Facile Synthesis of Photoluminescent Graphitic Carbon Nitride

Dec 6, 2017 - ... Phosphorescence from Benzo[2,1,3]thiadiazoles at Room Temperature J. Org. Chem. 2016, 81, 4789– 4796 DOI: 10.1021/acs.joc.6b00789...
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Research Article Cite This: ACS Sustainable Chem. Eng. 2018, 6, 1732−1743

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Facile Synthesis of Photoluminescent Graphitic Carbon Nitride Quantum Dots for Hg2+ Detection and Room Temperature Phosphorescence Khemnath Patir and Sonit Kumar Gogoi* Department of Chemistry, University of Gauhati, G. B. Nagar, Guwahati, 781014 Assam, India

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ABSTRACT: Carbon nitride materials have become highly explored carbon based nanomaterials since their rediscovery in the 1990s due to their semiconductor like behavior. Here, we report a facile one pot synthesis of sulfur and oxygen doped carbon nitride quantum dots (SCNQDs) from thiourea and ethylenediaminetetraacetic acid disodium salt by a thermal method. The SCNQDs prepared are characterized by UV− visible, FT-IR, X-ray photoelectron spectroscopy (XPS), powder X-ray diffraction, and transmission electron microscopy (TEM) imaging. We have demonstrated the Hg2+ sensing ability of SCNQDs in solution as well as in the solid phase, i.e. SCNQDs loaded onto filter paper. The Hg2+ sensing capability of the SCNQDs in the solution phase is the same for Hg2+ ion in double distilled water as well as in tap water, which gives the method a practical applicability in real conditions. The sensitivity of SCNQDs with Hg2+ follows a linear relationship in the range from 10 nM to 1 μM. The minimum detection limit is found to be 0.01 nM, which is lower than previous reports. Similarly the SCNQDs loaded onto filter paper also showed same sensing capability with Hg2+ spiked tap water as Hg2+ in double distilled water solutions. Thus, we have devised a ready to use system for Hg2+ detection with SCNQDs loaded filter paper to be used in biological fluids as well as in environmental samples, which is not available now, to the best of our knowledge. Further we have also made a composite of SCNQDs with poly(vinyl alcohol) (PVA) to observe room temperature phosphorescence (RTP) in SCNQDs. Therefore, RTP in SCNQDs, which is reported for the first time, will add to the exploration and development of high efficiency optoelectronic devices involving SCNQDs. KEYWORDS: Graphitic carbon nitride quantum dots, Blue fluorescence, Mercury(II) sensor, Filter paper based sensor, Room temperature phosphorescence

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INTRODUCTION Among the carbon based nanomaterials, graphitic carbon nitrides (GCNs) are a type of polymeric nanomaterial primarily composed of carbon and nitrogen. GCN has secured considerable attention in recent times due to its unique and tunable properties. Semiconductor like luminescence in GCN is one such property that has been explored for various applications, including photocatalysis, photoelectronic devices, sensing, imaging, solar energy conversion, etc.1−5 The doping of the GCN with SrCO3 or KNO3 has been reported and is found to be highly effective as a catalyst for photocatalytic degradation of NO.6−8 GCN has been prepared in bulk form, as nanosheets as well as in the form of quantum dots for different applications.9−14 At the same time heteroatoms, i.e. other than carbon and nitrogen, such as sulfur, oxygen, boron, etc. are also doped into these GCN in order to achieve new properties.15,16 Because of its unique structure, composed of tris-triazine units bridged by amino groups, with periodic lattice vacancies, GCN offers more maneuverability in terms of tuning its optical properties compared to graphene sheets. In quantum © 2017 American Chemical Society

dot form, GCN shows characteristic blue photoluminescence (PL) which is much applied in sensing and imaging applications.5,11,17 The graphitic carbon materials (GCMs) also have been used in medical sciences such as detection of prostate specific antigen (PSA), alkaline phosphate, and bleomycin, a drug used for treatment of cancer.18−20 Moreover, doping of GCNs on α-Fe3O4/Ti makes them effective catalysts for photocatalytic water oxidation.21 For synthesis of GCN quantum dots, several routes have been employed, with hydrothermal, microwave, thermal treatment, and chemical etching being popular.22−26 However, there is still scope for making the synthetic processes more simplistic and economical. It can be emphasized that an easy and economically viable synthetic route is an essential criteria for developing new application of a material and making the material a practically applicable product. For example, because of the simplistic Received: August 29, 2017 Revised: December 2, 2017 Published: December 6, 2017 1732

