Long and hollow glassy carbon microfibers were fabricated based on the pyrolysis of a natural product, human hair. The two fabrication steps of carbon microelectromechanical and carbon nanoelectromechanical systems, or C-MEMS and C-NEMS, are: (i) photolithography of a carbon-rich polymer precursor and (ii) pyrolysis of the patterned polymer precursor.
A wide range of carbon sources are available in nature, with a variety of micro-/nanostructure configurations. Here, a novel technique to fabricate long and hollow glassy carbon microfibers derived from human hairs is introduced. The long and hollow carbon structures were made by the pyrolysis of human hair at 900 °C in a N2 atmosphere. The morphology and chemical composition of natural and pyrolyzed human hairs were investigated using scanning electron microscopy (SEM) and electron-dispersive X-ray spectroscopy (EDX), respectively, to estimate the physical and chemical changes due to pyrolysis. Raman spectroscopy was used to confirm the glassy nature of the carbon microstructures. Pyrolyzed hair carbon was introduced to modify screen-printed carbon electrodes ; the modified electrodes were then applied to the electrochemical sensing of dopamine and ascorbic acid. Sensing performance of the modified sensors was improved as compared to the unmodified sensors. To obtain the desired carbon structure design, carbon micro-/nanoelectromechanical system (C-MEMS/C-NEMS) technology was developed. The most common C-MEMS/C-NEMS fabrication process consists of two steps: (i) the patterning of a carbon-rich base material, such as a photosensitive polymer, using photolithography; and (ii) carbonization through the pyrolysis of the patterned polymer in an oxygen-free environment. The C-MEMS/NEMS process has been widely used to develop microelectronic devices for various applications, including in micro-batteries, supercapacitors, glucose sensors, gas sensors, fuel cells, and triboelectric nanogenerators. Here, recent developments of a high-aspect ratio solid and hollow carbon microstructures with SU8 photoresists are discussed. The structural shrinkage during pyrolysis was investigated using confocal microscopy and SEM. Raman spectroscopy was used to confirm the crystallinity of the structure, and the atomic percentage of the elements present in the material before and after pyrolysis was measured using EDX.
Carbon has many allotropes and, depending on the particular application, one of the following allotropes can be chosen: carbon nanotubes (CNTs), graphite, diamond, amorphous carbon, lonsdaleite, buckminsterfullerene (C60), fullerite (C540), fullerene (C70), and glassy carbon1,2,3,4. Glassy carbon is one of the most widely used allotropes because of its physical properties, including high isotropy. It also has the following properties: good electrical conductivity, low thermal expansion coefficient, and gas impermeability.
There has been a continuous search for carbon-rich precursor materials to obtain carbon structures. These precursors can be manmade materials or natural products that are available in particular shapes, and even include waste products. A wide variety of micro/nanostructures are formed via biological or environmental processes in nature, resulting in unique features that are extremely difficult to create using conventional manufacturing tools. As patterning took place naturally in this case, the synthesis of nanomaterials using natural and waste hydrocarbon precursors could be carried out using an easy, one-step process of thermal decomposition in an inert or vacuum atmosphere, called pyrolysis5. High-quality graphene, single-walled CNTs, multi-walled CNTs, and carbon dots have been produced by thermal decomposition or the pyrolysis of plant-derived precursors and wastes, including seeds, fibers, and oils, such as turpentine oil, sesame oil, neem oil (Azadirachta indica), eucalyptus oil, palm oil, and jatropha oil. Also, camphor products, tea-tree extracts, waste foods, insects, agro wastes, and food products have been utilized6,7,8,9 Recently, researchers have even used silk cocoons as a precursor material to prepare porous carbon microfibers10. Human hair, usually considered a waste material, was recently used by this team. It is made up of approximately 91% polypeptides, which contain more than 50% carbon; the rest are elements such as oxygen, hydrogen, nitrogen, and sulphur11. Hair also comes with several interesting properties, such as very slow degradation, high tensile strength, high thermal insulation, and high elastic recovery. Recently, it has been used to prepare carbon flakes employed in supercapacitors12 and to create hollow carbon microfibers for electrochemical sensing13.
