Upload
lydat
View
217
Download
0
Embed Size (px)
Citation preview
Journal of Colloid and Interface Science 470 (2016) 92–99doi:10.1016/j.jcis.2016.02.052
Fabrication of polyelectrolyte multilayered
nano-capsules using a continuous layer-by-layer
approach
Iuliia S. Elizarova* and Paul F. Luckham
*Corresponding author
E-mail addresses: [email protected] (Iuliia Elizarova)
[email protected] (Paul Luckham)
Department of Chemical Engineering and Chemical Technology, Imperial College London,
Prince Consort Road, London SW7 2AZ, UK
Keywords: layer-by-later self-assembly, continuous, polydiallyldimethylammonium chloride
PDADMAC, polystyrenesulfonate PSS, lambda carrageenan, poly-L-lysine, nano-capsules,
calcium phosphate.
1
Journal of Colloid and Interface Science 470 (2016) 92–99doi:10.1016/j.jcis.2016.02.052
ABSTRACT
The layer-by-layer approach is a highly versatile method for the fabrication of multilayered
polymeric films and capsules. It has been widely investigated in research for various
polyelectrolyte pairs and core template particles. However, the fabrication of nano-sized
capsules at the larger scale is difficult and time consuming, due to the necessity of washing
and centrifugation steps before the deposition of each polyelectrolyte layer. This results not
only in a very long fabrication time, but also in the partial loss of particles during those
intermediate steps. In this study, we introduced a continuous approach for the fabrication of
multilayer polyelectrolyte based nano-capsules using calcium phosphate core nanoparticles
and a tubular flow type reactor with the potential for synthesizing tens of milligrams of
capsules per hour. Adsorption of the polyelectrolyte layer occurred in the tubing where
particles and polyelectrolyte solution of choice were mixed, creating a layer of
polyelectrolyte on the particles. After this, these newly surfaced-modified particles passed
into the next segment of tubing, where they were mixed with a second polyelectrolyte of
opposite charge. This process can be continuously repeated until the desired number of layers
is achieved. One potential problem with this method concerned the presence of any excess
polyelectrolyte in the tubing, so careful control of the amount of polymer added was crucial.
It was found that slightly under dosing the amount of added polyelectrolyte ensured that
negligible unadsorbed polyelectrolyte remained in solution. The particles created at each
deposition step were stable, as they all had a zeta potential of greater than +/-25 mV.
Furthermore the zeta potential measurements showed that charge reversal occurred at each
stage. Having achieved the necessary number of polyelectrolyte layers, the calcium phosphate
cores were easily removed via dissolution in either hydrochloric or acetic acid.
2
Journal of Colloid and Interface Science 470 (2016) 92–99doi:10.1016/j.jcis.2016.02.052
INTRODUCTION
The deposition of materials of opposite charge sequentially onto surfaces or particles has
become increasingly well studied in recent decades. The original studies date back to the
work of Iler[1] fifty years ago, who deposited alternating layers of positively-charged silica
and negatively-charged boehmite onto flat charged surfaces. Surprisingly, the work remained
largely in abeyance for twenty five years and it was not until Decher et al adapted the
approach to coat oppositely charged surfaces with polyelectrolyte multilayers [2–4] which led
to extensive study in this area. This method was later used to coat colloidal particles[5–7] and
subsequently construct microcapsules[8,9]. The microcapsules were prepared by the stepwise
adsorption of polyelectrolytes onto charged colloidal template particles, before the removal of
the core particles via dissolution in acid. This fabrication technique allowed for control over
the capsule size (by controlling the core template size), the capsule wall thickness (by
controlling the number of polymer layers), and the capsule wall composition. This process
has now emerged as a simple, versatile and inexpensive technique for the construction of
polymeric thin films on both surfaces and particles. This methodology has been used in many
studies to prepare capsules of different sizes using various polymer pairs (not all involving
opposite charged polyelectrolytes) and core template particles. However, there are a number
of associated issues with this technology. Firstly, although the preparation of micron-sized
capsules has been met with relative ease, the fabrication of nanosized capsules has proved to
be a challenge[10–12]. Micron sized particles using melamine cores were originally used
because the core can be easily removed via the immersion in acid, however difficulty arose
with particles less than 100 nm in size, because the core particles that small were
unavailable[7–9,13–18]. Fabrication of smaller capsules is possible with the use of silica or
polystyrene particles as core templates, but complete removal of the core is problematic[19–
23]. Polystyrene is commonly dissolved by immersing the coated particles in tetrahydrofuran,
3
Journal of Colloid and Interface Science 470 (2016) 92–99doi:10.1016/j.jcis.2016.02.052
THF, which is both miscible with water and a strong solvent for polystyrene. However, the
polystyrene would need to diffuse through the polyelectrolyte film and whether complete
dissolution occurs whilst maintaining the integrity of the polyelectrolyte film is uncertain.
Another issue related to the layer-by-layer manufacture of nano-capsules, concerns the time
consuming, labour intensive method currently used for producing the capsules, which inhibits
their use in any large scale commercial application. The commonly practiced procedure
involves mixing the core particles with a dilute polyelectrolyte solution until the adsorption
of the polyelectrolyte is complete. The resulting suspension is then centrifuged and the
particles redispersed in water, before the process is repeated a further two, or three, times to
ensure removal of any unadsorbed polyelectrolyte. The entire procedure is then repeated for
the addition of every polyelectrolyte layer. As well as being time consuming, each
centrifugation/redispersion cycle inevitably leads to the partial loss of particles, so that after
10-20 cycles, not many particles remain, even before the core removal process occurs.
