Controlled Growth of Gold Nanoparticles Preorganized in Langmuir

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Controlled growth of gold nanoparticles preorganized in Langmuir-Blodgett monolayers Jean-François Lemineur, and Anna M. Ritcey Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b02595 • Publication Date (Web): 27 Oct 2016 Downloaded from http://pubs.acs.org on November 1, 2016

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Controlled growth of gold nanoparticles pre-organized in Langmuir-Blodgett monolayers

Jean-François Lemineur, and Anna M. Ritcey* Department of Chemistry and CERMA, Université Laval Pavillon Alexandre-Vachon 1045, avenue de la Médecine Québec (Québec) G1V 0A6 Canada

Phone: 418-656-2368 [email protected]


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Abstract A method is described for the in situ growth of substrate-supported organized gold nanoparticles. Upon exposure to an aqueous solution of a gold salt and a mild reducing agent, particle size can be significantly increased without loss of superstructure organization. Furthermore, no secondary nucleation is observed. The surface-supported regrowth procedure can be combined with the Langmuir-Blodgett technique to produce a rich library of plasmonic nanoparticle assemblies. Controlled particle regrowth plays a crucial role in this assembly method because only relatively small metallic nanoparticles can be directly dispersed in polymeric Langmuir-Blodgett films. The versatility of the method is demonstrated through the fabrication of several specific nanoparticle structures, including contiguous plasmonic rings, core-satellite structures and necklace assemblies. Plasmon extinction spectra are presented for the various nanoparticle superstructures and illustrate the importance of controlling both particle size and assembly architecture in achieving targeted optical properties. The reported approach constitutes a viable bottom-up assembly route for the fabrication of surface-supported nanoparticle superstructures for plasmonic applications.

Introduction The assembly of nanoparticles (NPs) on a Langmuir trough is a simple and effective produce different kinds of nanostructured thin films.1,2 This technique gives access to variety of arrangements and can offer precise control of the organization of NPs dimensions.3,4,5,6,7 Furthermore, ordered assemblies formed at the air-water interface transferred to solid substrates by the Langmuir-Blodgett (LB) technique.

way to a wide in two can be

Noble metal NPs exhibit optical extinction due to the well-known localized surface plasmon resonance.8 For isolated particles, absorption and scattering intensities, as well as the frequency of the plasmon resonance, depend on several parameters such as the nature9, the size10, the shape11 and the surrounding environment of the particles12,13. For NPs assemblies, plasmon coupling between particles can appear. This interaction modifies the plasmon resonance frequency14 and enhances the electromagnetic field in the vicinity of the particles15. Furthermore, the strength of the coupling between NPs strongly depends on their dimensions and the size of the gap between them.16 Amphiphilic block copolymers with appropriate relative block sizes self-assemble into ordered structures known as surface micelles when spread at the air-water interface. Within these structures, the hydrophilic blocks typically spread to cover the water surface whereas the hydrophobic blocks form raised aggregates in order to minimise their interaction with the aqueous phase.17 Block copolymer surface micelles can serve as templates for the directed assembly of co-spread NPs.18 Although this approach offers a convenient bottom-up route to controlled NP assemblies, it is limited to particles with very small diameters. For example,


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Lamarre et al.19 reported the formation of novel ring structures through the assembly of gold NPs at the periphery of the hydrophobic domains of surface micelles formed by poly(styrene)-bpoly(methyl methacrylate). NP rings, however, are formed only for a very limited range of particle diameters, on the order of 6 nm. The co-spreading of larger NPs with the block copolymer leads rather to the formation of hexagonally packed island-like aggregates. Since similar aggregates have been reported for NPs spread alone on a Langmuir trough20, their formation is attributed to inter-particle van der Waals attractions that become stronger with increasing NP size and prohibit particle dispersion. In order to overcome the difficulty of dispersing larger NPs within block copolymer matrices, we have developed a procedure for in situ particle regrowth. With this approach, small NPs can be pre-organized in LB films and subsequently grown to dimensions more suitable for plasmonic sensing applications. Furthermore, in situ particle growth also reduces the gap between neighbouring particles, leading to increased inter-particle plasmonic coupling. Methods for the fabrication of libraries of substrate-supported of NPs employing surface growth have been previously reported.21,22,23 To date, however, such studies have been limited to single particles or simple architecture assemblies. The present article focuses rather on the production of more complex and less common structures, including clusters24, rings25,26, core-satellite assemblies27,28,29,30,31,32 and necklaces33,34,35,36. These multi-particle structures can support coupled plasmonic modes, which are of interest for optical applications such as biosensing37 or as waveguides38.

