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4. COLLOIDAL SYSTEMS

TUNABLE COLLOIDAL CRYSTALS

The following points are made by William B. Russel (Nature 2003 421:490):

1) Fundamental advances in colloid science often depend on physical models, which are made by dispersing carefully tailored particles, less than a micrometer in size, in pure aqueous or organic liquids. Such dispersions can be characterized by methods such as light scattering and confocal microscopy, and the physical and chemical interactions between the particles, responsible for intriguing phases such as colloidal crystals (which behave like atomic solids), can be precisely controlled. Yethiraj and van Blaaderen(1), for example, describe a new model system that can be tuned with an electric field to display phase transitions and unexpected crystalline structures.

2) Colloidal crystals first attracted interest in the 1960s. In studies of the light scattered from dilute dispersions, a transition was detected from a disordered fluid to an ordered body-centered-cubic (b.c.c.) crystal when the screened (or reduced) Coulomb repulsions between the colloidal particles extended to length scales greater than the lattice spacing(2). In fact, this transition can be controlled: adding a small amount of salt decreases the range of the repulsive force, because the salt dissociates into ions that enhance the screening. As a result, the volume fraction (or density) of particles at the transition increases, and a denser, face-centered-cubic (f.c.c.) crystal structure is favored. Adding even more salt leads to "hard-sphere" transitions -- as though the particles were effectively hard spheres, with no Coulomb repulsion. Then, entropy --generally considered to be a measure of disorder --favors the f.c.c. crystal, as the number of configurations available to a particle localized about a lattice site in the f.c.c. crystal exceeds those accessible in a disordered fluid or the b.c.c. crystal(3).

3) Although hard-sphere behavior of polymer-based colloids could be achieved in model systems, there was a drawback: those colloids were opaque at even moderate densities, so little could be learned about their structure from light scattering. More transparent dispersions were sought, such as silica spheres coated with short hydrocarbon chains in a nonpolar solvent that eliminates surface charge(4). In the 1980s, these organophilic silicas and the aqueous lattices sufficed for many studies of fluid-to-crystal transitions and other colloidal phenomena. But small silicas could not easily be made highly uniform in size and there can be extra, van der Waals attractions, between the larger ones, so better colloidal hard spheres were sought. Eventually a standard emerged: poly(methylmethacrylate) (PMMA) spheres coated with a low-molecular-weight polymer(5).

References (abridged):

1. Yethiraj, A. & van Blaaderen, A. Nature 421, 513-517 (2003)

2. Monovoukas, Y. & Gast, A. P. J. Colloid Interface Sci. 128, 533-548 (1989)

3. Hachisu, S. & Kobayashi, Y. J. Colloid Interface Sci. 46, 470-476 (1974)

4. de Kruif, C. G., Jansen, J. W. & Vrij, A. in Complex and Supramolecular Fluids (eds Safran, S. A. & Clark, N. A.) 315-343 (Wiley-Interscience, New York, 1987)

5. Antl, L., Goodwin, J., Ottewill, R. & Waters, J. Colloids Surf. 17, 67-78 (1986)

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A COLLOIDAL MODEL SYSTEM WITH AN INTERACTION TUNABLE FROM HARD SPHERE TO SOFT AND DIPOLAR

The following points are made by A. Yethiraj and A. Van Blaaderen (Nature 2003 421:513):

1) Monodisperse colloidal suspensions of micrometer-sized spheres are playing an increasingly important role as model systems to study, in real space, a variety of phenomena in condensed matter physics -- such as glass transitions and crystal nucleation(1-4). But to date, no quantitative real-space studies have been performed on crystal melting, or have investigated systems with long-range repulsive potentials.

2) Recent advances in quantitative three-dimensional (3D) real-space analysis of the structure and dynamics of colloids have been accompanied by progress in the synthesis of model spheres with a controllable size and surface chemistry enabling the manipulation of the interaction potentials between the spheres(5). Together with the well developed scattering techniques for studying colloidal systems, this has led to new insights into the glass transition of simple glass formers(2,3), crystal nucleation and growth(4), and the role of the attractive part of interaction potentials on phase behavior.

