Interface and gate bias dependence responses of sensing organic thin-film transistors

The effects of the exposure of organic thin-film transistors, comprising different organic semiconductors and gate dielectrics, to 1-pentanol are investigated. The transistor sensors exhibited an increase or a decrease of the transient source-drain current in the presence of the analyte, most likely as a result of a trapping or of a doping process of the organic active layer. The occurrence of these two effects, that can also coexist, depend on the gate-dielectric/organic semiconductor interface and on the applied gate field. Evidence of a systematic and sizable response enhancement for an OTFT sensor operated in the enhanced mode is also presented.     

Development of immunosensors for the analysis of 1-naphthol in organic media

Immunosensor systems have been developed for the rapid determination of 1-naphthol. In this work, the comparison of performance of immunosensors working in aqueous and organic media was done. Direct, indirect and capture formats were studied. Immunoreagents were immobilized on controlled pore glass (CPG), hidroxysuccinimide agarose gel or on azlactone Protein A/G supports. The Protein A/G-based sensor showed the best performance. In aqueous media, a LOD of 16.2 microg l(-1) and a DR of 33.7-586.6 microg l(-1) were achieved employing Tween 20 at a concentration ranging from 0.01 to 0.05% v/v. Maximum sensitivity was reached with 0.025% of surfactant. Binary mixtures of methanol or acetonitrile with aqueous buffer and ternary mixtures of methanol/isopropanol or ethyl acetate/methanol with the same buffer were studied as organic media. The mixture 50% MeOH-50% 20 mM sodium phosphate, pH 8, with 0.05% (v/v) Tween 20 resulted to be the best. A detection limit of 12.0 microg l(-1) and a dynamic range of 53.6-17,756.0 microg l(-1) were reached. The recycling of Protein A/G-based sensor working in this media was about 300 assays. Preconcentration factors around 250 were achieved using methanol as extracting solvent. It has been demonstrated that the technique can be successful in carrying out the analysis of low solubility in water analytes, such as 1-naphthol. The sensors developed can use higher concentrations of organic solvent (up to 50% methanol) compared to ELISA. On the other hand, the advantage of preconcentration can also be taken for the use of the same procedure as recommended for standard sample treatments.     

Labelfree fully electronic nucleic acid detection system based on a field-effect transistor device

The labelfree detection of nucleic acid sequences is one of the modern attempts to develop quick, cheap and miniaturised hand-held devices for the future genetic testing in biotechnology and medical diagnostics. We present an approach to detect the hybridisation of DNA sequences using electrolyte-oxide-semiconductor field-effect transistors (EOSFETs) with micrometer dimensions. These semiconductor devices are sensitive to electrical charge variations that occur at the surface/electrolyte interface, i.e. upon hybridisation of oligonucleotides with complementary single-stranded (ss) oligonucleotides, which are immobilised on the oxide surface of the transistor gate. This method allows direct, time-resolved and in situ detection of specific nucleic acid binding events without any labelling. We focus on the detection mechanism of our sensors by using oppositely charged polyelectrolytes (PAH and PSS) subsequently attached to the transistor structures. Our results indicate that the sensor output is charge sensitive and distance dependent from the gate surface, which pinpoints the need for very defined surface chemistry at the device surface. The hybridisation of natural 19 base-pair sequences has been successfully detected with the sensors. In combination with nano-transistors a PCR free detection system might be feasible in future.     

Ultra-thin Semiconductor/Metal Resonant Superabsorbers

The design of thin-film semiconductor absorbers is a long-sought-after goal of crucial importance for optoelectronic devices. We propose a new strategy that achieves multi-band optical absorption in an ultra-thin semiconductor-insulator-metal nanostructure. The whole thickness of the absorber is just 60 nm, which is less than λ/12. The ultra-thin semiconductor resonators are used as the photonic coupling elements. The plasmonic metal layer with the thickness about 15 nm simultaneously acts as the transmission cancel layer and the plasmon source for resonant coupling with the optical near-field energy. The combined semiconductor resonators and the thin metal film produce strong electromagnetic field coupling and confinement effects, which mainly contribute to the efficient light trapping for the multi-band strong light absorption. The semiconductors such as Si, GaAs, and Ge are confirmed with the capability to show high light absorption via this simple hybrid metal-semiconductor resonant system. These features pave new insight on ultra-thin semiconductor absorbers and hold potential applications for optoelectronics such as nonlinear optics, hot-electron excitation and extraction, and the related devices.

