· Web viewThe methods of determination of the optimum experiment parameters such as deposition time and polyelectrolyte concentration are presented as a supplementary data. This material

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.

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http://dx.doi.org/10.1016/j.jcis.2016.02.052

mailto:[email protected]


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.

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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,

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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,

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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

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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

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(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

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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.

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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.

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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

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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

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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).

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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

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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

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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

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(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

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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.

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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



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/

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· Web viewThe methods of determination of the optimum experiment parameters such as deposition time and polyelectrolyte concentration are presented as a supplementary data. This material