Fluorescent sulfur quantum dots for environmental monitoring

Kawan F. Kayani; Omer B. A. Shatery; Sewara J. Mohammed; Shujahadeen B. Aziz; Nian N. Mohammad; Gaylany H. Abdullah; Muhammad S. Mustafa; Dana A. Kader; Wrya O. Karim; Jamal Hassan

doi:10.1515/ntrev-2024-0138

Article Open Access

Fluorescent sulfur quantum dots for environmental monitoring

Kawan F. Kayani , Omer B. A. Shatery , Sewara J. Mohammed , Shujahadeen B. Aziz , Nian N. Mohammad , Gaylany H. Abdullah , Muhammad S. Mustafa , Dana A. Kader , Wrya O. Karim and Jamal Hassan

Published/Copyright: March 5, 2025

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From the journal Nanotechnology Reviews Volume 14 Issue 1

Abstract

The importance of environmental monitoring is on the rise, driven by the increased pressure on the natural environment during the age of urbanization and industrialization. To address this demand, it is necessary to have fast and dependable probes for real-time monitoring with precision and sensitivity. Analytical probes utilizing sulfur nanoparticles offer a modern alternative, exhibiting the ability to identify a range of environmental analytes. The discovery of zero-dimensional quantum dots, such as sulfur quantum dots (SQDs), with unique properties, including optical characteristics, high hydrophilicity, low toxicity, and cost-effectiveness, has positioned SQDs as advancing luminescent nanomaterials. SQDs hold great potential for fluorescence sensing, making them promising candidates for environmental monitoring. This article reviews recent studies on the synthesis of SQDs using various methods and highlights their applications as sensing materials for detecting heavy metal ions and other hazardous molecules. The article provides valuable insights into the production of high-quality SQDs tailored for environmental applications, offering guidance to researchers aiming to enhance sensing technologies for environmental monitoring and contamination detection.

Graphical abstract

Keywords: sulfur quantum dots; fluorescence sensing environmental monitoring; heavy metals; cytotoxicity

1 Introduction

Environmental pollution poses a significant global concern, impacting the safety of plants, animals, and humans alike [1,2,3,4]. Experts worldwide have increasingly focused on environmental challenges, particularly those associated with harmful substances in water and air. Human activities such as resource extraction, daily production, and the use of anti-inflammatory drugs have led to the improper disposal of chemical waste, contaminating lakes, rivers, oceans, and groundwater [5,6,7]. The aquatic environment has been found to harbor various contaminants, including surfactants, textile dyes, heavy metals, insecticides, and pesticides [8,9,10,11,12,13,14,15].

These pollutants can be broadly categorized as organic or inorganic based on their chemical composition. Inorganic contaminants typically encompass radioactive waste, halides, and heavy metals, with lead being a prominent contributor in terms of both frequency and elevated concentrations in contaminated waterways [16,17,18,19,20]. Fortunately, a potential solution to these environmental issues may lie in a recently developed substance.

Over the last decade, there has been significant attention directed toward sulfur nanoparticles, driven by their remarkable electrochemical capabilities [21,22], favorable photoluminescence [23], low toxicity [24], distinctive antibacterial qualities [25], and excellent biocompatibility [26]. The sulfur nanomaterial family, which includes sulfur cores and various surface functional groups such as sulfate, sulfite, and other passivators [27], has recently expanded to include sulfur quantum dots (SQDs). Unlike other sulfur-based nanomaterials, SQDs are distinguished by their excellent solubility and the versatility of their surface groups, which enable reliable and tunable optical properties [28,29,30,31,32,33]. This makes SQDs highly promising for a wide range of optical and electronic applications. In contrast to conventional semiconductor quantum dots, which have garnered interest across diverse research fields for their physical and chemical properties, SQDs offer a promising alternative [34,35,36,37,38,39]. Traditional quantum dots often incorporate heavy metal components, such as lead [40], mercury [41], and cadmium [42], known for their harmful effects on biological systems and the environment, even at low concentrations [43]. In response, researchers have been actively exploring heavy metal-free quantum dots with acceptable biocompatibility, low or non-toxicity, and chemical inertness over the past decade [44]. Carbon dots (CDs) [45,46,47,48,49,50,51,52], graphene quantum dots [53], silicon quantum dots [54], silver quantum dots [55], and polymer quantum dost [56].

Despite being relatively recent entrants, SQDs have demonstrated considerable potential for analytical sensing applications [57,58,59]. They have found applications in bio-imaging and sensing, thanks to their unique qualities [60,61]. Researchers are actively involved in the rapid production of SQDs, emphasizing functional design to modify their surfaces according to the specific requirements of particular analytes [62,63]. Notably, the modification of SQDs enables the attachment and detection of various specialized analytes on their surface through hydrogen bonds and electrostatic interactions, serving as platforms for specific sensing applications [64,65,66].

Two approaches can be used to synthesize SQDs: (i) the top-down method and (ii) the bottom-up technique. Using the top-down method, larger sulfur structures can be broken down through acid treatment [67], laser ablation [68], ultrasonication [69], and electrolysis [70]. However, the top-down approach has limitations, including the need for expensive sulfur supplies, high temperatures, toxic organic solvents, and extended reaction times [71]. On the other hand, the bottom-up strategy describes how smaller sulfur molecules are transformed into the appropriate size of SQDs. This technique begins with the dissolution of sulfur precursors, which are then synthesized into SQDs through hydrothermal [72], solvothermal [73], microwave [74], and pyrolysis methods [75]. Among all, the fluorescent SQDs and their applications have advanced significantly, and new opportunities have arisen for the creation of rapid, sensitive, and selective methods for detecting inorganic pollutants, particularly in water bodies.

Unlike many existing review articles that solely focus on the synthesis of SQDs, this article broadens its scope by introducing the applications of SQDs in environmental monitoring through FL sensing. It explores their potential applications in water remediation and discusses the challenges and opportunities associated with SQD synthesis. The work presents the design of environmentally friendly single- and dual-emission FL sensing methods that remain stable in aqueous environments. The objectives of these methods include achieving single-target detection, simultaneous detection of multiple targets, and enabling rapid, highly sensitive, and selective analysis for determining various typical inorganic pollutants in water environments [76,77,78]. In addition, this review unveils fluorescent SQDs and investigates the relationship between the raw materials used in their preparation, structure, and surface chemistry [72,73]. It seeks to examine their analytical performance in detecting environmental pollutants in water samples, especially heavy metal ions [79,80,81].

2 Synthesis methods

Several techniques have been developed to produce SQDs for detection purposes. Broadly, there are two primary methods for SQD synthesis, as depicted in Figure 1. The first method, referred to as the top-down approach, involves the fabrication of nanomaterials by disintegrating bulk materials using physical or chemical means. Conversely, the bottom-up approach involves the assembly of atoms or molecules to construct the desired nanomaterial structure [82,83]. The preparation of SQDs involves employing both bottom-up and top-down strategies, which encompass methods like hydrothermal, microwave-assisted, ultra-sonication, chemical oxidation, and self-assembly [23,60,84,85].

Figure 1

Schematic diagram depicting the synthesis of SQDs, incorporating both top-down and bottom-up approaches.

Hydrothermal synthesis is a widely used method for preparing nanoparticles. It involves the reaction of precursors under high-temperature and high-pressure conditions in a water-based solution [86]. This method is known for its simplicity, scalability, and ability to produce SQDs with excellent optical properties [87]. The rapid synthesis of SQDs was achieved through a one-step hydrothermal method, utilizing sulfur, sodium hydroxide, and polyethylene glycol, with the latter serving as the surface modifier. The prepared SQDs exhibited excellent performance characteristics, including monodispersity, water solubility, and a relatively high quantum yield (QY) of up to 4.02% [72].

The microwave-assisted method is regarded as a simple, safe, and affordable approach with the added advantage of a rapid reaction time [88]. Ma et al. synthesized SQDs using a microwave-assisted method. The precursor-containing solution was placed in a microwave oven and reacted at 200 W for 1.5 h. The resulting SQDs exhibited the highest fluorescence emission at 470 nm, with a size of 2 nm for the dots. These synthesized SQDs, in the presence of Cr(iv), were applied for the detection of alkaline phosphatase (ALP) in serum samples [89]. In this study, the synthesis of SQDs was accomplished using the bottom-up method, utilizing traditional chemistry techniques employing sodium thiosulfate (with sulfur in the S²⁺ state). This method produced elemental sulfur in situ via a reaction with an acidic solution. Subsequent to sulfur etching with NaOH, under the protection of Polyethylene glycol 400 (PEG-400), it transformed fluorescent SQDs. The generated SQDs demonstrated a QY of 2.5%, showcased photostability when exposed to UV light, exhibited remarkable dispersibility in aqueous mediums, and retained stability even over extended periods spanning several weeks. These SQDs were effectively utilized as probes for the sensing of multiple metal ions [30].

In 2018, Zhang et al. [90] proposed the “assemble-fission” method to synthesize nanostructured polymers and ultrafine sulfur particles at the quantum scale derived from powdered sulfur, employing PEG-400 as a passivation agent. Under alkaline conditions (pH > 7), in an alkaline environment (pH > 7), bulk sulfur powder initially dissolves, resulting in the formation of sodium polysulfide particles within 30 h. Subsequently, assembling and fission processes compete, with assembling predominating between 54 and 72 h, leading to a monodisperse state of sodium polysulfide particles. After 72 h, the fission effect becomes more pronounced, yielding clear and defined spherical particles. Additionally, the addition of nitric acid proves beneficial, aiding in achieving monodisperse states by etching the surface of sulfur particles and expediting the fission step [91]. Electrochemical exfoliation is another technique for preparing SQDs. In a recent study, Sun’s group employed a two-electrode system to synthesize SQDs and SQD-graphene nanohybrids. They used Na₂SO₄ and Na₂S dispersed in deionized water as the electrolyte for deposition. The resultant material functions as a metal-free electrocatalyst for ambient N₂-to-NH₃ conversion, demonstrating excellent selectivity [92].

The synthesis process involves forming larger aggregates, which then break apart over time. Initially, bulk sulfur converts into inhomogeneous polysulfide aggregates, which subsequently undergo fission to form individual SQD particles. A dynamic equilibrium between assembly and fission determines the morphology of the SQDs. Surface etching of polysulfides introduces functional groups that enable tunable emission. Lower temperatures (e.g., 70°C) extend the synthesis time, while ultrasonication or hydrothermal methods accelerate the process. Using sodium thiosulfate or conducting reactions in an O₂ atmosphere are also effective strategies [31].

To gain a deeper understanding of SQD synthesis, various factors such as specific reaction precursors, synthesis methods, reaction times, and QY from selected literature are summarized in Table 1.

Table 1

Comparison of some synthesis parameters of prepared SQDs from selected literature

Precursors	Synthetic method	Temperature	Reaction time (min and h)	QY (%)	Ref.
Sublimed sulfur and ethylenediamine	Top down	Room temperature	10 min	23.6	[93]
Sublimed sulfur and PEG-400	Top down	70°C	72 h	6.30	[94]
Sublimed sulfur and PEG-400	Top down	70°C	74 h	0.67	[63]
Sublimed sulfur and PEG-400	Top down	95°C	24 h	4.5	[95]
Sublimed sulfur and PEG-400	Top down	70°C	1 h	58.6	[39]
Sublimed sulfur and PEG-400	Top down	70°C	5–125 h	23	[69]
Sublimed sulfur and PEG-400	Top down	70°C	72 h	31.7	[96]
Sublimed sulfur and PEG-400	Top down	90°C	36 h	20.8	[97]

3 Characterization of SQDs

It is essential to characterize SQDs comprehensively to gain insights into various properties, including size, stability, morphology, surface composition, purity, and optical characteristics. A thorough analysis of gathered data is crucial for predicting mechanisms of action, deepening comprehension of parameters influencing behavior, and facilitating synthesis optimization. SEM is a conventional technique for evaluating the morphology of quantum dots [98]. Additionally, advanced visualization methods, such as transition electron microscopy (TEM) [99], high-resolution TEM, and field emission scanning electron microscopy [100], are employed [101]. Beyond these methods, TEM is utilized to ascertain size distribution, assess the homogeneity of the product, and quantify the extent of aggregation of SQDs [36]. Energy-dispersive X-ray spectroscopy and Fourier transform infrared spectroscopy (FTIR) are used to characterize the elemental constituents and surface functional groups, respectively. These techniques also help in examining the purity level of the samples [102,103]. X-ray diffraction (XRD) methods offer further understanding of the lattice configuration, phases, purity, and crystalline nature of the SQDs. Raman spectroscopy is employed to identify structural components, molecular bond vibrations, or lattices, shedding light on the mechanisms involved in sulfur dot formation [36,91].

