Enhanced stability of organic field-effect transistor biosensors bearing electrosynthesized ZnO nanoparticles

Herein electrosynthesized ZnO nanoparticles (ZnO NPs) agents to largely improve functional bio-interlayer organic field-effect transistor (FBI-OFET) biosensors stability are investigated. For a proof-of-principle, streptavidin (SA) was chosen as the capturing biomolecule to sense biotin and poly-3-hexylthiophene (P3HT) served as channel material. The ZnO NPs were prepared and integrated into the FBI-OFET architecture by means of a straightforward and versatile procedure. To this end, ZnO NPs were mixed with an SA solution and the resulting aqueous suspension was readily spin-coated onto the SiO2gate dielectric. The P3HT film was spin-coated on the SA-ZnO NPs layer afterwards with the whole fabrication procedure taking no more than 30 min. The FBI-OFET biosensors bearing the ZnO NPs exhibited a shelf life exceeding one year, while the bare ones failed to work after few weeks. Moreover, the ZnO NPs enabled a two orders of magnitude increase in field-effect mobility while the already proven very good sensing performances were retained. The electrical and XPS characterization of the ZnO NPs based functional bio-interlayer provided information about the role of the nanostructured oxide on the improved device stability and a plausible mechanism for this occurrence is derived accordingly

Solution Processed Electrolyte-Gated Thin Film Transistors and their Sensing applications

The thin film transistor (TFT) is one of the most important fundamental building blocks in modern electronic devices. Examples of TFT applications are in integrated circuits (ICs), amplifiers, addressing of flat panel displays, and also as chemical sensors, e.g. as ion- selective field effect transistors (ISFETs). Although the most common semiconductor materials used in thin film transistors (TFTs) is silicon (Si), the versatility of TFTs allows other semiconductors to be used instead of Si-based materials. Recent research effort has been directed towards alternative TFT semiconductors, for example solution-processable semiconductors that enable new manufacturing options. Since the 1990s, soluble semiconducting polymers have been intensely researched for TFT applications. The possibility to engineer their transport (HOMO/LUMO) levels via chemical synthesis, their low temperature processing from solution, and their mechanical flexibility offers the potential for economical device production, as well as large-area and flexible applications. In the last view years, the interest as promising materials for the next generation of TFTs has shifted towards oxide based semiconductors such as ZnO due to their high performance and relative stability to ambient conditions. The potential of ZnO was boosted by the discovery that it can derived by pyrolysis of an organic precursor that is soluble in polar organic solvents (e.g. alcohols, ketones) thus enabling solution processing of ZnO TFTs and other devices. In addition, it has been demonstrated that both organic thin films and ZnO films can be gated by a field effect with very low threshold using deionized (DI) water as an electrolytic gate medium, leading to the ‘water- gated thin film transistors’ (WGTFTs). This discovery marks the beginning of a new method for the sensing of waterborne analytes, which differs from the classic ISFET in one significant point: Here, the aqueous sample under test is an active portion of the transducer. A number of works have since been undertaken using this discovery as a novel sensor concept for detecting waterborne analytes. Such sensors rely in the integration of analyte- specific sensitisers into the TFT architecture. For the selective sensing of ions, one of the most prominent families of water- insoluble ion sensitisers (‘ionophores’) are the calixarenes, a family of organic macrocycles. Calixarenes can be designed to selectively to complex specific cations, anions and neutral molecules

Solution Processed Electrolyte-Gated Thin Film Transistors and their Sensing applications

The thin film transistor (TFT) is one of the most important fundamental building blocks in modern electronic devices. Examples of TFT applications are in integrated circuits (ICs), amplifiers, addressing of flat panel displays, and also as chemical sensors, e.g. as ion- selective field effect transistors (ISFETs). Although the most common semiconductor materials used in thin film transistors (TFTs) is silicon (Si), the versatility of TFTs allows other semiconductors to be used instead of Si-based materials. Recent research effort has been directed towards alternative TFT semiconductors, for example solution-processable semiconductors that enable new manufacturing options. Since the 1990s, soluble semiconducting polymers have been intensely researched for TFT applications. The possibility to engineer their transport (HOMO/LUMO) levels via chemical synthesis, their low temperature processing from solution, and their mechanical flexibility offers the potential for economical device production, as well as large-area and flexible applications. In the last view years, the interest as promising materials for the next generation of TFTs has shifted towards oxide based semiconductors such as ZnO due to their high performance and relative stability to ambient conditions. The potential of ZnO was boosted by the discovery that it can derived by pyrolysis of an organic precursor that is soluble in polar organic solvents (e.g. alcohols, ketones) thus enabling solution processing of ZnO TFTs and other devices. In addition, it has been demonstrated that both organic thin films and ZnO films can be gated by a field effect with very low threshold using deionized (DI) water as an electrolytic gate medium, leading to the ‘water- gated thin film transistors’ (WGTFTs). This discovery marks the beginning of a new method for the sensing of waterborne analytes, which differs from the classic ISFET in one significant point: Here, the aqueous sample under test is an active portion of the transducer. A number of works have since been undertaken using this discovery as a novel sensor concept for detecting waterborne analytes. Such sensors rely in the integration of analyte- specific sensitisers into the TFT architecture. For the selective sensing of ions, one of the most prominent families of water- insoluble ion sensitisers (‘ionophores’) are the calixarenes, a family of organic macrocycles. Calixarenes can be designed to selectively to complex specific cations, anions and neutral molecules

