Have you ever wondered if the “24K gold face cream” or a “Nanogold face mask” you can buy from your local drugstore really contains gold or is it just a marketing trick from cosmetic companies to make you buy a luxurious and costly product? And if it really contains gold, is using gold on your skin beneficial? Or can it be somehow dangerous and toxic? In this article, we will discuss the benefits, drawbacks, and safety precautions of the usage of nanogold in your everyday skincare.
What makes nanocosmeceuticals so special compared to “regular” ingredients?
In the EU, nanomaterials in cosmetics are officially defined as: “an insoluble or bio-persistent and intentionally manufactured material with one or more external dimensions, or an internal structure, on the scale from 1 to 100 nm” . Cosmetics that contain nanoparticles as ingredients show long-lasting effects and increased stability. Moreover, it has been discovered that the enhanced surface area of nanoparticles enables a more coherent conveyance of the active ingredients across the skin . However, they come with some disadvantages. Along with high reactivity, some nanocosmeceuticals can exhibit toxic effects and can represent potential health hazards if used without previous regulations and control [3-5].
Figure 1. Graphical abstract of nanocosmeceuticals used in the cosmetic industry (a) and advantages and disadvantages of nanocosmeceuticals  *API: Active Pharmaceutical Ingredient
Nanogold particles in cosmetics
Gold nanoparticles used in cosmetic products have a size range from 5-400 nm and can be found in many different morphological forms, such as nanocubes, nanostars, nanospheres, etc (Figure 1). The color of the nanogold depends on the particle’s size, shape, resonance frequency and can vary from red to purple or blue to almost black due to aggregation that can occur . Many research papers that have been recently published report various ways of eco-sustainable (or green) synthesis or formulation of gold-based nanoparticles and have evaluated their antioxidant, moisture-retaining, whitening, and dermo-protective properties [7,8]. Other major characteristics of nanogold that endorse its use in cosmetic products are the inertness, very low toxicity, biocompatibility, and highly stable nature of these particles. Gold nanoparticles have also shown excellent antibacterial and antifungal properties.
There are many different cosmeceutical products in the global market which contain gold nanoparticles, including anti-aging creams, face masks, toners, mists, toothpaste, etc. Nanogold is also very common in beauty care and make-up products because it was demonstrated that these particles exhibit antiseptic and anti-inflammatory properties, they can delay the aging process, enhance the elasticity as well as the tightness of the skin, and even dermal circulation [9,10]. Table 1., contains some examples of commercialized cosmetics including gold nanoparticles which are readily available in the market .
Table 1. Commercially available formulations containing gold nanoparticles with their uses 
What’s the role of skin in the delivery of nanogold particles?
To be able to understand the potential risks that involve the use of nanoparticles, first, we have to consider their routes of exposure and ask ourselves: “How deep can gold nanoparticles really penetrate our skin and wherein the skin can they exhibit potentially toxic effects?” There are three ways by which nanoparticles can enter our skin and they are:
intracellular (or transcellular) and
follicular penetration pathway (Figure 2a).
Permeation through the intercellular pathway is strenuous and is dependent on the ability of the cosmetic product to penetrate the protective hydrophobic layer on the skin. Intracellular permeation also depends on the physicochemical characteristics of used nanoparticles and whether they can overcome the tight matrix of the skin’s corneocytes . Recent studies have shown that the most dominant skin permeation pathway for the nanoparticles is a follicular penetration pathway. This is explained by the presumption that follicular openings can serve as entree points for nanoparticles, which can then be conveyed into the skin [6,12]. The skin consists of three main layers: dermis, epidermis and subcutaneous tissue, and many other thin sublayers in between (Figure 2b). The very top layer of human skin is called the stratum corneum (the first layer of the epidermis) and it represents the main protective barrier that nanoparticles have to overcome to achieve a positive, desirable effect on our skin. Negative and potentially toxic effects may occur in cases where there’s a possibility that nanoparticles successfully penetrate into deeper layers of our skin . The degree of penetration depends on many factors, such as shape, size, surface properties, and dosage of nanoparticles present in a cosmetic product. Generally, gold nanoparticles are reviewed to be of good biocompatibility, great control delivery properties, and of low toxicity . However, the low toxicity rule does not apply to high concentrations of certain sizes of gold nanoparticles. A group of scientists has shown that gold nanoparticles, roughly 1 nm in diameter, in both low and high concentrations, can penetrate the cell and nuclear membranes and attach themselves to the DNA molecule.  This strong interaction of gold nanoparticles with cell and nuclear membranes is interpreted as a result of their powerful electrostatic appeal to the cell membrane’s bilayer which is negatively charged .
Figure 2. a) Illustration of the possible pathways of skin penetration: intercellular, intracellular, and follicular penetration pathway ; b) Schematic of the structure of human skin with its layers which are considered vital for maintaining skin barrier function 
Regulations and safety precautions: Are we safe?
Without strict and specific instructions and guidelines, cosmetics containing nanoparticles are being commercialized quickly and are achieving large profits for the cosmetic industry. Regulative bodies such as European Commission together with the Scientific Committee on Consumer Products Safety (in EU) and Food and Drug Administration (in the US) are responsible for overseeing and systematically regulating the safety of used ingredients and cosmetic products containing nanoparticles available on the market [18-20]. For example, in the EU, it is mandatory that all nanocosmeceuticals ingredients must be specifically indicated with a word ‘nano’ on the label of the product . In the year 2007 , the FDA presented the Nanotechnology Task Force Report in which an evaluation of scientific as well as the regulatory parameter related to the reactivity and usefulness of nano-containing products were shown. The manufacturers were supposed to take these guidelines into consideration during the research and development phases of their products since they are held accountable for ensuring the safety of their cosmetics.
