Assembly Behavior of Organically Interlinked Gold Nanoparticle

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The Assembly Behavior of Organically Interlinked Gold Nanoparticle Composite Films: A Quartz Crystal Microbalance Investigation Yelyena Daskal, Tina Tauchnitz, Frederic Güth, Rosemarie Dittrich, and Yvonne Joseph Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b01974 • Publication Date (Web): 21 Sep 2017 Downloaded from http://pubs.acs.org on September 26, 2017

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The Assembly Behavior of Organically Interlinked Gold Nanoparticle Composite Films: A Quartz Crystal Microbalance Investigation Yelyena Daskal, Tina Tauchnitz†, Frederic Güth, Rosemarie Dittrich, and Yvonne Joseph* Institute of Electronic and Sensor Materials, TU Bergakademie Freiberg, Gustav-ZeunerStraße 3, 09599 Freiberg, Germany

ABSTRACT Thin films based on dodecylamine stabilized gold nanoparticles interlinked with different organic molecules are prepared by automatic layer-by-layer self-assembly in a microfluidic quartz crystal microbalance (QCM) cell, to obtain an in-situ insight on the film formation by ligand/linker exchange reactions. The influence of interlinking functional groups and the length of the organic linker molecule on assembly behavior is investigated. Namely alkyldithiols with different lengths are compared to alkyldiamines and alkylbisdithiocarbamates with a C8 alkylic molecular backbone. The stepwise layer-by-layer assembly occurs independently of the linker molecule, while the largest frequency changes always correspond to the gold nanoparticle step. During the solvent rinsing and ligand/linker exchange reaction step, the frequency is almost constant with slight increases or decreases dependent on the molar mass of the linker compared to the

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exchanged ligand. The assembly efficiency is higher for shorter molecules and for molecules with stronger interacting functional groups. The densities of the composite films are calculated from QCM data and independent thickness measurements. They reflect the higher fraction of organic material in the films comprising of longer organic linkers. The plasmon resonance band of the gold nanoparticles in the final assemblies is measured with UV/Vis spectroscopy. Band positions in films prepared from dithiols and diamines of comparable lengths are very similar, while the spectrum of the bisdithiocarbamate film exhibits a distinct blue-shift. This observation is explained by the longer molecular structure of the linker due to a larger binding group, in conjunction with a delocalization of particle charge on the organic molecule. Obtained results play an essential role in the understanding of thin film layer-by-layer self-assembly processes, and enable the formation of new gold nanoparticle networks with organic diamine and bisdithiocarbamate molecules.


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1 INTRODUCTION Over the past few years, one focus of materials research was the development of materials to be tailor-made for applications on the molecular level [1]. Nanocomposites based on nanoparticles (NP) embedded in an organic matrix are interesting candidates since their components can be chosen from a variety of options such as polyelectrolytes, dendrimers, polymers, small organic molecules, as well as semiconducting, oxide, and metal nanoparticles [2]. Depending on the exact composition, these nanocomposites show very different physical and chemical properties which have been exploited to demonstrate their uses as catalysts, capacitors, coatings, or in optics, drug release, and diagnostic applications. A well-researched example of nanocomposites is those based on gold nanoparticles and thiol functionalized organic linker molecules [3–7]. This is mainly due to the strong binding between the thiol groups of the linker and the surface of the gold nanoparticles, which result in stable particle networks that can be used as sensors for various gaseous organic analytes [6, 8–10]. Since the sorption behavior of volatile organic compounds (VOCs) depends on the structural characteristics of the organic linker, the sensitivity and selectivity of these sensors can be tuned by selecting appropriate molecules [6, 11, 12]. When a wide variety of such sensors utilizing different linker molecules are integrated into a sensor array, the combined response of all sensors can be used to distinguish between analytes and determine the composition of complex vapor mixtures via pattern recognition algorithms [13]. However, according to current knowledge, the selection of suitable organic linker molecules is often limited to commercial thiols, while non-commercial thiols are only accessible by complex and time consuming chemical synthesis procedures. Thus, an easy way to integrate different commercial non-thiol molecules in these composites would be beneficial. Furthermore, thiol based composites are prone to long term oxidation by ozone


