Synthesis and Coordinative Layer-by-Layer Deposition of Pyridine

Article pubs.acs.org/Langmuir

Synthesis and Coordinative Layer-by-Layer Deposition of PyridineFunctionalized Gold Nanoparticles and Tetralactam Macrocycles on Silicon Substrates Christoph H.-H. Traulsen,† Valentin Kunz,† Thomas Heinrich,†,‡ Sebastian Richter,† Markus Holzweber,‡ Andrea Schulz,† Larissa K. S. von Krbek,† Ulf T. J. Scheuschner,† Johannes Poppenberg,† Wolfgang E. S. Unger,*,‡ and Christoph A. Schalley*,† †

Institut für Chemie und Biochemie der Freien Universität Berlin, Takustraße 3, 14195 Berlin, Germany BAM Federal Institute for Materials Research and Testing, Unter den Eichen 44-46, 12203 Berlin, Germany

‡

S Supporting Information *

ABSTRACT: Coordination chemistry was applied to deposit pyridine-functionalized gold nanoparticles on silicon substrates. The particles were synthesized through the Brust/Schiffrin route with a subsequent ligand exchange reaction yielding well-defined particles of two different sizes. Multilayer deposition was carried out on a pyridineterminated SAM, anchored on a hydroxyl-terminated silicon surface. Analogously, Hunter/Vögtle-type tetralactam macrocycle multilayers were deposited as well as mixed layers containing both either in an alternating sequence or as a macrocycle multilayer with a terminating nanoparticle layer. These composite layers were examined with respect to their ability to bind squaraine axles in the macrocycle cavities. The amount of guest bound is higher for the composite layer with alternating macrocycles and nanoparticles.

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INTRODUCTION Modern supramolecular chemistry is no longer focused exclusively on the investigation of intermolecular processes, but aims at the applicability of weak interactions in order to solve technical problems. Due to the interdisciplinary nature, a huge scope of applications exists such as the construction of musclelike supramolecular polymers consisting of polyrotaxanes as synthesized by Du et al. and the development of a molecular peptide synthesizer which can mimic natural peptide synthesis as it has recently been reported by Leigh et al.1,2 A major task in this context is the transfer of processes to solid supports in order to construct smart materials such as surfacebound metal−organic frameworks (SurMOFs).3 Herein, the layer-by-layer approach represents a strategy which is easy-tomanage at the laboratory scale and can be used for the deposition of rotaxanes on solid supports as we have demonstrated recently.4 For this purpose, many template layers have been established in the past decade.5−8 Additionally, multilayers consisting of macrocyclic host molecules can be deposited into multilayers on gold substrates and can be used for reversible and switchable on-surface pseudorotaxane formation.9 These pseudorotaxanes can be used, for example, as chloride sensors.10,11 Besides the deposition of soft organic materials, surfacebound nanoparticles have gained attention.12,13 Gold nanoparticles deposited on glass have for example been used for plasmon-assisted catalysis (PAC) which can be utilized as a © 2013 American Chemical Society

photosynthetic device to autonomously generate hydrogen and oxygen or for the heterogeneous catalysis of Suzuki coupling reactions.14,15 Many other functions of ordered nanoparticle systems on surfaces can be imagined. In this context, Huskens, Reinhoudt , and co-workers developed a supramolecular layerby-layer assembly consisting of functionalized dendrimers and functionalized gold or silica nanoparticles.16−19 Here, we report the layer-by-layer deposition of pyridinefunctionalized gold nanoparticles on solid support. The integration of nanometer-sized tetralactam macrocycles into the nanoparticle multilayers leads to the incorporation of welldefined cavities with converging functional groups that can be used for on-surface host−guest chemistry. The uptake of a highly fluorescent squaraine dye into the macrocycles produces pseudorotaxanes by diffusion of the guest molecule into the pregenerated macrocycle layer. The surfaces prepared were analyzed carefully using a multitechnique analysis comprising AFM, SEM, XPS, ToF-SIMS, and transmission UV/vis spectroscopy.