The machining of a bulk carbon material to fabricate three-dimensional (3D) structures is a difficult task, as the material is very brittle. Focused ion beam14,15 or reactive ion etching16 may be useful in this context, but they are expensive and time-consuming processes. Carbon microelectromechanical system (C-MEMS) technology, which is based on the pyrolysis of patterned polymeric structures, represents a versatile alternative. In the past two decades, C-MEMS and carbon nanoelectromechanical systems (C-NEMS) have received much attention because of the simple and inexpensive fabrication steps involved. The conventional C-MEMS fabrication process is carried out in two steps: (i) patterning a polymer precursor (e.g., a photoresist) with photolithography and (ii) pyrolysis of the patterned structures. Ultraviolet (UV)-curable polymer precursors, such as SU8 photoresists, are often used to pattern structures based on photolithography. In general, the photolithography process includes steps for spin coating, soft bake, UV exposure, post bake, and development. In the case of C-MEMS; silicon; silicon dioxide; silicon nitride; quartz; and, more recently, sapphire have been used as substrates. The photo-patterned polymer structures are carbonized at a high temperature (800-1,100 °C) in an oxygen-free environment. At those elevated temperatures in a vacuum or inert atmosphere, all of the non-carbon elements are removed, leaving only carbon. This technique allows for the attainment of high-quality, glassy carbon structures, which are very useful for many applications, including electrochemical sensing17, energy storage18, triboelectric nanogeneration19, and electrokinetic particle manipulation20. Also, the fabrication of 3D microstructures with high aspect ratios using C-MEMS has become relatively easy and has led to a wide variety of carbon electrodes applications18,21,22,23, often replacing noble metal electrodes.
In this work, the recent development of a simple and cost-effective way to fabricate hollow carbon microfibers from human hair using non-conventional C-MEMS technology13 is introduced. The conventional SU8 polymer-based C-MEMS process is also described here. Specifically, the fabrication procedure for high-aspect ratio solids and hollow SU8 structures is described24.
1. 3D Human Hair-derived Carbon Structure Fabrication
NOTE: Use personal protective equipment. Follow laboratory instructions to use the instruments and to work inside the laboratory.
2. 3D Polymer Structure Fabrication: Photolithography
3. 3D Carbon Structure Fabrication: Pyrolysis
A schematic of the fabrication process for human hair-derived hollow carbon microfibers is shown in Figure 1. The carbonized human hair was characterized using SEM to estimate the shrinkage. The hair diameter shrank from 82.88 ± 0.003 µm to 31.42 ± 0.003 µm due to the pyrolysis. Scanning electron microscopic (SEM) images of various patterns made using hair-derived carbon microfibers are shown in Figure 2. The atomic percentages of the elements present in the hair before and after pyrolysis are presented in Table 1. The human hair used in this research was (atomic percent) 66.57% carbon, 16.19% oxygen, 7.94% nitrogen, 9.14% sulphur, and a small percentage of minerals such as calcium. The atomic percentage of carbon and oxygen was found to be 80.84%, and 14.83% respectively after the pyrolysis. Raman spectroscopic analysis of the hair before and after pyrolysis was also performed as shown in Figure 3. Only two broad peaks corresponding to the D- and G-bands were found for the hair after pyrolysis. The ratio of the intensities of the D-band to the G-band in the hair-derived carbon fibers was calculated to be 0.99, indicating that the hair-derived fibers are mostly glassy carbon.
The hair-derived carbon fibers were applied to detect dopamine and ascorbic acid using an electrochemical sensor. A screen-printed carbon electrode was modified with the hair-derived carbon fibers and used as the working electrode of the sensor. A schematic diagram of the carbon electrodes for electrochemical sensing is shown in Figure 4a. Cyclic voltammograms of 100 µM dopamine and 100 µM ascorbic acid in a 0.1 M phosphate buffer solution of pH 7.4 are shown in Figure 4b and c, respectively. A bare carbon electrode, a carbon electrode modified with the hair-derived carbon, and a carbon electrode modified with the CNTs were used as the working electrode of the sensors to compare the performance to detect dopamine and ascorbic acid. The oxidation peaks for dopamine were measured at 333 mV for the bare carbon electrode, 266 mV for the carbon electrode modified with the hair derived carbon fibers, and 96 mV for the electrode modified with the CNTs. The oxidation peaks for ascorbic acid were observed at 414 mV, 455 mV, and 297 mV for those electrodes, respectively.
