University of St Andrews

About: University of St Andrews is a education organization based out in St Andrews, Fife, United Kingdom. It is known for research contribution in the topics: Population & Laser. The organization has 16260 authors who have published 43364 publications receiving 1636072 citations. The organization is also known as: St Andrews University & University of St. Andrews.

Simultaneous micromanipulation in multiple planes using a self-reconstructing light beam

Abstract: Optical tweezers1 are commonly used for manipulating microscopic particles, with applications in cell manipulation2, colloid research3,4,5, manipulation of micromachines6 and studies of the properties of light beams7. Such tweezers work by the transfer of momentum from a tightly focused laser to the particle, which refracts and scatters the light and distorts the profile of the beam. The forces produced by this process cause the particle to be trapped near the beam focus. Conventional tweezers use gaussian light beams, which cannot trap particles in multiple locations more than a few micrometres apart in the axial direction, because of beam distortion by the particle and subsequent strong divergence from the focal plane. Bessel beams8,9, however, do not diverge and, furthermore, if part of the beam is obstructed or distorted the beam reconstructs itself after a characteristic propagation distance10. Here we show how this reconstructive property may be utilized within optical tweezers to trap particles in multiple, spatially separated sample cells with a single beam. Owing to the diffractionless nature of the Bessel beam, secondary trapped particles can reside in a second sample cell far removed (∼3 mm) from the first cell. Such tweezers could be used for the simultaneous study of identically prepared ensembles of colloids and biological matter, and potentially offer enhanced control of ‘lab-on-a-chip’ and optically driven microstructures.

α‐MnO2 Nanowires: A Catalyst for the O2 Electrode in Rechargeable Lithium Batteries

Abstract: Charge storage in rechargeable lithium batteries is limited by the positive electrode, usually the lithium intercalation compound LiCoO2, which can store 130 mAhg . Intense efforts are underway worldwide to discover new lithium intercalation compounds for use as positive electrodes which, it is hoped, may deliver specific capacities of about 300 mAhg . However, increasing the capacity significantly beyond this limit is a major challenge requiring a more radical approach, such as replacement of the intercalation electrode by an O2 electrode, in which Li + from the electrolyte and e from the external circuit combine reversibly with O2 from the air within a porous matrix containing a catalyst. Although it provides higher capacities than intercalation electrodes, much fundamental work is required to understand and optimize the performance of the O2 electrode for lithium batteries before it can be considered further for technological application. The nature of the catalyst plays a key role. It is important to identify good catalysts for the electrode reaction before focusing on other tasks, such as reducing the catalyst loading and optimizing porosity, binder, and electrolyte. Herein we show that a-MnO2 nanowires give the highest charge storage capacity yet reported for such an electrode, reaching 3000 mAh per gram of carbon, or 505 mAhg 1 if normalized by the total electrode mass. Furthermore, by avoiding deep discharge, excellent capacity retention has been demonstrated. Finally, the capacities delivered by an O2 electrode and a conventional intercalation compound are compared. The reversible oxygen electrode is shown schematically in Figure 1. On discharge, the Li ions (electrolyte) and e (external circuit) combine with O2 (air) to form Li2O2 within the pores of the porous carbon electrode. Previously, we demonstrated that rechargeability of the Li/O2 cell involves decomposition of Li2O2 back to Li and O2. [8] Our earlier studies on the rechargeable Li/O2 cell focused on electrolytic manganese dioxide (EMD) as catalyst in the oxygen electrode. Recently, we examined a number of other potential catalyst materials including Co3O4, Fe2O3, CuO, and CoFe2O4. [9] Such investigations served to demonstrate that the nature of the catalyst is a key factor controlling the performance of the oxygen electrode, especially the capacity, which is the primary reason for interest in the O2 electrode. Herein we report on the high capacities that an a-MnO2 nanowire catalyst can deliver. We also compare the performance of a-MnO2 with other manganese oxide compounds. Note that the specific capacities are normalized with respect to the mass of carbon in the electrode, as is usual for porous electrodes; this point is discussed at the end of the paper. Synthesis and characterization of the various MnOx catalysts and their incorporation into lithium cells with porous electrodes is described in the Experimental Section. Powder X-ray diffraction data were collected for all catalysts (see the Supporting Information) and confirmed their identities (a-MnO2 in bulk and nanowire form, b-MnO2 in bulk and nanowire form, g-MnO2, l-MnO2, Mn2O3, and Mn3O4). The variation of capacity with cycle number for a porous electrode containing a-MnO2 nanowires as catalyst is presented in Figure 2a, from which the superior behavior of the a-MnO2 catalyst is evident. The initial discharge capacity is 3000 mAhg , it then drops slightly, rises again to 3100 mAhg 1 on cycle 4, before declining steadily thereafter. This may be contrasted with previous reports for EMD, the capacity of which falls below 1000 mAhg 1 after one cycle (Figure 2a). The variation of potential with state of charge for several cycles of a-MnO2 is shown in Figure 2b. As observed previously for all other catalysts, the discharge voltage is around 2.6 V versus Li/Li. 9] Previous results have demonstrated that the charging potential varies according to the catalyst type. Values ranging from 4 to 4.7 V versus Li/Li have been observed, and a-MnO2 exhibits a charging potential at the lower end of this spectrum, at around 4.0 V. This is another advantage of the a-MnO2 nanowires, since it is important to minimize the charging potential. Note that a-MnO2, and many of the other MnOx compounds described herein, support some Li intercalation. However, Figure 1. Schematic representation of a rechargeable Li/O2 battery.

