pubs.acs.org/Langmuir © 2010 American Chemical Society
Controlling Gold Nanoparticle Stability with Triggerable Microgels Azwan Mat Lazim, Melanie Bradley, and Julian Eastoe* School of Chemistry, University of Bristol, Bristol BS8 1TS, U.K. Received May 12, 2010. Revised Manuscript Received June 10, 2010 The interaction of a photodegradable surfactant (PS, 4-hexylphenylazosulfonate, C6PAS) with microgels (MGs) of poly(2-vinyl pyridine) (MGA) in the protonated state (pH 3) has been investigated. Electrophoretic mobility measurements confirm that negatively charged PS interacts with positively charged MGA to form mixed PS-MG complexes. This was sensed by a decrease in the effective PS-MGA charge and a switch in sign of electrophoretic mobility, from positive to negative, with increasing PS concentration. After the addition of extra positive microgels (MGB), the system undergoes coflocculation. Incident UV irradiation was used to photolyze the anionic PS, effectively eliminating the headgroups, thereby lowering the electrostatic interactions between PS and MGA microgel networks. Consequently, a reversal of MGA charge occurred, leading to electrostatic repulsions and causing the MGs to reswell and redisperse, with both MGA and MGB now being positively charged and hence stabilized against coflocculation. Extending this approach, negatively charged gold nanoparticles (AuMES) have been incorporated into the PS-MGA complexes. Atomic absorption spectroscopy (AAS) showed that 100% of the AuMES particles were recovered after coflocculation of (PS-MGA)-AuMES complexes with MGB. Furthermore, approximately 75% of the AuMES could be redispersed after UV irradiation to restabilize the dispersion. This system provides an interesting method for phase separation and gold nanoparticle recovery for reuse and recycling.
1. Introduction Smart materials respond to changes in environmental stimuli such as pH, solvent composition, temperature, electric field, magnetic field, and light.1 Examples of smart materials include photoresponsive surfactants and polymers, which have gained attention owing to potential applications ranging from drug delivery and photopatterning to holographic sensors and mechanochromic materials.1-3 These polymers also offer new dimensions in the controlled release of highly valuable active compounds, which are being used in pharmaceuticals and drug delivery.4 Most common microgel systems (MGs) are based on poly(N-isopropylacrylamide) (PNIPAM) or the associated copolymers.5,6 As such, MGs can be designed to be responsive to a range of stimuli.1,3,5,7,8 For example, when functionalized with pH-ionizable groups such as carboxylic acids, the swelling/collapse behavior can be made pH-tunable. Temperature is another useful variable for controlling MGs, and even modest temperature changes can give rise to significant effects, especially above the lower critical solution temperature (LCST, ∼32 °C for PNIPAM). Above this LCST, the polymer undergoes a volume phase transition, and as a result, water is expelled from the microgel interior.3,6 Light-sensitive phenyl azo-containing surfactants can be photolyzed by UV, being first introduced by Sherrington et al.9 The *To whom correspondence should be addressed. E-mail: julian.eastoe@ bristol.ac.uk. (1) Nayak, S.; Lyon, L. A. Chem. Mater. 2004, 16, 2623–2627. (2) Sershen, S. R.; Westcott, S. L.; Halas, N. J.; West, J. L. J. Biomed. Mater. Res. 2000, 51, 293–298. (3) Saunders, B.; Vincent, B. In Encyclopedia of Surface and Colloid Science, 2nd ed.; Somasundaran, P., Ed.; Taylor & Francis: Boca Raton, FL, 2006, pp 5430-5444. (4) Eastoe, J.; Sanchez-Dominguez, M.; Cumber, H.; Burnett, G.; Wyatt, P.; Heenan, R. K. Langmuir 2003, 19, 6579–6581. (5) Hall, R. J.; Pinkrah, V. T.; Chowdhry, B. Z.; Snowden, M. J. Colloids Surf., A 2004, 233, 25–38. (6) Ballauff, M.; Lu, Y. Polymer 2007, 48, 1815–1823. (7) Pelton, R. Adv. Colloid Interface Sci. 2000, 85, 1–33. (8) Hui, D.; Nawaz, M.; Morris, D. P.; Edwards, M. R.; Saunders, B. R. J. Colloid Interface Sci. 2008, 324, 110–117. (9) Ian, R.; David, A.; Sherrington, C.; Paul, W. J. Chem. Soc. Chem. Commun. 1994, 19, 2245–2246.
