Facile Synthesis of Photoluminescent Graphitic Carbon Nitride

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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 ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03008 • Publication Date (Web): 06 Dec 2017 Downloaded from http://pubs.acs.org on December 7, 2017

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ACS Sustainable Chemistry & Engineering

Facile

Synthesis

of

Photoluminescent

Graphitic

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

Corresponding Author: Department of Chemistry, University of Gauhati, G. B. Nagar,

Guwahati -781014, Assam, India. e-mail:[email protected]

ABSTRACT: Carbon nitride materials have become one of the highly explored carbon based nanomaterial since its re-discovery in 1990s due to its semiconductor like behavior. Here, we report a facile one pot synthesis of sulphur and oxygen doped carbon nitride quantum dots (SCNQDs) from thiourea and ethylene diaminetetraacetic acid disodium salt by thermal method. 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 solid phase, i.e. SCNQDs loaded onto filter papers. Hg2+ sensing capability of the SCNQDs in solution phase is 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 10nM-1µM. The minimum detection limit is found to be 0.01nM 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 papers to be used in biological fluids as well as in environmental samples, which was not available earlier to the best of our knowledge. Further we have also made a composite of

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SCNQDs with polyvinyl 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,

INTRODUCTION Among the carbon based nanomaterials, graphitic carbon nitrides (GCN) are a type of polymeric nanomaterial primarily composed of carbon and nitrogen. GCN has secured considerable attention in recent times due to their unique and tunable properties. Semiconductor like luminescence in GCN is one such property that has been explored for various applications, including photo catalysis, 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 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 hetero atoms i.e. other than carbon and nitrogen, like sulphur, oxygen, boron etc. are also doped into these GCN in order to achieve new properties.15,16

Its unique structure,

composed of tri-s-triazine units bridged by amino groups, with periodic lattices vacancies GCN offer more maneuverability in terms of tuning its optical properties compared to graphene sheets. In quantum dot form, GCN show characteristic blue photoluminescence (PL) which is much applied in sensing and imaging applications.5,11,17 The graphitic carbon materials (GCM) also been used in medical sciences such as detection of prostate specific antigen (PSA), alkaline phosphate, bleomycin a drug used for treatment of cancer.18-20 Moreover, doping of GCM on αFe3O4/Ti make them effective catalyst 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, easy and economically viable synthetic route is an essential criteria for developing new application of a material and making the material to a practically applicable product. For example, because of the


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simplistic synthetic routes for preparing carbon nanoparticles, far many explorations for novel properties and applications have been reported as compared to graphitic carbon nitride quantum dots (CNQD). Toxic effect of Hg2+ is well established and in the past there have been several reports of disasters caused by mercury. Though the developed nations have put restrictions on mercury usage, increasing emission of mercury in developing world is a major concern.27,28 Pesticides, batteries, paper industries etc. being major contributors of mercury emission, fluorescent lamps have brought mercury to every household. With the ever increasing use of fluorescent lamps in the developing nations, there is growing possibility of mercury exposure to humans and animals due to lack of 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. In this perspective colorimetric and/or fluorescence detection system for Hg2+ can play an important role in Hg2+ sensing in water used for household chores due to inexpensive nature inherent to these techniques. Lot of effort is put 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 like CdSe/ZnS,31 Eu3+-doped CdS32 etc. and metal free semiconductors like 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, 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 high cost and toxicity of heavy metals, metal free alternatives are being explored. Therefore, organic RTP materials are on 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 nanomaterials,41,42 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. Here in we report a very facile method for preparation of photoluminescent sulphur and oxygen doped graphitic carbon nitride quantum dots (SCNQDs) by simple heating at 200oC 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 triplet state in this material which can have application in optoelectronics, photo catalysis to glow in dark paints for security signs.

EXPERIMENTAL SECTION Materials. Thiourea, ethylenediaminetetraacetic acid disodium salt dihydrate (EDTA.2Na), NaOH pellets, manganese sulphate, mercury chloride, copper sulphate, silver nitrate, lead acetate, nickel sulphate, cadmium chloride, zinc sulphate, calcium chloride, sodium chloride, ferric chloride, aluminum sulphate are procured from Merck Specialties Private Limited, India. Double distilled water is used for all the experiments unless mentioned otherwise. PVA (M.Wt1700-1800) was received from LOBA Chemie private limited, India, sulphuric acid, ethanol, methanol, DMF, DMSO, polyethylene glycol were received from Merck Specialties Private Limited, India.

