Spherical Blackberry-type Capsules Containing ... - ACS Publications

Feb 7, 2014 - ABSTRACT: The design, preparation, and properties of nanosized blackberry-like structures are described. These capsules are composed of ...
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Spherical Blackberry-type Capsules Containing Block Copolymer Aggregates Renata Vyhnalkova,†,∥ Lin Xiao,§,∥ Guang Yang,§ and Adi Eisenberg*,†,‡ †

Department of Chemistry, and ‡Centre for Self-Assembled Chemical Structures, McGill University, Otto Maass Building, 801 Sherbrooke Street W, Montreal, Quebec H3A 2K6, Canada § Department of Biomedical Engineering, College of Life Science and Technology, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China S Supporting Information *

ABSTRACT: The design, preparation, and properties of nanosized blackberry-like structures are described. These capsules are composed of two layers of individual block copolymer aggregates, relatively large core vesicles onto which is deposited a layer of smaller vesicles or micelles. The composition of the adjacent layers is such as to ensure strong electrostatic interactions between them. The core vesicles are typically composed of either PS-b-P4VP with a positively charged corona or of PS-b-PAA with a negatively charged corona, and are surrounded by a layer of smaller, oppositely charged block copolymer vesicles or micelles. These composite structures bear a strong resemblance to blackberries, hence the proposed name. The blackberry structures can be prepared in solution or on a flat surface, for example, a silicon wafer. Four compositional possibilities for the blackberries structures were studied, in which the positively or negatively charged core vesicles are covered either by a layer of oppositely charged micelles or by vesicles. These structures represent the earliest stage of a layer-by-layer approach of small spherical aggregates onto a larger spherical hollow core. The strong interaction between the contacting layers is achieved by electrostatic interactions or by complementary acid−base properties, for example, H-bonding. These multicompartmented capsules could be used potentially as delivery vehicles for multiple components; each layer of the capsules could be loaded with hydrophobic (in the core of the micelles or vesicle wall) or hydrophilic molecules (in the vesicle cavity). The overall size of such structures can vary, but in any case can be kept below 1 μm. charge, with wash steps between.29 One of the many attractive features of LbL self-assembly (on planar or spherical surfaces) is the remarkable nanoscale control that can be exercised over the properties of thin films (for example, thickness, roughness, wettability, and swelling behavior), by varying assembly parameters and conditions such as pH, ionic strength, polyelectrolyte (PE) functionality, and concentration.25,30 The LbL technique offers several advantages over other thin film deposition methods; that is, it is extremely simple and cheap, and can employ a wide variety of materials that can be deposited on planar or spherical surfaces (polyions, nanoparticles, and biological molecules). Another important advantage of the LbL method is the high degree of control over thickness, which is due to the near-linear growth of the film thickness with the number of bilayers. Because each bilayer can be as thin as 1 nm, this method offers easy control over the thickness with 1 nm resolution, with the overall thickness ranging from nanometers to several micrometers.27,28 The thickness of the film depends on the nature of the PEs, their deposition conditions, the number of deposited layers, and their possible functionalization.27 The bilayers and wash steps

1. INTRODUCTION Layer-by-layer (LbL) assembly techniques have shown themselves to be very versatile in the preparation of multilayer thin films with controlled structures and compositions on planar surfaces or on spherical cores.1−13 LbL methods are typically based on sequential deposition of materials with complementary functional groups employing electrostatic attraction,14−16 hydrogen bonding,2,17,18 or covalent interactions,19−21 among others.22−24 Because of their facile, inexpensive, and environmentally friendly nature, LbL assembly techniques offer a broad range of opportunities,1−13 varying from biological and therapeutic materials to energy and electrochemical devices. They are, in particular, commonly used as encapsulation methods for therapeutic agents,1−6 nanoreactors,7,8 in fields of biomimetics and tissue engineering,9 as photovoltaic materials,10,11 and in a range of other coatings.12,13 The use of LbL techniques in the preparation of drug delivery carriers, 1−6 for example, multilayer thin films14−16,18and colloidal capsules containing polymer chains, micelles, and vesicles or polymersomes, has shown significant potential to improve the therapeutic delivery of a range of drugs.1−6 For electrostatically interacting components, the sequential buildup of material is achieved in LbL systems through charge overcompensation after the deposition of each layer of opposite © 2014 American Chemical Society

