248 research outputs found
Salt-induced reentrant stability of polyion-decorated particles with tunable surface charge density
The electrostatic complexation between DOTAP-DOPC unilamellar liposomes and
an oppositely charged polyelectrolyte (NaPA) has been investigated in a wide
range of the liposome surface charge density. We systematically characterized
the "reentrant condensation" and the charge inversion of
polyelectrolyte-decorated liposomes by means of dynamic light scattering and
electrophoresis. We explored the stability of this model
polyelectrolyte/colloid system by fixing each time the charge of the bare
liposomes and by changing two independent control parameters of the
suspensions: the polyelectrolyte/colloid charge ratio and the ionic strength of
the aqueous suspending medium. The progressive addition of neutral DOPC lipid
within the liposome membrane gave rise to a new intriguing phenomenon: the
stability diagram of the suspensions showed a novel reentrance due to the
crossing of the desorption threshold of the polyelectrolyte. Indeed, at fixed
charge density of the bare DOTAP/DOPC liposomes and for a wide range of polyion
concentrations, we showed that the simple electrolyte addition first (low salt
regime) destabilizes the suspensions because of the enhanced screening of the
residual repulsion between the complexes, and then (high salt regime)
determines the onset of a new stable phase, originated by the absence of
polyelectrolyte adsorption on the particle surfaces. We show that the observed
phenomenology can be rationalized within the Velegol-Thwar model for
heterogeneously charged particles and that the polyelectrolyte desorption fits
well the predictions of the adsorption theory of Winkler and Cherstvy. Our
findings unambiguously support the picture of the reentrant condensation as
driven by the correlated adsorption of the polyelectrolyte chains on the
particle surface, providing interesting insights into possible mechanisms for
tailoring complex colloids via salt-induced effects.Comment: 34 pages, 7 figure
Interaction between like-charged polyelectrolyte-colloid complexes in electrolyte solutions: a Monte Carlo simulation study in the Debye-H\"uckel approximation
We study the effective interaction between differently charged
polyelectrolyte-colloid complexes in electrolyte solutions via Monte Carlo
simulations. These complexes are formed when short and flexible polyelectrolyte
chains adsorb onto oppositely charged colloidal spheres, dispersed in an
electrolyte solution. In our simulations the bending energy between adjacent
monomers is small compared to the electrostatic energy, and the chains, once
adsorbed, do not exchange with the solution, although they rearrange on the
particles surface to accomodate further adsorbing chains or due to the
electrostatic interaction with neighbor complexes. Rather unexpectedly, when
two interacting particles approach each others, the rearrangement of the
surface charge distribution invariably produces anti-parallel dipolar doublets,
that invert their orientation at the isoelectric point. These findings clearly
rule out a contribution of dipole-dipole interactions to the observed
attractive interaction between the complexes, pointing out that such
suspensions can not be considered dipolar fluids. On varying the ionic strength
of the electrolyte, we find that a screening length, short compared with the
size of the colloidal particles, is required in order to observe the attraction
between like charged complexes due to the non-uniform distribution of the
electric charge on their surface ('patch attraction'). On the other hand, by
changing the polyelectrolyte/particle charge ratio, the interaction between
like-charged polyelectrolyte-decorated (pd) particles, at short separations,
evolves from purely repulsive to strongly attractive. Hence, the effective
interaction between the complexes is characterized by a potential barrier,
whose height depends on the net charge and on the non-uniformity of their
surface charge distribution.Comment: 24 pages, 9 figure
Overcharging and reentrant condensation of thermoresponsive ionic microgels
We investigated the complexation of thermoresponsive anionic
poly(N-isopropylacrylamide) (PNiPAM) microgels and cationic
-polylysine (-PLL) chains. By combining electrophoresis,
light scattering, transmission electron microscopy (TEM) and dielectric
spectroscopy (DS) we studied the adsorption of -PLL onto the microgel
networks and its effect on the stability of the suspensions. We show that the
volume phase transition (VPT) of the microgels triggers a large polyion
adsorption. Two interesting phenomena with unique features occur: a
temperature-dependent microgel overcharging and a complex reentrant
condensation. The latter may occur at fixed polyion concentration, when
temperature is raised above the VPT of microgels, or by increasing the number
density of polycations at fixed temperature. TEM and DS measurements
unambiguously show that short PLL chains adsorb onto microgels and act as
electrostatic glue above the VPT. By performing thermal cycles, we further show
that polyion-induced clustering is a quasi-reversible process: within the time
of our experiments large clusters form above the VPT and partially re-dissolve
as the mixtures are cooled down. Finally we give a proof that the observed
phenomenology is purely electrostatic in nature: an increase of the ionic
strength gives rise to the polyion desorption from the microgel outer shell.Comment: 15 Figure
Study of network composition in interpenetrating polymer networks of poly(N isopropylacrylamide) microgels:the role of poly(acrylic acid)
Hypothesis: The peculiar swelling behaviour of poly(N-isopropylacrylamide)
(PNIPAM)-based responsive microgels provides the possibility to tune both
softness and volume fraction with temperature, making these systems of great
interest for technological applications and theoretical implications. Their
intriguing phase diagram can be even more complex if poly(acrylic acid) (PAAc)
is interpenetrated within PNIPAM network to form Interpenetrating Polymer
Network (IPN) microgels that exhibit an additional pH-sensitivity. The effect
of the PAAc/PNIPAM polymeric ratio on both swelling capability and dynamics is
still matter of investigation. Experiments: Here we investigate the role of
PAAc in the behaviour of IPN microgels across the volume phase transition
through dynamic light scattering (DLS), transmission electron microscopy (TEM)
and electrophoretic measurements as a function of microgel concentration and
pH. Findings: Our results highlight that aggregation is favored at increasing
weight concentration, PAAc content and pH and that a crossover PAAc content
C*_{PAAc} exists above which the ionic charges on the microgel become relevant.
