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Interfaces in advanced materials
Current Opinion in Colloid & Interface Science 19 (2014) 43–48
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Current Opinion in Colloid & Interface Science journal homepage: www.elsevier.com/locate/cocis
Editorial overview
Interfaces in advanced materials☆
The importance of interfaces in advanced materials is becoming increasingly recognized in diverse disciplines and industrial sectors as supported by an increasing number of monographs in the last several years addressing surface modification chemistry, physics, and engineering [1–7] and by a much greater number of topical reviews [8–18]. The past forty years have seen publication of about 410,000 journal articles addressing the concept of interfaces in materials or chemistry, according to a recent SciFinder® database analysis. Over the years since 1972 the number of such articles has increased geometrically with scaling exponent of about 0.4 from 1973 to 1980 and 2.3 from 1989 to 2013. The breakpoint between these linear behaviors occurred approximately in 1985. In 2012 this number appears to have peaked with about 27,500 articles, dropping to about 26,300 in 2013, after over twenty years of geometrical growth. At the time of this writing, about 5700 articles had appeared in 2014, and this suggests a possible further decrease for 2014. Interfaces may arguably be credited as the glue that holds composite materials or multiphase materials together, and they thereby provide a unifying context in which to discuss and ponder material properties. In this Applications issue eleven papers discuss diverse materials and processes that are controlled by the physics, chemistry, and biology of interfaces. Passivation and functionality are provided by appropriate decoration of the respective interfaces. This passivation or interfacial stabilization may be represented by interfacial material or phase densities illustrated in Fig. 1, wherein an interfacial modifier, ι, “matches” the surface free energies of the respective phases and is the main chemical tool we have to molecularly engineer interfaces. This “matching” may comprise nonspecific adsorption (van der Waals force driven alkane-surface interactions), specific adsorption (hydrogen bonding, coulombic ion pairing), specific absorption (oriented solubilization of modifier in each phase or material, as in emulsion droplets and in diblock copolymer stabilized mesophases), and covalent interfacial bridging, wherein a modifier is covalently attached to one or both phases. In practical systems this “matching” involves laterally heterogeneous (spatially varying in a plane parallel to the interface) interactions, involving mixtures of the above described interaction types and other interactions not mentioned. The thermodynamics of such heterogeneously modified interfaces are difficult to formulate in terms of specific interactions, although specific structures may be examined and characterized through tools involving Monte Carlo and molecular dynamics methods. The passivation resulting from interfacial chemical modification can be understood for an α phase, having a very small radius of curvature, in contact with a β phase and having a positive interfacial free energy, γαβ, ☆ This paper was edited by Professor Krister Holmberg, Chalmers University of Technology, Göteborg, Sweden.
http://dx.doi.org/10.1016/j.cocis.2014.04.003 1359-0294/© 2014 Elsevier Ltd. All rights reserved.
through the Ostwald–Freundlich equation [19,20] for the dependence of solubility or activity, Sr, on radius of curvature. In this equation V is the molar volume, R is the gas constant, and T is temperature:
RT ln
γ αβ V Sr ¼ r S∞
This equation shows why interfacial energy (with concomitant interfacial modification) is the most significant access we have experimentally to stabilize interfaces of high curvature (low r), without modifying the bulk activity or solubility. It has been convincingly proven effective in the formulation of microemulsions and the stabilization (solubilization) of one immiscible liquid in another, wherein particular surfactants reduce γαβ nearly to zero. For liquids in equilibrium with their vapor, γαβ is the respective liquid's surface tension, and this Ostwald–Freundlich equation becomes the Kelvin equation [21]. This equation also explains the thermodynamic basis of Ostwald ripening, how passivation may be used to slow such ripening, and also how modifying the solubility of one phase in the other can also mitigate such ripening. The significance of various kinds of surface modifying approaches is illustrated in most of the companion reviews in this issue. Some of these processes result in traditional stabilization, others in directing binding affinity and cellular targeting, and others in providing new functionality and stimuli responsiveness. 1. Inorganic nanoparticles for therapeutic delivery Rotello et al. [22] provide a perspective of the evolution of nanoparticles, particularly inorganic nanoparticles, in drug delivery and diagnostic applications. Of greater importance is their highlighting of some major problems limiting the application and practical use of such colloidal materials. They point out how upconverting nanoparticles derived from less toxic phases than selenides, sulfides, and telurides (as are more conventional quantum dots, QDs) are showing great promise for replacing such QDs and for developing therapies based upon through skin and through tissue photo-illumination. Such advances in upconverting particles are being reported at increasing frequency, and include anti-Stokes fluorescent nanoparticles that can be driven by incoherent radiation [23], near infrared (NIR) bioimaging [24], and multianalyte immunoassays [25]. The importance of interfacial modification is intrinsic in all of these QD systems, toxic or not, and specific use of lipoic acid stabilizers, polyethyleneglycol polymeric stabilziers, zwitterionic stabilizers, protein corona, and cell receptor targeting species are discussed Rotello et al.
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Editorial overview
Fig. 1. Material densities, ρ, for α and β phases and for an interfacial modifier, ι, as a function of the interfacial normal, ξ.
