ZnO-Based Sunscreen: The Perfect Example To Introduce

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Laboratory Experiment pubs.acs.org/jchemeduc

ZnO-Based Sunscreen: The Perfect Example To Introduce Nanoparticles in an Undergraduate or High School Chemistry Lab Wanda J. Guedens,*,†,‡,∥ Monique Reynders,‡ Heidi Van den Rul,§ Ken Elen,†,‡,∥ An Hardy,†,‡,∥ and Marlies K. Van Bael†,‡,∥ †

Institute for Materials Research, Chemistry Division, Hasselt University, Campus Diepenbeek, Agoralaan-building D, BE-3590 Diepenbeek, Belgium ‡ Faculty of Sciences, Hasselt University, Campus Diepenbeek, Agoralaan-building D, BE-3590 Diepenbeek, Belgium § Sirris Smart Coating Application Lab, Wetenschapspark 3, BE-3590 Diepenbeek, Belgium ∥ IMEC Division IMOMEC, Agoralaan-building D, BE-3590 Diepenbeek, Belgium S Supporting Information *

ABSTRACT: The experiment described in this article is designed for undergraduates as well as for high school students to help them understand nanoscience in a basic way. The attractive subject of a sunscreen is used to illustrate the properties of nanoparticles. The students prepare particles of Zn(OH)2 by the same reaction either in a microemulsion, a microemulsion contaminated with acetone, or in an aqueous solution. Hereby it is shown that Zn(OH)2 has different properties depending on its particle size. Furthermore, the students discover that, due to their higher surface-to-volume ratio, a nanopowder of ZnO dispersed in glycerin absorbs more UV light than the same mass of a micropowder ZnO dispersed in glycerin. Finally, a sunscreen based on a homemade hand cream containing ZnO particles is formulated, and its UV absorbing ability is demonstrated by an uncomplicated procedure. By incorporating more detailed characterization techniques and a more in-depth explanation, these laboratory experiments are also instructive for undergraduate students in the framework of a physical chemistry course. KEYWORDS: High School/Introductory Chemistry, First-Year Undergraduate/General, Inorganic Chemistry, Laboratory Instruction, Hands-On Learning/Manipulatives, Colloids, Micelles, Nanotechnology, Precipitation/Solubility, Aqueous Solution Chemistry anoscience and nanotechnology are very interesting fields of study. Because of their impact on tomorrow’s world, it is crucial that these recently developed areas of research are introduced at every level of teaching.1,2 Especially in high schools, the interdisciplinary character of this area can be exploited. Indeed, within the social environment of high school students, the appealing subject of cosmetics, with nanosunscreen as an example, is chosen as a framework to get students acquainted with some fundamental properties of nanoparticles and nanoparticle synthesis routes in chemistry lessons. Students learn about skin diseases on short and longer term due to UV sunrays in biology classes, while at the same time, in physics classes, the emission spectrum of sunlight versus that of a tanning bed is studied. The principles of a sunscreen, and nanosunscreen in particular, are described extensively and at a basic level in the NanoSense project, in the activity “Clear Sunscreen: How Light Interacts with Matter”.3 Depending on the level and the background of the students, the authors strongly suggest to use (parts of) this text as an introductory lesson before adopting the experiment described hereafter. Briefly, a nanosunscreen contains zinc oxide (ZnO) or titanium dioxide (TiO2) nanoparticles as active ingredients. The ability of ZnO and

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© 2013 American Chemical Society and Division of Chemical Education, Inc.

TiO2 to absorb not only UVB, but also the even more harmful UVA radiation makes these materials particularly interesting for use in recent sunscreens. By using nanoparticles, the sunscreen appears transparent when applied to the skin. Larger, microsized particles scatter more light and leave an undesired, white haze on the skin when used in a sunscreen. Nanoparticles can be manufactured by a variety of routes, commonly classified as wet-chemical, mechanical, form-inplace, or gas-phase synthesis.4 In wet-chemical processes, solutions of different ions are mixed in defined ratios under controlled external conditions to prepare a large variety of compounds.5 These kinds of routes can be carried out using simple equipment and are therefore ideally suited for student experiments. A straightforward example of a wet-chemical synthesis route for nanoparticles, applied in this experiment, is microemulsion processing. A microemulsion, defined as a dispersion made of water, oil, and surfactant(s), is an isotropic and thermodynamically stable system. In microemulsions, reagents can be encapsulated by a surfactant layer into spheres at the nanoscale, varying approximately from 1 to 100 nm, usually 10−50 nm.6−9 Published: October 10, 2013 259