Taking everything into consideration, there is no doubt that nanogold is an amazing bioactive ingredient with many beneficial effects on our skin. However, it is necessary to raise the awareness of the consumers in regards to both positive and potentially negative effects that can occur using nanoparticles in our everyday skincare so that they can safely explore the new era of cosmetic ingredients.
If you are interested in finding out more about what other types of nanoparticles are there in the market and in which cosmetic products you can find them, feel free to explore our other articles such as: “Delving into The Era of Nanotechnology Pursuing Nanomedicine” and "Science fiction meets dermatology: The use of Nanoemulsions in the cosmetic industry".
 Regulation (EC) No 1223/2009. Available online: https://ec.europa.eu/growth/sectors/cosmetics/products/nanomaterials_en (accessed on 26 November 2020).
 Ahmad, U., Ahmad, Z., Khan, A.A., Akhtar, J., Singh, S.P., Ahmad, F.J. Strategies in development and delivery of nanotechnology-based cosmetic products. Drug Research, 2018, 68, 545–552.
 Henkler, F., Tralau, T., Tentschert, J., Kneuer, C., Haase, A., Platzek, T., Götz, M.E. Risk assessment of nanomaterials in cosmetics: A European union perspective. Arch. Toxicol. 2012, 86,1641–1646.
 Bilal, M., Iqbal, H.M. An insight into toxicity and human-health-related adverse consequences of
cosmeceuticals—A review. Sci. Total Environ. 2019, 670, 555–568.
 Subramaniam, V.D., Prasad, S.V., Banerjee, A., Gopinath, M., Murugesan, R., Marotta, F., Pathak, S. Health hazards of nanoparticles: Understanding the toxicity mechanism of nanosized ZnO in cosmetic products. Drug Chem. Toxicol. 2019, 42, 84–93.  Dhapte-Pawar, V., Kadam, S., Saptarsi, S., Kenjale, P. P. Nanocosmeceuticals: facets and aspects. Future Science OA, 2020 FSO613.
 Khan, A.K., Rashid, R., Murtaza, G., Zahra, A. Gold nanoparticles: Synthesis and applications in drug delivery. Trop. J. Pharm. Res. 2014, 13, 1169–1177.
 Jiménez-Pérez, Z.E., Singh, P., Kim, Y.-J., Mathiyalagan, R., Kim, D.-H., Lee, M.H., Yang, D.C. Applications of Panax ginseng leaves-mediated gold nanoparticles in cosmetics relation to antioxidant, moisture retention, and whitening effect on B16BL6 cells. J. Ginseng Res. 2018, 42(3), 327–333.
 Yeh, Y.C., Creran, B., Rotello, V.M. Gold nanoparticles: Preparation, properties, and applications in bionanotechnology. Nanoscale, 2012, 4, 1871–1880.
 Thakor, A.S., Jokerst, J., Zavaleta, C., Massoud, T.F., Gambhir, S.S., Gold nanoparticles: A revival in precious metal administration to patients. Nano Lett. 2011, 11, 4029–4036.
 Antonio, J.R.; Antonio, C.R.; Cardeal, I.L.S.; Ballavenuto, J.M.A.; Oliveira, J.R. Nanotechnology
in dermatology. An. Bras. Dermatol. 2014, 89, 126–136.
 Otberg, N.; Richter, H.; Schaefer, H.; Blume-Peytavi, U.; Sterry, W.; Lademann, J. Variations of hair follicle size and distribution in different body sites. J. Investig. Dermatol. 2004, 122, 14–19.
 Katz, L.M., Dewan, K., Bronaugh, R.L. Nanotechnology in cosmetics. Food Chem. Toxicol. 2015, 85,127–137.
 Xu Z.P., Zeng Q.H., Lu G.Q., Yu A.B. Inorganic nanoparticles as carriers for efficient cellular delivery. Chem. Eng. Sci. 2006, 61, 1027-1040.
 Tsoli M., Kuhn H., Brandau W., Esche H., Schmid G. Cellular uptake and toxicity of AU(55) clusters. Small, 2005, 1, 841-844.
 Goodman C.M., McCusker C.D., Yilmaz T., Rotello V.M. Toxicity of gold nanoparticles functionalized with cationic and anionic side chains. Bioconjugate Chem. 2004, 15,897-900.
 Mihranyan, A.; Ferraz, N.; Strømme, M. Current status and future prospects of nanotechnology in cosmetics. Prog. Mater. Sci. 2012, 57, 875–910.
 Kumud, M., Sanju, N. Nanotechnology driven cosmetic products: commercial and regulatory milestones. Appl. Clin. Res. Clin. Trials Regul. Aff. 2018, 5(2), 112–12.
 European Commission, Brussels. Nanotechnology Emerging Needs (2007). http://cordis.europa.eu/nanotechnology/src/pressroom projects nmp6.htm
 F.T.F.R. Nanotechnology Task Force Report. 2007. Available online:
https://www.fda.gov/science-research/nanotechnology-programs-fda/nanotechnology-task-force-report-2007 (accessed on 08 December 2020).