A number of authors have proposed various solutions to these problems. Voigt[16] developed
a method of layer-by-layer polyelectrolyte adsorption on core latex particles by means of
semi-continuous membrane filtration. This method produced high quality microcapsules due
to reduced particle aggregation, but required constant control and repeated manual
adjustments, which make scaling-up a challenge. Kantak[24] presented a “microfluidic
pinball” technique for the continuous generation of microcapsules, which involved the
microfluidic production of oil droplets. The droplets were guided along a row of micropillars
and through three parallel laminar streams – two polymer solutions and one washing solution.
This method, however, was only capable of producing a relatively small number of capsules.
Richardson[25] presented an automated capsule preparation approach, whereby the core
particles were immobilized in a gel film which was then clamped and dipped into different
polyelectrolyte and rinsing solutions. The gel was then melted and the particles recovered,
4
Journal of Colloid and Interface Science 470 (2016) 92–99doi:10.1016/j.jcis.2016.02.052
however again only a few particles can be made at any one time. Later, Richardson
introduced another method based on the utilisation of immobilised particles in a hydrogel
[26], in a process called electrophoretic polymer assembly. The hydrogel-particle substrate
was placed between two electrodes so that the polyelectrolyte solution travelled through the
hydrogel-particle substrate from anode to cathode or vice versa, depending on the
polyelectrolyte charge. Richardson also developed a method called convective polymer
assembly[27], that involved using immobilised particles in a hydrogel and convection to
move the polyelectrolyte solutions through the gel. However, once again, only a small
number of capsules can be made using this method. One of the more recent methods for the
fabrication of layer-by-layer capsules was developed by Björnmalm[28], and is a solely flow-
based technique which used tangential flow filtration to aid the removal of excess
polyelectrolyte from the system.
The method we present in this paper addresses the common issues associated with the layer-
by-layer technique, but from a novel perspective. We have investigated the build-up of the
polyelectrolyte multilayers[29–35] onto the core particles using a tubular type reactor,
whereby polyelectrolytes of opposite charge are introduced at different points along a tube.
One problem that was anticipated in this work was the presence of excess polyelectrolyte
following the adsorption process. However, it is known that polymers and polyelectrolytes
exhibit a high affinity type adsorption isotherm, where initially almost all the added polymer
adsorbs to a surface of a particle, and only in the final stages, does any excess polymer
remain in solution[36–40]. Therefore, precise control over the concentration ratio of the
added polymer to core particles may allow for the buildup of multilayers of the
polyelectrolyte, without any significant polyelectrolyte complexation issues. Hence, the aim
of this work was to investigate the hypothesis by deliberately using fractionally less
polyelectrolyte than would be required to reach saturation conditions. We show that it is
5
Journal of Colloid and Interface Science 470 (2016) 92–99doi:10.1016/j.jcis.2016.02.052
possible to produce nano-capsules at a flow rate of 600 milliliters of colloidal suspension per
hour, enabling relatively large quantities of capsules to be easily prepared. More
significantly, we suggest an approach for the future large-scale, commercial production of
nano-capsules prepared from alternating layers of charged materials.
EXPERIMENTAL SECTION
Chemicals
Two types of core particles, both comprised of calcium phosphate coated with a positively
charged polyelectrolyte, were prepared. For the first type of particles, the following materials
were used: poly(diallyldimethylammonium chloride) (PDADMAC, average Mw 100000 –
200000 (low molecular weight), 20 wt. % in H2O, Sigma-Aldrich), calcium L-lactate hydrate
( ≥98 % KT calc. based on dry substance, water ~ 4 mol/mol, Sigma-Aldrich, Gillingham,
Dorset), ammonium phosphate (Hopkin & Williams Ltd). The same set of materials was
used for the second type of core particles, except that the polyelectrolyte – PDADMAC was
replaced with ε–Poly–L–Lysine (96,3%, Handary EpolylyTM HCL, Ultrapure). Three
polyelectrolyte pairs were used for layering in order to fabricate multilayered capsules. The
first pair was poly(sodium-4-styrenesulfonate) (PSS, average Mw ~70000, Sigma-Aldrich,
Gillingham, Dorset) and poly(diallyldimethylammonium chloride) (PDADMAC, average Mw
100000 – 200000 (low molecular weight), 20 wt. % in H2O, Sigma-Aldrich, Gillingham,
Dorset), the second pair was the poly(diallyldimethylammonium chloride) and lambda
carrageenan (Food Grade, Modernist Pantry), and the third pair was ε–Poly–L–Lysine and
lambda carrageenan. Sodium chloride (≥99.5 %, Sigma-Aldrich) was required for mixing
with the suspensions of resulting particles, hydrochloric acid (37%, VWR) and acetic acid
6
Journal of Colloid and Interface Science 470 (2016) 92–99doi:10.1016/j.jcis.2016.02.052
(Glacial ACS Grade, VWR) were used for core calcium phosphate removal. The molecular
structures of the polyelectrolytes used in this study are presented in Figure 1.
