Fluorescence sensing mechanism of contaminant Cu²⁺ in water by up-converting nanoparticles

The detection of Cu²⁺ trace ions in water is significant due to their environmental implications and potential consequences for human health. Recent methods utilized up-converting nanoparticles, which emit visible light under near-infrared (NIR) radiation and offer the advantages of simplicity and high sensitivity in detection. Most of these studies are based on monitoring the decrease in the up-conversion emission intensity of suitable bands due to the adhesion affinity of Cu²⁺ ions to specific coating molecules of such nanoparticles. In this work, we systematically investigated the evolution of excitation and emission spectra of NaYF₄: Yb³⁺/Ho³⁺ up-converting nanoparticles functionalized with branched polyethyleneimine (BPEI) as a function of Cu²⁺ concentration in water. Hence, the most suitable fluorescence parameters (i.e., emission bands, excitation intensity and excitation wavelength) for Cu²⁺ sensing have been determined. The most sensitive emission band (green emission of Ho³⁺) and the ratiometric sensitivity (ratio between the green and red Ho³⁺ emissions) are explained through internal reabsorption of the Ho³⁺ ion emitted bands by the Cu²⁺ ions located in the BPEI coating of the nanoparticles. The detection range (up to 200 μM) is discussed in terms of the limited number of Cu²⁺ ions that can be chelated by the PEI molecules at the nanoparticle surface. Thus, this work offers a novel overview to explain the Cu²⁺ sensing mechanism by other fluorescent nanoparticles.