Multiband Fluorescent Graphitic Carbon Nanoparticles from Queen of

Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Multiband Fluorescent Graphitic Carbon Nanoparticles from Queen of Oils Partha Pratim Das,†,§ Srikrishna Pramanik,† Sabyasachi Chatterjee,‡ Anurag Roy,† Arindam Saha,† Parukuttyamma Sujatha Devi,*,† and Gopinatha Suresh Kumar‡ †

Sensor & Actuator Division, CSIR-Central Glass and Ceramic Research Institute, 196 Raja SC Mullick Road, Kolkata 700032, India Biophysical Chemistry Laboratory, Chemistry Division, CSIR-Indian Institute of Chemical Biology, Kolkata 700032, India

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S Supporting Information *

ABSTRACT: Here, we report a facile and rapid approach for the synthesis of multiband fluorescent graphitic carbon nanoparticles (CNPs) from queen of oils exhibiting multifarious applications. The as-prepared and calcined CNPs exhibited excitation dependent multiband emission that has been explored for detection of deoxyribonucleic acid (DNA) and removal of water pollutant cationic dye. We report here selective detection of Escherichia coli (E. coli-DNA) using fluorescent graphitic carbon nanoparticles through fluorescence enhancement of the nanoparticle. On the basis of thermal melting, CD and calorimetric studies, we could conclude that the interaction of CNPs are stronger with E. coli-DNA resulting in preferential selectivity for E. coli-DNA. The as-derived CNPs could be an emerging cost-effective material for the selective detection of E. coli-DNA and a sorbent for removal of basic pollutant dyes from water. KEYWORDS: Graphitic carbon, Multiband emission, Escherichia coli DNA, Sorbent, Conductive

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INTRODUCTION The synthesis of carbon nanoparticles in various forms such as fullerene, graphene, nanotubes, nanofibers and nanoparticles has attracted immense interest due to their growing applications in various fields.1,2 Among these, carbon nanoparticles (CNPs) and C-dots have been more widely investigated recently due to their interesting photoluminescence properties and related applications.3−10 An unusually large number of methods are available for the preparation of CNPs using chemical reagents, catalysts and various high temperature pyrolysis techniques.11−14 In addition to the mentioned techniques, various green synthesis methods using candle soot, dopamine, orange waste, orange juice, egg white, hair, lamp soot and strawbery juice, etc. have also been explored for the synthesis of fluorescent carbon nanoparticles.15−22 In this work, we have prepared multifunctional graphitic carbon nanoparticles by a facile catalyst free green technique at room temperature using sesame oil as the source of carbon using a diya lamp. Sesame oil is an edible vegetable oil derived from sesame seeds. This oil has been used as a cooking oil in India and as a flavor enhancer in many Asian countries. The major components of sesame oil are poly unsaturated fatty acids such as oleic acid and linoleic acids. On buring, these organic components decomposes to produce functional carbon nanoparticles as revealed here through a series of characterization techniques. The as-formed virgin carbon nanoparticles have been characterized without further purification or processing. The powder was hydrophilic in nature and formed stable dispersions in water and dimethyl sulfoxide (DMSO) by © XXXX American Chemical Society

simple sonication in an ultrasonic bath. We have also evaluated the effect of postannealing temperatures on the various characteristics of carbon nanopaticles. The as-formed, calcined at 250 °C (2 h) and 350 °C (2 h) samples will be designated as CNP1, CNP2 and CNP3, respectively, hereafter. Understanding the interaction between nanoparticles and biomolecules like DNA is a radical feature in the design and development of new types of sensing probe, nanodevices and use of nanoparticles in drug delivery application. In recent years, research has been directed toward identification and design of sensitive and specific materials for sequence specific DNA recognition and nucleic acid detection. Among the various methods, fluorescence based techniques have been reported to be fast and less complicated assays for nucleic acid detection.23−25 Especially, carbon based nanostructres like nanotubes and particles have emerged as the most superior material due to their biocompatibility, less cytotoxicity and availability at a lower cost.26−29 In such studies, fluorophore dyes like FAM dye modified single stranded DNA were functionalized with nanoparticles and used as a fluorescence probe for recognition of complementary strand. To date, studies on the interactions of carbon nanoparticles with DNA and the detection method for long chain natural DNA have been scarcely explored. Recently, fluorescent carbon nanoparticles have emerged as a new fluorophore probe that can be used to recognize natural DNA, in addition to other small Received: March 30, 2018 Revised: May 20, 2018

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DOI: 10.1021/acssuschemeng.8b01443 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

Figure 1. Characteristics of the CNP particles: (a) TG-DTA of CNP1, (b) X-ray diffraction pattern and (c) Raman spectra where 1,2,3,4 represent CNP1, CNP2, CNP3 and standard graphite powder, respectively.

organic molecules.30,31 Recent findings from our group reported the interaction of metal nanoparticles, semiconducting oxides and carbon nanostructures and their application as label free DNA binding probes for the development of novel biosensors and white light emitting materials.32−34 In this work, intrinsic optical and surface properties of carbon nanoparticles derived from a cost-effective approach directed out attention toward the development of a label free fluorescence probe for the selective recognition of Escherichia coli (E. coli)-DNA. The binding affinity of CNP toward E. coliDNA has been compared with the calf thymus (CT)-DNA to understand the binding mechanism and selective sensitivity. In addition, surface chemistry of the synthesized CNP has been exploited to selectively remove cationic pollutant dyes from water with excellent re-usability.

