Controlling the Assembly of Nanoparticle Mixtures With Two

Jan 17, 2012 - Robert Pelton , Yuguo Cui , Dan Zhang , Yang Chen , Kate L. Thompson , Steven P. Armes , and Michael A. Brook. Langmuir 2013 29 (2), 59...
0 downloads 0 Views 2MB Size
Download PDF
Article pubs.acs.org/Langmuir

Controlling the Assembly of Nanoparticle Mixtures With Two Orthogonal Polymer Complexation Reactions Dan Zhang† and Robert Pelton*,‡ †

School of Materials Science and Engineering, Shijiazhuang Tiedao University, 17 Northeast, Second Inner Ring, Shijiazhuang, Hebei, PRC 050043 ‡ Department of Chemical Engineering, McMaster University, Hamilton, Ontario, Canada L8S 4L7 S Supporting Information *

ABSTRACT: Self-assembly from mixed dispersions of three sizes of monodisperse polystyrene nanoparticles, large (L), medium (M), and small (S), was controlled by coating each particle type with either a monofunctional or bifunctional polymer capable of participating in specific complexation reactions. The complexation reactions were (1) complexation between phenolic polymers and polyethylene glycol (PEG) containing polymers and (2) condensation of phenylboronic acid containing polymers with polyols. These complexation reactions function independently and can be “turned off” independently; phenylboronic acid complexation was reversed by lowering the pH, whereas the interactions of phenolic copolymers with PEG copolymers could be reversed by adding excess PEG homopolymer. The specificity and reversibility of the interactions was demonstrated by the formation of simple binary aggregates from mixtures. The bifunctional copolymers were poly(vinyl phenol-co-diallyldimethyl ammonium chloride), Ph-DADMAC, and poly(3-acrylamide phenylboronic acid-co-PEG methacrylate), PBA-PEG. The monofunctional polymer was polyvinylalcohol, PVA. Ph-DADMAC forms complexes with PBA-PEG (H-bonding) and with anionic surfaces or polymers (electrostatic/polyelectrolyte complexation). PBA-PEG complexes with Ph-DADMAC (H-bonding) and with PVA (boronate ester formation). PVA does not interact with Ph-DADMAC; therefore, PVA coated particles do not deposit onto Ph-DADMAC coated particles.

■

INTRODUCTION The ultimate applications of nanoscale objects often require assembly into macroscale surfaces or structures. For most practical applications, this assembly must be spontaneous or some form of directed assembly involving minimum intervention. For example, layer-by-layer (LbL) assembly of polymers and particles on surfaces is very popular in the scientific literature because it is easy and versatile.1 However, each layer requires an adsorption/deposition step followed by a washing step. For very low cost applications, sequential LbL is too complex. The controlled assembly of aqueous nanoparticle suspensions is particularly challenging because, unlike macroscopic surfaces, nanoparticle suspensions are difficult to isolate or wash between steps. We are interested in developing approaches to controlling the assembly in suspension of nanoparticle mixtures. Herein we describe approaches for modifying nanoparticle surfaces to facilitate the controlled interaction of three distinct types of nanoparticles, we call L (large), M (medium), and S (small). If we consider only doublet formation from mixtures of three particle types (i.e., S, M, L), the total number of possibilities is six (SS, MM, LL, SM, SL, ML). Herein we show how to design nanoparticle surfaces so that SM and ML combinations occur whereas other combinations (LS, LL, MM, SS) are not formed. Furthermore, we show that either the LM or SM interactions can be turned off independently by manipulation of the solution phase. Before introducing our work in more detail, the following paragraphs © 2012 American Chemical Society

summarize other approaches in the literature used to control nanoparticle interactions. More than 30 years ago, Luckham et al. published a series of papers describing the products obtained from mixing oppositely charged monodisperse latex of different sizes.2 The addition of low dosages of small particles to an oppositely charged suspension of large particles gave large flocs, with the smaller particles bridging the larger ones into aggregates. By contrast, high dosages of small particles gave completely coated large particles with a raspberry appearance. The capacity of the large latex to adsorb the smaller oppositely charged particles depended upon ionic strength. When the added salt concentration was low, adsorbed small particles occupied a large exclusion or blocked area due to electrostatic repulsion, limiting the packing density. The authors showed that the strong electrostatic interactions between oppositely charged particles could be attenuated by the presence of nonionic, surface polymers. The resulting weaker particle−particle adhesion gave shear sensitive structures.3 In summary, electrostatic attraction can be used to drive the self-assembly of oppositely charged particles. However, electrostatics or any other individual interaction mechanism will not allow for Received: November 15, 2011 Revised: January 7, 2012 Published: January 17, 2012 3112

