Application of thin-film transistors in label-free DNA biosensors

DNA sensors are the most important biosensors that have been popularly studied. The detection of nucleic acid is of great scientific and economic importance. Applications of DNA sensors include gene-expression monitoring, pharmacogenomic research and drug discovery, clinical diagnostics, and viral and bacterial identification. It is also important for the detection of biowarfare and bioterrorism agents, and for forensic and genetic identification. It has been recognized that the DNA sensors for the above applications should be highly sensitive, selective, integrated, portable and disposable. Therefore, DNA microarray (DNA chip) technology has been developed to offer unprecedented simultaneous and multiplexed analysis in a high-throughput screening format.

Today, the confocal fluorescence microscope is the standard detection platform for the analysis of DNA microarrays [1]. However, fluorescent scanning and imaging technology is not portable and has a limited spatial resolution and sensitivity. More importantly, this technique requires labeling of targets, which is time-consuming, more complex and expensive. Therefore, various technologies, such as electrochemical detection [2], surface vibration spectroscopy [3], atomic force microscopy [4], scanning Kelvin probe microscopy [5], genetic field effect transistor (FET) [6–9] and microcantilever [10] have been developed recently to realize highly sensitive and label-free DNA microarrays.

Genetic FET has many advantages over other label-free techniques. First, the fabrication of high-density FET arrays is a mature technology; it has been developed for more than half a century. For example, random access memory based on FETs is one of the highest density integrated circuits, and has billions of devices in a small chip. Second, a sensor based on a FET is the combination of a sensor and an amplifier: a small variation of gate voltage may change the channel current of the device by several orders of magnitude. Third, the device can be miniaturized without losing signal-to-noise ratio because the channel current of a FET is proportional to the ratio of the channel width to the channel length and not related to the area of the device [11]. Therefore, FET is ideal for use in small-sized, high-density and multifunctional microarray sensors. In addition, biosensors based on FETs can be easily integrated with circuitry to form self-supported and portable DNA detection platforms since FET is the key component of an integrated circuit.

DNA sensors have been developed based on various FETs, including single crystal silicon FETs, polycrystalline or amorphous silicon thin-film transistors (TFT), silicon nanowire transistors [12], carbon nanotube transistors [13,14], graphene transistors [15,16], organic TFTs (OTFT) [8,9,17] and so on. TFT is a FET fabricated on an insulating substrate, especially a low-cost or flexible one. Therefore, TFTs show great advantages over devices based on silicon wafers in disposable applications. Here, transistors based on carbon nanotubes, silicon nanowires and graphene can also be regarded as different types of TFTs. It is worth noting that polysilicon or amorphous silicon TFTs have been popularly used in flat-panel displays and thus they are mature technologies that can be directly transferred to sensing applications, including high-density DNA sensors. OTFT is another promising device for application in disposable DNA sensors, because it is much cheaper than any other FETs. OTFT will be discussed in more detail later in this article.

Although many types of TFTs have been used as DNA sensors, the operation principles of these devices are somewhat different. They can be classified by where the DNA molecules are immobilized on the transistor. The first type of TFT-based DNA sensor is normally modified with DNA molecules on gate dielectric, and similarly to other TFT-based biosensors, the DNA layer is exposed to an electrolyte during the measurement [18]. In this case, the applied gate voltage is modulated by the potential drop near the DNA layer due to the intrinsic charge of the DNA molecules. Bendriaa et al. reported a type of DNA sensor based on a suspended gate polysilicon TFT [19]. The hybridization of probe DNA with complementary target DNA is evidenced by a positive shift of the gate voltage as large as 0.35 V. Goncalves et al. used amorphous silicon TFTs for a pH and label-free DNA sensor [20]. A shift of the gate voltage by approximately 100 mV has been observed after the immobilization of DNA molecules on the gate insulator. Estrela et al. have developed DNA sensors based on extended gate polysilicon TFTs [7]. A parallel shift of transfer characteristics (drain current versus gate voltage) of 355 mV has been observed after the hybridization of single-strand DNA probes with its complementary strands. It is worth noting that, for DNA sensors based on single-crystal silicon FETs, the shift of gate voltage owing to DNA hybridization is normally tens of millivolts [6], which is much lower than that of the TFT-based DNA sensors. Since the DNA molecules in electrolytes are screened by mobile counter ions during a measurement, the induced interface potential drop is influenced by various factors, including the concentration of ions in the electrolyte, DNA concentration in the surface and the length of DNA strand, which may be the main reasons for the different shift of gate voltage in different reports.

