Directed Self-Assembly and Infrared Reflection Absorption

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Directed Self-Assembly and IRRAS Analysis of Amphiphilic and Zwitterionic Janus Gold Nanoparticles Svenja Bourone, Corinna Kaulen, Melanie Homberger, and Ulrich Simon Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b03897 • Publication Date (Web): 04 Jan 2016 Downloaded from http://pubs.acs.org on January 14, 2016

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Directed Self-Assembly and IRRAS Analysis of Amphiphilic and Zwitterionic Janus Gold Nanoparticles Svenja D.M. Bourone, Corinna Kaulen, * Melanie Homberger, Ulrich Simon 1. Institute of Inorganic Chemistry and 2. JARA – Fundamentals of Future Information Technologies, RWTH Aachen University, 52074 Aachen, Germany

KEYWORDS: IR, RAIRS, spectroscopic proof, anisotropic nanostructures, gold nanoparticles, Janus, directed self-assembly

ABSTRACT: Here, we report an approach to use infrared reflection absorption spectroscopy (IRRAS) for the unambiguous proof of the presence as well as the spatial distribution of organic ligands on the Janus AuNP´s surface. For this purpose we synthesized amphiphilic and zwitterionic Janus AuNP and immobilized these on pretreated gold surfaces by directed selfassembly, exploiting hydrophilic/hydrophobic or electrostatic interactions, respectively. Thus we obtained macroscopic two-dimensional arrays of Janus AuNP exhibiting a specific orientation. These arrays were investigated by IRRAS and the obtained spectra revealed only peaks of the

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ligands facing the IR beam, while the ligands facing the gold substrate were not detected due to reflection of the IR beam on the AuNP´s cores. Thus, we describe a straightforward spectroscopic procedure to prove the Janus character of zwitterionic and amphiphilic AuNP in the size range of 10-15 nm.

Introduction Gold nanoparticles (AuNP) with engineered surface functionalities gain increasing interest in nanoscience and -technology due to their versatile surface chemistry and the tunability of their optical properties.1, 2, 3 Applications of AuNP reach from theranostics including tumor detection and treatment,4, 5 chemical and biological sensing6 to nanoelectronics.7, 8, 9 In these applications, the AuNP’s cores guarantee functionality due to their electronic and optic properties, while the ligands add tailored function based on their chemical structure. Further, by means of symmetry breaking synthesis routes Janus AuNP which present different ligands on their hemispheres can be synthesized.10,

11

Owing to their anisotropic properties, Janus AuNP are suited e.g. as

Pickering type surfactants12, 13, as building blocks for functional self-assembled superstructures14, 15, 16

or as building blocks for nanoelectronic devices.17, 18

In the latter context we recently reported on the synthesis of zwitterionic Janus AuNP that were functionalized with 4-mercaptophenylamine (MPA) and 8-mercaptooctanoic acid (MOA).17 Individual MPA/MOA-Janus AuNP were successfully immobilized in between heterometallic nanogaps in a directed manner. Electric measurements performed with these PtMPA/AuNP/MOA-AuPd devices, revealed that the conductance depended on the voltage polarity, thus indicating an anisotropic distribution of ligands on the AuNP’s surface. In order to


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probe the assumed anisotropic distribution of the ligands as well as the directed adsorption, we performed XPS on MPA/MOA-AuNP adsorbed on Pt surfaces. By these measurements only MOA was detected, however it was not possible to selectively detect the MPA ligand and to directly prove the Janus character of the MPA/MOA-AuNP. In order to provide straightforward evidence of the Janus character, the respective ligands need to be detected selectively and undoubtedly. This task, necessary to promote the use of Janus AuNP in the abovementioned applications, remains challenging as a confined area of the particles has to be analyzed. For this purpose, a spectroscopic method that allows identifying the ligands with high molecular specificity, e.g. by detecting the chemical groups of the molecules, has to be applied. Hitherto NMR, namely nuclear Overhauser effect spectroscopy (NOESY), has been used successfully to characterize Janus AuNP19. However, NMR is limited to AuNP with diameters smaller than approximately 5 nm, as for bigger AuNP line broadening of the recorded signals impedes the precise identification of the ligands.20, 21 Another powerful method to spectroscopically characterize Janus AuNP constitutes infrared reflection absorption spectroscopy (IRRAS, also named RAIRS), as it is a fast and nondestructive surface sensitive technique which is run under ambient conditions.22, 23 Patnaik et al. used IRRAS to characterize Janus AuNP (average diameter 3.5 nm) immobilized at the air/water interface via the Langmuir-Blodget (LB) technique by applying different angles of incidence of the IR beam.24 Based on these measurements they were able to show that the probed AuNP were Janus-like. However, the IRRAS set-up used by Patnaik et al. is limited to the characterization of Janus AuNP smaller than approximately 10 nm in diameter, as for bigger AuNP the reflectivity of the gold core in the mid-infrared range would impede the detection of


