Green Fluorescent Onion-Like Carbon Nanoparticles from Flaxseed

Research Article pubs.acs.org/journal/ascecg

Green Fluorescent Onion-Like Carbon Nanoparticles from Flaxseed Oil for Visible Light Induced Photocatalytic Applications and LabelFree Detection of Al(III) Ions Kumud Malika Tripathi,† Tuan Sang Tran,† Yoon Jin Kim,‡ and TaeYoung Kim*,† †

Department of Bionanotechnology, Gachon University, 1342 Seongnam-daero, Sujeong-gu, Seongnam-si, Gyeonggi-do 461-701, South Korea ‡ Energy Nano Materials Research Center, Korea Electronics Technology Institute, 25 Saenari-ro, Bundang-gu, Seongnam-si, Gyeonggi-do 463-816, South Korea S Supporting Information *

ABSTRACT: Onion-like carbon nanoparticles (CNOs) were synthesized via traditional pyrolysis of flaxseed oil. Oxidative treatment of as-synthesized carbon soot introduced numerous carboxyl (−COOH) functionalities, rendering them hydrophilic and stable in aqueous phase. The water-soluble onion-like carbon nanoparticles (wsCNOs) were 4−8 nm in size and exhibited stable green photoluminescence (PL) emission. CNOs were explored as efficient photocatalysts for the degradation of methylene blue (MB) as model organic pollutant dye under visible light irradiation. The wsCNOs exhibited photocatalytic efficiency ∼9 times higher than CNOs for MB degradation. Enhanced photocatalytic efficiency of wsCNOs was attributed to their surface functionalities and nanostructure. The unique morphology (concentric nanographene shells) with considerable surface defects, increased the physisorption of MB on the wsCNOs surface and significantly enhanced the photocatalytic efficiency of wsCNOs. Furthermore, the wsCNOs enabled specific detection of Al(III), even with interference from high concentrations of other metal ions, with a detection limit of 0.77 μM, which compares favorably to other reported fluorescent probe. Altogether, the wsCNOs showed a significantly enhanced photocatalytic activity and were used as highly selective fluorescent probes for Al(III) ion detection, suggesting a potential use in environmental wastewater treatment. KEYWORDS: Carbon nanoonions, Green emissions, Enhanced photocatalysis, Fluorescence sensor, Sensing of Al(III)

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INTRODUCTION Nanostructures of carbon in all of its forms have been the focus of interdisciplinary research and have proved to be extremely useful. Among them, carbon nanoonions (CNOs) are one of the latest and least popular carbon nanostructures, although they exhibit unique physicochemical properties due to the pronounced edge effects.1−4 CNOs consist of concentric nanographene shells around a solid or hollow inner core and morphologically are in between fullerenes and multiwall nanotubes.3 This peculiar onion-like structure results in CNOs possessing small graphitic sp2 carbon domains with highly localized π electrons and peripheral defects in the form of dangling bonds.5 Consequently, CNOs exhibited unique optical, mechanical, and electrochemical properties that render them a potential candidate for myriad applications, including bioimaging,4,6,7 electrochemical energy storage devices,3,8 hydrogen storage,9 immunotherapy,10 drug delivery,11 sensing,12,13 plant productivity,14,15 and environment remediation.16,17 So far, a variety of methods have been developed for the synthesis of CNOs including arc discharge, annealing of nanodiamonds, chemical vapor deposition, flame pyrolysis, ion implantation, and microwave synthesis.1,4,18 However, the facile © 2017 American Chemical Society

fabrication of homogeneous water-soluble onion-like carbon nanoparticles (wsCNOs) with unique surface properties and aqueous stability is still in its infancy, while appropriate carbon precursors and synthetic routes are considerable factors in establishing targeted synthesis and characteristics of CNOs. Among various top-down and bottom-up synthetic approaches, traditional pyrolytic techniques have attracted considerable attention in recent years due to their simplicity, versatility, and high yield.5,19 The synthesis of fluorescent, homogeneous CNOs with outstanding aqueous stability from natural materials is important in biomedical applications because it is environment-friendly, less toxic, and compatible with mass production.5,7,15 From this perspective, we chose flaxseed oil as the green carbon precursor for the fabrication of homogeneous, hydrophilic, and fluorescent CNOs. Flaxseed oil, with an average global production of approximately 2.52 million metric tons (MMT), is mainly used in nutraceuticals and functional foods due to its structural characteristics.20,21 Received: December 27, 2016 Revised: March 22, 2017 Published: March 30, 2017 3982

