portion of C60, C70, and other fullerenes type nanocarbons, which are found to be appropriate for the growth of SWCNT as a carbon precursor when used in laser decomposition method. The diameters of as synthesized SWCNT by waste DPM lie majorly in the range of 1 nm.
and (c) effect of pH on fluorescence intensity of CNPs, ULCNPs intensity are shown by black line and LLCNPs intensities are shown by red lines [31].
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CDs isolated from DPM are hydrophobic in nature and larger in size, so inapt for biological applications. Tripathi et al., [30] adapted oxidative strategy for water solubilization of CDs. Oxidation of CDs in nitric acid leads to surface modification of CDs in terms of the incorporation of high density, surface functionalization by carboxylic and hydroxyl-type negative group (confirmed by the high-negative value of zeta potential –30.15 mV) [79], as
“surface defects.” Structure morphology and internal surface characterization of wsCDs was carried out with SEM and TEM techniques. It has been reported that acid treatment cuts the particles in the smaller size and incorporates water solubility [74, 80]. As separated CDs from DPM contains some amorphous carbon as impurity (Figure 5.2b), while, after oxidation,
wsCDs are homogenous in nature without showing any impurity (Figure 5.7a). wsCDs are ~10–30 nm in diameter, while CDs are ~60–100 nm in diameter (Figures 5.2b and 5.7d). TEM image reveals spherical morphology (Figure 5.7b), and HRTEM shows the presence of surface defects (Figure 5.7c). Surface functionalization of wsCDs was corroborated by using FTIR spectroscopy shows the presence of C–O (~1720 cm–1), C=O (~1720 cm–1), –OH (~3425 cm–1) stretching peaks related to carbonyl groups as shown in Figure 5.7e, whereas the varying degree of carbon nature (sp2 and sp3) in wsCDs was confirmed by Raman spectra (Figure 5.7f).
Figure 5.7 (a) SEM image of wsCDs after oxidative treatment, (b) low- resolution TEM image of wsCDs, (c) high-resolution TEM image of wsCDs showing the curvature and graphitic layers, (d) size distribution histogram of wsCDs after oxidative treatment, (e) FTIR spectrum, (f) Raman spectra (excited with 532 nm laser at room temperature) [30].
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The presence of heavy density surface defects promotes the formation of surface energy traps of different energy gaps, responsible for multicolor tunable fluorescent emissions. wsCDs exhibit light green color under UV
light excitation (Figure 5.8a). The optical behavior of wsCDs studied at different excitation wavelengths from 400 to 600 nm (Figure 5.8b). More significantly, fluorescent emissions of wsCNDs extended up to NIR region as shown in Figure 5.8c. Emissions in NIR window are assumed to be very important especially for bio-imaging purposes due to its deeper penetration ability and lack of autofluorescence in this region [50]. wsCDs are resistant to photo-bleaching confirmed by continuous excitation at 445 nm for five hours (Figure 5.8d). Green and red fluorescence images of aqueous solution of wsCDs after evaporation on a glass plate are shown in Figure 5.8e and f, respectively. We applied wsCDs for detection of cholesterol and cell imaging of E. coli. Ion pair conjugation of wsCDs-Methylene blue (MB) was utilized for the detection of cholesterol based upon fluorescence “turn off”/“turn on”
technique. The present method for the one-step isolation of wsCDs from the waste soot and further application for multicolored imaging purposes especially for biological imaging and sensing of biomolecules based upon fluorescent “turn on”/“turn off” is very simple, convenient, reproducible, and realistic approach for the high yield utilization of DPM.
Figure 5.8 Optical properties of wsCDs: (a) digital photograph of aqueous solution of wsCDs under daylight and UV light; (b) emission spectra of aqueous solution of wsCDs recorded at 20 nm progressive increment of excitation wavelength from 400 to 600 nm; (c) NIR emission profile of wsCDs, zoomed image of (b); (d) photostability tests of wsCDs at 440 nm excitation wavelength continuous for 5 h; (e and f) fluorescence images of the wsCDs on glass slides after evaporating a very dilute solution, under the band-pass filters (e) 488 nm and (f) 562 nm [30].