DOI: 10.1021/acssuschemeng.7b03008 ACS Sustainable Chem. Eng. 2018, 6, 1732−1743

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synthetic routes for preparing carbon nanoparticles, far more explorations for novel properties and applications have been reported as compared to graphitic carbon nitride quantum dots (CNQDs). The toxic effect of Hg2+ is well established, and in the past there have been several reports of disasters caused by mercury. Though developed nations have put restrictions on mercury usage, increasing emission of mercury in the developing world is a major concern.27,28 Pesticides, batteries, paper industries, etc. are major contributors of mercury emission, and fluorescent lamps have brought mercury to every household. With the ever increasing use of fluorescent lamps in developing nations, there is the growing possibility of mercury exposure to humans and animals due to lack of a proper disposal system for these fluorescent lamps after use. Therefore, it is very much essential to have a handy Hg2+ detection system for early detection and remediation. From this perspective colorimetric and/or fluorescence detection systems for Hg2+ can play an important role in Hg2+ sensing in water used for household chores due to the inexpensive nature inherent to these techniques. A lot of effort is put forth in this regard, and nanomaterial based sensing of Hg2+ has given promising results in recent years.29,30 Quantum dot systems, both metal containing semiconductors such as CdSe/ZnS,31 Eu3+doped CdS,32 etc. and metal free semiconductors such as carbon dots, CNQD, etc.,16,33,34 have been applied for this purpose. What is still missing is a high sensitivity, high selectivity, and low cost ready to use device for Hg2+ detection. Another aspect of this work is a room temperature phosphorescent material. Room temperature phosphorescence (RTP) boasts a host of opportunities for diverse applications from sensing to imaging to optoelectronics.35−37 Conventionally, RTP is observed through heavy metal doping into molecular or solid state systems to enhance spin orbit coupling, leading to long persistent phosphorescence.38 Due to the high cost and toxicity of heavy metals, metal free alternatives are being explored. Therefore, organic RTP materials are in high demand. There have been different successful strategies developed for obtaining RTP in organic, molecular, and supramolecular systems.39 However, one cannot deny the fact that the synthetic procedures followed for getting organic molecules showing RTP are often complicated, costly and not benign.38,40 Here carbon based nanomaterials41,42 offer a unique solution of long lasting RTP in metal free, organic systems. Among the different carbon nanomaterials, carbon dots are more popular for obtaining RTP. To the best of our knowledge, this is the first report on RTP observed in CNQDs obtained in a very simple synthetic procedure. Herein we report a very facile method for preparation of photoluminescent sulfur and oxygen doped graphitic carbon nitride quantum dots (SCNQDs) by simple heating at 200 °C in a laboratory oven. The PL from SCNQDs thus prepared is highly sensitive to Hg2+ ion concentration. These SCNQDs are further loaded onto filter paper to make a handy PL based detection device for Hg2+. Though there have been reports of sensing of Hg2+ by SCNQDs, there is no report on making it a ready to use device by loading SCNQDs onto filter paper. SCNQDs have also been incorporated into PVA films to get long afterglow RTP, as a proof of existence of the triplet state in this material, which can have application in optoelectronics and photocatalysis to glow in the dark paints for security signs.

Research Article

EXPERIMENTAL SECTION

Materials. Thiourea, ethylenediaminetetraacetic acid disodium salt dihydrate (EDTA.2Na), NaOH pellets, manganese sulfate, mercury chloride, copper sulfate, silver nitrate, lead acetate, nickel sulfate, cadmium chloride, zinc sulfate, calcium chloride, sodium chloride, ferric chloride, and aluminum sulfate are procured from Merck Specialties Private Limited, India. Double distilled water is used for all the experiments unless mentioned otherwise. PVA (M.Wt-1700− 1800) is received from LOBA Chemie private limited, India. Sulfuric acid, ethanol, methanol, DMF, DMSO, and polyethylene glycol are received from Merck Specialties Private Limited, India. Synthesis of sulfur doped carbon nitride quantum dots (SCNQDs). EDTA.2Na and thiourea are mixed thoroughly in a mortar in different molratios, such as (EDTA.2Na)1:1(thiourea), (EDTA.2Na)1:4(thiourea), (EDTA.2Na)1:8(thiourea), (EDTA.2Na)1:12(thiourea), and (EDTA.2Na)1:16(thiourea). The mixture is transferred to a silica crucible with a lid and heated at 150 °C to 250 °C in a laboratory oven from Bio craft Scientific System Pvt. Ltd. (Agra-5, India) for 2 h and allowed to cool to room temperature. The resultant brown jelly or powder product is purified by washing with ethanol several times and finally filtered with Whatman 40 filter paper. Sulfur and oxygen doped graphitic carbon nitride quantum dots thus prepared with a (EDTA.2Na)1:12(thiourea) molratio heated at 200 °C for 2 h are found to exhibit the highest PL intensity, as well as quantum yield (Figure S1a, 1b, and 1c). Henceforth all the characterization and studies are carried out with this sample. We will use the abbreviation SCNQDs for our prepared sulfur doped graphitic carbon nitride quantum dots prepared with mol ratio (EDTA.2Na)1:12(thiourea), heated at 200 °C for 2 h. Stability of the SCNQDs with time, ionic strengths and pH dependence of PL has been studied (Figure S2). Determination of quantum yield. The PL quantum yield (QY) of SCNQDs is measured by comparing the integrated photoluminescence intensities (excited at 360 nm) and the absorbance value (at 358 nm) using quinine sulfate in 0.1 mol L−1 H2SO4 (Φ = 0.54) as standard reference. Different concentrations of the SCNQDs and standard reference quinine sulfate is prepared so that its absorbance is less than 0.1 at their respective excitation wavelengths. The PL QY is calculated using the following equation.

