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Facile synthesis of sulfur and nitrogen codoped graphene quantum dots for optical sensing of Hg and Ag ions
Chemical Physics Letters 730 (2019) 436–444
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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Facile synthesis of sulfur and nitrogen codoped graphene quantum dots for optical sensing of Hg and Ag ions
T
Ekta Sharmaa, Devika Vashishtb, Aseem Vashishtc, Virender Kumar Vatsa, S.K. Mehtab, ⁎ Kulvinder Singha, a
Department of Chemistry, School of Basic and Applied Sciences, Maharaja Agrasen University, Baddi 174103, India Department of Chemistry, Panjab University, Chandigarh 160014, India c Department of Physics, Panjab University, Chandigarh 160014, India b
H I GH L IG H T S
and simple synthesis of S, N-GQDs using solid state carbonization. • Facile QDs shows excellent water stability, photoluminescence. • Synthesized is independent to the excitation wavelength. • Photoluminescence • Sensor shows high selectivity and sensitivity for Ag and Hg ions.
A R T I C LE I N FO
A B S T R A C T
Keywords: Graphene quantum dots Optical sensor Stern Volmer Water pollution
Facile and simple sulfur and nitrogen co-doped graphene Quantum Dots (S, N-GQDs) were fabricated via. calcination. UV–vis. spectrum reveals two peaks originates due to the transition from n and π to π* of the luminescent material. The photoluminescence emission of S, N-GQDs has been quenched upon addition of Hg and Ag ions. The higher affinity of sulfur for Hg and Ag ions assigned selectivity to the sensing protocol which was confirmed from the absence of any interference of the competitive ions. The S, N-GQDs could sense Hg and Ag ions as low as 9.14 µM and 12.90 µM respectively.
1. Introduction Quantum Dots (QDs) have attained a respectable place in different classes of semiconductor nanoparticles due to their inherent luminescent property that leads to applications in various fields [1] such as optoelectronic devices [2,3], bioimaging [4,5], matrix for surface clean metal NPs [6], catalysis [7], sensing [8–10] etc. Among all these applications, sensing of toxic chemicals and heavy metal ions has been widely explored by various research groups [11–13]. In this context, Yao et al. [14] fabricated color tunable fibers of CdTe QDs with the aid of wet spinning which worked quite well as glucose and pH sensor. In addition, Yan et al. have studied the aggregation along with stability issues of CsPbBr3 QDs and successfully overcomes this issue by ligand modification route via. replacing oleic acid with 2‐hexyldecanoic acid during synthesis [15]. Similarly, the role of surface functionalization on CdTe QDs for optical monitoring of Hg2+ has been explored by Labeb et al. [16]. Two different capping agents (thioglycolic acid and L-
⁎
cysteine) were utilized by Labeb and co-workers for the functionalization of CdTe QDs and the sensing studies carried by them revealed that the L-cysteine capped CdTe QDs had better sensing efficacy as compared to thioglycolic acid for Hg2+ ions in terms of the selectivity and sensitivity. Numerous metal and metal chalcogenide based semiconducting QDs have been utilized by various scientific groups for the optical detection of toxic chemicals/ions [11,17,18]. In addition to their application as optical sensors, these QDs carry certain limitations which include toxicity, cost, complicated synthetic route etc. [19,20]. A step further in QDs, a novel class of zero-dimensional QDs (Carbonaceous QDs) came into existence with several advantages over the conventional QDs [21,22]. Among these, various kinds of Carbonaceous QDs especially graphene QDs (GQDs) have gained the attention of scientific community due to their low toxicity, excellent photo stability, relatively smaller size, prodigious multi-photo excitation, chemical inertness, ease of functionalization, highly water stability, excellent photoluminescence (PL) and size-dependent luminescence emission
Corresponding author. E-mail address: [email protected] (K. Singh).
https://doi.org/10.1016/j.cplett.2019.06.040 Received 30 April 2019; Received in revised form 4 June 2019; Accepted 15 June 2019 Available online 17 June 2019 0009-2614/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. (a) UV–vis. Absorption spectrum of S, N-GQDs (b) Effect of excitation on emission of S, N-GQDs.
2. Experimental section
[23]. These properties made GQDs eligible for several applications like dye sensitized solar cells [24], energy storage and sensing [25], Drug Delivery [26], photovoltaic cells [27], photo-degradation [28], photodetector [29], catalysis [30], bioimaging [31] etc. The sensing-based applications of the GQDs have been explored extensively [32]. Particularly, Shehab et al. have fabricated GQDs by the carbonization of glucose and functionalized with phenylboronic acid for efficient detection of glucose [33]. Similarly, GQDs obtained from pyrolyzing citric acid have been effectively employed for the detection of chlorine content in water system by Dong et al. [34]. To further enhance the performance of GQDs as sensors and properties such as electronic characteristics, band gap, chemical reactivity, optical behavior, doping of GQDs have been done using many non-metals [25,35–38]. The introduction of heteroatom to the basic structure of GQDs assigns tunable characteristics to their PL emission. Zhang et al. [35] fabricated boron doped GQDs for monitoring the level of glucose. The doping of heteroatom like sulfur is relatively difficult in comparison to boron, nitrogen and phosphorous. This is because of a significant size difference in carbon & sulphur atoms and comparable values of their electronegativities which results in low polarization. In terms of sulphur doped GQDs (S-GQDs), Li et al. reported S-GQDs fabricated using sulphuric acid and fructose as starting precursor. Also, Zhu et al. described S-GQDs as novel luminescent material for the qualitative assesment of Ag+ ions by single step fabrication route electrochemically using graphite and sodium p-toluene sulphonate as source materials in water [36]. Li and co-workers [35] employed S-GQDs made via. electrolysis for efficient tagging of Ag+ ions as low as 4.2 nM. Similarly, Bain et al. [37] have developed a facile synthesis route for SGQDs as PL sensor for Ag+ ions but the synthetic route for the fabrication of S-GQDs is tedious and requires use of toxic chemicals. So herein, we report a facile synthesis of sulphur, nitrogen codoped GQDs (S, N-GQDs) using thiourea and citric acid as precursors through simple Solid-State Reaction via calcination. The fabricated S, N-GQDs revealed excellent dispersibility, highly luminescent, low polydispersity and excellent working pH range. The prepared QDs shows a prominent PL change in presence of Ag as well as Hg ions indicating its optical sensing efficiency for both the ions. The minimum concentration detectable for the present sensor i.e. limit of detection calculated to be 12.9 and 9.14 µM for Ag and Hg ion respectively. The present optical probe works quite well in wide range of pH indicating its workability efficiency in different acidic and basic solution. Interference studies reveals that except Hg no other metal ions effects the sensing of Ag ion.
