27 research outputs found
Thin Films of Zinc Oxide Nanoparticles and Poly(acrylic acid) Fabricated by the Layer-by-Layer Technique: a Facile Platform for Outstanding Properties
The incorporation of nanoparticles into polyelectrolytes
thin films
opens the way to a broad range of applications depending on the functionality
of the nanoparticles. In this work, thin films of ZnO nanoparticles
and poly(acrylic acid) (PAA) were built up using the layer-by-layer
technique. The thickness of a 20-bilayer film is about 120 nm with
a surface roughness of 22.9 nm as measured by atomic force microscopy
(AFM). Thin ZnO/PAA films block UV radiation starting at a wavelength
of 361 nm due to absorption by ZnO although the films are highly transparent.
Due to their high porosity, these thin films show a broadband antireflection
in the visible region, and thus they provide selective opacity in
the UV region and enhanced transmittance in the visible region up
to the near-infrared region. They are also superhydrophilic due to
their high porosity and surface roughness
Influence of Surface Charge Density and Morphology on the Formation of Polyelectrolyte Multilayers on Smooth Charged Cellulose Surfaces
To
clarify the importance of the surface charge for the formation
of polyelectrolyte multilayers, layer-by-layer (LbL) assemblies of
polydiallyldimethylammonium chloride (pDADMAC) and polystyrenesulfonate
(PSS) have been investigated on cellulose films with different carboxylic
acid contents (20, 350, 870, and 1200 μmol/g) regenerated from
oxidized cellulose. The wet cellulose films were thoroughly characterized
prior to multilayer deposition using quantitative nanomechanical mapping
(QNM), which showed that the mechanical properties were greatly affected
by the degree of oxidation of the cellulose. Atomic force microscopy
(AFM) force measurements were used to determine the surface potential
of the cellulose films by fitting the force data to the DLVO theory.
With the exception of the 1200 μmol/g film, the force measurements
showed a second-order polynomial increase in surface potential with
increasing degree of oxidation. The low surface potential for the
1200 μmol/g film was attributed to the low degree of regeneration
of the cellulose film in aqueous media due to increasing solubility
with increasing charge. The multilayer formation was characterized
using a quartz crystal microbalance with dissipation (QCM-D) and stagnation-point
adsorption reflectometry (SPAR). Extensive deswelling was observed
for the charged films when pDADMAC was adsorbed due to the reduced
osmotic pressure when ions inside the film were released, and the
1:1 charge compensation showed that all the charges in the films were
reached by the pDADMAC. The multilayer formation was not significantly
affected by the charge density above 350 μmol/g due to interlayer
repulsions, but it was strongly affected by the salt concentration
during the layer build-up
Nanometer Smooth, Macroscopic Spherical Cellulose Probes for Contact Adhesion Measurements
Cellulose spheres were prepared by
dissolving cellulose fibers and subsequently solidifying the solution
in a nonsolvent. Three different solution concentrations were tested
and several nonsolvents were evaluated for their effect on the formation
of spheres. Conditions were highlighted to create cellulose spheres
with a diameter of ∼1 mm and a root-mean-square surface roughness
of ∼1 nm. These solid spheres were shown to be easily chemically
modified without changing the mechanical properties significantly.
Contact adhesion measurements were then implemented with these spheres
against a poly(dimethylsiloxane) (PDMS) elastomer in order to quantify
the adhesion. Using Johnson–Kendall–Roberts (JKR) theory,
we quantified the adhesion for unmodified cellulose and hydrophobic
cellulose spheres. We highlight the ability of these spheres to report
more accurate adhesion information, compared to spin-coated thin films.
