A simple strategy to improve the interfacial activity of true Janus gold nanoparticles: a shorter hydrophilic capping ligand

A simple strategy to improve the interfacial activity of true Janus gold nanoparticles: a shorter hydrophilic capping ligand† Miguel Angel Fernandez-Rodriguez,aLimei Chen,bChristopher P. Deming,bMiguel Angel Rodriguez-Valverde,aShaowei Chen,bMiguel Angel Cabrerizo-Vilchez,aand Roque Hidalgo-Alvarez∗a Janus gold nanoparticles (JPs) of ∼4 nm-diameter half functionalized with 1-hexanethiol as hydrophobic capping ligand exhibit significantly higher interfacial activity, reproducibility and rheological response when the other half is functionalized with 1,2-mercaptopropanediol (JPs-MPD) than with 2-(2-mercaptoethoxy)ethanol (JPs-MEE), both acting as the hydrophilic capping ligand. The interfacial pressure measured by pendant drop tensiometry reaches 50 mN/m and 35 mN/m for the JPs-MPD at the water/air and water/decane interface, respectively. At the same area per particle, the JPs-MEE reveal significantly lower interfacial pressure: 15 mN/m and 5 mN/m at the water/air and water/decane interface, respectively. Interfacial dilatational rheology measurements also show an elastic shell behaviour at higher compression states for JPs-MPD while the JPsMEE present near-zero elasticity. The enhanced interfacial activity of JPs-MPD is explained in terms of chemical and hydration differences between the MPD and MEE ligands, where MPD has a shorter hydrocarbon chain and twice more hydroxyl terminal groups than MEE. Pickering emulsions can be thermodynamically stabilized by amphiphilic Janus nanoparticles (JPs) with a wettability anisotropy1–3.It is known that JPs show three times more adsorption energy than homogeneous nanoparticles2,4. Strong efforts have been made to synthesize and simulate JPs with different morphologies and surface chemistry to control the way in which these particles self-assemble at fluid interfaces5–9. Gold nanoparticles randmonly functionalized with 1undecanethiol and N,N,N-trimethyl (11-mercaptoundecyl) ammonium chloride are reported to become Janus-like when the capping ligands rearrange at the water/air interface10. Nevertheless, recently Reguera et al.11 demonstrated by neutron reflectivity that such rearrangement does not happen with gold nanoparticles functionalized with 1-octanethiol and 6-mercapto1-hexanol at the water/air interface. A way to obtain gold JPs with true separate domains is to selectively functionalize each hemisphere of the core with the desired capping ligands immobilizing the nanoparticles in a Langmuir balance12,13. It is fundamental to select the appropiate capping ligands that aBiocolloid and Fluid Physics Group, Applied Physics Department, Faculty of Sciences, University of Granada, Granada, Spain; E-mail: [email protected] bDepartment of Chemistry and Biochemistry, University of California, 1156 High Street, Santa Cruz, CA 95064, USA. †Electronic Supplementary Information (ESI) available: HRTEM micrographies and pendant drops pictures. See DOI: 10.1039/b000000x/ will confer the Janus character to the nanoparticles because of the significant dependence between these capping ligands and the interfacial activity of the final JPs. We propose a simple strategy to enhance the interfacial activity, rheological response and reproducibility through colloidal stability of true gold Janus nanoparticles. We synthesized Janus gold nanoparticles half capped by 1hexanethiol and the other half by 2-(2-mercapto-ethoxy)ethanol (JPs-MEE) or 1,2-mercaptopropanediol (JPs-MPD) in surfactantfree conditions as described in previous works (see Fig. 1)12–14. The sizes obtained by high resolution TEM measurements (see Fig. S1†) are 3.5±0.9nm and 3.7±1.9nm for the JPs-MEE and JPs-MPD, respectively. The electrophoretic mobility of both JPs was measured with a ZetaSizer Nano (Malvern) in a 10−2M sodium citrate MilliQ water solution to stabilize the electrical double layer obtaining µe,JPs−MEE = (−2.2±1.5)·10−8m2/(V·s) and µe,JPs−MPD = (−2.9±0.4)·10−8m2/(V·s). From previous works, the JPs-MEE showed an average macroscopic contact angle of (56.1±1.8)◦and (49.0±1.3)◦in each hemisphere, whereas the JPs-MPD showed (63.3±2.7)◦and (53.4±2.9)◦in their respective hydrophobic and hydrophilic hemispheres12,13. The contact angles are slightly lower for the JPs-MEE likely due to the fabrication process: for MEE adsorption, the close-packed monolayer of 1-hexanethiol covered gold nanoparticles was immersed in a water solution containing MEE13 and this might produce greater 1–5 | 1 SH OOH CH3 SH OH OH Fig. 1 MEE (left) and MPD (right) capping ligands. The SH group is the anchor group at the gold nanoparticle surface. 