Received: October 22, 2013 Revised: January 21, 2014 Published: February 7, 2014 2188

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detail in the Supporting Information, section 1.12. The adhesion of the deposited aggregates onto the core vesicle is accomplished in the blackberry structure via the use of corona compositions with complementary properties. For example, if the core vesicle has a corona of PS-b-PVP, the aggregates used for the first external layer to be deposited should have a corona consisting of PAA, which interacts strongly with the P4VP. Thus, the capsules consist of a relatively large core vesicle (ca. 500 nm in diameter), on which a layer of oppositely charged smaller vesicles or micelles, of a composition that assures strong interaction between neighboring layers, is deposited. In one of the examples to be discussed below, the central vesicle is composed of PS-b-P4VP, which is then surrounded by smaller vesicles of PS-b-PAA of ∼100 nm in diameter. These composite structures bear a strong resemblance to blackberries, hence the proposed name. While this publication deals with the single layer of small aggregates deposited on a large core vesicle, and thus represents the early stage of the LbL assembly, we refer even to this structure as LbL, in part because of the obvious potential extension to a considerably larger number of spherical layers and, more importantly, because the same mechanism of adhesion is involved as in the more classical planar LbL structures. A number of compositional possibilities for blackberries present themselves; the core vesicle can be covered with a layer of either oppositely charged micelles, vesicles, or other aggregate types. These structures represent essentially a layerby-layer construct on a spherical core. In the present study, two-layer BB capsules of four different combinations are described. The following nomenclature is suggested: The core vesicle is denoted by V, and subsequent layers, vesicles, or micelles are noted sequentially as V or M from left to right after the “BB”, which denotes the blackberry structure. For example, if one labels the central positively charged vesicle V+ and gives subsequent layer the designation V− in case of small negatively charged vesicles, a typical example of a simple blackberry could be BBV+V−, indicating that the blackberry is composed of the core vesicle, V+, which is surrounded by a layer of oppositely charged (smaller) vesicles V−. Three additional combinations studied in this work include BBV+M−, BBV−V+, and BBV−M+. It should be noted that to achieve strong interaction between successive layers, the layers must consist of particles that are of opposite charge or of alternating acid−base properties. It should be also pointed out that it is possible to prepare BB from solution or as deposited on flat surfaces, for example, on a silicon wafer. In the latter case, the BBs should not be expected to be spherical, but rather truncated spheres.

