# Recent Advances in Intrinsically Fluorescent
## 1 Introduction
first reported using hydrogen peroxide as an oxidant to prepare fluorescent polydopamine nanoparticles under normal temperature aerobic conditions, and the quantum yield reached 1.2% [22], opening a new chapter in the study of PDA intrinsic fluorescence and making the solution oxidation method of preparing fluorescent polydopamine attract great attention from researchers. Based on this pioneering work, researchers have successfully prepared fluorescent polydopamine using potassium permanganate and manganese dioxide as oxidants [23,24]. However, many mechanism characteristics of the fluorescence enhancement of polydopamine are still unknown. Studies have shown that PDA is composed of three parts: uncycled (catecholamine) and cyclized (indole) units, as well as novel pyrrole carboxylic acid moieties [13]. In these structures, the two cyclized fluorescent units of DHI and its reduced form dimers show remarkable emission characteristics [25]. Another study shows the polydopamine supramolecular aggregate of monomers (consisting primarily of 5.6-dihydroxycholine and its dione derivative dopaminergic), which are held together through a combination of charge transfer, \(\pi\)-stacking, and hydrogen-bonding interactions [19]. Therefore, it is now generally believed that reducing the accumulation of \(\pi\)-\(\pi\) stacking will increase the quantum yield and enhance the intrinsic fluorescence of PDA. It was reported that hydroxyl groups can reduce the \(\pi\)-\(\pi\) stacking interaction between oligomeric units in polydopamine nanoparticles. The hydroxyl radicals generated under the action of high concentrations of hydrogen peroxide and sodium hydroxide are added to the double bonds activated by the pyrrole ring and the aromatic ring [26, 27, 28], which laid a theoretical foundation for the research of solution oxidation. In addition to the method of oxidation by oxidizing agents, UV irradiation photooxidation is also a feasible way to prepare fluorescent polydopamine [29]. It was speculated that the uncyclized catecholamine units in PDA are oxidized by ultraviolet light, resulting in the formation of cyclized fluorescent units, which also confirms that the cyclic side chain can effectively reduce the \(\pi\)-\(\pi\) accumulation effect [20]. The researchers not only enhanced the quantum yield of the fluorescent copolymer through oxidation but also weakened the \(\pi\)-\(\pi\) stacking by doping with N atoms [30,31]. After branched-chain polyethyleneenimine (BPEI) achieved good results in enhancing fluorescence [32,33], it has also been used to enhance PDA fluorescence. The abundant amino groups of BPEI can form hydrogen bonds with groups containing O and N, destroying the intermolecular and intramolecular cross-linking of polydopamine. On the other hand, BPEI being alkaline in an aqueous solution is also beneficial to the formation of polydopamine [34]. Moreover, compared with the fluorescent polydopamine prepared by the solution oxidation method, the quantum yield of the PDA-PEI fluorescent polymer is higher (12.5%) [35; 36], and the particle size is smaller. The basic idea of solution oxidation and doping with N atoms is to weaken the \(\pi\)-\(\pi\) stacking interaction of PDA. The former has more abundant experiments and applications, but it also exists the problem of a low fluorescence quantum yield (about 0.58-1.2%) [37; 38]. The latter's current experimental method is single, and the research content needs to be further expanded. There are still many gaps in the mechanism and methods of FPDA enhancing fluorescence, and more solid theoretical research on the structure-property relationship is needed. This review aims to present a comprehensive overview of the relevant advances in the intrinsic fluorescence of PDA from 2012 until now; the research of other coupled fluorophores will not be discussed (Figure 1). At the beginning, we will introduce the fabrication methods of FPDA and discuss the mechanism of synthesis and performance improvements based on mainstream views.