DOI: 10.1021/acssuschemeng.6b03182 ACS Sustainable Chem. Eng. 2017, 5, 3982−3992

Research Article

ACS Sustainable Chemistry & Engineering

surface functionalities of carbon nanostructures, and oxidation−reduction following a photoinduced electron transfer (PET).39 So far, few reports on the fabrication of fluorescent probes for selective detection of Al(III) have been published.32,41−43 Those probes generally require complex synthetic protocols, fluorophores, and long incubation times, and suffer from photobleaching. Hence, it is still challenging to develop specific, simple fluorescence probes for the detection of Al(III). To our knowledge, no report exists on the detection of Al(III) using green fluorescent wsCNOs. In this work, we aimed to synthesize wsCNOs through a facile technique via traditional pyrolysis of flaxseed oil using a cotton wick, as a potentially valuable low-cost and green precursor. The synthesized wsCNOs exhibited good stability in aqueous phase and strong green fluorescence. CNOs were explored for the photocatalytic degradation of methylene blue (MB) as model pollutant dye under visible light illumination. The catalytic activities of CNOs were improved ∼9 times by simple acid oxidation. Moreover, the wsCNOs showed highly sensitive and selective detection of Al(III) in aqueous media based on fluorescence quenching effects and thus can serve as a fluorescent probe for label-free detection of Al(III).

To address the issue of scarcity of safe water with the rise in global population and associated environmental consequence is a topmost global concern. Organic pollutants such as textile dyes are a major factor that affects water bodies worldwide because of their release into water resources and low biodegradability.22 Because of their complex chemical nature, organic dyes can block both sunlight and oxygen dissolution in water bodies and are hard to treat with a biological technique.23,24 Therefore, a facile, economic and eco-friendly technique is needed to degrade organic dyes into nonhazardous compounds. Photocatalytic treatment of wastewater is advantageous over various physical and chemical techniques for water decontamination since it offers almost negligible or no harmful byproducts.25 In addition, it is an energy efficient, clean, and sustainable process. In recent years, carbon nanomaterials have been investigated for the photocatalytic treatment of wastewater and other pollutants since they offer band gaps tunable with visible spectrum.23,26−28 CNOs are particularly attractive for photocatalysis applications because of their high chemical stability, environmentally friendly nature, and large specific surface area with a high degree of curvature.29 To the best of our knowledge, CNOs have not been explored for photocatalytic applications. This led us to investigate the characteristic electron transfer properties of wsCNOs as a photocatalyst for dye degradation. Contamination of water with heavy metal ions has been another serious global concern for decades. Aluminum, the third most abundant element in the Earth’s crust after oxygen and silicon, widely exists in water as a result of acid rain.30 In addition, the increasing use of aluminum compounds in industry, water treatment, cosmetics, pharmaceuticals, antiperspirants, and food additives further increases its content in drinking water and foodstuffs.31 Aluminum in water has been found to be lethal to plants30 and a potential risk factor in elderly cognitive impairment.32 Furthermore, in humans, increasing exposure to aluminum is recognized as a significant cause of Alzheimer’s disease (AD), Parkinson’s disease (PD), dialysis encephalopathy, down syndrome,33 and even breast cancer.34 Therefore, the detection of Al(III) is crucial to control its concentration in the biosphere and limit human exposure levels. Several standard techniques have been established for the detection of Al(III), such as atomic absorption spectroscopy (AAS), hydride generation atomic fluorescence spectrometry (HG-AFS), inductively coupled plasma mass spectrometry (ICPMS), and square wave anodic stripping voltammetry (SWASV-CV).30,32 Although SWASV-CV and ICPMS meet the sensitivity and selectivity demands, their on-site and practical application is hampered by lengthy, complicated sample preparation steps, the requirement of trained manpower, and costly equipment. In recent years, fluorescence-based detection methods have attracted extensive attention due to their ease of operation, high sensitivity, rapid implementation, and low cost.35 The “off−on” fluorescence properties of carbon nanostructures have been exploited for the development of PL sensors for a diverse range of molecules.5,19,36,37 Carbon nanostructures are both electron donors and acceptors,38 and their fluorescence can be quenched (turned off) upon absorption of electron acceptor molecules, such as metal ions or conjugated systems,39,40 while PL emissions are further enhanced (turned on) upon absorption of electron donor molecules.39 Various factors direct the PL turn on/off and selectivity of ions or molecules, such as π−π interactions, Hbonding, adsorption affinity, metal−catechol interactions,

