N-Heterocyclic carbene-based gold etchants

Abstract

N-Heterocyclic carbenes (NHCs) are an emerging alternative to thiols for the formation of stable self-assembled monolayers (SAMs) on gold. We examined several different species that have been used to produce NHC-based monolayers on gold, namely 1,3-diisopropyl-5-nitrobenzimidazolium iodide, 1,3-diisopropyl-5-nitrobenzimidazolium hydrogen carbonate, bis(1,3-diisopropyl-5-nitrobenzimidazolium)gold(I) iodide, and 1,3-diisopropyl-5-nitrobenzimidazole-2-ylidene. Contrary to expectation, solutions containing the first two species in tetrahydrofuran and dichloromethane caused visible loss of gold from thin-film-coated glass slides. The use of toluene solutions of all species resulted in no apparent dissolution of gold. We present scanning electron micrographs and elemental imaging analyses by energy dispersive X-ray spectroscopy to examine the effect of solutions of each species on the gold film. This work highlights the risk of unwanted etching during some routes to NHC-based surface functionalization but also the potential for deliberate etching, with the outcome determined by choice of chemically synthesized organic species and solvent.

Keywords: N-heterocyclic carbenes; gold etchant; microfabrication; self-assembled monolayer (SAM); thin films.

Figure 1

Four species were examined, namely 1,3-diisopropyl-5-nitrobenzimidazolium iodide (1), 1,3-diisopropyl-5-nitrobenzimidazolium hydrogen carbonate (2), bis(1,3-diisopropyl-5-nitrobenzimidazolium)gold(I) iodide (3), and 1,3-diisopropyl-5-nitrobenzimidazole-2-ylidene (4).

Figure 2

Photograph of examples of gold-coated tokens before (left) and after (right) immersion for 2 h in a 3.60 × 10⁻² M solution of 1 in DCM. Scratches were introduced by handling, and the small gold-free rectangle was present on the as-supplied glass slides.

Figure 3

The scanning electron micrographs show the tokens after immersion in solutions of 1 in different solvents and for different durations. From left-to-right by column, the immersion times were 30 min, 1 h, and 2 h. The solvents and concentrations of 1 were (a) toluene (5.00 × 10⁻³ M), (b) DCM (3.60 × 10⁻² M), (c) THF (5.00 × 10⁻³ M), and (d) THF (2.32 × 10⁻³ M). In (a–c), the concentrations were chosen to be close to the solubility limit of 1 in each solvent.

Figure 4

Scanning electron micrographs of tokens after immersion in solution of 2 in (a) THF (3.25 × 10⁻² M) and (b) DCM (3.74 × 10⁻² M), 3 in (c) THF (1.95 × 10⁻³ M) and (d) DCM (7.80 × 10⁻³ M), and 4 in (e) THF (3.30 × 10⁻² M) and (f) DCM (3.30 × 10⁻² M). From left to right, immersion times were 30 min, 1 h, and 2 h. An EDS spectrum (not shown) indicates that chromium, used as an adhesion layer underneath the original gold film, was present on the surface in (a).

Figure 5

UV–vis spectra of 10 µL of 5 mM 1 in THF mixed with 10 mL of AuNPs (left) and 10 μL of 5 mM 2 in THF mixed with 10 mL of AuNPs (right) over 6 h of hourly measurements. The inset shows the peak at approx. 530 nm with the 0 h data corresponding to the left absorbance axis and the subsequent time point data corresponding to the right absorbance axis.