2.1. Materials and reagents Citric acid, thiourea and 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) were obtained from Avra Synthesis Ltd. Aqueous solution of various metal ions (Ca2+, Cd2+, Ba2+, Co2+, Cr2+, Na+, Pb2+, Cu2+ Mg2+, Ni2+, Zn2+) were prepared from metal acetates and nitrates. All the metal salts were purchased from AlfaAesar. The solutions were prepared using distilled water. 2.2. Instrumentations The Fluorescence emission spectra were monitored on Hitachi F7000 FL spectrophotometer. The emission results were recorded using 3.5 mL quartz cuvette with 10 mm path length. The PMT voltage of 600 V with slit width of 10 nm was fixed for all the experiments. For UV vis. Studies Lambda 750 UV/Vis/NIR spectrophotometer from Perkin Elmer was used accessed with quartz cuvette of 10 mm path length. Fourier Transform Infrared (FT-IR) spectra were scanned in the range 4000–400 cm−1 on Thermo Scientific Nicolet iS-50 spectrometer. pH measurements were done on a calibrated Cole Parmer P200 bench-top digital pH meter. 2.3. Preparation of S, N-GQDs S, N-GQDs were prepared by already published synthetic route with little modification using citric acid and thiourea by carbonization via solid state reaction, whereas the citric acid is used as a source of carbon and the thiourea is used as a source of sulfur and nitrogen [34]. Briefly, 5 g of citric acid and 5 g of thiourea were put into a 40 mL beaker and heated to 300 °C by placing in a Hot air oven for 20 min. until the color becomes dark brown. The prepared gel is then transferred in the mortar pestle and then grinded. 3. Results and discussions 3.1. Optical, morphological and crystalline studies of S, N-GQDs After the synthesis, the important part is to characterize the assynthesized QDs. For this, the easiest and the most important characterization is the UV–vis. adsorption studies. 437
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Fig. 2. (a) Effect of pH on emission of S, N-GQDs (b) FT-IR spectrum of synthesized S, N-GQDs.
Fig. 3. (a) TEM image of S, N-GQDs (low magnification) (b) TEM image of S, N-GQDs (high magnification) (c) Size histogram (d) XRD pattern of S, N-GQDs.
emission response of the synthesized GQDs, the prepared solution was then exposed to various excitation wavelength (300–400 nm) and the emission responses were monitored. Fig. 1(b) depicts the PL spectrum of doped GQDs at a different wavelength reveals that with increase in excitation wavelength from 300 to 360 nm, PL intensity of the emission increase but with further increase in excitation wavelength, intensity decreases. In addition to this, it was also observed that the no additional emission peaks appeared in the spectrum except at 436 nm. The
The synthesized QDs were dispersed in DDW to form a uniform solution of 0.1 mg/mL. Fig. 1 (a) depicts the UV–vis absorption results of prepared solution and revealed a clear absorption at 234 nm as well as a shoulder at around 333 nm. The appearance of the intense peak at 234 nm attributed to π → π* process originated because of C]C functionality present in the GQDs system [39]. The appearance of shoulder at 333 nm assigned to the n → π* process originated due to blockage of excited state by the defective surface sites [40]. To examine the 438
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Fig. 4. Selectivity of S, N-GQDs (b) Effect on various metal ions on emission intensity of S, N-GQDs.
3.2. Effect of pH and FTIR studies Also, the effect of pH on the PL emission was monitored as shown in the Fig. 2 (a). Almost similar intensities were observed when the prepared QDs were exposed with the pH ranging from 4.5 to 9.5, above and below this pH range, the PL intensity started diminishing revealing the instability of the QDs in the extreme acidic and basic medium. The phenomenon of lower intensity of PL at highly acidic condition arises due to the ionization of carboxyl groups present on the surface. While that in highly basic solution these functional groups get deprotonated the groups present on the surface. Both these process leads to change in charge density and electronic structure around S, N-GQDs which leads to the demolish or/and passivates the surface defective states and results in decrease in photoluminescence emission of S, N-GQDs [37,42–44]. In the pH range of 4.5 to 7.5, the maximum intensity of PL emission was monitored in the fabricated GQDs, which revealed it maximum applicability useful in numerous fields especially in sensing as well as other medical applications. The FT-IR spectrum of S, N-GQDs (Fig. 2(b)) was analyzed to get a detailed insight of the hyperfine chemical structure. The peak at 3380 and 3165 cm−1 was attributed to OeH and NeH bond respectively. The signals at 3015, 1710 and 1630 cm−1 were due to aromatic CeH bond and C]O and C]H respectively [36]. Peaks originated at 1455 and 1392 cm−1 appeared due to NeH, CeN, COO− bond [45]. The C]S and SO3− groups gave rise to peaks at 1192 and 1086 respectively [45]. The transmittance at 887 and 724 cm−1 originated due to CeS bond respectively [36,46]. These results revealed the presence of the functional groups i.e. NH2, COO−, C]S, eSH, eSO3H on the surface of GQDs. The presence of these functional groups not only helped in the formation of stable dispersion in water but also helpful in binding of metal ions on the surface. Both these properties extended to the sensing application of the QDs. 3.3. TEM and XRD pattern
Fig. 5. Time Studies for (a) Ag ion (b) Hg ion sensing.
Fig. 3(a, b) displays TEM histogram of synthesized GQDs and depict that the synthesized particles are below 10 nm of size with little polydispersity index in nature. This study reveal that fabricated particles are in QD region of size. Further deep insight of the QDs (Fig. 3(b)) demonstrate the characteristic lattice fringes corresponds 0.32 nm revealing the successful synthesis of S, N-GQDs [47]. The particle size was examined by ImageJ software as shown in Fig. 3(c). Form the fig. it is
maximum intensity of the system was observed when the GQDs are excited at 360 nm. The absence of dependence behavior of emission spectra on excitation revealed the uniform QDs size distribution, density, and absence of the defective state in the system [41].
