A microbubble cavity sensor using a Fano resonance for fast detection of ultralow concentrations of Cu2+

Copper is both an essential element in the human body and one of the common heavy metal pollutants in the environment. A fast, accurate, and low-cost detection method is needed to mitigate its impact and protect the human health and environment. In this paper, a high-sensitivity microbubble resonator (MBR) sensor is proposed and investigated for fast detection of ultralow concentration Cu2+ in liquid. The MBR is a high-quality spherical microcavity structure in which whispering gallery mode resonances can be induced. The unique design of the MBR sensor allows two light modes in different states to interfere with each other through different optical paths, resulting in Fano resonance and higher refractive index sensitivity (315.21 nm/RIU). The experimental results show that the optimized MBR sensor has a good linear semi-logarithmic response to Cu2+ over a wide concentration range of 10 - 106 pg/mL. The limit of detection is as low as 4.25 pg/mL, along with a rapid response time of less than 4 min. The sensor has been used for testing industrial wastewater samples, and the spike recovery rate is 101-114%, which provides a reliable method to assess the accuracy of detection in real-world applications. In addition, the regeneration ability of the sensor and its reusability were evaluated by removing the heavy metal ions adsorbed on the sensor surface using an eluent. The proposed MBR sensor has potential applications for effective Cu2+ monitoring in the fields of biomedical science, pollution control and chemical engineering.