The influence of molecular mobility on the properties of networks of gold nanoparticles and organic ligands

Abstract

SummaryWe prepare and investigate two-dimensional (2D) single-layer arrays and multilayered networks of gold nanoparticles derivatized with conjugated hetero-aromatic molecules, i.e., S-(4-{[2,6-bipyrazol-1-yl)pyrid-4-yl]ethynyl}phenyl)thiolate (herein S-BPP), as capping ligands. These structures are fabricated by a combination of self-assembly and microcontact printing techniques, and are characterized by electron microscopy, UV–visible spectroscopy and Raman spectroscopy. Selective binding of the S-BPP molecules to the gold nanoparticles through Au–S bonds is found, with no evidence for the formation of N–Au bonds between the pyridine or pyrazole groups of BPP and the gold surface. Subtle, but significant shifts with temperature of specific Raman S-BPP modes are also observed. We attribute these to dynamic changes in the orientation and/or increased mobility of the molecules on the gold nanoparticle facets. As for their conductance, the temperature-dependence for S-BPP networks differs significantly from standard alkanethiol-capped networks, especially above 220 K. Relating the latter two observations, we propose that dynamic changes in the molecular layers effectively lower the molecular tunnel barrier for BPP-based arrays at higher temperatures.

Spin Transition in Arrays of Gold Nanoparticles and Spin Crossover Molecules

“…Many of the observed changes are strongly reminiscent of our previous report on binding of the parent (S)-(4-{[2,6-(dipyrazol-1-yl)pyrid-4-yl]ethynyl}phenyl-)thioate (S-BPP) ligand at a gold nanoparticle array. 16 For example, the mode which dominates the SERS spectrum at 1574 cm À1 is attributed to the aryl inplane CÀC stretch mode of the benzenethiol moiety which is observed at approximately 1590 cm À1 in the unbound complex. 16,35 In the Raman spectrum of the SCO complex, this mode is superimposed on a more intense pyridine CÀC stretch mode.…”

“…Basically, a rod-like metal complex is displayed, based on an iron(II) ion and two organic ligands. 16 In bulk powder form, the formula is [Fe(AcS-BPP) 2 ](ClO 4 ) 2 (hereafter called molecule 1), including thioacetate protecting groups and counterions. After exposure to gold surfaces, deprotection of the AcÀS anchor groups occurs, leading to a formal composition of Fe(S-BPP) 2 under near surface conditions (hereafter called molecule 1 0 ).…”

“…In short, spherical gold nanoparticles (8.5 ( 1.5 nm diameter) are synthesized in-house and modified with octanethiol molecules. 16,32 Next, they are made to selfassemble in an ordered array on a water surface. Such an array can be transferred to a substrate of choice by microcontact printing.…”

“…For the second set, the nonmetalated S-BPP ligand (see Figure 1A) is used to coat the gold nanoparticles directly, after which array devices are created via self-assembly and microcontact printing. 16…”

Dielectrophoresis of gold nanoparticles conjugated to DNA origami structures

“…A physical measure of the polarizability is the dielectric constant, ε. Experimental data show that DNA molecules with ε = 8.5 are a less polarizable material than gold nanoparticles (ε → ∞) [ 33 – 34 ]. Hence, the higher polarizability of gold nanoparticles extends the electrical field strength and frequency range of DNA/gold-nanoparticle hybrid structures for pDEP [ 35 – 36 ].…”