Experimental methods Materials A poly(styrene)-b-poly(2-vinylpyridine) block copolymer, as well as poly(styrene) and poly(2vinylpyridine) homopolymers, were employed in the preparation of LB matrix films. All polymers were purchased from Polymer Source and their molecular weight characteristics are summarized in Table 1. Table 1: Molecular weight characteristics of the polymers used in the present study.

PS-b-P2VP PS P2VP

Mean molecular weight (g/mol) 54900-b-50100 52000 3700

Polydispersity index 1.07 1.07 1.44

All other chemicals were commercially obtained from the indicated sources: methanol (Fisher Scientific, 99,9%), chloroform (Sigma-Aldrich, ≥ 99%), gold (III) chloride trihydrate (SigmaAldrich, 98%), sodium borohydride (Sigma-Aldrich, 99%), tetraoctylammonium bromide


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(Sigma-Aldrich, 98%), Sulphuric acid (Caledon, 95% to 98%), 1-octanethiol (Sigma-Aldrich, 98%), 1-octadecanethiol (Sigma-Aldrich, 98%), cetyltrimethylammonium bromide (SigmaAldrich, ≥ 98%), ascorbic acid (Sigma-Aldrich, 99%). Water was purified using a Nanopure II filtration system. Synthesis of gold seed nanoparticles Gold NPs were prepared by a modified Brust synthesis.39 Gold (III) chloride trihydrate (0.6 g) was first dissolved in 50 mL of ultrapure water and then transferred to 100 mL of a chloroform solution containing 3.5 g of tetraoctylammonium bromide. An aqueous solution of NaBH4 (0.63 g in 20 mL) was then added to reduce the gold salt. The chloroform suspension of NPs was isolated by decantation and washed with dilute sulphuric acid (0.1 M) and ultrapure water. Alkanethiols were then added in excess to the suspension and stirring was maintained overnight. Particles were capped with either octanethiol or octadecanethiol and the resulting populations are designated as C8NPs and C18NPs, respectively. Finally, excess ligand was removed by three cycles of centrifugation and NP redispersion in a chloroform/methanol mixture. Particles were dried under vacuum until use. Langmuir-Blodgett film formation Spreading solutions were prepared by first dissolving block copolymer or hydrophilic homopolymer in chloroform at a concentration of 1.8 mg/mL. Seed NPs were then introduced in the polymer solution at concentrations between 1 and 2 mg/mL. NP dispersion was promoted by treatment of the solution in an ultrasonic bath for 1 minute. Solutions were used for LB film formation within one day of their preparation. Monolayers were formed by spreading 50-100 µL of solution on a sub-phase of ultrapure water in a KSV 3000 Langmuir bath. The chloroform was allowed to evaporate for 10 ± 5 minutes before monolayer compression to a surface pressure of 15 mN/m. Once the surface pressure stabilized, molecules and NPs were transferred to a glass microscope slide withdrawn though the interface at a speed of 5 mm/min. In-situ growth procedure A regrowth stock solution was prepared by dissolving 0.0197 g of gold (III) chloride trihydrate and 6 g of cetyltrimethylammonium bromide in 200 mL of ultrapure water. Higher concentrations lead to uncontrolled growth and particle fusion. LB monolayers were pre-treated by exposure to UV radiation (13 cm long 4W F4T5/BLB fluorescent tube) at 365 nm for at least 6 hours before particle growth. Samples were positioned 3 cm below the tube. The growth solution was prepared by mixing 10 mL of stock solution with 0.05 mL of a fresh aqueous solution of ascorbic acid (0.1 M) under vigorous agitation. Once the orange color had vanished, agitation was stopped and the solid substrate supporting the LB