Figure 1. Molecular structures of polyelectrolytes: (a) PDADMAC, (b) PSS, (c) lambda
carrageenan, (d) ε–Poly–L–Lysine
Preparation of template calcium phosphate nanoparticles
In order to fabricate core template nanoparticles, the methodology (with subtle modifications)
described by Urch[41] was used. It involved the rapid mixing of aqueous solutions of calcium
lactate (24 mM, 7 g∙L-1) and ammonium phosphate (15 mM, 2.5 g∙L-1), resulting in the
formation of calcium phosphate nanoparticles, which were further mixed with a preferred
cationic polyelectrolyte solution (PDADMAC (10 g∙L-1) or ε–Poly–L–Lysine (1 g∙L-1)) for
coating. This process was conducted under ambient conditions using a tubular-type flow
reactor, as shown schematically in Figure 2. Using a Watson Marlow Sci Q 400 peristaltic
pump, all three solutions were pumped through three individual channels (comprised of
silicon tubing) at a volume ratio of 1:1:1. The flow rate through each channel was maintained
at 10 ml∙min-1. The silicon tubing had an internal diameter of 3 mm. The first tube, in which
7
Journal of Colloid and Interface Science 470 (2016) 92–99doi:10.1016/j.jcis.2016.02.052
the calcium phosphate nanoparticles were formed, had a length of 10 cm, whilst the second
tube, where the positively charged polyelectrolyte was adsorbed to the calcium phosphate
nanoparticles, had a length of 100 cm. The resulting particles were collected in a conical flask
without filtering, then stirred for further 24 hours. The collected nanoparticles were then
dialysed against deionised water using dialysis membranes (MWCO 12-14000 Daltons,
Medicell International Ltd) for 12 hours. The particles were then centrifuged at a speed of
6000 rpm for 20 minutes (AccuSpin™ 400, Fisher Scientific), the supernatant was removed
and replaced with an equivalent volume of deionised water. The resulting system was
sonicated for 2 minutes (Ultrasonic Analog SRH Baths FB15048, Fisher Scientific) and
centrifuged at the speed of 6000 rpm for further 20 minutes. This process removed the
ammonium lactate from the solution and any excess calcium, or phosphate, ions in solution.
The supernatant was removed and replaced with an equivalent volume of deionised water
again. After that, 15mM NaCl solution was added to the system in a volume ratio 1:1 in
order to enhance subsequent polyelectrolyte coating (resulting in NaCl concentration in
solution of 7,5 mM), followed by sonication for a further 5 minutes. Afterwards, the system
was shaken with a mechanical shaker (Stuart™ SF1 Flask Shaker, Stuart Scientific) to
achieve homogeneity throughout the solution. These particles were then used as a core for
coating with further polyelectrolyte layers. The concentration of the resulting particles was
2.2 g∙L-1.
8
Journal of Colloid and Interface Science 470 (2016) 92–99doi:10.1016/j.jcis.2016.02.052
Figure 2. Preparation of polymer-functionalised calcium phosphate nanoparticles in a tubular
flow reactor
Addition of polyelectrolyte layers
In this study, the idea was to flow the particles and the polyelectrolyte through a tubular flow
type reactor, consisting of a peristaltic pump and silicon tubing. (Under the conditions of
operation, the Reynolds number was estimated to be 14 based on water at 20ºC, thus the flow
in the tube is laminar, pulsed). It was necessary to adjust the length of the tubing to allow
sufficient time for the complete adsorption of the polyelectrolyte onto a particle, before a
second polyelectrolyte of opposing charge was added and again the tube length was adjusted
to ensure this second polyelectrolyte fully adsorbed onto the particles. This process continued
until the required number of polyelectrolyte layers had been added. Since the elimination of
the purification of the particles between polyelectrolyte addition steps was a key concept
behind the approach, it was necessary to ensure that as little unadsorbed polyelectrolyte as
possible was present in the reactor. Excess amount of polyelectrolytes could cause
polyelectrolyte complexation issues, leading to the aggregation of the layered particles. The
minimum concentration of polyelectrolyte necessary to form a stable layer on the particle,
without causing any major irreversible aggregation or complexation, was denoted the
optimum polyelectrolyte concentration. Preliminary experiments were undertaken to
determine this optimum polyelectrolyte concentration, utilizing zeta potential measurements
as a monitoring tool for adsorption of polyelectrolytes onto the core particles. The full details
of this procedure are presented in the section 1 of the supplementary data of this paper.
9
Journal of Colloid and Interface Science 470 (2016) 92–99doi:10.1016/j.jcis.2016.02.052
Continuous layering of polyanion/polycation onto core particles
The experimental setup is presented in Figure 3 and consists of a Watson Marlow Sci Q 400
peristaltic pump with three individual channels, several pieces of silicon tubing (3 mm
diameter) and a set of opening/closing valves. All three channels pump solutions at the same
rate (1.9 ml∙min-1). Samples at each polyelectrolyte layer deposition step can be taken for
analysis through outlets in the tubing.
Figure 3. Schematic layout of continuous production
The core particles, polyanion and polycation solutions of the experimentally pre – defined
optimum concentrations entered the tubing through the three individual pumping channels.
Polyelectrolyte solutions were alternatively deposited onto the positively charged calcium
phosphate – PDADMAC/ε–Poly–L–Lysine nanoparticles starting with polyanion solution
(see Figure 3).
The length of tubing required for the stable deposition of polyelectrolytes was determined
according to the necessary deposition time, which was different for each polyelectrolyte, as
well as to the flow rate. The deposition time was determined by monitoring the zeta potential
of the mixture of the core particles and polyelectrolyte as a function of time. In this study,
zeta potential measurements were performed using a ZetaPALS Zeta Potential Analyser from
10
Journal of Colloid and Interface Science 470 (2016) 92–99doi:10.1016/j.jcis.2016.02.052
Brookhaven Instruments Corp, this instrument has been shown to be suitable to determine the
zeta potential of calcium phosphate particles[42]. The process involved mixing the particles
with polyelectrolyte in cuvettes in 1:1 volume ratio, before immediately taking zeta potential
measurements. The time taken to reach a zeta potential > ± 25 mV, (a value which is
frequently assumed to represent stable particles) was denoted the deposition time. The full
details of this procedure can be found in the section 2 of the supplementary data for this
publication.