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from the atmosphere [7]. Correspondingly, molecules with more stable linking functionalities are needed for long term stability of the composites. To realize such sensor arrays, simple, efficient, and cheap fabrication methods are needed to coat the sensitive composites selectively on the respective areas of suitable transducers, e.g. on wafers with interdigitated electrodes or quartz crystal microbalances. Many techniques have been explored with this aim [14–21]. Most frequently a layer-by-layer approach is used, in which the ligand protected nanoparticles and the linker molecules are alternately deposited on a substrate by a ligand-linker exchange reaction. This procedure allows good control over final film thickness and the resulting layers are of high quality due to self-assembly of the molecular building blocks. However, precautions must be taken to prevent composite exposure to air, layer-by-layer deposition is usually a time-consuming and/or manual process, and in most of the cases, the whole transducer is coated. Therefore, automated deposition setups with the ability to coat selective areas in an unsupervised fashion are favorable. Furthermore, an in-situ insight into film formation could help to understand the structure and stability of the nanocomposites and therefore facilitate the identification of linker molecules suitable for stable and reproducible sensing films. Optical (e.g. UV/Vis spectroscopy) [22–24] or electrical [5, 25–27] characterization methods but especially microgravimetry using a quartz crystal microbalance (QCM), has proven to be a suitable tool for this task due to its ability to sense material deposition in terms of mass changes in the ng domain [28,29]. In the literature, an associative exchange mechanism has been postulated for thiol/thiol ligand exchange reactions [30, 31]. Here, in a first step, the new (incoming) linker is adsorbed associatively in the ligand layer. The second step is the elimination of the (outgoing) ligand.


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In this study, we present a versatile automatic microfluidic setup for the layer-bylayer deposition of self-assembled nanocomposites in predefined areas of variable planar substrates. The parallel observation of the layer-by-layer self-assembly behavior by ligandlinker exchange was made with an in-line QCM module. In order to show the functionality of this design, the assembly behavior of nanocomposites comprised of the well-known combination of alkyl-dithiols and gold nanoparticles (AuNP) is discussed with respect to the linker lengths. New alkyl linker molecules in the form of diamines (DA) and bisdithiocarbamates (BDTC) were also used and their assembly behavior has been investigated. Many diamines are commercially available and bisdithiocarbamates can be synthesized from commercially available secondary diamines by an easy one pot synthesis procedure [32, 33]. For proof of concept, we have used a set of model linker molecules (1,6Hexanedithiol

(C6),

1,8-Octanedithiol

(C8),

1,16-Hexadecanedithiol

(C16),

1,8-

Octanediamine (C8A), and 1,8-Octanebisdithiocarbamate (C8B)), which are represented in Figure 1 and Table 1. C8, C8A, and C8B are compared due to similar lengths in the molecular alkyl backbone (C8), but different chemical structures of their functional end groups. Besides valuable insight in ligand-linker exchange reactions, this combined approach of automated layer-by-layer assembly with new commercial linker molecules can be easily up-scaled for sensing arrays with a number of different materials, allowing the cheap formation of sensor arrays based on novel nanocomposites with high structural quality.

2 EXPERIMENTAL 2.1. Materials Chemicals with reagent grade or higher purity were purchased from Sigma Aldrich, VWR and Acros Organics and are used as received. For the layer-by-layer self-assembly, the


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dithiol (DT) molecules (1,6-hexanedithiol (C6), 1,8-octanedithiol (C8), and 1,16hexadecanedithiol (C16)) were dissolved in toluene, whereas 2-propanol was chosen as the solvent for the 1,8-octanediamine (C8A). All samples had a concentration of 5 mmol/L. Because of its instability outside of a solvent medium, the 1,8-octanebisdithiocarbamate (C8B) had to be synthesized in-situ by a base catalysis induced addition of carbon disulfide in 2-propanol

from

N,N-dimethyl-1,8-octanediamine:

11.44

µl

N,N-dimethyl-1,8-

octanediamine was dissolved in 20 ml 2-propanol, yielding a solution with a concentration of 2.5 mmol/L. Furthermore, 6.06 µl carbon disulfide was mixed with 20 ml 2-propanol (5 mmol/L). Finally, a third solution consisting of 2-propanol saturated with potassium carbonate, acting as the base catalyst in the reaction, was prepared. These three solutions were combined in a ratio 2:2:1 by volume to yield the final solution containing the C8B in a concentration of 1 mmol/L. This lower concentration is used due to weaker solubility of the bisdithiocarbamate linker. However, due to the tiny amount of organic molecules needed to form the network, no impact on the film formation is expected. Dodecylamine(DDA)-stabilized AuNPs were prepared similarly to the wet-chemical method developed by Leff et al. [35] and had a core size of 3.7 ± 1.5 nm as determined by transmission electron microscopy (TEM). For the deposition experiments the as-prepared AuNP suspension was diluted with toluene. The final concentration was indirectly controlled using UV/Vis spectroscopy by assuring an absorbance of 0.4 at the maximum of the plasmon absorbance band (λmax = 512 nm; 1 cm path length).