56 60 64 68 72 0 200 400 600 800 1000 1200 Surface Tension γ (mN/m) Time (s) 1.6⋅1012 JPs−MPD 3.2⋅1012 JPs−MPD 4.8⋅1012 JPs−MPD 8.0⋅1012 JPs−MPD Fig. 2 Surface tension evolution over the time after the deposition of different number of JPs-MPD in THF on the surface of an initial 5µL MilliQ water pendant drop and growth up to 45 µL. Same color curves correspond to different experiments with the same number of deposited JPs-MPD. After the solvent evaporation, the surface tension remained stable. hydrophilic capping ligand exchange and thus lower contact angle than for the JPs-MPD in which the functionalization with MPD was performed directly in the Langmuir balance, exchanging the MPD ligands from the water subphase12. The contact angle values in the hydrophilic hemispheres are similar within errors, which reflects that the differences between MPD and MEA are not reflected macroscopically in the contact angle and the wettability contrast (i.e. contact angle differences between hemispheres) is similar in both JPs-MEE and JPs-MPD within errors. The pendant drop tensiometry was conducted as follows: different amounts of Janus nanoparticles (JPs-MEE and JPs-MPD) dispersed in tetrahydrofuran (THF, HPLC grade) were deposited on a water pendant drop with a handheld microsyringe and a micropositioner. The surface tension was obtained by axisymmetric drop shape analysis upon THF evaporation, while the pendant drop volume was kept constant. The results in Fig. 2 show a decrease in the final surface tension after evaporation of THF which is higher as the concentration of JPS-MPD is increased (refer to previous work for the JPs-MEE similar characterization15). The experiments are highly reproducible as can be seen in the different runs for a fixed concentration of JPs (different curves with same color in Fig. 2). After the THF evaporation, growing and shrinking experiments were performed at 0.08 µL/sfor each JP concentration. Next, the pendant drop was immersed in decane and the growing and shrinking experiments were performed again. We plot the interfacial pressure (Π=γ0−γ, where γ0is 72.5mN/mfor the water/air (W/A) and 52.3mN/mfor the water/decane (W/O) interfaces and γis the measured interfacial tension) against the drop area per particle (Ap, the area of the pendant drop divided by the number of deposited JPs). A piecewise compression isotherm can be seen in Fig. 3a and Fig. 3b for W/A and W/O interfaces, respectively. The first remarkable fact is the lower interfacial activity of JPs-MEE compared to JPs-MPD at the same Apvalues. At the lowest Apreached for JPs-MPD, Πis 50mN/mand 35mN/mand 15mN/mand 5mN/mfor the JPs-MEE at the W/A and W/O interfaces, respectively. The highest Πvalues obtained for JPs-MEE, after further compression, are 30mN/mand 20mN/mat the W/A and W/O interfaces, respectively. Contrary to JPs-MEE, the compression isotherms for JPs-MPD at the W/A interface exhibit open cycles at the beginning of the experiments pointing out that the colloidal monolayer rearranges into a final state that is preserved in further compression cycles. This behaviour is attenuated at the W/O interface as the hysteresis cycles are much smaller (i.e. the upper compression and lower expansion curves are closer) which might be due to the fact that enough energy is provided to reach a more relaxed state when it is immersed in decane. Moreover, the low hysteresis of the JPs-MEE compression cycles compared to the JPs-MPD might be due to the lower interfacial activity of these particles. Since the fabrication process, hydrophobic capping ligand, wettability contrast, size and charge were similar for both JPs-MEE and JPs-MPD, the differences between interfacial activity must come from the interfacial activity of the MEE and MPD hydrophilic capping ligands. Whereas MEE has a longer hydrocarbon chain (four CH2and one oxygen) and one hydroxyl terminal group, the MPD has a shorter hydrocarbon chain (three CH2) and two hydroxyl terminal groups (see Fig. 1). The lower number of hydrocarbon groups and higher number of hydroxyl groups of MPD might result in higher hydration of the hydrophilic hemisphere of the JPs (i.e. establishing hydrogen bonds between water and the hydrophilic capping ligands). These chemical differences might play a decisive role in the final interfacial activity of these JPs. The interfacial dilatational rheology of the JPs was evaluated by ten periodic volume variations of 1µLfor different periods. When a periodic injection/extraction of volume is performed to the pendant drop, the interface tries to re-establish