Synthesis of sulphur doped carbon nitride quantum dots (SCNQDs). EDTA.2Na and thiourea are mixed thoroughly in a mortar in different molar ratios, 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 silica crucible with a lid and heated at 1500C-2500C in a laboratory oven from Bio craft Scientific System Pvt. Ltd (Agra-5, India) for 2hr 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. Sulphur and oxygen doped graphitic carbon nitride quantum dots thus prepared with (EDTA.2Na)1:12(thiourea) molar ratio heated at 200oC for 2hr are found to exhibit highest PL


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intensity, as well as quantum yield, Figure S1a and 1b. Hence forth all the characterization and studies are carried out with this sample. We will use the abbreviation SCNQDs for our prepared sulphur

doped

graphitic

carbon

nitride

quantum

dots

prepared

with

molar

ratio

o

(EDTA.2Na)1:12(thiourea), heated at 200 C for 2hr.

Determination of Quantum Yield. PL quantum yield (QY) of SCNQDs was measured by comparing the integrated photoluminescence intensities (excited at 360 nm) and the absorbance value (at 358 nm) using quinine sulfate in 0.1 molL-1 H2SO4 (ф=0.54) as standard reference. Different concentrations of the SCNQDs and standard reference quinine sulphate were prepared so that its absorbance is less than 0.1 at their respective excitation wavelengths. The PL QY was calculated using the following equation no (1). ФX= ФST × (GX/GST) × ( ƞ2X /ƞ2ST)

--------------------------- (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 sulphate.( 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 5mg/mL of SCNQDs is prepared by dissolving 50mg of solid SCNQDs in 50 mL of water. For the PL quenching experiment 10µL of SCNQDs stock solution is diluted with 2mL water and different concentrations of 2µL Hg2+ solution is added and kept for 5 min in static conditions for equilibration. The resultant concentration of the Hg2+ is 100 times lesser than the initially added Hg2+. After 5min, PL of mixture solution with different amounts of Hg2+ are recorded with excitation wavelength 360nm at room temperature. The selectivity of SCNQDs towards 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 is 10µM. In order to study the sensitivity of Hg2+ in mixture of metal solutions, the SCNQD solution was mixed with Hg2+ in the absence and presence of other metal ions and kept in static condition for 5min. After 5 min PL emission spectra is recorded at room temperature.



PL sensing of Hg2+ ions in tap water. 50mg of solid SCNQDs is dissolved in 50 mL of tap water from our laboratory, to make a stock solution with concentration 5mg/mL. 1mM Hg2+ ions stock solution is also prepared in tap water. For the PL quenching experiment 10µL of SCNQDs solution is diluted with 2mL water and different concentrations of 2µL Hg2+ solution is added and kept for 5 min to equilibrate. After 5min PL of mixture solution were recorded with excitation wavelength 360nm at room temperature.

PL sensing of Hg2+ ions with filter paper. Filter paper was cut into rectangular piece of 5cm×1cm. Filter paper piece is dip to SCNQDs solution (5mg/mL) for 5 min. After that it is dried at 60oC for 10min. To these filter paper pieces loaded with SCNQDs 2µL (10µM) different metal ions are drop casted. After drying the PL emission of SCNQDs filter papers are observed and photographed under a UV-lamp with 365nm irradiation.

SCNQD loaded filter paper studies. Filter paper was cut into rectangular piece of 5cm×1cm. Filter paper piece is dip to SCNQDs solution (5mg/mL) for 5 min. After that it is dried at 60oC for 10min. The UV-visible diffuse reflectance spectrum (DRS) and the PL emission of the dried SCNQD loaded filter paper was recorded as it is. The PL emissions of the SCNQDs loaded filter paper are also recorded at excitation wavelength 360 nm. The photo stability test was performed both under day light and UV light. The SCNQD loaded filter paper was kept under day and UV light respectively for 1 hr.

Preparation of SCNQDs/PVA composite. 0.25g of poly vinyl alcohol is dissolved in 50mL water by stirring with magnetic stirrer at 800C for 30minutes. 20mL of PVA solution is mixed with 2mL (5mg/mL) of SCNQDs uniform solution is obtained. The mixture was transferred to petri dish and dried at 800C to form a SCNQDs/PVA composite film.