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EXPERIMENTAL SECTION

Materials and Reagents. Cold-pressed and unrefined flaxseed oil was obtained from a local market. All chemicals and solvents were of analytical reagent grade and used as obtained without any further purification. All metal ions were chloride salts, except K2Cr2O7 and Al(NO3)3·9H2O, and purchased from Alfa Aesar (South Korea). Methylene blue(MB) was purchased from Sigma-Aldrich (South Korea). Deionized (DI) water was used throughout for the preparation of solutions, unless stated otherwise. Instrumentation. Transmission electron microscopy (TEM) and high resolution TEM (HRTEM) images were taken with Tecnai G2 at a 200 kV operational voltage. Samples for TEM were prepared by casting droplets of an aqueous solution of wsCNOs onto a carboncoated copper grid, followed by drying under vacuum at 25 °C for 12 h. Imaging of wsCNOs was done with a DM 2500 inverted microscope (Leica, Heerbrugg, Switzerland). Samples for fluorescence microscope imaging were prepared by drop-casting an aqueous solution of wsCNOs on a glass slide, followed by drying under vacuum at 25 °C for 12 h. Fourier transform infrared (FTIR) spectra were determined in solid state using KBr pellets on a Bruker Vector 22 IR spectrometer in the range of 400−4000 cm−1. Raman analysis was done with a WITec Raman spectrometer at 532 nm laser excitation. Thermogravimetric analysis (TGA) was performed under nitrogen atmosphere with a Mettler thermal analyzer at a heating rate of 10 °C min−1, starting from room temperature up to 900 °C. Fluorescence spectra of aqueous solutions were recorded at room temperature with a Varian fluorescence spectrometer. The UV−vis absorption spectra of wsCNOs were analyzed with a Varian 50 Bio UV−vis spectrophotometer. The powder X-ray diffraction (XRD) patterns were acquired for 2θ values from 10 to 80° on a Rigaku model D X-ray diffractometer using Cu−Kα radiation. X-ray photoelectron spectroscopy (XPS) measurements for surface analysis were done with a ULVAC-PHI Xtool XPS spectrometer with an Al Kα X-ray source. Zeta potentials were measured by a zeta potential analyzer version 5.72 (Brookhaven instruments). Synthesis of Water-Soluble Carbon Nanoonions (wsCNOs). The wsCNOs were prepared following a recently reported method,19 using flaxseed oil as carbon source. An earthen pot was filled with flaxseed oil, which was then pyrolyzed in a traditional way using a cotton wick in stable air conditions. The as-synthesized soot was collected on a clean inverted borosilicate glass beaker to avoid any metal contamination. Control oxidation of as-obtained carbon soot under acidic conditions resulted in its water-soluble version, as wsCNOs.44,45 In brief, 1 g of soot was oxidized in 100 mL of nitric acid 3983


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Scheme 1. Representation of the Synthesis of Green Fluorescent Water-Soluble Carbon Nanoonions (wsCNOs) from Flaxseed Oil

Figure 1. (a,b) TEM images of wsCNOs; the inset shows the size distribution of wsCNOs; (c) HRTEM image of single wsCNO; (d) HRTEM image of wsCNO showing the lattice spacing with marked surface defects. (65%) by refluxing for 10 h. The resulted mixture was centrifuged (8000 rpm, 10 min), and water was decanted. The excess nitric acid was removed through repetitive solution evaporation in a water bath until pH reached 7. The resultant wsCNOs were dissolved in 200 mL of DI water, subsequently purified by filtration on a 0.2 μm microporous membrane, and finally vacuum-dried at room temperature. Photocatalytic Activity Measurement. The photocatalytic efficiency of CNOs and wsCNOs were evaluated by the decolorization of MB in aqueous solution under direct visible light illumination. MB solution (5 × 10−5 M/L) was prepared in DI water for photocatalysis measurements. Ten milligrams of CNOs and wsCNOs were separately added to 50 mL of MB solutions, and both solutions were stirred for 30 min in the dark to achieve an absorption equilibrium. Both MB solutions containing CNOs and wsCNOs were then sealed in a glass vial and exposed for visible light illumination from a 60 W tungsten

