Titanium dioxide (TiO2) and zinc oxide (ZnO) nanoparticles have been widely used as commercial sunscreen fillers due to their ability to absorb and scatter UV light [1–3]. TiO2 crystals absorb UVB radiation from 280 to 315 nm, while ZnO crystals absorb UVA radiation from 315 to 400 nm; therefore the combined use of both particles provides the UV protection in a broad spectra . Later, silicon (Si) nanoparticles were also proposed to achieve the same purpose . The advantage of using nanoparticles, as opposed to micro-sized particles, is the transparency of nanoparticles to visible light, which is more desirable than the white opaque appearance of micron sized metal oxide particles [1, 6]. Besides, after reducing size to the nano-scale, the performance (UV attenuation) of these particles can be enhanced, which has been verified by both theoretical and experimental studies [7–9]. However, in spite of their ability to efficiently block UV radiation, concerns have been raised about the environmental impact and potential toxicity of these metal oxide nanoparticles [9, 10].
It is well-known that the photocatalytic activity of metal oxide nanoparticles can result in free radical generation, which has been proven to damage DNA or tissues [1, 9, 11]. Although some metal oxide nanoparticles can be modified with non-semiconductor materials to reduce the generation of reactive oxygen species [1, 12], other biosafety concerns, such as uptake and the interaction of nanoparticles with biological tissues still exist . Though significant penetration of nanoparticles contained in sunscreens through the intact epidermal layer of skin has not been observed till now [1, 13], human skin is not an impenetrable barrier. Hair follicles and abrasions provide opportunities for penetration into the vasculature [9, 14]. A previous study indicated that 40 nm nanoparticles could penetrate through follicular openings and enter epidermal cells . The increased use of synthetic nanoparticles has also been reported as a source of environmental contamination that may affect the ecosystem. Studies showed that TiO2 nanoparticles released into the aquatic environment may have long-term toxic effects due to their prolonged stability [16, 17]. Besides, the toxic bioaccumulation caused by the transfer of nanomaterials among species raised more concerns [6, 18]. Although many studies have been carried out to understand the scope and breadth of these potential hazards, a consensus about nanoparticle toxicity has not been reached [19, 20].
Due to the potential hazards associated with the utilizing of metal oxide nanoparticles in sunscreen products, new green nanomaterials which are harmless to both human health and environment while providing similar UV protective effects are highly desirable. Naturally occurring ivy nanoparticles, which are secreted from the adventitious roots of English ivy (Hedera helix) , have been proposed for sunscreen applications [6, 22]. Inherent properties of the natural organic nanoparticles usually endow them with less biosafety or environmental compatibility concerns compared to inorganic counterparts. As one special case of naturally occurring nanostructures , ivy nanoparticles are secreted from the root hairs accompanying with the secretion process of the ivy adhesive , forming a matrix with other components to support the surface climbing [21, 25]. Both experimental and theoretical studies have shown that ivy nanoparticles have excellent transparency to visible light, and a strong ultraviolet extinction potential compared to TiO2 or ZnO nanoparticles . Moreover, previous studies have indicated that ivy nanoparticles could be degraded by proteolytic enzymes, showed low cytotoxicity to mammalian cells, and had a limited possibility of penetrating human skin, all of which make ivy nanoparticles a promising candidate for sunscreen fillers .
However, before practical application of ivy nanoparticles to cosmetic fields, the physicochemical properties of these nanoparticles should be investigated. Nanoparticles behave differently from other micro- or macro-scale materials, due to the larger surface-to-volume ratio, which allows more atoms or molecules to be displayed on the surfaces . The impact of particle size on the properties of materials can be illustrated by the case of TiO2 nanoparticles. TiO2 nanoparticles demonstrate rutile phase while particle size is above ~20 nm, whereas they exist in the form of anatase phase while particle size is below ~20 nm. [10, 26]. Rutile TiO2 nanoparticles are usually used for sunscreen fillers, while anatase nanoparticles have been applied to self-cleaning glasses. The anatase-to-rutile phase transition is not only dependent on the particle size, but also related to other parameters, such as temperature , reaction atmosphere , and synthesis conditions [26, 29]. Besides, the final nanomorphology of TiO2 depends upon the pH value, and hence its properties are sensitive to the resultant chemistry at the surface [26, 30]. Therefore, a detailed understanding of the relationship between the function of nanoparticles and their physicochemical properties must be carried out before related products could be produced and commercialized . Moreover, nanoparticles are also sensitive to the ambient environment. Environmental changes, such as temperature, pressure, or humidity, may alter the performance of nanomaterials . For example, electromagnetic irradiation can permanently alter the shape of colloidal silver nanoparticles, thus affecting the surface plasmon resonances [31, 32]. Due to the high surface-to-volume ratio of nanoparticles, it is challenging to maintain the stable surfaces of these nanomaterials both in device and in storage media. Changes of temperature or pH often influence the surface reactivity or desorb stabilizing surfactants on the surfaces, and hence cause the agglomeration of nanoparticles . As naturally occurring nanoparticles, ivy nanoparticles may offer more complicated composition than synthetic inorganic nanoparticles. Thus, in-depth investigation on their physicochemical properties is necessary before practical applications. This study is not only necessary for the efficient use of ivy nanoparticles in the sunscreen industry but also for eliminating potential biosafety concerns about utilizing ivy nanoparticles.
In this study, the UV extinction properties of ivy nanoparticles were measured and analyzed under various temperatures and pH values. In addition, the influence of prolonged UV radiation on ivy nanoparticles was also investigated. Methylthiazol tetrazolium (MTT) assay was employed to study the cytocompatibility of this naturally occurring nanomaterial. Different from our early study, this research focused on the physicochemical properties analysis of ivy nanoparticles and data from this study provided a comprehensive understanding about the relationship between the UV extinction ability and the properties. Information gained through this study will advance potential applications of the ivy nanoparticles in sunscreen products.