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Tian et al. synthesized fluorescent wsCNPs from the combustion of natural gas soot [81]. They collected the indoor BC generated routinely in our daily practice in an invert glass beaker placing upside of a natural gas burner. As obtained, CNPs were thermally refluxed in nitric acid and purified by dialysis, to achieve its fluorescent water-soluble version. Surface morphology and crystalline nature of fluorescent wsCNPs were characterized by TEM showing spherical and homogeneous size distribution ranged from 4.4 to 5.4 nm (Figure 5.9a) with its HRTEM image shown in Figure 5.9b illustrated the lattice plane of graphitic carbon with d-spacing equal to 0.208 nm. As well, wsCNPs with tunable fluorescent emissions were also obtained from candle soot [82].
Figure 5.9 (a) TEM micrograph of wsCNPs inset shows the size distribution histogram; (b) HRTEM image showing the interplanar distance of lattice planes [81].
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To explore more about the applications, they further synthesized nanocomposites of wsCNPs with metal by using metal salts [81]. In the presence of CNPs, metal salts were reduced to metal nanoparticles with ascorbic acid. They proposed that metal ions bound with surfacial carboxylic acid groups reduce to metal atoms on the addition of reducing agent. Which act as a nucleation core for the growth of metal nanoparticles. Composite formation between CNPs and metal nanoparticles was characterized by increase in size in TEM micrographs. TEM images of CNPs with three different metals Ag, Cu, and Pd along with their size distribution histogram are shown in Figure 5.10a–c, respectively, and their corresponding HRTEM images are shown in Figure 5.10d–f. As illustrated in Figure 5.10 insets, the average diameters of the CNPs-metal nanocomposites are 16.4 nm, 18.6 nm, and 20.4 nm, for Ag, Cu, and Pd, respectively.
Figure 5.10 Representative TEM micrographs of carbon nanoparticles
functionalized with different metal nanostructures: panels (a) and (d), silver;
panels (b) and (e), copper; and panels (c) and (f), palladium. The crystalline lattices were identified in the respective HRTEM images. The central insets are the corresponding histograms of the overall particle size distribution. The upper-right inset to panel (f) depicts the elemental mapping of a palladium nanoparticle. Scale bars are 20 nm in the left panels and 2 nm in the right ones [81].
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The fluorescent emissions of metal-CNPs nanocomposites did not exhibit any significant changes in comparison to wsCNPs and exhibited distinct photoluminescence properties (Figure 5.11a). They proposed that photoluminescence arises from the electronic transitions between surface energy traps of different energy states. More significantly, wsCNPs exhibit electrochemical properties and show two peaks –0.28 V (Peak I) and –0.06 V (Peak II) in the scan range of –0.1 to +0.1 V as shown in Figure 5.11b.
Electrochemical properties are pH dependent and on decreasing sweep rate splitting of anodic peak merge while cathodic peaks remain defined and well separated. Electrochemical measurement results are similar to the phenanthrenequinone derivatives, hence suggest the functionalization of CNPs surface with similar analogous.
Figure 5.11 (a) Excitation and emission spectra of wsCNPs and their
composite with Ag, Cu, and Pd; (b) cyclic voltammograms of a glassy carbon electrode (3 mm in diameter) in a water solution containing 0.1 MKCl and 1.5 mg/mL carbon nanoparticles (solid curves) at varied potential sweep rates (depicted as figure legends). The voltammogram of the same electrode in a 0.1 MKCl water solution at 0.1 V/s was also included (dashed curve). Inset shows the pH dependence of the formal potentials of Peaks I and II [81].
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5.4 Nano-Carbons from Pollutant Soot for Wastewater Treatment
5.4.1 Removal of Organic Pollutant
In these days, wastewater treatment for the removal of contaminated potentially known as toxic organic pollutants (organic dyes) is a serious environmental concern. Contamination of artificial dye in water even at very low level of concentration causes several adverse environmental impacts