# Methods for Detecting Picric Acid--A Review of Recent Progress
## 3 Sensors for Detecting and Measuring PA
### Polymers and Organic Molecules  
Detecting picric acid (PA) using organic compounds and polymers relies almost entirely on the use of those materials as fluorescent probes (Table 6) because both PA and other nitroaromatic compounds are strong fluorescence quenchers, but otherwise do not have a large impact on the readily-measurable properties of those materials. The recently reported fluorescence probes viable for this purpose are mostly \(\pi\)-conjugated polymers, including donor-acceptor copolymers [131], more classical polyfluorene derivatives [132] and rylene-based systems [133, 134]. Non-conjugated polymers [135] and non-polymer rylene derivatives [136] have also been reported as promising materials for detecting PA. Most of the fluorescent probes offer similarly high detection parameters, with limits of detection (LODs) in the submicromolar range, regardless of the specific classification of the employed material. It has also been shown that as the concentration of the analyte tested increases, the fluorescence signal read by the probes increases (Figure 5). The sole outlier with a subnanomolar LOD was a \(\pi\)-conjugated polymer [132]. The main drawback of these methods is that while they offer some measure of selectivity towards PA, their responses to other nitroaromatic compounds (NACs) are significant and few works investigate the effect of sample contamination with the heavy metals typically present alongside PA, particularly in the case of unexploded m  
Figure 5: Fluorescence spectra of FP1 in the presence of increasing PA concentration. Reprinted with permission of the Royal Society of Chemistry from [131]. Copyright 2018, permission conveyed through Copyright Clearance Center, Inc. \begin{table}
\begin{tabular}{l l l l l l} \hline \hline \multirow{2}{*}{**Main Substrates**} & \multirow{2}{*}{**Solvent**} & \multicolumn{2}{c}{**Quantum Yield of**} & \multicolumn{1}{l}{**Linear Range**} & \multicolumn{1}{l}{**Detection**} & \multirow{2}{*}{**Ref.**} \\  & & **C-Dots [\%]** & & **(\mu M)** & & **Limit (nM)** & \\ \hline Grapes, rhodamine 6G & H\({}_{2}\)O & 18.67 & 0.06–79.4 & 10 & [124] \\ citric acid anhydrous, ethylenediamine & H\({}_{2}\)O & 29.01 & 1–10 & 10 & [125] \\ malonic acid, urea & H\({}_{2}\)O & 12.6 & 0.1–26.5 & 51 & [126] \\ L-Lysine, thiourea & H\({}_{2}\)O & 53.19 & 1–10 & 240 & [127] \\ Betel leave & H\({}_{2}\)O & 4.21 & 0.3–3.3 & 110 & [128] \\ Gelatine, aniline & H\({}_{2}\)O & 17 & 0.37–1.42 & 56 & [129] \\ Sucrose, phosphoric acid & not applicable & 21.8 & 0.2–17.0 & 16.9 & [130] \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Summary of materials used for C-dot fabrication and their picric acid detection performance. while methods based on fluorescence quenching are an excellent tool for the detection of NACs in general, particularly as part of a suite of portable/on-site analyses, speciation analysis of the NACs needs to be carried out using different methodology. An altogether different approach to detecting PA is to use a Schiff base that can form an adduct with PA that exhibits different absorption in the UV-Vis spectral range [137]. Not only does this approach rely on a simplistic and readily portable measurement method (UV-Vis spectroscopy), but offers high sensitivity (LOD of 6.92 \(\times\) 10\({}^{-7}\) M), comparable with fluorescence quenching methods and may far exceed them in terms of selectivity towards PA. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Fluorescent Probe** & **LOD 1** & **Selectivity 2** & **Ref.** \\ \hline FP1 & 5.1 \(\times\) 10\({}^{-7}\) M & Limited for NACs 3; & [131] \\ FP2 & 3.09 \(\times\) 10\({}^{-11}\) M & Average & [132] \\ FP3a & 1.81 \(\times\) 10\({}^{-7}\) M & n/a & [133] \\ FP3b & 1.4 \(\times\) 10\({}^{-6}\) M & n/a & [133] \\ FP4 & n/a & Limited & [134] \\ FP5 & 44.0 ppb & Limited / Average & [135] \\ FP6a & n/a & Limited & [136] \\ FP6b & n/a & Limited & [136] \\ \hline \hline \end{tabular} 1  
\end{table} 
Table 6: Summary of polymer and organic fluorescent probes used for the detection of picric acid.