# Recent Advances in Intrinsically Fluorescent
## 2 Preparation Principle
### Redshift Phenomenon  
Oxidation of dopamine by strong oxidants in alkaline aqueous solutions is a common method for FPDA formation. However, uncontrolled autoxidation of dopamine in alkaline aqueous solutions causes the emission wavelength of FPDA to be dependent on the excitation wavelength [29; 49; 50]. Usually, the optimal emission wavelength increases with the excitation wavelength, which may cause unnecessary interference in multiple biological applications and greatly limit their application as fluorescent probes for biosensing and bioimaging. Especially for imaging, this phenomenon may cause background fluorescence or unclear images. Efforts have been taken to avoid this phenomenon and to inhibit uncontrollable self-polymerization of dopamine (and other catecholamines) in a solution to obtain photostability nanoparticles. One way which is easy for inhibiting the autoxidation of dopamine was to prepare the FPDA in acidic aqueous solutions. [PERSON] et al. employed hydrogen peroxide and metal ions to prepare fluorescent polydopamine under acidic conditions [39]. Inspired by the Fenton reaction, three metal ions were selected to react with hydrogen peroxide to generate hydroxyl radicals and crack the PDA unit [49]. When the excitation wavelength increases from 350 nm to 452 nm, the best emission light of Cu\({}^{2+}\)-polydopamine does not change and stabilizes at 484 nm. Transition metals can form stable complexes with dopamine through the one-electron oxidation bond in the catechol group [50], which may induce electron transfer to oxygen for the corresponding semiquinones. These corresponding semiquinone substances may continue to be oxidized to undergo disproportionation, coupling, and cyclization [51]. However, this process will be significantly inhibited under acidic conditions and avoid the production of various oxidation products. [PERSON] et al. chose ammonium persulfate ((NH\({}_{4}\))\({}_{2}\)S\({}_{2}\)O\({}_{8}\) or APS) as an oxidant to oxidize and polymerize. Dopamine polymerization happened in a citric acid aqueous solution (pH = 1.8) at 180 \({}^{\circ}\)C for 24 h in a 20-mL PPL-lined autoclave (Figure 3) [52]. The FPDA aqueous solution showed bright blue-green fluorescence under UV light, and the emission peak was stable when the excitation wavelength was increased from 310 to 360 nm, which indicated that the FPDA had unified chromophores. The small full width at half maximum (FWHM) also indicated unified chromophores in the FPADs. However, it is not always possible to obtain FPDA with good photostability by oxidizing DA under acidic conditions. [PERSON] et al. chose acidic (pH = 5.5) anaerobic conditions as the reaction conditions, and dopamine polymerization happened in plasma-activated water (PAW) [53]. PAW is rich in reactive oxygen species (ROS) (i.e., hydronium and hydroxyl ions), which has been reported to accelerate dopamine polymerization under acidic, basic, and neutral conditions [47; 54]. Under different optical properties from other reactions under acidic conditions, the optimal emission changed, with gradually increased excitation wavelengths from 360 to 480 nm. It is speculated that polydopamine formation proceeds via several concurrent pathways, favored by the exceedingly slow oxidation kinetics due to the oxidation ability of PAW being too weak [51]. Another interesting method to control FPDA formation and improve photostability is the use of other groups to occupy amino groups in dopamine that can prevent the generation of branched side chains. [PERSON] et al. performed dopamine polymerization in glutaraldehyde and glycine based on the well-known Schiff base reaction. The polymerization reaction was initiated by adding an ice-cold NaBH\({}_{4}\) solution and then terminated with excessive mercaptoethanol after 30 min [55]. When the obtained FPDA was excited at different wavelengths in the range of 360 to 440 nm, the purified oligomer had only one emission peak near 485 nm.