A novel upconversion nanoprobe-based approach for microalgae concentration determination†

Microalgae are unicellular microorganisms distributed in freshwater and seawater. Accurate detection of microalgae concentration is crucial in various practical applications, such as microalgae cultivation, ballast water monitoring and early warning of water body ecological conditions. Herein, a simple approach for quantifying microalgae concentration is established by using a polymer-free upconversion luminescence (UCL) nanoprobe. In this strategy, chlorophyll-a (CH-a), a common microalgae component often used as an indicator of microalgae concentration, is extracted and served as an energy acceptor due to its absorption in the red region. The polymer-free NaYF₄:Er³⁺,Tm³⁺ nanoprobe, which exhibits red UCL under 980 nm excitation, acts as the energy donor. By simply mixing, the UCL of the nanoprobes quenches gradually as the CH-a concentration increases. The fluorescence dynamics indicate that this UCL quenching may be due to the inner filter effect and fluorescence resonance energy transfer mechanisms. An experimentally determined exponential relationship between UCL intensity and CH-a concentration has been established and is used to quantify microalgae concentration. The results show that the measured concentrations of microalgae samples are 36, 41, 103, 153, 179, 250 and 374 pieces per mm³ (pcs mm⁻³), respectively. These quantification results agree very well with those obtained from the traditional microscope counting method for the same microalgae samples. Consequently, this approach can be applied in practical applications for measuring microalgae concentration, providing both sensitive and accurate results.