CMOS integration of inkjet-printed graphene for humidity sensing.

We report on the integration of inkjet-printed graphene with a CMOS micro-electro-mechanical-system (MEMS) microhotplate for humidity sensing. The graphene ink is produced via ultrasonic assisted liquid phase exfoliation in isopropyl alcohol (IPA) using polyvinyl pyrrolidone (PVP) polymer as the stabilizer. We formulate inks with different graphene concentrations, which are then deposited through inkjet printing over predefined interdigitated gold electrodes on a CMOS microhotplate. The graphene flakes form a percolating network to render the resultant graphene-PVP thin film conductive, which varies in presence of humidity due to swelling of the hygroscopic PVP host. When the sensors are exposed to relative humidity ranging from 10-80%, we observe significant changes in resistance with increasing sensitivity from the amount of graphene in the inks. Our sensors show excellent repeatability and stability, over a period of several weeks. The location specific deposition of functional graphene ink onto a low cost CMOS platform has the potential for high volume, economic manufacturing and application as a new generation of miniature, low power humidity sensors for the internet of things.S.S. acknowledges Department of Science and Technology (DST), India for Ramanujan Fellowship to support the work (project no. SR/S2/RJN-104/2011). This work was (partly) supported through the EU FP7 project MSP (611887). T.H. acknowledges support from the Royal Academy of Engineering through a fellowship (Graphlex).This is the final version of the article. It was first available from NPG via http://dx.doi.org/10.1038/srep1737

Enhancing the Performance of Poly(3-Hexylthiophene) Based Organic Thin-Film Transistors Using an Interface Engineering Method

An original design and photolithographic fabrication process for poly(3-hexylthiophene-2, 5-diyl) (P3HT) based organic thin-film transistors (OTFTs) is presented. The structure of the transistors was based on the bottom gate bottom contact OTFT. The fabrication process was efficient, cost-effective, and relatively straightforward to implement. Current–voltage (I-V) measurements were performed to characterize the primary electronic properties of the transistors. The measured mobility of these transistors was significantly higher than most results reported in the literature for other similar bottom gate bottom contact P3HT OTFTs. The higher mobility is explained primarily by the effectiveness of the fabrication process in keeping the interfacial layers free from contamination, as well as the proper annealing of the P3HT. An interface engineering method is investigated to further enhance the performance of the OTFTs. Three interfacial materials were used for this purpose: graphene oxide (GO), poly(oligo (ethylene glycol) methyl ether methacrylate- glycidyl methacrylate- lauryl methacrylate) (P(OEGMA-GMA-LMA)) or POGL, and a composite of GO and P(OEGMA-GMA-LMA) or GO-POGL. The OTFTs with a GO interfacial layer were observed to have a higher drain current and field-effect mobility than the OTFTs with no interfacial layer. The enhanced drain current and mobility are associated with the particular structure of the P3HT layer on the dielectric surface and the reduction in the contact resistance between the GO-covered electrodes and the P3HT. The OTFTs with a POGL interfacial layer were observed to have a smaller threshold voltage than the OTFTs with no interfacial layer. The POGL OTFTs were also observed to have much more ideal drain current saturation characteristics with very small I-V curve slope. This is explained by the deep trap states on the POGL surface and the reduction of the contact resistance at the electrode/organic semiconductor interface. The OTFTs with a GO-POGL composite layer were observed to have a higher drain current and mobility, and a smaller threshold voltage than the OTFTs without an interfacial layer, which is the optimum case for these two device parameters. The higher drain current and field-effect mobility are attributed to the larger interconnecting grains of the P3HT that is deposited onto the GO-POGL surface and the smaller interfacial tension between the GO-POGL and the P3HT. The smaller threshold voltage is attributed to the deep trap states on the GO-POGL layer and the smaller contact resistance between the GO-POGL modified electrodes and the P3HT. Furthermore, experiments that could be performed to advance this research work and enhance the performance of the OTFTs even further are proposed

Charge-Modulated Field-Effect Transistor: technologies and applications for biochemical sensing