     

Fabrication of ultra-thin polypyrrole-glucose oxidase film from supporting electrolyte-free monomer solution for potentiometric biosensing of glucose

A simple electropolymerisation process is described for the fabrication of an ultra-thin ( approximately 55 nm) polypyrrole (PPy)-glucose oxidase (GOD) film in a supporting electrolyte-free monomer solution for potentiometric biosensing of glucose. The optimum conditions for growing the ultra-thin film include 0.1 M pyrrole, 55-110 U/ml GOD, an applied current density of 0.05 mA/cm(2) and an electrical charge of 25 mC/cm(2). Long-term storage of the biosensor in acetate buffer improved the sensitivity of the biosensor by a factor of approximately two. The biosensor can also be used repeatedly for over 2 months with little or no loss in sensitivity. The interference effect of ascorbic acid was successfully reduced by inclusion of an outer PPy-Cl layer.     

Detection of clinically relevant levels of protein analyte under physiologic buffer using planar field effect transistors

Electrochemical detection of protein binding at physiological salt concentration by planar field effect transistor platforms has yet to be documented convincingly. Here we report detection of streptavidin and clinically relevant levels of biotinylated monokine induced by interferon gamma (MIG) at physiological salt concentrations with AlGaN heterojunction field effect transistors (HFETs). The AlGaN HFETs are functionalized with a silane linker and analyte-specific affinity elements. Polarity of sensor responses is as expected from n-type HFETs to negatively and positively charged analytes. Sensitivity of the HFET sensors increases when salt concentration decreases, and the devices also exhibit dose-dependent responses to analyte. Detection of clinically relevant MIG concentrations at physiological salt levels demonstrates the potential for AlGaN devices to be used in development of in vivo biosensors.     

On the application of the Clark oxygen electrode to the study of enzyme kinetics in apolar solvents: the catalase reaction.

A method for recording O2 concentrations in nonconducting organic media with the Clark oxygen electrode was developed. Spontaneous oxidation of Na2S2O4 and the enzymatic reduction of NaBO3 or H2O2 by bovine liver catalase trapped in hydrated micelles of dioctylsulfosuccinate (AOT)/toluene were used as model systems. O2 titration with the above systems showed that air-saturated 1.6 M H2O/0.2 M AOT/toluene media contain seven times more O2 (1.4 mM) than aqueous solutions (0.2 mM). The measured Km values of catalase for NaBO3 and H2O2 in organic media were Kmov = 15 and 17 mM, respectively, whereas in aqueous buffer the values were 45 and 54 mM. In the toluene media, catalase activity increased with the W0 (H2O/AOT molar ratio) of the micellar preparation, reaching maximal activity at W0 = 10-12; under this condition, the catalytic center activity (Kp) of H2O2 was 7 x 10(6) min-1, similar to that obtained in the aqueous buffer (H2O2 = 7 x 10(6) min-1). It was found that the optimal pH for catalase in toluene media (pH 8.0) was shifted 1.0 unit compared to that in the aqueous buffer (pH 7.0). On the other hand, catalase was severely inhibited by NaN3 in both media. Thus, polarography based on the Clark oxygen electrode seems to be an easy, rapid, and sensitive technique for studying enzyme reactions consuming or evolving O2 in apolar media.     

Random CNT network and regioregular poly(3-hexylthiophen) FETs for pH sensing applications: A comparison

Background

Nowadays, there is a tremendous need for cheap disposable sensing devices for medical applications. Materials such as Carbon Nanotubes (CNTs) and regioregular P3HT are proven to offer a huge potential as cost-effective and solution processable semiconductors for (bio)sensing applications.

Methods

CNT-based field-effect transistors (CNT-FETs) as well as regioregular P3HT-based ones (P3HT-FETs) are fabricated and operated in the so-called electrolyte-gated configuration. The active layer of the P3HT-FETs consists of a spin-coated regioregular P3HT layer, which serves on one hand as the active sensing element and on the other hand as passivation layer for the transistor's metal contacts. The active layer of the nanotube transistors consists of a randomly distributed single walled CNT-network (> 90% semiconducting tubes) deposited from a CNT-ink solution by spin-coating.

