Immobilization of Nanoscale Sunscreening Agents onto Natural Halloysite Micropowder

Immobilization of Nanoscale Sunscreening Agents

onto Natural Halloysite Micropowder

Yong Jae Suh

and Kuk Cho

1_{Mineral Resources Research Division, Korea Institute of Geoscience and Mineral Resources,}

124 Gwahang-no, Yuseong-gu, Daejeon 305-350, Republic of Korea

2_{Nanomaterials Science and Engineering, Korea University of Science and Technology,}

217 Gajeong-ro, Yuseong-gu, Daejeon 305-350, Republic of Korea 3_{Department of Environmental Engineering, Pusan National University,}

2 Busandaehak-ro 63beon-gil, Geumjeong-gu, Busan 609-735, Republic of Korea

Titanium dioxide (TiO2) nanoparticles contained in sunscreen products are typically smaller than 30 nm in order to enhance the efficacy of the sun protection. TiO2nanoparticles of this size range raise toxicity concerns because of the generation of reactive oxygen species (ROS) and the ability to penetrate into the skin tissue. To solve these problems, we immobilized the nanoparticles on a natural tubular powder, halloysite. The nanoparticle-halloysite hybrid powders are prepared by two synthetic methods: anex situimmobilization of TiO2nanoparticles (after separate synthesis of a nanoparticle batch), and anin situgeneration of TiO2nanoparticles inside halloysite. Notably, TiO2nanoparticles of pure rutile phase are synthesized so that the ROS generation is suppressed before immobilization. The ultraviolet-visible (UV-vis) light extinction results show that the sunscreening efficacy of the hybrid powder producedviatheex situroute appears to be 22%higher than that of the hybrid powder fabricated by thein situmethod. An increase in the hybrid powder concentration results in a sunscreening efficiency equivalent to that of the bare TiO2nanoparticles. Our results can be used to produce nanoparticle-halloysite hybrid powders suitable for UV light screening while reducing the potential toxicity concerns. [doi:10.2320/matertrans.M2015086]

(Received March 2, 2015; Accepted April 3, 2015; Published May 15, 2015)

Keywords: colloid, nanomaterials, penetration, absorption, scattering, functional cosmetics

1. Introduction

The use of inorganic ultraviolet (UV)filters, such as TiO2

and ZnO nanoparticles, has been increasing significantly,

especially in sunscreen products for children and people with sensitive skin.1)_{The inorganic UV-screening ingredients are}

contained in commercially available skin care creams and lotions, makeup foundations, eye shadows, and even in lip balms. Among the inorganic ingredients, titanium dioxide (TiO2) nanoparticles are typically adopted as the main

UV-screening agent.

The sunscreen is required to block the UV solar radiation

over the whole UVB (290­320 nm) and UVA (320­400 nm)

ranges, while being transparent to visible light for cosmetic

aesthetics. To satisfy the cosmetic efficacy, TiO2

nano-particles in the range of 20­50 nm have been considered as an optimal material.2­4)_{On the other hand, TiO}

2nanoparticles of

this size range have also been used as catalysts because of their high photoactive efficiency; in particular, TiO2particles

with a size of approximately 33 nm show the highest activity.3­5) _{The photoactive surfaces of the TiO}

2

nano-particles produce reactive oxygen species (ROS), such as

•_{OH, O}

2¹•, and H2O2,6­8) which are known to be cytotoxic.

For example, hydroxyl radicals (•OH) generated from

photoactive TiO2 nanoparticles cause damage to DNA

plasmids in vitro and to the whole human skin cells in

cultures.1,9) Accordingly, the generation of ROS has been

suppressed by modifying the TiO2nanoparticles surface with

other materials, e.g., Al2O3, SiO2, and ZrO2, which form

hydrated oxides that capture the hydroxyl radicals and thus reduce the photoactivity.1) _{For the same reason, the rutile}

phase is more advantageous than the anatase phase, as the

latter shows the highest activity among the TiO2

poly-morphs.6,10­14)_{The most effective phase at generating ROS is}

likely to exhibit the highest cytotoxicity.12)

Moreover, as the dermal exposure to TiO2 nanoparticles

occurs regularly during the use of sunscreen products, nanoparticles penetration through the skin has become a

growing concern.8,15­22) _{For example, the normal flexing}

movement of the skin facilitates the penetration of 0.5 and 1.0 µm particles into the dermis.22)Besides, when a sunscreen lotion containing TiO2nanoparticles is rubbed on a skin area

burned by the sun or damaged by acne, the TiO2

nano-particles may enter the human body through the skin and have harmful effects.

