Synthetic Approaches, Modification Strategies and the Application of Quantum Dots in the Sensing of Priority Pollutants
Synthetic Approaches, Modification Strategies and the Application of Quantum Dots in the Sensing...
Maluleke, Rodney;Oluwafemi, Oluwatobi Samuel
2021-12-07 00:00:00
applied sciences Review Synthetic Approaches, Modification Strategies and the Application of Quantum Dots in the Sensing of Priority Pollutants 1 , 2 1 , 2 , Rodney Maluleke and Oluwatobi Samuel Oluwafemi * Department of Chemical Sciences, University of Johannesburg, Doornfontein, Johannesburg 2028, South Africa; rodney.maluleke@gmail.com Center for Nanomaterials Science Research, University of Johannesburg, Doornfontein, Johannesburg 2028, South Africa * Correspondence: oluwafemi.oluwatobi@gmail.com Featured Application: Authors are encouraged to provide a concise description of the specific application or a potential application of the work. This section is not mandatory. Abstract: Polycyclic aromatic hydrocarbons (PAHs) and nitro-aromatic compounds (NACs) are two classifications of environmental pollutants that have become a source of health concerns. As a result, there have been several efforts towards the development of analytical methods that are efficient and affordable that can sense these pollutants. In recent decades, a wide range of techniques has been developed for the detection of pollutants present in the environment. Among these different techniques, the use of semiconductor nanomaterials, also known as quantum dots, has continued to gain more attention in sensing because of the optical properties that make them useful in the identification and differentiation of pollutants in water bodies. Reported studies have shown great improvement in the sensing of these pollutants. This review article starts with an introduction on two types of organic pollutants, namely polycyclic aromatic hydrocarbons and nitro-aromatic explosives. Citation: Maluleke, R.; This is then followed by different quantum dots used in sensing applications. Then, a detailed Oluwafemi, O.S. Synthetic discussion on different groups of quantum dots, such as carbon-based quantum dots, binary and Approaches, Modification Strategies ternary quantum dots and quantum dot composites, and their application in the sensing of organic and the Application of Quantum Dots pollutants is presented. Different studies on the comparison of water-soluble quantum dots and in the Sensing of Priority Pollutants. Appl. Sci. 2021, 11, 11580. https:// organic-soluble quantum dots of a fluorescence sensing mechanism are reviewed. Then, different doi.org/10.3390/app112411580 approaches on the improvement of their sensitivity and selectivity in addition to challenges associated with some of these approaches are also discussed. The review is concluded by looking at different Academic Editor: Al Meldrum mechanisms in the sensing of polycyclic aromatic hydrocarbons and nitro-aromatic compounds. Received: 4 October 2021 Keywords: detection; sensors; polycyclic aromatic hydrocarbons; nitro-aromatic explosives Accepted: 24 November 2021 Published: 7 December 2021 Publisher’s Note: MDPI stays neutral 1. Introduction with regard to jurisdictional claims in The contamination of water sources has become a global concern due to the increas- published maps and institutional affil- ing production of different chemical products, particularly organic species [1]. Organic iations. 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 Copyright: © 2021 by the authors. evidence of the health implications caused by the consumption of water contaminated by Licensee MDPI, Basel, Switzerland. these species. Therefore, the U.S. Environmental Protection Agency (EPA) has decided to This article is an open access article include them in the priority list of pollutants [1]. Priority pollutants are pollutants that distributed under the terms and have been identified to be very toxic, even at low levels or concentrations. Furthermore, conditions of the Creative Commons these pollutants have been identified as potential causes of cancer. Some of the PAHs that Attribution (CC BY) license (https:// occur in mixtures have been shown to exhibit higher toxicity than individual species [2]. creativecommons.org/licenses/by/ One of the major challenges with monitoring these pollutants is that they can occur at very 4.0/). Appl. Sci. 2021, 11, 11580. https://doi.org/10.3390/app112411580 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 11580 2 of 14 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 al- ternatives 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 exten- sively 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. Furthermore, different approaches on the improvement of their sensitivity and selectivity as well as challenges associated with some of these approaches were also discussed. 2. Quantum Dots in Sensing Applications An attractive and promising approach for the sensing of environmental pollutants involves using an optical method, which offers many advantages over other standard detec- tion techniques, including cost-effectiveness, good portability and high sensitivity as well as selectivity [14]. Among different materials used in sensing organic pollutants, QDs have appeared to be the most promising materials because of their unique properties. Quantum dots (QDs) are zero-dimensional nanostructured materials with excellent optoelectronic properties [14]. Zero-dimensional nanostructured materials exist in different groups, such as semiconductor quantum dots, carbon quantum dots and graphene quantum dots. These materials are characterized by optoelectronic properties such as broad excitation spectra, narrow bandwidth emission spectra, high resistance to photo-bleaching and good stability as well as biocompatibility [15,16]. 2.1. 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 Appl. Sci. 2021, 11, 11580 3 of 14 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 compounds in 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. [19–22]. Among these compositions, cadmium-based sensors have been mostly reported for the application of sensing aromatic compounds. In 2019, Cao et al. synthesized cucurbit-modified CdTe QDs for the fluorescence sensing of p-nitroaniline. In 2010, Yang et al. detected PAHs using TiO -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 -nanotube- modified CdTe QDs [23]. In 2016, Qian 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, Sharma 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 Appl. Sci. 2021, 11, x FOR PEER REVIEW 4 of 14 spectral overlapping between the absorption spectrum of 2,4,6 TNP and the excitation spectrum of ZnSe (Figure 1). Figure 1. Spectral overlapping of the absorption spectrum of 2,4,6 TNP with PL and the PL excitation Figure 1. Spectral overlapping of the absorption spectrum of 2,4,6 TNP with PL and the PL excita- spectra of ZnSe QDs [25]. tion spectra of ZnSe QDs [25]. Ternary QDs exist in different groups, such as CdSeTe, CdZnSe, CuInX (X = S, Se and Te) AgInX (X = S, Se and Te), etc. However, current research is being focused on 2.2. Carbon Quantum Dots cadmium-free ternary QDs because of their low toxicity [16,26]. Ternary QDs can be Carbon quantum dots are generally carbon-based nanoparticles that exhibit optoe- synthesized in two ways, namely an organic route and an aqueous route (which can be lectronic properties due to the quantum confinement effect [34]. Carbon quantum dots further divided into two approaches: direct synthesis and ligand exchange). However, (CQDs) have attracted much attention in many applications because of their advantages, there have been challenges in synthesizing ternary QDs due to the uncontrolled reactivity 3+ + + of the cationic precursors of In and Ag or Cu , which can lead to the formation of such as water solubility, fluorescence, low toxicity, cheap scale-up production, abundance undesired products such as indium sulfides. Different studies on their synthesis have and ease of modification [35]. CQDs can be prepared from different carbon sources, such shown that this challenge can be overcome by (1) the use of dual stabilizers and (2) the as food waste [36], waste