## 1 Introduction  
The contamination of water sources has become a global concern due to the increasing production of different chemical products, particularly organic species [1]. Organic pollutants occur in the form of sediments of different groups, such as polycyclic aromatic hydrocarbons (PAHs) and explosive residues [2, 3]. These pollutants are introduced into water bodies through two major ways: industrial activities and human activities. The regular detection of these pollutants continues to be paramount because there is clear evidence of the health implications caused by the consumption of water contaminated by these species. Therefore, the U.S. Environmental Protection Agency (EPA) has decided to include them in the priority list of pollutants [1]. Priority pollutants are pollutants that have been identified to be very toxic, even at low levels or concentrations. Furthermore, these pollutants have been identified as potential causes of cancer. Some of the PAHs that occur in mixtures have been shown to exhibit higher toxicity than individual species [2]. One of the major challenges with monitoring these pollutants is that they can occur at very low levels. Despite their occurrence in low concentrations, continuous exposure to them can result in adverse effects. This shows a necessity for the development of sensitive and selective methods for detecting these species [4, 5]. Techniques such as Raman scattering, infrared absorption spectroscopy, high-performance liquid chromatography (HPLC), gas chromatography-mass spectrometry (GC), quartz crystal microbalance, ion mobility spectroscopy (IMS) and metal detectors have been reported for the analysis of PAHs and explosive residues [6]. Though these methods have been successfully used to analyze PAHs, nitro-aromatic compounds and other organic contaminants in water, there are major drawbacks associated with many of them. These include relatively expensive instrumentation, highly skilled personnel necessary for their operation and difficult sample pre-treatment [7] to mention a few. These demerits have led to the search for other detection methods. One of the promising alternatives for the monitoring of organic pollutants in water is the group of techniques called optical methods, which include photoluminescence spectrometry and absorption spectrometry. The optical methods are characterized by a change in the emission intensity, peak position, and absorption of the fluorescent probe due to the interaction between the fluorescent probe and the target analyte. Optical detection techniques have been applied for the sensing of several organic pollutants, such as nitro-aromatic compounds (explosives), PAHs, pesticides and so on, with good sensitivity and selectivity being reported [8; 9]. The widely used fluorescent probes are molecularly imprinted polymers (MIP), surface-enhanced Raman scattering (SERS)-based nanoparticles, non-conjugated polymers, etc. SERS methods have been widely employed in the detection of PAHs and NACs; however, their use in sensing is limited because of the weak interaction between PAHs' rings and the surfaces of SERS-based nanoparticles [10; 11]. Polyurethanes (PUs) are a very versatile class of non-conjugated polymers with tunable properties, and have been used extensively in the sensing of explosives [5]. However, polyurethanes are insoluble in water; therefore, the reaction usually occurs in organic media, leading to further pollution of the environment [12]. Recently, small nanomaterials known as quantum dots have emerged as promising materials due to their superior optical properties, which can enhance the sensitivity and selective determination of PAHs and nitro-aromatic pollutants [4; 13]. In this review, the sensing of organic pollutants, namely polycyclic aromatic hydrocarbons and nitro-aromatic explosives, using different types of quantum dots was discussed. Different studies on the comparison of water-soluble quantum dots and organic soluble quantum dots on fluorescence sensing mechanism were also presented.