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prepare specimens for transmission electron microscopy (TEM) observation, which was performed on a TECNAI G230 highresolution transmission electron microscope operating at 300 kV. Fourier transform-infrared (FT-IR) measurements were performed on a Perkin Elmer-Spectrum two, FTIR spectrophotometer. Dynamic light scattering (DLS) study was performed on a HORIBA (SZ-100 OZ) dynamic light scattering particle analyzer. For DNA experiments, the steady state fluorescence was measured at 20 ± 0.5 °C on a Shimadzu RF-5301 PC spectrofluorophomete (Shimadzu Corporation, Kyoto, Japan) using 75W Xeron arc lamp. Time correlated single photon counting (TCSPC) measurement was performed with a Horiba Jobin Yvon IBH Fluorocube apparatus after exciting the sample with 340 nm excitation lamp. The fluorescence decay was collected with a Hamamatsu MCP (R3809 photomultiplier at 435 nm, and the fluorescence decay was analyzed with IBH DAS6 software. The surface wettability of the CNP2 film was accomplished by measuring the successive water contact angle on a drop shape analyzer (Krüss DSA25) using Young’s equation (sessile drop method). The X-ray photoemission spectroscopy (XPS) measurements of all the samples were conducted on a PHI 5000 Versa probe II scanning XPS microprobe (ULVAC-PHI, U.S.) with monochromatic Al Kα (hν = 1486.6 eV) radiations. Fluorescence quantum yields of the carbon nanoparticles were measured with respect to quinine sulfate (QY = 0.54) as a standard dye, using the following formula:

EXPERIMENTAL SECTION

Materials. We used high purity cooking grade sesame oil for the synthesis. The oil was burnt with the help of a diya lamp as shown in the Electronic Supplemenatry Information ESI, Figure SI using a cleaned cotton thread medium. Both calf thymus- and Escherichia coliDNA were purchased from Sigma Chemicals. DNA stock solutions were prepared in citrate−phosphate (CP) buffer of pH ∼ 7.4, containing 10 mM Na2HPO4. Concentrations of CT- and E. coli-DNA were calculated using the molar extinction absorption value of ε260 = 13 200 M−1 cm−1 and ε260 = 13 000 M−1 cm−1, respectively for CTand E. coli-DNA. Methyl Violet (MV), Methylene Blue (MB) and Methyl Orange (MO) dyes were purchased form Sigma Aldrich. Synthesis. The particles were collected by keeping a stainless steel or glass plate on top of the flame. The distance from the flame to the substrate was adjusted as 15−20 cm to deposit the particles. During pyrolysis, the sesame oil decomposes and forms functionalized carbon nanoparticles. To get appreciable deposition, substrate was exposed at different variations of time. The water dispersions of the particles were filtered through a MF-Millipore syringe filter of (0.2 μm pore size) and used for optical measurements. Methods. Thermo gravimetric analysis (TGA) and differential thermal analysis (DTA) studies of the as synthesized powder was carried out on a NETZSCH 449C simultaneous thermal analyser. The room temperature powder X-ray diffraction (XRD) was carried out using a Philips X-ray diffractometer (PW1730) with Cu Kα radiation at a 2θ scan rate of 2° per minute. The absorbance spectra were measured on a UV−Vis-NIR Spectrometer (SHIMADZU UV-3600). The fluorescence spectra of the dispersed solutions were recorded on a Steady State Spectrofluorometer (QM-40, Photon Technology International, PTI) using a 150 W xenon lamp as an excitation source, at various excitation wavelengths as specified in the Results and Discussion at a band-pass of 5 nm. Raman Spectra were obtained using a Renishaw Reflex micro Raman spectrometer with an argon ion (514 nm) laser. The spectra were collected with a resolution of 1 cm−1. Diluted CNP solution was dropped onto copper grids to

(QY)sample = (QY)standard (Isample/Istandard)(A standard /A sample) (nsample /nstandard)2

(1)

where I is the integrated peak area of the fluorescence curve, A is the absorbance at 350 excitation wavelength and n is refractive index of water. Optical densities were kept under 0.1 to evade any kind of inner filter effect. The thermal denaturation experiments of all the DNAs and DNA−CNP system were executed on a Shimadzu Pharmaspec 1700 unit equipped with the Peltier controlled TMSPC-8 model accessory (Shimadzu Corporation, Japan). To measure the melting temperature of DNAs, the changes in the absorbance value at 260 nm with respect to temperature were monitored from 20 to 110 °C at a heating rate 0.5 °C/min. Spectropolarimetric measurements of DNA−CNP complexes were studied with a Jasco J815 spectropolarimeter (Jasco International Co. Ltd., Hachioji, Japan) equipped with a Jasco temperature controller (model PFD 425L/15) interfaced with a HP PC at 20 ± 0.5 °C using instrument parameters as scanning speed 100 nm/min., bandwidth of 1.0 nm, and sensitivity of 100 milli degree. A buffer baseline scan was subtracted from the average scan for each sample. Five scans were averaged and smoothed within permissible limits using the Jasco software to improve signal-to-noise ratio. The thermodynamic parameters of the interaction and equilibrium binding affinity (Ka) of DNA−CNP systems were measured by isothermal micro calorimetric titration experiments using a VP-ITC unit (MicroCal, Inc., Northampton, MA, USA) at 20 °C. The obtained data were plotted as a function of the CNP/DNA ratio and fitted with a model for one set of binding sites and examined B


