Cancer nanotherapeutics have offered several advantages over traditional medicines in terms of better cellular uptake, selectivity, enhanced bioavailability, and higher therapeutic indices. One persisting challenge is to figure out the sub-cellular organelle at which the disassembly of nanomaterials occurs. Our lab has recently developed two Rhodamine based pH-responsive self-assembled probes, Rho-KK and Rho-KE (Rho=Rhodamine, K=lysine, E=glutamic acid). The objective is to validate if the developed probes Rho-KK/Rho-KE can enter the cells (HT-29, human colorectal adenocarcinoma cells) as self-assembled nanotubes. Rhodamine amide derivatives are known to exhibit acid or metal ion induced ring opening of its closed ring, non-fluorescent spirolactam form to open-ring, fluorescent amide form at low pH. Attachment of a β-sheet forming dipeptide motif enables the system to assemble into nanotubes. Rho-KK/Rho-KE can exist either in monomeric, open-ring state (l<sub>max</sub> 580 nm) at low pH (4.1/4.2) or in the assembled nanotubular state which exhibits aggregation induced fluorescence (l<sub>max</sub> 460 nm) at pH&gt; (5.8/6.3). The structural changes as a function of pH have been validated using UV-Vis, CD, and fluorescence spectroscopy. Between pH (4.1-5.8/4.2-6.2) respectively for Rho-KK/Rho-KE, the transition from fluorescent open-ring Rhodamine form to monomeric, non-fluorescent spirolactam form occurs. The hypothesis is that these probes upon entering the cancer cells would emit fluorescence corresponding to their location within the cells as intracellular organelles are known to have different pH values, from the early (pH 6.3) to the late endosome (pH 5.5) and ultimately the lysosome (pH 4.7). Relationship between pH and nanotube/monomer state of probes can be verified by recording the corresponding emitted wavelength and simultaneously visualizing their location within the cell using confocal microscopy.

Exploring the potential of rhodamine-dipeptide nanomaterials for tracking the assembly state within the cell

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