SQDs are known for their remarkable optical properties, which require the application of UV–Visible and photoluminescence spectroscopy to capture unique absorption and emission spectra [39]. Fluorescence (FL) lifetimes are measured using techniques such as FL spectroscopy and time correlated single photon counting [104,105]. X-ray photoelectron spectroscopy provides essential insights into the oxidation states of surface elements in SQDs [91]. This array of techniques plays a crucial role in providing a comprehensive understanding of SQDs and customizing them for specific applications based on their properties. Figure 2 shows a classification of the mentioned methods.

Figure 2

Illustration of the characterization of SQDs based on various techniques.

Li et al. [106] characterized the size and morphology of their synthesized SQDs using high-resolution transmission electron microscopy. The results, shown in Figure 3a, reveal the nanoscale dimensions and uniform morphology of the SQDs, demonstrating the precision of their synthesis process. The average diameter of the quantum dots observed in the field of view is 29.24 nm. The structural characteristics of SQDs were further analyzed using FTIR spectroscopy (Figure 3b). The peak at 3,471 cm⁻¹ corresponds to –OH groups, while the band centered at 1,650 cm⁻¹ is associated with C═O groups. An asymmetric stretching vibration absorption peak of –CH is present at 2,884 cm⁻¹. The bending vibration of C–H in SQDs appears at 1,450 cm⁻¹, and the peaks at 1,110 and 925 cm⁻¹ correspond to C–O–H or C–O–C stretching vibrations, respectively. These characteristic peaks align well with those of PEG-400. Additionally, sulfur-related absorption peaks in SQDs are observed at 663 and 570 cm⁻¹, corresponding to the stretching vibrations of S–O and S–S, respectively. Comparing the FTIR spectra of SQDs and PEG-400 reveals no new absorption peaks in the SQD spectra, indicating that the infrared absorption peaks of SQDs and PEG-400 are identical. This suggests that PEG is uniformly attached to the surface of SQDs through physical interaction during synthesis, with no chemical interaction between them [107]. Ma et al. [108] synthesized l-cysteine-protected SQDs, and their structural properties were analyzed using powder XRD, as shown in Figure 3c. The sharp diffraction peaks indicate that their SQDs exhibit good crystalline quality. The observed peak positions are consistent with the orthorhombic S8 phase, confirming the crystallinity and phase composition of the SQDs.

Figure 3

(a) TEM image [106], Copyright 2023, Elsevier; (b) FTIR spectra [107], Copyright 2022, Elsevier; (c) XRD spectrum [108], Copyright 2023, Elsevier; (d) UV−vis absorption, excitation, and emission spectrum of SQDs [109] Copyright 2023, American Chemical Society.

Jiang et al. [109] synthesized β-cyclodextrin-coated SQDs. The UV−vis absorption spectra of the material are displayed in Figure 3d. At high concentrations, the absorption spectrum shows a band at 212 nm, which likely originates from the n → σ* transition of heteroatoms (S, O) on the surface of the synthesized quantum dots. The absorption peaks at 340 and 360 nm are attributed to the direct bandgap transition of zerovalent sulfur and the presence of polysulfide ions ( S x 2 − ) , respectively. Additionally, the excitation spectrum with emission monitored at 420 nm is shown in Figure 3d, with the most intense emission occurring at an excitation wavelength of 312 nm.

Zhang et al. [110] utilized Raman spectroscopy to analyze their SQDs, revealing three prominent bands at 560, 800, and 1,100 cm⁻¹. The peak at 800 cm⁻¹ was attributed to C–O stretching vibrations, while the peak at 1,100 cm⁻¹ was associated with S–O vibrations. Additionally, the broad peaks observed between 520 and 650 cm⁻¹ were due to short-chain polysulfides. X-ray photoelectron spectroscopy analysis reveals the presence of various sulfur-related functional groups along with PEG on the SQD surface. Lian and colleagues identified five peaks in the high-resolution S2p spectrum at 160.8, 161.9, 162.8, 165.1, and 165.5 eV, which they attributed to atomic sulfur. Peaks corresponding to oxidized sulfur species were observed above 166 eV, consistent with previous studies. These findings indicate that the SQDs are primarily composed of atomic sulfur and oxidized sulfur [72].

4 Application of SQDs for environmental monitoring

Environmental monitoring entails the measurement of specific physical, chemical, and biological factors in the environment over a duration [111–115]. The objective is to evaluate the quality of the environment in a designated area. Several studies have showcased the effectiveness of utilizing SQDs for measuring certain environmental variables through FL sensing. Although these findings have been presented individually, when considered together, they unequivocally illustrate the competence and appropriateness of SQDs in the realm of environmental monitoring. The review below focuses on the integration of SQDs in fluorescent sensors for environmental monitoring, encompassing the detection of metal ions, phenol, and tetracycline (Figure 4).

Figure 4

Schematic diagram of SQDs for ion detections.

4.1 Metal ions

The predominant application of SQD lies in the detection of ions, with various targets developed, including Fe³⁺, Fe²⁺, Co²⁺, Hg²⁺, chromium ions, and others. In this section, a review of the most recent research work done in this field is summarized.

4.1.1 Detection of iron ions

As far as we know, fluorescent quantum dots have predominantly focused on ferric ions as the primary metal ion target in recent years. Iron is essential for life, and its deficiency is recognized as one of the three major global micronutrient deficiencies. Effectively regulating and monitoring the presence of ferric ions in both biological systems and the environment is crucial [116–118].

Many research papers have highlighted the use of fluorescent probes based on SQDs for the detection of Fe³⁺ ions, primarily leveraging the response of FL quenching. Seyedeh’s group synthesized highly fluorescent CDs through a solvothermal method using polyvinylpyrrolidone and sulfur powder as precursors, as shown in Figure 5a [119]. Ferric ions demonstrated the ability to sensitively and selectively quench the FL of SQDs, exhibiting a good linear correlation (R ² = 0.983) and a lower detection limit (LOD) of 48 nM. The analysis of Fe³⁺ in water samples exhibited a satisfactory recovery rate within the range of 90.5–111%, with a relative standard deviation ranging from 0.2 to 1.2%, as illustrated in Figure 5b.

Figure 5

(a) Schematic depiction of SQDs preparation through solvothermal method. (b) UV–vis spectra of SQDs at various concentrations of Fe³⁺, FL spectra of SQDs (ranging from 0 to 250 μM of Fe³⁺), and a calibration curve of Io/I versus various concentrations of Fe³⁺ [119], Copyright 2024, Elsevier.

Lei et al. presented a polyvinyl alcohol (PVA)-capped fluorescent SQDs sensor designed for the detection of Fe³⁺ ions [120]. The PVA-capped fluorescent SQDs can be produced easily and economically from elemental sulfur by using PVA as a ligand under an O₂ atmosphere, as illustrated in Figure 6a.

Figure 6

(a) Schematic representation of the synthesis of fluorescent PVA-capped SQDs. (b) FL emission spectra and the corresponding F/F _o values of SQDs upon the addition of varying concentrations of Fe³⁺ [120], Copyright 2021, Wiley.

The obtained SQD showed excellent water dispensability, low cytotoxicity, and tunable FL. In addition, the FL of the SQDs shows sensitivity to Fe³⁺ ions in water with an excellent detection range from 0 to 165 μM, with an LOD of 92 nM, as shown in Figure 6b.

Furthermore, Liu et al. introduced a new fluorescent probe designed for the sensitive and selective detection of Fe³⁺ in actual lake water. The probe, utilizing SQDs, exhibited effective FL quenching in the presence of Fe³⁺ attributed to both internal filter effect (IFE) and static quenching mechanisms. This nanoprobe demonstrated a broad detection range spanning from 2.5 to 700 μM, with a low LOD down to 53.6 nM [78].

Additionally, another form of active iron in the human body is present as iron(II) (Fe²⁺). An excess of Fe²⁺ can result in the accumulation of a significant quantity of reactive oxygen species and lipid peroxidation through the Fenton reaction, ultimately triggering cell death and contributing to the onset of neurodegenerative diseases [121]. Given these circumstances, it is highly meaningful to precisely and quantitatively assess the intake of Fe²⁺ in real samples as a preventive measure against diseases [122]. A dual-signal method employing colorimetry and FL, and relying on SQDs, was developed for the simultaneous determination of iron(II) and hydrogen peroxide (H₂O₂) in food samples. The complexation of Fe²⁺ with SQDs leads to FL quenching, presenting a linear relationship between the concentration of Fe²⁺ and the FL intensity over a range of 2.5–55 μM, with LOD as low as 1.41 μM. To validate the method, mineral water was utilized as one of the real samples [123].

4.1.2 Detection of cobalt ions

It is well known that cobalt (Co) is an important element in living organisms and is an important component in Vitamin B₁₂, needed in the synthesis of DNA. Therefore, any deficiency of this element is harmful, suppressing the formation of red blood cells, as well as myelin [124,125]. On the other hand, a high level of Co causes many health problems on skin, bone defects and even cancer [126]. Therefore, it is crucial to develop analytical techniques that are sensitive, selective, and reliable for accurately determining the concentrations of Co²⁺ in natural water samples [127].

Li et al. detailed in their 2020 research that they developed cysteine-decorated SQDs by straightforwardly modifying pristine SQDs [128]. These modified dots function as a sensor for the fluorometric detection of cobalt with exceptional sensitivity and specificity. The precise measurement of Co²⁺ levels in drinking water is crucial for assessing water quality. Notably, the study achieved highly sensitive detection of Co²⁺, with a remarkably low LOD at 0.16 μM and a broad detection range. Additionally, they created portable paper sensors using cysteine-decorated sulfur dots for Co²⁺ detection, demonstrating superior detection capabilities, as seen in Figure 7a. The presence of Co²⁺, the FL intensity of SQDs is extinguished due to aggregation-caused quenching. Optimized under specific conditions, these SQDs facilitate the identification of Co²⁺ ions within a concentration range of 0–9 × 10⁻⁵ mol L⁻¹ and have been applied for the detection of Co²⁺ ions in water samples (Figure 7b) [129].

Figure 7

(a) FL spectra of SQDs in water samples loaded with Co²⁺ and their calibration curve [128], Copyright 2020, Springer. (b) The sulfur dots sensing to Co²⁺ and calibration curve [129], Copyright 2019, Royal Society of Chemistry.

The synthesized samples of SQD using Ozone was successful in detecting Co²⁺ over the range between 19.6 and 56.6 μM [130].

4.1.3 Detection of mercury ions

Mercury(II) ion (Hg²⁺) stands out as a highly hazardous and widespread pollutant, raising environmental and health issues. Hg²⁺ is capable of permeating skin, respiratory, and gastrointestinal tissues, causing DNA damage, hindering mitosis, and resulting in lasting harm to the central nervous system. As per the US Environmental Protection Agency, the upper limit allowed level of Hg²⁺ in drinking water is set at 10 nM [131,132], therefore, monitoring its level is important to reduce health complications.

Wang et al. [133] used a new approach in using aptamer-functionalized SQDs as a highly sensitive detector of Hg²⁺ in water samples (Figure 8a). The developed SQDs not only preserved the favorable FL properties of quantum dots but also addressed the issue of poor selectivity of SQDs for heavy metal ions. The fluorescent apt sensor exhibited a robust response to Hg²⁺ within concentration ranges of 10⁻¹⁵–10⁻⁷ M, featuring an exceptionally low limit of detection at 0.3 f M.

Figure 8

(a) Illustration of the mechanism of the fluorescent aptamer-functionalized SQDs for Hg²⁺ [133], Copyright 2022, Elsevier. (b) Diagram illustrating the detection of Hg²⁺ using SQD [95], Copyright 2020, Elsevier.

