Article pubs.acs.org/Langmuir
Electrostatic Repulsion-Controlled Formation of Polydopamine− Gold Janus Particles Haolan Xu,*,† Xiaokong Liu,† Ge Su,† Bin Zhang,‡ and Dayang Wang*,† †
Ian Wark Research Institute, University of South Australia, SA 5095, Australia Department of Chemistry, Tianjin University, Tianjin, 300072, P. R. China
‡
S Supporting Information *
ABSTRACT: Polydopamine (PDA)−Au Janus particles were obtained by simply adding HAuCl4 to a PDA particle suspension, prepared via selfpolymerization of dopamine in basic solution at room temperature. The structures of the PDA−Au particles are readily controlled by electrostatic repulsion between the constituent particles, which can be realized simply via adjusting the environmental pH. PDA−Au Janus particles are formed only in a narrow pH range of 2.5−3.0 due to the properly enhanced electrostatic repulsion between the Au particles growing on as-prepared PDA particles and between the Au and PDA particles. The obtained PDA−Au Janus particles can become interfacially active and self-assemble at oil/water interfaces as a result of spatially well-separated hydrophilic (PDA) and hydrophobic (Au) domains on the surfaces, reminiscent of amphiphilic molecules.
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INTRODUCTION Janus particles are of burgeoning interest because their anisotropic structures and surface chemistry enable them to mimic the self-organization properties of molecules, which yields unprecedented superparticle materials.1,2 Many techniques have been successfully developed to prepare Janus particles,3−6 such as the microfluidic technique, controlled phase separation, region-selective modification, surface nucleation, self-assembly of block polymers, space-confined assembly, electrospinning, and Pickering emulsion interfacial synthesis. However, it is still imperative to develop simple, efficient, and reliable methods to prepare Janus particles with excellent colloidal stability and dispersibility in large scale.7 Herein we have demonstrated a facile pathway to produce Janus particles, composed of well-separated polydopamine (PDA) and Au particles, via control of electrostatic repulsions between the Au particles growing on the surfaces of host PDA particles and between the Au and PDA particles. This can be easily realized by adjusting the pH of the reaction environment. Electrostatic interactions have been widely used to modify the surfaces of charged particles, for instance via alternating deposition of oppositely charged species such as polyelectrolytes and charged nanoparticles (NPs).8 Nonetheless, the electrostatic repulsion between these identically charged adsorbing species usually leaves behind patchy coatings on the particle surfaces, exemplified by the low density and inhomogeneous distribution of their first layer or several layers adsorbed on the particles.9 These patchy coatings manifest a nonuniform and incomplete charge conversion or discrete charge distribution on the particle surfaces during or after coating the charged particles with oppositely charged species, which usually implies a vulnerable electrostatic repulsion © 2012 American Chemical Society
between the resulting particles and even incurs particle agglomeration.10 This patchy coating character is definitely a major disadvantage for surface modification of colloidal particles. In the past, therefore, electrostatic surface modification has been deliberately optimized to minimize the patchy character of the surface coating via, for instance, appropriately increasing the ionic strength of the coating systems to weaken electrostatic interactions.11 In contrast, here we took advantage of the patchy coating character of electrostatic surface modification to produce Janus particles. By significantly enhancing the electrostatic repulsion between the Au particles growing on preformed host particles of PDA, we have successfully decorated one PDA particle with only one Au particle. Although the patchy character of electrostatic surface coatings has been well-recognized for decades, it has hardly been adopted for construction of Janus or patchy particles. The reason is mainly 2-fold. First, the patchy patterns of electrostatic surface coatings are irregular and are difficult to define and manipulate. Second, the electrostatic surface coating is usually implemented via adsorption of oppositely charged species on charged particles, in which the incomplete surface charge conversion of the coated particles associated with the patchy surface coating may cause particle agglomeration. To circumvent these issues, we grew positively charged Au particles on preformed, positively charged PDA particles at room temperature on the basis of the high reductive activity of the PDA (Scheme 1). We utilized the pH of the reaction environment to properly enlarge the surface charge density of Received: June 13, 2012 Revised: August 20, 2012 Published: August 20, 2012 13060
