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Synthesis of Cone-Shaped Colloids from Rodlike Silica Colloids with a Gradient in Etching Rate Fabian Hagemans, Ernest Benjamin Van der Wee, Alfons van Blaaderen, and Arnout Imhof Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b00678 • Publication Date (Web): 05 Apr 2016 Downloaded from http://pubs.acs.org on April 11, 2016
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Synthesis of Cone-Shaped Colloids from Rod-like Silica Colloids with a Gradient in Etching Rate Fabian Hagemans*, Ernest B. van der Wee, Alfons van Blaaderen, and Arnout Imhof* Soft Condensed Matter, Debye Institute for NanoMaterials Science, Utrecht University, Princetonplein 1, 3584 CC, Utrecht, The Netherlands Colloids, cone, silica, chemical composition, confocal microscopy
Abstract: We present the synthesis of monodisperse cone-shaped silica colloids and their fluorescent labeling. Rod-like silica colloids prepared by ammonia catalyzed hydrolysis and condensation of tetraethyl orthosilicate in water droplets containing polyvinylpyrrolidone crosslinked by citrate ions in pentanol were found to transform into cone-shaped particles upon mild etching by NaOH in water. The diameter and length of the resulting particles were determined by those of the initial rod-like silica colloids. The mechanism responsible for the cone-shape involves silica etching taking place with varying rate along the particle’s length. Our experiments thus also lead to new insights into the variation of the local particle structure and composition. These are found to vary gradually along the length of the rod, as a result of the way the rod grows out of a water droplet that keeps itself attached to the flat end of the bullet-shaped particles. Subtle differences in composition and structure could also be resolved by high resolution stimulated-emission-depletion confocal microscopy on fluorescently labeled particles.
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The incorporation of a fluorescent dye chemically attached to an amine-based silane coupling agent resulted in a distribution of fluorophores mainly on the outside of the rod-shaped particles. In contrast, incorporation of the silane coupling agent alone resulted in a homogeneous distribution. Additionally, we show that etching rods where a silane coupling agent alone was incorporated and subsequently coupled to a fluorescent dye resulted in fluorescent silica cones, the orientation of which can be discerned using super-resolution confocal microscopy.
Introduction Synthetic methodologies towards anisotropic colloidal particles have attracted increased attention over the last decade because of their great importance in nanomaterial assembly strategies. The interest in particles with less symmetric shapes comes from their potential in chemical, electronic, and optical applications.1-3 Recent advances in the synthesis of anisotropic particles have supplied us with a large variety of anisotropic colloidal building blocks. Colloidal self-assembly of these anisotropic building blocks could lead to new functional materials with greater complexity than those currently available.4-6 Inorganic particles can be obtained in a large variety of shapes. In literature, many methods are available to synthesize spherical and polyhedral particles from a large range of materials. Some examples are: gold spheres7, silver polyhedra8, rhombohedral and cubic cadmium carbonate particles and tetrahedral SnS microcrystals9. Rod and board-like particles show special tunable optical properties upon self-assembling into colloidal liquid-crystalline phases, but can also exhibit interesting catalytic properties.10 These types of particles can be prepared from a large range of materials with varying size and aspect ratio. Some examples of these particles are; goethite boards11,12, gold nanorods13, carbon nanotubes14, CdSe@CdS rods15, silicon nanowires16 and CdSe/Au17 and CdSe/CsTe nanobarbells18.