In 2019, Qiao et al. [95] introduced a novel SQD designed as an intrinsic fluorescent sensor, with its signal transduction based on the interaction between mercury ions and sulfur atoms within the fluorophore’s internal system. The prepared SQD shows detection limits of Hg²⁺ ions at 65 nM, within a dynamic range spanning from 0 to 100 nM as shown in Figure 8b.

Kumar et al. [134] prepared novel SQDs with QYs of up to 58.7%. These synthesized fluorescent SQDs were used as fluorescent probes for the quantitative detection of Hg²⁺. The SQDs served as effective Hg²⁺ detection probes with a detection limit of 2.7 μM and a linear range of 0–250 μM due to the chemical interactions between metal ions and the sulfur group, particularly the SO group. The probe was applied to real water samples, including tap, bottled, and river water. Additionally, latent fingerprints were recognized using SQD powder, with fingerprints detected on various surfaces, such as paper, glass, and aluminum foil. This technique may find use in forensic investigations [134].

4.1.4 Detection of chromium ions

Given its significant role as an industrial pollutant and its toxicity to living organisms, chromium has become a focal point of interest for the scientific community [135]. Chromium typically exhibits two oxidation states, Cr(iii) and Cr(vi), where the latter being reported to as 100 times more hazardous than the former [135–137]. In many parts of the world, water bodies have been contaminated with Cr(vi) from different human activities including leather tanneries, pigment producers, mining operations, refining processes, and other industries involved in chromate production. Therefore, it is important to develop nanoprobes that are efficient, quick, selective, and sensitive for detecting chromium ions.

In 2021, Xia et al. [38] developed SQDs using an “assembling–fission” method to detect Cr(vi) via the “ON–OFF” principle. These SQDs serve as a fluorescent probe, offering linear detection ranges of 5–1,500 μM and detection limits of 1.5 μM in water, proving their effectiveness for environmental applications, as depicted in Figure 9a.

Figure 9

(a) Schematic illustration of the “ON–OFF” mechanism for a Cr(vi) sensor based on SQDs [38], Copyright 2021, Royal Society of Chemistry. (b) The fabrication of SQDs through a top-down method using H₂O₂, along with the mechanism for detecting Cr(vi) [138], Copyright 2021, Elsevier.

Tan et al. [138] synthesized SQDs using an H₂O₂-assisted top-down method, which demonstrated good water dispersion, stability, and photoluminescence, giving a QY of 11%. These SQDs have been successfully applied to water samples. With the introduction of Cr(vi) into the SQDs, there was a reduction in FL intensity attributed to the IFE, leading to a detection threshold of 0.36 μM within the concentration range of 10–120 μM (Figure 9b).

To control Cr(vi) pollution and mitigate its toxic effects, a straightforward and efficient approach has been introduced by Deng et al. [139] to fabricate a hydrogel composite using chitosan and SQDs. This composite hydrogel demonstrates remarkable sensing and absorption abilities specifically tailored for Cr(vi) ions. The FL stability of the chitosan-based SQDs remains remarkably high across various conditions, including fluctuations in pH, increased ionic strength, UV-light exposure, and prolonged storage durations. The chitosan SQDs’ LOD is 176.2 nM, with a dynamic range from 0 to 76 µM. In 2020, Zhang et al. [140] introduced a study focusing on the ultra-sensitive identification of Cr(vi) ions in water using a fluorescent sensor integrating metal-organic frameworks and SQD. The luminescence of SQDs@UiO-66-NH₂ hybrids in water was significantly diminished by over 90%, demonstrating a substantial reduction in ineffectiveness in the presence of Cr(vi) due to the IFE. Their detection setup has a swift response time of 10 s, with high sensitivity and a minimal detection threshold of 0.16 μM within an extensive linear scope of 0–200 μM for Cr₂O₇ ²⁻ and 0.17 μM for CrO₄ ²⁻ within a wide linear range of 0–220 μM.

4.1.5 Detection of other selected ions

SQDs find applications in the detection of many other ions, such as MoO₄ ²⁻, Pb²⁺, Cu²⁺, and P₂O₇ ⁴⁻. Lead ion (Pb²⁺) is among the harmful transition metals, capable of inducing various negative health effects such as mental disabilities, memory loss, migraines, and cognitive dullness, particularly in children [141,142].

Molybdenum is a vital trace element crucial for the well-being of all forms of living organisms. In animals, it plays a role in various redox enzymes, such as xanthine oxidase. Metal complexes containing thiosemicarbazone and semicarbazone ligands have shown activity against viruses, ulcers, and specific types of tumors. In plants, molybdenum is essential for the initial stages of protein synthesis, facilitating the fixation of atmospheric nitrogen by bacteria. However, excessive levels of molybdenum can be harmful [143–145]. Consequently, it is advisable and highly recommended to monitor the molybdenum content in environmental samples.

Li et al. [106] used chitosan oligosaccharide to assist in the synthesis of SQDs with excellent FL and peroxidase activity. MoO₄ ²⁻ was observed to significantly suppress the FL of the SQDs and hinder their peroxidase function. The changes in relative FL intensity of the prepared SQDs exhibited a well-defined linear relationship with the concentration of MoO₄ ²⁻, with a respective detection limit of 0.068 mM.

Tammina et al. [146] synthesized PVA-stabilized SQDs that exhibited favorable luminescence attributes (QY: 11%) and notable hydrophilicity, enhanced dispersion, and excellent photostability. As a Pb²⁺ sensor, it exhibits an LOD of 0.020 M. It is well known that copper (Cu²⁺) is a vital ion for biological systems, playing a pivotal role in both pathological and physiological processes. However, prolonged exposure to high concentrations of copper can be harmful to the body [147,148]. Copper significantly influences the function and development of the immune system and the central nervous system, serving as a key component in hemocyanin composition [148]. Elevated Cu²⁺ levels have been associated with several diseases, including Alzheimer’s and Wilson’s diseases [149]. Therefore, there is an urgent requirement for highly selective and sensitive methods to detect Cu²⁺ in the environment. Zhang et al. [90] developed a probe that works simultaneously with FL and colorimetric in order to detect Cu²⁺ in environmental samples. Their probe contains both SQDs and carbon quantum dots, leading to a decrease in the FL response of the SQDs due to interactions between the carbon quantum dots and copper ions. Under specific conditions, the dual-mode probe enabled the accurate measurement of Cu²⁺ concentration within a range of 0.1–5.0 μM, with a detection limit of approximately 31 nM.

Anions, such as pyrophosphate (PPi), are important in biochemistry, food preservation, and biological processes, including DNA polymerization, and cellular metabolism [150]. Too high or too low levels of PPi in cells have significant health consequences [151]. Therefore, it is important to develop an easy and cost-effective method for determining the concentrations of PPi in environmental samples.

Gong et al. [152] developed a switching probe for detecting PPi using Fe³⁺ as a tool to speed up the aggregation and disaggregation of the SQD in the samples under study, and mediated fluorescent SQDs for the detection of PPi. The SQDs, capped with bovine serum protein (BSA) for excellent water dispersibility and optical stability, were synthesized through an H₂O₂-assisted chemical etching reaction. Specifically, Fe³⁺ prompted strong aggregation of SQDs into larger sizes, leading to FL quenching. PPi selectively binds to Fe³⁺, emulating coordination and preventing SQDs’ aggregation, resulting in FL recovery. The nano switch’s PPi sensing capability was assessed using an FL assay. The FL response of the PPi-treated SQD-Fe³⁺ mixture was recorded at various PPi concentrations. As anticipated, the F/F _o value gradually increased with PPi concentration ranging from 0.1 to 200 μM, where C is the PPi concentration, and F/F _o is the corresponding signal-to-background ratio. The calculated LOD was 0.01 μM [152]. The research study on SQDs for metal ion sensing is summarized in Table 2.

Table 2

List of selected SQDs for different metal ion detection, including targets, sensing mechanism, QY, LOD, and dynamic range values

Target	Sensing mechanism	QY (%)	LOD	Dynamic range	Ref.
Fe³⁺	Complexation reaction	—	48 nM	0–250 μM	[119]
Fe³⁺	Complexation reaction	4.62	92 nM	0–165 μM	[120]
Fe³⁺	IFE and static quenching	—	53.6 nM	2.5–700 μM	[78]
Fe³⁺	Complexation reaction	1.1	1.41 μM	2.5–55 μM	[123]
Co²⁺	PET	2.74	0.16 μM	0–200 μM	[128]
Co²⁺	Aggregation-caused quenching	—	20 nM	0–9 × 10⁻⁵ mol L⁻¹	[129]
Co²⁺	FRET	9.26	2.44 μM	19.6–56.6 μM	[130]
Hg²⁺	Complexation reaction	5.3	0.3 fM	10⁻¹⁵–10⁻⁷ M	[133]
Hg²⁺	Complexation reaction	—	65 nM	0–100 nM	[95]
Hg²⁺	Complexation reaction	58.7	2.7 μM	0–250 μM	[134]
Cr(VI)	IFE	—	1.5 μM	5–100 and 200–1,500 μM	[38]
Cr(VI)	IFE	11	0.36 μM	10–120 μM	[138]
Cr(VI)	IFE	—	176.2 nM	0–76 µM	[139]
Cr(VI)	IFE	—	0.16 μM	0–220 μM	[140]
MoO₄ ²⁻	—	—	0.068 mM	0.075–1.125 mM	[106]
Pb²⁺	Static quenching and Complexation reaction	11	20 nM	0–175 μM	[146]
Cu²⁺	Redox reaction	9.2	31 nM	0.1–5.0 μM	[90]
P₂O₇ ⁴⁻	Complexation reaction	—	0.01 μM	0.1–200 μM	[152]

4.2 Detection of hazardous molecules

The incorporation of SQDs in FL sensors for environmental monitoring, including the detection of phenol, hydrazine, tetracycline, O-phenylenediamine (OPD), acridine orange, and 2,4-dichlorophenoxyacetic acid (2,4-D), is reviewed in Figure 10.

Figure 10

Schematic diagram of SQDs for target detection.

4.2.1 Detection of phenols

Current reports in petrochemical engineering, printing, food processing, and dyeing underscore the prevalence of various phenolic forms as significant pollutants [153]. Due to their toxicity and slow biodegradation, these substances present a significant environmental hazard. Hence, it is crucial to possess tools for detecting them [153].

In 2021, Peng et al. [154] developed fluorescent SQDs via a simple method (shown in Figure 11) to specifically detect p-nitrophenol in aqueous environments. They achieved a detection limit of 70 nM, exceeding that of previously reported FL sensing techniques.

Figure 11

Another research team [155] utilized SQDs incorporated with green-emitting sulfur, known as GSQDs, for the detection of O-nitrophenol, a highly pollutant substance [156]. When excited at 420 nm, the FL emission of GSQDs at 515 nm declined as the concentration of O-nitrophenol increased. The range of detection spanned from 1 to 240 μM, exhibiting a high correlation coefficient (R ² = 0.9993). Detection limits were achieved as low as 2.54 μM. To evaluate the practical utility of GSQDs for O-nitrophenol detection, real-world samples including dye wastewater and pesticide wastewater were selected.

Stable SQDs were synthesized hydrothermally and characterized using various techniques by Tammina et al. [157]. These SQDs displayed a monoclinic crystal structure with an average size of 5.5 ± 2.0 nm. Optical analysis revealed emission at 415 nm, independent of excitation at 342 nm. SQDs with a high QY (72%) detected nitrophenols with detection limits of 46 and 171 nM for 4- and 2-nitrophenol, respectively. The result reported by this research group using SQDs in water samples is shown in Figure 12.

Figure 12

(a) Schematic diagram for using SQDs in nitrophenol detections. (b) The FL spectra of SQDs at different concentrations of 4-nitrophenol and 2-nitrophenol, along with their respective calibration curves [157], Copyright 2023, Royal Society of Chemistry.

Gao et al. [158] prepared SQDs through an assembly-fission method for p-AP, which efficiently quenches the FL of SQDs through the IFE mechanism. The linear range of p-AP is between 0.5 and 120 μM, and the limit of detection is 0.15 μM. This proposed method, with high recoveries and low detection limits, has great potential for the determination of p-AP in various water samples, including tap water, river water, and reclaimed water [158]. The overall process is depicted in Figure 13.