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Fig. 6. (a) Effect of Ag ion on PL emission of S, N-GQDs (b) Effect of Hg2+ ion on the PL emission of S, N-GQDs (c) Detail effect of Ag on PL emission of S, N-GQDs (d) Detailed effect of Concentration of Hg ions on PL emission of S, N-GQDs.
3.5. Time studies for sensing
shown that the particles synthesized are in quantum dots range with very little polydispersity and the average particle size in range of 4–5 nm. The XRD pattern of the S, N-GQDs displayed good crystallinity with a broad peak at 2θ = 20° signifying the graphitic nature of QDs with plane 002 plane [48]. The broadness on the peak revealed the amorphous nature of the GQDs with little crystalline nature.
Fig. 5(a, b) displays the sensing mechanism dependence of Ag and Hg ions on time. The PL signal of GQDs was monitored at a fixed emission wavelength i.e. 440 nm in presence of Ag and Hg ions. In case of Ag ions, the PL quenches exponentially and is fitted with an exponential equation with R2 = 0.98735 (Fig. 5(a)). Although the PL intensity starts decreasing with the passage of time but after 30 min. the PL intensity remains stable so the optimum temperature for Ag ions sensing is 30 min. So, the rest of the Ag ion sensing experiments, PL was monitored after 30 min. of delay. Similarly, when time studies were carried out for Hg2+ ions irregular behavior was observed. The PL intensity decrease in minimum in the starting of the experiment after certain time it increases and then it decreases exponentially. After 20 min. of the time interval the PL intensity remains almost same revealing the optimum time for Hg ion sensing is 20 min. Furthermore, to analyze the trending behavior of increasing concentration on the quenching, the fixed concentration of S, N-GQDs was exposed with different concentrations of Ag+ and Hg2+ ions. With increase in concentration of Hg2+ and Ag+ ions, the PL intensity of S, N-GQDs decreased which confirmed the sensing capability of the sensor (Fig. 6 (a,b)). The mechanism behind the quenching of PL signal of S, N-GQDs in presence of Ag+ and Hg2+ ions was due to photo-induced electron transfer or energy shift from the conduction band of S, N-GQDs to the metal ions which were bound on the surface of S, N-GQDs [50,51]. The exponential decay of PL intensity with successive increasing concentration of Ag+ ions was observed by plotting PL intensity v/s concentration and was found to fit the exponential decay model (y = A1 * e(−x/t1) + A2 * e(−x/t2) + yo) with R2 of 0.99614,
3.4. Cation recognition studies of S, N-GQDs Monitoring the optimum level of Hg2+ and Ag+ ions is crucial because of their toxic nature and their wide existence in water, soil and even in food [49]. In the present report, we explored the sensing behavior of as prepared S, N-GQDs in aqueous solution for evaluation of Hg2+ and Ag+ ions. Firstly, the PL emission of S, N-GQDs was probed in the absence and presence of various toxic metal ions (Fig. 4(a, b)). Except Hg2+ and Ag+ ions, none of the other 11 metal ions i.e. Ca2+, Cd2+, Ba2+, Co2+, Cr2+, Na+, Pb2+, Cu2+ Mg2+, Ni2+, Zn2+ showed any noticeable quenching in the PL signal of S, N-GQDs. In addition, when the detailed selectivity was examined for various metal ions, it was discovered that the 100 µM of Hg2+ and Ag+ ions quenched the PL emission of S, NGQDs by 46.30% and 28.11% This revealed that the prepared luminescent S, N-GQDs were more prone to Ag+ as compared to Hg2+ ions and thereby can be utilized for the selective sensing of Hg2+ and Ag+ ions (Fig. 4(b)). The enhanced selectivity of the S, N-GQDs is attributed to the fact that the Ag+ and Hg2+ions have relatively faster and stronger binding affinity for the sulfur functionalities present on S, NGQDs as compared to other metal ions [50].
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Fig. 7. Stern Volmer on (a) Ag ions (b) Stern Volmer equation for Hg ion (c) Linearity in Stern Volmer for LOD of Ag (d) Linearity in Stern Volmer for LOD of Hg ions. Table 1 Comparison of proposed probe with different reported probes for Hg2+ and Ag+ detection. Sr. No.
Probe
Target ion
Limit of detection (µM)
Solvent system
Reference
1 2
S, N-C dots Quinoline based Schiff base
Ag+ Ag+
0.40 14.0
[56] [57]
3 4 5 6
Naphthalene based fluorescent probe Bis (5,6-dimethyl benzimidazole) derivative di-podal 1,3-calix[4]arene based sensor 1-(((4-([2,2′:5′,2″-terthiophen]-5-ylethynyl)phenyl)imin-o) methyl) naphthalen-2-ol (3TN Thiophene based Schiff base guanidine based bis-Schiff base 4,4′-difluoro-8-(methyl 4-benzoate)-1,7-dimethyl-2,6-diethyl-3,5,-di-styryl-(3,5-ditert-butyl-4-hydroxyphenyl)-4-bora-3a,4a-diaza-s-indacene S GQDs S GQDs
Ag+ Ag+ Ag+ Hg2+
17.2 0.42 23.3 0.11
H2O HEPES buffered MeOH-H2O (1:1, (v/v)) DMSO: H2O (1:1, (v/v)) (H2O/THF, v/v, 1/1) H2O/MeOH (30:70 v/v) THF/H2O (7/3, v/v)
[58] [59] [60] [61]
Hg2+ Hg2+ Hg2+
20.0 0.98 0.7
MeOH/H2O (8/2, v/v) MeOH-tris buffer THF/H2O (1/1, v/v)
[62] [63] [64]
Ag+ Hg2+
12.90 9.14
H2O H2O
Present Work Present Work
7 8 9 10 11
revealing the good fitting as shown in Fig. 6(c). Similarly, the trend of PL intensity of QDs was observed with successive increase in concentration of Hg2+ which displayed the linear quenching of QDs as shown in Fig. 6(d). Although the quenching data fit into linear model, but the R2 i.e. 0.9787 did not display a good fitting. At lower concentration of both the analyte, the PL decay was quite prominent but with increase in concentration of analyte, the relative intensity change was not so prominent. This may be due to a greater number of binding sites available initially, but with the increasing concentration of the analyte, the availability of binding sites decreased. To get a deep insight into the mechanism of the sensing, Stern-Volmer equation was used to examine the behavior of the quenching as given in Eq. (1) [9].
