# Recent Advances in Intrinsically Fluorescent
## 3 Application
### Detection of Biomolecules  
Owing to the excellent optical properties of PDA nanoparticles, they are becoming a kind of popular nanomaterial and have attracted a lot of attention from detecting biomolecules [103; 104; 105]. PDA nanoparticles as quenchers have been used to construct biosensors for detecting various biomolecules due to their broadband absorption of 300-800 nm [106; 107; 108; 109]. However, the biosensing application based on the intrinsic fluorescence of PDA nanoparticles is still in its infancy [110]. [PERSON] et al. reported that DA was oxidized to fluorescence polydopamine under alkaline conditions, and it could be used as a signal molecule to turn on fluorescence. The fluorescence biosensor was used for rapid and sensitive detection of DA with a detection limit of 40 nM [111]. DA sensing experiments showed that sensors still display a high selection to DA under the interference of many common human chemicals, even ascorbic acid and uric acid, which could greatly affect the selectivity of electrochemical methods. Turn-on fluorescence sensing shows huge potential for DA detection due to its simple operation and low cost compared with other detection methods (e.g., electrochemical methods and chromatographic methods). Thereafter, similar turn-on type fluorescent biosensors of DA have also been reported and simplified the experiment process [112]. In addition to the reaction's raw material as the signal molecule, other molecules also can turn on fluorescence. [PERSON] et al. designed a sensitive fluorescence sensor composed of fluorescent polydopamine and cobalt oxyhydroxoxide (CoOOH) with the determination of a-glucosidase activity [113]. The two materials are connected due to the electrostatic effect and meet the requirements of fluorescence resonance energy transfer (FRET), resulting in fluorescence quenching. The ascorbic acid produced by the hydrolysis of 2-O-\(\alpha\)-D glucopyranosyl-L-ascorbic acid by glucosidase can reduce CoOOH to Co\({}^{2+}\). Without the quenching effect of CoOOH, FPDA shows fluorescence intensity that varies with \(\alpha\)-glucosidase activity. It is worth noting that the selectivity and sensitivity of fluorescence biosensors were tested in human serum to investigate its practical application. The detection results showed high selectivity and a low detection limit (1.65 U L\({}^{-1}\)) for the sensor, which has the potential to be applied in biomolecular detection (Figure 7). The above discussions are turn-on fluorescence biosensors, which turn on fluorescence through the presence of signal molecules and perform quantitative detection through fluorescence  
Figure 6: (**A**) Schematic of detection of _P. aeruginosa_. (**B**) FL with different concentrations of _P. aeruginosa_. (**C**) Change in FL with some bacteria [36]. Reprinted/adapted with permission from Ref. [36]. 2020 Elsevier. recovery. Because of the good selectivity and sensitivity of this method, it has been used to detect a variety of biomolecules [114; 115; 116]. Another regular method for designing fluorescence biosensors is the turn-off mode detection method; that is, by inhibiting the synthesis of the fluorescent probe or quenching the fluorescent probe to realize the fluorescence response, the qualitative and quantitative detection of the substance to be tested is carried out. [PERSON] et al. employed CoOOH as an oxidant to synthesize fluorescence polydopamine by oxidizing dopamine [117]. Ascorbic acid (AA) can reduce CoOOH to Co\({}^{2+}\), which renders the DA unable to be oxidized to FPDA due to the absence of the oxidant CoOOH, resulting in a decrease in fluorescence intensity. Therefore, the fluorescence intensity of the PDA could be used to define the AA concentration. The sensing system shows a good linear relationship between fluorescence intensity and AA concentration in the detection range, with a detection limit of 4.8 mM.