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monolayer immersed at room temperature. The surface of the film became increasingly red after a few minutes of immersion in the growth solution. After the desired time of immersion, the monolayer was removed from the solution, dipped in 10 mL of ultrapure water for 15 minutes and finally dried under airflow. For step-by-step growth, the above procedure was repeated with fresh solutions and new ultrapure water for each subsequent immersion/washing cycle. Monolayer characterization Samples for transmission electron microscopy (TEM) observation were prepared on microscope grids glued to the glass substrates before LB deposition. Images were obtained on a Jeol 1230 microscope using an acceleration voltage of 80 kV. A Nanoscope III multimode microscope from Digital Instruments, operated in tapping mode with a cantilever resonance frequency of 325 kHz, was used to record atomic force microscopy (AFM) images. High resolution tips (HQ:NSC15/AL BS), specified to have a typical uncoated radius of 8 nm, were obtained from MikroMasch. Height, amplitude and phase images were collected at the same time during the surface scans. Finally, a Varian Cary 500 UV-Visible spectrophotometer served to record plasmon extinction spectra of the different monolayers.

Results and discussion Organization of gold seeds The modified Brust method described above yields alkanethiol-functionalized gold NPs with an average diameter of about 6 ± 1 nm. As described elsewhere,18,19 NPs of this size can be cospread with amphiphilic block copolymers at the air-water interface to form ordered assemblies. In addition, the exact location of the NPs within the block copolymer template depends on the length of the capping ligands. For longer alkyl chains (C18), the NPs are found within the hydrophobic cores of the block copolymer micelles, either as isolated particles or aggregates of variable size. This is illustrated in Figure 1a. Shorter chain (C8) alcanethiol-stabilized NPs of the same core size (6 nm) are organized in a strikingly different manner, being relegated to the periphery of the hydrophobic PS domains, as shown in Figure 1b. The difference in the spatial distribution of NPs in the block copolymer matrix can be attributed to differences in the packing density of the capping ligands and their capacity to interact with PS chains. This has been discussed in more detail in prior papers. 18,19 The plasmonic properties of NP ring assemblies such as those shown in Figure 1b depend on both ring diameter and particle size. As previously reported 40, ring size can be conveniently modulated by adding hydrophobic homopolymer to the block copolymer spreading solution. In the present case, the addition of PS homopolymer at a concentration of 0.75 mg/mL leads to


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swelling the surface micelle cores and a ring diameter of roughly 100 ± 30 nm, as illustrated in Figure 1c. In absence of PS homopolymer, the ring diameter of the assemblies is evaluated to be 55 ± 5 nm. Alkanethiol-capped gold NPs were also co-spread at the air-water interface with P2VP homopolymer, resulting in the formation of the necklace-like structures shown in Figures 1d-e. These structures are composed of quasi-circular aggregates strung together to form chains. The capping ligand length influences both the size of the aggregates and the length of the necklaces. C8NPs form well-defined 44 ± 10 nm aggregates connected into strings with an average length of 1.3 ± 0.7 µm. (It should be noted that the indicated uncertainties correspond to the standard deviation of the size of the structures and not to the error associated with their evaluation.) For C18NPs, the necklace structures have almost the same length (1.2 ± 0.6 µm), but the aggregates are larger and less uniform (56 ± 32 nm). The TEM images of the various NP assemblies shown in Figure 1 are provided at two different magnifications. High magnification allows for visualization of individual structures, whereas large-scale images demonstrate that NP assembly is uniform over relatively extensive surface areas and that the samples are exempt from sporadic NP aggregation. Additional, intermediatescale TEM images, permitting the evaluation of structure-to-structure variations, are provided as Supporting information.


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Figure 1: TEM images of composite LB films formed by the co-spreading of (a) C18NPs and PS-b-P2VP, (b) C8NPs and PS-b-P2VP, (c) C8NPs, PS-b-P2VP and PS (d) C18NPs and P2VP and (e) C8NPs and P2VP. Images are provided at two different magnifications for each sample.

