## 2 Quantum Dots in Sensing Applications
### Semiconductor Quantum Dots  
Semiconductor QDs are generally divided into different groups, such as binary and ternary QDs. These QDs are prepared through two approaches, namely organic and aqueous. Studies have shown that organic-soluble QDs have a higher affinity for organic pollutants than water-soluble QDs do [17]. However, there are environmental concerns with organic-soluble quantum dots, and so the focus is being shifted to water-soluble QDs [18]. These QDs have been recently used as chemical sensors for aromatic compoundsin fluorescence and electrochemical methods. These quantum dots exist in different classes, such as binary and ternary. Binary QDs may consist of different periodic table elements such as ZnSe, ZnS, CdTe, CdS, AgS, etc. [19; 20; 21; 22]. Among these compositions, cadmium-based sensors have been mostly reported for the application of sensing aromatic compounds. In 2019, [PERSON] et al. synthesized cucurbit-modified CdTe QDs for the fluorescence sensing of p-nitroaniline. In 2010, [PERSON] et al. detected PAHs using TiO\({}_{2}\)-nanotube-modified CdTe QDs. The detection of PAHs was observed via the intensity enhancement, which occurs as a result of fluorescence resonance energy transfer between PAHs and TiO\({}_{2}\)-nanotube-modified CdTe QDs [23]. In 2016, [PERSON] et al. functionalized CdTe with an amino acid for the determination of trace TNT explosive. L-cysteine served as a stabilizer, which enhanced the interaction between the functional groups of TNT and the QDs' surface [24]. However, this type of chemical sensor is being limited by the intrinsic toxicity of cadmium. Recently, binary cadmium-free QDs have been used as chemical sensors for nitro-aromatic explosives. In 2021, [PERSON] et al., synthesized blue, fluorescent, zinc selenide QDs for the recognition of nitro-aromatic compounds. The sensing was performed via fluorescence techniques. The quenching of mercaptopropanoic-acid-capped QDs has been shown to occur via the inner filter effect (IFE) [25]. The IFE mechanism can be observed through spectral overlapping between the absorption spectrum of 2,4,6 TNP and the excitation spectrum of ZnSe (Figure 1). Ternary QDs exist in different groups, such as CdSeTe, CdZnSe, CuInX\({}_{2}\) (X = S, Se and Te) AgInX\({}_{2}\) (X = S, Se and Te), etc. However, current research is being focused on cadmium-free ternary QDs because of their low toxicity [16; 26]. Ternary QDs can be synthesized in two ways, namely an organic route and an aqueous route (which can be further divided into two approaches: direct synthesis and ligand exchange). However, there have been challenges in synthesizing ternary QDs due to the uncontrolled reactivity of the cationic precursors of In\({}^{3+}\) and Ag\({}^{+}\) or Cu\({}^{+}\), which can lead to the formation of undesired products such as indium sulfides. Different studies on their synthesis have shown that this challenge can be overcome by (1) the use of dual stabilizers and (2) the use of a single precursor (that can produce both In and Cu or Ag) [27]. Furthermore, the reactivity of the cationic precursors in an organic medium can be balanced through the use of either single stabilizer or dual stabilizers [28]. Their optical properties are said to be  
Figure 1: Spectral overlapping of the absorption spectrum of 2,4,6 TNP with PL and the PL excitation spectra of ZnSe QDs [25]. dependent on the optimization of different parameters, including (i) the type of stabilizers, (ii) refluxing time, (iii) type or amount of dopants [29] and (iv) the type of synthetic method [30]. Apart from being environmentally friendly, these fluorescent probes exhibit two major outstanding properties: direct bandgaps and absorption coefficients as large as \(10^{-5}\) cm\({}^{-1}\)[16; 31]. However, these semiconductors suffer from surface defects, which are undesirable in sensing applications since they result in self-quenching. This challenge can be addressed by coating the core semiconductor with another semiconductor referred to as the shell [32].