What is Nano-characterization? Discuss the purpose ofmicroscopic analysis of soil. Characterizationof nanoparticles is based on the size, morphology and surface charge, usingsuch advanced microscopic techniques as atomic force microscopy (AFM), scanningelectron microscopy (SEM) and transmission electron microscopy (TEM).
Properties such as the size distribution, average particle diameter, chargeaffect the physical stability and the in vivo distribution of thenanoparticles. Properties like surface morphology, size and overall shape aredetermined by electron microscopy techniques. Features like physical stabilityand redispersibility of the polymer dispersion as well as their in vivo performanceare affected by the surface charge of the nanoparticles. Differentcharacterization tools and methods for nanoparticles are mentioned in Table.Therefore, it’s very important to evaluate the surface charge duringcharacterization of nanoparticles.
· Surface charge Surface charge and intensitydetermines the interaction of nanoparticles with the biological environment aswell as their electrostatic interaction with bioactive compounds. Stability ofcolloidal material is usually analysed through zeta potential of nanoparticles.Zeta potential is an indirect measure of the surface charge. It can be obtainedby evaluating the potential difference between the outer Helmholtz plane andthe surface of shear. Thus, zeta potential of colloidal based dispersionassists in directly evaluating its storage stability. Zeta potential values(high zeta potential values, either positive or negative) are achieved toensure stability and avoid aggregation of the particles. Zeta potential valuescan be utilized in evaluating surface hydrophobicity and the nature of materialencapsulated within the Nano capsules or coated onto the surface. Thepurpose of microscopic analysis of soil is to characterize it according to itsproperties.
And for the characterization following microscopic technologies areuse: 1. Photon-CorrelationSpectroscopy (PCS) or Dynamic Light Scattering (DLS) The fastest and most popular techniqueslike photon-correlation spectroscopy (PCS) or dynamic light scattering (DLS),widely used to determine the size of Brownian nanoparticles in colloidalsuspensions in the Nano and submicron ranges. In this technique solution ofspherical particles in Brownian motion causes a Doppler shift when they = areexposed against shining monochromatic light (laser). Such monochromatic lightexposure hits the moving particle which results in changing the wavelength of the incoming light.
Extent of thischange in wavelength determines the size ofthe particle. This parameter assists in evaluation of the size distribution, particle’s motion in the medium, which mayfurther assist in measuring the diffusioncoefficient of the particle and using the autocorrelation function. Dynamic light scattering (DLS) offer the most frequentlyused technique for accurate estimation of theparticle size and size distribution. 2. Scanning Electron Microscopy(SEM) This electron microscopy basedtechnique determines the size, shape and surface morphology with directvisualization of the nanoparticles. Therefore, scanning electron microscopyoffer several advantages in morphological and sizing analysis. However, theyprovide limited information about the size distribution and true populationaverage.
During the process of SEM characterization, solution of nanoparticlesshould be initially converted into a dry powder. This dry powder is thenfurther mounted on a sample holder followed by coating with a conductive metal(e.g. gold) using a sputter coater. Whole sample is then analysed by scanningwith a focused fine beam of electrons.
Secondary electrons emitted from thesample surface determine the surface characteristics of the sample. Thiselectron beam can often damage the polymer of the nanoparticles which must beable to withstand vacuum. Average mean size evaluated by SEM is comparable withresults obtained by dynamic light scattering. In addition, these techniques aretime consuming, costly and frequently need complementary information aboutsizing distribution. 3.
Transmission ElectronMicroscope Experimental difficulties instudying nanostructures stem from their small size, which limits the use oftraditional techniques for measuring their physical properties. Transmissionelectron microscopy techniques can provide imaging, diffraction andspectroscopic information, either simultaneously or in a serial manner, of the specimenwith an atomic or a sub-nanometre spatial resolution. TEM operates on differentprinciple than SEM, yet it often brings same type of data. The sample preparationfor TEM is complex and time consuming because of its requirement to be ultra-thinfor the electron transmittance. High-resolution TEM imaging, when combined withNano diffraction, atomic resolution electron energy-loss spectroscopy and nanometreresolution X-ray energy dispersive spectroscopy techniques, is critical to thefundamental studies of importance to nanoscience and nanotechnology. During theTEM characterization nanoparticles dispersion is deposited onto support gridsor films. After dispersion, they are fixed using either a negative staining material(phosphotungstic acid or derivatives, uranyl acetate, etc.
, or by plastic embedding).This is done to make nanoparticles withstand against the instrument vacuum andfacilitate handling. Alternatively, nanoparticles sample can also be exposingto liquid nitrogen temperatures after embedding in vitreous ice. When a beam ofelectrons is transmitted through an ultra-thin sample it interacts with the sampleas it passes through the surface characteristics of the sample are obtained.TEM imaging mode has certain benefits compared with the broad-beam illuminationmode: • Collection of theinformation about the specimen using a high angular annular dark field (HAADF)detector (in which the images registered have different levels of contrast relatedto the chemical composition of the sample)• It can be utilized for theanalysis of biological samples is its contrast for thick stained sections,since high angular annular dark field images (samples with thickness of 100–120nm) have better contrast than those obtained by other techniques.
• Combining HAADF-TEM imagingleads to imaging the atomistic structure and composition of nanostructures at asub-angstrom resolution.• Availability of sub-nanometreor sub-angstrom electron probes in a TEM instrument, due to the use of a fieldemission gun and aberration correctors, ensures the greatest capabilities forstudies of sizes, shapes, defects, crystal and surface structures, andcompositions and electronic states of nanometre-size regions of thin films,nanoparticles and nanoparticle systems. 4. Atomic Force Microscopy This technique is also knownas scanning force microscopy (technique that forms images of surfaces using a probethat scans the specimen), very high-resolution type of scanning probemicroscopy, with reported resolution on the order of fractions of a nanometre,more than 100 times better than the optical diffraction limit. The atomic forcemicroscopy is based on a physical scanning of samples at sub-micron level usinga probe tip of atomic scale and offers ultra-high resolution in particle sizemeasurement. Depending upon properties, samples are usually scanned in contactor noncontact mode. During contact mode, the topographical map is generated bytapping the probe on to the surface across the sample and probe hovers over theconducting surface in non-contact mode.
One of the prime advantage of AFM is itsability to image non-conducting samples without any specific treatment. This featureallows the imaging of delicate biological and polymeric Nano andmicrostructures. Moreover AFM (without any mathematical calculation) provides themost accurate description of size, size distribution and real picture whichhelps in understanding the effect of various biological conditions.