dx.doi.org/10.1021/la302394e | Langmuir 2012, 28, 13060−13065
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Scheme 1. Formation of Different PDA−Au Structures via Control of the Electrostatic Repulsion between the Particles: (a) Multidecorated PDA particle, (b) PDA−Au Janus Particle, and (c) Detached PDA and Au Particles
were shaken for 10 min and then kept still for another 3−4 h. The products were collected via centrifugation of the suspensions at 3000 rpm for 5 min. Preparation of Au-Decorated PDA Hollow Capsules. PDA hollow capsules were prepared according to the modified method reported previously.12g Briefly, 0.2 mL of hexadecane was added to 10 mL of NaOH (0.6 mM) solution. The mixture was then homogenized to produce hexadecane-in-water emulsions. Then 5 mg of dopamine was added to the resulting emulsions. After 2 h self-polymerization of dopamine at emulsion surfaces, PDA hollow capsules were formed. Different amounts of 1 M HCl (0 and 15 μL) and 0.2 mL of HAuCl4 (1 wt %) were then added to the PDA hollow capsule suspensions. The pH value of the reaction media was set as 2.98 and 2.67, respectively. After being shaken 10 min, the reaction media were kept still for 3−4 h. The products were collected via centrifugation and washed by ethanol three times to remove the hexadecane. Characterization. UV−vis adsorption spectra were recorded with a Cary 50 UV−vis adsorption spectrophotometer. TEM imaging was implemented with FEI CM100 at an acceleration voltage of 100 kV. SEM images were conducted with FEI Quanta 450 operated at 5 kV. The zeta potential of the particles was measured on Malvern zetasizer nano ZS.
both the growing Au and the PDA host particles to grow only one Au particle on one PDA particle (Scheme 1b). PDA has recently attracted intense attention due to its excellent biocompatibility,12a strong adhesion to a diversity of substrates,12b,c potential application in drug delivery,12d,e the free-radical-scavenging property,12f pH environment sensitivity and selectivity,12g and a range of secondary reactions with different inorganic and organic molecules.12b PDA particles have been simply synthesized via self-polymerization of dopamine in basic aqueous medium under ambient conditions.12f,g To date, however, PDA particles have been rarely utilized as a reactive platform to template the growth of inorganic particles and especially Janus particles.
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EXPERIMENTAL SECTION
Materials. Hydrogen tetrachloroaurate(III) (99.9%, metal basis) was purchased from Alfa Aesar. Dopamine hydrochloride, hexadecane, and NaOH were purchased form Sigma-Aldrich and used without purification. Milli-Q water used has a resistance of higher than 18.2 MΩ cm−1. Preparation of PDA Particle Suspension. Five milligrams of dopamine was dissolved in 10 mL of NaOH (0.6 mM) solution. The resulting solutions were shaken for 18 h, yielding PDA particle suspensions. The as-prepared PDA particle suspensions were directly used to synthesize PDA−Au Janus particles. Preparation of PDA−Au Janus Particles. Fifteen microliters of HCl (1 M) was added into the as-prepared PDA particle suspension, followed by adding 0.2 mL of HAuCl4 (1 wt %). The pH value of the suspension was lowered to ∼2.65. After being shaken 10 min, the reaction medium was kept still for 3−4 h. After centrifugation of the resulting suspension at 3000 rpm for 5 min, the obtained particles were redispersed in water and then dropped on a copper grid and Si substrate for transmission electron microscopy (TEM) and scanning electron microscopy (SEM) (without any Au, Pt, or C coating) characterization. When the PDA−Au Janus particles were washed with water four times, the Pickering emulsions were formed simply by shaking the mixture of the aqueous particle dispersion