Rhodamine-dipeptide derived nanomaterials as a cellular probe for self-reporting assembly state

Cancer, a global healthcare challenge, demands innovative approaches in drug delivery and intracellular imaging. This research focuses on the development and validation of two pH-responsive self-assembled probes, Rho-KK and Rho-KE, derived from Rhodamine B and incorporating a β-sheet-forming dipeptide motif. The primary objective is to assess the probes' capability to penetrate cancer cells (HT-29, human colorectal adenocarcinoma cells) in a self-assembled nanotubular structure, and then to explore its ability to self-report its assembly state (nanotube vs monomer). Rho-KK/Rho-KE can exist either in a monomeric, open-ring state (λmax 580 nm) at low pH (4.1/4.2) or in the assembled nanotubular state which exhibits aggregation-induced emission (λmax 460 nm) at pH> (5.8/6.3). 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 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). Structural validation involved a comprehensive analysis through UV-Vis, circular dichroism (CD), and fluorescence spectroscopy, complemented by imaging nanotubes via Atomic Force Microscopy (AFM) and Transmission Electron Microscopy (TEM). Confocal microscopy is utilized to visualize the probes within the cell, allowing for a correlation between changes in emitted wavelength and specific cellular compartments or organelles. The future direction is to create a theranostic agent by linking these nanoprobes with a chemotherapeutic, paving the way for targeted drug delivery and imaging.