Introduction
The interest on the chemical detection in situ of nitroaromatic compounds (NACs) accurately and reliably, is mainly associated with security, environment pollution, and human health. This fact has currently encouraged researches to focus on the design of new chemical sensors that show selective detection of NACs such as 2,4,6-trinitrophenol (TNP),1 nitrobenzene (NB)2, 2,6-dinitrotoluene (DNT)3⁠, among others.4⁠ Furthermore, these compounds are extensively used in many industrial processes and some of them are highly toxic or carcinogenic. Moreover, they are potent pollutants of soils and groundwaters.5-6⁠ On the other hand, a highlighted fact about these chemical compounds is that they represent a potential risk to security, due to their high explosive power as well as the easy access to them; being also used in the manufacture of improvised explosive devices.7-8 In this sense, more research on their effective detection is needed as well as the implementation and improvement of devices used to this end. The performance principle of a chemical sensors is based on the change of an observable response upon interaction with analytes of interest. The observables are based on physical principles such as absorbance, transmittance, the polarization of light, luminescence, among others. In this context, Metal-Organic Frameworks (MOFs) have emerged as a very interesting alternative for the design of chemosensors due to their structural and photophysical features; luminescence specifically.9 MOFs are composed of metal centers or cluster-like arrangements called nodes connected by linkers, which are organic ligands.10This wide range of possibilities to get different structures of MOFs, with different nodes and linkers, confers to these materials different photophysical responses including luminescent properties.11 In this regard, light emission can arise from either the ligand or the nodes, i.e. from the metal ions or metal clusters. The luminescence mechanism can involve energy transfer processes, ligand-to-ligand charge transfer (LLCT), ligand-to-metal charge transfer (LMCT) or metal-to-ligand charge transfer (MLCT) as well as metal-to-metal charge transfer (MMCT).12 These inorganic and organic species confers them flexible coordination environment that leads to different secondary building units (SBUs) and then a variety of topologies of the network (i.e. IRMOF-n series based on the renowned MOF-5, the UiO series such as UiO-66, UiO-67, UiO-68, etc). 13-14 The importance of the porosity and the large surface area of MOFs is recognized as the system acts as a pre-concentrator of the targeted analyte, displaying host-guest interactions which can be modulated because of their synthetic versatility.⁠15-16⁠ When the MOF encapsulates the analytes, the interactions that occur (host-guest) might induce changes in the photophysical properties, causing the activation or deactivation of the luminescent signal of the whole system.17⁠-18 This response to the emission intensity gives rise to an important classification that groups them as luminescent turn-off or turn-on chemosensor. In this sense, when the emission is very weakly or the system does not emit but the resulting luminescence is enhanced by adding the analyte, it is called a Turn-on chemosensor. While Turn-off chemosensors are systems whose luminescence is quenched after interaction with the chemical species of interest (Scheme 1). 19-20