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Nanomaterials that respond to intracellular signals, such as pH, have potential for many biomedical applications, such as drug delivery, because the assembly/disassembly process can be tailored to respond to a stimulus characteristic of a specific subcellular location. In this work, two rhodamine-peptides that form stable nanotubes at physiological pH but dissociate into highly fluorescent monomers within the acidified interior of endosomal/lysosomal cellular compartments has been developed. The rhodamine dipeptide conjugates, NH2-KK(RhB)-NH2 (RhB-KK) and NH2-EK(RhB)-NH2 (RhB-KE) with rhodamine B chromophores appended at the ε-amino position of a lysine residue, were shown to assemble into well-defined nanotubes at pH values above ~4-5 and to dissociate into a fluorescent monomer state at lower pH values. The pH-dependence of the assembly process was investigated using CD and fluorescence spectroscopy along with TEM, AFM and confocal imaging. Although the ring opening/closing transition of the rhodamine chromophore took place at pH 4.1 for both peptides, the onset of assembly began at pH 4.6 for RhB-KE and at a comparatively more basic pH (5.8) for RhB-KK. Accordingly, the rhodamine-peptides interconverted between three, pH-dependent states: an open-ring, monomeric state (max 580 nm, 𝜆ex 550 nm) at pH values at or below ~4.6; a closed-ring, nanotube form that exhibits AIEE (max 460 nm, 𝜆ex = 330 nm) at higher pH values and a closed-ring, non-emissive monomeric state that emerged below the CMC. The pH-responsive features of the peptides were evaluated by live-cell imaging in three cancer cell lines using confocal laser scanning microscopy (CLSM). Visualizing the cells after incubation with either RhB-KE or RhB-KK produced CLSM images with a punctate appearance in the Texas Red channel that colocalized with the lysosomes. These experiments indicating that the nanotubes were rapidly trafficked into the acidic lysosomal compartments within the cells, which induced dissociation into a monomeric, open state. Uptake inhibition studies suggested that cellular uptake was mediated by either or both caveolae- and clathrin-mediated endocytosis, depending on the cell line studied. 

In this work, a polydopamine-coated, camptothecin nanotube functionalized with a targeting anti-TAG-72 scFv exhibited enhanced cytotoxicity against TAG-72 positive, LS-174T colon cancer cells, compared with HT-29 cells negative for the antigen.