Scalable, Shape-Specific, Top-Down Fabrication Methods for the Synthesis de particulas coloidais

Scalable, Shape-Specific, Top-Down Fabrication Methods for the Synthesis de...

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DOI: 10.1021/la903890h ALangmuir X, X(X), X–X ©XXXX American Chemical Society

Scalable, Shape-Specific, Top-Down Fabrication Methods for the Synthesis of Engineered Colloidal Particles

Timothy J. Merkel,† Kevin P. Herlihy,† Janine Nunes,† Ryan M. Orgel,† Jason P. Rolland,‡ and Joseph M. DeSimone*,†,‡,§

†Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, ‡Liquidia Technologies, Research Triangle Park, North Carolina 27709, and §Department of Chemical and Biomolecular Engineering, North Carolina State University, Raleigh, North Carolina 27695

Received October 13, 2009. Revised Manuscript Received November 25, 2009

The search for a method to fabricate nonspherical colloidal particles from a variety of materials is of growing interest.

As the commercialization of nanotechnology continues to expand, the ability to translate particle-fabrication methods from alaboratorytoan industrial scaleisof increasing significance. In this feature article, weexamine several ofthemost readily scalable top-down methods for the fabrication of such shape-specific particles and compare their capabilities with respect to particle composition, size, shape, and complexity as well as the scalability of the method. We offer an extensive examination of particle replication in nonwetting templates (PRINT) with regard to the versatility and scalability of this technique. We also detail the specific methods used in PRINT particle fabrication, including harvesting, purification, and surface-modification techniques, with an examination of both past and current methods.


The design and fabrication of particles with dimensions on the colloidal size scale (<10 μm) has become an area of great and continually growing interestfor manyapplications. Particleshave been researched and developed for such varied applications as catalysis, microelectronics, photovoltaics, coatings, cosmetics, smart fluids, and nanomedicine, among others. These small particles are of fundamental interest not only because of the changes in optical, electrical, and other material properties that occur when the materialis reduced tothenanometersizescale but also because of other factors such as improved solubility properties or the capability of encapsulating bioactive materials (e.g., therepeutics or imaging agents), which can be poorly soluble or toxic, lacking the protection of the particle. Anisotropic colloidal particlescanassembletoformstructuresthataredistinctfromthe hexagonal close-packed arrangements favored by spherical particles and are of interest in the fields of memory storage, optical electronics, photonics, and sensors.1

Considerable effort has been devoted to the development of fabrication methods that can mass produce colloidal particles with fine control of size, shape, composition, cargo, and surface chemistry.Wesubmitparticlereplicationinnonwettingtemplates (PRINT) as a readily scalable method for the fabrication of such monodisperse colloids with precise control over size and shape and with broad capabilities with regard to material composition and chemical anisotropy. In addition to PRINT, we will present several other top-down particle-fabrication methods that have excellent potential for the scalable production of monodisperse, shape-specific particles on the colloidal size scale.

Two broad approaches are amenable to the fabrication of anisotropic particles: bottom-up and top-down techniques. Bottom-upapproachesbeginattheatomicormolecularscaleand build up to the desired particle size, whereas top-down methods process a material on the desired size scale. The most commonly employed methods for the production of mass quantities of particles on the colloidal length scale are bottom-up synthesis approachessuchasemulsionpolymerization.Inatypicalprocess, a monomer is emulsified via rapid stirring in a mobile phase that contains an initiator and a surfactant. Upon heating to activate the initiator, spherical particles are nucleated in the surfactant micelles and grow to the desired size. Particles obtained by emulsion methods are typically spherical and can vary in size from tens of nanometers to as large as several micrometers in diameter. The particle size and the molecular weight of the polymers formed in emulsions are controlled with parameters such as the surfactant concentration and reaction temperature. Surfactant adsorbed onto the surface of these particles can be difficult to remove if undesired. Although this method is extremely scalable, the particles fabricated in this way are typically spherical in shape and are fairly polydisperse.

There is a growing need to generate particles with a diversity of nonspherical shapes. Complex particle shapes are desirable for a range of applications, including self-assembly,2 photonic materials,3 and microelectromechanical systems (MEMS).4 Particles with nonspherical shapes are of increasing interest in biomedical applicationssuchasdrugdelivery,where therodlikeorcorkscrew morphologies possessed by many viruses and bacteria are suspected to have derived an evolutionary advantage from their specific shapes.5 For many applications, the ability to control particle size and shape is of vital importance, and the ability to manufacture large quantities of such size- and shape-specific particles is a crucial factor in the utility of the fabrication method. For a particle-fabrication method to prove industrially useful, it must be scalable and capable of producing usefully large quantities of particles. For example, a particle tested for efficacy as a drug carrier will have very different production needs in a

*To whom correspondence should be addressed. E-mail: desimone@ (1) Glotzer, S. C.; Solomon, M. J. Nat. Mater. 2007, 6, 557.