Having obtained the deposition time for the polyelectrolytes in these static experiments the
same time period for polyelectrolyte deposition was assumed for the flow experiments. The
flow rate was set to the slowest setting on the peristaltic pump in order to minimize the length
of tubing whilst providing the necessary amount of time for polyelectrolyte deposition.
Furthermore, with the deposition time and the flow rate being known, the length of 3 mm in
diameter tubing required was readily established (see details in the section 3 of the
supplementary data).
After reaching the necessary number of polyelectrolyte layers, which was selected to be 9 to
ensure the particles were positively charged, the suspensions were purified by dialysis against
distilled water for 24 hours using dialysis membranes (MWCO 12-14000 Daltons, Medicell
International Ltd). This was followed by two centrifugation (6000 rpm for 20 minutes) and
redispersion (sonication for 2 minutes) steps. After the final stage the particles were re-
dispersed in 7.5 mM NaCl solution followed by sonication for 20 minutes.
Core removal
In order to remove core calcium phosphate particles from the polyelectrolyte coating, a
suspension of particles with 9 adsorbed layers of polyelectrolytes was added to an equal
volume of 0.1 M solutions of HCl (for all three pairs) or 0.02 M acetic acid (for lambda
11
Journal of Colloid and Interface Science 470 (2016) 92–99doi:10.1016/j.jcis.2016.02.052
carrageenan/ε–Poly–L–Lysine pair). The resulting systems were slowly stirred for 1 hour,
then dialysed against deionised water for 24 hours using dialysis membranes (MWCO 12-
14000 Daltons, Medicell International Ltd). The resulting suspensions were translucent rather
than opaque, indicating the removal of the core calcium phosphate particles, and stored
without any further treatment.
Characterisation
In order to determine the optimum polyelectrolyte concentration, zeta potential measurements
were used to monitor the adsorption of polyelectrolytes onto particles. Zeta potential is the
potential difference between the dispersion medium and the stationary layer of fluid attached
to the dispersed particle. The principles of zeta potential, and the electro-kinetic effects
associated with its measurements, are well described by Hunter[43]. Since the stability of the
layered particles could indicate full polyelectrolyte coverage[44], it was considered justifiable
to use zeta potential technique for the purpose of the determination of the optimum
polyelectrolyte concentration. The details of this procedure are presented in the section 1 of
the supplementary data of this paper. The use of this technique to determine the deposition
time of the polyelectrolytes was governed by the same considerations associated with the
determination of the optimum polyelectrolyte concentrations – the stability of the layered
particles would mean a high level of adsorption of the polyelectrolyte. The full details of this
procedure can be found in the section 2 of the supplementary data for this publication. Zeta
potential measurements were also taken for samples at each deposition step in order to ensure
that the multilayered nanoparticles formed were stable and the charge on their surface was
reversed. The results presented are the average of three sets of readings (each of which were
themselves averages of 20 readings).
12
Journal of Colloid and Interface Science 470 (2016) 92–99doi:10.1016/j.jcis.2016.02.052
Transmission electron microscopy (TEM JEOL 2000 FX) was used in order to determine the
size, and morphology of the particles and the polyelectrolyte layered nanocapsules. The TEM
samples were prepared by drying a drop of the suspension onto Carbon Films on 300 Mesh
Copper Grids (Agar Scientific).
RESULTS AND DISCUSSION
Determination of the optimum polyelectrolyte concentrations
Using the aforementioned zeta potential measurement approach, the optimum polyelectrolyte
concentrations were determined and are shown in Table 1. A ten times dilution of the core
particles was performed in order to reduce aggregation and to assist the fabrication process in
case of two polyelectrolyte pairs – PSS/PDADMAC and lambda carrageenan/ε–Poly–L–
Lysine. However, there was no observable aggregation in case of lambda
carrageenan/PDADMAC pair, hence dilution was not necessary.
Since layer-by-layer polyelectrolyte deposition is believed to be governed by electrostatic
interactions between the polyelectrolytes[45], one would expect the ratio between anionic and
cationic polyelectrolyte concentrations to be related to the ratio of the amount of charge
present in each polyelectrolyte solution. Based on chemical formulas and accounting for the
specifics of stock materials (for example, stock solution of PDADMAC is 20 wt. % in H2O,
not 100 wt. %), these ratios have been approximately calculated and are also presented in
Table 1. Full details of calculations can be found in subsection 1.4 of the supplementary data
of this paper.
Experimental polyelectrolyte concentration ratios were found to be in a good correlation with
the calculated values.
Table 1. The optimum concentrations of cationic and anionic polyelectrolytes
Polyelectrolyte pair
Core particles type and
concentration, g∙L-1
Cation concentration,
g∙L-1
Anion concentration,
g∙L-1
Experimental cation/anion concentration
ratio
Calculated cation/anion ratio based on charge
PSS/PDADMAC Calcium 0.06 0.02 3 3.3
13
Journal of Colloid and Interface Science 470 (2016) 92–99doi:10.1016/j.jcis.2016.02.052
phosphate with one layer of
PDADMAC, 0.22 g∙L-1
Lambda carrageenan/PDADMAC
Calcium phosphate with
one layer of PDADMAC, 2.2
g∙L-1
0.06 0.02 3 3.7
Lambda carrageenan/ε–Poly–L–Lysine
Calcium phosphate with
one layer of PDADMA ε–
Poly–L–Lysine, 0.22 g∙L-1
0.08 0.07 1.14 1.1
Determination of the polyelectrolyte deposition times
The determination of the deposition time in this study was performed by measuring of the
zeta potential as a function of time for samples in cuvettes. In order to account for any
potential time differences of the polyelectrolyte adsorption process within the flow in the
tubular reactor, compared to a cuvette, it was determined that more time was required for
polyelectrolyte to adsorb within the system of tubing to ensure there was negligible
unadsorbed polyelectrolyte in solution before the next polyelectrolyte was added (see section
2 of the supplementary data).
