Synthesis of carbon nanoparticles using one step green approach and their application as mercuric ion sensor

Journal of Luminescence 161 (2015) 117–122

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Synthesis of carbon nanoparticles using one step green approach and their application as mercuric ion sensor V. Roshni, Divya Ottoor n Department of Chemistry, Savitribai Phule Pune University, Ganeshkhind Road, Pune 411007, India

art ic l e i nf o

a b s t r a c t

Article history: Received 26 September 2014 Received in revised form 19 December 2014 Accepted 23 December 2014 Available online 2 January 2015

Carbon nanoparticles (CNPs) have been evolved as a promising candidate for the metal sensing applications due to their synthesis from naturally occurring and easily available non-toxic molecular precursors by green chemistry. A simple and one step procedure is reported here for the synthesis of CNPs from coconut milk by thermal pyrolysis at a temperature of 120–150 1C for 2–5 min without using any carbonizing or passivating agent. On pyrolysis the coconut oil is separated from the carbon rich residue and the residue when dissolved in water showed blue fluorescence under UV light. The CNPs produced are found to show an emission maximum at 440 nm when excited at 360 nm. Synthesis by green approach makes CNPs a promising substitute for the metal sensing applications. Series of metal ions which have a hazardous impact on the ecological system have been taken for the analysis and it is observed that the fluorescence of CNPs gets remarkably quenched by mercuric ions. Fluorescence quenching was studied using standard Stern–Volmer quenching model. Limit of detection was found to be 16.5 nM Hg2 þ concentration. & 2014 Elsevier B.V. All rights reserved.

Keywords: Carbon nanoparticles (CNPs) Fluorescence Coconut milk Metal ions Quenching Limit of detection (LOD)

1. Introduction The scope of optical sensing has been broadened with the invention of highly fluorescent non-toxic carbon nanoparticles (CNPs) or carbon nanodots as the sensing probes [1–3]. Historically, starting from the natural organic dye to the recent semiconductor nanoparticles, the researchers have experimented with various advances in this area. The latest one among them was the use of semiconductor nanoparticles which shows some remarkable properties like high emission quantum yields, size-tuneable emission, chemical and physical stability, narrow spectral bands, possibility of surface modification for a specific sensing application, etc. [4]. But they suffer from the serious limitation of major health problems caused by the toxic effect of the heavy metal elements from which they are produced [5]. In this scenario, the non-toxic CNPs are benign alternative to semiconductor nanoparticles. Apart from the high photo-stability and lack of any cytotoxicity, the size and the excitation dependent photoluminescence are the versatile characteristics of these carbon nanoparticles. Sun and co-workers had synthesised carbon nitride dots from organic amines, N,N-dimethylformamide, CCl4, etc. and these were successfully used for specialised catalytic applications [6–11]. Yu and co-workers had reported a lasing emission from carbon nanodots in organic solvents [12].

n

Corresponding author. Tel.: þ 91 20 25601395; fax: þ 91 20 25601728. E-mail address: [email protected] (D. Ottoor).

http://dx.doi.org/10.1016/j.jlumin.2014.12.048 0022-2313/& 2014 Elsevier B.V. All rights reserved.

Hu and co-workers had prepared cabon nanodots from single chain polymeric nanoparticles and investigated their photoluminescence mechanism in organic solvents [13]. Various efforts were carried out to synthesise carbon nanoparticles from natural precursors. Peng and coworkers had used naturally occurring carbohydrates like glucose [14], Yang and co-workers used sucrose, and citric acid [15], and Sun and co-workers used pomelo peel [16], willow bark [17], etc. for the synthesis of carbon dots. It is reported that the optical, physical and chemical properties of produced CNPs are influenced by the molecular precursors used, methodology and the pre-treatment performed. Apart from optical sensing CNPs have found widespread applications in the areas of bioimaging [18,19], photocatalysis [20], optoelectronics [21], etc. Synthesis of CNPs from naturally occurring and economically viable molecular precursors and their promising application towards various fields are important areas worth looking at. Application of fluorescent nanomaterials as a luminescent probe is the current trend. There are various reports on the use of the above mentioned CNPs for the metal sensing application [22]. Still the application of CNPs for the selective detection of metal ions is in the developing stage. The environmental and health hazards caused by the presence of toxic chemical waste containing heavy metals are of great concern for the modern industrial world. Presence of even traces of mercuric ions is a threat to the ecosystem due to their toxic nature [23]. Various sensitive and selective methods like atomic absorption spectroscopy, liquid chromatography, adsorptive stripping voltammetry, electrochemical, spectrophotometry and spectrofluorimetry