Transmission UV-visible extinction spectra were recorded for the various NP assemblies transferred to glass substrates. Since the spot size is on the scale of a mm2, a large population of nanostructures is analyzed and an average optical property is measured. Even if the extinction intensity of 6 nm NPs is very weak, a plasmon peak can be distinguished at about 550 nm for all assemblies. UV-visible extinction spectra of the different assemblies, reported in Figure S2 of the supporting information, are all very similar and do not show significant coupling, presumably because of the small particle size. Growth of NPs organized by surface micelles The size of NPs was increased by dipping glass-supported films of the assembled structures into fresh growth solutions for various immersion times. Typically, the red color characteristic of the gold plasmon appears at the glass surface almost immediately upon immersion, becoming more intense with time. The immersion solution, on the other hand, remains colorless during the entire regrowth process, indicating that the ascorbic acid reduction of gold ions dissolved in solution


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occurs only at the glass surface. Ascorbic acid is known to be a very weak reducing agent that which reacts with gold salt only in the presence of a metallic surface and surfactant.41 The in situ growth of NPs organized into small rings is illustrated by the TEM images of Figure 2 for four separate LB monolayers, each treated for a different immersion time. Particles are observed to increase in size without being displaced from the rings.

Figure 2: TEM images of composite LB monolayers of PS-b-P2VP containing gold C8NPs organized into rings. After UV irradiation, the films were immersed in a growth solution for (a)10, (b) 20, (c) 30 and (d) 40 minutes.

After ten minutes of immersion in the growth solution, the mean diameter of particles increases by almost 4 nm (10 nm as compared with the initial 6 nm). It can also be noted that after regrowth, the number of particles in each ring is significantly smaller than in the original nanorings shown in Figure 1b. This observation can be attributed primarily to the fusion of adjacent seeds during growth. As expected, longer immersion times lead to larger NPs as shown in the Figures 2b-d. Average diameters of 11 nm, 14 nm, and 16 nm are found for 20 min, 30 min, and 40 min of immersion, respectively. However, as the immersion time is further increased, uncontrolled particle growth becomes more frequent and the monolayers contain an increasing number of defects. This is illustrated by the AFM images of Figure S3 provided as Supporting information. After 50 minutes of immersion, the image in Figure S3d reveals the presence of holes in the monolayers. This observation is attributed to desorption of entire micelles once the NPs become too large. The desorption of large particles limits the growth procedure to particle diameters of about 20 nm, corresponding to immersion times of less than 1 hour.


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Reproducibility tests also reveal that the growth sometimes significantly increases the size dispersity of NPs. This observation suggests that all particles are not equally susceptible to secondary growth. In an attempt to improve reproducibility, two pre-treatments were considered: UV irradiation and heating at 90°C during 6 hours. The best results are always obtained after UV treatment, which greatly improves the homogeneity of particle populations after the immersion step. On the contrary, heating the monolayers has no effect. The improved reproducibility after UV treatment can be tentatively attributed to destabilization of the thiol-gold bond, rendering the particles more accessible for growth. Comparison of Figures 3a, 3b and/or 3c indicates that the morphology of the hydrophobic domains of the polymer matrix do not appear to be modified by UV irradiation. Although possible changes in the hydrophilic regions of the films cannot be probed by AFM images, they would not be anticipated to influence particle growth. AFM images and height profiles are provided in Figure 3 for PS-b-P2VP surface micelles without NPs and with C8NPs both before and after in situ particle growth. These images show that the introduction of the original 6 nm NPs does not significantly disrupt the block copolymer micellar morphology. These NPs have a diameter that is similar to the height of the micelle core and they can thus be easily accommodated at the edge of the PS domains. The mean diameter of polymer micelles does, however, increase significantly (from 52 nm to 75nm) after 10 minutes of immersion in the growth solution. The AFM profiles of Figure 3c also show that although the larger particles exceed the vertical dimensions of the PS domains, they remain at the micelle periphery. Figure 3c furthermore reflects the significant polydispersity of the gold NPs, consistent with the TEM images of Figure 2. AFM images for composite LB after 20, 30, 40 and 50 minutes of growth are provided as Supporting information.


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Figure 3: AFM images and height profiles of PS-b-P2VP LB films (a) without NPs, (b) with 6 nm seed NPs and (c) after 10 minutes of immersion in the growth solution. The image in (a) was recorded prior to UV irradiation whereas samples shown in (b) and (c) both received UV treatment. Schematic representations of the idealized particle position are provided below the AFM profiles.

