Design of an Acidic pH‐Activated NIR Fluorescent Convertible Rhodamine−Hemicyanine Probe‐Peptide Conjugate for Living Cancer Cell Active Targeted Selective Tracking of Lysosomes

Abstract We have synthesized an acidic pH‐activatable dual targeting ratiometric fluorescent probe‐peptide conjugate using the SPPS protocol on Rink amide AM resin. Living carcinoma cell specific active targeting, successive cell penetration, and selective staining of lysosomes are accomplished. Real‐time monitoring of lysosomes, 3D, and multicolor cancer cell imaging are also attained. The de novo design consists of the integration of multifunctionality into a single molecular scaffold, e. g., RGDS peptide residue to target cancer cell surface overexpressed receptor α V β 3 integrin, live‐cell penetrating organic unsymmetrical rhodamine‐hemicyanine chromophore comprising a lysosome targeting morpholine group, and an acidic pH openable spiro‐lactam ring for a visible‐to‐NIR switchable ratiometric response. Water‐soluble fluorescent probe‐peptide conjugate exhibits intramolecular spirolactamization at basic pH through Arg amide N. The visible spirolactam state predominantly exists at physiological and basic pH and can be switched to the highly conjugated NIR open amide state ( λ em =735 nm) through spiro‐lactam ring opening triggered by acidic pH with a huge bathochromic shift (Δ λ abs =336 nm, Δ λ FL =265 nm). Moreover, pH‐sensitive ratiometric optical switching is achieved. This in situ acidic cancer cell lysosome activatable multifunctional fluorophore‐peptide conjugate shows augmented molar absorptivity, enhanced quantum yield, and improved fluorescence lifetime at acidic lysosomal pH; negligible cytotoxicity; and dual targeted ratiometric imaging capability of living cancer cell selective lysosomes with a p K a value of 5.1.