Laboratorio de Química Orgánica y Biomolecular, CMN, Universidad Industrial de Santander, Parque Tecnológico Guatiguará, Km 2 vía refugio, Piedecuesta, A.A. 681011, Colombia
Laboratorio de Química Orgánica Aplicada, Universidad Manuela Beltrán, Cl. 33 # 26-34, Bucaramanga, A.A. 680002, Colombia
† These authors contributed equally to this work.
Academic Editor: Hossein Hosseinkhani
Special Issue: Nanosensors: Recent Advances and Future Trends
Received: December 01, 2020 | Accepted: April 15, 2021 | Published: April 28, 2021
Recent Progress in Materials 2021, Volume 3, Issue 2, doi:10.21926/rpm.2102015
Recommended citation: Echeverry-Gonzalez CA, Kouznetsov VV. Carbon D ots: An Insight into Their Application in Heavy Metal Sensing. Recent Progress in Materials 2021;3(2):20; doi:10.21926/rpm.2102015.
© 2021 by the authors. This is an open access article distributed under the conditions of the Creative Commons by Attribution License, which permits unrestricted use, distribution, and reproduction in any medium or format, provided the original work is correctly cited.
In the last decade, the development of novel functional electronic devices had gained much attention owing to the increasing problems regarding energy consumption and fossil fuels. These electronic devices are made from organic, inorganic, and combined organic–inorganic materials, and offer the possibility of tunable long-term stability, conductivity, and photoluminescence properties through subtle structural changes that resulted in wide-range applications, such as photovoltaic (PV) , artificial photosynthesis , and Organic Light-Emitting Diodes (OLEDs) .
Among these electronic devices, nanomaterial-based devices gained increasing interest because their sizes are in the quantum-confined regimen with at least one dimension less than 10 nm and their excitons are confined in spatial dimensions with quantized energy states resulting in great physicochemical properties [4,5]. An example of this kind of nanomaterials would be the fluorescent quantum dots (QDs), which are applicable in a broad range of fields, such as bio-labelling , bio-imaging , drug delivery , biochemical sensing , and photodynamic therapy . QDs were first described by Ekimov and Onushenko  by exploring the size effects on the exciton absorption spectrum of CuCl crystals’ growth in a transparent dielectric matrix (glass matrix). However, the term “Quantum Dot” was coined by Reed and co-workers, referring to nanoparticle-based semiconductors [4,12]. Thus, the first class of these nanomaterials corresponded to semiconductor quantum dots (SQDs) that were obtained from the group 13 and 14 elements (e.g., gallium-, indium-, silicon-, germanium-, and lead-based QDs). However, colloidal methods were used for synthesizing the QDs by combining a transition metal, such as zinc, cadmium, and mercury, with the representative elements such as sulphur, selenium, or tellurium [13,14]. Another class of QDs is based on carbon (CQDs), which have received great attention in the last two decades because of their optoelectronic properties. According to Valcárcel and co-workers , CDs could be classified in carbon nanodots (CNDs, nanodots that lack quantum confinement), carbon quantum dots (CQDs, nanodots with quantum confinement and a crystalline structure), and graphene quantum dots (GQDs, which use a p-conjugated single sheet of graphene with quantum confinement).
The increasing interest in QDs has been reflected in the huge number of papers published in this field since the first original report by Ekimov and Onushenko. According to the SciFinder search performed by our team, in the last two decades, the number of publications introducing the term QDs in the articles’ titles has increased considerably, with a peak of 9563 publications reached in 2019. Likewise, the number of publications using the term CQDs in their titles has also increased since their discovery in 2004 (Figure 1).
Figure 1 Number of publications on QDs and CQDs since 2000, according to SciFinder database
This review is specifically focused on the fluorescent CDs, which include all three groups mentioned above (CNDs, CQDs, and GQDs), and their application in heavy metal fluorescence sensing. We conducted a comprehensive survey on the important aspects of CDs, including their design, fabrication, principal features, optical properties, and the applications of their fluorescence emission for the detection of metal ions in aqueous solutions with demonstrated toxicological and environmental impact. Accordingly, we could provide an overview of the status of these applications.
