Alizarin Grafting onto Ultrasmall ZnO Nanoparticles: Mode of Binding, Stability, and Colorant Studies

Abstract

The demand is rising for colorants that are obtained from natural resources, tolerant to industrial processing methods, and meet color quality demands. Herein, we report how relevant properties such as thermal stability and photostability of the natural colorant alizarin can be improved by grafting it onto ZnO nanoparticles (NPs), allowing application in a warm extrusion process for the fabrication of polyamide fibers. For this study, ZnO NPs (diameter 2.0 ± 0.6 nm) were synthesized and subsequently functionalized with alizarin. The alizarin-coated ZnO NPs (i.e., dyed nanoparticles, DNPs) were characterized. Thermogravimetric analysis and ultraviolet–visible (UV–vis) studies revealed that alizarin coating accounts for ∼65% (w/w) of the total mass of the DNPs. A subsequent detailed characterization with Fourier transform infrared (FT-IR), 1H nuclear magnetic resonance (NMR), 13C cross-polarization magic angle spinning (CP-MAS) NMR, X-ray photoelectron spectroscopy (XPS), and quantum chemistry studies using various density functional theory (DFT) functionals and basis sets indicated that binding onto the ZnO NPs occurred predominantly via the catechol moiety of alizarin. Importantly, this grafting increased the thermal stability of alizarin with >100 °C, which allowed the processing of the DNPs into polyamide fibers by warm extrusion at 260 °C. Evaluation of the lightfastness of the DNP-dyed nylon fibers revealed that the changes in color quantified via the distance metric ΔE* of alizarin when embedded in a hybrid material were 2.6-fold better compared to nylon fibers that were directly dyed with alizarin. This reveals that the process of immobilization of a natural dye onto ZnO nanoparticles indeed improves the dye properties significantly and opens the way for a wide range of further studies into surface-immobilized dyes.

Solution NMR methods for structural and thermodynamic investigation of nanoparticle adsorption equilibria

“…For example, de Haan et al found that the changes in the UV/Vis spectrum of alizarin in the presence of ZnO NPs are consistent with the absorbance spectrum observed for deprotonated alizarin, suggesting that alizarin binds to ZnO NPs in its deprotonated form. 8 …”

“…Of note, the latter approach to the determination of K does not require estimation of q max and is, therefore, the preferred method to obtain the Langmuir equilibrium constant for weak ligand-NP interactions. 26 However, it should be noted that eqn (8) only applies to sorption equilibria based on elementary reactions. In the case of more complex equilibria involving one or more intermediate states, the ratio of the rate constants in eqn (8) would not provide an accurate estimate of K. 23…”

“…26 However, it should be noted that eqn (8) only applies to sorption equilibria based on elementary reactions. In the case of more complex equilibria involving one or more intermediate states, the ratio of the rate constants in eqn (8) would not provide an accurate estimate of K. 23…”

“…For example, de Haan et al found that the changes in the UV/Vis spectrum of alizarin in the presence of ZnO NPs are consistent with the absorbance spectrum observed for deprotonated alizarin, suggesting that alizarin binds to ZnO NPs in its deprotonated form. 8 Fluorescence Fluorescence spectroscopy and its variations, such as Fluorescence Correlation Spectroscopy (FCS), are convenient techniques for investigating adsorption processes due to their high intrinsic sensitivity and their ability to provide input experimental data for the Langmuir isotherm model. The uorescence of ligands is highly sensitive to quenching by the formation of NP-ligand conjugates.…”

“…Nanoparticle (NP) catalysts have found many applications in numerous elds such as energy, 1,2 petrochemical, 3,4 and medicine, 5,6 due to their high surface-area-to-volume ratio and sizedependent properties that make them highly efficient and tunable catalysts. Since sorption equilibria play an essential role in determining the efficiency and selectivity of NP catalysis, many analytical techniques have been utilized to characterize ligand-NP surface interactions, including ultraviolet-visible (UV/Vis) spectroscopy, [7][8][9][10] uorescence, 11,12 vibrational spectroscopy, [11][12][13] and isothermal titration calorimetry (ITC). 14 However, the outcomes of these analytical methods are generally dependent on the specic system under investigation and experimental conditions.…”