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Morphology and Kinetics of Aggregation of Ag Nanoparticles Induced with Regioregular Cationic Polythiophene Samrana Kazim, Alessandro Jager, Miloš Steinhart, Jiri Pfleger, Jiri Vohlidal, Dmitrij Bondarev, and Petr Stepanek Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b03365 • Publication Date (Web): 02 Dec 2015 Downloaded from http://pubs.acs.org on December 7, 2015

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Morphology and Kinetics of Aggregation of Ag Nanoparticles Induced with Regioregular Cationic Polythiophene Samrana Kazim1†, Alessandro Jӓger1, Miloš Steinhart1, Jiří Pfleger1*, Jiří Vohlídal2, Dmitrij Bondarev2††, Petr Stěpánek1 1

Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, v.v.i., Heyrovsky Sq. 2, 162 06 Prague 6, Czech Republic

2

Charles University in Prague, Faculty of Science, Department of Physical and Macromolecular Chemistry, Hlavova 2030, 128 40 Prague 2, Czech Republic

Keywords: Conjugated polyelectrolyte,

silver nanoparticles, dynamic light scattering, TR-

SAXS, zeta potential

ABSTRACT:

The aggregation kinetics of negatively charged borate-stabilized Ag

nanoparticles (NPs) induced by the cationic regioregular polythiophene polyelectrolyte: poly{3[6-(1-methylimidazolium-3-yl)hexyl]thiophene-2,5-diyl

bromide},

PMHT-Br,

and

the

morphology of formed aggregates have been investigated by the UV-Vis spectroscopy, transmission electron microscopy, ζ–potential measurements, dynamic light scattering (DLS) and

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time-resolved small-angle X-ray scattering (SAXS). Two to three populations of NPs are formed within milliseconds upon mixing the components, which differ in the mean size, extent of polymer coating and time stability. These characteristics are primarily controlled by the PHMTBr to Ag-NPs ratio. Population of single NPs of the mean size about 5 nm is present in every system and is mostly long time stable. At the low ratios, the single NPs are most probably almost free of polymer chains and the second population includes slowly but in a limited extent growing NPs in which single NPs might be interconnected by polymer chains. At the ratios corresponding to the charge balance in the system (ca zero ζ–potential of NPs), the NPs aggregate forming the second population continuously growing in size, and finally undergo sedimentation. At the high ratios, three long-time stable populations of NPs are observed of the mean size of ca 5, 13 and 35 nm; all NPs should be fully coated with PHMT-Br giving them stabilizing positively charged shell.

INTRODUCTION Conjugated polyelectrolytes (CPEs) have received a great attention as perspective materials for electronic and optoelectronic devices such as the field effect transistors, bulk heterojunction photovoltaic cells, and light emitting diodes.1, 2 In addition to the unique electrical and optical properties based on the combination of π-conjugated system and pendant ionic groups they namely exhibit a good solubility in “green” solvents including water, which gives them high processing advantages and makes them attractive for sensing in biological environment.2, 3 Due to their large optical cross-section and strong fluorescence CPEs can be used as effective fluorophores for chemical or biological sensors.4-6 They can be also used as materials for the


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layer-by-layer building of optically and electrically responsive three-dimensional structures,7 providing binding sites for metal ions, ionic chromophores or metal nanoparticles.8-11 Comparably intensive research takes place in the area of metal nanoparticles (NPs) and their aggregates, which is driven by their unique optical, catalytic and electronic properties12 and targeted towards a wide range of chemical, medical, and biological applications.1,

13, 14

As

functional properties of these systems strongly depend on their morphology,15-18 understanding and controlling the process of aggregation of NPs is of high interest. The aggregation is controlled, e.g., by the adsorbate chemical nature and concentration16,19 charge density,20 pH of the surroundings21 or the presence of a pre-aggregating agent.22 In as prepared sols, metal NPs are mostly stabilized electrostatically by positive or negative surface charges and their aggregation can be initiated by ionic or nonionic adsorbates that are able to disturb the stabilizing charged surface layer. Here we focus on the aggregation induced by ionic adsorbates of the polyelectrolyte class.14 Any polyelectrolyte added into a sol of electrostatically stabilized NPs affects the original thermodynamic equilibrium between aqueous phase and diffuse layers of NPs.23,

24

When

polyelectrolyte chains are charged oppositely to NPs or form complexes with metal ions constituting the NPs, the chains can readily coat NPs and a fast aggregation usually takes place.25 The effective range of electrostatic interactions in aqueous solutions is generally low, up to 10 nm. Interparticle bridging by polyelectrolyte chains can also destabilize a sol of NPs.26 The destabilizing effect of a polyelectrolyte depends on its molar mass, charge density and concentration in the sol, concentration and surface charge of NPs, and the ionic strength, pH and temperature of the sol.25-27,28, 29 The curvature radius of NPs could be also important.30


