Fiber-based hybrid nanogenerators for/as self-powered systems in biological liquid.

A goal of nanotechnology is to create nanosystems that are intelligent, multifunctional, super-small, extremely sensitive, and low power consuming. The search for sustainable power sources for driving such nanosystems is an emerging field in today s energy research, and harvesting energy from multiple sources available in the environment is highly desirable for creating self-powered nanosystems. For implanted nanodevices, such as a glucose sensor used to monitoring diabetes, it is rather challenging to power them since the solar energy is not available inside the body and thermal energy cannot be used because there is no temperature gradient. The only available energy in vivo is mechanical and biochemical energy. Nanogenerators (NGs) were demonstrated to convert low( Hz) and high-frequency ( 50 kHz) mechanical energy into electricity by means of piezoelectric zinc oxide (ZnO) nanowires (NWs). Following this landmark discovery, direct current (DC) and alternative current (AC) NGs, single-wire and multi-nanowire arrays-based NGs have been developed. On the other hand, biofuel cells have been demonstrated to convert biochemical energy into electricity by using active enzymes as catalyst and glucose as fuel. We have previously demonstrated that biochemical and mechanical generators can work together to harvest multiple kinds of energy in bio-liquid, however, the two units were separately arranged on plastic substrate without integration, and the output was too low and the size was too large to be used for real applications. Here we demonstrate a flexible fiberbased hybrid nanogenerater (hybrid NG) consisting of a fiber nanogenerator (FNG) and a fiber biofuel cell (FBFC), which can be used in bio-liquid (such as blood) for energy harvesting. The FNG and FBFC are totally integrated on a single carbon fiber for the first time for simultaneously or independently harvesting mechanical and biochemical energy. In addition, the hybrid NG can also serve as a self-powered pressure sensor for detecting pressure variation in bio-liquid. Our fiber-based hybrid NG is an outstanding example of selfpowered nanotechnology for applications in biological sciences, environmental monitoring, defense technology, and even personalized electronics. A hybrid nanogenerater made up of a fiber nanogenerater (FNG) and a fiber biofuel cell (FBFC) is designed onto a carbon fiber. The design of the FNG is based on the textured ZnO NW film grown on the surface of the carbon fiber. The carbon fiber serves not only as the substrate on which the ZnONW film is grown, but also as an electrode (noted as core electrode). In previous work we have fabricated a textured ZnO NW film by using physical vapor deposition. The FNG was fabricated by etching the ZnO NW film at one end of the carbon fiber, contacting the top surface using silver paste and tape, and leading out two electrodes from the surface and the core electrodes (left-hand in Figure 1a). An FBFC, which is used for converting chemical energy from bio-fluid such as glucose/blood into electricity, is fabricated at the other end of the carbon fiber (Figure 1a). A layer of soft epoxy polymer is coated on the carbon fiber as an insulator, then two gold electrodes are patterned onto it and coated with carbon nanotubes (CNTs), followed by immobilization of glucose oxidase (GOx) and laccase to form the anode and cathode, respectively. In comparison to conventional biofuel cells and miniature biofuel cells, the FBFCs described here were integrated with the NG (or nanodevices) on an individual carbon fiber, forming a self-powered nanosystem. And the size of the FBFCs shrank a lot due to eliminating the separator membrane and mediator. For easy handling and fabrication, we created our hybrid NG on individual carbon fibers, and our measurements were performed on a bundle of (ca. 1000) carbon fibers. The performance of the hybrid NG is characterized by measuring the short-circuit current Isc and the open-circuit voltage Voc. The FBFC outputs are given as VFBFC and IFBFC, the AC FNG outputs as VFNG and IFNG, and the hybrid NG outputs as VHNG and IHNG. When the hybrid NG is immersed into bio-liquid containing glucose, the FBFC generates a DC output. A typical FBFC output is shown in Figure 2a and b with IFBFC of ca. 100 nA and VFBFC of ca. 100 mV. When a pressure is periodically applied to the bio-liquid, the FNG starts to generate an AC output. The general output of VFNG is 3.0 Vat an output current of IFNG= 200 nA (Figure 2c and d) for an FNG made of ca. 1000 carbon fibers, and the corresponding current density is 0.06 mAcm . By integrating the AC FNG and DC FBFC, a hybrid NG is obtained with the output close to the sum of the FBFC and the FNG (Figure 2e and f). The shape and frequency of the AC FNG output are the same before and after the hybrid[*] Dr. C. Pan, Z. Li, W. Guo, Prof. Z. L. Wang School of Materials Science and Engineering Georgia Institute of Technology, Atlanta, GA 30332-0245 (USA) E-mail: zlwang@gatech.edu Homepage: http://www.nanoscience. gatech.edu/zlwang