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These approaches are mostly limited to crystalline materials, but amorphous particles such as silica can be produced using a template-based synthesis. These shapes include cubes, peanuts and ellipsoids.19-21 These particles were synthesized by coating a hematite template with a layer of amorphous silica. These silica cubes have been observed to self-assemble in the presence of depletion attractions into a cubic or hexagonal lattice depending on the depletant size. In the absence of attractive interactions, these particles were able to form hexagonal crystals with hollow site stacking. Silica, in particular, has the advantage that it can be easily chemically modified with various types of functional groups.22 Recently, a new colloidal system of rod-like silica colloids was developed that does not require the use of a template that must be removed after the synthesis.23,24 These particles can be produced in batch synthesis, have sufficiently low polydispersity to assemble into ordered phases, and can be easily functionalized by grafting or by the incorporation of a silane coupling agent. This system allows the quantitative real-space 3D study of their self-assembly into various liquid crystalline phases.25,26 The procedure starts with the synthesis of silica rods, as described by Kuijk et al.23 These particles can be prepared in a simple one-pot synthesis by mixing ethanol, water, sodium citrate, ammonia and tetraethyl orthosilicate (TEOS) with a solution of polyvinylpyrrolidone (PVP) in 1-pentanol. The rods grow from a water-rich droplet that serves as the locus for silica growth. Each rod grows from a single water droplet containing PVP, in a pentanol phase (water-in-oil emulsion). In this emulsion droplet, which is stabilized by citrate ions, silica condensation takes place due to the presence of water, which is required for the hydrolysis of TEOS. Due to the anisotropic supply of hydrolyzed TEOS, the particle predominantly grows from one side only. The particles have a bullet shape; the round end having moved away from the water droplet during growth while the flat end remained in contact with the aqueous phase until the end of the synthesis.
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This mechanism has shown to be robust enough that it allows modification of the shape and of interactions of these silica rods. For example, it was shown that a hydrophobic segment could be added by the delayed addition of a hydrophobic silica precursor.27 This yielded particles that consisted of a hydrophilic and a hydrophobic segment that self-assembled into micelle-like structures. A manganese oxide segment was introduced by in situ prepared nanoparticles that decorated the water droplet and resulted in self-propelling particles upon the addition of hydrogen peroxide.28,29 A gold tip was introduced by the introduction of an Au(III)polyvinylpyrrolidone complex in the water droplet.30 The shape of the particles could also be easily modified by changing the reaction conditions during particle growth.31,32 Reaction temperature, ethanol concentration and reagent addition time affected the particle’s diameter allowing the synthesis of segmented silica rods. The concentration of base was shown to influence the locus of condensation of silicon alkoxide; at high concentration of base, condensation took place predominantly at the droplet surface leading to hollow silica rods.30 Kuijk et al.33 noticed that the addition of a fluorescent dye that is incorporated after being chemically attached to the amine functionalized end of a so-called silane coupling agent into the silica at the beginning of the reaction resulted in rods with a gradual decrease in fluorescence along their length. This was explained by a decreasing availability of the dye during growth, leading to a gradient in dye concentration. It was shown that such a gradient in fluorescence can be used to determine the orientation of the rods even in concentrated systems.25,26 On the contrary, the addition of the silane coupling agent 3-aminopropyltriethoxysilane (APTES) without a fluorescent dye chemically attached resulted in a homogeneous incorporation of amino groups throughout the particle. In addition, it is well known that the base catalyzed hydrolysis and condensation of silane coupling agents is slower than that of silicon-tetraalkoxides in water
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alcohol mixtures.34 Even without any dye present, reaction conditions are expected to change during particle growth, as silicon alkoxide and water are consumed, and ethanol is formed. This may be expected to lead to a subtle chemical gradient as expressed in the level of condensation of the siloxane structure in the rods. Here, we confirm this by demonstrating that the silica produced early in the growth etches faster than the silica produced at the end of the reaction. Moreover, we make use of this chemical gradient to transform rods into cone-shaped particles. We further show that these particles can be made fluorescent allowing them to be studied in realspace. The fluorescent silica cones were imaged using 2D continuous wave (CW)-gated stimulated emission depletion (STED) confocal microscopy. This technique allows significant improvement of resolution in xy, where z is parallel to the optical axis, compared to conventional confocal microscopy, with a decrease in minimum separation of resolvable sources from ~250nm to