dx.doi.org/10.1021/la204514y | Langmuir 2012, 28, 3112−3119

Langmuir

Article

groups (cellulose-PBA) in our previous paper.18 Although individual borate condensation complexes are weak, we showed that the cumulative effects of multiple contacts are sufficient to bind large colloidal particles to surfaces. Borate condensation with a polyol can be reversed by lowering the pH below the borate pKa or by adding a sugar with a higher binding constant.19 In the following sections, we describe the preparation of bifunctional copolymers that can participate in two of the above complexation reactions. In addition, we demonstrate the controlled assembly of up to three nanoparticle types coated with interacting copolymers.

controlled assembly of complex structures from a mixture of three particle types. Biology offers many examples of complex self-assembly over a range of distance scales. The tools of biological assembly, including antibody−antigen binding, DNA duplex formation, and enzyme−substrate complexation, are extensively reported for applications including biosensing and controlled drug delivery. Perhaps the most common example of a bioinspired tool is the biotin−streptavidin complex.4 We have used many of these approaches, and they can show spectacular selectivity. The two key limitations of the bioassembly tools are (1) they are usually sensitive to degradation by heat, desiccation, and enzymes; and (2) they are expensive. For high value-added biomedical applications in controlled environments, these are not serious limitations. For more widespread applications in lower cost products such as smart packaging, solar cells, or performance coating, these limitations are serious. To control the assembly of more than two types of particles, there are two approaches. One is sequential assembly, where each new particle type is added stepwise, possibly with intervening cleaning steps; this is analogous to LbL assembly on a surface. The other method, and the one we have pursued, is to invoke multiple, independent interactions leading to “one pot” controlled assembly. Our approach is to modify particle surfaces by adsorbing a water-soluble polymer capable of one (monofunctional) or two (bifunctional) complexation reactions with a complementary polymer. In preliminary work, we demonstrated three independent complexation reactions for driving the assembly of water-soluble polymers on wafer surfaces.5 The complexation reactions were as follows: (1) Polyelectrolyte complexation between oppositely charged water-soluble polymers (electrostatic attraction). Polyelectrolyte complexation has been intensively investigated and was the interaction exploited by Decher in his original LbL work.1 (2) Polyethylene glycol (PEG)−phenolic copolymer complex formation. These complexes are an important aspect of the mechanism by which very high molecular weight polyethylene oxide performs as a flocculant. We have reported extensively on these complexes both in solution6−12 and on surfaces.9 NMR suggests that the polyether segments are sitting over the phenolic pie rings. This type of complex formation is insensitive to pH values less than 118 and to ionic strength, making it a robust candidate for driving assembly. (3) Complex formation between polyols and polymers bearing phenylboronic acid groups. Phenylboronate groups condense with polyols to form five or six membered rings. The key features of these complexes are that the individual binding constants are low (typically 102 to 103 M−1) and are sensitive to the diol stereochemistry.13−15 In addition, the borate ion is the reactive species, thus complex formation is normally associated with alkaline pH. Neutral pH reactions are possible in the presence of amine groups near the boronic acid groups.16 Ivanov and co-workers have described the adhesion strengths of cells to boronate-terminated polymer brushes.17 Bound cells were washed away by elution with fructose solution. We reported specific, pH-dependent adsorption, and desorption of poly(glycerol monomethacrylate)-stabilized polystyrene latex onto regenerated cellulose film bearing phenylboronic acid