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advantage of photosurfactants (PS) arises from the fact that photolysis triggers dramatic changes in properties (surfactant on/off switching) leading to many potential applications.10 For example, the much-researched PS sodium 4-hexylphenyl-azosulfonate (C6PAS) has been employed in various scenarios for triggering transformations of colloidal properties and phase stability,11,12 including photoflocculation of gold nanoparticles (AuNPs).13 For a surfactant to influence MG physical properties there must be favorable interactions, and many studies of polymer-surfactant systems have been conducted.14-16 The combination of positively charged vinyl pyridine MGs with a photolyzable anionic surfactant (sodium 4-hexylphenyl-azosulfonate C6PAS) was also investigated recently, generating light-sensitive photosurfactant microgel (PSMG) complexes.17 Initially, the systems are in a shrunken state, owing to neutralization of the positive MG charges by strong association with the negatively charged surfactants. After irradiation, these systems reswell because PS photolysis results in the elimination of the surfactant charged groups, splitting them away from the positively charged vinyl pyridine groups in the MG networks. These are quite straightforward systems to generate because standard MGs and PSs can be mixed together. It should be noted that other ways to obtain photoresponsive MGs have been demonstrated with Irie et al.18-20 pioneering the (10) Eastoe, J.; Vesperinas, A. Soft Matter 2005, 1, 338–347. (11) Vesperinas, A.; Eastoe, J.; Wyatt, P.; Grillo, I.; Heenan, R. K.; Richards, J. M.; Bell, G. A. J. Am. Chem. Soc. 2006, 128, 1468–1469. (12) Salabat, A.; Eastoe, J.; Vesperinas, A.; Tabor, R. F.; Mutch, K. J. Langmuir 2008, 24, 1829–1832. (13) Vesperinas, A.; Eastoe, J.; Jackson, S.; Wyatt, P. Chem. Commun. 2007, 3912–3914. (14) Thompson, L.; Bos, M.; Varga, I.; Gilanyi, T. Langmuir 2003, 19, 609–615. (15) Bradley, M.; Liu, D.; Keddie, J. L.; Vincent, B.; Burnett, G. Langmuir 2009, 25, 9677–9683. (16) Bradley, M.; Vincent, B. Langmuir 2008, 24, 2421–2425. (17) Bradley, M.; Vincent, B.; Warren, N.; Eastoe, J.; Vesperinas, A. Langmuir 2005, 22, 101–105. (18) Irie, M. Macromolecules 1986, 19, 2890–2892. (19) Irie, M.; Kunwatchakun, D. Macromolecules 2002, 19, 2476–2480. (20) Mamada, A.; Tanaka, T.; Kungwatchakun, D.; Irie, M. Macromolecules 2002, 23, 1517–1519.
Published on Web 06/24/2010
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Mat Lazim et al. Scheme 1. Photoresponsive Surfactant Microgels Denoted (PS-MGA) þ MGBa
a (A) C6PAS (PS) complexes with positively charged poly(2-vinyl pyridine) (MGA) to form negative PS-MGA complexes. (B) Coflocculation occurs when adding new positive microgels (MGB) to the anionic PS-MGA complexes. (C) UV irradiation photolyzes C6PAS, thereby switching the charge on the PS-MGA complex, causing reswelling and furthermore restabilising the separate MGA and MGB microgels.