3) The authors demonstrate a charge- and sterically-stabilized colloidal suspension -- poly(methyl methacrylate) spheres in a mixture of cycloheptyl (or cyclohexyl) bromide and decalin --where both the repulsive range and the anisotropy of the interparticle interaction potential can be controlled. This combination of two independent tuning parameters gives rise to a rich phase behavior, with several unusual colloidal (liquid) crystalline phases, which the authors explore in real space by confocal microscopy. The softness of the interaction is tuned in this colloidal suspension by varying the solvent salt concentration. The anisotropic (dipolar) contribution to the interaction potential can be independently controlled with an external electric field ranging from a small perturbation to the point where it completely determines the phase behavior. The authors also demonstrate that the electric field can be used as a pseudo-thermodynamic temperature switch to enable real-space studies of melting transitions. The authors expect studies of this colloidal model system to contribute to our understanding of, for example, electro- and magneto-rheological fluids.

References (abridged):

1. Grier, D. G. & Murray, C. A. The microscopic dynamics of freezing in supercooled colloidal fluids. J. Chem. Phys. 100, 9088-9095 (1994)

2. van Blaaderen, A. & Wiltzius, P. Real-space structure of colloidal hard-sphere glasses. Science 270, 1177-1179 (1995)

3. Kegel, W. K. & van Blaaderen, A. Direct observation of dynamical heterogeneities in colloidal hard-sphere suspensions. Science 287, 290-293 (2000)

4. Gasser, U., Weeks, E. R., Schofield, A., Pusey, P. N. & Weitz, D. A. Real-space imaging of nucleation and growth in colloidal crystallization. Science 292, 258-262 (2001)

5. Antl, L., Goodwin, J., Hill, R., Ottewill, R. & Waters, J. The preparation of poly(methyl methacrylate) lattices in non-aqueous media. Colloids Surf. 17, 67-78 (1986)

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COLLOIDOSOMES: SELECTIVELY PERMEABLE CAPSULES COMPOSED OF COLLOIDAL PARTICLES

The following points are made by A.D. Dinsmore et al (Science 2002 298:1006):

1) Efficient encapsulation of active ingredients such as drugs, proteins, vitamins, flavors, gas bubbles, or even living cells is becoming increasingly important for a wide variety of applications and technologies, ranging from functional foods to drug delivery to biomedical applications (1-5). Increasingly sophisticated techniques are being developed to create physical structures that can meet the demanding requirements of these applications. A versatile technique should provide efficient encapsulation in structures whose size, permeability, mechanical strength, and compatibility can be easily controlled. Control of the size allows flexibility in applications and choice of encapsulated materials; control of the permeability allows selective and timed release; control of the mechanical strength allows the yield stress to be adjusted to withstand varying of mechanical loads and to enable release by defined shear rates; and control of compatibility allows encapsulation of fragile and sensitive ingredients, such as biomolecules and cells. Precise control of all these features would allow the strategic design of possible release mechanisms. Ideally, it should be feasible to construct these capsules from a wide variety of inorganic, organic, or polymeric materials to provide flexibility in their uses.

2) A variety of techniques has been developed to address specific encapsulation requirements: Coacervation, or controlled gelation, of polymers at the surface of water drops can be used to fabricate nano- or microporous capsules (1-5); other fluid extrusion methods can also be used to create the polymer coating. Coating immiscible templates by electrostatic deposition of alternating layers of charged polymers or particles can be used to fabricate nanoporous capsules. Microfabrication technology can be used to create submillimeter-sized silicon capsules with exquisitely precise nanometer-scale holes for selective permeability and slow release. However, despite the enormous progress in encapsulation technologies, these methods can be limited in their applicability, in the range of materials that can be used, in the uniformity of pore sizes, in the accessible permeabilities and elasticities, or in the ease of synthesis, filling efficiency, and yield.

3) In summary: The authors present an approach to fabricate solid capsules with precise control of size, permeability, mechanical strength, and compatibility. The capsules are fabricated by the self-assembly of colloidal particles onto the interface of emulsion droplets. After the particles are locked together to form elastic shells, the emulsion droplets are transferred to a fresh continuous-phase fluid that is the same as that inside the droplets. The resultant structures, which the authors call "colloidosomes", are hollow, elastic shells whose permeability and elasticity can be precisely controlled. The generality and robustness of these structures and their potential for cellular immunoisolation are demonstrated by the use of a variety of solvents, particles, and contents.