2. Synthesis and General Properties of the CDs
CDs are novel nano-sized carbon particles (size < 10nm) with extraordinary features, such as non-toxicity, high quantum yield (QY) with a tunable bandgap, photostability, low cost, and high surface passivation . Despite these advantages, it is worth mentioning that CDs show some disadvantages in comparison to SQDs that limit their applications, such as difficulties in preparation, purification, and characterization. In addition, they often provide low quantum yields and certain ambiguity in their geometry and structure [16,17].
CDs can be classified as CNDs, CQDs, and GQDs depending on their nature, crystalline structure, and quantum confinement as mentioned above, which, in turn, depend on the different precursors and synthesis methods used for their fabrication (Figure 2) . CNDs are carbon-based nanoparticles that lack crystalline structure as well as quantum confinement; the carbon core is a disordered mixture of mostly sp3 carbons. Different top-down and bottom-up strategies have been reported for the synthesis of CNDs. However, it is very difficult to control their structure and size [19,20]. CQDs have a quasi-spherical structure with a carbon crystalline core based on sp2 and sp3 carbons and quantum confinement. In contrast, GQDs are 0 D materials that possess a well-defined crystalline structure comprising metallic atoms and nanosheets of sp2 carbons  with the specific characteristics that are derived from graphene and CDs . Therefore, GQDs have gained special attention in the scientific community. Certain characteristics of GQDs, such as FLE properties (excitation-dependent FLE, functionalization-dependent FLE), biocompatibility, low toxicity, very high thermal conductivity, good electron mobility, superior mechanical flexibility, and high photo-stability (non-photo-bleaching and non-photo-blinking) have increased their application as biomedical sensors , environmental detection of heavy metal ions , and drug delivery systems for anticancer therapy .
Figure 2 Schematic example of top-down and bottom-up methodologies for producing CDs
Regarding preparation, different synthetic methods using either top-down or bottom-up methodologies have been reported for obtaining high yields of CDs with high purity, suitable particle size, and better luminescence behaviour (Figure 2) . Top-down methodologies include protocols in which carbon sources break down to generate CDs, while bottom-up methodologies happen via the chemical fusion of small aromatic molecules through pyrolysis or carbonization. In the top-down methodologies, the most commonly used methods are laser ablation [26,27], chemical oxidation using acid-refluxing methods , ultrasonic treatment [29,30], and electrochemical oxidation [31,32]. On the other hand, the bottom-up methodologies include the classic pyrolytic process and microwave technique [33,34,35,36].
The investigation of novel CDs with different passivation agents has improved their photophysical properties while maintaining their low toxicity. For example, the application of CDs in light-emitting diode (LED) technology is an area of increasing interest due to its efficiency in energy consumption [37,38,39]. The most important component of LED devices is a sandwich-type structure known as the emitting layer, which is made of different organic and organic-inorganic materials with extraordinary emission properties, high photostability, and optimal external quantum efficiency (EQE) [3,40]. Recently, the use of organic materials for fabricating OLEDs has gained immense research interest as these materials are cheaper, thinner, more efficient, and more flexible than the other technologies used before. However, the use of fluorescent organic compounds leads to poor colour purity because these materials exhibit a broad fluorescence emission with a full-width at half-maximum (FWHM) of >60 nm . Therefore, the synthesis of novel materials showing narrow FWHM with high EQE is the current field of research with several pitfalls. As the first approach, colloidal SQDs based on Cd2+ were obtained with an efficient deep-blue emission, and quantum yields (QYs) of over 70%, EQE of 8.05%, and brightness of 62,600 cd m–2; however, the toxicity of Cd limited their applications . Considering that, Sargent and co-workers synthesized deep-blue CDs from citric acid as the carbon source and diaminonaphthalene as the passivation agent, which afforded a QY of 70% ±10%, EQE of 4%, and brightness of 2,240 cd m-2. Interestingly, these CDs showed high colour purity considering the narrow FWHM of ~35 nm .
