7.3.6. Toxins

Among the biggest threats to our community are toxins, whose quick and sensitive detection in aqueous solutions, body fluids, food, and drinking water enables the immediate application of appropriate remedies. Among the most common sources of toxins are bacteria whose toxins can be detected in trace amounts in urine or blood after poisoning. However, saliva and nasal swabs can be tested to confirm exposure to toxins even before the onset of symptoms. Another source of toxins in the environment is industrial, military or agricultural activity. Pollutants, such as pesticides and residues from explosives, pollute soil and groundwater and can easily enter the human body [103–107]. Table 8 shows an exemplary QDs-based sensors for toxins determination.



A new type of molecularly imprinted silica layers appended to CdS/CdSe/ZnS QDs (MIP-QDs) for saxitoxin were fabricated through the surface grafting technique. Sun et al. demonstrated that the synthesized MIP-QDs exhibited excellent selective fluorescence quenching to saxitoxin because of the complementary imprinted cavities on the surface of MIP-QDs. Such constructed MIP-QDs sensor exhibited excellent linearity in the range of 20.0–100.0 μg/L with LOD 0.3 μg/kg [103].

Wang et al. [104] reported a novel FRET-based nanobiosensor that uses luminescent QDs and dark quencher-labelled peptide probes to rapidly (on the order of hours) detect and quantify biologically active Botulinum neurotoxin (BoNT) and differentiate serotypes A and B, which is based on quantifiable differences in the photoluminescence (PL) intensity of QD reporters. The biorecognition elements for these probes are peptides that contain an amino acid sequence specific for BoNT/A or /B cleavage, a poly(histidine) sequence at the C-terminal for assembly on the QDs, and a dark quencher label (a dye

with no native fluorescence) that quenches the QD PL only when the peptide chain is uncleaved (i.e., in the absence of the target BoNT). The sensor signal scaled linearly with the analyte concentration over a range of 8–200 nM, with 4 pM as the LOD.

#### 7.3.7. Volatile Substances

Volatile organic compounds (VOCs) are organic chemical compounds with an evaporation temperature close to room temperature. These substances are commonly used in industry and as household products. Too high exposure to volatile organic compounds can have both short- and long-term adverse effects on health, such as respiratory failure [108–111]. An exemplary QDs-based sensors for volatile substances determination are presented in Table 9.


**Table 9.** QDs-based sensors for volatile substances determination.

Liu et al. [109] has presented work in which PbS-QDs/TiO2-nanotubes arrays (PbS QDs/TiO2 NTARs) are prepared by successive ionic layer adsorption and reaction, which are used to fabricate the gas sensor. The gas sensing performance shows that PbS QDs/TiO2 NTARs possess a good response towards ammonia gas at room temperature. The enhanced sensing mechanism lies in the fact that PbS QDs in PbS QDs/TiO2 NTARs may provide more sites to absorb the ammonia molecules and increase the depletion layer. The well-combined interface may provide effective transportation of the electrons as well as the direct transportation of the electrons along the TiO2 NTARs axis. This sensing strategy exhibited linearity in the range from 2 to 100 ppm at room temperature, with a LOD 2 ppm.

Barroso et al. [110] presented a new strategy for the detection of methanol using fluorescence spectroscopy and photoelectrochemical (PEC) analysis. The analytical system is based on the oxidation of cysteine (CSH) with hydrogen peroxide (H2O2) enzymatically generated by alcohol oxidase (AOx). H2O2 oxidizes capping agen<sup>t</sup> CSH, modulating the growth of CSH-stabilized CdS QDs. Disposable screen-printed carbon electrodes (SPCEs) modified with a conductive osmium polymer (Os-PVP) complex were employed to quantify resulting CdS QDs. This polymer facilitates the "wiring" of in situ enzymatically generated CdS QDs, which photocatalyzed oxidation of 1-thioglycerol (TG), generating photocurrent as the readout signal. As a result, an increase of the fluorescence intensity was observed.

Sotelo-Gonzalez et al. [111] has prepared colloidal Mn2+-doped ZnS nanoparticles exhibiting room temperature phosphorescence (RTP) emission and water solubilized by capping the QDs surface with l-cysteine. Such coating of the nanoparticle with cysteine groups allows their analytical application for acetone determination (selected as model ketone species) in aqueous media (by measuring the quenching on the RTP emission of such QDs after direct interaction with the analyte). It was observed that the rise of acetone concentration efficiently quenches of the phosphorescence emission. The linear range of the developed methodology turned out to be at least up to 600 mg·L−<sup>1</sup> with the LOD for acetone dissolved in an aqueous medium of 0.2 mg·L−1.