The magnetic nature of solar flares

Abstract: The main challenge for the theory of solar eruptions has been to understand two basic aspects of large flares. These are the cause of the flare itself and the nature of the morphological features which form during its evolution. Such features include separating ribbons of H $\alpha$ emission joined by a rising arcade of soft x-ray loops, with hard x-ray emission at their summits and at their feet. Two major advances in our understanding of the theory of solar flares have recently occurred. The first is the realisation that a magnetohydrodynamic (MHD) catastrophe is probably responsible for the basic eruption and the second is that the eruption is likely to drive a reconnection process in the field lines stretched out by the eruption. The reconnection is responsible for the ribbons and the set of rising soft x-ray loops, and such a process is well supported by numerical experiments and detailed observations from the Japanese satellite Yohkoh. Magnetic energy conversion by reconnection in two dimensions is relatively well understood, but in three dimensions we are only starting to understand the complexity of the magnetic topology and the MHD dynamics which are involved. How the dynamics lead to particle acceleration is even less well understood. Particle acceleration in flares may in principle occur in a variety of ways, such as stochastic acceleration by MHD turbulence, acceleration by direct electric fields at the reconnection site, or diffusive shock acceleration at the different kinds of MHD shock waves that are produced during the flare. However, which of these processes is most important for producing the energetic particles that strike the solar surface remains a mystery.

An efficient one-step site-directed deletion, insertion, single and multiple-site plasmid mutagenesis protocol.

Abstract: Mutagenesis plays an essential role in molecular biology and biochemistry. It has also been used in enzymology and protein science to generate proteins which are more tractable for biophysical techniques. The ability to quickly and specifically mutate a residue(s) in protein is important for mechanistic and functional studies. Although many site-directed mutagenesis methods have been developed, a simple, quick and multi-applicable method is still desirable. We have developed a site-directed plasmid mutagenesis protocol that preserved the simple one step procedure of the QuikChange™ site-directed mutagenesis but enhanced its efficiency and extended its capability for multi-site mutagenesis. This modified protocol used a new primer design that promoted primer-template annealing by eliminating primer dimerization and also permitted the newly synthesized DNA to be used as the template in subsequent amplification cycles. These two factors we believe are the main reasons for the enhanced amplification efficiency and for its applications in multi-site mutagenesis. Our modified protocol significantly increased the efficiency of single mutation and also allowed facile large single insertions, deletions/truncations and multiple mutations in a single experiment, an option incompatible with the standard QuikChange™. Furthermore the new protocol required significantly less parental DNA which facilitated the DpnI digestion after the PCR amplification and enhanced the overall efficiency and reliability. Using our protocol, we generated single site, multiple single-site mutations and a combined insertion/deletion mutations. The results demonstrated that this new protocol imposed no additional reagent costs (beyond basic QuikChange™) but increased the overall success rates.

Contemporary Guidance for Stated Preference Studies

Abstract: This article proposes contemporary best-practice recommendations for stated preference (SP) studies used to inform decision making, grounded in the accumulated body of peer-reviewed literature. These recommendations consider the use of SP methods to estimate both use and non-use (passive-use) values, and cover the broad SP domain, including contingent valuation and discrete choice experiments. We focus on applications to public goods in the context of the environment and human health but also consider ways in which the proposed recommendations might apply to other common areas of application. The recommendations recognize that SP results may be used and reused (benefit transfers) by governmental agencies and nongovernmental organizations, and that all such applications must be considered. The intended result is a set of guidelines for SP studies that is more comprehensive than that of the original National Oceanic and Atmospheric Administration (NOAA) Blue Ribbon Panel on contingent valuation, is more germane to contemporary applications, and reflects the two decades of research since that time. We also distinguish between practices for which accumulated research is sufficient to support recommendations and those for which greater uncertainty remains. The goal of this article is to raise the quality of SP studies used to support decision making and promote research that will further enhance the practice of these studies worldwide.