Influence of the Anionic Zinc-Adeninate Metal–Organic Framework Structure on the Luminescent Detection of Rare Earth Ions in Aqueous Streams

Abstract

Rare earth elements (REEs) are critical to numerous technologies; however, a combination of increasing demand, environmental concerns, and monopolistic marketplace conditions has spurred interest in boosting the domestic REE production from sources such as coal utilization byproducts. The economic viability of this approach requires rapid, inexpensive, and sensitive analytical techniques capable of characterizing the REE content during resource exploration and downstream REE processing (e.g., analyzing REE separation, concentration, and purification production steps). Luminescence-based sensors are attractive because many REEs may be sensitized to produce element-specific emission. Hence, a single material may simultaneously detect and distinguish multiple REEs. Metal–organic frameworks (MOFs) can sensitize multiple REEs, but their viability has been hindered by sensitivity and selectivity challenges. Understanding how the MOF structure impacts the REE sensing efficacy is critical to the rational design of new sensors. Here, we evaluate the sensing performance of seven different anionic zinc-adeninate MOFs with different organic linkers and/or structures for the visible-emitting REEs Tb, Dy, Sm, and Eu. The choice of a linker determines which REEs are sensitized and significantly influences their sensitivity and selectivity against competing species (here, Fe­(II) and HCl). For a given linker, structural changes to the MOF can further fine-tune the performance. The MOFs produce some of the lowest detection limits (sub-10 ppb for Tb) reported for the aqueous sensitization-based REE detection. Importantly, the most selective MOFs demonstrated the ability to sensitize the REE signal at sub-ppm levels in a REE-spiked acid mine drainage matrix, highlighting their potential for use in real-world sensing applications.

Probing Lanmodulin’s Lanthanide Recognition via Sensitized Luminescence Yields a Platform for Quantification of Terbium in Acid Mine Drainage

“…The LODs of the best-performing sensor at low pH, T90W, are below 5 ppb over the pH 3–7 range, even at 1 μM T90W (Table ). Even at pH 3, our LOD values are similar to (or better than) the lowest values reported for other complexes, which are often reported at pH 7. , These results suggested that T90W-LanM might be robust enough to detect Tb III even in challenging environmental samples.…”

“…However, when this sensor was applied in an AMD sample containing ∼1 ppb Tb III at pH 3.4, the sample had to be spiked with 800 ppb (5 μM) Tb III to be able to observe a signal. 42 By contrast, simply adding T90W-LanM directly to AMD detects and quantifies 3 ppb Tb III present in AMD at pH 3.24 without any spiking. This comparison shows that the high affinity and selectivity of LanM for lanthanides provides a significant advantage over conventional REE sensitizer ligands, which (while they may have high affinity) usually do not have sufficient selectivity to work well in complex solutions like AMD.…”

“…Terbium is one of five REEs deemed by the United States as the most economically critical, for its utility including in phosphors in energy-efficient lighting as well as for its domestic supply risk . Despite recent progress to decrease limits of detection (LODs) for Tb III even at low pH and in AMD matrix, , popular approaches utilizing small molecules or metal–organic frameworks (MOFs) still lack the necessary selectivity, requiring spiking of samples with Tb III for detection. By contrast, biomolecular luminescence-based sensors for Tb III based on Trp-containing EF hands such as lanthanide binding tags (LBTs) exhibit better selectivity, but their affinities ( K d ≈ 50 nM , or weaker at pH 7) are insufficient for applications below pH ≈ 6 .…”

Metal–organic framework thin films as versatile chemical sensing materials

“…84,85 Photofunctionality can be introduced into MOF thin films through a variety of ways: 86 the use of chromophoric organic linkers within the MOF structures, 87 optically-active metal centers (particularly emissive lanthanides), 88 or via host-guest interactions, in which photoactive ions, 89,90 metal complexes, 91,92 molecules, 93-96 nanoparticles, 97-100 and/or other species are encapsulated by the MOF. MOF-based optical sensors are intriguing because they typically exhibit rapid response times (often on the order of seconds), [101][102][103][104] high sensitivity (ppb or lower LOD), 103,105,106 recyclability (via removal of analyte from the MOF after each sensing cycle), 103,104,[107][108][109] and usually can be integrated with inexpensive, portable equipment, 90 such as fiber optic-based platforms. 104,110,111 Indeed, many optical sensors allow for naked-eye detection of analytes and require only simple laser pointers, lightemitting diodes, UV lamps, or other portable excitation sources for operation.…”

“…82 For example, Yang and co-workers demonstrated that Cu 2+ selectively undergoes partial ion exchange with terbium in a terbium-succinate MOF, which disrupts the sensitization of terbium and reduces emission (Figure 8H). 129 For "turn off" sensors, insights into the emission quenching mechanism may be gained through a Stern-Volmer analysis of the quenching 106,168 and through timeresolved studies. 169 A key challenge is the development of sensors that exhibit a selective emission response only in the presence of a target ion, hence it is crucial for the sensor to be tested on a multitude of potential interferants that may be encountered in "real-world" systems, such as environmental waters and biological matrices.…”