lamp placed 25 cm away. Aliquots were taken at a set 10 min time interval in a quartz cell for UV−vis absorbance measurements. The heat effect was minimized by employing air conditioning of the room. Detection of Al(III) Ions. Detection of Al(III) was performed at room temperature in DI water. A 0.001 M stock solution of Al(NO3)2 was prepared. Fluorescence intensity of 2 mL of wsCNOs solution (5 × 10−3 mg/mL) was measured at 390 nm excitation wavelength, denoted as Io fluorescence intensity of the starting point. Following that, 10 μL of Al(NO3)2 solution was added to the wsCNOs solution, and fluorescence intensity was measured after 1 min. The excitation and emission slits were kept at 15 throughout the experiment. The sensitivity and selectivity for Al(III) and Fe(III) were measured in triplicate under identical experimental conditions. The effect of metal ions (Co2+, Ag+, Cr2+, Cr3+, Cr6+, Mg2+, Ca2+, Na+, K+, Ba2+, Fe2+, Cu+, Cu2+, Ni2+, Hg+, Hg2+, Cd2+, Mn2+, and Zn2+) on fluorescence sensing of Al(III) were also analyzed under identical experimental conditions. 3984


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Figure 2. Comparative analysis for the structural characteristics of as-synthesized CNOs (black line) and wsCNOs (blue line). (a) FTIR spectra of as-synthesized CNOs and wsCNOs; (b) Raman spectra of as-synthesized CNOs and wsCNOs; (c) thermograph of as-synthesized CNOs and wsCNOs as obtained under an inert atmosphere; (d) X-ray diffraction patterns of as-synthesized CNOs and wsCNOs.

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is due to adsorbed water. A strong peak around 1607 cm−1 arose due to CC stretching vibrations5 and a broad band at 550 cm−1 due to C−H bending.45 In the IR spectrum of wsCNOs (Figure 2a blue line), the sharp peak at 1720 cm−1 is attributed to stretching vibrations of the carboxyl groups on the carbon surface. A main absorption band characteristic of − O− H stretching appears at 3430 cm−1 and a −C−H stretching band at 3017 cm−1.7 A sharp band at 1595 cm−1 is attributed to vibrational stretching of CC, and a peak at 1236 cm−1 to −C−O stretching. A high density of −COOH on the surface of wsCNOs is likely due to the breaking of CC and C−C carbon clusters by −OH during oxidation, which were simultaneously oxidized into −COOH groups. Raman spectroscopy was used for characterization of the intrinsic properties and quality of wsCNOs. The Raman spectra in Figure 2b show two prominent peaks, corresponding to characteristic D and G bands. The G band of wsCNOs (Figure 2b, blue line), which indicates the in-plane vibration mode of sp2 carbons, is centered at 1591 cm−1. Existence of a welldefined sharp G band in both CNOs and wsCNOs corroborates the high quality of material.50 The D band, which corresponds to disruptions in sp2 bonding, such as dangling bonds, sp3 bonding, vacancies, and carbon rings other than hexagon, is located at 1335 cm−1.51,52 The existence of a broad and intense D band results from surface oxidation of CNO’s heptagon and pentagon rings, known to produce bent or curved shapes from flat two-dimensional graphene (2Dgraphene) sheets as found in carbon nanoparticles.37,53 From Figure 2b, it is evident that the peak position of the D band for

RESULTS AND DISCUSSION The wick pyrolysis of flaxseed oil initially formed channel-type carbon clusters possessing graphitic domains, which further assembled to form CNOs. The oxidation of as-obtained CNOs with nitric acid rendered them water-soluble as wsCNOs with no precipitation for several months, thus making them suitable for various biological and environmental applications. In addition, acid treatment oxidized the defective sites of larger particles into smaller ones with spherical shape and high crystallinity.46 The formation of wsCNOs via carbonization and fragmentation of carbon clusters during traditional pyrolysis is depicted in Scheme 1. The present synthetic method has the advantages of gram-scale fabrication at considerably lower costs. Structural and Morphological Characterization. TEM images, shown in Figure 1a and b, indicate the homogeneous and monodisperse nature of wsCNOs. The wsCNOs exhibited relatively narrow size distribution in a range of 2−10 nm in diameter, as evaluated statistically through the measurement of about 100 particles. Most of the wsCNOs are 4−8 nm in size. The structure of wsCNOs was further examined with HRTEM. The HRTEM image in Figure 1c reveals the well-defined spherical shape with concentric nanographene shells.47 Figure 1d shows the crystalline structure of wsCNOs with obvious surface defects and an in-plane lattice spacing of 0.26 nm.15,48,49 The surface functionalization with hydrophilic moieties was confirmed with FTIR. Figure 2a shows the FTIR spectra of CNOs (black line) and wsCNOs (blue line) with distinguishable features of a carboxylic group in wsCNOs. The broad peak at 3430 cm−1 in the as-synthesized CNOs (Figure 2a black line) 3985