2.2. Substrate Preparation The gold coated 4.95 MHz QCM sensor crystals were purchased from Q-Sense. For UV/Vis and film thickness measurements, borosilicate glass cover slips (VWR) with a defined thickness of 170 ± 5 µm and an edge length of 22 mm were used as substrates. Prior


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to the film deposition, the substrates required cleaning. For the QCM crystals this procedure consisted of a 10 minute ozone cleaner conditioning followed by a 5 minute treatment in a Standard Clean solution (NH4OH:H2O2:H2O, 1:1:4, at 75 °C) and a DI-water rinse. Finally the crystals underwent another 10 minute ozone treatment. The glass cover slips were immersed in an acetone bath under ultra-sonication treatment for 15 minutes and subsequently rinsed with DI-water. They were subjected to Piranha solution (H2SO4:H2O2, 3:1 at 90 °C) for 15 minutes and were then thoroughly rinsed with DI-water and 2-propanol and blow dried with argon. Afterwards, the glass substrates were immediately placed in a custom-build

sealable

glass

apparatus

for

gasphase-silanization

with

3-

mercaptopropyltriethoxysilane (MPTES). In this apparatus, the MPTES was heated to 120 °C in an oil bath, and the resulting silane vapor was guided over the room temperature substrates. This procedure was performed in order to functionalize the glass surface of the substrate for a better nanoparticle adhesion [36].

2.3. Apparatus and coating The layer-by-layer deposition of the nanocomposites was performed in an automated custom-build setup as depicted in Figure 2. In this modular setup, the solutions containing the molecular building blocks of the final nanocomposite are sequentially pumped through a computer-controlled six-way valve (Hamilton MVP), and a coating cell holding the respective substrate by a peristaltic pump (Ismatec, ISM935C). Since the six ports of the valve can be addressed in any order and opened for arbitrary periods of time by a custom written LabVIEW program, full automatization of the setup is achieved. The insets in Figure 2 show a detailed view of the custom-made coating cell used for the glass substrates and the commercial microfluidic quartz coating cell (Q-Sense E1, Q-Sense) with laminar flow and a volume of 140µl (40µl above the quartz). The electronics allows the readout of the change in


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resonance frequency of a QCM with a limit of detection of 0.5 Hz (twice the standard deviation of the noise) together with the change in dissipation factor during mass uptake. In both cells, the coating areas are defined through an o-ring embedded in the bottom part of the cell. By pressing the part holding the glass substrate or quartz crystal against the oring, a sealed compartment is created. During the deposition process, it is filled and drained through the inlet and outlet holes in the bottom. As reported by Günthel et al., the custom built microfluidic setup allows the coating with homogenous layers [37]. If required, additional coating cells can be connected in series to increase the yield of coated substrates. For the coating, in the QCM cell a cleaned gold coated quartz crystal was mounted. This enabled the desired stepwise in-situ analysis of the assembly behavior of the nanocomposite. During the experiments, the temperature of the coating cell was kept constant at 25 °C while frequency changes were recorded. The silanized glass substrates were placed into the custom-build microfluidic cell and connected in series with the commercial quartz crystal microbalance cell. During the layer-by-layer self-assembly process, AuNP solution, organic solvent, linker solution, and organic solvent were sequentially pumped through both cells with a flow rate of 100 µL/min. The flow sequences and times for the different linker are given in Figure 1. The times were selected with respect to the volume of the microfluidic cells to minimize the consumption of chemicals while gaining high assembly efficiencies. Especially the concentration and time of the AuNP solution in the cell is important because this will lead to the highest increase in mass. Under the selected conditions 97% of the possible gold per cycle has been assembled (compare Supporting Information). As shown in the lower part of Figure 1, the experiments with the 2-propanol based C8A and C8B solutions needed further rinsing steps to ensure a complete exchange of the different organic solvents in the tubing. Thus, the organic solvent had to be changed automatically from toluene to 2propanol twice within an assembly cycle. In order to evaluate the layer-by-layer assembly


