Non Conventional Energy Resources Book By G D Rai Free 808

Focusing on the bioimaging applications of RE-based NPs, different imaging approaches can be found in the literature, most of them based on LI and, to a lesser extent, on magnetic resonance imaging (MRI) and X-ray computed tomography (CT). The use of RE-based NPs for LI is based on the luminescent properties of Ln cations, which are included as dopants in a large variety of inorganic matrices such as oxides, fluorides, phosphates, vanadates, molybdates and wolframates. Such inorganic matrices do not necessarily have to be based on RE, although RE compounds normally facilitate the doping process, since the incorporation of the doping Ln cations with the same charge and very similar size is favored [18]. Two great groups of luminescent RE-based NPs can be established according with the selected dopant cations. On the one hand, downconversion nanoparticles (DCNPs) convert higher-energy photons into photons with lower energy (i.e. conventional Stokes luminescence) and often contain Eu3+, Tb3+ (codoped with Ce3+) and Dy3+, which are excited by ultraviolet (UV) radiation and emit in the visible region, and Nd3+, which is excited and emits in the near infrared (NIR) region. On the other hand, upconversion nanoparticles (UCNPs), mostly containing Er3+ or Tm3+ codoped with Yb3+, are able to emit shorter wavelength light after having been excited by long wavelength radiation (i.e. anti-Stokes luminescence). Both UCNPs and Nd3+ doped DCNPs have attracted high research interest, since they are excited by NIR radiation, avoiding thus photodamage and background fluorescence of biological systems and enabling a higher penetration depth into biological tissues [19].

As mentioned in Section 1, through the selection of the dopants cations, downconverting (DC) and upconverting (UC) phosphors can be obtained. Downconversion is a conventional Stokes luminescence process in which higher-energy photons are converted into lower-energy photons. Although DC luminescence is expected for most of the Ln cations, the majority of the luminescent DCNPs used for biomedical applications contain Eu3+, Tb3+ (often codoped with Ce3+) and Dy3+, which produce red, green and yellow luminescence, respectively [167], [168], [169], [170], [171]. The large energy difference between the lowest lying excited (emissive) state and the highest sublevel of the ground multiplet of these Ln ions minimize non-radiative (NR) processes and thus make them very appropriate for luminescent applications [172]. Nd3+is a DC ion of especial interest since it produces NIR luminescence after NIR excitation and has also been often employed. UC luminescence is a nonlinear optical process that converts two or more low-energy pump photons into a higher-energy output photon [34], [35], [173]. Several upconversion mechanisms have been reported in the literature, including excited-state absorption, energy transfer UC (ETU), photon avalanche, cooperative UC and energy migration-mediated UC. The mechanism mainly involved in the development of luminescent NPs for bioapplications is ETU, which will be described later. Readers interested in the other mechanisms may refer to more specialized reviews on this topic [31], [174], [175]. The most important Ln cations for UCNPs are Ho3+, Er3+ and Tm3+, in most cases codoped with Yb3+. Typical concentrations used to optimize brightness by increasing photon absorption and minimizing luminescence quenching are 2% mol active lanthanide cation (Eu3+, Ho3+, Tm3+) and 20% mol of the sensitizer cation (Yb3+) [19], [80], [176].

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Although the contrast resolution of CT is much higher than that of conventional radiography, it is convenient to use CAs with a high capacity to absorb X-rays for a better delineation of regions of interest. The interested readers can refer to two recent reviews on this subject by Liu et al. [317] and Lusic and Grinstaff [36]. CT CAs are substances containing high Z elements because of the Z4 dependence of the X-ray absorption coefficient of an atom (Î=ÏZ4/AE3, where Ï=density of the material, Z=atomic number, A=atomic mass and E=energy of the X-ray beam). Iodine-based chelates (ZIodine=53) and barium sulfate (ZBarium=56) suspensions are classical CT CAs approved for human use. Both of them present, however, several inconveniences such as the inherent toxicity of Ba2+ ions, which hinders use other than in gastrointestinal imaging, and the short circulation time and high osmolality of iodinated compounds, which prevent wider application. These disadvantages are strengthened by the fact that CT imaging requires high dosages of CAs.

