# Methods for Detecting Picric Acid--A Review of Recent Progress
## 3 Sensors for Detecting and Measuring PA
### Metal-Organic Frameworks Sensor
#### 3.1.2 Lanthanide-MOFs  
Unlike transition metals, the electrons of metals belonging to the lanthanide group allow them to have a larger coordination sphere. Lanthanide ions are mostly trivalent, except for Eu\({}^{2+}\) and Ce\({}^{4+}\), with a ground-state electron configuration of [Xe]4 fn which can generate a rich diversity of electron levels and translations [65]. Based on considerations for hard soft acids and bases [66], the lanthanides show affinity in relatively hard oxygen-containing linkers to other functional groups. Due to the structural stability provided by the bond between the ligand and the central ionic unit of the metal due to the transition metal (paramagnetic) ion, the luminescence of MOFs is focused on the metal, as metal ions with an f orbital can act as junctions. In addition, the metal centre ion can combine with the ligand to provide high absorption of metal centre-coordinated bands, facilitating the inter-system transition from singlet to triplet excited states in the Ln-MOF. This phenomenon is known as the antenna effect, and it provides good emission enhancement properties [67; 68] (Figure 3). Investigations were carried out for different types of MOFs, in the context of the possibility of their ability to detect aromatic nitrocompounds, with particular emphasis on PA. Positive luminescence quenching selectivity results were obtained for PA in most of the Ln-MOFs tested (Table 3). Research on Ln-MOFs as sensors for high-energy nitro compounds in aqueous solutions has shown that in most cases, MOFs can perform well as selective PA sensors. For other solvents, the affinity of some MOFs for PA is much lower than for other nitroaromatic compounds Figure 4. This suggests that the presence of electron-rich molecules and the flexible coordination geometry of lanthanide ions may provide many opportunities towards specific interactions between MOFs and selective analytes. \begin{table}
\begin{tabular}{l l l l l} \hline
**MOFs** & **Structure** & **Solvent** & **Detection Limit** & **K\({}_{\text{SV}}\)\({}^{1}\) [M\({}^{-1}\)]** & **Ref.** \\ \hline \(Zn(NDC)(H_{2}O)|_{n}\) & 3D Porous & H\({}_{2}\)O & \(1\times 10^{-6}\) & \(6\times 10^{4}\) & [59] \\ \([Zn_{4}(DMF)(Ur)_{2}(NDC)_{4}]\) & 3D Porous & H\({}_{2}\)O & \(1.63\) ppm & \(10.83\times 10^{4}\) & [60] \\ \([Zn_{2}(NDC)_{2}(pypy)]\cdot Gx\) & 3D framework & ethanol & not applicable & \(4.22\times 10^{3}\) & [61] \\ \([Cd(NDC)(H2O)|_{n}\) & 3D Porous & H\({}_{2}\)O & \(4\times 10^{-6}\) & \(2.385\times 10^{4}\) & [59] \\ \(([Cd_{4}(1,1^{\prime}:3^{\prime},1^{\prime\prime}-terphenyl]-4,4^{\prime},4 ^{\prime\prime},6^{\prime}-tetracarboxylic)_{2}(2-amino-4,4^{\prime}-bipyridim 3)(H_{2}O_{2})_{2}\cdot 8 DMF\cdot 8H_{2}O)\) & & \\ \([Cu_{2}(tpt)_{2}(tda_{2})]\cdot H_{2}O\) & 3D network & H\({}_{2}\)O & \(2.71\times 10^{-7}\) & \(1.36\times 10^{5}\) & [63] \\ \([Zr_{6}O_{4}(OH)_{4}(BTDB)_{6}]\cdot 8H_{2}O\cdot 6 DMF\) & 3D Porous & methanol & \(1.63\times 10^{-6}\) & \(2.49\times 10^{4}\) & [64] \\ \hline \end{tabular} \({}^{1}\) Fluorescence quenching constant.  
\end{table} 
Table 2: Summary of d-block metal–organic frameworks used for the detection of picric acid.