Results

We compare both devices concerning their stability in aqueous environment and their response when exposed to buffers with different pH. We found that even if P3HT shows lower stability its pH sensitivity is reproducible even after long-term measurements.

Conclusion

CNT-FETs and P3HT-FETs offer different advantages and drawbacks concerning their stability in solution and the ease of fabrication. A discussion of their different sensing mechanisms as well as sensitivity is given here.

General Significance

This work reports on fast and cost-effective production of solution processable thin-film transistors based on carbon nanotubes and regioregular P3HT and demonstrates their suitability as reliable pH sensors. This article is part of a Special Issue entitled Organic Bioelectronics — Novel Applications in Biomedicine.     

A sensor for superoxide in aqueous and organic/aqueous media based on immobilized cytochrome c on binary self-assembled monolayers

A method for the electrochemical detection of superoxide radical was developed, based on cytochrome c (cyt c) immobilized on the binary self-assembled monolayers (SAMs) of thioctic acid (T-COOH) and thioctic amide (T-NH2) on gold electrode. The sensor works by electrochemically detecting cyt c reduced by the superoxide radical generated by a xanthine-XOD system. The electrochemical properties of immobilized cyt c were investigated in aqueous buffer and in a mixture of aqueous and organic solvents. The interaction of superoxide radical with the modified electrode was characterized in phosphate buffer solution (PBS) and in the mixtures of both PBS and dimethyl sulfoxide (DMSO) and PBS and glycerol (Gly). The results showed that the sensors responded immediately to superoxide radical in PBS and gave a steady-state anodic current within 10s during the generation of superoxide radical. In 40% DMSO and in 30% Gly solution, the current response reached a steady-state anodic current within 20s. The sensor could also be used to estimate superoxide dismutase (SOD).     

Reproducible fabrication of miniaturized glucose sensors: preparation of sensing membranes for continuous monitoring.

The immobilization process of glucose oxidase(GOx) in the poly(1,3-diaminobenzene) (poly(1,3-DAB)) network was closely investigated in situ using an electrochemical quartz crystal microbalance(EQCM). GOx captured in approximately 50 nm thick poly-1,3-DAB layer causes a 514 Hz frequency increase, corresponding to 541 ng, and distributes mostly in the outer part of the polymer film. The presence of poly-L-lysine and glutaraldehyde during electropolymerization of poly(1,3-DAB) improves sensitivity by raising the amount of GOx immobilized. Adding a protective membrane on to the enzyme layer from poly(tetrafluoroethylene) (PTFE) dispersed in aqueous media lets the entire fabrication procedure finish perfectly without nonaqueous solvent. The finalized needle-type glucose sensors show competent functions in sensitivity, stability, biocompatibility, lifetime, interference and reproducibility.     

Using silicon nanowire devices to detect adenosine triphosphate liberated from electrically stimulated HeLa cells

In this study, we used a biosensor chip featuring Abl tyrosine kinase-modified silicon nanowire field-effect transistors (SiNW-FETs) to detect adenosine triphosphate (ATP) liberated from HeLa cells that had been electrically stimulated. Cells that are cultured in high-ionic-strength media or buffer environments usually undermine the sensitivity and selectively of SiNW-FET-based sensors. Therefore, we first examined the performance of the biosensor chip incorporating the SiNW-FETs in both low- and high-ionic-strength buffer solutions. Next, we stimulated, using a sinusoidal wave (1.0 V, 50 Hz, 10 min), HeLa cells that had been cultured on a cell-culture chip featuring interdigitated electrodes. The extracellular ATP concentration increased by ca. 18.4-fold after electrical stimulation. Finally, we detected the presence of extracellular ATP after removing a small amount of buffer solution from the cell-cultured chip and introducing it into the biosensor chip.     