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are used for the qualitative and quantitative estimation of mercuric ions [24,25]. Among them, fluorescence spectroscopy has some added advantages such as low cost, facile sample preparation, high selectivity and easy detection [26]. So far many fluorescent probes especially based on carbon dots are employed for the detection of mercuric ions using various types of molecular precursors [27–31]. It has been reported that unmodified CDs can be used as a selective and sensitive fluorescence probe for rapid detection of Hg2 þ . A good linear correlation was observed over the concentration range of 0–3 mM, with a detection limit of 4.2 nM based on a 3δ/slope [32]. However, many of the methodologies used for the synthesis of CNPs have the drawbacks of unfavourable reaction conditions like high temperature, prolonged reaction duration, usage of oxidising agents, the lack of aqueous solubility, etc. Coconut milk which contains a high percentage of saturated fat (lauric acid) is a rich source of coconut oil. Coconut oil is extracted from coconut milk by thermal pyrolysis which is a traditional method to synthesise the virgin coconut oil. In the food industry normally the black residue obtained after the separation of coconut oil is discarded. Herein, we report a novel approach for the synthesis of CNPs from the waste by-product obtained by the thermal pyrolysis of coconut milk. This pyrolysis procedure does not involve any acid treatment or any surface passivating reagents. The obtained residue is found to be water soluble and the solution contains carbon nanoparticles of fluorescent characteristics. These CNPs are highly fluorescent and photostable, dissolves readily in water and other organic polar solvents and show excitation tuneable emission spectra. The efficiency of CNPs for metal ions sensing was tested using fluorescence quenching approach and the analytical characteristics are discussed.

wavelength was kept at 360 nm and emission was recorded from 365 nm to 600 nm. The slit width for excitation and emission was kept at 2.5 nm for all the measurements. An Infrared spectrometer, FTIR-8400 (Shimadzu) was used to characterise the functional groups on the CNPs. A transmission electron microscope (TEM, Model TECNAI G2-20 U-Twin) with an operating voltage of 200 kV was used for the physical characterisation of the synthesised CNPs. The fluorescence life time measurements were carried out using a FLTCSPC fluorescence spectrometer (Horiba Jobin Yvon Inc., France). 2.3. Synthesis Fig. 1 shows the facile and one step synthesis of carbon nanoparticles from coconut milk in a cost effective and greener method. This involves the thermal pyrolysis of coconut milk under a mild temperature range of 120–150 1C for 2–5 min. On heating the sample, coconut oil was separated leaving behind a black residue. This residue obtained after the separation of coconut oil was air dried. The dried sample was easily dispersed in water and an appreciable amount of the substance get dissolved in water and shows a yellowish brown colour as shown in Fig. 1C. The undissolved particles are removed by filtration and the centrifuged solution showed an appreciable blue fluorescence when exposed to UV light. These lyophilised CNPs were diluted with water till the appropriate concentration was reached and used for further analysis and sensing applications.