ΦX = ΦST × (G X /GST) × (η2 X /η2 ST)

(1)

where G is the gradient of the plot, η is the refractive index of the solvent, Φ is the quantum yield, X refers to SCNQDs, and ST refers to quinine sulfate (refractive index: 1.33). The fluorescence quantum yield is found to be 13.4% (Figure S1b, Table S1). PL sensing of Hg2+ ions. A stock solution of 1 mg/mL of SCNQDs is prepared by dissolving 50 mg of solid SCNQDs in 50 mL of water. For the PL quenching experiment, 10 μL of SCNQDs stock solution is diluted with 2 mL of water and different concentrations of 2 μL Hg2+ solution are added and kept for 5 min under static conditions for equilibration. The resultant concentration of the Hg2+ is 100 times lesser than the initially added Hg2+. After 5 min, PL of the mixture solution with different amounts of Hg2+ is recorded with excitation wavelength 360 nm at room temperature. The selectivity of SCNQDs toward Hg2+ is compared with other metal ions such as Ag+, Al3+, Cd2+, Cu2+, Fe3+, Mn2+, Na+, Ni2+, Pb2+, Ca2+, and Zn2+ under identical conditions. The concentrations of Hg2+ and other metal ions taken are 10 μM. In order to study the sensitivity of Hg2+ in a mixture of metal solutions, the SCNQD solution is mixed with Hg2+ in the absence and presence of other metal ions and kept under static conditions for 5 min. After 5 min, PL emission spectra are recorded at room temperature. PL sensing of Hg2+ ions in tap water. 50 mg of solid SCNQDs is dissolved in 50 mL of tap water from our laboratory, to make a stock solution with concentration 1 mg/mL. 1 mM Hg2+ ions stock solution is also prepared in tap water. For the PL quenching experiment, 10 μL of SCNQDs solution is diluted with 2 mL of water, and different concentrations of 2 μL of Hg2+ solution are added and kept for 5 min 1733

DOI: 10.1021/acssuschemeng.7b03008 ACS Sustainable Chem. Eng. 2018, 6, 1732−1743

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ACS Sustainable Chemistry & Engineering Scheme 1. Proposed Synthetic Scheme of SCNQDs

Figure 1. (a) UV−visible absorption (black) and emission (blue) spectrum of SCNQDs. Inset: photograph of SCNQDs solution in white light (left) and under 365 nm UV (right) irradiation. (b) TEM image of the SCNQDs. Inset: lattice planes of an individual SCNQD. (c) Particle size distribution of the SCNQDs calculated from TEM images. (d) Powder X-ray diffraction pattern of SCNQDs. mixture is transferred to a Petri dish and dried at 80 °C to form a SCNQD/PVA composite film. Instrumentations. The powder X-ray diffraction (PXRD) patterns of the sample are recorded from 5° to 80° 2θ on a Rigaku Ultima IV instrument with Cu Kα radiation (λ = 1.54 Å) at room temperature. DRS for samples are recorded with a Hitachi U-4100 spectrophotometer in the range 200 nm to 800 nm. A UV-1800 spectrophotometer (SHIMADZU, Japan) is used for recording UV−visible absorption (200 to 1000 nm) spectrum. FT-IR spectra are recorded in SHIMADZU IR Affinity, in the range from (500 to 4000) cm−1 with KBr pellets. PL measurements are carried out with Hitachi F-2500 (Hitachi, Japan) and Fluoromax-4, Horiba spectrophotometers. Time-resolved fluorescence decay is measured in a picoseconds time-resolved cum steady-state luminescence spectrometer, FSP920 from Eddinburg Instruments. Phosphorescence spectra and lifetime are recorded in a Quanta Master-30plus phosphorescence spectrophotometer (PTI, USA). The phosphorescence spectrum for SCNQDs is measured by applying a 200 μs delay. The sizes of the SCNQDs are observed with a transmission electron microscope (JEM2100, JEOL) with an accelerating voltage of 60−200 kV. The particle

to equilibrate. After 5 min the PL of the mixture solution is recorded with excitation wavelength 360 nm at room temperature. PL sensing of Hg2+ ions with filter paper. Filter paper is cut into rectangular pieces of 5 cm × 1 cm. A filter paper piece is dipped into an SCNQDs solution (1 mg/mL) for 5 min. After that it is dried at 60 °C for 10 min. To these filter paper pieces loaded with SCNQDs, 2 μL of (10 μM) different metal ions are drop casted. After drying, PL emission of SCNQDs filter papers is observed and photographed under a UV-lamp with 365 nm irradiation. SCNQD loaded filter paper stability studies. The UV−visible diffuse reflectance spectrum (DRS) and the PL emission of the dried SCNQD loaded filter paper are recorded as it is. The PL emissions of the SCNQDs loaded filter paper are also recorded at excitation wavelength 360 nm. The photostability test is performed under both day light and UV light. The SCNQD loaded filter paper is kept under day and UV light respectively for 1 h. Preparation of SCNQD/PVA Composite. 0.25 g of poly vinyl alcohol is dissolved in 50 mL of water by stirring with a magnetic stirrer at 80 °C for 30 min. 20 mL of PVA solution is mixed with 2 mL (1 mg/mL) of SCNQDs until a uniform solution is obtained. The 1734

DOI: 10.1021/acssuschemeng.7b03008 ACS Sustainable Chem. Eng. 2018, 6, 1732−1743

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Figure 2. (a) XPS survey spectrum of SCNQDs, corresponding high resolution XPS spectra of (b) C 1s, (c) N 1s, (d) O 1s, and (e) S 2p. (f) FTIR spectrum of the SCNQDs taken in a KBr pellet.

