# Recent Advances in Intrinsically Fluorescent
## 3 Application
### Bioimaging  
With the rapid development and utilization of optical instruments and equipment, fluorescence imaging is an important auxiliary modality for biological and medical investigations [78]. Therefore, many kinds of fluorescent probes have been proposed for the visualization of the structure and function in cells [79; 80; 81; 82]. However, most of the commonly used fluorescent probes have disadvantages, such as high cytotoxicity to live cells, poor water dispersibility, and background fluorescence [83; 84; 85]. PDAs have been proposed as a good material for bioimaging not only because of their excellent water dispersibility and remarkable biocompatibility but also because of their abundant surface groups that allow for connecting other functional groups (e.g., drugs and peptides). Despite many advantages, unfortunately, the low quantum yield and poor photostability of polydopamine limit its use for applications in fluorescence imaging. [PERSON] et al. synthesized starch-based PDA-PEI copolymer dots with strong and stable green-yellow fluorescence using the green and mild methods [86]. Bright green-yellow fluorescence was observed when the cell and copolymer dots culture medium was irradiated by a 405-nm laser, and the fluorescence signals showed that a starch-based PDA-PEI copolymer could be internalized by cellsthrough non-specific cell endocytosis. It is worth noting that with the high incubation concentration of the starch-based PDA-PEI copolymer for 1 day, the cell viability value was still as high as 99.69%, which is an important factor in deciding their suitability for bioimaging. In addition, the researchers also found that the fluorescence intensity was related to the physical and chemical properties of the cell. [PERSON] et al. synthesized a label-free ratiometric pH sensor in an alkaline alcohol solution under the ultrasonic radiation method [87]. The as-prepared fluorescent polydopamine showed different intensities of green fluorescence under different pH levels, which could be applied to bioimaging for detecting intracellular pH in live cells in real time (Figure 4). Further research shows the remarkable specificity of FPDA to the pH level. The fluorescence intensity of FPDA is hardly affected by other types of metal ions and biological macromolecules. The change in fluorescence intensity with the pH can be attributed to \(\pi\)-\(\pi\) interactions and decreased electrostatic repulsive interactions between the FPDA in an alkaline environment. Using low-concentration NaOH can easily form 20-30-nm size nanoparticles. As the concentration of NaOH increases, the particle size of the product gradually increases until the concentration of NaOH reaches 2.25 nM, which can be attributed to originating from the congregation of small particles under high-pH conditions. Similar situations have been reported in other studies of PDA [88]. The pH has a great influence on the polymerization mechanism, leading to changes in particle size. In addition to the changes in the fluorescence intensity of FPDA caused by the pH level, other intracellular substances may also affect the fluorescence intensity. [PERSON] et al. dropped NH\({}_{4}\)OH into a mixture of alcohol and water to obtain PDA nanodots which could be further oxidized by hydrogen peroxide to fluorescent polydopamine [89]. Based on the fact that H\({}_{2}\)O\({}_{2}\) is the main component of ROS in cells when PDA nanodots get into the cells, there should be a different fluorescence response due to the different concentrations of ROS in tumor cells (high) and normal cells (low), making the PDA effectively distinguish cancer cells from normal cells. When incubating PDA nanodots with normal cells and Hela cells for 4 h, the normal cells displayed weak fluorescence, while the Hela cells had strong fluorescence, which shows it is feasible to distinguish normal cells from cancer cells by the difference in the concentration of ROS in the cell.