, and thin-film transistors are already well developed and are currently being commercialized for a variety of applications. More recently, organic transistors have found new applications in the field of biosensors. The progress made in this direction is the topic of this review. Various configurations are presented, with their detection principle, and illustrated by examples from the literature.     

Enzyme based amperometric biosensors

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High-performance organic field-effect transistors based on pi-extended tetrathiafulvalene derivatives

Aromatic ring-condensed TTF derivatives exhibited excellent p-type FET performances in thin films. Introduction of fused benzene and pyrazine rings to the TTF skeleton was effective to enhance the intermolecular interactions and stability to oxygen. Ordered molecular alignment was confirmed by XRD studies. A pi-stacking structure was observed in the single crystal of diquinoxalinoTTF.     

Air-stable ambipolar field-effect transistors based on copper phthalocyanine and tetracyanoquinodimethane

The fabrication of air-stable ambipolar organic field-effect transistors with sandwich-type and bilayer architecture based on copper phthalocyanine and 7,7,8,8-tetracyanoquin odimethane is reported. In comparison with bilayer devices, the sandwich-type configuration enhances the performance and reproducibility of ambipolar devices, which is mainly ascribed to the double conductive channels in the sandwich-type structure. The measured p-channel and n-channel mobility of sandwich-type devices are comparable to that of copper phthalocyanine and 7,7,8,8-tetracyanoquinodimethane single layer devices, respectively.     

DNA sensing by field-effect transistors based on networks of carbon nanotubes

We report on the sensing mechanism of electrical detection of deoxyribonucleic acid (DNA) hybridization for Au- and Cr-contacted field effect transistors based on single-walled carbon nanotube (SWCNT) networks. Barrier height extraction via low-temperature electrical measurement provides direct evidence for the notion that the energy level alignment between electrode and SWCNTs can be affected by DNA immobilization and hybridization. The study of location-selective capping using photoresist provides comprehensive evidence that the sensing of DNA is dominated by the change in metal-SWCNT junctions rather than the channel conductance.     

Monolayer organic field-effect transistors

Monolayer organic field-effect transistors(OFETs) are attracting worldwide interest in device physics and novel applications due to their ultrathin active layer for two-dimensional charge transport. The monolayer films are generally prepared by thermal evaporation, the Langmuir technique or self-assembly process, etc., but their electrical performance is relatively lower than corresponding thick films. From 2011, the performance of monolayer OFETs has been boosted by using the monolayer molecular crystals(MMCs) as active channels, which opened up a new era for monolayer OFETs. In this review, recent progress of monolayer OFETs, including the preparation of monolayer films, their OFET performance and applications are summarized.Finally, perspectives of monolayer OFETs in the near future are also discussed.     

High performance n- and p-type field-effect transistors based on tetrathiafulvalene derivatives

The first n-type FET based on TTF derivatives was prepared. TTF derivatives with halogeno-substituted quinoxaline rings showed excellent n- or p-type performances with high carrier mobilities. Introduction of halogen groups determined the FET polarity by controlling the HOMO and LUMO levels of the molecules. The pi-stacking structures were observed in the single crystals of tetrahalogeno-TTFs.     

Photoresponsive n-channel organic field-effect transistors based on a tri-component active layer

Photoresponsive OFETs were fabricated based on a tri-component active layer (NDI2OD-DTYM2, spiropyran and polystyrene). The results demonstrated that these OFETs displayed photoresponsive feature to alternate UV and vis light due to the photoisomerization of spiropyran between the closed-ring state and ionic open-ring state.     

Novel highly stable semiconductors based on phenanthrene for organic field-effect transistors

Two novel phenanthrene-based conjugated oligomers were synthesized and used as p-channel semiconductors in field-effect transistors; they exhibit high mobility and excellent stability during long-time ambient storage and under UV irradiation.     

