Photoresist-derived Carbon Research Articles

Highly Sensitive Lactic Acid Biosensors Based on Photoresist Derived Carbon

In this study, a highly sensitive electrochemical capacitive lactic acid enzymatic sensor based on a carbon-microelectromechanical system (C-MEMS) is developed. The sensing platform of the biosensor, which is photoresist-derived carbon (PDC) microelectrode, was synthesized by pyrolysis of photo-patterned photoresist polymer in oxygen-free and high-temperature conditions. The surfaces of the microelectrodes were functionalized by the oxygen-plasma pretreatment technique to form –COOH functional groups. The lactate oxidase enzyme was immobilized on the carboxylic group covered surfaces of the microelectrodes. The sensor demonstrated the detection of lactic acid over a wide dynamic range of 0.1- $5000~\mu \text{M}$ with a low detection limit of $1.45~\mu \text{M}$ (S/N=3). The sensitivity found to be 33.48 mF.cm−2. (Log (M))−1. This mediator-free lactic acid sensor exhibited 80.07% accuracy in the presence of uric and ascorbic acid and only a 2.35% decrease of measured capacitance in 20 days, which implies good selectivity and high stability. The presented results show that the C-MEMS based lactic acid biosensor is a promising contender for the highly demanding point of care health monitoring applications.

Methodology and fabrication of adherent and crack-free SU-8 photoresist-derived carbon MEMS on fused silica transparent substrates

Development of carbon based micro electromechanical systems (C-MEMS) has enabled the fabrication of durable, low cost and biocompatible micro devices for specific applications. Thermochemical decomposition of SU-8 (a common photoresist) is often used to fabricate C-MEMS. However, this technique has yielded unreliable results when fabrication on transparent substrates is required due to cracking and detachment of the produced carbon micro structures. We present a methodology for the fabrication of photopatterned carbon films based on SU-8 deposited on transparent fused silica substrates. Specifically, we developed and implemented this methodology for carbon microstructure fabrication derived from SU-8 2035 and SU-8 3035. It was found that SU-8 3035 derived carbon microstructures were crack free and adhered well to the substrate, while SU-8 2035 resulted in fractured and detached carbon microstructures. In addition, we characterized the produced SU-8 3035 derived carbon by measuring its electrical resistivity (1.412 ± 0.011 mΩ m), inter-structure electrical resistance, contact angle (35.7° ± 6.0°), Raman spectrum and adhesion strength to the substrate. In brief, even though SU-8 2035 and SU-8 3035 are useful materials for C-MEMS fabrication, we found that SU-8 3035 is more suitable for the fabrication of crack free and adherent carbon microstructures on transparent fused silica substrates.

High performance microsupercapacitors based on a nano-micro hierarchical carbon electrode by direct laser writing

We have demonstrated a direct laser writing (DLW) process that uses a femtosecond laser to fabricate a nano-micro hierarchical structure for a large capacitance microsupercapacitor (MSC) electrode. By applying a two-photon polymerization-based DLW technique, a photoresist (PR) nano-pillar pattern was fabricated on a pre-defined PR interdigitated electrode (PR-IDE) pattern to form a nano-micro hierarchical structure. Carbon pyrolysis converted a PR-IDE with a nano-micro hierarchical structure to a PR derived carbon (PRC)-IDE while maintaining the aspect ratio of the pillar structure. The electrochemical performance of the PRC-MSC is improved by introducing the nano-pillar pattern to the PRC-IDE, resulting in larger areal capacitance of the as-fabricated PRC-IDE compared to the PRC-IDE with a micropattern only. The PRC-IDE with a nano-micro hierarchical structure in this study could be further applied as a backbone electrode structure for a high power pseudo-capacitor.

Transparent Electric Heaters Based on Photoresist‐Derived Carbon Micropatterns on Quartz Plates

AbstractIn this study, transparent electric heaters (TEHs) are developed based on photoresist‐derived carbon micropatterns (CMs). Well‐defined SU‐8 micropatterns formed by deep UV lithography are carbonized in a tube furnace under an inert atmosphere. The resulting CMs are found to have pseudo‐graphitic structures containing both graphitic and disordered structures. The optical transmittance and electrical conductivity of the CMs increase as their thickness increases. The 102 nm thick CMs exhibit a good transmittance of 78% at 550 nm and a high electrical conductivity of 2.51 × 102 S cm−1. As TEHs, the 102 nm thick CMs demonstrate a maximum temperature of 116 °C with a maximum heating rate of 8.78 °C s−1 at an applied voltage of 60 V and a low electricity consumption of 1.02 W cm−2, which are comparable to the electric heating performance of TEHs prepared with carbon nanomaterials. Therefore, the CMs fabricated in this study can be utilized as electrodes for TEHs in various medical, industrial, consumer, and automobile applications.

Structure and Electrochemical Properties of Pyrolyzed Magnesium Citrate/SU-8 Nanocomposites for Supercapacitators

