Like-charged nanoparticles, NPs, can assemble in water into large, faceted crystals, each made of several million particles. These NPs are functionalized with mixed monolayers comprising ligands terminating in carboxylic acid group ligands as well as positively charged quaternary ammonium ligands. The latter groups give rise to electrostatic interparticle repulsions which partly offset the hydrogen bonding between the carboxylic acids. It is the balance between these two interactions that ultimately enables self-assembly. Depending on the pH, the particles can crystallize, form aggregates, remain unaggregated or even-in mixtures of two particle types-can choose whether to crystallize with like-charged or oppositely charged particles.open

Pillai, Pramod P.

Kowalczyk, Bartlomiej

Grzybowski, Bartosz A.

Nanoscale

Piersma, T

University of Groningen Research Database

Estimating energy reserves of Great Crested Grebes <i>Podiceps cristatus</i> on the basis of body dimensions

Piersma, T.

ARTS repository - University of Groningen

ScholarWorks@UNIST

NanoscaleCOMMUNICATIONCite this: Nanoscale, 2016, 8, 157Received 9th October 2015,Accepted 10th November 2015DOI: 10.1039/c5nr06983awww.rsc.org/nanoscaleSelf-assembly of like-charged nanoparticles intomicroscopic crystals†Pramod P. Pillai,a,b Bartlomiej Kowalczyk*a,c and Bartosz A. Grzybowski*dLike-charged nanoparticles, NPs, can assemble in water into large,faceted crystals, each made of several million particles. These NPsare functionalized with mixed monolayers comprising ligands ter-minating in carboxylic acid group ligands as well as positivelycharged quaternary ammonium ligands. The latter groups give riseto electrostatic interparticle repulsions which partly oﬀset thehydrogen bonding between the carboxylic acids. It is the balancebetween these two interactions that ultimately enables self-assem-bly. Depending on the pH, the particles can crystallize, form aggre-gates, remain unaggregated or even – in mixtures of two particletypes – can “choose” whether to crystallize with like-charged oroppositely charged particles.Self-assembly of nanoparticles into three-dimensional (3D)crystals is motivated by the potential uses of these structuresin bio-sensing,1 optoelectronics,2 chemical amplifiers,3 or cata-lysis.4 The methods of obtaining such structures employcovalent or non-covalent molecular interactions,5,6 dipole–dipole interactions,7 or electrostatic forces,3,8–13 many exhibit-ing eﬀects peculiar to the nanoscale.14–19 While in numerouscases, the formation of 3D NP crystals was achieved using onlyone type of particle, these systems were typically based on NPssuspended in non-polar solvents and interacting via van derWaals forces.20 On the other hand, crystallization in aqueoussolutions has generally required the use of binary NP mixtures– e.g., mixtures of particles functionalized with oppositelycharged ligands3,5,8,12,15 or with complementary DNAstrands.9,21–24 Here, we attempted to assemble NP crystals inaqueous solutions from only one type of particle. The distinc-tive feature of our method is that rather than having diﬀerent/complimentary chemical functionalities on diﬀerent NPs, ourparticles combine two types of functionalities on each NP(Fig. 1a). One of these functionalities (carboxylic acids) iscapable of hydrogen bonding whereas the other (quaternaryammonium salts) gives rise to electrostatic repulsions betweenthe NPs. Perhaps counterintuitively, micron-sized NP crystalsFig. 1 (a) Scheme of nanoparticles decorated with mixed SAMs com-prising ligands illustrated in the legend. Crystals form when carboxylicacid groups are protonated and capable of hydrogen bonding. (b) Owingto the presence of both acidic and basic groups, the NPs are stable inwater both at low and high pH values and precipitate only at the pH atwhich the net charge on the NPs is zero (see ref. 26).