2. Mesocrystals Ma and Cölfen [26] illustrate the evolution of mesocrystals as a class of supra-assemblies of nanoparticles emanating from the renaissance in templated zeolite synthesis of the last century [27]. The influence of bioinspired form for function [28,29] has provided not only insight into how nanoparticles can be assembled via appropriate templates [30], but insight and mechanisms very firmly founded in the role of speciation at the ionic and molecular level have been developed for these mesocrystals [31]. This area of materials is closely associated with two related but independently developed classes of three-dimensional particle-based materials, photonic crystals [32–34] and metamaterials and superlattices derived from nanocrystals and multiple types of nanocrystals [35,36]. Some insight into face centered cubic (fcc) assembly [37] competitive with that derived for calcium carbonate is available, but mechanisms deriving from speciation and additive effects on the molecular scale provide stimulating understanding and paths for modification [38–40]. Mention must be made of the pioneering contributions of Matijevic et al. in formulating inorganic microparticles composed of selfassembled nanoparticles. Over the course of 65 years or so Matijevic developed many approaches to precipitate inorganic nanoparticles and microparticles, and many of these were found to form quite uniformly sized and shaped particles, even though in some cases they were composed of much smaller nanoparticles. Some of these aspects were reviewed a few years ago [41], and some examples of such composite particles are illustrated in Fig. 2 and include Sr-doped barium titanate [41], hematite [41], and CuO [42]. A theory to explain the resulting aggreage or mesocrystal size has been presented [43,44].
3. Polymeric colloids Stimuli-responsive particles and particle assemblies are providing new materials for encapsulation and controlled release, sensors and diagnostics, thermoreversible stabilizers, phase transfer agents, targeted delivery, among other applications. Urban [45] highlights a variety of studies aimed at producing stimuli responsive coatings and films. These include hydrophobic–hydrophilic transitions accompanying stratified film formation, synthetic cilia formation and propulsion, and self-repairing films. Yuan et al. [46] present a comprehensive view of polymerized ionic liquid (PIL) particles, microparticles, and nanoparticles, 20 nm to 300 μm in diameter, derived by radical and condensation polymerization of imidazolium-based and phosphonium-based ionic liquid (IL)
Fig. 2. SEM of inorganic mesocrystals composed of inorganic nanoparticles (collected on filters): (a) Sr-doped barium titanate [41]; (b) hematite [41]; (c) CuO [42]. The scale bars in (a) and (b) corresponds to 1 μm and to 100 nm in (c). These frames are reproduced by permission; ©1996 Elsevier (frames a and b) and ©1997 Elsevier (frame c).
monomers. These PIL [47] represent a collection of new materials that exhibit stimuli responsiveness of various types. Thermal responsiveness accompanying lower critical solution temperatures (LCST) and anion responsiveness are discussed. Also discussed are early efforts to use PIL and PIL particles as stabilizers of nanocarbons such as SWCNT (single wall carbon nanotubes) and graphene. Another class of colloidal PIL derived from organotrialkoxysilanes are solvent-free nanofluids. These materials were introduced about eight years ago by the Giannelis group [48–58] and others [59–65]. They are usually composed of an inorganic core and are surface functionalized with organic salts that look like ionic liquids. The organotrialkoxysilanes themselves are usually ionic liquids of the tetraalkylammonium or sulfo type. In an early paper, nanofluids based on silica and on hematite cores were produced using an ammonium trialkoxysilane and a soft sulfonate counter ion exchanged for the chloride that accompanied the ammonium trialkoxysilane precursor. Similar materials (sometimes referred to as nano ionic materials, NIMs) have been developed using anionic
Editorial overview
surface groups [66] with diverse cationic counter ions (e.g., ammonium, imidazolium), where these materials also are well described as supramolecular ionic liquids. A core-free solvent free nanofluid was reported that exhibited crystallization frustration due to polydispersity and a pair of lambda transitions, one traversing the glass transition and another traversing the freezing transition [67]. In addition to these supramolecular ionic liquids or solvent-free nanofluids, applications in the development of ionic and nonionic nanofluids have also been developed by the groups of Xiong [68,69], Zheng [70–75], Nakanishi [76–78], and others [79–82]. Luminescent nanoparticle ZnO has been stabilized as a nanofluid using a zwitterionic acetate ammonium surface stabilizer [79]. Metal oxides such as SnO2 and TiO2 have been stabilized with corona of 350 Da PEGMe to produce nanofluids [80]. CdTe q-dots surface modified with mercaptoacetate and ion-exchanged with octadecyldi-PEG ammonium cations show that emission wavelengths can be thermally controlled by modifying the association of such nanofluids particles in the neat nanofluids during melting–freezing transitions [82]. Mann et al. have recently developed a nanofluid exhibiting a liquid crystalline phase and a liquid phase of a cationic ferritin protein, a solvent-free liquid protein [83]. Lin and Park introduced nanosilica templated nanofluids, ionic and nonionic, that exhibit CO2 sequestration activity [84]. Yu and Koch recently introduced a theoretical treatment of nanofluids cores attached to a corona of oligomers and demonstrated derivation of radial distribution functions among other results. This appears to be the first theoretical treatment of inorganic–polymer hybrid nanofluid liquid structure [85]. Further applications to produce nanocarbon nanofluids have also been made [86–89]. Combinations of ionic liquid and reactive-group monomers were used to create several new classes of resins, including UV-cured clearcoats and air-cured clearcoats [90–92]. Volume phase transitions are an important class of thermoreversible stimuli responsive phenomena for a variety of solvated polymeric systems, and these responses have been tied to a variety of diagnostic and sensing phenomena [93–96]. Co-nonsolvency in binary liquid mixtures, wherein solvency exists in each