air-eau
Assembly of polystyrene-coated gold nanoparticles at the air-
water interface
Résumé
Des nanoparticules d’or décorées de chaînes de polystyrène de différentes masses molaires ont été ajoutées à une monocouche de PS-b-PMMA à l’interface air-eau. Les films composites ainsi obtenus ont été transférés sur substrat solide à l’aide de la technique de Langmuir-Blodgett. Grâce aux techniques TEM et AFM, la formation d’agrégats en forme d’îlots contenant un réseau ordonné hexagonal de nanoparticules en deux dimensions a été observée dans la plupart des cas. Lorsque les chaînes de PS passivant les NPs sont dans le même ordre de longueur que les chaînes PS du copolymère, la formation d’agrégats tel que mentionné précédemment n’est plus observée. Ces résultats sont comparés avec ceux obtenus pour des systèmes analogues où les NPs sont déposées seules directement à la surface de l’eau ou sur une surface de verre dénudée. La distance entre les particules à l’intérieur des agrégats dépend de la surface sur laquelle elles ont été déposées, de la présence ou non d’une monocouche de copolymère et de la longueur des ligands fixés sur les NPs.
Abstract
Gold nanoparticles (NPs) coated with thiol-terminated polystyrene chains of varying molar mass were added to polystyrene-b-polymethylmethacrylate (PS-b-PMMA) block copolymer monolayers at the air-water interface. Composite films were transferred to solid substrates by the Langmuir-Blodgett technique. For most of the investigated systems, TEM micrographs and AFM images reveal the formation of 2D island-like aggregates of particles organized on a close-packed hexagonal lattice. This characteristic aggregate formation is lost when PS ligands are within the same length regime as the PS block from the copolymer. The results are compared with those obtained for analogous systems containing no copolymer where NPs are deposited on either a bare water surface or bare glass. Interparticle distance between NPs is found to depend on the surface on which they are deposited, the presence or not of the copolymer monolayer, and ligand length.
Introduction
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nanoparticle placement in different architectures is also of practical importance and is key to the optimization of various devices such as solar cells49-53 and sensors54, 55. For example, NP sensing strategies based on
surface-enhanced Raman spectroscopy, metal-enhanced fluorescence and surface plasmon resonance are all sensitive to details of particle organization at the nanoscale27.
To date, the most efficient and reliable methods for the fabrication of ordered NP arrays on a substrate are top- down techniques, such as electron beam or ion beam lithography56. However, top-down patterning has
intrinsic disadvantages such as the minimal size of the attainable features and a high cost per unit area nanopatterned. On the other hand, bottom-up techniques have always attracted interest because they are low cost, are able to cover large patterned areas and are not time consuming. In this paper we investigate a promising bottom-up way to pattern colloidal Au NPs on a solid substrate, namely Langmuir-Blodgett (LB) transfer. To achieve patterning in a Langmuir film, NPs are co-deposited at the air-water interface with chemical species known to adopt ordered morphologies at the nanoscale level. Composite Langmuir monolayers made of fatty acids mixed with various NPs have been previously studied57-59. However, the
objective of the present work is to add a level of order to the NP films through the use of an amphiphilic block- copolymer3, 18, 60, 61. More specifically, this study focuses on a class of diblock copolymers known to form
circular nanodomains, known as surface micelles, when spread at the air-water interface4, 6-10, 62. Surface
micelles are typically formed by symmetric block copolymers that contain a hydrophobic segment and a hydrophilic, non-water soluble block, such as the poly(styrene)-b-poly(methyl methacrylate) (PS-b-PMMA) employed here. When PS-b-PMMA is spread at the air-water interface, the hydrophobic PS segments aggregate to form hemispherical domains that constitute the core of the surface micelles. The PMMA chains spread at the air-water interface as a pseudo-monolayer that both lies under the PS domains and forms a corona around them.
