Gold nanoparticles assembled with dithiocarbamate-anchored molecular wires.

Nini E A Reeler,Jozsef Speder,Matthias Arenz,Kasper Nørgaard,Søren V Petersen,Walter Somerville,Knud A Lerstrup,Xiaohui Qiu,Tom Vosch,Bo W Laursen

doi:10.1038/srep15273

Abstract

A protocol for the bottom-up self-assembly of nanogaps is developed through molecular linking of gold nanoparticles (AuNPs). Two π-conjugated oligo(phenylene ethynylene) molecules (OPE) with dithiocarbamate anchoring groups are used as ligands for the AuNPs. OPE-4S with a dithiocarbamate in each end of the molecule and a reference molecule OPE-2S with only a single dithiocarbamate end group. The linking mechanism of OPE-4S is investigated by using a combination of TEM, UV-Vis absorption and surface enhanced Raman spectroscopy (SERS) as well as studying the effect of varying the OPE-4S to AuNP concentration ratio. UV-Vis absorption confirms the formation of AuNP aggregates by the appearance of an extended plasmon band (EPB) for which the red shift and intensity depend on the OPE-4S:AuNP ratio. SERS confirms the presence of OPE-4S and shows a gradual increase of the signal intensity with increasing OPE-4S:AuNP ratios up to a ratio of about 4000, after which the SERS intensity does not increase significantly. For OPE-2S, no linking is observed below full coverage of the AuNPs indicating that the observed aggregate formation at high OPE-2S:AuNP ratios, above full AuNP coverage, is most likely of a physical nature (van der Waals forces or π-π interactions).

Highlights

An oligo(phenylene ethynylene) molecules (OPE) molecule with two dithiocarbamate anchoring groups was used to study the linking of AuNPs suspended in water
The latter could indicate that at high OPE-4S:AuNP ratio, the linking is more difficult, but since the covering and linking is time dependent, some molecular linking can occur before the AuNPs are fully covered, perhaps limiting the aggregates to smaller numbers
The reason for this assumption is that full coverage makes interdigitation much harder and the negatively charged dithiocarbamate end groups will tend to repel each other

Summary

Introduction

This approach requires that a functional, typically π -conjugated, molecule is inserted into the nanogap, which severely hampers the self-assembly process due to solubility issues[15,16,17]. The effect of changing the ratio of linker to AuNP concentration was investigated As shown, this ratio can impact the amount and distribution of AuNPs that are linked together. The charged nature of the sodium dithiocarbamate group(s) facilitates dissolution of the OPE wires in methanol and therefrom in the aqueous colloid solution, while – at the same time – ensuring a stable covalent binding to the AuNPs. The assembly process is characterized using surface enhanced Raman spectroscopy (SERS) in combination with absorption spectroscopy (UV-Vis), transmission electron microscopy (TEM), and investigated as a function of the ratio between linker molecule and AuNP concentrations

Methods

Results

Conclusion