biomass sources [37], corncobs [38] and weak organic acids [39]. Carbon dots are typically prepared via chemical methods and physical methods. Chemi- cal methods include electrochemical synthesis, combustion/thermal/hydrothermal/acidic oxidation, supported synthesis, microwave/ultrasonic and solution chemistry methods. Physical methods include arc discharge, laser ablation/passivation and plasma treatment [40,41]. These materials have been investigated for the fluorescence sensing of aromatic compounds due to their fluorescence properties. However, they have not been investi- gated for PAHs such as phenanthrene, naphthalene, pyrene, anthracene, etc. Recently, Hu et al. synthesized CQDs for the fluorescence sensing of para-nitrophenol. The sensor and the phenol interacted through the IFE mechanism. This mechanism was proven via spec- tral overlap [42]. In 2015, Cheng et al. also prepared CQDs for the fluorescence sensing of 2,4,6-trinitrophenol. The quenching of the CQDs by the phenol was suspected to be due to the IFE [43]. Fan et al. reported the fluorescence detection of 2,4,6-trinitrophenol (TNP) using manganese (Mn)-doped CDs. The probe showed fluorescence quenching with an increasing concentration of TNP and high selectivity in the midst of other nitroaromatic compounds [41]. 2.3. Graphene Quantum Dots Graphene quantum dots are zero-dimensional (0D) nanomaterials that are prepared by converting 2D graphene. The particles of these materials are non-quasi-spherical, which is the feature that makes them different from CQDs [44]. These QDs can be synthe- sized from different sources, such as coal [45], natural graphite [46], rice husk biomass [47], etc. Recently, GQDs have also emerged as fluorescent probes because of their ad- vantages, such as a large surface area, biocompatibility, lower toxicity and ease of surface modification [48]. These QDs are prepared via two types of methods, namely the “top- down” approach and the “bottom-up” approach. The “top-down” approach involves breaking down large-scale macroscopic carbon-based materials such as natural graphite, Appl. Sci. 2021, 11, 11580 4 of 14 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 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 5 1 10 cm [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]. While QDs have great potential as fluorescent probes in the sensing of organic molecules, the affinity of their surfaces for the target molecules must be improved. Recently, a few studies on the improvement of their affinity for aromatic compounds have been reported. In 2021, Maluleke et al. reported the synthesis of graphene oxide (GO)-modified CuInS/ZnS QDs for the sensing of PAHs. The addition of different concentrations of PAHs to the GO-modified QDs has shown enhancement of photoluminescence (PL) intensity. This enhancement was ascribed to the - stacking between the chemical sensor and the PAHs [33]. 2.2. Carbon Quantum Dots Carbon quantum dots are generally carbon-based nanoparticles that exhibit optoelec- tronic properties due to the quantum confinement effect [34]. Carbon quantum dots (CQDs) have attracted much attention in many applications because of their advantages, such as water solubility, fluorescence, low toxicity, cheap scale-up production, abundance and ease of modification [35]. CQDs can be prepared from different carbon sources, such as food waste [36], waste biomass sources [37], corncobs [38] and weak organic acids [39]. Carbon dots are typically prepared via chemical methods and physical methods. Chemical methods include electrochemical synthesis, combustion/thermal/hydrothermal/acidic oxidation, supported synthesis, microwave/ultrasonic and solution chemistry methods. Physical methods include arc discharge, laser ablation/passivation and plasma treatment [40,41]. These materials have been investigated for the fluorescence sensing of aromatic com- pounds due to their fluorescence properties. However, they have not been investigated for PAHs such as phenanthrene, naphthalene, pyrene, anthracene, etc. Recently, Hu et al. synthesized CQDs for the fluorescence sensing of para-nitrophenol. The sensor and the phenol interacted through the IFE mechanism. This mechanism was proven via spectral overlap [42]. In 2015, Cheng et al. also prepared CQDs for the fluorescence sensing of 2,4,6-trinitrophenol. The quenching of the CQDs by the phenol was suspected to be due to the IFE [43]. Fan et al. reported the fluorescence detection of 2,4,6-trinitrophenol (TNP) using manganese (Mn)-doped CDs. The probe showed fluorescence quenching with an increasing concentration of TNP and high selectivity in the midst of other nitroaromatic compounds [41]. 2.3. Graphene Quantum Dots Graphene quantum dots are zero-dimensional (0D) nanomaterials that are prepared by converting 2D graphene. The particles of these materials are non-quasi-spherical, which is the feature that makes them different from CQDs [44]. These QDs can be synthesized from different sources, such as coal [45], natural graphite [46], rice husk biomass [47], etc. Recently, GQDs have also emerged as fluorescent probes because of their advantages, such as a large surface area, biocompatibility, lower toxicity and ease of surface modification [48]. These QDs are prepared via two types of methods, namely the “top-down” approach and the “bottom-up” approach. The “top-down” approach involves breaking down large-scale macroscopic carbon-based materials such as natural graphite, carbon fibers, graphene oxide, and metal–organic frameworks and so on. The “bottom-up” method involves the growing of GQDs from different sources, such as plant extracts [49,50], rice husks [47], Appl. Sci. 2021, 11, x FOR PEER REVIEW 5 of 14 Appl. Sci. 2021, 11, x FOR PEER REVIEW 5 of 14 carbon fibers, graphene oxide, and metal–organic frameworks and so on. The “bottom- up” method involves the growing of GQDs from different sources, such as plant extracts carbon fibers, graphene oxide, and metal–organic frameworks and so on. The “bottom- Appl. Sci. 2021, 11, 11580 5 of 14 [49,50], rice husks [47], wood charcoal [51], rice grains [52], coffee grounds [53], etc. Chen up” method involves the growing of GQDs from different sources, such as plant extracts et al. explored the synthesis of GQDs through a “bottom-up” route using starch as the [49,50], rice husks [47], wood charcoal [51], rice grains [52], coffee grounds [53], etc. Chen precursor [54]. Though these QDs have been mostly investigated for the fluorescence sens- et al. explored the synthesis of GQDs through a “bottom-up” route using starch as the wood charcoal [51], rice grains [52], coffee grounds [53], etc. Chen et al. explored the ing of PAHs and NACs, there are few reports on their use in sensing PAHs. Furthermore, precursor [54]. Though these QDs have been mostly investigated for the fluorescence sens- synthesis of GQDs through a “bottom-up” route using starch as the precursor [54]. Though these materials’ sensitivity and selectivity have been shown to depend on size and shape, ing of PAHs and NACs, there are few reports on their use in sensing PAHs. Furthermore, these QDs have been mostly investigated for the fluorescence sensing of PAHs and NACs, surface modification and heteroatom doping [55]. Kaur et al. explored the fluorescence these materials’ sensitivity and selectivity have been shown to depend on size and shape, there are few reports on their use in sensing PAHs. Furthermore, these materials’ sensitivity detection of trinitrophenol (TNP) based on nitrogen-doped GQDs [56]. The doping with sur and face selectivity modifica have tion been and shown heteroat toom depend dopion ng size [55]and . Kashape, ur et al. surface explored t modification he fluoand rescence nitrogen atoms enhanced the interaction between the QDs and the phenol. Fluorescence det heter ection o oatom f t doping rinitropheno [55]. Kaur l (TNP) et al. bexplor ased on ed the nitrogen fluorescence -doped GQDs [56]. The detection of trinitr do ophenol ping with q (TNP) uenchi based ng wa on s observed nitrogen-doped owinGQDs g to the tra [56]. Th nsf eer of en doping ergy from N-GQDs, which with nitrogen atoms enhanced act as a nitrogen atoms enhanced the