†Electronic supplementary information (ESI) available: Further experimentaldetails, images of crystals as well as NMR and EDX spectra. See DOI: 10.1039/c5nr06983aaDepartment of Chemistry and Department of Chemical Engineering, NorthwesternUniversity, 2145 Sheridan Road, Evanston, Illinois, USAbDepartment of Chemistry, Indian Institute of Science Education and Research(IISER) Pune, Dr. Homi Bhabha Road, Pashan, Pune, 411008, Indiac3M Purification Inc., 400 Research Parkway, Meriden, CT 064503, USA.E-mail: bkowalczyk@mmm.comdIBS Center for Soft and Living Matter and the Department of Chemistry, UlsanNational Institute of Science and Technology (UNIST), Ulsan, South Korea.E-mail: grzybor72@unist.ac.krThis journal is © The Royal Society of Chemistry 2016 Nanoscale, 2016, 8, 157–161 | 157Published on 30 November 2015. Downloaded on 14/01/2016 10:49:18. View Article OnlineView Journal  | View Issueform when the NPs are all like-charged. Under these con-ditions, however, interparticle electrostatic repulsions serve topartly oﬀset and “balance” the hydrogen bonding (attractive)interactions that would – by themselves – have resulted inaggregation/flocculation rather than crystallization. Interest-ingly, as the pH and the balance between H-bonding andelectrostatic interactions change, the mixed charge (MC) NPscan “choose” whether and with which other types of NPspresent in solution to co-crystallize. Overall, our approachillustrates that by appropriately designing mixed-ligand shellson nanoscale objects it is possible to modify interparticlepotentials and flexibly guide nanoscale self-assembly involvingseveral types of interactions.We used gold nanoparticles (AuNPs) synthesized accordingto a previously published procedure.25 These NPs were initiallystabilized with dodecylamine (DDA) ligands and were preparedin five batches characterized by diﬀerent core diameters, 4.2 ±0.5 nm, 5.5 ± 0.5 nm, 8.0 ± 0.5 nm, 9.5 ± 0.8 nm and 11.5 ±0.5 nm (average sizes and size distributions were determinedby TEM analysis of ∼200 nanoparticles for each size). Next, theNPs were functionalized via a ligand exchange reaction withmixed self-assembled monolayers (mSAMs)26 comprisingneutral 11-mercaptoundecanoic acid (MUA) and positivelycharged N,N,N-trimethyl(11-mercaptoundecyl)ammoniumchloride (TMA). This was done by soaking the DDA/AuNPs in a2 : 1 mol : mol mixture of MUA and TMA thiols (αsoln = CMUAsoln /CTMAsoln = 2) in toluene/dichloromethane (20 mL/10 mL) for 15 h.The concentration of AuNPs (0.15 mM in terms of Au atoms)and the total concentration of thiols (0.15 mM) were kept con-stant, and the thiols were in ∼40 fold excess with respect tothe number of adsorption sites on the NPs. After ligandexchange, the unbound thiols were removed by several cyclesof precipitation using dichloromethane followed by washingwith copious amounts of acetone. The purified particles werethen dissolved in water and the pH of all the solutions wasadjusted to either pH = 4 (by addition of 0.2 M HCl) or pH =11 (using 0.2 M tetramethylammonium hydroxide). As illus-trated in Fig. 1b, the NPs were stable in water at both of thesepH values. The composition of the two ligands in the mSAMson the NP surfaces was determined by dissolving NP coresusing molecular I2 followed by1H NMR analysis (for details,see ESI†); it was found that the ratio of surface concentrationsof MUA to TMA thiols was αsurf = CMUAsurf /CTMAsurf = 1.6. NPs fromall batches were also characterized by TEM, UV-Vis, DLS andzeta potential measurements.For each crystallization, a solution of AuNPs (2–3 μmol interms of gold atoms; prepared by diluting 150 μL of 15 mMstock with DI water to 2 mL) was diluted by adding half of thesample’s volume (i.e., 1 mL) of dimethyl sulfoxide (DMSO).The solution was heated to 65 °C to slowly evaporate water andinitialize the crystallization process. After ∼36 h, the super-natant solution became colorless and a black powder settled atthe bottom of the vial. The remaining DMSO was decantedcarefully and the powder was washed several times with an-hydrous acetonitrile to remove excess salts. SEM, TEM andSmall Angle X-ray Diﬀraction Spectroscopy (SAXS) were used todetermine whether the powder contained NP crystals (cf. ESI†for further experimental details).The main result of this work is that NP crystals could formeven when the particles were all positively charged. Specifically,at pH = 4, 4.2 nm, 5.5 nm and 8.0 nm NPs all formed regularlyfaceted crystals, each composed of several million NPs andwith dimensions up to 3 μm in each direction (Fig. 2a andFig. 