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Aggregates of Ag- and Au- NPs are known to contain “hot spots” where the optical field is extremely enhanced giving rise to a strong enhancement of the Raman scattering intensity and related photophysical phenomena31. Such aggregate with a conjugated chromophore can act as an antenna concentrating optical field to photoactive molecules, which could increase the sensitivity of SERS probes and enhance or bring better functionality of artificial photosynthetic systems.32 π-Conjugated polyelectrolytes (CPEs) can facilitate interactions between electrons of metal NPs and π-electrons of surrounding conjugated organic system.33 Mutual interactions between surface plasmons of NPs and π-electrons of conjugated chains adsorbed on or located in close proximity of the NPs surfaces can affect electronic processes in the chains, as it was shown for assemblies of Au- and Ag-NPs with the cationic polythiophene polyelectrolyte: poly{3-[6-(1methylimidazolium-3-yl)hexyl]thiophene-2,5-diyl bromide}, PMHT-Br.34,

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The properties of

the systems, such as optical absorption, Raman scattering and temperature dependence of electrical conductivity, were found to be strongly influenced by the present NPs, particularly, by the extent of their aggregation. However, a course of this aggregation has not been studied and thus still remains unclear. Nanoparticle aggregates in hydrosol form complex two-phase (water– solid) materials with 3D structures, which is difficult to visualize directly by conventional electron or X-ray imaging methods. However, nowadays the transient small angle X-ray scattering (SAXS) method is available and, according to literature,17,

18

applicable for in situ

investigation of such systems. The time resolved SAXS technique is commonly used to extract information on molecular architecture, size and shape of nano-objects in solution. This technique is sensitive to scattering density inhomogeneities in the range from 1 to 100 nm, just in the range of sizes of metal nanoparticles in hydrosols. Recently, it has been used for the in-situ studies of formation of metal NPs detecting their size and volume content, for monitoring the nucleation


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and growth processes of metal nanoparticles and their aggregates formation, and for observations of the effect of the different capping agents on metal nanoparticles growth.36-42 However, to our best knowledge, similar time resolved SAXS studies on metal nanoparticles using CPEs seem to be scarce. The present study aims to approach the process of formation of the Ag-NPs aggregates facilitated by the PMHT-Br polycations and its relation to the optical properties of the formed sol. The influence of the size distribution of the Ag-NPs aggregates as a function of the cationic polymer concentration was studied in detail by the UV-Vis spectroscopy, ζ–potential measurements, dynamic light scattering (DLS), transmission electron microscopy (TEM) and the kinetics of aggregation by the time-resolved small-angle X-ray scattering (TR-SAXS).

EXPERIMENTAL SECTION Materials. Analytical grade solvents as well as chemicals supplied by Merck and Aldrich were used. Water of the specific resistance 18 MΩcm obtained using a Milli-Q-plus system (M/S Millipore Corporation, USA) was twice distilled in a quartz apparatus before being used in the preparation of metal colloids and all aqueous solutions. All glassware was thoroughly cleaned and rinsed with deionized water and dried prior to use. Regioregular poly{3-[6-(1methylimidazolium-3-yl)hexyl]thiophene-2,5-diyl bromide}, PMHT-Br (for formula see chart 1) has been prepared using the procedure described in ref.43 Characteristics of the used PMHT-Br sample were as follows: regioregularity 92 %; number- and weight-average molar masses: Mn = 6500, Mw = 9100 based on polystyrene standards.


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Chart 1: Chemical structure of cationic polythiophene polyelecrolyte, poly{3-[6-(1methylimidazolium-3-yl)hexyl]-thiophene-2,5-diyl bromide}, PMHT-Br.

Synthesis of Ag colloid. Borate-stabilized Ag-NPs colloids were prepared by reduction of aqueous AgNO3 solution with NaBH4 in deionized and double-distilled water using slightly modified method described in refs.44 Briefly, 7 mg of NaBH4 was dissolved in water (75 ml) and cooled down to 2 ºC. Then an aqueous solution of AgNO3 (9 mL, 2.2×10-3 M) cooled to ~ 2 ºC was dropwise added to the NaBH4 solution under vigorous stirring, which was continued for next 45 min. A yellow transparent Ag hydrosol was obtained with the Ag content 25 mg/l, which showed the maximum optical extinction at 400 nm. The hydrosol was allowed to ripen for several weeks; its ripening was monitored by recording its optical absorption spectrum.

Mixing protocol for Ag-NPs/PMHT-Br assemblies. The Ag-NPs/PMHT-Br sols were prepared at room temperature by adding measured volumes (using a Hamilton microsyringe) of DMSO and a stock solution of PMHT-Br in DMSO (cpol = 1×10-2 M based on monomeric units) into a ripe Ag hydrosol (2 mL) to obtain a series of sols with cpol values equal to: 5.0×10-7 M, 1.0×10-6 M, 2.5×10-6 M, 5.0×10-6 M, 1.0×10-5 M, and 1.0×10-4 M, respectively, and a constant fraction of DMSO. DMSO was used for the stock solution because PMHT-Br seems to be molecularly dissolved in this solvent, while in aqueous solution of cpol = 1×10-2 M it forms associates that undergo a slow dissociation upon dilution, which lasts up to one week.43


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Instrumentation. UV-Vis optical absorption spectra were recorded in the spectral range 200 – 850 nm using a double beam Perkin Elmer Lambda 950 UV-Vis spectrometer. The samples were measured in a 1 cm quartz cuvette. Transmission electron microscopy (TEM) images were recorded using a JEOL JEM 200 CX transmission electron microscope operating at 100 kV with the point-to-point resolution of 0.2 nm. TEM samples of Ag-NPs clusters without and with adsorbed polymer (PMHT-Br) were prepared as reported previously.35 Small drop of the freshly prepared hydrosol was transferred onto a copper-mesh grid covered by a carbon foil and left to dry in air. The NPs size analyses were done using the ImageJ software. Dynamic light scattering experiments (DLS) and zeta potential measurements were performed on a Zetasizer Nano ZS instrument (Malvern Instruments, U.K.) at constant temperature of 25 ± 1 °C. The hydrodynamic radii (RH) of Ag-NPs were determined from the intensity correlation function g2 (t) converted to the distribution of relaxation times, τ, and further to the size distributions using the Stokes−Einstein equation (eq 1).