The parameters of the process for different polyelectrolyte pairs were set as follows: first pair
– PSS deposition time 2 minutes/PDADMAC deposition time 3 minutes; second pair –
lambda carrageenan, deposition time 3 minutes/PDADMAC deposition time 6 minutes; third
pair (using calcium phosphate - ε–Poly–L–Lysine core particles) – ε–Poly–L–Lysine
deposition time is less than 1 minute/lambda carrageenan deposition time is less than 1
minute (see section 2 of the supplementary data for details).
The deposition time for lambda carrageenan when paired with PDADMAC was found to be
longer, than when paired with ε–Poly–L–Lysine. The reason for this was not entirely clear,
although the authors see a possible explanation in the difference of the core particles used in
14
Journal of Colloid and Interface Science 470 (2016) 92–99doi:10.1016/j.jcis.2016.02.052
these two experiments. For lambda carrageenan/PDADMAC pair the core particles were
coated with one layer of PDADMAC, for lambda carrageenan/ε–Poly–L–Lysine pair the core
particles were layered with one layer of ε–Poly–L–Lysine, but the stock material used for
layering was not pure ε–Poly–L–Lysine, but ε–Poly–L–Lysine hydrochloride. Even though
the concentration of hydrochloride in the stock solution was not high enough to dissolve the
newly formed core calcium phosphate particles, there is a possibility that hydrochloride
caused a slight decrease in their size, hence less time would be required to coat the smaller
particles.
Experimental setup parameters
Since the minimum deposition time for all three pairs of polyelectrolytes was found to be
different, it was determined that the setup should be assembled for the pair with the longest
deposition times, as this would provide the necessary amount of time for the deposition of all
the polyelectrolytes. The flow rate of the peristaltic pump was set to the slowest setting of 1.9
ml∙min-1 in order to decrease the length of tubing. The average total residence time of the
suspension in the setup was found to be approximately 20 minutes, counted from the entrance
point to the end point, after creating particles with five layers of polyelectrolyte onto them
(including the starting layer of the core particles). In order to produce more layers these
particles were simply transferred to the starting “core particles” container of the setup, before
undergoing further layering until nine layers were formed. There was no visible back mixing
of the particles into the polyelectrolyte streams observed.
The design of the setup corresponds to the scheme presented in Figure 3, whilst an additional
photograph of the setup is presented in the section 3 of the supplementary data.
Stability measurements for the layered particles
The stability of the multilayered particles after each deposition step is another significant
aspect of the continuous layer – by – layer approach. It is important that layered particles
15
Journal of Colloid and Interface Science 470 (2016) 92–99doi:10.1016/j.jcis.2016.02.052
(with any number of layers) do not undergo any major stability changes, as particle
aggregation would prevent uniform polyelectrolyte coating of each particle, resulting in
poorly dispersed final nanoparticles, and, consequently, nanocapsules.
The graphs for the step-by-step change of the zeta potential of the particles layered with
polyelectrolytes of previously determined optimum concentrations are presented in Figure 4.
It can be observed that the zeta potential exceeds values of ± 25 mV throughout, which
demonstrates that this continuous method of deposition of polyelectrolyte layers produces
stable particles.
Figure 4. Zeta potential as a function of number of layers for the particles layered with
different polyelectrolyte pairs. Unless visible, error bars are within the size of the data points.
Size and morphology of the layered particles and capsules
Figure 5 shows TEM images of both 9 layered particles and the resulting capsules for three
pairs of polyelectrolytes. For the PSS/PDADMAC pair (a) it can be seen that the size of the
16
Journal of Colloid and Interface Science 470 (2016) 92–99doi:10.1016/j.jcis.2016.02.052
particles and capsules varies from 60 to 200 nm, the shape of both particles and capsules is
spherical. Arrows on the picture show dark spots in some of the capsules which maybe the
remains of the core, but overall we may conclude that core removal was successful. The size
of the capsules made with lambda carrageenan/PDADMAC pair (b) varies at the same range
of 60 – 200 nm, they are also spherical. The situation is different when it comes to capsules
for the third pair, lambda carrageenan/ε – poly –l – lysine. With the use of hydrochloric acid
for the core removal capsules seem to shrink which can be possibly explained by ε – poly –l –
lysine hydrolysis in presence of the strong acid, HCl, while with the use of acetic acid they do
not change their size but change their shape which may be due to the mechanical properties
of capsules made of this polyelectrolyte pair being weaker than those of capsuled made of the
other two pairs.