C18NP clusters (Figure 1a) and C8NPs organised into larger rings (Figure 1c) were also subjected to particle regrowth solutions. As shown by the TEM images of Figure 4, the results are analogous to those obtained for the smaller ring assemblies.


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Figure 4: TEM images of PS-b-P2VP monolayers containing C8NPs organized into large rings (a-d) or C18NPs organized in clusters (e-h). All samples were irradiated with UV prior to immersion in in a growth solution for 10 (a, e), 20 (b, f), 30 (c, g) or 40 minutes (d, h). Images are provided at two different magnifications for each sample.

The growth rate for gold NPs in both ring and cluster arrangements, expressed as average particle size as a function of immersion time, is provided in Figure 5a. Despite the relatively high size polydispersity of particles, growth appears to be faster in the clusters than in the rings. It is important to note that the error bars in Figure 5a correspond to the standard deviation of NP diameter and reflect the width of the size distribution rather than the uncertainty associated with the mean diameter. Growth rates can be evaluated as 0.4 ± 0.1 and 0.2 ± 0.1 nm per minute, for clusters and rings, respectively. Particle growth may be faster in the case of clusters because of less dense packing of the longer chain alkanethiol ligands (C18 versus C8) or because of more frequent NP fusion, as suggested, for example, by the TEM image of Figure 4h.


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Figure 5: (a) Mean NP diameter as a function of immersion time in a growth solution for clusters (black symbols) and small ring arrangements (red symbols). The vertical bars correspond to the standard deviation of particle size evaluated from TEM images of hundreds of NPs. Extinction spectra of PS-b-P2VP monolayers containing gold NPs organized into (b) small rings, (c) large rings and (d) clusters after different growth solution immersion times as indicated.

The extinction spectra of the various NP assemblies as a function of particle growth time are provided in Figures 5b-d. In the case of small ring structures (Figure 5b), the wavelength of maximum extinction is slightly, but consistently, red-shifted by particle growth: The plasmon band moves from 540 nm for 6 nm NPs, to 550 for 10 nm NPs and to 560 nm for 16 nm NPs. In addition, an unstructured tail in the longer wavelength region is also observed in comparison with the monolayer without regrowth. This extinction characteristic is consistent with the appearance of coupled modes and is presumably not the result of isolated particles of bigger size.42 The assignment of extinction at longer wavelengths to coupled modes is further supported by the fact that its intensity does not vary systematically with immersion time. Plasmon coupling within the rings does not only depend on NP size, but also on the number of particles per ring and the exact distance between them. As particles grow, coupling will initially increase, both because of a decrease in particle separation and the longer coupling range of larger particles.


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However, upon further growth, these effects can be negated by particle fusion. This point, discussed further below, is more clearly illustrated by the spectra of Figure 5d. Because of initial ring-to-ring variations in NP distribution, particle fusion will not occur at the same immersion time for each ring, leading to sample heterogeneity and the absence of a systematic variation of coupling intensity with immersion time. Small ring extinction spectra are particularly sensitive to this variability because coupling extends over relatively short NP chains.40 Much stronger red-shifts are observed upon the growth of NPs organized into larger rings (Figure 5c), with the position of maximum extinction reaching 640 nm after 40 min of immersion. Two contributions to the extinction spectra are apparent. The longer wavelength component can be assigned to a coupled mode, which becomes increasingly important at longer immersion times. This is to be expected since the strength of plasmon coupling increases with particle size. The difference between the spectra of small and large rings can be attributed to a difference in the number of particles involved in the coupled mode. Previously reported discrete dipole approximation calculations for these structures indicate that plasmon coupling extends over an increasing number of NPs as the rings are enlarged.40 Extinction spectra of clusters subjected to in situ particle regrowth are presented in Figure 5d. After 10 minutes of immersion, the NPs are presumably not close enough together to produce coupling and the optical properties remain essentially identical to those of the monolayer containing 6 nm seeds. After 20 and 30 minutes of immersion, a coupled mode does appear at 650 nm. This shoulder, however, disappears at immersion times over 40 minutes. The TEM images indicate that at longer immersion times many of the original NPs have merged and the majority of the surface micelles contain only single particles. In the absence of coupling, the plasmon peak maximum, originally at 550 nm, is not significantly shifted after 40 minutes of immersion. A small red-shift of about 3 nm is observed and can be attributed to the increase of the mean NP diameter from 6 to 22 nm. Core-satellite assemblies The different assembly behavior of C8NPs and C18NPs in the block copolymer surface micelle template can be exploited to fabricate core-satellite nano-structures. As illustrated in Figure 6a, when roughly equal quantities of the two NP populations are mixed and co-spread with the PS-bP2VP, particles are observed both within the PS domains and at their periphery. The two types of particles organize independently, with C8NPs forming ring structures and the C18NPs being dispersed within the micelle cores.


