Touhidur Rahman

A Mediator Free Amperometric Bienzymatic Glucose Biosensor Using Vertically Aligned Carbon Nanofibers (VACNFs)

A biosensor is proposed for selective and sensitive detection of glucose. This electrochemical amperometric bienzymatic glucose biosensor is constructed by co-immobilization of horseradish peroxidase and glucose oxidase on vertically aligned carbon nanofibers (VACNFs). An enzyme wiring technique is used to plug the enzymes with the metal electrode using VACNFs, which facilitates an effective way to transfer electrons from the electrode to the electrochemical reaction centers. Direct electron transfer of horseradish peroxidase at the VACNF electrode is observed. Mediator or membrane free operation of this biosensor can potentially result in the application of these sensors in environmental monitoring, healthcare, as well as in varieties of scientific experiments. Experimental results indicate that the proposed biosensor can detect very low level of glucose (as low as 0.4 μM). Operational characteristics of the bienzymatic biosensor in terms of detection limit, sensitivity, and selectivity are also examined.

A vertically aligned carbon nanofiber (VACNF) based amperometric glucose sensor

Due of the clinical significance of measuring blood glucose levels of diabetic patients substantial research and development efforts have been devoted to the realization of reliable glucose sensors for in vitro or in vivo applications [1]. Wang et al introduced a mediator-free and membrane-free biosensor which provided a novel direction for biosensor development [2]. On the other hand the presence of defect sites in vertically aligned carbon nanofiber (VACNF) results in better mechanical stability, sensitivity and response compared to carbon nanotubes (CNT) for sensor applications [3–4]. Based on these scientific observations a mediator-free, bienzyme, glucose sensor incorporating VACNF is proposed in this paper.

Integration of a dose control circuit with a vertically aligned nanofiber field emission device

This paper discusses the complete integration of the prototype digital electrostatic focused e-beam array direct-write lithography (DEAL) device with the dose control circuitry (DCC). The DCC regulates charge emission from the vertically aligned carbon nanofibers (VACNFs) and prevents resists from being over exposed during the e-beam lithography process. The emission of electrons from the VACNF tip requires relatively high voltage. The I-V characteristic of a typical VACNF based device is presented with threshold voltage of ~75 V. The DCC built using a standard 5 V digital CMOS process cannot handle such voltage levels

A Precision Dose Control Circuit for Maskless E-Beam Lithography With Massively Parallel Vertically Aligned Carbon Nanofibers

This paper describes a highly accurate dose control circuit (DCC) for the emission of a desired number of electrons from vertically aligned carbon nanofibers (VACNFs) in a massively parallel maskless e-beam lithography system. The parasitic components within the VACNF device cause a premature termination of the electron emission, resulting in underexposure of the photoresist. In this paper, we compensate for the effects of the parasitic components and noise while reducing the area of the chip and achieving a precise count of emitted electrons from the VACNFs to obtain the optimum dose for the e-beam lithography.

A highly selective mediator less glucose detector employing vertically aligned carbon nanofiber (VACNF)

This work reports a vertically aligned carbon nanofiber (VACNF) based glucose detector which shows excellent selectivity without employing any mediator or artificial membrane. Forests of VACNF are fabricated on silicon (Si) substrate using plasma enhanced chemical vapor deposition (PECVD) process and a metal layer is fabricated over silicon to serve as the electrode. VACNFs demonstrate superior conductive and structural properties compared to other carbon nano-materials and serve as an excellent location for charge transfer in electrochemical reaction process. Measurement results show that this glucose sensor can detect very low level of glucose with a high degree of linearity with respect to glucose concentration. Glucose detection is often interfered by several electro-active compounds and mediators/membranes are used to improve the performance of the detectors. Test results demonstrate that the proposed glucose sensor works well in presence of the interferer materials.

A precision dose control circuit for vertically aligned carbon nanofiber based maskless lithography

This paper presents a precision control circuit for the emission of desired number of electrons from vertically aligned carbon nanofibers (VACNFs) for the realization of a massively parallel maskless e-beam lithography system. The digitally addressable field emission arrays (DAFEAs) of the VACNFs function as the lithography heads for massively parallel e-beam exposure of resist eliminating the cost of photomasks [1]. A dose control circuit (DCC) to prevent under- or over-exposure of the resist has been integrated [2] by our research group. This paper describes further improvements in dose control electronics to reduce the parasitic effects while reducing the area of the chip and lowering the count of electrons for achieving the optimum dose from each of the nanofiber emitters.

Numerical modeling and implementation in circuit simulator of SOI four-gate transistor (G4FET) using multidimensional Lagrange and Bernstein polynomial

This paper presents two efficient numerical models developed for simulating circuits containing silicon-on-insulator four-gate transistors (G4FET). First the model is developed using one set of available data and then it is validated using another set of test data. These models provide a single multivariate polynomial expression that is valid across different biasing regimes as long as it falls within the range of data set used to develop the model. Lagrange form of interpolating polynomial and Bernstein Form of approximating polynomial are used to generate these models. Both forms have the advantage of being computed from a limited number of data points. Since these polynomial models and their derivatives are continuous, they are suitable for implementation in circuit simulator. The models have been developed and validated for both n-channel and p-channel G4FETs using both TCAD and experimental data and are successfully implemented in SPICE simulator for simulating circuits containing G4FETs.

Digitally addressable vertically aligned carbon nanofibers for implementation of massively parallel maskless lithography

Field emission (FE) of electrons from nanostructured graphitic carbon-based materials has been an area of intense investigation in recent years. Each field emitting device has control gates and an electron emitting cathode, which emits electron when a sufficient voltage is applied at the gate electrode. Recently, a technique for fabricating gated cathode structures that uses a single in situ grown vertically aligned carbon nanofiber (VACNF) as a FE element has been reported. This paper presents digitally addressable VACNFs for implementation of massively parallel maskless lithography.

Detection of Alcohol with Vertically Aligned Carbon Nanofiber (VACNF)

A reagentless amperometric enzymatic biosensor is constructed using Vertically Aligned Carbon Nanofiber (VACNF) for the detection of alcohol. Yeast alcohol dehydrogenase (YADH), and its cofactor nicotinamide adenine dinuleotide (NAD+) are immobilized by covalent attachment and adsorption to the carbon substrate. Electrochemical techniques are employed to test the function and performance of the constructed biosensor. Characterization of the electrode is performed using NADH. Subsequently, amperometric measurements are conducted with the electrodes for the detection of ethanol to determine the response of the electrical current due to an increase in analyte concentration. The detection range, storage stability, reusability, and response time of the biosensor is also examined.

7.4: Dose control circuits for digitally addressable VACNF based maskless lithography

This paper presents dose control electronics and a digital addressing method for the vertically aligned carbon nanofiber (VACNF) based massively parallel maskless e-beam lithography system. The Digital Electrostatically focused e-beam Array direct-write Lithography (DEAL) developed by our research group in Oak Ridge National Laboratory [1] incorporates digitally addressable field emission arrays (DAFEAs) of the VACNFs which function as the lithography heads during the exposure of the resist. A logic and memory control circuit (LMC) and a dose control circuit (DCC) have been designed to write a desired pattern and control the dose of electrons, respectively. This paper summarizes our previous works on different versions of the DCCs [2–4] designed and optimized in the effort of obtaining a fixed and optimum dosage with the smaller circuit area.