field. They showed that UV light could cause the reversible bending of rod-shaped polyacrylamide gels, resulting in a contraction of MG networks containing triphenylmethane leucocyanides. Photothermal light-responsive MGs that deswelled with increased temperature were developed by Tanaka et al.21 A related system was introduced by Nayak and Lyon1 using lightsensitive chromophore malachite green in phototriggerable PNIPAM microgels. Expanding these principles, Kumacheva et al.22 invented photothermally responsive composite MGs doped with gold nanorods, which were very sensitive to near-infrared light. In colloidal systems, heteroflocculation offers the scope for applications in various fields such as oil and mineral recovery, ceramic materials, and wastewater treatment.5 Heteroflocculation relates to aggregation processes that are primarily driven by a difference in particle charge.5,8,23-25 The majority of the literature describes heteroflocculation induced by pH,5,8,23 ionic strength,5,26 and temperature,5 but no reports of heteroflocculation driven by UV light owing to photoresponsive surfactants have been found. Gold nanoparticles (AuNPs) continue to attract attention owing to numerous potential applications, including catalysis. For the entrapment and recovery of these high-value nanomaterials, different sorbents have been tested, including activated carbon,27 persimmon tannin gel,28,29 ion-exchange resins,30 tannin,31 and fungal biomass.32 Although the techniques above offer approaches for the separation of AuNPs, it is of interest to explore and develop alternative strategies that may be advantageous under certain circumstances. Interestingly, pH-adjustable heteroflocculating MGs have recently been introduced, which are also effective for AuNP recovery/reuse cycling.33 The findings show that the heteroflocculation of mixed MGs is reversible and also suitable for the efficient entrapment and recovery of AuNPs. In this paper, the general concept of MG heteroflocculation in combination with photoresponsive surfactants is applied to access new light-sensitive materials, as conceptualized in Scheme 1. With (21) Tanaka, T.; Suzuki, A. Nature 1990, 346, 345–347. (22) Gorelikov, I.; Field, L. M.; Kumacheva, E. J. Am. Chem. Soc. 2004, 126, 15938–15939. (23) Starck, P.; Ducker, W. A. Langmuir 2009, 25, 2114–2120. (24) Wang, Q.; Heiskanen, K. Int. J. Miner. Process. 1992, 35, 121–131. (25) Rohrsetzer, S. Colloid Polym. Sci. 1982, 260, 1129–1132. (26) Vincent, B.; Young, C.; Tadros, T. Faraday Discuss. Chem. Soc. 1978, 65, 296–305. (27) Navarro, P.; Vargas, C.; Alonso, M.; Alguacil, F. J. Gold Bull. 2006, 39, 93–97. (28) Nakajima, A.; Ohe, K.; Baba, Y.; Kijima, T. Anal. Sci. 2003, 19, 1075–1077. (29) Zhang, H.; Garris, J.; Charles, A. ASME Conf. Proc. 2004, 225–234. (30) Warshawsky, A.; Kahana, N.; Kampel, V.; Rogachev, I.; Meinhardt, E.; Kautzmann, R.; Cortina, J. L.; Sampaio, C. Macromol. Mater. Eng. 2000, 283, 103–114. (31) Ogata, T.; Nakano, Y. Water Res. 2005, 39, 4281–4286. (32) Khoo, K. M.; Ting, Y. P. Biochem. Eng. J. 2001, 8, 51–59. (33) Lazim, A. M.; Eastoe, J.; Bradley, M.; Trickett, K.; Mohamed, A.; Rogers, S. E. Soft Matter 2010, 6, 2050–2055.
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Table 1. Nomenclature for the Different Separate Chemical Components shorthand PS (C6PAS) MGA (P2VP) MGB (PNIPAMþ) AuMES
identity sodium 4-hexylphenyl-azosulfonate anionic photosurfactant positively charged microgels of poly(2-vinylpyridine) positively charged microgels of PNIPAM negatively charged gold nanoparticles
these PS-MG complexes, stability is controlled by both charge and light. First, anionic PS is complexed with cationic MGA microgels to form stable but shrunken PS-MGA complexes, which are then negatively charged. Next, the addition of another cationic microgel (MGB) induces instability by driving coflocculation with the PSMGA complexes. Finally, UV irradiation is used to photolyze the complexed PS, reinstating the original positive charges on MGA, reswelling that microgel component, and also stabilizing the mixed MGA/MGB dispersion. The heteroflocculation behavior was studied as a function of PS concentration and to demonstrate photoswitchable heteroflocculation/stabilization. An application was explored whereby light was used to restabilize an initially coflocculated system bearing incorporated AuNPs. The initially separated AuNPs, trapped in the flocculate, could then be dispersed through the entire system by photoinduced MG restabilization. The nomenclature for the different chemical systems used in this article is declared in Table 1, with shorthand used for mixtures and complexes found in Supporting Information.