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Figure 2. TEM images of the carbon nanoparticles produced under different conditions: (a) CNP1, (b) HRTEM of CNP1, (c) CNP2, (d) HRTEM of CNP2, (e) CNP3 and (f) HRTEM of CNP3, respectively. Insets of panels a, c and e show the corresponding size distribution.

starts oxidizing above 600 °C. The XRD patterns of the ascollected and calcined carbon nanoparticles along with that of a standard graphite powder is shown in Figure 1b. One prominent reflection around 2θ = 24.2° and two weak reflections around 44.2° and 77°, respectively, are seen for the CNPs. These peaks correspond to the (002), (101) and (110) reflections, respectively, of graphitic carbon. Compared to the standard graphite powder, the XRD patterns of CNPs are broad and the interlayer spacing in graphite (0.348 nm) becomes 0.336, 0.363 and 0.362 nm, respectively, for CNP1, CNP2 and CNP3. These reflections suggest the CNPs derived from sesame oil are nanocrystalline graphitic carbon particles. Interestingly, the CNPs heated at 250 °C exhibited an increase in the intensity of both 002 and 101 reflections compared to as-derived and 350 °C calcined samples. The crystallite size calculated from the X-ray line broadening was 2.7, 2.1 and 2.5 nm, respectively, for CNP1, CNP2 and CNP3 samples. The Raman spectra of the CNPs along with that of the standard graphite are presented in Figure 1c. Usually, three significant bands namely G for graphite band appearing at 1580 cm−1, D for disorder band (sp3) appearing at 1350 cm−1 and G′ or 2D band at 2700 cm−1 dominate in the Raman spectrum of graphitic carbon nanoparticles.35,36 The G band corresponds to the high frequency Raman active E2g mode (sp2

to obtain the binding stoichiometry (N), binding affinity (Ka) and the enthalpy of binding (ΔH°). The binding Gibbs energy change (ΔG°) and the entropic contribution to the binding (TΔS°) were calculated from the standard thermodynamic equations: ΔG° = − RT ln Ka

(2)

ΔG° = ΔH ° − T ΔS°

(3)

The dye removal efficiency and adsorption−desorption rate were evaluated based on the difference in the MV concentration in the aqueous solution before and after adsorption, according to the following equation:

% of dye removed =

C0 − Ct × 100 C0

(4)

−1

where C0 (mg L ) is the initial concentration of dye in the solution and Ct (mg L−1) is the equilibrium concentration at time t (min) during the measurement.

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RESULTS AND DISCUSSION Characterization of CNP Structure and Morphology. In order to find out the stability of the nanoparticles, we carried out simultaneous thermogravimetric-differential thermal analysis (TGA-DTA) studies on the as-derived virgin powder as shown in Figure 1a. It is evident from the TGA that the as-collected carbon nanoparticle is stable up to 550 °C and C


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separate peaks reveal different degrees of stacking order along the c-direction. This could again be a clear indication of the formation of disordered graphitic carbon with more graphene layers. The transmission electron microscopic images obtained from the as-prepared and calcined carbon particles are shown in Figure 2. The particles exhibited chain-like structures as evident from Figure 2a,c,e. The size of the individual carbon nanoparticles varied in the range of 20−30 nm as evident in the particle size distribution calculated from the corresponding images (inset in Figure 2a,c,e). Average particle sizes calculated from the bright field TEM images using imageJ software were 33.5 ± 2, 36.9 ± 3 and 31.9 ± 5 nm for CNP1, CNP2 and CNP3, respectively. The crystallite size calculated from the Xray line broadening was only 2.7, 2.1 and 2.5 nm for CNP1, CNP2 and CNP3 samples, respectively. The single particle scanned under TEM may be a cluster of many crystallites leading to a higher size distributions. The characterization of the individual particles by high resolution transmission electron microscopy (HRTEM) suggests the absence of distinguishable lattice structures for CNP1 indicating a partially crystalline nature of individual carbon nanoparticles. More importantly, the particles of CNP1 appeared to be porous in the high resolution image (Figure 2b,d,f). However, the HRTEM of CNP2 and CNP3 indicates the presence of few graphene layers, a clear-cut indication of the formation of crystalline graphitic layers during calcination. Optical Characteristics of the CNP Nanoparticles. To gain insight on the optical properties of the prepared carbon nanoparticles, we carried out absorption and fluorescence studies on the water dispersions of the particles at pH ∼ 6.8. The UV−Vis absorption spectra of the carbon nanoparticles and the graphite powder are shown in ESI, Figure S3a. All the samples exhibited a strong absorption band in the 250−300 nm region. The observed band maxima varied as 257, 258 and 254 and 271 nm for CNP1, CNP2, CNP3 and standard graphite, respectively. This strong UV absorption is attributed to the π−π* transition of aromatic sp2 CC bonds. The prepared CNPs exhibited a hypsochromic shift compared to the graphite nanoparticles dispersion. The observed absorption band is consistent with the previous reports on carbon nanoparticles prepared from natural sources.17,20 The FT-IR spectra of CNP shown in the Supporting Information (Figure S3b) were used to identify the functional groups present in our samples. The peak around 3420 cm−1 is assigned to the C OH group and that at 1620 cm−1 corresponds to the CO stretching bond. This indicates the presence of hydrophilic functional groups such as −OH and COO− on the surface of the CNPs.17,20 These functional groups covalently bound to the carbon framework improve the hydrophobicity and stability of the nanoparticles in polar media like water. The dispersions of the nanoparticles in water exhibited zeta potentials of −11, −35 and −43 mV (Figure S3c) for CNP1, CNP2 and CNP3, respectively, confirming the good stability in aqueous solution and high affinity with positively charged groups. The hydrodynamic radius of the synthesized nanoparticles was measured by dynamic light scattering. The average hydrodynamic radius was found to be 115, 117 and 108 nm for CNP1, CNP2 and CNP3, respectively (Figure S4). We also carried out fluorescence measurements on the CNPs dispersed in different solvents. In most cases, the excitation wavelength was varied from 250 to 400 nm. The emission data were collected at various excitation wavelengths