3). Under these conditions, all carboxylic acids on NP sur-faces were protonated27 and the NPs had net positive chargesas evidenced by the values of the zeta potential being greaterthan zero (typically, tens of mV; see Fig. 2a). The crystals wereexamined by small angle X-ray diﬀraction spectroscopy (SAXS)exhibiting four distinct peaks located at scattering vectors, q =0.097 A−1, 0.108 A−1, 0.1728 A−1, and 0.2021 A−1 for 5.5 NPs(Fig. 3d). This diﬀraction pattern characterizes the sphaleritestructure8 with lattice constant a = 19.2 nm and with peakpositions corresponding to the Bragg reflections on planesspecified by Miller indices (111), (200), (220), and (311),respectively. In contrast, larger (9.5 nm and 11.5 nm) NPsformed disordered assemblies such as those illustrated inFig. 2c and d.Finally, at high pH – when the carboxylic acids were fullydeprotonated – the NPs were all negatively charged, had nega-tive zeta potentials (Fig. 2b) and remained stable in solutionrather than form crystals or disordered aggregates.These observations can be rationalized based on thebalance between attractive and repulsive interactions acting inthe system (Fig. 4). The former include vdW and hydrogenbonding, while the latter are electrostatic forces. As discussedin detail in our previous studies,14,27b the energies of vdWinteractions at small interparticle separations for ∼6 nmAuNPs covered with 1.5 nm-thick SAMs in water are up to∼10kTs whereas the hydrogen bonding between such sphericalparticles contributes few additional kTs. The electrostaticFig. 2 Four major types of structures observed for diﬀerent particlesizes and diﬀerent pH values: (a) smaller NPs (up to ca. 8 nm) form regu-larly faceted crystals at pH = 4; (b) unaggregated NPs are observed forall particle sizes at pH = 11; (c, d) non-crystalline aggregates areobtained from larger NPs at pH = 4.Communication Nanoscale158 | Nanoscale, 2016, 8, 157–161 This journal is © The Royal Society of Chemistry 2016Published on 30 November 2015. Downloaded on 14/01/2016 10:49:18. View Article Onlinerepulsions for such charged particles are ca. 10–20kTs strong.It follows that at low pH, the superposition of attractive andrepulsive potentials gives rise to a “shallow” net potential (fea-turing a small energetic barrier; see for example ref. 27b).Eﬀectively, the NPs are weakly attractive on the thermal, kT,scale and their self-assembly process is partly reversible allow-ing for the formation of ordered assemblies rather than therapid aggregation that would be expected for strong particleattractions.8 Such an aggregation is observed for larger par-ticles because the vdW forces scale with particle size. At highpH values there is no hydrogen bonding between the fullydeprotonated carboxylates and the electrostatic repulsionsdominate.Few further remarks are in order. One question that arisesis whether the mixed SAMs are indeed necessary to observecrystallization – in particular, one might imagine NPs coveredwith only COOH terminated thiols for which, at some inter-mediate pH, enough carboxylic acids would be deprotonatedto give rise to electrostatic interparticle repulsions that balanceout the sum of vdW and (remaining) hydrogen bonding attrac-tions. In such a system, however, water solubility becomes aproblem as the particles precipitate at lower pH values – thereare potentially ways around this complication (e.g., adjustingthe dielectric constant of the solvent) but formation of crystalsremains problematic as we have indeed experienced in manyunsuccessful experiments with such particles. In contrast, asillustrated in Fig. 1b, the mixed-SAM NPs are readily soluble atboth low and high pH values and at either negative or positivenet particle charges. The only regime where these NPs precipi-tate is when the negative and positive charges are compen-sated and the net charge is zero.The second question to address is the composition of themSAMs – here, we used αsurf = CMUAsurf /CTMAsurf = 1.6 but otherratios can be prepared, as described in detail in ref. 26a. Atlower ratios, when there are more TMAs than MUAs on the par-ticles’ surfaces, the NPs do not crystallize. On the other hand,at ratios higher than 1.6, the hydrogen bonding at low pHdominates electrostatic repulsions and rapid precipitationrather