Carbon Microelectromechanical Systems Research Articles

Modeling and Experimental Study of Micro Enzymatic Biofuel Cells

Miniaturized enzymatic biofuel cells (EBFCs) converting biological energy into electrical energy by using enzyme-modified electrodes are considered as a candidate to power implantable medical devices and portable electronics. In this study, modeling of the EBFC system has been conducted using finite element analysis based on COMSOL Multiphysics 4.3a in terms of cell performance, efficiency and optimum cell configurations. Two modules have been applied to obtain the maximum theoretical cell performance of on-chip microelectrodes: 1) diffusion module to incorporate the mass transport and enzymatic kinetics; 2) conductive module to integrate concentration and potential. The design rule has been developed in the steady state. The effect of orientation of the microelectrode arrays in blood artery has been studied. In the experimental part, integration of the high surface area nanomaterials such as carbon nanotubes (CNTs) and graphene could be one of the effective solutions to further improve the performance of current EBFCs. The EBFC systems employing 3D carbon micropillar arrays integrated with different nanomaterials, such as graphene, reduced graphene oxide (rGO) and rGO/CNTs composite have been investigated. The fabrication process of this micro-system combined top-down carbon microelectromechanical system (C-MEMS) technology to fabricate the 3D micropillar arrays platform and bottom-up electrophoretic deposition (EPD) to deposit various carbon based nanomaterials onto the 3D micropillar arrays. The materials characterization, electrochemical characterization and device evaluation have been conducted. Upon comparison among bioelectrodes integrated with different nanomaterials, the amperometric response of the rGO/CNTs based bioelectrodes exhibited the best electrochemical performance and the as-resulted 3D micro EBFC generated a maximum power density of 196.04 µWcm-2 at 0.61 V. In addition, modeling of the rGO/CNTs EBFC system based on finite element analysis has also been conducted to predict the theoretical cell performance. The power density of actual device has been noted to be 71.1% of the modeling result.

Pyrolyzed carbon with embedded NiO/Ni nanospheres for applications in microelectrodes

Photoresist, a frequently used material in existing microfabrication processes, can be utilized in carbon micro electro mechanical system (C-MEMS) since the patterned carbon micro/nano structures can be formed by pyrolysis of a patterned photoresist.

Patternable Nanoporous Carbon Electrodes for Use as Supercapacitors

Capacity of electrochemical supercapacitors strongly depends on the specific surface area of electrodes that can be significantly enlarged by incorporating nanoporous morphology. We present patternable nanoporous carbon microstructures, fabricated using simple O2 microwave plasma etching and carbon-microelectromechanical systems processes, for application as supercapacitors. By incorporating a simple O2 microwave plasma etching process prior to the pyrolysis process, nanoporous surfaces with nanoporous sponge-like networks are formed on the pre-patterned negative photoresist structures due to a self-masking effect. These O2-plasma-etched photoresist structures are converted into nanoporous glassy carbon electrodes (average pore size <1 nm) via pyrolysis while retaining the morphology of sponge-like networks. The pore size and surface morphology are controllable depending on O2 plasma etching time. As a result, the surface area and electrochemical capacitance of the nanoporous carbon surface are increased, respectively, by up to 795 times (∼994 m2 g−1) and 28.1 times (23.9 mF cm−2) compared to a bare carbon surface. In addition, arrays of 3D nanoporous carbon microstructures such as cylindrical posts can be simply integrated on the planar carbon electrode surface by adding a photoresist patterning process prior to the O2 plasma etching, enabling a further improvement in capacitance by a factor of up to 37.3.