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the bottom ligand. Moreover, the method is restricted to amphiphilic Janus AuNP which can be immobilized at the air/water interface. In this study we report for the first time on the direct spectroscopic validation of the asymmetric ligand distribution by IRRAS (i) on Janus AuNP in the size range of 10-15 nm, (ii) on Janus AuNP which cannot be assembled at the air/water interface and (iii) without recourse to LB technique. In our approach we applied directed self-assembly (DSA) as the two ligands of the Janus AuNP exhibited different terminal groups which selectively adsorbed to pretreated surfaces, so that the Janus AuNP were immobilized exhibiting a specific orientation. Subsequently we characterized these oriented two-dimensional (2D) arrays of Janus AuNP by IRRAS which allowed selectively detecting one specific ligand of the Janus AuNP as it was exposed to the IR beam, while the second one was shielded by the AuNP´s cores. In detail, we synthesized two types of Janus AuNP, i.e. amphiphilic Janus AuNP functionalized with 1-octanethiol (OT) and MOA12 and zwitterionic MPA/MOA-Janus AuNP.17 The amphiphilic OT/MOA-AuNP were deposited by hydrophilic/hydrophobic interactions on pretreated polar and apolar gold substrates, whereas the zwitterionic MPA/MOA-AuNP were deposited by electrostatic interactions on gold substrates functionalized with a positively or negatively charged self-assembled monolayer (SAM). For reference purposes the IRRAS spectra of deposited monofunctionalized AuNP (OT-, MOA- or MPA-AuNP) were recorded. For the directionally assembled Janus AuNP only the expected vibrations assignable to the ligand oriented towards the IR beam were detected.

Experimental Section


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Materials and Instrumentation. The following chemicals were purchased from SigmaAldrich Chemie GmbH and used as received: hydrogen tetrachloroaurate(III) trihydrate (HAuCl4∙3H2O), trisodium citrate dihydrate (Na3C6H5O7∙H2O), 11-amino-1-undecanethiol hydrochloride (MUA), 8-mercaptooctanoic acid (MOA), 1-octanethiol (OT), sodium-3mercapto-1-propanesulfonate piperazineethanesulfonic

(MPS),

acid

4-Aminothiophenol

(HEPES),

(MPA),

4-(2-hydroxyethyl)-1-

3-triethoxysilylpropylamine

(APTES),

and

tris(hydroxymethyl)aminomethane (Tris). 1-octadecanethiol (ODT) and Trimethoxymethylsilane (MPTES) were purchased from Fluka. Glass beads were purchased from Carl Roth GmbH + Co. KG. 8-Aminooctanethiol hydrochloride (AOT) was purchased from Dojindo Molecular Technologies, Inc. Toluene p.A. was purchased from VWR. Absolute (abs.) ethanol, hydrogen peroxide (H2O2) and ammonia solution 25% were purchased from Th. Geyer. Hydrochloric acid was purchased from Grüssing. All glassware was cleaned with aqua regia and rinsed with copious amount of water prior to use. Ultrapure water with a conductivity < 55 nScm-1 was used for all procedures. IRRAS measurements were performed on a FT-IR spectroscope Vertex 70, Bruker Optics equipped with a high sensitivity Hg-Cd-Te (MCT) detector and an A513/Q variable angle reflection accessory including an automatic rotational holder for MIR polarizer. The IR beam was polarized with a KRS-5 polarizer with 99% degree of polarization. Double sided interferograms were collected with a sample frequency of 20 kHz, an aperture of 1.5 mm and a nominal spectral resolution of 4 cm-1. The interferograms were apodized by a Blackmann-Harris 3 term apodization and zerofilled with a zerofilling factor of 2. The angle of incidence was set to 80° and p-polarized IR radiation was used to record the spectra. For the background measurements the sample chamber was purged with argon during 5 min, then 1024 scans were


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collected while continuing to purge. For the sample measurements argon purging was started at the moment the first scan was recorded. The scans were averaged until the peaks arising from the water vapor in the sample chamber were compensated for what typically 800-1500 scans were necessary. The spectra were processed using the OPUS software (Bruker). Where necessary, scatter correction was applied to the spectra. UV-vis measurements were performed with a JASCO V-630 spectrophotometer. Dynamic light scattering (DLS) measurements and ζ-potential measurements were performed with a Malvern Zetasizer Nano S, He-Ne-Laser λ = 633 nm, P = 4 mW, θ = 173° in order to determine the hydrodynamic radii (z-average). Scanning electron microscopy (SEM) in transmission mode was performed with a Zeiss LEO Supra 35 VP. Contact angle measurements were conducted with a homemade contact angle goniometer using the sessile drop method and droplets of 5 µL of ultrapure water. Drop curvatures were fitted with LB-ADSA plug-in for ImageJ.25, 26 Preparation of Reference Samples. Synthesis, purification and characterization of the monofunctionalized AuNP (citrate-, OT-, MOA- and MPA-AuNP) for reference purposes is displayed in the Supporting Information 1 and 2. Preparation of Janus AuNP. OT/MOA-AuNP. OT/MOA-AuNP were prepared adopting procedures described by Andala12 and Ciesa14. A two phase ligand exchange reaction was applied by combining 5 mL of water dispersed citrate-AuNP (mean particle diameter = 13.4 ± 0.9 nm (evaluated from more than 150 particles), c = 5.7 × 10-9 mol L-1) and 5 mL of toluene in a polypropylene centrifuge tube. To this two phase system a mixture of 10 µL of OT (1 mM solution in toluene, 4.0 × 102 fold excess with respect to the initial citrate-AuNP concentration) and 10 µL of MOA (1 mM solution in toluene, 4.0 × 102 fold excess with respect to the initial citrate-AuNP concentration) was added. After gently shaking for 1h, the AuNP were densely