17
Journal of Colloid and Interface Science 470 (2016) 92–99doi:10.1016/j.jcis.2016.02.052
Figure 5. TEM images of layered particles and capsules: (a) calcium phosphate particles
coated with 9 layers of PSS/PDADMAC (1 and 2) and the resulting capsules (3); (b) calcium
phosphate particles coated with 9 layers of lambda carrageenan/PDADMAC (4) and the
resulting capsules (5); (c) calcium phosphate particles coated with 9 layers of lambda
carrageenan/ ε–Poly–L–Lysine (6) and the resulting capsules formed using HCl (7) and using
acetic acid (8)
SUMMARY AND CONCLUSION
A continuous process to produce nanocapsules utilizing the layer-by-layer adsorption
mechanism has been developed. This method produced a suspension of nanoparticles at a rate
of 600 milliliters per hour, resulting in a concentration of multilayered particles of 0.22 g∙L-1
for the lambda carrageenan/ε–Poly–L–Lysine pair, 0.022 g∙L-1 for the poly(sodium-4-
styrenesulfonate)/poly(diallyldimethylammonium chloride pair and 0.022 g∙L-1 for the
lambda carrageenan/poly(diallyldimethylammonium chloride) pair. This was achieved by
preparing the capsules in a continuous flow tubular reactor consisting of a peristaltic pump
and a set of tubing; by controlling the deposition time of the polyelectrolyte (via the length of
tubing and the flow rate of the pump) and by deliberately under dosing the polyelectrolyte
concentrations to ensure that there was negligible unadsorbed polyelectrolyte remaining in
the reactor. Since this method contained no purification steps, precise determination of
polyelectrolyte concentration in relation to the concentration of the core particles was
therefore key to successful layering and creating stable particles and hence capsules. This
method significantly reduced production time and material usage in comparison to previous
approaches for the synthesis of nano- and micro-capsules, whilst managing to produce a
relatively large amount of capsules, making it more suitable for scale-up than any previously
reported strategies[5–9,16,24–28]. In this study, very narrow silicon tubing (3 millimeter in
18
Journal of Colloid and Interface Science 470 (2016) 92–99doi:10.1016/j.jcis.2016.02.052
diameter) and low flow rates (to reduce the required length of tubing) were used, and we
were able to produce 130 mg of 9 layered particles per hour. Future work will focus on
further scalability of the approach by using more concentrated core particles, as well as larger
diameter tubing and faster flow rates.
Acknowledgments
One of the authors (Iuliia S. Elizarova) would like to thank Imperial College London for the
provision of a student bursary. We would also like to greatly acknowledge Andrew W.
Russell for his kind assistance with our study, and to thank the referees of this paper for some
very helpful comments.
Supplementary information
The methods of determination of the optimum experiment parameters such as deposition time
and polyelectrolyte concentration are presented as a supplementary data. This material is
available free of charge via the Internet at http://www.journals.elsevier.com/journal-of-
colloid-and-interface-science/
References
[1] R.K. Iler, Multilayers of Colloidal Particles, J. Colloid Interface Sci.,. 21 (1966) 569–580.
[2] G. Decher, J.D. Hong, Buildup of Ultrathin Multilayer films by a self assembly Process. 1. Consecutive Adsorption of Anionic and Cationic Bipolar Amphiphiles on charged Surfaces , Makromol. Chemie-Macromolecular Symp. 46 (1991) 321–327. doi:10.1002/masy.19910460145.
[3] G. Decher, J.D. Hong, Buildup of Ultrathin Multilayer films by a self assembly Process. 2. Consecutive assembly of Anionic and Cationic bipolar Amphipliles and Polyelectrolytes on Charged Surfaces, Berichte Der Bunsen-Gesellschaft-Physical Chem. Chem. Phys. 95 (1991) 1430–1434. <Go to ISI>://WOS:A1991GX48500020.
[4] G. Decher, J. Schmitt, Fine-tuning of the film thickness of ultrathin multilayer films of consecutively Adsorbed alternating layers of Anionic and Cationic polyelectrolytes, in:
19
Journal of Colloid and Interface Science 470 (2016) 92–99doi:10.1016/j.jcis.2016.02.052
C. Helm, M. Losche, H. Mohwald (Eds.), Trends Colloid Interface Sci. VI, 1992: pp. 160–164. <Go to ISI>://WOS:A1992BW94Z00036.
[5] E. Donath, D. Walther, V.N. Shilov, E. Knippel, A. Budde, K. Lowack, et al., Nonlinear hairy layer theory of electrophoretic fingerprinting applied to consecutive layer by layer polyelectrolyte adsorption onto charged polystyrene latex particles, Langmuir. 13 (1997) 5294–5305. doi:10.1021/la970090u.
[6] P.T. Hammond, Recent explorations in electrostatic multilayer thin film assembly, Curr. Opin. Colloid Interface Sci. 4 (1999) 430–442. doi:10.1016/s1359-0294(00)00022-4.
[7] G.B. Sukhorukov, E. Donath, H. Lichtenfeld, E. Knippel, M. Knippel, A. Budde, et al., Layer-by-layer self assembly of polyelectrolytes on colloidal particles, Colloids Surfaces a-Physicochemical Eng. Asp. 137 (1998) 253–266. doi:10.1016/s0927-7757(98)00213-1.
[8] A. Voigt, E. Donath, H. Möhwald, Preparation of microcapsules of strong polyelectrolyte couples by one-step complex surface precipitation, Macromol. Mater. Eng. 282 (2000) 13–16. doi:10.1002/1439-2054(20001001)282:1<13::AID-MAME13>3.0.CO;2-W.
[9] I. Radtchenko, G. Sukhorukov, S. Leporatti, G. Khomutov, E. Donath, H. Möhwald, Assembly of Alternated Multivalent Ion/Polyelectrolyte Layers on Colloidal Particles. Stability of the Multilayers and Encapsulation of Macromolecules into Polyelectrolyte Capsules., J. Colloid Interface Sci. 230 (2000) 272–280. doi:10.1006/jcis.2000.7068.
[10] A. Biswas, A.T. Nagaraja, M.J. McShane, Fabrication of Nanocapsule Carriers from Multilayer-Coated Vaterite Calcium Carbonate Nanoparticles, ACS Appl. Mater. Interfaces. 6 (2014) 21193–21201. doi:10.1021/am5061195.
