Kinetics of the luminol oxidation reaction with sodium hypochlorite and prospects for enhancing the intensity of the accompanying chemiluminescence in the presence of metal nanoparticles

Abstract

The chemiluminescence accompanying the oxidation of luminol has found diverse applications, yet the mechanism of the light-producing pathway requires further clarification. Investigating the dependence of luminescence intensity on the time elapsed since the reaction began offers a valuable opportunity to monitor the reaction's progress and compare observational results with existing models of its mechanism.
We measured the chemiluminescence kinetics of luminol at various ratios of initial luminol and sodium hypochlorite concentrations in a phosphate buffer. A theoretical model accounting for changes in reactant concentrations during the reaction has been proposed to describe experimentally obtained kinetic curves. Model parameters—specifically, the rate constant for the formation of the critical product along the reaction's optical path and its relaxation rate—were determined by fitting the model to experimental data. The dependencies of these parameters on the initial oxidant concentration were established, and avenues for further model development to incorporate these findings were outlined. This will enable us to elucidate the characteristics of the chemiluminescent reaction and expand its scope of application in the study of chemical reactions involving short-lived radicals.
The dependence of chemiluminescence intensity on the concentrations of luminol and sodium hypochlorite was also investigated. While the dependence of chemiluminescence intensity on luminol concentration is linear over a wide concentration range, the dependence on sodium hypochlorite concentration is more complex. Upon diluting the hypochlorite from 1 mM to 20 µM, the decrease in chemiluminescence intensity is well described by a power-law dependence on hypochlorite concentration with an exponent of 2.4; however, with further dilution, the intensity drops more sharply than such a dependence would predict, becoming indistinguishable from the noise level at a hypochlorite concentration of 10 µM.
A well-known approach to enhancing chemiluminescence intensity by introducing metal nanoparticles—which exhibit plasmon resonance within the luminol chemiluminescence band—into the reaction mixture is discussed. Calculations are presented for the potential chemiluminescence intensity enhancement factors for bulk samples as a function of the concentration of the introduced metal nanoparticles, as well as for the maximum enhancement achieved at the optimal distance between the emitting molecule and the metal nanoparticle.
This work was supported by the Russian Science Foundation (Project 23-72-00045).

Speaker

Tigran Vartanyan
ITMO University
Russia

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