The NPs selected to be included in the PS-b-PMMA matrix are composed of a gold core decorated with thiol- terminated PS chains. Gold NPs are of interest to our group for their plasmonic properties28, 63. While the
literature covering composite films of block copolymers and NPs is very large, only a few experiments, to our knowledge, have been conducted at the air-water interface. Cheyne et al.19, 20 successfully incorporated PS-
coated cadmium sulfide quantum dots into 1D poly(stryrene)-block-poly(ethylene oxide) structures at the air- water interface and were able to transfer the resulting composite films to solid substrates by the LB technique. Li et al. 64 mixed iron oxide (Fe2O3) NPs with a poly(ethylene oxide)-block-poly(isobutylene) and reported either
NP dispersion or NP aggregation, depending on the number of particles incorporated within the Langmuir film. We recently reported the use of PS-b-PMMA surface micelles as a template for the controlled 2D assembly of alkanethiol capped gold NPs65. The way in which the NPs organized within the polymer matrix was found to
assembly of NPs that are coated with polymer chains of the same chemical composition as one of the copolymer blocks. The results presented below indicate that PS-capped NPs exhibit behavior that is significantly different from that of their alkanethiol-capped counterparts. Furthermore, particle assembly is found to depend on the nature of the substrate surface, with the densest arrays being obtained with the LB technique. The results presented below indicate that the Langmuir-Blodgett technique can be used to prepare well-separated, island-like arrays of ordered metal NPs that are of potential interest for sensing applications based on plasmon enhancement.
Experimental Section
Au NP Synthesis and Functionalization
Gold cores of two different sizes were obtained by different synthetic routes. The smaller ones (r = 2.7 nm) were synthesized via a phase-transfer reduction method derived from Brust66. Larger particles (r = 5.5 nm)
were prepared by citrate reduction in an aqueous medium. Both NP populations were functionalized with thiol- terminated polystyrene chains via a ligand transfer reaction. In all, nine thiol-terminated PS samples of different chain lengths were used to functionalize the gold cores: 3770, 4300, 6500, 8000, 8700, 12700, 23000, 43000 and 66000 g/mol. Experimental details concerning the preparation of the NPs, the synthesis of the thiol-terminated PS and the ligand exchange procedure are provided elsewhere11.
Copolymer
A PS-b-PMMA block copolymer, with a mean molecular weight of 50000 and 54000 g/mol for the PS and the PMMA blocks, respectively, was obtained from Polymer Source Inc. (Montréal, Canada).
LB isotherms
Composite monolayers were obtained by co-spreading the copolymer and PS-coated gold NPs at the air-water interface from a single solution in HPLC grade chloroform (Laboratoires MAT Inc., Québec, Canada). PS-b- PMMA concentrations ranged from 0.7 to 1.2 mg/mL, whereas NP concentrations varied from 0.3 to 2.9 mg/mL, depending on the size of the gold core and the chain length of the ligand. Solutions were deposited dropwise, in a grid pattern, on Nanopure water in a KSV 3000 Langmuir trough. Spreading volumes varied from 55 to 100 µL. Exact concentrations and spreading volumes for each sample are provided as supporting information. After a delay of 5 minutes to allow for solvent evaporation, the monolayer was symmetrically compressed by two mobile barriers advancing at a speed of 1mm/min.
LB transfers
Monolayers were spread as described above and compressed to a target surface pressure of 17 mN/m before transfer. Monolayers were transferred to glass microscope cover slips (Fisher Scientific) during vertical
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surface pressure. Transferred Langmuir monolayers were imaged by transmission electron microscopy and atomic force microscopy.
Transmission Electron Microscopy
Monolayers were transferred onto carbon coated TEM grids, glued to glass substrates. TEM images were obtained with a JEOL 1230 microscope operated at 80 kV. TEM image analysis was carried out with ImageJ software.
Atomic Force Microscopy
AFM analysis was carried out with a Digital Instruments NanoScope MultiMode in tapping mode. Silicon cantilevers (MikroMasch) with a typical resonance frequency of 325 kHz and a force constant of 40 N/m were used. Tips had a radius of curvature of 10 nm.
Results and Discussion
Block copolymer – NP composite films
In all, 18 different particle populations were investigated. Since the qualitative morphology of the system doesn’t change for PS ligands with molecular masses ranging from 3770 to 43000 g/mol, the general features of the composites will be discussed only for NPs coated with 12700 g/mol PS chains. TEM and AFM images of composite films are presented in Figure 30 for the two different particle sizes. In both cases, the NPs form 2D circular island-like aggregates. The average diameter of the island domains varies considerably with both particle size and ligand length (see Table 9 of Supporting Information). Comparison of domain size with various experimental parameters indicates that it can be correlated with the absolute quantity of NPs deposited, but this feature was not investigated in detail. Within the aggregates, NPs are arranged in an ordered close-packed 2D hexagonal lattice. Furthermore, larger NPs are found to concentrate in the center of the aggregates whereas smaller ones are relegated to the periphery. This size fractionation is due to the increase in the strength of van der Waals attractions with increasing particle size, leading to larger NPs serving as the primary nucleation sites for the formation of clusters13.
