Nano Crystals

[Amabella Batton]

Nanomaterialswhich have all dimensions in the nanoscale are classified as zero-dimensional. Most common 0D nanomaterials are nanoparticles. They offer more active edge sites per unit mass due to higher surface to volume ratio. Their improved properties such as high quantum efficiency and chemiluminescence are attributed to their quantum confinement effects (Pirzada and Altintas, 2019a).

Zero dimensional structures based on carbon have been studied extensively for their excellent electrical and optical properties, low production cost, low toxicity, and their ability to undergo functionalization. These properties cater to many applications such as bioimaging, sensing, drug delivery and electrochemical devices (Zhou et al., 2019). The following section enumerates the most widely studied carbon-based 0D nanostructures.

Carbon quantum dots or sometimes called carbon dots (CD) were first obtained as a product of impure carbon soot during the arc discharge synthesis of carbon nanotubes, in 2004 by Xu et al. (Xu et al., 2004) and were named CDs by Sun et al. in 2006, a name given to fluorescent carbon nanomaterials (Sun et al., 2006). These carbon-based nanomaterials are smaller than 10 nm and are quasi- spherical. They may be amorphous or nanocrystalline composed of graphitic or turbostratic carbon (sp2 carbon) or graphene and graphene oxide sheets that are joined by sp3hybridized carbon (Demchenko and Dekaliuk, 2013). As a result of low proportion of crystalline sp2 carbon and higher surface defects, CDs have low crystallinity compared to GQDs (Pirzada and Altintas, 2019b). CDs are endowed with excellent properties in fluorescence, electrochemiluminescence (ECL) and chemiluminescence (CL) which make them useful in bioimaging, biosensing, and drug delivery (Atabaev, 2018). In addition, they also show good water solubility, high chemical stability, photobleaching resistance and large-scale production (Yang et al., 2014). The presence of carboxyl moieties at their surface enhances the solubility of CDs in aqueous and non-aqueous solvents, and provides scope for surface passivation and functionalization using organic, inorganic or polymeric reactive groups (Lim et al., 2015). As a result of these outstanding properties, the applications of CDs have been immensely popular in fields such as biology, chemical sensing, nanomedicine, and photoelectrocatalysis (Yuan et al., 2016).

Metal nanoparticles encapsulated inside carbon shells is a new class of 0D carbon nanomaterials that forms core-shell nanostructures. The shell is composed of multilayer-graphitized carbon which protects the polyhedral metallic core from the external environment, corrosion, and magnetic coupling between individual particles (Mostofizadeh et al., 2011). (Ruoff et al., 1993) and (Tomita et al., 1993) published the first report on polyhedral carbon encapsulated LaC2 and since then several research groups have been involved in the encapsulation of different materials in a hollow graphitic cage using arc discharge method. Similar research was carried out by Saito in 1995 wherein 13 rare earth metals and iron group metals were encapsulated in graphitic carbon (Popov, 2004). The synthesis of nanocrystals of nickel and cobalt, coated with carbon, using tungsten arc technique, was reported by Host et al. (1998).

Nanoscale diamonds or nanodiamonds are cubic structural diamonds which possess the structure and properties of diamond with the average size of 5 nm diameter (Fig. 2). Broadly, they are composed of a variety of diamond-based nanoscale materials with size smaller than 100 nm, such as pure phase diamond films, diamond particles, and their structural assemblies. There are two main methods for the synthesis on NDs: transformation of graphite under high temperature and high pressure and detonation of carbon explosive materials (Mostofizadeh et al., 2011). The method of detonation for synthesis of nanoscale diamonds was first discovered in 1963 in USSR (Danilenko, 2004) but remained unknown to the world until the late 1980s when they became commercially available in the USA (Mochalin et al., 2012). In addition to these methods, an array of nanodiamonds have also been synthesized by laser ablation (Yang et al., 1998), plasma-assisted chemical vapor deposition (CVD) (Frenklach et al., 1991), autoclave synthesis from supercritical fluids (Gogotsi et al., 1996), chlorination of carbides (Welz et al., 2003), ion irradiation of graphite (Daulton et al., 2001) and ultrasound cavitation (Galimov et al., 2004). Nanodiamonds have remarkable properties such as superior hardness and Youngs modulus, high thermal conductivity, chemical stability, and resistance to harsh environments (Mochalin et al., 2012). Their biocompatibility and optical properties such as fluorescence, act as incentives for their use in biomedicine including nanoscale magnetic resonance imaging cancer therapy (Passeri et al., 2015), orthopedic engineering (Suliman et al., 2015), synthesis of contact lenses (Kim et al., 2014) and microscopy or image diagnosis (Pham et al., 2017). The ability to functionalize the surface shell by functional groups such as carboxyl, hydroxyl (Passeri et al., 2015) or biomolecules such as lysosomes (Perevedentseva et al., 2011), allows NDs to be used for the delivery of a number of drugs, antigens and antibodies (Pham et al., 2017).

