# Recent Advances in Intrinsically Fluorescent
## 1 Introduction  
Compared with conventional fluorescent materials, fluorescent nanoparticles are receiving increasing interest due to their strong fluorescence, high photostability, water dispersibility, and remarkable biocompatibility, making them widely used in chemical sensing, optical materials, and biological detection [1; 2; 3]. Fluorescent nanoparticles can be simply defined as fluorescent inorganic nanoparticles (FINs) and fluorescent organic nanoparticles (FONs). Metal nanoparticles, semiconductor nanoparticles, and noble-metal nanoclaters are classic FINs. They generally present a strong fluorescent intensity and high quantum yield but low biocompatibility, which are not suitable for biomedical applications. For example, CdSe quantum dots, which represent semiconductor nanocrystals, have cytotoxicity without surface modification [4], and gold nanoclaters, as one of the typical noble-metal nanoclaters, are also characterized by bad stability and dispersion [5]. In addition, the preparation of FONs also currently has some limitations, including high-temperature conditions and long operation times. Dopamine (DA), a catecholamine neurotransmitter, plays an important role in regulating many biological processes in the central nervous neurotransmitter [6]. It also can be used as an adhesive material, which can be self-assembled under alkaline and aerobic conditions to generate polydopamine (PDA) [7]. Due to its remarkable biocompatibility and abundant functional groups on the surface, PDA has been widely used in drug delivery, biosensing, and other fields [8; 9; 10; 11]. For instance, [PERSON] et al. designed a drug delivery system based on PDA NPs as the photothermal agent and drug delivery carrier for tumor therapy [12]. Furthermore, the endothelial/lysosomal membrane is destroyed by the photothermal effect, thereby overcoming the endothelial/lysosomal barrier and improving the therapeutic effect. However, due to the complex auto-oxidation process and the various oxidation products, the chemical structure and formation mechanism of polydopamine are still controversial, and there are few reports on the development and utilization of polydopamine intrinsic fluorescence. Therefore, researchers are more focused on experimental experience to improve the quantum yield and fluorescence intensity, sincethe structure-property relationship lacks a solid theoretical foundation. What is more, the PDA structure is similar to a natural Eumelanin polymer, which can be analyzed and studied from a melanin analogy to PDA [13]. As is known to all, melanin is a chemical substance with high photothermal conversion efficiency, where more than 99.9% of the ultraviolet and visible radiation is dissipated by melanin in a non-radiative manner [14]. Therefore, melanin has a low quantum yield, and it is generally considered non-fluorescent even in the early stages of research. With the enrichment and improvement of research methods and measures, melanin is proven to exhibit fluorescence under UV excitation or oxidation conditions [15]. In addition, the quantum yield is a function of the excitation wavelength, and the optimal emission wavelength is at 400-550 nm, which depends on the excitation wavelength [16]. PDA has similar physical and chemical properties to melanin [17]. Most of the absorbed energy is converted into heat in a non-radiative way, which can be attributed to the \(\pi\)-\(\pi\) stacking interaction between the oligomeric units in PDA [18,19]. \(\pi\)-\(\pi\) stacking is a weak interaction (also called a non-covalent interaction) that exists in or between the molecules of some compounds with conjugated structures and can induce fluorescence quenching. In the melanin field, it has been reported to prepare fluorescent polymers by separating the p-conjugated skeleton and inhibiting the interaction between polymers [20]. There are also experiments using hydrogen peroxide to oxidize and cleave melanin polymers to enhance fluorescence [21]. In 2012, [PERSON] et al.