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assembled at the water/toluene interface thereby completely surrounding the water phase (see Supporting Information 3 for further details). Isolation of the particles was achieved as follows: First, we removed the toluene from the reaction mixture, which led to the adhesion of AuNP to the wall of the centrifuge tube. Next, we removed the water phase which contained excess ligand as well as free single AuNP (evident by the red color of the water phase) and rinsed the tube twice with ultrapure water and abs. ethanol. Upon addition of ultrapure water and toluene we observed that the AuNP were detached from the centrifuge tube wall and assembled at the water/toluene interface again. The AuNP were transferred onto plasma-cleaned silicon oxide transmission electron microscopy (TEM) membranes by dipping the membranes into the toluene/water interface. Successful deposition of AuNP was evident from the metallic lustre of the substrates. TEM investigations revealed individual, AuNP with a mean particle diameter = 14.8 ± 1.1 nm (evaluated from more than 150 particles, see Supporting Information 3 for photographs of the TEM membrane and TEM results). MPA/MOA-AuNP were prepared applying a solid phase support as published recently.17 Infrared Reflection Absorption Spectroscopy (IRRAS) Setup. General Procedure for the Preparation of Prefunctionalized Au Substrates. The Au substrates (see Supporting Information 4 for specifications) were cleaned in oxygen plasma [p(O2) = 0.4 mbar, f = 40kHz and P = 75 W] for 4 min immediately prior to immersion in an ethanolic solution (c = 10-3 mol L-1) of the respective thiol derivative. After 24 h the Au substrate was removed from the solution, washed with copious amounts of ethanol and dried in an argon stream. See Table 1 for identification of the Au substrates and the respectively used thiols. Table 1: Pretreatment of Au substrates for deposition of AuNP Identification

Applied pretreatment


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Polar substrate

Used immediately after plasma cleaning

Apolar substrate

1-Octadecanethiol (ODT)

Positively charged end group

11-Amino-1-undecanethiol hydrochloride (MUA) or

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8-Amino-1-octanethiol hydrochloride (AOT) Negatively charged end group

Sodium-3-mercapto-1-propanesulfonate (MPS)

General Procedure for the Deposition of Isotropically Functionalized AuNP on Au substrates. 350 µL AuNP dispersion were drop-cast on the respective gold substrate and incubated in a H2O saturated atmosphere for 1h. During incubation the sample was gently swayed in order to generate inward capillary flow and to prevent deposition of the AuNP at the edge of the drop. After that the substrate was thoroughly rinsed with ultrapure water in order to remove unbound AuNP, dried in an argon stream and stored in dry atmosphere until the IRRAS measurement. See Supporting Information 5 for the respective conditions of deposition. Deposition of amphiphilic Janus OT/MOA-AuNP. OT/MOA-AuNP were deposited either on polar or on apolar Au substrates. The polar Au substrates were dipped into the water/toluene interface at which the OT/MOA-AuNP were densely packed. The apolar Au substrates were placed in a dropping funnel, which was filled with water until the Au substrates were completely immersed. With an Eppendorf pipette OT/MOA-AuNP were taken off the liquid/liquid interface and deposited on the water phase in the dropping funnel. After evaporation of toluene the air water/interface was completely covered by densely packed OT/MOA-AuNP. The water was slowly drained so that the AuNP were brought in contact with the surface of the Au substrates leading to their deposition. The samples were rinsed with ultrapure water and ethanol, dried in an argon stream and stored in dry atmosphere until the IRRAS measurements.


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Deposition of zwitterionic Janus MPA/MOA-AuNP. MPA/MOA-AuNP were deposited on Au substrates functionalized with thiols bearing positively and negatively charged end groups, respectively. 1 ml of freshly prepared MPA/MOA-AuNP with a mean diameter of 14.7 ±1.1 nm (evaluated from more than 150 particles) was centrifuged twice at 9,000 g for 15 min. For deposition on Au substrates with positively charged end groups the MPA/MOA-AuNP were redispersed in 100 µl HEPES/TRIS/NaCl buffer at pH 7.5 and for deposition on Au substrates with negatively charged end groups in 100 µl HEPES/citric acid/NaCl buffer at pH 4.8 (see Supporting Information 6 for preparation of the buffer solutions). The violet AuNP dispersion was drop-cast on a gold substrate functionalized with the respective thiol (AOT or MPS) and kept in a H2O saturated atmosphere. After 45 min the Au substrates were immersed in a beaker filled with water to remove the AuNP dispersion and dried in an argon stream. The samples were stored in dry atmosphere until the IRRAS measurement.