separate liquid component, provides an intriguing window through which to view polymeric solvation– desolvaton processes. Richtering et al. [97] examine the manifestation of water and methanol co-nonsolvency of poly(N-isopropylacrylamide) (PNIPAM) as a function of molecular weight and in particle morphologies of various size scales. These co-nonsolvency phenomena really force us to focus upon polymer solvation, desolvation, and solvent interactions at the molecular scale, and Richtering et al. include significant molecular dynamics studies in their discussions. Voronov et al. present a comprehensive view of some new reactive polymeric surfactant (RPS) chemistry based on polymeric surfactants having reactive peroxide functionality [98]. We are used to incorporation of anhydride and amine functionality, for general modification of surfaces, but this detailed review shows how peroxide functionality can be used in many different ways. These chemistries offer some new tools for modifying polymeric particle surfaces as well as modifying polymeric films. Drug, diagnostic, and nutraceutical delivery are intensive areas of research and range from formulating dispersions of nanocrystallne active [99], homogenization [100], precipitation [101,102] to the loading of vesicles [103], microgels [104], nano and microcapsules [105], polyelectrolyte encapsulation of actives, and other nanocarriers [106]. Lapitsky [107] addresses this last topic focused on polyelectrolytes that are crosslinked via multivalent ions. These ions range from divalent ones such as Ca+ to polyphosphate and polyethylenimine. Particle assembly and particle–particle interactions are discussed, as well as non-DLVO (Deryagin, Lifshitz, Verway, and Overbeek) theoretica approaches. 4. Self-assembled π-compounds Alkylated π compounds where rigid π-conjugated molecules are functionalized with flexible alkyl chains may be viewed as composing
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a class of amphiphiles. The rigid and flexible parts each have preferred solvents. Such systems are discussed by Nakanishi et al. [108], wherein applications are being made in flexible electronics, so charge transport properties are of interest along with phase and ordering transitions. These complicated molecules, similarly to appropriately functionalized nanoparticles [48], can be made as solvent free liquids. Here coupling light emission and molecular orientation lead to new OLED (organic light emitting diode) applications. Combinations of molecular rigidity with fluidity lead to new thermotropic liquid crystal phases. Molecular engineering here can be used to create diverse new materials, and the self-assembly of such amphiphiles produces new architectures Two-dimensional assembly of diverse molecular architectures is discussed by Kunitake et al. [109]. They emphasize development of supramolecular order from single (benzene) and multiple (porphyrin) ring systems. Chemical lipid deposition (CLD) from solution is contrasted with CVD (chemical vapor deposition). The self-assembly is driven by facile Schiff base coupling between aldehydes and, for example, amines. The thermodynamic driving force for adsorption from solution emanates from the π system of the structural molecules. Open and close-packed structures are illustrated with melem and other systems, and interesting transitions are illustrated in melamine–melem binary mixtures. Schiff-base coupling is then illustrated as a function of pH and shown to yield surface assemblies that are close packed or “coagulated”, depending on pH. Condensation polymerization on surfaces is illustrated and open and close packed porphyrin assemblies are created. Poly(azomethine) films of various types are created, demonstrating optical tuning throughout the visible. Alterations of these materials are used to create bottom-up open porous assemblies, including two-dimensional net structures that can then be controllably grown in the third dimension. These demonstrations are very stimulating, particularly from the standpoint of bottom-up thin film design and construction. 5. Nanoinks Plastic electronics and flexible electronics have become reality, even if market penetration is relatively small. This arena has become a major focal point for entrepreneurs as well as major corporate development efforts, and it is producing new interdisciplinary relationships between organic and inorganic fields. Gysling [110] links these areas with catalysis in presenting a picture of an early approach to metallization emanating from catalytic reduction chemistries to the current increasing trend in developing additive coating and printing with nanoparticle inks. Other important associated topics deal with the additive fabrication of circuit elements such as capacitors, diodes, transistors, batteries, and other devices. Additive printing, processing, and manufacturing are becoming increasingly significant and are the basic enabling elements for flexible electronics [111–116] and rapid prototyping [117,118]. Display development [119], capacitor printing [120,121], transistor fabrication [122, 123], battery formulation [124,125], and electrode printing [126,127], among other applications, are being advanced by additive deposition methods. 6. Graphene dispersions The production of aqueous and non-aqueous dispersions of nanocarbons such as SWCNT (single wall carbon nanotubes), MWCNT (multiwall carbon nanotubes), graphene, hydrothermal carbon, and carbon black encompasses the oldest known application of nanotechnology in China [128] through the most recently discovered form of nanocarbon, graphene [129]. The discussion [130] of graphene dispersions illustrates very rapid advances made with this very new material. A key result presented is an experimental connection between the visible absorption of macroscopic single layer sheets and that measured in dispersion of randomly oriented micron and sub-micron dimensional
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Editorial overview
graphene platelets. Practical dispersion processing is now available to make disperse graphene available for large scale coating applications, but a persisting problem is that even the best dispersing aids may degrade electrical and thermal conductivities of thin coatings. The development of good dispersing aids that do not insulate significantly against electron or phonon transport will be useful additions to dispersion technology.
