Ultraviolet Extinction and Visible Transparency by Ivy Nanoparticles
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Though much research has been conducted for nanoparticles, naturally occurring nanoparticles have not yet been well explored for their diverse properties and potential applications. This paper reports the optical absorption and scattering properties of nanoparticles secreted by English ivy. Both experimental and theoretical studies have been conducted. Strong ultraviolet extinction and excellent visible transparency are observed, compared to the inorganic TiO2 and ZnO nanoparticles at similar concentrations. The contributions of absorption and scattering to the total extinction are quantified by simulation of the Mie scattering theory.
KeywordsIvy nanoparticle Optical absorption Light scattering
Optical properties of naturally occurring organic nanoparticles have not been well explored. Their potential applications are largely unknown. This may be due to two reasons. First, research in the field of natural organic nanoparticles is new to material science. Although natural nanostructures with biological functions have been found for many years [1, 2], natural organic nanoparticles have not been investigated for material properties such as mechanical and optical properties until recent years . Second, most attention is attached to artificial organic nanoparticles [4, 5]. These nanoparticles formed of organic molecules and polymers offer variable optical properties due to the abundant alternative components. However, in the case of biological applications, organic nanoparticles formed in nature are also a promising alternative due to their compatibility and non-toxicity. Natural nanoparticles can be used as the functional materials just as the artificial nanoparticles. Optical absorption and light scattering are the two main optical properties that induce the extinction effect. Optical absorption and light scattering of materials have many applications. Nanoparticles with ultraviolet extinction properties have the potential to be used in sunscreen. The ingredients of modern sunscreens usually contain organic compounds , inorganic nanoparticles  and organic nanoparticles . The inorganic nanoparticles (such as ZnO and TiO2) and organic nanoparticles can reflect, absorb and scatter the solar light. Though inorganic nanoparticles have been widely used in cosmetic products, there are still concerns about the toxicity of these inorganic materials. TiO2 has been reported to induce DNA [9, 10] and RNA  damage through the process of chemical oxidation. Considering these situations, if nature-derived harmless organic nanoparticles have strong ultraviolet absorption, they will be a potential promising alternative for sunscreen. In this paper, we reported the optical absorption and light scattering of ivy nanoparticles. These nanoparticles were isolated by two approaches, size exclusion chromatography and filtration. Optical absorption and light scattering properties of these samples were characterized. The advantages of these nanoparticles in their application to sunscreen compared to inorganic nanoparticles were also demonstrated.
Fresh-grown rootlets of English ivy were collected around the University of Tennessee, Knoxville Campus, before attaching themselves to solid surfaces. The collected rootlets were meshed using tweezers in 5-ml clean tube containing 500µl of 20-nm-filtered water after washing them three times with ddH2O. The solution was then centrifuged at 3000 RPM for 5 min, and the supernatant was collected. Two-thirds of the solution was dialyzed overnight at 4 degrees with ddH2O through a cellulose membrane (Sigma–Aldrich, D9277) allowing free pass of chemicals less than 12,500 Daltons. Size exclusion chromatography and high-performance liquid chromatography (SEC–HPLC) was used to isolate the nanoparticles from the solution. This technology can separate the nanoparticles in solution based on their sizes . The Varian HPLC system consists of three components including Varian Prostar 210, the Varian Prostar 335 detector and the Varian Prostar 430 autosampler. Next, 200 μl of dialysis solution was loaded to the column (Phenomenex ® BIOSEP-SEC-S3000 column with an attached Security Guard ® cartridge system). The running speed of the mobile phase was set to 0.5 ml/min, and the back pressure of the column was 580 psi. Samples were collected depending on the retention time. These samples we investigated in this study were named SEC-1, SEC-2 and SEC-3 with the time sequence. Filtration was another method used to reveal the optical properties of ivy nanoparticles. The dialyzed solution was filtered through the Millipore membrane (220-nm Nylon filters). This solution was marked as D220D1. Half of this solution was filtered again through 20-nm filters. The filtered solution was marked as D20D1. Furthermore, a portion of each solution was diluted into 1/3 concentrations. The diluted solutions were named D220D2 and D20D2, respectively. The non-dialysis solutions were also prepared and named ND220 and ND20, respectively.
The morphologies of the ivy nanoparticles were observed by Agilent 5500 atomic force microscope (Agilent Technologies, Santa Clara, CA). The instrument was operated at room temperature (20°C) using Picoview™ in AC mode, which means little or no contact between the tip and the sample.
Dynamic light scattering (DLS) measurements were performed by a Brookhaven Instruments BI-200SM goniometer equipped with a PCI BI-9000AT digital correlator. The operating wavelength is 633 nm. The detector is located at the scattering angle of 90º.
The ultraviolet and visible (UV–Vis) extinction (absorption and scattering) spectra were measured by the Thermo Scientific Evolution 600 UV-Visible spectrophotometers. The optical length of the quartz cuvette is 10 mm.
Results and Discussion
Natural or biological nanoparticles have not been investigated in detail for their material properties. In this study, we investigate these type nanoparticles as the functional materials just like usual inorganic and organic nanoparticles. The ivy nanoparticles are not monodisperse in solutions. This effect is due to the biological process of the nanoparticle formation. Nanoparticles from the original tiny ones to the mature ones all exit in the rootlets of ivy. The SEC technology isolated the nanoparticles from the mixed solutions including molecules, but it did not give us uniform size of nanoparticles. The effective methods to obtain the monodisperse ivy nanoparticles are in the future investigation. The modifications of ivy nanoparticles to improve the optical properties are expected. Recently, in order to increase refractive index of organic polymers, surface modification methods, such as deep UV modification and ion-beam implantation, have been reported [22, 23]. If similar modifications are applied to ivy nanoparticles, the refractive index mismatch between ivy nanoparticles and the medium will become larger. This effect can increase the light scattering of ivy nanoparticles and, furthermore, benefit the extinction.
In conclusion, ivy nanoparticles have been isolated by the SEC technology and the filtration methods. Optical absorption and scattering properties of these nanoparticles have been characterized. The extinction spectra of the ivy nanoparticles display strong ultraviolet and weak visible extinction with a sharp transition edge. Compared to inorganic nanoparticles, ivy nanoparticles can be used in ultraviolet protection due to their optical properties and harmless nature.
This work was support by the University of Tennessee. We also thank the help of Dr. Scott Lenaghan, Dr. Ziling Xue, Dr. Bin Zhao, Mr. Royce Dansby-Sparks and Mr. Xueguang Jiang on the absorption and DLS measurement.
This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
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