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Figure 6: TEM images of PS-b-P2VP monolayers containing both C8NPs and C18NPs organized into core-satellite structures, with UV pre-treatment and after being dipped in a growth solution for (a) 0, (b) 10, (c) 20, (d) 30 and (e) 40 minutes. For each sample, images are provided at both low and high magnification.

As shown by the TEM images of Figures 6b-e, both NP populations are affected upon immersion in the particle growth solution. Occasionally, this procedure leads to core-satellite structures such as the one presented in Figure 6e. Intermediate scale TEM images, provided in Figure S7 of the Supporting information, show that the number of central particles varies from one ring to another. The percentage of core-satellite structures, as opposed to empty rings, can be evaluated to be on the order of 70% ± 5%. This kind of arrangement is of a particular interest for the modification of ring secondary resonant modes. For example, other groups have utilized coresatellite structures to produce plasmonic Fano-like resonances and the fabrication of high figureof-merit sensors.42,43 Although further optimization will be required to consistently obtain a central NP with a more appropriate diameter for the observation of a Fano resonance, preliminary results suggest that it is possible to tune the size of the central NP by adjusting the seed concentration. Step-by-step growth One of the goals of this study is the formation of complete ring structures. Unfortunately, the one-step growth procedure fails to achieve this outcome. For this reason, a step-by-step process, inspired by the method reported by Jana et al.44,45 for the regrowth of particles in suspension, was also investigated. In the present study, substrate-supported monolayers were successively immersed in the growth solution and in ultra-pure water. For each immersion/washing cycle, fresh solutions were employed. Results obtained for 2 and 3 cycles are presented in Figure 7 below.


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Figure 7: TEM images of PS-b-P2VP monolayers with nanorings of C8NPs after (a) 2 and (b) 3 successive regrowths cycles. Mean NP diameter as a function of total immersion time is plotted in (c). Error bars correspond to the standard deviation of particle size and not to the uncertainty associated with the mean diameter.

As shown in the Figure 7c, the step-by-step procedure results in larger NPs than single immersion for the same total time. After two immersion/washing cycles, particles reach an average diameter of 20 nm which roughly corresponds to the size obtained only after 50 minutes of constant immersion. Furthermore, the step-wise growth procedure results in a narrower particle size distribution. Importantly, after three successive steps the NPs come into contact within the rings, as shown in Figure 7b. The mean NP diameter after three successive growth steps is difficult to evaluate because of particle merging. Nevertheless, it was estimated to be on the order of 40 nm. The difference between single-step and multi-step growth kinetics suggests that the process is diffusion controlled, as previously reported for substrate supported NP regrowth.21 Growth of NPs organized on P2VP homopolymer monolayers TEM images of the necklace structures after particle regrowth are presented in Figure 8. Under the same regrowth conditions, NPs organized in necklace structures grow much faster than those co-spread with block copolymers. After immersion times of only 10 minutes, NPs in necklaces reach average diameters of 18 ± 7 and 26 ± 14 nm when capped with octanethiols and octadecanethiols, respectively. The difference between the growth rate of NP necklaces and corresponding clusters and rings can be attributed to the position of the particles in the supporting polymer matrix. P2VP is hydrophilic and spreads at the air-water interface to form an ultrathin monolayer, with a thickness significantly smaller than the diameter of the NPs. Furthermore, since the NPs are capped with hydrophobic ligands, they will not be miscible with P2VP. In the composite films, the NPs can therefore be assumed to either ride on top of the polymer monolayer or form a border between neighboring domains. In either case, the polymer


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matrix does not shield the NPs from the regrowth solution. In contrast, NPs incorporated in the block copolymer matrix are embedded in the PS domains and thus less accessible for particle regrowth. Finally, the difference in the growth rate observed for the C8 and C18 NPs in the necklaces can be attributed to the difference in ligand packing as mentioned above. Since significant particle fusion is observed, particularly at longer immersion times, final particle size may also be influenced by the initial number of particles within the aggregates.