7. Summary These perspectives show that interfaces provide a unifying focus for contemplating diverse materials having varied applications. Inorganic colloids and dispersions have been discussed with applications ranging from medical diagnostics to inkjet printed circuitry. Supramolecular nanoparticle aggregation provides particulate and monolithic mesocrystalline materials that can be expected to further expand catalysis and sequestration applications. A diverse mixture of stimuli responsive polymeric colloids highlights the power of interfacial modification in making new materials. The development of π-conjugated amphiphiles and two-dimensional and three-dimensional supramolecular assembly of π-conjugated compounds is expanding the zoology of liquid crystalline materials and planar assemblies. These materials also have promise for applications in flexible electronics, as also do nanoinks and graphene inks (dispersions).
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Effects of bonding yypes and functional groups on CO2 capture using novel multiphase systems of liquid-like nanoparticle organic hybrid materials. Environ Sci Technol 2011;45:6633–9. [85] Yu HY, Koch DL. Structure of solvent-free nanoparticle-organic hybrid materials. Langmuir 2010;26:16801–11. [86] Lei Y, Xiong CX, Guo H, Yao J, Dong L, Su X. Controlled viscoelastic carbon nanotube fluids. J Am Chem Soc 2008;130:3256–7. [87] Li Q, Dong L, Deng W, Zhu Q, Liu Y, Xiong CX. Solvent-free fluids based on rhombohedral nanoparticles of calcium carbonate. J Am Chem Soc 2009;131: 9148–9. [88] Lei Y, Xiong CX, Dong L, Guo H, Su X, Yao F, et al. Ionic liquid of ultralong carbon nanotubes. Small 2007;3:1889–93. [89] Feng Q, Dong L, Huang J, Li Q, Fa Y, Xiong J, et al. Fluxible [sic] monodisperse quantum dots with efficient luminescence. Angew Chem Int Ed 2010;49: 9943–6. [90] Texter J, Qiu Z, Crombez R, Byrom J, Shen W. Solvent-free supramolecular liquids for novel resins & applications. China Coat J 2010:44–8 [January (Part 1), 38–42; 2010, March (Part 2)]. [91] Texter J, Qiu Z, Crombez R, Byrom J, Shen W. Nanofluid acrylate composite resins— initial preparation and characterization. Polym Chem 2011;2:1778–87. [92] Texter J, Qiu Z, Crombez R, Shen W. Nanofluid polyurethane/polyurea resins—thin films and clearcoats. J Polym Sci A Polym Chem 2013;51:3439–48. [93] Oishi M, Nagasaki Y. Stimuli-responsive smart nanogels for cancer diagnostics and therapy. Nanomedicine 2010;5:451–68.
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[94] Hu XB, Huang J, Zhang W, Li MH, Tao CG, Li GT. Photonic ionic liquids polymer for naked-eye detection of anions. Adv Mater 2008;20:4074–8. [95] Pan GQ, Guo QP, Cao CB, Yang HL, Li B. Thermo-responsive molecularly imprinted nanogels for specific recognition and controlled release of proteins. Soft Matter 2013;9:3940–50. [96] Heppner IN, Serpe MJ. Poly(N-isopropylacrylamide) microgel-based etalons constructed from various metal layers. Colloid Polym Sci 2014;291:1557–62. [97] Scherzinger C, Schwarz A, Bardow A, Leonhard K, Richtering W. Cononsolvency of poly-N-isopropyl acryl amide (PNIPAM): microgels versus linear chains and macrogels. Curr Opin Colloid Interface Sci 2014;19:83–93 [this issue]. [98] Voronov S, Kohut A, Tarnavchyk I, Voronov A. Advances in reactive polymeric surfactants for interface modification. Curr Opin Colloid Interface Sci 2014;19:94–120 [this issue]. [99] Merisko-Liversidge E, Liversidge G. Nanosizing for oral and parenteral drug delivery: a perspective on formulating poorly-water soluble compounds using wet media milling technology. Adv Drug Deliv Rev 2011;63:427–40. [100] Moeschwitzer JP. Drug nanocrystals in the commercial pharmaceutical development process. Int J Pharmacogn 2013;453:142–56. [101] D’Addio SM, Prud'homme RK. Controlling drug nanoparticle formation by rapid precipitation. Adv Drug Deliv Rev 2011;63:417–26. [102] Texter J. Precipitation and condensation of organic particles. J Dispers Sci Technol 2001;22:49–527. [103] Kraft JC, Freeling JP, Wang ZY, Ho RJY. Emerging research and clinical development trends of liposome and lipid nanoparticle drug delivery systems. J Pharm Sci 2014;103:29–52. [104] Shewan HM, Stokes JR. Review of techniques to manufacture micro-hydrogel particles for the food industry and their applications. J Food Eng 2013;119: 781–92. [105] Skorb EV, Mohwald H. 25th anniversary article: dynamic interfaces for responsive encapsulation systems. Adv Mater 2013;25:5029–42. [106] Kowalczuk A, Trzcinska R, Trzebicka