interaction between the QDs and the phenol. Fluorescence the interaction between the QDs and the phenol. Fluorescence quenching was observed donor to TNP, which is electron-deficient due to nitro-groups (Figure 2). Chen et al. also quenching was observed owing to the transfer of energy from N-GQDs, which act as a owing to the transfer of energy from N-GQDs, which act as a donor to TNP, which is did a similar study using creatinine-capped nitrogen-doped GQDs (Figure 3) [57]. Re- donor to TNP, which is electron-deficient due to nitro-groups (Figure 2). Chen et al. also electron-deficient due to nitro-groups (Figure 2). Chen et al. also did a similar study cently, Nsibande et al. synthesized ferric-ion-modified GQDs for the recognition of pyrene did a similar study using creatinine-capped nitrogen-doped GQDs (Figure 3) [57]. Re- using creatinine-capped nitrogen-doped GQDs (Figure 3) [57]. Recently, Nsibande et al. in an aqueous medium. The ferric ions have been reported to turn off the fluorescence cently, Nsibande et al. synthesized ferric-ion-modified GQDs for the recognition of pyrene synthesized ferric-ion-modified GQDs for the recognition of pyrene in an aqueous medium. intensity of the QDs, while pyrene turned on the intensity [58]. in an aqueous medium. The ferric ions have been reported to turn off the fluorescence The ferric ions have been reported to turn off the fluorescence intensity of the QDs, while intensity of the QDs, while pyrene turned on the intensity [58]. pyrene turned on the intensity [58]. Figure 2. (A) Fluorescence emission spectra of BSA/creatinine/N-GQDs–chitosan solid film in the Figure 2. (A) Fluorescence emission spectra of BSA/creatinine/N-GQDs–chitosan solid film in Figure 2. (A) Fluorescence emission spectra of BSA/creatinine/N-GQDs–chitosan solid film in the presence of PA at 0 (red line I0), 2, 5, 10, 20, 80, 100, 150 and 200 ng/mL (from top to bottom). (B) the presence of PA at 0 (red line I0), 2, 5, 10, 20, 80, 100, 150 and 200 ng/mL (from top to bottom). presence of PA at 0 (red line I0), 2, 5, 10, 20, 80, 100, 150 and 200 ng/mL (from top to bottom). (B) Stern–Volmer curve. I and I0 were the fluorescence intensities of BSA/creatinine/N-GQDs–chi- (B) Stern–Volmer curve. I and I0 were the fluorescence intensities of BSA/creatinine/N-GQDs– Stern–Volmer curve. I and I0 were the fluorescence intensities of BSA/creatinine/N-GQDs–chi- tosan fluorescent solid film at 360 nm in the presence and absence of PA, respectively [57]. chitosan fluorescent solid film at 360 nm in the presence and absence of PA, respectively [57]. tosan fluorescent solid film at 360 nm in the presence and absence of PA, respectively [57]. Figure 3. (a) FL behavior of N-GQDs in the presence of various TNP concentrations (from top to Figure 3. (a) FL behavior of N-GQDs in the presence of various TNP concentrations (from top to Figure 3. (a) FL behavior of N-GQDs in the presence of various TNP concentrations (from top to bottom (0–50 M)). (b) FL emission curve of N-GQDs with increased concentration of TNP (inset: bottom (0–50 M)). (b) FL emission curve of N-GQDs with increased concentration of TNP (inset: bottom (0–50 M)). (b) FL emission curve of N-GQDs with increased concentration of TNP (inset: Stern–Volmer plot for the Stern–Volmer plot for the quenching quenching process) [59]. process) [59]. Stern–Volmer plot for the quenching process) [59]. Appl. Sci. 2021, 11, 11580 6 of 14 3. Effect of Surface Chemistry on Sensitivity and Selectivity Appl. Sci. 2021, 11, x FOR PEER REVIEW 6 of 14 The versatile surface chemistry of semiconductor QDs provides a platform for mod- ification through different strategies. This has enabled researchers to anchor and tailor various receptors for specific target analytes, thereby enhancing their sensitivity. QDs can 3. Effect of Surface Chemistry on Sensitivity and Selectivity therefore work as signal transducers and receptors in composite materials. It is expected The versatile surface chemistry of semiconductor QDs provides a platform for mod- that changes in the surface charge or ligand components of the QDs would affect the ification through different strategies. This has enabled researchers to anchor and tailor various receptors for specific target analytes, thereby enhancing their sensitivity. QDs can efficiency of the core electron–hole recombination and consequently the luminescence therefore work as signal transducers and receptors in composite materials. It is expected efficiency. Therefore, a chemical sensing system based on QDs can be developed by us- that changes in the surface charge or ligand components of the QDs would affect the effi- ing fluorescence changes, which are induced by either the direct physical adsorption or ciency of the core electron–hole recombination and consequently the luminescence effi- chelating of ions and small molecules on the surface of the QDs activated by the exchanged ciency. Therefore, a chemical sensing system based on QDs can be developed by using ligand [60,61]. fluorescence changes, which are induced by either the direct physical adsorption or che- Reports have shown that the selectivity and sensitivity of sensors can be enhanced lating of ions and small molecules on the surface of the QDs activated by the exchanged lithr gan ough d [60,61]. surface modification, which can be done by functionalizing the material or conju- Reports have shown that the selectivity and sensitivity of sensors can be enhanced gating it to a material that has an affinity for the pollutants of interest [62]. The effective through surface modification, which can be done by functionalizing the material or con- detection and differentiation of pollutants can be done by conjugating QDs to differ- jugating it to a material that has an affinity for the pollutants of interest [62]. The effective ent molecules such as graphene derivatives, polymers, proteins, enzymes, nucleic acids, detection and differentiation of pollutants can be done by conjugating QDs to different etc. [27]. Peveler et al. studied the multi-channel detection and differentiation of explo- molecules such as graphene derivatives, polymers, proteins, enzymes, nucleic acids, etc. sives [63]. The functionalization of QDs’ surfaces has been reported to affect the recognition [27]. Peveler et al. studied the multi-channel detection and differentiation of explosives properties, which have an effect on the sensitivity and selectivity [63]. Recently, Aswathy [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, Aswathy et al. exploited the quenching efficiency of sulfur-containing amino acids such as L-cysteine et al. exploited the quenching efficiency of sulfur-containing amino acids such as L-cyste- and L-methionine. The modified QDs were found to be selective and more sensitive to- ine and L-methionine. The modified QDs were found to be selective and more sensitive wards picric acid. The energy transfer between the QDs and the TNP is ascribed to the towards picric acid. The energy transfer between the QDs and the TNP is ascribed to the 2+ acid–base reaction. From Figure 4, it was observed that L-cysteine-capped ZnS-Mn QDs 2+ acid–base reaction. From Figure 4, it was observed that L-cysteine-capped ZnS-Mn QDs gave a higher quenching efficiency than those that were L-methionine-capped. However, 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 the cause of this difference was not explained. These results could mean that the quench- ing effic efficiency iency is is dependent dependent on the structure on the structur of the functionality or e of the functionality the surface c or the surface hemistry chemistry of the of the amino acids [64]. amino acids [64]. Figure 4. Effect of various aromatic compounds on the fluorescent intensity of L-cysteine/L- Figure 4. Effect of various aromatic compounds on the fluorescent intensity of L-cysteine/L-methi- 2+ 7 2+ −7 methionine-capped ZnS-Mn QDs (concentration of picric acid is 2 10 M and other aromatics onine-capped ZnS-Mn QDs (concentration of picric acid is 2 × 10 M and other aromatics are 2 × −5 10 M). are 2 10 M). 