In another work, monodisperse CdSe nanoclusters were prepared and functionalized with L-cysteine (Cys-capped CdSe).32 These functionalized CdSe QDs exhibited strong specific affinity for through QDs interface functional groups. Based on the quenching of fluorescence signals of functionalized CdSe QDs, a simple, rapid, and specific array for was developed achieving a very low-detection limit (6 nM). The usefulness of this method was successfully demonstrated for detection in real samples (human urine and river water), as the results were in good agreement with those obtained by cold vapor atomic fluorescence spectrometry (CV-AFS). In a similar way, a method for the determination of was developed based on quenching of the fluorescence of TGA (thioglycolic acid)-capped CdTe QDs by in aqueous solutions.34 To demonstrate their practical application, the proposed method was successfully applied to the analysis of in food samples (popcorn and instant noodles) with a concentration over (ppm), and the results were satisfactory, i.e., consistent with those of flame atomic absorption spectrometry. According to EU legislation (EC/1881/2006), which sets maximum levels for chemical contaminants in foodstuffs, the content limits are 1.0– (ppm) depending on kind of food. Therefore, the proposed method presented enough sensitivity to fulfill the legal requirements. Among thiols, the use of glutathione (GSH), a linear tripeptide synthesized in the body, as QDs capping agent or ligand for the functionalization of QDs has stimulated recent research interests due to the biological significance of this molecule. GSH is not only an important water-phase antioxidant and essential cofactor for antioxidant enzymes, but it also plays roles in catalysis, metabolism, signal transduction, and gene expression. Thus, GSH-capped QDs as biological probe should be more biocompatible than other thiol-capping ligands. Moreover, GSH seems to be a very promising molecule, since GSH and its polymeric form, phytochelatin, are employed by nature to detoxify heavy metal ions in organisms. A sensor for was developed based on a selective fluorescence quenching of CdTe and CdZnSe QDs capped with GSH shells.35 As a result of specific interaction, the fluorescence intensity of GSH-capped QDs was selectively reduced in the presence of , while no quenching response to alkaline and alkaline earth metal ions, , , , and , was observed. A limitation of this sensor was the strong interference of and because they also showed a similar quenching effect as at similar concentration levels. In regard to the sensitivity, the detection limit was quite good (20 nM), although it was affected by the presence of a concentrated ionic mixture. In the presence of ionic mixtures, the system was still capable of detection with a detection limit as low as 40 nM, and only became less sensitive when the ionic mixture was as high as 50 μm. Similarly, GSH-capped ZnSe QDs were synthesized and used equally for the detection of .36 This method presented improved sensitive and selective characteristics for the detection of trace in water, reaching a very low detection limit (0.71 nM). Since the U.S. Environmental Protection Agency (EPA) permits the maximum level of lead in drinking water to be 72 nM (15 ppb), this method amply complies with legal requirements. Gonçalves et al. also used GHS-capped CdTe QDs for detecting the presence of micromolar quantities of .37 They carried out a parallel factor (PARAFAC) analysis of the fluorescence spectra to study the system. PARAFAC analysis of the excitation emission matrices of QDs acquired as a function of the ion showed that only one linearly independent component describes the quenching of the QDs by the ion, allowing robust estimation of the excitation and emission spectra and of the quenching profiles.