A schematic diagram of the conventional C-MEMS process, the photolithographic patterning of a polymer precursor and the subsequent pyrolysis, is shown in Figure 5. These fabricated structures were characterized using confocal microscopy and SEM to estimate the shrinkage due to pyrolysis. Five cylindrical structures of the same height (250 µm) and outer diameter (150 µm), but with various inner diameters (i.e., 0 (solid), 30, 50, 75, and 100 µm (hollow)) were fabricated for this study. The geometric changes of the cylindrical structures due to pyrolysis were measured. The percentage of shrinkage was varied for different inner and outer diameter structures. When the inner diameter was 0 µm (a solid microstructure), the outer diameter shrank around 35%. When the the inner diameters were 30, 50, and 75 µm, the outer diameters and inner diameters shrank around 42% and 30 – 35%, respectively. In the case of a 100-µm inner diameter, the inner diameter expanded 12%, rather than shrinking, and the outer diameter shrank only 15%. In Figure 6, the SEM images of solid and hollow carbon microstructures, of which the original inner diameters were 30, 40, and 75 µm, are shown. 3D optical images of hollow microstructures before and after pyrolysis are also shown in Figure 7. All the microstructures shrank almost 30% in height (250 to 175 µm) according to the confocal microscopic images.
Figure 1: Schematic Diagram of the Fabrication Process for Human Hair-derived Hollow Carbon Microfibers. Reproduced with permission from reference13. Copyright 2016, Elsevier Ltd. Please click here to view a larger version of this figure.
Figure 2: SEM Images of Different Patterns of Hair-derived Carbon Microfibers. (a and b) Aligned straight carbon microfibers. Scale bar = 50 µm. (c and d) A coiled carbon microfiber. Scale bars = 20 and 100 µm, respectively. (e and f) A broken hollow carbon microfiber. Reproduced with permission from reference13. Copyright 2016, Elsevier Ltd. Please click here to view a larger version of this figure.
Figure 3: Raman Spectra of Human Hair Before and After Pyrolysis. Reproduced with permission from reference13. Copyright 2016, Elsevier Ltd. Please click here to view a larger version of this figure.
Figure 4: Electrochemical Sensing of Dopamine and Ascorbic Acid. (a) Schematic diagram of carbon electrodes for electrochemical sensing. Cyclic voltammograms of (b) 100 µM dopamine and (c) 100 µM ascorbic acid. Reproduced with permission from reference13. Copyright 2016, Elsevier Ltd. Please click here to view a larger version of this figure.
Figure 5: A Schematic Diagram of the Conventional C-MEMS Process Based on Photolithography and pyrolysis. (a) Spin coating of photoresist, (b) UV-exposure, (c) development, and (d) pyrolysis. Please click here to view a larger version of this figure.
Figure 6: Scanning Electron Microscopic Images of Carbon Microstructures Fabricated by the Conventional C-MEMS Process. (a) Solid and (b) hollow structures with various inner diameters. Scale bars = 500 µm. Enlarged images for (c) the solid and the hollow structures, of which the inner diameters before pyrolysis were (d) 30, (e) 50, and (f) 75 µm. Scale bars = 50 µm. Reproduced with permission from reference24. Copyright 2016, The Electrochemical Society. Please click here to view a larger version of this figure.
Figure 7: 3D Optical Images of Hollow Microstructures. (a) Before and (b) after pyrolysis. Reproduced with permission from refernce24. Copyright 2016, The Electrochemical Society. Please click here to view a larger version of this figure.
Element | Before pyrolysis | After pyrolysis |
Atomic % | Atomic % | |
Carbon | 66.57 | 80.84 |
Oxygen | 16.19 | 14.83 |
Nitrogen | 7.94 | 0 |
Sulfur | 9.14 | 0.21 |
Calcium | 0.16 | 0.21 |
Sodium | 0 | 0.22 |
Silicon | 1.82 | 3.69 |
Table 1: Chemical Composition Analyzed by EDX for Human Hair Before and After Pyrolysis. Reproduced with permission from reference13. Copyright 2016, Elsevier Ltd.
In this paper the methods for manufacturing a variety of carbon microstructures based on the pyrolysis of natural precursor materials or photo-patterned polymer structures were reported. The carbon materials resulting from both the traditional and non-conventional C-MEMS/C-NEMS processes are typically found to be glassy carbons. Glassy carbon is a widely used electrode material for electrochemistry and also for high-temperature applications. The microstructure of glassy carbon is composed of both crystalline and amorphous regions. Glassy carbon has high conductivity, good resistance to high temperatures, low density, low electrical resistance, and relatively high hardness and high resistance to chemical attack.
