Yiheng Qin

Microfabricated electrochemical pH and free chlorine sensors for water quality monitoring: recent advances and research challenges

Continuous, real-time monitoring of the level of pH and free chlorine in drinking water is of great importance to public health. However, it is challenging when conventional analytical instruments, such as bulky pH electrodes and expensive free chlorine meters, are used. These instruments have slow response, are difficult to use, prone to interference from operators, and require frequent maintenance. In contrast, microfabricated electrochemical sensors are cheaper, smaller in size, and highly sensitive. Therefore, these sensors are desirable for online monitoring of pH and free chlorine in water. In this review, we discuss different physical configurations of microfabricated sensors. These configurations include potentiometric electrodes, ion-sensitive field-effect transistors, and chemo-resistors/transistors for electrochemical pH sensing. Also, we identified that micro-amperometric sensors are the dominant ones used for free chlorine sensing. We summarized and compared the structure, operation/sensing mechanism, applicable materials, and performance parameters in terms of sensitivity, sensing range, response time and stability of each type of sensor. We observed that novel sensor structures fabricated by solution processing and operated by smart sensing methodologies may be used for developing pH and free chlorine sensors with high performance and low cost. Finally, we highlighted the importance of the concurrent design of materials, fabrication processes, and electronics for future sensors.

Low-temperature solution processing of palladium/palladium oxide films and their pH sensing performance

Highly sensitive, easy-to-fabricate, and low-cost pH sensors with small dimensions are required to monitor human bodily fluids, drinking water quality and chemical/biological processes. In this study, a low-temperature, solution-based process is developed to prepare palladium/palladium oxide (Pd/PdO) thin films for pH sensing. A precursor solution for Pd is spin coated onto pre-cleaned glass substrates and annealed at low temperature to generate Pd and PdO. The percentages of PdO at the surface and in the bulk of the electrodes are correlated to their sensing performance, which was studied by using the X-ray photoelectron spectroscope. Large amounts of PdO introduced by prolonged annealing improve the electrodes sensitivity and long-term stability. Atomic force microscopy study showed that the low-temperature annealing results in a smooth electrode surface, which contributes to a fast response. Nano-voids at the electrode surfaces were observed by scanning electron microscope, indicating a reason for the long-term degradation of the pH sensitivity. Using the optimized annealing parameters of 200°C for 48 h, a linear pH response with sensitivity of 64.71±0.56 mV/pH is obtained for pH between 2 and 12. These electrodes show a response time shorter than 18 s, hysteresis less than 8 mV and stability over 60 days. High reproducibility in the sensing performance is achieved. This low-temperature solution-processed sensing electrode shows the potential for the development of pH sensing systems on flexible substrates over a large area at low cost without using vacuum equipment.

Direct bonding of liquid crystal polymer to glass

In this paper, a direct bonding technology for liquid crystal polymer (LCP) and glass is developed for the first time by using sequential plasma-activated bonding which is based on physical sputtering followed by the formation of chemically reactive surfaces. The sequential-plasma-activated surfaces of glass and LCP show high hydrophilicity with moderate roughness. The adhesion between the activated LCP and glass surfaces is governed by hydroxyl group-mediated interfacial Si–OH–C covalent bonds. The post-bonding anodic treatment increases the amount of interfacial oxides by generating more singly-bonded oxides on the glass surface. The post-bonding thermal treatment rearranges the plasma-induced reactive sites and improves the conformal contact between the LCP and glass. A high bonding strength of 6.3 MPa is obtained between the LCP and glass when both anodic and thermal treatments are used. This simple and low-temperature direct bonding process for LCP and glass provides insights for future bonding between polymers and thin glass films.

Morphology and electrical properties of inkjet-printed palladium/palladium oxide

Inkjet printing is used to deposit palladium films with different morphologies and electrical properties using different precursor thermolysis atmospheres. First, the precursor is reductively decomposed into amine-stabilized palladium clusters. In air, oxygen assists the decomposition of the organoamine ligands for the palladium clusters. Small nanoparticles are formed and fused to smooth films. In nitrogen, the residual ligands facilitate the formation of sub-micron spherical aggregates. In low vacuum, a bilayer film containing a bottom layer with fused nanoparticles and a top layer with spherical aggregates is formed. Such morphology is caused by the competition between the film formation processes in air and in nitrogen. The electrical properties of the films are determined by the whole film for the smooth film and by the bottom layer for the bilayer film. The temperature coefficient of resistance of the films can be adjusted from 0.067% °C−1 to −0.189% °C−1 by tuning the amount of semiconductive palladium oxide in the conduction path. In addition, humid air increases the resistance drift of the films by forming surface-adsorbed hydroxyl groups and/or molecular water. This study highlights the importance of precursor thermolysis atmospheres on the morphology and electrical properties of inkjet-printed palladium/palladium oxide films.