■

EXPERIMENTAL SECTION

Materials. Diallyldimethyl ammonium chloride (DADMAC), 4acetoxystyrene, poly(ethylene glycol) methacrylate (PEG, Mw = 360 Da), and monodispersed polystyrene latex suspensions with the diameters 200 nm, 500 nm, 1 μm, 10 μm (red), and 100 μm (blue) were all obtained from Aldrich. Poly(vinyl alcohol) (PVA, Mw = 72k and 97.5−99.5% degree of hydrolysis) was purchased from Fluka. The PEG macromonomer was PEG-methacrylate, 360 Da, purified with an Aldrich inhibitor removal column (product no. 306312) before use. Other chemicals were used without further purification. Details about the preparation and purification of monomer 3-acrylamide phenylboronic acid (PBA) have been described previously.5 Copolymer Synthesis and Characterization. Copolymer PhDADMAC from 4-acetoxystyrene (Ph) and diallyldimethyl ammonium chloride (DADMAC) and copolymer PBA-PEG from 3acrylamide phenylboronic acid (PBA) and poly(ethylene glycol) methacrylate (PEG) were prepared by free radical polymerization. The PBA-PEG copolymers were prepared from a 0.10 mol/L monomer (PBA and PEG, 20:80 mol %) mixture in 100 mL of water using a 0.4 mmol/L potassium persulfate initiator at 65 °C for 8 h. To prepare the copolymer, Ph-DADMAC, 0.1 mol/L monomer 4-acetoxystyrene was added dropwise over 120 min into 0.10 mol/L monomer DADMAC dissolved in 100 mL of methanol/water (60:40 vol %) mixture in a three-neck glass flask suspended in a 65 °C oil bath and mixed with a magnetic stirrer. The reaction was started by the introduction of 0.4 mmol/L 2,2′-azobis(2-methylpropionamidine)·2HCl initiator. After 10 h of polymerization time, the product was hydrolyzed in 0.6 mol/L aqueous sodium hydroxide at 55 °C for 30 h to remove the acetate protecting groups. The products were dialyzed against water for 2 weeks and were subsequently freeze-dried. The polymer compositions were determined by proton NMR to be Ph-DADMAC (63: 37 mol %) and PBA-PEG (13: 87 mol %). The refractive index increments (dn/dc) of the copolymers PhDADMAC and PBA-PEG dissolved in deionized water were determined with a Bausch & Lomb Abbe-3 L refractometer. The dn/dc for Ph-DADMAC is 0.144 mL/g, and the dn/dc for CMC is PBA-PEG 0.167 mL/g. The molecular weights were determined by static light scattering (Brookhaven BI-ISTW equipped with a 5 mW Ne−He laser). Copolymers were dissolved in deionized water and filtered through a syringe filter (Millipore size 5 μm, filter diameter 25 mm, product no. Z227439) to remove dust particles. The data analysis was performed with Brookhaven’s Zimm plot analysis software, and the molecular weights are 6.6 ± 1.9 × 105 g/mol (Ph-DADMAC) and 8.3 ± 0.86 × 105 g/mol (PBA-PEG). More details and Zimm plots are available in a thesis.20 LbL Assembly on Silicon Wafers. Silicon wafers (El-Cat Inc. Waldwick, NJ) were rinsed with 50 mL of methanol and toluene, and then treated with a buffered 1% HF aqueous solution (7:1 NH4F/HF) to remove the oxidation layer before use. Caution! Hydrof luoric (HF) acid is extremely corrosive and toxic. All work associated with its use should be caref ully conducted in a f ume hood. Face protection and gloves are required when handling it. The disposal of HF should follow the procedures provided by the supplier. Multilayer polymer layers were assembled on a silicon wafer by a layer-by-layer method (LbL). The silicon wafers were immersed in 0.1 wt % Ph-DADMAC solution with 2 mM NaCl 3113