Moreover we show that the softness of IPN microgels can be tuned ad hoc by
changing the PAAc/PNIPAM ratio. These findings provide new insights into the
possibility to control experimentally aggregation properties, charge and
softness of IPN microgels by varying PAAc content.Comment: preprint versio
On the effect of temperature on the reentrant condensation in polyelectrolyte-liposome complexation
In systems of highly charged linear polyelectrolytes and oppositely charged
colloidal particles, long-lived clusters of polyelectrolyte-decorated particles
form in an interval of concentrations around the isoelectric point, where
reentrant condensation connected to charge inversion of cluster is observed.
The mechanisms that drive the aggregation and stabilize, at the different
polymer/particle ratios, a well defined size of the aggregates are not
completely understood. Moreover, a central question still remains unanswered,
i.e., whether the clusters are true equilibrium or metastable aggregates. To
elucidate this point, in this work, we have investigated the effect of the
temperature on the formation of the clusters. We employed liposomes built up by
DOTAP lipid interacting with a simple anionic polyion, sodium polyacrylate,
over an extended concentration range below and over the isoelectric condition.
Our results show that the aggregation process can be described by a
thermally-activated mechanism.Comment: Submitted Langmui
Biophysical and biological contributions of polyamine-coated carbon nanotubes and bidimensional buckypapers in the delivery of miRNAs to human cells
Recent findings in nanomedicine have revealed that carbon nanotubes (CNTs) can be used as potential drug carriers, therapeutic agents and diagnostics tools. Moreover, due to their ability to cross cellular membranes, their nanosize dimension, high surface area and relatively good biocompatibility, CNTs have also been employed as a novel gene delivery vector system. In our previous work, we functionalized CNTs with two polyamine polymers, polyethyleneimine (PEI) and polyamidoamine dendrimer (PAMAM). These compounds have low cytotoxicity, ability to conjugate microRNAs (such as miR-503) and, at the same time, transfect efficiently endothelial cells. The parameters contributing to the good efficiency of transfection that we observed were not investigated in detail. In fact, the diameter and length of CNTs are important parameters to be taken into account when evaluating the effects on drug delivery efficiency. In order to investigate the biophysical and biological contributions of polymer-coated CNTs in delivery of miRNAs to human cells, we decided to investigate three different preparations, characterized by different dimensions and aspect ratios. In particular, we took into account very small CNTs, a suspension of CNTs starting from the commercial product and a 2D material based on CNTs (ie, buckypapers [BPs]) to examine the transfection efficiency of a rigid scaffold. In conclusion, we extensively investigated the biophysical and biological contributions of polyamine-coated CNTs and bidimensional BPs in the delivery of miRNAs to human cells, in order to optimize the transfection efficiency of these compounds to be employed as efficient drug delivery vectors in biomedical applications
Conductometric properties of linear polyelectrolytes in poor-solvent condition: The necklace model
Comparative treatments of a green tattoo ink with Ruby, Nd: YAG nano- and picosecond lasers in normal and array mode
The tattoos removal has become an issue upon spread of the tattooing practice worldwide and
hindsight regrets. Lasers are typically used for the purpose, though some colours such as green are
considered “recalcitrant” to the treatment. In the current investigation, we aim at determining the
efcacy of removal of a green ink water dispersion, using 5 laser treatments: Nd:YAG nano- and
picosecond lasers in normal and array mode and Ruby nanosecond laser, keeping the total irradiated
energy constant. The UV–Vis spectroscopy of the treated samples indicate that Nd:YAG picosecond
laser is most efective, and the Ruby nanosecond laser is the least efcient. Fragment compounds
generated from the pigment and siloxanes are common to all treatments, whereas hydrocarbon
emerge by a larger amount upon Nd:YAG nanosecond treatment. Fibres are formed upon picosecond
treatments and when operating in array mode, and lamellae are achieved by Ruby nanosecond laser
treatment. Residual particles suspensions are very heterogeneous upon nanosecond treatments
Proton-driven patterning of bulk transition metal dichalcogenides
At the few-atom-thick limit, transition metal dichalcogenides (TMDs) exhibit
a host of attractive electronic optical, and structural properties. The
possibility to pattern these properties has a great impact on applied and
fundamental research. Here, we demonstrate spatial control over the light
emission, lattice deformation, and hydrogen storage in bulk TMDs. By low-energy
proton irradiation, we create uniquely favorable conditions for the production
and accumulation of molecular hydrogen just one or few monolayers beneath the
crystal basal plane of bulk WS2, WSe2, WTe2, MoSe2, and MoS2 samples. H2
therein produced coalesces to form bubbles, which lead to the localized
swelling of one X-M-X plane prevalently. This results eventually in the
creation of atomically thin domes filled with molecular hydrogen at 10 atm. The
domes emit light strongly well above room temperature and can store H2
indefinitely. They can be produced with the desired density, well-ordered
positions, and size tunable from the nanometer to the micrometer scale, thus
providing a template for the manageable and durable mechanical and electronic
structuring of two-dimensional materials
- …