# Recent Advances in Intrinsically Fluorescent
## 3 Application
### Detection of Metal Ions  
Fluorescent nanoparticles are receiving great attention nowadays due to their orthodihydroxy phenyl structure, which is beneficial for chelating metal ions, making them useful in metal ion detection in the environment and the human body [70; 71]. The metal ions chelate with PDA to make the fluorescent polydopamine produce a fluorescent response so that the qualitative and quantitative analysis of metal ions can be performed quickly and sensitively [39; 72]. Fluorescence quenching is one of the most common fluorescence responses. There are many reasons for this phenomenon, and the mechanism is also complicated. Studies have shown that the formation of metal ion amine complexes and the quenching activity of the complexes for the FPDA may be the main reason for the fluorescence quenching of FPDA [73]. For instance, [PERSON] et al. synthesized high-photostability PDA-PEI copolymer dots with high selectivity toward Cu\({}^{2+}\) using a facile one-pot method. The detection of copper ions with a detection limit of 1.6 nM and the FPDA in real samples was evaluated in the detection of copper ions. The fluorescence quenching of the FPDA could be caused by the complexation effect between Cu\({}^{2+}\) and the amine group, which led to electron transfer from the probe to Cu\({}^{2+}\)[35]. EDTA (a strong metal ion chelator) was added to the mixture containing PDA-PEI and Cu\({}^{2+}\). The fluorescence recovered since EDTA has a stronger chelation ability with Cu\({}^{2+}\) than the amino groups in PDA-PEI. The results confirmed the mechanism for the Cu\({}^{2+}\) fluorescence quenching the formation of cupric amine complexes and led to electron transfer from the probe to Cu\({}^{2+}\)[74]. [PERSON] et al. synthesized FPDA with selectivity toward Fe\({}^{3+}\) using the solution oxidation method. The detection of ferric ions was performed a detection limit of 4.6 nM and confirmed the previous speculation [75]. Due to the oxidative and coordinative properties of Fe\({}^{3+}\), the intermolecular hydrogen bonds of FPDA are destroyed, and the oxidation produces a planar iron (II) benzo semiquinone structure layered by \(\pi\)-\(\pi\) interactions that can quench fluorescence. In addition to the fluorescence responses of fluorescence quenching, there are also studies showing that metal ions could enhance the fluorescence intensity of FPDA. One study showed that zinc ions can enhance the intensity of fluorescent polydopamine to achieve the sensitive detection of zinc ions with a detection limit of 60 nM, and the measurement was carried out in a complex mixture [38]. It is important to note that the ortho-dihydroxy phenyl structure of PDA is easy to chelate with metal ions, but few metal ions can cause a large fluorescence response mainly due to the large association constants of only a few metal ions with PDA. In practical applications, the samples to be tested often contain multiple metal ions which need to be sensitively detected at the same time. Recently, [PERSON] et al. prepared three PEI-PDA sensors with three different concentrations of BPEI to form a new chemical sensor array that could distinguish different concentrations of metal ions in biofluids, even in the mixtures with different valence states [76]. The study was inspired not only to start from the fluorescence intensity response but also to develop and utilize the fluorescence lifetime, response mode, etc. and combine some data processing methods for computer analysis to achieve large-scale rapid and sensitive detection. From the above discussion, it can be concluded that FPDA obtained by doping or solution oxidation can produce a good fluorescence response with metal ions. Furthermore, fluorescent polydopamine shows great potential in detecting metal ions due to mild preparation conditions, meets the requirements of green chemistry, and is easy to carry out compared with other methods.