RH =

k BTq 2 τ 6πη

(1)

where kB is the Boltzmann constant, T absolute temperature, η the solvent viscosity, τ the relaxation time related to the diffusion movement of NPs, and q the magnitude of the scattering vector given by θ=

4πnλ

λ

θ  sin  2

(2)

where nλ is the refractive index of the solvent (nλ = 1.33 for water), λ the wavelength of the incident beam (λ = 633 nm), and q the scattering angle (θ = 173°). The intensity distribution was


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recalculated to the size distribution using the values nAg = 0.13 and kAg = 3.9 for the refractive index and absorption coefficient of silver NPs, respectively. The average zeta potential (ζ) of NPs was determined by measuring their electrophoretic mobility (UE) that was converted to ζ-potential through the Henry equation:

ζ =

3ηU E 2εf (ka)

(3)

where ε is the relative permittivity of the medium and f(ka) the Henry function that was set to be 1.5 within the Smoluchowski approximation. For the DLS and ζ-potential measurements, the Ag hydrosol and PMHT-Br solution were filtered through a PVDF and PTFE syringe filter (porosity of 0.20 µm), respectively. The filtered sol and solution were mixed directly in a cuvette just before the start of an experiment, each lasting 60 min. The equilibration time was set to 10 min to stabilize the sample temperature (25 °C) before the first run. The actual measurement consisted of a series of consecutive runs, each lasting 2 min. For determining the particle size distribution of Ag-NPs in the pure hydrosol, only three measurements were done, each taking 60 sec, and the equilibration time was set to 60 sec. Time-Resolved Small-Angle X-ray Scattering experiments and data analyses. Kinetics of the interaction between Ag-NPs and PMHT-Br was measured using the SAXS beam-line of the synchrotron Elettra in Trieste, Italy. The computer-controlled Bio-LOGIC stopped flow apparatus (SMF-400) was employed for mixing. It comprises four pneumatically driven feed syringes, which were kept at 25.0 ± 0.1◦C by means of a water bath circulator. The solutions to be mixed (Ag hydrosol and PMHT-Br polymer, respectively) were driven using two syringes through a mixer into a quartz observation capillary. The mixtures were prepared in advance to yield resulting molar concentrations of PMHT-Br in the sols corresponding to 5.0×10-7 M,


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2.5×10-6 M and 1.0×10-4 M, respectively. The primary radiation wavelength was 0.155 nm (8keV). A Pilatus100K detector was positioned to probe the q-range from 0.05 to 5 nm-1 in order to reach the resolution of 1 – 140 nm in the real-space, which corresponds to the expected range of the NPs sizes. The acquisition times were 19 ms for the first 499 time frames, 994 ms for the next 500 frames and 10 s for the last 24 frames. Data transfer time after each frame was 6 ms. Data treatment should always begin with the model-independent analysis (MIA), which yields general parameters introduced already by Guinier, Kratky and Porod.45 In the case of timeresolved measurements, the MIA should be performed if possible even during the experiment (beam time). It can validate the experiment and may indicate that changes in the experimental parameters are required. The advantage of MIA is that it typically uses only experimental data and no or only minimum of presumptions. Even in the course of sophisticated analysis MIA helps to find an appropriate model. More detailed parameters can be obtained using modeldependent analysis (MDA) performed on the basis of a properly selected structural model that can fit the experimental data of sufficiently high quality. The analysis in this case is affected by the assumptions imposed by the selected model. The model must be built using all available information obtained from other techniques. Obtaining an excellent quality of the fit of the SAXS data itself is just necessary but not sufficient proof that the used model is appropriate. In a favorable case the scattering from nanostructures shows three different regions of the dependence of the scattering intensity, I(q), on the magnitude of the scattering vector q defined by the equation (2), in which the refraction index nλ =1 for X-rays. In the low q-range, i.e. in the ‘Guinier region’ (qRg < 1), the scattering dependence has the Gaussian shape, from which the values of the radius of gyration and of the forward intensity (the intensity extrapolated to zero angle), can be evaluated. Intermediate q-range, where often I(q) ~ q-α, is sensitive to the overall


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shape and surface of the scattering objects. For well-defined, i.e., sharp and smooth surfaces between the phases, value of α = 4 stands for globular particles, α = 2 for lamellar and α = 1 for rod-like structures. For the systems of Gaussian chains the slope is α = 2. However, if the condition of well-defined interphase is not fulfilled, the experiment often shows a non-integer α value in the intermediate q-range. Such behavior can also be the sign for partial scale symmetry in the system, so called fractal structure. In the high q-values range, local stiffness of macromolecules (due to shorter length scales probed) can be revealed with I(q) ~ q-1. MIA was based on Guinier analysis and on the calculation of zeroth (integral intensity), first and second moments of the scattering dependence. Using these data we evaluated parameters such as Porod invariant or correlation length. It must be stressed, however, that particularly in the case of MIA performed on-site during the measurements the resulting parameters may not have their proper meaning since there is no time or all the necessary information available during the experiment to subtract the background signal properly or perform necessary extrapolations. These parameters still can give us rough estimates and indicate probable changes in the studied system. For the model dependent analysis we used the program SASfit to test various models and to find good set of starting parameters. Then mass-fitting was performed using our own software written in Matlab. The best model was based on up to three populations of spheres with log-normal distribution. Intensity of a sphere of radius R and excess scattering density ∆η is usually written as

I sph (q; R, ∆h ) = ( 43 pR 3 ∆h ) 2 [3

sin( qR) − qR cos(qR) 2 ] (qR) 3


(4).