dx.doi.org/10.1021/la204514y | Langmuir 2012, 28, 3112−3119

Langmuir

Article

first for 3 h and rinsed gently by dipping 5 times into 2 mM NaCl solution to remove weakly adsorbed polymers. The pH of the solutions was adjusted with either 0.1 M NaOH or HCl. The same procedures were sequentially carried out with 0.1 wt % PBA-PEG and with 0.1 wt % PVA. The multilayer thin films on the silicon wafers were then dried at room temperature under a nitrogen atmosphere. The thicknesses of the dried polymer films on the silicon wafers were measured using Waterloo Digital Electronics Extra 2000 faradaymodulated self-nulling ellipsometer that measured two parameters, Δ (phase difference) and tan(Ψ) (the amplitude ratio upon reflection), from which the film thickness and refractive index of dry films can be calculated. In principle, both the thickness and the refractive index can be calculated from a single test. However, the measurement of refractive index is insensitive when the thickness of a dry film is less than 30 nm. Therefore, the refractive indices of the three copolymers were separately measured from their thick films, which were obtained by spin coating at 3000 rpm with acceleration for 100 rpm (P-6000 spin coater from Specialty Coating System, Inc.). Silicon wafers were cleaned before use with the same procedure described above. Three 1.0 g/L polymer solutions were prepared for spin coating. Thick polymer films on wafers were annealed under vacuum at 115 °C for 3 h. The results show that the copolymer refractive indices are PhDADMAC (nL1) 1.566, PBA-PEG (nL2) 1.516, and PVA (nL3) 1.557. The refractive indices of multilayer thin films are the arithmetic mean values of the components. Therefore, the refractive index values were nL1 for a one-layer film, 0.5nL1 + 0.5nL2 for the two-layer films, and 1/3nL1 + 1/3nL2 + 1/3nL3 for the three-layer films. The densities of dried Ph-DADMAC and PBA-PEG copolymers were also measured with the spin coated wafers. The coating mass was weighed and a plasma beam was employed to cut the polymer film from top to bottom to facilitate measuring the film thickness by SEM. The polymer densities of Ph-DADMAC and PBA-PEG are 1.09 and 1.17 g/cm3, respectively. The density of PVA is 1.27 g/cm3, which was obtained from Sigma. Examples of calculations are available in a thesis.20 LbL Assembly on Latex. Multilayer films were also assembled on the surface of a 500 nm diameter polystyrene latex suspension. All steps were performed in 2 mM NaCl solutions with controlled pH. Concentrated latex was added into 0.1 wt % Ph-DADMAC solution to give a latex concentration of 0.1 g/L. After 3 h, the polystyrene particles were centrifuged at 6000 rpm, which corresponds to a centrifugal force of 4000g, for 40 min using a Beckman Allegra 25R bench centrifuge, and then redispersed in salt solution by manually shaking the centrifuge tube. Two more centrifugation plus rinsing cycles were performed to remove weakly adsorbed polymers on the particle surfaces. Subsequently, the second layer polymers (0.1 wt % PBA-PEG) and the third layer (0.1 wt % PVA) were added by the same procedures. Nanoparticle Assembly from Mixtures. Monodisperse, anionic, surfactant-free polystyrene latexes were given a saturated adsorbed layer of Ph-DADMAC (1 and 100 μm particles), PBA-PEG (500 nm or 10 μm), or PVA (200 nm and 1 μm). In a typical experiment, 1 μm latex dispersion was added into 0.1 wt % Ph-DADMAC dissolved in 2 mM NaCl to give a latex concentration of 0.1 g/L. After gentle mixing for 60 min, the suspension was centrifuged at 6000 rpm for 30 min, the supernatant decanted, and the particles were redispersed in a 2 mM NaCl solution. The serum replacement washing was repeated for a total of three times. For the assembly step, three 0.05 g/L latex suspensions in 2 mM NaCl were gently stirred at room temperature for 3 h. A 20 μL mixed latex suspension was dried on a copper grid coated with Formvar at room temperature for TEM characterization using a JEOL JEM1200EX transmission electron microscope. Optical micrographs of nanoparticle assembly in aqueous environment were taken from a drop of mixed latex suspension on a glass slide using an Olympus BX51 optical microscope with a Q-Imaging Retiga EXi digital camera (ImagePro software). Dynamic light scattering (DLS) was employed to measure the size of latex particles. The concentration of polystyrene latex particles was 0.002 wt %, which is high enough to obtain reasonable scattering

signals and also advantageous to prevent latex aggregation. All the measurements were carried out at 25 °C with a Brookhaven 256 channel BI-9000 AT digital correlator and a Lexel 35 mW He−Ne laser (633 nm wavelength) at a scattering angle of 90°. The data were analyzed using the program CONTIN to give the particle sizes. The reported particle diameters are an average of three measurements. Electrophoretic mobility measurements were performed with a Brookhaven ZetaPlus zeta potential analyzer using phase analysis light scattering (PALS) mode with BIC Pals zeta potential analyzer software (version 2.5). All data were averaged over 10 cycles with 15 scans for each.

■

RESULTS AND DISCUSSION 1.1. Functional Polymers. Two water-soluble and bifunctional polymers were prepared and characterized. Ph-

Figure 1. Structures of the bifunctional copolymers.