Results and Discussion Characterization of isotropic AuNP by IRRAS. IRRAS-samples of isotropically functionalized AuNP, namely citrate-AuNP, OT-AuNP, MOA-AuNP and MPA-AuNP, were prepared for reference purposes by incubating the respective AuNP dispersions on pretreated gold substrates. The corresponding IRRAS spectra, where characteristic vibrations were highlighted, are shown in Figure 1. For a better overview the band positions of the recorded vibrations are summarized in Table 1.


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Figure 1. Details of the IRRAS-spectra of citrate-AuNP, OT-AuNP, MOA-AuNP and MPAAuNP. The characteristic ν(C=O) vibration of citrate and MOA around 1730 cm-1 is highlighted in grey. The region around 1380 cm-1 δip(OH) characteristic of citrate is highlighted in grey as well (see Supporting Information S7 for complete spectra). All presented absorption intensities are reflectance absorbance intensities.


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Table 1. Band positions of the vibrations detected by IRRAS for isotropically functionalized AuNP and Janus AuNP. νas/s(CH2)*

ν(C=O)† δ(NH3+)

citrateAuNP

2921/ 2851

1716

MOA-AuNP

2914/ 2845

1735, 1721

νas/s(CH3)*

OT-AuNP

2950/ 2871

νar(C=C)+ νas/s(COO-)* δs(CH2) δas(CH3) 1575/ 1471

δip(OH) δs(CH3) 1386

2910/ 2843

1459

MPA-AuNP

1618

OT/MOA-AuNP on Au

2959/ 2871

2926/ 2854

OT/MOA-AuNP on ODT

2958/ 2875

2919/ 2850

1731, 1715, 1695

MPA/MOA-AuNP on AOT

2927 /2849

1734, 1716

MPA/MOA-AuNP on MPS

2927/ 2849

1734, 1716

1263

1587, 1486 1463

1463, 1416

1618

ν(C-C)

1589

1379

1455

1264

1379

1575

*) The frequencies of the asymmetric and symmetric vibrations are separated by a slash. †) The carboxyl group can be present as monomer, asymmetric dimer and/or symmetric dimer, therefore up to three frequencies of vibration were detected for ν(C=O). +) For the ring vibrations different vibrational modes were detected as indicated by the comma.

Citrate-AuNP on MUA-functionalized Au-substrates. Citrate-AuNP were immobilized on MUA-functionalized gold surfaces at pH 4.5 by electrostatic interactions between the carboxyl groups of the ligands and the amino groups of the MUA-SAM. SEM investigations revealed that the citrate-AuNP were deposited as an array of single particles, further called sub-monolayer (see Supporting Information Figure S8a). The IRRAS spectrum of citrate-AuNP (see Figure 1) was recorded using a MUA-functionalized Au substrate for the reference measurement. In the upper wavenumber region the asymmetric and symmetric CH2 stretching vibrations (νas(CH2) and


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νs(CH2)) were detected. The shoulder at 2962 cm-1 may arise from an overlap of the O-H stretching vibration (ν(OH)) and ν(CH2), which results in ν(CH2) having broad wings.27 Furthermore, the carbonyl (ν(C=O)) and the carboxylate stretching vibrations (νas/s(COO-)) were observed, revealing that the acidic groups of the citrate ligands were partly deprotonated which is consistent with the pH of 4.5 adjusted during the deposition of citrate-AuNP. Characteristic for the citrate-AuNP in comparison to the MOA-AuNP are the vibrations arising from the tertiary alcohol group. In fact the in-plane deformation vibration of the O-H bond (δip(OH)) in tertiary alcohols absorbs between 1420 cm-1 and 1330 cm-1, while in carboxylic acids it absorbs between 1440 cm-1 and 1395 cm-1.28 Therefore the peak at 1386 cm-1 was assigned to δip(OH) of the alcohol group, being characteristic of the citrate molecules. The peak at 1646 cm-1 can be assigned to water adsorbed onto the sample.29

MOA-AuNP on polar Au-substrates. MOA-AuNP were immobilized on polar Au substrates by incubation at and SEM investigations revealed that they were deposited as a monolayer of individual and patches of AuNP further called a discontinuous monolayer (see Supporting Information Figure S8c). For immobilized MOA-AuNP at pH 1 the peaks at 2914 cm-1 and 2845 cm-1 corresponding to νas(CH2) and νs(CH2) indicate highly ordered alkyl chains.30 This observation corresponds well to the detection of ν(C=O) and the absence of νas(COO-) and νs(COO-) which means that the acidic terminal group is protonated. Indeed, partly charged end groups seem to disturb the formation of highly ordered alkyl chains on AuNP.31 For ν(C=O) two vibration frequencies were observed. The first one at 1735 cm-1 was attributed to monomeric acidic end groups and the second one at 1721 cm-1 to acyclic dimers of the carboxyl group, which weaken the carbonyl bond. The peak at 1653 cm-1 revealed that additional water


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molecules were adsorbed onto the sample.29, 32 Furthermore, for MOA-AuNP immobilized at pH 5 the carboxylate stretching vibrations were observed (spectrum not shown).

