Solution-gated Field-effect Transistors Research Articles

Aptamer-based biosensor for sensitive PDGF detection using diamond transistor

The detection of platelet-derived growth factor (PDGF) via a solution-gate field-effect transistor (SGFET) has been demonstrated for the first time using aptamers immobilized on a diamond surface. Upon introduction of PDGF to the immobilized aptamer, a shift of 31.7 mV in the negative direction is observed at a source-drain current of −50 μA. A shift of 32.3 mV in the positive direction is detected after regeneration by SDS solution, indicating that the static measurement returns to its original value. These SGFETs operate stably within the large potential window of diamond (>3.0 V), and hence the surface channel does not need passivating with a thick insulating layer. Thereof, the immobilized aptamer channels have been exposed directly to the electrolyte solution without a gate insulator. Immobilization is achieved via aptamers covalently bonding to amine sites, thereby increasing the sensitivity of the biosensors. Diamond SGFETs have potential for the detection of PDGF and show durability against biological degradation after repeated usage and regeneration.

Directly Grown Nanocrystalline Diamond Field-Effect Transistor Microstructures

Nanocrystalline diamond microscopic structures (5 m width) are grown directly on Si/SiO2 substrates by patterning of a nucleation layer using photolithography prior to the diamond growth. The diamond in a thickness of 300 nm is grown by a microwave chemical vapor deposition on the nucleation patterns. Morphology and material composition of the microstructures are characterized by scanning electron microscopy and atomic force microscopy. The diamond microstructures exhibit clear transistor characteristics in both solid-state and solution-gated field-effect transistor configurations. The conductivity is generated by a transfer doping of hydrogen-terminated diamond surface. Gating of the transistors is realized by a deposition of Al electrode or by an immersion into a pH buffer in contact with Ag/AgCl electrode. The solution-gated field-effect transistors are sensitive to pH.

Fabrication of calcium ion sensitive diamond field effect transistors (FETs) based on immobilized calmodulin

We have developed calcium (Ca2+) ion sensitive solution-gate field effect transistors (SGFETs) on polycrystalline diamond by using partial amination and immobilization of calmodulin (CaM), a specific protein to calcium ions. The CaM is covalently immobilized on functionalized diamond surface, and the functionalized surface was analyzed by X-ray photoelectron spectroscopy, respectively. Also, the high performance of the CaM-immobilized diamond SGFET for detecting Ca2+ were studied with respect to high selectivity and sensitivity in various concentrations and pH.

Ultrathin GaN/AlN/GaN solution-gate field effect transistor with enhanced resolution at low source-gate voltage

The pH response of a GaN/AlN/GaN solution-gate field effect transistor (SGFET), with a GaN/AlN barrier of 7.5 nm thick, is analyzed and compared with standard GaN/AlGaN/GaN SGFETs with total barrier thicknesses of 19 and 23 nm. While all types of SGFETs show a similar surface sensitivity to H + ions, a significant improvement in the transducive sensitivity of the SGFET source-drain current under pH changes is found when decreasing the barrier thickness, due to the increased transconductance of the FET structure. Resolution better than 0.005 pH can be estimated in the case of the ultrathin SGFET. Moreover, the maximum transconductance value shifts to gate-drain voltage close to 0 V, which eventually involves no need of reference electrode in less demanding applications, simplifying the final design of the device and making AlN barrier-based SGFETs highly recommended in the broad field of chemical sensors.

Solution-Gated Epitaxial Graphene as pH Sensor

A solution-gate field effect transistor (SGFET) has been fabricated on few-layer graphene (FLG). The ideally polarizable graphene/aqueous electrolyte interface allows the capacitive charging of the surface by hydroxyl (OH-) and hydroxonium ions (H3O+). The conductivity versus gate potential curve exhibits "V" shaped ambipolar transfer characteristics of graphene, with hole and electron mobilities of 3600 cm2/Vs and 2100 cm2/Vs, respectively. The shift of the negative gate potential with pH shows a supra-Nernstian response of 99 meV/pH. Our work points to the potential application of graphene in ultrafast and ultralow noise chemical or biological sensors.

