3,565 research outputs found
Hexagonal microlasers based on organic dyes in nanoporous crystals
Molecular sieves, such as nanoporous AlPO_4-5, can host a wide variety of
laser active dyes. We embedded pyridine 2 molecules as a representative of a
commercially available dye which fits into the channel pores of the host
matrix. Many efficient dye molecules, such as rhodamines, do not fit into the
pores. But the amount of encapsulated dyes can be increased by modifying the
structure of the dyes such that they match the host templates. The resulting
microlasers have properties that depend on size and shape of the
microresonators, and we discuss a model for microscopic hexagonal ring
resonators. In terms of pump needed to reach lasing threshold molecular sieve
microlasers are comparable to VCSELs. For dyes which fit into the pores we
observed a partial regeneration of photo-induced damage.Comment: 10 pages, 16 figure
The influence of geometry, surface character and flexibility on the permeation of ions and water through biological pores
A hydrophobic constriction site can act as an efficient barrier to ion and
water permeation if its diameter is less than the diameter of an ion's first
hydration shell. This hydrophobic gating mechanism is thought to operate in a
number of ion channels, e.g. the nicotinic receptor, bacterial mechanosensitive
channels (MscL and MscS) and perhaps in some potassium channels (e.g. KcsA,
MthK, and KvAP). Simplified pore models allow one to investigate the primary
characteristics of a conduction pathway, namely its geometry (shape, pore
length, and radius), the chemical character of the pore wall surface, and its
local flexibility and surface roughness. Our extended (ca. 0.1 \mu s) molecular
dynamic simulations show that a short hydrophobic pore is closed to water for
radii smaller than 0.45 nm. By increasing the polarity of the pore wall (and
thus reducing its hydrophobicity) the transition radius can be decreased until
for hydrophilic pores liquid water is stable down to a radius comparable to a
water molecule's radius. Ions behave similarly but the transition from
conducting to non-conducting pores is even steeper and occurs at a radius of
0.65 nm for hydrophobic pores. The presence of water vapour in a constriction
zone indicates a barrier for ion permeation. A thermodynamic model can explain
the behaviour of water in nanopores in terms of the surface tensions, which
leads to a simple measure of "hydrophobicity" in this context. Furthermore,
increased local flexibility decreases the permeability of polar species. An
increase in temperature has the same effect, and we hypothesise that both
effects can be explained by a decrease in the effective solvent-surface
attraction which in turn leads to an increase in the solvent-wall surface free
energy.Comment: Peer reviewed article appeared in Physical Biology
http://www.iop.org/EJ/abstract/1478-3975/1/1/005
Water-mediated interactions between hydrophobic and ionic species in cylindrical nanopores
We use Metropolis Monte Carlo and umbrella sampling to calculate the free
energies of interaction of two methane molecules and their charged derivatives
in cylindrical water-filled pores. Confinement strongly alters the interactions
between the nonpolar solutes, and completely eliminates the solvent separated
minimum (SSM) that is seen in bulk water. The free energy profiles show that
the methane molecules are either in contact or at separations corresponding to
the diameter and the length of the cylindrical pore. Analytic calculations that
estimate the entropy of the solutes, which are solvated at the pore surface,
qualitatively explain the shape of the free energy profiles. Adding charges of
opposite sign and magnitude or (where is the electronic charge)
to the methane molecules decreases their tendency for surface solvation and
restores the SSM. We show that confinement induced ion-pair formation occurs
whenever , where is the Bjerrum length, and is the
pore diameter. The extent of stabilization of the SSM increases with ion charge
density as long as . In pores with nm, in which the
water is strongly layered, increasing the charge magnitude from to
reduces the stability of the SSM. As a result, ion-pair formation which occurs
with negligible probability in the bulk, is promoted. In larger diameter pores
that can accomodate a complete hydration layer around the solutes, the
stability of the SSM is enhanced.Comment: 23 pages, 8 figures. To be published in The Journal of Chemical
Physic
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