B, Mueller AHE, Dworak A, Tsvetanov CB. Loading of polymer nanocarriers: factors, mechanisms and applications. Prog Polym Sci 2014;39:43–86. [107] Lapitsky Y. Ionically crosslinked polyelectrolyte nanocarriers: recent advances and open problems. Curr Opin Colloid Interface Sci 2014;19:121–9 [this issue]. [108] Zielenska A, Leonowicz M, Li HG, Nakanishi T. Controlled self-assembly of alkylated-π compounds—towards optical and optoelectronic applications. Curr Opin Colloid Interface Sci 2014;19:130–8 [this issue]. [109] Kunitake M, Higuchi R, Tanoue R, Unemura S. Self-assembled π-conjugated macromolecular architectures—a soft solution process based on Schiff-base coupling. Curr Opin Colloid Interface Sci 2014;19:139–53 [this issue]. [110] Gysling HJ. Nanoinks in inkjet metallization—evolution of simple additive-type metal patterning. Curr Opin Colloid Interface Sci 2014;19:154–61 [this issue]. [111] Huang YA, Bu NB, Duan YQ, Pan YQ, Liu HM, Yin ZP, et al. Electrohydrodynamic direct-writing. Nanoscale 2013;5:12007–17. [112] Hoffman J, Hwang S, Ortega A, Kim NS, Moon KS. The standardization of printable materials and direct writing systems. J Electron Packag 2013;135:011006 [8 pp.]. [113] Koskinen S, Pykari L, Mantysalo M. Electrical performance characterization of an inkjet-printed flexible circuit in a mobile application. IEEE Trans Components Packag Manuf Technol 2013;3:1604–10. [114] Pabat O, Perelaer J, Beckert E, Schubert US, Eberhardt R, Tunnemann A. All inkjetprinted piezoelectric polymer actuators: characterization and applications for micropumps in lab-on-a-chip systems. Org Electron 2013;14:3423–9. [115] Li YX, Hu XJ, Zhou SY, Yang L, Yan J, Sun CH, et al. A facile process to produce highly conductive poly(3,4-ethylenedioxythiophene) films for ITO-free flexible OLED devices. J Mater Chem C 2013;2:916–24. [116] Sun J, Fuh JYH, Thian ES, Hong GS, Wong YS, Yang R, et al. Fabrication of electronic devices with multi-material drop-on-demand dispensing system. Int J Comput Integr Manuf 2013;26:897–906. [117] Ivanova O, Williams C, Campbell T. Additive manufacturing (AM) and nanotechnology: promises and challenges. Rapid Prototyp J 2013;19:353–64. [118] Mannoor MS, Jiang ZW, James T, Kong YL, Malatesta KA, Soboyejo WO, et al. 3D printed bionic ears. Nano Lett 2013;13:2634–9. [119] Alsaid D, Rebrosova R, Joyce M, Rebros M, Atashbar M, Bazuin B. Gravure printing of ITO transparent electrodes for applications in flexible electronics. J Disp Technol 2012;8:391–6. [120] Cook BS, Cooper JR, Tentzenis MM. Multi-layer RF capacitors on flexible substrates utilizing inkjet printed dielectric polymers. IEEE Microw Wirel Components Lett 2013;23:353–5. [121] Bhansali US, Khan MA, Alshareef HN. Organic ferroelectric memory devices with inkjet-printed polymer electrodes on flexible substrates. Microelectron Eng 2013;105:68–73. [122] Jung SW, Na BS, You IK, Koo JB, Yang BD, Oh JM. Inkjet-printed organic thin-film transistor and antifuse capacitor for flexible one-time programmable memory applications. J Korean Phys Soc 2014;64:74–8. [123] Kang B, Lee WH, Cho K. Recent advances in organic transistor printing processes. ACS Appl Mater Interfaces 2013;5:2302–15. [124] Xu YF, Hennig I, Freyberg D, Strudwick AJ, Schwab MG, Weitz T, et al. Inkjet-printed energy storage device using graphene/polyaniline inks. J Power Sources 2014;248:483–8. [125] Palacios-Aguilera NB, Visser HA, Sridhar A, Balda-Irurzun U, Vargas-Liona LD, Zhou J, et al. Reliable inkjet-printed interconnections on foil-type Li-ion batteries. IEEE Trans Device Mater Reliab 2013;13:136–45. [126] Finn DJ, Lotya M, Cunningham G, Smith RJ, McCloskey D, Donegan JF, et al. Inkjet deposition of liquid-exfoliated graphene and MoS2 nanosheets for printed device applications. J Mater Chem C 2014;2:925–32.
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[127] Abulikemu M, Da’as ES, Haverinen H, Cha D, Malik MA, Jabbour GE. In situ synthesis of self-assembled gold nanoparticles on glass or silicon substrates through reactive inkjet printing. Angew Chem Int Ed 2014;53:420–3. [128] Winter J. Chinese ink. Expedition 1989;31:52–3. [129] Novoselv KS, Geim AK, Morozov SV, Jian D, Zhang Y, Dubonos SV, et al. Electric field effect in atomically thin carbon films. Science 2004;366:666–9. [130] Texter J. Graphene dispersions. Curr Opin Colloid Interface Sci 2014;19:162–73 [this issue].
John Texter School of Engineering Technology, Eastern Michigan University, Ypsilanti, MI 48197, USA E-mail address: [email protected].
