Size-Dependent Phase Transfer Functionalization ... - ACS Publications

Subscriber access provided by Universitaetsbibliothek | Johann Christian Senckenberg

Article

Size-Dependent Phase Transfer Functionalization of Gold Nanoparticles to Promote Well-Ordered Self-Assembly Florian Schulz, Steffen Tober, and Holger Lange Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03600 • Publication Date (Web): 01 Dec 2017 Downloaded from http://pubs.acs.org on December 2, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Langmuir is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Size-Dependent Phase Transfer Functionalization of Gold Nanoparticles to Promote Well-Ordered SelfAssembly Florian Schulz,*,†, ‡ Steffen Tober,†,ǁ Holger Lange†, ‡

†

Institute for Physical Chemistry, University of Hamburg, Grindelallee 117, 20146 Hamburg, Germany.

‡

The Hamburg Centre for Ultrafast Imaging (CUI), Luruper Chaussee 149, 22761 Hamburg, Germany.

keywords: gold nanoparticles, phase transfer, self-assembly, functionalization, polystyrene

We present a route for the functionalization of gold nanoparticles (AuNP) based on phase transfer functionalization in order to optimize the stability and the potential for self-assembly. Depending on the desired size, different ligand exchanges have to be employed: The maximum AuNP size that can be stabilized without concentration loss is 46 nm for polystyrene-based ligands with 5 and 10 kDa. Small particles 10 nm) is known to be more difficult to control.37–39 Thus, the functionalization of AuNP@Citrate extends the addressable size range for self-assembly assisted by PS-ligands. Smaller PS-ligands were reported to increase the longrange order of AuNP-monolayers.40 Based on these ideas we tested different strategies for the ligand exchange of AuNP@Citrate with thiol terminated PS-ligands (PSSH) with molecular weights in the lower range (2kDa, 5kDa and 10kDa) and the stabilization provided by these ligands for AuNP@Citrate with diameters ranging from 6-90 nm. The usual functionalization strategy used in the mentioned studies is based on the dropwise addition of concentrated AuNPsolution to a solution of the PSSH-ligand in an organic solvent (acetone, THF, chloroform or toluene), often assisted by sonication. Herein, we explore variations of a phase-transfer based

ACS Paragon Plus Environment

3

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 31

protocol that is straightforward and does not require large excess of PSSH ligands. We show that AuNP@Citrate can reliably be transferred into the toluene phase with PSSH-ligands. Larger AuNP require larger PSSH-ligands and we identified which polymer size is required to stabilize AuNP of certain diameters. In test experiments we observed that, despite comparably low particle concentrations, extended well-ordered AuNP monolayers can be obtained by selfassembly of the PSSH-functionalized AuNP on DEG.

2. Results and Discussion AuNP@Citrate with diameters of 6.4, 12.3, 23.1, 40.0, 46.8, 61.3, 79.6 and 89.8 nm were synthesized based on published protocols.32–34 To functionalize the AuNP with PSSH ligands, different strategies were tested: the direct phase transfer of the AuNP@Citrate from water to toluene containing the PSSH ligand or the phase transfer to toluene containing just oleylamine (OLAM) and subsequent reaction of the OLAM coated AuNP, AuNP@OLAM, with PSSH ligands. In any case, the phase transfer from water to toluene was assisted by ethanol, which is known to improve the process.41 UV/Vis spectroscopy was used to evaluate the ligand exchange by comparing the concentrations before and after ligand exchange, as reflected by the absorbance in the range 400-450 nm.42,43 Additionally, broadening of the localized surface plasmon resonance band (LSPR) can indicate aggregation.44 However, because of the different refractive indices of water and toluene and the possibility of turbidity caused by residual water in the toluene phase, the spectra have to be interpreted with care.

2.1 AuNP@OLAM


4

Page 5 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Oleylamine (OLAM) is a common ligand for AuNP from organic syntheses.45–47 It is known to facilitate the self-organization of small nanoparticles into highly ordered structures.48,49 Upon that, it has the advantage of being a good stabilizer that is still replaceable, e.g. by thiols, in analogy to the citrate ligand in aqueous systems. The phase transfer with OLAM is straightforward and reproducible. It is possible to concentrate AuNP in the toluene phase by replacing the water with fresh AuNP@Citrate in water after complete phase transfer.50 Up to ten repetitions were performed and reproducibly yielded AuNP@OLAM with the expected tenfold concentration of the AuNP@Citrate as tested with 6.4 and 12.3 nm AuNP (Figure 1a-d).


5

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 31

Figure 1. Concentration of AuNP@Citrate in the toluene phase by phase transfer with oleylamine (OLAM). After complete phase transfer in a water:toluene:ethanol 1:1:1 mixture, the aqueous phase can be exchanged with more AuNP@Citrate. This was repeated ten times. For UV/Vis measurements the more concentrated solutions were diluted and the absorbances in a (dAuNP = 6.4 nm) and c (dAuNP = 12.3 nm) are corrected with the dilution factors. The


6

Page 7 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

AuNP@Citrate addition steps are indicated at the color code. The concentrations were estimated based on the absorbance at 450 nm as described by Haiss et al.43 The experimental values are higher than the theoretical ones because of evaporation losses. The values are shown in b (dAuNP = 6.4 nm) and d (dAuNP = 12.3 nm). e: Exemplarily volume-weighted distributions of hydrodynamic diameters dH obtained by DLS for different batches of AuNP@OLAM including batches concentrated by repeated phase transfer and centrifugation at 20,000 g. f: Exemplarily TEM measurement of AuNP@OLAM after concentration by repeated phase transfer and centrifugation. No indications of aggregation were observed.