2. Experimental Section 2.1. Materials. All reagents, unless otherwise stated, were purchased from Aldrich Ltd. The poly(2-vinyl pyridine) (MGA) microgel particles were synthesized using 2-vinyl pyridine (2VP, 97%), 2,20 -azobis(2-methylamidinopropane)dichloride initiator (V50, Waco 97%), and divinylbenzene (DVB, 80%). The positive PNIPAM microgel particles (MGB) were synthesized using N-isopropyl-acrylamide (NIPAM, 97%), N,N-methylenebisacrylamide (BA), and V50. Photoresponsive surfactant C6PAS was synthesized in house.4,10 All solutions were prepared in water purified by PureLab, Elga with a resistivity of 18.2 MΩ cm. For all samples, the pH was measured by using a pH meter (HI98127, pHep Hanna) and the pH was adjusted with small aliquots of concentrated HCl. 2.2. Preparation of Microgels. A surfactant-free emulsion polymerization (SFEP) method was used to prepare 1 wt % MGA dispersions as described previously.17 The synthesis was carried out in a three-necked round-bottomed flask equipped with a condenser and a nitrogen inlet. After adding 2VP (5 g) and DVB (0.025 g) to 475 mL of Milli-Q water, the mixture was Langmuir 2010, 26(14), 11779–11783
Mat Lazim et al. heated to 70 °C under nitrogen for about 30 min. A V50 solution (0.2 g in 20 mL water) was injected to initiate the reaction, which was left to proceed for 24 h. After synthesis, the MGs were then purified by dialysis against daily water changes for a week. The MGB particles were synthesized using methods described by Snowden et al.,5 with minor adjustments of reagents and quantities. In a 1 L three-necked round-bottomed reaction flask equipped with a stirrer, 0.50 g of cationic initiator V50 was added to 800 mL of water. Separately, 5.0 g of NIPAM and 0.51 g BA were dissolved in 200 mL of water. The dissolved reagents were added to the reaction flask and polymerization, filtration, and dialysis were carried out prior to the method described above.5,34 2.3. Preparation of PS-MG Complexes. All MG dispersions were prepared by diluting the MG product by a factor of 10 with a 1 mM KCl solution as the background electrolyte. Samples were then adjusted to pH 3. A stock PS solution (20 mM) was made up by placing 0.538 g of PS in a KCl solution (1 mM) and making it up to 100 mL in a volumetric flask. This stock solution was then diluted to the desired surfactant concentration in the range of 0.5-10.0 mM and added to the appropriate MG at a concentration of 0.10 wt % (MGA) or 0.05 wt % (MGB). Samples were adjusted to pH 3 and stirred for 30 min to obtain equilibrated surfactant-microgel complexes (Figure S8). All samples containing PS were covered with aluminum foil and kept in the dark in the refrigerator to minimize photodegradation.17 2.4. Preparation of (PS-MGA)-MGB Dispersions. To prepare mixed (PS-MGA)-MGB dispersions, samples were prepared by adding MGB to PS-MGA complexes and then the mixed dispersions were stirred for 30 min. The concentration of PS was kept constant (2.5 mM) while the concentrations of other components were varied. KCl solution (1 mM) was used to make up all samples to the same total volume. The component volumes used for sample preparation are shown in Table S1 in Supporting Information. 2.5. Gold Sol Synthesis. The preparation of AuMES followed the method reported by Davies et al.34 Two milliliters of a stock solution of HAuCl4 3 3H2O (1 wt %) was diluted to 180 mL with water. Twenty milliliters of an MES aqueous solution (10-3 M) was then added and stirred at 700 rpm. After 5 min, freshly dissolved NaBH4 (0.03 g) in a minimum of water was added to the reaction mixture. An immediate color change from pale yellow to brown was observed, indicating the formation of Au nanoparticles stabilized by MES (AuMES). Stirring was continued overnight to ensure complete reaction. The characterization of the particles is described elsewhere33 (e.g., TEM confirmed the existence of small, relatively polydisperse and spherical gold nanoparticles with an average diameter of 3.5 ( 1.3 nm). 2.6. Preparation of (PS-MGA)-AuMES Complexes. The (PS-MGA)-AuMES complexes were prepared on the basis of the quantities shown in Table S1 in Supporting Information. First, MGA was added to AuMES and stirred for 30 min before adding PS, and then stirring was continued for another 30 min. The samples were then covered with aluminum foil and kept in the dark in the refrigerator to minimize PS degradation.
2.7. Preparation of (PS-MGA)-AuMES þ MGB Dispersions. The procedures in section 2.6 were followed with the addition of MGB (Table S1 in Supporting Information). It took approximately about 2 to 3 h to resolve the two separate phases, with a yellow-tinged concentrated precipitate separating from a pale-yellowish upper majority portion.