Bias-stress effects in organic field-effect transistors based on self-assembled monolayer nanodielectrics

The electrical stability of low-voltage organic transistors based on phosphonic acid self-assembled monolayer (SAM) dielectrics is investigated using four different semiconductors. The threshold voltage shift in these devices shows a stretched-exponential time dependence under continuous gate bias with a relaxation time in the range of 10(3)-10(5) s, at room temperature. Differences in the bias instability of transistors based on different self-assembled monolayers and organic semiconductors suggest that charge trapping into localized states in the semiconductor is not the only mechanism responsible for the observed instability. By applying 1-5 s long programming voltage pulses of 2-3 V in amplitude, a large reversible threshold voltage shift can be produced. The retention time of the programmed state was measured to be on the order of 30 h. The combination of low voltage operation and relatively long retention times makes these devices interesting for ultra-low power memory applications.     

Organic field-effect transistors based on heterocyclic co-oligomers containing a pyrazine ring

New oligomers containing a pyrazine unit have been prepared: the bithienyl derivatives afforded p-type FET devices whereas the trifluoromethylphenyl derivatives showed n-type FET behavior.     

High mobility n-channel single-crystal field-effect transistors based on 5,7,12,14-tetrachloro-6,13-diazapentacene

Heteroaromatic oligomer 5,7,12,14-tetrachloro-6,13-diazapentacene (TCDAP) was characterized and assessed as n-channel material in field-effect transistor applications. A single-crystal transistor based on TCDAP as the channel material exhibits a very high electron mobility of 3.39 cm(2) V(-1) s(-1) and an on/off ratio of ～1.08 × 10(4) respectively.     

New n-type semiconductor material based on styryl fullerene for organic field-effect transistors

Organic field-effect transistors with styryl fullerene as a semi conductor layer applied by centrifugation are considered. Electron mobility in the transistors was 0.067 ± 10% cm² V⁻¹ s⁻¹, whereas the mobility of electrons in these devices after the vacuum deposition of a semiconductor layer was much lower (0.023 ± 10% cm² V⁻¹ s⁻¹).     

Biosensors based on enzyme field-effect transistors for determination of some substrates and inhibitors

This paper is a review of the authors' publications concerning the development of biosensors based on enzyme field-effect transistors (ENFETs) for direct substrates or inhibitors analysis. Such biosensors were designed by using immobilised enzymes and ion-selective field-effect transistors (ISFETs). Highly specific, sensitive, simple, fast and cheap determination of different substances renders them as promising tools in medicine, biotechnology, environmental control, agriculture and the food industry.The biosensors based on ENFETs and direct enzyme analysis for determination of concentrations of different substrates (glucose, urea, penicillin, formaldehyde, creatinine, etc.) have been developed and their laboratory prototypes were fabricated. Improvement of the analytical characteristics of such biosensors may be achieved by using a differential mode of measurement, working solutions with different buffer concentrations and specific agents, negatively or positively charged additional membranes, or genetically modified enzymes. These approaches allow one to decrease the effect of the buffer capacity influence on the sensor response in an aim to increase the sensitivity of the biosensors and to extend their dynamic ranges.Biosensors for the determination of concentrations of different toxic substances (organophosphorous pesticides, heavy metal ions, hypochlorite, glycoalkaloids, etc.) were designed on the basis of reversible and/or irreversible enzyme inhibition effect(s). The conception of an enzymatic multibiosensor for the determination of different toxic substances based on the enzyme inhibition effect is also described.We will discuss the respective advantages and disadvantages of biosensors based on the ENFETs developed and also demonstrate their practical application.     

A highly pi-stacked organic semiconductor for field-effect transistors based on linearly condensed pentathienoacene

We present the synthesis and characterization of a fused-ring compound, dithieno[2,3-d:2',3'-d']thieno[3,2-b:4,5-b']dithiophene (pentathienoacene, PTA). In contrast to pentacene, PTA has a larger band gap than most semiconductors used in organic field-effect transistors (OFETs) and therefore is expected to be stable in air. The large pi-conjugated and planar molecular structure of PTA would also form higher molecular orders that are conductive for carrier transport. X-ray diffraction and atomic force microscopy experiments on its films show that the molecules stack in layers with their long axis upright from the surface. X-ray photoelectron spectroscopy suggests that there are no chemical bonds at the PTA/Au interface. OFETs based on the PTA have been constructed, and their performances as p-type semiconductors are also presented. A high mobility of 0.045 cm(2)/V s and an on/off ratio of 10(3) for a PTA OFET have been achieved, demonstrating the potential of PTA for application in future organic electronics.