Depending on their surface chemistry and/or structure, and therefore the mechanism involved in the sensing response, three main types of sensing systems can be distinguished: (i) ligand-free Ln-doped NPs, that is NPs without any recognition ligand on their surface, (ii) functionalized Ln-doped NPs, where specific ligands, groups or motifs have a recognition capability of the target analyte, and (iii) Ln ions are embedded in an organic framework (MOFs) or polymer. In the former case the response of the system is mainly governed by a physical mechanism, whereas in the second case there is a combination of chemical (i.e. analyte-recognition ligand interaction) and physical processes. In the last case physico-chemical processes (i.e. analyte-metal/analyte-ligand interactions and porosity) together with physical mechanisms provoke the response. Concerning the physical mechanisms behind the optical response, they can be roughly divided into three classes: (i) luminescence resonance energy transfer (LRET), (ii) ET mediated by a ligand and (iii) emission-reabsorption (inner filter effect, IFE), see Figure 21. The differences and pros/cons of each type of system will be explained by means of selected examples.

An interesting example of using ligand-free Ln-doped NPs for sensing purposes was reported recently by Guo et al. [327]. They proposed the detection of water molecules based on the intrinsically nonlinear upconversion amplification process of NaYF4:Yb3+, Er3+ NPs, being able to reach a detection limit as low as 0.008 vol % (80 ppm) in an organic solvent. The presence of water causes the quenching of the upconversion emission process, which is mainly attributed to efficient ET between upconversion NPs and water molecules as well as water-absorption-induced excitation energy attenuation. Thanks to the high photostability of the upconversion emission, an important advantage of this sensing system is its potential to be used for long-term and real-time monitoring of water content. However, owing to the fact that several quenching effects are involved in the response of this system (i.e. ET and direct water absorption of excitation energy) and the contribution of each one is still not well understood, a deeper understanding is required before opening the door to the development of other optical sensors relying on leveraging nonlinear upconversion process of Ln-doped NPs.

The effects of pelleting on the extent of the Maillard reaction (MR) and on calcium, magnesium and zinc solubility and absorption were analysed in a conventional pre-starter diet for suckling piglets. Development was tested measuring colour, absorbance (280/420 nm), fluorescence, residual free lysine, furosine, hydroxymethylfurfural (HMF) and furfural contents before and after pelleting. Fluorescence, absorbance and mineral solubility were also measured after in vitro digestion of diets. The effects on mineral absorption were tested using Caco-2 cells. MR indexes confirmed the development of the reaction during the pelleting of this particular diet compared with the meal diet. The CIE-Lab colour parameters showed a decrease in luminosity (L*) and progress of the colour to the red zone (a*) in the pelleted diet. A 36% decrease in free lysine content was observed. Significant correlations were observed between fluorescence intensity and furosine levels, HMF and furfural. The pelleting process did not modify calcium and magnesium solubility after in vitro digestion, but soluble zinc increased. The efficiency of calcium and zinc transport across Caco-2 cell monolayers was greater in the pelleted diet. Evidence of MR development is shown, resulting in various nutritional consequences. Optimisation of pelleting could result in a better formulation of diets for feedstuffs. (c) 2010 Society of Chemical Industry.

The conservation and transformation of energy is essential to the survival of mankind, and thus concerns every modern society. Solar energy, as an everlasting source of energy, holds one of the key solutions to some of the most urgent problems the world now faces, such as global warming and the oil crisis. Advances in technologies utilizing clean, abundant solar energy, could be the steering wheel of our societies. Solar cells, one of the major advances in converting solar energy into electricity, are now capturing people's interest all over the globe. While solar cells have been commercially available for many years, the manufacturing of solar cells is quite expensive, limiting their broad based implementation. The cost of solar cell based electricity is 15-50 cents per kilowatt hour (Â/kwh), depending on the type of solar cell, compared to 0.7 Â/kwh for fossil fuel based electricity. Clearly, decreasing the cost of electricity from solar cells is critical for their wide spread deployment. This will require a decrease in the cost of light absorbing materials and material processing used in fabricating the cells. Organic photovoltaics (OPVs) utilize organic materials such as polymers and small molecules. These devices have the advantage of being flexible and lower cost than conventional solar cells built from inorganic semiconductors (e.g. silicon). The low cost of OPVs is tied to lower materials and fabrication costs of organic cells. However, the current power conversion efficiencies of OPVs are still below 15%, while convention crystalline Si cells have efficiencies of 20-25%. A key limitation in OPVs today is their inability to utilize the near infrared (NIR) portion of the solar spectrum. This part of the spectrum comprises nearly half of the energy in sunlight that could be used to make electricity. The first and foremost step in conversion solar energy conversion is the absorption of light, which nature has provided us optimal model of, which is

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