carbon) of single crystal graphite and the broad D band corresponds to A1g mode arising out of disordered graphite and indicates the presence of nanocrystalline graphitic nanoparticles. In our samples, we observed the G band around 1591 cm−1 and D band around 1346 cm−1. The standard graphite powder exhibited the G band at 1580 cm−1. The G band positions of CNPs, on the other hand, were observed at 1592, 1586 and 1585 cm−1, respectively, due to the presence of sp3 carbon in these particles. From the usual position of the G band, the CNPs exhibited a shift of ∼12 cm−1 toward the higher wavenumber, thereby confirming the existence of nanocrystalline graphite or sp2 clusters.35,36 Similarly, the standard graphite exhibited the D band at 1350 cm−1. The D band positions of the prepared CNPs, on the other hand, were at 1344, 1350 and 1347 cm−1 for CNP1, CNP2 and CNP3, respectively. The intensity ratio between D and G peaks has been used to evaluate the graphitization of carbon nanoparticles. Our samples exhibited ID/IG ratios of 0.9293, 0.8808 and 0.9271 for CNP1, CNP2 and CNP3 samples, respectively, indicating a high degree of graphitization of our samples. In fact, this ratio is also a measure of the extent of disorder and sp3/sp2 carbon atoms. There is also a stark difference in the full width at half maxima (fwhm) of both G and D bands of the synthesized CNPs compared to that of the standard graphite powder. This broadening is a direct indication of an incresaed level of disorder due to the presence of sp3-C and a decrease in the graphitic domain size. This was also evident in the XRD patterns of the CNPs (Figure 1b) where the two major reflections of graphitic carbon were clearly evident, but the reflections were considerably broadened due to size. The fwhm of the D band varied from 0.228 for the standard carbon to 0.456 for CNP1, to 0.417 for CNP2 to 0.431 cm for CNP3, respectively. The broadening of the D band also implies an increased level of disorder and a decresae in the graphitic domain size. The recombination of carbon bonds during carbon nanoparticle formation introduces smaller graphitic domains with different bonds.35 The distribution of clusters with different size could lead to superposition of the different Raman modes resulting in a broader line width and shift of the Raman peaks as discussed above. We have also calculated the crystallite size “La” from the Raman data as 2.6, 2.1 and 3.5 nm for CNP1, CNP2 and CNP3, respectively.35 These results almost match with the crystallite size calculated from X-ray line broadening. In both cases, CNP2 exhibited finer sized crystallites compared to CNP1 and CNP3. In addition to the G and D bands, the standard graphite exhibited a strong G′ or 2D band around 2699.8 cm−1. This is the second most intense feature in the Raman spectra of a completely ordered 3D graphite or single layer graphene. Because 2D(G′) is the second order of the D band, it has been reported to be sensitive to the stacking order of the graphene sheets along the c-axis. Interestingly, our samples exhibited a broad 2D band that could be deconvoluted into two Lorentzian peaks as shown in the Supporting Information, Figure S2a. In samples, where there is no stacking order along the c-axis, the 2D band appears as a single Lorentzian peak similar to the 2D band exhibited by the standard graphite. Even a single layer graphene exhibits only a single sharp 2D band as reported by many researchers.35 On the contrary, the evolution of the two peaks became clear in the CNPs synthesized here (ESI, Figure S2a), which becomes more prominent with increase in calcination temperature. The change in the 2D band shape from a single sharp band to a broad band consisiting of two D


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Figure 3. Photoluminescence spectra of a water dispersion of (a) CNP1, (b) CNP2, (c) CNP3 at different excitation, digital images of the dispersion of all the samples in water on excitation with a UV lamp are shown in panel d (i), (ii) and (iii) for CNP1, CNP2 and CNP3, respectively.

Figure 4. Emission spectra of a DMSO dispersion of (a) CNP1, (b) CNP2 and (c) CNP3 at various excitation wavelengths as shown in the inset; (d) normalized PL intensity of CNP2 dipsersion in different solvents such as DMSO, water, DMF and toulene.

as shown in Figure 3a−c. The as-obtained virgin and calcined CNPs exhibited excellent blue emission without any surface functionalization or surface treatment. Interestingly, all the CNP samples exhibited two different emission bands on excitation between 250 and 290 nm giving rise to a stronger band at 320 nm and a weaker band around 430 nm. On further increase of the excitation wavelength beyond 290 nm, the intensity of the UV emission band decreases while the intensity of the visible emission band increases as evident in Figure 3a− c. The maximum UV emission occurred on 280 nm excitation and the maximum visible emission occurred on 350 nm

excitation. The most interesting feature of the PL of these samples is the clear dependence of the excitation wavelength on both emission wavelength and intensity. Fluorescence quantum yields of the nanoparticles were found to be 0.65%, 2.5% and 1.46% for CNP1, CNP2 and CNP3, respectively. We found a solvent dependent variation of the visible blue emission as clear from the decrease in intensity of the emission band. Among the solvents used, polar aprotic solvent like DMSO shows the most intense emission spectrum with a slight shift in the band maxima toward green. In Figure 4, the emission spectra recorded for the DMSO dispersions of CNP1, E


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Figure 5. Comparison of photoluminescence lifetime decay between different CNPs at 295 nm excitations and 325 nm collection windows.