The toxicity of the sunscreen nanoparticles can be prevented by encapsulating them in a micro-container, reducing the chance of direct exposure of the skin to the nanoparticles themselves. As a result, the ROS generated by the UV light remain in the proximity of the nanoparticles inside the container without entering in contact with the skin. In addition, the nanoparticles are located inside the container and cannot infiltrate through the container walls, excluding the possibility of their penetration. Even the nanoparticles strongly adsorbed onto the outer surface of the container cannot penetrate through the damaged skin. A good candidate for the fabrication of such containers is a naturally occurring mineral: halloysite.

Halloysite is an aluminosilicate clay belonging to the kaolin group and with a composition of Al2Si2O5(OH)4·

2H2O; the ratio of aluminum to silicon is 1 : 1.23)Halloysite

is mostly found in the form of a tubular structure with a 15­

125 nm internal lumen, a 30­250 nm outer diameter, and a

length of less than a few microns (Fig. 1(a)). The outer surface of the halloysite comprises a silicate SiO2¹ layer,

whereas the inner surface comprises an alumina Al2O3+

+_{Corresponding author, E-mail: kukcho@pusan.ac.kr}

layer. Inside the tube, inorganic or organic materials can be encapsulated to provide new functionalities.24)_Furthermore,

the cytotoxicity testing of halloysite shows no toxic effect on

fibroblast cells and human breast cancer cells for over 48 h.25)

Thus, the micron-sized tubular halloysite is considered a benign carrier suitable for delivering active substances such as fragrances, cosmetics, and drugs.25­29)

To date, a hybrid powder, in which the light-scattering nanoparticles are loaded into natural clay minerals similar to halloysite, has not yet been developed for sunscreen purposes. Presumably, immobilizing the light-scattering nanoparticles into halloysite and using them as UV-screening agents may reduce or eliminate the unwanted side effects on the skin. Here, to fabricate hybrid sunscreen powders, we

provide two synthetic methods in which TiO2 nanoparticles

are encapsulated into or coated on halloysite. An ex situ

immobilization of the nanoparticles both on the inner and outer surfaces of halloysite is achieved by vacuum pulling of

TiO2nanoparticles prepared in advance. On the other hand,

anin situgeneration of the nanoparticles inside halloysite is conducted by loading the reactants first, and then initiating the particle formation. The nanoparticle synthetic process is controlled to produce pure rutile phase, which generates a smaller amount of ROS compared to the anatase phase. The

sunscreening efficacy is examined by comparing the

UV-visible (UV-vis) extinction efficiency with that of the bare

TiO2 nanoparticles at the same concentration. The hybrid

powder produced by theex situmethod shows higher UV-vis

extinction than that fabricated by the in situ method. An

increase in hybrid powder concentration makes the extinction efficiency equivalent to that of the bare TiO2 nanoparticles.

Our study enables the synthesis of nanoparticle-halloysite hybrids suitable for use as UV-screening agents while mitigating the toxicity concerns.

2. Experimental Procedure

A sample of halloysite was donated by Atlas Mining Company, USA. Titanium tetraisopropoxide (TTIP, Aldrich,

97%), HCl (Junsei, GR, 35.0­37.0%), NaOH (Kanto, Cica

reagent, min. 97%), anhydrous ethanol (Sigma-Aldrich,

>99.5%), and T805 TiO2 nanoparticles (AEROXIDEμ

TiO2 T 805, Evonik, Germany) were used as received. All

the chemical solutions not specifically mentioned were

prepared with ultra-high-purity deionized water.

