# Theoretical Investigation of Charge Modulation Effects in Two Pyridine-Based Fluorescence Probes for Nerve Agent and Acetylcholinesterase (AChE) Detection

## TL;DR

This paper uses theoretical methods to study how two fluorescent probes detect nerve agents and acetylcholinesterase by analyzing changes in their fluorescence due to charge transfer and chemical interactions.

## Contribution

A novel theoretical framework is introduced to analyze fluorescence quenching and charge transfer in pyridine-based probes for nerve agent detection.

## Key findings

- HBQ-AE and NMU-1 show strong fluorescence quenching when interacting with DCP due to charge transfer and formation of a positively charged nitrogen.
- HBQ-AE acts as a 'turn-on' sensor via its hydrolysis product HBQ+H, while NMU-1 is a 'turn-off' sensor.
- Theoretical calculations align closely with experimental data, showing blue shifts of 0.23 eV and 0.24 eV for HBQ-AE and NMU-1, respectively.

## Abstract

The efficient detection
of nerve agents is paramount
in civilian
and war contexts. In this work, we investigated theoretically the
mechanisms of fluorescence quenching and charge transfer (CT) in two
recently synthesized small molecule fluorescent probes for detecting
nerve agents, NMU-1 and 10-hydroxybenzo­[h]­quinoline
(HBQ-AE), which are based on the pyridine group as the identifying
unit. These fluorescent molecules change their emission pattern upon
binding to an organophosphorus compound. We employed density functional
theory (DFT), implicit methods for describing the water solvent, and
a path integral approach to calculate fluorescence rates and emission
spectra from first-principles for analyzing the interaction between
the nerve agent simulant diethyl chlorophosphite (DCP) and NMU-1 or
HBQ-AE. Upon interacting with DCP, both HBQ-AE and NMU-1 experience
strong fluorescence quenching, with intensity reductions of ∼99.65
and ∼99.95%, respectively, due to enhanced CT induced by DCP’s
electron-accepting nature and formation of a positively charged nitrogen
within the probes. Although the measured fluorescence enhancement
in HBQ-AE/DCP systems was attributed to the HBQ-DCP product, our results
show a different picture: the true fluorescent species is the hydrolysis
product HBQ + H, whose calculated emission differs by a blue shift
of only 0.23 eV from the experimental data, rather than the HBQ-DCP
product as previously thought. For acetylcholinesterase (AChE) detection,
fluorescence is confirmed to originate from the HBQ-Keto, with a blue
shift deviation of 0.12 eV. The NMU-1 calculations also confirm the
experiments, showing a 0.24 eV deviation, thus confirming its quenching-based
detection of DCP. Therefore, HBQ-AE is a fluorescence turn-on sensor
via its emissive hydrolysis product HBQ+H. In contrast, NMU-1 is a
turn-off sensor, as both NMU-DCP and NMU+H are essentially nonemissive.
Our theoretical approach is general, accurate, and can be applied
to different problems involving fluorescent probes and binding agents.