The process with 10 steps was possible with OLAM concentrations down to 10 mM. A single transfer step was possible with OLAM concentrations down to 1 mM. At OLAM concentrations 12 nm were not stabilized by OLAM. AuNP@OLAM can also be obtained directly by syntheses in organic solvents.45,47 Citrate-stabilized AuNP offer the advantage of tunable size33 and very robust protocols,34 AuNP@OLAM from organic syntheses yield higher concentrations. It was not possible to synthesize stable AuNP@OLAM films on DEG, probably because the attractive intermolecular forces of the OLAM-ligand layers are not sufficient. Drop-casting of 12 nm AuNP@OLAM onto TEM-grids, silicon wafers or other substrates usually leads to well-ordered assemblies. Extended monolayers were not obtained but patches, monolayers with large voids and double-layer “islands” or multiple layers with ~ ± 2 layers formed, depending on the particle concentration. The high degree of order was confirmed with SEM, TEM and GISAXS measurements. The edge-to-edge distances determined by


7

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 31

nearest-neighbor- and Fourier analysis of the SEM and TEM-data and by refinement of the scattering data were in the range 1.6-2.0 nm, quite similar to values recently reported for superlattices from dodecanethiol coated AuNP.51 For AuNP@OLAM with 6.4 nm core diameter, less order was observed. The characterization of drop-casted AuNP@OLAM superstructures is provided as Supporting Information (Figure S1).

2.2 Size dependent direct phase transfer Figure 2 shows the results of the direct phase transfer from water to toluene for different particle sizes and OLAM, PSSH with 2 kDa (PSSH2k), 5 kDa (PSSH5k) and 10 kDa (PSSH10k). The spectra of the AuNP@Citrate before phase transfer are shown for comparison. The change in refractive index due to ligand and solvent exchange causes LSPR redshifts of up to ∆λ ~ 18 nm.

Figure 2. UV/Vis-spectra of AuNP with diameters as indicated after phase transfer ligand exchange. AuNP with diameters >12 nm were not stabilized by OLAM (dark red lines) and


8

Page 9 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

OLAM was not tested for AuNP >23.1 nm. The stabilization of AuNP with increasing diameters by PSSH2k (orange lines), PSSH5k (green lines) and PSSH10k (blue lines) correlates with the ligand size. The spectra of the AuNP@Citrate educt particles are shown for comparison (grey dashed lines). For larger AuNP, the solutions had to be diluted for UV/Vis spectroscopy and the absorbances were corrected with the dilution factors. The ligand exchange requires the addition of ethanol that is known to improve the mixing of the water and toluene phase.41 After ethanol addition, the phase transfer is completed quickly. An important finding is, that low concentrations of PSSH-ligands have to be used for the phase transfer functionalization. We routinely used 0.16 mM in the toluene phase. With 1.0 mM PSSHligands aggregation of the AuNP at the liquid-liquid interface was observed. The reason is probably an imbalance of phase transfer and functionalization kinetics leading to incompletely coated AuNP in the organic phase that aggregate. A similar mechanism was discussed by Yang et al.52 Another reason might be solubility effects; methanol or ethanol are typically used to precipitate PSSH functionalized particles. At higher PSSH concentrations solvent induced precipitation might become dominant. This effect of the PSSH-ligands is in contrast to OLAM assisted phase transfer, where high OLAM concentrations are not problematic. Compared to the PSSH ligands, OLAM is a small molecule that can quickly form a full ligand layer. Its binding group, a primary amine, is binding much weaker than the thiol groups of the PSSH ligands (amines: ~40-80 kJ/mol; thiolates: ~120-190 kJ/mol)53–55 and the according on-off equilibrium requires free OLAM, i.e. OLAM excess, for sufficient stabilization of the AuNP.49 The size of the ligands correlated with the AuNP-size that could be stabilized. OLAM stabilized AuNP up to 12 nm, PSSH2k up to 21.3 nm and PSSH5k and PSSH10k up to 46.3 nm without significant concentration losses. Phase transfers of AuNP up to 61.3 nm for PSSH5k and up to 79.6 nm for


9

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 31

PSSH10k were observed, but with significant concentration losses indicating insufficient stabilization. The concentrations by UV/Vis-spectroscopy have to be interpreted as a rough estimate because of the different solvents and turbidity effects. In fact, in reproductions of the phase transfer experiments the stability trends were reproducible but the obtained concentrations could differ.

2.3 Comparison of direct and indirect phase transfer functionalization Since the phase transfer of AuNP@Citrate up to 12 nm in diameter with OLAM is robust, the comparison of direct and indirect phase transfer functionalization with PSSH ligands is of interest. We performed the direct transfer as described in the previous section. For the indirect phase transfer, the AuNP were first transferred to toluene with OLAM. The obtained AuNP@OLAM were then reacted with PSSH ligands. The normalized absorbance spectra indicate that the indirect functionalization might provide a slightly better phase transfer functionalization without formation of any aggregates (Figure 3a). DLS measurements support this interpretation. Indications of some aggregate formation were observed in the directly exchanged samples but not in those prepared by indirect phase transfer functionalization (Figure 3b and c). The DLS measurements also confirm the expected increase of hydrodynamic diameter with ligand size, thus confirming the ligand exchange PSSH vs. OLAM (Figure 3b). The intensity weighted distributions of hydrodynamic diameters dH are shown in Figure 3b and c because these are most strongly affected by the presence of aggregates. The volume weighted mean dH of AuNP@OLAM and AuNP@Citrate, however, were in better agreement with the AuNP diameters determined by TEM (TEM: dAuNP: 11.9 nm; DLS: dH(AuNP@OLAM) = 14.3 nm, dH(AuNP@Citrate) = 14.0 nm)), considering the small contribution of the OLAM or Citrate


10

Page 11 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

layer to dH. The scaling behavior and thicknesses (PSSH2k ~ 3 nm, PSSH5k ~ 5 nm, PSSH10k ~ 8 nm; volume weighted distributions of hydrodynamic diameters dH are provided in Figure S2) of the polymer ligand layers based on the volume weighted mean dH agree well with those determined by Ye et al. in their recent work on small nanoparticles (d < 8 nm) coated with similar ligands.18 A detailed discussion of the polymers’ structure on spherical nanoparticles can be found therein.

