Estimation of the limit of detection using information theory measures

Definitions of the limit of detection (LOD) based on the probability of false positive and/or false negative errors have been proposed over the past years. Although such definitions are straightforward and valid for any kind of analytical system, proposed methodologies to estimate the LOD are usually simplified to signals with Gaussian noise. Additionally, there is a general misconception that two systems with the same LOD provide the same amount of information on the source regardless of the prior probability of presenting a blank/analyte sample. Based upon an analogy between an analytical system and a binary communication channel, in this paper we show that the amount of information that can be extracted from an analytical system depends on the probability of presenting the two different possible states. We propose a new definition of LOD utilizing information theory tools that deals with noise of any kind and allows the introduction of prior knowledge easily. Unlike most traditional LOD estimation approaches, the proposed definition is based on the amount of information that the chemical instrumentation system provides on the chemical information source. Our findings indicate that the benchmark of analytical systems based on the ability to provide information about the presence/absence of the analyte (our proposed approach) is a more general and proper framework, while converging to the usual values when dealing with Gaussian noise.     

Thermal stability studies on clay nanocomposites prepared from a degradable poly(ester amide) constituted by glycolic acid and 6-aminohexanoic acid

An intercalated nanocomposite of the organically modified montmorillonite Cloisite C25A and a degradable poly(ester amide) based on glycolic acid and 6-aminohexanoic acid units (poly(glc-alt-amh)) was prepared using a twin-screw co-rotating extruder. The non-isothermal degradation kinetics was investigated by thermogravimetric analysis (TG and DTG) in the temperature range of 50–600 °C at five heating rates (2, 5, 10, 20 and 40 °C/min) and compared with the neat polymer. Significant differences were found since the nanocomposite showed three degradation steps instead of the two decomposition processes detected in the pristine sample. The onset mass loss temperature decreased in the nanocomposite due to the presence of the organo-modifier compound, but the presence of the silicate layers significantly decreased the degradation rate at the last stages of decomposition. Kinetic analysis was performed using the Kissinger method and the isoconversional (Kissinger–Akahira–Sunose and Friedman) methods. The true kinetic triplets (E, A, f(α)) were determined for the first two steps of degradation through the Coats–Redfern and the Invariant Kinetic Parameters methods. The results clearly indicated that the presence of the organo-modified clay modified the mechanisms of degradation.     

Macroporous polymers obtained in highly concentrated emulsions stabilized solely with magnetic nanoparticles

Magnetic macroporous polymers have been successfully prepared using Pickering high internal phase ratio emulsions (HIPEs) as templates. To stabilize the HIPEs, two types of oleic acid-modified iron oxide nanoparticles (NPs) were used as emulsifiers. The results revealed that partially hydrophobic NPs could stabilize W/O HIPEs with an internal phase above 90%. Depending upon the oleic acid content, the nanoparticles showed either an arrangement at the oil-water interface or a partial dispersion into the oil phase. Such different abilities to migrate to the interface had significant effects on the maximum internal phase fraction achievable and the droplet size distribution of the emulsions. Highly macroporous composite polymers were obtained by polymerization in the external phase of these emulsions. The density, porosity, pore morphology and magnetic properties were characterized as a function of the oleic acid content, concentration of NPs, and internal phase volume of the initial HIPEs. SEM imaging indicated that a close-cell structure was obtained. Furthermore, the composite materials showed superparamagnetic behavior and a relatively high magnetic moment.     

Two step, hydrolytic-solvothermal synthesis of redispersible titania nanocrystals and their gas-sensing properties

A titanium chloromethoxide solution was prepared by reacting TiCl₄ with methanol, followed by water addition. The starting solutions were characterized by Fourier Transform Infrared (FTIR) spectroscopy, evidencing that the in situ generated water results in early hydrolysis of the chloroalkoxide. The solution was reacted with molten dodecylamine at room temperature, obtaining a white slurry of amorphous titania nanoparticles. Stable, redispersible TiO₂ nanocrystals could be prepared by subsequent solvothermal treatment in oleic acid at 250???C. The use of oleic acid was essential for obtaining crystalline structures, while other surfactants prevented crystallization. The nanocrystals were characterized by X-ray Diffraction and Transmission Electron Microscopy, confirming the formation of anatase TiO₂ nanocrystals with a mean size of 3.3?nm. The TiO₂ nanocrystals were used for fabricating gas-sensing devices, which were tested towards ethanol vapors. The initial small size of the nanocrystals, and the limited size growth during the high-temperature sensor operation, result in remarkable sensing performances if compared with bulk titania sensors.     