2.8. Particle Size and Electrophoretic Mobility Measurements. The diffusion coefficients of the samples were measured by dynamic light scattering (DLS) using a Brookhaven Instruments Zeta Plus apparatus fitted with a 15 mW laser (λ = 678 nm). The Stokes-Einstein equation was used to calculate the effective particle hydrodynamic diameters from the diffusion coefficients. Electrophoretic mobilities were obtained using a Nano Z(Zen 2500) Malvern Instrument. (34) Davies, P. T.; Vincent, B. Colloids Surf., A 2010, 354, 99–105.
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Figure 1. Effective diameter of MGA (9) and MGB (2) particles at pH 3 as a function of added PS (C6PAS) concentration.
2.9. UV Irradiation. Samples were subjected to UV irradiation with a polychromatic 80 W Lot-Oriel mercury vapor arc lamp for 1 h. The spectroscopic characterization of PS C6PAS can be found elsewhere.9-13 The lamp was warmed for 20 min before starting the photolysis experiments. The samples were placed in UV-transparent quartz cuvettes along with a magnetic flea. The stirrer was then turned on, and the light was pointed directly at the sample at a distance of approximately 15 cm. The irradiation time is a function of many competing and interrelated factors (e.g., path length, concentration, optical density, incident light intensity, sample-to-source distance, lamp age, etc.). As already noted in the literature on photodestructible surfactants (e.g., refs 9-13), the irradiation time is simply effective: it is how long it takes to get the job done with the system of study and the light source used. 2.10. AAS. To determine gold levels, atomic absorption spectroscopy (AAS) was employed (Unicam 919 AA Spectrometer Solar instrument). For calibration, a series of aqueous solutions of HAuCl4 3 3H2O with concentrations in the range of 1-50 ppm were prepared. Aqua regia was used as a leaching agent to digest Au out of the MG complexes. All measurements were carried out in an air-acetylene flame. The wavelength was set at 242 nm, employing standard calibration and operating procedures for Au. The sample total volumes were 10 mL, and each Au determination was made twice, with the cycle being repeated using five separately made samples. The uncertainty in the AAS measurements is (0.10 ppm.
3. Results and Discussion 3.1. Characterization of PS-MG Complexes. Figure 1 shows the dependence of the effective diameter on PS concentration for both MGA and MGB microgels at pH 3, showing opposite trends. The effective diameter for PS-MGB complexes rose with PS concentration. This is consistent with reports in the literature on the interaction of charged surfactants with PNIPAM microgels35,36 and is the result of PS adsorption onto the microgel network, through hydrophobic interactions, which increases the concentration of sulfonate charged groups in the particles, causing microgel swelling through electrostatic repulsion. However, the hydrodynamic diameter of MGA microgels rapidly declined between 0 and 2.5 mM PS, consistent with trends reported previously.17 In this case, the anionic PS binds by electrostatic attraction with the positively charged microgel network, reducing charge repulsions within the particle, hence leading to particle deswelling with increasing surfactant concentration. (35) Tam, K. C.; Ragaram, S.; Pelton, R. H. Langmuir 1994, 10, 418–422. (36) Nerapusri, V.; Keddie, J. L.; Vincent, B.; Bushnak, I. A. Langmuir 2006, 22, 5036–5041.
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Figure 4. PS-MGA (A1) þ MGB to form (PS-MGA) þ MGB before (A2) and after (A3) UV irradiation. All dispersions were prepared at pH 3 and with 2.5 mM PS. Table 2. PS-MG Sample Properties at pH 3 and 2.5 mM PS before and after Irradiation effective diameter/nm
Figure 2. Electrophoretic mobility of MGA (9) and MGB (2) systems in pH 3 solution as a function of PS concentration.
Figure 3. Complexes of mixed (PS-MGA) þ MGB microgel systems at pH 3 as a function of PS concentraton in the range of 0.5-10 mM.