Three to eight closed graphitic shell structures combine together with a hollow core to form an onion-like structure, known as onion-like carbons (OLCs) (Fig. 2). The outer diameters vary from 20 to 100 nm (Mostofizadeh et al., 2011). OLCs have been shown to be formed during the annealing of nanodiamonds along with the formation of intermediates, particles with a diamond core covered by graphitic shells (graphite diamond nanocomposites) (Kuznetsov et al., 1994). Bulusheva and group, in 2007, using X ray absorption spectroscopy, were the first ones to characterize and provide the electronic structure of OLCs. They also succeeded in vacuum annealing nanodiamond particles to synthesize quasispherical and polyhedral OLCs (Bulusheva et al., 2007).

Quantum dots are semiconductor crystals made from atoms in group II-VI or III-V in the periodic table. They were first synthesized in the 1980s (Ekimov and Onushchenko, 1981) and since then have been used extensively for a number of applications such as optoelectronic devices and for bio sensing techniques. Examples of QDs include CdTe, CdSe, and InP, with applicability in various bio medicine fields including bioimaging, biosensing, and therapy (Wegner and Hildebrandt, 2015). QDs suspended in solution show improved biocompatibility, water solubility and stability and have been applied in in vivo and in vitro imaging, labelling, and sensing techniques. The cytoxicity of commercial CdTe and CdSe QDs has been shown to be overcome by the introduction of heavy metal-free QDs such as Si QDs (Keshavarz et al., 2018). The interesting optical properties of QDs such as enhanced brightness, photobleaching resistivity, size tunable light emission, and large absorption coefficient, make them capable for the development of bioluminescence and chemiluminescence sensors (Ma et al., 2019).

Ways have been developed to chemically stabilize naked MNPs to protect them against degradation during or after synthesis and to prevent their oxidation in air and subsequent loss of magnetism. Functionalized NPs obtained may be useful for applications in catalysis, biolabeling, and bioseparation (Lu et al., 2007). This process may be carried out using two approaches, (i) attachment of MNPs to antibodies, proteins, and dyes, (ii) integrating MNPs with functional nanoparticles, such as metallic nanoparticles or quantum dots (Euliss et al., 2003).

Nanomaterials with two nanoscale dimensions and with the third dimension in the microscale, are said to be 1D nanostructures. They include structures such as nanofibers, nanotubes, nanowires, and nanorods (Fig. 3).

Carbon nanofibers are made of graphene layers that could be stacked or curved, from a quasi-1D filament. They are cylindrical or conical with diameters ranging from 1 to 100 nm and lengths less than a micrometer to millimeters. The angle between the graphene layers and the fiber axis decides the morphological structure and the nanofibers could be either plate CNFs, ribbon like CNFs, or herringbone CNFs (Huang et al., 2010). Several methods exist for the synthesis of CNFs, which are, traditional vapor growth method (Tan et al., 2005), cocatalyst deoxidation process (Tan et al., 2005), catalytic combustion (Tan and Lim, 2006), plasma enhanced chemical vapor deposition (Tanaka et al., 2004), hot filament assisted sputtering (Bezemer et al., 2006), ultrasonic spray pyrolysis (Takenaka et al., 2004)and ion beam irradiation (Tu et al., 2003). Since the discovery of the first carbon fiber in 1879 by Thomas Edison, CNFs have been applied in fields like energy storage and conversion, composites reinforcement and sensing devices (Wu et al., 2004).

They have the highest aspect ratio with length to width ratio >=1000 which allows the lateral confinement of electrons. Nanowires are typically single-crystalline, have high anisotropy, and could be semiconducting, insulating and/or metallic. They have uniform cross section which could be cylindrical, hexagonal, square, or triangular. Control over the growth conditions and understanding of their growth mechanism allows them to be applied in different compositions in a wide range of devices, such as field effect transistors, light-emitting diodes, bipolar junction transistors, nanoscale lasers. Their applicability has also extended to gas sensors, nanoresonators and nanogenerators.

Metal NPs are composed of pure metal precursors. They show unique optoelectronic properties owing to the characteristic localized surface plasmon resonance. Alkali and noble metal NPs such as Cu, Au, and Ag show broad absorption peaks in the visible regime. Ag NPs have found usage in various medical and optoelectronic applications (Caldern-Jimnez et al., 2017). Gold is widely used for drug delivery and cancer detection (Paciotti et al., 2004). Copper is known for its use as catalysts (Gawande et al., 2016), electrical and thermal conductors, sintering and lubricant additive and as antibacterial agents (Yoon et al., 2007). Alloys of aluminum-magnesium and titanium-aluminum being very light and strong are used in aerospace and for high-temperature applications (Morris, 2010) while alloys of iron-silicon-boron owing to their excellent magnetic properties are utilized for magnetic application (Yardley et al., 2007).

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