can be performed in many different ways, including dip-coating, spin-coating, spray-coating, as well as flow-based techniques.14−16,18 Characterization of LbL film deposition is typically done by optical techniques such as dual polarization interferometry or ellipsometry or mechanical techniques such as a quartz crystal microbalance.14−16,18 The simplicity of this self-assembled film fabrication approach has led to the extension of this concept to the preparation of multilayer systems incorporating more than just simple PEs, such as small molecule dyes, colloidal particles, liposomes, and self-assembled aggregates onto planar and spherical surfaces.2,3,12,34−37 For example, it was shown that it is possible to incorporate phospholipid vesicles, which could act as reservoirs for biologically active components, into LbL multilayers, containing several polyanion/polycation bilayers.27 Caruso et al. prepared self-assembled nanoporous multilayerd films employing block copolymer micelles of PS-b-PAA and PSb-P4VP as building blocks.34 The growth of such nanoporous films, which could potentially serve as carriers of various hydrophobic and hydrophilic materials, is governed by electrostatic or hydrogen-bonding interactions between the interacting block copolymer micelles.34 Another way of using the LbL nanotechnology to obtain carriers for various materials is through the use of multilayered capsules, prepared via a removable core.1,4,7,36,38−42 As reported previously, hollow capsule-like structures can also be prepared from various charged polymers, deposited on a soluble core.43,44 For example, Caruso et al.44 prepared drug delivery capsules of ∼4 μm, intended for DNA release, composed of vesicles or polymersomes deposited by LbL techniques on soluble silica core templates. The size of the resulting hollow capsules depends on the size of the removed silica template.44 As discussed by the authors, these polymersomes are formed under physiological conditions and change to unimeric polymer chains by acidification to cellular endocytic pH levels.44 Self-assembled aggregates prepared from amphiphilic block copolymers in aqueous solution received considerable attention over the past few decades, in part because of their potential application in many fields such as drug carriers and reaction containers.45−53 Self-assembled aggregates, such as micelles, rods, vesicles, LCMs, and others, can be prepared from amphiphilic diblock copolymers by the addition of a selective precipitant such as water to a solution of the diblock.45−53 These aggregates consist of a hydrophobic core (micelles, LCMs) or wall (vesicles) and a hydrophilic corona, which makes them stable in aqueous solution.45−53 The properties of these aggregates, including the size, loading and release kinetics, phase diagrams, and thermodynamics in solution, had been well explored.54 One of the most attractive potential applications involves the use of these systems as drug delivery agents or as carriers for a range of small molecules, including hydrophobic and hydrophilic species.50−53,55,56 The hydrophobic materials can be incorporated into the vesicle wall or into the micelle core, while the hydrophilic molecules can be loaded into the cavity of the vesicle. This Article describes the preparation and characterization of capsules of a novel block copolymer aggregates structure, referred to as blackberries (BB), consisting of core vesicles, which are subsequently coated with smaller sized vesicles or micelles. A similar structure, described in the literature, is the so-called raspberry.57,58 The structures prepared here are referred to as blackberries, because, structurally, they resemble more closely blackberries than raspberries, as described in more

2. RESULTS AND DISCUSSION A schematic representation of the designations employed for the aggregate layers of the capsules in solution and on Si wafers is shown in Figure 1A. As can be seen from the figure for both environments, in solution and on the Si wafer, the central core vesicle is always noted as layer 1, and the layer attached to the core vesicle is called layer 2, and can be composed of either vesicles or micelles. Figure 1B defines the symbols, which describe each blackberry. The symbol BB indicates a blackberry structure, that is, a capsule, the shape of which is reminiscent of the blackberry fruit, and is shown schematically in Figure 1A. The subscripts “S” or “W” designate preparation in solution or on a Si wafer, respectively. The capital letter “V+” indicates the type of structure functioning as a central aggregate, or layer 1, which is always a vesicle in the present work. Because the Si 2189