Figure 3. Comparison of direct or indirect phase transfer ligand exchange. a: Normalized absorbance spectra for AuNP (dAuNP = 11.9 nm in a, b and c) functionalized by direct phase transfer ligand exchange of AuNP@Citrate with PSSH2k, -5k and 10k as indicated (dashed lines), or by indirect phase transfer ligand exchange (solid lines). Lower panel: intensityweighted distributions of hydrodynamic diameters dH obtained by DLS for b: AuNP@OLAM


11

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 31

(indirect phase transfer ligand exchange) or c: AuNP@Citrate (direct phase transfer ligand exchange) reacted with PSSH ligands as indicated. In previous studies we observed the kinetics of ligand exchange of small thiols vs. OLAM to depend on the OLAM concentration.50 We tested the effect of OLAM concentration with 6 nm and 12 nm AuNP. The experiments are shown in Figure S3 and S4. A clear conclusion regarding the kinetics could not be drawn, but the experiments revealed that ligand exchange with reduced free OLAM concentration is beneficial in terms of aggregate formation. Larger PSSH ligands seem to favor the formation of some clusters or aggregates that cause a slight broadening and redshift of the absorbance spectra. This clustering effect is stronger for the smaller AuNP. It can be concluded that large PSSH ligands allow to stabilize larger AuNP, but that for small AuNP, small ligands are favorable for maximum stabilization. The functionalization of small AuNP with PSSH ligands can be significantly improved by using AuNP@OLAM with reduced free OLAM concentration. This indicates that the clustering is a kinetic effect. Incomplete functionalization of AuNP@OLAM with PSSH might promote encapsulation of more than one AuNP in a PSSH ligand shell. This could explain why the effect is more pronounced for smaller AuNP and larger PSSH ligands. With reduced OLAM concentration the PSSH functionalization is completed faster and no aggregation occurs.

2.4 Film formation on DEG In preliminary experiments we tested if monolayer film formation on DEG is possible with the AuNP@PSSH obtained by phase transfer functionalization. Reliable and reproducible formation of large ordered monolayer films was not achieved but in several experiments such films were obtained, demonstrating the potential of the approach. Figure 4 shows the films formed by AuNP


12

Page 13 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

with 28 nm diameter coated with PSSH2k, PSSH5k and PSSH10k. For the films of AuNP@PSSH5k several large areas >400 µm2 with high degree of order were observed by TEM. It has to be noted that on the same TEM grids areas with less order and fractured areas were found. It is not clear how much of the film fracture results from the transfer from the DEG onto the TEM grid. With other AuNP@PSSH2k (d = 36 nm) even larger monolayer film areas were found, but with slightly less order (Figure S5). AuNP@PSSH10k yielded smaller wellordered areas, ~40 µm2. With these particles, higher concentrations were tested, resulting in mostly two- or multilayer films.

Figure 4. TEM-images of self-assembled AuNP@PSSH films (dAuNP = 28 nm). a: Large area view of self-assembled AuNP@PSSH5k. The light grey area is a well-ordered monolayer, the darker regions consist of multiple layers. b: magnification of the highlighted area in a showing one, two, three and four ordered layers of assembled AuNP from right to left. c: Exemplarily magnification of the light grey area in a demonstrating the homogeneity of the monolayer. Lower panel: Monolayer structure of AuNP@PSSH2k (d), AuNP@PSSH5k (e) and AuNP@PSSH10k (f) with the according digital FFT of the entire image in the insets.


13

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 31

It was evident from all samples and complementary experiments, that PS, or the PSSH ligands respectively, forms thin films on DEG and other substrates and subphases, regardless of the number and order of particles within the PS-matrix. The interparticle distances (edge to edge) were quite small (PSSH2k and 5k: 2-3 nm, PSSH10k: 9 nm) and in accordance with expectations based on the ligand length (Figure 5 and Figure S5f). The correlation is not linear, the interparticle distances do not differ strongly between AuNP@PSSH2k and AuNP@PSSH5k. Chen et al. demonstrated freestanding film formation of AuNP with 13 and 28 nm diameter coated with PSSH10k at a chloroform-water interface.29 However, the films were of limited order. A reason might be the low uniformity of the AuNP that were used. Interestingly, the mean interparticle distance in that study was 2.1 nm in contrast to the 9 nm found in this study for a comparable system. This points at the role of the involved solvents and the liquid subphase.


14

Page 15 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 5. TEM-images of self-assembled AuNP@PSSH5k (a) and AuNP@PSSH10k (b) (dAuNP

= 28 nm). Ullrich et al. demonstrated, that large areas on indium tin oxide (ITO) substrates can be covered with AuNP (d ~ 10 or 50 nm) coated with comparably large PSSH ligands (25 or 50 kDa). Quasi-hexagonal monolayer structures were obtained but with no long-range order. At least for the smaller AuNP (d ~ 10 nm) the limited order can probably be attributed to the large ligand size. For the larger AuNP (d ~ 50 nm) in that study, again, the dispersity might also have played a role. Another reason might be the preparation method itself. The slow self-assembly on a liquid subphase favours the crystallization-like formation of 2D-superlattice sheets as described for small nanoparticles.11,18 We can confirm the formation of such highly ordered sheets with small AuNP (d = 12 nm) coated with PSSH2k. TEM-images of such sheets are shown in Figure 6. At low magnification, wrinkles and cracks can be observed, higher magnifications reveal hexagonal order. When two superlattice sheets overlap slightly twisted, Moiré patterns result, similar to those recently described for dendron-stabilized AuNP.56

Figure 6. TEM-images of self-assembled AuNP@PSSH2k (dAuNP = 12 nm). At low magnification (a), cracks and wrinkles can be observed. The hexagonal order (b) leads to Moirépatterns (c) when two monolayers overlap slightly twisted.