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RESULTS AND DISCUSSION Synthesis and Nanoparticle Preparation. 4-(Dodec-11enyl)pyridine (PD), 12-(pyridine-4-yl)dodecane-1-thiol Received: August 20, 2013 Revised: October 1, 2013 Published: October 25, 2013 14284

dx.doi.org/10.1021/la403222x | Langmuir 2013, 29, 14284−14292

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Figure 1. (a) Synthesis of PDT and PDS and preparation of mixed-shell PDT/HT-NPs from hexanethiol-capped HT-NPs. (b) Deposition of PDS on a hydroxylated silicon surface to yield a surface covered with a PDS-SAM. (c) Terpyridine-functionalized tetralactam macrocycle MC. (d) Metal ion precursor complexes used as the linking elements in the layer-by-layer deposition procedure. The cartoons defined here are used in subsequent figures.

the larger particles (Figure 2e), reflecting the typical size dependence of the collective oscillation of electrons in gold nanoparticles.27 Dynamic light scattering (DLS) in toluene includes the organic layer and part of the solvent shell and thus gives rise to somewhat larger diameters (Figure 2f) of 2.9 nm for HT-NP1 and 4.5 nm for HT-NP2 that are consistent with the TEM experiments. Initial attempts to prepare PDT-capped nanoparticles with pure PDT through the same procedure yielded particles insoluble in both water and organic solvents that only became soluble in water and acetonitrile after protonation with formic acid. However, pyridine protonation hampers complex formation with transition metal ions and renders the resulting electrosterically stabilized pyridine-terminated particles useless for the desired metal-mediated layer-by-layer multilayer growth. To obtain suitable well-soluble NPs, we therefore applied a ligand exchange reaction of HT against PDT as an alternative by reacting HT-NP1 or HT-NP2 in a solution of PDT in dichloromethane (DCM) for 24 h. The presence of bands of both HT-NP and PDT in the IR spectrum of mixed-shell PDT/ HT-NP qualitatively reveals PDT to be bound to the nanoparticles (Figure 3a), for example in the range around 3000 (C−H) and ∼1560 (CC and CN) cm−1. These bands can clearly be assigned to PDT (upper spectrum). Despite the significant peak broadening in the 1H NMR spectra of the nanoparticles, the ligand ratio can roughly be estimated

(PDT), and the terpyridine-functionalized tetralactam macrocycle MC (Figure 1) were synthesized as described earlier.8,9,20 4-(12-(Triethoxysilyl)dodecyl)pyridine (PDS) was obtained from PD by hydrosilylation with triethoxysilane using Karstedt’s catalyst.21 Hexanethiol-capped HT-NP nanoparticles served as the precursors for the desired pyridine-functionalized PDT-NP nanoparticles and were synthesized using the Brust/Schiffrin route by formation of Au-HT clusters in a phase-transfer reaction with HAuCl4·3H2O and tetraoctylammonium bromide (TOAB).22,23 After addition of hexanethiol and subsequent reduction with sodium borohydride, the smaller HT-NP1 particles were obtained.24 In order to increase the nanoparticle size, HT-NP1s were thermally annealed according to Maye et al. by heating them together with HT and TOAB at 150 °C in toluene for one hour in a sealed tube yielding larger HTNP2s.25 According to transmission electron microscopy (TEM) images depicted in Figure 2a,c, both particles are almost spherical. The ImageJ program is used to determine the particle size distributions.26 With a data set of more than 1500 particles each, the size distribution of the Au-core diameter is found to be within 1.8 ± 0.2 nm and 3.8 ± 0.2 nm for HT-NP1 and HTNP2, respectively (Figure 2b,d). The size difference of the HTNPs can also be monitored by UV/vis spectroscopy: The plasmon absorption band at 520 nm is more pronounced for 14285


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Figure 2. Left: Analysis of hexanethiol-capped nanoparticles HT-NPs. (a) TEM image of HT-NP1. (b) Particle size histogram by TEM image analysis (ImageJ) of HT-NP1. (c) TEM image of HT-NP2. (d) Particle size histogram by TEM image analysis (ImageJ) of HT-NP2. (e) UV/vis spectra of HT-NPs. (f) DLS volume-weighted size distributions of HT-NPs. Right: Analysis of mixed-shell nanoparticles PDT/HT-NPs. (g) TEM image of PDT/HT-NP1. (h) Particle size histogram by TEM image analysis (ImageJ) of PDT/HT-NP1. (i) TEM image of PDT/HT-NP2. (j) Particle size histogram by TEM image analysis (ImageJ) of PDT/HT-NP2. (k) UV/vis spectra of PDT/HT-NPs. (l) DLS volume-weighted size distributions of PDT/HT-NPs.