Figure 6. (a, b and c) Deconvoluted C1s XPS spectra of CNP1, CNP2 and CNP3, respectively, (d, e and f) deconvoluted O1s XPS spectrum of CNP1, CNP2 and CNP3, respectively.

and a lifetime of 1.8 ns (11.47%) as a minor component. CNP2, on the other hand, exhibited a short lifetime of 23 ps (90.48%) as a major component whereas two minor components exhibited lifetimes of 2.2 ns (2.95%) and 5.8 ns (6.57%), respectively. CNP1 exhibited lifetimes of 0.21 ns (35.88%) and 0.42 ns (60.42%) as major components and 3.6 ns (3.70%) as a minor component (Figure 5a). When these samples were excited at 295 nm excitation laser and collected at 325 nm, they exhibited somewhat larger lifetime. CNP3 exhibits 0.26 ns (77.52%) as a major component and 1.2 ns (14.94%) and 5.2 ns (7.54%) as minor components. On the

CNP2 and CNP3 are presented. The emission was weak in nonpolar solvents like hexane (Figure 4d). The probable reason for the high intensity emission in DMSO may be due to the interaction of the unpaired electrons on CNP with the vacant d-orbitals of sulfur. The fluorescence lifetime (τ) of the derived carbon nanoparticles was measured by time-resolved photoluminescence measurements (Figure 5). All the CNPs exhibited thirdorder exponential decay with short lifetime when excited at 340 nm and collected at 420 nm. CNP3 exhibited a lifetime of 0.15 ns (65.34%) and 6.6 ns (23.18%) as major components F


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Figure 7. Fluorescence intensity variation on addition of (a) CT-DNA, (b) E. coli-DNA, (c) dependence of fluorescence intensity on concentration of E. coli-DNA used, (d and e) changes in the CD spectrum of CT- and E. coli-DNA, respectively, (f and g) thermal melting profiles of CT-DNA and E. coli-DNA, respectively, in the presence of CNP2 and (h) schematic picture of the interaction.

at 284.6, 286.3, 288.48 and 290.7 eV corresponding to the band energies of the C1s states of graphitic sp2, CC, CO and CO of COOH groups, respectively. The deconvoluted XPS spectrum of CNP2 (Figure 6b) also consists of four peaks at 284.67, 286.23, 288.7 and 290.8 eV corresponding to the C1s states of graphitic sp2 CC, CO, CO and COOH groups, respectively.38−41 In the case of CNP3 (Figure 6c), the binding energies of these groups are observed at 284.82, 286.06, 288.48 and 290.8 eV. The deconvoluted O1s spectrum of (Figure 6d,e) shows four peaks at 530.2, 532, 533.5 and 536.2 eV, confirming the presence of CO group, oxygen atoms bonded in COC group, COH bonding in the surface and chemisorbed water molecules, respectively.42,43 Similarly, the O1s spectrum (Figure 6f) of CNP3 shows the binding energy of the above groups at 530.1, 532.1, 533.67 and 536.4, respectively. The XPS analysis of CNP samples confirmed the graphitic nature of all the synthesized CNPs. In addition, the nature and peak intensities of the deconvoluted O1s spectrum indicate the presence of different degrees of oxygenated defect states in CNP2 and CNP3. Application of CNP Nanoparticles. Finally, the fluorescent characteristics of the derived CNPs have been used in the detection of nucleic acids. For this purpose, we have chosen the CNP2 sample. A finely dispersed filtered aqueous solution of CNP2 was treated with CT-DNA and Escherichia coli-DNA solutions. With gradual addition of both DNAs, the emission maxima at 330 and 420 nm increased regularly without significant change in the peak maxima as shown in Figure 7a,b. Interestingly, the fluorescence enhancement was different for the two different DNA samples. In the case of CTDNA, the emission peak around 330 nm increased regularly but the latter peak around 420 nm exhibited only a minute change as shown in Figure 7a. Surprisingly, in the case of E.

other hand, CNP2 exhibits 0.23 ns (61.29%) and 0.37 ns (30.18%) as major components and 1.4 ns (8.53%) as a minor component (Figure 5b). Under this condition, the lifetime of CNP1 was too weak to be measured. In general, the fluorescence property of carbon nanoparticles synthesized at lower temperature is mainly controlled by different molecular states of fluorophores on the surface. High temperature pyrolysis >230 °C, on the other hand, can induce a more carbogenic domain and enhance the crystallinity by removal of organic components.37 The sp2 centers of the carbogenic domain along with different defect states created by the functional groups at high temperature can act as a luminescence center for excited dependent emission property. We also tried to understand the effect of high annealing temperature on the emission property of the synthesized nanoparticles. Apparently, when the as-prepared carbon nanoparticles were heated at 450 °C, no significant fluorescence property was observed (Figure S5a,b). Because of the absence of any characteristic fluorescence property at higher temperature calcined CNP, we have not used high temperature calcined sample for DNA interaction studies. As evident from the FT-IR, all the synthesized CNPs contain various functional groups like COH, CO, etc. on the surface resulting in emissive sites between π and π* states of carbon. The presence of oxygenated defect states vary from one particle to another. With change in the excitation wavelength various surface trap states present on the surface of different particles gets excited resulting in multiband and tunable broad emission as shown above. XPS measurement was performed to elucidate the elemental component and structural characterization of carbon nanoparticles. The XPS data unveiled that the synthesized CNP is made of carbon, and oxygen atoms (Figure 6). In Figure 6a, the deconvoluted XPS spectrum of CNP1 displays four peaks G


