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Synthesis of Silver Nanocubes for use in Self Assembled Monolayer Formation

Jason McHannChemical Engineering Dept.

University of Utah

ABSTRACT RESULTS/DISCUSSION

The synthesis of nanoparticles, while well documented, requires specific attention to reaction parameters and lab technique in order to achieve high yields and repeatability. A mono-disperse population is important for the formation of tightly packed 2-dimensional array (monolayer) and subsequent processing. Presented here are the advances made in synthesis and a novel method for the formation of a monolayer with surface area reducing funnel.

INTRODUCTION

Nanoparticles have garnered much attention during the last decade because they hold the promise of manipulating light and matter on a scale previously unconquered. The Hoepfner Group, in collaboration with Prof. Marc Porter, desires to use nanocubes as a basis for low concentration detection of pathogens or other target molecules [1,2]. By functionalizing a single face of a silver nanocube (AgNC) with a molecules that with high affinity for the target, it is possible to detect the presence of the target by the absorbed UV-visible light spectrum. Because AgNC, sized 40 to 110 nm in edge lenght, are smaller than the wavelengths of UV-vis, they act as antenna sensitive to the surrounding solution and surface bound molecules [3]. The next step is the formation of a close packed two dimensional array (monolayer) so that a single face can be functionalized.

During two semester of funding from the Undergraduate Research Opportunities Program (UROP) the following objectives have been reached.• Attaining a yield of 75% AgNC with tunable edge length.• Design of novel method for monolayer formation. • Construction of a high throughput apparatus.

METHODS

A conventional polyol reduction of ethylene glycol was preformed at, elevated temperature, in the presence of Sodium Hydrogen Sulfide (NaSH) [4]. Cubes ranging from 45 to 110 nm were synthesized by varying the reaction times from 45 to 90 minutes.

This reaction is sensitive to a number of variables not limited to temperature, trace metal ions, dust, and in situ concentration and molecular ratios. In order to increase the throughput testing of reaction parameters, a custom apparatus was built in house as pictured below (fig 3).

Cubes were dispersed in n-butanol and deposited drop wise to the surface of a saline solution and allowed to equilibrate for 20 minutes. Evaporation of n-butanol deposits cubes at the interface rather absorptioninto bulk solution. Controlledrelease of the saline from the funnel (Fig 4) resulted in the reduction of air-saline surface area and the compression ofthe monolayer into a dense configuration.

The production of AgNC that are homogenous in structure and size is highly dependent on molecular ratio [5]. The technique has resulted in a current yield of ~70% up from <20% four months ago. The following molar ratios were found to be optimal for our reagents.

Use of SARF has resulted in monolayer formation that is observed visually as compression changes the optical response from a light yellow to a mirror finish (Fig. 2 & 5). Independent of surface area reduction, force needed for monolayer assembly is controlled through the charge screening offered by the concentration of the saline solution.

Moving forward we desire to attain synthesis yield of 95% with monodisperse structure and selectable edge lengths ±3nm. We believe that this method is ultimately scalable and will validate with construction of a SARF that produces a monolayer on the order of 100cm2.

REFERENCES

[1] M. Porter et al. Chem. Soc. Rev., vol. 33, Mar. 2008[2] C. Moran et al. Nanotechnology, 25, 2014.[3] A. rose et al. Nano Letters, 14, 4797-4802, 2014.[4] Q. Zhang et al. Chem. Eur. J., 16, 10234-10239, 2010.[5] E. Panfilova et al. Colloid Journal, 74, 99-109, 2012.

ACKNOWLEDGEMENTS

• Undergraduate Research Opportunities Program (UROP)• Professor Michael Hoepfner• Professor Marc Porter

O-ring Supported Vial

Aluminum Plate

Silicone Oil Bath

Stir Hot Plate

fig 1. Synthesis Flowchart fig 2. Surface Area Reducing Funnel (SARF)

fig 4. Monolayer in funnel

fig 6. AgNC Synthesis – With Aqua Regia

Surface Area Reducing Funnel(SARF)

Reflective Monolayer

Needle Valve

fig 3. Synthesis Apparatus

fig 5. AgNC Synthesis – Without Aqua Regia

Cube is capped with poly vinylpyrrolidone(PVP) to induce the formation of cube and protect from oxidation. It is readily removed with thiol chemistry.

Sodium Sulfide (HS-) : Silver Ion (Ag2+) 620:1

PVP Repeating Unit : Silver Ion (Ag2+) 2.4:1