Figure 13

4.2.2 Detection of hydrazine

Hydrazine serves critical roles in various industries [159] due to its reactivity and reducing properties. However, its highly toxic nature poses significant environmental and health risks, impacting multiple organs and systems upon exposure. Given its widespread use and potential hazards [160,161], there is an urgent demand for a straightforward method to assess hydrazine levels in environmental samples. Li et al. [162] utilized an SQDs@MnO₂ NS composite as a probe to create an innovative fluorescent sensor for detecting hydrazine (N₂H₄) with sensitivity and selectivity. The sensor demonstrated remarkable selectivity and sensitivity, detecting N₂H₄ concentrations ranging from 0.1 to 10 μM, with a low detection limit of 0.072 μM. Furthermore, successful detection in real samples indicates the potential application of this fluorescent sensor for monitoring N₂H₄ levels in water.

4.2.3 Detection of tetracycline (TC)

TCs, a class of antibiotics well-established for their therapeutic properties, have been widely deployed in the treatment of infections in both humans and animals caused by Gram-positive and Gram-negative bacteria. The effectiveness of TCs in combating rickettsia and viral infections is attributed to their broad-spectrum efficacy against various pathogenic microorganisms [12]. Additionally, their favorable oral absorption, relatively low toxicity, and cost-effectiveness contribute to their therapeutic value. Despite these advantages, the improper and excessive use of TCs poses a potential risk, resulting in the accumulation of residues in the human body [163–165]. Therefore, it is crucial to assess and determine TC levels in environmental samples to mitigate these risks. Chang et al. [166] designed a sensor utilizing SQDs coated with a positively charged passivator to detect TC within a concentration range of 0.20–100 μM, achieving a limit of detection of 0.15 μM. They devised an optical sensing system that employs a smartphone as the detector. This system is capable of visualizing the color shift from blue to yellow-green upon the introduction of TC, with a subsequent transition to red in the presence of Eu³⁺. Practical applications, such as TC detection (refer to Figure 14), confirmed the efficacy of their sensor.

Figure 14

4.2.4 Detection of OPD

OPD serves a vital function as a chemical intermediate in the synthesis of pesticides such as carbendazim and thiophanate methyl. Its wide-ranging applications include the production of cationic and reductive dyes, as well as various chemical products like surfactants, rubber antioxidants, and fur dyes. Despite its widespread applications, OPD is known for its high toxicity, carcinogenicity [170], and teratogenicity, posing a significant risk of contaminating groundwater and surface water due to its solubility. Thus, it is essential to monitor OPD levels in both drinking and environmental water to mitigate potential hazards. Ye et al. [167] utilized SQDs@CMC in conjunction with Cu²⁺, inducing a reaction to OPD that enhances blue-shift FL (Figure 15). The FL intensity of SQDs@CMC-Cu²⁺ demonstrates a strong correlation, showing R ² = 0.995, within the 0–60 μM OPD concentration range, with an LOD of 0.083 μM, similar to other probes in the literature. This combination effectively enables selective monitoring of OPD across diverse water samples.

Figure 15

4.2.5 Detection of acridine orange

Most dyes are composed of azo compounds, which the body breaks down into aromatic amines, recognized for their ability to induce tissue damage and cancer. Nevertheless, despite being classified as a carcinogenic substance by the World Health Organization, acridine orange, a nitrogen-containing heterocyclic dye, is widely used in various applications including biomedicine and dye engineering [169,171]. Therefore, it is important to have tools to detect their existence in these applications.

Yan et al. [169] synthesized blue luminescent SQDs using a solvothermal method with sublimated sulfur as the source and β-cyclodextrin as the passivator. The assay demonstrated reliable accuracy and recovery in analyzing real samples, highlighting the importance of blue luminescent SQDs in environmental analysis.

4.2.6 Detection of 2,4-dichlorophenoxyacetic acid

2,4-Dichlorophenoxyacetic acid (2,4-D), an auxin-type herbicide, is classified as potentially carcinogenic to humans and toxic to various aquatic and mammalian species, potentially causing organ damage with prolonged exposure [172,173]. Therefore, the precise and efficient assessment of 2,4-D is crucial to support environmental surveillance, addressing growing concerns about public health and environmental quality. Li et al. [168] developed an SQD detector using Rhodamine B for the detection of 2,4-D (Figure 16). Their detector showed excellent FL and remained stable when emitting light at 455 and 580 nm. ALP helped break down a substance called p-nitrophenyl phosphate into p-nitrophenol, which made the FL of the SQDs weaker at 455 nm. However, it did not affect the brightness of the SQDs at 580 nm. When 2,4-D was present, ALP could not work as well, which stopped the breakdown process and restored the brightness of the SQDs at 455 nm. This method accurately detected 2,4-D in water and vegetables, showing great precision and selectivity. Table 3 provides a summary of research studies on SQDs used for the detection of hazardous molecules.

Figure 16

Table 3

List of selected SQDs for different hazardous molecules detection, including targets, sensing mechanism, QY, LOD, and dynamic range values

Target	Sensing mechanism	QY (%)	LOD	Dynamic range	Ref.
p-Nitrophenol	IFE	—	70 nM	0.2–30 µM and 30–90 µM	[154]
o-Nitrophenol	IFE	14.22	2.54 μM	1–240 μM	[155]
4- and 2-Nitrophenol	IFE	72	46 and 171 nM	0.125–500 μM	[157]
P-AP	IFE	—	0.15 μM	0.5–120 μM	[158]
Hydrazine	IFE	—	0.072 μM	0.1–10 μM	[162]
Tetracycline	IFE	—	0.15 μM	0.20–100 μM	[166]
OPD	ICT	9.31	0.083 μM	0–60 μM	[167]
Acridine orange	IFE and static quenching	—	0.41 μM	0–90 μM	[169]
2,4-Dichlorophenoxyacetic acid	IFE	—	17.3 ng mL⁻¹	0.050–0.500 μg mL⁻¹	[168]

5 Conclusion

Chemists are currently focused on developing analyte detectors that are both selective and sensitive, as they offer a cost-effective approach to environmental sensing. This review highlights the importance of SQDs in this field. The straightforward and cost-effective production of SQDs, along with their impressive reactivity towards various substances such as metal ions and phenols, positions them as robust candidates for detecting diverse environmental pollutants, particularly heavy metal ions. Additionally, these sensors demonstrate minimal detection limits, indicating their enhanced efficiency. However, there is still potential for improvement in these fluorescent sensors. Ongoing efforts will be necessary to develop new SQD sensors capable of detecting additional analytes in environmental studies.

In conclusion, integrating SQDs into fluorescent sensors for environmental monitoring has shown remarkable potential across various applications. SQDs have proven versatile and effective in assessing environmental quality through the detection of metal ions (iron, cobalt, mercury, chromium, molybdenum), phenols, hydrazine, acridine orange, TC, OPD, and 2,4-D. The reviewed studies collectively emphasize SQDs’ importance and suitability in environmental monitoring, SQDs consistently exhibit high sensitivity, selectivity, and reliability. Moreover, innovative approaches such as smartphone-based optical sensing systems and dual-mode probes enhance the practicality and feasibility of SQD-based environmental monitoring. Overall, these findings underscore SQDs’ significant contribution to advancing environmental monitoring techniques, promising solutions for addressing contemporary environmental challenges.

Future research in SQD-based environmental monitoring holds promise in several areas. Novel synthesis methods could enhance SQDs’ properties, including QY and stability, thereby improving sensor performance. Exploring SQDs’ potential in detecting emerging contaminants and their application in real-time monitoring systems represents another promising avenue of research.

Funding information: J. H. acknowledges the financial support received from Khalifa University, Fund # 847001.
Author contributions: All authors have accepted responsibility for the entire content of this manuscript and approved its submission.
Conflict of interest: The authors state no conflict of interest.
Data availability statement: All data generated or analysed during this study are included in this published article.

References

[1] Kayani KF. Bimetallic metal–organic frameworks (BMOFs) for dye removal: a review. RSC Adv. 2024;14(43):31777–96. 10.1039/D4RA06626J.Search in Google Scholar PubMed PubMed Central

[2] Skalny AV, Aschner M, Bobrovnitsky IP, Chen P, Tsatsakis A, Paoliello MMB, et al. Environmental and health hazards of military metal pollution. Env Res. 2021;201(June):111568. 10.1016/j.envres.2021.111568.Search in Google Scholar PubMed

[3] Khan S, Naushad M, Govarthanan M, Iqbal J, Alfadul SM. Emerging contaminants of high concern for the environment: Current trends and future research. Env Res. 2022 May;207:112609.10.1016/j.envres.2021.112609Search in Google Scholar PubMed

[4] Mishra RK, Mentha SS, Misra Y, Dwivedi N. Emerging pollutants of severe environmental concern in water and wastewater: A comprehensive review on current developments and future research. Water-Energy Nexus. 2023;6:74–95. 10.1016/j.wen.2023.08.002.Search in Google Scholar

[5] Briggs D. Environmental pollution and the global burden of disease. Br Med Bull. 2003;68:1–24.10.1093/bmb/ldg019Search in Google Scholar PubMed

[6] Kayani KF, Shatery OBA, Mohammed SJ, Ahmed HR, Hamarawf RF, Mustafa MS. Synthesis and applications of luminescent metal organic frameworks (MOFs) for sensing dipicolinic acid in biological and water samples: a review. Nanoscale Adv. 2024;7:13–41. 10.1039/D4NA00652F.Search in Google Scholar

[7] Kayani KF. Nanozyme based on bimetallic metal–organic frameworks and their applications: A review. Microchem J. 2025;208:112363, https://www.sciencedirect.com/science/article/pii/S0026265X24024767.10.1016/j.microc.2024.112363Search in Google Scholar

[8] Ansari MA, Shoaib S, Chauhan W, Gahtani RM, Hani U, Alomary MN, et al. Nanozymes and carbon-dots based nanoplatforms for cancer imaging, diagnosis and therapeutics: Current trends and challenges. Env Res. 2024;241:117522. 10.1016/j.envres.2023.117522.Search in Google Scholar PubMed

[9] Kayani KF, Mohammad NN, Kader DA, Mohammed SJ, Shukur DA, Alshatteri AH, et al. Ratiometric lanthanide metal-organic frameworks (MOFs) for smartphone-assisted visual detection of food contaminants and water: a review. ChemistrySelect. 2023 Dec 18;8(47):e202303472. 10.1002/slct.202303472.Search in Google Scholar

[10] Kordbacheh F, Heidari G. Materials chemistry horizons water pollutants and approaches for their removal. Mater Chem Horiz. 2023;2(2):139–53.Search in Google Scholar

[11] Mustafa MS, Mohammad NN, Radha FH, Kayani KF, Ghareeb HO, Mohammed SJ. Eco-friendly spectrophotometric methods for concurrent analysis of phenol, 2-aminophenol, and 4-aminophenol in ternary mixtures and water samples: assessment of environmental sustainability. RSC Adv. 2024;14(23):16045–55.10.1039/D4RA01094ASearch in Google Scholar

[12] Kayani KF, Mohammed SJ, Mohammad NN, Abdullah GH, Kader DA, Hamad Mustafa NS. Ratiometric fluorescence detection of tetracycline in milk and tap water with smartphone assistance for visual pH sensing using innovative dual-emissive phosphorus-doped carbon dots. Food Control. 2024 Oct;164:110611.10.1016/j.foodcont.2024.110611Search in Google Scholar

[13] Kayani KF, Rahim MK, Mohammed SJ, Ahmed HR, Mustafa MS, Aziz SB. Recent progress in folic acid detection based on fluorescent carbon dots as sensors: a review. J Fluoresc. 2024. 10.1007/s10895-024-03728-3.Search in Google Scholar PubMed

[14] Rashid Ahmed H, Kayani KF. A comparative review of Fenton-like processes and advanced oxidation processes for Methylene Blue degradation. Inorg Chem Commun. 2024;113467, https://www.sciencedirect.com/science/article/pii/S1387700324014576.10.1016/j.inoche.2024.113467Search in Google Scholar

