From Himba indigenous knowledge to engineered Fe2O3 UV-blocking green nanocosmetics

This contribution reports on the physical properties of the natural Namibian red Ochre used by the Himba Community in a form of a formulation, so called Otjize as a skin protective and beauty cream. The morphological and crystallographic studies of this red ochre validated its nano-scaled dominating phase of rhombohedral α-Fe2O3 nanocrystals with an additional hydrolized oxide component in a form of γ-FeOOH. The optical investigations showed that such a red ochre exhibits an exceptional UV filtration and a significant IR reflectivity substantiating its effectiveness as an effective UV-blocking & solar heat IR reflector in support of the low skin cancer rate within the Namibian Himba community. In addition, such nanocrystals exhibited a non-negligible antibacterial response against E. Coli & S. Aurus. This study seems confirming the effectiveness of the indigenous Otjize as an effective skin UV protection cream with a sound antimicrobial efficacy against e-Coli & S-Aurus.

In addition to the recently discovered Zeta-Fe 2 O 3 structure 1 , Iron (III) oxide represents one of the most rich and multifunctional oxide families 2 . It has four standard principal polymorphs, hematite (α-Fe 2 O 3 ), maghemite (γ-Fe 2 O 3 ), β-Fe 2 O 3 and ε-Fe 2 O 3 ; iron (III) oxy-hydroxide also has four polymorphs known as goethite (α-FeOOH), akaganeite (β-FeOOH), lepidocrocite (γ-FeOOH) and feroxyhyte (δ-FeOOH) 2 . Of a special interest, α-Fe2O3 is a prominent n-type semiconductor with an interesting bandgap Eg = 2.1 eV, conjugated to an excellent chemical stability and low toxicity under ambient conditions. Among these oxides, α-FeOOH and α-Fe2O3 have been commonly investigated for magnetic devices, catalysts, gas sensors and photoelectrodes applications 3-10 in addition to pigment production. However, it is worth noting that this latter pigment aspect has been identified as early as the birth of humanity.
Indeed, early human used several marine resources and pigments to express artistically on their natural surroundings & believes ranging from prehistoric, ochre-pigmented images on cave walls to paintings on canvasses in addition to their symbolic behaviors. Its usage in symbolic behavior was found to date164,000 years ago, far earlier than previously documented 11 .
More precisely, the earliest evidence of the ochre pigment's usage by ancient humans dates to the Paleolithic, about 285,000 years ago, at a Homo erectus site called GnJh-03 in Kenya. In this location, archaeologists discovered about 70 pieces of ochre weighing about 5 kg 12 .
Early Homo-sapiens also illustrated extensively with ochre pigment. At Blombos Cave, in South Africa, archaeologists discovered an abalone shell containing finely ground ochre, charcoal as well as fat that may have made up a likely painting kit dating to about 100,000 years ago. The earliest human-made drawing is a red hashtag on small rock flake that dates to about 73,000 years ago, also at Blombos and Klein Kliphuis Caves in South Africa. In addition, about 266,000 years ago, early hominins at a site called Twin Rivers in Zambia collected a type of hematite ochre that has significantly reflective metallic flakes in it making it glittering [13][14][15] .
As highlighted previously, while Ochre was used as an effective artistic & behavioral pigment in various locations within the African continent, the Himba ethnic group specifically, has genuinely extended its usage towards cosmetic & skin-protection applications (Fig. 1a). With the Nama & the Herero communities, the Himba society is an additional indigenous social group in Namibia. Living in the northern part within the Kunene region, they are hunter-gatherers, semi-nomadics & pastoralists. Exposed to harsh atmospheric conditions in general www.nature.com/scientificreports/ and daily solar radiations especially where the Direct Normal irradiation is among the highest globally. In the quasi-absence of clouds, they are exposed through out the year to the standard 5% UV (300-400 nm), 43% VIS (400-700 nm) and 52% NIR-FIR (700-2500 nm) solar radiations (Fig. 1b). Accustomed to a semi-arid climate, the Himba women are famous for covering their skin & hairs with the so called Otjize paste 16 ; a cosmetic mixture of goat-butterfat and ochre pigment. Otjize cleanses the skin over long periods due to water scarcity and considered as an effective protection from the hot and dry climate, as well as from insect bites. It gives Himba people's skin and hair plaits a distinctive texture, style, and orange or red tinge, and is often perfumed with the aromatic resin of the indigenous omuzumba shrub (Commiphora multijuga) 17 . From anthropological viewpoint, Otjize is considered foremost a highly desirable aesthetic beauty cosmetic, symbolizing earth's rich red colour and blood, the essence of life, and is consistent with the Himba ideal of beauty (Fig. 1a).
To validate the optical filtering & selectivity as well as the health skin protection of the Ochre pigment used in the Himba Otjize, this contribution reports on its physical properties with a special focus on their optical selectivity in the UV-VIS spectral solar range and their antibacterial efficacy. If so, this would be the originality & novelty of this contribution.   Figure 2a reports the HRSEM of the Himba ochre.