15

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 31

Taken together these experiments demonstrate that large AuNP@PSSH can form extended ordered monolayer films by self-assembly on DEG. Future work is required to improve the reproducibility and to understand and optimize the involved parameters. The particle concentration is one key parameter for the optimization and the correlation of particle size and interparticle distance is an interesting point to address. Although the dispersity of the particles used in this study was already quite low (CV ~8 %) it stands to reason that a significantly lower dispersity might improve the formation of well-ordered areas. At this point it can only be speculated if the larger PSSH ligands 5k and 10k might have promoted the formation of wellordered areas more efficiently compared to PSSH2k because they can better compensate the dispersity of the large AuNP. In fact, for smaller AuNP (d = 12 nm) with low dispersity (CV ~56 %) well-ordered films were obtained with PSSH2k. Once a robust protocol for film formation is established, more complex superlattices, e.g. binary structures or including other particle shapes, can be envisaged. Also, technical aspects like the film transfer to substrates could be improved.

3. Conclusion Our study provides an alternative and effective strategy for the preparation of PSfunctionalized AuNP, in particular with diameters >12 nm. Such AuNP are especially useful for studies addressing self-assembly, controlled assembly in solution or the synthesis of surfacepatterned (“patchy”) particles. The presented phase-transfer functionalization is simple and fast and we tested and discussed the critical parameters AuNP and ligand size and ligand concentration. We demonstrate that the strategy of self-assembly on DEG can be utilized for large AuNP@PSSH. In self-assembly studies, large areas >400 µm² of well-ordered monolayers


16

Page 17 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

were obtained and even larger areas with slightly less order. This approach has the potential to synthesize large, well-ordered AuNP@PSSH mono- and multilayer films with tunable interparticle distance and freestanding films. Future work is required to improve the reproducibility and to optimize the approach.

4. Experimental Section Materials: Tetrachloroauric(III) acid (≥99.9 % trace metals basis), potassium carbonate, oleylamine (98%), toluene, chloroform and ethanol (denat.) were from Sigma-Aldrich (USA). Trisodium citrate and diethylene glycol (DEG) were from Merck (Germany), tannic acid was purchased from Dr. K. Hollborn und Söhne (Germany). Thiolated polystyrenes (PSSH, PSSH2k: Mn = 2000, Mw = 2300; PSSH5k: Mn = 5300, Mw = 5800; PSSH10k: Mn = 11500, Mw = 12400) were from Polymer Source (Canada). All reagents were analytical grade and used without further treatment. For AuNP syntheses ultrapure water (18.2 Ω) was used. AuNP syntheses: Citrate stabilized gold nanoparticles (AuNP@Citrate) with diameters ranging from 6 nm to 90 nm were synthesized following different modified Turkevich protocols. Particles with 6 nm diameter were synthesized with an upscaled version of the protocol by Piella et al.33 AuNP@Citrate with 12 nm diameter were synthesized as described previously.34 The seeded-growth protocol by Bastús et al. was used with slight modifications to grow AuNP@Citrate with larger diameters.32 AuNP concentrations were determined based on their absorbance at 450 nm as described by Haiss et al.43 Details of the AuNP syntheses are provided as Supporting Information.


17

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 31

Increasing the concentration in the organic phase: For enrichment experiments, 0.1 M solutions of oleylamine in toluene were used. As described in the main text, ligand solution, AuNP@Citrate and ethanol were mixed in a volume ratio of 1:1:1. Separation funnel were employed instead of a flask. AuNP@Citrate volumes up to 150 mL were transferred. When the aqueous phase became clear, it was replaced by AuNP@Citrate and ethanol. For each step, 0.1 mL of the organic solution was removed for UV/Vis spectroscopy. Variations of the protocol were tested, e.g. with reduced oleylamine concentration. Phase transfer into chloroform was done without ethanol addition. AuNP@Citrate (100 mL, dAuNP = 12 nm, c(AuNP) = 4 nM) were mixed with oleylamine in chloroform (10 mM, 100 mL). After phase separation the aqueous phase was replaced with AuNP@Citrate. Three repetitions were tested, no detrimental effects on AuNP stability were observed. Ligand exchange: For all particles and ligands comparable ligand exchange protocols were applied. PSSH (0.16 mM) or oleylamine (0.1 M or 0.16 mM) in toluene were mixed with aqueous AuNP@Citrate and ethanol was added in a volume ratio of 1:1:1. AuNP@Citrate with diameters 6.4 nm (c(AuNP) = 28 nM), 12.3 nm (c(AuNP) = 3 nM), 23.1 nm (c(AuNP) = 2 nM), 40.0 nm (c(AuNP) = 0.7 nM), 46.8 nm (c(AuNP) = 0.3 nM), 61.3 nm (c(AuNP) = 0.2 nM), 79.6 nm (c(AuNP) = 0.1 nM), 89.8 nm (c(AuNP) = 0.03 nM) were used as synthesized. The solutions were thoroughly mixed in a closed flask until a homogeneous pink solution had formed. The reaction was allowed to take place over night at room temperature before the organic phase was carefully extracted and purified by syringe filtration (0.22 µm PTFE, Carl Roth, Germany). If necessary for characterization, the organic AuNP solutions were diluted with toluene. The solutions were stored in the fridge over night to allow remaining water to settle down. Residual water causes turbidity that strongly affects UV/Vis- and DLS measurements.