Contents lists available at ScienceDirect
Current Opinion in Colloid & Interface Science journal homepage: www.elsevier.com/locate/cocis
Editorial overview
Interfaces in advanced materials☆
The importance of interfaces in advanced materials is becoming increasingly recognized in diverse disciplines and industrial sectors as supported by an increasing number of monographs in the last several years addressing surface modification chemistry, physics, and engineering [1–7] and by a much greater number of topical reviews [8–18]. The past forty years have seen publication of about 410,000 journal articles addressing the concept of interfaces in materials or chemistry, according to a recent SciFinder® database analysis. Over the years since 1972 the number of such articles has increased geometrically with scaling exponent of about 0.4 from 1973 to 1980 and 2.3 from 1989 to 2013. The breakpoint between these linear behaviors occurred approximately in 1985. In 2012 this number appears to have peaked with about 27,500 articles, dropping to about 26,300 in 2013, after over twenty years of geometrical growth. At the time of this writing, about 5700 articles had appeared in 2014, and this suggests a possible further decrease for 2014. Interfaces may arguably be credited as the glue that holds composite materials or multiphase materials together, and they thereby provide a unifying context in which to discuss and ponder material properties. In this Applications issue eleven papers discuss diverse materials and processes that are controlled by the physics, chemistry, and biology of interfaces. Passivation and functionality are provided by appropriate decoration of the respective interfaces. This passivation or interfacial stabilization may be represented by interfacial material or phase densities illustrated in Fig. 1, wherein an interfacial modifier, ι, “matches” the surface free energies of the respective phases and is the main chemical tool we have to molecularly engineer interfaces. This “matching” may comprise nonspecific adsorption (van der Waals force driven alkane-surface interactions), specific adsorption (hydrogen bonding, coulombic ion pairing), specific absorption (oriented solubilization of modifier in each phase or material, as in emulsion droplets and in diblock copolymer stabilized mesophases), and covalent interfacial bridging, wherein a modifier is covalently attached to one or both phases. In practical systems this “matching” involves laterally heterogeneous (spatially varying in a plane parallel to the interface) interactions, involving mixtures of the above described interaction types and other interactions not mentioned. The thermodynamics of such heterogeneously modified interfaces are difficult to formulate in terms of specific interactions, although specific structures may be examined and characterized through tools involving Monte Carlo and molecular dynamics methods. The passivation resulting from interfacial chemical modification can be understood for an α phase, having a very small radius of curvature, in contact with a β phase and having a positive interfacial free energy, γαβ, ☆ This paper was edited by Professor Krister Holmberg, Chalmers University of Technology, Göteborg, Sweden.
http://dx.doi.org/10.1016/j.cocis.2014.04.003 1359-0294/© 2014 Elsevier Ltd. All rights reserved.
through the Ostwald–Freundlich equation [19,20] for the dependence of solubility or activity, Sr, on radius of curvature. In this equation V is the molar volume, R is the gas constant, and T is temperature:
RT ln
γ αβ V Sr ¼ r S∞
This equation shows why interfacial energy (with concomitant interfacial modification) is the most significant access we have experimentally to stabilize interfaces of high curvature (low r), without modifying the bulk activity or solubility. It has been convincingly proven effective in the formulation of microemulsions and the stabilization (solubilization) of one immiscible liquid in another, wherein particular surfactants reduce γαβ nearly to zero. For liquids in equilibrium with their vapor, γαβ is the respective liquid's surface tension, and this Ostwald–Freundlich equation becomes the Kelvin equation [21]. This equation also explains the thermodynamic basis of Ostwald ripening, how passivation may be used to slow such ripening, and also how modifying the solubility of one phase in the other can also mitigate such ripening. The significance of various kinds of surface modifying approaches is illustrated in most of the companion reviews in this issue. Some of these processes result in traditional stabilization, others in directing binding affinity and cellular targeting, and others in providing new functionality and stimuli responsiveness. 1. Inorganic nanoparticles for therapeutic delivery Rotello et al. [22] provide a perspective of the evolution of nanoparticles, particularly inorganic nanoparticles, in drug delivery and diagnostic applications. Of greater importance is their highlighting of some major problems limiting the application and practical use of such colloidal materials. They point out how upconverting nanoparticles derived from less toxic phases than selenides, sulfides, and telurides (as are more conventional quantum dots, QDs) are showing great promise for replacing such QDs and for developing therapies based upon through skin and through tissue photo-illumination. Such advances in upconverting particles are being reported at increasing frequency, and include anti-Stokes fluorescent nanoparticles that can be driven by incoherent radiation [23], near infrared (NIR) bioimaging [24], and multianalyte immunoassays [25]. The importance of interfacial modification is intrinsic in all of these QD systems, toxic or not, and specific use of lipoic acid stabilizers, polyethyleneglycol polymeric stabilziers, zwitterionic stabilizers, protein corona, and cell receptor targeting species are discussed Rotello et al.
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Editorial overview
Fig. 1. Material densities, ρ, for α and β phases and for an interfacial modifier, ι, as a function of the interfacial normal, ξ.
2. Mesocrystals Ma and Cölfen [26] illustrate the evolution of mesocrystals as a class of supra-assemblies of nanoparticles emanating from the renaissance in templated zeolite synthesis of the last century [27]. The influence of bioinspired form for function [28,29] has provided not only insight into how nanoparticles can be assembled via appropriate templates [30], but insight and mechanisms very firmly founded in the role of speciation at the ionic and molecular level have been developed for these mesocrystals [31]. This area of materials is closely associated with two related but independently developed classes of three-dimensional particle-based materials, photonic crystals [32–34] and metamaterials and superlattices derived from nanocrystals and multiple types of nanocrystals [35,36]. Some insight into face centered cubic (fcc) assembly [37] competitive with that derived for calcium carbonate is available, but mechanisms deriving from speciation and additive effects on the molecular scale provide stimulating understanding and paths for modification [38–40]. Mention must be made of the pioneering contributions of Matijevic et al. in formulating inorganic microparticles composed of selfassembled nanoparticles. Over the course of 65 years or so Matijevic developed many approaches to precipitate inorganic nanoparticles and microparticles, and many of these were found to form quite uniformly sized and shaped particles, even though in some cases they were composed of much smaller nanoparticles. Some of these aspects were reviewed a few years ago [41], and some examples of such composite particles are illustrated in Fig. 2 and include Sr-doped barium titanate [41], hematite [41], and CuO [42]. A theory to explain the resulting aggreage or mesocrystal size has been presented [43,44].