than controlled NP crystallization ensues. This is illus-trated in ESI Fig. S3† where for αsurf = CMUAsurf /CTMAsurf = 2.5 the NPsform some disordered/non-crystalline aggregates whereas forαsurf = CMUAsurf /CTMAsurf = 7.7, only a completely amorphous precipi-tate is observed. It follows that precipitation requires the“right” balance between electrostatic repulsions and vdW/H-bonding attractions and there must be enough TMA groupson the surface to oﬀset the hydrogen bonding.The dependence of net charge on our NPs on the pH opensadditional possibilities for controlling not only the self-assem-bly of such particles but also co-assembly with other chargedspecies present. This is illustrated by experiments in whichour TMA/MUA AuNPs were mixed with like-sized, TMA-coveredAg NPs (positively charged at all pH values). The use ofdiﬀerent metal cores allowed us to quantify the elementalcomposition of the crystals that formed by Energy DispersiveX-ray (EDX) spectroscopy. At pH ∼ 4, the MUA/TMA AuNPs hadnet positive charges (cf. Fig. 2a) and were capable of interact-ing with each other through hydrogen bonds. Under these con-ditions, the MUA/TMA AuNPs assembled predominantlybetween themselves rather than with the like-charged but non-hydrogen-bonding Ag TMA particles. Consequently, the crys-tals that formed (Fig. 5a) comprised ∼85% of Au, with only∼12% of Ag (rest is S, Cl, C etc.; see Fig. S4† for elementalmapping). The presence of this small percentage of Ag NPscould be attributed to their physisorption onto the crystals pre-cipitating from DMSO since TMA NPs are poorly soluble inthis solvent. In contrast, at pH ∼ 11, the MUA/TMA particleshad negative charges, and were not able to assemble with eachother, but readily co-crystallized with the positively chargedTMA NPs to give crystals that contained roughly equalFig. 4 Graph plotting the average sizes of the forming structures as afunction of NP size for low (red line) and high (blue line) pH.Fig. 3 Characterization of NP crystals. SEM images of 3D crystalsresulting from self- assembly of (a) 4.2 nm, (b) 5.5 nm and (c) 8.0 nmAuNPs (αsurf = CMUAsurf /CTMAsurf = 1.6) at pH ∼ 4. Individual NPs are clearlyvisible at the crystals’ surfaces. Corresponding large area SEM imagesare shown in the insets while (d) gives the small angle powder XRDspectra of the crystals. These spectra are congruent with the sphaleritecrystal structure.Nanoscale CommunicationThis journal is © The Royal Society of Chemistry 2016 Nanoscale, 2016, 8, 157–161 | 159Published on 30 November 2015. Downloaded on 14/01/2016 10:49:18. View Article Onlineproportions of each type of NPs (Fig. 5b and Fig. S5† forelemental mapping).This last example illustrates the flexibility of mixed-chargeNPs in the design of nanomaterials – the major virtue of theseparticles is that because they remain water-soluble at both lowand high pH values, their charges and propensity to interactby hydrogen bonds can be tuned by adjusting the pH. An inter-esting avenue for future research, one that would build on theresults from Fig. 5 of the current paper, would be to performsequential self-assembly from mixtures of mixed-charged par-ticles and other charged objects – for instance, by first assem-bling all-AuNP crystals (held tightly by vdW interactions onceformed, see ref. 27b) at low pH and then depositing onto them– from the same solution – Au/Ag shells at high pH. This andsimilar strategies could underlie a new family of one-batchsyntheses leading to nano-assemblies of complex internalstructures.AcknowledgementsThe authors would like to thank Dr Scott Warren and Dr YongYan for helping with SAXS and EDXS measurements. Thiswork was supported by the Nonequilibrium Energy ResearchCenter, which is an Energy Frontier Research Center fundedby the U.S. Department of Energy, Oﬃce of Science, Oﬃce ofBasic Energy Sciences under Award DESC0000989. B. A. G. alsogratefully acknowledges personal support from the Institutefor Basic Science Korea, Project Code IBS-R020-D1.Notes and references1 M. Zayats, A. B. Kharitonov, S. P. Pogorelova,O. Lioubashevski, E. Katz and I. Willner, J. Am. Chem. Soc.,2003, 125, 16006.2 S. A. Maier, P. G. Kik, H. A. Atwater, S. Meltzer, E. Harel,B. E. Koel and A. A. G. Requicha, Nat. Mater., 