High Quantum Yield Shortwave Infrared Luminescent Tracers for Improved Sorting of Plastic Waste.

More complete recycling of plastic waste is possible only if new technologies that go beyond state-of-the-art near-infrared (NIR) sorting are developed. For example, tracer-based sorting is a new technology that explores the upconversion or down-shift luminescence of special tracers based on inorganic materials codoped with lanthanide ions. Specifically, down-shift tracers emit in the shortwave infrared (SWIR) spectral range and can be detected using a SWIR camera preinstalled in a state-of-the-art sorting machine for NIR sorting. In this study, we synthesized a very efficient SWIR tracer by codoping Li 3 Ba 2 Gd 3 (MoO 4 ) 8 with Yb 3+ and Er 3+ , where Yb 3+ is a synthesizer ion (excited near 976 nm) and Er 3+ emits near 1550 nm. Fine-tuning of the doping concentration resulted in a tracer (Li 3 Ba 2 Gd (3- x-y ) (MoO 4 ) 8 : x Yb 3+ , yEr 3+ , where x = 0.2 and y = 0.4) with a high photoluminescence quantum yield for 1550 nm emission of 70% (using 976 nm excitation). This tracer was used to mark plastic objects. When the object was illuminated by a halogen lamp and a 976 nm laser, the three parts could be easily distinguished based on reflectance and luminescence spectra in the SWIR range: a plastic bottle made of polyethylene terephthalate, a bottle cap made of high-density polyethylene, and a label made of the tracer Li 3 Ba 2 Gd 3 (MoO 4 ) 8 :Yb 3+ , Er 3+ . Importantly, the use of the tracer in sorting may require only the installation of a 976 nm laser in a state-of-the-art NIR sorting system.