Formation of Reversible Clusters with Controlled Degree of Aggregation

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Formation of Reversible Clusters with Controlled Degree of Aggregation Saba Lotfizadeh, Hassan Aljama, Daniel T Reilly, and Themis Matsoukas Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b00746 • Publication Date (Web): 28 Apr 2016 Downloaded from http://pubs.acs.org on April 30, 2016

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Formation of Reversible Clusters with Controlled Degree of Aggregation Saba Lotfizadeh,∗ Hassan Aljama, Dan Reilly, and Themis Matsoukas∗ Department of Chemical Engineering, The Pennsylvania State University, University Park E-mail: [email protected]; [email protected]

Abstract We develop a reversible colloidal system of silica nanoparticles whose state of aggregation is controlled reproducibly from a state of fully dispersed nanoparticles to that of a colloidal gel and back. The surface of silica nanoparticles is coated with various amino silanes to identify a silane capable of forming a monolayer on surface of the particles without causing irreversible aggregation. Of the three silanes used in this study, N-[3-(trimethoxysilyl)propyl]ethylenediamine (TMPE) was found to be capable of producing monolayers up to full surface coverage without inducing irreversible aggregation of the nanoparticles. At near full surface coverage the electrokinetic behavior of the functionalized silica is completely determined by that of the aminosilane. At acidic pH the ionization of the amino groups provides electrosteric stabilization and the system is fully dispersed. At basic pH, the dispersion state is dominated by the hydrophobic interaction between the uncharged aminosilane chains in the aqueous environment and the system forms a colloidal gel. At intermediate pH values the dispersion state is dominated by the balance between electrostatic and hydrophobic interactions and the system exists in clusters whose size is determined solely by the pH. The transformation between states of aggregation is reversible and a reproducible function of pH. The

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rate of gelation can be controlled to be as fast as minutes while deaggregation is much slower and takes several hours to complete.

Introduction The synthesis of stable nanoclusters is of great interest for wide range of applications in biotechnology, medical imaging, sensing and catalysis. 1–3 Clusters exhibit enhanced physical properties relative to primary particles and provide features that do not exist in monomers 4,5 such as improved optic, electronic, and magnetic properties. 6–9 The preparation of colloidal clusters in a well-controlled manner is however a challenging proposition. Colloidal aggregation can be easily initiated by destabilizing an electrostatically stabilized colloid but the result is clusters that are irreversibly bonded and a process that proceeds until aggregates precipitate out of solution or form a gel. Reversible aggregates are formed when the interparticle potential is tuned to produce a shallow secondary minimum, but such systems form small clusters composed of few primary particles. 10 To produce clusters that can be redispersed, some type of steric stabilization is required to prevent contact of the bare surface. Aggregation may be then induced by a number of methods, by increasing the ionic strength, 11–13 via macromolecules and polyelectrolytes, 14,15 by depletion attraction 16,17 , by tuning the steric interactions to produce a van der Waals force that is longer range than the range of the steric repulsion, 18 or by reducing repulsion via the adsorption of an oppositely charged polyelectrolyte that reverses the particle charge. 19 Another special class is that of thermoreversible colloids. They are produced by grafting hydrocarbon chains onto silica particles, which are then dispersed in a non-polar solvent whose interaction with the chains changes from poor solvent to good solvent depending on temperature. These colloids can be turned into a gel and then back to a well-dispersed state by changing temperature. 20,21 While some evidence suggests the presence of clusters, 22 the transition between the fully dispersed and fully gelled states is generally abrupt.

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The formation of equilibrium clusters under the balance of a long-range repulsion and short-range attraction carries analogies to thermodynamic phase transitions and as such it has become the focus of theoretical developments. 23–25 The experimental challenge is to tune interactions so as to produce a colloid whose aggregation state can be varied over a wide range under an external stimulus. Here we report on a system that undergoes reversible aggregation and forms clusters whose size ranges from fully dispersed nanoparticles to a state of a colloidal gel. We accomplish this by coating silica nanoparticles with a short aminosilane. Particle interactions are governed by the balance between electrostatic repulsion arising from the ionization of the amino groups, and short-range attraction between the grafted chains, and the relative magnitude of these forces we control via the solution pH. We identify silanes that can produce high surface coverage without causing aggregation, characterize the electrokinetic behavior of the modified particles, and study the reversible formation of clusters as a function of pH.