2003, 2, 229.3 B. Kowalczyk, D. A. Walker, S. Soh and B. A. Grzybowski,Angew. Chem., Int. Ed., 2010, 49, 5737.4 J. Grunes, J. Zhu, E. A. Anderson and G. A. Somorjai,J. Phys. Chem. B, 2002, 106, 11463.5 C. J. Kiely, J. Fink, M. Brust, D. Bethell and D. J. Schiﬀrin,Nature, 1998, 396, 444.6 Y. Zhao, K. Thorkelsson, A. J. Mastroianni, T. Schilling,J. M. Luther, B. J. Rancatore, K. Matsunaga, H. Jinnai,Y. Wu, D. Poulsen, J. M. J. Fréchet, P. Alivisatos and T. Xu,Nat. Mater., 2009, 8, 979.7 D. A. Walker, K. P. Browne, B. Kowalczyk andB. A. Grzybowski, Angew. Chem., Int. Ed., 2010, 49,6760.8 A. M. Kalsin, M. Fialkowski, M. Paszewski, S. K. Smoukov,K. J. M. Bishop and B. A. Grzybowski, Science, 2006, 312,420.9 R. J. Macfarlane, B. Lee, M. R. Jones, N. Harris, G. C. Schatzand C. A. Mirkin, Science, 2011, 334, 204.10 E. V. Shevchenko, D. V. Talapin, N. A. Kotov, S. O’Brien andC. B. Murray, Nature, 2006, 439, 55.11 S. Srivastava, A. Santos, K. Critchley, K. S. Kim,P. Podsiadlo, K. Sun, J. Lee, C. Xu, G. D. Lilly, S. C. Glotzerand N. A. Kotov, Science, 2010, 327, 1355.12 B. Kowalczyk, K. J. M. Bishop, I. Lagzi, D. Wang, Y. Wei,S. Han and B. A. Grzybowski, Nat. Mater., 2012, 11,227.13 S. M. Rupich, E. V. Shevchenko, M. I. Bodnarchuk, B. Leeand D. V. Talapin, J. Am. Chem. Soc., 2009, 132, 289.14 K. J. M. Bishop, C. E. Wilmer, S. Soh and B. A. Grzybowski,Small, 2009, 5, 1600.15 D. A. Walker, B. Kowalczyk, M. O. de la Cruz andB. A. Grzybowski, Nanoscale, 2011, 3, 1316.16 P. Podsiadlo, G. Krylova, B. Lee, K. Critchley, D. J. Gosztola,D. V. Talapin, P. D. Ashby and E. V. Shevchenko, J. Am.Chem. 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Ed., 2010,49, 4589.Fig. 5 Characterization of crystals formed from mixtures of MUA/TMAAuNPs and TMA AgNPs at (a) pH ∼ 4 and (b) pH ∼ 11, respectively. EDXSmapping conﬁrms the presence of large quantities of Ag in crystalsformed at pH ∼ 11. Scale bar for (a) is 200 nm and for (b) is 1 μm.Communication Nanoscale160 | Nanoscale, 2016, 8, 157–161 This journal is © The Royal Society of Chemistry 2016Published on 30 November 2015. Downloaded on 14/01/2016 10:49:18. View Article Online25 N. R. Jana and X. Peng, J. Am. Chem. Soc., 2003, 125,14280–14281.26 (a) P. P. Pillai, S. Huda, B. Kowalczyk and B. A. Grzybowski,J. Am. Chem. Soc., 2013, 135, 6392; (b) D. Witt, R. Klajn,P. Barski and B. A. Grzybowski, Curr. Org. Chem., 2004, 8, 1763.27 (a) D. W. Wang, R. J. Nap, I. Lagzi, B. Kowalczyk,S. B. Han, B. A. Grzybowski and I. Szleifer, J. Am. Chem.Soc., 2011, 133, 2192; (b) D. W. Wang, B. Kowalczyk,I. Lagzi and B. A. Grzybowski, J. Phys. Chem. Lett., 2010,1, 1459.Nanoscale CommunicationThis journal is © The Royal Society of Chemistry 2016 Nanoscale, 2016, 8, 157–161 | 161Published on 30 November 2015. Downloaded on 14/01/2016 10:49:18. View Article Online

Self-assembly of like-charged nanoparticles into microscopic crystals

Dissertations of the University of Groningen

University of Groningen

Like-charged nanoparticles, NPs, can assemble in water into large, faceted crystals, each made of several million particles. These NPs are functionalized with mixed monolayers comprising ligands terminating in carboxylic acid group ligands as well as positively charged quaternary ammonium ligands. The latter groups give rise to electrostatic interparticle repulsions which partly offset the hydrogen bonding between the carboxylic acids. It is the balance between these two interactions that ultimately enables self-assembly. Depending on the pH, the particles can crystallize, form aggregates, remain unaggregated or even - in mixtures of two particle types - can "choose" whether to crystallize with like-charged or oppositely charged particles. © The Royal Society of Chemistry 2016112121sciescopu

Pillai, PP

Kowalczyk, B

Bartosz A. Grzybowski

IBS Publications Repository

Proceedings - University of Groningen

NARCIS 

Estimating energy reserves of Great Crested Grebes Podiceps cristatus on the basis of body dimensions

Self-assembly of like-charged nanoparticles into microscopic crystals

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ARTS repository - University of Groningen

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Proceedings - University of Groningen