pH-Responsive nanoprobes for tumour fluorescence imaging based on spirolactam rhodamine

Fluorescence imaging has gained wide attention due to its high sensitivity and resolution and can be used to direct tumour resection. The contrast in image formation is an important factor in tumour fluorescence imaging. Therefore, designing a responsive fluorescence imaging material is crucial for guiding the surgical resection of tumours. Spirolactam rhodamine has shown great potential in cell imaging and ion detection due to its excellent acid-responsive fluorescence. Nevertheless, poor water solubility seriously confines its further application in living organisms. In this study, to improve both solubilization and tumour targeting, hyaluronic acid was modified with spirolactam rhodamine, self-assembling into nanoparticles (HA-RdB-EDS NPs) with a narrow diameter size distribution of 200 nm through hydrophilic and hydrophobic interactions. The results showed that HA-RdB-EDS NPs could be turned on in response to the low pH of the tumour microenvironment. The pK(a) value of the NPs was calculated to be about 5.5, suitable for the detection of tumours. Besides, the HA-RdB-EDS NPs have fluorescence turn-on/off reversibility between pH = 5.5 and 7.4. According to the results of cell fluorescence stability experiments, about 50% of the fluorescence intensity of HA-RdB-EDS NPs was maintained after 5 minutes of persistent excitation. Endocytosis experiments demonstrated that HepG2 cells showed greater selection for HA-RDB-EDS NPs than 3T3 cells, about 2.7 times. After the intravenous injection of NPs into tumour-bearing mice, fluorescence could be turned on at the tumour. All these results indicate that HA-RdB-EDS NPs have the potential for clinical use.