Glucose sensor based on redox-cycling between selectively modified and unmodified combs of carbon interdigitated array nanoelectrodes

We present a novel electrochemical glucose sensor employing an interdigitated array (IDA) of 1:1 aspect ratio carbon nanoelectrodes for the electrochemical-enzymatic redox cycling of redox species (ferricyanide/ferrocyanide) between glucose oxidase (GOx) and the two comb-shaped nanoelectrodes of the IDA. The carbon nanoelectrodes were fabricated using a simple, cost-effective, reproducible microfabrication technology known as the carbon-microelectromechanical-systems (C-MEMS) process. One comb (comb 1) of the IDA was selectively modified with GOx via the electrochemical reduction of an aryl diazonium salt, while the other comb (comb 2) remained unmodified; this facilitates electrochemically more active surface of comb 2, resulting in sensitive glucose detection. Ferricyanide is reduced to ferrocyanide by the GOx in the presence of glucose, and ferrocyanide diffuses to both combs of the IDA where it is oxidized. The limited electrochemical current collection at the surface-modified comb 1 is counterbalanced by the efficient redox cycling between the enzyme sites at comb 1 and the bare carbon surface of comb 2. Reducing the electrode-to-electrode gap between the two combs (gap = 1.9 μm) increases the diffusion flux of redox species at comb 2 hence, enhanced the sensitivity and limit of detection of the glucose sensor by ∼2.3 and ∼295 times, respectively at comb 2 compared to comb 1. The developed IDA-based glucose sensor demonstrated good amperometric response to glucose, affording two linear ranges from 0.001 to 1 mM and from 1 to 10 mM, with limits of detection of 0.4 and 61 μM and sensitivities of 823.2 and 70.0 μA mM−1 cm−2, respectively.

Highly sensitive hydrogen gas sensor based on a suspended palladium/carbon nanowire fabricated via batch microfabrication processes

We present a highly sensitive H2 sensor based on a single Pd-functionalized carbon nanowire suspended ∼10μm above a substrate while monolithically bridging two carbon posts. The carbon nanowire and posts were fabricated using a batch carbon-microelectromechanical system (MEMS) process consisting of photolithography and pyrolysis. The pyrolysis process resulted in a significant volume reduction in which the nanowire diameter was reduced to ∼120nm from the original 1μm×1.5μm photoresist wire. Because of SiO2 eaves formed by isotropic etching of the Si substrate, as well as the suspended geometry of the carbon nanowire, a single palladium evaporation process enabled selective palladium coating on the carbon nanowire without any nano-patterning processes. The electrical conductivity of the carbon nanowire was modulated by varying the pyrolysis temperature so that the electrical resistance along the suspended Pd/C nanowire was affected predominantly by hydrogen chemisorption onto the thin Pd layer enclosing the carbon core. The suspended functionalized wire showed better sensing capability compared to a Pd nanowire fabricated on the substrate with similar dimensions, because of enhanced mass transport. The suspended nanowire-based H2 sensor quantified from 10 to 500ppm H2 with percentile resistance changes of 5.9–129%.

Graphene/enzyme-encrusted three-dimensional carbon micropillar arrays for mediatorless micro-biofuel cells.

Two-dimensional graphene is a promising candidate material for use in high-performance enzymatic biofuel cells (EBFCs). In this work, graphene/enzyme has been integrated onto three-dimensional (3D) micropillar arrays in order to obtain efficient enzyme immobilisation and enhanced enzyme loading and facilitate direct electron transfer. The fabrication process of this system combines top-down carbon microelectromechanical systems (C-MEMS) to fabricate the 3D micropillar array platform and bottom-up electrophoretic deposition (EPD) to deposit the graphene/enzyme onto the electrode surface. The amperometric response of the graphene-based bioelectrode exhibited excellent electrochemical activity, which indicated the successful co-deposition of graphene with the enzymes. The developed 3D graphene/enzyme network-based EBFC generated a maximum power density of 136.3 μW cm(-2) at 0.59 V, which is almost seven times the maximum power density of the bare 3D carbon micropillar array-based EBFC.

Surface Engineered Pyrolyzed Carbon Electrode Arrays for Bio-Sensing

There is an urgent need to develop highly selective, sensitive and reproducible miniaturized bio-sensing platforms based on reliable interfaces is compatible with microfabrication processing.. Our objective is to first create pyrolyzed carbon micropillar arrays with high surface area as bio-sensing electrodes, second to develop functionalization methods in order to improve the biomolecule immobilization efficiency and finally to understand the electrochemical phenomena occurring at the biomolecule/carbon interface. The carbon micropillar arrays with high aspect ratios and hierarchical porous surface have been fabricated by carbon microelectromechanical systems (C-MEMS). Nanomaterials have been integrated onto the micropillar arrays to further increase the accessible surface area and in order to achieve efficient covalent immobilization of biomolecules, various functionalization methods have been investigated. The developed bio-sensing platform was then applied to several applications including DNA sensors; H2O2 sensors, aptamer sensors and HIV sensors.