Morphology & crystallographic investigations.
As one could notice, it consists of agglomerated lamellar nanoplatelets. Their longitudinal & basal dimensions are relatively wide spread in terms of dimension. The average basal dimension is estimated within the range of 17-31 nm while the longitudinal average dimension is above 100 nm. Figure 2b reports an HRTEM of the Himba Ochre after sonication. The lamellar nano-platelets seem consisting of nanoscale relatively crystalline grains with preferred crystalline orientation and a platelets type morphology. The major observed atomic ordering correponds to an inter-reticular distance of 5.03 Å in agreement with the (104) atomic orientation of pure single phase hematite α-Fe 2 O 3 . Figure 2c displays the size distribution of the nanoparticles obtained by standard light scattering. The corresponding profile indicates a relatively polydisperse nanosystem with an average size of <ϕ> = 148.3 nm. In this analysis, the nano-platelets were approximated as spheroids with a diameter ϕ. This latter parameter could be almost be the equivalent of the size of the nanoplatelets.  27 and Blake et al. 28  exhibits several absorbance bands centered at about 347 nm, 543 nm assigned to metal charge transfer direct transitions and to the double excitation processes 6 A1(6 s) + 6 A 1 (6 s) to 4 T 1 ( 4 G) + 4 T 1 ( 4 G) respectively 29 . Those in the range of 478-550 nm are caused by 6 A 1 (6 s) to 4 E, 4 A 1 ( 4 G) ligand field transitions 30 . In addition to these various absorbance bands, there is a strong absorbance at 543 nm which at the origin of the red color of hematite. Figure 4c reports the Reflectance of the Otjize ochre sample in the spectral range of 190-2500 nm. As one can notice, there is a sharp spectral cut-off at about 570 nm. The reflectance is minimal below such a cut-off; < 3%. Because of the opacity of the sample, the spectral range in the UV & part of the VIS is fully absorbed by the Otjize  Antibacterial studies. Relatively to the standard nano-TiO 2 & nano ZnO possessing cut-off wavelengths at the UV-Bleu spectral range at the vicinity of 380 & 390 nm respectively, the one of the current Himba Otjize red Ochre is by contrast in the visible range at the vicinity of 570 nm as per (Fig. 4c). Consequentially, the nanoscaled α -Fe 2 O 3 of the Himba Otjize red Ochre should exhibit an antibacterial response. Indeed, as reported in Fig. 5a   www.nature.com/scientificreports/ compared to the standard Streptomycine. However, the efficacy of the Himba Otjize red Ochre α-Fe 2 O 3 nanoparticles become effective at higher concentrations. This trend is comparable to those obtained by Rufus et al. 35 on pure hematite bio-synthesized using natural extract of Psidium guajava's leaves as an effective chelating agent. In terms of the antibacterial activity of the nano-scaled α-Fe 2 O 3 of the Himba Otjize red Ochre, one can distinguish 3 potentials mechanisms which are schematically represented in Fig. 5b 36-43 ; (i) Photocatalytic mechanism, (ii)Reactive Oxygen Species mechanism & or (iii)Cell surface interaction with the α-Fe 2 O 3 nanoparticles mechanism. In the first mechanism, once the the α-Fe 2 O 3 nanoparticles are excited with a light energy larger than the bandgap, an e _ −H + excitonic pair is created. This e _ -H + pair reacts with O 2 , hydroxyl groups as well as the adsorbed H 2 O molecules to produce sturdy oxidative compounds such as OH _ , O 2_ and H 2 O 2 . The H + and OH _ are characterized by strong oxidative properties, able to break the chemical bonds of various compounds inducing the degradation of the micro-organisms and therefore antibacterial effectiveness. The second potential mechanism involves Reactive oxygen species. Once diffused in the cell, the nanoscaled α-Fe 2 O 3 nanoparticles produce ROS species inducing a severe damage to bacterial membranes and hence bacteriolysis & aggregation of α-Fe 2 O 3 nanoparticles within the bacteria leading their death. In the third mechanism, and because of the high surface to volume ratio of the α-Fe 2 O 3 nanoparticles, their interaction with the cell surface leads to a significant bacterial membrane damage and the release of ion channels, resulting in internal ionic imbalance of the cells, and eventually death 43,44 .
As a major pre-conclusion of this study, it is worth pointing that the effective UV filtration of the red ochre used by the Himba Women could explain the low skin cancer within such a community in Namibia 45 .

Conclusion
This study was geared towards the investigation of the bio-physical properties of Red Ochre pigment used by the Himba in their Otjize formulation to protect their skin from solar radiations. It was found that the Red Ochre pigment consisted of nanoscaled particles of mainly α-Fe 2 O 3 with γ-FeOOH nanoparticles. Such a Red Ochre pigment exhibited a significant UVA & UVB blocking optical properties hence is an effective skin cancer protection. In addition, it exhibits an effective reflectivity in the IR region hence minimizing solar heat burning of the Himba bodies in the harsh Namibian desertic climate. In addition, it was found that such a red Ochre pigment exhibit additional antibacterial efficacy against E-coli & Staphylococcus Aurus. As a major conclusion of this study, it is worth pointing that the effective UV filtration of the red ochre used by the Himba Women could explain the low skin cancer within such a community in Namibia. www.nature.com/scientificreports/ Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/.