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Figure 3. XPS full scan spectra of (a) CNOs and (d) wsCNOs. Corresponding C 1s short scan of (b) CNOs and (e) wsCNOs. O 1s scan of (c) CNOs and (f) wsCNOs.

Figure 4. (a) Ultraviolet−visible (UV−vis) absorption spectrum and digital images of an aqueous solution of wsCNOs under (1) daylight and (2) 385 nm excitation wavelength. (b) Photoluminescence (PL) emission spectra at different wavelengths; (c) PL excitation spectrum at a 523 nm emission wavelength; (d) fluorescence image of wsCNOs at 488 nm band-pass filter; scale bar = 20 μm.

wsCNOs downshifted from 1341 to 1335 cm−1, while that of the G band shifted upward from 1586 to 1591 cm−1. The

shifting of peak positions is induced by the strain of curved graphitic layers in spherical wsCNOs.18 The ratio of the D and 3986


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Figure 5. Extent of (a) MB photodegradation at different visible light irradiation times with CNOs and wsCNOs; inset change in color of MB (1) before and (2) after photodegradation. (b) Plot of ln(C/Co) for MB photodegradation with CNOs and wsCNOs under visible light irradiation.

zeta potential value of −77.91 mV at pH 7 indicates excellent stability without any sign of coagulation or precipitation for months. Optical Properties. The optical properties of wsCNOs were examined by UV−vis and PL spectroscopy. Aqueous solution of wsCNOs exhibited bright green fluorescence under illumination with 365 nm UV light (Figure 4a inset). The wsCNOs exhibited broad optical absorption at 246 nm with a long tail toward the visible light region (Figure 4a), assigned to the overlapping of aromatic polycyclic hydrocarbon backbone and oxygenous functional groups.56,57 The fluorescence emitted from the aqueous solution of wsCNOs was recorded at different excitation wavelengths, and the corresponding PL spectra are shown in Figure 4b. Fluorescent emission peaks were red-shifted with an increase in excitation wavelengths. The maximum emission peak was centered at 523 nm on excitation with the 380 nm wavelength. The corresponding full width at half-maximum (FWHM) was ∼56 nm, which suggests a relatively narrow size distribution of particles consistent with the TEM results.56 Green emissions correspond to the previous reports and are assigned to the large number of surfacial carbons with −CO functional groups.57 The excitation spectrum at a 523 nm emission wavelength shows a broad band at 335 nm (Figure 4c), revealing the presence of one emission species. The quantum yield, measured using quinine sulfate as the standard was ∼5%. The fluorescence image of an aqueous solution of wsCNOs obtained with a 488 nm band pass filter is shown in Figure 4d. The PL mechanism of CNOs has been extensively studied but remains controversial and debatable due to the existence of several synthetic routes and diverse size distributions. As CNOs have no defined band gap, surface functional groups on CNOs have a significant impact on tuning of the surface energy traps and the formation of localized electronic states. The most accepted mechanism for tunable emissions is radiative recombination of electron−hole pairs/ quantum confinement effects.6,58−60 A recent study by Sun and co-workers argued that edge functional groups affected the electron trapping states to compete with the radiative fluorescence emission states.57 Some studies have argued that variations in surface energy states, chemical defects, and surface functionalities significantly contribute to the emissive properties of CNOs.5,56,61,62 One often stated conjecture is that CNOs act as electric dipoles and that recombination of photogenerated charges on defect centers leads to their emission.63 A strong