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Relatively large, positively charged PS-b-P4VP vesicles (∼500 nm) (Figure 2A − Ia and Ib) or negatively charged PS-b-PAA vesicles (400 nm) (Figure 2D − Ia and Ib) were chosen as the central vesicles, that is, the cores of the capsules. The aqueous solution of core vesicles was then mixed with the aqueous solution of the aggregates to be deposited as the second layer. In case of the PS-b-P4VP core vesicles, the second layer is negative and consists of much smaller PS-b-PAA vesicles (Figure 2A-IIa) or PS-b-PAA micelles (Figure 2A-IIb). These structures are denoted as BBSV+V− and BBSV+M−, respectively. In case of PS-b-PAA serving as a core vesicle, the second layer has a positively charged corona and consists of PS-b-P4VP vesicles (Figure 2D-IIa) or PS-b-P4VP micelles (Figure 2DIIb). These structures are denoted as BBSV−V+ and BBSV−M+, respectively. Because the appearance of the structures after the deposition of the second layer on the core vesicle strongly resembles that of real blackberries, they are referred to as blackberries. A typical example of a simple BB could be BBSV+V−, indicating that the positively charged core vesicle V+ is surrounded by a layer of negatively charged small vesicles V−. The second approach to the preparation multilayered capsules is presented schematically in Figure 2B, C, E, and F. It deals with the LbL deposition of the various aggregates of opposite charge on the flat surfaces (Si wafer). In the case of positively charged PS-b-P4VP core vesicles, the preparation starts first with immersion of the overall negatively charged Si wafer into the solution of the large PS-b-P4VP core vesicles (Figure 2B-Ia and Figure 2C-Ia), during which the positively charged aggregates are deposited on a negatively charged Si wafer surface. The second layer is deposited on the Si wafer surface by using the same technique. The Si wafer containing the core vesicles is immersed again into an aqueous solution of the smaller aggregates of the opposite charge to deposit the second layer, that is, of aggregates onto the first layer of the core vesicles. In this case, it is a solution containing either vesicles of PS-b-PAA (Figure 2B-IIa) or micelles of PS-b-PAA (Figure 2CIIa). The PS-b-PAA vesicles or micelles are deposited on the PS-b-P4VP core of the capsules and held by electrostatic interactions as they form blackberries of the BBWV+V− or BBWV+M− type, respectively. In case of negatively charged PS-b-PAA core vesicles, the structure preparation is different from that involving core vesicles with a positive charge. Because the structures are to be deposited on a Si wafer, the surface of which is overall negative, and the negative core vesicle would not deposit on the surface of the same charge, the charge of the Si wafer has to be changed first. The modification is achieved by the deposition of 11 alternating layers of PAH and PAA PEs. The top layer of the PE multilayer consists of a positive PAH layer. Because the surface of the Si wafer was modified to a positive one, the next step of the preparation of the blackberries with negatively charged core vesicles involves immersion of the Si wafer containing the PE multilayer with a positively charged top layer into the solution of the large PS-b-PAA core vesicles (Figure 2E-Ia and Figure 2F-Ia); the negatively charged vesicles are deposited on the positively charged PAH layer on the Si wafer surface. The second layer is deposited on the modified Si wafer surface by using the same technique. The modified Si wafer containing the core vesicles is immersed again into the aqueous solution containing the aggregates of the second morphology of opposite charge to deposit the second layer of aggregates onto the first layer of core vesicles. In this case, it is a solution

Figure 1. (A) Schematic of the description of the layers in capsules prepared in solution and on Si wafers. (B) Example of blackberry nomenclature and definition of symbols. Subscript “S” on blackberry “BB” indicates preparation in solution; subscript “W” indicates preparation on Si wafer (not shown on the scheme); “V” indicates vesicles, and “M” indicates micelles (not shown on the scheme). “+” indicates positively charged corona, and “−” indicates negatively charged corona, respectively. An overview of figures related to all of the blackberry structures, prepared in this study from solution or by the deposition on a Si wafer, is shown in Table 3 in the Supporting Information.

wafer is of overall negative charge in water, to attach the core vesicle to the Si wafer by electrostatic interactions, the core vesicle either must have a positive charge, that is, contain a P4VP corona, or the overall charge on the Si wafer has to be changed to positive by deposition of a PE multilayer, containing PAH as the top layer to allow the attachment of negatively charged PS-b-PAA core vesicles. In Figure 1B, following the BBS or BBW designation, the layers are listed sequentially by capital letters. In this particular case, the symbol V+ denotes the core vesicle (formally layer 1), and the next letter on the right (V− or possibly M−) indicates that on top of the positively charged core vesicle is deposited a layer of negatively charged vesicles or micelles. The preparation of the bilayer blackberry capsules was accomplished in two ways, as shown schematically in Figure 2.

Figure 2. Schematic representation of successive steps (I, II) in blackberry structure formation: (A) BBSV+V− and BBSV+M−, (D) BBSV−V+ and BBSV−M+ types in solution; (B) BBWV+V−, (C) BBWV+M−, (E) BBWV−V+, and (F) BBWV−M+ type on Si wafer surfaces.