The PS-b-PMMA monolayer morphology in itself is not significantly altered by the presence of NPs. The organized array of PS domains appears in both the TEM and AFM images of Figure 30 and has the same characteristics as that observed in the absence of NPs. These observations suggest that the island-like NP aggregates are formed upon deposition of the spreading solution whereas PS domains are formed during compression of the monolayer, as they would in a non-composite monolayer13.
This behavior differs significantly from that found for an analogous system, composed of NPs of similar size, but coated with alkanethiols rather than PS chains65. As reported elsewhere, alcanethiol coated particles adopt
an organization dictated by the block copolymer, with the preferred location of the particles within the block copolymer template depending on both nanoparticle size and the length of the aliphatic capping agent.
Figure 30. LB transfer of composite monolayers containing phase-transfer reduced NPs a), c) and citrate reduced NPs b), d). Both NP populations are coated with 12 700 g/mol PS ligands. a) and b) are TEM micrograph while c) and d) are AFM images.
The NP aggregates diameter is 160 ± 60 nm in a) and 90 ± 30 nm in b).
NP films in the absence of block copolymer
The conclusion that the NPs and the block copolymer assemble independently at the interface can be tested by spreading the NPs alone. In the absence of the block copolymer matrix, no increase in surface pressure is observed upon compression to areas equivalent to those of the transferred composite films. The assembled particles are thus transferred at a surface pressure of 0 mN/m. As shown in Figure 31 and Table 5, the NPs spontaneously assemble into 2D circular aggregates of similar size to those formed in the presence of a PS-b- PMMA matrix. This supports the suggestion that the copolymer monolayer plays little role in island-like aggregate formation and size.
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Table 5. Average diameter of the island-like aggregates formed by NPs coated with 12700 g/mol PS-SH ligand for deposition in the presence and absence of copolymer. The indicated uncertainty corresponds to the standard deviation.
The tendency of hydrophobic metallic NPs to self-assemble into disks at the air-water interface has previously been observed for silver NPs passivated with short chain alcanesthiols13. This is an additional indication that
this type of structure is not induced by the PS-b-PMMA layer, but occurs spontaneously at the air-water interface.
Although the presence of the block copolymer does not direct NP organization, it does facilitate LB transfer. In the absence of polymer, NP transfer to glass substrates is irregular and irreproducible, probably because of the low surface pressure and the absence of a hydrophilic moiety in the film. The presence of the block copolymer also appears to improve the uniformity of the island-like NPs aggregates. Comparison of the TEM images of Figure 30 with those of Figure 31 indicates a greater variability in the size of the NPs domains, as well as a larger number of isolated particles, in the absence of a polymer matrix. This, however, was not investigated in a systematic way.
Figure 31. Phase-transfer reduced NPs a) and citrate reduced NPs b) transferred from a bare water surface onto carbon grids. Both NP populations are coated with 12 700 g/mol PS ligands. The NP aggregates diameter is 130 ± 60 nm in a) and 70 ± 20
nm in b).
Edge-to-edge distance between close-packed NPs
TEM images of the ordered NP domains formed in the presence of PS-b-PMMA are provided in Figure 32 for particles of two different sizes and coated with PS chains of varying length. As can be expected, the distance of separation between NPs increases with increasing ligand chain length, although the two samples with the shortest ligand (Figure 32a and Figure 32i) do not respect this trend. The reason for this is not immediately
phase-transfer reduced 210 ± 60 170 ± 80 citrate reduced 90 ± 30 60 ± 20 NP type Aggregate diameter with PS-b -PMMA (nm) Aggregate diameter on bare water surfac
evident, but may be related to more rapid solvent evaporation from the shorter PS chains. Interestingly, for a given NP sample, the edge-to-edge distance between particles differs in the presence (Figure 30) and absence (Figure 31) of the copolymer matrix. As indicated by the values provided in Table 6, the NPs are significantly closer together when transferred from a bare water surface. This means that there is at least minimal interaction between the NPs and the subjacent PS-b-PMMA monolayer. This could be due to the physical presence of PS chains from the monolayer between NPs or because PS-b-PMMA layer hinders the mobility of the NPs so the aggregates cannot reach the closest packing possible after solvent evaporation.
Figure 32. Island-like aggregates from composite monolayer observed by TEM. From a) to h) are phase-transferred NPs. From
i) to p) are citrate reduced NPs. Mean molecular mass of PS ligand chain in g/mol for each micrograph is as follow : a) and i)
3770, b) and j) 4300, c) and k) 6500, d) and l) 8000, e) and m) 8700, f) and n) 12700, g) and o) 23000, h) and p) 43000.
