Lead contamination in drinking water has become an increasingly serious global risk because a small concentration of lead can cause serious health problems, particularly for children. It is critical to frequently monitor lead concentration in drinking water, which can be challenging when using traditional centralized systems. In this study, we present an inexpensive, portable detection system for point-of-care (POC) monitoring of lead concentration in drinking water. The sensing mechanism is based on the interaction between the water sample flowing through a microchannel and a planar microwave resonator-based sensor that is integrated with the microfluidic chip. The microwave sensor has a double-T structure with a gap in between through which the microchannel is aligned and can be coated with gold nanoparticles to enhance its sensing performance. For proof-of-concept, the sample under test (SUT) was a small volume of deionized (DI) water or tap water spiked with lead ions at different concentrations. Results show that the gold nanoparticle-coated microwave sensor presents a much higher sensitivity than bare sensors with a detectable frequency shift of 5 MHz for a Pb2+ solution with a concentration of 1 ppb. The success of the system for testing lead ions in typical tap water which contains many different mineral ions confirms its real-world application. To highlight the potential for POC applications, a low-cost, portable vector network analyzer is used to capture the frequency shift of the sensor. The developed method offers a promising approach for POC monitoring of lead contamination in drinking water impactful for environmental and public health protection.
Abstract Non-invasive contactless simultaneous sensing and heating of individual droplets would allow droplet microfluidics to empower a wide range of applications. However, it is challenging to realize simultaneous sensing and heating of individual droplets as the resonance frequency of the droplet fluid, which is decided by its permittivity, must be known so that energy is only supplied at this frequency for droplet heating with one resonator. To tailor the energy transfer in real-life heating applications, the droplet has to be sensed first to identify its corresponding resonance frequency, which is used to dynamically tune the frequency for supplying the required energy for heating this particular droplet. To achieve this goal, two resonators are needed, with one for sensing and one for heating. Integrating multiple resonators into one typical microfluidic device limits placement of the resonators to be as close as possible, which would raise the concern of crosstalk between them. The crosstalk would result in inaccurate sensing and heating. This study focuses on numerically and experimentally investigating the effect of influencing parameters on the crosstalk between two adjacent resonators with the ultimate goal of providing guidance for multiplexing the resonators in a typical microfluidic device. ANSYS HFSS is used to perform the electromagnetic analysis based on the finite element method. Experimental studies are conducted on a microfluidic chip integrated with two resonators to validate the numerical results. An optimal distance between two resonators is suggested, with the recommendation for the resonator size and heating power towards simultaneous sensing and heating of individual droplets.
Heavy metal pollution on earth has evolved into a global issue causing serious risks to human health and other living entities and having an impact on sustainability. Accurate identification of metal contamination is often carried out in centralized facilities involving sampling, transportation, and the need for highly trained personnel, which becomes expensive, often causes delays in response to potential tragedies, and is prone to sample properties changes. Rapid, affordable methods for point-of-care (POC) detection of heavy metals with reasonable accuracy are ideal to address these challenges enabling diligent monitoring of metal pollution. There have been many POC systems reported, however, the systems that could work with real samples in which heavy metals are present in a complex form at a low concentration are limited. Sample preparation is often needed for the accurate identification of metal ions. Microfluidics offers tremendous potential for sample preparation and integration with various detection methods such as optical and electrochemical methods for POC detection of heavy metals. This review is limited to reviewing the reported microfluidic-based POC devices for heavy metal sensing and providing a brief perspective on the integration of microwave sensing methods with microfluidic devices for heavy metal detection. This review starts with introducing microfluidic-based heavy metal sensing using optical and electrochemical methods and then focuses on briefly discussing the development and potential of integrating microwave sensing with microfluidic devices for heavy metal sensing. The principle of each method and the limit of detection are briefly discussed.