Experimental Materials Silica nanoparticles (LUDOX TM-40, 40 wt.% suspension in water) were supplied by Sigma Aldrich CO., ST. Louis, MO. The diameter of the particles is 30 nm as determined by dynamic light scattering (DLS) and confirmed by transmission electron microscopy (TEM). Three silanes used in this study: 3-aminopropyltriethoxysilane (APES), aminopropyldimethylethoxysilane (APMS), and N-[3-(trimethoxysilyl)propyl]ethylenediamine (TMPE), were supplied by Aldrich Co., Milwaukee, WI. The pH of the suspension was adjusted using hydrochloric acid (HCl). Characterization Techniques Particle sizes were measured by dynamic light scattering (Brookhaven model A2039 AT using a He-Ne laser with λ = 632.8 nm). Samples were diluted in water and the autocorrelation function was recorded at 25◦ C and at a scattering angle θ = 90◦ . The average hydrodynamic diameter was obtained by cumulant analysis and 3


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the error bars reported represent the standard deviation of 10 measurements. Particles were also studied under transmission electron microscopy(TEM) using Philips 420 with minimum resolution of 3.4.˚ A. TEM grids were immersed into very diluted samples of particles at different pH values and let to dry under room temperature. Measurements of the zeta potential were conducted in a Brookhaven ZETAPALS analyzer and the results were analyzed using the Smoluchowski equation. 26 Values reported in the paper are the average of 10 measurements. Surface Modification The following procedure was used to graft the silane onto the particles. Ludox was diluted to 6 v% in deionized water (pH=9.5) and the prescribed amount of silane was added drop-wise under vigorous stirring. The concentration of the silane is fixed in proportion to that of silica particles and is reported as surface coverage by taking the area of the silane to be 20 ˚ A2 , 27 and a surface area of 110 m2 /g, as reported by the manufacturer. For example a sample of Ludox 6 v% treated with 0.029 g of silane per g of silica corresponds to 20% surface coverage. After addition of the full amount of the silane, the sample was stirred at room temperature for 24 hours at 1000 RPM. Gelation and Redispersion Experiments The particles used in the gelation experiments were obtained from silica treated with TMPE at surface coverage of 60% according to the procedure discussed above. At the end of the surface treatment the pH is 10.8, corresponding to a slightly negative zeta potential. The pH was then changed to 9.8 (±0.1) by addition of HCl. At this pH the zeta potential is nearly zero and the sample gels within minutes. The gelled samples were redispersed by adjusting their pH to one of the following values, 9.2, 8.6, 7.0, 5.6, 4.0 and 3.0. Samples were sonicated for 20 min at 25◦ C in a bath sonicator and their sizes were monitored by dynamic light scattering for several hours. After the size was stabilized, periodic measurements of size were repeated over a period of one month.

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APMS

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Figure 1: Chemical structure of silanes used in this study. Reversibility Experiments After surface modification by TMPE the pH was brought to pH = 3, at which point the system forms a stable dispersion of fully dispersed nanoparticles. Subsequently the pH was increased to the isoelectric point (pH = 9.8) and the sample was allowed to gel. After gelation the pH was adjusted to one of the following values: 3, 4, 5.6, 7, and the size was monitored by light scattering. When the size reached a stable value, the pH for all samples was changed back to 9.8 and all samples were observed to gel within minutes.

Results and Discussion The colloidal behavior of bare silica is determined by the amphoteric character of surface hydroxyls whose isoelectric point is in the pH range 1.5 – 3.5. 28 Except in strongly acidic environments, silica is negatively charged with moderate to good colloidal stability. If stability is destroyed, for example by shifting the pH to the isoelectric point or by adding an electrolyte, silica undergoes irreversible and uncontrollable aggregation via condensation of surface hydroxyl groups. Surface modification with aminosilanes accomplishes two things, it protects against irreversible aggregation by blocking the surface hydroxyls, and simultaneously provides a steric layer with ionizable groups that add electrostatic repulsion depending on the pH. Typical silane coupling agents contain alkoxysilanes groups of the 5