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Figure 8. (a and b) FT-IR spectra of CNPs (1) in the presence of different concentration (20 and 150 μM) of E. coli-DNA (2 and 3) and CT-DNA (2 and 3), respectively. (c and d) Absorbance spectra of E. coli-DNA and CT-DNA in the presence of different amounts of carbon nanoparticles.

coli-DNA both the emission maxima simultaneously increased (Figure 7b). The saturation effect was observed around 220 μM concentration of CT-DNA, whereas in the case of E. coliDNA, even up to this high concentration no saturtaion was observed in the emission spectra. It may be noted that under identical concentrations of DNA, the change in the fluorescence band around 420 nm was much higher for E. coli-DNA in comparison to CT-DNA. Furthermore, a continuous increase in the fluorescence intensity was obseved with increase in DNA concentration (Figure 7c). Perturbations in the secondary structure of DNA conformation on interaction with CNP binding has been monitored by circular dichroism (CD) spectral change as shown in Figure 7d,e. The far UV-CD spectrum of both the DNAs exhibited a maxima at ∼275 nm (due to base stacking) and a minima at ∼245 nm (due to helicity) corresponding to the right-handed B-form.44 With the addition of increasing concentration of CNPs, there was a gradual decrease in the intensities of both the positive and negative bands without any considerable shift of the positions. The change in the CD signal intensity with the addition of CNP is noteworthy. Although the structure of Bform of CT- and E. coli-DNA appears to be similar, the more changes in the CD spectrum of E. coli-DNA may be due to the interaction of the CNPs to the GC base pairs. To investigate whether the CNP binding altered the melting profile of DNA, we carried out thermal melting studies. It can be seen from Figure 7f,g that the melting curves of both DNAs exhibited a transition over a temperature of about 65 °C with a hyperchromicity change of about ∼40%. But a surprising result was observed in the melting profiles of the two different DNAs with CNP complex. In case of CT-DNA, there is nominal change in melting temperature due to complexation with CNP, but around 7 °C stabilization was observed in case of E. coli-DNA (71.97 °C). The possible reason for the enhancement of melting temperature (Tm) in the case of E. coli-DNA could be due to increase interaction and stabilization of DNA structure.

Isothermal titration calorimetry (ITC) is a sensitive technique that measures the heat of reaction of two aqueous solutions when one is titrated against the other and it used to directly obtain the thermodynamic parameters of complexation between the DNA solution and CNP.45 From the analysis of ITC data (Figure S6), it is observed that the affinity of the CNP toward E. coli-DNA (N is 0.172, Ka = 3.28 × 105 M−1, ΔH° = −0.17 kcal/mol, TΔS = 7.23 kcal/mol and ΔG° = −7.37 kcal/mol) is higher than that of CT-DNA (N is 0.0655, Ka = 5.67 × 104 M−1, ΔH° = −0.32 kcal/mol, TΔS° = 6.03 kcal/mol and ΔG° = −6.37 kcal/mol). The free energy of interaction (ΔG) values of CT-DNA and E. coli-DNA with CNP2 particles are −6.37 and −7.37 kcal/mol, respectively. These results confirm that the E. coli-DNA has more binding sites compared to CT-DNA, resulting in stronger and better interaction and selectivity. Our previous work on interaction of the silver and carbon nanoparticles with DNA also exhibited selective binding of such nanoparticles with bacterial DNA.32,33 Different types of weak interactions such as electrostatic, Hbonding, hydrophobic and van der Waals forces are the major driving forces between DNA and nanoparticles. All the above noncovalent interactions occurr through either intercalation or by groove binding and external surface binding. As the synthesized carbon nanoparticles exhibited negative surface charge, the possibilities of electrostatic binding with negatively charged phosphate backbone of the DNA can be eliminated. In addition, owing to the larger size of the nanoparticles, the intercalation binding mode is also unfavorable. The binding through grooves facilitated by promoting van der Waals interactions and H-bonding. The ITC data revealed that the DNA nanoparticles binding process is exothermic in nature and favored by significant entropy contribution (TΔS°) with a negative enthalpy changes. The negative ΔH° values signify that the binding process occurs by means of hydrogen bonding and van der Waals interactions.46 To get further evidence on the H-bonding interaction between CNP2 and DNA, FT-IR spectra of CNP2 were recorded in the presence of different H


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Figure 9. Deconvoluted (a) C 1s and (b) O 1s XPS spectrum of CNP2 in the presence of E. coli-DNA, (c) C 1s and (d) O 1s XPS spectra of bare CNP2 and CNP2 in the presence of E. coli-DNA.

Figure 10. Adsorption characteristics (a) MV, (b) MB, (c) MO dye in the presence of CNP samples, respectively, (inset: respective chemical structures of the pollutant dye), (d) absorption kinetics plot for CNP1 sample for MV, MB and MO dyes along with the digital image of respective dye absorption at a particular interval, (e) reusability studies of all CNP samples with MV dye for five consecutive cycles and (f) reusability studies of CNP1 sample with MV dye in terms of its adsorption and desorption efficiency in each measurement for five consecutive cycles.