The second type of TFT-based DNA sensor is a device modified with DNA probes on the active layer of the transistor. Since DNA molecules have a negative charge and can interact with the active layer directly, this type of device is much more sensitive than the first type described above. Most of the DNA sensors based on carbon nanotube transistors, graphene transistors, silicon nanowire transistors and OTFTs can be regarded as the second type. Wu et al. reported a highly sensitive DNA sensor based on a silicon nanowire transistor, which showed the detection limit of subfemtomolar level [12]. This sensor distinguished 1-bp mismatched DNA strands. Star et al. reported DNA sensors based on carbon nanotube network FET for label-free DNA detection at picomolar to micromolar concentrations [13]. Although in another paper Tang et al. mentioned that DNA hybridization could not occur near carbon nanotubes, their carbon nanotube FET was still a successful DNA sensor because the contact resistances of the transistor were changed by DNA hybridization [14]. Dong et al. reported a DNA sensor that used a graphene transistor [15]. The device shows a high sensitivity to DNA with a concentration as low as 0.01 nM, which was attributed to electronic n-doping to the device. Mohanty et al. reported a DNA sensor based on a graphene transistor, whose sensing ability was attributed to the electrostatic interaction between DNA molecules and the graphene layer [16]. Although all of the DNA sensors based on the novel transistors use different sensing mechanisms, the gate voltages of the transfer characteristics of the devices corresponding to DNA hybridization normally shift for hundreds of millivolts to several volts, which are much larger than that of the first type of DNA sensor. The detection limit to DNA concentration has also been extended down to a much lower level.

However, it is not easy to use the above transistors based on nanomaterials in disposable sensing arrays. The fabrication of uniform devices based on carbon nanotubes or graphene remains a significant technological challenge. Silicon nanowire transistors require access to advanced clean-room facilities similar to those used for microelectronics and thus are not suitable for low-cost and disposable sensors. In this aspect, OTFTs are excellent candidates for disposable DNA sensors for their easy and cheap fabrication compared with their inorganic counterparts. OTFT has been studied extensively for various applications, including flexible displays, organic circuits, memories and sensors. Organic materials can be dissolved in various solvents, so that transistors can be coated or printed on many different substrates by various technologies. In addition, some organic semiconductors are biocompatible and flexible, and thus they can be integrated with biological systems. Until now, there are several types of OTFT-based DNA sensors that have been reported.

One type of OTFT-based DNA sensor was reported by Zhang et al., in which DNA molecules were immobilized on the surface of semiconductor layer (pentacene) and an unambiguous doping-induced threshold voltage shift up to 20 V has been observed [8]. A drawback of this technique is that pentacene is not stable when it is exposed to DNA solution. In addition, it is not convenient to firmly bind biomolecules to the surface of pentacene. Another type of DNA sensor based on OTFT was suggested by Yan et al., in which DNA molecules were immobilized on a gold source with drain electrodes before coating with an organic semiconductor layer (poly[3-hexylthiophe]) [9]. The DNA layer changed the work function of the source/drain electrodes and increased the contact resistances of the transistor. Single- and double-stranded DNA have been differentiated successfully in these experiments. Although this technique cannot be used for in situ DNA sensing because the transistor is fabricated after the hybridization of DNA, it is expected to be suitable for application in high-density DNA arrays because the whole fabrication process is very convenient and low cost.

Another important OTFT is organic electrochemical transistor (OECT), which has attracted much attention recently because the device can be used in electrolyte with very stable performance and can provide in situ DNA detection [21]. Second, the device has a very low operating voltage, which is normally less than 1 V. Krishnamoorthy et al. fabricated OECTs based on poly(3,4-ethylenedioxythiophene) (PEDOT) by electropolymerization from 3,4-ethylenedioxythiophene solution in the presence of probe single-stranded DNA [17]. The expected conductivity changes were observed when the PEDOT-based TFT sensors were exposed to complementary single-stranded DNA solution and, as predicted, no response was observed when the device was exposed to a noncomplementary target sequence. However, the fabrication process is not suitable for high-density DNA arrays.

Therefore, OTFTs have already shown promising application in DNA sensors; however, several important issues, including the stability, reproducibility and specificity, of the devices must be further optimized.

Financial & competing interests disclosure

The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.

No writing assistance was utilized in the production of this manuscript.