# Methods for Detecting Picric Acid--A Review of Recent Progress
## 3 Sensors for Detecting and Measuring PA
### Covalent Organic Polymers (COPs) and Covalent Organic Frameworks (COFs)  
Recently, there have been an increasing number of reports in the literature on the exploitation of the action of covalent organic polymers (COPs) and covalent organic frameworks (COFs) [80] as novel sensor materials. Most COP- and COF-based high-energy materials sensors work based on phenomena such as RET, PET or dynamic quenching, among others. Some materials serve as sensors for high-energy nitromaterials in gen  
Figure 4: (**1**) The luminescence responses of large-porous Eu-MOF dispersed in DMF for nitroo compounds. Reprinted with permission of the Royal Society of Chemistry from [79]. Copyright 2013, permission conveyed through Copyright Clearance Center, Inc.; (**2**) Luminescence quenching efficiencies of [Dy(\(\mu_{2}\)-FcDCA)1.5(MeOH)-(H\({}_{2}\)O)]-0.5H\({}_{2}\)O]n. Reprinted with permission from [71]. Copyright 2019, American Chemical Society; (**3**) Luminescence responses of Eu-MOF to various nitroexplosives (5 mM) in DMF. Reprinted with permission from [69]. Copyright 2020, American Chemical Society.  
eral [81] while others are tailored for specific high-energy compounds [82; 83]. Nitrophenols contain a polar hydroxyl group capable of forming hydrogen bonds, making COP and COF-based sensors more selective for nitrophenols than for other compounds, for example, nitrotoluenes in general. The selectivity of these sensors was explained by the formation of hydrogen bonds between nitrophenols and O- and N-rich COP and COF and the overlap of absorption spectra between them.  
The reason for this phenomenon may be because nitrophenols have an acidic hydroxyl group and will be more susceptible to interactions with basic groups, as in the case of curcumin derivatives [84]. No specific mechanism of interactions has, however, been proposed. Due to the non-selective and non-specific nature of these interactions, likely, many other interferents bearing acidic hydroxyl groups will also be detected by this method.  
Hydrogen bonding is also responsible for the dominance of the ACQ mechanism in the fluorescence response.  
During a detailed study of the mechanism of detection of high-energy materials by COPs, two main factors affecting selectivity for PA were demonstrated using COP-301 as an example. The first was the relative positions of HOMO and LUMO and the electron-withdrawing effect of the functional groups of the NO\({}_{2}\)[82]. COP and nitro-aromatic high-energy materials interact via \(\Pi-\Pi\) interactions. Upon excitation, electrons in the COP conduction band are transferred to the LUMO PA, resulting in quenching. Due to the electron-withdrawing effect of the nitro groups, PA has the lowest LUMO, so its quenching ability is the highest. The main difference observed between COF and COP sensors is in the magnitude of the extinction constants. COFs can exhibit K\({}_{\text{SV}}\) values in the order of \(\times 10^{6}\) to \(\times 10^{7}\), while most COPs reach maximum values in the order of \(\times 10^{5}\) (Table 4). This suggests that the long-range order in COFs contributes to higher extinction degrees for a given analyte concentration and initial fluorescence signal.