The application of these new cellulose probes also opens up new possibilities
for direct, accurate measurement of adhesion between cellulose and
other materials instead of using uncertain surface energy determinations
to calculate the theoretical work of adhesion between cellulose and
different solid materials
Immunoselective Cellulose Nanospheres: A Versatile Platform for Nanotheranostics
This paper describes a novel route
for the preparation and functionalization
of perfectly spherical cellulose nanospheres (CNSs), ranging from
100 to 400 nm with a typical diameter of 160–170 nm, for use
in theranostics. The method of preparation enables both surface and
interior bulk functionalization, and this presumably also makes the
CNSs suitable for use in end-use applications other than theranostics.
Surface functionalization was here demonstrated by antibody conjugation
with an antibody specific toward the epidermal growth factor receptor
(EGFR) protein, i.e., facilitating interaction with cancer cells having
the EGFR. Besides showing specificity, the CNS–antibody conjugates
showed a very low nonspecific binding. The CNSs could easily be bulk
functionalized by embedding gold nanoparticles in the cellulose sphere
matrix during CNS preparation to provide imaging contrast for diagnostic
purposes
Spin-Assisted Multilayers of Poly(methyl methacrylate) and Zinc Oxide Quantum Dots for Ultraviolet-Blocking Applications
Thin UV-blocking films of poly(methyl methacrylate) (PMMA)
and
ZnO quantum dots (QDs) were built-up by spin-coating. Ellipsometry
reveals average thicknesses of 9.5 and 8.6 nm per bilayer before and
after heating at 100 °C for one hour, respectively. The surface
roughness measured by Atomic force microscopy (AFM) was 3.6 and 8.4
nm for the one and ten bilayer films, respectively. The linear increase
in thickness as well as the low surface roughness increment per bilayer
indicates a stratified multilayer structure and a smooth interface
without aggregation. The absorption of UV radiation increased with
increasing number of bilayers. At the same time, transmission was
damped at wavelengths shorter than 375 nm. The thin films had a high
and constant transparency in the visible region. Green-light emitting
QDs could be detected by confocal microscopy at a concentration of
20% in a single layer of PMMA/ZnO. PMMA/ZnO QDs thin films are hydrophobic,
as indicated by contact angle measurements
Rapid Development of Wet Adhesion between Carboxymethylcellulose Modified Cellulose Surfaces Laminated with Polyvinylamine Adhesive
The surface of regenerated cellulose
membranes was modified by irreversible adsorption of carboxymethylcellulose
(CMC). Pairs of wet CMC-modified membranes were laminated with polyvinylamine
(PVAm) at room temperature, and the delamination force for wet membranes
was measured for both dried and never-dried laminates. The wet adhesion
was studied as a function of PVAm molecular weight, amine content,
and deposition pH of the polyelectrolyte. Surprisingly the PVAm–CMC
system gave substantial wet adhesion that exceeded that of TEMPO-oxidized
membranes with PVAm for both dried and never-dried laminates. The
greatest wet adhesion was achieved for fully hydrolyzed high molecular
weight PVAm. Bulk carboxymethylation of cellulose membranes gave inferior
wet adhesion combined with PVAm as compared to CMC adsorption which
indicates that a CMC layer of the order of 10 nm was necessary. There
are no obvious covalent cross-linking reactions between CMC and PVAm
at room temperature, and on the basis of our results, we are instead
attributing the wet adhesion to complex formation between the PVAm
and the irreversibly adsorbed CMC at the cellulose surface. We propose
that interdigitation of PVAm chains into the CMC layer is responsible
for the high wet adhesion values
Nanometer-Thick Hyaluronic Acid Self-Assemblies with Strong Adhesive Properties
The adhesive characteristics of poly(allylamine
hydrochloride)
(PAH)/hyaluronic acid (HA) self-assemblies were investigated using
contact adhesion testing. Poly(dimethylsiloxane) spheres and silicon
wafers were coated with layer-by-layer (LbL) assemblies of PAH/HA.