Hydrogen-terminated diamond electrodes. I. Charges, potentials, and energies

Hydrogen-terminated intrinsic diamond is a most unusual insulator since in contact with air or electrolyte it develops a hole accumulation layer just below the surface. When immersed in electrolyte the hole concentration responds to ionic charges and potentials and this response can be monitored by measuring the conductance of the hole accumulation layer without interference form the bulk conductivity. This feature has been widely used for chemical sensors in the form of solution-gate field effect transistors (SGFET). Here we analyze the charge and the potential profiles in the diamond and the electrolyte, as well as the static differential capacitance of the diamond electrode under controlled potential conditions. From this analysis we derive expressions for the transfer characteristics of diamond-based SGFET's that faithfully describe experimental data presented here as well. This holds in particular for the threshold region of the transfer characteristics that can only be modeled if the unusual semiconducting properties of the diamond electrode are taken into account properly. From fits to our data we derive (among other things) a value of chi=-0.50+/-0.02eV for the electron affinity of the hydrogen-terminated diamond surface in contact with aqueous electrolyte.

Detection of Mismatched DNA on Partially Negatively Charged Diamond Surfaces by Optical and Potentiometric Methods

The effects of surface charge density on DNA hybridization have been investigated on a mixture of hydrogen-, oxygen-, and amine-terminated diamond surfaces. A difference in the hybridization efficiencies of complementary and mismatched DNA was clearly observed by fluorescence and potentiometric observations at a particular coverage of oxygen. In the fluorescence observation, singly mismatched DNA was detected with high contrast after appropriate hybridization on the surface with 10-20% oxygen coverage. The amount of oxygen in the form of C-O(-) (deprotonated C-OH) producing the surface negative-charge density was estimated by X-ray photoelectron spectroscopy. Electrolyte solution gate field-effect transistors (SGFETs) were used for potentiometric observations. The signal difference (change in gate potential) on the SGFET, which was as large as approximately 20 mV, was caused by the difference in the hybridization efficiencies of complementary target DNA (cDNA) and singly mismatched (1MM) target DNA with a common probe DNA immobilized on the same SGFET. The reversible change in gate potential caused by the hybridization and denaturation cycles and discriminating between the complementary and 1MM DNA targets was very stable throughout the cyclical detections. Moreover, the ratio of signals caused by hybridization of the cDNA and 1MM DNA targets with the probe DNA immobilized on the SGFET was determined to be 3:1 when hybridization had occurred (after 15 min on SGFET), as determined by real-time measurements. From the viewpoint of hybridization kinetics, the rate constant for hybridization of singly mismatched DNA was a factor of approximately 3 smaller than that of cDNA on this functionalized (oxidized and aminated) diamond surface.

Deoxyribonucleic acid sensor using electrolyte solution gate field effect transistors

Deoxyribonucleic acid sensor using electrolyte solution gate field effect transistors

Miniaturized diamond field-effect transistors for application in biosensors in electrolyte solution

The authors fabricated diamond solution-gate field-effect transistors (SGFETs) with miniaturization of the channel length to 5μm by photolithography. The channel surface was directly functionalized with amine by ultraviolet irradiation in an ammonia gas for 4h and aminated diamond SGFETs were sensitive to pH by 40mV∕pH. Urease was immobilized on the amine-modified channel surface, which was sensitive to urea by 27μA/decade from 10−5M to 10−2M. The authors fabricated submicron-sized (500nm) diamond SGFETs using electron-beam lithography. The transconductance (gm) was 56mS∕mm, which was 930-fold greater than that of the 500μm channel length.