EDTA·2Na and thiourea at 200 °C the basic structural unit consisting of the three tri-s-triazine rings is formed, which is repeated in two dimensions forming a constituent layer of SCNQD. These layers will stack one over other, forming the SCNQDs. The functional groups are supposed to be at the edges of the layers. SCNQDs prepared and washed with alcohol are characterized by UV−visible, FT-IR, X-ray photoelectron spectroscopy (XPS), powder X-ray diffraction, and transmission electron microscopy (TEM) imaging. Two UV− visible absorption peaks centered at 263 and 358 nm (Figure 1a (black line)) are observed, which is similar to graphitic CNQDs

size distribution through dynamic light scattering (DLS) and surface charge of the nanoparticles are measured with a Zeta Seizer, Nano Series Nano-ZS90 (Malvern, United Kingdom). X-ray photoelectron spectroscopy is recorded in an XPS, AXIS ULTRA (Kratos Analytical Ltd., United Kingdom) system. The elemental percentages of carbon, nitrogen, sulfur, and hydrogen in SCNQDs are also determined with a Leco CHNS 932 elemental analyzer.

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RESULTS AND DISCUSSION The proposed scheme for synthesis of SCNQDs based on our experimental findings is shown in Scheme 1. On reaction of 1735

DOI: 10.1021/acssuschemeng.7b03008 ACS Sustainable Chem. Eng. 2018, 6, 1732−1743

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Figure 3. (a) PL spectra of SCNQDs, (b) normalized PL spectra of SCNQDs as a function of changing excitation wavelength from 320 to 500 nm.

in earlier reports.11,14,43 The 263 nm peak indicating the presence of s-triazine rings and/or carbonyl groups, may be assigned to the π−π* electronic transition, while the 358 nm peak is assigned to n−π*electronic transitions due to the heteroatoms present, such as nitrogen, oxygen, and sulfur. A broad emission peak centered at 433 nm with a hump around 495 nm is observed on excitation at 360 nm (Figure 1a (blue line)). SCNQDs in aqueous medium are yellow in color under white light and a blue glow is seen under a 365 nm UV lamp (inset: Figure 1a), characteristic of the carbon nanomaterials, such as carbon dots, graphene quantum dots, or graphitic CNQDs. The particle sizes of the SCNQDs prepared are found to be in the range of 3−13 nm, as shown in the TEM image, Figure 1b, with the particle size distribution given in Figure 1c (particle size distributions are measured for 200 particles using ImageJ software). SCNQDs are of irregular shapes; some of them are circular while some are elongated. High resolution TEM image of a SCNQD, Figure 1b inset, showing an interplanar distance of 0.31 nm for the (002) planes of graphitic carbon nitride, comparable to existing reports.2,44 The HRTEM image showing lattice planes in different SCNQD particles is shown in Figure S3a. The interplanar distance is calculated by taking the average over six lattice fringes (Figure S3b). The DLS measurement of SCNQDs shown in Figure S3c also proves the formation of nanoparticles with size below 10 nm, which is in good agreement with TEM results. The powder Xray diffraction is shown in Figure 1d. It shows characteristic peaks at 2θ value 13.6° and 28.20°.14,23,45 The peak at 28.20° is assigned to the interlayer stacking reflection of graphite like materials, with d = 0.316 nm, comparable to the findings from the HRTEM image and earlier reports. While the peak at 13.6° with d = 0.653 corresponds to in-plane structural repeating units. In crystalline polymeric carbon nitride materials, the variation in XRD peak positions and systematic absence occur depending on the packing of the structural motifs, buckling process temperature, etc.46 The surface charge on the SCNQDs is found to be −29.8 mV from the zeta potential measurement (Figure S3d). These studies confirm the formation of graphitic carbon nitride quantum dots with negatively charged surface. The surface composition of SCNQDs is characterized with XPS and FT-IR spectroscopy. The XPS survey (Figure 2a) shows three peaks at 283.6, 398.9, and 530.8 eV, respectively, for C1s, N1s and O1s. Two more peaks appearing at 224.6 and