3. Polymeric colloids Stimuli-responsive particles and particle assemblies are providing new materials for encapsulation and controlled release, sensors and diagnostics, thermoreversible stabilizers, phase transfer agents, targeted delivery, among other applications. Urban [45] highlights a variety of studies aimed at producing stimuli responsive coatings and films. These include hydrophobic–hydrophilic transitions accompanying stratified film formation, synthetic cilia formation and propulsion, and self-repairing films. Yuan et al. [46] present a comprehensive view of polymerized ionic liquid (PIL) particles, microparticles, and nanoparticles, 20 nm to 300 μm in diameter, derived by radical and condensation polymerization of imidazolium-based and phosphonium-based ionic liquid (IL)
Fig. 2. SEM of inorganic mesocrystals composed of inorganic nanoparticles (collected on filters): (a) Sr-doped barium titanate [41]; (b) hematite [41]; (c) CuO [42]. The scale bars in (a) and (b) corresponds to 1 μm and to 100 nm in (c). These frames are reproduced by permission; ©1996 Elsevier (frames a and b) and ©1997 Elsevier (frame c).
monomers. These PIL [47] represent a collection of new materials that exhibit stimuli responsiveness of various types. Thermal responsiveness accompanying lower critical solution temperatures (LCST) and anion responsiveness are discussed. Also discussed are early efforts to use PIL and PIL particles as stabilizers of nanocarbons such as SWCNT (single wall carbon nanotubes) and graphene. Another class of colloidal PIL derived from organotrialkoxysilanes are solvent-free nanofluids. These materials were introduced about eight years ago by the Giannelis group [48–58] and others [59–65]. They are usually composed of an inorganic core and are surface functionalized with organic salts that look like ionic liquids. The organotrialkoxysilanes themselves are usually ionic liquids of the tetraalkylammonium or sulfo type. In an early paper, nanofluids based on silica and on hematite cores were produced using an ammonium trialkoxysilane and a soft sulfonate counter ion exchanged for the chloride that accompanied the ammonium trialkoxysilane precursor. Similar materials (sometimes referred to as nano ionic materials, NIMs) have been developed using anionic
Editorial overview
surface groups [66] with diverse cationic counter ions (e.g., ammonium, imidazolium), where these materials also are well described as supramolecular ionic liquids. A core-free solvent free nanofluid was reported that exhibited crystallization frustration due to polydispersity and a pair of lambda transitions, one traversing the glass transition and another traversing the freezing transition [67]. In addition to these supramolecular ionic liquids or solvent-free nanofluids, applications in the development of ionic and nonionic nanofluids have also been developed by the groups of Xiong [68,69], Zheng [70–75], Nakanishi [76–78], and others [79–82]. Luminescent nanoparticle ZnO has been stabilized as a nanofluid using a zwitterionic acetate ammonium surface stabilizer [79]. Metal oxides such as SnO2 and TiO2 have been stabilized with corona of 350 Da PEGMe to produce nanofluids [80]. CdTe q-dots surface modified with mercaptoacetate and ion-exchanged with octadecyldi-PEG ammonium cations show that emission wavelengths can be thermally controlled by modifying the association of such nanofluids particles in the neat nanofluids during melting–freezing transitions [82]. Mann et al. have recently developed a nanofluid exhibiting a liquid crystalline phase and a liquid phase of a cationic ferritin protein, a solvent-free liquid protein [83]. Lin and Park introduced nanosilica templated nanofluids, ionic and nonionic, that exhibit CO2 sequestration activity [84]. Yu and Koch recently introduced a theoretical treatment of nanofluids cores attached to a corona of oligomers and demonstrated derivation of radial distribution functions among other results. This appears to be the first theoretical treatment of inorganic–polymer hybrid nanofluid liquid structure [85]. Further applications to produce nanocarbon nanofluids have also been made [86–89]. Combinations of ionic liquid and reactive-group monomers were used to create several new classes of resins, including UV-cured clearcoats and air-cured clearcoats [90–92]. Volume phase transitions are an important class of thermoreversible stimuli responsive phenomena for a variety of solvated polymeric systems, and these responses have been tied to a variety of diagnostic and sensing phenomena [93–96]. Co-nonsolvency in binary liquid mixtures, wherein solvency exists in each separate liquid component, provides an intriguing window through which to view polymeric solvation– desolvaton processes. Richtering et al. [97] examine the manifestation of water and methanol co-nonsolvency of poly(N-isopropylacrylamide) (PNIPAM) as a function of molecular weight and in particle morphologies of various size scales. These co-nonsolvency phenomena really force us to focus upon polymer solvation, desolvation, and solvent interactions at the molecular scale, and Richtering et al. include significant molecular dynamics studies in their discussions. Voronov et al. present a comprehensive view of some new reactive polymeric surfactant (RPS) chemistry based on polymeric surfactants having reactive peroxide functionality [98]. We are used to incorporation of anhydride and amine functionality, for general modification of surfaces, but this detailed review shows how peroxide functionality can be used in many different ways. These chemistries offer some new tools for modifying polymeric particle surfaces as well as modifying polymeric films. Drug, diagnostic, and nutraceutical delivery are intensive areas of research and range from formulating dispersions of nanocrystallne active [99], homogenization [100], precipitation [101,102] to the loading of vesicles [103], microgels [104], nano and microcapsules [105], polyelectrolyte encapsulation of actives, and other nanocarriers [106]. Lapitsky [107] addresses this last topic focused on polyelectrolytes that are crosslinked via multivalent ions. These ions range from divalent ones such as Ca+ to polyphosphate and polyethylenimine. Particle assembly and particle–particle interactions are discussed, as well as non-DLVO (Deryagin, Lifshitz, Verway, and Overbeek) theoretica approaches. 4. Self-assembled π-compounds Alkylated π compounds where rigid π-conjugated molecules are functionalized with flexible alkyl chains may be viewed as composing
