Swipe and wipe: The next generation phosphate sensing platform based on NH2-MIL-88B(Fe) and mid-infrared attenuated total reflection spectroscopy

Abstract

The excess use of phosphates in agriculture and industry as well as in households led to an accumulation of nutrients in aquatic ecosystems around the world. Increased phosphate concentrations in freshwater among other factors induces the eutrophication of lakes and rivers, resulting in a severe loss in biodiversity. Thus, the detection of traces of phosphates in water is a topic of great importance. Mid-infrared (IR) techniques have proven in the past to be potent in the surveillance of water streams. However, strong water absorption in the region of the phosphate bands between 900 and 1200 cm-1 impede the use of long interaction lengths, thereby limiting the sensitivity which is needed for trace analysis. Here, the versatility and defined porosity of the novel material class of metal-organic frameworks (MOFs) come into play by selectively enriching phosphate from the aqueous phase into the probed volume of evanescent field spectroscopy. Using this technique, we present a mid-IR based sensing platform, capable of the detection of sub-ppm traces of phosphate in water without the need of sample preparation. We opted for a concept of easy applicable and removable single-use sensing layers on attenuated total reflection (ATR) prisms to overcome the struggle of desorbing chemisorbed phosphates, while still guaranteeing robust results. For that, a facile, reproducible workflow for the preparation of thin NH2-MIL-88B(Fe) layers onto the ATR substrates was developed. A standardised sampling procedure for the aqueous sample phosphate solutions was established, while internal referencing allowed for stable results regardless of the applied MOF layer.