## 3 Effect of Surface Chemistry on Sensitivity and Selectivity  
The versatile surface chemistry of semiconductor QDs provides a platform for modification through different strategies. This has enabled researchers to anchor and tailor various receptors for specific target analytes, thereby enhancing their sensitivity. QDs can therefore work as signal transducers and receptors in composite materials. It is expected that changes in the surface charge or ligand components of the QDs would affect the efficiency of the core electron-hole recombination and consequently the luminescence efficiency. Therefore, a chemical sensing system based on QDs can be developed by using fluorescence changes, which are induced by either the direct physical adsorption or chelating of ions and small molecules on the surface of the QDs activated by the exchanged ligand [60, 61]. Reports have shown that the selectivity and sensitivity of sensors can be enhanced through surface modification, which can be done by functionalizing the material or conjugating it to a material that has an affinity for the pollutants of interest [62]. The effective detection and differentiation of pollutants can be done by conjugating QDs to different molecules such as graphene derivatives, polymers, proteins, enzymes, nucleic acids, etc. [27]. [PERSON] et al. studied the multi-channel detection and differentiation of explosives [63]. The functionalization of QDs' surfaces has been reported to affect the recognition properties, which have an effect on the sensitivity and selectivity [63]. Recently, [PERSON] et al. exploited the quenching efficiency of sulfur-containing amino acids such as L-cysteine and L-methionine. The modified QDs were found to be selective and more sensitive towards picric acid. The energy transfer between the QDs and the TNP is ascribed to the acid-base reaction. From Figure 4, it was observed that L-cysteine-capped ZnS-Mn\({}^{2+}\) QDs gave a higher quenching efficiency than those that were L-methionine-capped. However, the cause of this difference was not explained. These results could mean that the quenching efficiency is dependent on the structure of the functionality or the surface chemistry of the amino acids [64]. Surface-modified QDs have been extensively employed as fluorescent probes for the detection of organic pollutants. This detection takes place through two basic photo-physical mechanisms: fluorescence resonance energy transfer (FRET) and quenching. The concentration of the nitroaromatic substrates on QDs' surfaces through donor-acceptor complexes  
Figure 4: Effect of various aromatic compounds on the fluorescent intensity of L-cysteine/L-methionine-capped ZnS-Mn\({}^{2+}\) QDs (concentration of picric acid is \(2\times 10^{-7}\) M and other aromatics are \(2\times 10^{-5}\) M). leads to the quenching of QDs' luminescence. In 2012, [PERSON] et al. reported the use of chemically modified CdSe/ZnS to detect nitro-aromatic explosives such as trinitrotoluene (TNT) and trinitrotriazine (RDX). They observed that the luminescence of the mercapto (MPA)-functionalized CdSe/ZnS could not be quenched by nitro-aromatic compounds, and this observation has been attributed to a lack of donor units. In order to optimize the quenching efficiency, MPA-functionalized QDs were covalently linked to different electron donors, such as tyramine, dopamine, 5-hydroxydopamine and 6-hydroxydopamine. The fluorescence quenching of the QDs modified with these capping ligands followed the order tyramine \(<\) dopamine \(<\) 5-hydroxydopamine \(<\) 6-hydroxydopamine. This order demonstrated that the quenching efficiency is dependent on the donating properties of the capping ligands. Another aspect to consider when modifying QDs is the pH of the medium. [PERSON] et al. demonstrated that the luminescence of the electron-donor-modified QDs decreases as the pH of the medium increases [65]. Another study on the detection of PAHs by semiconductor QDs was carried out by [PERSON] et al. in 2014.