# Mechanistic Insights into NO Releasing by Functionalized Carbon Quantum Dots: A DFT Study

## TL;DR

This study uses computational methods to understand how carbon quantum dots release nitric oxide under light, revealing how functional groups and pH affect the process.

## Contribution

The paper provides a mechanistic DFT-based explanation of NO release from functionalized carbon quantum dots, including the role of cysteine deprotonation and excited-state electron transfer.

## Key findings

- Cysteine deprotonation is critical for forming the S–NO bond in functionalized carbon quantum dots.
- A low-energy excited state drives electron transfer from sulfur to nitrogen, weakening the S–NO bond under visible light.
- The system remains stable under physiological pH, but acidic conditions can destabilize it.

## Abstract

The development of
carbon quantum dots (CQDs) for photoresponsive
nitric oxide (NO) delivery is a rapidly advancing field, with experimental
reports demonstrating promising release under visible light irradiation.
However, a mechanistic understanding of the photolytic process, the
explicit role of CQD functionalization, and the influence of key physiological
variables such as pH has been lacking, hindering rational design.
This study aims to unravel the atomistic details of the NO release
mechanism in functionalized CQD systems. Using density functional
theory (DFT) and time-dependent DFT (TD-DFT) calculations, we systematically
investigated a model system (CQDCA
CYS+TPP···NO)
to probe ground-state reactivity, protonation effects, and excited-state
properties. Our results reveal that cysteine deprotonation is a critical
effect for S–NO bond formation. TD-DFT calculations evidence
a low-energy SNO-localized excited state with predominant n → π* character, which drives direct photoinduced
electron transfer from sulfur to nitrogen, weakening the S–NO
bond and rationalizing the experimental photodynamic response. While
very acidic conditions can destabilize the system, it remains stable
under physiological pH. These findings provide a mechanistic framework
that clarifies the synergistic roles of the CQD, cysteine, TPP, and
NO moieties on the nanocomposite, offering foundational principles
for engineering next-generation CQD-based platforms with enhanced
stability, controlled release efficiency, and reduced off-target effects
for biomedical applications.