Preparation and characterization of 5,10,15,20‐tetraphenylporphyrin Langmuir films for gas chemsensor applications

In this work is reported the preparation and characterization of 5,10,15,20‐tetraphenylporphyrin (H₂TPP) films at the water‐air interfaces. The surface pressure‐area isotherms (π‐A) and UV‐Vis spectroscopy were used to investigate the effect of the spreading methods and parameters on the porphyrin monolayer formation. Also, Langmuir‐Blodgett (LB) and Langmuir‐Schaefer (LS) films were deposited onto glass substrates in order to study the conformation changes in porphyrin molecular packing. Quartz crystal microbalance (QCM) was utilized as the active solid substrate for the development of the NO₂ gas sensor based on the H₂TPP molecular films. The results of π‐A curves have clearly shown the significant contribution of the preparation methods and processing parameters on the conformation of porphyrin molecular films. The UV‐Vis spectroscopy results using polarized absorption dichroism have indicated different molecular packing for porphyrin films deposited by LB and LS methods, with relative tilted angles of 50° ± 5° and 35° ± 5°, respectively. Moreover, the QCM response has given strong evidence that H₂TPP porphyrin molecular films have performed as NO₂ chemsensor. Copyright © 2011 John Wiley & Sons, Ltd.     

Diastereoselective preparation of (S)-(1,4,4-trimethylpyrrolidin-3-yl)amine, a new chiral 1,2-diamine for thiourea-type organocatalysts

The enantioselective synthesis of the title compound, its conversion into a thiourea-type organocatalyst and the behavior of this organocatalyst in several enantioselective Michael reactions are described.     

Interaction of monovalent ions with hydrophobic and hydrophilic colloids: charge inversion and ionic specificity

Here we study experimentally and by simulations the interaction of monovalent organic and inorganic anions with hydrophobic and hydrophilic colloids. In the case of hydrophobic colloids, our experiments show that charge inversion is induced by chaotropic inorganic monovalent ions but it is not induced by kosmotropic inorganic anions. For organic anions, giant charge inversion is observed at very low electrolyte concentrations. In addition, charge inversion disappears for both organic and inorganic ions when turning to hydrophilic colloids. These results provide an experimental evidence for the hydrophobic effect as the driving force for both ion specific effects and charge inversion. In the case of organic anions, our molecular dynamics (MD) simulations with full atomic detail show explicitly how the large adsorption free energies found for hydrophobic colloids are transformed into large repulsive barriers for hydrophilic colloids. Simulations confirm that solvation free energy (and hence the hydrophobic effect) is responsible for the build up of a Stern layer of adsorbed ions and charge inversion in hydrophobic colloids and it is also the mechanism preventing charge inversion in hydrophilic colloids. Overall, our experimental and simulation results suggest that the interaction of monovalent ions with interfaces is dominated by solvation thermodynamics, that is, the chaotropic/kosmotropic character of ions and the hydrophobic/hydrophilic character of surfaces.     

Measuring screw-sense preference in a helical oligomer by comparison of 13C NMR signal separation at slow and fast exchange

While an unequal population of rapidly interconverting left- and right-handed conformers of a helical oligomer can be detected by circular dichroism, precise quantification of a conformer ratio has not previously been achieved. We demonstrate, using a set of labeled peptide analogues, that simple analysis of peak separation in their (13)C NMR spectra at slow and fast exchange allows an accurate value for the ratio of helical conformers to be obtained. The method reports the ratio of conformers at the site of the label and can therefore be used to investigate local variations in helical conformational control.     

6,6-Spiroimine analogs of (-)-gymnodimine A: synthesis and biological evaluation on nicotinic acetylcholine receptors

Simple models of the spiroimine core of (-)-gymnodimine A have been synthesized in racemic and optically active forms. The quaternary carbon of the racemic spiroimines was created by Michael addition of a β-ketoester to acrolein, whereas the asymmetric allylic alkylation of the same β-ketoester was used to access the spiroimines in an enantioselective fashion. Both racemic and enantio-enriched mixtures were tested for their biological activities on Xenopus oocytes either expressing (human α4β2) or having incorporated (Torpedoα1(2)βγδ) nicotinic acetylcholine receptors (nAChRs). These spiroimine analogs of (-)-gymnodimine A inhibited acetylcholine-evoked nicotinic currents, but were less active than the phycotoxin. Our results reveal that the 6,6-spiroimine moiety is important for the blockade of nAChRs and support the hypothesis that it is one of the pharmacophores of this group of toxins.     

Synthetic strategies for the surface functionalisation of gold nanoparticles with metals and metal clusters

The reaction of the new ditopic thiol-phosphine compound HS(CH(2))(11)OOCC(6)H(4)PPh(2) (L) with an excess of dodecanethiol-protected gold nanoparticles gave the asymmetric gold complex [CH(3)(CH(2))(11)SAuPPh(2)C(6)H(4)COO(CH(2))(11)SH] (4), but no phosphine-protected gold nanoparticles were formed. However, by blocking the phosphine function in L with metal fragments, we have been able to produce gold nanoparticles functionalised with AuCl- and cluster [Fe(2)(CO)(7)Au] units on the surface by the method of ligand-place exchange reaction.