Mikrochimica Acta 

About: Mikrochimica Acta is an academic journal. The journal publishes majorly in the area(s): Detection limit & Cyclic voltammetry. It has an ISSN identifier of 0026-3672. Over the lifetime, 12907 publications have been published receiving 170783 citations.

The quenching of the fluorescence of carbon dots: A review on mechanisms and applications

Abstract: Carbon dots (CDs) possess unique optical properties such as tunable photoluminescence (PL) and excitation dependent multicolor emission. The quenching and recovery of the fluorescence of CDs can be utilized for detecting analytes. The PL mechanisms of CDs have been discussed in previous articles, but the quenching mechanisms of CDs have not been summarized so far. Quenching mechanisms include static quenching, dynamic quenching, Forster resonance energy transfer (FRET), photoinduced electron transfer (PET), surface energy transfer (SET), Dexter energy transfer (DET) and inner filter effect (IFE). Following an introduction, the review (with 88 refs.) first summarizes the various kinds of quenching mechanisms of CDs (including static quenching, dynamic quenching, FRET, PET and IFE), the principles of these quenching mechanisms, and the methods of distinguishing these quenching mechanisms. This is followed by an overview on applications of the various quenching mechanisms in detection and imaging.

On the polymorphism of pharmaceuticals and other molecular crystals. II@@@ber die Polymorphie von Arzneistoffen und anderen Moleklkristallen. II: Applicability of thermodynamic rules

Abstract: SummaryOn the basis of statistical mechanics, it is shown that the following rules concerning the relative stability of different polymorphic modifications are usually obeyed. (1) The enthalpy of transition of two enantiotropic forms is always positive at temperatures above their transition point, and negative at temperatures below the transition point or between monotropic forms (heat-of-transition rule). (2) Instead of the heat of transition, the difference in the heats of fusion may generally be used (heat-of-fusion rule). (3) If a modification is less dense, it is less stable at absolute zero. (4) For hydrogen-bonded crystals, the modification for which the first absorption band in the infrared spectrum is at higher frequencies has the larger entropy. The possibility of exceptions is discussed.ZusammenfassungMit Argumenten der statistischen Mechanik wird gezeigt, daß im Hinblick auf die relative Stabilität polymorpher Modifikationen von Molekülkristallen im allgemeinen folgende Regeln gelten. 1. Die Umwandlungsenthalpie zweier enantiotroper Formen ist über ihrem Umwandlungspunkt immer positiv, unter dem Umwandlungspunkt oder zwischen monotropen Formen negativ (Umwandlungswärme-Regel). 2. Statt der Umwandlungswärme kann meist auch die Differenz der Schmelzwärmen verwendet werden (Schmelzwärme-Regel). 3. Eine Modifikation von geringerer Dichte weist am absoluten Nullpunkt die geringere Stabilität auf. 4. Bilden zwei Modifikationen Kristalle mit Wasserstoffbrücken, so hat diejenige die größere Entropie, deren erste Bande im IR-Spektrum bei höheren Frequenzen liegt. Auf mögliche Ausnahmen von diesen Regeln wird hingewiesen.

Micro determination of iodine by a catalytic method

Abstract: A procedure is described for the determination of quantities of iodide of the order of 0,05 to 3γ in 1 ml. solution or in a suitable amount of solid sample (sodium chloride). The method is based upon the strong catalytic effect of iodide upon the reaction between ceric sulfate and an excess of arsenious acid in dilute sulfuric acid (1,5–2N). The effect of temperature and foreign substances has been investigated and procedures have been given which are applicable in the presence of salts affecting the rate of the catalyzed reaction. By refinement of the experimental technique it should be possible to determine quantities of iodine much smaller than 0,05γ.

On the polymorphism of pharmaceuticals and other molecular crystals. II

Abstract: The heat-of-transition rule, heat-of-fusion rule, infrared rule and density rule, which concern the relative stability of polymorphic molecular crystals, have been checked experimentally. Among the 228 transitions examined, there are only few exceptions, and these mainly concern the infrared and density rules. The heat-of-transition and heat-of-fusion rules, however, give the right result in more than 99% of cases. If certain restrictions are observed, the other rules are also fairly reliable. By their means the stability relations in polymorphic systems can easily be clarified. Data are given for 113 polymorphic substances.

Mechanism of antioxidant capacity assays and the CUPRAC (cupric ion reducing antioxidant capacity) assay

Abstract: We report on the application of a simple and versatile antioxidant capacity assay for dietary polyphenols, vitamin C and vitamin E utilizing the copper(II)-neocuproine (Cu(II)-Nc) reagent as the chromogenic oxidant, which we term the CUPRAC (cupric reducing antioxidant capacity) method. It involves mixing the antioxidant solution (directly or after acid hydrolysis) with solutions of CuCl2, neocuproine, and ammonium acetate at pH 7, and measuring the absorbance at 450 nm after 30 min. Slowly reacting antioxidants required an incubation at 50 °C for 20 min for color development. The flavonoid glycosides were hydrolyzed to their corresponding aglycones by refluxing in 1.2 M HCl-containing 50% MeOH for fully exhibiting their antioxidant potencies. Certain compounds also needed incubation after acid hydrolysis for color development. The CUPRAC absorbances of mixture constituents were additive, indicating lack of chemical deviations from Beer’s law. The CUPRAC antioxidant capacities of a wide range of polyphenolics are reported in this work and compared to those found by ABTS/persulfate and Folin assays. The trolox-equivalent capacities of the antioxidants were linearly correlated (r = 0.8) to those found by ABTS but not to those of Folin. The highest antioxidant capacities in the CUPRAC method were observed for epicatechin gallate, epigallocatechin gallate, quercetin, fisetin, epigallocatechin, catechin, caffeic acid, epicatechin, gallic acid, rutin, and chlorogenic acid in this order, in accordance with theoretical expectations. The experiences of other CUPRAC users also are summarized.