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Figure 2: Hydrodynamic diameter of silica as a function of surface coverage. TMPE allows full coverage without aggregation. form −Si(OR)x (x=1, 2, 3) that attach to the silica surface via hydrolysis of the siloxane bond and subsequent condensation with hydroxyls on the silica surface. 29,30 Condensation reactions may also take place between the alkoxides themselves. 31 This increases the risk of forming siloxane oligomers that can attach to multiple particles and cause aggregation. As a result, surface treatment is often accompanied by an undesirable increase of the particle size. 32,33 Comparison of silanes The goal of the first set of experiments was to assess the performance of three different silanes with respect to allowing high surface coverage without causing aggregation. As shown in Fig. 1, the selected silanes have different structural properties whose effect we want to evaluate. APMS is a short aminosilane with one primary amine and a single alkoxy group. It cannot form oligomers larger than a dimer and the dimer contains no reactive groups that can attach to silica. This silane minimizes the risk of aggregation from oligomers but attaches to the surface via a single bond, which may compromise the mechanical stability of the steric layer. APES has the same amino chain as APMS, but three alkoxy groups. It can form three-dimensional oligomers but can also attach to the surface more strongly via three siloxane bonds. TMPE has similarly three alkoxy groups but a longer tail with two amines. With two ionizable groups, TMPE is ca6


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pable of providing stronger electrostatic stabilization in comparison to the other two silanes. Figure 2 shows the hydrodynamic diameter of the treated silica as a function of surface coverage for the three silanes of this study. The three silanes behave quite differently. APES produces very poor samples that aggregate even when treated with small amounts of the silane. This is likely due to formation of oligomers that we cannot control. APMS performs well up to 25% coverage but causes aggregation at higher amounts. TMPE shows the best behavior among the three silanes: it reacts up to to complete coverage without producing any detectable aggregation beyond a small increase of the diameter by 7.5 nm, a value that remains constant at high coverage. Comparing the results APES with APMS we conclude that formation of oligomers is the most likely cause of high aggregation in the case of APES. When the ability of the silane to polymerize is removed, as with APES, or the repulsion is increased, as with TMPE, this behavior is suppressed by removing the capacity of the silane to polymerize (APMS) or by increasing the electrostatic repulsion between coated particles (TMPE). Increasing repulsion is clearly more effective and permits coverage in the full range from 0 to 100%. Zeta potential Amino groups are positively charged below pKa and neutral above. As the silane reacts with surface hydroxyls (negatively charged in the pH range of these experiments) the electrokinetic profile varies continuously from that of bare silica at zero coverage, to that of the amine at full coverage. This systematic shift offers a simple way to characterize the degree of surface coverage. Figure 3 shows the zeta potential of the particles, before and after treatment with various amounts of silane, as a function of pH. For partially covered surfaces, the zeta potential represents the net charge arising from both the coated and uncoated parts of the surface. The full ionization of the aminogroups at low pH shifts the zeta potential into the positive territory. At high pH the amines are neutral and the zeta potential arises from the hydroxyls that have not reacted. In this region the zeta potential is raised upward toward the zero axis. The isoelectric point moves accordingly to higher pH. These shifts

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Figure 3: Zeta potential of bare and surface-modified silica as function of pH. are observed to occur systematically with increasing amount of silane. It is notable that even with the smallest amounts of silane, the zeta potential in the acidic region crosses into positive territory. The electrokinetic shifts are most clearly seen with TMPE, which allows us to study the system over the full range of surface coverage. At 88% surface coverage, the zeta potential is shifted entirely into the positive region and remains positive at least up to pH = 11, indicting that fraction of uncoated hydroxyls is indeed very small. This behavior confirms the ability of TMPE to provide complete coverage of the particle surface without causing aggregation.

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nearly neutral and form a gel, then we adjust the pH to a desired value in the range from 3 to 10. The samples are sonicated for 20 min to disperse the particles and the size is monitored by dynamic light scattering until it reaches a value that does not vary with time. The results of these experiments are shown in Fig. 4. At pH = 3, the size we measure by dynamic light scattering matches the size of the particles at the end of the surface treatment. At this pH the system is fully dispersed in the form of unattached primary particles. Upon increasing the pH the measured size increases, indicating the formation of aggregates. The size increase is monotonic with pH and is confirmed by TEM (4). Size measurements after one month of storage in closed vials at room temperature were reproducible except for one sample near the isoelectric point that shows signs of continuing growth. Measurements were conducted at two different volume fractions of the colloid, 6% and 8.4%, but we could not detect a systematic difference between the two systems. The results are consistent with the behavior we expect based on the zeta potential measurements. At pH below 4, the strong ionization of the amines renders the system stable against aggregation and disperses the particles fully, as indicated by the hydrodynamic diameter that matches that of the coated particles. At higher pH the fraction of ionized groups decreases and particles form clusters whose size is determined by the balance between electrostatic repulsion and the hydrophobic/van der Waals interaction between silane chains. This balance is shifted towards attraction with pH, leading to larger clusters and eventually to the formation of a gel. Reversible response to pH At pH = 9.8 the particles are uncharged and in the absence of repulsion, aggregation proceeds rapidly and leads to the formation of a colloidal gel network. This gel is held together by physical attraction between the chains on the particle surface and can be broken down by sonication or even by shaking, but reestablishes rapidly once the system is left to rest. Redispersion can be achieved in a controlled and reproducible manner by decreasing the pH to protonate the surface and create repulsion between the clustered particles. Figure 5 summarizes the reversible nature of this process. Starting with