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a class of amphiphiles. The rigid and flexible parts each have preferred solvents. Such systems are discussed by Nakanishi et al. [108], wherein applications are being made in flexible electronics, so charge transport properties are of interest along with phase and ordering transitions. These complicated molecules, similarly to appropriately functionalized nanoparticles [48], can be made as solvent free liquids. Here coupling light emission and molecular orientation lead to new OLED (organic light emitting diode) applications. Combinations of molecular rigidity with fluidity lead to new thermotropic liquid crystal phases. Molecular engineering here can be used to create diverse new materials, and the self-assembly of such amphiphiles produces new architectures Two-dimensional assembly of diverse molecular architectures is discussed by Kunitake et al. [109]. They emphasize development of supramolecular order from single (benzene) and multiple (porphyrin) ring systems. Chemical lipid deposition (CLD) from solution is contrasted with CVD (chemical vapor deposition). The self-assembly is driven by facile Schiff base coupling between aldehydes and, for example, amines. The thermodynamic driving force for adsorption from solution emanates from the π system of the structural molecules. Open and close-packed structures are illustrated with melem and other systems, and interesting transitions are illustrated in melamine–melem binary mixtures. Schiff-base coupling is then illustrated as a function of pH and shown to yield surface assemblies that are close packed or “coagulated”, depending on pH. Condensation polymerization on surfaces is illustrated and open and close packed porphyrin assemblies are created. Poly(azomethine) films of various types are created, demonstrating optical tuning throughout the visible. Alterations of these materials are used to create bottom-up open porous assemblies, including two-dimensional net structures that can then be controllably grown in the third dimension. These demonstrations are very stimulating, particularly from the standpoint of bottom-up thin film design and construction. 5. Nanoinks Plastic electronics and flexible electronics have become reality, even if market penetration is relatively small. This arena has become a major focal point for entrepreneurs as well as major corporate development efforts, and it is producing new interdisciplinary relationships between organic and inorganic fields. Gysling [110] links these areas with catalysis in presenting a picture of an early approach to metallization emanating from catalytic reduction chemistries to the current increasing trend in developing additive coating and printing with nanoparticle inks. Other important associated topics deal with the additive fabrication of circuit elements such as capacitors, diodes, transistors, batteries, and other devices. Additive printing, processing, and manufacturing are becoming increasingly significant and are the basic enabling elements for flexible electronics [111–116] and rapid prototyping [117,118]. Display development [119], capacitor printing [120,121], transistor fabrication [122, 123], battery formulation [124,125], and electrode printing [126,127], among other applications, are being advanced by additive deposition methods. 6. Graphene dispersions The production of aqueous and non-aqueous dispersions of nanocarbons such as SWCNT (single wall carbon nanotubes), MWCNT (multiwall carbon nanotubes), graphene, hydrothermal carbon, and carbon black encompasses the oldest known application of nanotechnology in China [128] through the most recently discovered form of nanocarbon, graphene [129]. The discussion [130] of graphene dispersions illustrates very rapid advances made with this very new material. A key result presented is an experimental connection between the visible absorption of macroscopic single layer sheets and that measured in dispersion of randomly oriented micron and sub-micron dimensional
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Editorial overview
graphene platelets. Practical dispersion processing is now available to make disperse graphene available for large scale coating applications, but a persisting problem is that even the best dispersing aids may degrade electrical and thermal conductivities of thin coatings. The development of good dispersing aids that do not insulate significantly against electron or phonon transport will be useful additions to dispersion technology.
7. Summary These perspectives show that interfaces provide a unifying focus for contemplating diverse materials having varied applications. Inorganic colloids and dispersions have been discussed with applications ranging from medical diagnostics to inkjet printed circuitry. Supramolecular nanoparticle aggregation provides particulate and monolithic mesocrystalline materials that can be expected to further expand catalysis and sequestration applications. A diverse mixture of stimuli responsive polymeric colloids highlights the power of interfacial modification in making new materials. The development of π-conjugated amphiphiles and two-dimensional and three-dimensional supramolecular assembly of π-conjugated compounds is expanding the zoology of liquid crystalline materials and planar assemblies. These materials also have promise for applications in flexible electronics, as also do nanoinks and graphene inks (dispersions).
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Editorial overview
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John Texter School of Engineering Technology, Eastern Michigan University, Ypsilanti, MI 48197, USA E-mail address: [email protected].