161.03 eV are due to S2s and S2p, respectively. These results show the presence of carbon, nitrogen, oxygen, and sulfur groups on the surface of SCNQDs. Further, deconvolution of the high resolution XPS spectrum for C1s (Figure 2b) shows four peaks at 283.6 eV, 284.5 eV, 285.5 eV, and 287.4 eV attributed to CC, NCN, C−OH/C−O−O, and CO/ COOH functional groups. The N1s spectrum (Figure 2c) shows two peaks at 398 eV and 399.36 eV due to −NH and NC− N, respectively. The spectrum of O1s (Figure 2d) consists of two peaks at 530.8 eV and 531.6 eV which are assigned to be C−O and CO bands. In addition, three peaks are observed in the S2p spectrum (Figure 2e) which are associated with −SH (161.03 eV), −CS (162.19 eV), and sulfoxide (164.2 eV).23,47 The representative FT-IR spectrum of SCNQDs is shown in Figure 2f. The peak at 3400−3500 cm−1 is due to the stretching modes of −OH/NH,14,23 which substantiate the hydrophilic nature of SCNQDs. The peak at 2200 cm−1 is assigned to C−N,2 2120 cm−1 to the S−H group,23 1670 cm−1 to CO, and 1410 cm−1 to the −COO group. Peaks at 1180− 1080 cm−1 are due to CS/CN vibrations and the characteristic 809 cm−1 peak is due to the breathing mode of striazine rings.11,14 The elemental analysis of the SCNQDs from XPS shows the presence of C = 40.68%, N = 25.01%, O = 26.94%, S = 3.15%, and H = 4.22% by atomic %. This result indicates that SCNQDs possess rich functional groups on the edges of the graphitic carbon nitride structure.11 Therefore, the SCNQDs may be called functionalized carbon nitride quantum dots. These functional groups make them highly soluble in polar solvents. The results of elemental analysis through XPS and elemental analyzer for SCNQDs are shown in Tables S2 and S3. The ratio of unsaturated to saturated bonds in SCNQDs is found to be approximately 2:1 (1.97652:1 to be exact). The excitation wavelength dependent red shift of the PL λmax, in the excitation wavelength range 320−500 nm, is observed to be similar to the carbon dots47 and graphene quantum dots,48 as show in Figure 3a and b. A gradual increase in the emission intensity as a function of excitation wavelength with maximum emission at 360 nm excitation, then a gradual decrease in intensity up to 500 nm excitation is observed. The fluorescence quantum yield of SCNQDs is found to be 13.4%, taking quinine sulfate reference, comparable to the earlier reports.14 The quantum yield calculation for the all synthesized SCNQDs is shown in Figure S1b and Table S1. 1736

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Figure 4. (a) Responses of PL from SCNQDs in aqueous medium (double distilled water) on addition of different amounts of Hg2+, λex = 360 nm; (b) bar diagram showing the effect on PL of SCNQDs on addition of different metal ions (λex = 360 nm, [Mn+] = 10 μM); (c) photograph of SCNQDs in aqueous medium (double distilled water) under white light (upper panel), under 365 nm light (middle panel); and in the presence of different metal ions (10 μM) under 365 nm light (bottom panel); (d) responses of PL from SCNQDs aqueous medium on addition of different amounts of Hg2+ in tap water, λex = 360 nm.

the WHO standard limit of 10 nM.14 The high selectivity of SCNQDs toward Hg2+ ions is observed as compared to that of the other metal ions, can be seen in the bar diagram in Figure 4b and the photographic representation in Figure 4c with 10 μM of metal ion concentration, where it can be clearly observed that only Hg2+ quenches the PL from SCNQDs while other metals have no visible effect on the PL. On adding other metal ions first to SCNQDs solution and then adding Hg2+, also quenching of PL of SCNQDs by Hg2+ is observed (Figure S6, and discussion therein); that is, in the presence of other metal ions, SCNQDs are capable of sensing Hg2+. The extent of quenching by Hg2+ in this case is less as compared to the Hg2+ being present alone. This may be due to blocking of the binding sites of SCNQDs by other metal ions. This however does not affect the detection of Hg2+ as none of the other metals tested is able to quench the PL of SCNQDs as seen from the bar diagram in Figure S6b. In order to demonstrate the practical applicability of the SCNQDs as Hg2+ sensor we have also studied its’ sensing capabilities in tap water spiked with Hg2+. To our delight the results are similar to the results obtained

To design an efficient detection system for Hg2+, solubility (monitored with UV−visible spectra) as well as the PL of SCNQDs in different solvents, such as ethylene glycol, DMSO, DMF, ethanol, and methanol, is tested, Figure S5 (and discussion therein). It is observed that the SCNQDs show maximum PL in water. Hence aqueous solutions (actually dispersion) of SCNQDs are taken for the Hg2+ detection. PL from SCNQDs prepared is found to be sensitive and selective for Hg2+ ions. A gradual quenching of the PL from SCNQDs is observed on addition of 2 μL of Hg2+ solution in the concentration range 0.001 μM to 20 μM (Figure 4a). It may be mentioned that we have added 2 μL of 0.001−20 μM Hg2+ to 2 mL of water and 10 μL of standard SCNQDs solution. Therefore, the concentration of Hg2+ in a reaction mixture of Hg2+ and SCNQDs is effectively (2 μL × bμM/2000 μL = bnM; b is the concentration of Hg2+ solution added) 10−2 times less (Refer to the Experimental Section). Thus, the detection limit of Hg2+ is 0.01 nM in the Hg2+ solution. This makes our system highly sensitive. Even with the detection limit of 1 nM of Hg2+ added, the concentration is much lower than 1737

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Figure 5. (a) pH dependent PL emission of SCNQDs in the absence and presence of 10 μM Hg2+; (b) corresponding bar diagram; (c) UV−visible spectra of SCNQD solution at different pH values, pH-1(acidic), pH-7 (neutral), pH-14 (basic), and corresponding spectra after addition of 10 μL of 10 μM Hg2+. (d) PL emission spectra of SCNQDs at different ionic strengths of NaCl.