2.1 Fluorescence Emission in CDs
At this point, it is important to mention that all CDs carry similar absorption and fluorescence properties regardless of their synthetic procedure; a typical absorption in the UV region (230–320 nm) corresponding to π-π* transitions of the carbon core and n-π* from the different connected groups, such as C=O , suggests a similarity in the structure and peripheral groups in various CDs . The fluorescence emission (FLE) in CDs is currently an important topic of discussion because the exact mechanism of this emission is not yet completely understood. Likewise, another important feature of CDs corresponds to the dependence of the fluorescence emission on the excitation wavelength (λexc), which is known as wavelength-dependent behaviour or giant red-edge effect . This behaviour in organic dyes and inorganic complexes results from a variety of solute–solvent interactions in the ground and excited states under the conditions of restricted mobility [47,48]. This phenomenon does not fit with Vavilov’s law and Kasha’s rule, which state that the emission energy is independent of the excitation energy within the absorption band and that fluorescence normally occurs from the lowest vibrational level of the first excited electronic state, respectively . This could be attributed to different factors, such as solvation effects and wide distributions of the sized dots and surface . Figure 3 summarizes a schematic mechanism for the tunable FLE in CDs in which the presence of the particles of different sizes and different emissive sites on the surface of the CDs result in multi-colour emission; it also shows the dependent behaviour of the emission peaks on lexc. It is worth noting that this tunable emission in a wide range of the visible spectrum shown by the CDs results in surface passivation wherein the surface defects are stabilized, leading to the emission with high quantum yields .
Figure 3 Emission features of CDs.
Different origins for FLE in CDs have been explored; however, those from the presence of different particles and multiple-core shell structures have a high grade of acceptance (Figure 3). The first one is based on the band-gap transitions caused by conjugated p-domains, while the second one considers that the presence of surface defects in CDs are responsible for the high efficient emission . Gao and co-workers  studied the origin of the multi-emission luminescence in CDs, which was obtained from a mixture of 2,4-diamine toluene, ethylenediamine, and phosphoric acid through the bottom-up approach by heating at 195 ℃ for 9.5 h. Multi-emission CDs showed three different emission peaks at 350, 420, and 520 nm, which were lexc-dependent, with excitation ranging from 260–480 nm. The FLE mechanisms were clarified mainly using time-resolved emission spectra, which revealed that the high-energy peak was associated with the band-gap transitions caused by the conjugated p-domains (carbon core); the peak at 420 nm was attributed to the surface defects and the peak at 520 nm corresponded to the aggregated molecular state. In this case, an important conclusion was that the multi-emission luminescence originated from multi-energy states resulting from the presence of three kinds of CDs. However, these emission phenomena not only restricted the presence of different particles but also prevented the presence of multiple-core shell structures (different emissive sites, Figure 3). Lü and co-workers reported the synthesis of multiple-core shell structured CDs via the hydrothermal method from 5-amino-1,10-phenanthroline and citric acid, which showed triple emission in blue, green, and red regions coming from the core, edge, and surface bands .
Thus, FLE is the main feature of CDs and responsible for their application in different fields. Considering that, we highlight the applications of CD fluorescence properties, specifically the applications in biochemical sensing . Fluorescence-based sensing has become an active area of interest in recent years due to its ability to easily recognize varied target molecules or ions using versatile sensory materials with high sensitivity and excellent linear response. Different organic and organic-inorganic materials have been employed as fluorescent sensors, including complexes [53,54], dyes [55,56], heterocyclic compounds [57,58], etc. Likewise, it has been demonstrated that the use of these organic sensors improves the catalytic activity for detecting the target metals .
A recognition unit contains within the structure of sensory material, which results in certain fluorescence changes upon contacting the target species; the changes in fluorescence include changes in the intensity, emission peak position, total quenching, anisotropy, or lifetime . Total quenching of fluorescence (turn-off mode) and fluorescence recovery (turn-on mode) are the most extensively used principles for sensory applications, in which different mechanisms leading to fluorescence turn-on and turn-off may occur depending on the interaction with the target ions or molecules.
Figure 4 summarizes the mechanisms that commonly lead to fluorescence turn-on and turn-off, which include: i) The photo-induced electron transfer, PET; it occurs through an internal deactivation process involving electronic interaction between the excited state of the recognition unit (fluorophore) and the target molecule; ii) The photo-induced charge transfer, PCT; the fluorescence changes occur by the interruption of the electronic coupling between the donor and acceptor moieties; iii) the Föster resonance energy transfer, FRET; this is a process in which deactivation of an excited molecule transfers the energy to another molecule to excite it; iv) Ratiometric dual emission, RDE; It happens in the presence of two non-overlapping emission peaks that respond independently to the target analytes, and v) Inner filter effect, IFE; it occurs when the absorption of excitation and/or emission radiation happens by a sample matrix and reduces the fluorescence intensity, thereby resulting in a lower QY [61,62].