[15] Ahmed HR, Kayani KF, Ealias AM, George G. Eco-friendly biocatalysis: Innovative approaches for the sustainable removal of diverse dyes from aqueous solutions. Inorg Chem Commun. 2024;113447, https://www.sciencedirect.com/science/article/pii/S1387700324014370.10.1016/j.inoche.2024.113447Search in Google Scholar

[16] Zou L, Gu Z, Sun M. Review of the application of quantum dots in the heavy-metal detection. Toxicol Env Chem. 2015;97(3–4):477–90.10.1080/02772248.2015.1050201Search in Google Scholar

[17] Jaishankar M, Tseten T, Anbalagan N, Mathew BB, Beeregowda KN. Toxicity, mechanism and health effects of some heavy metals. Interdiscip Toxicol. 2014;7(2):60–72.10.2478/intox-2014-0009Search in Google Scholar PubMed PubMed Central

[18] Rabiee N, Dokmeci MR, Zarrabi A, Makvandi P, Saeb MR, Karimi-Maleh H, et al. Green Biomaterials: fundamental principles. Green Biomater. 2023;1(1):1–4. 10.1080/29934168.2023.2268943.Search in Google Scholar

[19] Kayani KF, Abdullah AM. Eco-friendly fluorescent detection method for Cu2+ ions combined with smartphone-integrated paper strip sensors based on highly fluorescent 2-aminoterephthalic acid in milk samples. J Food Compos Anal. 2024;135:106577.10.1016/j.jfca.2024.106577Search in Google Scholar

[20] Ahmed HR, Salih MA, Agha NNM, Tofiq DI, Karim MAH, Kayani KF, et al. Enhanced methyl orange removal in aqueous solutions using bio-catalytic metal oxides derived from pomegranate peel waste: a green chemistry evaluation. React Kinet Mech Catal. 2024;137:3417–40. 10.1007/s11144-024-02685-z.Search in Google Scholar

[21] Fu L, Wang A, Xie K, Zhu J, Chen F, Wang H, et al. Sensors and Actuators B: Chemical Electrochemical detection of silver ions by using sulfur quantum dots modi fi ed gold electrode. Sens Actuators B Chem. 2019;314:127390.10.1016/j.snb.2019.127390Search in Google Scholar

[22] Liu L, Zhang Y, Yuan R, Wang H. Ultrasensitive electrochemiluminescence biosensor using sulfur quantum dots as an emitter and an efficient DNA walking machine with triple-stranded DNA as a signal amplifier. Anal Chem. 2020;92(22):15112–9.10.1021/acs.analchem.0c03311Search in Google Scholar PubMed

[23] Liu S, Wang H, Feng A, Chang J, Zhang C, Shi YE, et al. Photoluminescence investigations of sulfur quantum dots synthesized by a bubbling-assisted strategy. Nanoscale Adv. 2021;3(14):4271–5.10.1039/D1NA00282ASearch in Google Scholar PubMed PubMed Central

[24] Gao P, Wang G, Zhou L. Luminescent sulfur quantum dots: synthesis, properties and potential applications. ChemPhotoChem. 2020;4(11):5235–44.10.1002/cptc.202000158Search in Google Scholar

[25] Shivalkar S, Arshad F, Sahoo AK. Visible light-mediated photoactivated sulfur quantum dots as heightened antibacterial agents. ACS Omega. 2022;7:33358–64.10.1021/acsomega.2c03968Search in Google Scholar PubMed PubMed Central

[26] Ning K, Sun Y, Liu J, Fu Y, Ye K, Liang J, et al. Research update of emergent sulfur quantum dots in synthesis and sensing/bioimaging applications. Molecules. 2022;27(9):2822.10.3390/molecules27092822Search in Google Scholar PubMed PubMed Central

[27] Nanomaterials S, Li C, Wang Y, Jiang H, Wang X. Biosensors based on advanced sulfur-containing nanomaterials. Sensors. 2020;20(12):3488.10.3390/s20123488Search in Google Scholar PubMed PubMed Central

[28] Bai Z, Shen L, Wei J, Li Y, Abbas A, Li Y, et al. Layered sulfur nanosheets prepared by assembly of sulfur quantum dots: implications for wide optical absorption and multiwavelength photoluminescence. ACS Appl Nano Mater. 2020;3(11):10749–56.10.1021/acsanm.0c02024Search in Google Scholar

[29] Li S, Chen D, Zheng F, Zhou H, Jiang S, Wu Y. Water-soluble and lowly toxic sulphur quantum dots. Adv Funct Mater. 2014;24(45):7133–8.10.1002/adfm.201402087Search in Google Scholar

[30] Arshad F, Sk MP. Luminescent sulfur quantum dots for colorimetric discrimination of multiple metal ions. ACS Appl Nano Mater. 2020;3(3):3044–9.10.1021/acsanm.0c00394Search in Google Scholar

[31] Pal A, Arshad F, Sk MP. Emergence of sulfur quantum dots: Unfolding their synthesis, properties, and applications. Adv Colloid Interface Sci. 2020;285:102274. 10.1016/j.cis.2020.102274.Search in Google Scholar PubMed

[32] Siomra A. Two-photon excited luminescence of sulfur quantum dots for heavy metal ion detection. RSC Adv. 2024;14:2439–46.10.1039/D3RA07521DSearch in Google Scholar PubMed PubMed Central

[33] Xu N, Wen Q. Single element material sulfur quantum dots nonlinear optics and ultrafast photonic applications. Opt Laser Technol. 2021;138:106858.10.1016/j.optlastec.2020.106858Search in Google Scholar

[34] Cotta MA. Quantum dots and their applications: what lies ahead? ACS Appl. 2020;3(6):4920–4.10.1021/acsanm.0c01386Search in Google Scholar

[35] Shao L, Gao Y, Yan F. Semiconductor quantum dots for biomedicial applications. Sensors. 2011;11(12):11736–51.10.3390/s111211736Search in Google Scholar PubMed PubMed Central

[36] Arshad F, Sk MP, Maurya SK, Siddique HR. Mechanochemical synthesis of sulfur quantum dots for cellular imaging. ACS Appl Nano Mater. 2021;4(4):3339–44.10.1021/acsanm.1c00509Search in Google Scholar

[37] Deng H, Yang Y, Chen W, Xia X. Size-focusing results in highly photoluminescent sulfur quantum dots with steadfast emission wavelength. Nanoscale. 2021;13:2519–26.Search in Google Scholar

[38] Xia M, Mei H, Qian Q, Dahlgren RA, Gao M, Wang X. Sulfur quantum dot-based “ON–OFF–ON” fluorescence platform for detection and bioimaging of Cr(VI) and ascorbic acid in complex environmental matrices and biological tissues. RSC Adv. 2021;11(18):10572–81.10.1039/D1RA00401HSearch in Google Scholar

[39] Sheng Y, Huang Z, Zhong Q, Deng H, Lai M, Yang Y, et al. Size-focusing results in highly photoluminescent sulfur quantum dots with a stable emission wavelength. Nanoscale. 2021;13(4):2519–26.10.1039/D0NR07251FSearch in Google Scholar PubMed

[40] Tang H, Zhong J, Chen W, Shi K, Mei G, Zhang Y, et al. Lead sulfide quantum dot photodetector with enhanced responsivity through a two-step ligand-exchange method. ACS Appl Nano Mater. 2019;2(10):6135–43.10.1021/acsanm.9b00889Search in Google Scholar

[41] Mohammed NJ, Dagher HF. Synthesis and characterization of mercuric sulfide nanoparticles thin films by pulsed laser ablation (PLA) in distilled water (DW). Thin Films Laboratory, Coll Sci, Mustansiriyah University, Baghdad, Iraq. 2020;17(3):11–6.Search in Google Scholar

[42] Devi RA, Latha M, Velumani S, Oza G, Rohini M, Lee J, et al. Synthesis and characterization of cadmium sulfide nanoparticles by chemical precipitation method. J Nanosci Nanotechnol. 2015;15:8434–9.10.1166/jnn.2015.11472Search in Google Scholar PubMed

[43] Le N, Zhang M, Kim K. Quantum dots and their interaction with biological systems. Int J Mol Sci. 2022;23(18):1–27.10.3390/ijms231810763Search in Google Scholar PubMed PubMed Central

[44] Zhu C, Chen Z, Gao S, Leng B, Bin I, Wen K, et al. Progress in natural science: materials international recent advances in non-toxic quantum dots and their biomedical applications. Prog Nat Sci Mater Int. 2020;29(6):628–40.10.1016/j.pnsc.2019.11.007Search in Google Scholar

[45] Kayani KF, Mohammed SJ, Ghafoor D, Rahim MK, Ahmed HR. Carbon dot as fluorescence sensor for glutathione in human serum samples: a review. Mater Adv. 2024;5(11):4618–33. 10.1039/D4MA00185K.Search in Google Scholar

[46] Kayani KF, Shatery OBA, Mustafa MS, Alshatteri AH, Mohammed SJ, Aziz SB. Environmentally sustainable synthesis of whey-based carbon dots for ferric ion detection in human serum and water samples: evaluating the greenness of the method. RSC Adv. 2024;14(8):5012–21.10.1039/D3RA08680ASearch in Google Scholar

[47] Kayani KF, Abdullah CN. A dual-mode detection sensor based on nitrogen-doped carbon dots for visual detection of Fe(III) and ascorbic acid via a smartphone. J Fluoresc. 2024.10.1007/s10895-024-03604-0Search in Google Scholar PubMed

[48] Mansuriya BD. Carbon dots: classification, properties, synthesis, characterization, and applications in health care – an updated review (2018–2021). Nanomater. 2021;11:2525.10.3390/nano11102525Search in Google Scholar PubMed PubMed Central

[49] Mohammed SJ, Hawaiz FE, Aziz SB, Al-Jaf SH. Organic soluble nitrogen-doped carbon dots (ONCDs) to reduce the optical band gap of PVC polymer: Breakthrough in polymer composites with improved optical properties. Opt Mater (Amst). 2024;149:1–16.10.1016/j.optmat.2024.115014Search in Google Scholar

[50] Shatery OBA, Kayani KF, Mustafa MS, Mohammed SJ. Rational design for enhancing sensitivity and robustness of a probe via encapsulation of carbon dots into a zeolitic imidazolate framework-8 for quantification of tetracycline in milk with greenness evaluation. Res Chem Intermed. 2024;50:2291–306. 10.1007/s11164-024-05271-z.Search in Google Scholar

[51] Kayani KF, Ghafoor D, Mohammed SJ, Shatery OBA. Carbon dots: synthesis, sensing mechanisms, and potential applications as promising materials for glucose sensors. Nanoscale Adv. 2024;7:42–59. 10.1039/D4NA00763H.Search in Google Scholar PubMed PubMed Central

[52] Mohammed SJ, Sidiq MK, Najmuldeen HH, Kayani KF, Kader DA, Aziz SB. A Comprehensive review on nitrogen-doped carbon dots for antibacterial applications. J Env Chem Eng. 2024;12(6):114444, https://www.sciencedirect.com/science/article/pii/S2213343724025752.10.1016/j.jece.2024.114444Search in Google Scholar

[53] Zhao C, Song X, Liu Y, Fu Y, Ye L, Wang N, et al. Synthesis of graphene quantum dots and their applications in drug delivery. J Nanobiotechnol. 2020;18:1–32.10.1186/s12951-020-00698-zSearch in Google Scholar PubMed PubMed Central

[54] Zhang Y, Cai N, Chan V. Recent advances in silicon quantum dot-based fluorescent biosensors. Biosensors. 2023;13:1–16.10.3390/bios13030311Search in Google Scholar PubMed PubMed Central

[55] Ren Q, Ma Y, Zhang S, Ga L, Ai J. One-step synthesis of water-soluble silver sulfide quantum dots and their application to bioimaging. ACS Omega. 2021;6(9):6361–7.10.1021/acsomega.0c06276Search in Google Scholar PubMed PubMed Central

[56] Vishnoi P, Mazumder M, Barua M, Pati SK, Rao CNR. Phosphorene quantum dots. Chem Phys Lett. 2018;699:223–8.10.1016/j.cplett.2018.03.069Search in Google Scholar

