Giacovazzo C.,CNR Institute of Crystallography
Acta Crystallographica Section A: Foundations and Advances | Year: 2015
For a given unknown crystal structure (the target), n random structures, arbitrarily designed without any care for their chemical consistency and usually uncorrelated with the target, are sheltered in the same unit cell as the target structure and submitted to the same space-group symmetry. (These are called ancil structures.) The composite structures, whose electron densities are the sum of the target and of the ancil electron densities, are denoted derivatives. No observed diffraction amplitudes are available for them: in order to emphasize their unreal nature, the term phantom is added. The paper describes the theoretical basis by which the phantom derivative method may be used to phase the target structure. It may be guessed that 100-300 ancil structures may be sufficient for phasing a target structure, so that the phasing technique may be denoted as the multiple phantom derivative method. Ancil phases and amplitudes may be initially combined with observed target magnitudes to estimate amplitudes and phases of the corresponding phantom derivative. From them suitable algorithms allow one to obtain poor target phase estimates, which are often improved by combining the indications arising from each derivative. Probabilistic criteria are described to recognize the most reliable target phase estimates. The method is cyclic: the target phase estimates just obtained are used to improve amplitudes and phases of each derivative, which, in their turn, are employed to provide better target phase estimates. The method is a fully ab initio method, because it needs only the experimental data of the target structure. The term derivative is maintained with reference to SIR-MIR (single isomorphous replacement-multiple isomorphous replacement) techniques, even if its meaning is different: therefore the reader should think of the phantom derivative method more as a new method than as a variant of SIR-MIR techniques. The differences are much greater than the analogies. The paper also describes how phantom derivatives may be used for improving structure models obtained via other ab initio or non-ab initio techniques. The method is expected to be insensitive to the structural complexity of the target and to the target experimental data resolution, provided it is better than 4-6 Å. © 2015 International Union of Crystallograph.
Scognamiglio V.,CNR Institute of Crystallography
Biosensors and Bioelectronics | Year: 2013
In the last decades, a wide multitude of research activity has been focused on the development of biosensors for glucose monitoring, devoted to overcome the challenges associated with smart analytical performances with commercial implications. Crucial issues still nowadays elude biosensors to enter the market, such as sensitivity, stability, miniaturisation, continuous and in situ monitoring in a complex matrix. A noteworthy tendency of biosensor technology is likely to push towards nanotechnology, which allows to reduce dimensions at the nanoscale, consenting the construction of arrays for high throughput analysis with the integration of microfluidics, and enhancing the performance of the biological components by using new nanomaterials. This review aims to highlight current trends in biosensors for glucose monitoring based on nanotechnology, reporting widespread representative examples of the recent approaches for nanobiosensors over the past 10 years. Progress in nanotechnology for the development of biosensing systems for blood glucose level monitoring will be discussed, in view of their design and construction on the bases of the new materials offered by nanotechnology. © 2013 Elsevier B.V.
Li H.,Italian Institute of Technology |
Zanella M.,Italian Institute of Technology |
Genovese A.,Italian Institute of Technology |
Povia M.,Italian Institute of Technology |
And 3 more authors.
Nano Letters | Year: 2011
We demonstrate that it is possible to convert CdSe nanocrystals of a given size, shape (either spherical or rod shaped), and crystal structure (either hexagonal wurtzite, i.e., hexagonal close packed (hcp), or cubic sphalerite, i.e., face-centered cubic (fcc)), into ZnSe nanocrystals that preserve all these characteristics of the starting particles (i.e., size, shape, and crystal structure), via a sequence of two cation exchange reactions, namely, Cd 2+ →Cu+ →Zn2+. When starting from hexagonal wurtzite CdSe nanocrystals, the exchange of Cd2+ with Cu+ yields Cu2Se nanocrystals in a metastable hexagonal phase, of which we could follow the transformation to the more stable fcc phase for a single nanorod, under the electron microscope. Remarkably, these metastable hcp Cu2Se nanocrystals can be converted in solution into ZnSe nanocrystals, which yields ZnSe nanocrystals in a pure hcp phase. © 2011 American Chemical Society.
Giacovazzo C.,CNR Institute of Crystallography
Acta Crystallographica Section A: Foundations of Crystallography | Year: 2015
Crystallographic least squares are a fundamental tool for crystal structure analysis. In this paper their properties are derived from functions estimating the degree of similarity between two electron-density maps. The new approach leads also to modifications of the standard least-squares procedures, potentially able to improve their efficiency. The role of the scaling factor between observed and model amplitudes is analysed: the concept of unlocated model is discussed and its scattering contribution is combined with that arising from the located model. Also, the possible use of an ancillary parameter, to be associated with the classical weight related to the variance of the observed amplitudes, is studied. The crystallographic discrepancy factors, basic tools often combined with least-squares procedures in phasing approaches, are analysed. The mathematical approach here described includes, as a special case, the so-called vector refinement, used when accurate estimates of the target phases are available. © 2015 International Union of Crystallography.
Rea G.,CNR Institute of Crystallography |
Antonacci A.,CNR Institute of Crystallography |
Giardi M.T.,CNR Institute of Crystallography
Critical Reviews in Food Science and Nutrition | Year: 2013
In recent years, both food quality and its effect on human health have become a fundamental issue all over the world. As a consequence of this new and increased awareness, American, European, and Asian policymakers have strongly encouraged the research programs on food quality and safety thematic. Attempts to improve human health and to satisfy people's desire for healthcare without intake of pharmaceuticals, has led the food industry to focus attention on functional or nutraceutical food. For a long time, compounds with nutraceutical activity have been produced chemically, but the new demands for a sustainable life have gradually led the food industry to move towards natural compounds, mainly those derived from plants. Many phytochemicals are known to promote good health, but, sometimes, undesirable effects are also reported. Furthermore, several products present on the market show few benefits and sometimes even the reverse - unhealthy effects; the evidence of efficacy is often unconvincing and epidemiological studies are necessary to prove the truth of their claims. Therefore, there is a need for reliable analytical control systems to measure the bioactivity, content, and quality of these additives in the complex food matrix. This review describes the most widespread nutraceutics and an analytical control of the same using recently developed biosensors which are promising candidates for routine control of functional foods. © 2013 Copyright Taylor and Francis Group, LLC.