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Probability that a sphere from a set with Log-normal distribution has radius R is

f ( R; m, s ) =

1 sR 2p

exp{− 12 [

ln( mR ) 2 ]} s

(5).

Here m is the location parameter coincidently equal to the median of the distribution and s is the width parameter. Other statistical parameters such as moments of the distribution can be obtained using m and s. It proved efficient to start fitting with the best data in the last frame of each experiment and then going backwards to shorter time and use the final parameters of the previous fit as starting parameters for the next one.

RESULTS AND DISCUSSION TEM and UV-visible extinction spectra of borate-stabilized AgNPs. For the present study, small-size AgNPs with the diameter from 3 to 4 nm were intentionally chosen to see how PMHT-Br added in various concentration affects assemblies of these small NPs and to compare these results with those previously published35 on the system of aggregated Ag-NPs having ca twofold diameter. The desired decrease in the mean diameter of Ag-NPs was achieved by increasing the amount of reducing agent NaBH4 and reducing the reaction temperature.46 The prepared borate-stabilized hydrosols of Ag-NPs exhibit an intense extinction band centered at λSPE1 ~ 400 nm (Figure 1a), which results from the resonance excitation of the dipolar surface plasmon band of isolated spherical Ag-NPs in water.47 The mean diameter of these Ag-NPs was obtained through the Gaussian fits of size distribution histograms determined by the TEM image analysis and dynamic light scattering (DLS) data (see Figure 1b). The mean size of Ag-NPs was determined to be 3.7 ± 2.3 nm (by TEM) and 3.2 ± 0.4 nm (DLS).


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Figure 1. (a): UV-Vis absorption spectrum of Ag hydrosol and TEM image of Ag-NPs. (b) Size distribution histograms of the number distribution of Ag NPs in pure Ag hydrosol as obtained from TEM image analysis (blue, dashed line) and from DLS measurement (black, full line).

Surprisingly, DLS provided smaller mean size and narrower size distribution compared to the TEM analysis. However, direct comparison of the size distributions obtained by DLS and TEM is reliable only in the case of monomodal narrow distribution. The intensity size distribution acquired by the DLS shows a bimodal distribution for the AgNPs (see Figure 5a). In order to better compare the results of both methods the intensity distribution was transformed to the number size distribution, which is the result of TEM image analysis. The conversion from intensity to number distribution emphasizes the smaller particles in the distribution by DLS. Therefore the width becomes narrow and the particles size distribution is shifted to lower values. Moreover, the number size distribution obtained by TEM was obtained by the analysis of several TEM images, some of them showing bigger nanoparticles, which may occur due to accidental aggregation and hydrosol ripening during the sample preparation. Since the processes under study proceed in solution, DLS method seems to us more appropriate for the determination of the size distribution.


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Influence of the PMHT-Br concentration on the surface plasmon band of assembled Ag-NPs and stability of the colloidal system. Changes in the UV-Vis extinction spectra of the aqueous Ag-NPs colloid caused by the added PMHT-Br polyelectrolyte are shown in Figure 2. As can be seen, the surface plasmon extinction (SPE) band of pure Ag-NPs at λSPE1 = 400 nm (Fig. 2, curve a) attenuates and broadens with increasing PMHT-Br concentration (cpol) in the system (Fig. 2, curves b, c). These changes of the SPE band can be attributed to the formation of very small and stable aggregates of Ag-NPs, since similar UV-Vis spectral behavior was observed for deliberately prepared dimers and trimers of colloidal Ag NPs upon adsorption of PMHT-Br.13,

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It should be stressed that, at these low

polymer concentrations, the composite sol is stable without any visible precipitation of Ag-NPs even for several weeks after mixing. When cpol is increased to 2.5 x 10-6 M, which can be assigned as a critical concentration, the intensity of the original SPE band drops to less than half of its original value and the shoulder develops to a longer wavelengths second SPE band at λSPE2 about 505 nm (Fig. 2, curve d). It was shown in our earlier article35 that the appearance of the second SPE band is related to the presence of large aggregates of NPs in the system and that these large aggregates strongly enhance optical field as confirmed by intensive surface-enhanced Raman scattering (SERS). In accord with the presence of large aggregates, the sol of the critical cpol is unstable showing a clear tendency to precipitate (see Figures S1 and S2 in the Supporting Information). Further increase in cpol above the critical value results in (i) disappearance of the second SPE band, (ii) an increase in the intensity and overall broadening of the extinction band (Fig. 2, curves e, f, g), and (iii) restoring the stability of the composite sol. The extinction band observed for these systems has to be regarded as an overlap of the SPE band of Ag-NPS and the