Hierarchically structured suspended TiO2 nanofibers for use in UV and pH sensor devices.

Photoelectrochemical sensors based on hierarchically structured titanium dioxide (TiO2) nanofibers (NFs) were fabricated by combination of electrospinning, carbon microelectromechanical systems (MEMS), and hydrothermal reaction. During the electrospinning step, a rotating drum collector was used to align and position NFs of titanium tetraisopropoxide (TTIP) in polyvinylpyrrolidone (PVP) on top of a carbon-MEMS structure. Following calcination under vacuum, a stable ohmic contact was obtained between suspended TiO2-carbon NFs (TiO2/C NF) and the carbon electrodes. Subsequent to this, a hierarchical nanostructure of TiO2 nanowires (TiO2 NWs) was hydrothermally synthesized onto the TiO2/C NFs and successfully utilized as UV and pH sensors. This is the first demonstration of a semiconductor-based nanofiber sensor suspended on carbon electrodes that has been achieved by a relatively simple and cost-effective electrospinning method. Furthermore, these sensors demonstrate a high sensitivity, as well as a stable ohmic contact, due to the large surface area of the TiO2 NWs and the carbon-carbon contact between the suspended TiO2/C NFs and carbon electrodes.

Surface Engineered Pyrolyzed Carbon Electrode Arrays for Bio-Sensing

There is urgent need to develop highly selective, sensitive and reproducible miniaturized bio-sensing platform based on reliable interface that is compatible with microfabrication processing. Our research objective is to advance fundamental research by fabricating pyrolyzed carbon arrays with high surface area as a bio-sensing electrode, developing the functionalization methods to increasing biomolecules immobilization efficiency and further understanding electrochemical phenomena occurring at bio/carbon interfaces. The carbon microelectrode arrays with high aspect ratio and porous surface have been fabricated by carbon microelectromechanical systems (C-MEMS) while the nanomaterials such as graphene have been integrated to further increase surface area. To achieve the efficient covalent immobilization of biomolecules, various oxidation and reduction functionalization methods have been investigated. The oxidation treatment we used in this study includes vacuum ultraviolet, electrochemical activation, UV/Ozone and oxygen RIE. The reduction treatment includes direct amination and diazonium grafting. The functionalized surface has been characterized using XPS, CV and FTIR to confirm and calculate the surface coverage of different functional groups. The developed bio-sensing platform was then applied for several applications, such as: DNA sensor; Glucose sensor; H2O2 sensor; Aptamer sensor and HIV sensor. The performance and sensitivity of each biosensor will be discussed in the talk.

C-MEMS Based Microsupercapacitors and Microsensors

Micro-supercapacitors with high pulse power and long cycle lifetime have attracted a great deal of attention for various emerging micropower applications. Recently, there have been several efforts to improve device architecture for the next generation high power and energy density micro-supercapacitors. These devices are usually fabricated by depositing the active materials on interdigitally arranged current collectors of micron-size. We have explored various techniques, materials and structural designs for micro-supercapacitors. In this talk, Carbon-Microelectromechanical systems (C-MEMS) based various supercapacitor systems will be briefly introduced. Both two and three dimensional micro-electrodes where fabricated and various electroactive materials where integrated on the microelectrodes. These include carbon nanotubes (CNTs), Polypyrrole and manganese oxide. More recently we have also examined the integration of Graphene sheets into micro-electrodes as promising high power and energy density material.

SU8 Derived Glassy Carbon for Lithium Ion Batteries

Glassy carbon (GC) is an amorphous yet sp 2 rich allotrope of carbon that has received a lot of attention as an electrode material in Li ion batteries. In this contribution we explore the microstructural, thermal and mechanical properties of GC derived from the pyrolysis of lithographically patterned SU8, which is the most common precursor polymer in CMEMS (carbon micro-electromechanical systems). Microstructural characterization of CMEMS carbon in terms of interlayer separation of the characteristic turbostratic (randomly arranged) graphene layers, which influences the mechanism of lithiation and accompanying swelling and heating during battery operation, is carried out employing electron microscopy. Mechanical properties of the thus obtained GC are also investigated using nanoindentation for films, and microcompression tests for micropillars. Rapid heating/cooling of the miniaturized GC structures due to their relatively large surface area is demonstrated by Joule heating of a carbon nanowire (CNW) suspended onto CMEMS walls.

