Versatile one-pot synthesis of gold nanoclusters and nanoparticles using 3,6-(dipyridin-2-yl)-(1,2,4,5)-tetrazine

Abstract

A one-pot synthesis of gold nano-objects is described by simply mixing a gold salt (HAuCl₄), dodecanethiol and 3,6-di-2-pyridyl-1,2,4,5-tetrazine. When a large excess of thiol is used, gold nanoclusters of 2 nm are obtained in a large amount and with a narrow size distribution. The reaction mechanism was investigated by absorption and emission spectroscopies and shows the in situ formation of dihydrotetrazine acting as the reductant of Au(iii) to make Au(0). Au nanoclusters were isolated from the molecular precursors by HPLC. The nature of the ligands stabilizing Au nanoclusters was investigated by various techniques such as mass spectrometry, SEM-EDS, XPS and NMR. Thiol and tetrazine are shown to play both the role of ligand stabilizing the clusters. Finally, when a much smaller amount of thiol is used, a mixture of Au nanoclusters and Au nanoparticles of 10-15 nm, sometimes aggregated into clusters of 50 nm is obtained. The formation of larger nanoobjects is explained by the lower amount of thiol available to block the growth at the early stage as shown by UV-vis absorption monitoring.

Scheme 1. Formulae of bptz and its reduced form H₂bptz.

Fig. 1. Color changes during the synthesis: (a) initial (left) and final (right) reactional mixtures; (b) DDT; (c) DDT + HAuCl₄; (d) DDT + HAuCl₄ + bptz and following pictures taken every 7 minutes over a period of 160 minutes.

Fig. 2. (A) TEM snapshots (two different areas) of a crude sample directly after synthesis without purification; (B) HRTEM snapshot of the supernatant after purification by centrifugation. (C) HRTEM snapshot of the fraction isolated by HPLC.

Fig. 3. TEM pictures at different magnifications of Au NPs obtained from the mixture of DDT and HAuCl₄ in ethanol, in absence of bptz.

Fig. 4. UV-vis absorption spectra evolution upon synthesis of Au NCs (reaction time in minutes). Inset shows the magnification of the high wavelength region.

Fig. 5. (A) Emission (λ^exc = 368 nm, red trace) and excitation (λ^em = 630 nm, black trace) spectra of the final solution of clusters in ethanol; (B) Emission spectra (λ^exc = 368 nm) at various reaction times from 0 to 180 minutes.

Scheme 2. Mechanism of Au NC formation starting from tetrazine (Tz) and alkanethiol (R-SH) and global outcome.

Fig. 6. MALDI-TOF mass spectrum of the Au NC sample just after synthesis. No peaks have been observed at higher m/z values.

Fig. 7. HPLC chromatograms of the sample containing Au NCs (top), and the comparison of another Au NC sample with a blank sample containing no gold (bottom). Conditions: MeOH : H₂O (50 : 50), Phenomenex Luna C-18, 1 mL min⁻¹, 110 bar.

Fig. 8. ¹H (top) and ¹³C (bottom) NMR spectra in MeOD of Au NCs isolated from HPLC (black line), bptz + DDT mixture leading to H₂bptz (blue line) and bptz from crystal (red line).

Fig. 9. TEM pictures of Au NPs synthesized for sample 2 (see Table 1).

Fig. 10. (A) UV-vis absorption monitoring of the reaction with the following proportions of reactants: Au : bptz : DDT 1 : 1 : 2 (sample 2 in Table 1). Times in minutes. (B) UV-vis absorption spectra after 5 minutes of reaction and pictures at end of the reaction for the five samples (see Table 1 for the proportions corresponding to each sample).