Solvatochromic Surface-Passivated Carbon Dots for Fluorometric Moisture Sensing in Organic Solvents

Herein, we reported the synthesis of solvatochromic, red fluorescence-emitting carbon dots (rCDs) from para-phenylenediamine (p-PD) by a one-step microwave-assisted solvothermal method. The effect of surface passivation with hydroxyl-enriched HBPs of varying generation numbers (G2 and G4) on the optical, physicochemical, and solvatochromic properties of the rCDs was evaluated. The quantum yield (QY) of rCDs was altered from 31.4% to 79.8% in EtOH when their surface was passivated with HBP-G4 (rCD-bis64); however, surface passivation of rCDs with HPBs did not alter the solvatochromic property of rCDs, where only a slight decay was observed in rCD-bis64. The solvatochromism of rCDs with a red-shifted fluorescence emission from green to red as a response to increased solvent polarity was used as a simple and economical quantitative analysis tool for naked-eye, fluorometric, and smartphone-applicable on-site detection of moisture in organic solvents by converting the digital images to RGB. The polarity-dependent fluorescence color switchability of rCDs and rCDs-bis64 was then applied for the visual analysis of surface characteristics of paper strips. The visual response of both CDs to the changing paper type was transformed to a color map of the paper surface that was further used for visual and quantitative analyses of surface properties of the filter papers.