The lead hydroxide stain of Watson (1958) used for increasing contrast in thin sections for electron microscopy has found acceptance in many laboratories. However, this stain has an unfortunate tendency to form precipitates (probably of lead carbonate (5)) on exposure to the air, thus contaminating the sections and irritating the observer. This drawback has led to the development of several modifications (2, 3) of the original method of staining and the use of ingenious devices (4, 5) for preventing exposure to air and consequent precipitate formation. We offer the following alternative methods which, we believe, are simpler to perform than those hitherto described. They have the additional advantages mentioned below. The methods are based on the observation that highly alkaline solutions of lead salts (pH > 11.5) yield relatively stable solutions which stain rapidly and intensely, thus obviating the hazard of precipitation to a marked degree. The methods have these additional advantages: the staining solutions are easily and rapidly prepared, are simply stored, and are stable for long periods of time. Furthermore, they can be efficiently used, many grids being treated simultaneously, without excessive precautions being taken against lead carbonate precipitation. Finally, "difficult" material, embedded in media which characteristically yield rather low contrast, such as epoxide resins, can be rapidly and easily stained. "C lean" preparations, of high contrast, are routinely obtained. As will be discussed later, it is thought that in these highly alkaline staining solutions lead is present as an hydroxide complex anion (plumbite ion) and that this anion is responsible for the staining. The methods of preparation are based on this hypothesis. Two methods for preparing the staining solutions have been found useful:
The capacity of bacteria to survive and grow at alkaline pH values is of widespread importance in the epidemiology of pathogenic bacteria, in remediation and industrial settings, as well as in marine, plant-associated and extremely alkaline ecological niches. Alkali-tolerance and alkaliphily, in turn, strongly depend upon mechanisms for alkaline pH homeostasis, as shown in pH shift experiments and growth experiments in chemostats at different external pH values. Transcriptome and proteome analyses have recently complemented physiological and genetic studies, revealing numerous adaptations that contribute to alkaline pH homeostasis. These include elevated levels of transporters and enzymes that promote proton capture and retention (e.g., the ATP synthase and monovalent cation/proton antiporters), metabolic changes that lead to increased acid production, and changes in the cell surface layers that contribute to cytoplasmic proton retention. Targeted studies over the past decade have followed up the long-recognized importance of monovalent cations in active pH homeostasis. These studies show the centrality of monovalent cation/proton antiporters in this process while microbial genomics provides information about the constellation of such antiporters in individual strains. A comprehensive phylogenetic analysis of both eukaryotic and prokaryotic genome databases has identified orthologs from bacteria to humans that allow better understanding of the specific functions and physiological roles of the antiporters. Detailed information about the properties of multiple antiporters in individual strains is starting to explain how specific monovalent cation/proton antiporters play dominant roles in alkaline pH homeostasis in cells that have several additional antiporters catalyzing ostensibly similar reactions. New insights into the pH-dependent Na(+)/H(+) antiporter NhaA that plays an important role in Escherichia coli have recently emerged from the determination of the structure of NhaA. This review highlights the approaches, major findings and unresolved problems in alkaline pH homeostasis, focusing on the small number of well-characterized alkali-tolerant and extremely alkaliphilic bacteria.
Stimuli-responsive hydrogels are materials with great potential for development of active functionalities in fluidics and micro-fluidics. Based on the current state of research on pH sensors, hydrogel sensors are described qualitatively and quantitatively for the first time. The review introduces the physical background of the special properties of stimuli-responsive hydrogels. Following, transducers are described which are able to convert the non-electrical changes of the physical properties of stimuli-responsive hydrogels into an electrical signal. Finally, the specific sensor properties, design rules and general conditions for sensor applications are discussed.
Abstract Solar flares emit intense X‐ray and ultraviolet radiation, causing strong ionization in the neutral atmosphere and increasing the electron density in the ionospheric D‐region. These variations affect the propagation of very low frequency (VLF) radio signals, observed as perturbations in amplitude and phase. This study investigates the D‐region response to selected M and X‐class solar flares through VLF signal perturbation analysis. Data were recorded by a VLF receiver in Algeria (36.75N, 3.48E, Boumerdes), monitoring two transmitters (ICV and NSC) propagating over the Mediterranean Sea under similar conditions. The Long Wavelength Propagation Capability (LWPC) code was used to solve the inverse problem and derive Wait's parameters ( and ) and electron density variations. Nine flare events from the rising phase of Solar Cycle 24 (2011–2014) were analyzed, including eight M‐class and one X‐class flare. For the X2.8 flare on 13 May 2013, LWPC simulations showed that along the ICV–Algiers path, decreased from 74 to 54.71 km and increased from 0.3 to 0.485 . Along the NSC–Algiers path, decreased to 57.95 km and increased to 0.46 . These small differences are attributed to transmitter frequency. Averaging the results obtained from both paths improved electron density estimation. At 74 km, the electron density during the M1.0 flare increased from 216.10 to 2.9 × , while for the X2.8 flare it reached 91.3 × . Finally, ionization enhancement was simulated by solving the continuity equations using the Glukhov–Pasko–Inan model. The results are consistent with those derived from LWPC simulations.