These structures represent, essentially, a low generation layerby-layer approach on a spherical core. The first method involves preparation of capsules from solution, represented by schemes in Figure 2A and D. First, the individual aggregates, such as vesicles and micelles, were self-assembled in aqueous solution. The preparation of these self-assembled aggregates was described briefly above and can be found elsewhere in greater detail.59 2190

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200 nm, which is in agreement with the data for the individual aggregates shown in the Supporting Information. The size of this complex structure is between 600 and 800 nm in diameter. When a small amount of solution was dropped onto the Si wafer and imaged by AFM after drying, a similar complex structure is seen. As shown in Figure 3B, a dense layer of PS188b-PAA34 vesicles surrounds a PS300-b-P4VP30 core vesicle, although the PS297-b-P4VP30 core vesicle is not visible because of the density of the outer small-vesicle layer. The size of the outer vesicles, shown in the AFM image in Figure 3B, is in agreement with those in the Supporting Information. The overall size of the structure is about 600 nm in diameter, which agrees with that shown in Figure 3A. 2.1.3. Blackberries Containing a Positively Charged Core and Negatively Charged Micelles BBSV+M−. Blackberries of the structure BBSV+M−, containing positively charged PS-bP4VP core vesicles surrounded by a layer of negatively charged PS-b-PAA micelles, were prepared in solution. The TEM and AFM images of these blackberry structures are shown in Figure 4. Figure 4A shows blackberry structures composed of a PS297-

containing vesicles of either positively charged PS-b-P4VP (Figure 2E-IIa) or positively charged micelles of PS-b-P4VP (Figure 2F-IIa). It should be noted that in the present study, structures with only two layers were prepared, as shown schematically in Figure 2. A very large number of potential structures could be obtained using the same preparative approach based on the deposition of individual aggregate layers alternating in charge deposited on top of each other to prepare structures containing more than two layers. Only the simplest types are described here. An overview of the experimental conditions together with the sizes of individual aggregates for the blackberry construction is given in the Supporting Information in section 1.6. 2.1. Blackberries in Solution. 2.1.1. Vesicles and Micelles in Solution. The TEM images of aggregates, used as the building materials to assemble the blackberry structures, together with the detailed listing of the sizes of the aggregates and experimental conditions of their preparation, are given in detail in the Supporting Information. It is worth pointing out that in water at room temperature the individual vesicles are very stable. The wall material (i.e., polystyrene) is ca. 70 °C below its glass transition temperature, and, in pure water, no solvent is present to plasticize the wall. Furthermore, in solutions of vesicles containing only one type of corona material, that is, PAA or P4VP, the interactions would be repulsive, which would improve the stability. In solutions containing both types of vesicles, the interactions between the PAA and the P4VP corona materials would be strong and effectively collapse and cross-link the regions between the PS walls, which would prevent the PS walls from approaching each other. Again, this would make the system very stable. 2.1.2. Blackberries Containing a Positively Charged Core Surrounded by Negatively Charged Vesicles BBSV+V−. Blackberries of the structure BBSV+V−, containing positively charged PS-b-P4VP core vesicles surrounded by a layer of negatively charged PS-b-PAA vesicles, were prepared in solution. The TEM and AFM images of these blackberry structures are shown in Figure 3.

Figure 4. (A) TEM and (B,B′) AFM images of the blackberry structure BBSV+M− composed of PS297-b-P4VP30 vesicles surrounded by PS217-b-PAA45 micelles in solution.

b-P4VP30 core vesicle and PS217-b-PAA45 micelles. The sizes of the core vesicles are in the range of 200−500 nm, and the sizes of the micelles range between 20 and 50 nm. These size data are in agreement with those obtained by TEM and shown in the Supporting Information. The overall sizes of the structures are between 300 and 500 nm in diameter. The AFM images in Figure 4B and B′ also show similar blackberry-like bilayer structures, with similar sizes of the individual aggregates and of the overall structure. 2.1.4. Blackberries with Negatively Charged Core Vesicles. TEM and AFM images and a detailed description of the blackberries from solution are given in the Supporting Information in section 1.7. These blackberries contain a negatively charged central core vesicle, which is surrounded by vesicles (BBSV−V+) or micelles (BBSV−M+). 2.2. Blackberries on a Si Wafer Surface. 2.2.1. Vesicles and Micelles on a Si Wafer Surface. The AFM images of aggregates, used as the building material to assemble the blackberry structures, together with the detailed explanation of the sizes of the aggregates and the experimental conditions of their preparation, are given in detail in the Supporting Information in section 1.8. 2.2.2. Blackberries Containing a Positively Charged Core and Negatively Charged Outer Vesicles (BBWV+V−). Blackberries of the structure BBWV+V−, containing positively charged PS297-b-P4VP30 core vesicles surrounded by a layer of negatively

Figure 3. (A) TEM and (B) AFM images of the blackberry structure BBSV+V− composed of PS297-b-P4VP30 vesicles surrounded by PS190-bPAA34 vesicles in solution.