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behavior of the nanocomposites, the deposition cycle was repeated 15 times. After preparation, the films were purged with toluene for 10 minutes inside the flow cell and dried by extracting all the residual solvent through the peristaltic pump. To control the concentration of the used NP solution and to observe the qualitative dependency of the surface plasmon band in the composite films, a Zeiss Specord S10 spectrometer was used. In all cases, the absorbance and peak position of the plasmon band were determined. For measurements of liquid samples, the path length was equal to 1 cm. Appropriate baseline corrections using cleaned quartz glass substrates or cuvettes filled with toluene were performed prior to each measurement. To determine the film thickness, a KLA Tencor P-15 profilometer was employed. The measurements were conducted at scratches made in the film using an established procedure [15] by moving a plastic tweezer with gentle pressure over the quartz glass substrates after film deposition. By this procedure the film, that was optically visible has been completely removed resulting in an transparent scratch while due to the softness of the plastic tweezer the quartz glass is unchanged. During the profilometric measurements, a scan velocity of 50 µm/s and an applied force of 5 mg were used.


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3 RESULTS 3.1. Layer-by-layer growth of the DT-composites The layer-by-layer self-assembly of all composite materials was analyzed using the quartz crystal microbalance setup previously described. The theory behind such microgravity experiments is established in the Sauerbrey equation, stating a linear relationship between any adsorbed mass on a quartz sensor and its resonance frequency. 2

∆f =

2

− 2 f 0 ∆m − 2 f 0 = ∆ ρd ρ q mq A ρ q mq

(1)

Where, ∆f is the observed change in the resonance frequency, f0 the resonance frequency of the pristine quartz, A is the piezoelectrically active crystal area, ρq is the density of quartz, mq is the shear modulus of quartz, ∆m is the mass change and ρ and d are the density and thickness of the deposited thin film respectively. Accordingly, QCM is a powerful tool in the analysis of the layer-by-layer assembly including ligand-linker exchange processes. If the experiments are carried out in liquid media, an additional damping term has to be considered [38], when viscoelastic properties are present:

∆f =

− 2 f0

2

A ρ q mq

∆m − f 0

3/ 2

ηl ρl πmq ρ q

(2)

The damping term also includes ηl, the dynamic viscosity and ρl, the density of the liquid medium [36]. However, in accordance with other publications [39], the validity of the Sauerbrey equation (1) for these experiments is assumed.

The required conditions for this

assumption are: Firstly, a homogenously distributed mass, which was optically proven by the homogeneously coloring of the thin film. Secondly, the frequency changes (∆f) should be smaller than 2% of the resonance frequency (f0) of the quartz. As provided by the data in all measurements the changes are far below 99000 Hz for 4,95MHz crystals (∆f/f < 0,001). Finally the film can be assumed as behaving rigid, because all measured changes in the


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dissipation factor (as discussed in the Supporting Information) are usually small with respect to the frequency changes (∆f/∆D > 25). Figure 3 depicts the change in resonance frequency of a quartz sensor due to adsorbed mass over fifteen deposition cycles of AuNP interlinked with the indicated DT molecules in conjunction with the removal of the DDA ligands from the particle´s surface. The curves have a distinct shape reflecting the stepwise layer-by-layer deposition scheme with large frequency changes during the AuNP step and an almost constant frequency during the toluene rinsing and linker steps. This observation can be explained by the much higher mass of the AuNPs compared to the light organic linker molecules. Single measuring points at the end of each cycle were fitted with a straight line. The stepwise increase is reproducible within ±15% when using the same batch of particles (c.f. Supporting Information). As given in the Table 2, the comparison of the slopes of these linear fits for the three different dithiols reveals that the frequency shift per cycle, and therefore the deposited mass, varies strongly. The smallest slope is observed for the longest linker molecule, C16, whereas the frequency change per cycle for C6 is about three times higher. This may be due to the lower density of the longer chain composites. Furthermore, due to enhanced steric hindrance of the longer molecules, this supports the assumption that an associative ligand-linker exchange mechanism occurs also for amine/thiol exchange similar to thiol/thiol exchange [30, 31]. In order to further elucidate the mechanism taking place during the individual injection steps, detailed views of the frequency shift over one cycle for each DT linker type is shown in Figure 4. It depicts the change of the resonance frequency of a quartz sensor due to adsorbed mass over the eighth deposition cycle. A detailed view of the assembly behavior with C6 as the linker molecules is shown (upper graph in Figure 4). It is clearly separated into two parts. After the addition of AuNPs, the resonance frequency decreases, that indicates an increase in deposited mass due to nanoparticle sorption. During the following rinsing and interlinking


