pH-sensitive spontaneous decay of functionalized carbon dots in solutions

Area of Science:

• Materials Science
• Nanotechnology
• Physical Chemistry

Background:

• Carbon quantum dots (CQDs) are versatile nanomaterials with applications in drug delivery, sensing, and energy.
• Luminescence lifetime measurements of surface-functionalized CQDs offer potential for autonomous pH sensing due to their long lifetimes.

Purpose of the Study:

• To theoretically investigate the relationship between the luminescence lifetime of CQDs and the pH of their surrounding environment.
• To develop a model for predicting pH sensitivity based on the protonation/deprotonation of functional groups on CQDs.

Main Methods:

• A theoretical separation approach was employed, treating functional groups and the CQD core independently.
• Calculations were performed for specific functional groups (m-phenylenediamine, phloroglucinol, tethered disperse blue 1).
• The model considers the CQD's optical spectrum as an environmental factor influencing functional group transition rates.

Main Results:

• A direct relationship was derived between pH, pKa, and the mixed fluorescence lifetime of CQDs.
• Optimal pH sensitivity is achieved when the difference in transition rates between protonated and deprotonated states is maximized.
• Maximum sensitivity occurs when the pKa of the functional group aligns with the target pH range.

Conclusions:

• The developed theoretical model accurately describes pH sensing using CQD luminescence lifetime.
• The model provides a framework for designing CQDs with enhanced pH sensitivity for specific applications.
• The approach is extendable to complex systems with multiple protonation states and multicomponent liquids.