such as chloroform, dichloromethane and ethanol. To determine the diameters of the PDT/HT-NPs, TEM, DLS and UV/vis spectroscopy have been applied as described above (Figure 2g−l). The mean diameters of the particle cores measured after the ligand exchange reaction are 2.0 ± 0.2 nm and 3.9 ± 0.5 nm for PDT/HT-NP1 and PDT/HT-NP2, respectively, and agree within the error margins with those of the corresponding HT-NPs. UV/vis spectroscopy reveals the plasmon resonance band to increase again with particle size (Figure 2k).14,15 The plasmon coupling is affected by the thicker ligand shell giving rise to an absorption band shift.28 The hydrodynamic diameter of both particles increases as determined by DLS (Figure 2l) to 4.5 nm for PDT/HT-NP1 and 7.5 nm for PDT/HT-NP2 in line with the significantly longer PDT molecules. In addition, X-ray photoelectron spectroscopy (XPS) revealed all expected species to be present in the sample including carbon (C 1s), gold (Au 4f), and nitrogen (N 1s). Only uncomplexed pyridine nitrogen atoms were observed at a binding energy of BE = 399.8 eV, in line with prior studies for PDT SAMs on gold (Supporting Information (SI), Figures S15−S17).8 This observation is also consistent with a variety of other studies by Wöll et al., Kolb et al., Zharnikov et al. and Evans et al.. It therefore unambiguously identifies pristine pyridine.29−32 Pyridine and terpyridine species complexed to transition metal ions exhibit a significant N 1s BE shift toward higher energies (SI, Figure S20).33 Thus, complex formation between the gold core and pyridine tail groups can be excluded for the presented nanoparticles. Preparation and Characterization of the Templating PDS Self-Assembled Monolayer (SAM) on Silicon. To investigate whether triethoxy-, trimethoxy-, or trichlorosilane is preferable for SAM preparation, we initially carried out

Figure 3. (a) IR spectra and (b) 1H NMR spectra of PDT, PDT/HTNP1, PDT/HT-NP2, and HT-NP.

from the integration of pyridine and aliphatic proton signals (Figure 3b) to be 2:1 for PDT/HT-NP1 and 1.3:1 for PDT/ HT-NP2. Both particles are well soluble in organic solvents 14286