[57] Kayani KF, Mohammed SJ, Mohammad NN, Abdullah AM, Tofiq DI, Mustafa MS, et al. Sulfur quantum dots for fluorescence sensing in biological and pharmaceutical samples: a review. Mater Adv. 2024;5:6351–67. 10.1039/D4MA00502C.Search in Google Scholar

[58] Wang R, Xu Y, Zhang T, Jiang Y. Rapid and sensitive detection of Salmonella typhimurium using aptamer-conjugated carbon dots as fluorescence probe. Anal Methods. 2015;7(5):1701–6.10.1039/C4AY02880ESearch in Google Scholar

[59] Ma ZB, Zhang Y, Ren XH, He XW, Li WY, Zhang YK. Dual-reverse-signal ratiometric fluorescence method for malachite green detection based on multi-mechanism synergistic effect. Spectrochim Acta - Part A Mol Biomol Spectrosc. 2022;276:121196.10.1016/j.saa.2022.121196Search in Google Scholar PubMed

[60] Ruan H, Zhou L. Synthesis of fluorescent sulfur quantum dots for bioimaging and biosensing. Front Bioeng Biotechnol. 2022;10:1–7.10.3389/fbioe.2022.909727Search in Google Scholar PubMed PubMed Central

[61] Huang Z, Gao Y, Huang Z, Chen D, Sun J, Zhou L. Sulfur quantum dots: A novel fluorescent probe for sensitive and selective detection of Fe3+ and phytic acid. Microchem J. 2021;170(May):106656. 10.1016/j.microc.2021.106656.Search in Google Scholar

[62] Winzell A, Winzell A. Surface modification of CdSe (ZnS) quantum dots for biomedical applications surface modification of CdSe (ZnS) quantum dots for biomedical applications. Thin Solid Films. 2010;519:1202–12.10.1016/j.tsf.2010.08.070Search in Google Scholar

[63] Zhao J, Fan Z. Using zinc ion-enhanced fluorescence of sulfur quantum dots to improve the detection of the zinc (II) -binding antifungal drug clioquinol. Microchim Acta. 2020;187(3):1–8.10.1007/s00604-019-4020-6Search in Google Scholar PubMed

[64] Kayani KF, Omer KM. A red luminescent europium metal organic framework (Eu-MOF) integrated with a paper strip using smartphone visual detection for determination of folic acid in pharmaceutical formulations. N J Chem. 2022;46(17):8152–61. 10.1039/D2NJ00601D.Search in Google Scholar

[65] Li H, Zhu H, Sun M, Yan Y, Zhang K, Huang D. Manipulating surface chemistry of quantum dots for sensitive ratiometric fluorescence detection of sulfur dioxide. Langmuir. 2015;31(31):8667–71.10.1021/acs.langmuir.5b02340Search in Google Scholar PubMed

[66] Wang F, Zhang H, Xu B. Physical chemistry of nanoclusters nitrogen and sulfur quantum dot co-modified graphene nanosheet with enhanced photocatalytic activity for methyl orange degradation. Russian J Phys Chem A. 2020;94(11):2299–305.10.1134/S0036024420110333Search in Google Scholar

[67] Shen L, Wang H, Liu S, Shen L, Wang H, Liu S, et al. Assembling of sulphur quantum dots in fission of sublimed sulphur assembling of sulphur quantum dots in fission of sublimed sulphur. J Am Chem Soc. 2018;140(25):7878–84.Search in Google Scholar

[68] Mendonça LTB, Azevedo WMDe. A fast bottom-up route for preparing CdS quantum dots using laser ablation in a liquid environment. J Lumin. 2015;171:79–84.10.1016/j.jlumin.2015.10.031Search in Google Scholar

[69] Wang H, Wang Z, Xiong Y, Kershaw SV, Li T, Wang Y, et al. Hydrogen peroxide assisted synthesis of highly luminescent sulfur quantum dots. Angew Chem – Int Ed. 2019;58(21):7040–4.10.1002/anie.201902344Search in Google Scholar PubMed

[70] Navarro M. Toward greener electrochemical synthesis of composition-tunable luminescent CdX-based (X = Te, Se, S) quantum dots for bioimaging cancer cells. Sens Actuators B Chem. 2017;250:233–43.10.1016/j.snb.2017.04.185Search in Google Scholar

[71] Saleh HM. Synthesis and characterization of nanomaterials for application in cost-effective electrochemical devices. Sustainability. 2023;15:10891.10.3390/su151410891Search in Google Scholar

[72] Xiao L, Du Q, Huang Y, Wang L, Cheng S, Wang Z, et al. Rapid synthesis of sulfur nanodots by one-step hydrothermal reaction for luminescence-based applications. ACS Appl Nano Mater. 2019;2(10):6622–8.10.1021/acsanm.9b01507Search in Google Scholar

[73] Saravanan K, Selladurai S, Ananthakumar S, Suriakarthick R. Materials science in semiconductor processing solvothermal synthesis of copper cadmium sulphide ( CuCdS 2) nanoparticles and its structural, optical and morphological properties. Mater Sci Semicond Process. 2019;93:345–56.10.1016/j.mssp.2019.01.024Search in Google Scholar

[74] Bhargava SK. Easy, one- step synthesis of CdTe quantum dots via microwave irradiation for fingerprinting application. Mater Res Bull. 2017;90:260–5.10.1016/j.materresbull.2017.03.003Search in Google Scholar

[75] Safeera TA, Johns N, Anila EI, Martinez AI, Sreenivasan PV, Reshmi R, et al. Low temperature fabrication and characterization of wurtzite structured ZnS quantum dots by chemical spray pyrolysis. J Anal Appl Pyrolysis. 2015;115:96–102.10.1016/j.jaap.2015.07.005Search in Google Scholar

[76] Nair VR, Shanthil M, Sandeep K, Savitha KU, Archana A, Deepamol V, et al. Quantum dot-based fluorometric sensor for Hg(II) in water customizable for onsite visual detection. ACS Omega. 2023;8(32):29468–74.10.1021/acsomega.3c03125Search in Google Scholar PubMed PubMed Central

[77] Wang Z, Xing X, Yan Y, Zhao R, Zou T, Wang Z. One-step hydrothermal synthesis of thioglycolic acid capped CdS quantum dots as fluorescence determination of cobalt ion. Sci Rep. 2018;8(8953):1–12.10.1038/s41598-018-27244-0Search in Google Scholar PubMed PubMed Central

[78] Liu Y, Shao X, Gao Z, Zhu X, Pan Z, Ying Y, et al. Sulfur quantum dot as a fluorescent nanoprobe for Fe3+ ions: Uncovering of detection mechanism, high sensitivity, and large detection range. J Lumin. 2023;257:119693. 10.1016/j.jlumin.2023.119693.Search in Google Scholar

[79] Kadian S, Chaulagain N. Probe sonication-assisted rapid synthesis of highly fl uorescent sulfur quantum dots. Nanotechnology. 2023;34(30):1–9.10.1088/1361-6528/acd00aSearch in Google Scholar PubMed

[80] Du W, Liao L, Yang L, Qin A, Liang A. Aqueous synthesis of functionalized copper sulfide quantum dots as near-infrared luminescent probes for detection of Hg2+, Ag+ and Au3+. Sci Rep. 2017;7(11451):1–12.10.1038/s41598-017-10904-ySearch in Google Scholar PubMed PubMed Central

[81] Zhou J, Liu Y, Tang J, Tang W. Surface ligands engineering of semiconductor quantum dots for chemosensory and biological applications. Biochem Pharmacol. 2017;20(7):360–76.10.1016/j.mattod.2017.02.006Search in Google Scholar

[82] Baig N, Kammakakam I, Falath W, Kammakakam I. Nanomaterials: A review of synthesis methods, properties, recent progress, and challenges. Mater Adv. 2021;2(6):1821–71.10.1039/D0MA00807ASearch in Google Scholar

[83] Khayal A, Dawane V, Amin MA, Tirth V, Yadav VK, Algahtani A, et al. Advances in the methods for the synthesis of carbon dots and their emerging applications. Polym (Basel). 2021;13(18):1–31.10.3390/polym13183190Search in Google Scholar PubMed PubMed Central

[84] Gao P, Huang Z, Tan J, Lv G, Zhou L. Efficient conversion of elemental sulfur to robust ultrabright fluorescent sulfur quantum dots using sulfur-ethylenediamine precursor. ACS Sustain Chem Eng. 2022;10(14):4634–41.10.1021/acssuschemeng.2c00036Search in Google Scholar

[85] Liu J, Ba D, Chen Y, Wen S, Cheng G. Synthesis of 3-(2-quinolyl) chromones from ynones and quinoline N-oxides via tandem reactions under transition metal-and additive-free conditions. Chem Commun. 2020;56(29):4078–81.10.1039/C9CC09460ASearch in Google Scholar

[86] Wang Y, Hu A. Carbon quantum dots: Synthesis, properties and applications. J Mater Chem C. 2014;2(34):6921–39.10.1039/C4TC00988FSearch in Google Scholar

[87] Shen L, Chen X, Chen Y, Peng J, Abbas A, Wei J, et al. One-step hydrothermal synthesis of sulfur quantum dots for photoelectrochemical catalysis for dye degradation. J Electron Mater. 2022;51(6):3092–100. 10.1007/s11664-022-09528-4.Search in Google Scholar

[88] Ayyıldız MF, Karaman DN, Kartoğlu B, Şaylan M, Chormey DS, Bakırdere S. A simple microwave-assisted synthesis of cobalt ferrite nanoparticles and its application for the determination of lead ions in rooibos (Aspalathus linearis) tea. Food Chem. 2023;429:136862.10.1016/j.foodchem.2023.136862Search in Google Scholar PubMed

[89] Ma F, Zhou Q, Yang M, Zhang J, Chen X. Microwave-assisted synthesis of sulfur quantum dots for detection of alkaline phosphatase activity. Nanomaterials. 2022;12(16):1–10.10.3390/nano12162787Search in Google Scholar PubMed PubMed Central

[90] Zhang H, Li Y, Lu H, Gan F. A ratiometric fluorescence and colorimetric dual-mode sensing platform based on sulfur quantum dots and carbon quantum dots for selective detection of Cu2+. Anal Bioanal Chem. 2022;414(7):2471–80. 10.1007/s00216-022-03888-w.Search in Google Scholar PubMed

[91] Shen L, Wang H, Liu S, Bai Z, Zhang S, Zhang X, et al. Assembling of sulfur quantum dots in fission of sublimed sulfur. J Am Chem Soc. 2018;140(25):7878–84.10.1021/jacs.8b02792Search in Google Scholar PubMed

[92] Chen H, Zhu X, Huang H, Wang H, Wang T, Zhao R, et al. Sulfur dots-graphene nanohybrid: A metal-free electrocatalyst for efficient N2-to-NH3 fixation under ambient conditions. Chem Commun. 2019;55(21):3152–5.10.1039/C9CC00461KSearch in Google Scholar

[93] Gao P, Zhong W, Li T, Liu W, Zhou L. Room temperature, ultrafast and one-step synthesis of highly fluorescent sulfur quantum dots probe and their logic gate operation. J Colloid Interface Sci. 2024;666:221–31.10.1016/j.jcis.2024.04.033Search in Google Scholar PubMed

[94] Lu H, Zhang H, Li Y, Gan F. Sensitive and selective determination of tetracycline in milk based on sulfur quantum dot probes. RSC Adv. 2021;11(37):22960–8.10.1039/D1RA03745ESearch in Google Scholar

[95] Qiao G, Liu L, Hao X, Zheng J, Liu W, Gao J, et al. Signal transduction from small particles: Sulfur nanodots featuring mercury sensing, cell entry mechanism and in vitro tracking performance. Chem Eng J. 2020;382:1–32. 10.1016/j.cej.2019.122907.Search in Google Scholar

[96] Tan J, Deng P, Wang L, Bi X, Zhou Y, Wang Y, et al. Signal-on and selective detection of anti-tumor adjunctive drug zoledronic acid using fluorescent sulfur quantum dots. Dye Pigment. 2022;205:110520. 10.1016/j.dyepig.2022.110520.Search in Google Scholar