Supercapacitors have attracted significant research interest due to their fast cycling and high power densities, charging and discharging time for millions of cycle [1]. Highly porous carbon materials are commonly used as supercapacitor electrodes due to their high specific surface area and good conductivity. To realize MEMS supercapacitors, a scalable technique for on-chip integrated high-performance microelectrodes are in urgent required. Pyrolyzed patterned photoresist is one attractive way to deposit porous carbon electrode for electrochemical applications. However, current research on pore structure control of photoresist-derived carbon electrodes with large specific capacitance and good cycling performance is rather limited including the problem of uneven porosity [2, 3]. For porous structure control of pyrolyzed photoresist-derived carbon, 200 mg, 300 mg and 400 mg of magnesium citrate nanoparticles were mixed in 10 ml SU-8 photoresist during pyrolysis process respectively.The ultra-thick SU-8 photoresist layer mixed with magnesium citrate was deposited on silicon wafer with multiple spin-coating methods at the rotation speed of 1500 rpm for 35s. Then, the doped SU-8 photoresist was pyrolyzed in a horizontal tube furnace under inert forming gas (95% N2, 5% H2) atmosphere with flow rate of 2000 sccm, which the temperature was programmed from room temperature to 950 oC. Fig. 1 shows the SEM images of pyrolyzed SU-8 photoresist before and after magnesium citrate doping. Fig. 2 presents the Current-Voltage (C-V) curves of carbon microelectrodes at different scan rate in 1M Na2SO4 electrolyte. Clearly, the C-V curves of the carbon electrodes with magnesium citrate doping is more close to symmetric rectangular shape than that without doping, which optimized capacitance is 205mF/cm-2capacitance for the sample (30 mg/ml). Fig. 3 plots the galvanostatic charging and discharging curves of carbon microelectrodes at different current densities. The charging and discharging curve exhibits a near isosceles triangle, which indicates a better irreversible discharging and discharging performance. However, the charging/discharging time of the carbon electrodes with magnesium citrate doping is much longer than that without doping, which indicates the specific surface area of the carbon electrodes was greatly increased after magnesium citrate doping. In summary, carbon microelectrodes derived from pyrolyzed SU-8 photoresist with different doping concentration have been prepared and characterized by SEM and electrochemical technique. The optimized concentration of magnesium citrate nanoparticles in SU-8 photoresist is 30 mg/ml, and optimized capacitance is 205mF/cm-2correspondingly. The high capacitance could be associated with the highly porous structure.

Characteristics of Photoresist-derived Carbon Nanofibers for Li-ion Full Cell Electrode

Carbon nanofiber electrode has been fabricated for energy storage systems by the electrospinning of SU-8 precursor and subsequent pyrolysis. Various parameters including the applied voltage, the distance between syringe tip and target collector and the flow rate of the polymer affect the diameter of SU-8 electrospun nanofibers. Shrinkage during pyrolysis decreases the fiber diameter. As the pyrolysis temperature increases, the resistivity decreases dramatically. Low resistivity is one of the important characteristics of the electrodes of an energy storage device. Given the advantages of carbon nanofibers having high external surface area, electrical conductivity, and lithium intercalation ability, SU-8 derived carbon nanofibers were applied to the anode of a full lithium ion cell. In this paper, we studied the physical properties of carbon fiber electrode by scanning transmission microscopy, thermal gravimetric analysis, and four-point probe. The electrochemical characteristics of the electrode were investigated by cyclic voltammogram and electrochemical impedance spectroscopy plots.

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.

Stress-Driven and Carbon-Assisted Growth of $ \hbox{SiO}_{x}\hbox{N}_{y}$ Nanowires on Photoresist-Derived Carbon Microelectrode

An integration strategy is devised for a reliable, scalable, and catalyst-free assembly of SiOxNy nanowires in photoresist-derived carbon microelectrodes. The approach involved UV photolithography process of SU8 photoresist, followed by high-temperature carbonization, and was versatile in yielding various 3-D micro-nano integrated carbon microelectrode arrays (CMEAs). The morphology of the SiOxNy nanowires and the nanowire-integrated CMEA was characterized by scanning electron microscopy and high-resolution transmission electron microscopy. The chemical composition of the SiOxNy nanowires was confirmed by energy-dispersive X-ray and X-ray photoelectron spectroscopy. A synergetic growth mechanism is proposed based on our experimental observations, in which both carbon-assisted and stress-driven mechanisms were identified to interpret the formation of SiOxNy nanowires. Further study revealed that the nanowire-integrated 3-D CMEA showed improved electrical and electrochemical performances than the blank ones, demonstrating potential applications in electrochemical, biological, and energy-related fields. Meanwhile, the developed method represents a low-cost and easy way to mass production of SiOxNy nanowire-integrated CMEA.

Concentration gradient induced morphology evolution of silica nanostructure growth on photoresist-derived carbon micropatterns

The evolution of silica nanostructure morphology induced by local Si vapor source concentration gradient has been investigated by a smart design of experiments. Silica nanostructure or their assemblies with different morphologies are obtained on photoresist-derived three-dimensional carbon microelectrode array. At a temperature of 1,000°C, rope-, feather-, and octopus-like nanowire assemblies can be obtained along with the Si vapor source concentration gradient flow. While at 950°C, stringlike assemblies, bamboo-like nanostructures with large joints, and hollow structures with smaller sizes can be obtained along with the Si vapor source concentration gradient flow. Both vapor–liquid-solid and vapor-quasiliquid-solid growth mechanisms have been applied to explain the diverse morphologies involving branching, connecting, and batch growth behaviors. The present approach offers a potential method for precise design and controlled synthesis of nanostructures with different features.

An optimized process for fabrication of high-aspect-ratio photoresist-derived carbon microelectrode array on silicon substrate

An optimized process was developed for fabrication of high-aspect-ratio photoresist-derived carbon microelectrode array on silicon substrate. This process consisted of conventional photolithography, three-step linear pyrolyzing process and micromechanical interlocking. Comparing with previous two-step pyrolysis, three-step linear pyrolysis process can better preserve the geometry of microstructure during pyrolysis, and micromechanical interlocking was introduced to improve bonding strength between carbon microstructure and substrate. As a result, it can be achieved that high-aspect-ratio carbon microelectrode array remained upright and robustly with substrate. The experimental results confirmed that the optimized process is very effective, and the technology will be helpful for integration of 3-dimensional carbon-based devices in the fields of bioMEMS and electrochemical analysis.