3. Results and discussion 3.1. Morphology and fluorescence characteristics

2. Experimental 2.1. Materials Coconut milk was extracted from grated coconut. This was used for the study without any preservatives. A.R. Grade Cupric nitrate (Cu(NO3)2), cobalt nitrate (Co(NO3)2), cadmium nitrate Cd(NO3)2, mercuric chloride (HgCl2), nickel nitrate (Ni(NO3)2, lead nitrate Pb (NO3)2, manganese sulphate (MnSO4) and iron sulphate (FeSO4) were purchased from SDFCL and used as received without any further purification. Triply distilled water was used as a solvent throughout the experiment. 2.2. Instrumentation The absorbance and fluorescence measurements were recorded using a UV–vis spectrophotometer (Shimdzu spectrophotometer) and spectrofluorimeter (JascoFP-8300), respectively. The absorbance of the sample was monitored between 200 nm and 600 nm. Excitation

A few drops of CNPs aqueous solution is placed on a copper grid and dried and viewed under TEM. Fig. 2 shows the TEM and HRTEM images of the sample which contains spherical shaped carbon nanoparticles with an average size distribution ranging from 20 nm to 50 nm. This large size range could be the result of inhomogeneous pyrolysis process adopted in the synthetic step. Fourier Transform Infrared spectroscopy was used to determine the functional groups present in carbon nanoparticles. FTIR spectrum of carbon nanoparticles produced from coconut milk is shown in Supporting information (Fig. S1). A characteristic absorption peak is observed at 3354 cm  1 which is due to –OH in the alcohol or phenol group. The stretching vibration bands at 2929 cm  1, 1647 cm  1 and 1284 cm  1 are due to alkyl C–H bond, C–O bond in the amide groups and C–O bond in carboxylic acid, respectively. The bending vibration band of aromatic C–H bond was found at 887 cm  1, suggesting the presence of alcohol or phenol, alkyl, carboxylic acid, amide and aromatic groups. Fig. 3 shows the UV–vis absorbance spectrum of CNPs when dispersed in water. The absorption spectrum shows an edge at

Fig. 1. (A, B) Digital images of the coconut milk and crude CNPs produced by the thermal pyrolysis of coconut milk, respectively. (C) Photographs of CNPs excited under daylight at 360 nm. (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)

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Fig. 2. The TEM image of the CNPs produced by the thermal pyrolysis of coconut milk at 120–150 1C for 2–5 min.

3.2. As a metal sensing probe

Fig. 3. UV/vis absorption spectra of the produced CNPs.

276 nm which can be attributed to the π–π* transitions of the functional groups. The emission spectra (Fig. 4A and B) of CNPs are broad, ranging from 423 nm to 484 nm depending on the excitation wavelength which varied from 320 nm to 420 nm. As the excitation wavelength is increased the emission peak position get shifted to longer wavelength. The maximum emission intensity is obtained at 440 nm when exited at 360 nm. Then intensity of fluorescence spectrum decreases and shows a red-shift when excitation wavelength changes from 360 nm onwards. The red-shift in fluorescence emission spectra may be due to different sized carbon nanoparticles in the sample. These excitation tunable emission spectra are considered to be the versatile characteristic of CNPs and this property may be due to quantum confinement, size distribution or the presence of emissive traps on the surface [33]. The effect of pH on the fluorescent property of CNPs was also studied. The fluorescence characteristic of the CNPs strongly depends upon the pH value as shown in Fig. 5. Appreciable quenching is evident in the acidic and basic pHs with maximum emission intensity at the neutral medium. This pH effect enables the CNPs to be used as an efficient sensing probe in the neutral pH especially in biological systems. Fig. 5 shows the fluorescence response of the CNPs when irradiated continuously for 1 h with an excitation wavelength of 360 nm. The CNPs were able to retain about almost 100% of fluorescence after 1 h proving its good photo-stability.

