Laser Reduced Graphene and its Applications for Strain Sensor and Biosensor

Abstract

Laser reduction method for graphene has attracted significant interest in recent years, attributing to its distinctive advantages in selective and localized reduction, direct micro- nanoscale patterning, and no requirement for chemicals. Upon the laser irradiation, the precursor, typically graphene oxide or polyimide, is converted in situ into reduced graphene. This laser reduced graphene (LRG) has recently found its way into various applications including supercapacitor, sensor, field effect transistors, flat lens, lithium-ion batteries, water purification etc., due to its fascinating properties, such as large surface area, high electrical and thermal conductivity. This PhD project is dedicated to four correlated studies of LRG: (1) the investigation and summary of the most recent research progress on LRG; (2) the mechanism study of the laser reduction process; (3) the strain sensor application based on LRG doped with Au nanoparticles; (4) the biosensor application for RNA detection based on laser induced graphene. Firstly, recent research progress on the aspects of fabrication, properties and applications of LRG was reviewed. In these researches, different precursors, mainly graphene oxide (GO) film, GO solution and polymers, were employed for laser reduction. The mechanism of LRG formation, essentially involving with the three main theories: photochemical process, photothermal process, and a combination of both processes was investigated and concluded to be strongly related with the wavelength of the laser. Diverse strategies such as the adjustment of laser parameters, chemical doping, structure modulation and environment control, which are effective to tune the properties and performance of LRG had been summarized. A broad range of published applications based on LRG was reviewed and potential opportunities for new applications and their improved performances were also discussed. Secondly, the mechanism of this laser reduction was systematically investigated and studied to provide more profound insights into the overall process. The graphene oxide film was irradiated and effectively reduced by a fs laser (780 nm). After the laser treatment, the surface morphology, the degree of graphitization, crystal structure chemical components and electrical properties of LRG were characterized. It can be concluded that the two coexisting sub-processes during laser reduction, namely the direct conversion of sp3 carbon into sp2 carbon and the removal of oxygen can be tuned by adjusting the laser parameters. The different oxygen-contained groups can also be selectively reduced by controlling the power of the laser. These findings improve the understanding of the fundamental mechanism of laser reduction and expand the vision to effectively tune the properties of LRG. Thirdly, laser reduced graphene sheets decorated with Au nanoparticles in situ (LRG/Au) were synthesized and applied for strain sensor application. In this study, the composites of HAuCl4 and GO were irradiated with a milliwatt fs laser (780 nm). It was revealed that both the reduction of graphene oxide and the nucleation and growth of the Au nanoparticles were enhanced due to the multi-stage interactions among the precursors and the laser. The strain sensor based on LRG/Au exhibits high sensitivity (gauge factor 52.5) in large strain range of 25.4% and good stability in 500 cycles. It was demonstrated to detect human motions such as folding and unfolding of wrist and finger, indicating great potential for artificial skin and wearable electronics. Furthermore, the mechanisms underneath the improved strain sensing performance were explored from fundamental physical chemistry, electronic properties and mechanical mechanisms. In the end, a simple and inexpensive biosensor based on laser induced graphene (LIG) is explored for ultrasensitive RNA detection. The LIG was prepared from commercial polyimide by direct CO2 laser writing and patterned as the electrode of the biosensor for electrochemical measurement. The purified RNAs were adsorbed to the surface of LIG via graphene-RNA affinity, which hinders the access of the [Fe(CN)6]3-/4- system to the electrode surface and eventually results in a lower electrochemical response in the ferricyanide redox system. The LIG biosensor demonstrates good response and high sensitivity (10 fM) for RNA detection, showing strong potential for cancer detection and gene screening in practical.