Instrumentations. The powder X-Ray diffraction (PXRD) patterns of the sample were recorded from 5° to 80° 2θ on Rigaku Ultima IV instrument with CuKα radiation ( λ = 1.54Ǻ) at room temperature. DRS samples were recorded with Hitachi U-4100 Spectrophotometer (Solid) in the range of 200 nm to 800nm. UV 1800 spectrophotometer (SHIMADZU, Japan) is used for recording UV-VIS absorption (200 to 1000 nm). FT-IR spectra is recorded in SHIMADZU IR Affinity, in the range from (500 to 4000) cm-1 with KBr pellets. PL


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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 Quanta Master-30plus phosphorescence spectrophotometer (PTI, USA). The phosphorescence spectrum for SCNQDs is measured by applying 200µs delay. The size of the SCNQDs are observed by transmission electron microscope (JEM-2100, JEOL) with an accelerating voltage of 60-200kV. The particle size distribution through dynamic light scattering (DLS) and surface charge of the nanoparticles are measured with Zeta Seizer, Nano Series Nano-ZS90 (Malvern, United Kingdom). X-ray photoelectron spectroscopy is recorded in XPS, AXIS ULTRA (Kratos Analytical Ltd, United Kingdom) system. Elemental percentage of carbon, nitrogen, sulphur and hydrogen in SCNQDs is also determined with Leco CHNS 932 elemental Analyzer.

Scheme 1 Proposed synthetic scheme of SCNQDs

RESULTS AND DISCUSSION The proposed scheme for synthesis of SCNQDs based on our experimental findings is shown in the Scheme 1. On reaction of EDTA.2Na and thiourea at 200oC 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 in 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 UVVisible absorption peaks centered at 263nm and 358nm, Figure 1a (black line), are observed which is similar to graphitic CNQDs in earlier reports.11,14,43 263nm peak indicating the presence of s-triazine rings and/or carbonyl groups, may be assigned to π-π* electronic transition, while 358nm peak is assigned to n- π*electronic transitions due to the hetero atoms present, such as nitrogen, oxygen and sulphur. A broad emission peak centered at 433nm with a hump around 495nm is observed on excitation at 360nm, Figure 1a (blue line). SCNQDs in aqueous medium is yellow in color under white light and a blue glow is seen under a 365nm UV lamp (inset: Figure 1a), characteristic of the carbon nanomaterials like carbon dots, graphene quantum dots or graphitic CNQDs.

(a)

(b)

(c)

(d)

35 30

Frequency %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

25 20 15 10 5 0 3

4

5

6

7

8

9

10

11

12

13

Size(nm)


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Figure 1. (a) UV-Visible absorption (black) and emission (blue) spectrum of SCNQDs. Insets photograph of SCNQDs solution in white light (left) and under 365nm UV (right) irradiation. (b) TEM image of the SCNQDs and inset shows the lattice planes of an individual SCNQD. (c) Particle size distribution of the SCNQDs calculated from TEM images. (d) Powder X-ray diffraction of SCNQDs

Particle size of the SCNQDs prepared are found to be in the range of 3-13nm as shown in the TEM image, Figure 1b, with the particle size distribution given in the Figure 1c (Particle size distribution were measure 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, shows inter-planer distance of 0.31 nm for the (002) planes of graphitic carbon nitride, comparable to existing reports.2,44 HRTEM image showing lattice planes in different SCNQD particles is shown in Figure S2a. The inter-planer distance is calculated by taking average over six lattice fringes, Figure S2b. The DLS measurement of SCNQDs is shown in the Figure S2c also prove the formation of nanoparticles with size below 10nm, which is in good agreement with TEM results. The powder X-ray diffraction is shown in the Figure 1d. It shows characteristic peaks at 2Ɵ value 13.6o and 28.20o.14,23,45 Peak at 28.20o is assigned to the interlayer stacking reflection of graphite like materials with d=0.316nm, comparable to the findings from the HRTEM image and earlier reports. While peak at 13.6o with d=0.653 corresponds to in-plane structural repeating units. In crystalline polymeric carbon nitride materials, variation in XRD peaks positions and systematic absence occur depending on the packing of the structural motifs, buckling process temperature etc.46 Surface charge on the SCNQDs is found to be -29.8mV from the Zeta potential measurement, Figure S2d. These studies confirm the formation of graphitic carbon nitride quantum dots with negatively charged surface.