Although there are a large number of reports of Hg2 þ sensors, simple and effective detection of the ion still remains a challenge. Considering its remarkable fluorescent property the thought of utilising CNPs as a metal sensing probe came into the picture. This study was encouraged by the reports which suggested that irrespective of the sources and method of preparation, the CNPs could be used as potential sensing probes [34–39]. In view of this, a detailed procedure was carried out to find the interaction of CNPs with various metals. The metal ions were selected by keeping in mind their presence in the industrial areas as well as their hazardous impact on the environment. The metal ions selected were Pb (II), Ni (II), Co (II), Cd (II), Hg (II), Zn (II), Mn (II) and Fe (II). The metal ion concentration in the solution was maintained at 100 mM. From Fig. 6, it is evident that the fluorescence of CNPs has been quenched in the presence of metal ions. Out of these, Hg (II) shows appreciable quenching of fluorescence. This observation is very interesting because the presence of Hg (II) is considered to be the major culprit for industrial pollution. The fluorescence quenching of CNPs with Hg (II) ions could be due to the facilitation of non radiative electron/hole recombination through an effective charge transfer process [32]. Recombination of excited electrons in the conduction band with the holes in the valence band could be a possible reason for this metal mediated fluorescence quenching. This charge transfer process may be a strong binding interaction between Hg (II) and the functional groups mainly hydroxyl and carboxyl or carbonyl groups present on CNPs surface. The fact that without doing any surface modification these CNPs are selective towards Hg (II) makes this study interesting.

3.3. Analytical characteristics of fluorescence quenching For analysing the fluorescence quenching characteristics of CNPs by metal ions, three series of solutions were prepared. The metal ions concentrations in these three sets vary from 1 mM to 10 mM, 10 mM to 100 mM and 10 nM to 100 nM. Fluorescence intensity of CNPs in the absence and in the presence of metal ions was recorded. Fluorescence spectrum of CNPs with various metal ions concentrations is shown in Fig. 7. From this figure, it is evident that the quenching is very less in nanomolar concentrations and becomes appreciable in micromolar concentrations. Standard Stern–Volmer relationship explains the phenomenon of bimolecular fluorescence quenching. A linear relationship was observed showing an increased quenching with concentration. From the Stern–Volmer plots the analytical characteristic of quenching is

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Fig. 4. (A) Excitation and emission spectra of CNPs. (B) Normalised emission spectra of CNPs when excited at wavelengths ranging from 320 nm to 420 nm in 10 nm increment. Inset: emission spectra of CNPs without normalisation.

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Fig. 5. (A) Fluorescence spectra of CNPs in different pH conditions. (B) Fluorescence intensity of CNPs with respect to time on continuous irradiation for 1 h.

determined. The Stern–Volmer equation is represented as follows: F0 ¼ 1 þ K SV ½C  F

ð1Þ

where F0 and F are fluorescence intensities of the CNPs before and after addition of metal ions, respectively. [C] is the concentration of metal ions and KSV is the Stern–Volmer quenching constant. The Stern–Volmer quenching plot based on the fluorescence data is generated to evaluate KSV, which is the Stern–Volmer quenching constant (Fig. 8). A linear relation is obtained when concentration of metal ion is plotted against F0/F value. Correlation coefficient (R2) and slope of the line (KSV value) were obtained on fitting the data and were found to be 0.98 and 3.1  105 M  1 respectively. The equation used to find out limit of detection (LOD) is 3σ/s where σ denote the standard deviation of CNPs corrected blank signals and s denote the slope of the linear curve derived from Stern–Volmer equation.

In accordance to the linear relationship obtained by adding the different concentration of Hg (II) ions to the CNPs, the limit of detection (LOD) of the synthesised CNPs towards Hg (II) was calculated. LOD was found to be 16.5 nM. This value is slightly greater than the acceptable value mandated by the U.S. EPA for the concentration of mercury in the drinking water (10 nM, 2 ppb); however, the sensor can be useful in detecting inorganic mercury in samples of biological products, rivers, lakes and also in samples where regulations for mercury are less stringent. The bimolecular quenching constant, kq has to be evaluated using the average lifetime value of carbon nanoparticles and KSV value. The relation between bimolecular quenching constant and Stern–Volmer constant is given in the following equation: K SV ¼ kq τ 0

ð2Þ

where KSV is the Stern–Volmer quenching constant and τ0 is the fluorescence lifetime in the absence of metal ion. τ0 value was

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obtained using fluorescence lifetime measurements. The decay profile of CNPs shows tri-exponential decay with an average lifetime of 1.81 ns. (Fig. S2, Supporting information).