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freshly coated particles we adjust the pH to 9.8 and observe the system to turn viscous within seconds and form a gel within minutes. This process is too fast to follow by light scattering. We then add HCl to decrease the pH to 3, 4, 5.6 or 7, and track the size as a function of time without any sonication. After decreasing the pH the gel network breaks down, initially very rapidly, followed by a slower decrease in size that can take several days to reach equilibrium. The lower the pH the faster the rate of deaggregation and the smaller the final size. At pH = 3, the final size is reached within about 10 hr and matches the size of the primary particles, indicating full dispersion. This behavior is reversible: when the pH of all the samples is brought back to pH=9.8, all suspensions gelled rapidly, as with the freshly coated particles. Similar behavior is observed in transitions between any two values of pH. Figure 6 shows that by decreasing the pH of a stable suspension from pH=9.8 in steps, first to pH=5.6, then to pH=4, and finally to pH=3 the particle size decreases also in steps, first to 100 nm, and then to around 58 nm and finally to 40 nm. These values match those in Fig. 4 at the corresponding pH. Therefore, the size of the clusters responds to pH and adjusts to it as the pH is decreased. If the pH is increased in small increments, we generally find the reaggregation process to be very slow. However, if the pH is increased to the gel point, gelation is always observed and the redispersion under acidification can be repeated. Above the isoelectric point the particles carry sufficiently negative charge to disrupt the gel structure and remain in dispersed form. The gelation/redispersion cycle can be repeated several times. The only practical limitation is the effect of ionic strength: with each addition of electrolyte the ionic strength increases, leading to reduced repulsion at the same pH. The reversible response to pH changes suggest that the size of the clusters is determined only by the final pH of the system and not its prior history. If this is indeed so, other factors that affect size, such as sonication, should have only temporary effect on the size of the clusters if pH remains constant. To test this hypothesis we started with 5 samples equilibrated to their pH in the range pH=4 to pH=9.5. These samples were then sonicated for an hour in a water bath sonicator at 25◦ C. As shown in Figure 7, immediately after

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sonication, all samples exhibited a decreased size that approached but did not reach the size of the fully dispersed particles. Clearly, the forces that hold clusters together are weak enough to be disrupted, at least partially, under sonication. After sonication, all samples exhibit a slow growth in size, and after 10 days they had reached the original size before sonication. The rate of growth depends on the pH. At the extreme case where pH was set right at the isoelectric point, minutes after sonication was stopped the system became a gel again. At intermediate pH values, we find that the rate of growth correlates with the distance between the size of the sonicated clusters and that at equilibrium. This difference acts as a driving force: the larger it is, the faster the rate of growth. The experiment was repeated at three other volume fractions, 6% and 22%, and the results were very similar (see figures in the supplement).

Figure 8: Schematic representation of reversible model. The picture that emerges from these experiments is that the aggregation state of the clusters is solely determined by pH of the suspending liquid. It depends neither on the volume fraction of the particles, nor on the method used to disperse the clusters (mechanical or electrostatic), nor on the size of the clusters before a pH adjustment is made. These traits point towards a processes controlled by the equilibrium established under the competing effects of electrostatic repulsion, and a short-range attraction due to the hydrophobic interaction of the aminosilane chains on the particle surface, as schematically shown in Fig. (8). The attractive interaction is short range. It is proportional to the contact points between particles and to a first approximation may be taken to be proportional to the number of 14