G band intensities (ID/IG) was used to quantify oxidation since they are directly related to the extent of defects in the form of sp3 carbons. The higher ID/IG for the oxidized CNOs (1.66) than for as-synthesized CNOs (1.40) reflects their higher degree of surface functionalization in terms of carboxylic acid groups.5,54 Subsequently, the functionalization of CNOs was determined by TGA. The comparative thermal stability of assynthesized CNOs and wsCNOs, based on their respective decomposition, is shown in Figure 2c. The as-synthesized CNOs exhibited the weight loss of ∼12% at 900 °C due to the loss of incorporated oxygenated species, while the wsCNOs loses 27.51% of its total weight, ascribed to the thermal decomposition of surface functional groups.19 A higher density of oxygenated functional groups on the surface of wsCNOs resulted in a larger weight loss as compared to that of the CNOs. The structure of CNOs was further examined by XRD. In Figure 2d, the CNOs showed a broad diffraction peak centered at 2θ = 23°, corresponding to (002) reflections of graphitic carbon, and a peak at 44°, indexed to the (100) reflections of the hexagonal graphite crystal structure. The XRD patterns of as-synthesized CNOs and wsCNOs were similar. X-ray photoelectron spectroscopy (XPS) was performed on CNOs and wsCNOs to examine their composition. In Figure 3a, two prominent peaks for C 1s and O 1s were clearly observed in the full survey spectrum of CNOs, with a C/O ratio of 12.27. The C 1s XPS spectrum of CNOs showed distinct peaks for CC (284.5 eV), C−C (285.4 eV), and three small peaks for C−O (286.2 eV), CO (287 eV), and COO− (287.9 eV) (Figure 3b).55 Three oxygen signals for C O (531.7 eV), C−O (532.5 eV), and COO− (535.2 eV) were observed in a short survey scan for O 1s as shown in Figure 3c. In Figure 3d, the wsCNOs exhibited two clear peaks for C 1s and O 1s in full scan spectrum, showing an increased oxygen content (C/O ratio = 4.63). The C 1s XPS spectrum of wsCNOs was deconvoluted into CC (284.4 eV), C−C (285.4 eV), C−O (286.2 eV), CO (287 eV), and COO− (287.9 eV), as shown in Figure 3e.40 The O 1s XPS spectrum of wsCNOs was fitted to CO (531.7 eV), C−O (532.5 eV), and COO− (535.2 eV), with three carbon−oxygen bindings modes (Figure 3f). Taken together, the CNOs are carbon-rich nanoonions, which contain aromatic and aliphatic carbon cores with surface adsorbed moisture, while wsCNOs are oxidized with carboxylic and hydroxyl groups present on the surface. The zeta potential was measured to assess the stability and solubility of wsCNOs in aqueous solution. The negative 3987


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ACS Sustainable Chemistry & Engineering H 2O/O2

coupling between the electronic transition and collective vibrations of the lattice contributes to tuning of the emission. Photocatalytic Dye Degradation under Visible Light Illumination. The photocatalytic behavior of as-prepared CNOs and wsCNOs was studied by measuring photodegradation of model organic pollutant dye (i.e., MB). Absorption changes from the initial concentration (C0) to the final concentration (C) of the dye were analyzed at a set time interval of 10 min under visible light irradiation. The rate constants (k) of MB degradation in the presence of CNOs and wsCNOs were calculated using the Langmuir−Hinshelwood model formula26

dyes· + O2 → dyes‐OO· ⎯⎯⎯⎯⎯⎯⎯⎯→ degradation H 2O/O2

dyes‐H + ·OH → H‐dyes‐OH· ⎯⎯⎯⎯⎯⎯⎯⎯→ degradation

where C0 and C represent the concentrations of organic dye at 0 and t minutes of irradiation times, respectively. Figure 5a showed a variation in the concentration of MB versus visible light irradiation time. Drawing assessment between photocatalysis activity clearly shows that wsCNOs exhibited enhanced degradation efficiency than CNOs, as demonstrated in Figure 5a. The intensity of the characteristic MB absorbance peak (663 nm) was decreased to 14.5% from its initial measured value (100%), after 120 min of exposure to visible light in the presence of wsCNOs (Figure S1 a). This indicates the photodegradation of MB by 85.5% (5 × 10−3 M/L) from its initial concentration. In sharp contrast, a 78.6% decrease in MB absorbance intensity was observed for CNOs (Figure S1b) from its initial measured values (100%). No significant photodegradation of MB was observed even after 120 min of visible light irradiation in the absence of CNOs and wsCNOs, as shown in Figure 5b. These results suggest that wsCNOs play a vital role in the photocatalytic process. The whole catalytic experiment was repeated in dark conditions to confirm the dye degradation via photocatalysis instead of the adsorption process. Figure S2 demonstrates a variation in the concentration of MB in the presence and absence of light with CNOs and wsCNOs. In the absence of visible light irradiation, the degradation of MB was significantly diminished, and only ∼31.1% degradation was observed by wsCNOs after 120 min. These results give direct evidence that wsCNOs facilitate the degradation of MB in the presence of visible light. The wsCNOs have the ability to act as photosensitizers due to their broad and extended absorption in visible light.64 Electrons in wsCNOs are excited by visible light absorption to the conduction band, and electron−hole pairs are generated (eq 1).23 Photosensitized holes on wsCNOs interacted with surface adsorbed water molecules (eq 2) or directly oxidized the dye molecules (eq 3), resulting in hydroxyl or organic radicals, respectively. Aerial oxygen acts as an electron scavenger to oxidize the activated organic dyes. This reaction pathway possessed low or nonexistent barriers for facile oxidative degradation of organic dyes (eqs 4−6).46 The proposed photocatalytic mechanism for the degradation of MB is as follows:65 (1)