Figure 4 Schematic example of the application of CDs in fluorescence-based sensing.
In some cases, the design of the sensory materials that recognize the target analytes with high sensibility is a complicated aim owing to the difficulties of building the right structure using the correct energy-transfer mechanisms. A good example of the design of a sensing probe was reported by Wang and co-workers . For the detection of tetracyclines, they described the synthesis and characterization of a sensor that was based on boron nitride QDs and Europium ions (Eu3+). Tetracyclines are broad-spectrum antibiotic compounds extensively used for the prevention and control of microorganisms. The QDs showed a maximum FLE peak at 414 nm with a QY of 5.6%, and the presence of Eu3+ did not influence the FLE of the QDs. This sensor recognized the target molecules by three different mechanisms: i) Excitation spectrum of QDs that was overlapped with the absorption spectrum of tetracyclines, which might cause the fluorescence turn-off from the IFE of the QDs towards tetracyclines; ii) the conduction bands of the QDs were calculated to be –2.1 eV, which is higher than the conduction bands of different tetracyclines derivatives; therefore, a PET process could take place from QDs towards the target molecules; and iii) tetracyclines had a b-diketonate configuration within their structures that enabled them to chelate with Eu3+, which was accompanied by the energy transfer, defined by the authors as the antenna effect; this caused the enhanced red fluorescence of Eu3+ at 616 nm (ratiometric dual emission). Likewise, the above-mentioned mechanisms were used in designing the sensors for detecting heavy metal ions. For example, Wang and co-workers reported the synthesis of dual-emission SQDs from colloidal methods using a binary mixture of Cd and Te for the detection of Cu ions. These green emissive SQDs showed two well-resolved emission peaks at 550 and 650 nm in the presence of Cu2+ under a single wavelength excitation, which allowed them to explore a ratiometric probe with a linear range, which resided between 5 × 10–2 to 5 × 10–1 µM, with a LOD of 0.001 µM .
3. Application of CDs in Heavy Metal Sensing
One of the most important sensing applications of CDs corresponds to the recognition of heavy metals with environmental and biological concerns. Some recent studies reported that the use of CDs with passivation agents could detect Hg2+, Pb2+, Cu2+, Fe3+, Cd2+, and Cr6+ ions, as summarized in Table 1. Next, we describe the recent advancements for each metal ion regarding this important issue.
3.1 CDs in Mercury Ion Sensing
The detection of Hg2+ ions is very important due to their toxicity and environmental risks. Diverse diseases are associated with the accumulation of Hg2+ ions in the human body, such as Minamata disease, which is related to severe neurological pathologies including motion disorders, ataxia, and paroxysmal convulsion . Different studies have been focused on Hg2+ detection in aqueous solutions , food , and living cells . Yang and co-workers reported the bright blue-fluorescent N-doped CDs that were synthesized from citric and folic acid . CDs were demonstrated to be a sensitive fluorescent system for detecting Hg2+ in an aqueous solution through an ultrafast electron transfer reaction, with a low limit of detection (LOD) of 0.124 µM. Interestingly, the fluorescence intensities exhibited a pH-dependent behaviour when CDs were excited at the lexc of 390 nm, with an excellent correlation between FLE and pH ranging from 6.8–7.8. Considering the tunable emission of CDs, different fluorescent systems for the detection of Hg2+ have been synthesized, with emission in many regions of the visible spectrum. Teng and co-workers developed N-doped CDs with an efficient green emission that was quenched (turn-off) through PET mechanism in the presence of Hg2+ ions, showing a LOD of 0.89 µM in water. Interestingly, the fluorescence could be recovered (turn-on) when the iodide ions were present in the solution, which could produce HgI2, leading to a LOD for iodide ion detection of 0.50 µM . In recent years, several studies demonstrating natural products as a highly abundant carbon source for fabricating CDs have been reported. Citrus juice is a good alternative because of its high content of citric acid. Belachew and co-workers synthesized the bright blue-fluorescent N-doped CDs from citrus lemon juice as a carbon source and ethylenediamine as the passivation agent via hydrothermal treatment at 200 ℃ . These CDs showed a narrow size distribution (particle size of ~3 nm), with a high QY of 31%, high selectivity and sensitivity towards Hg2+ ions, a LOD of 0.0053 µM, and a limit of quantification (LOQ) of 0.0183 µM. Other CDs have also been synthesized for Hg2+ detection and are summarized in Table 1 [65,66,67,68].