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deposition experiments with these species, revealing triethoxysilane to be most preferable as it combines a higher stability of the reactant with high coverage of the surface (SI, page S16). For the deposition of PDT/HT-NPs on solid support, PDS monolayers were deposited on hydroxyl-terminated silicon wafers (for XPS and NEXAFS experiments) or glass slides (for UV/vis spectroscopy).34 Silicon or glass was selected instead of the gold surfaces used in our earlier studies,9 because the use of gold would not only complicate XPS analysis, but also the detection of the nanoparticles through transmission UV/vis spectroscopy due to plasmon coupling. Characterization by time-of-flight secondary-ion mass spectrometry (ToF-SIMS, SI, Figure S10) and XPS (SI, Figure S11) confirmed the deposition of the PDS molecules on the surface. The BE region of the N 1s reveals unmodified nitrogen at 399.5 eV to be the major component accompanied with traces of protonated and hydrogen-bonded pyridine nitrogen.30 Therefore, a partial binding of the tail group to the substrate cannot be rigorously ruled out, but clearly represents a fraction smaller than 5% of all pyridine nitrogen atoms. Linear dichroism effects in angleresolved near-edge X-ray absorption fine structure (NEXAFS) spectroscopic experiments (SI, Figure S11) revealed the SAM to be densely packed with a preferential upright orientation of both, the aliphatic backbone and the pyridine groups. Nanoparticle Mono- and Multilayers. Layer-by-layer deposition of trans-PdCl2(NCC6H5) (NCC6H5 = benzonitrile) and PDT/HT-NP2s was performed on these PDS-SAMs on glass and silicon by alternatingly immersing the template layer in a 1 mM solution of trans-PdCl2 in acetonitrile (deposition step 1) and PDT/HT-NP2 in ethanol (deposition step 2a). After each deposition step, the samples were rinsed with the same solvents. Prior to multilayer deposition, the deposition conditions were optimized for maximum surface coverage in a NP monolayer. Particle deposition from a 0.1 mg/mL solution within 24 h at room temperature (RT) yielded incomplete particle coverage (Figure 4a). Extending the deposition time to 3 days or increasing the nanoparticle concentration to 0.25 mg/ mL (Figure 4b) resulted in densely packed NP-monolayers. Using the higher concentration for all subsequent experiments, the metal ion and nanoparticle deposition steps were alternatingly repeated to produce nanoparticle multilayers. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) analysis after three cycles (equivalent to a total of seven deposition steps including SAM formation) revealed the resulting multilayer to be densely packed (Figure 4c,d). Transmission UV/vis spectroscopy (Figure 5a) reveals the plasmon resonance band of the particles (520 nm) to increase linearly with every deposition cycle (Figure 5b).35,36 Consequently, a constant number of particles are immobilized during each single deposition cycle, indicating the number of defects to be small. Unspecific NP binding to the surface can be ruled out by a simple control experiment: After immersing a PdCl2-free PDS-SAM into a solution of PDT/HT-NP2, no plasmon resonance band is detected. Consequently, the nanoparticles are bound to the surface through metal coordination, while unspecifically bound NPs are washed away. All expected species are also observed in the XP spectra. The nearly quantitative coordination of trans-PdCl2 to the PDSSAM and/or the nanoparticle layer can be followed by the BE shift of the pyridine nitrogen to a binding energy of BE = 400.5 eV (SI, Figures S17 and S20). The C 1s/Si 2p peak area ratio does not change with the deposition of a PdCl2 layer, but

Figure 4. (a) SEM analysis of a single layer of PDT/HT-NP2 with (a) submonolayer coverage (0.1 mg/mL, 24 h, RT) and (b) an almost complete monolayer (0.25 mg/mL, 24 h, RT). (c) SEM and (d) AFM analysis of a multilayer after the third deposition of PDT/HT-NP2 nanoparticles (seven deposition steps: SAM → (PdCl2 → NPs)3) on silicon substrate.

Figure 5. Analytical data of nanoparticle multilayers on a PDS-SAM. (a) Transmission UV/vis spectra recorded after each PDT/HT-NP2 deposition. (b) Intensity of the plasmon band over the number of deposited nanoparticle layers. (c) XPS C 1s/Si 2p peak area ratio determined for the templating PDS SAM, after deposition of the first metal and the first nanoparticle layer and after the deposition of the third and fifth nanoparticle layers.

increases, as expected, significantly after the deposition of a nanoparticle layer (Figure 5c).9 These data provide evidence 14287


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In order to test whether the deposition of a nanoparticle layer on top of the macrocycle multilayer is possible, PdII ions were deposited exemplarily on a 10-layer stack of macrocycles followed by the deposition of PDT/HT-NP2 (red spectrum in Figure 6a). Clearly, a plasmon resonance band is observed after deposition and thorough washing indicating the presence of the terminating nanoparticle layer. Alternating Macrocycle and Nanoparticle Layers. Based on these results, PDT/HT-NP2 and MC were deposited alternatingly using PdII ions as the metal ion suitable to connect one pyridine with one terpyridine. The first nanoparticle layer is fixed to the PDS template layer with trans-PdCl2 as described above (pyridine−pyridine connection). In line with expectation, the transmission UV/vis spectra (Figure 7a) exhibit three pronounced absorption bands, each of which is specifically indicative of one of the deposited building blocks. The π → π*

