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Light-Directed Patchy Particle Fabrication and Assembly from Isotropic Silver Nanoparticles Erich Michael See, Cheryl L. Peck, Webster L Santos, and Hans D. Robinson Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02307 • Publication Date (Web): 14 Sep 2017 Downloaded from http://pubs.acs.org on September 22, 2017
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Light-Directed Patchy Particle Fabrication and Assembly from Isotropic Silver Nanoparticles Erich M. See‡, Cheryl L. Peck†, Webster Santos†, and Hans D. Robinson*,‡ †Department of Chemistry, Virginia Tech, Blacksburg, Virginia 24061, United States ‡Department of Physics, Virginia Tech, Blacksburg, Virginia 24061, United States
ABSTRACT: We demonstrate the creation of anisotropic patchy silver-nanospheroids (AgNSs) using linearly polarized UV light and a photouncaging o-nitrobenzyl-based ligand, which anchors to the AgNSs by two sulfur bonds. Exposure to a 1 J/cm2 dose of UV light induces a photouncaging reaction in the ligand that reveals a primary amine on the surface. By using linearly polarized UV light, we meter the exposure dose such that only the poles of the nanoparticle receive a full dose, limiting the photouncaging reaction primarily to the particle’s plasmonic hot spots. We reveal this anisotropy by preferentially adhering negatively charged gold nanospheres (AuNSs) to the AgNSs’ poles by using the electrostatic attraction between them and the positively charged primary amines generated by photouncaging. When the assembly is performed onto silver particles that are immobilized on a substrate, it results in nanoscale structures with a strong tendency to align with the polarization of the exposing light.
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This manifests in polarimetric spectroscopy as a linear dichroism aligned with the polarization direction.
1. INTRODUCTION The self-assembly of nanoparticles into larger-scale structures is a natural step in the advance of nanotechnology. One paradigm of controlled self-assembly hinges on the ability to create nanoparticles with differentiated regions or patches whose surface properties differ from the rest of the nanoparticle.1 The patches allow for anisotropic interaction with other particles or surfaces. Once created, the anisotropies can be used and manipulated to cause the patchy particles to self-assemble into larger scale structures, relying either solely on the properties of the particle patches, such as hydrophobicity, polarity, and solvency, or using external modalities such as magnetic or electric fields.2-9 The methods for creating patchy particles are vast and varied, and so too are the ways in which to classify them.2,
4, 10, 11
One very simple approach is to break them down into two broad
categories: liquid phase deposition (LPD), in which the patches are assembled or applied via some wet chemical process, and vapor phase deposition (VPD), which involves vapor-based deposition or removal of chemical or structural patches on the nanoparticle.2, 10 Techniques that fall under the broad umbrella of VPD include glancing angle deposition, chemical vapor deposition, and metal evaporation onto a monolayer of particles. LPD techniques include, but are in no way limited to, colloidal assembly, capillary flow techniques, and DNA modification.2, 10 Templating is the oldest technique for creating patchy particles and involves the manufacture of patches via the manipulation of interface boundaries, by chemically or physically partially trapping a nanoparticle within a surface or along a boundary (the template) and then altering its
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surface at one of the boundaries.2,
4, 10, 12-15
Because of this, templating can be thought of as
straddling the boundary between LPD and VPD, as it falls under both umbrellas. In this paper we approach patchy particle creation through the selective photoreaction of cleavable ligands functionalized on the nanoparticle surface. Self-assembled monolayers (SAMs) of such photocleavable ligands have already been the subject of extensive work, which has investigated photosensitive groups such as coumarin-4-ylmethyl groups, arylcarbonylmethyl groups, metal-based photocleaving groups, and nitroaryl moieties.16 These SAMs allow the creation of functionalized surfaces that can be activated via exposure to light, altering the chemistry of the surface in response to the light. In this way, an initially uniform surface can be textured via light exposure—in our case, an isotropic nanoparticle functionalized with a photocleavable SAM could be transformed into a patchy particle by exposing only portions of the nanoparticle to the incident light. Given that nanoparticles are, by definition, smaller than the wavelength of visible and UV light, selectively photocleaving areas on their surface would at first appear to be a rather difficult proposition. One way to achieve the desired resolution is to use metal nanoparticles, which have the ability to enhance and redirect light incident upon them into very small regions through two mechanisms: surface plasmon resonance (SPR) and the lightning rod effect. The former is a collective electron oscillation, excited by the electric fields of the incident light.17, 18 This causes the electrons in the nanosphere to collectively act like a damped, driven oscillator, which, when excited on or near resonance, can result in significant enhancements to the intensity of the electric field. These enhancements and their locations depend on a variety of factors, including the shape of the particle, and the polarization and wavelength of the incident light.
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The lightning rod effect is a geometric effect caused by field line crowding along the edges of nanoscale metal surfaces.19, 20 In the lightning rod effect, sharp or narrow metal features redirect incident electric fields, concentrating them in specific regions. Both SPR and the lightning rod effect result in hot spots—areas where the intensity of the light is significantly greater than that of the light incident upon the particle as a whole. By combining these phenomena with photocleavable surface-bound ligands, it becomes possible to create patchy particles from isotropically functionalized nanoparticles solely via the introduction of polarized monochromatic light. In this paper, we describe our first attempt at creating and characterizing such patchiness in silver nanospheroids (AgNSs). While this technique is largely limited to conducting or plasmonically active nanoparticles, it constitutes a novel way of creating patchy particles that provides several advantages over existing methods. First, the source of anisotropy is polarization rather than orientation relative to a surface, which means that anisotropic particles can in principle be made throughout the threedimensional volume of a suspension rather than on a two-dimensional surface or interface, which would enable highly efficient scale-up to large volume production. Second, more complex metal nanoparticles can possess multiple SPRs at different wavelengths, which opens the possibility of using our technique to sequentially pattern different sets of hotspots with different ligands simply by changing the wavelength of the exposing light. Third, the hot spots on metal nanostructures are precisely the areas where reactants and analytes need to be brought to optimally implement techniques such as plasmon-enhanced catalysis21 and surface-enhanced Raman spectroscopy (SERS).22-24 The efficiency of those techniques may therefore be enhanced by modifying the hot spots to specifically bind molecules of interest while binding elsewhere is avoided.
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Previous work by our group explored the properties of an o-nitrobenzyl-based photocleavable ligand,
1-(6-nitrobenzo[d][1,3]dioxol-5-yl)ethyl(4-(1,2-dithiolan-3-yl)butyl)
carbamate
(1),
shown in Scheme 1. In Daengngam et al.25 we demonstrated how this ligand undergoes a photouncaging reaction into 1a when exposed to a dose of 1 J/cm2 of 365 nm light. Furthermore, a 2 mM solution of 1 in ethanol forms a self-assembled monolayer (SAM) when exposed to a gold surface by binding to the gold with its double sulfide group. The SAM can then be patterned with UV light, which creates areas of 1a bound to the gold surface. Because 1a SAMs are positively charged at low (
