Europium and cobalt modified MOF-808: a humidity-responsive fluorescent barcode

Abstract

Stimuli-responsive luminescent materials can be used to design anti-counterfeit barcodes because even small changes in environmental conditions, such as temperature or the molecules present, can lead to changes in the fluorescence behavior of the materials, enabling dynamic information encoding. Metal–organic frameworks (MOFs) are promising materials for the design of stimuli-responsive barcodes, as their modular structure allows fine tuning of the fluorescence, which in turn can be switched in response to external stimuli. In this work, we present a humidity-responsive fluorescent barcode prototype based on europium- and cobalt-modified MOF-808, which exhibits fluorescence switching in response to changes in relative humidity. The combination of cobalt and europium cations in the same material, along with the coordination ligands on the cobalt cations, are crucial for the functionality of this barcode. By selectively suppressing the fluorescence of europium through the absorption of cobalt complexes in the same energy range, the initial fluorescence intensity of the material can be adjusted. The additional coordination of water molecules to the cobalt centers leads to the formation of cobalt complexes with higher ligand field splitting energy, resulting in a stepwise reactivation of the emission of the europium cations. This can be readily accomplished by exposing the material to different relative humidities. As a result, the barcode provides a higher level of security, as its decoding must be performed under controlled atmospheric conditions, and it is user-friendly since no harmful chemicals are required during the reading process. Additionally, the initial state must be restored before each decoding by heating, as the fluorescence changes dynamically according to the environmental conditions. These characteristics of the material underline the anti-counterfeiting capabilities of the barcode.