As the TEM image in Figure 3A confirms, a complex blackberry structure composed of core vesicles surrounded by a shell outer vesicles was formed. The PS297-b-P4VP30 core vesicles are about 500 nm in diameter, surrounded by a layer of PS188-b-PAA34 vesicles, which are much smaller in size. The diameters of the PS188-b-PAA34 vesicles are in a range of 80− 2191

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2.2.3. Blackberries Containing a Positively Charged Core and Negatively Charged Outer Micelles (BBWV+M−). Blackberries of the structure BBWV+M−, containing positively charged PS297-b-P4VP30 core vesicles surrounded by a layer of negatively charged PS217-b-PAA45 outer micelles, were prepared on a Si wafer. The AFM images of the BBWV+M− blackberry structures are shown in Figure 6. As can be seen from Figure 6A, these particular blackberrytype structures are smaller in size as compared to those of the BBWV+V− type; the diameters range from 100 to 500 nm. The polydispersity in the size of these complex structures arises from the size polydispersity of PS297-b-P4VP30 core vesicles. Figure 6A′ shows an enlarged AFM image of one of the blackberry structures shown in Figure 6A. As can be seen from the image Figure 6A′, the blackberry structure bears a remarkably strong resemblance in appearance to a real blackberry, Rubus canadensis − Canadian Blackberry (Figure 6B). The small PS217-b-PAA45 micelles in the outer layer are ca. 30 nm in diameter, while the diameter of the PS297-b-P4VP30 core vesicle is ca. 150 nm. The overall size of the BBWV+M− blackberry structure is about 200 nm, which is about 105 times smaller than a real blackberry. 2.2.4. Blackberries with Negatively Charged Core Vesicles. The results, AFM images, and description of the blackberries deposited on the Si wafer modified by PE multilayer, which contains a negatively charged core vesicle and is surrounded by vesicles (BBWV−V+) or micelles (BBWV−M+), are given in the Supporting Information in section 1.9. 2.3. Relationship between the Various Blackberries and between Blackberries and Planar Structures − Summary. Since a series of spherical blackberry capsules, both in solution and on Si wafers, has been prepared and characterized, it is useful to compare these various structures, which is the subject of the present section. It is also useful to compare the present blackberry structures with analogous planar structures described in the previous publication.60 The connection is illustrated schematically in Figure 7. The blackberry structure in solution consists of a charged large central or core vesicle and a surrounding layer of oppositely charged small aggregates (vesicles or micelles) attached via electrostatic interactions. A typical example of blackberry structure in solution, BBSV+V−, is shown in Figure 7B, where the core vesicle V+ is prepared from positively charged PS-b-P4VP, while the outer layer of small vesicles is composed of PS-b-PAA chains, which give it a negatively charged corona. Similar blackberry structures can be prepared from the same materials also on a solid planar surface, such as a Si wafer. For example, BBWV+V− is composed of a positively charged PS-bP4VP core vesicle and a layer of negatively charged PS-b-PAA small vesicles as shown in Figure 7A, which is similar in composition to BBSV+V− except for the preparative conditions (on a Si wafer as opposed to a solution environment as indicated by the subscript “W” after BB). In the preparation of blackberry structures on a Si wafer, however, when a negatively charged large vesicle is employed as the core, a coating, typically a PE multilayer, is required to change the negatively charged Si surface to a positively charged one to ensure electrostatic attraction. After that step, the negatively charged large core vesicle and the positively charged aggregates (vesicles or micelles) can be deposited sequentially to form the blackberry structure on the Si wafer, as shown schematically in Figure 7C. As an example, BBWV−V+ is

charged PS188-b-PAA34 vesicles, were prepared on a Si wafer. The AFM images of the BBWV+V− structures are shown in Figure 5.