Figure 8: TEM images of P2VP monolayers containing C18NP (a-d) and C8NP (e-h) gold NPs. Samples were irradiated with UV before immersion in growth solution for 10 (a, e), 20 (b, f), 30 (c, g) and 40 (d,h) minutes.


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As shown in Figure 9, the extinction spectra of the necklace assemblies are dramatically redshifted after only 10 minutes of immersion with maxima appearing at 700 and 800 nm, for C8NPs and C18NPs, respectively. (Spectra before NP regrowth are provided as Supporting information and resemble to those of isolated particles.) Longer immersion times lead to further extension of the extinction spectra into the near-IR. This can be attributed to strong longitudinal plasmon coupling along the NPs chains.35 The plasmon shift exhibited by the C18NPs necklaces is more pronounced than that of the corresponding C8NPs chains, probably because the anisotropy of the structures is less disrupted by particle regrowth. The TEM image of Figure 8h does indeed show that after 40 min of regrowth, little remains of the original chainlike aggregates. Furthermore, large isolated NPs are clearly present, as also indicated by the appearance of a local maximum at 570 nm in the corresponding extinction spectra (blue curves of Figure 9).

Figure 9: Extinction spectra of P2VP monolayers containing (a) C8NP and (b) C18NP gold NPs organized into necklaces after particle regrowth for 10, 20, 30 or 40 minutes as indicated.

The plasmon bands of Figure 9 are extremely broad in comparison to spectra obtained for ring and cluster structures. This can be attributed to the large heterogeneity in chain morphology over the sampled area. Chain disorder in the form of zig-zags and other non-linearities, length polydispersity, looping and branching points all contribute to broadening of the longitudinal plasmon resonance.34,46 Furthermore, the size polydispersity of NPs after regrowth is higher for the necklace structures than the rings and clusters, possibly because the NPs are not embedded in a protective PS matrix that helps control growth. The broad extinction spectra obtained for the necklace assemblies may be advantageous for certain applications where plasmon-enhanced fields active over a relatively large frequency range are required. The necklace structures are particularly interesting because of their NIR activity. For example, such substrates can be used in metal-enhanced spectroscopies, such as surface-enhanced fluorescence or infrared absorption, or as localized heat sources.35,47


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Conclusions The in situ regrowth of substrate-supported gold NPs greatly enhances the versatility of the LB approach for the preparation of ordered particle assemblies. When co-spread with appropriate amphiphilic block copolymers or hydrophilic homopolymers at the air-water interface, NPs selfassemble to form organized structures such as rings, clusters and necklaces. This method of NP assembly is, however, limited to relatively small particles, on the order of 6 nm in diameter. The regrowth procedure established herein allows for particle size to be significantly increased (in some cases up to diameters of 40 nm) without modification of particle location. In addition, no secondary nucleation of a new particle population is observed. When compared with a single step procedure, step-wise growth is found to offer better control over particle size. The ability to control particle size is crucial for all potential plasmonic-based applications of NP assemblies. Since interparticle-coupling is highly sensitive to particle size, particle separation and the details of particle organization, the results presented here are particularly relevant to applications based on coupled plasmonic modes. Several specific examples of NP superstructures accessible through the combination of the LB technique and substrate-supported NP growth are demonstrated. These include contiguous rings relevant to the development of metamaterials, core-satellite structures with potential Fano-like resonances and necklace assemblies that exhibit significant plasmon extinction in the NIR. The reported approach therefore constitutes a viable bottom-up assembly route for the fabrication of surface-supported NP superstructures for plasmonic applications.

Acknowledgements The authors would like to acknowledge the financial support of le Fonds de recherche du Québec – Nature et technologies (FRQNT) and the National Sciences and Engineering Research Council of Canada (NSERC).

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Graphical abstract


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