References (abridged):

1. E.L. Chaikof, Annu. Rev. Biomed. Eng. 1, 103 (1999)

2. B.F. Gibbs, S. Kermasha, I. Alli, C. N. Mulligan, Int. J. Food Sci. Nutr. 50, 213 (May 1999)

3. R.P. Lanza, R. Langer, J. Vacanti, Principles of Tissue Engineering (Academic Press, San Diego, CA, 2000)

4. T-A. Read, et al., Nature Biotechnol. 19, 29 (2001)

5. T. Joki, et al., Nature Biotechnol. 19, 35 (2001)

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MICROFLUIDIC CONTROL USING COLLOIDAL DEVICES

The following points are made by A. Terray et al (Science 2002 296:1841):

1) Microscale devices designed to accomplish specific tasks have repeatedly demonstrated superiority over their macroscale analogs (1,2). The advantages of such devices are due largely to unique transport properties resulting from laminar flows and vastly increased surface-to-volume ratios (3) and have enabled microscale sensors (4) and fabrication schemes (5) not possible on the macroscale. Additionally, microfluidic processes may be easily parallelized for high throughput and require vastly smaller sample volumes, a major benefit for applications in which reagents or analytes are either hazardous or expensive.

2) The utility, speed, and performance of microsystems typically increase as the overall device size decreases. The need to pump and direct fluids at very small length scales, however, has long been the limiting factor in the development of microscale systems, thus generating a tremendous amount of interest in microfluidics development. As improved actuation techniques have become available, conventional valving and pumping schemes have been miniaturized, yet continue to dwarf microchannels and other chip-top features.

3) Recently, several approaches conceived explicitly for the microscale have been developed, including platforms based upon electrohydrodynamics, electroosmosis, interfacial phenomena, conjugated materials, magnetic materials, and multilayer soft lithography. Although these microfluid handling techniques enable functional devices on microscopic length scales, they also impose unique constraints upon potential device capability, flexibility, and performance. To fully integrate multiple fluidic processes within a single microsystem, methods for microfluid handling must be developed that can accommodate fluids of complex and dynamic composition and are of comparable size to that of the processes into which they are embedded.

4) In summary: The authors report that by manipulating colloidal microspheres within customized channels, they have created micrometer-scale fluid pumps and particulate valves. The authors describe two positive-displacement designs, a gear and a peristaltic pump, both of which are about the size of a human red blood cell. Two colloidal valve designs are also demonstrated, one actuated and one passive, for the direction of cells or small particles. The authors suggest that the use of colloids as both valves and pumps will allow device integration at a density far beyond what is currently achievable by other approaches and may provide a link between fluid manipulation at the macro- and nanoscale.

References (abridged):

1. C. S. Effenhauser, A. Manz, H. M. Widmer, Anal. Chem. 65, 2637 (1993)

2. P. Wilding, M. A. Shoffner, L. J. Kricka, Clin. Chem. 40, 1815 (1994)

3. G. M. Whitesides, A. D. Stroock, Phys. Today 54, 42 (June 2001)

4. A. E. Kamholz, B. H. Weigl, B. A. Finlayson, P. Yager, Anal. Chem. 71, 5340 (1999)

5. P. J. A. Kenis, R. F. Ismagilov, G. M. Whitesides, Science 285, 83 (1999)

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ON DESIGNER COLLOIDAL "ATOMS"

The following points are made by Daan Frenkel (Science 2002 296:65):

1) Tailor-made, submicrometer particles will be the building blocks of a new generation of nanostructured materials with unique physical properties. The basis of this prediction is that the macroscopic physical properties of colloidal suspensions --solutions of particles with diameters ranging from tens of nanometers to micrometers -- can be influenced dramatically by tuning the interactions between their building blocks.