The electrophoretic mobility (μe) of MGA and MGB microgels as a function of PS concentration is shown in Figure 2. For both systems, the addition of surfactant results in a decrease in μe and charge reversal from positive to negative. The magnitude of negative charge reached is larger for MGA than for MGB, suggesting that surfactant binding is favored for the MGA particles. Higher adsorption of PS to the MGA microgel is also suggested by the effective particle diameters (Figure 1), indicating a larger surface charge contribution from surfactant on the smaller MGA microgels than on the larger MGB particles. 3.2. Mixed (PS-MGA) þ MGB Complexes and UV Irradiation. Mixed (PS-MGA) þ MGB microgels were prepared using formulations declared in Table S1 in Supporting Information. It should be noted that MGB was added to the initially formed PSMGA complexes. These mixed microgels were found to be stable at a PS concentration of 0.5 mM, and no flocculation was observed below 2.5 mM PS (Figure 3). Between 2.5 and 5.0 mM PS, the mixed microgels flocculated, taking approximately 3 h to achieve complete resolution. After that, there were two phases in equibrium, a lower yellow-tinged turbid concentrate separated from a paleyellowish upper majority portion (Figure 3). As shown in Figure 3, at 10 mM PS the system remained stable. The addition of MGB to the PS-MGA complex could potentially result in repartitioning of PS surfactant in the mixtures. Focusing on the region of instability (the flocculated samples), if the PS-MGA complex dispersion is diluted with electrolyte solution instead of MGB, then these dispersions are stable (Figure S2). Therefore, the MGB microgel drives the instability. These results indicate that the adsorption of PS onto the MGB network draws a sufficient concentration of surfactant away from 11782 DOI: 10.1021/la1018955
mobility (10-8 m2 V-1 s-1)
sample
before irradiation
after irradiation
before irradiation
after irradiation
PS-MGA PS-MGB
221 1000
447 875
-3.1 -1.1
0.5 -0.01
the PS-MGA particles, shifting them closer to the isoelectric point (Figure 2). At the same time, the adsorption of PS onto MGB also pushes these particles toward their isoelectric point (Figure 2); hence, flocculation is due to the loss of charge MG stability. Any surfactant concentration outside the range of 2.5-5.0 mM results in the mixed microgels having sufficient charge to remain stable. The novel approach to using a photoresponsive surfactant in these mixed MG systems is that UV irradiation can be used to change properties and stabilities quickly and noninvasively. When the flocculated (PS-MGA) þ MGB systems were subjected to UV irradiation, the samples were restabilized as shown in Figure 4, changing from two distinct phases into a homogeneous dispersion. Irradiation of the PS resulted in selective photolysis, generating predominantly 4-hexylphenol11 (mechanism in Supporting Information). The differences in particle size and mobility for individual MGs in the presence of 2.5 mM PS before and after irradiation are shown in Table 2. It is clear that UV irradiation causes a shift in particle size toward the native particles (without surfactant present, see Figure 1) and a shift in mobility from negative to positive.17 It is interesting that after UV-irradiation the byproduct of the surfactant degradation must contribute to the MG dispersion properties, although at the concentration investigated these effects are not significant. Using these individual dispersion properties as a guide, UV irradiation of the (PSMGA) þ MGB dispersions eliminates charge neutralization of the MGA and MGB particles, thereby switching the net MG charge from effectively zero to positive. The separate cationic MGA and MGB MGs therefore coexist in the dispersion through net electrostatic repulsion.
4. Application of Photoresponsive Surfactant Microgels for Controlling Nanoparticle Stability The efficient photoinduced separation and recovery of both C6PAS-stabilized AuNPs13 and silica NPs12 have already been demonstrated on the basis of the selective photolysis of the PS. The MGA MGs used here have already been shown to form complexes with both NPs34,37 and surfactants.15,17 In this section, it is shown that the coflocculation process outlined above can also be used as a method to control the dispersion state of AuNPs incorporated into the mixtures. (37) Yusa, S. I.; Yamago, S.; Sugahara, M.; Morikawa, S.; Yamamoto, T.; Morishima, Y. Macromolecules 2007, 40, 5907–5915.