and hexadecane. When the resulting Janus particles were washed with water more than five times, the particle aggregates spontaneously formed. Residual Solution and Pure PDA Particle Suspension. After 18 h polymerization of dopamine in 0.6 mM of NaOH solution, the asprepared PDA particle suspension was centrifuged at 10000 rpm for 10 min. The supernatant solution was collected and is referred to as residual solution. The volume was brought to ∼10 mL by adding water. The PDA particles in the sediments were washed with water three times to remove the residual reactants and then redispersed in 10 mL of water, which is referred to as pure PDA particle suspension. Fifteen microliters of HCl (1 M) was added to 10 mL of residual solution and 10 mL of pure PDA particle suspension, respectively, followed by adding 0.2 mL of HAuCl4 (1 wt %). The reaction media
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RESULTS AND DISCUSSION As shown in Scheme 1, we first synthesized ∼230 nm PDA particles by 18 h self-polymerization of dopamine in water at pH 8.2 at room temperature (Figure 1a). After consecutive addition of HCl and HAuCl4 to the resulting PDA particle suspension (the pH of the resulting mixture medium lowered to 2.65), PDA−Au Janus particles were obtained. The considerable charge contrast observed in SEM images indicates that the resulting particles comprise spatially well-separated Au (bright due to the high conductivity of Au) and PDA parts (dark) (Figure 1b and 1c). This spatial surface chemical anisotropy can be readily identified by the element analysis (SEM line scanning) of the resulting particles; the bright part is predominantly composed of Au while the dark part is composed of carbon (Figure 1e and 1f). The Janus character of the resulting PDA−Au composite particle is further verified by TEM imaging (Figure 1d). The dark black part in the TEM image is Au because Au can adsorb more electrons in TEM due to its larger atomic number. We observed fairly rapid reduction of HAuCl4 to Au in the PDA particle suspension. Figure 2a shows that about 10 nm Au NPs are formed on PDA particles immediately after consecutive addition of HCl and HAuCl4 into the as-prepared PDA particle suspension. The sizes rapidly increase to 125−250 nm within 5 min (Figure 2b). The particle sizes remain little changed by further prolonging the reaction time. The low magnification SEM imaging suggests that most 13061
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both PDA particles and residual solution in the as-prepared PDA suspension were reductive. Pure PDA particles and residual solution were separated from the as-prepared PDA particle suspension via centrifugation. By analysis of the absorption spectra, the residual solution was found to contain a considerable amount of free dopamine and its slightly oxidized derivative, dopa-quinone (Figure S2, Supporting Information). Addition of HAuCl4 into the aqueous dispersion of pure PDA particles caused a number of 25−50 nm Au NPs on one PDA particle (Figure 3a). On the other hand, addition
Figure 3. (a) SEM image of the PDA−Au particles obtained via adding HAuCl4 into pure PDA particle suspension without residual molecules such as free dopamine and dopa-quinone (inset: corresponding TEM image). (b) SEM image of the Au particles obtained via adding HAuCl4 into residual solution containing dopamine and dopa-quinone molecules (inset: corresponding TEM image).
of HAuCl4 into the residual solutions yielded only 250−550 nm Au particles (Figure 3b). Thus, we can conclude that both PDA particles and, residual free dopamine and dopa-quinone in the as-prepared PDA particle suspension can reduce HAuCl4 to Au, but neither of them alone allows the formation of PDA−Au Janus particles. Figure 4 and Table 1 reveal a noticeable effect of the pH of the reaction medium on formation of PDA−Au particles. When
Figure 1. (a) SEM image of PDA particles via 18 h self-polymerization of dopamine. (b) Low and (c) high magnification SEM images of PDA−Au Janus particles obtained by HAuCl4 reduction in the asprepared PDA particle suspension. (d) TEM image of a PDA−Au Janus particle. (e, f) SEM line-scan element analysis of a PDA−Au Janus particle.
Figure 2. TEM images of PDA−Au Janus particles formed (a) immediately and (b) 5 min after consecutively adding HCl and HAuCl4 into the as-prepared PDA particle suspensions.
