Addressing the plasmonic hotspot region by site-specific functionalization of nanostructures.

Eric S A Goerlitzer,Nicolas Vogel,Reza Mohammadi,Lutz E Speichermann,Talha A Mirza

doi:10.1039/c9na00757a

Eric S A Goerlitzer, Nicolas Vogel + Show 3 more

Open Access

https://doi.org/10.1039/c9na00757a

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Abstract

Strong electromagnetic fields emerge around resonant plasmonic nanostructures, focusing the light in tiny volumes, usually referred to as hotspots. These hotspots are the key regions governing plasmonic applications since they strongly enhance properties, signals or energies arising from the interaction with light. For a maximum efficiency, target molecules or objects would be exclusively placed within hotspot regions. Here, we propose a reliable, universal and high-throughput method for the site-specific functionalization of hotspot regions over macroscopic areas. We demonstrate the feasibility of the approach using crescent-shaped nanostructures, which can be fabricated using colloidal lithography. These structures feature polarization-dependent resonances and near-field enhancement at their tips, which we use as target regions for the site-selective functionalization. We modify the fabrication process and introduce a defined passivation layer covering the central parts of the gold nanocrescent, which, in turn, selectively uncovers the tips and thus enables a localized functionalization with thiol molecules. We demonstrate and visualize a successful targeting of the hotspot regions by binding small gold nanoparticles and show a targeting efficiency of 90%. Finally, we demonstrate the versatility of the method exemplarily by translating the principle to different nanostructure geometries and architectures.

Highlights

Electromagnetic radiation impinging on noble metal nanostructures excites collective oscillations of the free electron gas, which are known as localized surface plasmon resonances (LSPR)
To allow the desired deposition of material. We reduced their size by exposing them to an oxygen plasma.[42]
We demonstrated a reliable, yet simple method to selectively address the tips of nanocrescents, where their near- eld enhancement is maximized

Summary

Introduction

Electromagnetic radiation impinging on noble metal nanostructures excites collective oscillations of the free electron gas, which are known as localized surface plasmon resonances (LSPR). We employ the established fabrication method of nanocrescents[24,25,26] as analogues of split-ring resonators,[27,28,29] with polarization-dependent resonances,[24,30] that can be controlled over a large spectral range,[25] using various materials such as silver, gold or aluminum.[31,32,33,34] Such nanocrescents are attractive structures to demonstrate our concept as they provide sharp tips with high near- eld enhancements (Fig. 1a) with a simple fabrication process,[35,36] and as they have been successfully employed as substrates for dielectric sensing,[35,37,38] SERS,[39,40] and to enable concentration measurements in micro uidics.[41]

Results

Conclusion