Determination and Characterization of Gold Nanoparticles in Liquor Using Asymmetric Flow Field-Flow Fractionation Hyphenated with Inductively Coupled Plasma Mass Spectrometry

Abstract

The EU has approved the usage of gold as a food additive (E175) and it has been applied in numerous foods for coloring and decoration purposes. Different from the general assumption that edible gold is mainly present in the form of flakes or external coating in foods, this work demonstrated that gold nanoparticles (Au NPs) can be released from gold flakes and extracted under optimized conditions. To support future risk assessment associated with the exposure of Au NPs to human health, an effective approach was established in this study for both size characterization and mass determination of Au NPs released in a commercial gold-containing liquor using Asymmetric Flow Field-flow Fractionation (AF4) hyphenated with Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Our results showed that no Au NPs were detected in the original liquor product and only after ultrasonication for several minutes did Au NPs occur in the ultrasound-treated liquor. Particularly, Au NPs released in the liquor can be well extracted after 100-fold enrichment of gold flakes and the subsequent ultrasonication for 25 min. Size characterization of Au NPs was conducted by AF4-ICP-MS under calibration with Au NP standards. The gold particle sizes detected ranged from 8.3-398.0 nm and the dominant size of the released Au NPs was around 123.7 nm in the processed liquor. The mass concentration of gold particles determined in the liquor sample with gold flakes concentrated and subsequently sonicated was 48.1 μg L^-1 by pre-channel calibration and the overall detection recoveries ranged over 82-95%. For the comparison control samples without ultrasonication, there was no detection of Au NPs. The established method was demonstrated to be useful for monitoring Au NPs in liquor and is possibly applied to other similar foodstuffs.

Keywords: AF4-ICP-MS; food additive; gold flakes; gold nanoparticles; gold-containing liquor; ultrasound.

Figure 1

Effects of ethanol on stabilization of Au NPs in alcoholic suspensions and their separation for AF4-ICP-MS analysis, demonstrated by the overlaid fractograms of typical Au NPs size standards for comparisons under different solution environments and varied Au NPs concentrations, including in 0.2% NC vs. 43.5% ethanol at individual concentrations of (a) 50 µg L⁻¹ or (b) 10,000 µg L⁻¹, as well as (c) in 43.5% ethanol at 5000 µg L⁻¹ for each of spiked Au NPs with wider size range.

Figure 1

Effects of ethanol on stabilization of Au NPs in alcoholic suspensions and their separation for AF4-ICP-MS analysis, demonstrated by the overlaid fractograms of typical Au NPs size standards for comparisons under different solution environments and varied Au NPs concentrations, including in 0.2% NC vs. 43.5% ethanol at individual concentrations of (a) 50 µg L⁻¹ or (b) 10,000 µg L⁻¹, as well as (c) in 43.5% ethanol at 5000 µg L⁻¹ for each of spiked Au NPs with wider size range.

Figure 2

Overlaid AF4-ICP-MS fractograms of Au NPs standards dispersions (size-ladder) spiked in the blank liquor matrix (at 1000 µg L⁻¹ for each), with the modeling equation displayed from corresponding size calibration curve (i.e., retention times of the AF4 peak maxima (min) (x) vs. nanoparticle diameters (nm) (y)).

Figure 3

Profiling and characterization of the Au NPs with 100-fold enrichment and subsequent release in the studied gold-containing liquor after varied treatments of ultrasonic processing. (a) Overlaid AF4-ICP-MS fractograms of Au NPs in the processed liquor products and its blank liquor matrix under different scenarios, with the comparison of other blank control samples included as well. (b) Number-based size distribution of Au NPs detected in the concentrated gold-containing liquor based on ultrasound process for 25 min, as determined with AF4-ICP-MS and the following transformation of the mass-based to number-based distribution.