of the PDA−Au particles (>90%) are of Janus type (Figure S1a, Supporting Information) while some of the large Au particles (>350 nm) detached from the Janus particles (Figure S1b and S1c). Because the cavities on the detached Au particles matched well to the profiles of the PDA particles, one can conclude that the Au particles grow on preformed PDA particles. It has been reported that PDA can efficiently reduce noble metal ions of Au, Ag, Pt, etc.12b In our work, it was found that
Figure 4. SEM images of the samples obtained by reduction of HAuCl4 in the as-prepared PDA particle suspensions at pH 2.3 (a) and 2.0 (b). TEM images of the samples obtained by reduction of HAuCl4 in the as-prepared PDA particle suspensions at pH 3.0 (c) and 3.1 (d). 13062
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to the positively charged surfaces of PDA particles. We also expect that the Au particles formed on host PDA particles are positively charged due to adsorption of free dopamine and dopa-quinone from the PDA particle suspension. This has been clearly demonstrated by zeta potential (+40.1 mV at pH 2.7) of Au particles obtained via the reaction of HAuCl4 and residual solution containing free dopamine and dopa-quinone (Figure S4, Supporting Information). The zeta potential of Au particles is very close to that of PDA particles. Hence, the zeta potential of the growing Au particles is expected to be the same as those of PDA particles shown in Figure 5. In this scenario, there are electrostatic repulsions between the growing Au particles and between the Au and PDA particles. Exclusive growth of one Au particle on one PDA particle needs a sufficiently large surface charge density of a single Au particle to create a strong electrostatic hindrance to prevent other Au particles growing on the same PDA particles (Scheme 1b). Lowering this electrostatic hindrance (via the pH increase as shown in Figures 5 and 4c,d) leads to multiple Au particles decorated on the PDA particles (Scheme 1a). Similarly, in the pure PDA particle suspension without residual free dopamine and dopa-quinone molecules, because there is no charged species adsorbing on the surfaces of the growing Au particles, the electrostatic repulsion between the growing Au particles is weak, thus leading to PDA particles decorated by a number of Au particles (Figure 3a). Because PDA host particles and the Au particles growing on their surfaces are identically charged, growth of multiple Au particles on one PDA particle is unfavorable when the electrostatic repulsion between the growing Au particles is very strong. As shown in Scheme 1c, on the other hand, the growing Au particles may detach from the PDA host particles when the electrostatic repulsion between the Au and PDA particles becomes too strong (pH < 2.3, Figure 4a,b). This should also account for the fact that when the resulting Janus particles bear a large Au particle, the detachment of the Au particle is occasionally observed (Figure S1b,c). Thus, we can conclude that formation of PDA−Au Janus particles needs a pH window in which electrostatic repulsion both between the Au particles growing on PDA particles and between the Au and PDA particles is sufficiently strong (pH 2.5−3.0). As mentioned above, the presence of free dopamine and dopa-quinone in the as-prepared PDA particle suspension is essential for the genesis of PDA−Au Janus particles. We found that the free dopamine and dopa-quinone can not only adsorb on the Au particles growing on host PDA particles and make them positively charged but can also significantly increase the size of Au particles. The size of Au particles growing on PDA particles increases from an initial 10 nm to a final 125−250 nm within 5 min (Figure 2). Under the same conditions, however, only small Au NPs with sizes of 25−50 nm were obtained on PDA particles in the absence of free dopamine and dopaquinone after several hours of incubation (Figure 3a). The fast growth of a single large Au particle on an individual PDA particle should be favorable to create large electrostatic hindrance to block the entire surface of the PDA particles for additional Au particles to grow, thus guaranteeing formation of Janus particles. Otherwise, formation of multiple Au particles on one PDA particle should be inevitable. To further demonstrate the effect of the electrostatic repulsion between the Au particles growing on PDA host particles, we generated Au particles on the surfaces of PDA hollow capsules with sizes of several micrometers instead of PDA particles (∼230 nm), while other experimental conditions
Table 1. Effects of HCl and NaOH Concentrations and Environmental pH Values on the Structure of the PDA−Au Particles Obtained Thereof PDA solution, mL
HCl (1 M), mL
HAuCl4 (1 wt %), mL
NaOH (0.02 M), mL
pH
product multipatched particles multipatched particles Janus particles Janus particles Janus particles detached particles detached particles
10
0
0.2
0.6
3.12
10
0
0.2
0.1
3.02
10 10 10 10
0 0.015 0.02 0.1
0.2 0.2 0.2 0.2
0 0 0 0
2.96 2.65 2.52 2.31
10
0.2
0.2
0
2.04
the pH of the reaction medium is 2.3, the predominant products are Au particles with sizes of ∼600 nm in coexistence with original 230 nm PDA particles; only a tiny amount of PAD−Au Janus particles are visible (Figure 4a). Only large, separated Au particles with sizes of ∼1 μm are obtained at the reaction medium pH of