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steps a slight increase in frequency is visible due to the ligand exchange occurring through the exposure of the immobilized nanoparticles to the linker solution. As the molar mass of the C6 is slightly lower than the molar mass of the exchanged DDA ligands on the particle surface (DDA 185.25 g/mol, C6 150.31 g/mol), a slight decrease in mass is detected. The same argumentation of the molar mass (C8 178.36 g/mol, C16 258.51 g/mol) applies in order to explain the almost constant frequency (Fig. 4 inset C8) and decrease of the frequency (Fig. 4 inset C16) for C8 and C16 respectively.

3.2. Layer-by-layer growth of the C8B- and C8A composites Since the product of ηl and ρl is larger for 2-propanol than for toluene, the solvent change from toluene to 2-propanol results in a stronger decrease in the measured frequency, as can be seen in Figure 5. This decrease is not the result of an increase in mass adsorbed on the quartz, but only of the different viscoelastic properties of the solvent, according to the second term in equation 2. Similar to the dithiol results, single measuring points at the end of each cycle can be fitted with a straight line, ignoring the strong effects related to solvent change. As given in Table 2, the comparison of the slopes of these linear fits for the three linkers with comparable alkyl chain lengths reveals that the frequency shift per cycle, and therefore the deposited mass, varies significantly. The largest slope has been observed for the C8B, the smallest for C8A and an intermediate slope for C8. This order reflects the strength of the bonds between the functional groups and the AuNP surface. While the amine/gold bond is considered as a weak covalent bond, the thiol/gold bond is stronger [31]. Many publications have shown that thiol molecules usually chemisorb to the surface of AuNPs through a strong S-Au bond (40 to 50 kcal mol-1), between sulfur atoms of the organic molecules that link the hydrocarbon chains and the metal surface [40–41]. This phenomenon depends neither on the AuNP surface structure nor on the aromatic or aliphatic character of


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the molecule backbone. The dithiocarbamate/gold bond shows an even higher thermal stability than the thiol/gold bond and can be regarded as the most stable.[42] This effect could be explained by a bidentate binding between the gold surface and C8B by its CS2- function (at both molecular ends). The evaluation of the assembly behavior of the AuNPs by ligand exchange of DDA against the C8A and C8B linkers are shown in detail in Figure 6. Both of the assembly reactions correlate to a negative frequency shift, or an increase in deposited mass, when depositing the nanoparticles as well as during the ligand linker exchange. The mass increase after the AuNP step is comparable to the dithiol experiments but a further frequency decrease after the addition of the linker was observed for the C8A and C8B. A possible explanation for this in case of the C8B is the significantly higher molar mass of the linker molecule compared to the DDA (C8B 324.61 g/mol, DDA 185.25 g/mol). However, the C8A has a smaller molar mass than DDA (C8A 144.26 g/mol) and therefore a simple linker exchange should lead to a positive frequency change. Thus, in the linker step physisorption of excess linker likely occurs. In the following rinsing step the mass decreases again, as shown in Figure 6, presumably indicating the removal of weakly bound amine linker molecules. Additionally in the literature, there are investigations that use molecules in the assembly solutions to molecular imprint the network.[43] The change in dissipation factors in the QCM experiments (c.f. Supporting Information) revealed that for short thiol linker as well as for the amine and bisdithiocarbamate based materials the incorporation of solvents or unbound linker molecule are more likely to occur, while for the materials with the longer thiol linker molecules the incorporation of foreign molecules in the network are less probable. Thus molecular imprinting should be easier for amine and bisdithiocarbamate interlinked network, while the former is less strong bound and the latter forms a less dense network due to the larger bidentate binding group.