OT-AuNP on polar Au-substrates. OT-AuNP were immobilized on polar Au substrates by incubation. SEM investigations revealed that they were deposited as clusters with patches of monolayer (see Supporting Information Figure S8b). In the corresponding IRRAS spectrum (see Figure 1) only the peaks arising from the stretching vibrations of the methyl and methylene groups (νas/s(CH3) and νas/s(CH2), respectively) as well as from the asymmetric deformation vibration of the methyl groups (δas(CH3)) were detected. No vibrations from citrate (ν(C=O), νas/s(COO-) or δip(OH)) were observed; hence the ligand shell of the analyzed particles was composed of OT molecules. From the vibration frequencies of νas(CH2) and νs(CH2) the order of the alkyl chains can be deduced.30 For OT-AuNP they occurred at 2910 cm-1 and 2843 cm-1 indicating highly ordered chains which is probably due to the fact that the particles were dispersed in water during deposition which promoted the van der Waals’ interaction between the alkyl chains. Moreover, it is highly unlikely that water molecules are included between the methylene groups of OT, so that the order of the alkyl chains is not disturbed by the dispersion medium, meaning that it is even increased further. This assumption is corroborated by the fact that in the IRRAS spectrum no peak around 1650 cm-1, arising from water molecules adsorbed onto the sample, was detected in contrast to citrate-AuNP and MOA-AuNP.

MPA-AuNP on polar gold substrates. MPA-AuNP were immobilized on polar Au substrates and SEM investigations revealed that they were deposited as a monolayer with areas of multilayer (see Supporting Information Figure S8d). By IRRAS the deformation vibration of the


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ammonium group (δ(NH3+)) as well as the aromatic C=C stretching vibrations (var(C=C)) were detected. The peak at 1486 cm-1 is assigned to a combination of var(C=C) and the deformation vibration of the C-H bond.33 The peaks in the upper wavenumber region around 2900 cm-1 are less evident in the original spectrum without baseline correction and arise from impurities deposited from the laboratory atmosphere (see Supporting Information S7d for original spectrum). Characterization of Janus AuNP by IRRAS. In order to utilize IRRAS for the selective detection of ligands on Janus AuNP it is necessary to prepare large (1 × 2 cm) ordered 2D arrays of these particles by directed self-assembly, because the measuring spot is relatively large due to the grazing incidence of the IR beam. Self-assembly of ligand stabilized AuNP is based on electrostatic, hydrophilic/hydrophobic and van der Waals’ interactions as well as on hydrogen and coordinative bond formation thus being applicable to a broad variety of ligand molecules used for the functionalization of Janus AuNP.34 According to the definition of Grzelczak et al. self-assembly achieved by external stimuli such as pH, temperature, redox activity etc. is called directed self-assembly (DSA).35 Thus, by carefully adjusting the interaction between surfaces and AuNP large ordered 2D arrays of AuNP can be obtained. We applied DSA guided by hydrophilic/hydrophobic or by electrostatic interactions to obtain macroscopic monolayers of amphiphilic and zwitterionic Janus AuNP exhibiting a specific orientation. By this means it is possible to selectively detect the top ligand of the Janus AuNP by IRRAS, as the bottom ligand, which interacts with the substrate, is shielded from the IR-beam by the AuNP´s cores. Nevertheless this set-up is subject to the following measurement conditions. First, the size of the metal NP has to be in a range where the incident IR beam is reflected, which is the case for AuNP bigger than approximately 10 nm in diameter. Second, the spectroscopic signatures of the


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ligands need to be distinguishable and third the Janus AuNP must exhibit different terminal groups suitable for DSA on pretreated gold substrates, which allow for directed immobilization. Yet, these two last points are of minor concern, as a large range of chemical groups is distinguishable by IRRAS and simultaneously suitable for DSA. The advantage of our amended method is that it can be applied to directly probe the Janus character of various metal nanoparticles which are big enough to block the IR-beam.

OT/MOA-AuNP on polar and apolar Au-substrates. OT/MOA-AuNP were synthesized from citrate-AuNP by a two phase ligand exchange reaction at the toluene/water interface. Briefly, citrate-AuNP and toluene were filled in a centrifuge tube thereby forming a two-phase system to which an equimolar mixture of MOA, which is hydrophilic, and OT, which is hydrophobic, was added (see Figure 2). After shaking for 60 minutes AuNP assembled at the water/toluene interface. The OT/MOA-AuNP were purified carefully in order to detect only the ligands attached to AuNP, as described in the experimental part. This precise purification is crucial for the spectral characterization of OT/MOA-AuNP by IRRAS, since unbound ligand molecules also contribute to the IRRAS signals.


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Figure 2. Schematic drawing of the synthesis of amphiphilic Janus AuNP functionalized with OT as hydrophobic and MOA as hydrophilic ligand by a two phase ligand exchange reaction at the liquid/liquid interface. Directed self-assembly onto hydrophobic and hydrophilic gold substrates was applied to obtain macroscopic arrays of directionally deposited amphiphilic AuNP which allowed to validate their Janus character by IRRAS.