H 2O + h+ → ·OH + H+

(2)

dyes‐H + h+ → dyes‐H·+ ⇌ dyes + H+

(3)

dyes‐H + ·OH → dyes· + H 2O

(4)

(6)

The apparent rate constant of wsCNOs is 0.0175 min−1 for MB, which is ∼9 times higher than that of the CNOs (Figure 5b). The enhancement in photocatalytic activity of wsCNOs is attributed to the increased adsorption affinity for positively charged MB on wsCNOs.46 Effective adsorption of organic dyes on photocatalysts is a key step to mineralization by visible light induced electron transfer reactions. The unique morphology (concentric multilayered shell) of wsCNOs with considerable surface defects in the form of dangling bonds is likely to enhance MB adsorption. Additionally, wsCNOs with negatively charged carboxylic groups present on the surface provide higher binding affinity for hydrophilic MB molecules as compared to CNOs. Oxygenated surface functional groups such as −OH, −COOH, −COO−, and −CO facilitate the charge transfer between organic dyes and wsCNOs, resulting in a diminished photoexcited electron−hole pair recombination and high upward band bending with enhanced hole transport.46 Generation of active oxygen species (·O2) and hydroxyl (·OH) radicals are supposed to be promoted due to this charge separation process. The dye molecules could be degraded by these species.64 In addition, the active sites of wsCNOs also directed the increased generation of holes, contributing to photocatalytic activity.65 The possible photocatalytic mechanism for dye degradation is shown in Scheme 2.

k = ln(C /C0)/t

wsCNOs + h v → e− + h+

(5)

Scheme 2. Possible Photocatalytic Mechanism of wsCNOs under Visible Light Irradiation

Zeta potential measurements were further carried out to confirm the increased photocatalytic efficiency of wsCNOs. An increase in zeta potential value from −77.91 mV to −12.08 mV for wsCNOs before and after photocatalytic degradation of wsCNOs-MB solution, respectively, was observed. Zeta potential measurements confirm that the increased physisorption of positively charged MB on the negatively charged wsCNOs surface simply boost the photocatalytic efficiency of wsCNOs. Detection of Al(III) Ions. Encouraged by the strong photoluminescence and abundant surfacial oxygenous functionalities, the feasibility of wsCNOs as a fluorescent probe for the specific detection of Al(III) ions was demonstrated. The fluorescence of wsCNOs toward varying concentrations of Al(III) was analyzed at 390 nm excitation wavelength. With increasing Al(III) concentration from 10 to 100 μL (1 × 10−3 M), the PL emission of wsCNOs centered at 531 nm was 3988


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Figure 6. (a) Fluorescence quenching of wsCNOs by stepwise addition of 100 μL Al(III) (λex = 390 nm); (b) Stern−Volmer plot for linear fitting of the values of Io/I at 390 nm versus the concentrations of Al(III). (c) Effect of various anions on the relative PL intensity of wsCNOs. (d) Effect of various metal ions on the relative PL intensity of wsCNOs. (e) Fluorescence quenching of wsCNOs by stepwise addition of 100 μL of Fe(III) (λex = 390 nm).