after addition of 10 μM Hg2+ ions when excited with a 375 nm laser and emission monitored at 425 nm, (Figure S8).14,52 The increase in lifetime of SCNQDs on addition of Hg2+ is attributed to external heavy atom effect. The PL lifetime values are depicted in the Table S4−S6. Presence of heavy atom Hg2+ leads to better intersystem crossing between singlet and triplet electronic states, hence increase in lifetime, (please refer to Figure S8 and the discussion therein). Selective quenching of PL by Hg2+ may be attributed to Hg2+ binding to the sulfur and nitrogen functionalities due to greater preference of soft Hg2+ toward soft sulfur and moderately hard nitrogen atoms.53 The interaction between the SCNQDs and Hg2+ also has an electrostatic contribution. This is supported by our pH dependent sensing of Hg2+ ions with SCNQDs. It is observed that PL of SCNQDs is considerably more sensitive to Hg2+ ions at pH-7(neutral) than acidic (pH-1) or basic (pH-14) as shown in the Figure 5a and b. Maximum quenching of PL is observed at pH 7. After a look at the absorbance spectra of SCNQDs at pH 1, 7, and 14, in the absence and presence of Hg2+, Figure 5c, it is observed that at pH 1 and 14 there is no change in the absorbance at 263 and 358 nm on addition of Hg2+. While at pH 7, two peaks at 263 and 358 nm disappear on addition of Hg2+, indicative of ground state adduct formation between

under standard condition with double distilled water (Figure 4d). A plot of F0/F (where F0 = PL intensity of SCNQDs; F= PL intensity of SCNQDs in the presence of different concentrations of Hg2+) vs Hg2+ concentration (quencher concentration) shows a similar pattern for both the cases where Hg2+ is in double distilled water and tap water (Figure S7a and b, respectively). A linear part is observed at lower concentrations of the quencher (Hg2+) while a curved part reaching saturation is observed for higher concentrations of the quencher. The linear range is the same, 10 nM to 1 μM for Hg2+ in double distilled water and tap water. This type of F0/F versus concentration of quencher pattern relates the quenching process to a static mechanism.49 This is also supported by the UV−visible absorbance of SCNQDs, at 263 and 358 nm, on addition of different concentrations of Hg2+, which gradually disappear (Figure S7c), thus confirming the formation of ground state adduct between the SCNQDs and Hg2+. The complexation constant for SCNQD and Hg2+ is found to be 9.35 and the number of binding sites is determined to be 1.51 from the slope (Figure S7d and related discussion in the SI).50,51 Further, the PL emission average lifetime is measured to be 3.154 ns and 3.972 ns, respectively, for SCNQDs before and 1738

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Figure 6. Photograph of SCNQDs loaded onto filter paper (a) under white light, (b) under 365 nm UV light, and (c) on drop casting of 2 μL of 10 μM metal ion solutions (Mn2+, Zn2+, Hg2+, Pb2+, Al3+, Cu2+, Ca2+, Fe3+, Ni2+, Cd2+, Ag+, and Na+).

Hg2+ and SCNQDs. On the other hand, there is no such adduct formation at pH 1 and 14. At highly acidic conditions, generation of positive charges on the surface of SCNQDs reduces the association with the same charge Hg2+ ions leading to less sensitivity. While under highly basic conditions OH− may combine with Hg2+ to form Hg(OH)2 or H-bonding by agglomerates resulting in decreased sensitivity. A comparison of SCNQDs sensitivity toward Hg2+ with selected literature reports is shown in Table S7, and our system is found to exhibit a lower detection limit (0.01 nM) for Hg2+. PL from the SCNQDs prepared is found to be independent of the ionic strength of the solution used in the range of 0−500 mM NaCl solutions as seen in Figure 5d. However, the PL intensity of the SCNQDs is found to be sensitive to pH as shown in Figure S3d. Both on increasing and decreasing the pH from 1−7 and 7−13, there is an observed decrease in PL intensity similar to that for graphitic CNQDs earlier reported.43,52 With the aim to make a ready to use litmus paper like device for detection of Hg2+, we have loaded SCNQDs onto uniformly cut filter papers and used them for easy detection of Hg2+. In Figure 6a the filter papers loaded with SCNQDs and a blank under white light can be seen. Figure 6b shows the same set of filter papers under 365 nm illumination, and a bright glow from the SCNQDs loaded ones is visible while the blank filter paper appears dark. In Figure 6c, when 2 μL of 10 μM metal ion solutions is drop casted onto the SCNQDs loaded filter papers and observed under 365 nm light, clear quenching of PL is observed for Hg2+ while other metals do not show any such effect. For these SCNQDs loaded filter papers to be practically useful, we have further carried out the Hg2+ detection in normal tap water collected from our laboratory spiked with Hg2+. To our pleasure, the system worked with the tap water also Figure S9. The properties of SCNQD loaded filter paper are studied with UV−visible and PL measurements, Figure S10. In the UV−visible DRS of the SCNQDs loaded filter paper, Figure S10a absorbance peaks (263 and 358 nm) due to SCNQDs are retained. However, an additional peak at ∼480 nm appears, which is absent in the SCNQDs in solution. This peak may be attributed to energy states arising from interaction of the SCNQDs with the filter paper or due to solid phase aggregation of SCNQDs on the filter paper. In the PL spectrum and digital photograph of the filter paper SCNQDs composite, no