We proposed a simple and low-cost method to fabricate hollow glassy carbon microfibers from human hair and described their application to the electrochemical sensing of ascorbic acid and dopamine. The manufacturing of long, hollow, and electrically conductive carbon microfibers was possible with the low-cost, one-step thermal treatment of human hair, which is a waste material. A human hair composed of medulla, cortex, and cuticle, produced a hollow carbon fiber after pyrolysis. The medulla disappeared, whereas the cuticle and cortex combined together and create long, hollow carbon fibers. The unique anatomy of human hair, particularly its hollow structure after pyrolysis, is characterized by a significant increase in surface area, leading to their use in electrochemical sensing. For the working electrode modified with the hair-derived carbons, the peak potentials for dopamine and ascorbic acid were shifted towards more negative and positive values respectively. The surface of the hair-derived carbon is little negatively charged which attracs positively charged dopamine and therefore, the peak currents were increased for dopamine and decreased for ascorbic acid. As a result, human hair-derived hollow carbon microfibers could be useful to detect dopamine in the presence of ascorbic acid. A comparative study of electrochemical sensing performance of bare electrodes and modified electrodes (with hair-derived carbon and CNTs) has been carried out . The hair-derived carbon modified electrode shows better sensing performance than the unmodified electrode, but the CNT-coated electrodes showed the best performance for sensing dopamine and ascorbic acid in these experimental conditions, as was expected. The CNTs shows higher sensitivity due to their greater surface-area-to-volume ratio over hair-derived hollow carbon fibers. However, the major advantage of using hair-derived carbon microfibers over CNTs is the extremely low cost. Ease of fabrication of the hair-derived carbons also makes them dominate over CNTs. It is possible to enhance the sensing performance by further modification or functionalization of the human hairs before or after pyrolysis.
The shrinkage effect due to the pyrolysis is unavoidable, as non-carbon atoms detach from the carbon bonds at high temperatures, causing a substantial loss of mass. Before designing a desired structure, one should have a good estimate off the expected shrinkage of the patterned structure. Here, the shrinkage of the carbon structures fabricated using a conventional C-MEMS process, which includes two steps for the photo-patterning and pyrolysis of polymer structures, was studied. Five strucutres with different inner diameters (0, 30, 50, 75, and 100 µm) were considered for the shrinkage study, whereas the outer diameter was kept constant at 150 µm for all structures. The structures with inner diameters of 0, 30, 50, and 75 µm showed more shrinkage on the outer diameter than the inner diameter after pyrolysis. In these cases, the outer surface areas were larger than the inner surface areas. Thus, the outer surfaces were dominant during the shrinking process, allowing more non-carbon atoms to leave the structures. In the structure with a 100 µm inner diameter, where the inner surface area also made up a significant portion of the total surface area, the shrinkage ocurred at both the inner and the outer surfaces.
The parameters for pyrolysis, such as the pyrolysis time, the maximum pyrolysis temperature, and the atmospheric environment, significantly affect the properties of the carbon structure, such as the electrical conductivity, the chemical composition, the electrochemical property, etc. Ongoing research focuses on revealing the effects of pyrolysis conditions on the qualities and properties of carbon structures. In the case of conventional C-MEMS, carbon structures with diverse electrical and mechanical properties can also be fabricated, resulting in a wide variety of applications, by changing the photolithography conditions, such as the baking times, the baking temperature, the type of photoresist, and the additives. It is our belief that this research can provide substantial and useful information to researchers in the field of carbon structure nano/microfabrication for a variety of applications.
The authors have nothing to disclose.
This work was supported by Technologico de Monterrey and the University of California at Irvine.
SU8-2100 | Microchem | Product number-Y1110750500L | |
Spinner | Laurell Technologies Corporation | Model-WS650HZB-23NPP/UD3 | |
Hotplate | Torrey Pines Scientific | HS61 | |
UV-exposer | Mercury Lamp, SYLVANIA | H44GS-100M, P/N-34-0054-01 | |
Photomask | CAD/Art | No number | |
Developer | Microchem | Y020100 4000L | |
DI water system | Milli Q | ZOOQOVOTO | |
IPA | CTR Sientific | CTR 01244 | |
N2 gas | AOC Mexico | No number | |
Furnace | PEO 601, ATV Technologie GMBH | Model-PEO 601, Serial no.-195 | |
Si/SiO2 | Noel Technologies |