Field-effect devices for detecting cellular signals

The integration of living cells together with silicon field-effect devices challenges a new generation of biosensors and bioelectronic devices. Cells are representing highly organised complex systems, optimised by millions of years of evolution and offering a broad spectrum of bioanalytical receptor “tools” such as enzymes, nucleic acids proteins, etc. Their combination with semiconductor-based electronic chips allows the construction of functional hybrid systems with unique functional and electronic properties for both fundamental studies and biosensoric applications. This review article summarises recent advances and trends in research and development of cell/transistor hybrids (cell-based field-effect transistors) as well as light-addressable potentiometric sensors.     

The effects of reformulating buffer species and their concentrations on subtilisin-catalyzed optical resolution of racemic 1-phenylethylamine in 3-methyl-3-pentanol

For the optical resolution of racemic 1-phenylethylamine in 3-methyl-3-pentanol, subtilisin was reformulated by lyophilization with buffer salts. The amide synthesis activity of subtilisin in organic solvent was compared with the hydrolysis activity in aqueous buffer when different buffer species and their concentrations were used in lyophilization. The enzyme activity in organic solvent showed a different pattern from that of the hydrolysis depending upon the species and the concentrations of buffers. Morphology of the reformulated subtilisin was examined by scanning electron microscopy (SEM). The porosity of reformulated subtilisin particles increased up to the optimal buffer concentrations for the amide synthesis in organic solvent. Glassy looks and decrease in porosity developed at high (i.e. above the optimal) buffer concentrations appear to affect the decrease in the synthetic activity in organic media.     

Organic charge transfer complex based printable biosensor

This paper describes the preparation of an organic charge transfer complex (CTC) based printable enzyme electrode. CTC crystals were prepared by mixing TCNQ powder with TTF solution (in acetonitrile). Glucose oxidase (GOD) was adsorbed at the CTC crystal surface in a monolayer. A printable paste was prepared by mixing GOD-adsorbed crystals with a binder and a solvent. This paste was applied to an electrode cavity and vacuum dried. A thin layer of gelatin was cast on the paste filled dried electrode, and cross-linked with glutaraldehyde in the dry condition. The sensors were fixed in a flow injection system, and continuously polarized at 0·15 V and 37°C, and the samples were automatically injected every 30 min. The developed sensors produced a huge response curren with an extended linear range of detection (0–100 mM) and the response was unaffected by the presence of normal oxygen in the buffer solution. The sensor showed excellent stability. The performance of the sensors was significantly influenced by the binder used.     

Preparation of the gemini detergent-lipase complexes and their high enzymatic activities in the transesterifications in homogeneous organic solvents

Lipase and newly synthesized gemini-type detergent were mixed in an aqueous phosphate buffer solution and lyophilized. Due to complex formation between the substances it was possible to dissolve or at least finely disperse these preparations in organic media. The lipase-detergent complexes were used to catalyse the irreversible transesterfication of 6-methyl-5-hepten-2-ol (sulcatol) or 2,2-dimethyl-1,3-dioxolane-4-methanol (solketal) with vinyl or isopropenyl carboxylate in organic solvents. The complexes were considerably more active than enzyme powder or the complexes prepared with conventional synthesized detergents in organic media. © Rapid Science Ltd. 1998     

A nano-sized Au electrode fabricated using lithographic technology for electrochemical detection of dopamine

One big challenge of fabricating nanosensors for spatially resolved electrochemical detection of neurochemicals, such as dopamine (DA), is the difficulty to assembly nanometer-scale patternable and integrated sensors. In this work we develop a novel approach to precisely manufacture nano-Au-electrode (NAE) using lithographic fabrication technique, and characterize the NAE for DA detection. A negative photoresist, SU-8, is used as a substrate and protection layer for the 127-nm Au active sensing layer. The cross surface morphology and thickness of the Au layer are imaged by scanning electron microscopy and an interference microscopy. This NAE could be precisely controlled, repeatedly fabricated and conveniently renewed for several times. The electrochemical sensitivity and selectivity of the NAE towards DA detection are significantly higher than those of a standard Au thin-film electrode. This work demonstrates that the NAE could be used as an attractive means for electrochemically sensing and recording DA.     