The surface composition of SCNQDs was characterized with XPS and FT-IR spectroscopy. The XPS survey, Figure 2a shows three peaks at 283.6eV, 398.9eV, and 530.8eV respectively for C1s, N1s and O1s. Two more peaks appearing at 224.6eV and 161.03eV are due to S2s and S2p respectively. These results show the presence of carbon, nitrogen, oxygen and sulphur groups on



the surface of SCNQDs. Further, deconvolution of the high resolution XPS spectrum for C1s, Figure 2b shows four peaks at 283.6eV,284.5eV,285.5eV,287.4eV attributed to C=C, N=C=N, C-OH/C-O-O and C=O/COOH functional groups. The N1s spectrum, Figure 2c show two peaks at 398eV, 399.36eV due to –NH and N=C-N respectively. The spectrum of O1s, Figure 2d consists of two peaks at 530.8eV, 531.6eV which is assigned to be C-O and C=O bands. In addition, three peaks are observed in S2p spectrum, Figure 2e which are associated with –SH (161.03eV), -C=S (162.19eV) and sulfoxide (164.2eV).23,47 Representative FT-IR spectrum of SCNQDs is shown in the Figure 2f. Peak in 3400-3500cm-1 is due to the stretching modes of – OH/NH14,23 which substantiate the hydrophilic nature of SCNQDs. The peak at 2200cm-1is assigned to C-N,2 2120 cm-1 to S-H group,23 1670 cm-1 to C=O and 1410 cm-1 to the –COO group. Peaks at 1180–1080 cm-1 to C=S/C=N vibrations and characteristic 809 cm-1 peak is due to the breathing mode of s-triazine rings.11,14


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(a)

(c)

(b)

(d)



75

(e)

(f)

70 65

% Transmittance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60

S-H C=S C=N

55 50

C-N

45

-

COO

40

-NH/OH

C=O

35 30 0

500

1000

1500

2000

2500

3000

3500

4000

4500

-1 Wavenumber(cm )

Figure 2. (a) XPS survey spectrum of SCNQDs, corresponding high resolution XPS spectra of (b) C1s, (C) N 1s, (d) O1s and (e) S 2p. (f) FTIR spectrum of the SCNQDs taken in a KBr pallet. 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 as 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 Table 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). Excitation wavelength dependent red shift of the PL λmax, in the excitation wavelength range 320-500nm is observed similar to the carbon dots 47 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 360nm excitation, then gradual decrease in intensity up to 500nm excitation is observed. Fluorescence quantum yield of SCNQDs is found to be 13.4% taking quinine sulphate reference, comparable to the earlier reports.14 The quantum yield calculation for the all synthesized SCNQDs is shown in the Figure S1b and Table S1.


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(a)

(b)

Figure 3. (a) PL spectra of SCNQDs, (b) Normalized PL spectra of SCNQDs as a function of changing excitation wavelength from 320nm to 500nm. In order to design an efficient detection system for Hg2+, solubility as well as (monitored with UV-Visible spectra) the PL of SCNQDs in different solvents such as ethylene glycol, DMSO, DMF, ethanol, 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 are found to be sensitive and selective to Hg2+ ions. A gradual quenching of the PL from SCNQDs is observed on addition of 2µL Hg2+ solution in the concentration range 0.001µM to 20µM, Figure 4a. It may be mentioned that, we have added 2µL of 0.001 -20µM Hg2+ to 2mL of water and 10µL of standard SCNQDs solution. Therefore, the concentration of Hg2+ in reaction mixture of Hg2+ and SCNQDs is effectively (2µL X bµM /2000µL=bnM; b is the concentration of Hg2+ solution added) 10-2 times less.(Refer to the experimental section) Thus detection limit of Hg2+ being 0.01nM in the Hg2+ solution.This makes our system highly sensitive. Even with detection limit of 1nM of Hg2+ added, the concentration is much lower than WHO standard limit of 10nM.14 High selectivity of SCNQDs towards Hg2+ ions is observed as compared to the other metal ions, as seen in the bar diagram in Figure 4b and the photographic representation in Figure 4c with 10µM 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 13 ACS Paragon Plus Environment


S6, and discussion therein) i.e. in 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 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 presence of different concentrations of Hg2+) Vs Hg2+ concentration (quencher concentration) shows 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 being same, 10nM-1µM for Hg2+ in double distilled water and tap water. This type of F0/F versus concentration of quencher relates the quenching process to a static mechanism.49 It is also supported by the UV-Visible absorbance of SCNQDs, at 263nm and 358nm, on addition of different concentrations of Hg2+, it gradually disappear (Figure S7c). Thus confirming the formation of ground state adduct between the SCNQDs and Hg2+. Complexation constant for SCNQD and Hg2+ is found to be 9.35 and number of binding site is determine to be 1.51 from the slope (Figure S7d and related discussion in ESI).50,51 Further, PL emission average life time is measured to be 3.154ns and 3.972ns respectively for SCNQDs before and after addition of 10 µM Hg2+ ions when excited with a 375nm laser and emission monitored at 425nm, (Figure S8).14,52 The increase in life time of SCNQDs on addition of Hg2+ is attributed to external heavy atom effect. The PL life time 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 sulphur and nitrogen functionalities due to greater preference of soft Hg2+ towards soft sulphur 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


