Nanoparticles (NPs) exist in different forms. NPs are tiny materials having size ranges from 1 to 100 nm. They can be classified into different classes based on their properties, shapes or sizes. The different groups include fullerenes, metal NPs, ceramic NPs, and polymeric NPs. NPs possess unique physical and chemical properties due to their high surface area and nanoscale size. Their optical properties are reported to be dependent on the size, which imparts different colors due to absorption in the visible region. Their reactivity, toughness and other properties are also dependent on their unique size, shape and structure. Due to these characteristics, they are suitable candidates for various commercial and domestic applications, which include catalysis, imaging, medical applications, energy-based research, and environmental applications. Heavy metal NPs of lead, mercury and tin are reported to be so rigid and stable that their degradation is not easily achievable, which can lead to many environmental toxicities.

Nanotechnology is a known field of research since last century. Since “nanotechnology” was presented by Nobel laureate Richard P. Feynman during his well famous 1959 lecture “There’s Plenty of Room at the Bottom” (Feynman, 1960), there have been made various revolutionary developments in the field of nanotechnology. Nanotechnology produced materials of various types at nanoscale level. Nanoparticles (NPs) are wide class of materials that include particulate substances, which have one dimension less than 100 nm at least (Laurent et al., 2010). Depending on the overall shape these materials can be 0D, 1D, 2D or 3D (Tiwari et al., 2012). The importance of these materials realized when researchers found that size can influence the physiochemical properties of a substance e.g. the optical properties. A 20-nm gold (Au), platinum (Pt), silver (Ag), and palladium (Pd) NPs have characteristic wine red color, yellowish gray, black and dark black colors, respectively. Fig. 1 shows an example of this illustration, in which Au NPs synthesized with different sizes. These NPs showed characteristic colors and properties with the variation of size and shape, which can be utilized in bioimaging applications (Dreaden et al., 2012). As Fig. 1 indicates, the color of the solution changes due to variation in aspect ratio, nanoshell thickness and % gold concentration. The alteration of any of the above discussed factor influences the absorption properties of the NPs and hence different absorption colors are observed.

NPs are not simple molecules itself and therefore composed of three layers i.e. (a) The surface layer, which may be functionalized with a variety of small molecules, metal ions, surfactants and polymers. (b) The shell layer, which is chemically different material from the core in all aspects, and (c) The core, which is essentially the central portion of the NP and usually refers the NP itself (Shin et al., 2016). Owing to such exceptional characteristics, these materials got immense interest of researchers in multidisciplinary fields. Fig. 2 shows scanning electron microscopy (SEM) and transmittance electron microscope (TEM) images of mesoporous and nonporous methacrylate-functionalized silica (MA-SiO2).

NPs are broadly divided into various categories depending on their morphology, size and chemical properties. Based on physical and chemical characteristics, some of the well-known classes of NPs are given as below.

Carbon-based NPs-Fullerenes and carbon nanotubes (CNTs) represent two major classes of carbon-based NPs. Fullerenes contain nanomaterial that are made of globular hollow cage such as allotropic forms of carbon. They have created noteworthy commercial interest due to their electrical conductivity, high strength, structure, electron affinity, and versatility (Astefanei et al., 2015). These materials possess arranged pentagonal and hexagonal carbon units, while each carbon is sp2 hybridized. Fig. 3 shows some of the well-known fullerenes consisting of C60 and C70 with the diameter of 7.114 and 7.648 nm, respectively.

CNTs are elongated, tubular structure, 1–2 nm in diameter (Ibrahim, 2013). These can be predicted as metallic or semiconducting reliant on their diameter telicity (Aqel et al., 2012). These are structurally resembling to graphite sheet rolling upon itself (Fig. 4). The rolled sheets can be single, double or many walls and therefore they named as single-walled (SWNTs), double-walled (DWNTs) or multi-walled carbon nanotubes (MWNTs), respectively. They are widely synthesized by deposition of carbon precursors especially the atomic carbons, vaporized from graphite by laser or by electric arc on to metal particles. Lately, they have been synthesized via chemical vapor deposition (CVD) technique.

Metal NPs are purely made of the metals precursors. Due to well-known localized surface plasmon resonance (LSPR) characteristics, these NPs possess unique optoelectrical properties. NPs of the alkali and noble metals i.e. Cu, Ag and Au have a broad absorption band in the visible zone of the electromagnetic solar spectrum. The facet, size and shape controlled synthesis of metal NPs is important in present day cutting-edge materials (Dreaden et al., 2012). Due to their advanced optical properties, metal NPs find applications in many research areas. Gold NPs coating is widely used for the sampling of SEM, to enhance the electronic stream, which helps in obtaining high quality SEM.