F 0/ F = Ksv [Q] + 1
(1)
where F0 and F are the PL signal of blank and after the addition of analyte, respectively, Ksv is the Stern-Volmer constant, [Q] is the quencher concentration. The graph obtained for F0/F − 1 vs. concentration revealed upward curvature for both the analytes fitted with exponential function as shown in Fig. 7(a, b). Both the analytes show excellent exponential fitting with R2 values of 0.99706 and 0.99528 for Ag and Hg ion respectively. Generally, PL decrease of the luminescent materials with addition of the quencher emerges due to various pathways such as photo induced electron or energy transfer, Fourier Resonance Energy Transfer, induced the delay of the fluorescence intensity etc.[52–54]. 441
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plot indicated the single class of fluorescent material. Also, the absence of any deviations from the linearity revealed that only one type of quenching mechanism was operative in both the systems and hence static quenching was found responsible for the sensing of both the metal ions. The R2 value (0.98929) of fitting exposed the excellent linearity in the response of Ag ions with the linear range of 12–125 µM with the equation of I = −0.20768 + 0.01217 [Q] while in case of Hg2+ ions the linearity in the response ranging from 12 to 125 µM (I = −0.26496 + 0.01176 [Q]) with R2 of 0.99227 revealing the good agreement with the Stern-Volmer equation. Stern-Volmer results on both the analytes revealed that the linearity range in case of Hg2+ ions was wider in comparison to the Ag+ ions. Also, the Ksv values in both cases were comparable i.e. 12,170 and 11760 M−1 for Ag+ and Hg2+ ion respectively which signified almost similar kind of binding as well as strength of both the analytes towards S, N-GQDs. The limit of detection (LOD) is calculated to be 12.90 and 9.14 µM for both Ag and Hg ions respectively (S/N). Table 1 shows sensing profile of various probes used for Ag and Hg ion sensing. Some probes have higher detection limit for the present sensors but also there are some probe cited having lower detection limit than the present work but the present probe for detection of Ag and Hg ions carries various advantages then other like aqueous system, facile and easy synthesis route, less toxic etc. 3.6. Effect of pH on sensing To examine the effect of pH on the sensing of both the analytes, pH studies were carried. Fig. 8(a, b) displays the effect of the pH on the relative change in PL intensity in the presence of fixed concentration of the analyte at different pH range. It was observed in both the studies that the present sensor works well from 5.0 to 12.0 pH revealing a wide pH working range. In both the metal ions i.e. Ag+ and Hg2+ maximum intensity change (F0/F) was observed to be at 10–12 pH range. This is ascribed due to the deportation of the functional groups present over the surface that leads to the binding of metal ions on the surface via. ionic bonding. While at strongly acidic as well as basic medium the photoluminescence response of QDs itself is very less as shown in Fig. 2 (a) therefore the respective change in PL intensity is less.
Fig. 8. (a) Effect of pH on Ag ion sensing (b) Effect of pH on Hg ion sensing.
3.7. Interference A systematic study was performed to examine the ability of GQDs for differentiating Ag+ ion in the presence of other interfering ions that may coexist in sample (Fig. 9). For this the PL intensity were monitored for bare S, N-GQDs as well as by addition of 100 µM Ag ions in QDs. Almost 50% quenching was shown by the Ag ions but when the PL intensity of QDs and Ag ions in presence of other metal ions (Ba, Ca, Cd, Co, Cu, Mg, Na, Ni, Pb, Zn) were monitored almost similar response was obtained revealing their minimum interference in the sensing mechanism of GQDs for Ag ions except Hg. When 100 µM of Hg ions were added to system i.e. Ag ion GQDs almost 90% decrease in PL intensity was monitored revealing the interference of Hg ion in the sensing of Ag ions.
Fig. 9. Interference studies.
The PL quenching response of fluorescent materials due the presence of quencher occurs via. dynamic and/or static quenching. Static quenching happens due to the complex formation of luminescent material in the ground state with the analyte while the dynamic quenching occurs via. collision of the excited luminescent molecule with quencher. When closely observed the Stern Volmer plot of both the analyte it was observed that at lower concentration of the both the analyte reveals the linear behavior but with increase in concentration of analyte upward curvature was obtained. The upward curvature in the Stern Volmer response developed due to competition between static as well as dynamic quenching [10,55]. The linearity at lower concentration of both the analyte are shown in Fig. 7(c, d). A linear response in Stern-Volmer
4. Conclusions The present research article focuses on the simple route to fabricate S, N-GQDs via. calcination of citric acid and thiourea in 1:1 ratio. The synthesized GQDs show bright blue luminescence in UV light. XRD results reveals the graphitic nature of the synthesis QDs. Functionality on the surface was examined by FITR spectroscopy confirming the presence of SH, SO3, COOH, NH functional groups. TEM studies reveals the successful control of size in quantum dot reign with good control over poly dispersibility. Optical studies reveal that the presence probe undergoes π → π* and n → π* transition responsible for PL properties of GQDs. The emission response is independent to the excitation 442
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wavelength shows that the synthesized QDs are free from any defective states. When the synthesized QDs were exposed with different metal ions under PL emission, the fabricated S, N-GQDs exhibited excellent selectivity and sensitivity towards Ag and Hg ions. The enhanced affinity of GQDs for Hg and Ag ions assigned due to the presence of sulfur functional groups on the surface of QDs. LOD for Hg and Ag ions comes out to be 9.14 µM and 12.90 µM respectively. The proposed sensors work well in a wide range of pH from 5 to 12. The paramount performance of the probe was found at pH 12.0. The practical applicability of the S-GQDs to real water samples was also investigated.
[17]
[18]
[19]
[20]
Declaration of Competing Interest
[21]
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[22]
Acknowledgements
[23]
KS would like to thanks CIL Panjab University for carrying out XRD and FTIR studies. ES would like to thanks IIT Mandi for TEM studies. DV is thankful to DST for Purse grant II.