2) Just over a century ago, J.H. van't Hoff (1852-1911) received the first Nobel prize in Chemistry for his groundbreaking work on osmotic pressure. He had found that the osmotic pressure of dilute solutions depends on the concentration of dissolved molecules, in the same way that the pressure of a gas depends on the concentration of gas molecules. Subsequent theoretical work by Onsager (3,4) and others showed that the analogy that van't Hoff had noted for dilute solutions is valid at arbitrary densities. This implies that just as atomic gases may condense at high pressures to form a liquid and eventually a solid, macromolecules in solution may undergo a transition to a "liquid" or "crystalline" solution state at sufficiently high osmotic pressure.

3) This phenomenon is often observed in colloidal suspensions. Although colloidal particles are much larger than atoms (a micrometer-sized colloid may contain billions of atoms), they nevertheless exhibit the same phases as, say, argon: vapor, liquid, and crystal. Like simple molecules, the phase behavior of macromolecules in solution is completely determined by the nature of the forces acting between the constituent particles.

4) The forces acting between small, spherical molecules all depend in a qualitatively similar way on intermolecular separation. As a result, all such substances exhibit very similar phase behavior, an observation already quantified in 1880 by Van der Waals (1837-1923) in his "Law of Corresponding States". If the forces between macromolecules in solution were similar in shape to those between gas atoms, then all colloidal suspensions would exhibit the same phase diagram as an assembly of argon atoms. However, the forces between macromolecules in solution come in all sorts and shapes. Furthermore, we can "tune" these forces by choosing an appropriate combination of solvent, solute, and additives. Hence, far from being simply a scale model for atomic fluids, colloidal suspensions can form new states of matter, the building blocks of which are large "designer atoms."(5)

References (abridged):

1. K. P. Velikov et al., Science 296, 106 (2002)

2. X. Pham et al., Science 296, 104 (2002)

3. L. Onsager, Chem. Rev. 13, 73 (1933)

4. L. Onsager, Proc. N.Y. Acad. Sci. 51, 627 (1949)

5. B. J. Alder, T. E. Wainwright, J. Chem. Phys. 27, 1208 (1957)

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A NEW COLLOID STABILIZATION MECHANISM

The following points are made by V. Tohver et al (Proc. Nat. Acad. Sci. 2001 98:8950):

1) Colloidal suspensions have widespread use in applications ranging from advanced materials to drug delivery. By tailoring interactions between colloidal particles, one can design stable fluids, gels, or colloidal crystals needed for ceramics processing, coatings, direct-write photonic devices, and pharmaceutical applications. Long-range attractive van der Waals forces are ubiquitous and must be balanced by Coulombic, steric, or other repulsive interactions to engineer the desired degree of colloidal stability.

2) The authors report the discovery of a new mechanism for regulating the stability of colloidal particles. Colloidal microspheres with negligible charge, which flocculate when suspended alone in aqueous solution, undergo a remarkable stabilizing transition upon the addition of a critical volume fraction of highly charged nanoparticle species. Zeta potential analysis reveals that these microspheres exhibit an effective charge build-up in the presence of such nanoparticle species. Reflectometry measurements, however, indicate that these nanoparticle species do not adsorb on the microspheres under the experimental conditions. The authors therefore propose that highly charged nanoparticles segregate to regions of near negligibly charged microspheres because of their repulsive Coulombic interactions in solution, and that this type of nanoparticle haloing provides a previously unreported method for tailoring the behavior of complex fluids.

Notes:

Van der Waals forces: Considering molecules that have permanent dipoles, and molecules that can have dipoles induced by the electric fields of other molecules, there are three possible mechanisms recognized in the formation of the van der Waals bonds: 1) the orientation effect, in which molecules rearrange themselves in their mutual electrical fields, the rearrangements involving reorientations of whole molecules; 2) the static induction effect, in which molecules that are static monopoles (ions) or dipoles may induce a static rearrangement of the electron distribution of other molecules; 3) the dynamic induction effect, or "dispersion" effect, in which any molecule, polar or nonpolar, may induce in other molecules transient electron distribution rearrangements that are time-variant. All these mechanism involve interaction energies, and they are "bonds" in the sense that they all involve energetic couplings between molecules.