that the layer-by-layer deposition of densely packed nanoparticle multilayers is possible. Preparation of Macrocycle Multilayers and Deposition of a Terminal Nanoparticle Layer on Top. Hunter/ Vögtle-type tetralactam-macrocycles form densely packed and preferentially oriented mono- and multilayers on gold surfaces as described previously.4,9 To prepare the stage for mixed multilayers comprising macrocycles as well as nanoparticles, we applied this approach to the PDS-SAM on glass under study here. For the deposition of the first MC layer on PDS, PdII from Pd(NCCH3)4(BF4)2 as the precursor is used as a squareplanar coordinating metal-ion connecting the SAM’s pyridines to the macrocycle terpyridines. FeII from Fe(BF4)2·6H2O with its octahedral coordination sphere is then applied to connect two terpyridines and thus to deposit the subsequent layers.37 Transmission UV/vis spectra clearly demonstrate the controlled MC layer growth in close analogy to our previous results (Figure 6): The π → π* absorption band at 290 nm increases

Figure 6. (a) Transmission UV/vis spectra of MC multilayers on a PDS-SAM recorded after each MC deposition step. The spectrum depicted in red corresponds to a 10-layer stack of macrocycles on top of which PdII ions followed by a layer of PDT/HT-NP2 nanoparticles were deposited. (b) Absorption intensities at 290 nm over the number of deposited macrocycles.

linearly with the number of deposited macrocycles, whereas the ligand-centered (LC) band at 360 nm and the metal-to-ligand charge transfer (MLCT) band at 575 nm grow only, when FeII is deposited. As the same intensity increase at 290 nm is observed with every deposited MC layer, the layer stack can be regarded as densely packed as found for the gold surfaces in our earlier experiments.9 Consequently, the deposition of macrocycle multilayers is easily achieved irrespective whether gold with pyridine-functionalized thiols or glass with the corresponding pyridine-terminated triethoxysilanes is used as the substrate.

Figure 7. Analytical data of an alternating MC/PDT/HT-NP2 multilayer on a PDS-SAM. (a) Transmission UV/vis spectra recorded after each NP and MC deposition. (b) Absorption intensity at 540 nm (plasmon resonance band) plotted over the number of deposited PDT/HT-NP2 layers. (c) Absorption intensity at 289 nm (π → π*) plotted over the number of deposited MC layers. (d) XPS C 1s/Si 2p peak area ratio determined after different deposition steps. (e) XPS C 1s/Au 4f peak area ratios indicating the alternating deposition sequence. 14288


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absorption band at 289 nm indicates MC deposition, the LC band at 360 nm that of the PdII ion, and the plasmon resonance at 560 nm that of PDT/HT-NP2. Every deposition of a macrocycle, metal or nanoparticle layer induces an increase of the corresponding absorption band. The intensity of each band can be plotted over the number of the corresponding layers deposited resulting in a nearly linear trend as shown in Figure 7b,c. In line with these results and those discussed above, XPS experiments exhibit an increasing C 1s/Si 2p ratio (Figure 7d). Furthermore, the C 1s/Au 4f peak area ratio (Figure 7e) alternates as expected with the alternating deposition of nanoparticles and macrocycles. These results thus provide evidence for the successful alternating deposition of macrocycles and nanoparticles on the surface. As the nanoparticles have a size distribution, it is likely that the surface roughness increases with the number of deposited nanoparticle layers. Consequently, one would expect defects to form rather than well-ordered surfaces, as the increasing roughness certainly disturbs well-packed macrocycles in between. The roughness has been determined using AFM measurements and the results support this hypothesis (Table 1).38

Figure 8. (a) SEM and (b) AFM analysis of a SAM→PdCl2→PDT/ HT-NP2→PdII→MC→ PdII→PDT/HT-NP2 multilayer.

Table 1. Height Deviations Obtained by AFM Measurements (1 μm) Performed with Different Samplesa layer

Rq [nm]

SAD [%]

Si-blank SAM (PDS) SAM-PdCl2-PDT/HT-NP2 SAM-PdCl2-PDT/HT-NP2-PdII-MC SAM-PdCl2-PDT/HT-NP2-PdII-MC- PdII‑PDT/HT-NP2

0.2 1.1 1.7 1.4 1.7

0.1 1.5 7.8 1.9 5.0

a

All Rq values represent the root-mean-square (RMS) average of height deviations Zi from the mean data plane: Rq = √(Σ(Zi)2/n). Zi = current Z value, and n = number of data points. The surface area difference (SAD) is defined as the difference between the threedimensional surface area and its two-dimensional projection: SAD = 100% × (A3D − A2D)/(A2D). Values represent the mean value of at least three data sets.