The hybrid powder of TiO2 nanoparticles and halloysite

was prepared by either of the following two methods. The

first method is anex situTiO2colloidal method based on the

direct loading of TiO2 nanoparticles into halloysite. The

second entails the in situ TTIP-HCl solution method that

generates TiO2 nanoparticles inside halloysite by

vacuum-pulling the nanoparticle precursor into the halloysite lumen. The TiO2colloidal method is used to immobilize the TiO2

nanoparticles, which are prepared separately, directly into halloysite. The synthetic process for the rutile phase TiO2

nanoparticles is developed based on the study conducted by

Han’s group.30) In detail, a 1 M HCl (80 mL) solution is

placed in a 250 mL beaker and stirred using a magnetic bar; TTIP (20 mL) is then slowly added and stirred at 60°C for 3 h. When the TiO2 nanoparticles are produced, the solution

is left to cool to room temperature, and the TiO2nanoparticles

are collected by centrifugation at 10000 rpm (=15260©g)

for 3 min. The collected TiO2 nanoparticles are added to

distilled water (20 mL) and dispersed with ultrasonic waves for 30 min to prepare a colloidal solution. Then, halloysite powder (3 g) is added to the colloidal solution and the resulting mixture is vacuum pulled. Subsequently, the hybrid powder is collected by centrifugation, washed with a pH 1 solution twice, with distilled water once, and then dried at 60°C.

Thein situTTIP-HCl solution method is used to load the TiO2nanoparticles into halloysite by adding halloysite in the

early formation stages of the TiO2 nanoparticles. In detail,

a TTIP solution (20 mL) is added to a 1 M HCl solution (80 mL) and stirred at 60°C for 30 min. Next, the halloysite powder (3 g) is added to the resulting solution and then vacuum pulled. Subsequently, the powder is collected by centrifugation and washed with anhydrous ethanol once. Then, the collected powder is dispersed in a 1 M HCl solution (80 mL), and the resulting mixture is stirred at 60°C for 3 h. After cooling the resulting solution to room temperature, the hybrid powder is collected by centrifugation and washed with a solution with pH 1 once, with distilled water once, and then dried at 60°C.

The size and shape of the nanoparticles and hybrid powders were characterized by transmission electron

mi-croscopy (TEM, Philips, CM12, Netherlands) and field

emission scanning electron microscopy (FE-SEM, FEI, Quanta650F, Netherlands). The crystalline structure of the nanoparticles was characterized by X-ray diffractometry (XRD, Rigaku Denki Co. Ltd., RU-200B, Japan). The UV-screening efficiencies of the TiO2 nanoparticles, halloysite,

and hybrid powders are measured as follows. T805 and

synthesized TiO2 nanoparticles are dispersed in anhydrous

ethanol, whereas raw halloysite and the hybrid powders are

dispersed in a pH 10 aqueous solution. All the samples with a

concentration of 0.0025 mass% are sonicated for 10 min to

disperse uniformly the particles. The measurement sample is injected into a 1 cm path length quartz cuvette and mounted in a UV-vis spectrophotometer (Scinco, S3100, Korea). Then, the light transmittance of the sample is measured in the wavelength (­) range of 250­500 nm.

3. Results and Discussion

To obtain the rutile phase TiO2nanoparticles, we modified

the synthetic process reported by Han’s group.30) _The

reaction time was shortened from 48 to 3 h, the reaction temperature was raised from room temperature to 60°C, and no stabilizing agents were utilized. On the other hand, hydrochloric acid was used to control the hydrolysis and condensation of the TTIP precursor. The use of stabilizing agents, such as the Pluronic P-123 polymer, should be avoided, as their bulky structure prevents the movement of the TiO2nanoparticles into the lumen of halloysite; besides,

they may be unsuitable for cosmetics. As a result, oblong

TiO2 nanoparticles with an average size of 15 nm©70 nm

were fabricated (Fig. 1(b)). This size appears to be similar to that of T805, which exhibits an average size of 21 nm (Fig. 1(c)). T805 is a fumed TiO2 nanoparticles treated with

octylsilane to form a hydrophobic surface and widely used as a physical UV-filter in sunscreen and daily care products.

The crystalline structures of the synthesized TiO2

nano-particles were confirmed by the XRD patterns (Fig. 2). The

synthesized particles show sharp peaks at the characteristic angles of rutile phase and no peaks at those of anatase phase, confirming the formation of a pure rutile phase. On the other hand, T805 shows a mixed pattern revealing the presence of both the rutile and anatase phases, and exhibiting sharper peaks at the characteristic angles of the anatase phase.

