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October 8, Latest eBook. Then an electron from the outer shell with higher-energy fills the hole, which releases the energy in difference in between the outer shell and the inner shell in the form of X-ray [ 26 ]. As each element has a unique set of peaks on electromagnetic emission spectrum determined by its unique atomic structure, through measuring the number and energy of the X-rays emitted from the sample by an EDS instrument, the information about elemental composition of the sample is obtained [ 26 ].
There are two main types of X-ray spectroscopy-based techniques that can be used to analyze speciation and localization of NPs within the plant tissue: X-ray fluorescence XRF spectrometry and X-ray absorption XAS spectrometry.
Both of them are based on measuring the spectra of emission or absorption of X-radiation. The absorption of X-ray photons by element is controlled by the photo-electric effect. When sample is subjected to X-ray radiation, incident X-rays photons of a definite energy shine on the samples. If the energy of incident X-rays that reaches the sample is lower than the binding energy E0 of the core electrons of the element, the atoms of this element do not participate in the absorption process.
While with increasing energy of the incident X-ray photons, a point will be reached where their energy is approximately equal to the binding energy of the core electrons. At this point a sharp increase in absorption of the X-ray photons occurs [ 27 ]. The energy absorbed by the core electron elevates it into a higher energy state or electron orbital, which is unoccupied.
This excited core electron is referred to as a photoelectron. At the binding energy E0 , the photoelectron is ejected from the atom into the continuum [ 28 ]. As a result, a vacancy in the shell of the core electron is created in a core orbital.
In order for the atom to return to the ground state, an electron from a higher energy orbital e. These X-ray photons have characteristic energies for each element in the periodic table, confers element-specificity to the absorption and fluorescence spectroscopy.
During XRF process, the emitted fluorescence signal can be recorded at each position and used to generate XRF elemental maps. XRF is a nondestructive technique, it can be used to identify and determine the concentrations of elements present in biological samples, as well as providing information of in situ localization of elements in the samples.
During XAS process, the energy of the incident X-ray beam is progressively increased beyond the binding energy, thus the emission of fluorescence and absorption of the incident X-ray progressively increases, generating a characteristic X-ray absorption spectrum by detecting and recording the absorption or fluorescence at each energy point.
The main feature of the XAS spectrum is a sharp, step-like curve called the absorption edge [ 28 , 29 ]. The XAS spectrum is conventionally divided into two parts according to the energy region. XANES is particularly sensitive to the oxidation states of elements and the electronegativity of the ligands, and it provides electronic structural information about the oxidation state and local geometry of the absorbing metal atom.
EXAFS can provide information about the element coordination such as the identity and number of the coordinating atoms, and the interatomic distance between the central absorbing atom and its next nearest neighbors. XAS is an element specific spectroscopic technique that provides specific qualitative information about chemical species at very high subatomic spatial resolution and is able to analyze almost any type of samples including amorphous non-crystalline materials in situ, requiring minor or no sample preparation prone to modify the chemical species.
XAS experiments require an intense and polychromatic X-ray source. Nowadays, owing to the development of synchrotron radiation facilities, the combination of synchrotron radiation with XAS is proved to be a powerful technique for speciation analysis of chemical elements.
XAS spectroscopy can be performed as bulk analyses to assess the overall speciation of the chemical of interest in the sample usually homogenized. In this case, bulk-XAS analysis is insufficient for us to obtain specific information from a complex and heterogeneous mixture of biological sample. This limitation can be overcome by decreasing the beam size to the range of tens of nm to a few um, and using thin section of sample. Therefore, a trade-off exists between detecting minor species and obtaining the overall speciation of the analyte in the whole sample.
Recently, a new approach termed XANES imaging has been developed with the capacity to analyze element speciation and full lateral distribution over large areas of sample.
These resulting maps can be aligned and stacked, then the XANES spectra can be extracted from individual pixels or groups of pixels over regions of interest, eventually, the spatial distribution of both major and minor species within the sample will be obtained [ 28 ]. X-ray diffraction XRD is a nondestructive technique for characterizing crystallographic structure or elemental composition of crystalline materials.
It can reveal information about the crystal structure, crystalline phase, preferred crystal orientation texture , average crystallite size and strain of materials. The constructive interference of a monochromatic beam of X-rays diffracted at specific angles from each set of lattice planes in the crystalline sample will produce X-ray diffraction peaks, intensities of which are determined by the distribution of atoms within the lattice, therefore, an X-ray diffraction pattern will be generated which reflects the periodic atomic arrangements in the sample.
For synchrotron-based X-ray diffraction SR-XRD technique, the high intensity and well-defined wavelength of the incident synchrotron radiation will generate a better resolution of diffraction peaks and make SR-XRD capable in detecting minor constituents in a sample [ 27 ]. Using mathematical principles of tomography, this series of images is then reconstructed to produce a 3D image, thus a 3D distribution of the element of interest within the sample is obtained [ 27 , 32 ].