CT-DNA (20 nm), suggesting a strong interaction with E. coliDNA (Figure 8a). The reduced intensity and broadening of OH stretching band in the presence of DNA also indicates the existence of strong H-bonding interaction. Overall, the reduction in the intensity and shift in the COH stretching band confirmed the presence of strong hydrogen bonding interaction between CNP2 and E. coli-DNA system. Balanced nucleobase pair composition of E. coli-DNA makes them more flexible and accessible to the CNP2 surface for favorable H-

concentrations of DNA. FT-IR spectrum of the synthesized CNP2 confirmed the presence of various functional groups like COH, CO, etc. on the surface and IR stretching frequency of these functional groups are highly sensitive toward the surrounding environment and H-bonding interaction. In Figure 8a,b, a prominent decrease in the intensity and shift in the 1620 cm−1 peak position, corresponding to the CO stretching bond can be observed. The observed shifting was higher in the presence of E. coli-DNA (40 nm) than that of I


Research Article

ACS Sustainable Chemistry & Engineering Table 1. Comparative Study of Different Carbon Materials for Dye Adsorption Application Typea

Source

Dyeb

Morphology

AC

Sawdust

Large particle

CNT MC

CVD Cotton

Nanotube Fiber

AC

Coconut shell

−

GC Graphene

Orange peel Graphite

Nanoparticle Sheet

GO

Graphite

Sheet

CM HPTC

Coal based Wood

Sheet Tube

AC GO CNT GO GO

Algae Graphite Pyrolysis Graphite Graphite/PVP

− Sheet Tube Hydrogel Hybridized with hyper-cross-linked porous polymers

GO

Graphite

Sheet

Cellulose

Fruit peel

−

AC AC GC

Rice husk Coconut choir Queens of oil

− − Nanoparticle

DB 2B DB GB Alizarin S MB MO GRL DY 12 MB MV MB RB AO8 DR23 MB MB MO MB

MB MB RB EY MB RB MB Alcian Blue MG MV MB

Adsorption (%)

Time (min)

Reference

70

120

52

50 90

200 >200

53 54

80

60

55

90 99

60 140

56 57

>90

>300

58

94 >90

10 30

59 60

>90

200

61

62 63

∼100

240 20 40 35 180

64

95−100

250

65

95 93.5 99

60 120 100

66 67 Current Work

80 ∼100 ∼100

a

AC, activated carbon; GC, graphitic carbon; CNT, carbon nanotube; CM, carbon monolith; GO, graphene oxide; MC, mesoporous carbon; HPTC, hierarchically porous tubular carbon. bCommercial name of the model pollutant dye.

bonding. The nucleobase pairs in double stranded DNA have a number of nitrogen containing groups that can build additional H-bonds with the surrounding media.34 As reported earlier, Hbonding interaction and surface interaction with nitrogen containing groups can enhance the emission property of the carbon nanoparticles.47−50 In order to further confirm the interaction with nucleobases, absorbance spectra of DNAs in the presence of CNP2 were recorded. The absorbance band of the E. coli-DNA around 260 nm exhibited a strong hyperchromicity effect in the presence of CNP2 (Figure 8c). The observed hyperchromic effect was higher for E. coli-DNA compared to the CT-DNA (Figure 8d). Generally, the hyperchromic effect is attributed to the structural decomposition of ds DNA to ss DNA from the more exposed nucleobases. Here, the hyperchromic effect could be attributed to the exposed nucleobases resulting from the selective extended H-bonding interaction with CNP2. The Raman intensity of both D band and G bands in the presence of CTand E. coli-DNA also confirmed the selective interaction of carbon nanoparticles toward E. coli-DNA (Figure S7). To obtain further evidence on H-bonding interaction, XPS spectra of CNP2 were recorded in the presence of E. coli-DNA. The deconvoluted XPS spectra of both C 1s and O 1s in the presence of DNA are presented in Figure 9. The CC, C OC and CO bonding of the deconvoluted C 1s spectrum

were observed at 284.5, 286.17 and 288.7 eV, respectively. From Figure 9b, it can be seen that the nature of the O 1s spectrum becomes more asymmetric in the presence of DNA and consists of five peaks. The binding energy of the O1s spectrum corresponds to the CO, COC and COH groups observed at 529, 532.27 and 533.26 eV, respectively. The observed binding energy values are little shifted compare to the bare CNP2 (Figure 9c,d). The observed kind of shift in the XPS binding energies also specifies the involvement of Hbonding interaction with chemical groups present on the surface of CNP2.51 Finally, we also used these CNPs as a sorbent material to study the adsorption property of the prepared CNPs toward pollutant dye molecules. To monitor the dye removal capability of CNPs, 5 mg of the nanoparticles was added to 20 mL of 10−5 M aqueous solution of methyl violet (MV) and methylene blue (MB) dyes in the dark. We chose the cationic dye, MV and MB, in anticipation of the electrostatic interaction of the dye molecules with the functional groups present on the surface of CNPs. Figure 10a,b shows respective absorption maximum at 578 nm of MV and 664 nm of MB were explicitly monitored during the controlled degradation experiments presented here (Figure 10a,b) with an effective treatment of CNP1, CNP2 and CNP3. The adsorption of the dye using CNP1 was fast, probably due to the porous nature of J



