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Layered Silicate Clays as Templates for Anisotropic Gold Nanoparticle Growth Eric H. Hill, Nathalie Claes, Sara Bals, and Luis M. Liz-Marzán Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b02186 • Publication Date (Web): 02 Jul 2016 Downloaded from http://pubs.acs.org on July 3, 2016
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Chemistry of Materials
Layered Silicate Clays as Templates for Anisotropic Gold Nanoparticle Growth Eric H. Hill1, Nathalie Claes2, Sara Bals2, Luis M. Liz-Marzán1,3 1
Bionanoplasmonics Laboratory, CIC biomaGUNE, 20009 Donostia-San Sebastián,
Spain 2
EMAT-University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium
3
Ikerbasque, Basque Foundation for Science, 48013 Bilbao, Spain
Abstract Clay minerals are abundant natural materials arising in the presence of water, and are composed of small particles of different sizes and shapes. The interlamellar space between layered silicate clays can also be used to host a variety of different organic and inorganic guest molecules or particles. Recent studies of clay-metal hybrids formed by impregnation of nanoparticles into the interlayer spaces of the clays have not demonstrated the ability for templated growth following the shape of the particles. Following this line of interest, a method for the synthesis of gold nanoparticles on the synthetic layered silicate clay laponite was developed. This approach can be used to make metal-clay nanoparticles with a variety of morphologies while retaining the molecular adsorption properties of the clay. The SERS enhancement of these particles was also found to be greater than that obtained from metal nanoparticles of a similar morphology, likely due to increased dye adsorption from the presence of the clay. The hybrid particles presented herein will contribute to further study of plasmonic sensing, catalysis, dye aggregation, and novel composite materials.
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Introduction Metal nanoparticles (NPs) are of great interest due to their myriad applications arising from nanoscale properties, which deviate from those of bulk materials.1 Noble metals are particularly interesting and have been well-studied due to their shape- and size-dependent physical and electronic properties.2 Many of these properties are granted or influenced by the occurrence of plasmons, which are oscillations of the electron density in the nanoparticle upon excitation by light.3 In addition to non-linear optical effects and improved catalysis, surface plasmons can greatly enhance the scattering of molecules in the vicinity of the metal surface where the plasmon is generated, and this effect has given rise to highly-sensitive analytical techniques such as surface enhanced Raman scattering (SERS), which can even reach single-molecule detection under certain conditions.4 SERS alone has led to a plethora of interesting advances in sensing of molecular species, ions,5 disease biomarkers, and other compounds, leading to breakthroughs in medical diagnostics,6 monitoring of environmental pollutants,7 and monitoring the dynamics of bacterial communication.8 In addition to analytical techniques, metal nanoparticles have been shown to have improved catalytic properties compared with bulk.9 A number of reviews cover the history of these developments, which have guided the synthetic efforts of metal NPs over the last 20 years.1,2,9-11 Through control of the twinning and crystal structure of metal seed particles, the growth of the particle along different crystal facets can be directed to form anisotropic shapes such as triangles and rods.12,13 Recently, studies on the use of template materials for the growth of metal nanoparticles to conform to a certain shape have been performed using inorganic templates such as silica and biological templates such as DNA and viral capsids, though many of these techniques still rely on the use of seeds for some part of the synthesis.9,10 Clay particles have also been used to direct nanoparticle growth,14 with recent emphasis on layered silicate clays due to their interesting electrostatic properties and anisotropic shapes. Clay minerals are abundant natural materials arising in the presence of water, which can be dispersed in liquids to form suspensions consisting of particles with a variety of sizes and shapes such as rods15 and plates.16 These clay particles are able to self-organize into complex structures17 and can have rheological effects resulting in a variety of phases such as gels18, glasses,19,20 and “ringing” gels.21 In addition to their abundance and low cost, properties of clays such as high surface area, good ion2 ACS Paragon Plus Environment
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Chemistry of Materials
exchange capacity, and recyclability have led to their use in numerous industrial processes. Layered silicate clays, such as montmorillonite and laponite, can delaminate in water to a suspension of platelets with a high aspect ratio (300:1).19,22–24 Studies have shown that through loading of organic molecules on clay particles, the intermolecular interactions and resulting molecular aggregates can be altered.25 This can result in different photophysical and chemical properties leading to alteration of photochemical processes26,27 such as the enhancement of organic laser dyes25 and reducing photochemical degradation.28 Additionally, their potential biomedical applications, such as drug-delivery and release and wound healing, have been the subject of recent study.29–33 We expect that the combination of plasmonic properties of anisotropic metal nanoparticles with the molecular adsorption of the clay will lead to improved SERS enhancement over a purely metal particle. Laponite RD (laponite) is a synthetic layered silicate clay with the chemical formula Na+0.7[(Si8Mg5.5Li0.3)O20(OH)4]−0.7. Its specific surface area determined by nitrogen adsorption is 370 m2/g, and its density is ~2.53 g/cm3.34 A smectite-type clay, it is composed of disc-like particles with a thickness of ~1 nm and a diameter of 25-30 nm.35 While the cation exchange capacity is 0.75 meq/g, the discs can be charged heterogeneously in aqueous suspensions depending on the pH.36 While similar to other natural smectite clays such as montmorillonite, the synthetic nature of laponite provides a clay with a narrower distribution of particle diameter and cation exchange capacity. Scheme 1 gives a visual representation of laponite and its crystal structure.
Scheme 1. Schematic view of the morphology, surface charge distribution and crystal structure of a laponite clay particle.
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While previous studies have demonstrated the potential for layered silicate clays to be used as supports for metal nanoparticle growth and intercalation,26,27 the ability to use the delaminated clay particles as shaped templates for nanoparticle growth has not been achieved. In previous studies, the resulting particles are either spherical and/or small (