3.2 CDs in Lead Ion Sensing
Pb2+ ions are of great concern as they cause high toxicity in the human body and environmental damage . Kumar and co-workers reported the synthesis of biocompatible CDs for Pb2+ detection using a green methodology from Pearl Millet Seed by a hydrothermal approach . The optical properties of the CDs allowed them to recognize Pb2+ selectively with a LOD of 0.18 nM through either a colorimetric or fluorometric methodology, which resulted in the modulation of a PCT process. As can be seen, the use of biomass is a great alternative for obtaining CDs; for example, it has been reported that the use of extracts of bamboo leaves for the development of nanohybrids of CDs with multi-emission luminescence for highly sensitive detection of sub-nanomolar Pb2+ and Hg2+ ions with a LOD of 0.14 and 0.22 nM (ratiometric probe), respectively . Likewise, several studies using fluorescence sensors based on CDs for the detection of Pb2+ ions have been reported so far (Table 1) [69,70].
3.3 CDs in Copper Ion Sensing
The oxidized copper species (Cu1+ and Cu2+ ions), which are among the most studied and versatile catalytic systems , play a key role in many biological processes and result in neurodegenerative diseases when the cellular homeostasis is altered by the copper ions . However, an excess of Cu2+ ions in the human body, which could enter through contaminated water, can result in different diseases including liver or kidney damage and gastrointestinal disturbance . Solanki and co-workers reported the synthesis of highly fluorescent N, S co-doped CDs from banana (Musa acuminata) juice for the detection of Cu2+ ions. The spherical CDs showed high QY (32%), displaying the detection of Cu2+ through a “turn-off” response, with a LOD of 0.3 µg/mL in a concentration range of 1–800 µg/mL. This “turn-off” response likely resulted from the PCT mechanism; upon coordination of Cu2+ to the functional groups (-OH,-COOH, C =O) on the CD surface, a charge transfer between NS-CDs and Cu2+ ions was possible, showing non-radiative recombination, and, therefore, the fluorescence quenching happened . The activation of Metal-Organic Frameworks (MOFs) by treatment with CDs was explored in recent years for the development of selective sensors towards the metal ions. Thus, the activation of MOFs based on 2-amino-terephthalic acid using the CDs obtained from glucan by a hydrothermal method led to enhanced fluorescence, which was quenched with the removal of CDs by Cu2+ and Fe3+ ions. The detection method using this fluorescence “on/off” probe showed a LOD of 1.3 ppb and 2.3 ppb for Cu2+ and Fe3+ ions, respectively . Also, other different fluorescence modes were used to detect Cu2+ ions. Liu and co-workers synthesized Si-doped CDs using a fluorescent “on-off-on” mode for the detection of Cu2+ and L-cysteine (L-Cys), wherein, upon recognition of Cu2+, the fluorescence was quenched by an electron transfer process. However, fluorescence could be recovered by the addition of L-Cys (Table 1, ref ).
3.4 CDs in Iron Ion Sensing
Similar to the Cu2+ ions, the detection of Fe3+ ions is also very important because they play a key role in different biological processes, such as oxygen transportation, enzymatic catalysis, and cell proliferation . Considering that a disproportion of this metal ion can lead to many diseases, it is important to evaluate Fe3+ ion levels in the drinking water . A fluorescent “on-off-on” mode was employed to determine Fe3+ ions using N and P co-doped CDs with a high QY of 84%. Interestingly, Fe3+ led to quenching of the fluorescence through PET from CDs toward the 3d orbital of Fe3+, which was recovered when catecholamine neurotransmitters were employed, demonstrating the versatility of these CDs as sensors. For the detection of Fe3+, co-doped CDs showed a linear range between 0–40 µM and a LOD of 0.2 µM that was below the recommended Fe3+ levels in drinking water (5.4 µM) . For dopamine, one of the most important catecholamine neurotransmitters, the linear range was between 4–15 µM, with a LOD of 0.1 µM. Recently, Geng and co-workers also reported the synthesis of co-doped CDs for the detection of Fe3+ ions together with Escherichia coli. CDs were co-doped with nitrogen and boron and resulted in a blue FLE. The recognition of E. coli was possible due to the presence of charged groups on the CD surface that could bind to the bacterial cells through electrostatic interaction. The LOD for Fe3+ was 0.74 µM and that for E. coli was 165 CFUs (colony forming units)/mL. The sensor was tested with real samples of tap water and orange juice, proving to be practical and effective . Recently, several other studies on the fluorescence sensors based on doped CDs have been reported using different carbon sources, such as chitosan and dansyl , maize starch , citric acid and L-cysteine , trisodium citrate and phosphoric acid , benzene sulfonic and phenylboronic acid derivatives , etc. A brief description of these studies is provided in Table 1.