18

Page 19 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Indirect ligand exchange: For indirect ligand exchange, AuNP@OLAM (dAuNP = 12 nm) were mixed with the according PSSH-ligand solutions (1 mM) to yield 0.14 mM. Functionalized particles with the same final PSSH concentration were prepared by direct phase transfer functionalization for comparison. Self-assembly of AuNP@PSSH at the liquid-liquid interface: Self-assembly of AuNP@PSSH was tested with the method described by the Murray group.19 AuNP@PSSH were prepared by direct phase transfer functionalization as described. 300 µL AuNP@PSSH solution in toluene were pipetted onto 300 µL DEG in a Teflon well. The well was covered with a glass slide and left undisturbed at room temperature until the solvent was evaporated and golden films had formed. The waiting time was at least 24 h. AuNP@PSSH2k and 5k with dAuNP = 28 nm were 1.9 nM, AuNP@PSSH10k with dAuNP = 28 nm were 12 nM and just 200 µL solution were used. 300 µL were also tested and comparable films were obtained. AuNP@PSSH2k with dAuNP = 36 nm were 0.4 nM (films shown in Figure S5). Transmission Electron Microscopy (TEM). TEM measurements were performed using a Jeol JEM-1011 instrument operating at 100 kV. The mean diameters of AuNP@Citrate were determined as described previously.34 Samples of self-assembled AuNP@PSSH films were carefully skimmed off with a carbon coated copper grid held by a tweezer. The grid was then dried on filter paper for at least 24 h. DLS measurements. DLS measurements were performed with a Zetasizer Nano ZS (Malvern Instruments). The instrument uses a He-Ne-Laser (4.0 mW, 633 nm). Data were analyzed using the software Dispersion Technology Software (Version 5.10). The number of runs per measurement was set to 30. At least two measurements were recorded for each sample, the standard deviations of the volume means were below 3 %.


19

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 31

UV/Vis Spectroscopy. Absorbance measurements were carried out using a Perkin-Elmer Lambda 25 or a Varian Cary 50 spectrometer. UV microcuvettes sealed with lids (Plastibrand, Carl Roth GmbH, Karlsruhe, Germany) were used for all experiments with aqueous solutions, quartz cuvettes (Hellma QS, Hellma, Germany) were used for solutions in toluene.

ASSOCIATED CONTENT Additional experimental information, SEM- and GISAXS-characterization of dried AuNP@OLAM films, additional DLS-data, absorbance spectra recorded during reaction of AuNP@OLAM (dAuNP = 6.4 nm and 11.9 nm) with PSSH-ligands, TEM characterization of selfassembled AuNP@PSSH2k (dAuNP = 36 nm). This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Florian Schulz, E-mail: [email protected] Present Addresses † Deutsches Elektronen-Synchrotron DESY, Notkestr. 85, 22607 Hamburg, Germany

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


20

Page 21 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

ACKNOWLEDGMENT We acknowledge financial support from the German Research Foundation (DFG) via the Cluster of Excellence “Centre for Ultrafast Imaging” (CUI). F.S. is supported by the DFG via the project SCHU 3019/2-1. We thank Dr. Andreas Meyer for GISAXS measurements and data processing, Robert Schön for SEM measurements and Michael Deffner and Sönke Wengler-Rust for supporting experiments on AuNP@OLAM synthesis.

REFERENCES (1) Bigioni, T. P.; Lin, X.-M.; Nguyen, T. T.; Corwin, E. I.; Witten, T. A.; Jaeger, H. M. Kinetically driven self assembly of highly ordered nanoparticle monolayers. Nat. Mater. 2006, 5, 265–270. (2) Bishop, K. J. M.; Wilmer, C. E.; Soh, S.; Grzybowski, B. A. Nanoscale Forces and Their Uses in Self-Assembly. Small 2009, 5, 1600–1630. (3) Gong, J.; Newman, R. S.; Engel, M.; Zhao, M.; Bian, F.; Glotzer, S. C.; Tang, Z. Shapedependent ordering of gold nanocrystals into large-scale superlattices. Nat. Commun. 2017, 8, 14038. (4) Grzelczak, M.; Vermant, J.; Furst, E. M.; Liz-Marzán, L. M. Directed Self-Assembly of Nanoparticles. ACS Nano 2010, 4, 3591–3605. (5) Luo, D.; Yan, C.; Wang, T. Interparticle Forces Underlying Nanoparticle Self-Assemblies. Small 2015, 11, 5984–6008. (6) Ye, X.; Chen, J.; Eric Irrgang, M.; Engel, M.; Dong, A.; Glotzer, S. C.; Murray, C. B. Quasicrystalline nanocrystal superlattice with partial matching rules. Nat. Mater. 2017, 16, 214– 219. (7) Shevchenko, E. V.; Talapin, D. V.; Kotov, N. A.; O’Brien, S.; Murray, C. B. Structural diversity in binary nanoparticle superlattices. Nature 2006, 439, 55–59. (8) Boles, M. A.; Engel, M.; Talapin, D. V. Self-Assembly of Colloidal Nanocrystals: From Intricate Structures to Functional Materials. Chem. Rev. 2016, 116, 11220–11289. (9) Collier, C. P.; Vossmeyer, T.; Heath, a. J. R. NANOCRYSTAL SUPERLATTICES. Annu. Rev. Phys. Chem. 1998, 49, 371–404. (10) Cargnello, M.; Johnston-Peck, A. C.; Diroll, B. T.; Wong, E.; Datta, B.; Damodhar, D.; Doan-Nguyen, V. V. T.; Herzing, A. A.; Kagan, C. R.; Murray, C. B. Substitutional doping in nanocrystal superlattices. Nature 2015, 524, 450–453. (11) Gu, X. W.; Ye, X.; Koshy, D. M.; Vachhani, S.; Hosemann, P.; Alivisatos, A. P. Tolerance to structural disorder and tunable mechanical behavior in self-assembled superlattices of polymer-grafted nanocrystals. Proc. Natl. Acad. Sci. U.S.A. 2017, 114, 2836–2841. (12) Klinkova, A.; Choueiri, R. M.; Kumacheva, E. Self-assembled plasmonic nanostructures. Chem. Soc. Rev. 2014, 43, 3976–3991.