Quantitative Real-Time Measurements of DNA Hybridization with Alkylated Nonoxidized Silicon Nanowires in Electrolyte Solution

The quantitative, real-time detection of single-stranded oligonucleotides with silicon nanowires (SiNWs) in physiologically relevant electrolyte solution is demonstrated. Debye screening of the hybridization event is circumvented by utilizing electrostatically adsorbed primary DNA on an amine-terminated NW surface. Two surface functionalization chemistries are compared: an amine-terminated siloxane monolayer on the native SiO2 surface of the SiNW, and an amine-terminated alkyl monolayer grown directly on a hydrogen-terminated SiNW surface. The SiNWs without the native oxide exhibit improved solution-gated field-effect transistor characteristics and a significantly enhanced sensitivity to single-stranded DNA detection, with an accompanying 2 orders of magnitude improvement in the dynamic range of sensing. A model for the detection of analyte by SiNW sensors is developed and utilized to extract DNA-binding kinetic parameters. Those values are directly compared with values obtained by the standard method of surface plasmon resonance (SPR) and demonstrated to be similar. The nanowires, however, are characterized by higher detection sensitivity. The implication is that SiNWs can be utilized to quantitate the solution-phase concentration of biomolecules at low concentrations. This work also demonstrates the importance of surface chemistry for optimizing biomolecular sensing with silicon nanowires.

Catalytic activity of enzymes immobilized on AlGaN∕GaN solution gate field-effect transistors

Enzyme-modified field-effect transistors (EnFETs) were prepared by immobilization of penicillinase on AlGaN∕GaN solution gate field-effect transistors. The influence of the immobilization process on enzyme functionality was analyzed by comparing covalent immobilization and physisorption. Covalent immobilization by Schiff base formation on GaN surfaces modified with an aminopropyltriethoxysilane monolayer exhibits high reproducibility with respect to the enzyme/substrate affinity. Reductive amination of the Schiff base bonds to secondary amines significantly increases the stability of the enzyme layer. Electronic characterization of the EnFET response to penicillin G indicates that covalent immobilization leads to the formation of an enzyme (sub)monolayer.

Label-free DNA sensors using ultrasensitive diamond field-effect transistors in solution

Charge detection biosensors have recently become the focal point of biosensor research, especially field-effect-transistors (FETs) that combine compactness, low cost, high input, and low output impedances, to realize simple and stable in vivo diagnostic systems. However, critical evaluation of the possibility and limitations of charge detection of label-free DNA hybridization using silicon-based ion-sensitive FETs (ISFETs) has been introduced recently. The channel surface of these devices must be covered by relatively thick insulating layers ( SiO2, Si3N4, Al2O3, or Ta2O5) to protect against the invasion of ions from solution. These thick insulating layers are not suitable for charge detection of DNA and miniaturization, as the small capacitance of thick insulating layers restricts translation of the negative DNA charge from the electrolyte to the channel surface. To overcome these difficulties, thin-gate-insulator FET sensors should be developed. Here, we report diamond solution-gate FETs (SGFETs), where the DNA-immobilized channels are exposed directly to the electrolyte solution without gate insulator. These SGFETs operate stably within the large potential window of diamond (>3.0 V). Thus, the channel surface does not need to be covered by thick insulating layers, and DNA is immobilized directly through amine sites, which is a factor of 30 more sensitive than existing Si-ISFET DNA sensors. Diamond SGFETs can rapidly detect complementary, 3-mer mismatched (10 pM) and has a potential for the detection of single-base mismatched oligonucleotide DNA, without biological degradation by cyclically repeated hybridization and denature.

PH-sensitive diamond field-effect transistors (FETs) with directly aminated channel surface

We have introduced pH sensors fabricated on diamond thin films through modification of the surface-terminated atom. We directly modified the diamond surface from hydrogen to amine or oxygen with ultraviolet (UV) irradiation under ammonia gas. The quantified amine site based on the spectra obtained by X-ray photoelectron spectroscopy (XPS) is 26% (2.6 × 10 14 cm −2) with UV irradiation for 8 h and its coverage is dependent on the UV irradiation time. This directly aminated diamond surface is stable with long-term exposure in air and electrolyte solution. We fabricated diamond solution-gate field-effect transistors (SGFETs) without insulating layers on the channel surface. These diamond SGFETs with amine modified by direct amination are sensitive to pH (45 mV/pH) over a wide range from pH 2 to 12 and their sensitivity is dependent on the density of binding sites corresponding to UV irradiation time on the channel surface.