For the validation of the Janus character by IRRAS the differently functionalized hemispheres of OT/MOA-AuNP need to be selectively exposed to the IR beam. For this purpose OT/MOAAuNP were deposited as monolayer on polar and apolar gold substrates by DSA. Polar, hydrophilic gold surfaces were obtained by cleaning the gold substrates with oxygen plasma. The measured contact angle of water droplets on these surfaces amounted to 39°, which corroborates their hydrophilicity and is due to a partly oxidized Au-surface.36 Therefore the OT/MOA-AuNP


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are expected to adhere with the hydrophilic MOA ligand oriented towards the hydrophilic gold surface while the hydrophobic OT ligand is oriented upwards towards the IR beam thereby enabling its selective detection (see Figure 2). OT/MOA-AuNP were deposited onto polar Au substrates by dipping these into the water/toluene interface at which the AuNP were assembled after synthesis and purification. SEM investigations revealed that they were deposited as a discontinuous monolayer (see Figure 3). Due to the orientation of OT/MOA-AuNP immobilized on polar Au substrates only vibrations arising from OT, particularly ν(CH3) and δ(CH3) should be detected by IRRAS, whereas peaks from MOA, particularly ν(C=O), νas(COO-) and νs(COO), should not be. Indeed, νas(CH3), νs(CH3) and δs(CH3) revealed the presence of methyl groups, while peaks arising from the carboxyl group of MOA were absent (see Figure 3). The peak arising from δs(CH3) is sharper than the broad peak at 1379 cm-1 in the spectrum of OT-AuNP, which is consistent with the assumption that for the OT-AuNP this peak is due to an overlap of δs(CH3) and δip(OH). δas(CH3) is probably overlapped by δs(CH2) which appears at 1463 cm-1. In summary, only vibrations from OT were detected as expected, while ν(C=O) which occurs around 1700 cm-1, as well as νas(COO-) and νs(COO-) which lead to strong broad peaks around 1570 cm-1 and 1450 cm-1 were not observed. These findings imply that the MOA ligand was efficaciously shielded from the IR beam by the AuNP´s cores. However, according to Thue´s theorem the highest packing density of non-overlapping circles on a plane amounts to 0.9137 meaning that even if the sample consists of a perfect monolayer of AuNP the ligand pointing downwards to the gold surface could theoretically be detected through the holes between the particles. In practice the detection of the bottom ligand is impeded, as for AuNP with a diameter of 14.5 nm 4 particles densely packed in a row sufficiently absorb the IR beam, so that it does not reach the detector. Hence, only IR radiation which is reflected on the AuNP´s cores and


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excites the top ligand is not absorbed, thereby allowing the selective detection of that ligand (see Supporting Information Figure S9 for a detailed illustration). This way the OT ligand on the OT/MOA-AuNP was selectively detected by IRRAS. The orientation of OT/MOA-AuNP was reversed using apolar ODT-functionalized Au substrates that are hydrophobic as verified by the contact angle of water droplets which amounted to 109°. In this case the OT/MOA-AuNP are expected to adhere to the hydrophobic Au substrates simply by van der Waals interactions between the hydrophobic OT ligand and the ODT-SAM, so that the hydrophilic MOA ligand is exposed to the IR beam thereby enabling its selective detection (see Figure 2). OT/MOA-AuNP were deposited on apolar Au substrates by placing these in a homemade dropping funnel adapted to the measures of the substrates so that they stand upright in it. After filling the dropping funnel with ultrapure water, OT/MOA-AuNP from the water/toluene interface were deposited on top with a pipette. By cautiously opening the tap of the funnel, the particles slowly moved along the substrates and adhered to them. SEM investigations revealed that the OT/MOA-AuNP were largely deposited as a monolayer with a few patches of multilayer (see Figure 3). For OT/MOA-AuNP deposited on apolar Au substrates only vibrations arising from MOA, particularly from the carboxyl group, should be detected by IRRAS. Indeed, the peaks at 1731 cm-1, 1715 cm-1 and 1695 cm-1 in the corresponding IRRAS spectrum (see Figure 3) arise from ν(C=O) of the protonated carboxyl group which is present as monomer, as acyclic dimer and as cyclic dimer, respectively. Hence, by depositing OT/MOAAuNP on a hydrophobic substrate MOA is detected by IRRAS as opposed to IRRAS measurements of OT/MOA-AuNP immobilized on a hydrophilic substrate, where only OT was identified. Spectra of the OT/MOA-AuNP on the ODT-functionalized gold surface were recorded using cleaned (see Figure 3) as well as ODT-modified gold substrates (see Supporting


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Information Figure S10) for the reference measurements. For purposes of comparison with the spectrum of the OT/MOA-AuNP deposited on a polar Au substrate, the spectrum of the OT/MOA-AuNP deposited on an apolar ODT-modified Au substrate shown in Figure 3 was recorded using a plasma-cleaned Au substrate for the reference measurement. Therefore also the vibrations of the ODT-monolayer, namely the stretching and deformation vibrations of methyl and methylene groups, were detected, which explains why they absorb strongly compared to the vibrations arising solely from the MOA-ligand. Spectra recorded using ODT-functionalized Au substrates for the reference measurements show essentially the same results, except for the peaks of the methyl and methylene stretching vibrations which are reversed, probably because the ODT-monolayer is partly shielded by the deposited AuNP (see Supporting Information Figure S10). In summary, we showed, that it is possible to deposit amphiphilic OT/MOA-AuNP in a directed manner, so that the ligands are selectively detected by IRRAS thereby proving their Janus character.