21

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 31

(13) Murray, C. B.; Kagan, a. C. R.; Bawendi, M. G. Synthesis and Characterization of Monodisperse Nanocrystals and Close-Packed Nanocrystal Assemblies. Annu. Rev. Mater. Sci. 2000, 30, 545–610. (14) Nie, Z.; Petukhova, A.; Kumacheva, E. Properties and emerging applications of selfassembled structures made from inorganic nanoparticles. Nat. Nanotechnol. 2010, 5, 15–25. (15) Pileni, M. P. Self organization of inorganic nanocrystals: Unexpected chemical and physical properties. J. Colloid Interface Sci. 2012, 388, 1–8. (16) Si, K. J.; Chen, Y.; Shi, Q.; Cheng, W. Nanoparticle Superlattices: The Roles of Soft Ligands. Adv. Sci. 2017, 1700179-n/a. (17) Travesset, A. Self-Assembly Enters the Design Era. Science 2011, 334, 183–184. (18) Ye, X.; Zhu, C.; Ercius, P.; Raja, S. N.; He, B.; Jones, M. R.; Hauwiller, M. R.; Liu, Y.; Xu, T.; Alivisatos, A. P. Structural diversity in binary superlattices self-assembled from polymer-grafted nanocrystals. Nat. Commun. 2015, 6, 10052. (19) Dong, A.; Chen, J.; Vora, P. M.; Kikkawa, J. M.; Murray, C. B. Binary nanocrystal superlattice membranes self-assembled at the liquid-air interface. Nature 2010, 466, 474–477. (20) Choueiri, R. M.; Klinkova, A.; Thérien-Aubin, H.; Rubinstein, M.; Kumacheva, E. Structural Transitions in Nanoparticle Assemblies Governed by Competing Nanoscale Forces. J. Am. Chem. Soc. 2013, 135, 10262–10265. (21) Klinkova, A.; Thérien-Aubin, H.; Ahmed, A.; Nykypanchuk, D.; Choueiri, R. M.; Gagnon, B.; Muntyanu, A.; Gang, O.; Walker, G. C.; Kumacheva, E. Structural and Optical Properties of Self-Assembled Chains of Plasmonic Nanocubes. Nano Lett. 2014, 14, 6314–6321. (22) Choueiri, R. M.; Galati, E.; Thérien-Aubin, H.; Klinkova, A.; Larin, E. M.; QuerejetaFernández, A.; Han, L.; Xin, H. L.; Gang, O.; Zhulina, E. B. et al. Surface patterning of nanoparticles with polymer patches. Nature 2016, 538, 79–83. (23) Che, J.; Jawaid, A.; Grabowski, C. A.; Yi, Y.-J.; Louis, G. C.; Ramakrishnan, S.; Vaia, R. A. Stability of Polymer Grafted Nanoparticle Monolayers: Impact of Architecture and Polymer– Substrate Interactions on Dewetting. ACS Macro Lett. 2016, 5, 1369–1374. (24) Ullrich, S.; Scheeler, S. P.; Pacholski, C.; Spatz, J. P.; Kudera, S. Formation of Large 2D Arrays of Shape-Controlled Colloidal Nanoparticles at Variable Interparticle Distances. Part. Part. Syst. Char. 2013, 30, 102–108. (25) Scheeler, S. P.; Mühlig, S.; Rockstuhl, C.; Hasan, S. B.; Ullrich, S.; Neubrech, F.; Kudera, S.; Pacholski, C. Plasmon Coupling in Self-Assembled Gold Nanoparticle-Based Honeycomb Islands. J. Phys. Chem. C 2013, 117, 18634–18641. (26) Sánchez-Iglesias, A.; Grzelczak, M.; Altantzis, T.; Goris, B.; Pérez-Juste, J.; Bals, S.; van Tendeloo, G.; Donaldson, S. H.; Chmelka, B. F.; Israelachvili, J. N. et al. Hydrophobic Interactions Modulate Self-Assembly of Nanoparticles. ACS Nano 2012, 6, 11059–11065. (27) Yockell-Lelièvre, H.; Lussier, F.; Masson, J.-F. Influence of the Particle Shape and Density of Self-Assembled Gold Nanoparticle Sensors on LSPR and SERS. J. Phys. Chem. C 2015, 119, 28577–28585. (28) Yockell-Leliévre, H.; Desbiens, J.; Ritcey, A. M. Two-Dimensional Self-Organization of Polystyrene-Capped Gold Nanoparticles. Langmuir 2007, 23, 2843–2850. (29) Chen, Y.; Fu, J.; Ng, K. C.; Tang, Y.; Cheng, W. Free-Standing Polymer-Nanoparticle Superlattice Sheets Self-Assembled at the Air-Liquid Interface. Cryst. Growth Des. 2011, 11, 4742–4746.


