Materials for the photoluminescent sensing of rare earth elements: challenges and opportunities

Rare earth elements (REEs) are widely used in high-performance technologies including wind turbine magnets, electric vehicle batteries, lighting displays, circuitry, and national defense systems. A combination of projected increasing demand for REEs, monopolistic economic conditions, and environmental hazards associated with the mining and separation of REEs has led to significant interest in recovering REEs from alternative sources such as coal waste streams. However, rapidly locating high-value waste streams in the field remains a significant challenge primarily because of slow analytical methods, and existing techniques with low limits of detection such as inductively-coupled plasma mass spectrometry suffer from high equipment and operating costs and a lack of portability. Alternatively, luminescence-based sensors for REEs present a potential path for sensitive, portable, low-cost detection. The development and design of materials suitable for the luminescence-based detection of REEs are crucial to realizing this potential. Here, we review a broad range of materials used (or that have the potential to be used) for REE luminescence-based detection, including organic compounds, biomolecules, polymers, metal complexes, nanoparticles, and metal-organic frameworks. A general overview of REE optoelectronic properties and luminescent sensing protocols is first presented, followed by analyses of material-specific sensing mechanisms, emphasizing sensing figures of merit including sensitivity, selectivity, reusability and portability. The review concludes with a discussion of remaining barriers to luminescent REE sensing, how each sensor class may be best deployed, and directions for future material and spectrometer design. Taken together, this review provides a broad overview of sensing materials and methods that should be foundational for the continued development of high-performance sensors.