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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Facile synthesis of sulfur and nitrogen codoped graphene quantum dots for optical sensing of Hg and Ag ions
T
Ekta Sharmaa, Devika Vashishtb, Aseem Vashishtc, Virender Kumar Vatsa, S.K. Mehtab, ⁎ Kulvinder Singha, a
Department of Chemistry, School of Basic and Applied Sciences, Maharaja Agrasen University, Baddi 174103, India Department of Chemistry, Panjab University, Chandigarh 160014, India c Department of Physics, Panjab University, Chandigarh 160014, India b
H I GH L IG H T S
and simple synthesis of S, N-GQDs using solid state carbonization. • Facile QDs shows excellent water stability, photoluminescence. • Synthesized is independent to the excitation wavelength. • Photoluminescence • Sensor shows high selectivity and sensitivity for Ag and Hg ions.
A R T I C LE I N FO
A B S T R A C T
Keywords: Graphene quantum dots Optical sensor Stern Volmer Water pollution
Facile and simple sulfur and nitrogen co-doped graphene Quantum Dots (S, N-GQDs) were fabricated via. calcination. UV–vis. spectrum reveals two peaks originates due to the transition from n and π to π* of the luminescent material. The photoluminescence emission of S, N-GQDs has been quenched upon addition of Hg and Ag ions. The higher affinity of sulfur for Hg and Ag ions assigned selectivity to the sensing protocol which was confirmed from the absence of any interference of the competitive ions. The S, N-GQDs could sense Hg and Ag ions as low as 9.14 µM and 12.90 µM respectively.
1. Introduction Quantum Dots (QDs) have attained a respectable place in different classes of semiconductor nanoparticles due to their inherent luminescent property that leads to applications in various fields [1] such as optoelectronic devices [2,3], bioimaging [4,5], matrix for surface clean metal NPs [6], catalysis [7], sensing [8–10] etc. Among all these applications, sensing of toxic chemicals and heavy metal ions has been widely explored by various research groups [11–13]. In this context, Yao et al. [14] fabricated color tunable fibers of CdTe QDs with the aid of wet spinning which worked quite well as glucose and pH sensor. In addition, Yan et al. have studied the aggregation along with stability issues of CsPbBr3 QDs and successfully overcomes this issue by ligand modification route via. replacing oleic acid with 2‐hexyldecanoic acid during synthesis [15]. Similarly, the role of surface functionalization on CdTe QDs for optical monitoring of Hg2+ has been explored by Labeb et al. [16]. Two different capping agents (thioglycolic acid and L-
⁎
cysteine) were utilized by Labeb and co-workers for the functionalization of CdTe QDs and the sensing studies carried by them revealed that the L-cysteine capped CdTe QDs had better sensing efficacy as compared to thioglycolic acid for Hg2+ ions in terms of the selectivity and sensitivity. Numerous metal and metal chalcogenide based semiconducting QDs have been utilized by various scientific groups for the optical detection of toxic chemicals/ions [11,17,18]. In addition to their application as optical sensors, these QDs carry certain limitations which include toxicity, cost, complicated synthetic route etc. [19,20]. A step further in QDs, a novel class of zero-dimensional QDs (Carbonaceous QDs) came into existence with several advantages over the conventional QDs [21,22]. Among these, various kinds of Carbonaceous QDs especially graphene QDs (GQDs) have gained the attention of scientific community due to their low toxicity, excellent photo stability, relatively smaller size, prodigious multi-photo excitation, chemical inertness, ease of functionalization, highly water stability, excellent photoluminescence (PL) and size-dependent luminescence emission
Corresponding author. E-mail address: [email protected] (K. Singh).
https://doi.org/10.1016/j.cplett.2019.06.040 Received 30 April 2019; Received in revised form 4 June 2019; Accepted 15 June 2019 Available online 17 June 2019 0009-2614/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. (a) UV–vis. Absorption spectrum of S, N-GQDs (b) Effect of excitation on emission of S, N-GQDs.
2. Experimental section
[23]. These properties made GQDs eligible for several applications like dye sensitized solar cells [24], energy storage and sensing [25], Drug Delivery [26], photovoltaic cells [27], photo-degradation [28], photodetector [29], catalysis [30], bioimaging [31] etc. The sensing-based applications of the GQDs have been explored extensively [32]. Particularly, Shehab et al. have fabricated GQDs by the carbonization of glucose and functionalized with phenylboronic acid for efficient detection of glucose [33]. Similarly, GQDs obtained from pyrolyzing citric acid have been effectively employed for the detection of chlorine content in water system by Dong et al. [34]. To further enhance the performance of GQDs as sensors and properties such as electronic characteristics, band gap, chemical reactivity, optical behavior, doping of GQDs have been done using many non-metals [25,35–38]. The introduction of heteroatom to the basic structure of GQDs assigns tunable characteristics to their PL emission. Zhang et al. [35] fabricated boron doped GQDs for monitoring the level of glucose. The doping of heteroatom like sulfur is relatively difficult in comparison to boron, nitrogen and phosphorous. This is because of a significant size difference in carbon & sulphur atoms and comparable values of their electronegativities which results in low polarization. In terms of sulphur doped GQDs (S-GQDs), Li et al. reported S-GQDs fabricated using sulphuric acid and fructose as starting precursor. Also, Zhu et al. described S-GQDs as novel luminescent material for the qualitative assesment of Ag+ ions by single step fabrication route electrochemically using graphite and sodium p-toluene sulphonate as source materials in water [36]. Li and co-workers [35] employed S-GQDs made via. electrolysis for efficient tagging of Ag+ ions as low as 4.2 nM. Similarly, Bain et al. [37] have developed a facile synthesis route for SGQDs as PL sensor for Ag+ ions but the synthetic route for the fabrication of S-GQDs is tedious and requires use of toxic chemicals. So herein, we report a facile synthesis of sulphur, nitrogen codoped GQDs (S, N-GQDs) using thiourea and citric acid as precursors through simple Solid-State Reaction via calcination. The fabricated S, N-GQDs revealed excellent dispersibility, highly luminescent, low polydispersity and excellent working pH range. The prepared QDs shows a prominent PL change in presence of Ag as well as Hg ions indicating its optical sensing efficiency for both the ions. The minimum concentration detectable for the present sensor i.e. limit of detection calculated to be 12.9 and 9.14 µM for Ag and Hg ion respectively. The present optical probe works quite well in wide range of pH indicating its workability efficiency in different acidic and basic solution. Interference studies reveals that except Hg no other metal ions effects the sensing of Ag ion.