(2) Glotzer, S. C. Science 2004, 306, 419. (3) Lu, Y.; Yin, Y.; et al. Adv. Mater. 2001, 13, 415. (4) Beebe, D. J.; Moore, J. S.; et al. Nature 2000, 404, 588. (5) Champion, J. A.; Katare, Y. K.; et al. J. Controlled Release 2007, 121,3 .

B DOI: 10.1021/la903890h Langmuir X, X(X), X–X

Invited Feature Article Merkel et al.

laboratory setting than on the industrial scale. The fabrication of milligram quantities of particles suffices for testing in the laboratory, whereas the commercialization of this technology would require the production of gram and kilogram quantities to satisfy the needs of phase 1 or 2 clinical trials.

Although the focus of this feature article is on top-down methods, a brief review of bottom-up particle-fabrication methods may be instructive by way of comparison. Bottom-up approaches are readily scalable but can lack fine control of particle size and dispersity and can be limited in the variety of shapes that can be produced. Crystals of many metals and metal oxides can be grown into both spherical and anisotropic shapes such as cubes, rods, discs, and faceted polyhedra using nucleation and arrested growth strategies.1 More complex branched structures can be formed by the sequential growth of dots and rods of different materials.6 Selective crystallization and deposition methods have been developed to produce prisms, rods, arrows, and teardrop-shaped crystals of gold, silver, and cadmium selenide, respectively.7-9 Such nucleation and growth methods are limitedtoinorganics, and theshape selectivity ishighlydependent on the material and its crystal structure. Furthermore, these inorganic materials often lack the capability to encapsulate cargo or undergo surface modification.

Bottom-up approaches to the fabrication of organic particles have received a great deal of attention for use in biomedical applications such as imaging, gene or drug delivery. These methods chiefly rely on self-assembly to create particles such as micelles, vesicles, liposomes, and polymersomes that are able to encapsulate cargo in their otherwise hollow interior. In a typical preparation, molecules are synthesized that possess both hydrophilic and hydrophobic domains. Upon exposure to water, the molecules orient themselves to present their hydrophilic portion to the water and the hydrophobic domains orient inward, resulting in a sphere with a hydrophobic pocket. The hydrophobic interior is able to encapsulate hydrophobic cargo. Although spherical particles have been the standard, increasing attention has been directed toward higher-aspect-ratio particles that are achievable with control of the relative length of hydrophobic and hydrophilic domains.5,10 Recently, Discher et al. explored filamentous polymersomes called filomicelles for their circulation persistence and ability to encapsulate and deliver chemotherepeutics. The filomicelles were prepared from block copolymers with lipid-like amphiphilicity but a more symmetric ratio of hydrophilic to hydrophobic blocks than is found in lipids. Filomicelles were prepared with a hydrophilic block of poly(ethylene oxide) and a hydrophobic block of either poly(ethylethylene) or poly- (caprolactone). These filamentous particles persisted in circulation about 10 times longer than spheres with similar surface chemistry and were successful in encapsulating and delivering a hydrophobic chemotherapeutic to tumored mice.10 One criticism of these self-assembled systems is the dynamic nature of particles so assembled. With membrane components held together with attractive forces rather than with covalent bonds, the generated structures can add or lose components, making their size and shape more fluid than may be desired. The dynamic nature of these self-assembled systems has been addressed by Wooley et al. with shell-cross-linked knedel-like particles (SCKs).1 These blockcopolymermicellesaremodifiedwithreactivegroupsonthe surface that can be chemically cross linked after self-assembly, giving superior stability to their structure.

Top Down, Scalable Methods for Fabricating Shape-Specific Particles

Bottom-up fabrication methods tend to be readily scalable to large batches but typically offer limited control over size, size distribution, and particle shape. Whereas “crude” top-down methods, such as milling and grinding of bulk materials, have found utility in industry, they typically offer very little control over these same parameters of particle size, size distribution, and shapedeviatinggreatlyfromspherical.Severaltop-downparticlefabrication methods are excellent targets for the mass production of shape-specific particles, having the potential for superior control over the above parameters with the potential for scalability. In addition to the PRINT method favored by us, hardtemplate methods, microfluidic reactors, and particle stretching all show promise in this regard (Table 1).