No increase in adhesion was observed when surfaces covered with dried
LbL films were placed in contact. However, bringing the coated surfaces
in contact while wet and separating them after drying resulted in
an increase by a factor of 100 in the work of adhesion (from one to
three bilayers). Herein we discuss the adhesion in PAH/HA and PAH/poly(acrylic
acid) assemblies. PAH/HA assemblies have potential application as
strong biomedical adhesives
Adhesive Layer-by-Layer Films of Carboxymethylated Cellulose Nanofibril–Dopamine Covalent Bioconjugates Inspired by Marine Mussel Threads
The preparation of multifunctional films and coatings from sustainable, low-cost raw materials has attracted considerable interest during the past decade. In this respect, cellulose-based products possess great promise due not only to the availability of large amounts of cellulose in nature but also to the new classes of nanosized and well-characterized building blocks of cellulose being prepared from trees or annual plants. However, to fully utilize the inherent properties of these nanomaterials, facile and also sustainable preparation routes are needed. In this work, bioinspired hybrid conjugates of carboxymethylated cellulose nanofibrils (CNFC) and dopamine (DOPA) have been prepared and layer-by-layer (LbL) films of these modified nanofibrils have been built up in combination with a branched polyelectrolyte, polyethyleneimine (PEI), to obtain robust, adhesive, and wet-stable nanocoatings on solid surfaces. It is shown that the chemical functionalization of CNFCs with DOPA molecules alters their conventional properties both in liquid dispersion and at the interface and also influences the LbL film formation by reducing the electrostatic interaction. Although the CNFC–DOPA conjugates show a lower colloidal stability in aqueous dispersions due to charge suppression, it was possible to prepare the LbL films through the consecutive deposition of the building blocks. Adhesive forces between multilayer films prepared using chemically functionalized CNFCs and a silica probe are much stronger in the presence of Fe<sup>3+</sup> than those between a multilayer film prepared from unmodified nanofibrils and a silica probe. The present work demonstrates a facile way to prepare chemically functionalized cellulose nanofibrils whereby more extended applications can produce novel cellulose-based materials with different functionalities
Formation of Colloidal Nanocellulose Glasses and Gels
Nanocellulose
(NC) suspensions can form rigid volume-spanning arrested
states (VASs) at very low volume fractions. The transition from a
free-flowing dispersion to a VAS can be the result of either an increase
in particle concentration or a reduction in interparticle repulsion.
In this work, the concentration-induced transition has been studied
with a special focus on the influence of the particle aspect ratio
and surface charge density, and an attempt is made to classify these
VASs. The results show that for these types of systems two general
states can be identified: glasses and gels. These NC suspensions had
threshold concentrations inversely proportional to the particle aspect
ratio. This dependence indicates that the main reason for the transition
is a mobility constraint that, together with the reversibility of
the transition, classifies the VASs as colloidal glasses. If the interparticle
repulsion is reduced, then the glasses can transform into gels. Thus,
depending on the preparation route, either soft and reversible glasses
or stiff and irreversible gels can be formed
Direct Adhesive Measurements between Wood Biopolymer Model Surfaces
For the first time the dry adhesion was measured for
an all-wood
biopolymer system using Johnson–Kendall–Roberts (JKR)
contact mechanics. The polydimethylsiloxane hemisphere was successfully
surface-modified with a Cellulose I model surface using layer-by-layer
assembly of nanofibrillated cellulose and polyethyleneimine. Flat
surfaces of cellulose were equally prepared on silicon dioxide substrates,
and model surfaces of glucomannan and lignin were prepared on silicon
dioxide using spin-coating. The measured work of adhesion on loading
and the adhesion hysteresis was found to be very similar between cellulose
and all three wood polymers, suggesting that the interaction between
these biopolymers do not differ greatly. Surface energy calculations
from contact angle measurements indicated similar dispersive surface
energy components for the model surfaces. The dispersive component
was dominating the surface energy for all surfaces. The JKR work of
adhesion was lower than that calculated from contact angle measurements,
which partially can be ascribed to surface roughness of the model
surfaces and overestimation of the surface energies from contact angle
determinations