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3.3. Thickness measurements and density calculations It would be desired to gain further information on the film thickness increase during film formation. However breaks in the assembly may change the interface chemistry and possibly lead to oxidation of the film,[7] thus only non-destructive in-situ measurements of the thickness during film assembly are favorable. A microfluidic cell for in-situ ellipsometry measurements during QCM experiments is commercially available, but the optical constants of the composite materials which are needed in the model to reveal the film thickness are not known. For that reason, the film thicknesses of all assembled composites were measured only mechanically using a profilometer at the end of the assembly process. Both, the increase in film thickness and density, ∆dρ, determined by QCM as well as the final film thickness, d, investigated by the profilometer are shown in Table 3. By taking into account the final thickness and number of performed assembly cycles and considering a particle size of 3.7 ± 0.5 nm as determined with TEM, it has been found that a sub-monolayer of particles has been deposited per cycle, in accordance with the literature.[15] From the measured thicknesses, the densities as well as the ratio of gold (ρAu = 19.3 g/cm3) of the composite materials were calculated. It can be clearly seen that the thickness of the C8B-composite is more than four times that of the film interlinked with C8. This may be due to longer molecular structure because of the larger functional groups of the C8B (thiol versus dithiocarbamate), as well as the higher assembly efficiency due to the bidentate binding. Another key observation is that the density of the composites decreases in the case of interlinking with longer molecules. Table 3 shows that the ratio of Au in the composites also decreases in agreement with the literature.[15] Even if the absolute values of the composition are strongly dependent on the particles size, this finding may be explained by a larger size of the functional groups (Figure 1) of the C8B


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that may lead to higher steric hindrance at the nanoparticle surface and thus a lower density of the composite.

3.4. Absorbance measurements on nanocomposite thin films Figure 7 shows the plasmon resonance bands of the AuNP films interlinked with C6, C8, C16, C8A and C8B.

Considering the usual broad plasmon resonance bands of

interlinked AuNP film, the maximum of wavelength for the DT linker assemblies is in quite good agreement with the literature [15]. The plasmon band position of C8 and C8A is virtually the same while the spectrum of the C8B exhibits a distinct blue-shift. This observation can be explained by the molecular structure of the linker molecules. The molecular length for C8 and C8A are comparable (11.9 and 11.2 Å, compare Table 1) while the C8B molecule is significantly longer (16.1 Å) owing to its spacious functional group. The longer molecule increases the distance between individual NPs which in turn decreases the particle-particle interaction resulting in the blue-shift of the surface plasmon band. The interactions between two nanoparticles can be described with a well-known dipoledipole interaction model [46, 47]: Irradiating light resonantly interacts with the confined electrons in the nanoparticles and charges the surface in a way that the particles can be considered as dipoles. Correspondingly, when two nanoparticles are situated close to each other, there is an attractive interaction between the electrical dipoles of the particles. The corresponding resonance frequency shifts to larger wavelengths. Accordingly, it has been shown that the resonance peak shift decreases with interparticle distance [23, 48–51]. The plasmon resonance bands of nanoparticles also depend on surface electron density [52–53]. The resonance peak wavelength is proportional to the electron density:

∆λmax = −

∆N 1− L λ ε+ εm 2N L


(3)

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Here, ∆λmax is the wavelength peak shift, N is the electron density of the AuNP, L is the AuNP shape factor, λ is the wavelength of incident light, ε and εm are the dielectric constants of the AuNP and surrounding media respectively. As proposed by von Wrochem et al. for dithiocarbamate gold bonds, the charge on the nanoparticle and the linking group of C8B delocalizes, enabling lower contact resistances compared to thiols [45]. Thus, we can assume that the electron density for the C8-/C8AAuNP is more localized at the particle, and the surface plasmon band (SPB) is located at higher wavelengths compared to C8B. The dependencies of surface plasmon peak positions and composite density from the linker length are depicted in the Figure 8. Both parameters change with the linker length in an exponential manner.

4. Summary and Conclusion This study was undertaken to examine the assembly behavior of AuNP composites interlinked with commercially available linkers using a QCM to obtain an in-situ insight on the film formation by ligand/linker exchange reactions. Namely alkyldithiols with different length are compared with alkyldiamines and alkylbisdithiocarbamates in order to study the effect of different linker groups and lengths on assembly behavior. For the layer-by-layer deposition and the in-situ investigation of the self-assembly of nanocomposites, a versatile automated microfluidic setup was presented. This setup allows the coating of planar substrates and to simultaneously follow the assembly behavior with an in-line QCM module. It was observed that during the layer-by-layer assembly the stepwise resonance frequency changes, while the largest frequency changes correspond always to the AuNP step. During the solvent rinsing and ligand/linker exchange reaction step, frequency remains


