By measuring the frequency of the scattered light fluctuation, the intensity of the Doppler shift can be determined by the use of a photon detector, depending on time and fluctuation. Fluctuations in light intensity are greatly affected by the viscosity of the solution and the temperature; hence, the particle size of dispersion can vary [91, 92–94].
Scanning electro-microscopy (SEM) and transmission electron microscopy (TEM) are the most needed tools to analyze morphological characteristics [95]. SEM and TEM characterization of nanoparticles provides shapes and surface facets, and it determines the accurate particle size of a particular particle in dispersion [96]. The morphological shape details can be taken from the SEM image for a large nanocrystal with a normal design. SEM works on the basis of scattered electrons and provides particulate morphology. The shape of a small nanocrystal cannot be analyzed by SEM because of its restricted resolution. Although TEM analysis is able to represent the morphology of such particles that are not suitable for SEM by passing electrons through in resultant, it differentiates the chemical entities on the basis of electron density [97].
Polarized light microscopy (PLM) is usually used to observed lyotropic liquid crystalline structures [98]. It can be used to confirm the presence of liposomes and similar structures in parenteral dispersion systems. The coordinated structure of the phospholipid surfaces results in anisotropical properties. Anisotropic systems deviate polarized light in the plane, which helps in imaging and gives a standard black and white or colored image using a λ-plate [98, 99].
SEM imaging only analyzes the samples in 2-D, that is, x- and y-axes, but cannot measure the z-axis to provide 3-D morphological information of a particle surface. Atomic force microscopy (AFM) overcomes this problem by analyzing the surface on the z-axis along with the x- and y-axes using deflection of a fine leaf spring. AFM is widely used for crystallinity studies of a sample. The technique uses fixed wavelengths to provide information on the molecular organization of crystals. In AFM imaging, with resolution up to 0.01 nm, the force between the surface of the material and the sensor tip is used [100]. Silicon wafers are usually used for sample fixation. The sample surface should be extra smooth for AFM imaging [101–106].
1.6.2 Surface Charge
Zeta potential is an essential characteristic of particles that can be easily calculated with a zetasizer tool [92]. It is an important aspect to understand particle stability. In drug delivery applications, the particle charge is an important factor. “Zeta potential is the potential differentiation between the surface and the stationary fluid layer connected to the dispersed sample.” The particle surface is surrounded by ions charged opposite to each other, which generates the thin stern layer adsorbing non-hydrated co-ions and counter ions at the surface. The next layer consists of the hydrated/partially hydrated counter ions, and the last layer consists of co-ions; this layer is known as the diffuse layer. This layer is attached to the slip plane, an imaginary plane that separates mobile ions from immovable ions on the surface. A stable dispersion has zeta potential ideally from -30 to +30 mV. Repulsion occurs between particles with a strong positive or negative surface charge; it results in reduced stability span of dispersion through flocculation or aggregation by particles [107]. The pH of the dispersion is the factor that mostly affects the zeta potential [108].
1.6.3 Thermal Analysis
To examine lipid and lipid carriers’ “thermal behavior,” a thermal gravimetrical analysis (TGA) and differential scanning calorimetry (DSC) are used. Melting point, crystallinity, and endothermal and exothermal characteristics of the sample are the main data generated by thermal analysis [109]. This technique involves samples being analyzed in unusual atmospheres consisting of nitrogen, oxygen, and argon warmed at a controlled heating rate. The thermal analysis can be easily predicted for phase transition, crystallization, and lipid sample amorphication by the enthalpy or entropy of the carrier system/sample, which involves change in free energy in phase transition during thermal analysis [109, 110].
1.6.4 X-Ray Diffraction
Various X-ray diffraction analyses have been reported over 40 years for lipid systems and lipid dispersions. X-ray diffraction (XRD) emphasizes the crystalline nature and polymorphic transitions of lipids [111]. In the formulation development of most lipids, surfactants and drugs are polymorphic, and transformation may occur. Therefore, XRD provides a clear-cut demonstration of polymorphic transition of lipid samples, which is useful in the dispersion stability [111]. The physical attributes are the fundamental feature of lipid carriers for stable dispersions. The relation between physical aspects and dispersion stability and drug loading capacity inside the carrier system changes with the change in polymorphic forms. It is evident in literature that XRD analysis is employed in several lipidic drug delivery nanosystems including lamellar, hexagonal, and cubic phases [112]. A specialized x-ray scattering technique is available to identify the crystal lattice in aqueous dispersion. This technique is known as small-angle x-ray scattering.
1.6.5 Spectroscopic Analysis
Spectroscopic analyses including Fourier-transform infrared spectroscopy (FT-IR), nuclear magnetic resonance (NMR), and mass spectrometry (MS) are used to determine the fundamental functional groups, content and purity of the sample, and the mass-to-charge ratio of ions (molecular weight) of samples, respectively [113, 114]. They help in identifying the modification of functional groups present in the carrier system or identification of conjugated or modified groups of sample under examination undergoing chemical or physical reactions [115–118]. It is more effective in the field of chemical synthesis but also helpful for the qualitative analysis of lipid nanocarrier systems and interactions between lipids, surfactants, and drug compounds.
1.7 Application of Lipid-Based Nanocarriers
1.7.1 Application in Drug Delivery
Liposome drug delivery has created a greater opportunity to formulate a large variety of drugs, which causes difficulty during delivery. Various kinds of marketed formulations are present in the form of liposomes such as Atragen, Amphotec, Ambisome, Amphocil, Abclcet, ALEC, Avian retrovirus vaccine, DaunoXome, Depoeyt, Doxil, Estrasorb, Evacet, Fungizone, Mikasome, Nyotran, Topex Br, Ventus, VincaXome, etc. [119].
Doxil (ALZA, Mountain View, CA), which was approved by US-FDA in 1995, was the first liposomal delivery to anti-cancer medication for breast cancer. Doxil is a PEGylated liposomal nanoformulation of encapsulated doxorubicin at the range of 75 nm [120]. Recently, a research study done by Dong et al. reported that PEGylated liposome doxorubicin was prepared by microfluidic mixing in the size range of 50- and 70-nm diameter on tumor retention and penetration [121].
Hua et al. (2013) reported that celecoxib in the form of hyaluronate gel improves the therapeutic benefit of lipid nanocarriers for pain management [122]. Hua and Cabot (2013) discussed in their study that Endomorphin-I opioids in the form of NPs (Polysorbate 80 coated NPs) enhance the penetration across BBB and improve therapeutic benefits. In addition, few studies mentioned that application of liposomal preparation for the administration of anti-retroviral drugs (ARVs), such as stavudine and zidovudine, was specifically targeted for HIV-related CNS diseases [123].
The SLNs are also very ideal for parenteral delivery due to their tiny size, lipidic nature, and high storage capacity after sterilization [124]. SLN intravenous administration is favored for the delivery of viral and non-viral genes because they can circulate quickly in the blood, for example, doxorubicin-loaded stealth and nonstealth SLNs. It was noticed that nonstealth nanoparticles are present at a lower concentration than stealth nanoparticles after 24 hours of intravenous administration [125].
Researchers demonstrated that SLNs by nasal routes depicted extremely positive results, e.g., SLNs of donepezil (DPL) for delivery to brain via the nasal route. The oral
