연구홍보포털

논문 초록 (Abstract)

Microfluidic reaction systems (e.g., lab-on-a-chip systems)1, 2 have emerged to improve conventional bulk reaction systems by reducing fluid consumption levels and by offering elaborate controllability of the experimental conditions and faster response times.3 Based on these advantages, various applications using microfluidic systems have been demonstrated, such as quantum dots,4 nanoparticle synthesis,5, 6 chemical analysis,7 cellomics,8 and point-of-care diagnostic testing (POCT).9 To implement a sensing component into a microfluidic system, an optical spectroscopic method and an electrochemical method have been used. Spectroscopic methods such as laser-induced fluorescence (LIF) and UV absorption offer high sensitivity and accuracy; however, bulky and high-maintenance equipment limit the miniaturization capabilities and the range of application of these approaches. Electrochemical methods such as amperometry and conductimetry, which convert chemical signals into electrical signals, allow miniaturization due to the simple electrode-based structures involved;10 however, electrochemical detection methods inevitably introduce external bias, thus demanding an external power supply and conversion circuit components. Moreover, the direct electrical contact between a sensitive electrode and target analytes can be problematic as it degrades the stability of the system. Recently, apart from well-developed thermoelectric,11 piezoelectric, and pyroelectric energy conversion approaches,12 triboelectric approaches,13-15 which generate electrical output via contact electrification16-18 and electrostatic induction, have come to the fore as an attractive candidate for realizing self-powered electronics19, 20 and cost-effective self-powered active sensors for chemical detection,21, 22 mechanical sensing,23-27 and environmental monitoring.28, 29 Most triboelectric devices rely on solid–solid contact electrification; however, devices based on liquid–solid contact electrification30-32 were recently demonstrated to be capable of harvesting ambient mechanical energy33, 34 and electrostatic energy under various water environments.35-40 In particular, triboelectric devices based on liquid–solid contact electrification have great potential as chemical sensors because the generated triboelectricity is closely influenced by additive chemicals or factors of the ambient environment, such as the temperature.31, 35 In this paper, a triboelectric self-powered ion concentration sensor (TSICS) embedded in a microfluidic system is demonstrated. The TSICS generates electrical signals via contact electrification between a polymeric microfluidic channel surface and a moving fluid. A simple configuration involving two remote electrodes with indirect contact between the target liquid and the electrodes offers improved stability. It also provides scalability because the output signal is generated not from an external power source but from the induced triboelectric charges of the target liquid in the microfluidic channel. Based on the phenomenon by which the amount of charge induced by liquid–solid contact electrification depends strongly on the ion concentration of the liquid,30, 32 the TSICS successfully serves as a self-powered ion concentration sensor. The sensing resolution of the TSICS was found to be comparable to that of a commercialized handheld salinity sensor, though the TSICS has much simpler structure and does not require a bulky monitoring system. This makes the TSICS a suitable candidate for a miniaturized self-powered microfluidic sensor component, which can further widen the application spectrum of microfluidic systems. The TSICS consists of two parts: a bottom part with a pair of electrodes and a top part with an aligned microfluidic channel. The overall fabrication process is depicted in Figure 1a,b. The microfluidic channel was fabricated by soft-lithography using a master mold which was created by conventional photolithography (Figure 1a). The pattern of the master mold is fabricated using SU-841 due to the high mechanical strength and thermal endurance of this material. For the fabrication of the master mold, an SU-8 layer was spin-coated onto an Si wafer and subsequently annealed (Figure 1a, i). By UV exposure and development (Figure 1a, ii), the master mold with the microfluidic channel pattern was prepared (Figure 1a, iii), after which polydimethylsiloxane (PDMS) was poured onto the master mold (Figure 1a, iv). After curing, the PDMS layer, which was used as the top part, was peeled off from the master mold. To fabricate the bottom part, a stack structure composed of Si/SiO2/Ni was prepared (Figure 1b, i). By photolithography with a film mask (Figure 1b, ii) and subsequent Ni etching (Figure 1b, iii), a pair of isolated Ni electrodes was formed (Figure 1b, iv). On the Ni electrodes, PDMS was poured and cured for integration with the top part. The top part and the bottom part were chemically bonded using a conventional plasma bonding method42 to ensure a closed microfluidic channel. Further details about the fabrication process are described in the Experimental Section.