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almost constant with slight increases or decreases depending on the molar mass of the linker with respect to the molar mass of the exchanged ligand. The assembly efficiency (represented by the frequency shift per deposition cycle of the QCM) is higher for shorter molecules and for molecules with stronger interacting linking groups, resulting in best efficiencies for shortchain bisdithiocarbamate linkers. The calculated mass density of the composites reflects the higher ratio of organic material with respect to gold for longer organic linkers, as expected from the literature [15]. Another significant finding of this study is that the plasmon resonance bands of assemblies prepared from dithiols and diamines with comparable length of the aliphatic backbone are very similar, while the spectrum of the bisdithiocarbamate composite exhibits a distinct blue-shift. This observation was explained by the longer molecular structure of the linker due to a larger binding group in conjunction with a delocalization of particle charge on the organic molecule. It can be concluded, that the proposed method is useful to understand ligand/linker exchange processes by following the thin film layer-by-layer self-assembly of gold nanoparticle composites with QCM measurements. Furthermore, all results suggest that novel AuNP nanocomposites based on interlinkage with organic diamine and bisdithiocarbamate molecules could be very convenient in creating sensor-arrays for applications in numerous fields.


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Figures:

Figure 1: (Left) Overview of the different organic linker molecules used: 1,6-hexanedithiol (C6), 1,8-octanedithiol (C8), 1,16-hexadecanedithiol (C16), 1,8-octanediamine (C8A), and 1,8-octanebisdithiocarbamate (C8B). Colors correspond to the following elements: white – hydrogen, black – carbon, yellow – sulfur, and blue – nitrogen. Images were rendered using the Avogadro molecule editor [34]. (Right) A schematic explanation of the fluidic layer-bylayer self-assembly process, using the different linker molecules in the microfluidic quartz crystal microbalance cell. One 360° sequence corresponds to one assembly cycle.


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Figure 2. Scheme of the used automated setup for the nanocomposite preparation. A photograph of the setup can be found in the Supporting Information.


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Figure 3. Frequency change over time for in-situ QCM measurements of the layer-by-layer assembly of AuNP and DT linker nanocomposites. The large steps stem from the adsorption of AuNPs on the quartz substrate. A linear fit was performed on the last data point of each cycle to evaluate the linearity of the deposition and to compare the dependency of the frequency shifts on linker molecules.


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Figure 4. Detailed view of the frequency shift over time during the 8th deposition cycle for three dithiol linker molecules (C6, C8, C16). The respective injection steps for the AuNPs, the linker solution, and the toluene rinse are labeled. The insets magnify the curve progression during the linker injection and subsequent rinsing. The colors in the background


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indicate the liquid in the microfluidic cell (blue: toluene, yellow: gold nanoparticles in toluene, red: linker in toluene).

Figure 5. Frequency change over time for in-situ QCM measurements of the layer-by-layer assembly of AuNPs, C8A, and C8B linker nanocomposites. The large reversible shifts of the frequency are the result of the change of the solvent between the NP and linker deposition. A linear fit (black line) was performed on the last data point of each cycle to evaluate the linearity of the deposition and to compare the dependency of the frequency shifts on linker molecule.


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Langmuir

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Figure 6. Detailed view of the frequency shift over time during the 8th deposition cycle for the C8A and C8B linker molecules. The respective injection steps for the AuNPs, the linker solution, and the solvent rinse are labeled. The insets magnify the curve progression during the linker injection and subsequent rinsing. The colors in the background indicate the liquid in the microfluidic cell (light blue: toluene, dark blue 2-propanol, yellow: gold nanoparticles in toluene, red: linker in 2-propanol).

Figure 7. UV-Vis spectra of AuNP networks interlinked with 1,6-hexanedithiol (C6), 1,8octanedithiol (C8), 1,16-hexadecanedithiol (C16), 1,8-octanediamine (C8A), and 1,8octanebisdithiocarbamate (C8B).


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C6

15

C8B

C8A C8

C16

590

580

Density (g/cm3)

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10

570

560 5 550

Position of SPB peak (nm)

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540 0 8

10

12

14

16

18

20

22

Molecule length (Å) Figure 8. Density and SPB position of AuNP networks depending on the length of the interlinking molecules.