Figure 5. (A,A′) AFM phase image of the BBSV+V− blackberry structure composed of PS297-b-P4VP30 core vesicles surrounded by PS190-b-PAA34 outer vesicles on a Si wafer surface.

As can be seen in Figure 5A′, which contains an enlarged image of one structure from Figure 5, the diameters of PS188-bPAA34 (second layer) vesicles are in the range of 80−200 nm. The overall diameter of this structure is ca. 600 nm. The PS297b-P4VP30 core vesicle located inside is estimated to be ca. 400 nm in diameter. The sizes of both the small PS188-b-PAA34 vesicles and the large core PS297-b-P4VP30 vesicles are in agreement with data obtained from the TEM images of the individual aggregates presented in the Supporting Information. According to the AFM image in Figure 5, the sizes of the blackberry structures are highly polydisperse, which is due to the polydispersity of the sizes of PS297-b-P4VP30 core vesicles. The “blackberry” structure, seen in the present study and shown in Figures 5 and 6, appears to be a new morphology,

Figure 6. (A,A′) AFM phase image of blackberry BBWV+M− structure composed of PS297-b-P4VP30 core vesicles surrounded by PS217-bPAA45 micelles on a Si wafer, and (B) Rubus canadensis − Canadian Blackberry (difference in size A′ vs B ∼105). Note difference in scale between Figures 5A′ and 6A′ (the BB in Figure 5A′ is ca. 3× larger than the BB in Figure 6A′).

reminiscent of a naturally grown blackberry, which to our knowledge has not been described previously. The present structure bears a stronger resemblance to a blackberry than to a raspberry; therefore, that name was chosen. A brief description of the difference between the blackberry and the raspberry morphologies is given in the Supporting Information. 2192

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these structures to blackberries found in nature, they were named blackberries. These structures represent essentially the earliest stage of a layer-by-layer approach of small spherical aggregates on a larger spherical hollow core. More complex structures are, obviously, possible. The adhesion between successive layers is achieved by electrostatic interactions between the oppositely charged aggregates. It was shown that the size of such blackberry capsules can vary, but in any case can be kept well below 1 μm for a low number of layers. Capsules of this type are of interest because it is useful to prepare systems for protection and controlled release of encapsulated materials in fields such as medicine, food technology, biotechnology, or materials science. Such multicompartmental systems are of great interest because of their possibilities of simultaneous delivery of various drugs, or other functional materials. As described in the Introduction, capsules prepared by a layer-by-layer method had been prepared previously.31−33 The approach used in those studies involved a template used as the core of the system, onto which the additional layer or layers of the smaller particles or aggregates were deposited. Subsequently, the temporary core was dissolved and removed. As opposed to that approach, the present method does not involve a removable core template, but one that is built in permanently and thus does not involve the additional dissolution step. In addition, it uses the core of the system as an integral part of the structure, which can be used as a storage space for hydrophobic and/or hydrophilic materials. It should be mentioned that the present approach is, to our knowledge, the first example involving the deposition of block copolymer aggregates onto block copolymer aggregates. In the potential applications as delivery vehicles, the cavity of the central vesicle as well as those in the subsequent layer can be filled with different hydrophilic materials, while the walls can be similarly filled with different hydrophobic ingredients. The thicknesses and compositions of the walls can possibly be used to control the release rates of the various ingredients. Finally, it is worth noting that a number of systems exist in which small spherical structures are arranged on a spherical shell.39−41 However, none of those are composed entirely or even largely of block copolymers. Thus, the present blackberry structure can be considered new block copolymer morphology.

