Sensitive detection of dissolved oxygen in seawater by fluorometric sensing based on ZnO-NH2@SiO2 QDs

Abstract

Monitoring dissolved oxygen (DO) levels in seawater, particularly through fluorometric sensing, is vital for the well-being of marine ecosystems and fisheries. Stable and reliable oxygen-responsive fluorescent materials are required to enhance sensor longevity and minimize maintenance costs. Semiconductor nanocrystals, particularly zinc oxide (ZnO) nanoparticles, have the advantages of low cost, easy preparation, nontoxicity, and good stability. In this study, SiO₂-wrapped ZnO-NH₂ quantum dots (ZnO-NH₂@SiO₂ QDs, ZNS) were successfully synthesized by introducing (3-aminopropyl)triethoxysilane (APTEs) on the surface of nanocrystals for detection of DO in seawater. The structural analysis and optical characterization indicated that the introduction of –NH₂ could effectively enhance the fluorescence emission intensity of the nanocrystals, and the encapsulation of SiO₂ could effectively inhibit the permeation of Cl⁻ in seawater, thus avoiding their aggregation and fluorescence quenching. The electron paramagnetic resonance and optical characterization results further indicated that the synthesized ZNS possesses a high concentration of oxygen vacancies and exhibits exceptional stability in alkaline environments. These properties were complemented by superior anti-interference capabilities. Consequently, the sensing performance of ZNS was systematically investigated. For the detection of DO, the response of ZNS to DO showed a linear Stern–Volmer relationship (K_sv = 1334.83 M⁻¹) with the concentration of DO (0.56–20.44 mg L⁻¹) from deoxidation to oxidation; this response is superior to that of traditional molecular compound materials or other sensing materials reported in the literature. The findings of this research offer a straightforward approach to DO detection in seawater, holding promise for enhanced monitoring of marine water quality.