Table 6. Edge-to-edge distance between Au NPs coated with 12700 g/mol SH-terminated PS in 2D hexagonal close-packed lattice in different systems
phase transfer-reduced 6.1 7.3 14
NP type NPs on water(nm) LB composite(nm) NPs on glass11
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The nanoparticle assemblies prepared at the air-water interface can furthermore be compared with those obtained by direct casting on a solid substrate. In a previously reported study11, the identical PS-coated Au
NPs were deposited on glass from chloroform suspensions. Higher NP concentrations were employed and continuous 2D hexagonal arrays were obtained instead of disk-shaped islands. The edge-to-edge distance of separation is plotted in Figure 33 as a function of PS ligand molecular weight, for assemblies prepared at the air-water surface (open circles) and for assemblies cast on glass (filled diamonds). Clearly, much closer particle packing is achieved in the Langmuir-Blodgett films. In addition, TEM images of assemblies obtained by direct casting on glass11, show that hexagonal ordering is lost for PS ligands with molar masses exceeding
20000 g/mol. Preparation of NP assemblies at the air-water interface thus leads to denser, more highly ordered arrays. This is probably due to the high mobility of hydrophobic NPs at the water surface that persists even after solvent evaporation. In contrast, the same NPs are only mobile on a glass substrate when the solvent layer is still present.
Figure 33. Edge-to-edge distance between Au NPs in island-like aggregates in composite monolayers (open circles) and in dense monolayer deposited on glass surface11 (filled diamonds). Graph in a) is for phase-transfer reduced NPs series and b)
is for citrate reduced NPs series.
Composite film cross-sections
Height profiles of composite monolayers were determined from AFM images and are shown in Figure 34a and Figure 34b. The island-like NP domains are observed to be significantly thicker than the surrounding block copolymer monolayer. Peak-to-valley distances, as indicated by the colored markers on the profiles, were evaluated as 16 nm for the smaller, phase-transfer reduced NPs and 21 nm for the larger citrate reduced NPs. Since the NPs pack into dense hexagonal arrays, half of the edge-to-edge distance, as determined by TEM, can be taken as a measure of the thickness of the PS ligand shell surrounding the Au core. Following this, the height of an individual NP can be estimated. In virtually all cases, the estimated NP height is significantly smaller than the height of the particle aggregates measured by AFM (See data provided as supporting information). From this analysis, we can deduce the presence of a layer of PS-b-PMMA underneath the disk- like particle aggregates. By subtracting the NP diameter (Au core and PS shell) from the height of the island- like aggregates (Figure 34) for all PS ligand lengths, the thickness of the underlying copolymer layer can be
0 5 10 15 20 0 10000 20000 30000 40000 Ed g e -to -ed g e d istan ce (n m)
Ligand mean molecular mass (g/mol)
0 5 10 15 20 25 30 0 10000 20000 30000 40000 50000 60000 Ed g e -to -ed g e d istan ce (n m)
Ligand mean molecular mass (g/mol)
evaluated as 2 ± 1 nm in the case of the phase transfer-reduced NPs and 3 ± 1 in the case of citrate reduced NPs. This layer is much thinner than the PS domains of the unperturbed surface micelles, indicating that the block copolymer organization is significantly altered under the Au NP islands. Schematic representations of the composite films are provided in Figure 34c and Figure 34d. This analysis admittedly depends on several assumptions, the most important being that the PS shell coating the NPs is symmetrical. Although this may not be strictly true, it can be argued that the shell thickness evaluated from the lateral interparticle separation represents an upper limit, since closer packing is observed at the bare water surface. This means that the thickness of the underlying polymer layer may be underestimated. In light of surface energy considerations, the presence of a polymer layer beneath the NP domains in not surprising since this configuration allows the hydrophilic PMMA segments to form the layer in contact with the water surface. Finally, it is interesting to note that there appears to be no miscibility between the underlying PS segments of the block copolymer and the PS ligands coating the NPs. If mixing were present, one would not expect to observe the close distances of separation between the particles and the systematic dependence of this distance on ligand chain length.
Figure 34. AFM cross sectional analysis of composite LB monolayers containing phase-transfer reduced NPs a) and citrate reduced NPs b). Both NP populations are coated with 12 700 g/mol PS ligands. Inserts in a) and b) show the horizontal scan
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Loss of order at higher ligand length
As shown in Figure 35, composite films composed of PS-b-PMMA and 2.7 nm Au NPs coated with 66000 g/mol PS ligands adopt a morphology that is very different from all of the other samples. Instead of close-