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In this study, two novel fluorescent probes, probe A and probe B were designed, synthesized and characterized, based on Microthrix parvicella (M. parvicella) preferring to utilize long-chain fatty acid (LCFA), for the labeling of M. parvicella in activated sludge. The molecular structure of probe A and probe B include long-chain alkane and LCFA, respectively. The results indicated that probe A and probe B had a large stokes shift of 118 nm and 120 nm and high quantum yield of 0.1043 and 0.1058, respectively, which were significantly helpful for the fluorescent labeling. As probe A was more stable than probe B in activated sludge, and the fluorescence intensity keep stable during 24 h, probe A was more suitable for labeling M. parvicella in situ. In addition, through the Image Pro Plus 6 (IPP 6) analysis, a quantitative relationship was established between sludge volume index (SVI) and integral optical density (IOD) of the labeled M. parvicella in activated sludge samples. The relationship between IOD and SVI conforms to Logistic curve (R2 = 0.94).

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Although the fluorescent in situ hybridization (FISH) has been widely used to identify the Microthrix parvicella (M. parvicella), there are a few disadvantages and difficulties, such as complicated process, time consuming, etc. In this work, a series of fluorescent probes, which were modified by long-chain alkane with hydrophobic property and based on the property of M. parvicella utilizing long-chain fatty acids (LCFA), for the labeling of M. parvicella in bulking sludge were designed, synthesized, and characterized. The probes were characterized by ultraviolet-visible (UV-Vis) absorption spectra, fluorescence spectra, (1)H NMR spectra, and mass spectra, and the photostability and hydrophobic property of probes were investigated. All the results showed that the probes were quite stable and suitable for the fluorescent labeling. The probes had a large stoke shift of 98-137 nm, which was benefit for the fluorescent labeling. In the fluorescent labeling of M. parvicella by the synthesized probes, the probes had excellent labeling effects. By comparison of the images and the Image Pro Plus 6.0 analysis, the optimal concentration of the probes in the activated sludge sample for labeling was 0.010 mmol/L and the probe 3d had the best labeling. In addition, the effect of the duration time of probes was also investigated, and the results showed that the fluorescent intensity of probes hardly changed in a long period of time and it was suitable for labeling.