[97] Jiang X-X, Li P, Zhao M-Y, Chen R-C, Wang Z-G, Xie J-X, et al. In situ encapsulation of SQDs by zinc ion-induced ZIF-8 growth strategy for fluorescent and colorimetric dual-signal detection of alkaline phosphatase. Anal Chim Acta. 2022;1221:340103.10.1016/j.aca.2022.340103Search in Google Scholar PubMed

[98] Karimi H, Rajabi HR, Kavoshi L. Application of decorated magnetic nanophotocatalysts for efficient photodegradation of organic dye: A comparison study on photocatalytic activity of magnetic zinc sulfide and graphene quantum dots. J Photochem Photobiol A Chem. 2020;397:112534. 10.1016/j.jphotochem.2020.112534.Search in Google Scholar

[99] Sheik Mydeen S, Raj Kumar R, Sivakumar R, Sambathkumar S, Kottaisamy M, Vasantha VS. Graphene quantum dots/ZnO nanocomposite: Synthesis, characterization, mechanistic investigations of photocatalytic and antibacterial activities. Chem Phys Lett. 2020;761:138009. 10.1016/j.cplett.2020.138009.Search in Google Scholar

[100] Borah DJ, Mostako ATT, Malakar A. Tailoring the crystalline phase and size of the MoO3 quantum dots via sonication induced modified top-down method. J Alloy Compd. 2022;891:161870. 10.1016/j.jallcom.2021.161870.Search in Google Scholar

[101] Skapas M, Stanionytė S, Paulauskas T, Butkutė R. HRTEM study of size-controlled Bi quantum dots in annealed GaAsBi/AlAs multiple quantum well structure. Phys Status Solidi Basic Res. 2019;256(5):2–7.10.1002/pssb.201800365Search in Google Scholar

[102] Lv G, Dai X, Lu G, Ye L, Wang G, Zhou L. Facile fabrication of portable electrospun poly(vinyl alcohol)/sulfur quantum dots film sensor for sensitive and selective detection of Fe3+. Opt Mater (Amst). 2023;135:113227. 10.1016/j.optmat.2022.113227.Search in Google Scholar

[103] Gliniak J, Lin JH, Chen YT, Li CR, Jokar E, Chang CH, et al. Sulfur-doped graphene oxide quantum dots as photocatalysts for hydrogen generation in the aqueous phase. ChemSusChem. 2017;10(16):3260–7.10.1002/cssc.201700910Search in Google Scholar PubMed

[104] Jagannathan M, Dhinasekaran D, Soundharraj P, Rajendran S, Vo DVN, Prakasarao A, et al. Green synthesis of white light emitting carbon quantum dots: Fabrication of white fluorescent film and optical sensor applications. J Hazard Mater. 2021;416:125091. 10.1016/j.jhazmat.2021.125091.Search in Google Scholar PubMed

[105] Róding M, Bradley SJ, Nydén M, Nann T. Fluorescence lifetime analysis of graphene quantum dots. J Phys Chem C. 2014;118(51):30282–90.10.1021/jp510436rSearch in Google Scholar

[106] Li Y, Jia W, Yang Z, Yang J, Chen Y, Yang J, et al. A fluorescent and colorimetric probe for MoO42- based on sulfur quantum dots nanozyme. J Alloy Compd. 2023;934:167926. 10.1016/j.jallcom.2022.167926.Search in Google Scholar

[107] Huang A, Yang X, Xia T, He D, Zhang R, Li Z, et al. A fluorescence probe of sulfur quantum dots for sensitive detection of copper ions in Paris polyphylla var. yunnanensis. Microchem J. 2022;179:107639.10.1016/j.microc.2022.107639Search in Google Scholar

[108] Ma Q, Zu Y, Guan Y, Ma P, Li S, Song J, et al. Small molecule L-cysteine assisted synthesis of green fluorescent sulfur quantum dots for nimesulide sensing and bioimaging. Microchem J. 2023;190:108734.10.1016/j.microc.2023.108734Search in Google Scholar

[109] Jiang W, He R, Lv H, He X, Wang L, Wei Y. Chiral sensing of tryptophan enantiomers based on the enzyme mimics of β-cyclodextrin-modified sulfur quantum dots. ACS Sens. 2023;8(11):4264–71.10.1021/acssensors.3c01616Search in Google Scholar PubMed

[110] Zhang C, Zhang P, Ji X, Wang H, Kuang H, Cao W, et al. Ultrasonication-promoted synthesis of luminescent sulfur nano-dots for cellular imaging applications. Chem Commun. 2019;55(86):13004–7.10.1039/C9CC06586ESearch in Google Scholar

[111] Chaghaghazardi M, Kashanian S, Nazari M, Omidfar K, Joseph Y, Rahimi P. Nitrogen and sulfur co-doped carbon quantum dots fluorescence quenching assay for detection of mercury (II). Spectrochim Acta Part A Mol Biomol Spectrosc. 2023;293:122448.10.1016/j.saa.2023.122448Search in Google Scholar PubMed

[112] Shamsipur M, Nasirian V, Mansouri K, Barati A, Veisi-Raygani A, Kashanian S. A highly sensitive quantum dots-DNA nanobiosensor based on fluorescence resonance energy transfer for rapid detection of nanomolar amounts of human papillomavirus 18. J Pharm Biomed Anal. 2017;136:140–7.10.1016/j.jpba.2017.01.002Search in Google Scholar PubMed

[113] Long C, Jiang Z, Shangguan J, Qing T, Zhang P, Feng B. Applications of carbon dots in environmental pollution control: A review. Chem Eng J. 2021;406:126848.10.1016/j.cej.2020.126848Search in Google Scholar

[114] Pavlicek A, Neubauer S, Zafiu C, Huber-Humer M, Ehmoser E-K, Part F. The use and detection of quantum dots as nanotracers in environmental fate studies of engineered nanoparticles. Env Pollut. 2023;317:120461.10.1016/j.envpol.2022.120461Search in Google Scholar PubMed

[115] Kader DA, Aziz DM, Mohammed SJ, Maarof NNN, Karim WO, Mhamad SA, et al. Green synthesis of ZnO/catechin nanocomposite: Comprehensive characterization, optical study, computational analysis, biological applications and molecular docking. Mater Chem Phys. 2024 Jun 1;319:129408.10.1016/j.matchemphys.2024.129408Search in Google Scholar

[116] Liu B, Tao J, Pan J, Li C, Li F, Zheng Y. Construction of green fluorescent carbon dots with high quantum yield for cancer cell recognition and Fe3+ detection. Opt Mater (Amst). 2021;113:110892.10.1016/j.optmat.2021.110892Search in Google Scholar

[117] Issa MA, Abidin ZZ, Sobri S, Rashid SA, Mahdi MA, Ibrahim NA. Fluorescent recognition of Fe3 + in acidic environment by enhanced-quantum yield N-doped carbon dots: optimization of variables using central composite design. Sci Rep. 2020;10(1):1–18. 10.1038/s41598-020-68390-8.Search in Google Scholar PubMed PubMed Central

[118] Zhou M, Zhou Z, Gong A, Zhang Y, Li Q. Synthesis of highly photoluminescent carbon dots via citric acid and Tris for iron(III) ions sensors and bioimaging. Talanta. 2015;143:107–13. 10.1016/j.talanta.2015.04.015.Search in Google Scholar PubMed

[119] Longardner K, Litvan I, Wang J. A highly selective fluorescent biosensor based on sulfur quantum dots for iron(III) detection. Mater Today Commun. 2024;38:131234. 10.1016/j.snb.2021.131234.Search in Google Scholar

[120] Lei J, Huang Z, Gao P, Sun J, Zhou L. Polyvinyl alcohol enhanced fluorescent sulfur quantum dots for highly sensitive detection of Fe3+ and temperature in cells. Part Part Syst Charact. 2021;38(4):1–8.10.1002/ppsc.202000332Search in Google Scholar

[121] Ashraf A, So PW. Spotlight on ferroptosis: Iron-dependent cell death in Alzheimer’s disease. Front Aging Neurosci. 2020;12:1–10.10.3389/fnagi.2020.00196Search in Google Scholar PubMed PubMed Central

[122] Iqbal A, Tian Y, Wang X, Gong D, Guo Y, Iqbal K, et al. Carbon dots prepared by solid state method via citric acid and 1,10-phenanthroline for selective and sensing detection of Fe2+ and Fe3+. Sens Actuators, B Chem. 2016;237:408–15.10.1016/j.snb.2016.06.126Search in Google Scholar

[123] Lu C, Wang Y, Xu B, Zhang W, Xie Y, Chen Y, et al. A colorimetric and fluorescence dual-signal determination for iron(II) and H2O2 in food based on sulfur quantum dots. Food Chem. 2022;366:130613. 10.1016/j.foodchem.2021.130613.Search in Google Scholar PubMed

[124] Barras F, Fontecave M. Cobalt stress in Escherichia coli and Salmonella enterica: Molecular bases for toxicity and resistance. Metallomics. 2011;3(11):1130–4.10.1039/c1mt00099cSearch in Google Scholar PubMed

[125] Wen X, Wen G, Li W, Zhao Z, Duan X, Yan W, et al. Carbon dots for specific “off-on” sensing of Co2+ and EDTA for in vivo bioimaging. Mater Sci Eng C. 2021;123:112022. 10.1016/j.msec.2021.112022.Search in Google Scholar PubMed

[126] Awual MR, Ismael M, Yaita T. Efficient detection and extraction of cobalt(II) from lithium ion batteries and wastewater by novel composite adsorbent. Sens Actuators, B Chem. 2014;191:9–18. 10.1016/j.snb.2013.09.076.Search in Google Scholar

[127] Li CL, Huang CC, Periasamy AP, Roy P, Wu WC, Hsu CL, et al. Synthesis of photoluminescent carbon dots for the detection of cobalt ions. RSC Adv. 2015;5(3):2285–91.10.1039/C4RA11704BSearch in Google Scholar

[128] Li L, Yang C, Li Y, Nie Y, Tian X. Sulfur quantum dot-based portable paper sensors for fluorometric and colorimetric dual-channel detection of cobalt. J Mater Sci. 2021;56(7):4782–96.10.1007/s10853-020-05544-zSearch in Google Scholar

[129] Wang S, Bao X, Gao B, Li M. A novel sulfur quantum dot for the detection of cobalt ions and norfloxacin as a fluorescent “switch.”. Dalt Trans. 2019;48(23):8288–96.10.1039/C9DT01186BSearch in Google Scholar PubMed

[130] Reji RV, Biju V. Rapid and scalable synthesis of sulfur quantum dots through ozone etching: photoluminescence and FRET-mediated Co2+ sensing. N J Chem. 2023;47(19):9113–23.10.1039/D2NJ06195CSearch in Google Scholar

[131] Sun X, Lei Y. Fluorescent carbon dots and their sensing applications. TrAC - Trends Anal Chem. 2017;89:163–80. 10.1016/j.trac.2017.02.001.Search in Google Scholar

[132] Keerthana S, Sam B, George L, Sudhakar YN, Varghese A. Fluorescein based fluorescence sensors for the selective sensing of various analytes. J Fluoresc. 2021;31(5):1251–76. 10.1007/s10895-021-02770-9.Search in Google Scholar PubMed

[133] Wang X, Guo W, Wang X, Hua Q, Tang F, Li X, et al. Dual recognition strategy for ultra-sensitive fluorescent detection of Hg2+ at femto-molar level based on aptamer functionalized sulfur quantum dots. Arab J Chem. 2022;15(9):104080. 10.1016/j.arabjc.2022.104080.Search in Google Scholar

[134] Kumar JV, Tammina SK, Rhim J-W. One-step hydrothermal synthesis of sulfur quantum dots for detection of Hg2+ ions and latent fingerprints. Colloids Surf A Physicochem Eng Asp. 2024;690:133682.10.1016/j.colsurfa.2024.133682Search in Google Scholar

[135] Singh P, Arpita N, Kumar S, Kumar P, Kataria N, Bhankar V, et al. Assessment of biomass-derived carbon dots as highly sensitive and selective templates for the sensing of hazardous ions. Nanoscale. 2023;15(40):16241–67.10.1039/D3NR01966GSearch in Google Scholar PubMed