Scanning transmission X-ray microscopy STXM is a type of X-ray microscopy that allows in situ mapping of elements at high lateral resolution within a specimen. STXM uses a Fresnel zone plate to focuses synchrotron soft X-ray absorption beamline into a small spot, the sample is placed at the focus of the zone plate and scanned by X-ray, then a film or charged coupled device detector is used to detecting the transmitted X-rays intensity that pass through the specimen [ 33 ].
Secondary ion mass spectrometry SIMS uses an energetic ion beam to bombard a sample, particles from the top few atomic layers of the sample surface are then removed, resulting in the consequent liberation of ions, known as secondary ions.
It is capable of measuring most elements in the periodic table, from hydrogen to uranium, as well as their different isotopes.
These advantages of NanoSIMS make it one of the most powerful tools to quantitatively investigate elemental distribution in organisms at the cellular level [ 36 , 37 ]. In addition, it is a destructive technique, which can be a disadvantage for some samples. This problem can be overcome by using high-pressure freezing followed by freeze substitution to preserve cellular and subcellular structures as well as elemental distributions of plant cells [ 29 ].
Inductively coupled plasma ICP based analytical techniques can provide quantitative elemental composition of a wide variety of sample types, including solids, liquids, and suspensions.
Inductively coupled plasma-optical emission Spectrometry ICP-OES can be used to measure nanoparticle number concentration and elemental composition within a sample. As ICP-based techniques involve the use of liquid phases, suspensions could be analyzed directly, but solid samples have to be pretreated for the digestion of the matrix [ 21 ]. Generally, solid samples are dissolved or digested using acid in a microwave to get volatile analytic species. The sample solution is then nebulized into the core of inductively coupled argon plasma, where a flame temperature in a range from to 10, K vaporizes the nebulized solution, thus the analytic species are atomized, ionized and thermally excited.
The excited atoms and ions return to low energy position, emitting electromagnetic radiation at wavelengths characteristic of a particular element, then the analytic species can be detected and quantified with an optical emission spectrometer OES through measuring the intensity of radiation, which is converted to elemental concentration by comparison with calibration standards.
Inductively coupled plasma-mass spectrometry ICP-MS is an inorganic elemental analysis technique based on atomic mass spectrometry. ICP-MS consists of an ion source, a sampling interface, ion lens, a mass spectrophotometer and a detector system [ 18 ].
ICP sources are mainly used for metal analysis. Mass spectrophotometer e. The ions generated in the high temperature argon plasma core are subsequently sorted by mass with the mass spectrophotometer and subjected to further elemental and isotopic analysis.
The identities of the ions are determined by their mass-to-charge ratio using a mass analyzer, while the ions intensity is measured at ppt to ppm levels using the ion detector, then the intensity measurements are converted to elemental concentration by comparison with calibration standards.
With the high sensitivity and specificity, ICP-MS has been widely used for the detection, characterization, and quantification of nanoparticles [ 38 ]. During SP-ICP-MS process, the sample is first suspended in a nebulized liquid and subsequently carried to argon plasma, where the sample is sequentially desolvated and atomized and ionized, creating a plume of ions.
The ions pass through the mass spectrometer where they are separated by mass-to-charge ratio and detected using a time resolved analysis acquisition. The sample solution needs to be diluted sufficiently to ensure low concentrations ppt to ppb that no more than one particle will enter the plasma at a time.
By using sufficiently short integration dwell time which is a duration for the instrument to take a reading, thousands of fast and individual readings are generated to capture nanoparticle event as a discrete signal pulse, each pulse is assumed to correlate to one nanoparticle event [ 39 , 40 , 41 ]. Based on ionic calibration standard, the particle mass can be determined by the intensity of the ICP-MS response. If the density of the elemental constituents of the particle is known, the theoretical size of the particle can be determined.
If the transport efficiency from the nebulizer to the plasma is known, then the particle number concentration can be further calculated [ 38 , 42 ]. SP-ICP-MS has been widely applied to measure particle size, size distribution, number concentration and elemental composition of nanoparticles in biological samples, demonstrating it as a powerful tool in quantifying NPs.
To deal with biological tissues, a strong acid extraction procedure is required to release the NPs from the matrix. This introduces the possible dissolution of metal NPs which challenges the accuracy of the final analytical data. To solve this problem, Dan et al. With the aid of enzymatic digestion, we have applied SP-ICP-MS analysis to characterize Ag NPs internalized by Arabidopsis, thus having established a new technique and opened up new research domain in our lab [ 44 ].
Overview of these analytical techniques including advantages and limitations with examples of application in plant-NP interaction studies is provided in Table 1. Overview of analytical techniques discussed in this review with examples of application.
Although a range of techniques are available to detect and characterize uptake, translocation and biotransformation of NPs in plant tissue, no single technique can provide all information regarding plant-NP interaction. Sufficient information is often obtained by the combination of these analytical techniques, which could provide complementary information mutually.
Electron microscopes yield much greater resolution than the older light microscopes; they can obtain magnifications of up to 1 million times, while the best light microscopes can magnify an image only about 1, times.
The scanning tunneling microscope STM is among a number of instruments that allows scientists to view and manipulate nanoscale particles, atoms, and small molecules. Atomic force microscopes AFMs gather information by "feeling" the surface with a mechanical probe. National Nanotechnology Initiative.
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