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Table 3. (PS-MGA)-AuMES Properties at pH 3 and 2.5 mM PS before and after UV Irradiation effective diameter/nm
mobility (10-8 m2 V-1 s-1)
order of component before after before after addition irradiation irradiation irradiation irradiation MGA-AuMES MGB-AuMES (MGA-AuMES)-PS (PS-MGA)-AuMES
367 534 270 243
1.4 0.5 -3.2 -3.0
320 300
0.9 1.0
Table 4. AAS Results Following Gold Recovery ( 10-5 g mL-1) [Au] in heteroflocculated systems order of component addition
initial sample [Au]
(MGA-AuMES)-PS 1.5 (PS-MGA)-AuMES 1.5
upper phase
lower phase
[Au] in redispersed sample/Au recovery efficiency
0.0 1.44
1.5 0.07
1.1/75% 1.3/0%
The adsorption of AuMES nanoparticles into MGA microgels was recently studied,26 and previously observed trends in effective diameters and electrophoretic mobilities shown in Table 3 for MGA-AuMES complexes are replicated here. The addition of negatively charged AuMES causes a decrease in the apparent MGA hydrodynamic diameter (initial value in Figure 1) because of the uptake of AuMES into the positively charged network and a consequent decrease in the magnitude of MG positive charge (initial value in Figure 2). Alternatively, the effect of adding AuMES into MGB dispersions causes a reduction in MG surface charge without greatly influencing the hydrodynamic diameter because only the surface of MGB is charged as a result of polymerization initiator groups (Table 3). Therefore, AuMES adsorbs preferentially to the surface of MGB, causing some charge neutralization. To investigate the recovery of AuMES by MG coflocculation, the order of addition of AuMES and PS to MGA dispersions was studied. In the first mixing approach, MGA was added to AuMES, followed by PS (to give (MGA-AuMES)-PS), and in the second formulation, PS was added to MGA followed by AuMES (denoted (PS-MGA)-AuMES). The effective diameter and mobility results for these two dispersions are shown in Table 3 before and after UV irradiation. Significantly, the MGA-AuMES complex bears a positive charge, whereas the PS-MGA complex is negative (Table 2); therefore, PS can interact through electrostatic attractions with the first formulation and AuMES is repelled from the second. As expected, this determines the capacity for the mixed microgel systems to recover gold as shown by the results in Table 4 and discussed below. Regardless of the order of addition of AuMES and PS to MGA, the addition of MGB results in coflocculation. The appearance of the samples is similar to those shown in Figure 4 before and after UV irradiation. Before irradiation, the samples separate into two phases (a lower flocculated phase and a yellow upper supernatant): each of the phases was collected separately for AAS gold determinations. Table 4 shows Au levels for three
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different situations: (i) initial sample, (ii) upper and lower phases for separated coflocculated samples prior to UV exposure, and (iii) redispersed systems after UV irradiation. For the flocculated system where MGB was added to (MGAAuMES)-PS, AAS showed the absence of gold in the upper phase and the apparent gold levels were ∼1.5 10-5 g mL-1 in the lower phase. This confirms that the negatively charged AuMES nanoparticles have been separated and partitioned to the lower phase. After irradiation, the mixture was redispersed and the total gold content was approximately 1.1 10-5 g mL-1. Therefore, by this flocculation process it is feasible to trap in a densely separated phase ∼100%, and then after irradiation disperse about 75%, of the AuMES. However, if MGB is added to (PS-MGA)-AuMES, then AuMES remains dispersed because it is not adsorbed to MGA.
5. Conclusions This work extends recent applications of responsive microgels1,17-22,33 showing that microgels (MGs) and photosurfactants (PSs) can be combined to generate sophisticated multifunctional systems. Of particular interest here is the new observation that the photolysis of an appropriately charged PS, preadsorbed to an oppositely charged MG, can be used to control the charge sign and magnitude of the PS-MG complex and hence the overall electrostatic stability of a mixed system. The characteristics of mixed microgel dispersions bearing different charges with added anionic photosurfactant have been investigated. Electrophoretic mobility measurements show that adding photodegradable anionic surfactant PS (C6PAS) to cationic microgels changes the net MG charge from positive to negative with increasing PS concentration. Therefore, it is possible to switch the charge on initially positive MGA microgels by adding anionic PS to generate PS-MGA complexes with net negative charges. As depicted in Scheme 1, when positively charged microgels (MGB) were introduced into the PS-MGA dispersions, heteroflocculation occurred, leading to dispersion instability. After UV irradiation, stable MG dispersions could be recovered, owing to the elimination of the negative surfactants and the reestablishment of positive charges on both MGs (Figure 4, Scheme 1). On the basis of these promising findings, the concept of phototriggered microgels was extended to systems loaded with gold nanoparticles. These gold-containing MGs were shown to be effective systems for the support and entrapment of AuNPs, permitting one-shot triggerable changes in the stability of the encapsulated/dispersed gold. Therefore, these mixed complexes offer interesting and effective approaches to controlling gold nanoparticle dispersions. Acknowledgment. A.M.L thanks the Ministry of Higher Education of Malaysia and Universiti Kebangsaan Malaysia for the provision of Ph.D. scholarships. M.B. acknowledges funding from the Royal Society for a Dorothy Hodgkin Fellowship. Supporting Information Available: Additional details of the photosurfactant degradation mechanism, sample formulation, and control experiments. This material is available free of charge via the Internet at http://pubs.acs.org.
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