dx.doi.org/10.1021/ed300851a | J. Chem. Educ. 2014, 91, 259−263

Journal of Chemical Education

Laboratory Experiment

Figure 1. During mixing of two microemulsions, containing the reagents in the nanosized inverse micelles (left), very short inverse micelles’ fusion occurs (middle). At that point the reagents are mixed and the precipitation reaction takes place. The final particles (right) have dimensions equal to or smaller than the spheres in the inverse micelles in microemulsion 1 and 2. Note that AOT is a two-tailed surfactant, but the representation emphasizes the polar head and nonpolar tail.

ZnCl2 with water. The microemulsion used in this experiment, a commonly used microemulsion composition in research,7 was a mixture of 53.33 g of AOT and 80 mL of heptane. AOT is dioctyl sulfosuccinate sodium salt, a two-tailed surfactant. The microemulsion was usually prepared by the instructor. If time permits, students can prepare the microemulsion from the raw materials AOT and heptane. However, dissolving AOT in heptane can be a time-consuming activity. To compare the precipitation of Zn(OH)2 particles in water with their precipitation in a microemulsion, the students made two solutions. For the aqueous solution, they pipetted 5 mL of NaOH (0.50 M) into a beaker filled with 5 mL of ZnCl2 (0.25 M). According to the precipitation reaction, Zn(OH)2 precipitates in the aqueous solution by mixing ZnCl2 and NaOH:

The tiny spheres are stabilized in the water-in-oil (w/o) microemulsion by this interfacial film of surfactant molecules, called inverse micelles, and dispersed in the continuous oil phase. These inverse micelles at sizes smaller than ∼100 nm can act as nanoreactors, preventing particle growth beyond the nanoregime. Indeed, performing, for example, a precipitation reaction is possible when two different w/o microemulsions, that is, one containing aqueous nanospheres of zinc chloride, ZnCl2, and another one containing aqueous nanospheres of sodium hydroxide, NaOH, are mixed (Figure 1). Precipitation of zinc hydroxide, Zn(OH)2, in newly formed micelles occurs in the mixed w/o microemulsion when, during a small time interval, colliding inverse micelles of ZnCl2 and NaOH merge. By making use of such a microemulsion-based precipitation environment, the experiment described allows students to prepare nanoparticles of the active ingredient, Zn(OH)2, in a simple way. The experiment was divided into four steps, required approximately 2−3 h, and was successfully performed by 24 students working in pairs in the laboratory.

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ZnCl 2(aq) + 2NaOH(aq) → Zn(OH)2 (s) + 2NaCl(aq) (1)

To prepare Zn(OH)2 in a microemulsion, the students added 5 mL of ZnCl2 (0.25 M) to one beaker filled with 40 mL of microemulsion (microemulsion 1) with stirring, and to the other beaker also filled with 40 mL of microemulsion, 5 mL of NaOH (0.50 M) (microemulsion 2) was added with stirring. Finally, they stirred the content of the beaker filled with microemulsion 2 into the beaker filled with microemulsion 1 to form Zn(OH)2 (Figure 1).10,11 A striking difference in appearance between the visible Zn(OH)2 precipitation in water and the microemulsion with Zn(OH)2 nanoparticles was observed.

EXPERIMENTAL PROCEDURE

Materials

Raw materials used in the experiment are purchased from chemical suppliers: dioctyl sulfosuccinate sodium salt (AOT) 96%, glycerin 99+%, hexadecan-1-ol p.a., sodium dodecyl sulfate (SDS) 99%, HCl (1 M), and ZnCl2 p.a. from Acros Organics (Geel, Belgium); heptane p.a., NaOH (0.5 M), acetone p.a., and microsized ZnO powder p.a. from VWR International (Haasrode, Belgium); and nanosized ZnO powder (particle size