significant change in PL intensity is observed after one week, as shown in Figure S10b. The excitation dependent PL emission of SCNQDs is retained in the filter paper (Figure S10c and d). Similar to the solution phase sensing experiments, the filter paper based sensing of metal ions is found to be equally selective toward Hg2+, as can be concluded from Figure S10e and f. Therefore, the visual observation of fluorescence quenching on SCNQDs loaded filter papers is substantiated by the PL spectroscopic results. In order to find out the stability of SCNQDs loaded filter paper, the photostability is tested by keeping it under day light and UV light for 1 h. The PL emission spectra of SCNQDs-filter paper composite along with their respective intensity versus time plot (inset) are shown in Figure S10g and h. The SCNQDs loaded filter papers are found to be stable for at least 1 week. Though there are some reports14,23 on sensing of Hg2+ ions in aqueous medium, there are none on solid matrix to the best of our knowledge. We have used SCNQDs for selective detection of Hg2+ ions in both the solid and aqueous phase. Due to high sensitivity and selectivity, SCNQDs may find wide application in biological and environmental fields. The filter papers loaded with SCNQDs can be used for preliminary detection of Hg2+ at the sites or in a laboratory, and then if required further studies may be carried out for quantification of Hg2+. This filter paper based detection system for Hg2+ is cost-effective and can play a vital role in water quality monitoring for public health. We would also like to report here our findings on RTP observed for SCNQD/PVA composite. SCNQD/PVA composite and made into a film, when excited with 365 nm light exhibits cool blue PL, Figure 7a and b (also refer to the chromaticity diagrams of SCNQDs shown in Figure S11a and b, substantiating the visual observation). On turning off the 365 nm light the SCNQD/PVA composite and film shows green phosphorescence. This is observed for few seconds with naked eye (Refer to the video in the SI). Cool blue PL from the SCNQD/PVA composite film (Figure 7c.) has a broad band emission from ∼400 nm-700 nm, fluorescence emission centered at λ emF = 425 nm, green line while the phosphorescence emission band starts at ∼450 nm, extending up to 700 nm with maxima at λemP = 535 nm, red line. The phosphorescence decay profile of SCNQD/PVA film is fitted to a multiexponential function with three lifetimes, 101.8, 447.6, 1739

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Figure 7. (a) Photograph of the SCNQD/PVA composite in white light, 365 nm UVlight on and off. (b) Photograph of the SCNQD/PVA composite film in white light, 365 nm UVlight on and off. (c) PL excitation spectrum (black) and fluorescence (green) and phosphorescence (red) emission spectra of a SCNQD/PVA composite film (λex = 360 nm, λemF = 425 nm, λemP = 535 nm) (d)Time resolved phosphorescence study of a SCNQD/PVA composite film (λex = 360 nm, λem = 535 nm).

assigned to the surface CN/CO groups as various organic molecules have been known to exhibit phosphorescence.54−57 The role of the PVA matrix is to provide structural rigidity and exclusion of oxygen, stabilizing the triplet state, as no RTP observed in aqueous dispersion of the SCNQDs. Observation of RTP in heavy atom free organic systems is attributed to the outcome of a few competing factors, the effective intersystem crossing between the excited singlet and triplet states, and the nonradiative decay and quenching of triplet state.42 Intersystem crossing, which promotes the phosphorescence, is enhanced due to the spin orbit coupling between the states involved in emission process. Spin orbit coupling is affected by the proximity of singlet and triplet states and internal or external heavy atoms. As there is no heavy atom present in our SCNQD/PVA composite, we believe closely spaced singlet and triplet states lead to effective intersystem crossing. Nonradiative processes, such as vibrational relaxation and collisonal quenching effects that prohibit the observation of RTP, are

and 960.9 ms, with an average lifetime of 503.43 ms (Figure 7d). However, for SCNQDs in aqueous medium no RTP is observed, but a blue fluorescence with lifetime of about 3.154 ns at 425 nm on excitation at 375 nm is seen. The average lifetime of both fluorescence and phosphorescence is calculated using eq 2 ⟨τ ⟩ =

∑ αiτi 2/∑ αiτi

(2)

In the PL excitation spectrum of SCNQD/PVA, (black line in Figure 7c) two peaks at 300 and 360 nm are observed. It can be seen that the higher contribution to the PL at 425 nm is from the 300 nm excitation as compared to the 360 nm excitation, which can also be seen in the excitation dependent RTP spectra of SCNQD/PVA composite film (Figure S11c). On changing the excitation wavelength from 300 to 380 nm, it is observed that there is a gradual decrease in the phosphorescence intensity, however with no change in the emission center λmax ∼ 535 nm. The origin of the RTP can be 1740

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Figure 8. Partial energy level diagram for PL in a molecular system for explanation of phosphorescence in an SCNQD/PVA composite film.

application in phosphorescence OLED devices and glow in the dark paints. It cannot be overemphasized that the SCNQDs reported here are one of the cheapest RTP materials reported so far. The ease of synthesis and inexpensive precursors for obtaining SCNQDs are the added advantages. GCN composites have already been used for fabrication of light emitting devices.59 Now with the phosphorescent SCNQDs, the possibility of higher light harvesting devices can be designed at low cost. Moreover, phosphorescence OLEDs can be explored due to the tunability of emission through size modification of SCNQDs and functionalization.