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absorption band of PMHT-Br, the intensity of which becomes comparable to that of the SPE band at these concentrations. A time stability of composite sols was also investigated more in detail (see Figs. S1 and S2 in the Supporting Information). All sols were found to be long time stable except for that with the critical

Cpol

= 2.5×10-6 M, the extinction of which decreased to a half within two hours due to

precipitation. At low cpol values (5×10-8 to 5×10-7 M) only a small part of the negative surface charge of Ag-NPs is compensated by the cationic PMHT-Br chains, which is not enough to promote significant formation of large aggregates. At around the critical cpol, polycationic chains compensate the surface charge of Ag-NPs thus promoting their aggregation to large particles, the Brownian motion of which already will not hold them floating. At cpol above the critical value, Ag-NPs should be coated with polycations, thus having a positive surface charge preventing them from aggregation (see ζ-potential measurements). The aggregation ability of NPs can be correlated with the polymer concentration and overall surface of NPs: At the critical cpol = 2.5 × 10-6 M the polymer should cover about 1/3 of the NPs surface, which suggests possibility of the nearest contact of two Ag NPs through a single polymer chain.


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Figure 2. Extinction spectra of the parent Ag colloid (curve a), and composite Ag-NPs sols with different overall concentrations of PMHT-Br: 5×10-7 M (curve b), 1×10-6 M (curve c), 2.5×10-6 M (curve d), 5×10-6 M (curve e), 1.0×10-5 M (curve f), 1.0×10-4 M (curve g).

Zeta potential measurements. The dependence of ζ-potential on the PMHT-Br concentration is shown in Figure 3. The ζpotential determined for pure Ag-NPs hydrosol was -41±4 mV showing a good electrostatic stabilization of the sol. This colloid can be assumed to remain in principal non-aggregated when no positively charged species are added. As can be seen, the ζ-potential increased upon addition of the cationic PMHT-Br reaching positive values at highest cpol, passing zero at the critical concentration corresponding to the concentration at which the long-wavelength SPE band reached its maximum intensity.


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40 30

Zeta potential, ζ (mV)

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~~

20 10 0 -10 -20 -30 -40 -50 0

1

2

3

4

5

20

40

60

80

100

Concentration of PMHT-Br,Cpol (µM)

Figure 3. Zeta potential of the Ag NPs and Ag-NPs/PMHT-Br sol system as a function of PMHT-Br concentration. At critical concentration,

Cpol=

2.5 × 10-6 M the zeta potential

approaches zero.

At the polymer concentration at which the isoelectric point is reached, each Ag NP might contain maximum of 11-17 monomer unit (depending on average diameter dTEM and dLS respectively and suppose that all monomeric units are adsorbed on the Ag-NPs surface; see Supporting Information for calculation). At PMHT-Br concentrations higher than the critical one, the positive sign of ζ-potential implies that Ag NPs are coated by several polycationic


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chains, which minimizes the NPs aggregation. Further addition of the PMHT-Br increases the zeta potential above +30 mV.

Morphology of pure PMHT-Br solutions. Optical absorption and fluorescence spectra of PMHT-Br were found to be solvent and/or time dependent due to the presence of slowly dissociating aggregates,43 which is typical of aqueous solutions of conjugated polyelectrolytes.49-58 A polymer similar to PHMT-Br, polythiophene with ammonium side groups, was recently reported59 to form aggregates in a mixed DMSO/water solvent, the size of whose depends on the volume ratio of the solvents. In order to minimize the presence of large aggregates the measured sols were prepared by adding a polymer stock solution in pure DMSO. The DMSO/water ratio in the aggregating sol was kept constant by adding a corresponding volume of pure DMSO to a sol of cpol < 1 x 10-4 (the highest used cpol). The results of characterization of pure PMHT-Br in DMSO/water mixture (1:100, v/v) in the concentration range from 5×10-7 to 1×10-4 M by the DLS technique are shown in Figure 4. The DLS data indicate the presence of aggregates even in the most dilute solutions (5×10-7 M), which is in a good agreement with the recent observations on sols of polythiophene with ammonium side groups in water, DMSO and mixed DMSO/water solvents.59 In water, the authors found the most populated aggregates of the size of 160 to 430 nm (by DLS) resulting from strong π-π interchain interactions as it was seen from NMR spectra. The size of the aggregates decreased with increasing DMSO content and decreasing cpol. In pure DMSO, smaller aggregates (DLS) of flexible random coil chains were observed, as indicated also by NMR. In order to examine the presence of π-π stacked PMHT-Br chains in our sols, the concentration dependence of the UV-Vis spectra of PMHT-Br sols in DMSO/water mixture (1:100,v/v) was measured. The spectra normalized to cpol were identical, not indicating the presence of π-π


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stacked aggregates of polymer chains even at the highest used cpol. Thus it can be concluded that the overwhelming majority of observed aggregates is composed of random-coil polymer chains, which suggests that the electrostatic and hydrophobic interactions play dominant role in the aggregation rather than the π −π interchain interactions.

Figure 4. Average intensity distribution in the equal area representation, RhA(Rh), of the hydrodynamic radius (Rh) of PMHT-Br aggregates in DMSO/water (1:100 v/v) sol as a function of the polymer concentration. (Note that a real number fraction of large particles in sols is rather low since their contribution to the overall scattering intensity is incomparably higher than contribution of the smaller particles).