[11] H. Chen, X.H. Wang, D. Li, Y.Z. Guo, R.C. Sun, Preparation and characterization of quaternary chitosan/sodium alginate self-assembled microcapsules, in: S. Chen, Z.T. Liu, Q.Z. Zeng (Eds.), Adv. Chem. Res. Ii, Pts 1-3, 2012: pp. 263–267. doi:10.4028/www.scientific.net/AMR.554-556.263.
[12] F. Cuomo, F. Lopez, A. Ceglie, L. Maiuro, M.G. Miguel, B. Lindman, pH-responsive liposome-templated polyelectrolyte nanocapsules, Soft Matter. 8 (2012) 4415–4420. doi:10.1039/c2sm07388a.
[13] S.S. Liu, C.Y. Wang, X.X. Liu, Z. Tong, B.Y. Ren, F. Zeng, NRET from naphthalene labels in multilayer shell wall on melamine formaldehyde microparticles fabricated with layer-by-layer self-assembly to pyrene-labeled polyelectrolyte in solution, Eur. Polym. J. 42 (2006) 161–166. doi:10.1016/j.eurpolymj.2005.06.027.
[14] T. Pavlitschek, M. Gretz, J. Plank, Microcapsules Prepared from a Polycondensate-Based Cement Dispersant via Layer-by-Layer Self-Assembly on Melamine-Formaldehyde Core Templates, J. Appl. Polym. Sci. 127 (2013) 3705–3711. doi:10.1002/app.37981.
[15] M.B. Thomas, K. Radhakrishnan, D.P. Gnanadhas, D. Chakravortty, A.M. Raichur, Intracellular delivery of doxorubicin encapsulated in novel pH-responsive chitosan/heparin nanocapsules, Int. J. Nanomedicine. 8 (2013) 267–273. doi:10.2147/IJN.S37737.
20
Journal of Colloid and Interface Science 470 (2016) 92–99doi:10.1016/j.jcis.2016.02.052
[16] A. Voigt, H. Lichtenfeld, G.B. Sukhorukov, H. Zastrow, E. Donath, H. Baumler, et al., Membrane filtration for microencapsulation and microcapsules fabrication by layer-by-layer polyelectrolyte adsorption, Ind. Eng. Chem. Res. 38 (1999) 4037–4043. doi:10.1021/ie9900925.
[17] S.Q. Ye, C.Y. Wang, X.X. Liu, Z. Tong, Multilayer nanocapsules of polysaccharide chitosan and alginate through layer-by-layer assembly directly on PS nanoparticles for release, J. Biomater. Sci. Ed. 16 (2005) 909–923. doi:10.1163/1568562054255691.
[18] S.Q. Ye, C.Y. Wang, X.X. Liu, Z. Tong, B. Ren, F. Zeng, New loading process and release properties of insulin from polysaccharide microcapsules fabricated through layer-by-layer assembly, J. Control. Release. 112 (2006) 79–87. doi:10.1016/j.jconrel.2006.01.015.
[19] H.J. Dong, J.D. Brennan, Rapid fabrication of core-shell silica particles using a multilayer-by-multilayer approach, Chem. Commun. 47 (2011) 1207–1209. doi:10.1039/c0cc04221h.
[20] I. Pastoriza-Santos, B. Scholer, F. Caruso, Core-shell colloids and hollow polyelectrolyte capsules based on diazoresins, Adv. Funct. Mater. 11 (2001) 122–128. doi:10.1002/1616-3028(200104)11:2<122::aid-adfm122>3.0.co;2-n.
[21] X.Y. Shi, A.L. Briseno, R.J. Sanedrin, F.M. Zhou, Formation of uniform polyaniline thin shells and hollow capsules using polyelectrolyte-coated microspheres as templates, Macromolecules. 36 (2003) 4093–4098. doi:10.1021/ma034185g.
[22] Y. Wang, A.S. Angelatos, F. Caruso, Template synthesis of nanostructured materials via layer-by-layer assembly, Chem. Mater. 20 (2008) 848–858. doi:10.1021/cm7024813.
[23] X.L. Yang, X. Han, Y.H. Zhu, (PAH/PSS)(5) microcapsules templated on silica core: Encapsulation of anticancer drug DOX and controlled release study, Colloids Surfaces a-Physicochemical Eng. Asp. 264 (2005) 49–54. doi:10.1016/j.colsurfa.2005.05.017.
[24] C. Kantak, S. Beyer, L. Yobas, T. Bansal, D. Trau, A “microfluidic pinball” for on-chip generation of Layer-by-Layer polyelectrolyte microcapsules, Lab Chip. 11 (2011) 1030. doi:10.1039/c0lc00381f.
[25] J.J. Richardson, K. Liang, K. Kempe, H. Ejima, J. Cui, F. Caruso, Immersive Polymer Assembly on Immobilized Particles for Automated Capsule Preparation, Adv. Mater. 25 (2013) 6874–6878. doi:10.1002/adma.201302696.
[26] J.J. Richardson, H. Ejima, S.L. Lörcher, K. Liang, P. Senn, J. Cui, et al., Preparation of Nano- and Microcapsules by Electrophoretic Polymer Assembly, Angew. Chemie Int. Ed. 52 (2013) 6455–6458. doi:10.1002/anie.201302092.
[27] J.J. Richardson, M. Björnmalm, S.T. Gunawan, J. Guo, K. Liang, B. Tardy, et al., Convective polymer assembly for the deposition of nanostructures and polymer thin films on immobilized particles, Nanoscale. 6 (2014) 13416–13420. doi:10.1039/C4NR04348K.