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primary particles in the cluster, n. 17 The repulsion is a long-range interaction among all particle pairs in the cluster, and it roughly scales as n2 . This non-linear dependence of the cluster energy on n establishes a critical cluster size that minimizes energy. This size can be obtained by balancing the two interactions. Following Bordi et al 19 , who applied the same approach to equilibrium clusters of vesicles whose repulsion was tuned by adsorbing an oppositely charged polyelectrolyte, we use the Yukawa screened potential to describe the interaction between charged particles. The cluster size that minimizes energy, R∗ , is given by 19 , E (R∗ /R0 )df e2 z 2 = kT 4πǫǫ0 kT (R∗ + R0 ) (1 + κR∗ )(1 + κR0 ) where E is the strength of attraction per particle, z is the charge per primary particle, e is the electron charge, ǫ is the relative dielectric constant of the medium, R0 is the radius of the primary particle, κ is the inverse screening length and df is the fractal dimension of the cluster. A simpler but qualitatively similar expression was given by 17 for gold nanoclusters. The above equation provides a qualitative description of the behavior of the system and correctly predicts that the critical cluster size increases as the strength of repulsion decreases. Using typical values for the ionic strength, particle charge and fractal dimension of our clusters, the model correctly shows that the magnitude of the attraction is essentially the same at all pH. It is more difficult to obtain quantitative agreement, given the simplicity of the model (entropic effects are not accounted for in the calculation of the cluster size, structure is described by a simple fractal model), and uncertainty on the precise magnitude of the experimental values (the model requires the particle charge whereas in the experiment we measure the zeta potential). These theoretical considerations provide a useful starting point for further and more detailed treatments of reversible colloidal aggregation.

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Conclusions In summary, we have shown that not all silanes are suitable for surface modification. Aggregation during surface treatment is irreversible and uncontrollable. TMPE was shown to provide enhanced stability that allows us to produce any surface coverage, from 0 to 100%. The colloidal behavior of the modified silica is remarkably different from that of native silica: it exists in well-dispersed form below pH ≈ 3, and as a colloidal gel at pH = 9.8. In the intermediate pH range it forms clusters whose size depends on the pH. Thus the state of aggregation is tunable. This provides us with a model colloid that exhibits a unique continuous transition between two phases, fully dispersed and fully gelled, in response to an external stimulus.

Acknowledgement This material is based upon work supported by the National Science Foundation under Grant No. CTS 1132220.

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(23) Bentz, J.; Nir, S. Aggregation of colloidal particles modeled as a dynamical process. Proc Natl Acad Sci U S A 1981, 78, 1634–1637. (24) Groenewold, J.; Kegel, W. K. Anomalously Large Equilibrium Clusters of Colloids. J. Phys. Chem. B 2001, 105, 11702–11709. (25) Law, B. M.; Petit, J.-M.; Beysens, D. Adsorption-induced reversible colloidal aggregation. Phys. Rev. E 1998, 57, 5782–5794. (26) Hunter, R. J. Zeta potential in colloid science; Academic press New York, 1981. (27) Iler, R. The Chemistry of Silica: Solubility, Polymerization, Colloid and Surface Properties and Biochemistry of Silica; A Wiley-Interscience publication; Wiley, 1979. (28) Parks, G. A. The Isoelectric Points of Solid Oxides, Solid Hydroxides, and Aqueous Hydroxo Complex Systems. Chem. Rev. 1965, 65, 177–198. (29) van Blaaderen A.,; Vrij, A. The colloid chemistry of silica; American Chemical Society, 1994; Chapter 4. (30) Kaas, R. L.; Kardos, J. L. The interaction of alkoxy silane coupling agents with silica surfaces. Polym. Eng. Sci. 1971, 11, 11–18. (31) Arkles, B.; Steinmetz, J.; Zazyczny, J.; Mehta, P. Factors contributing to the stability of alkoxysilanes in aqueous solution. J. Adhes. Sci. Technol. 1992, 6, 193–206. (32) Choi, H.; Chen, I.-W. Surface-modified silica colloid for diagnostic imaging. J. Colloid Interface Sci. 2003, 258, 435–437. (33) Cs˝ogör, Z.; Nacken, M.; Sameti, M.; Lehr, C.-M.; Schmidt, H. Modified silica particles for gene delivery. Mater. Sci. Eng., C 2003, 23, 93 – 97, Current Trends in Nanotechnologies: From Materials to Systems. Proceedings of Symposium Q, E-MRS Spring Meeting 2002. June 18-21 2002. 19


Langmuir

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TOC Graphic

pH Figure 9: For Table of Contents Only

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Formation of Reversible Clusters with Controlled Degree of Aggregation

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