Compared to linear polyelectrolytes with flexible chains, characterization of solutions of conjugated polyelectrolytes with stiffer chains is always a challenging problem as their chains tend to aggregate in solutions, and also for other reasons than the presence of long-range


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electrostatic interactions.50-53, 60-64 Large aggregates can easily exist in dilute aqueous solutions, even in the presence of an added salt that increases the ionic strength and thus shields the longrange electrostatic interactions.62 As can be seen from Fig. 4, aggregates are present even in very diluted polymer solutions (5×10-7 M), where the counter ion concentration is very low, electrostatic repulsion between polycationic chains is stronger and thus the chains should aggregate to a lesser extent. No aggregates were observed for linear polyelectrolytes with flexible saturated chain backbones in aqueous solutions of similar low concentrations by the DLS.65 Aggregation of PMHT-Br in very dilute aqueous solutions strongly indicates importance of the hydrophobic interactions between polymer chains. Dobrynin and Rubinstein described such behavior by a scaling model for the dilute solution conformation of a uniformly charged polymer in a poor solvent.66 Authors concluded that a polymer backbone in a poor solvent, such as a conjugated backbone in water, tends to collapse to spherical globules. Upon increasing the globule charge, e.g., by increasing ionization of the side chains, a critical value can be reached, above which the globule splits to several smaller ones connected by linear segments. The semidilute solutions show even more complex behavior since an avalanche condensation of counter ions on polyionic chains can lead to a phase separation of the solution into the dilute and concentrated phases.67 This obviously happens in medium concentrated aqueous PMHT-Br solutions where the increased number of aggregates should be a result of both electrostatic and hydrophobic interactions.

TEM and DLS morphology of Ag NPs / PMHT-Br systems Possibility of tuning the extent of aggregation of Ag-NPs from the isolated ones to large aggregates by the amount of PMHT-Br added to Ag-NPs sol was further confirmed by the TEM


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imaging (Figures 5). A pure silver hydrosols contain mainly individual Ag-NPs of the average diameter, dTEM ~ 3.7 ± 2.3 nm (Fig. 5 a’). For the cpol range from 5×10-7 M to 1×10-6 M, small and stable aggregates are observed (Figs 5 b’, c’), which in turn are replaced by larger aggregates at the critical concentration of PMHT-Br, cpol = 2.5×10-6 M (Fig. 5 d’). The large aggregates consist of randomly distributed, most probably single-layer polymer coated Ag-NPs. The systems of cpol > 2.5×10-6 M (Figs 5 e’, f’) show only individual Ag-NPs and their small aggregates well separated by PMHT-Br multilayers. This demonstrates feasibility of tuning the inter particle distance by optimizing the polymer concentration. For DLS data on Ag NPs/PMHT-Br systems to be evaluated correctly, the contribution of non-adsorbed PMHT-Br should be known. The DLS signals of pure PMHT-Br and Ag-NPs + PMHT-Br systems are compared in Figure S3 in the Supporting Information, which proves that the contribution of PMHT-Br is negligible regardless the cpol value. Therefore, the polymer chains shall not be seen as a population in the correlation distribution function and the timeaveraged hydrodynamic radius (RH) distribution shall be related exclusively to the pure AgNPs or AgNPs/PMHT-Br aggregates present in solution. The time evolution of RH during interaction of Ag-NPs with PMHT-Br at different cpol (Figure S4, Supporting Information) indicates that the aggregation process is very fast and actually completed before the DLS measurements is started, except for the system with critical


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Figure 5. Average intensity-weighted distributions of hydrodynamic radius (Rh) (a-f) and corresponding TEM images (a‘-f‘). (a) Pure hydrosol of AgNPs, CAgNP =2.3 × 10-7 M (according to dLS) and AgNPs/PMHT-Br sols with PMHT-Br concentrations: (b) 5.0 × 10-7 M, (c) 1.0 × 10-6 M, (d) 2.5 ×10-6 M, (e) 1.0 × 10-5 M and (f) 1.0 × 10-4 M.


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cpol = 2.5 ×10-6 M, in which the size of bigger aggregates steadily increases up to RH ~ 1 µm, which results in their visible macroscopic sedimentation. The DLS intensity-weighted representations of the size distribution of NPs in AgNPs/PMHTBr systems of different cpol are shown in Figures 5a-f. For pure AgNPs (Figure 5a), the intensity distribution clearly shows two populations centered: (i) at RH = 1.6 nm that essentially represents isolated Ag NPs, and (ii) at RH = 13 nm that belongs to aggregates of a few Ag-NPs; Note that the intensity of the light scattered by spherical particles is proportional to the sixth power of their radius: Isc ~ R6 ! Thus the intensity contribution of a sphere of RH = 13 nm is ca 3 x 105 times higher than that of a sphere of the same nature of RH = 1.6 nm. When the intensity-weighted size distribution is converted to the number-weighted one, the fraction of AgNPs aggregates with RH = 13 nm is actually very small (see Fig. S8 in the Supporting Information). For very low PMHTBr/AgNPs ratios (cpol from 5×10-7 to 1×10-6 M, Figures 5b,c), the DLS intensity distribution exhibits two small peaks at RH ~ 1.6 nm and RH ~ 10 nm and a prominent peak centered at RH ~ 40 nm that can be assigned to small aggregates of Ag-NPs partly coated with PMHT-Br chains. For the critical cpol = 2.5×10-6 M, the DLS distribution is dominated by a peak centered at RH ~ 400 nm belonging to large aggregates that continuously grow till they achieve a micrometer range and get precipitated (Figure 5d). At this cpol, single Ag NPs are absent but small populations of aggregates with RH ~ 6 nm and 50 nm are observed. For cpol from 5×10-6 to 1×10-4 M (Fig. 5e, 5f), smaller polymer-coated aggregates of NPs and isolated Ag-NPs, are observed together with surplus polymer (see TEM images). These data are in accord with the above mentioned broadening of the SPE band observed for systems of higher cpol.