DADMAC with a weight average molecular weight of 6.6 ± 1.9 × 105 Da is a copolymer of p-acetoxystyrene (63 mol %) and diallyldimethyl ammonium chloride (37 mol %), which after hydrolysis yields the corresponding phenolic copolymer; see Figure 1 for structures. Ph-DADMAC binds to anionic surfaces and polymers because of the quaternary ammonium groups on DADMAC. The phenolic groups on the copolymer form complexes with PEG copolymers. The second bifunctional copolymer, PBA-PEG, is copolymer of 3-acrylamide phenylboronic acid (13 mol %) and poly(ethylene glycol) monomethyl methacylate (87 mol %), with a weight averaged molecular weight of 8.3 ± 0.9 × 105 Da. The phenylboronate (PBA) groups condense with diols and will also be attracted to cationic surfaces and polymers. The PEG chains complex with Ph groups on Ph-DADMAC. Both PBA-PEG and Ph-DADMAC were water-soluble from pH 3 to 11, and both copolymers spontaneously adsorbed onto polystyrene latex. In addition to the bifunctional polymers, we employed poly(vinyl alcohol), PVA, as a monofunctional coating polymer that could only bind to the PBA groups on PBA-PEG. 1.2. LbL Assembly. The two bifunctional copolymers and PVA were consecutively deposited (LbL, layer by layer) on an oxidized silicon wafer and on polystyrene support particles (500 3114

dx.doi.org/10.1021/la204514y | Langmuir 2012, 28, 3112−3119

Langmuir

Article

Figure 4. Electrophoretic mobilities of coated 500 nm polystyrene particles as functions of pH. All measurements were performed in 2 mM NaCl aqueous solutions at 25 °C. L1, L2, and L3 samples are individual polymer coatings adsorbed on the polystyrene, whereas with the L1 + L2 + L3 results polymers were adsorbed consecutively (LbL method).

Figure 2. Schematic illustration of LbL assembly of Ph-DADMAC, PBA-PEG, and PVA sequentially adsorbed on anionic silica coated wafers or on anionic polystyrene latex.

Figure 5. (A) Cumulative total thicknesses as L1 (Ph-DADMAC), L2 (PBA-PEG), and L3 (PVA) are consecutively adsorbed onto 500 nm polystyrene latex. (B) Shows the displacement of the L2 (PBA-PEG) by the addition of excess PEG homopolymer. All measurements were conducted in 2 mM NaCl solutions at 25 °C.

nm) to probe the pH dependence of layer thickness and surface charge. In each case, Ph-DADMAC (layer 1), PBA-PEG (layer 2), and PVA (layer 3) were sequentially adsorbed and rinsed under conditions of constant pH and ionic strength. Figure 2 shows the proposed structures of the LbL assemblies. Adsorption of the first layer, Ph-DADMAC, is driven primarily by electrostatic attraction between the cationic DADMAC moieties and the anionic surface. In the case of the polystyrene latex, hydrophobic interactions may also promote adsorption. Layer 2, PBA-PEG, interacts with the phenolic groups on Ph-DADMAC. Although there is a possibility of electrostatic attraction between the anionic phenylboronate groups layer 2 PEG-PBA with ammonium groups on layer 1 Ph-DADMAC, evidence will be presented to show that this is

Figure 3. Ellipsometric thickness (A) and the corresponding coverage (B) of dry films formed by LbL assembly, as a function of the pH used for consecutive adsorption of the three dry polymer layers on oxidized silicon wafers. The measurements were performed under ambient laboratory conditions, and all data are an average of three measurements. 3115

dx.doi.org/10.1021/la204514y | Langmuir 2012, 28, 3112−3119

Langmuir

Article

Figure 6. Binary mixtures of large (L) and medium (M) particles. The L particles were coated with Ph-DADMAC, and the M particle coating was PBA-PEG. (A,B) Transmission electron microscopy images of L 1000 nm + M 500 nm particles. (C,D) Optical micrographs of L 100 μm + M 10 μm polystyrene particles. All suspensions were in 2 mM NaCl at pH 7.

Figure 7. Binary mixtures of medium (M) particles, coated with APA-PEG, and small (S) particles, coated with PVA. (A) SEM micrograph and (B) TEM micrograph of mixtures of M 500 nm and S 200 nm particles. (C,D) Optical micrographs of M 10 μm + S 1 μm particles. All solutions were made with 2 mM NaCl at pH 10.

not significant. Finally, layer 3, PVA, interacts with layer 2 by borate condensation forming weak covalent bonds. The coated wafers were dried and characterized after each adsorption step by ellipsometry. Figure 3A shows the dry film thickness of each layer as a function of application pH. Figure

