In situ reduction of chloroauric acid (HAuCl4) for generation of catalytic Au nanoparticle embedded triazine based covalent organic polymer networks

Abstract

Covalent-organic polymer networks (COPNs) have been used as catalyst supports due to their stable and favorable structure. Herein, a simple synthetic route was applied to generate Au@COPN-1 hybrids via in situ reduction of gold ions with no additional reducing agent. Synthesized novel COPN-1 was mixed with different concentrations of HAuCl₄ which resulted in Au@COPN-1 with varying sizes of Au nanoparticles in a controlled manner. The microstructural and morphological features of COPN-1 and Au@COPN-1 were characterized in detail using FT-IR, C-NMR, elemental analysis, UV-Vis, XRD, TEM, BET, and TGA. It is noteworthy that the red-shifted LSPR peaks of Au nanoparticles produced with increasing concentrations of HAuCl₄ indicated an increase in the particle size of the Au nanoparticles as justified by TEM images. The optimum catalytic activity of Au@COPN-1 was obtained when 4.6 × 10^-3 mM HAuCl₄ was used, which led to the complete reduction of 4-nitrophenol within 16 minutes with excellent recyclability for more than 5 catalytic cycles, giving yields over 94%. Moreover, the non-aggregation of nanoparticles in the reused catalyst further confirmed the stability of the prepared catalysts. Consequently, these results indicated that in situ synthesis of AuNPs inside the COPN-1 matrix produces a promising catalyst platform for the reduction of aromatic nitro compounds, for example, for the degradation of one of the most common persistent organic pollutants 4-nitrophenol, as shown here. In addition, the Au@COPN-1 hybrid system showed good biocompatibility at appropriate doses confirmed by a dynamic real-time cell analysis system which can be used in various medical applications, such as drug delivery, in the future.

Scheme 1. COPN-1 synthesized via reacting tris(2,3-epoxypropyl)isocyanurate with melamine.

Fig. 1. (a) FTIR and (b) C-NMR spectra of COPN-1.

Fig. 2. (a) UV spectra and (b) XRD Pattern of COPN-1 and Au@COPN-1 (4.6 × 10⁻³ mM Au).

Fig. 3. (a and c) TEM images of Au@COPN-1 obtained from different concentrations of Au(+3) (4.6 × 10⁻⁴ mM and 4.6 × 10⁻³ mM, respectively) with the corresponding particle size distribution histograms (b and d) The HR-TEM images of Au nanoparticles showing interplanar spacing.

Fig. 4. (a) UV spectra of COPN-1 and Au@COPN-1 at different concentration (4.6 × 10⁻⁴, 9.2 × 10⁻⁴, 4.6 × 10⁻³ and 9.2 × 10⁻³ mM Au) (b) TGA thermograms of COPN-1 and Au@COPN-1 (4.6 × 10⁻³ mM Au) in N₂.

Scheme 2. Proposed mechanism for HAuCl₄ reduction with COPN-1 in water.

Fig. 5. Catalytic reduction of 4-nitrophenol by using NaBH₄, (a) COPN-1, (b) with Au@COPN-1 (4.6 × 10⁻³ mM Au), (c) kinetics of reduction process of 4-Nph solution (4-Aph formation) and (d) fitted data of the pseudo-first-order kinetics (Au@COPN-1, HAuCl₄·3H₂O, and COPN-1).

Fig. 6. (a) Recycling experiment of Au@COPN-1 for the conversion of 4-Nph during 5 cycles of reaction and (b) real-time cell analysis results: effect of Au@COPN-1 treatments on L929 cell line proliferation.