Coulombic force: In general, the electrical force between two charged particles.

flocculate: In general, flocculation is the process in which particles in a colloid aggregate into larger clumps.

Zeta potential: (electrokinetic potential) The electric potential associated with an electrical double layer around a colloid.

Reflectometry: In general, measurement of the optical reflectance of a surface.

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INTERACTION BETWEEN LIKE-CHARGED COLLOIDAL SPHERES

The interactions between colloidal particles in electrolyte solutions play an apparently central role in the phase behavior and agglomeration kinetics of colloidal dispersions. These interactions are thus of fundamental importance for understanding the properties of inorganic materials (e.g., ceramics composed of nanoparticles), foods such as milk, and solutions of biomacromolecules such as globular proteins. After decades of theoretical and experimental efforts, the long-accepted theories for describing the interactions of colloidal particles in electrolyte solutions have been challenged by results from recent experiments. Included in this challenge is the issue of apparent attractive electrostatic forces between like-charged colloidal particles in an electrolyte solution.

The following points are made by J. Wu et al (Proc. Nat. Acad. Sci. 1998 95:15169):

1) The authors report Monte Carlo simulation studies that indicate the existence of a short-range attractive force between identical macroions in electrolyte solutions containing divalent counterions. The authors report strong evidence (complementing recent and related results by others) of attraction between a pair of spherical macroions in the presence of added salt ions for the conditions where the interacting macroion pair is not affected by any other macroions that may be in the solution.

2) The authors state that classical theories fail to describe the attractive interactions found in their simulations, with one set of classical theories (Derjaguin-Landau-Verwey-Overbeek) predicting only repulsive interactions and another set of theories (Sogami-Ise) predicting a long-range attraction that is too weak and that occurs at too large macroion separations. The authors suggest their simulations provide fundamental "data" for an improved theory of colloidal interactions in electrolyte solutions.

Notes:

In this context, the term "phase" refers to any part of a system which is uniform in chemical composition and physical properties and separated from other homogeneous parts of the system by boundary surfaces. Also in this context, the term "phase behavior" refers to the equilibrium relationships between water, the dispersed colloid, and dissolved non-colloidal electrolytes.

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INCORPORATION OF QUANTUM DOTS IN COLLOIDS

The following points are made by W. Wang and S.A. Asher (J. Am. Chem. Soc. 2001 123:12528):

1) Nanoscale metal and semiconductor particles are of current interest because they mark a material transition range between quantum and bulk properties. With decreasing particle size, bulk properties are lost as the continuum of electronic states becomes discrete (the quantum size effect) and as the fraction of surface atoms becomes large. The electronic and magnetic properties of metallic nanoparticles and nanoclusters show new characteristics that can be utilized in novel applications in areas that range from nonlinear optical switching and catalysis to high-density information storage.

2) Numerous methods have been developed to synthesize metal nanoparticles. A major difficulty with scale-up of these methods is that the metal colloid stability is often controlled by electrostatic interactions across the Debye double layer and sterically through adsorption of steric stabilizing agents such as polymers and surfactants. As a consequence, such metal colloids are extremely sensitive to their environment.

3) One way to improve the stability of metal nanoparticles is to coat them with silica, which is very resistant to coagulation, even at high volume fractions. It has been reported that particles of noble metals such as Ag and Au can be coated with silica shells, but these procedures usually involve a multi-step process, and only single metal particle cores could be coated.

4) The authors report the development of a new method to fabricate nanocomposite silicon dioxide spheres (approximately 100 nanometers in diameter) containing homogeneously dispersed Ag quantum dots (2 to 5 nanometers in diameter). The inclusion morphology is controlled through the timing of the photochemical reduction of silver ions during hydrolysis of tetraethoxysilane in a microemulsion.

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OSMOTIC PRESSURE AND COLLOIDAL INTERACTIONS

Osmosis is the passage of a solvent through a semipermeable membrane separating two solutions of different concentrations. A semipermeable membrane is one through which the molecules of a solvent can pass but the molecules of most solutes cannot. There is a thermodynamic tendency for solutions separated by such a membrane to become equal in concentration, the water (or other solvent) flowing from the weaker to the stronger solution [Note #1]. Osmosis will stop when the two solutions reach equal concentrations, and can also be stopped by applying a pressure to the liquid on the stronger solution side of the membrane. The pressure required to stop the flow from a pure solvent into a solution is a characteristic of the solution, and is called the "osmotic pressure". Osmotic pressure depends only on the concentration of particles in the solution, not on their nature (i.e., it is a "colligative" property).