Figure 7. Schematic summary of blackberry and planar multilayer structures on a spherical base (A−C) or a planar base (D) or in solution (B), or on a Si wafer (A,C,D). The blackberry (B) could also be prepared from oppositely charged components (a negatively charged core vesicle with a positively charged outer vesicle layer).

composed of a negatively charged PS370-b-PAA47 core vesicle and a layer of positively charged PS473-b-P4VP36 small vesicles. To ensure the electrostatic attraction between the negatively charged PS370-b-PAA47 core vesicle and the Si surface, a PE multilayer, composed of 11 alternating PAH and PAA layers with PAH as the top layer, was used to modify the surface (Figure 7C). In addition, it has been observed in our previous work60 that when the small negatively charged PS190-b-PAA34 vesicles were deposited onto the same PE multilayer-modified Si surface, a relatively dense layer with a high PS-b-PAA vesicle coverage can be achieved. On the basis of this concept, a series of planar multilayer structures containing various sequences of layers of vesicles, micelles, and large compound micelles have been obtained. As a simple example, the WL11+V−V+ planar LbL structure, shown schematically in Figure 7D, represents a system composed of a layer of negatively charged vesicles and a layer of positively charged vesicles, sequentially deposited on a Si wafer modified with a PE multilayer composed of 11 alternating PAH and PAA layers with positively charged PAH on top.60 The “W” in the system name indicates preparation on a Si wafer, while the L11+ indicates a multilayer composed of 11 alternating PAH/PAA layers with PAH on top. A detailed description of the planar structures is given in a previous publication.60 Similar to the planar multilayer structures, spherical multilayer structures containing layers of vesicles, micelles, or other aggregates can be also constructed. In this case, a large core vesicle instead of a planar Si surface is used as the supporting surface, as mentioned above. The blackberry structures discussed in this work represent a low generation series of such spherical multilayer structures.

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ASSOCIATED CONTENT

S Supporting Information *

The materials to prepare blackberry structures are described in detail in section 1.1. The preparation procedures of the individual aggregates, the blackberries from solution, and on silicon wafer surfaces are also given in detail in sections 1.2, 1.3, and 1.4. These descriptions of the preparation of blackberries include the detailed elaboration of the assembly of blackberries containing a positively charged core vesicle surrounded by the negatively charged outer vesicles or micelles, as well as assembly of the blackberries containing a negatively charged core vesicle surrounded by positively charged vesicles or micelles. The blackberries with negatively charged cores deposited on the silicon wafer must be placed on a PE multilayer, which is applied to change the charge of the silicon wafer surface. Detailed explanation of the surface charge modification procedure is included. The description of the characterization techniques can be found in section 1.5. This material is available free of charge via the Internet at http://pubs.acs.org.

3. CONCLUSIONS Structures composed of a core vesicle surrounded by an outer layer consisting of micelles or vesicles were studied in solution and on Si wafer surfaces. Because of the strong resemblance of 2193

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AUTHOR INFORMATION

Corresponding Author

*Tel.: (514) 398-6934. Fax: (514) 398-3797. E-mail: adi. [email protected]. Author Contributions ∥

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by NSERC is gratefully acknowledged. We would like to thank Dr. Futian Liu for synthesizing the PS297-bP4VP30 block copolymer and Dr. Tony Azzam for synthesizing PS190-b-PAA34 and PS217-b-PAA45 block copolymers, which were prepared in connection with other projects. We would also like to thank Mohini Ramkaran for help with AFM imaging. We are indebted to Professor C. J. Barrett for valuable comments. L.X. would like to thank the China Scholarship Council and McGill University for financial assistance during his stay at McGill University. This work was performed while L.X. was a registered Ph.D. student at Huazhong University of Science and Technology and a visiting student at McGill University, supported in part by the China Scholarship Council and in part by McGill University.

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ABBREVIATIONS

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REFERENCES

PE, polyelectrolyte; PS-b-PAA, poly(styrene)-block-poly(acrylic acid); PS-b-P4VP, poly(styrene)-block-poly(4-vinyl pyridine); LCMs, large compound micelles; LCVs, large compound vesicles; PAH, poly(allyl hydrochloride); PAA, poly(acrylic acid); LbL, layer-by-layer; AFM, atomic force microscopy; TEM, transmission electron microscopy; THF, tetrahydrofuran

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