3B shows the corresponding estimates of coverage (mass/ surface area) for each layer as a function of pH. An example calculation is given in the Supporting Information. Layers 1 and 2 are 2−3 nm thick (dried films), and the values are independent of pH. These dried film thickness values are 3116

dx.doi.org/10.1021/la204514y | Langmuir 2012, 28, 3112−3119

Langmuir

Article

adsorption below pH 7, whereas a thick adsorbed PVA layer forms by pH 10. These results are consistent with the known pH dependence of the borate-polyol condensation reaction.14 Ph-DADMAC (layer 1 polymer) was adsorbed onto anionic polystyrene latex, and the electrophoretic mobility is shown in Figure 4 as a function of the pH used to adsorb the polymer and to perform the pH measurement. At low pH, the mobility was highly positive, indicating overcharging by the adsorbed cationic polymer. However, as the pH rose above 8, the particles became less positive due to the ionization of the phenolic groups.23 By contrast, adsorption of the nonionic PVA decreased the magnitude of the negative mobility of the support particle. This is a common observation, and the usual explanation is that the layer of nonionic polymer moves the shear plane away from the charged surface.24 At low pH, PBA-PEG decreased the negative mobility, much like PVA. However, ionization of the phenylboronic acid groups at alkaline conditions increased the negative charge density giving more negative mobilities. The final curve shown in Figure 4 shows the electrokinetic behavior of the trilayer assembly Ph-DADMAC + PBA-PEG + PVA. The resulting particles had a net negative charge over the whole pH range and the curve fell between that for latex coated only with PBAPEG and latex only coated with PVA. Dynamic light scattering was employed to measure the change in latex diameter with sequential polymer adsorption. Figure 5A shows the effect of adsorption pH on the adsorbed layer thickness after each step. The pH dependence of the wet layer thickness values followed the same trends as the dry layer thickness values in Figure 3A. The PVA adsorption (layer 3) was sensitive to pH, reflecting the boronic acid chemistry, whereas the other layer thicknesses were rather insensitive to pH. One of our goals was to develop techniques to turn off or control independently the various polymer complexation mechanisms. As explained above, PVA complexation with PBA was turned off simply by lowering the pH. The Ph-PEG complexation was attenuated by addition of a 10-fold (based on PEG copolymer) excess of 600 Da PEG. The free PEG effectively competed with the PBA-PEG copolymer. Figure 5B shows a sequence of experiments where Ph-DADMAC (layer 1) was first adsorbed and washed by serum replacement. In the next step, PBA-PEG (layer 2) was adsorbed and the suspension washed. Up to this stage, the experiments in parts A and B in Figure 5 were identical. In the final step, PEG homopolymer was added, and after 1 h the light scattering measurements were repeated. The final adsorbed layer thickness was decreased, suggesting the low molecular weight PEG homopolymer displaced the much higher molecular weight PBA-PEG copolymer. Finally, the layer 1 and layer 1 + 2 thicknesses are a little greater for the A series (upper figure) and the error bars were smaller compared to the B series (lower figure). The preparation conditions were the same for both sets of experiments, and we do not have a good explanation for this small discrepancy. 1.3. Using Complexation to Control Nanoparticle Aggregation. The following results involve two sets of monodisperse particles, a small set (L, 1 μm;M, 500 nm; S, 200 nm) and a large set (L, 100 μm; M, 10 μm; S, 1 μm). The L particles in each set were coated with adsorbed Ph-DADMAC (i.e., layer 1 polymer in the last experiments). Similarly, the M particles were coated with PBA-PEG and the S particles were coated with adsorbed PVA. For binary experiments, SM, SL,

Figure 8. Structures formed from ternary mixtures of large (L) particles, coated with Ph-DADMAC, medium (M), coated with PBAPEG, and small (S) particles coated with PVA. (A,B) TEM images of L1000 nm + M 500 nm+ S 200 nm mixtures; (C) L 100 μm + M 10 μm + S1 μm mixtures.

typical.5,21,22 The density of ionized silanol groups on the wafer and the charge density of quaternary ammonium polymer PhDADMAC are nearly independent of pH over the range investigated. The estimated coverage of layer 2 polymer is also not very dependent on pH. It is well-known that complexation between phenolic polymers and PEG copolymers is not pH sensitive.8 On the other hand, the ionization of the boric acid groups on the PBA moieties is very pH sensitive; the pKa of phenylboronic acid is 8.8. However, this ionization does not seem to influence the layer 2 adsorption on layer 1, suggesting that electrostatic attraction is not a significant driver for PBA-PEG adsorption onto Ph-DADMAC. The final step, the adsorption of PVA onto the PBA-PEG layer, was very pH sensitive. There was essentially no 3117

dx.doi.org/10.1021/la204514y | Langmuir 2012, 28, 3112−3119

Langmuir

Article

Figure 9. TEM micrograph taken from a mixture of L, M, and S coated latex particles at pH 7.0 where there are no PBA−PVA interactions preventing MS pair formation.