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SCNQDs. It is observed that PL of SCNQDs is considerably more sensitive to Hg2+ ions at pH7(neutral) than acidic (pH-1) or basic (a)

(b) 1.0

0.8

F /F 0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.6

0.4

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0.0 2+

2+

2+

2+

3+

SCNQD Mn Zn Hg Pb Al

(c)

2+

2+

3+

2+

2+

+

+

Cu Ca Fe Ni Cd Ag Na

(d)

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



(pH-14) as shown in the Figure 5a and b. Maximum quenching of PL is observed at pH 7. A look at the absorbance spectra of SCNQDs at pH 1, 7 and 14, in absence and presence of Hg2+, Figure 5c, it is observed that at pH 1 and 14 there is no change in the absorbance at 263nm and 358nm on addition of Hg2+. While at pH 7, two peaks at 263nm and 358nm disappear on addition of Hg2+, indicative of ground state adduct formation between Hg2+ and SCNQDs. On the other hand

140

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Figure 5. (A) pH dependent PL emission of SCNQDs in absence and presence of 10µM Hg2+ (B) corresponding bar diagram (C) UV-Visible spectra of SCNQD solution at different pH1(acidic),pH-7 (neutral), pH-14 (basic) and corresponding spectra after addition of 10µL of 10µM Hg2+ (D) PL emission spectra of SCNQDs at different ionic strength of NaCl.


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there is no such adduct formation at pH 1 and 14. At highly acidic condition, generation of positive charges on the surface of SCNQDs reduces the association with same charge Hg2+ ions leading to less sensitivity. While in highly basic condition OH– may combine with Hg2+ to form Hg(OH)2 or H-bonding by agglomerates resulting in decreased sensitivity. A comparision of SCNQDs sensitivity towards Hg2+ with selected literature reports, is shown in Table S7, and our system is found to exhibit lower detection limit (0.01nM) for Hg2+. PL from the SCNQDs prepared is found to be independent of the ionic strength of the the solution used in the range of 0-500mM NaCl solutions as seen in Figure 5d. However the PL intensity of the SCNQDs is found to be sensitive to pH as shown in the Figure S3d. Both on increasing and decreasing the pH from1-7 and 7-13, there is observed decrease in PL intensity similar to 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 filter papers under 365nm illumination and bright glow from the SCNQDs loaded ones are is visible while the blank filter paper appears dark. In Figure 6c, when 2µL of 10µM metal ion solutions drop casted onto the SCNQDs loaded filter papers and observed under 365nm 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 filter paper composite 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 (263nm and 358nm) due to SCNQDs is retained. However additional peak at ~480nm appears, which was 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. The PL spectra and digital photograph of filter paper SCNQDs composite, no significant change in PL intensity observed after 1week, as shown in Figure S10b. The wavelength dependent 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 towards 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



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SCNQDs loaded filter paper, the photo stability is tested by keeping it under day light and UV light for 1hour. The PL emission spectra of SCNQDs-filter paper composite along with their respective intensity versus time plot (inset) is shown in Figure S10g and h. The SCNQDs loaded filter papers are found to be stable for at least one week. Though there are some reports14,23 on sensing of Hg2+ ions in aqueous medium, but no report on solid matrix to the best of our knowledge. We have used SCNQDs for selective detection of Hg2+ ions both in solid and aqueous phase. Due to the 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 laboratory, 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.

(a)

(b)

Blank Mn2+ Zn2+

Hg2+ Pb2+

Al3+

Cu2+

Ca2+

Fe3+

Ni2+

Cd2+ Ag+ Na+

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


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We would also like to report here our findings on RTP observed for SCNQD/Polyvinylacohol (PVA) composite. SCNQD/PVA composite and made into a film, when excited with 365nm 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 365nm light the SCNQD/PVA composite and film shows green phosphorescence. This is observed for few seconds with naked eye (Refer to the video in SI). Cool blue PL from the SCNQD/PVA composite film (Figure 7c.) has a broad band emission from ~400nm-700nm, fluorescence emission centered at λemF = 425nm, green line while the green phosphorescence emission band starts at ~450nm, extending up to 700nm with maxima at λemP = 535nm, red line. Phosphorescence decay profile of SCNQD/PVA film fit to multi-exponential function with three life time 101.8ms, 447.6ms, and 960.9ms with average life time of 503.43ms, Figure 7d. However for SCNQDs in aqueous medium no RTP is observed, but a blue fluorescence with life time of about 3.154ns at 425nm on excitation at 375nm is seen. The average life time of both fluorescence and phosphorescence is calculated using equation (2)