The well-ordered PDS-SAM exhibits a roughness of Rq = 1.06 nm. Deposition of a (PdCl2→PDT/HT-NP2) layer on top increases the Rq value significantly; deposition of PdII→MC results in a roughness decrease, giving rise to a surface which is still significantly rougher than the unmodified PDS-SAM. Another nanoparticle layer deposited on top leads again to an increased roughness which is comparable to the SAM-PdCl2PDT/HT-NP2 layer. The corresponding SEM and AFM images obtained from the mixed NP/MC multilayers further support this hypothesis (Figure 8). Host−Guest Chemistry of Mixed Nanoparticle/Macrocycle Multilayers: Pseudorotaxane Formation with Squaraine Axles. Supramolecular interactions such as hydrophobic effects can be used to bind suitable guest molecules on solid supports.39 In this context, highly fluorescent squaraine dyes such as SqA (Figure 9) have been shown previously to reversibly bind to MC multilayers on gold substrates.9 These axles bind in the tetralactam macrocycle cavity through four MC−NH···O−SqA hydrogen bonds. Hydrogen bonding leads to a structure-indicative shift of the SqA absorption band from 632 to 667 nm so that on-surface pseudorotaxane formation with SqA can easily be detected using transmission UV/vis

Figure 9. (a) Transmission UV/vis spectra of the (MC)10→PDT/HTNP2 multilayer before (blue line) and after (black line) immersion into a 1 mM solution of SqA in DCM. Intensities were normalized to the MLCT band to facilitate comparison. (b) Transmission UV/vis absorption intensity increases at 667 nm over SqA deposition time for (MC)10→PDT/HT-NP2 (blue line) and (PDT/HT-NP2→MC)10→ PDT/HT-NP2 (black line) multilayers. Langmuir adsorption kinetics was used to fit the obtained data points using eq 1. Both multilayers contain the same number of macrocycle layers. The UV/vis spectra obtained before SqA deposition have been subtracted.

spectroscopy.10,40 Two of the surfaces described above have been tested with respect to their ability to bind the squaraine axle inside the macrocycle cavities: (i) a multilayer containing 10 layers of macrocycles that are covered by one terminal PDT/HT-NP2 layer (abbreviated as (MC)10→PDT/HT-NP2) and (ii) an alternating multilayer stack also containing a total of ten macrocycle layers interlaced and topped by nanoparticle layers ((PDT/HT-NP2→MC)10→PDT/HT-NP2). Anticipating slow uptake kinetics, the (MC)10→PDT/HTNP2 multilayer was immersed in a preliminary test in a 1 mM solution of SqA in DCM for six days. A clearly visible 14289


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bound within the layers is due to an increased amount of immobilized MC.

absorption band arises at 667 nm, which can be assigned to the squaraine dye hydrogen-bonded inside the macrocycle cavities (Figure 9a). To exclude unspecific SqA adsorption by the nanoparticles, the same experiment preformed with a multilayer consisting of ten PDT/HT-NP2 layers in the absence of any macrocycles did not result in an SqA absorption band (SI Figure S19). These results provide qualitative evidence for pseudorotaxane formation on a pregenerated surface. In order to examine the time required for reaching the binding equilibrium, the intensity increase of the band at 667 nm was followed over time (Figure 9b, blue line). Interestingly, saturation of the surface was reached already after much shorter times than anticipated (ca. 3 h). Afterwards, the intensity at 667 nm remains more or less constant. The same experiment was repeated with the alternating (PDT/HT-NP2→MC)10→PDT/ HT-NP2 layer stack (black line). To determine the uptake rates, we plotted the obtained data points together with the fitting curves obtained with a Langmuir adsorption kinetics curve (eq 1) assuming the adsorptions follows second-order kinetics. A = E*(1 − e−kSt )

