# Recent Advances in Intrinsically Fluorescent
## 2 Preparation Principle
### Size Distribution  
Thanks to its remarkable physical and chemical properties, abundant surface groups, and good in vivo degradations [56; 57], PDA stands out for the development of drug delivery and bioimaging. As a nanomaterial, the particle size of PDA is an important factor affecting its performance. It was found that the particle size of PDA not only affects its loading ability but also its cytotoxicity [58; 59; 60; 61]. The particle size of nanomaterials is related to the cell death pathways. Gold nanoparticles with a smaller particle size (<2 nm) can cross the nuclear membrane and enter the nucleus, resulting in significant cytotoxicity [62]. Furthermore, it was reported that the cancer endothelium has fenestrations between the cells of the vascular system. The size of these gaps ranges from 100 to 780 nm, depending on the tumor type [63; 64; 65]. Therefore, it is generally considered that the small size of PDA is in favor of impressing its rate of cell endocytosis and its accumulation in cancer cells. For these reasons, it is necessary to reduce the particle size of FPDA. The particle size of nanomaterials is related to many factors (preparation method, pH, temperature, etc.). Fluorescent polydopamine is mostly prepared under alkaline conditions at room temperature and pressure. Thus, it is necessary to consider reducing the particle size of PDA from the type and ratio of raw materials. [PERSON] et al. proposed the use of hydroxyl radical-induced degradation of polydopamine nanoparticles to obtain fluorescent polydopamine with a particle size of 22 \(\pm\) 3 nm [28]. This is attributed to the addition of hydroxyl groups to the 5,6-dihydroxyindole (DHI) unit of the polydopamine unit, effectively dividing the polydopamine nanoparticles into oligomeric units and able to reduce the \(\pi\)-\(\pi\) stacking interaction between the oligomeric units and inhibit the autonomy of DA and DHI. However, this method is a multi-step and time-consuming process, which limits its preparation and application. Fortunately, there are many ways to generate hydroxyl radicals, including  
Figure 3: (**A**) Schematic of the preparation of PDAs. (**B**) PL spectra [52]. Reprinted/adapted with permission from Ref. [52]. 2017 Wiley-VCH. the commonly used tendon method, electrolytic oxidation method, high-power sonication (HPS), and photoelectric catalytic method [66; 67; 68]. [PERSON] et al. used a micro-plasma anode to treat the dopamine solution and oxidized polydopamine with reactive oxygen species (ROS) generated by the anode to obtain nanoparticles with a uniform particle size of 3.1 nm [37]. At the same time, acidic conditions also inhibited the further growth of FPDA. Hydroxyl radicals are used in these methods to oxidatively cleave the PDA group, which successfully reduces the particle size. It is speculated that the hydroxyl radicals generated by other methods will have a similar effect. In addition to using hydroxyl radicals to crack the PDA, the method of doping N atoms is also a good way to reduce the particle size. [PERSON] et al. mixed DA, ammonium sulfate, and citric acid using a hydrothermal method to react at 180 \({}^{\circ}\)C to obtain an average FPDA particle size of 3.02 nm [52]. However, it is important to note that the material particle size was not as small as possible, as different applications require different particle sizes. When used as a drug carrier, small nanoparticles have a strong tissue-penetrating ability but have a short circulation time in the blood. Although large-diameter nanoparticles have a longer circulation time in the body, the penetration ability in the tissue is slightly weaker. Designing a nanodrug system requires considering the influence of different aspects and organizing the advantages of each component. Therefore, when deciding to construct functional materials, it is necessary to study the optimized size range corresponding to each application.