Xiao et al. reported a novel process of laser ablation in liquid (LAL) as a simple green top-down strategy for the synthesis of NDs from various coals like anthracite, bitumite, and coke.202 Coal is the most abundant and cheap energy source of carbon and is used for the eco-friendly synthesis of NDs. Anthracite coal consists of micro-sized particles with irregular sizes and shape distributions. While bitumite and coke coals are of similar shapes.

The energy dispersive X-ray spectroscopy (EDS) analysis of anthracite and coke are of purely carbon, while bitumite shows the additional presence of oxygen. The three samples were ablated with a Q-switched Nd:YAG laser device of laser-pulse energy of 200 mJ with a wavelength of 532 nm, having a pulse width of 10 ns, and possessing a repetition frequency of 10 Hz. The different laser irradiations involves a series of colloidal color changes from opaque grayish to dark reddish brown and finally to a transparent yellow, which indicates the formation of various types of products. During all these stages, amorphous carbon particles act as intermediate phase which gets finally transformed into NDs. The green synthesized cubic crystalline NDs show a uniform particle size with a mean size of about 3 nm. These monodisperse colloidal particles are devoid of agglomeration and show a stable bright green fluorescence. The selected area electron diffraction (SAED) pattern shows three rings which corresponds to the (111), (200), and (311) planes of diamonds. The (111) twinning plane of the diamond is often considered as a characteristic of cubic diamonds.

A low-power ultrasound stimulation of low-grade Indian coal mixed with H,02 in an ultrasonic processor (Sonapros; Model: PR-1000 M) at atmospheric pressure and frequency of 20 kHz for 3 h was reported.203 The filtrate after ultra sonication is neutralized and filtered. During the ultrasound cavitation, veiy high temperature of 5000 К and pressure of 2000 atm can be attained, which act as initiation point for the formation of NDs.204-205 Ultrasonicated filtrate containing ND phases show bright blue fluorescence under UY light (at 365 nm) due to size-induced quantum- confinement effect, which finds practical applications in biomedical imaging field. UV-vis. absorption bands appear at 250-350 nm and a shoulder appears around 300 nm due to the я—>я* transition of the aromatic я system and п-л* transition of C=0 bonds, respectively. The broad peak at 26.7° in XRD indicates that the NDs are embedded within an amorphous carbon matrix. ТЕМ analysis gives the planner spacing in the range of 2.0-2.3 A, which is in good agreement with the lattice planes of various diamond phases including cubic diamond and lonsdaleite. Two Raman peaks at 1600 and 1350 cm-1 at positions of G band and D band infer the presence of sp2 carbons in higher ratio in the product than the characteristic ND sp3 peaks.

Another novel method for the synthesis of nanaodiamonds from the carbon black precursor was reported.206 Carbon black is a quasi zero-dimensional structure with very small size. Carbon black is inexpensive and is an ideal source of carbon for the preparation of pure NDs. The procedure involves a long-pulse-width laser irradiation of carbon black in water suspension. The Nd:YAG pulsed laser with a power density of 9 x 106 Wcnr2, having a wavelength length of 1064 urn, frequency 20 Hz, a pulse width of 0.4ms, and an irradiation time of 4 h is used. The laser beam irradiation resulted in vaporization of the surface of carbon black along with a small amount of the surrounding liquid to form bubbles within the water suspension.207 These species within the bubbles are believed to be subjected to high temperature and pressure conditions, which resulted in the formation of high-pressure-phase structure of nanaodiamonds.208 The irradiated product is purified by boiling in perchloric acid and passivated by PEG,ooon. The purified product is heated for 70 h at a temperature of 120°C, is then cooled to room temperature and centrifuged. The SAED pattern of purified product shows rings corresponding to (111), (220), (311), (400), (331), (422), and (511) planes, which indicates diamond type structure. The ТЕМ analysis shows a crystalline diameter of 3.6 nm and an interplanar distance of 0.206 mn concur with (111) plane of cubic diamond. The Raman spectrum shows two intense peaks at 13 31 and 1611 cm"1. The former peak corresponds to the first order diamond Raman line which ratifies the presence of diamond particles. The peak at 1611 cm'1 was probably due to the paired threefold coordinated defects.

A novel, simple, and efficient extraction of NDs from carbonaceous waste deposits present on the roof of Hawaii Kund in Indian Temples was also reported.209 Metallic impurities of the raw materials were removed by treatment with a mixture of concentrated sulphuric acid (H,S04) and fuming nitric acid (HN03). After 3 h of heat treatment of the mixture at 250°C in air, deionised water was added to maintain the pH as 5 and was allowed for sedimentation at room temperature. The decanted phase was dried, crushed into powder, and was then treated with HNO,/H,0, mixture and heated at 150°C. The final product was washed with deionized water to remove any undesirable carbon from the raw material and the color of the sample changes to grayish black. EDS of extracted NDs show the presence of silicon, oxygen, and carbon. Raman spectrum shows a sharp and intense peak at 1332 cm'1 corresponds to high purity of ND core sp3 character. The ТЕМ image of NDs have a size range of about 4-5 nm. XRD peaks at 43.9° and 75.27° corresponds to (111) and (221) planes of sp3 diamond core, and a broad diffraction up to 26° corresponds to sp2 structure. The extracted NDs contain various functional groups on its surface which offers the binding of different biomolecules, which make it suitable for various applications.

Less expensive natural sources can be used as the green precursors for the cost effective synthesis of NDs. NDs are identified as less toxic than all other nanocarbons210-213 and they emerged as a novel platform for nanoscience and nanotechnology.214 Due to the nontoxicity, biological compatibility, and luminescent properties, NDs possess a wide range of applications in the fields of biomedical imaging, biology and medicine.210-213 They also shows considerable potential in the fields of photo- voltaics, microelectronics, optoelectronics and biosensing.211212 Due to the chemical resistance, hardness, and abrasive nature, they are also used in novel wear resistant polymers, metal coatings213-214 and lubricant additives.215 Green synthesized carbon NDs and then rapid functionalization owe wide applications in the current global trend.

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