= Σαi࣎i2/Σαi࣎i ------------------------- (2) In the PL excitation spectrum of SCNQDs/PVA, two peaks at 300nm and 360nm are observed. It can be seen that the higher contribution to the PL at 425nm is from the 300nm excitation as compared to 360nm 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 300nm to 380 nm, it is observed that there is gradual decrease in the phosphorescence intensity, however with no change in the emission centre λmax ~535nm. The origin of the RTP can be assigned to the surface C=N/C=O groups as in various organic molecules have been known to exhibit phosphorescence.54-57 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 non-radiative decay and quenching of triplet state.42 Intersystem crossing, which promotes the phosphorescence, is enhanced due to the close proximity of singlet and triplet states and spin orbit coupling caused by heavy atoms. As there is no heavy atom present in



our SCNQDs/PVA composite, we believe closely spaced singlet and triplet states lead to effective intersystem crossing. Secondly the non radiative process like vibrational relaxation and collisonal quenching effects that prohibit the observation of RTP, is 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 the Figure S11e. 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 state 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 state respectively. Therefore electronic excitations, S0-S1 and S0-S2, in Figure 8 can be correlated to absorbance bands 358nm and 263nm in Figure 1a, for SCNQDs respectively. It is also supported by the two excitation peaks observed at 300nm and 360nm in Figure 7c (black line). Higher intensity of the 300nm peak in the PL excitation spectrum, Figure 7c and higher phosphorescence on excitation at 300nm, Figure S11c, suggest that S0-S2 i.e. π −π∗ transition is dominant in the contribution to the fluorescence and phosphorescence process for SCNQD/PVA composite film. At present we can suggest that the phosphorescence observed in SCNQD/PVA composite /film is from the T1 (n-π*) to S0 energy state, though S0-S2 (π −π∗) is the dominant excitation. 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 is can be justified if we consider the 425nm peak in the PL spectrum of SCNQD/PVA composite film to be its fluorescence peak and notice that phosphorescence starts approximately from 450nm, i.e. very small energy difference between the first singlet and triplet excited state.


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



off. (d) Photograph of the SCNQDs/PVA composite film in white light, 365nm UV- light on and off. To the best of our knowledge there are only two more reports 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. Ability to disperse SCNQDs into a PVA solution and then make a film out of it can have potential application in phosphorescence OLED devices and glow in a 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, possibility of higher light harvesting devices can be designed at low costs. Moreover, phosphorescence in SCNQDs can also help in exploration of newer emitters for phosphorescence OLEDs can be explored due to the tunability of emission through size modification of SCNQDs and functionalization.


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

CONCLUSIONS In summary, we report an easy one pot synthesis of photoluminescent SCNQDs from low cost precursors, thiourea and EDTA.2Na salt with quantum yield of about 13.4%. The SCNQDs prepared by this method have average size of about 6nm and it is applied for selective and sensitive detection of Hg2+ ion to the limit of about 0.01nM 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 efficiency is similar to the standard conditions. Spectroscopic investigations suggest a primarily static quenching mechanism. PL sensitivity of SCNQDs towards 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 towards 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.

ASSOCIATED CONTENT Supporting Information. The

following

files

are

available

free

of

PL Spectra, quantum yield calculation, HTEM, Zeta potential measurements,

charge. DLS

measurements, Photo stability and pH dependent study, solvent dependent study, PL quenching



experiments in 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 photo stability test of SCNQD-filter paper composite, fluorescence and phosphorescence CIE diagram, Mechanism of phosphorescence in SCNQDs/PVA composite, Fluorescence Hg2+ sensing efficiency comparison for different materials. AUTHOR INFORMATION Corresponding Author * Email:[email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT 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, SAIF-Gauhati 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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For Table of Contents Use Only Synopsis: Sulphur doped carbon nitride quantum dots (SCNQDs) have been prepared in a facile pyrolysis method. SCNQDs exhibit selective sensitivity towards toxic Hg2+ and room temperature phosphorescence.


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Facile Synthesis of Photoluminescent Graphitic Carbon Nitride

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