Abstract
Over the years there has been an increase in the number of cases of individuals dealing
with issues such as pruritus, erythema, premature skin ageing, sun burn and even skin
cancer. All of these issues are directly linked to exposure to ultra-violet (UV) radiation
without adequate protection from the suns’ harmful rays. As a result, the widespread
use of sunscreen agents has become more prevalent on a global scale. However, the
efficacy and safety of these sunscreen products has been the centre of numerous
discussions within the field of dermatological research. In particular, focus has been
placed on organic UV filters which have been known to generate harmful by-products
and undergo photoreactive degradation which ultimately poses a great threat to
consumers. Inorganic filters on the other hand, such as zinc oxide (ZnO) and titanium
dioxide (TiO2), are also commonly used within sunscreen formulations. Although the
safer option, specifically for children and individuals with sensitive skin, these filters are
not without threat. As known photocatalysts, the use of ZnO and TiO2 within cosmetic
formulations as they may also have negative effects on human health due to their
photocatalytic nature and ability to generate reactive oxygen species. However, they do
provide good UV-shielding properties which are ultimately desirable for any kind of sun
screening agent, and therefore, they are still being used within sunscreen formulations. Therefore, this study aimed at developing a Zn-Ti layered double hydroxide (LDH) to be
used as a novel inorganic UV filter in sunscreen formulations. We investigated the
physical and photochemical properties of the Zn-Ti LDH, as well as the properties of a
Zn-Al LDH and Mg-Al LDH. In this study the Zn-Ti LDHs were synthesized using Zinc
Nitrate Hexahydrate and Titanium Isopropoxide as precursors which were kept constant
in a (3:1) – Zn: Ti ratio. Synthetic parameters such as reaction temperature and time,
drying conditions and ageing time were varied in pursuit of obtaining the most optimized
Zn-Ti LDH.
To understand the effect of varying the synthetic parameters on the structural,
morphological and physical properties of the Zn-Ti LDH, X-ray diffraction (XRD),
Scanning electron microscopy(SEM), Fourier Transform infrared (FTIR) spectroscopy,
Thermogravimetric analysis (TGA), Brunauer-Emmett-Teller (BET), X-ray photoelectron
spectroscopy (XPS) and Transmission electron microscopy (TEM) were employed as
characterization techniques. Structural analyses revealed that the reaction temperature
was of paramount importance for achieving the desired layered structure of an LDH and
facilitating the incorporation of tetravalent titanium between the layers. Microscopy
showed that the shapes of the LDH particles were indeed affected by varying the
synthetic parameters and the variation of reaction temperature warranted the greatest
effect. XPS, TGA, BET and FTIR indicated that the optimum reaction parameters for the
synthesis of a Zn-Ti LDH were as follows; 120°C under reflux for 24h, followed by drying
at 120°C for 24h in the vacuum oven. As ageing had the least effect on the structure,
the ageing time may vary.
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Furthermore, the optical analyses by diffuse reflectance UV-Vis (DR UV-Vis) analysis
revealed that the synthesized Zn-Ti LDH had a band gap of 3.72 eV and in turn
exhibited reflective properties which proves favourable for application as a UV filter
within sunscreen. The photocatalytic activity of this material also proved favourable as
there were minimal traces of methylene blue degradation under both UVA and UVB
radiations. The cytotoxicity results further affirmed the viability of the synthesized Zn-Ti
LDH as it exhibited no cytotoxicity towards human malignant melanoma cells, human
keratinocyte cells, as well as normal human lung fibroblast cells. Finally, the Sun
protection factor (SPF) value of 18 obtained on Zn-Ti LDH from DR UV-Vis
measurements confirmed our initial hypothesis. In vitro Ultraviolet-A protection factor
(UVA-PF) analysis indicated that the Zn-Ti LDH possess a valid claim of “broad
spectrum” and UVA protection with a UVA balance of 41% and UVA-PF value of 2.65 ±
0.06.
To affirm the efficacy of the Zn-Ti LDH for cosmetic applications the LDH was compared
to Zn-Al and Mg-Al LDH. These LDHs were characterized accordingly. Structural
analyses revealed that the incorporation of Ti4+ caused an increase in inter-gallery
spacing which translated to shapely flake-like structures which had favourable surface
area (77.03m2/g). Optical properties of the Zn-Al and Zn-Ti LDHs were similar due to the
presence of Zn in the structure. This ultimately resulted in similar SPF /UVA-PF
readings. Zn-Al LDH had a UVA-PF value of 3.82 ± 0.08 with valid claims for UVA
protection and “broad spectrum” protection.