Hard-Template Synthesis of Nanoparticles

For the last 20 years, hard-template methods have been used extensively to prepare nanowires, nanotubes, and nanorods.28 In this technology, porous templates are filled with one or more materials to fabricate monodisperse nanoparticles. The two most common templates used are anodized aluminum oxide (AAO) and track-etched polymer membranes. AAO, formed from the electrochemical oxidation of aluminum in an acid electrolyte, has a high density of uniform, parallel pores (as high as 1011 pores/ cm2)regularlyspacedonahexagonallattice.Templateshavebeen fabricated with pore diameters as small as 5 nm.29 Track-etched polymermembranesareformedviathebombardmentofpolymer films, such as polycarbonate, with nuclear fission fragments to create damage tracks that are then chemically etched into pores. These polymer templates contain randomly distributed, uniformdiameter pores, with pore densities approaching 109 pores/cm2 and pore diameters as low as 10 nm. In this case, however, the pores are not parallel and so may intersect. Traditionally, the templates are filled via electrochemical means. This is accomplished by first depositing a sacrificial metal layer, such as gold or silver, on one side of the template for electrical contact. Then, the material or materials of interest are electrochemically deposited into the pores of the template; for example, metals can be synthesized using the appropriate electroplating solution, and conducting polymers can be synthesized via oxidative polymerization. After deposition, the template can be dissolved; for example, AAO dissolves readily in acidic or basic solutions. This yields an ensemble of nanoparticles connected at the base to the

(6) Milliron, D. J.; Hughes, S. M.; et al. Nature 2004, 430, 190. (7) Jana, N. R.; Gearheart, L.; Murphy, C. J. J. Phys. Chem. B 2001, 105, 4065. (8) Manna, L.; Scher, E. C.; Alivisatos, A. P. J. Am. Chem. Soc. 2000, 122, 12700. (9) Jin, R.; Cao, Y.; et al. Science 2001, 294, 1901. (10) Geng, Y.; Dalhaimer, P.; et al. Nat. Nanotechnol. 2007, 2, 249. (1) Thurmond, K. B., 2nd; Remsen, E. E.; et al. Nucleic Acids Res. 1999, 27, 2966. (12) Hurst, S. J.; Payne, E. K.; et al. Angew. Chem., Int. Ed. 2006, 45, 2672.

(13) Hou, S.; Wang, J.; et al. J. Am. Chem. Soc. 2005, 127, 8586. (14) Hou, S.; Wang, J.; et al. Nano Lett. 2005, 5, 231. (15) Dendukuri, D.; Doyle, P. S. Adv. Mater. 2009, 21,1 . (16) Nisisako, T.; Torii, T. Lab Chip 2008, 8, 287. (17) Xu, S.; Nie, Z.; et al. Angew. Chem., Int. Ed. 2005, 4, 724. (18) Shepherd, R. F.; Conrad, J. C.; et al. Langmuir 2006, 2, 8618. (19) De Geest, B. G.; Urbanski, J. P.; et al. Langmuir 2005, 21, 10275. (20) Dendukuri, D.; Pregibon, D. C.; et al. Nat. Mater. 2006, 5, 365. (21) Shepherd, R. F.; Panda, P.; et al. Adv. Mater. 2008, 20, 4734. (2) Pregibon, D. C.; Toner, M.; et al. Science 2007, 315, 1393. (23) Hernandez, C. J.; Mason, T. G. J. Phys. Chem. C 2007, 1, 47. (24) Badaire, S.; Cottin-Bizonne, C.; et al. J. Am. Chem. Soc. 2007, 129, 40. (25), 2009. (26) Canelas, D. A.; Herlihy, K. P.; et al. Wiley Interdis Rev: Nanomed.

