Visual and Real-Time Monitoring of Cd²⁺ in Water, Rice, and Rice Soil with Test Paper Based on [2 + 2] Lanthanide Clusters

Cadmium ions (Cd²⁺) are highly toxic to animal and human health, especially through the drinking of Cd²⁺-contaminated water and eating Cd²⁺-contaminated rice. Therefore, accurate detection of Cd²⁺ in water, rice, and rice soil is urgent. In this work, two [2 + 2] lanthanide clusters of <b>Tb₂Tb₂</b> and <b>Eu₂Eu₂</b> were synthesized and characterized in detail. Interestingly, <b>Tb2Tb2</b> is a rapid sensor for Cd²⁺ through luminescence “turn-off”. Further studies show that <b>Tb2Tb2</b> is a highly sensitive and selective sensor toward Cd²⁺ in water, rice supernatants, and rice soil supernatants, with a very short response time of 20 s. The limit of detection (LOD) in the above three real samples is as low as 0.0112, 1.1240, and 0.1124 ppb, respectively, which is lower than the national standards for food safety in China (GB 2762-2022). More interestingly, a portable sensing device of test paper based on <b>Tb2Tb2</b> is developed with a facile method, which shows visible, highly sensitive, and selective sensing toward Cd²⁺ in real samples of water, rice supernatants, and rice soil supernatants. <b>Tb2Tb2</b> and its sensing device of test paper are an on-site analysis sensor for potentially non-expert users, especially for people in remote rural areas.