Since the discovery of Quantum Dots (QDs) by Alexey I. Ekimov in 1981, the interest of researchers in that particular type of nanomaterials (NMs) with unique optical and electrical properties has been increasing year by year. Thus, since 2009, the number of scientific articles published on this topic has not been less than a thousand a year. The increasing use of QDs due to their biomedical, pharmaceutical, biological, photovoltaics or computing applications, as well as many other high-tech uses such as for displays and solid-state lighting (SSL), has given rise to a considerable number of studies about its potential toxicity. However, there are a really low number of reported studies on the detection and quantification of QDs, and these include ICP–MS and electrochemical analysis, which are the most common quantification techniques employed for this purpose. The knowledge of chemical phenomena occurring on the surface of QDs is crucial for understanding the interactions of QDs with species dissolved in the dispersion medium, while it paves the way for a widespread use of chemosensors to facilitate its detection. Keeping in mind both human health and environmental risks of QDs as well as the scarcity of analytical techniques and methodological approaches for their detection, the adaptation of existing techniques and methods used with other NMs appears necessary. In order to provide a multidisciplinary perspective on QD detection, this review focused on three interrelated key aspects of QDs: properties, surface chemistry and detection.

We studied the interaction of Cu²⁺ ions and CuO nanoparticles with the fluorescent Schiff base ligand H₃L, which derives from the condensation of 4-formyl-3-hydroxybenzoic acid with N-(2-aminobenzyl)-5-(dimethylamino)naphthalene-1-sulfonamide (DsA). A detailed assignment of the most significant bands of the electronic and infrared spectra of H₃L and DsA was performed using DFT methods, based on both crystal structures. The affinity of H₃L to react with Cu²⁺ ions in solution (K_B = 9.01 10³ L mol⁻¹) is similar to that found for the Cu²⁺ ions present on the surface of CuO NPs (K_B = 9.84 10³ L mol⁻¹). Fluorescence spectroscopic measurements suggest five binding sites for H₃L on the surface of the CuO NPs used. The µ-XRF analysis indicates that a polycrystalline sample of CuO-H₃L NPs contains 15:1 Cu:S molar ratio (CuO:H₃L). ATR-FTIR spectroscopy, supported by DFT calculations, showed that the HL²⁻ (as a phenolate and sulfonamide anion) is coordinated to superficial Cu²⁺ ions of the CuO NPs through their azomethine, sulphonamide, and phenolic groups. A solution of H₃L (126 ppb) shows sensitive responses to CuO NPs, with a limit of detection (LOD) of 330 ppb. The working range for detection of CuO NPs with [H₃L] = 126 ppb was 1.1–9.5 ppm. Common metal ions in water, such as Na⁺, K⁺, Mg²⁺, Ca²⁺, Fe³⁺, and Al³⁺ species, do not interfere significantly with the detection of CuO NPs.

With the aim of investigating the development of an easily synthesized dansyl-based fluorescent probe for detecting heavy-metal-based nanomaterials in aqueous solution, we synthetized and characterized a Schiff-base ligand, which we named H₃L. It was derived from the reaction of 4-formyl-3-hydroxybenzoic acid with N-(2-aminobenzyl)-5-(dimethylamino)naphthalene-1-sulfonamide. H₃L incorporates a suitable O,N-binding domain that can bind heavy-metal ions at the surface of the nanoparticles.

We investigated the easily synthesized ligand H₃L as a fluorescent chemosensor for the detection of CdSe nanoparticles (CdSe NPs) and L-cysteine-capped CdSe quantum dots (CdSe-Cys QDs) in ethanol–water samples. A drastic quenching of the fluorescence emission of H₃L at 510 nm occurred, as a result of the addition of CdSe NPs and CdSe-Cys QDs. A solution of H₃L (1.26 ppb) showed sensitive responses to both CdSe NPs and CdSe-Cys QDs, with limits of detection (LOD) as low as 40 and 62 ppb, respectively. Moreover, using a smartphone color recognizer application, the fluorescence intensity response of H₃L-modified cellulose paper to CdSe-Cys QDs was recorded on a red channel (R), which allowed us to detect CdSe-Cys QDs with LOD = 15 ppb. Interference of some common metal nanomaterials (NMs), as well as metal ions, in the determination of CdSe NMs in solution was studied. The affinity of H₃L to CdSe NPs and CdSe-Cys QDs was spectroscopically determined. Scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDX), micro-X-ray fluorescence (µ-XRF), ¹H-NMR, attenuated total reflection infrared spectroscopy (ATR-IR), and density functional theory (DFT) were also used to investigate the interaction of H₃L with CdSe NMs.

The Schiff base ligand H₂SB derived from the condensation of N-(2-aminobenzyl)-5-(dimethylamino)naphthalene-1-sulfonamide with 4-formyl-3-hydroxybenzoic acid has been investigated as a chemosensor for the detection of CdSe-Cys QDs in water samples. We immobilized H₂SB onto cellulose paper by forming an amide bond, which results from the condensation of a carboxylic acid and an amine. Three dominant signals located around 270, 330, and 420 nm in the diffuse reflectance spectrum of the H₂SB-modified paper demonstrated its immobilization. A linear decrease can be observed in the absorbance of the 270 nm band with the increase of the CdSe-Cys QDs concentration from 100 ppb to 2 ppm. The LOD and LOQ show values of 245 and 815 ppb, respectively. An interaction via metal–ligand coordination between CdSe-Cys QDs and H₂SB has been demonstrated with ¹H NMR, ATR-FTIR, and UV-Vis spectroscopy.

A novel approach is reported to quantify CuO NPs in aqueous samples by using digital image colorimetry (DIC), which is a rapid, low-cost, and facile method. We have used a colour recogniser application on a smartphone to register the gradual colour changes experimented, under λ _ex = 365 nm radiation, by a dansyl-based chemosensor immobilised on cellulose paper, after interacting with aqueous dispersions of CuO NPs with concentrations between 5 and 100 µg L⁻¹. The low limits of detection (LOD) and quantification (LOQ) (10.5 and 34.9 µg L⁻¹, respectively) show a relevant sensitivity for this simple method, which also displays a good precision and accuracy (analytical recovery, 99 ± 1%). A comparison of the results similarly obtained for the modified paper through diffuse reflectance (DR) revealed worse detection parameters (LOD = 100.0 µg L⁻¹ and LOQ = 333.4 µg L⁻¹). In addition, fluorescence spectroscopy (λ _em = 520 nm) demonstrated the ability of the free chemosensor (1.26 µg L⁻¹ in 20:80 (v/v) water–ethanol solutions) for the quantification of CuO NPs (LOD = 30 µg L⁻¹ and LOQ = 101 µg L⁻¹), without improvement of the results obtained with DIC on the modified papers. The dissolved free chemosensor demonstrated selectivity towards CuO NPs in the presence both of metal ions common in aqueous samples as K⁺, Na⁺, Ca²⁺, Mg²⁺, Fe³⁺, or Al³⁺ and of other usual nanomaterials as those of Cu, CdSe, TiO₂, and ZnO NPs. Graphical Abstract