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Figure 3. Details of the IRRAS spectra of OT/MOA-AuNP deposited on a polar as well as on an apolar Au substrate (see Supporting Information S11 for complete spectra) along with corresponding schematic drawings of the AuNP´s orientation and representative TEM images.


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IRRAS of MPA/MOA-AuNP Zwitterionic MPA/MOA-AuNP were synthesized by a solid phase supported approach as reported before.17 During the solid phase synthesis, citrate-AuNP are immobilized on aminosilanized glass beads and addition of MOA to the immobilized AuNP leads to coating of the accessible part of the AuNP with MOA. After removing excess ligand and adding the second ligand, namely MPA, the AuNP are detached from the solid support by ultrasonication and thereby ligand exchange on the formerly shielded side of the AuNP takes place. By this means, patchy particles are formed, where the ligand added first covers most of the AuNP´s surface and only a small part of the AuNP is coated by the second ligand. It is important to note that for the order of ligand addition the binding strength of the applied thiol ligands has to be considered. In our case the alkyl thiol MOA binds stronger to the gold surface than the phenyl thiol MPA due to the electron withdrawing effect of the phenyl spacer on the thiol group.38 Therefore the stronger binding alkyl thiol has to be added first, because otherwise ligand substitution of the less strongly bound phenyl ligand would occur. Solutions of these MPA/MOA-AuNP in HEPES/NaCl buffer at pH 5-7 exhibit only small aggregates as revealed by DLS.17 Formation of small aggregates in solution is expected due to the electrostatic interactions between the oppositely charged end groups of the applied ligands. Consequently, for the deposition of the MPA/MOA-AuNP as a monolayer on gold substrates the adhesion forces to the gold surface have to overcome the interparticle forces. For this purpose SAMs of thiols with charged end groups on the gold surfaces are utilized. Functionalization with positively charged AOT leads to DSA of MPA/MOA-AuNP with the negatively charged MOA ligand oriented towards the gold surface, due to electrostatic interactions (see Figure 4). We adjusted the pH of the immobilization solution to 7.5 to ensure that the carboxyl groups of the MPA/MOA-AuNP were deprotonated


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and therefore negatively charged, whereas the amino groups of the AOT-monolayer are still protonated and therefore positively charged, leading to the zwitterionic state of the particles. At pH 7.5 the alkalinity of alkyl substituted amino groups in AOT is higher than that of the phenyl substituted amino groups of MPA, leading to protonated, mostly positively charged amino groups at the AOT-functionalized gold surface but partly neutral amino groups on the MPA/MOA-AuNP. Thus the adsorption of MOA/MPA-AuNP on the gold surface is facilitated and formation of agglomerates of the zwitterionic AuNP is hampered.

Figure 4. Schematic drawing of the solid phase supported synthesis of zwitterionic Janus AuNP functionalized with MOA and MPA. Directed self-assembly onto MPS- and AOT-functionalized gold substrates was applied to obtain macroscopic arrays of directionally deposited zwitterionic AuNP which allowed to validate their Janus character by IRRAS.


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SEM images of the prepared gold substrates showed single particles with an interparticle distance of 15-50 nm or 2D arrays of a small number of particles spread over the entire surface (see Figure 5). Hence, the electrostatic interactions between the Janus AuNP in solution are small in comparison to the adhesion forces to the AOT-functionalized gold surface. For the IRRAS measurements an AOT-functionalized gold substrate was used as reference. As expected for MPA/MOA-AuNP deposited on an AOT-monolayer, vibrations from MPA, namely νar(C=C) and δ(NH3+), as well as from MOA, namely νas/s(CH2), ν(C=O) and νas(COO-) were detected. The peak at 1649 cm-1 near that of δ(NH3+) arises from water molecules adsorbed onto the sample.29, 32 These findings show that both ligands are present on the AuNP´s surface and that they are oriented with the MPA ligand towards the IR beam. For the deposition of the MPA/MOA-AuNP with the amino group facing the gold surface, we chose MPS for the functionalization of the gold substrate. The acidic sulfonate groups are negatively charged at pH > 3. We incubated the MPA/MOA-AuNP dispersed in HEPES/NaCl buffer at pH 4.8 on the MPS-functionalized Au substrate. At this pH MPA/MOA-AuNP are mainly present as single particles.17 Additionally, the amino groups of MPA are protonated and therefore positively charged, enabling attraction to the negatively charged MPS groups on the gold substrate (see Figure 4). SEM investigations revealed that MPA/MOA-AuNP are distributed over the entire sample surface either as single AuNP or as patches of a densely packed monolayer. For the IRRAS measurements of MPA/MOA-AuNP immobilized on MPS, a MPSfunctionalized gold substrate was used as reference. Again, in the high wavenumber region νas/s(CH2) and in the low wavenumber region range ν(C=O) of MOA were detected (see Figure 5). However, the strong νar(C=C) and δ(NH3+) of MPA were not detected, as opposed to the IRRAS measurements of MPA/MOA-AuNP on AOT. From these findings we conclude


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successful DSA of MPA/MOA-AuNP resulting in an orientation with MPA pointing towards the gold surface. In this orientation MPA is effectively shielded by the AuNP´s cores and not accessible by the IR beam. In summary, it was possible to control the deposition of zwitterionic MPA/MOA-AuNP and to selectively detect the MPA ligand, thereby straightforwardly proving the Janus character of the MPA/MOA-AuNP by IRRAS.