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 concen- tration of the nitroaromatic substrates on QDs’ surfaces through donor–acceptor complexes Appl. Sci. 2021, 11, x FOR PEER REVIEW 7 of 14 Appl. Sci. 2021, 11, 11580 7 of 14 Surface-modified QDs have been extensively employed as fluorescent probes for the detection of organic pollutants. This detection takes place through two basic photo-phys- ical mechanisms: fluorescence resonance energy transfer (FRET) and quenching. The con- centration of the nitroaromatic substrates on QDs’ surfaces through donor–acceptor com- leads to the quenching of QDs’ luminescence. In 2012, Freeman et al. reported the use of plexes leads to the quenching of QDs’ luminescence. In 2012, Freeman et al. reported the chemically modified CdSe/ZnS to detect nitro-aromatic explosives such as trinitrotoluene use of chemically modified CdSe/ZnS to detect nitro-aromatic explosives such as trinitro- (TNT) and trinitrotriazine (RDX). They observed that the luminescence of the mercapto toluene (TNT) and trinitrotriazine (RDX). They observed that the luminescence of the mer- (MPA)-functionalized CdSe/ZnS could not be quenched by nitro-aromatic compounds, capto (MPA)-functionalized CdSe/ZnS could not be quenched by nitro-aromatic com- and this observation has been attributed to a lack of donor units. In order to optimize the pounds, and this observation has been attributed to a lack of donor units. In order to op- quenching efficiency, MPA-functionalized QDs were covalently linked to different elec- timize the quenching efficiency, MPA-functionalized QDs were covalently linked to dif- tron donors, such as tyramine, dopamine, 5-hydroxydopamine and 6-hydroxydopamine. ferent electron donors, such as tyramine, dopamine, 5-hydroxydopamine and 6-hy- The fluorescence quenching of the QDs modified with these capping ligands followed droxydopamine. The fluorescence quenching of the QDs modified with these capping lig- the order tyramine < dopamine < 5-hydroxydopamine < 6-hydroxydopamine. This or- ands followed the order tyramine < dopamine < 5-hydroxydopamine < 6-hydroxydopa- der demonstrated that the quenching efficiency is dependent on the donating properties mine. This order demonstrated that the quenching efficiency is dependent on the donating of the capping ligands. properties o Another f the capping aspect ligan to consider ds. Another aspect to c when modifying onsider QDs when is modifyin the pH of g QDs is the the medium. Freeman pH of the et al. medemonstrated dium. Freeman that et al. d theemonstrat luminescence ed that the lumine of the electr scence o on-donor f the - electron- donor-modified QDs decreases as the pH of the medium increases [65]. Another study on modified QDs decreases as the pH of the medium increases [65]. Another study on the the detection of PAHs by semiconductor QDs was carried out by Baslak et al. in 2014. In detection of PAHs by semiconductor QDs was carried out by Baslak et al. in 2014. In their their study, different PAHs—2-hyroxy-1-naphthaldehyde (2H–1N), 9,10-phenanthraqui- study, different PAHs—2-hyroxy-1-naphthaldehyde (2H–1N), 9,10-phenanthraquinone none (PQ), 9-anthracenecarboxaldehyde(9-AC) and quinolone (Q)—showed different (PQ), 9-anthracenecarboxaldehyde(9-AC) and quinolone (Q)—showed different quenching quenching effects, and this has been attributed to the molecular structure or functional effects, and this has been attributed to the molecular structure or functional groups of the groups of the PAHs [66]. The interaction of the surface-modified QDs, PAHs and NACs PAHs [66]. The interaction of the surface-modified QDs, PAHs and NACs is schematically is schematically illustrated in Schemes 1 and 2. illustrated in Schemes 1 and 2. Scheme 1. Fluorescence detection of different polycyclic aromatic hydrocarbons by surface-modified QDs [33]. Surface modification is also believed to improve the lifetime of a sensor. Mitra et al. (2016) explored the effect of lifetime on the sensing of organic pollutants such as bisphenol A, 1-napthol, phenol and picric acid (Figure 5) [67]. Apart from the functionalization and conjugation, reaction parameters were said to cause an effect on the sensing of the pollu- tants. Liu et al. (2016) investigated the effect of pH in the reaction between AgInZnS and Cu ion [68]. Furthermore, the sensitivity of the sensors is also dependent on the types of functional groups of the sensors in addition to the functionalization approaches [69,70]. Different results on sensitivity of different sensors towards PAHs and NACs are shown in Table 1. Apart from surface functional groups, the sensitivity of fluorescent probes has been shown to be dependent on the type of synthetic method. Recently, Nsibande et al. Appl. Sci. 2021, 11, x FOR PEER REVIEW 8 of 14 Scheme 1. Fluorescence detection of different polycyclic aromatic hydrocarbons by surface-modified QDs [33]. Scheme 2. Fluorescence detection of different nitroaromatic compounds by surface-modified QDs. Appl. Sci. 2021, 11, 11580 8 of 14 Surface modification is also believed to improve the lifetime of a sensor. Mitra et al. (2016) explored the effect of lifetime on the sensing of organic pollutants such as bisphenol A, 1-napthol, phenol and picric acid (Figure 5) [67]. Apart from the functionalization and prepared GQDs using GO and citric acid via the “top-down” approach and “bottom-up” approach, respectively. The as-prepared materials successfully detected pyrene in an aque- conjugation, reaction parameters were said to cause an effect on the sensing of the pollu- ous medium. The GQDs prepared via the “bottom-up” route gave a better limit of detection tants. Liu et al. (2016) investigated the effect of pH in the reaction between AgInZnS and (LOD) as compared to the result obtained by the GQDs synthesized through the “top-down” Appl. Sci. 2021, 11, x FOR PEER REVIEW 8 of 14 route. Furthermore, the two types of GQDs exhibited different optical properties, and this Cu ion [68]. Furthermore, the sensitivity of the sensors is also dependent on the types of can be observed from their emission spectra (Figure 5) [58]. QD-based sensors can also be functional groups of the sensors in addition to the functionalization approaches [69,70]. modified through doping with a metal or non-metal. Surface doping with a metal has been reported to enhance the electrochemical properties of QD sensors; however, there have been Different results on sensitivity of different sensors towards PAHs and NACs are shown few reports on the doping of fluorescent probes and electrochemical sensors [56,58,71,72]. Scheme 1. Fluorescence detection of different polycyclic aromatic hydrocarbons by surface-modified QDs [33]. in Table 1. Apart from surface functional groups, the sensitivity of fluorescent probes has been shown to be dependent on the type of synthetic method. Recently, Nsibande et al. prepared GQDs using GO and citric acid via the “top-down” approach and “bottom-up” approach, respectively. The as-prepared materials successfully detected pyrene in an aqueous medium. The GQDs prepared via the “bottom-up” route gave a better limit of detection (LOD) as compared to the result obtained by the GQDs synthesized through the “top-down” route. Furthermore, the two types of GQDs exhibited different optical prop- erties, and this can be observed from their emission spectra (Figure 5) [58]. QD-based sen- sors can also be modified through doping with a metal or non-metal. Surface doping with a metal has been reported to enhance the electrochemical properties of QD sensors; how- ever, there have been few reports on the doping of fluorescent probes and electrochemical sensors [56,58,71,72]. Scheme 2. Fluorescence detection of different nitroaromatic compounds by surface-modified QDs. Scheme 2. Fluorescence detection of different nitroaromatic compounds by surface-modified QDs. Surface modification is also believed to improve the lifetime of a sensor. Mitra et al. (2016) explored the effect of lifetime on the sensing of organic pollutants such as bisphenol A, 1-napthol, phenol and picric acid (Figure 5) [67]. Apart from the functionalization and conjugation, reaction parameters were said to cause an effect on the sensing of the pollu- tants. Liu et al. (2016) investigated the effect of pH in the reaction between AgInZnS and Cu ion [68]. Furthermore, the sensitivity of the sensors is also dependent on the types of functional groups of the sensors in addition to the functionalization approaches [69,70]. Different results on sensitivity of different sensors towards PAHs and NACs are shown in Table 1. Apart from surface functional groups, the sensitivity of fluorescent probes has been shown to be dependent on the type of synthetic method. Recently, Nsibande et al. prepared GQDs using GO and citric acid via the “top-down” approach and “bottom-up” approach, respectively. The as-prepared materials successfully detected pyrene in an aqueous medium. The GQDs prepared via the “bottom-up” route gave a better limit of Figure 5. The effect of excitation wavelength on the emission spectra of (A) GO-GQDs and (C) CA-GQDs. (B,D) the effect of excitation wavelength on the maximum PL intensity (PLmax) detection (LOD) as compared to the result obtained by the GQDs synthesized through the in each case. “top-down” route. Furthermore, the two types of GQDs exhibited different optical prop- erties, and this can be observed from their emission spectra (Figure 5) [58]. QD-based sen- sors can also be modified through doping with a metal or non-metal. Surface doping with a metal has been reported to enhance the electrochemical properties of QD sensors; how- ever, there have been few reports on the doping of fluorescent probes and electrochemical sensors [56,58,71,72]. Appl. Sci. 2021, 11, 11580 9 of 14 Table 1. Effect of different materials on the limit of detection of polycyclic aromatic hydrocarbons and nitro-aromatic compounds. Detection LOD Detection Range Semiconductor Sensor Modifier Pollutant/Analyte References 1 1 Technique (mol L ) (mol L ) 9 6 GO-CdSeTeS/ZnS L-cysteine Fluorescence 2.26 10 M 0.1–0.5 10 PAHs [4] GO-CdSeTe/ZnSe/ZnS L-cysteine Fluorescence 0.19 mg/L PAHs [60] 0.1–0.5 10 8 6 CuInS2 BSA Fluorescence 28 nmol/L 5.0 10 –3 10 Nitro-aromatics [61] CdTe L-cysteine [41] Fluorescence 1.1 nM Nitroaromatics [73] 7 5 CdS lysozyme Fluorescence 0.1 5 10 –1.5 10 Nitro-aromatics [48] CdTe(S) polyacrylamide Fluorescence 2.1 nmol/L Nitro-aromatics [74] 0–7.0 10 7 5 CdTe (TGA)/CD Fluorescence 0.085 M 5 10 –7.5 10 PAHs [40] CdSe oleylamine Fluorescence 2.1 10 mol/L Nitro-aromatics [75] 1,4-dihydro-nicotinamide CdSe/ZnS Fluorescence 0.1 nM RDX [76] adenine dinucleotide (NADH) 0.4 M 7 5 CQDs -NH 1.0 10 –1.58 10 [75] 27 nM 8 7 CdSe PAMAM-G4 dendrimer [76] 5.5 10 –5.5 10 7 6 CQDs -NH 2.13 10 0–1.0 10 [77] 8 7 5 GQDs Sulfur 9.3 10 1.0 10 –9.9 10 4. Nanocomposite as Sensors A nanocomposite can be defined as a material that is composed of two or more nanomaterials, with enhanced chemical properties and physical properties. The purpose of a composite is to achieve a synergy effect in the sensing process. Nanocomposites present an alternative approach to overcome the current limitations of individual nanomaterials [78]. 4.1. Mesoporous-Silica-Coated-QD Composites The phrase “mesoporous materials” refers to solids based on either ordered or disor- dered networks with a broad or narrow distribution of pores within the range of 20 to 50 nm, which is good for the incorporation of QDs. These materials have the potential to be used in surface modification for both fluorescence sensing and electrochemical sensing [79,80]. The porous structure of mesoporous silica is known for its large surface area and volume. These properties make it easy for the QDs to be embedded into the mesoporous silica for the formation of a nanocomposite with enhanced properties of QDs. There have been few reports on the development of mesoporous-silica-coated QDs for the detection of pollutants. For instance, a chiral nematic mesoporous-silica-encapsulated CdS film was used for the detection of trace TNT [81]. 4.2. QD–GO Composites/Hybrids Graphene and its derivatives have attracted much attention in different areas of research because of their good physical and chemical properties [82]. Graphene oxide (GO) can be best described as a single-layer planar hexagonal array of carbon atoms to which different functional groups such as carboxylic acid, hydroxyl and epoxy are attached [83]. These carbon-based materials are emerging as the most promising platform for the preparation of nanocomposites for different sensing of NACs and PAHs [84]. Graphene, graphene oxide (GO) and reduced graphene oxide (rGO) can be easily combined with semiconductor QDs to form composites with enhanced sensitivity and selec- tivity towards the detection of organic pollutants [85]. Graphene, GO and their composites have been widely used for wastewater treatment because of their high adsorption capacity. The -electron system of graphene and its derivative make them suitable modifiers of QDs for the fluorescence detection of -electron-rich organic pollutants. Mitra et al. (2016) explored rGO-based sensors for the fluorescence detection of both PAH analytes and NACs. The results of their study showed an enhancement of PL intensity of the sensor by the pollutants (Figure 6) [67]. In 2016 and 2017 Adegoke et al. investigated the selectivity and sensitivity of a QD–GO composite for the detection of PAHs [4,60]. Their study showed that PAHs could be detected by a QD–GO composite through an enhancement mecha- nism. Recently, Liu et al. synthesized CuInS quantum dot (QD) and graphene oxide (GO) nanocomposites as a fluorescent sensor for the detection of kanamycin [82]. Appl. Sci. 2021, 11, x FOR PEER REVIEW 10 of 14 These carbon-based materials are emerging as the most promising platform for the prep- aration of nanocomposites for different sensing of NACs and PAHs [84]. Graphene, graphene oxide (GO) and reduced graphene oxide (rGO) can be easily combined with semiconductor QDs to form composites with enhanced sensitivity and se- lectivity towards the detection of organic pollutants [85]. Graphene, GO and their compo- sites have been widely used for wastewater treatment because of their high adsorption capacity. The π-electron system of graphene and its derivative make them suitable modi- fiers of QDs for the fluorescence detection of π-electron-rich organic pollutants. Mitra et al. (2016) explored rGO-based sensors for the fluorescence detection of both PAH analytes and NACs. The results of their study showed an enhancement of PL intensity of the sensor by the pollutants (Figure 6) [67]. In 2016 and 2017 Adegoke et al. investigated the selectiv- ity and sensitivity of a QD–GO composite for the detection of PAHs [4,60]. Their study showed that PAHs could be detected by a QD–GO composite through an enhancement mechanism. Recently, Liu et al. synthesized CuInS2 quantum dot (QD) and graphene ox- Appl. Sci. 2021, 11, 11580 10 of 14 ide (GO) nanocomposites as a fluorescent sensor for the detection of kanamycin [82]. Figure 6. “Turn-On” fluorescence response of four different organic pollutants; picric acid (a), Figure 6. “Turn-On” fluorescence response of four different organic pollutants; picric acid (a), phenol (b), bisphenol A (c) and 1-napthol (d) toward a RGPD-fl composite probe [67]. phenol (b), bisphenol A (c) and 1-napthol (d) toward a RGPD-fl composite probe [67]. 