Osmotic pressure is a subtle ubiquitous phenomenon that is relevant at scales extending from molecules to biological cells and tissues. For sufficiently low concentrations, the osmotic pressure follows an "ideal gas" law (van't Hoff Law): p = N/V x kT, where p is the osmotic pressure, N/V is the number of particles per unit volume, k is the Boltzmann constant, and T is the temperature. In polymer physics, osmotic pressure measurements provide a useful method of determining molecular weights. In biological systems, a proper osmotic pressure balance is essential for the proper functioning of cells and tissues; in fact, cells have apparently evolved special mechanisms ("ion pumps") to regulate the osmotic pressure build-up that results from the presence in the cytoplasm of cells of a large concentration of proteins and other charged molecules with their associated counterions. In theory, in a solution with macromolecules present, two surfaces that come in close contact (close enough to exclude the solute particles from the gap between them) experience an attractive force ("osmotic depletion interaction") caused by the osmotic pressure.

The following points are made by M. Singh-Zocchi et al (Proc. Nat. Acad. Sci. 1999 96:6711):

1) The authors report a measurement of the osmotic depletion interaction by using a micron-sized glass sphere bound to a flat glass plate through a single molecular attachment in an albumin-containing solution [Note #2]. The technique involves the use of laser beam scattering to follow the confined Brownian motion of the sphere and determine the mean distance between the sphere and plate. The total interaction potential is derived from this distance, and the osmotic contribution is obtained by varying the solute concentration.

2) The authors report they obtain the osmotic part of the interaction potential in experiments with a resolution of sphere-wall distance less than 1 nanometer and a resolution of total interaction potential less than 1 kT (room temperature). The osmotic part of the attractive interaction has a range related to the size of the albumin molecule-counterion entity (which is experimentally manipulated by altering pH), and the authors report the experimental results are in broad agreement with the geometric model first proposed by Asakura and Oosawa (1954).

Notes:

Note #1: From the standpoint of density gradients, the essential considerations are as follows: A solution of particles in a solvent, where the particles are unequally distributed, contains two density gradients: one density gradient of particles and one density gradient of solvent. If the system contains no barrier to solute or solvent movement, both solute and solvent are redistributed by Brownian motion with a resultant equalization of their gradients. If the system contains a barrier to solute movement but no barrier to solvent movement (e.g., a semipermeable membrane), solvent movement will occur according to the solvent density gradient. In real systems, the two gradients are always interactive, since the solubility of a solute in a solvent implies an interaction with that solvent.

albumin: The protein albumin has a special role in regulating the osmotic pressure balance at the level of blood vessels, since it is the largest protein constituent of blood plasma and is present at a concentration of approximately 0.6 millimolar (approximately 40 milligrams/milliliter). An abnormal deficiency of albumin can lead to water passing from the bloodstream into the tissues (edema).

Note #2: In this experimental system, the glass sphere is bound to the glass plate at a single point through a *biotin-avidin-biotin connection (essentially a pivot). The surface of the sphere and the surface of the plate are both covered with a monolayer of albumin, and the solution contains the same albumin (bovine serum albumin). The experiment is conducted in a range of pH in which the albumin is negatively charged. The experimental system involves a large number of such attached spheres, but the photodiode-optical apparatus measures the behavior of only a single sphere as observed with a microscope.

biotin-avidin-biotin: Biotin (coenzyme R, vitamin H) is cis-hexahydro-2-oxo-1H-thieno[3,4,-d]-imidazoline-4-valeric acid. It is the D-isomer component of the vitamin B2 complex, and is a small molecule with a high affinity for the glycoprotein *avidin.

avidin: Avidin is a glycoprotein obtained from egg white with a high affinity for biotin. A number of reaction amplification and visualization methods involve labeled avidin binding to macromolecules tagged with biotin.

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