N, N-Dimethylaminoethyl methacrylate-based core/shell microgels loaded with silver nanoparticles for catalysis

Abstract

In this work, poly(styrene)@poly(N-isopropylmethacrylamide-co-2-(N,N-dimethyl)aminoethyl methacrylate [p(sty)@p(NIPMAM-DMAEMA)] core/shell microgel particles were produced by a two-step free-radical precipitation polymerization process. Ag nanoparticles were successfully embedded inside the sieves of a crosslinked network by using silver nitrate as the precursor salt and NaBH₄ as the reductant. The synthesized pure and hybrid microgels were analyzed by various characterization tools, including Fourier transform infrared (FTIR) and UV-visible (UV-vis) spectroscopies, transmission electron microscopy (TEM) and dynamic light scattering (DLS). Results indicate the successful fabrication of spherical silver nanoparticles with diameters ranging from 10 to 15 nm within the sieves of the poly(styrene)@poly(N-isopropylmethacrylamide-co-2-(N,N-dimethyl)aminoethyl methacrylate) core/shell microgels, which have a hydrodynamic diameter of 155 ± 25 nm. The Ag nanomaterial exhibited long-term stability in the p(sty)@p(NIPMAM-DMAEMA) system due to the strong donor-acceptor relationship between the lone pair of the amide moiety in the polymer microgels and the Ag nanomaterial. The catalytic activity of the Ag-p(sty)@p(NIPMAM-DMAEMA) material was determined by performing the catalytic reduction of p-nitrophenol (4-NPh) as a model reaction under diverse concentrations of the catalyst. UV-vis spectrophotometry was used to check the progress of the reaction. The apparent rate constant (k _app) was measured by applying the pseudo-first-order kinetics model. It was observed that k _app increased with increasing catalyst dose, demonstrating occurrence of the reaction on the surface of the catalyst.

Fig. 1. Graphical representation of the synthesis of p(sty) core, p(sty)@p(NIPMAM-DMAEMA) core/shell microgels and Ag-nanoparticle-loaded p(sty)@p(NIPMAM-DMAEMA) core/shell hybrid microgels.

Fig. 2. FT-IR spectra of p(sty) core, pure p(sty)@p(NIPMAM-DMAEMA) microgel and Ag-p(sty)@p(NIPMAM-DMAEMA) hybrid microgels.

Fig. 3. UV-vis spectra of dilute dispersions of p(sty), p(sty)@p(NIPMAM-DMAEMA) and Ag-p(sty)@p(NIPMAM-DMAEMA) at room temperature in an aqueous medium.

Fig. 4. TEM images of p(sty) (A), p(sty)@p(NIPMAM-DMAEMA) (B and C) and Ag-p(sty)@p(NIPMAM-DMAEMA) (D).

Fig. 5. Hydrodynamic radius of p(sty) core particles and p(sty)@p(NIPMAM-DMAEMA) core/shell particles as a function of temperature (A) and particle size distribution of the p(sty)@p(NIPMAM-DMAEMA) core/shell particles at two different temperatures (B).

Fig. 6. UV-vis spectra of the freshly prepared dilute dispersion of Ag-p(sty)@p(NIPMAM-DMAEMA) hybrid microgels and hybrid microgels stored for four months.

Fig. 7. Time-dependent UV-visible spectrophotometric analysis of the reduction of 0.06 mM 4-NPh by 15 mM NaBH₄ in an aqueous medium in the absence of the Ag-p(sty)@p(NIPMAM-DMAEMA) hybrid microgels.

Fig. 8. Time-dependent UV-visible spectrophotometric analysis of the reduction of 0.06 mM 4-NPh by 15 mM NaBH₄ in water in the presence of p(sty)@p(NIPMAM-DMAEMA) pure microgels.

Fig. 9. Time-dependent UV-visible spectrophotometric analysis of the reduction of 0.06 mM 4-NPh to 4-APh by 15 mM NaBH₄ in water in the presence of 0.2020 mg mL⁻¹ Ag-p(sty)@p(NIPMAM-DMAEMA) pure microgels.

Fig. 10. Plot of ln(C_t/C₀) versus time for the catalytic reduction of 4-NPh (0.06 mM) to 4-APh in water medium using NaBH₄ (15 mM) in the presence of 0.0404–0.202 mg mL⁻¹ Ag-p(sty)@p(NIPMAM-DMAEMA) hybrid microgel at 14 °C.

Fig. 11. Graph of ln(C_t/C₀) versus post-induction time for the catalytic reduction of 4-NPh (0.06 mM) to 4-APh in water medium using NaBH₄ (15 mM) in the presence of 0.0404–0.202 mg mL⁻¹ Ag-p(sty)@p(NIPMAM-DMAEMA) hybrid microgel at 14 °C.

Fig. 12. Plot of k_appvs. concentration of Ag-p(sty)@p(NIPMAM-DMAEMA) for the catalytic reduction of 4-NPh (0.06 mM) to 4-APh in an aqueous medium using NaBH₄ (15 mM) in the presence of 0.0404–0.202 mg mL⁻¹ Ag-p(sty)@p(NIPMAM-DMAEMA) hybrid microgel at 14 °C.