Figure 5. Details of the IRRAS spectra of MPA/MOA-AuNP deposited on a positively charged AOT- and on a negatively charged MPS-functionalized Au substrate (see Supporting


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Information S12 for complete spectra) along with corresponding schematic drawings of the AuNP´s orientation and representative TEM images. For MPA/MOA-AuNP deposited on MPS only ν(C=O) was detected in the low wavenumber region, whereas for MPA/MOA-AuNP deposited on AOT also δ(NH3+) and νar(C=C) from MPA as well as νas(COO-) from deprotonated MOA were detected.

Conclusion This experimental study provides spectroscopic evidence of the Janus-like ligand distribution on amphiphilic and zwitterionic Janus AuNP in the size range of 10-15 nm by IRRAS. For this purpose the particles have been deposited by DSA on pretreated gold surfaces. Amphiphilic Janus OT/MOA-AuNP and zwitterionic Janus MPA/MOA-AuNP were immobilized exploiting hydrophilic/hydrophobic and electrostatic interactions, respectively. Thus it was possible to obtain 2D arrays of Janus AuNP exhibiting a preferred orientation on Au surfaces which were investigated by IRRAS. Thereby the ligand facing the IR beam was selectively detected. Hence, this procedure enabled us to directly prove the Janus character of amphiphilic and zwitterionic anisotropic AuNP bigger than 10 nm, which up to now has not been accomplished. Moreover, our approach will also be applicable to the characterization of further metallic nanoparticles (e.g AgNP or PtNP), big enough to block the IR-beam and impede the detection of the bottom ligand.


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ASSOCIATED CONTENT Supporting Information. Synthesis and characterization of isotropically functionalized AuNP, synthesis and characterization of OT/MOA-AuNP, Au substrates used for the deposition of AuNP, the deposition of isotropic AuNP on Au substrates and respective SEM images, the selective detection of one ligand on the Janus AuNP as well as complete IRRAS spectra. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources This work was supported by German Research Foundation (DFG, contract Si 609/16-1). S.B. acknowledges personal funding by the Fonds National de la Recherche, Luxembourg.


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ACKNOWLEDGMENT We thank Jutta Kiesgen for support in synthetic work, Birgit Hahn for performing SEM measurements and Claudia Klöser for preparing the Au substrates by sputtering technique. Further, we thank Dr. Michael Noyong for helpful discussions on the manuscript.

REFERENCES

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Figure 1. Details of the IRRAS-spectra of citrate-AuNP, OT-AuNP, MOA-AuNP and MPA-AuNP. The characteristic ν(C=O) vibration of citrate and MOA around 1730 cm-1 is highlighted in grey. The region around 1380 cm-1 νip(OH) characteristic of citrate is highlighted in grey as well (see Supporting Information S7 for complete spectra). All presented absorption intensities are reflectance absorbance intensities. 124x152mm (300 x 300 DPI)


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Figure 2. Schematic drawing of the synthesis of amphiphilic Janus AuNP functionalized with OT as hydrophobic and MOA as hydrophilic ligand by a two phase ligand exchange reaction at the liquid/liquid interface. Directed self-assembly onto hydrophobic and hydrophilic gold substrates was applied to obtain macroscopic arrays of directionally deposited amphiphilic AuNP which allowed to validate their Janus character by IRRAS. 99x54mm (300 x 300 DPI)


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Figure 3. Details of the IRRAS spectra of OT/MOA-AuNP deposited on a polar as well as on an apolar Au substrate (see Supporting Information S11 for complete spectra) along with corresponding schematic drawings of the AuNP´s orientation and representative TEM images. 148x135mm (300 x 300 DPI)


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Figure 4. Schematic drawing of the solid phase supported synthesis of zwitterionic Janus AuNP functionalized with MOA and MPA. Directed self-assembly onto MPS- and AOT-functionalized gold substrates was applied to obtain macroscopic arrays of directionally deposited zwitterionic AuNP which allowed to validate their Janus character by IRRAS. 107x63mm (300 x 300 DPI)


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Figure 5. Details of the IRRAS spectra of MPA/MOA-AuNP deposited on a positively charged AOT- and on a negatively charged MPS-functionalized Au substrate (see Supporting Information S12 for complete spectra) along with corresponding schematic drawings of the AuNP´s orientation and representative TEM images. For MPA/MOA-AuNP deposited on MPS only ν(C=O) was detected in the low wavenumber region, whereas for MPA/MOA-AuNP deposited on AOT also δ(NH3+) and νar(C=C) from MPA as well as νas(COO-) from deprotonated MOA were detected. 159x144mm (300 x 300 DPI)


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Directed Self-Assembly and Infrared Reflection Absorption

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