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Table 1. Characteristics of 1,6-hexanedithiol (C6), 1,8-octanedithiol (C8), 1,16hexadecanedithiol (C16), 1,8-octanediamine (C8A), and 1,8-octanebisdithiocarbamate (C8B) linker molecules. The lengths of the molecules were calculated by minimizing the energy in a molecular force field approach (MMFF94) using the Avogadro molecule editor. [31]

Linker

M, g/mol

Length, Å

ρ, g/cm3

C6

150.31

9.4

0.98

C8

178.36

11.9

0.97

C16

290.57

21.9

0.90

C8A

144.26

11.2

0.98

Used Solvent

toluene

2-propanol C8B

324.61

16.1


0.94

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Table 2. Comparison of the linear fits slopes (mean frequency decrease per cycle) for 1,6hexanedithiol (C6), 1,8-octanedithiol (C8), 1,16-hexadecanedithiol (C16), 1,8-octanediamine (C8A), and 1,8-octanebisdithiocarbamate (C8B) linker molecules. Linker

Mean frequency decrease per cycle

C6

-35.1 ± 5.2

C8

-31.3 ± 1.2

C16

-11.1 ± 4.1

C8A

-22.7 ± 2.5

C8B

-40.8 ± 2.3


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Table 3. Change in the product of thickness and density measured by QCM and thickness measured by a profilometer, resulting in calculated densities and Au contents of nanoparticle networks

interlinked

with

1,6-hexanedithiol

(C6),

1,8-octanedithiol

(C8),

1,16-

hexadecanedithiol (C16), 1,8-octanediamine (C8A), and 1,8-octanebisdithiocarbamate (C8B). ∆dρ, nm g/cm3

d, nm

ρ, g/cm3

Au V %

QCM

Profilometer

Calculated

Calculated

C6

261 ± 39

23.8 ± 2.4

11.0 ± 2.8

54.7 ± 13.7

C8

186 ± 30

19.2 ± 2.0

9.7 ± 2.6

47.0 ± 12.3

C16

84 ± 18

41.3 ± 4.2

2.0 ± 0.6

5.9 ± 1.9

C8A

144 ± 21

24.0 ± 2.4

6.0 ± 1.5

27.4 ± 6.9

C8B

253 ± 40

92.0 ± 9.2

2.8 ± 0.8

10.1 ± 2.7

Linker


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ACKNOWLEDGMENT The authors would like to thank A. Reichel for the AuNP synthesis, S. Rabe for the technical support in film deposition and C. Ashworth for proofreading. .

Corresponding Author * [email protected]

Present Addresses †* Present Address: Helmholtz-Zentrum Dresden-Rossendorf, Bautzner Landstraße 400, 01328 Dresden

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. REFERENCES (1) Shipway, A. N.; Katz, E.; Willner, I. Nanoparticle Arrays on Surfaces for electronic, optical, and sensor applications. Chem. Phy. Chem. 2000, 1, 18–52. (2) Saha, K.; Agasti, S. S.; Kim, C.; Li, X.; Rotello, V. M. Gold nanoparticles in chemical and biological sensing. Chemical reviews 2012, 112, 2739–2779, DOI: 10.1021/cr2001178. (3) Vossmeyer, T.; Stolte, C.; Ijeh, M.; Kornowski, A.; Weller, H. Networked GoldNanoparticle Coatings on Polyethylene: Charge Transport and Strain Sensitivity. Advanced Functional Materials 2008, 18, 1611–1616. (4) Shi, C.; Seok Choi, H.; Armani, A. M. Optical microcavities with a thiol-functionalized gold nanoparticle polymer thin film coating. Applied Physics Letters 2012, 100, 13305. (5) Qu, D.; Kim, B.-C.; Lee, C.-W. J.; Ito, M.; Noguchi, H.; Uosaki, K. 1,6-Hexanedithiol Self-Assembled Monolayers on Au(111) Investigated by Electrochemical, Spectroscopic, and Molecular Mechanics Methods. The Journal of Physical Chemistry C 2010, 114, 497–505. (6) Joseph, Y.; Krasteva, N.; Besnard, I.; Guse, B.; Rosenberger, M.; Wild, U.; KnopGericke, A.; Schlögl, R.; Krustev, R.; Yasuda, A. et al. Gold-nanoparticle/organic linker films: self-assembly, electronic and structural characterisation, composition and vapour sensitivity. Faraday Discussions 2004, 125, 77. (7) Joseph, Y.; Guse, B.; Nelles, G. Aging of 1,ω-Alkyldithiol Interlinked Au Nanoparticle Networks. Chemistry of Materials 2009, 21, 1670–1676.


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Table of Contents Graphic


Assembly Behavior of Organically Interlinked Gold Nanoparticle

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