Polymer Solar Cells with Efficiency >10% Enabled via a Facile Solution‐Processed Al‐Doped ZnO Electron Transporting Layer

A facile and low‐temperature (125 °C) solution‐processed Al‐doped ZnO (AZO) buffer layer functioning very effectively as electron accepting/hole blocking layer for a wide range of polymer:fullerene bulk heterojunction systems, yielding power conversion efficiency in excess of 10% (8%) on glass (plastic) substrates is described. The ammonia‐treatment of the aqueous AZO nanoparticle solution produces compact, crystalline, and smooth thin films, which retain the aluminum doping, and eliminates/reduces the native defects by nitrogen incorporation, making them good electron transporters and energetically matched with the fullerene acceptor. It is demonstrated that highly efficient solar cells can be achieved without the need for additional surface chemical modifications of the buffer layer, which is a common requirement for many metal oxide buffer layers to yield efficient solar cells. Also highly efficient solar cells are achieved with thick AZO films (>50 nm), highlighting the suitability of this material for roll‐to‐roll coating. Preliminary results on the applicability of AZO as electron injection layer in F8BT‐based polymer light emitting diode are also presented.     

Chloroperoxidase-catalyzed Epoxidation of Styrene in Aqueous and Nonaqueous Media

To overcome poor product yields and stability in aqueous solution, we have examined the chloroperoxidase (CPO from Caldariomyces fumago ) catalyzed oxidation of styrene in organic media using tert -butyl hydroperoxide as external oxidant. CPO's intrinsic catalytic activity in tert -butanol , as reflected in its k cat value, was ca. one-fourth of that in aqueous buffer, indicating that the enzyme remains highly active in the organic solvent. Styrene epoxidation reactions were modeled in both aqueous and nonaqueous media to provide global kinetic information, which dominates non-initial rate conditions and is heavily influenced by continuous deactivation of the CPO. Deactivation studies revealed that the enzyme is deactivated quickly by the combination of the tert -butyl hydroperoxide and styrene, possibly due to the styrenic free radicals generated during the enzymatic reaction. These results may enable catalyst-engineering strategies to be initiated to improve the prospects of using CPO in nonaqueous media for large-scale epoxidation reactions.     

The determination of phospholipase D activity in emulsion systems

Although phospholipase D (PLD) is often used in emulsion systems consisting of buffer and a nonpolar organic solvent, most activity assays have been designed to work in purely aqueous milieu. Here a method is described for the determination of PLD activity in emulsion systems. The assay is based on the transphosphatidylation of phosphatidylcholine with 1-butanol in dichloromethane/buffer with the subsequent densitometric quantification of the products after their separation by HPTLC and staining with a CuSO4/H3PO4 reagent. The method is particularly appropriate for the determination of enzymes such as PLD from Streptomyces sp. that prefer the exchange of the head group in glycerophospholipids to their hydrolysis. Since the application of an organic solvent in the PLD assay allows the determination of the enzyme in analytes insoluble in aqueous media, the method can also be used to determine PLD activity in the presence of high concentrations of phospholipids.     

Lipase-catalyzed cellulose acetylation in aqueous and organic media

Screening for lipases capable of catalyzing acetylation of cellulosic substrates was conducted in aqueous buffer solution using water-soluble carboxymethyl cellulose (CMC) as substrate. Lipase A12 from Aspergillus niger (A. niger) showed the most promising acetylation activity among 11 tested commercial microbial lipases and was further applied to catalyzing acetylation of solid cellulose in aqueous solution. This reaction was shown to be feasible with an acetylation extent of 0.16 wt % achieved compared with no detectable acetylation in the absence of enzyme. Pretreatments on cellulose substrate by ultrasonic irradiation and surfactant solution only slightly improved the acetylation extent by 44 and 27%, respectively. Alternatively, this lipase-catalyzed acetylation was remarkably improved with solubilized cellulose as substrate in the dimethyl sulfoxide/paraformaldehyde solvent system, with an acetylation extent (7.87 wt %) nearly 50 times higher than that achieved in aqueous solution. This improvement was attributed to (1) the absence of bulk water and the increase in substrate solubility by the transition of reaction media from aqueous solution to organic solvents and (2) the ability of lipase A12 to remain catalytically active in highly polar DMSO. This discovery that the A. niger lipase was capable of surviving its contact with polar solvents was further confirmed by its considerably preserved catalytic activity on CMC acetylation in aqueous media after enzyme pretreatments with organic solvents of various polarities and in mixture media with the aqueous phase partially replaced by organic solvents.