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Kinetics of the Ag-NPs aggregation induced by PMHT-Br: time-resolved SAXS measurements. Kinetic measurements in general address collective changes in a system rather than its precise structure details. The only possibility for time-resolved measurements is to make a compromise between the data quality and time-resolution. Due to main interest in the initial kinetics (at the earliest times after mixing polymer solution with Ag-hydrosol) we have chosen 25 ms frames (consisting of 19 ms and 6 ms of the data transfer time) for the first 500 frames of the measurement. Consequently, data were very noisy within the earliest time period in the beginning of the experiment. In order to gain maximum information from these data we first performed a model-independent analysis (MIA). We employed the Guinier analysis, calculated the first three moments of the scattering curves, evaluated corresponding parameters and looked at their time development. We are aware that the exact structural meaning of these parameters is not clear. One of the reasons is that the moments are calculated only from the experimental data without extrapolation to zero and infinity. But it is well proved that these pseudo-parameters often serve as good estimates and their change in the course of the experiment strongly indicates that there is a real change running in the system. And vice-versa, if none of the pseudoparameters changes visibly in time, it is certain that there is no change in the system, or the experiment is not sufficiently sensitive to see it due to, e.g., wrong probing time or angle region. As an illustration of the MIA approach, the time dependences of Rg for three PMHT-Br/AgNPs systems differing in cpol (at constant concentration of Ag-NPs) are shown in Figure 6. Very similar behavior was also observed for the integral intensity, Porod's invariant Q, or correlation length. To smooth noisy data at the start of the measurements we used moving time average. We


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averaged a group of 1-5 frames into a single frame without a substantial effect on the slope of the dependences. The illustration of this approach is in Figure S7 in the Supporting Information. Comparing these results and taking into account those from other techniques we consider that the time course of Rg at the very beginning of the experiment is real and it shows the decay of primary particles probably created at times below the time resolution of our measurements. A chaotic initial stage of mixing can be explained by the interfacial turbulences and “diffusionstranding” processes involving two not equilibrated liquid phases during the mixing process.68 The turbulences generate collisions that cause desintegration and merging of the primary assemblies formed during the first contacts of PMHT-Br chains and Ag-NPs. Once turbulences subside, the time dependence of Rg values of aggregates becomes evident. Shorter accumulation time, however, also contributes to the noisy signal at the initial stage of the experiment. The semilogarithmic (Rg vs time) dependences displayed in Fig. 6 show a medium fast increase of Rg for the lowest cpol = 5.0 x 10-7, rapid increase for critical cpol= 2.5 x 10-6 and practically no increase for the highest cpol = 1.0 x 10-4. These results are in a good agreement with the results of the DLS, ζ-potential and UV-Vis spectroscopic measurements. No increase observed for the latter system indicates fast coverage of AgNPs and their small aggregates by a large amount (probably several layers) of PMHT-Br. The adsorbed polycations switch the charge of NPs from negative into the positive one (+40 mV; see chapter ζ-potential) which prevents aggregation of so coated composite NPs. Analyses of the early SAXS frames of composite systems strongly suggested the presence of significant amounts of single and low aggregated Ag-NPs in the systems (Fig. S5, Supporting Information). Static measurements done on pure Ag-sol provided a log-normal distribution of Ag-NPs with parameters: Rg ~ 2.6 nm, δ = 0.55. The next step was the model-dependent analysis


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(MDA), i.e. finding an appropriate model that could fit well the experimental data, giving us deeper insight into the changes of the system during the time course of the measurements and, at the same time, yielding parameters with a reasonable physical meaning. Similarly as in the previously published paper18 we used the program SASfit for selecting a proper model. From the rich offer of SASfit we tested several models that could describe the quite different character of the system during the measurements. We tested e.g. the Beaucage cutoff, or combination of various distributions of spheres or ellipsoids with fractal decay. Though these fits were often very good, it was difficult to understand the meaning of the obtained parameters and their changes in relation to our system. As a reasonable compromise we finally selected that model that included two or three populations (distributions) of spheres with a scattering background. Based on this model we show in the Figure 7 the development of the mean sizes of the populations. The fits of the time evolution of frames revealed a set of distinct Ag-NPs populations occurring in each system, as well as their cpol-dependent development (Figure 7). The smallest observed population of Ag-NPs with Rg ~ 3 nm (blue circles, Fig. 7) remains unchanged during the whole experiment in all systems, and it can be identified as the single Ag-NPs either low or completely coated with PMHT-Br chains. The second observed population of NPs shows a cpol-dependent behavior similar as that revealed by the Guinier MIA analysis (Fig. 6). A reliable growth of aggregates can be identified after at least five seconds of the experiment (Figure 7) for the above mentioned reasons. This population of Ag-NPs grows in size from ca 20 to 30 nm for cpol = 5 x 10-7 M and from ca 20 to 36 nm for cpol = 2.5×10-6 M, within ten minutes. For the system with cpol = 1 x 10-4 M, this NPs


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Figure 7. Time development of R for the population of (○) small and (○) large Ag-NPs and for the (○) Ag-NPs aggregates fitted by log-normal distribution of polydisperse spheres at PMHT-Br concentrations of (a) 5.0 × 10-7 M, (b) 2.5 × 10-6 M and (c) 1.0 × 10-4 M. The slope is represented by a red line.


