Synthesis of pH-Sensitive Fluorescein Grafted Cellulose Nanocrystals with an Amino Acid Spacer

Fluorescent labeled nanoparticles are expected to have potential applications in biological and imaging systems. In this study, in order to construct the fluorescent nanoparticles for pH-sensing, L-leucine amino acid is used as a spacer linker between cellulose nanocrystals (CNCs) and a pH-indicator dye (5 (and 6)-carboxy-2',7'-dichloro-fluorescein, CDCF). Characterization by Fourier transform infrared (FTIR), nuclear magnetic resonance (NMR), and X-ray photoelectron spectroscopy (XPS) is adopted to follow the chemical modification, whereas the morphology is determined by atomic force microscopy (AFM). The crystallinity determined by NMR for the CNCs, A-CNCs, and F-A-CNCs is 78.32, 79.84, and 79.52, respectively. The quantitative analysis by elemental analysis revealed that the degree of cellulose hydroxyl group substitution (DS) of A-CNCs and F-A-CNCs is 0.05 and 0.27, respectively. According to the obtained results, CDCF fluorescein could be successfully labeled onto CNCs via L-leucine amino acid spacer linking, and the cellulose structure is stable during the labeling reaction. The fluorescent properties of the yielding pH-sensitive fluorescent CNCs (F-A-CNCs) are characterized by fluorescence spectrometer and imaged by confocal laser scanning microscopy (CLSM). The result shows that the fluorescence intensity of F-A-CNCs increases with increasing pH of the buffer from 2.28 to 10.84.