5. Detection Mechanisms Different approaches based on surface modification with different molecules or ligands 5. Detection Mechanisms are said to cause different detection mechanisms. For instance, mechanisms of fluorescence detection are not only based on quenching but also based on mechanisms such as fluores- Different approaches based on surface modification with different molecules or lig- cence turn-on, spectral shift, lifetime and anisotropy [5]. A study on the determination ands are said to cause different detection mechanisms. For instance, mechanisms of fluo- of 2,4,6-trinitrophenol (TNP) by Liu et al. demonstrated that the quenching mechanism for the detection of electron-deficient NACs occurs through acid–base pairing between rescence detection are not only based on quenching but also based on mechanisms such amino functional groups and the electron-deficient rings [61]. The quenching mechanism as fluorescence turn-on, spectral shift, lifetime and anisotropy [5]. A study on the deter- can occur in two ways: static and collisional quenching. These are differentiated through mination of 2,4,6-trinitrophenol (TNP) by Liu et al. demonstrated that the quenching time-resolved measurements of fluorescence lifetime. In the static quenching process, the fluorescence lifetime of the sensor remains unchanged as the concentration of the quencher mechanism for the detection of electron-deficient NACs occurs through acid–base pairing is increased. In the collisional quenching, the fluorescence lifetime becomes shorter as the between amino functional groups and the electron-deficient rings [61]. The quenching concentration of the quencher increased. The quenching mechanism is normally triggered by the transfer of electrons from electron donors to electron-deficient compounds [73,86]. mechanism can occur in two ways: static and collisional quenching. These are differenti- Theoretically, it means that the fluorescence enhancement, also known as fluorescence ated through time-resolved measurements of fluorescence lifetime. In the static quenching “turn-on” is encountered for the detection of explosive residues or pollutants that are not process, the fluorescence lifetime of the sensor remains unchanged as the concentration of electron-rich [79]. The detection of PAHs also occurs through fluorescence “turn-on” [40]. This type of mechanism is said to be triggered by energy transfer between the sensor and the quencher is increased. In the collisional quenching, the fluorescence lifetime becomes the species. Energy transfer of this nature can be due to different factors, such as hydrogen shorter as the concentration of the quencher increased. The quenching mechanism is nor- bonding and – interaction [58]. mally triggered by the transfer of electrons from electron donors to electron-deficient com- 6. Conclusions pounds [73,86]. Theoretically, it means that the fluorescence enhancement, also known as The continuous discharge of PAHs and NACs into the environment through man- fluorescence “turn-on” is encountered for the detection of explosive residues or pollutants made and natural processes suggests that there should be regular monitoring of these pollutants in water sources. Fluorescence detection has turned out to be an important that are not electron-rich [79]. The detection of PAHs also occurs through fluorescence technique for monitoring PAHs and NACs. Semiconductor nanocrystals have shown better “turn-on” [40]. This type of mechanism is said to be triggered by energy transfer between sensitivity and selectivity due to their versatile surface chemistry. Reports from several studies have shown that binary quantum dots (QDs) have attracted much more attention the sensor and the species. Energy transfer of this nature can be due to different factors, in sensing applications and the detection of PAHs. However, these have been based on the such as hydrogen bonding and π–π interaction [58]. use of heavy-metal-based QDs. Furthermore, the literature has also shown that there are challenges in detecting PAHs without substituents because many modifiers cannot react with their - system. As far as the environment is concerned, we suggest developing fluorescent probes or chemical sensors based on ternary QDs for monitoring PAHs and Appl. Sci. 2021, 11, 11580 11 of 14 nitro-aromatics (explosives). Ternary QDs and carbon-based QDs are emerging as the better alternative for sensing aromatic organic pollutants, due to their low toxicity. Furthermore, different modification approaches have been shown to enhance the sensitivity of the fluorescent probes. This review of different studies has also shown that organic-soluble QDs exhibit better properties for sensing applications. However, aqueous-synthesized QDs will be preferred due to environmental concerns based on the toxicity of organic-soluble ternary QDs. Conjugation of the ternary QDs with a good selection of appropriate ligands or carbon- based material conjugates has emerged as the better way for the improvement of stability and sensitivity. This is due to their excellent optical properties, such as narrow emission peaks, high quantum yield (QY), large stokes shift and tunable emissions from short wavelengths (visible region) to longer wavelengths (near-infrared region). Nevertheless, group I-III-VI ternary quantum dots (QDs) are emerging as alternative fluorescent probes or chemical sensors for the monitoring of nitroaromatics (explosives). However, there is no report on the sensing of PAHs using these fluorescent probes. Furthermore, the proper selection of conjugate may enhance the selectivity and sensitivity of the probes. Although group I-III-VI QDs are potential environmentally friendly sensors, their surface defects are a challenge in FRET-based studies. Nevertheless, different sensing mechanisms derived from surface modification of these materials can improve their efficiency for energy transfer, which is an essential part of sensing organic pollutants. Author Contributions: R.M. and O.S.O. contributed equally and have given approval to the final version of the manuscript. All authors have read and agreed to the published version of the manuscript. Funding: National Research Foundation (N.R.F.) under Competitive Programme for Rated Re- searchers (CPRR), grants no 106060 and 129290, the University of Johannesburg (URC) and the Faculty of Science (FRC). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Acknowledgments: The authors would like to thank the National Research Foundation (N.R.F) under its Competitive Programme for Rated Researchers (CPRR), grants no. 106060 and 129290, the University of Johannesburg (URC) and the Faculty of Science (FRC) for financial support. Conflicts of Interest: The authors declare no conflict of interest. References 1. Ji, Y.; Huang, W.; Lu, X.; Feng, X.; Yang, Z. Theoretical limit of energy consumption for removal of organic contaminants in US EPA Priority Pollutant List by NRTL, UNIQUAC and Wilson models. Fluid Phase Equilibria 2010, 297, 210–214. [CrossRef] 2. Nsibande, S.; Montaseri, H.; Forbes, P. Advances in the application of nanomaterial-based sensors for detection of polycyclic aromatic hydrocarbons in aquatic systems. TrAC Trends Anal. Chem. 2019, 115, 52–69. [CrossRef] 3. Chatterjee, S.; Deb, U.; Datta, S.; Walther, C.; Gupta, D.K. Common explosives (TNT, RDX, HMX) and their fate in the environment: Emphasizing bioremediation. Chemosphere 2017, 184, 438–451. [CrossRef] 4. Adegoke, O.; Montaseri, H.; Nsibande, S.A.; Forbes, P.B. Alloyed quaternary/binary core/shell quantum dot-graphene oxide nanocomposite: Preparation, characterization and application as a fluorescence “switch ON” probe for environmental pollutants. J. Alloys Compd. 2017, 720, 70–78. [CrossRef] 5. Sun, X.; Wang, Y.; Lei, Y. Fluorescence based explosive detection: From mechanisms to sensory materials. Chem. Soc. Rev. 2015, 44, 8019–8061. [CrossRef] [PubMed] 6. Lebedev, A.T. Environmental mass spectrometry. Annu. Rev. Anal. Chem. 2013, 6, 163–189. [CrossRef] 7. Li, H.; Wang, L. Highly Selective Detection of Polycyclic Aromatic Hydrocarbons Using Multifunctional Magnetic–Luminescent Molecularly Imprinted Polymers. ACS Appl. Mater. Interfaces 2013, 5, 10502–10509. [CrossRef] 8. Bapat, G.; Labade, C.; Chaudhari, A.; Zinjarde, S. Silica nanoparticle based techniques for extraction, detection, and degradation of pesticides. Adv. Colloid Interface Sci. 2016, 237, 1–14. [CrossRef] 9. Fan, Y.; Liu, L.; Sun, D.; Lan, H.; Fu, H.; Yang, T.; She, Y.; Ni, C. “Turn-off” fluorescent data array sensor based on double quantum dots coupled with chemometrics for highly sensitive and selective detection of multicomponent pesticides. Anal. Chim. Acta 2016, 916, 84–91. [CrossRef] Appl. Sci. 2021, 11, 11580 12 of 14 10. Zhou, Z.; Lu, J.; Wang, J.; Zou, Y.; Liu, T.; Zhang, Y.; Liu, G.; Tian, Z. Trace detection of polycyclic aromatic hydrocarbons in environmental waters by SERS. Spectrochim. Acta Part A Mol. Biomol. Spectrosc. 2020, 234, 118250. [CrossRef] [PubMed] 11. Lin, D.; Dong, R.; Li, P.; Li, S.; Ge, M.; Zhang, Y.; Yang, L.; Xu, W. A novel SERS selective detection sensor for trace trinitrotoluene based on meisenheimer complex of monoethanolamine molecule. Talanta 2020, 218, 121157. [CrossRef] 12. Jiang, N.; Li, G.; Che, W.; Zhu, D.; Su, Z.; Bryce, M.R. Polyurethane derivatives for highly sensitive and selective fluorescence detection of 2, 4, 6-trinitrophenol (TNP). J. Mater. Chem. C 2018, 6, 11287–11291. [CrossRef] 13. Yi, K.