C-MEMS Based Microsupercapacitors and Microsensors

The rapid development of miniaturized electronic devices has led to a growing need for rechargeable micro-power sources with high performances. Miniaturized micro-supercapacitors have great potential to replace batteries and electrolytic capacitors in different types of applications. Carbon microelectromechanical systems (C-MEMS) technique, in which patterned photoresist is pyrolyzed in an inert environment at a high temperature is a powerful approach to fabricate high aspect ratio carbon microelectrode arrays, which could be a promising platform for various micro-power applications. To further improve the electrochemical properties of the carbon microelectrodes, different nanomaterials could be integrated by electrostatic spray deposition (ESD). This review provides an overview of our research at FIU on micro-supercapacitors with different structural designs and active materials.

Growth of nano-wrinkles on photoresist-derived carbon microelectrode array

A novel fabrication method for integrating nano-wrinkles to carbon microelectromechanical systems (C-MEMS) posts on a silicon substrate is proposed in this paper. In the fabrication of carbon microelectrode array, SU-8, a negative photoresist is patterned by photolithography and subsequently pyrolysed at high temperatures in an oxygen-free environment. In this process, the SU-8 posts are slowly converted to the desired carbon posts with properties resembling those of glassy carbon, and the carbon posts have shrinkage in both vertical and radial directions. As we deposit a thin layer (nanoscale order) of carbon before pyrolysis, nano-wrinkle patterns can be generated with the shrinking of the photoresist due to a large difference in the elastic moduli as well as high compressive stress. The nano-wrinkles generated in carbon films can greatly enlarge specific surface area, which significantly enhances the electrochemical performance compared to the blank posts. Since the carbon nano-wrinkles have the same nature as the carbon posts, the stability and biological compatibility of nano-wrinkles integrated microelectrodes are greatly enhanced.

Integration of MnO2 thin film and carbon nanotubes to three-dimensional carbon microelectrodes for electrochemical microcapacitors

Large-scale three-dimensional (3D) hybrid microelectrodes have been fabricated through modified carbon microelectromechanical systems (Carbon-MEMS) process and electrochemical deposition method. Greatly improved electrochemical performance has been shown for the 3D photoresist-derived carbon microelectrodes with the integration of carbon nanotubes (CNTs) and manganese dioxide (MnO2). The electrochemical measurements of the microelectrodes indicate that the specific geometric capacitance can reach up to 238 mF cm−2 at the current density of 0.5 mA cm−2. The capacitance loss is less than 18.2% of the original value after 6000 charge–discharge cycles. This study shows that stacking of MnO2 film and integrating of CNTs to the 3D glassy carbon microelectrodes have great potential for on-chip microcapacitors as energy storage devices, and the presented approach is promising for large-scale and low-cost manufacturing.

Tailoring diffraction-induced light distribution toward controllable fabrication of suspended C-MEMS

A simple and controllable method is proposed to fabricate suspended three-dimensional carbon microelectromechanical systems (C-MEMS) structures by tailoring diffraction-induced light distribution in photolithography process. An optical model is set up and the corresponding affecting parameters are analyzed to interpret and predict the formation of suspended structures based on Fresnel diffraction theory. It is identified that mask pattern dimensions, gap distance between the photomask and photoresist, and the exposure time are critical to the final suspended structures, which have also been verified through experimental demonstrations. The fabricated biocompatible suspended C-MEMS structures could find wide applications in electrochemical and biological areas.

Surface Modification of Carbon Post Arrays by Atomic Layer Deposition of ZnO Film

The applicability of atomic layer deposition (ALD) process to the carbon microelectromechanical system technology was studied for a surface modification method of the carbon post electrodes. A conformal coating of the ALD-ZnO film was successfully demonstrated on the carbon post arrays which were fabricated by the traditional photolithography and subsequent two-step pyrolysis. A significant Zn diffusion into the underlying carbon posts was observed during the ALD process. The addition of a sputter-deposited ZnO interfacial layer efficiently blocked the Zn diffusion without altering the microstructure and surface morphology of the ALD-ZnO film.