blocked by the rigid matrix provided due to the strong hydrogen bonding between SCNQDs and PVA chains. Intermolecular hydrogen bonding is also reported to influence the effective intersystem crossing in organic molecules, thus in SCNQD/PVA composite helping in observation of RTP. Further, PVA is known to exhibit excellent oxygen barrier property. This helps in preventing oxygen from quenching of the triplet state, thus leading to RTP observation. The mechanism of phosphorescence generation in SCNQD-PVA composite is depicted in Figure S11e. The phosphorescence process here could also be understood by correlating the data available and the partial energy level diagram shown in Figure 8. The electronic excited states S1 and T1 are an n−π* type singlet and triplet excited state, respectively, while S2 and T2 are π−π* type singlet and triplet states, respectively. Therefore, the electronic excitations S0−S1 and S0−S2 in Figure 8 can be correlated to absorbance bands 358 and 263 nm in Figure 1a, for SCNQDs, respectively. This is also supported by the two excitation peaks observed at 300 and 360 nm in Figure 7c (black line). The higher intensity of the 300 nm peak in the PL excitation spectrum (Figure 7c) and the higher phosphorescence on the excitation at 300 nm (Figure S11c) suggest that S0-S2, i.e. the π−π*transition, is dominant in the contributing to the fluorescence and phosphorescence process for the SCNQD/PVA composite film. At present we can suggest that the phosphorescence observed in the SCNQD/PVA composite/film is from the T1 (n−π*) to the S0 energy state, though S0−S2 (π−π*) is the dominant excitation. The RTP observed is attributed to the close proximity of the singlet and the triplet excited states, as there is no possibility of enhanced spin orbit coupling caused by the presence of heavy atoms. This can be justified if we consider the 425 nm peak in the PL spectrum of the SCNQD/PVA composite film to be its fluorescence peak and notice that phosphorescence starts approximately from 450 nm, i.e. a very small energy difference between the first singlet and triplet excited states. To the best of our knowledge there have only been two other reports up till now on phosphorescence from bulk g-C3N4, and this is the first report on RTP exhibited by SCNQDs. Existing reports involve high temperature routes for preparation of bulk g-C3N4,57,58 contrary to our low temperature synthesis of SCNQDs. The ability to disperse SCNQDs into a PVA solution and then make a film out of it can have potential

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CONCLUSIONS

In summary, we report an easy one pot synthesis of photoluminescent SCNQDs from low cost precursors, thiourea, and EDTA.2Na salt with a quantum yield of about 13.4%. The SCNQDs prepared by this method have average size of about 6 nm and are applied for selective and sensitive detection of Hg2+ ion to the limit of about 0.01 nM in solution. Hg2+ detection is through quenching of PL of SCNQDs by Hg2+ ions. This sensing system is also tested for normal tap water spiked with Hg2+ ions, and the efficiency is similar to the standard conditions. Spectroscopic investigations suggest a primarily static quenching mechanism. The PL sensitivity of SCNQDs toward Hg2+ is maximum at pH-7, making it a potential tool for Hg2+ detection in biological fluid. Further, we have developed a SCNQDs loaded filter paper based detection system for Hg2+ ions which works with equal efficiency toward Hg2+ present in tap water as well as in distilled water. Thus, we have developed an easy and cost-effective detection device for Hg2+ ions. A composite of SCNQDs and PVA is made to stabilize the triplet state in SCNQDs for getting RTP. This is a simple and inexpensive method to prepare purely organic RTP material so far. Therefore, we believe this will lead to more exploration/ exploitation of graphitic CNQDs for more efficient organic light emitting devices, photocatalysis, and many other novel applications. 1741

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b03008. PL spectra, quantum yield calculation, HRTEM, zeta potential measurements, DLS measurements, photostability and pH dependent study, solvent dependent study, PL quenching experiments in the presence of other metal ions, Stern−Volmer plot, time-resolved PL measurements, digital photograph of PL quenching experiments of SCNQDs on filter paper, PL emission, PL quenching experiments and PL photostability test of SCNQD-filter paper composite, fluorescence and phosphorescence CIE diagram, mechanism of phosphorescence in SCNQD/PVA composite, and fluorescence Hg2+ sensing efficiency comparison for different materials (PDF) (MPG)SCNQD/PVA composite shows green phosphorescence

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Sonit Kumar Gogoi: 0000-0001-9030-1162 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.K.G. and K.P thank the SERB-DST SB/S1/PC-105/2012, UGC-NFST-2015-17-ST-ASS-2321, Govt. of India, for the financial assistance. The authors also thank SAIF-NEHU, SAIFGauhati University, Department of Chemistry-IIT Guwahati, CIF-IIT Guwahati, CeNSE-IISc Bangalore, IASST-Guwahati, and Numaligarh Refinery Limited (NRL)-Golaghat for extending their instrumental support. We are thankful to Dr. D. Mahanta, Gauhati University, Dr. D. Choudhury, IASST, and Mr. Subrat Jyoti Borah for helping us with XPS, DLS, and CHNS recording, respectively.

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