-Y. Application of CdSe quantum dots for the direct detection of TNT. Forensic Sci. Int. 2016, 259, 101–105. [CrossRef] 14. May, B.M.; Parani, S.; Oluwafemi, O.S. Detection of ascorbic acid using green synthesized AgInS quantum dots. Mater. Lett. 2019, 236, 432–435. [CrossRef] 15. Tsolekile, N.; Parani, S.; Matoetoe, M.C.; Songca, S.P.; Oluwafemi, O.S. Evolution of ternary I–III–VI QDs: Synthesis, characteriza- tion and application. Nano-Struct. Nano-Objects 2017, 12, 46–56. [CrossRef] 16. Zikalala, N.Z.; Sundararajan, P.; Tsolekile, N.; Oluwafemi, O.S. Facile green synthesis of ZnInS quantum dots: Temporal evolution of its optical properties and cell viability against normal and cancerous cells. J. Mater. Chem. C 2020, 8, 9329–9336. [CrossRef] 17. Li, J.; Li, P.; Wang, D.; Dong, C. One-pot synthesis of aqueous soluble and organic soluble carbon dots and their multi-functional applications. Talanta 2019, 202, 375–383. [CrossRef] [PubMed] 18. Ncube, S.; Madikizela, L.; Cukrowska, E.; Chimuka, L. Recent advances in the adsorbents for isolation of polycyclic aromatic hydrocarbons (PAHs) from environmental sample solutions. TrAC Trends Anal. Chem. 2018, 99, 101–116. [CrossRef] 19. Ncapayi, V.; Oluwafemi, S.O.; Songca, S.P.; Kodama, T. Optical and cytotoxicity properties of water soluble type II CdTe/CdSe nanoparticles synthesised via a green method. MRS Online Proc. Libr. Arch. 2015, 1748, 69–75. [CrossRef] 20. Ncapayi, V.; Parani, S.; Oluwafemi, O.S. Facile Size Tunable Green Synthesis of Highly Fluorescent Isotropic and Anisotropic CdSe Nanostructures via a Non-phosphine Based Route. J. Clust. Sci. 2017, 28, 2891–2903. [CrossRef] 21. Shikha, P.; Kang, T.S. Facile and green one pot synthesis of zinc sulphide quantum dots employing zinc-based ionic liquids and their photocatalytic activity. New J. Chem. 2017, 41, 7407–7416. 22. Sharma, A.; Sharma, R.; Bhatia, N.; Kumari, A. Review on Synthesis, Characterization and Applications of Silver Sulphide Quantum Dots. J. Mater. Sci. Res. Rev. 2021, 7, 42–58. 23. Yang, L.; Chen, B.; Luo, S.; Li, J.; Liu, R.; Cai, Q. Sensitive detection of polycyclic aromatic hydrocarbons using CdTe quantum dot-modified TiO2 nanotube array through fluorescence resonance energy transfer. Environ. Sci. Technol. 2010, 44, 7884–7889. [CrossRef] 24. Qian, J.; Hua, M.; Wang, C.; Wang, K.; Liu, Q.; Hao, N.; Wang, K. Fabrication of l-cysteine-capped CdTe quantum dots based ratiometric fluorescence nanosensor for onsite visual determination of trace TNT explosive. Anal. Chim. Acta 2016, 946, 80–87. [CrossRef] 25. Sharma, V.; Mehata, M.S. Rapid optical sensor for recognition of explosive 2, 4, 6-TNP traces in water through fluorescent ZnSe quantum dots. Spectrochim. Acta Part A Mol. Biomol. Spectrosc. 2021, 260, 119937. [CrossRef] 26. Varghese, R.J.; Parani, S.; Adeyemi, O.O.; Remya, V.; Maluleke, R.; Thomas, S.; Oluwafemi, O.S. Green Synthesis of Sodium Alginate Capped-CuInS 2 Quantum Dots with Improved Fluorescence Properties. J. Fluoresc. 2020, 30, 1331–1335. [CrossRef] 27. Aladesuyi, O.A.; Oluwafemi, O.S. Synthesis strategies and application of ternary quantum dots—in cancer therapy. Nano-Struct. Nano-Objects 2020, 24, 100568. [CrossRef] 28. Zhang, J.; Chen, Q.; Zhang, W.; Mei, S.; He, L.; Zhu, J.; Chen, G.; Guo, R. Microwave-assisted aqueous synthesis of transition metal ions doped ZnSe/ZnS core/shell quantum dots with tunable white-light emission. Appl. Surf. Sci. 2015, 351, 655–661. [CrossRef] 29. Patel, J.; Jain, B.; Singh, A.K.; Susan, M.A.B.H.; Jean-Paul, L. Mn-Doped ZnS Quantum dots–An EffectiveNanoscale Sensor. Microchem. J. 2020, 105, 104755. [CrossRef] 30. Mubiayi, K.P.; Neto, D.M.G.; Morais, A.; Nogueira, H.P.; de Almeida Santos, T.E.; Mazon, T.; Moloto, N.; Moloto, M.J.; Freitas, J.N. Microwave assisted synthesis of CuInGaSe quantum dots and spray deposition of their composites with graphene oxide derivatives. Mater. Chem. Phys. 2020, 242, 122449. [CrossRef] 31. Kays, J.C.; Saeboe, A.M.; Toufanian, R.; Kurant, D.E.; Dennis, A.M. Shell-free copper indium sulfide quantum dots induce toxicity in vitro and in vivo. Nano Lett. 2020, 20, 1980–1991. [CrossRef] 32. Tsolekile, N.; Parani, S.; Vuyelwa, N.; Maluleke, R.; Matoetoe, M.; Songca, S.; Oluwafemi, O.S. Synthesis, structural and fluorescence optimization of ternary Cu–In–S quantum dots passivated with ZnS. J. Lumin. 2020, 227, 117541. [CrossRef] 33. Maluleke, R.; Parani, S.; Oluwafemi, O.S. Preparation of Graphene oxide-CuInS /ZnS Quantum dots Nanocomposite as “Turn-On” Fluorescent Probe for the Detection of Polycyclic Aromatic Hydrocarbons in Aqueous Medium. J. Fluoresc. 2021, 31, 1297–1302. [CrossRef] 34. Jelinek, R. Carbon Quantum Dots; Springer International Publishing: Cham, Switzerland, 2017; pp. 29–46. 35. Devi, P.; Saini, S.; Kim, K.-H. The advanced role of carbon quantum dots in nanomedical applications. Biosens. Bioelectron. 2019, 141, 111158. [CrossRef] [PubMed] 36. Fan, H.; Zhang, M.; Bhandari, B.; Yang, C.-H. Food waste as a carbon source in carbon quantum dots technology and their applications in food safety detection. Trends Food Sci. Technol. 2020, 95, 86–96. [CrossRef] Appl. Sci. 2021, 11, 11580 13 of 14 37. Boruah, A.; Saikia, M.; Das, T.; Goswamee, R.L.; Saikia, B.K. Blue-emitting fluorescent carbon quantum dots from waste biomass sources and their application in fluoride ion detection in water. J. Photochem. Photobiol. B Biol. 2020, 209, 111940. [CrossRef] [PubMed] 38. Zhang, L.; Wang, Y.; Liu, W.; Ni, Y.; Hou, Q. Corncob residues as carbon quantum dots sources and their application in detection of metal ions. Ind. Crops Prod. 2019, 133, 18–25. [CrossRef] 39. Pu, Z.-F.; Wen, Q.-L.; Yang, Y.-J.; Cui, X.-M.; Ling, J.; Liu, P.; Cao, Q.-E. Fluorescent carbon quantum dots synthesized using 3+ phenylalanine and citric acid for selective detection of Fe ions. Spectrochim. Acta Part A Mol. Biomol. Spectrosc. 2020, 229, 117944. [CrossRef] 40. Qu, F.; Li, H. Selective molecular recognition of polycyclic aromatic hydrocarbons using CdTe quantum dots with cyclodextrin as supramolecular nano-sensitizers in water. Sens. Actuators B Chem. 2009, 135, 499–505. [CrossRef] 41. Chen, Y.; Chen, Z.; He, Y.; Lin, H.; Sheng, P.; Liu, C.; Luo, S.; Cai, Q. L-cysteine-capped CdTe QD-based sensor for simple and selective detection oftrinitrotoluene. Nanotechnology 2010, 21, 125502. [CrossRef] 42. Hu, Y.; Gao, Z. Sewage sludge in microwave oven: A sustainable synthetic approach toward carbon dots for fluorescent sensing of para-Nitrophenol. J. Hazard. Mater. 2020, 382, 121048. [CrossRef] 43. Cheng, F.; An, X.; Zheng, C.; Cao, S. Green synthesis of fluorescent hydrophobic carbon quantum dots and their use for 2, 4, 6-trinitrophenol detection. RSC Adv. 2015, 5, 93360–93363. [CrossRef] 44. Haque, E.; Kim, J.; Malgras, V.; Reddy, K.R.; Ward, A.C.; You, J.; Bando, Y.; Hossain, M.S.A.; Yamauchi, Y. Recent advances in graphene quantum dots: Synthesis, properties, and applications. Small Methods 2018, 2, 1800050. [CrossRef] 45. Ye, R.; Xiang, C.; Lin, J.; Peng, Z.; Huang, K.; Yan, Z.; Cook, N.P.; Samuel, E.L.; Hwang, C.-C.; Ruan, G. Coal as an abundant source of graphene quantum dots. Nat. Commun. 2013, 4, 2943. [CrossRef] [PubMed] 46. Zhao, Y.; Wu, X.; Sun, S.; Ma, L.; Zhang, L.; Lin, H. A facile and high-efficient approach to yellow emissive graphene quantum dots from graphene oxide. Carbon 2017, 124, 342–347. [CrossRef] 47. Wang, Z.; Yu, J.; Zhang, X.; Li, N.; Liu, B.; Li, Y.; Wang, Y.; Wang, W.; Li, Y.; Zhang, L. Large-scale and controllable synthesis of graphene quantum dots from rice husk biomass: A comprehensive utilization strategy. ACS Appl. Mater. Interfaces 2016, 8, 1434–1439. [CrossRef] [PubMed] 48. Na, W.; Liu, X.; Pang, S.; Su, X. Highly sensitive detection of 2, 4, 6-trinitrophenol (TNP) based on lysozyme capped CdS quantum dots. RSC Adv. 2015, 5, 51428–51434. [CrossRef] 49. Roy, P.; Periasamy, A.P.; Chuang, C.; Liou, Y.-R.; Chen, Y.-F.; Joly, J.; Liang, C.-T.; Chang, H.