Utilization of nanomaterials to develop analytical methods of prohibited substance in skincare products

Abstract

Recently, there has been a growing concern regarding the potential presence of prohibited substances in skincare products, especially preservatives, and mercury. Mercury is a toxic metal that can accumulate and be absorbed into the bloodstream. Preservatives are substances that are added to prevent microorganisms and prolong the shelf-life of the products. However, extended use of skin care products containing mercury and preservatives can have harmful side effects on the skin and internal organs. The usage of mercury and preservatives in skincare products is legally restricted in numerous countries owing to their damaging effects. There are various traditional methods employed for mercury and preservative detection, such as atomic absorption spectrophotometry (AAS), inductively coupled plasma optical emission spectroscopy (ICP-OES), inductively coupled plasma mass spectrometry (ICP-MS), and high-performance liquid chromatography (HPLC). However, despite their high accuracy and reliability, these techniques require costly equipment, skilled professionals, and complex sample preparation. Therefore, this study aimed to develop a fast, affordable, simple, and efficient approach to analyzing mercury and preservatives in skincare samples by using nanomaterials as key substances in the analysis reactions, followed by fluorometric detection. This work was divided into two sections.

First, determining mercury in skincare products using carbon dots (CDs) as a reagent combined with a sequential injection analysis (SIA) and spectrofluorometric detection was proposed. The fluorescence intensity of CDs is significantly decreased due to mercury ions. The CDs have been successfully synthesized using the microwave-assisted method. The properties of the CDs were characterized by transmission electron microscopy (TEM), X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FT-IR), UV−vis spectrometry, and spectrofluorometry. This proposed method showed a linear range from 0.5 to 600 ppm with a detection limit of 0.1 ppm and an acceptable percentage recovery. The relative standard deviation was 1.53% (n = 12) with a 20-sample per hour sample throughput. By comparison with ICP-MS, the accuracy of our approach was validated, and the results between the two methods are not significantly different. This work was the first time to present the use of CDs to determine mercury ions in skincare samples using the SIA method with an easy, automatic, and cost-effective detection.

Next, the fluorometric detection of preservatives in skincare products using Ni-MnFe-layered double hydroxides (Ni-MnFe-LDHs) as peroxidase-like mimicking was developed. 4-Hydroxybenzoic acid (PHBA) and benzoic acid were studied as preservative model targets. In the presence of preservatives, Ni-MnFe-LDHs can catalyze the oxidation of H2O2. The generated hydroxy radical (•OH) was then consumed by PHBA or benzoic acid to form phenoxy radical, leading to less of •OH to catalyze o-phenylenediamine (OPD) into the yellow-fluorescent product of 2, 3-diamino phenazine (DAP). The yellow fluorescence signal of DAP significantly decreased, corresponding to the concentration of preservatives in skincare products. A smartphone captured the color of the solution under a UV-controlled lightbox within 20 minutes. Under the optimum conditions, this developed method showed a linear range of 0.008-1.0 and 0.008-1.0 mM for PHBA and benzoic acid, with a limit of detection of 0.0072 and 0.0042 mM for PHBA and benzoic acid, respectively. Our proposed method was validated with the HPLC-DAD and showed an acceptable percentage recovery.

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