be turned off simply by lowering pH. Second, the absence of S−S interaction in this case gives further support to the suggestion that S−S aggregation in Figure 8 could be due to the removal of weakly adsorbed PBA-PEG on the M particles. Chemical grafting of surface polymers would prevent artifacts from polymer desorption.

and ML combinations were prepared for both the small set and the large set. The resulting suspensions were characterized by electron microscopy for the small set and by optical microscopy for the large set. TEM gave the clearest pictures whereas optical microscopy did not involve sample drying, thus avoiding possible drying artifacts. Based on our LbL results and our knowledge of the complexation mechanisms, we hypothesized that the L particles will bind to M particles and the S particles should also only bind to M particles, whereas LS, LL, MM, and SS combinations should not occur. Binary mixtures of L + M and M + S were first investigated, and sample results are shown in Figures 6 and 7, showing TEM micrographs of the smaller series and optical micrographs of the larger series in suspension. There is no evidence of LL, MM, or SS combinations as expected for colloidal stable polystyrene particles. Both sets of L + M combinations (Figure 6) and M + S combinations (Figure 7) showed aggregates between the different sized spheres, indicating attractive interactions. These images are reminiscent of results of Vincent et al. for mixing oppositely charged latex particles in water.2 Figure 8 shows electron micrographs and an optical micrograph of the two sets of L + M + S mixtures at pH 10. The electron micrographs were for the small set, and the optical micrograph was for the larger set in suspension. Generally, we see the desired interactions L−M and M−S with no evidence of L−L or M−M. On the other hand, there was some S−S aggregation. We propose that the S particles, coated with PVA, were stripping some PBA-PEG from the M particles, leading to S−S aggregation. In further support of this explanation, we mixed L + M + S at pH 7.0 where M−S complexation (boronate ester formation) should not occur. The results in Figure 9 show that L−M assemblies form, whereas the S−S aggregates do not form. This observation is important from two perspectives. First, it demonstrates that M−S interactions can

■

CONCLUSIONS

■

ASSOCIATED CONTENT

(1) The assembly of structures from mixtures of three sizes of polystyrene nanoparticles was directed by coating the particles with bifunctional or monofunctional polymers that can undergo interactions based on (1) electrostatic attraction, (2) complexation of PEG copolymers with phenolic copolymers, or (3) boronate ester formation. (2) The three interaction types are orthogonal; that is, they function independently of the presence or absence of the other type of interaction. (3) Boronate ester formation and PEG/phenolic complexation can be “turned off” independently. Boronate esters are hydrolyzed by lowering pH, whereas complexation between PEG-copolymer and phenolic copolymer is disrupted by low molecular weight PEG addition.

S Supporting Information *

Additional figures; calculations of densities of individual polymers and coverages of individual polymers. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Telephone: (905) 529 7070 ext. 27045. Fax: (905) 528 5114. E-mail [email protected]. 3118

dx.doi.org/10.1021/la204514y | Langmuir 2012, 28, 3112−3119

Langmuir

■

ACKNOWLEDGMENTS

■

REFERENCES

Article

(21) Kharlampieva, E.; Kozlovskaya, V.; Tyutina, J.; Sukhishvili, S. A. Hydrogen-Bonded Multilayers of Thermoresponsive Polymers. Macromolecules 2005, 38, 10523−10531. (22) Zhang, H. Y.; Wang, Z. Q.; Zhang, Y. Q.; Zhang, X. HydrogenBonding-Directed Layer-by-Layer Assembly of Poly(4-vinylpyridine) and Poly(4-vinylphenol): Effect of Solvent Composition on Multilayer Buildup. Langmuir 2004, 20, 9366−9370. (23) Lu, C.; Pelton, R. Preparation and Characterization of Polystyrene-Poly(P-acetoxystyrene) and Polystyrene-Poly(P-vinylphenol) Composite Latex Particles. Colloids Surf., A 2002, 201, 161−171. (24) Hunter, R. Zeta Potential in Colloid Science, Principles and Applications; Academic Press: London, 1981; p 386.

The authors acknowledge the Natural Sciences and Engineering Research Council of Canada for funding. R.P. holds the Canada Research Chair in Interfacial Technologies.