22

Page 23 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(30) Si, K. J.; Sikdar, D.; Chen, Y.; Eftekhari, F.; Xu, Z.; Tang, Y.; Xiong, W.; Guo, P.; Zhang, S.; Lu, Y. et al. Giant Plasmene Nanosheets, Nanoribbons, and Origami. ACS Nano 2014, 8, 11086–11093. (31) Ng, K. C.; Udagedara, I. B.; Rukhlenko, I. D.; Chen, Y.; Tang, Y.; Premaratne, M.; Cheng, W. Free-Standing Plasmonic-Nanorod Superlattice Sheets. ACS Nano 2012, 6, 925–934. (32) Bastús, N. G.; Comenge, J.; Puntes, V. Kinetically Controlled Seeded Growth Synthesis of Citrate-Stabilized Gold Nanoparticles of up to 200 nm: Size Focusing versus Ostwald Ripening. Langmuir 2011, 27, 11098–11105. (33) Piella, J.; Bastús, N. G.; Puntes, V. Size-Controlled Synthesis of Sub-10-nanometer CitrateStabilized Gold Nanoparticles and Related Optical Properties. Chem. Mater. 2016, 28, 1066– 1075. (34) Schulz, F.; Homolka, T.; Bastús, N. G.; Puntes, V.; Weller, H.; Vossmeyer, T. Little Adjustments Significantly Improve the Turkevich Synthesis of Gold Nanoparticles. Langmuir 2014, 30, 10779–10784. (35) Ziegler, C.; Eychmüller, A. Seeded Growth Synthesis of Uniform Gold Nanoparticles with Diameters of 15-300 nm. J. Phys. Chem. C 2011, 115, 4502–4506. (36) Hühn, J.; Carrillo-Carrion, C.; Soliman, M. G.; Pfeiffer, C.; Valdeperez, D.; Masood, A.; Chakraborty, I.; Zhu, L.; Gallego, M.; Yue, Z. et al. Selected Standard Protocols for the Synthesis, Phase Transfer, and Characterization of Inorganic Colloidal Nanoparticles. Chem. Mater. 2017, 29, 399–461. (37) Guo, Q.; Xu, M.; Yuan, Y.; Gu, R.; Yao, J. Self-Assembled Large-Scale Monolayer of Au Nanoparticles at the Air/Water Interface Used as a SERS Substrate. Langmuir 2016, 32, 4530– 4537. (38) Kim, B.; Tripp, S. L.; Wei, A. Self-Organization of Large Gold Nanoparticle Arrays. J. Am. Chem. Soc. 2001, 123, 7955–7956. (39) Wang, H.; Levin, C. S.; Halas, N. J. Nanosphere Arrays with Controlled Sub-10-nm Gaps as Surface-Enhanced Raman Spectroscopy Substrates. J. Am. Chem. Soc. 2005, 127, 14992– 14993. (40) Choi, J.; Hui, C. M.; Schmitt, M.; Pietrasik, J.; Margel, S.; Matyjazsewski, K.; Bockstaller, M. R. Effect of Polymer-Graft Modification on the Order Formation in Particle Assembly Structures. Langmuir 2013, 29, 6452–6459. (41) Yang, J.; Lee, J. Y.; Deivaraj, T. C.; Too, H.-P. A highly efficient phase transfer method for preparing alkylamine-stabilized Ru, Pt, and Au nanoparticles. J. Colloid Interface Sci. 2004, 277, 95–99. (42) Scarabelli, L.; Sánchez-Iglesias, A.; Pérez-Juste, J.; Liz-Marzán, L. M. A "Tips and Tricks" Practical Guide to the Synthesis of Gold Nanorods. J. Phys. Chem. Lett. 2015, 6, 4270–4279. (43) Haiss, W.; Thanh, N. T. K.; Aveyard, J.; Fernig, D. G. Determination of Size and Concentration of Gold Nanoparticles from UV-Vis Spectra. Anal. Chem. 2007, 79, 4215–4221. (44) Ghosh, S. K.; Pal, T. Interparticle Coupling Effect on the Surface Plasmon Resonance of Gold Nanoparticles: From Theory to Applications. Chem. Rev. 2007, 107, 4797–4862. (45) Lu, X.; Tuan, H.-Y.; Korgel, B. A.; Xia, Y. Facile Synthesis of Gold Nanoparticles with Narrow Size Distribution by Using AuCl or AuBr as the Precursor. Chem. Eur. J. 2008, 14, 1584–1591. (46) Mourdikoudis, S.; Liz-Marzán, L. M. Oleylamine in Nanoparticle Synthesis. Chem. Mater. 2013, 25, 1465–1476.