intensity. Quenching of PL was more significant for Al(III) than Fe(III). These results revealed the selectivity of Al(III) even in the presence of several metal ions. The higher selectivity of wsCNOs for Al(III) than for Fe(III) is probably due to the high affinity of carboxylic groups toward Al(III) (hard acid−hard base interactions).41 The possible mechanism for the quenching of PL emissions of wsCNOs upon the addition of Al(III) is explained on the basis of their ionselective chemical nature and charge/energy transfer effects. The carboxylic groups on the surface of CNOs can interact with Al(III) ions via coordination bonds and form a complex. This complex affects the distribution of surface energy traps and enhances the nonradiative recombination of electrons and holes, and the consequences of fluorescence quenching (Scheme 3).66

effectively quenched, without change in peak position as shown in Figure 6a. The binding of Al(III) with surface functional groups of wsCNOs led to the quenching of PL emission. There is a linear relationship between the concentration of Al(III) ions and fluorescence quenching, as shown in Figure 6b, in which Io and I are the concentrations of wsCNOs in the absence and presence of Al(III), respectively. The detection limit of Al(III), based on the definition of 3 times the standard deviation of the blank signal (3σ), was calculated as 0.77 μM from the fitted curve with a correlation coefficient of 0.998.5 This limit of detection is approximately 10 times lower than the maximum permitted level of Al(III) (7.4 μM) in drinking water by the World Health Organization (WHO).5 The wsCNOsbased fluorescence sensing system could provide a green, rapid, sensitive, and reliable method for Al(III) detection in drinking water. On account of the complexity of real samples, selectivity is also crucial for fluorescent probes. The selectivity of Al(III) was therefore examined with respect to other metal ions and anions under identical experimental conditions. Varying types of anions did not result in any significant change in the relative PL intensity (Figure 6c), and no obvious effects were observed upon the addition of other metal ions either (except Fe3+), as illustrated in Figure 6d. The selectivity for Al(III) is remarkably higher than that for Fe(III) or other metal ions. The quenching effect of Al(III) is more prominent than that of Fe(III), as shown in Figure 6e. The selectivity toward Al(III) was further investigated in the presence of several metal ions, as shown in Figure S3. No remarkable change in the PL intensity was observed after the addition of several metal ions. However, further addition of Al(III) and Fe(III) caused a decrease in PL

Scheme 3. Schematic Representation of the Sensing of Al(III) with a wsCNO Based Fluorescent Probe

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The absorption spectra of wsCNOs before and after the addition of 100 μL Al(III) is shown in Figure S4. The absorption intensity and peak position are significantly affected by Al(III). A strong and wide adsorption band appears at 298 nm, while the absorption band of wsCNOs at 246 nm disappears after the addition of Al(III). The UV−vis spectrum thus considerably changes, which confirms the coordination of Al(III) with wsCNOs. This suggests that co-ordination of Al(III) with wsCNOs is responsible for the fluorescence quenching and hence selective sensing of Al(III).

CONCLUSIONS Herein, an economically viable and facile synthetic route for the synthesis of wsCNOs by traditional pyrolysis of flaxseed oil as the carbon precursor is reported. The wsCNOs have a homogeneous and narrow distribution of particle sizes ranging from 4 to 8 nm. The wsCNOs exhibited excellent visible light driven photocatalytic performance toward degradation of organic dyes. Compared with as-synthesized CNOs, the wsCNOs exhibited improved photocatalytic performance ∼9 times for MB degradation in water. Furthermore, the stable, green emissions of wsCNOs are employed as a fluorescent probe for the sensitive detection of Al(III) ions based upon the florescence turn-off technique. The wsCNO-based fluorescence probe of Al(III) is highly specific, even in the presence of other metal ions and anions, with a 0.77 μM detection limit. The wsCNO-based PL sensor reported here may lead to a novel pathway for the specific detection of metal ions by the design of functional nanomaterials. Altogether, wsCNOs could be a promising material for sustainable treatment of wastewater and environmental therapy. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b03182. UV−vis absorbance spectra of MB and photocatalytic properties of wsCNOs and CNOs (PDF)

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REFERENCES

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Research Article

AUTHOR INFORMATION

Corresponding Author

*Tel: +821071237516. E-mail: [email protected]. ORCID

Tuan Sang Tran: 0000-0002-0047-9571 TaeYoung Kim: 0000-0001-8156-4438 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Collaborative Research Program among Industry, Academia and Research Institute through the Ministry of Science, ICT, and Future Planning (MSIP) and Korea Industrial Technology Association (KOITA) (grant no. KOITA-2014-4). This research was also supported by Nano Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning (NRF-2016M3A7B4027712) 3990


Research Article

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Green Fluorescent Onion-Like Carbon Nanoparticles from Flaxseed

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