Shark's olfactory system inspired mems-based and flexible LCP-based chemical sensors for heavy metal detection
Date of Issue2017
School of Mechanical and Aerospace Engineering
Singapore-MIT Alliance for Research and Technology
Water pollution has become an issue of serious concern on a global scale. Among different pollutants, heavy metal ions are considered as one of the most hazardous contaminants since they cannot be biologically degraded and will accumulate in the human body for a long period of time. Conventional water quality monitoring methods typically rely on collecting water samples in the field and then bringing them to localized laboratories for testing. However, there are some inevitable disadvantages such as labor-intensive sample collection, time-consuming preparation and analysis, and costly experimental apparatus. Real-time data regarding the level of target metal ions in concerned sites cannot be attained through the conventional approaches. Therefore, the development of miniaturized, inexpensive, and portable chemical sensors for on-site/in-situ heavy metal monitoring is highly desirable. In this thesis, lead (Pb) and zinc (Zn) are selected as target heavy metal ions in consideration of the historical situation of heavy metal contamination in Singapore. The environmental regulatory values for Pb and Zn ions in drinking water are 10 ppb (µg/L) and 3 ppm (mg/L). For Pb ions detection, a miniaturized, lab-on-a-chip chemical sensor is designed by drawing inspiration from the morphological structure of shark’s olfactory system. The sensor features a bottom sensor base with three thin-film, photolithographically-fabricated electrodes, and a top microfluidic channel. A three-dimensional, free-standing micropillar working electrode array is incorporated into the sensor, which mimics the configuration of shark’s sensory cells. To the best of our knowledge, this is the first time that a shark’s olfactory system inspired chemical sensor has been developed. The sensor was successfully fabricated by means of microelectromechanical systems (MEMS) techniques. The analytical performance of the biomimetic chemical sensor was comprehensively investigated through electrochemical experiments. Under optimal conditions, an analytical sensitivity of 32 nA/(µg/L) and favorable limit of detection (LOD) of 0.2 ppb were obtained under a short deposition time of 30 s. The sensor exhibited a linear response to Pb ions in the concentration range from 1 to 130 µg/L. It was found that the bio-inspired design significantly enhanced collection efficiency of the sensor towards target metal ions. This is both due to the enlargement of electrode surface area and the interaction between protruding micropillar working electrodes and moving solution. The proposed sensor eliminates the involvement of mechanical stirring of testing solution, manifesting great potential to be deployed for the application of on-site Pb ions determination. Driven by the attempt to achieve in-situ Zn ions detection, a flexible chemical sensor integrated on a liquid crystal polymer (LCP) substrate is developed. Incorporation of the LCP material renders improved operational reliability, high durability, as well as extraordinary flexibility to the sensor, which allows it to be mounted on any flat or curved surface for harsh environmental sensing. The fabrication of the sensor was achieved through standard MEMS techniques. Experimental evaluation showed that the sensor was able to detect Zn ions down to 0.08 ppb. In addition, the sensor showed an analytical sensitivity of 92 nA/(µg/L), gratifying detection range from 0.3 to 70 µg/L, and favorable reproducibility with relative standard deviation of 1.64% (n = 6). Real-time application of the sensor was also demonstrated by measuring Zn ions in seawater using a sensor array attached to the hull of a remotely-controlled autonomous kayak. The response of the sensor array indicated a clear trace of Zn ions present in the seawater, which was subsequently confirmed by performing inductively coupled plasma mass spectrometry analysis with collected seawater samples. The laboratory and field investigations suggest promising application of the proposed flexible chemical sensor for in-situ Zn determination.