-T. Plant leaf-derived graphene quantum dots and applications for white LEDs. New J. Chem. 2014, 38, 4946–4951. [CrossRef] 50. Suryawanshi, A.; Biswal, M.; Mhamane, D.; Gokhale, R.; Patil, S.; Guin, D.; Ogale, S. Large scale synthesis of graphene quantum dots (GQDs) from waste biomass and their use as an efficient and selective photoluminescence on–off–on probe for Ag ions. Nanoscale 2014, 6, 11664–11670. [CrossRef] 51. Nirala, N.R.; Khandelwal, G.; Kumar, B.; Prakash, R.; Kumar, V. One step electro-oxidative preparation of graphene quantum dots from wood charcoal as a peroxidase mimetic. Talanta 2017, 173, 36–43. [CrossRef] [PubMed] 52. Kalita, H.; Mohapatra, J.; Pradhan, L.; Mitra, A.; Bahadur, D.; Aslam, M. Efficient synthesis of rice based graphene quantum dots and their fluorescent properties. RSC Adv. 2016, 6, 23518–23524. [CrossRef] 53. Wang, L.; Li, W.; Wu, B.; Li, Z.; Wang, S.; Liu, Y.; Pan, D.; Wu, M. Facile synthesis of fluorescent graphene quantum dots from coffee grounds for bioimaging and sensing. Chem. Eng. J. 2016, 300, 75–82. [CrossRef] 54. Dervishi, E.; Ji, Z.; Htoon, H.; Sykora, M.; Doorn, S.K. Raman spectroscopy of bottom-up synthesized graphene quantum dots: Size and structure dependence. Nanoscale 2019, 11, 16571–16581. [CrossRef] [PubMed] 55. Abbas, A.; Mariana, L.T.; Phan, A.N. Biomass-waste derived graphene quantum dots and their applications. Carbon 2018, 140, 77–99. [CrossRef] 56. Kaur, M.; Mehta, S.K.; Kansal, S.K. Nitrogen doped graphene quantum dots: Efficient fluorescent chemosensor for the selective and sensitive detection of 2, 4, 6-trinitrophenol. Sens. Actuators B Chem. 2017, 245, 938–945. [CrossRef] 57. Chen, S.; Song, Y.; Shi, F.; Liu, Y.; Ma, Q. Sensitive detection of picric acid based on creatinine-capped solid film assembled by nitrogen-doped graphene quantum dots and chitosan. Sens. Actuators B Chem. 2016, 231, 634–640. [CrossRef] 58. Nsibande, S.; Forbes, P. Development of a turn-on graphene quantum dot-based fluorescent probe for sensing of pyrene in water. RSC Adv. 2020, 10, 12119–12128. 59. Kaur, M.; Mehta, S.K.; Kansal, S.K. A fluorescent probe based on nitrogen doped graphene quantum dots for turn off sensing of explosive and detrimental water pollutant, TNP in aqueous medium. Spectrochim. Acta Part A Mol. Biomol. Spectrosc. 2017, 180, 37–43. [CrossRef] [PubMed] 60. Adegoke, O.; Forbes, P.B. l-Cysteine-capped core/shell/shell quantum dot–graphene oxide nanocomposite fluorescence probe for polycyclic aromatic hydrocarbon detection. Talanta 2016, 146, 780–788. [CrossRef] [PubMed] 61. Liu, S.; Shi, F.; Chen, L.; Su, X. Bovine serum albumin coated CuInS2 quantum dots as a near-infrared fluorescence probe for 2, 4, 6-trinitrophenol detection. Talanta 2013, 116, 870–875. [CrossRef] [PubMed] 62. Ting, S.L.; Ee, S.J.; Ananthanarayanan, A.; Leong, K.C.; Chen, P. Graphene quantum dots functionalized gold nanoparticles for sensitive electrochemical detection of heavy metal ions. Electrochim. Acta 2015, 172, 7–11. [CrossRef] 63. Peveler, W.J.; Roldan, A.; Hollingsworth, N.; Porter, M.J.; Parkin, I.P. Multichannel detection and differentiation of explosives with a quantum dot array. ACS Nano 2016, 10, 1139–1146. [CrossRef] [PubMed] Appl. Sci. 2021, 11, 11580 14 of 14 64. Aswathy, S.; Vaisakh, S.; George, S. Thio amino acids-modified manganese doped ZnS quantum dot probes for the photolumines- cent sensing of hazardous nitro aromatics. Mater. Today Proc. 2020, 41, 628–637. [CrossRef] 65. Freeman, R.; Finder, T.; Bahshi, L.; Gill, R.; Willner, I. Functionalized CdSe/ZnS QDs for the detection of nitroaromatic or RDX explosives. Adv. Mater. 2012, 24, 6416–6421. [CrossRef] 66. Baslak, C.; Kus, M.; Cengeloglu, Y.; Ersoz, M. A comparative study on fluorescence quenching of CdTe nanocrystals with a serial of polycyclic aromatic hydrocarbons. J. Lumin. 2014, 153, 177–181. [CrossRef] 67. Mitra, R.; Saha, A. Reduced Graphene Oxide Based “Turn-On” Fluorescence Sensor for Highly Reproducible and Sensitive Detection of Small Organic Pollutants. ACS Sustain. Chem. Eng. 2016, 5, 604–615. [CrossRef] 68. Liu, Y.; Deng, M.; Tang, X.; Zhu, T.; Zang, Z.; Zeng, X.; Han, S. Luminescent AIZS-GO nanocomposites as fluorescent probe for detecting copper (II) ion. Sens. Actuators B Chem. 2016, 233, 25–30. [CrossRef] 69. Tyrakowski, C.M.; Snee, P.T. A primer on the synthesis, water-solubilization, and functionalization of quantum dots, their use as biological sensing agents, and present status. Phys. Chem. Chem. Phys. 2014, 16, 837–855. [CrossRef] [PubMed] 70. Karakoti, A.S.; Shukla, R.; Shanker, R.; Singh, S. Surface functionalization of quantum dots for biological applications. Adv. Colloid Interface Sci. 2015, 215, 28–45. [CrossRef] [PubMed] 71. Jadoon, T.; Mahmood, T.; Ayub, K. Silver-graphene quantum dots based electrochemical sensor for trinitrotoluene and p-nitrophenol. J. Mol. Liq. 2020, 306, 112878. [CrossRef] 72. Cayuela, A.; Carrillo-Carrión, C.; Soriano, M.L.; Parak, W.J.; Valcárcel, M. One-step synthesis and characterization of N-doped carbon nanodots for sensing in organic media. Anal. Chem. 2016, 88, 3178–3185. [CrossRef] [PubMed] 73. Peng, Y.; Dong, W.; Wan, L.; Quan, X. Determination of folic acid via its quenching effect on the fluorescence of MoS 2 quantum dots. Microchim. Acta 2019, 186, 605. [CrossRef] 74. Hu, F.; Huang, Y.; Zhang, G.; Zhao, R.; Zhang, D. A highly selective fluorescence turn-on detection of hydrogen peroxide and d-glucose based on the aggregation/de-aggregation of a modified tetraphenylethylene. Tetrahedron Lett. 2014, 55, 1471–1474. [CrossRef] 75. Fan, Y.Z.; Tang, Q.; Liu, S.G.; Yang, Y.Z.; Ju, Y.J.; Xiao, N.; Luo, H.Q.; Li, N.B. A smartphone-integrated dual-mode nanosensor based on novel green-fluorescent carbon quantum dots for rapid and highly selective detection of 2, 4, 6-trinitrophenol and pH. Appl. Surf. Sci. 2019, 492, 550–557. [CrossRef] 76. Algarra, M.; Campos, B.; Miranda, M.; da Silva, J.C.E. CdSe quantum dots capped PAMAM dendrimer nanocomposites for sensing nitroaromatic compounds. Talanta 2011, 83, 1335–1340. [CrossRef] 77. Tian, X.; Peng, H.; Li, Y.; Yang, C.; Zhou, Z.; Wang, Y. Highly sensitive and selective paper sensor based on carbon quantum dots for visual detection of TNT residues in groundwater. Sens. Actuators B Chem. 2017, 243, 1002–1009. [CrossRef] 78. Omanovic-Mikli ´ canin, ˇ E.; Badnjevic, ´ A.; Kazlagic, ´ A.; Hajlovac, M. Nanocomposites: A brief review. Health Technol. 2020, 10, 51–59. [CrossRef] 79. Walcarius, A. Mesoporous materials and electrochemistry. Chem. Soc. Rev. 2013, 42, 4098–4140. [CrossRef] [PubMed] 80. Wang, M.; Xia, Y.; Qiu, J.; Ren, X. Carbon quantum dots embedded mesoporous silica for rapid fluorescent detection of acidic gas. Spectrochim. Acta Part A Mol. Biomol. Spectrosc. 2019, 206, 170–176. [CrossRef] [PubMed] 81. Nguyen, T.D.; Hamad, W.Y.; MacLachlan, M.J. CdS quantum dots encapsulated in chiral nematic mesoporous silica: New iridescent and luminescent materials. Adv. Funct. Mater. 2014, 24, 777–783. [CrossRef] 82. Liu, Z.; Tian, C.; Lu, L.; Su, X. A novel aptamer-mediated CuInS 2 quantum dots@ graphene oxide nanocomposites-based fluorescence “turn off–on” nanosensor for highly sensitive and selective detection of kanamycin. RSC Adv. 2016, 6, 10205–10214. [CrossRef] 83. Li, Z.; Young, R.J.; Wang, R.; Yang, F.; Hao, L.; Jiao, W.; Liu, W. The role of functional groups on graphene oxide in epoxy nanocomposites. Polymer 2013, 54, 5821–5829. [CrossRef] 84. Pan, Z.; He, L.; Qiu, L.; Korayem, A.H.; Li, G.; Zhu, J.W.; Collins, F.; Li, D.; Duan, W.H.; Wang, M.C. Mechanical properties and microstructure of a graphene oxide–cement composite. Cem. Concr. Compos. 2015, 58, 140–147. [CrossRef] 85. Huang, Y.-L.; Walker, A.S.; Miller, E.W. A photostable silicon rhodamine platform for optical voltage sensing. J. Am. Chem. Soc. 2015, 137, 10767–10776. [CrossRef] 86. Lakowicz, J. Mechanisms and dynamics of fluorescence quenching. Princ. Fluoresc. Spectrosc. 2006, 3, 331–351.
http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png
Applied Sciences
Multidisciplinary Digital Publishing Institute
http://www.deepdyve.com/lp/multidisciplinary-digital-publishing-institute/synthetic-approaches-modification-strategies-and-the-application-of-0ZRNdB9yk4