23

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 31

(47) Peng, S.; Lee, Y.; Wang, C.; Yin, H.; Dai, S.; Sun, S. A facile synthesis of monodisperse Au nanoparticles and their catalysis of CO oxidation. Nano Res. 2008, 1, 229–234. (48) Harada, T.; Hatton, T. A. Formation of Highly Ordered Rectangular Nanoparticle Superlattices by the Cooperative Self-Assembly of Nanoparticles and Fatty Molecules. Langmuir 2009, 25, 6407–6412. (49) Lau, C. Y.; Duan, H.; Wang, F.; He, C. B.; Low, H. Y.; Yang, J. K. W. Enhanced Ordering in Gold Nanoparticles Self-Assembly through Excess Free Ligands. Langmuir 2011, 27, 3355– 3360. (50) Deffner, M.; Schulz, F.; Lange, H. Impact of the Crosslinker’s Molecular Structure on the Aggregation of Gold Nanoparticles. Z. Phys. Chem. 2016, 231, 19–31. (51) Olichwer, N.; Koschine, T.; Meyer, A.; Egger, W.; Ratzke, K.; Vossmeyer, T. Gold nanoparticle superlattices: Structure and cavities studied by GISAXS and PALS. RSC Adv 2016, 6, 113163–113172. (52) Yang, J.; Lee, J. Y.; Ying, J. Y. Phase transfer and its applications in nanotechnology. Chem. Soc. Rev. 2011, 40, 1672–1696. (53) Hoft, R. C.; Ford, M. J.; McDonagh, A. M.; Cortie, M. B. Adsorption of Amine Compounds on the Au(111) Surface: A Density Functional Study. J. Phys. Chem. C 2007, 111, 13886–13891. (54) Lavrich, D. J.; Wetterer, S. M.; Bernasek, S. L.; Scoles, G. Physisorption and Chemisorption of Alkanethiols and Alkyl Sulfides on Au(111). J. Phys. Chem. B 1998, 102, 3456–3465. (55) Tang, Q.; Jiang, D.-e. Comprehensive View of the Ligand-Gold Interface from First Principles. Chem. Mater. 2017, 29, 6908–6915. (56) Elbert, K. C.; Jishkariani, D.; Wu, Y.; Lee, J. D.; Donnio, B.; Murray, C. B. Design, SelfAssembly, and Switchable Wettability in Hydrophobic, Hydrophilic, and Janus Dendritic Ligand–Gold Nanoparticle Hybrid Materials. Chem. Mater. 2017, 29, 8737–8746.


24

Page 25 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Synopsis: Polystyrene-based ligands have great potential in facilitating self-assembly of nanoparticles. The functionalization of citrate-stabilized gold nanoparticles, 6-90 nm in diameter, with such ligands by phase-transfer is studied. The presented strategy extends the addressable size range for self-assembly at a liquid interface. Well-ordered AuNP monolayer films >400 µm2 can be synthesized.


25

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1. Concentration of AuNP@Citrate in the toluene phase by phase transfer with oleylamine (OLAM). After complete phase transfer in a water:toluene:ethanol 1:1:1 mixture, the aqueous phase can be exchanged with more AuNP@Citrate. This was repeated ten times. For UV/Vis measurements the more concentrated solutions were diluted and the absorbances in a (dAuNP = 6.4 nm) and c (dAuNP = 12.3 nm) are corrected with the dilution factors. The AuNP@Citrate addition steps are indicated at the color code. The concentrations were estimated based on the absorbance at 450 nm as described by Haiss et al.43 The experimental values are higher than the theoretical ones because of evaporation losses. The values are shown in b (dAuNP = 6.4 nm) and d (dAuNP = 12.3 nm). e: Exemplarily volume-weighted distributions of hydrodynamic diameters dH obtained by DLS for different batches of AuNP@OLAM including batches concentrated by repeated phase transfer and centrifugation at 20,000 g. f: Exemplarily TEM measurement of AuNP@OLAM after concentration by repeated phase transfer and centrifugation. No indications of aggregation were observed. 170x198mm (300 x 300 DPI)


Page 26 of 31

Page 27 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2. UV/Vis-spectra of AuNP with diameters as indicated after phase transfer ligand exchange. AuNP with diameters >12 nm were not stabilized by OLAM (dark red lines) and OLAM was not tested for AuNP >23.1 nm. The stabilization of AuNP with increasing diameters by PSSH2k (orange lines), PSSH5k (green lines) and PSSH10k (blue lines) correlates with the ligand size. The spectra of the AuNP@Citrate educt particles are shown for comparison (grey dashed lines). For larger AuNP, the solutions had to be diluted for UV/Vis spectroscopy and the absorbances were corrected with the dilution factors. 90x50mm (300 x 300 DPI)


Page 28 of 31

Page 29 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 3. Comparison of direct or indirect phase transfer ligand exchange. a: Normalized absorbance spectra for AuNP (dAuNP = 11.9 nm in a, b and c) functionalized by direct phase transfer ligand exchange of AuNP@Citrate with PSSH2k, -5k and 10k as indicated (dashed lines), or by indirect phase transfer ligand exchange (solid lines). Lower panel: intensity-weighted distributions of hydrodynamic diameters dH obtained by DLS for b: AuNP@OLAM (indirect phase transfer ligand exchange) or c: AuNP@Citrate (direct phase transfer ligand exchange) reacted with PSSH ligands as indicated. 109x85mm (300 x 300 DPI)


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 4. TEM-images of self-assembled AuNP@PSSH films (dAuNP = 28 nm). a: Large area view of selfassembled AuNP@PSSH5k. The light grey area is a well-ordered monolayer, the darker regions consist of multiple layers. b: magnification of the highlighted area in a showing one, two, three and four ordered layers of assembled AuNP from right to left. c: Exemplarily magnification of the light grey area in a demonstrating the homogeneity of the monolayer. Lower panel: Monolayer structure of AuNP@PSSH2k (d), AuNP@PSSH5k (e) and AuNP@PSSH10k (f) with the according digital FFT of the entire image in the insets. 529x264mm (200 x 200 DPI)


Page 30 of 31

Page 31 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 5. TEM-images of self-assembled AuNP@PSSH5k (a) and AuNP@PSSH10k (b) (dAuNP = 28 nm). 120x180mm (300 x 300 DPI)


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 6. TEM-images of self-assembled AuNP@PSSH2k (dAuNP = 12 nm). At low magnification (a), cracks and wrinkles can be observed. The hexagonal order (b) leads to Moiré-patterns (c) when two monolayers overlap slightly twisted. 526x131mm (200 x 200 DPI)


Page 32 of 31

Size-Dependent Phase Transfer Functionalization ... - ACS Publications

Recommend Documents