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population shows a broad distribution (Rg ≅ 20 – 45 nm) without any systematic growth. The observed differences in the slopes time dependences are not as high as those obtained from Guinier analysis of SAXS data, but qualitatively equal. A fit of the data for the system (cpol = 1.0×10-4 M, Fig. 7c) revealed the significant population of NPs with R ~ 12 nm, which might be related to NPs with RH = 13 nm detected by the DLS method (Fig. 5). The exclusive presence of this population in the most concentrated system throughout the whole experiment suggests that this fraction mainly consists of single AgNPs and their small assemblies multi-layer coated with PMHT-Br chains. This result also demonstrates high rapidity of the coating process, since this population of AgNPs is already observed within a few initial frames, i.e. within milliseconds after mixing. A question might arise as to the effect of the small fraction of aggregated AgNPs present in pure Ag hydrosol on the overall aggregation process. Simple calculations based on the numberweighted size distribution of AgNPs (Fig. S8, Supporting Information) and considering spherical shape of the NPs give the total surface area of the isolated AgNPs (RH = 1.6 nm) ca 870 times bigger than the total surface area of the fraction of their aggregates (RH = 13 nm) present in the used Ag-hydrosol. Hence it seems clear that the studied process of aggregation of PMHT-Br chains with AgNPs have to be driven by the isolated AgNPs, contribution of aggregated AgNPs being negligible.

CONCLUSIONS A combination of the scattering methods (DLS, ζ-potential and time-resolved SAXS) with the UV-Vis spectroscopy and TEM imaging provided insight into the course of assembly of small borate-stabilized Ag-NPs with cationic polyelectrolyte chains and morphology of resulting


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assemblies. The aggregation kinetics of Ag NPs induced by the regular cationic polythiophene polyelectrolyte, PMHT-Br as well as the morphology of resulting NPs assemblies was found to depend on the polymer concentration in the system. The time resolved SAXS experiments demonstrated that the coating of AgNPs by polymer with PMHT-Br chains is completed within a few milliseconds and the growth of the AgNPs aggregates is kinetically dependent of the charge compensation equilibrium given by the polymer concentration. Two to three populations of NPs are formed within milliseconds upon mixing the components, which differ in the mean size, extent of polymer coating and time stability. Population of single NPs of the mean size about 5 nm is present in every system and is mostly long time stable. At the low polymer to NPs concentration ratios most NPs are probably almost free of polymer chains and the second population includes slowly but in a limited extent growing NPs, in which single NPs might be interconnected by polymer chains. When charge balance is achieved the NPs aggregate and finally undergo sedimentation. At the high ratios, three long-time stable populations of NPs are observed of the mean size of ca 5, 13 and 35 nm; all NPs should be fully coated with PHMT-Br giving them stabilizing positively charged shell.

ASSOCIATED CONTENT Supporting Information. Calculation of the maximum possible coverage of the nanoparticles by the polymer, Time dependence of surface plasmon extinction spectra of Ag NPs/PMHT-Br composite sols, Plot of absorbance as a function of time, Plots of Average light scattering intensity versus the log of polymer concentrations, Aggregation kinetics for AgNPs with different concentration of PMHT-Br showing hydrodynamic radius versus time, Comparison between the scattering curves of specific time frames from the kinetic measurements, Comparison between the as-obtained scattering data and data after averaging over 5 frames,


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Number-weighted distribution of hydrodynamic radius (RH) of pure hydrosol AgNPs. “This material is available free of charge via the Internet at http://pubs.acs.org.” AUTHOR INFORMATION Corresponding Author *J. Pfleger, email:[email protected] , Tel: +420 296809571 Present Addresses † Abengoa Research, Abengoa, C/Energia Solar No.1, Palmas Altas-41014, Seville, Spain, E-mail: [email protected] ††Tomas Bata University in Zlín, Faculty of Technology, Department of Polymer Engineering, Vavrečkova 5669, 760 01 Zlín, Czech Republic E-mail: [email protected]

ACKNOWLEDGMENT Financial support of the Czech Science Foundation (P108/12/1143) is gratefully acknowledged. The authors also thank Ms. J. Hromadkova for performing TEM measurements.

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Synopsis: The aggregation kinetics of the negatively charged borate-stabilized Ag nanoparticles induced by the cationic regioregular polythiophene polyelectrolyte studied by DLS and TR-SAXS shows a competition between the nanoparticle aggregation and the cationic polymer adsorption, which provides an electrostatic stabilization.


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Morphology and Kinetics of Aggregation of Silver ... - ACS Publications

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