Research Article

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CONCLUSION In summary, we have designed a facile catalyst free green technique at room temperature to prepare multifunctional carbon nanoparticles (CNPs) from a highly cost-effective starting material. The essence of this process is the green synthesis adopted for preparing hydrophilic multiband emitting conductive CNPs that could be explored for multifarious applications. We project the CNPs produced by this process as an emerging material for the selective detection of E. coli-DNA. On the basis of thermal melting, CD and calorimetric studies, we could conclude that the interactions of CNPs are stronger with E. coli-DNA resulting in preferential selectivity. To our understanding, this report on selective detection of E. coli-DNA using carbon could find potential application in the direct detection of E. coli bacteria in water for environmental monitering. Our results are different and more interesting compared to the earlier reports on fluorescence quenching effects of tagged graphene, CNT and carbon nanoparticles in the presence of DNA. Future work will focus on utilizing the full potential of these nanomaterials for other sensing and supercapacitor applications. In addition, the synthesized CNPs exhibited excellent adsorption property and their efficiency further support future application for selective removal of cationic pollutant dyes in water remediation.

the particles compared to others. In order to confirm the selective cationic dye adsorption property of the CNPs, we also performed the dye adsorption experiments with methyl orange (MO), an anionic dye and interestingly. No change was observed for MO absorbance after treatment with the CNP1, CNP2 and CNP3 samples (Figure 10c). This experiment strongly suggests the adsorption behavior originated from the electrostatic interaction of negatively charged CNPs and cationic dyes. The dye removal efficiency of CNP1 was calculated as ∼99% for both the MB and MV dyes in 90 min. In contrast, there was insignificant change observed for MO dye, even after 90 min, which indicates selective absorption property of CNP1 toward cationic dye, as shown in Figure 10d. The digital pictures of the MV, MB and MO dye after treatment with CNP1 are shown in inset of Figure 10d at particular time intervals. From the digital images, it can be also determined the dye adsorption is fast in the cases of MB and MV dye whereas for MO it does not make any impact even after 90 min of adsorption. However, the overall absorption performances of the prepared samples are quite considerable among the reported performances of such kind of carbon material as summarized in Table 1. It is worth mentioning that all the CNPs exhibited insignificant loss in absorption efficiency even after recycling for five times, which highlights the effective reusability of the samples as photocatalysts (Figure 10e). Also, a desorption study was conducted for the MV adsorbed CNP1 (where adsorption study was conducted at pH 6.5) using phosphate buffer of pH ∼ 7.5, keeping the other experimental conditions constant. A 20 mL aliquot of each buffer solution was used for desorption of MV dye from MV adsorbed CNP (∼5 mg), and the experiments were carried out on an incubator shaker with a shaking speed of 180 rpm for 4 h in the dark. After the desorption processes, the adsorbents were completely separated by centrifugation at 10,000 rpm followed by washing two times using ethanol with 0.1 M NaOH (1 wt %) and the absorbance spectra of the supernatant were instantly checked by UV−Vis spectrophotometer. The MV dye desorption study also suggests an almost equal amount of adsorbed dye was removed during the desorption process, and the process shows excellent durability of the CNP1 up to 5 cycles for MV dye adsorption (Figure 10f). We could also demonstrate that these CNPs could make conducting papers as shown in Figure S8a where a filter paper coated with the CNP clearly shows a surface resistance of 278.73 kΩ. The water contact angle measured on this surface was 48.9° (shown in Figure S8b), which further confirm the hydrophilic nature of the synthesized graphitic carbon. Because the water dispersion of CNP is negatively charged, the CNPs may not get bound to the negatively charged phosphate backbone of the DNA. Instead, they may bind through the grooves by hydrogen bonding interaction and π−π interactions. Because the guanine−cytosine (GC) mol percentage of CT-DNA is 42% and that in E. coli-DNA is 50%,68 it is likely that H-bonding interactions with the amino groups are more favored in the E. coli-DNA resulting in a stronger interaction and enhancement of the blue emission leading to selectivity for E. coli-DNA. Our results are contrary to the previous reports on fluorescenec quenching effects of graphene, CNT and carbon nanoparticles in the presence of DNA and a fluorophore. 26,69 On the basis of these observations, we could confirm that this proof of concept could be used for selctive detection of E. coli-DNA.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b01443.

■

Lamp used for the synthesis; deconvoluted 2D band of the CNP samples; UV−Vis spectra of the CNP samples in water, IR spectra of the CNP samples and the ζpotential value of the CNP samples; hydrodynamic radius of the synthesized carbon nanoparticles; fluorescence spectra of carbon nanoparticles in water and DMSO after heated at 450 °C; ITC profiles of the binding of CNP with DNA; Raman spectra of carbon nanoparticles in the presence of E. coli-DNA and CTDNA; CNP2 particles on MF-Millipore membrane filter paper exhibiting resistance on a multimeter and the contact angle measured on the Millipore filter paper (PDF)

AUTHOR INFORMATION

Corresponding Author

*P. S. Devi. Tel.: +91-33-2483-8082, Fax: +91 33 2473 0957, Email: [email protected], [email protected]. ORCID

Sabyasachi Chatterjee: 0000-0001-8507-7696 Arindam Saha: 0000-0002-0207-7971 Parukuttyamma Sujatha Devi: 0000-0002-6224-7821 Gopinatha Suresh Kumar: 0000-0002-3596-979X Present Address §

Crystallography Lab, Department of Earth System Sciences, Yonsei University, Yonseiro 50, Seoul 03722, Korea (P.P.D.) Notes

The authors declare no competing financial interest. K


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ACKNOWLEDGMENTS P.S.D. acknowledges Council of Scientific and Industrial Research for support through the network program on MULTIFUN CSC0101. S.P., S.C., and A.R. acknowledge CSIR, UGC and DST, respectively for awarding Senior Research Fellowship.

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Multiband Fluorescent Graphitic Carbon Nanoparticles from Queen of

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