the equilibrium. This counteraction is represented by a complex quantity composed by a storage part and a loss part: E=Ed+iωηd(1) where Eis the surface dilatational modulus that accounts for the change in surface tension produced by a small change in a surface area, Edis the interfacial dilatational elasticity, ωis the oscillation frequency and ηdis the interfacial dilatational viscosity16. If the viscosity is negligible during the relaxation process after perturbation of the interface, the interface present an essentially elastic behavior. The extraordinary interfacial activity of JPs-MPD is reflected in the rheology results in Fig. 4a and 4b (see Fig. S2 and S3†). For both W/A and W/O interfaces, E decreases slightly and ηincreases for increasing periods. For the W/A interface, Edand ηdincreases clearly with the compression state of the colloidal monolayer. This trend is also observed for the W/O inter2 | 1–5 (a) Water/air interface 0 10 20 30 40 50 0 5 10 15 20 25 30 35 Π (mN/m) Areaparticle (nm2/particle) 1.6⋅1012 JPs−MEE 3.3⋅1012 JPs−MEE 5.0⋅1012 JPs−MEE 8.3⋅1012 JPs−MEE 16.6⋅1012 JPs−MEE 33.1⋅1012 JPs−MEE 49.7⋅1012 JPs−MEE 1.6⋅1012 JPs−MPD 3.2⋅1012 JPs−MPD 4.8⋅1012 JPs−MPD 8.0⋅1012 JPs−MPD (b) Water/decane interface 0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 35 Π (mN/m) Areaparticle (nm2/particle) 1.7⋅1012 JPs−MEE 5.0⋅1012 JPs−MEE 16.6⋅1012 JPs−MEE 1.6⋅1012 JPs−MPD 3.2⋅1012 JPs−MPD 4.8⋅1012 JPs−MPD 8.0⋅1012 JPs−MPD Fig. 3 Surface pressure against the area per particle for different number of JPs-MEE and JPs-MPD deposited at the (a) W/A and (b) W/O interfaces. For a more detailed characterization of the JPs-MEE, please refer to Fernandez et al.15 face but with lower values of both Edand ηd. The high Edvalue is a signal that the colloidal monolayer creates an elastic shell on the pendant drop at higher compression states. This elastic behaviour again suggests the ability of the JPs-MPD as emulsifiers. Further rheology experiments were performed for a fixed period of 10scomparing the response of JPs-MEE and JPs-MPD. The results in Fig. 4c and 4d point out that the JPs-MPD reach significantly higher Edand ηdvalues upon compression (i.e. lower Ap) than the JPs-MEE (∼10 times higher Edand ηdfor JPs-MPD than JPs-MEE at the W/A interface and 2times at the W/O interface), suggesting that the elastic shell behaviour is not present for the JPs-MEE. A final consideration must be taken into account for gold nanoparticles in the range of a few nanometers (i.e. less than 10nm), the adsorption energy at the interface is of the order of KBT17 and they are expected to easily leave the interface. Nevertheless, the stable interfacial tension over time after the THF evaporation, the closed growing/shrinking cycles and the dilatational rheology seem to point out that the JPs-MEE and JPs-MPD are irreversibly anchored at the W/A and W/O interfaces, probably due to its Janus character. In conclusion, the JPs-MEE and JPs-MPD are similar in fabrication process, hydrophobic capping ligand, wettability contrast, size and charge, but are functionalized with different hydrophilic capping ligand. The JPs-MPD exhibit a significantly higher interfacial activity at W/A and W/O interfaces. Moreover, the dilatational rheology suggests an elastic shell-like behaviour of the pendant drop when the JPs-MPD are deposited at W/A and W/O interfaces. This elastic shell behaviour seems to be absent with the JPs-MEE. This points out the importance of the chemical structure of the capping ligands in JPs to predict the interfacial activity and therefore their ability as emulsifiers. Shorter hydrocarbon chain and more hydroxyl terminal groups in the hydrophilic capping ligands seems to be a route to obtain enhanced interfacial activity of this kind of JPs via enhanced hydration of the hydrophilic hemisphere of the JPs. To the best of our knowledge, it is the first time that such high interfacial activity is obtained with ∼4nmdiameter gold nanoparticles in surfactant-free conditions. Acknowledgements This work was supported by the Spanish MINECO (projects MAT2013-44429-R and MAT2014-60615R), by “Junta de Andalucía” and FEDER (projects P10-FQM-5977 and P12-FQM1443) and by US National Science Foundation (DMR-1409396). Authors thank to Dr. J.A. Holgado-Terriza for the software Contacto c used for surface tension measurements. References 1 J. Hu, S. Zhou, Y. Sun, X. Fang and L. Wu, Chem. Soc. Rev., 2012, 41, 4356–4378. 2 A. Kumar, B. J. Park, F. Tu and D. Lee, Soft Matter, 2013, 9, 6604–6617. 3 B. T. T. Pham, C. H. Such and B. S. Hawkett, Polym. 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