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