(1) Decher, G. Fuzzy Nanoassemblies: Toward Layered Polymeric Multicomposites. Science 1997, 277, 1232−1237. (2) Luckham, P.; Vincent, B.; Hart, C. A.; Tadros, T. F. The Controlled Flocculation of Particulate Dispersions Using Small Particles of Opposite Charge 0.1. Sediment Volumes and Morphology. Colloids Surf. 1980, 1, 281−293. (3) Luckham, P. F.; Vincent, B.; Tadros, T. F. The Controlled Flocculation of Particulate Dispersions Using Small Particles of Opposite Charge 0.4. Effect of Surface Coverage of Adsorbed Polymer on Heteroflocculation. Colloids Surf. 1983, 6, 119−133. (4) Hermanson, G. T. Bioconjugate Techniques; Academic Press: San Diego, CA, 1996. (5) Zhang, D.; Tanaka, H.; Pelton, R. Polymer Assembly Exploiting Three Independent Interactions. Langmuir 2007, 23, 8806−8809. (6) Lu, C.; Pelton, R. Peo Flocculation of Polystyrene-Core Poly(vinylphenol)-Shell Latex: An Example of Ideal Bridging. Langmuir 2001, 17, 7770−7776. (7) Lu, C.; Pelton, R. Factors Influencing the Size of Peo Complexes with a Tyrosine-Rich Polypeptide. Langmuir 2004, 20, 3962−3968. (8) Cong, R. J.; Bain, A. D.; Pelton, R. An Nmr Investigation of the Interaction of Polyethylene Oxide with Water-Soluble Poly(Vinyl Phenol-co-Potassium Styrene Sulfonate). J. Polym. Sci., Part B: Polym. Phys. 2000, 38, 1276−1284. (9) Lu, C.; Richardson, R.; Pelton, R.; Cosgrove, T.; Dalnoki-Veress, K. Peo Penetration into Water-Plasticized Poly(vinylphenol) Thin Films. Macromolecules 2004, 37, 494−500. (10) Cong, R. J.; Pelton, R. The Influence of PEO/Poly(vinyl phenol-co-styrene sulfonate) Aqueous Complex Structure on Flocculation. J. Colloid Interface Sci. 2003, 261, 65−73. (11) Cong, R. J.; Pelton, R.; Russo, P.; Doucet, G. Factors Affecting the Size of Aqueous Poly(vinylphenol-co-potassium styrenesulfonate)/ Poly(ethylene oxide) Complexes. Macromolecules 2003, 36, 204−209. (12) Pelton, R.; Xiao, H. N.; Brook, N. A.; Hamielec, A. Flocculation of Polystyrene Latex with Mixtures of Poly(P-vinylphenol) and Poly(ethylene oxide). Langmuir 1996, 12, 5756−5762. (13) Pezron, E.; Ricard, A.; Lafuma, F.; Audebert, R. Reversible Gel Formation Induced by Ion Complexation 0.1. Borax Galactomannan Interactions. Macromolecules 1988, 21, 1121−1125. (14) Springsteen, G.; Wang, B. H. A Detailed Examination of Boronic Acid-Diol Complexation. Tetrahedron 2002, 58, 5291−5300. (15) Kuzimenkova, M. V.; Ivanov, A. E.; Galaev, I. Y. BoronateContaining Copolymers: Polyelectrolyte Properties and Sugar-Specific Interaction with Agarose Gel. Macromol. Biosci. 2006, 6, 170−178. (16) Niwa, M.; Sawada, T.; Higashi, N. Surface Monolayers of Polymeric Amphiphiles Carrying a Copolymer Segment Composed of Phenylboronic Acid and Amine. Interaction with Saccharides at the Air-Water Interface. Langmuir 1998, 14, 3916−3920. (17) Ivanov, A. E.; Galaev, I. Y.; Mattiasson, B. Interaction of Sugars, Polysaccharides and Cells with Boronate-Containing Copolymers: From Solution to Polymer Brushes. J. Mol. Recognit. 2006, 19, 322− 331. (18) Zhang, D.; Thompson, K. L.; Pelton, R.; Armes, S. P. Controlling Deposition and Release of Polyol-Stabilized Latex on Boronic Acid-Derivatized Cellulose. Langmuir 2010, 26, 17237− 17241. (19) Pelton, R.; Hu, Z.; Ketelson, H.; Meadows, D. Reversible Flocculation with Hydroxypropyl Guar-Borate, a Labile Anionic Polyelectrolyte. Langmuir 2009, 25, 192−195. (20) Zhang, D. Controllable Self-Assembly Based on Interaction of Boronic Acids and Diols; McMaster University: Hamilton, 2011. 3119

dx.doi.org/10.1021/la204514y | Langmuir 2012, 28, 3112−3119