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The kq value is calculated using the above expression and was found out to be 1.7  1014 M  1 s  1 indicating the presence of both diffusion as well as static quenching in the system. The kq value obtained is considerably larger than those possible for a diffusioncontrolled quenching in solution (about 1010 M  1 s  1) hence it is

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Fig. 8. Stern–Volmer plot of fluorescence quenching of CNPs with mercuric ions (10–100 nm).

Fig. 6. The effect of different metal ions (concentration of 100 mM) on the emission intensity of the CNPs (excitation wavelength at 360 nm).

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Fig. 7. Fluorescence quenching of CNPs in the presence of mercuric ions of concentration (A) 1–10 mM, (B) 10–100 mM and (C) 10–100 nM.

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Table 1 Comparison of different fluorescent probes for Hg (II) detection. Sr. no

Fluorescence probe

Detection limit

Linear range

Reference

1 2 3 4 5 6 7 8 9 10

Carbon dots Carbon dots CPs Histidine based C dots Carbon dots Carbon dots Carbon dots Carbon dots N-CQDs CNPs

2.69 μM 8.2 nM 0.23 nM 1.03  10  7 M (0.02 ppm) Submicron molar 0.5 nM 4.2 nM 20 nM 0.23 μM 16.5 nM

0.5–1 μM 50 nM to 100 μM 0.5  10 μM

[34] [35] [36] [37] [38] [39] [32] [40] [41] This work

evident that some types of binding interaction is existing between fluorophore and quencher. The response of CNPs as a sensor for mercuric ions is compared with the other recently reported mercuric sensor based on carbon dots/nanoparticles. The details regarding the fluorescent probe employed, detection limit, linear range and reference are provided in Table 1. The feasibility of CNPs for detecting Hg (II) in real water samples was explored by taking tap water sample spiked with standard solutions containing different concentrations of Hg (II). Fluorescence intensity decreases with increased concentration of Hg (II) from 10 to 100 nM. Calibration plot is developed with F0/F along y axis and metal ion concentration along x axis. The solution which is spiked with 50 nM Hg (II) is predicted using the calibration graph and a concentration of 57 nM Hg (II) is achieved with a percentage error of 14.6%. The experiment was performed thrice and a standard deviation of 7.4 was obtained. These results show that the fluorescent probe obtained is capable of detecting mercuric ion and can be effectively used for further research. 4. Conclusion We have adopted a general strategy for preparing the CNPs from a bio-precursor which is economically cheap and naturally abundant. The conversion method involved was simple thermal pyrolysis without any passivating or stabilizing agents. The CNPs produced were highly fluorescent and show a blue fluorescence when excited at 360 nm with λem at 440 nm. Due to their excellent photoluminescent property the synthesised CNPs were utilised for the metal sensing application where the intensity of fluorescence of CNPs was effectively quenched by Hg (II) ions. Acknowledgements This work is financially supported by the Science and Engineering Research Board (SERB), Department of Science and Technology (DST), New Delhi, India (SB/FT/CS-109/2012) and the authors are thankful to IISER Pune and Physics Dept., SPPU for TCSPC and TEM facilities, respectively. Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jlumin.2014.12.048. References [1] S.N. Baker, G.A. Baker, Angew. Chem. Int. Ed. 49 (2010) 6726.

0.1–2.69 nM 0.0005–0.01 μM 0–3 μM 0–3 μM 0–25 μM 30–50 nM

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