3.5 CDs in Cadmium and Chromium Ion Sensing
Among other metal ions with toxicological and environmental concerns, Cd2+ and Cr6+ are important [98,99]. The investigation of fluorescence sensors for the detection of Cd2+ ions has not been widely explored. Nanohybrids based on CDs and gold nanocluster (AuNCs) have been developed as ratiometric fluorescent probes for the detection of Cd2+ ions and ascorbic acid . CDs obtained from alanine and histidine by hydrothermal treatment afforded blue emissive particles with a QY of 40.8%. The CDs/AuNCs-based system exhibited high selectivity toward Cd2+ with a LOD of 32.5 nM, leading to fluorescence quenching. However, the fluorescence was recovered in the presence of ascorbic acid (fluorescence “on-off-on” mode). The Cr6+ ions have been more extensively studied in comparison to the Cd2+ ions. Li and co-workers reported the green synthesis of CDs from flax straw, which showed a QY of 20.7%. The CDs/AuNCs used for the detection of Cd2+ led to Cr6+ detection through a “turn-off” response. Using a fluorescence “on-off-on” mode, these CDs also detected ascorbic acid based on the reduction of Cr6+ to Cr3+ by this acid, and the FLE was recovered as well . LOD values for Cr6+ ions and ascorbic acid were 0.19 and 0.35 µM, respectively. On the other hand, the use of P/N dual-doped CDs for the development of a sensor for Cr6+ ions and ascorbic acid through a fluorescence “on-off-on” mode led to a lower LOD for Cr6+ (LOD = 0.023 µM), while the LOD was higher for ascorbic acid (LOD = 1.35 µM) . Likewise, Chen and co-workers reported the synthesis of N-doped CQDs that were immobilized in hydrophilic silica hydrogels for the selective detection of Cr6+ and Re4+ ions by a turn-off fluorescent sensing platform based on the IFE mechanism. LOD values for Cr6+ and Re4+ ions were about 65 nM and 2.3 µM, respectively . It is worth noting that more serious efforts were put into the detection of Cr6+ ions instead of Cr3+ ions because the former shows higher toxicity. A GQD sensor based on the FLE turn-off mechanism was reported for the detection of Fe3+, Cr3+, and Pb2+ ions in aqueous media, with low LOD values of 50, 100, and 100 nM, respectively . Different sensing systems for Cr6+ ions have been reported using different carbon sources and presenting low LOD [77,78,79].
The monitoring of heavy metal ions having toxicological and environmental impacts is an important issue nowadays. The increase in anthropogenic activities, such as mining, agriculture, and crop eradication, can release huge amounts of these metal ions into the ecosystems. CDs have emerged as an efficient alternative to detect heavy metal ions due to their extraordinary FLE properties; in addition, they can perform without generating a negative impact on the environment, considering their low toxicity and biocompatibility. Although tremendous progress has been made for decreasing the LOD values in aqueous solution for heavy metal detection, there are still many barriers to overcome, which include difficult preparation, purification, and characterization, low quantum yields, selectivity, and reproducibility.
CAEG acknowledges the postdoctoral program VIE-UIS-Conv. 2019.
Authors contributed equally to this work.
This work was supported by Colombian Institute for Science and Research (COLCIENCIAS) under the project No. RC-007-2017, Cod. 110274558597.
The authors have declared that no competing interests exist.
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