*7.2. pH Detection*

Among the group of chemical sensors, the pH sensor is the object of greatest interest of scientists, because pH is one of the most important parameters in biochemical industrial processes [66–70]. Properly modified QDs, using organic ligands, may gain sensitivity to changes in pH. This property has a promising application in the design of a variety of luminescence sensors, examples of which are shown in Table 2.


**Table 2.** QDs-based sensors for pH determination.

In the work of Tomasulo et al. [66], the adsorption of pH-sensitive 1,3-oxazine on the surface of CdSe/ZnS QDs gives the possibility of changing the luminescence of inorganic nanoparticles by means

of chemical stimulation: 1,3-oxazine rings open in an acidic or basic environment to form nitrobenzyl phenolate chromophores. This transformation activates the energy-transfer path from excited quantum dots to ligands and facilitates energy transfer in the opposite direction. As a result, the intensity of PL QDs decline. Such a system can be used in aqueous solutions for pH changes in the range of 3–11.

The Snee group proposed a sensor based on signal transduction by FRET between the QDs and a fluorescent pH-sensitive squaraine dye attached to the surface of the QDs. The detection system process consisted in modulating the FRET efficiency resulting from the overlap of the absorption spectrum of squaraine with the emission of QDs. The emission of QDs (donor) was inhibited by the presence of squaraine, acting as an acceptor. Lowering the pH value caused a rise in the photoluminescence intensity of QDs [67].

Many works present pH-sensitive sensors that use semiconductor QDs combined with thiol compounds [6]. The emission of QDs fluorescence with mercaptoacetic acid on the surface allowed in vitro detection (but only in an acidic environment) and in live cells. The increase in intracellular pH has given an increased intensity signal of PL QDs [12]. In contrast, QDs combined with mercaptosuccinic acid (MSA) proved to be a simple system for the detection of urea. Hydrolysis of urea generates hydroxide anions, gradually raising the pH of the solution. With increasing urea concentration, the intensity of PL QDs increased [70].

#### *7.3. Detection of Organic Compounds*