2.1. Materials and reagents Citric acid, thiourea and 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) were obtained from Avra Synthesis Ltd. Aqueous solution of various metal ions (Ca2+, Cd2+, Ba2+, Co2+, Cr2+, Na+, Pb2+, Cu2+ Mg2+, Ni2+, Zn2+) were prepared from metal acetates and nitrates. All the metal salts were purchased from AlfaAesar. The solutions were prepared using distilled water. 2.2. Instrumentations The Fluorescence emission spectra were monitored on Hitachi F7000 FL spectrophotometer. The emission results were recorded using 3.5 mL quartz cuvette with 10 mm path length. The PMT voltage of 600 V with slit width of 10 nm was fixed for all the experiments. For UV vis. Studies Lambda 750 UV/Vis/NIR spectrophotometer from Perkin Elmer was used accessed with quartz cuvette of 10 mm path length. Fourier Transform Infrared (FT-IR) spectra were scanned in the range 4000–400 cm−1 on Thermo Scientific Nicolet iS-50 spectrometer. pH measurements were done on a calibrated Cole Parmer P200 bench-top digital pH meter. 2.3. Preparation of S, N-GQDs S, N-GQDs were prepared by already published synthetic route with little modification using citric acid and thiourea by carbonization via solid state reaction, whereas the citric acid is used as a source of carbon and the thiourea is used as a source of sulfur and nitrogen [34]. Briefly, 5 g of citric acid and 5 g of thiourea were put into a 40 mL beaker and heated to 300 °C by placing in a Hot air oven for 20 min. until the color becomes dark brown. The prepared gel is then transferred in the mortar pestle and then grinded. 3. Results and discussions 3.1. Optical, morphological and crystalline studies of S, N-GQDs After the synthesis, the important part is to characterize the assynthesized QDs. For this, the easiest and the most important characterization is the UV–vis. adsorption studies. 437
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Fig. 2. (a) Effect of pH on emission of S, N-GQDs (b) FT-IR spectrum of synthesized S, N-GQDs.
Fig. 3. (a) TEM image of S, N-GQDs (low magnification) (b) TEM image of S, N-GQDs (high magnification) (c) Size histogram (d) XRD pattern of S, N-GQDs.
emission response of the synthesized GQDs, the prepared solution was then exposed to various excitation wavelength (300–400 nm) and the emission responses were monitored. Fig. 1(b) depicts the PL spectrum of doped GQDs at a different wavelength reveals that with increase in excitation wavelength from 300 to 360 nm, PL intensity of the emission increase but with further increase in excitation wavelength, intensity decreases. In addition to this, it was also observed that the no additional emission peaks appeared in the spectrum except at 436 nm. The
The synthesized QDs were dispersed in DDW to form a uniform solution of 0.1 mg/mL. Fig. 1 (a) depicts the UV–vis absorption results of prepared solution and revealed a clear absorption at 234 nm as well as a shoulder at around 333 nm. The appearance of the intense peak at 234 nm attributed to π → π* process originated because of C]C functionality present in the GQDs system [39]. The appearance of shoulder at 333 nm assigned to the n → π* process originated due to blockage of excited state by the defective surface sites [40]. To examine the 438
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Fig. 4. Selectivity of S, N-GQDs (b) Effect on various metal ions on emission intensity of S, N-GQDs.
3.2. Effect of pH and FTIR studies Also, the effect of pH on the PL emission was monitored as shown in the Fig. 2 (a). Almost similar intensities were observed when the prepared QDs were exposed with the pH ranging from 4.5 to 9.5, above and below this pH range, the PL intensity started diminishing revealing the instability of the QDs in the extreme acidic and basic medium. The phenomenon of lower intensity of PL at highly acidic condition arises due to the ionization of carboxyl groups present on the surface. While that in highly basic solution these functional groups get deprotonated the groups present on the surface. Both these process leads to change in charge density and electronic structure around S, N-GQDs which leads to the demolish or/and passivates the surface defective states and results in decrease in photoluminescence emission of S, N-GQDs [37,42–44]. In the pH range of 4.5 to 7.5, the maximum intensity of PL emission was monitored in the fabricated GQDs, which revealed it maximum applicability useful in numerous fields especially in sensing as well as other medical applications. The FT-IR spectrum of S, N-GQDs (Fig. 2(b)) was analyzed to get a detailed insight of the hyperfine chemical structure. The peak at 3380 and 3165 cm−1 was attributed to OeH and NeH bond respectively. The signals at 3015, 1710 and 1630 cm−1 were due to aromatic CeH bond and C]O and C]H respectively [36]. Peaks originated at 1455 and 1392 cm−1 appeared due to NeH, CeN, COO− bond [45]. The C]S and SO3− groups gave rise to peaks at 1192 and 1086 respectively [45]. The transmittance at 887 and 724 cm−1 originated due to CeS bond respectively [36,46]. These results revealed the presence of the functional groups i.e. NH2, COO−, C]S, eSH, eSO3H on the surface of GQDs. The presence of these functional groups not only helped in the formation of stable dispersion in water but also helpful in binding of metal ions on the surface. Both these properties extended to the sensing application of the QDs. 3.3. TEM and XRD pattern
Fig. 5. Time Studies for (a) Ag ion (b) Hg ion sensing.
Fig. 3(a, b) displays TEM histogram of synthesized GQDs and depict that the synthesized particles are below 10 nm of size with little polydispersity index in nature. This study reveal that fabricated particles are in QD region of size. Further deep insight of the QDs (Fig. 3(b)) demonstrate the characteristic lattice fringes corresponds 0.32 nm revealing the successful synthesis of S, N-GQDs [47]. The particle size was examined by ImageJ software as shown in Fig. 3(c). Form the fig. it is
maximum intensity of the system was observed when the GQDs are excited at 360 nm. The absence of dependence behavior of emission spectra on excitation revealed the uniform QDs size distribution, density, and absence of the defective state in the system [41].