[136] Humaera NA, Fahri AN, Armynah B, Tahir D. Natural source of carbon dots from part of a plant and its applications: a review. Luminescence. 2021;36(6):1354–64.10.1002/bio.4084Search in Google Scholar PubMed

[137] Harish R, Samuel J, Mishra R, Chandrasekaran N, Mukherjee A. Bio-reduction of Cr(VI) by exopolysaccharides (EPS) from indigenous bacterial species of Sukinda chromite mine, India. Biodegradation. 2012;23(4):487–96.10.1007/s10532-011-9527-4Search in Google Scholar PubMed

[138] Tan Q, An X, Pan S, Liu H, Hu X. Hydrogen peroxide assisted synthesis of sulfur quantum dots for the detection of chromium(VI) and ascorbic acid. Spectrochim Acta – Part A Mol Biomol Spectrosc. 2021;247:119122. 10.1016/j.saa.2020.119122.Search in Google Scholar PubMed

[139] Deng S, Long J, Dai X, Wang G, Zhou L. Simultaneous detection and adsorptive removal of Cr(VI) ions by fluorescent sulfur quantum dots embedded in chitosan hydrogels. ACS Appl Nano Mater. 2023;6(3):1817–27.10.1021/acsanm.2c04768Search in Google Scholar

[140] Zhang Y, Liu J, Wu X, Tao W, Li Z. Ultrasensitive detection of Cr(VI) (Cr2O72−/CrO42−) ions in water environment with a fluorescent sensor based on metal-organic frameworks combined with sulfur quantum dots. Anal Chim Acta. 2020;1131:68–79. 10.1016/j.aca.2020.07.026.Search in Google Scholar PubMed

[141] Batool M, Junaid HM, Tabassum S, Kanwal F, Abid K, Fatima Z, et al. Metal ion detection by carbon dots – a review. Crit Rev Anal Chem. 2022;52(4):756–67. 10.1080/10408347.2020.1824117.Search in Google Scholar PubMed

[142] Kumar JV, Rhim J. Fluorescent carbon quantum dots for food contaminants detection applications. J Env Chem Eng. 2024;12:111999. 10.1016/j.jece.2024.111999.Search in Google Scholar

[143] Madrakian T, Ghazizadeh F. Cloud-point preconcentration and spectrophotometric determination of trace amounts of molybdenum(VI) in steels and water samples. J Hazard Mater. 2008;153(1–2):695–700.10.1016/j.jhazmat.2007.09.013Search in Google Scholar PubMed

[144] Wang X, Su Z, Li L, Tu Y, Yan J. Sensitive detection of molybdenum through its catalysis and quenching of gold nanocluster fluorescence. Spectrochim Acta - Part A Mol Biomol Spectrosc. 2020;229:117909. 10.1016/j.saa.2019.117909.Search in Google Scholar PubMed

[145] Pyrzynska K. Determination of molybdenum in environmental samples. Anal Chim Acta. 2007;590(1):40–8.10.1016/j.aca.2007.03.013Search in Google Scholar PubMed

[146] Tammina SK, Priyadarshi R, Rhim JW. Dual functions of metal ion detection and antibacterial activity of sulfur quantum dots. N J Chem. 2023;47(16):7733–45.10.1039/D3NJ00373FSearch in Google Scholar

[147] Liu W, Zhang R, Kang Y, Zhang X, Wang H, Li L, et al. Preparation of nitrogen-doped carbon dots with a high fluorescence quantum yield for the highly sensitive detection of Cu2+ ions, drawing anti-counterfeit patterns and imaging live cells. N Carbon Mater. 2019;34(4):390–402. 10.1016/S1872-5805(19)30024-1.Search in Google Scholar

[148] Kumari N, Dey N, Bhattacharya S. Remarkable role of positional isomers in the design of sensors for the ratiometric detection of copper and mercury ions in water. RSC Adv. 2014;4(9):4230–8.10.1039/C3RA45054FSearch in Google Scholar

[149] Sonaimuthu M, Ganesan S, Anand S, Kumar AJ, Palanisamy S, You SG, et al. Multiple heteroatom dopant carbon dots as a novel photoluminescent probe for the sensitive detection of Cu2 + and Fe3 + ions in living cells and environmental sample analysis. Env Res. 2023 Feb;219:115106.10.1016/j.envres.2022.115106Search in Google Scholar PubMed

[150] Du J, Yang Y, Shao T, Qi S, Zhang P, Zhuo S, et al. Yellow emission carbon dots for highly selective and sensitive OFF-ON sensing of ferric and pyrophosphate ions in living cells. J Colloid Interface Sci. 2021;587:376–84. 10.1016/j.jcis.2020.11.108.Search in Google Scholar PubMed

[151] Wang ZX, Yu XH, Li F, Kong FY, Lv WX, Fan DH, et al. Preparation of boron-doped carbon dots for fluorometric determination of Pb(II), Cu(II) and pyrophosphate ions. Microchim Acta. 2017;184(12):4775–83.10.1007/s00604-017-2526-3Search in Google Scholar

[152] Gong X, Cai Q, Zhang J, Min Y, Fu W, Wang Y, et al. Protein-directed synthesis of fluorescent sulfur quantum dots for highly robust detection of pyrophosphate. Microchim Acta. 2023;190(3):104. 10.1007/s00604-023-05686-2.Search in Google Scholar PubMed

[153] Omar NAS, Fen YW, Ramli I, Hashim HS, Ramdzan NSM, Fauzi NIM. A review on carbon dots: synthesis, characterization and its application in optical sensor for environmental monitoring. Nanomaterials. 2022;12(14):2365.10.3390/nano12142365Search in Google Scholar PubMed PubMed Central

[154] Peng X, Wang Y, Luo Z, Zhang B, Mei X, Yang X. Facile synthesis of fluorescent sulfur quantum dots for selective detection of p-nitrophenol in water samples. Microchem J. 2021;170:106735. 10.1016/j.microc.2021.106735.Search in Google Scholar

[155] Wang X, Yan F, Xu M, Ning J, Wei X, Bai X. Facile synthesis of multicolor emitting sulfur quantum dots and their applications in light blocking field, anti-counterfeiting and sensing. J Colloid Interface Sci. 2024;653(PB):1137–49. 10.1016/j.jcis.2023.09.140.Search in Google Scholar PubMed

[156] Qu Y, Yu L, Zhu B, Chai F, Su Z. Green synthesis of carbon dots by celery leaves for use as fluorescent paper sensors for the detection of nitrophenols. N J Chem. 2020;44(4):1500–7.10.1039/C9NJ05285BSearch in Google Scholar

[157] Tammina SK, Priyadarshi R, Rhim JW. Sensing nitrophenols and luminescent ink applications of sulfur quantum dots. N J Chem. 2023;47(41):18945–56.10.1039/D3NJ03447JSearch in Google Scholar

[158] Gao M, Bian C, Zhao H, Wang H, Wang X. Assembly-fission synthesized fluorescent sulfur quantum dots combined with tyrosinase catalytic oxidation for selective detection of p-aminophenol in water samples. N J Chem. 2024;48(5):2311–20.10.1039/D3NJ04152BSearch in Google Scholar

[159] Sanabria-Chinchilla J, Asazawa K, Sakamoto T, Yamada K, Tanaka H, Strasser P. Noble metal-free hydrazine fuel cell catalysts: EPOC effect in competing chemical and electrochemical reaction pathways. J Am Chem Soc. 2011;133(14):5425–31.10.1021/ja111160rSearch in Google Scholar PubMed

[160] An Z, Li Z, He Y, Shi B, Wei L, Yu M. Ratiometric luminescence detection of hydrazine with a carbon dots-hemicyanine nanohybrid system. RSC Adv. 2017;7(18):10875–80.10.1039/C6RA27844BSearch in Google Scholar

[161] Hiremath SD, Priyadarshi B, Banerjee M, Chatterjee A. Carbon dots-MnO2 based turn-on fluorescent probe for rapid and sensitive detection of hydrazine in water. J Photochem Photobiol A Chem. 2020;389:112258. 10.1016/j.jphotochem.2019.112258.Search in Google Scholar

[162] Li X, Wang X, Guo W, Luan F, Tian C, Zhuang X, et al. A novel turn-on fluorescent sensor based on sulfur quantum dots and MnO2 nanosheet architectures for detection of hydrazine. Nanomaterials. 2022;12(13):2207.10.3390/nano12132207Search in Google Scholar PubMed PubMed Central

[163] Feng Y, Zhong D, Miao H, Yang X. Carbon dots derived from rose flowers for tetracycline sensing. Talanta. 2015;140:128–33. 10.1016/j.talanta.2015.03.038.Search in Google Scholar PubMed

[164] Qu F, Sun Z, Liu D, Zhao X, You J. Direct and indirect fluorescent detection of tetracyclines using dually emitting carbon dots. Microchim Acta. 2016;183(9):2547–53. 10.1007/s00604-016-1901-9.Search in Google Scholar

[165] Yan Y, Liu JH, Li RS, Li YF, Huang CZ, Zhen SJ. Carbon dots synthesized at room temperature for detection of tetracycline hydrochloride. Anal Chim Acta. 2019;1063:144–51. 10.1016/j.aca.2019.02.047.Search in Google Scholar PubMed

[166] Chang Y, He R, Wei Y, Wang L. Polyethyleneimine-sulfur quantum dot composites for dual-channel detection of tetracycline by colorimetric and fluorescence sensing. ACS Appl Nano Mater. 2023;6(9):7414–21.10.1021/acsanm.3c00506Search in Google Scholar

[167] Ye C, Yu M, Wang Z. Fabrication of sulfur quantum dots via a bottom-up strategy and its application for enhanced fluorescence monitoring of o-phenylenediamine. Microchem J. 2022;183:107978. 10.1016/j.microc.2022.107978.Search in Google Scholar

[168] Li X, Chen C, Xu F, Liang Z, Xu G, Wei F, et al. Novel dual-emission sulfur quantum dot sensing platform for quantitative monitoring of pesticide 2,4-dichlorophenoxyacetic acid. Talanta. 2023;260:124639. 10.1016/j.talanta.2023.124639.Search in Google Scholar PubMed

[169] Yan F, Zang Y, Xu J, Guan S, Chen L, Sun J. Facile preparation of sulfur quantum dots involving β-cyclodextrin for ratiometric fluorescence/scattered light detection of acridine orange. Mater Res Bull. 2022;150:111765. 10.1016/j.materresbull.2022.111765.Search in Google Scholar

[170] Lai X, Qiu G, Ye Q, Wang R, Liu JB. A reaction-based fluorescent probe for detecting o-phenylenediamine in water and lateritic soil samples. J Photochem Photobiol A Chem. 2020;386:112101. 10.1016/j.jphotochem.2019.112101.Search in Google Scholar

[171] Liu J, Zhang W, Kumar A, Rong X, Yang W, Chen H, et al. Acridine orange encapsulated mesoporous manganese dioxide nanoparticles to enhance radiotherapy. Bioconjug Chem. 2020;31(1):82–92.10.1021/acs.bioconjchem.9b00751Search in Google Scholar PubMed

[172] Wang X, Yu J, Wu X, Fu J, Kang Q, Shen D, et al. A molecular imprinting-based turn-on Ratiometric fluorescence sensor for highly selective and sensitive detection of 2,4-dichlorophenoxyacetic acid (2,4-D). Biosens Bioelectron. 2016;81:438–44. 10.1016/j.bios.2016.03.031.Search in Google Scholar PubMed

[173] Su D, Han X, Yan X, Jin R, Li H, Kong D, et al. Smartphone-assisted robust sensing platform for on-site quantitation of 2,4-dichlorophenoxyacetic acid using red emissive carbon dots. Anal Chem. 2020;92(18):12716–24.10.1021/acs.analchem.0c03275Search in Google Scholar PubMed

Received: 2024-03-12

Revised: 2024-09-25

Accepted: 2024-12-27

Published Online: 2025-03-05

This work is licensed under the Creative Commons Attribution 4.0 International License.

Articles in the same Issue

https://doi.org/10.1515/ntrev-2024-0138

Keywords for this article

sulfur quantum dots; fluorescence sensing environmental monitoring; heavy metals; cytotoxicity

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