DOI: 10.1021/la903890h CLangmuir X, X(X), X–X

Merkel et al. Invited Feature Article substratemetal. It is occasionally usefulto leave the nanoparticles connected for electronic applications; however, the connecting sacrificial metal layer can also be dissolved to generate a monodisperse suspensionof high-aspect-ratio nanoparticles. Both tubes and solid wires or rods can be prepared depending on the surface treatment of the template. For example, gold nanowires can be synthesizedgalvanostaticallyin AAO; however,if the templateis first treated with (2-cyanoethyl)triethoxysilane, then the gold preferentiallydepositson the walls of the template poresand thus forms nanotubes instead of solid nanowires.30 Additionally, whereasthenanotubesaregenerallypreparedwithuncappedends, theycan be synthesizedwithoneend capped.31Thishas interesting implications with respect to the ability of nanotubes to act as effective biological delivery vectors.31 Nonelectrochemical methodshavealsobeendevelopedtofillthetemplates,suchassol-gel32 and layer-by-layer33 approaches to the fabrication of semiconductor(cadmiumselenide,zincoxide,andgraphitic) nanoparticlesand nonconducting polymer particles.34

Remarkably, Martin and co-workers13,14 have extended the template method to the fabrication of biological nanoparticles such as DNA and protein nanotubes. For the case of DNA nanotubes, this was made possible by pretreating the template pores with alternating R,ω-diorganophosphate (R,ω-DOP) Zr- (IV) chemistry. The first DNA segment binds to Zr(IV) through the phosphonate end of the DNA strand. Subsequent DNA segments are incorporated into the nanotube through hybridization with the preceding DNA segment. Thus, the resulting DNA nanotube has an outer skin composed of the (R,ω-DOP) Zr(IV) layer.13 A similar alternating chemistry approach was used in the fabrication of protein nanotubes;the AAO pores were first treated with3-amino propylphosphonicacid, followedbyprotein immobilization agent glutaraldehyde. Protein then reacted with glutaraldehyde, and additional alternating layers of glutaraldehyde and protein were deposited to thicken the nanotube walls.14 One crucial factor in these biological systems is the release of nanotubes from the template because the AAO template is dissolved under acidic or basic conditions. For the release of these nanotubes, the template was dissolved under very mild acidic conditions (1.5-5% phosphoric acid)13,14 at 0 C for 24 h. The biological activity of the nanotubes was confirmed after release. However, in the case of the protein nanotubes, it was noted that many of the tubes were broken during the dissolution and filtration steps.14

Onemajoradvantageofthetemplatemethodisthatsegmented or multicomponent nanoparticles can be straightforwardly fabricated (Figure 1).12,35 This has led to an impressive breadth of research into the area of postfabrication modifications of segmented nanoparticles. Multicomponent nanorods can be selectively functionalized for different multiplexing, sensing, and selfassembly applications.36-38 Mirkin et al. developed on-wire lithography (OWL) to create nanowires with nanoscale gaps along the length of the wire. For example, a segmented nanowire with alternating gold and nickel segments was synthesized electrochemically in an AAO template.37 After the nanowires were released and dispersed onto a substrate, gold or silicon dioxide wascoatedontoone side. The rodswere thenredispersed,


1. Comparison of Several

Highly Scalable,




Methods size limits shape specific ity par ticle comp osition/mate rial limitation s fabri cation capac ity cargo hard tem plate me thods

10 nm m for track- etched membran es, nm for

AAO 12 limited to rod/ cylind er morp holog ies:

high aspec t rati os possible me tals, po lymers, ino rganic compo unds , semicon ducto rs, 12 acid/ base-co mpatib le materials tem plates with up to pores/cm and protein microfluidics drop let-ba sed spher ical derivativ es, defi ned by chan nel architecture ph otopolym erizab le materials, low-m elting-po int oils, solub le polym ers mL/ m acrylate par ticles quantum do ts, colloid s, 18 prot eins 19 micoflu idics - flow met hods define b y channe l archit ecture and lithographic mas k ph otopolym erizab le materials only,

PDM S-com patible up to par ticles/min for μ m par ticles colloids,

21 DN parti cle stretchin spher ical derivativ es with comp lex geom etries po lystyrene,

10 parti cles pe r stretchin g appara tus proteins 5 photo lithography thic kness nm,

23 lat eral dimensio nm 24 define b y mask

-any arb itrary sha pe dra wn with compu ter- aided de sign ph otopolym erizab le materials but optima l for photore sists

8 parti cles/ min dyes, ino rganic nano parti cles 23 μ m shape defi ned a mas ter tem plate with nano met er resolu tion pro teins, active therepeu tics, solub le or melt processa ble polym ers, polym erizable materials

400 feet of mold/h mg/ min of sub-200

-nm particles, mg/m in of

5 μ m particles colloids and ima ging age nts,

26 fluoroph ores, prot eins,

27 drugs a The scalability of these methods has not been demon strated

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