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Fig. 6. (a) Effect of Ag ion on PL emission of S, N-GQDs (b) Effect of Hg2+ ion on the PL emission of S, N-GQDs (c) Detail effect of Ag on PL emission of S, N-GQDs (d) Detailed effect of Concentration of Hg ions on PL emission of S, N-GQDs.
3.5. Time studies for sensing
shown that the particles synthesized are in quantum dots range with very little polydispersity and the average particle size in range of 4–5 nm. The XRD pattern of the S, N-GQDs displayed good crystallinity with a broad peak at 2θ = 20° signifying the graphitic nature of QDs with plane 002 plane [48]. The broadness on the peak revealed the amorphous nature of the GQDs with little crystalline nature.
Fig. 5(a, b) displays the sensing mechanism dependence of Ag and Hg ions on time. The PL signal of GQDs was monitored at a fixed emission wavelength i.e. 440 nm in presence of Ag and Hg ions. In case of Ag ions, the PL quenches exponentially and is fitted with an exponential equation with R2 = 0.98735 (Fig. 5(a)). Although the PL intensity starts decreasing with the passage of time but after 30 min. the PL intensity remains stable so the optimum temperature for Ag ions sensing is 30 min. So, the rest of the Ag ion sensing experiments, PL was monitored after 30 min. of delay. Similarly, when time studies were carried out for Hg2+ ions irregular behavior was observed. The PL intensity decrease in minimum in the starting of the experiment after certain time it increases and then it decreases exponentially. After 20 min. of the time interval the PL intensity remains almost same revealing the optimum time for Hg ion sensing is 20 min. Furthermore, to analyze the trending behavior of increasing concentration on the quenching, the fixed concentration of S, N-GQDs was exposed with different concentrations of Ag+ and Hg2+ ions. With increase in concentration of Hg2+ and Ag+ ions, the PL intensity of S, N-GQDs decreased which confirmed the sensing capability of the sensor (Fig. 6 (a,b)). The mechanism behind the quenching of PL signal of S, N-GQDs in presence of Ag+ and Hg2+ ions was due to photo-induced electron transfer or energy shift from the conduction band of S, N-GQDs to the metal ions which were bound on the surface of S, N-GQDs [50,51]. The exponential decay of PL intensity with successive increasing concentration of Ag+ ions was observed by plotting PL intensity v/s concentration and was found to fit the exponential decay model (y = A1 * e(−x/t1) + A2 * e(−x/t2) + yo) with R2 of 0.99614,
3.4. Cation recognition studies of S, N-GQDs Monitoring the optimum level of Hg2+ and Ag+ ions is crucial because of their toxic nature and their wide existence in water, soil and even in food [49]. In the present report, we explored the sensing behavior of as prepared S, N-GQDs in aqueous solution for evaluation of Hg2+ and Ag+ ions. Firstly, the PL emission of S, N-GQDs was probed in the absence and presence of various toxic metal ions (Fig. 4(a, b)). Except Hg2+ and Ag+ ions, none of the other 11 metal ions i.e. Ca2+, Cd2+, Ba2+, Co2+, Cr2+, Na+, Pb2+, Cu2+ Mg2+, Ni2+, Zn2+ showed any noticeable quenching in the PL signal of S, N-GQDs. In addition, when the detailed selectivity was examined for various metal ions, it was discovered that the 100 µM of Hg2+ and Ag+ ions quenched the PL emission of S, NGQDs by 46.30% and 28.11% This revealed that the prepared luminescent S, N-GQDs were more prone to Ag+ as compared to Hg2+ ions and thereby can be utilized for the selective sensing of Hg2+ and Ag+ ions (Fig. 4(b)). The enhanced selectivity of the S, N-GQDs is attributed to the fact that the Ag+ and Hg2+ions have relatively faster and stronger binding affinity for the sulfur functionalities present on S, NGQDs as compared to other metal ions [50].
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Fig. 7. Stern Volmer on (a) Ag ions (b) Stern Volmer equation for Hg ion (c) Linearity in Stern Volmer for LOD of Ag (d) Linearity in Stern Volmer for LOD of Hg ions. Table 1 Comparison of proposed probe with different reported probes for Hg2+ and Ag+ detection. Sr. No.
Probe
Target ion
Limit of detection (µM)
Solvent system
Reference
1 2
S, N-C dots Quinoline based Schiff base
Ag+ Ag+
0.40 14.0
[56] [57]
3 4 5 6
Naphthalene based fluorescent probe Bis (5,6-dimethyl benzimidazole) derivative di-podal 1,3-calix[4]arene based sensor 1-(((4-([2,2′:5′,2″-terthiophen]-5-ylethynyl)phenyl)imin-o) methyl) naphthalen-2-ol (3TN Thiophene based Schiff base guanidine based bis-Schiff base 4,4′-difluoro-8-(methyl 4-benzoate)-1,7-dimethyl-2,6-diethyl-3,5,-di-styryl-(3,5-ditert-butyl-4-hydroxyphenyl)-4-bora-3a,4a-diaza-s-indacene S GQDs S GQDs
Ag+ Ag+ Ag+ Hg2+
17.2 0.42 23.3 0.11
H2O HEPES buffered MeOH-H2O (1:1, (v/v)) DMSO: H2O (1:1, (v/v)) (H2O/THF, v/v, 1/1) H2O/MeOH (30:70 v/v) THF/H2O (7/3, v/v)
[58] [59] [60] [61]
Hg2+ Hg2+ Hg2+
20.0 0.98 0.7
MeOH/H2O (8/2, v/v) MeOH-tris buffer THF/H2O (1/1, v/v)
[62] [63] [64]
Ag+ Hg2+
12.90 9.14
H2O H2O
Present Work Present Work
7 8 9 10 11
revealing the good fitting as shown in Fig. 6(c). Similarly, the trend of PL intensity of QDs was observed with successive increase in concentration of Hg2+ which displayed the linear quenching of QDs as shown in Fig. 6(d). Although the quenching data fit into linear model, but the R2 i.e. 0.9787 did not display a good fitting. At lower concentration of both the analyte, the PL decay was quite prominent but with increase in concentration of analyte, the relative intensity change was not so prominent. This may be due to a greater number of binding sites available initially, but with the increasing concentration of the analyte, the availability of binding sites decreased. To get a deep insight into the mechanism of the sensing, Stern-Volmer equation was used to examine the behavior of the quenching as given in Eq. (1) [9].