The research activity described in the attached dissertation focused on the development, fabrication and characterization of field-effect transistor-based biochemical sensor (bioFET) developed in different technologies. Such a research field has been attracting a significant interest in the last decades, as electronic sensors can represent as valuable, portable and low cost alternative to the bulk, expensive laboratory instrumentation. Among the biochemical reactions, genetic processes have been thoroughly investigated in literature: in particular, DNA hybridization detection represents a basic biological reaction for several, more sophisticated analysis in medical, pharmaceutical and forensic fields. The development of the research activity was centered on a specific biosensor, namely Charge-Modulated Field-Effect Transistor (CMFET), originally proposed in 2005 by the Electronic Department at the University of Cagliari. In particular, the aim of the activity was to make a significant step forward with respect to the results already presented in literature for DNA hybridization detection, employing two different technologies: CMOS process and organic electronics. As regards CMOS process, the activity mainly focused on the testing of a Lab-on-a-Chip (LoC), hosting several CMFET structures, developed and fabricated before but never tested. The activity carried out allowed to develop a precise electrical model of the device, validated by actual measurements, by which the basic performances of the device were derived. Subsequently, the application of the LoC for DNA hybridization detection was demonstrated: a reliable biochemical protocol for the modification of the chip surface with DNA strands was developed, as well as a precise measurement procedure. A complete evaluation of the sensitivity and selectivity of the device with respect to DNA hybridization was obtained; from the obtained results, several consideration about the relationship between the chip layout and the performances of the device were inferred. In conclusion, a road-map for the development of a new chip, customized for the application as DNA hybridization sensor, was developed. As regards the Organic CMFET (OCMFET), the activity comprised design, fabrication and testing of devices particularly conceived as disposable DNA hybridization sensors for field-measurement kits. Such a task required the development of innovative technological processes for the fabrication of high-performances organic transistors, i.e. transistors capable to be operated at low voltages (about 1 V) with quasi-ideal electrical performances. In particular, a highly reliable fabrication process, compatible with plastic electronics and easily up-scalable to an industrial size, was determined. Consequently, novel OCMFET were fabricated and tested. World record results in terms of sensitivity and selectivity among the organic transistor-based DNA sensors were reproducibly obtained. Thanks to the reliability of the results, the performances of the OCMFET were carefully studied, and design rules for the optimization of the device were inferred; an optimized, low voltage OCMFET allowed to further enhance the result, determining final performances even better than the one of silicon-based sensors. Finally, thanks to an innovative analysis on the influence of the device polarization to the characteristics of the bioreceptor layer at a micro-nanometrical size, a physical effect related to a tilting of the DNA molecules with respect to the surface was observed. This feature, possibly related to the CMFET working principle, can allow to overcome a general limitation of the bioFET technologies that have limited so far the application of these devices in vivo, thus opening novel possible applications for the CMFET working principle beyond the measurements in vitr

Sewage sludge heavy metal analysis and agricultural prospects for Fiji

Insoluble residues produced in Waste Water Treatment Plants (WWTP) as by products are known as sewage sludge (SS). Land application of SS, particularly in agricultural lands, is becoming an alternative disposal method in Fiji. However, currently there is no legislative framework governing its use. SS together with its high nutrient and organic matter contents, constitutes some undesired pollutants such as heavy metals, which may limit its extensive use. The focus of this study therefore was to determine the total concentrations of Pb, Zn, Cd, Cu, Cr, Ni and Mn in the SS produced at the Kinoya WWTP (Fiji) and in the non-fertile soil amended with the SS at 20, 40, 60, 80% application rates and in the control (100% Soil). The bioavailable heavy metals were also determined as it depicts the true extent of metal contamination. The treatment mixtures were then used to cultivate cabbage plants in which the total heavy metal uptake was investigated. Total Zn (695.6 mg/kg) was present in the highest amounts in the 100% SS (control), followed by Pb (370.9 mg/kg), Mn (35.0 mg/kg), Cu (65.5 mg/kg), Cr (20.5 mg/kg) and finally Cd (13.5 mg/kg) and hence a similar trend was seen in all treatment mixtures. The potential mobility of sludgeborne heavy metals can be classified as Ni > Cu > Cd > Zn > Mn > Cr > Pb. Total metal uptake in plant leaves and stems showed only the bioavailable metals Cu, Cd, Zn and Mn, with maximum uptake occurring in the leaves. Ni, despite being highly mobile was not detected, due to minute concentrations in the SS treatments. Optimum growth occurred in the 20 and 40% SS treatments. However maximum Cu and Mn uptake occurred in the 40% SS treatment thereby making the 20% treatment the most feasible. Furthermore the total and bioavailable metal concentrations observed were within the safe and permitted limits of the EEC and USEPA legislations