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CONCLUSION Pyridine-functionalized nanoparticles have been synthesized and used to construct multilayers through the layer-by-layer deposition approach. As the underlying template layer, pyridine-functionalized silicon or glass substrates were applied. Coordination chemistry served to connect the individual layers. After characterizing multilayers of nanoparticles by a multitechnique approach using XPS, NEXAFS, ToF-SIMS, SEM, and AFM, mixed layers have been prepared that bear nanometer-sized tetralactam macrocycles and nanoparticles. An alternating multilayer was compared to a macrocycle multilayer stack terminated by one nanoparticle layer on top. These mixed layers exhibit functionality in that they are capable of binding squaraine axles that form pseudorotaxanes on the pregenerated surfaces through hydrogen bonding inside the macrocycle cavities. SEM analysis reveals the nanoparticle multilayers to be homogeneous and densely packed. Multilayers with a nanoparticle layer on top exhibit an increased roughness as determined using AFM. The two mixed layers show only slight differences in squaraine uptake rates, although the maximum amount of squaraine adsorbed differs significantly at saturation. Overall, these experiments complement our earlier studies on multilayers of tetralactam macrocycles that do not contain any nanoparticles. They are consistent with and thus support these earlier results.

(1)

with E* =

kaCA max kaC + kd

ks = kaC + kd

(2)

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(3)

A is the obtained UV/vis absorption intensity at 667 nm, Amax is the maximum absorption possible, ka is the rate constant for SqA adsorption, and kd is the rate constant for its desorption. Equation 1 was derived as described by Hu et al.41 The uptake rate is ks(1) = 0.35 ± 0.03 h−1 for the (MC)10→PDT/HT-NP2 multilayer and ks(2) = 0.29 ± 0.02 h−1 for (PDT/HT-NP2→ MC)10→PDT/HT-NP2. Two observations are made: (i) The amount of the squaraine axle adsorbed under saturation conditions is significantly higher for the alternating multilayer. (ii) The uptake rates are nevertheless quite similar and differ by about 20%. In our previous studies, we found SqA to be able to reach not only the top macrocycle layers, but also to diffuse into the lower layers.9 The SqA/MC ratio was estimated to be in the range of ca. 1:3.9 The new results obtained for the nanoparticle multilayers here agree quite nicely with these previous results. Several effects may contribute to the higher SqA loading of the alternating multilayer: (i) As each nanoparticle layer presents a rougher and therefore somewhat larger surface, the alternating multilayer may contain more macrocycles as compared to a densely packed macrocycle multilayer. However, in view of the similar sizes of nanoparticles (≈4 nm) and macrocycles (≈3.5 nm), this effect is likely not the only one as it is difficult to imagine how this effect would lead to a three times higher amount of deposited SqA. (ii) The packing of the macrocycles in the alternating layer stack is likely less ordered than that in the (MC)10→PDT/HT-NP2 multilayer. While the SqA side chains dive into the cavities of the two adjacent macrocycles in an ordered layer in agreement with a 1:3 SqA/MC ratio, they can use voids in a less ordered, alternating multilayer, thus leaving more MC cavities open for SqA binding. Consequently, the overall amount of SqA bound is higher for the alternating stack. However, it is also possible that the increased SqA amount

ASSOCIATED CONTENT

S Supporting Information *

Additional analytical data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *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. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to D. Treu for operating the XPS instrument at BAM 6.8. Support by A. Lippitz (BAM), A. Nefedov (KIT), and staff at BESSY II (M. Mast) during our activities at the HESGM beamline is gratefully acknowledged. The authors gratefully acknowledge the Deutsche Forschungsgemeinschaft (SCHA 893/9-1, UN 80/8-1), the Freie Universität Berlin, and the Fonds der Chemischen Industrie (FCI) for financial support. S.R. thanks the FCI for a Chemiefonds Ph.D. fellowship. M.H. is grateful for financial support by BAM through the Adolf-Martens fellowship program.

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REFERENCES

(1) Lewandowski, B.; De Bo, G.; Ward, J. W.; Papmeyer, M.; Kuschel, S.; Aldegunde, M. J.; Gramlich, P. M. E.; Heckmann, D.; Goldup, S. M.; D’Souza, D. M.; Fernandes, A. E.; Leigh, D. A.

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Synthesis and Coordinative Layer-by-Layer Deposition of Pyridine

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