Ceramics NPs are inorganic nonmetallic solids, synthesized via heat and successive cooling. They can be found in amorphous, polycrystalline, dense, porous or hollow forms (Sigmund et al., 2006). Therefore, these NPs are getting great attention of researchers due to their use in applications such as catalysis, photocatalysis, photodegradation of dyes.

Semiconductor materials possess properties between metals and nonmetals and therefore they found various applications in the literature due to this property (Ali et al., 2017, Khan et al., 2017a). Semiconductor NPs possess wide bandgaps and therefore showed significant alteration in their properties with bandgap tuning. Therefore, they are very important materials in photocatalysis, photo optics and electronic devices (Sun, 2000). As an example, variety of semiconductor NPs are found exceptionally efficient in water splitting applications, due to their suitable bandgap and bandedge positions.

NPs and in the literature a special term polymer nanoparticle (PNP) collective used for it. They are mostly nanospheres or nanocapsular shaped (Mansha et al., 2017). The former are matrix particles whose overall mass is generally solid and the other molecules are adsorbed at the outer boundary of the spherical surface. In the latter case the solid mass is encapsulated within the particle completely.

NPs contain lipid moieties and effectively using in many biomedical applications. Generally, a lipid NP is characteristically spherical with diameter ranging from 10 to 1000 nm. Like polymeric NPs, lipid NPs possess a solid core made of lipid and a matrix contains soluble lipophilic molecules. Surfactants or emulsifiers stabilized the external core of these NPs.

One study revealed the spherical magnetite NPs synthesis from natural iron oxide (Fe2O3) ore by top-down destructive approach with a particle size varies from ∼20 to ∼50 nm in the presence of organic oleic acid (Priyadarshana et al., 2015). A simple top-down route was employed to synthesize colloidal carbon spherical particles with control size. The synthesis technique was based on the continuous chemical adsorption of polyoxometalates (POM) on the carbon interfacial surface. Adsorption made the carbon black aggregates into relatively smaller spherical particles, with high dispersion capacity and narrow size distribution.

NPs are formed from relatively simpler substances, therefore this approach is also called building up approach. Examples of this case are sedimentation and reduction techniques. It includes sol gel, green synthesis, spinning, and biochemical synthesis. (Iravani, 2011). Mogilevsky et al. synthesized TiO2 anatase NPs with graphene domains through this technique (Mogilevsky et al., 2014). They used alizarin and titanium isopropoxide precursors to synthesize the photoactive composite for photocatalytic degradation of methylene blue. Alizarin was selected as it offers strong binding capacity with TiO2 through their axial hydroxyl terminal groups. The anatase form was confirmed by XRD pattern. The SEM images taken for different samples with reaction scheme are provided in scheme 2. SEM indicates that with temperature elevation, the size of NPs also increases.

Different characterization techniques have been practiced for the analysis of various physicochemical properties of NPs. These include techniques such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), infrared (IR), SEM, TEM, Brunauer–Emmett–Teller (BET), and particle size analysis.

Morphological features of NPs always attain great interest since morphology always influences most of the properties of the NPs. There are different characterization techniques for morphological studies, but microscopic techniques such as polarized optical microscopy (POM), SEM and TEM are the most important of these.

SEM technique is based on electron scanning principle, and it provides all available information about the NPs at nanoscale level. Wide literature is available, where people used this technique to study not only the morphology of their nanomaterials, but also the dispersion of NPs in the bulk or matrix. The dispersion of SWNTs in the polymer matrix poly(butylene) terephthalate (PBT) and nylon-6 revealed through this technique (Saeed and Khan, 2014, Saeed and Khan, 2016). The same group also provides POM study of their materials, which showed star-like spherulites of the formed materials, whose size was decreased with the incremental filling of SWNTs. The morphological features of ZnO modified metal organic frameworks (MOFs) were studied through SEM technique, which indicates the ZnO NPs dispersion and morphologies of MOFs at different reaction conditions.

