br As the key factor of the assembly
As the key factor of the assembly between CPF and FA is the elec-trostatic interaction between quaternary ammonium cation and nega-tive folate anions, the interaction would be reliant on the pH of the system. The eﬀect of pH was elucidated by the pH dependent photo-luminescence study by taking initial CPF-FA complex at pH 9. The Fig. 4a reveals the quenched fluorescence of CPF in CPF-FA complex is gradually recovered with the lowering of pH. At very high acidic pH (at pH 2) it restores back its 50% photoluminescence intensity. At acidic pH, the -COO- of FA can procedures back -COOH and the interaction with CPF is getting lower. The reversibility study of this pH dependent luminescence “turn oﬀ/turn on” in respective pHs is also carried out by alternate pH changing in alkaline pH (pH 9) and acidic pH (pH 2) of the CPF-FA solution system and the respective luminescence changes are shown in Fig. 4b. We have performed up to the 5th MIK665 (S-64315) and it exhibits
Fig. 4. (a) The pH dependent photoluminescence spectra of CPF-FA assembly (with 200 μM of FA). (b) Reversibility study of CPF luminescence in CPF-FA assembly upon successive changing of pH between pH 2 and pH 9.
very promising repeatability of the luminescence and the trend of re-versibility clearly provides the idea that beyond fifth cycle the system should be continuing its reversibility.
The pH dependent assembly and disassembly between CPF and FA will also aﬀect the corresponding morphology of the polymer CPF. To get better insight about the morphological change during the as-sembled/disassembled process, we studied the FESEM image study of CPF and CPF-FA assembly in pH 2 and pH 9. The FESEM image of CPF in Fig. 5a reveals the self-assembled nanosphere morphology for the polymer in water/THF solvent mixture (9:1). The self-assembly of the amphiphilic polymeric system having hydrophilic ionic alkyl group and hydrophobic polyfluorene main chain eﬀectively provides the nano-sphere of ˜100−400 nm diameter. The TEM image in Fig. 5b divulges that the aggregated nanospheres are solid. In a self-assembled polymer structure, the aromatic fluorene moieties in main chain provides a higher degree of π-π stacking which results in the solid nanosphere formation [44,45,52]. Again after complex formation with FA at pH 9, nanosphere morphology of the CPF is completely destroyed and an
interconnected assembled nanostructured network is prominent in Fig. 5c. However, when the pH decreases to 2, it reinstates the nano-sphere morphology of CPF shown in Fig. 5d. The complexation of CPF and FA at pH 9 generates the interconnected network structure which is decomplexed at acidic pH 2 to produce the CPF back in its original nanosphere shape . So, the morphological study directly proves the reversible nature of the complexation/decomplexation between CPF-FA depending on the pH values.
The photophysical and morphological experiments discussed above envisioned the pH dependent electrostatic interaction between the CPF and FA. Taking the electrostatic interaction based luminescence “turn oﬀ” into account, we can deploy the CPF polymer nanosphere for cancer cell imaging by interacting with the FA in a folate receptor overexpressed cancer cell pre-treated with the FA. However, before going to imaging experiment, we have studied again the photo-luminescence quenching of CPF with FA in PBS buﬀer of pH 7.4 which is biological pH condition. The Fig. 6 depicts the luminescence of CPF is quenched by ˜91% in presence of FA at pH 7.4. So, the fluorescence
Fig. 6. Fluorescence quenching of CPF with the interaction of FA at pH 7.4 in PBS buﬀer condition.
probe CPF would be highly applicable for cancer cell imaging in bio-logical condition using fluorescence “turn oﬀ” mechanism.