F 0/ F = Ksv [Q] + 1
(1)
where F0 and F are the PL signal of blank and after the addition of analyte, respectively, Ksv is the Stern-Volmer constant, [Q] is the quencher concentration. The graph obtained for F0/F − 1 vs. concentration revealed upward curvature for both the analytes fitted with exponential function as shown in Fig. 7(a, b). Both the analytes show excellent exponential fitting with R2 values of 0.99706 and 0.99528 for Ag and Hg ion respectively. Generally, PL decrease of the luminescent materials with addition of the quencher emerges due to various pathways such as photo induced electron or energy transfer, Fourier Resonance Energy Transfer, induced the delay of the fluorescence intensity etc.[52–54]. 441
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plot indicated the single class of fluorescent material. Also, the absence of any deviations from the linearity revealed that only one type of quenching mechanism was operative in both the systems and hence static quenching was found responsible for the sensing of both the metal ions. The R2 value (0.98929) of fitting exposed the excellent linearity in the response of Ag ions with the linear range of 12–125 µM with the equation of I = −0.20768 + 0.01217 [Q] while in case of Hg2+ ions the linearity in the response ranging from 12 to 125 µM (I = −0.26496 + 0.01176 [Q]) with R2 of 0.99227 revealing the good agreement with the Stern-Volmer equation. Stern-Volmer results on both the analytes revealed that the linearity range in case of Hg2+ ions was wider in comparison to the Ag+ ions. Also, the Ksv values in both cases were comparable i.e. 12,170 and 11760 M−1 for Ag+ and Hg2+ ion respectively which signified almost similar kind of binding as well as strength of both the analytes towards S, N-GQDs. The limit of detection (LOD) is calculated to be 12.90 and 9.14 µM for both Ag and Hg ions respectively (S/N). Table 1 shows sensing profile of various probes used for Ag and Hg ion sensing. Some probes have higher detection limit for the present sensors but also there are some probe cited having lower detection limit than the present work but the present probe for detection of Ag and Hg ions carries various advantages then other like aqueous system, facile and easy synthesis route, less toxic etc. 3.6. Effect of pH on sensing To examine the effect of pH on the sensing of both the analytes, pH studies were carried. Fig. 8(a, b) displays the effect of the pH on the relative change in PL intensity in the presence of fixed concentration of the analyte at different pH range. It was observed in both the studies that the present sensor works well from 5.0 to 12.0 pH revealing a wide pH working range. In both the metal ions i.e. Ag+ and Hg2+ maximum intensity change (F0/F) was observed to be at 10–12 pH range. This is ascribed due to the deportation of the functional groups present over the surface that leads to the binding of metal ions on the surface via. ionic bonding. While at strongly acidic as well as basic medium the photoluminescence response of QDs itself is very less as shown in Fig. 2 (a) therefore the respective change in PL intensity is less.
Fig. 8. (a) Effect of pH on Ag ion sensing (b) Effect of pH on Hg ion sensing.
3.7. Interference A systematic study was performed to examine the ability of GQDs for differentiating Ag+ ion in the presence of other interfering ions that may coexist in sample (Fig. 9). For this the PL intensity were monitored for bare S, N-GQDs as well as by addition of 100 µM Ag ions in QDs. Almost 50% quenching was shown by the Ag ions but when the PL intensity of QDs and Ag ions in presence of other metal ions (Ba, Ca, Cd, Co, Cu, Mg, Na, Ni, Pb, Zn) were monitored almost similar response was obtained revealing their minimum interference in the sensing mechanism of GQDs for Ag ions except Hg. When 100 µM of Hg ions were added to system i.e. Ag ion GQDs almost 90% decrease in PL intensity was monitored revealing the interference of Hg ion in the sensing of Ag ions.
Fig. 9. Interference studies.
The PL quenching response of fluorescent materials due the presence of quencher occurs via. dynamic and/or static quenching. Static quenching happens due to the complex formation of luminescent material in the ground state with the analyte while the dynamic quenching occurs via. collision of the excited luminescent molecule with quencher. When closely observed the Stern Volmer plot of both the analyte it was observed that at lower concentration of the both the analyte reveals the linear behavior but with increase in concentration of analyte upward curvature was obtained. The upward curvature in the Stern Volmer response developed due to competition between static as well as dynamic quenching [10,55]. The linearity at lower concentration of both the analyte are shown in Fig. 7(c, d). A linear response in Stern-Volmer
4. Conclusions The present research article focuses on the simple route to fabricate S, N-GQDs via. calcination of citric acid and thiourea in 1:1 ratio. The synthesized GQDs show bright blue luminescence in UV light. XRD results reveals the graphitic nature of the synthesis QDs. Functionality on the surface was examined by FITR spectroscopy confirming the presence of SH, SO3, COOH, NH functional groups. TEM studies reveals the successful control of size in quantum dot reign with good control over poly dispersibility. Optical studies reveal that the presence probe undergoes π → π* and n → π* transition responsible for PL properties of GQDs. The emission response is independent to the excitation 442
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wavelength shows that the synthesized QDs are free from any defective states. When the synthesized QDs were exposed with different metal ions under PL emission, the fabricated S, N-GQDs exhibited excellent selectivity and sensitivity towards Ag and Hg ions. The enhanced affinity of GQDs for Hg and Ag ions assigned due to the presence of sulfur functional groups on the surface of QDs. LOD for Hg and Ag ions comes out to be 9.14 µM and 12.90 µM respectively. The proposed sensors work well in a wide range of pH from 5 to 12. The paramount performance of the probe was found at pH 12.0. The practical applicability of the S-GQDs to real water samples was also investigated.
[17]
[18]
[19]
[20]
Declaration of Competing Interest
[21]
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Acknowledgements
[23]
KS would like to thanks CIL Panjab University for carrying out XRD and FTIR studies. ES would like to thanks IIT Mandi for TEM studies. DV is thankful to DST for Purse grant II.
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