Different techniques can be used to estimate the size of the NPs. These include SEM, TEM, XRD, AFM, and dynamic light scattering (DLS). SEM, TEM, XRD and AFM can give better idea about the particle size (Kestens et al., 2016), but the zeta potential size analyzer/DLS can be used to find the NPs size at extremely low level. In one study Sikora et al. used DLS technique to investigate the size variation of silica NPs with absorption of proteins from serum. The results showed that size increased with acquisition of protein layer. However, in case of agglomeration and hydrophilicity, DLS might prove incapable of accurate measurement, so in that case we should rely on the high-resolution technique of differential centrifugal sedimentation (DCS) (Sikora et al., 2016). Beside DSC, nanoparticle tracking analysis (NTA) is relatively newer and special technique, which can be helpful in case of biological systems such as proteins, and DNA. In NTA method, we can visualize and analyze the NPs in liquids media that relates the Brownian motion rate to particle size. This technique allows us to find the size distribution profile of NPs with diameter ranging from 10 to 1000 nm in a liquid medium (Filipe et al., 2010). This technique produced some good results as compared to DLS and found to be very precise for sizing monodisperse as well as polydisperse samples, with substantially better peak resolution. Gross et al. detected the particle size and concentration of different sized NPs in suspensions of polymer and protein samples and provided an overview on the effect of experimental and data evaluation parameters (Gross et al., 2016).

Large surface area of nanomaterials offers great room for various applications and BET is the best technique to determine the surface area of NPs materials. This technique is based on adsorption and desorption principle and Brunauer–Emmett–Teller (BET) theorem. Normally nitrogen gas is used for this purpose. BET produces four types of isotherm specifically, which are labeled as Type-I, Type-II, Type-III and Type-IV (Fagerlund, 1973). The fresh 7Cu3Ce/ZSM-5 showed typical features of Type-I isotherm obtain from nitrogen adsorption/desorption. It was discovered that N2 adsorption volume is progressively increased with relative pressure until certain limit signifying the availability of pores. The BET specific surface area for this material was 133–144 m2/g, while the total pore volume was 0.112–0.185 cm3/g. But after sulphidation process, the BET surface area reduced to 110 m2/mg and the pore volume decreased to 0.096 cm3/g, respectively (Liu et al., 2016).

Optical properties are of great concerned in photocatalytic applications and therefore, photo-chemists acquired good knowledge of this technique to reveal the mechanism of their photochemical processes. These characterizations are based on the famous beer-lambert law and basic light principles.

The optical and electronic properties of NPs are inter-dependent to greater extent. For instance, noble metals NPs have size dependent optical properties and exhibit a strong UV–visible extinction band that is not present in the spectrum of the bulk metal. This excitation band results when the incident photon frequency is constant with the collective excitation of the conduction electrons and is known as the localized surface plasma resonance (LSPR). LSPR excitation results in the wavelength selection absorption with extremely large molar excitation coefficient resonance Ray light scattering with efficiency equivalent to that of ten fluorophores and enhanced local electromagnetic fields near the surface of NPs that enhanced spectroscopies. It is well established that the peak wavelength of the LSPR spectrum is dependent upon the size, shape and interparticle spacing of the NPs as well as its own dielectric properties and those of its local environment including the substrate, solvents and adsorbates.

Magnetic NPs are of great curiosity for investigators from an eclectic range of disciplines, which include heterogenous and homogenous catalysis, biomedicine, magnetic fluids, data storage magnetic resonance imaging (MRI), and environmental remediation such as water decontamination. The literature revealed that NPs perform best when the size is <critical value i.e. 10–20 nm (Reiss and Hütten, 2005). At such low scale the magnetic properties of NPs.

NPs have thermal conductivities higher than those of fluids in solid form. For example, the thermal conductivity of copper at room temperature is about 700 times greater than that of water and about 3000 times greater than that of engine oil. Even oxides such as alumina (Al2O3) have thermal conductivity higher than that of water. Therefore, the fluids containing suspended solid particles are expected to display significantly enhanced thermal conductivities relative to those of conventional heat transfer fluids. Nanofluids are produced by dispersing the nanometric scales solid particles into liquid such as water, ethylene glycol or oils. Nanofluids are expected to exhibit superior properties relative to those of conventional heat transfer fluids and fluids containing microscopic sized particles.

Nanocrystalline materials provide very interesting substances for material science since their properties deviate from respective bulk material in a size dependent manner. Manufacture NPs display physicochemical characteristics that induce unique electrical, mechanical, optical and imaging properties that are extremely looked-for in certain applications within the medical, commercial, and ecological sectors.

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