Trapping and Manipulation of Bioparticles by a 3-D Optimal Multiple-Designed Offset Carbon-Microelectrode Array in C-MEMS Fabrication

A dielectrophoretic approach with latest developed three-dimensional (3-D) carbon micro-electro-mechanical system (C-MEMS) has been extended as a potential route with idyllic solution to recommend a low-cost, biocompatible and high throughput manipulation and positioning for bio-particles as compared to 2D-planar microelectrodes. Presented in this paper is a novel platform for modelling and simulation of C-MEMS microfabrication process for dielectrophoresis (DEP) force based on various 3-D offset-microelectrode configurations. Numerical solutions are employed to investigate the upshots of multi-designed microelectrodes, applied voltage, electrode edge-to-edge gap and geometric size of microelectrodes on the electric field intensity gradient, induced by an AC voltage for the deployment of broad categories of bioparticles creation, utilization and their manipulation (separation, concentration, transportation and focusing). Sharp edge electrodes are the principle focus of this paper for DEP manipulation that is more convenient to enhance the electric field intensity distribution. The results show that square column electrodes configuration comparatively create large gradient magnitude in electric field intensity as compared to all other configurations. It is also observed that electric field extends drastically with increases in microelectrode height. These findings are consistent with literature experimental reports and will provide vital strategy for optimal design of DEP devices with 3-D C-MEMS.

Carbon Interdigitated Array Nanoelectrodes for Electrochemical Applications

Redox current amplification has been demonstrated using carbon interdigitated array (IDA) nanoelectrodes derived from precursor polymer microstructures through conventional photolithography and pyrolysis. This simple conversion process, also known as carbon-microelectromechanical systems, enables nanometer-level fabrication of carbon materials in a reproducible and an economic manner. We demonstrated that with carbon IDA nanoelectrodes fabricated in two mask processes a current amplification factor of 25 can be obtained. This high amplification factor is a result of the efficient recycling of redox species between the 1:1 aspect ratio carbon nanoelectrodes. This type of a current amplification value is hard to obtain when using more traditional flat nanometer level spaced noble metal IDA electrodes fabricated with more expensive nanopatterning processes such as E-beam lithography.

Integration of Carbon Nanotubes to C-MEMS for On-chip Supercapacitors

Carbon nanotubes (CNTs)/carbon microelectromechanical systems (C-MEMS) composites were fabricated as electrode materials for on-chip supercapacitors. By using photolithography and pyrolysis process, 3-D C-MEMS architectures were prepared. The iron catalyst particles were conformally coated on the C-MEMS by electrostatic spray deposition (ESD) and CNTs were synthesized on the surfaces of C-MEMS by catalytic CVD method. The CNT/C-MEMS composites exhibited higher specific capacitance than C-MEMS. Furthermore, the composites with more homogeneous CNTs showed better capacitance. After treatment of oxygen plasma, the specific capacitance of the composite increased due to the contribution of oxygen functional groups.

Electrochemically activated carbon micro-electrode arrays for electrochemical micro-capacitors

Interdigitated carbon micro-electrode arrays for micro-capacitors are fabricated through the carbon microelectromechanical systems (C-MEMS) technique which is based on the carbonization of patterned photoresist. To improve the capacitive behavior, electrochemical activation is performed on carbon micro-electrode arrays. Cyclic voltammetry (CV) and galvanostatic charge–discharge results demonstrate that the electrochemical activation effectively increases the capacitance of the micro-electrode arrays by three orders of magnitude. Although the charge–discharge experiments show the non-ideal behavior of micro-capacitors, the specific geometric capacitance reaches as high as 75 mF cm −2 at a scan rate of 5 mV s −1 after electrochemical activation for 30 min. The capacitance loss is less than 13% after 1000 CV cycles. These results indicate that electrochemically activated C-MEMS micro-electrode arrays are promising candidates for on-chip electrochemical micro-capacitor application.