[28] M. Björnmalm, A. Roozmand, K.F. Noi, J. Guo, J. Cui, J.J. Richardson, et al., Flow-Based Assembly of Layer-by-Layer Capsules through Tangential Flow Filtration, Langmuir. 31 (2015) 9054–9060. doi:10.1021/acs.langmuir.5b02099.
21
Journal of Colloid and Interface Science 470 (2016) 92–99doi:10.1016/j.jcis.2016.02.052
[29] T.S. Chung, J. Howell, First Order Consecutive Reactions in a Tubular Reactor with Axial Dispersion, Chem. Eng. Sci. 25 (1970) 1954–&. doi:10.1016/0009-2509(70)87017-8.
[30] V. Daniloska, P. Carretero, R. Tomovska, J.M. Asua, High performance pressure sensitive adhesives by miniemulsion photopolymerization in a continuous tubular reactor, Polymer (Guildf). 55 (2014) 5050–5056. doi:10.1016/j.polymer.2014.08.038.
[31] P.J. Dowding, J.W. Goodwin, B. Vincent, Production of porous suspension polymer beads with a narrow size distribution using a cross-flow membrane and a continuous tubular reactor, Colloids Surfaces a-Physicochemical Eng. Asp. 180 (2001) 301–309. doi:10.1016/s0927-7757(00)00777-9.
[32] N. Hordy, T.F.L. McKenna, A continuous tubular reactor for core-shell latex particles, Can. J. Chem. Eng. 90 (2012) 437–441. doi:10.1002/cjce.20562.
[33] I. Kraus, S.N. Li, A. Knauer, M. Schmutz, J. Faerber, C.A. Serra, et al., Continuous-Microflow Synthesis and Morphological Characterization of Multiscale Composite Materials Based on Polymer Microparticles and Inorganic Nanoparticles, J. Flow Chem. 4 (2014) 72–78. doi:10.1556/jfc-d-13-00029.
[34] A.I. Lopez-Lorente, M. Valcarcel, B. Mizaikoff, Continuous flow synthesis and characterization of tailor-made bare gold nanoparticles for use in SERS, Microchim. Acta. 181 (2014) 1101–1108. doi:10.1007/s00604-014-1215-8.
[35] R. Tomovska, J.C. de la Cal, J.M. Asua, Miniemulsion Photo-Copolymerization of Styrene/Butyl Acrylate in a Continuous Tubular Reactor, Ind. Eng. Chem. Res. 53 (2014) 7313–7320. doi:10.1021/ie402779y.
[36] M. Balastre, J. Persello, A. Foissy, J.F. Argillier, Binding and ion-exchange analysis in the process of adsorption of anionic polyelectrolytes on barium sulfate, J. Colloid Interface Sci. 219 (1999) 155–162. doi:10.1006/jcis.1999.6469.
[37] K.E. Bremmell, G.J. Jameson, S. Biggs, Polyelectrolyte adsorption at the solid/liquid interface - Interaction forces and stability, Colloids Surfaces a-Physicochemical Eng. Asp. 139 (1998) 199–211. doi:10.1016/s0927-7757(98)00281-7.
[38] M.A.G. Dahlgren, P.M. Claesson, R. Audebert, Highly chanrged Cationic polyelectrolytes on Mica - Influence of Polyelectrolyte concentration on Surface Forces, J. Colloid Interface Sci. 166 (1994) 343–349. doi:10.1006/jcis.1994.1304.
[39] C. Dange, T.N.T. Phan, V. Andre, J. Rieger, J. Persello, A. Foissy, Adsorption mechanism and dispersion efficiency of three anionic additives poly(acrylic acid), poly(styrene sulfonate) and HEDP on zinc oxide, J. Colloid Interface Sci. 315 (2007) 107–115. doi:10.1016/j.jcis.2007.03.068.
[40] T. Afsharrad, A.I. Bailey, P.F. Luckham, W. Macnaughtan, D. Chapman, Forces between Poly-L-Lysine of Molecular-Weight Range 4,000-75,000 Adsorbed on Mica Surfaces, Colloids and Surfaces. 25 (1987) 263–277. <Go to ISI>://A1987J854900014.
[41] H. Urch, M. Vallet-Regi, L. Ruiz, J.M. Gonzalez-Calbet, M. Epple, Calcium phosphate nanoparticles with adjustable dispersability and crystallinity, J. Mater. Chem. 19 (2009) 2166. doi:10.1039/b810026h.
[42] M. Tsui, S.B. Heiwagei, Synthesis of PDADMAC/PSS Nanocapsules Using Calcium
22
Journal of Colloid and Interface Science 470 (2016) 92–99doi:10.1016/j.jcis.2016.02.052
Phosphate Templates, Imperial College London, 2010.
[43] R.J. Hunter, Zeta Potential in Colloid Science, Academic Press Inc. (London) Ltd., 1981.
[44] I. Szilagyi, G. Trefalt, A. Tiraferri, P. Maroni, M. Borkovec, Polyelectrolyte adsorption, interparticle forces, and colloidal aggregation, Soft Matter. 10 (2014) 2479–2502. doi:10.1039/C3SM52132J.
[45] K. Ariga, Y. Yamauchi, G. Rydzek, Q. Ji, Y. Yonamine, K.C.-W. Wu, et al., Layer-by-layer Nanoarchitectonics: Invention, Innovation, and Evolution, Chem. Lett. 43 (2014) 36–68. doi:10.1246/cl.130987.
23