The UV-vis extinction of the nanoparticle suspensions can be determined by measuring the transmittance of the

radiation between 250 and 500 nm.31)The absorbance, A=

¹log(I/I0), is calculated from the Beer­Lambert law:32)

A¼¾cl ð1Þ

where I/I0, I, I0, ¾, c, and l represent the transmittance,

intensity of the light transmitted through the sample, intensity of the incident light, extinction coefficient, concentration of the nanoparticles, and path length of the light passing through the sample, respectively.

First, the UV extinctions of the raw materials before

hybridization were examined (Fig. 3). All the TiO2

nano-particles show a strong extinction tendency across most of the UVB and UVA spectra, whereas halloysite is nearly trans-parent. This result indicates the dependence of the light scattering efficiency on the material optical properties, such as the refractive index.2,32) The broad-spectrum protection

shown by the TiO2 nanoparticles can impart high values of

sun protection factor (SPF) at relatively low concentra-tions.33)_{The highest absorbance of T805 may be attributed to}

the large presence of the anatase crystalline phase, as anatase

phase is more photoactive than rutile phase.6,10,11,13,14)

The TiO2 nanoparticles synthesized in this study show a

dependence of the absorbance on the reaction time. This

variation is due to their size difference, as the particle size is the key parameter that determines the light scattering characteristics.32) _{By prolonging the particle growth time,}

the average size of rutile nanoparticles increased from

12 nm©50 nm (data not shown) to 15 nm©70 nm

(Fig. 1(b)) as the reaction time increased from 2 to 3 h. Next, the UV extinctions of the hybrid powders were examined and compared with that of halloysite and bare TiO2

nanoparticles synthesized in this study (Fig. 4). The hybrid powders show an intermediate absorbance between halloysite and bare TiO2nanoparticles, indicating the extent of the TiO2

nanoparticle immobilization on halloysite, as the contribution of halloysite is very low. Between the two hybrid powders, that prepared by following the TiO2colloidal method exhibits

a higher UV extinction efficiency. The amounts of TiO2

nanoparticles immobilized on halloysite in the hybrid powders can be qualitatively estimated by inspecting the SEM micrographs (Fig. 5). The outer surface of halloysite in

the hybrid powder prepared by thein situTTIP-HCl solution

method is covered by a small number of nanoparticles (Fig. 5(b)). Conversely, the surface of halloysite in the hybrid

Fig. 2 X-ray diffraction (XRD) patterns of TiO2nanoparticles prepared at 60°C in this study and Degussa T805. The XRD data of the indexed bars in grey and black are obtained from JCPSD#21-1276 and#21-1272 for the rutile and anatase crystalline phases, respectively.

powder prepared by the ex situ TiO2 colloidal method is

covered by a large number of nanoparticles (Fig. 5(c)). Considering a rather narrow halloysite inner lumen and the

oblong TiO2 particle shape, a higher amount of TiO2

nanoparticles can be immobilized in the case of the TiO2

colloidal method. Despite the lower sunscreening efficiency, however, the TTIP-HCl solution method would be consid-ered more advantageous if the contact between the photo-catalytic particles and the skin has to be avoided.

It is noted that raw halloysite is a natural clay mineral exhibiting a broad spectrum of size distributions. Although the SEM micrograph of Fig. 5(a) shows many smaller particles, not shown in the TEM micrograph of Fig. 1(a), all the particles are natural. In the cosmetic applications, if needed, these small particles can be easily removed by a simple separation technique such as centrifugation.

To compare the overall sunscreening performance, we defined a representative UV light extinction efficiency by integrating the UV light absorbance with respect to

wave-length over the UVB (290­320 nm) and UVA (320­400 nm)

ranges. The order of the representative extinction perform-ance is TiO2nanoparticles>hybrid powder produced by the

TiO2 colloidal method>hybrid powder produced by the

TTIP-HCl solution method (Fig. 6). This order is the same for both the UVB and UVA wavelength ranges. The representative extinctions of the hybrid powders produced

by the colloidal method and solution method are 31.3%and

25.6% of that of the bare TiO2 nanoparticles, respectively.

According to the Beer­Lambert law, the light extinction is proportional to the concentration, and thus it is expected that a four times higher concentration of the hybrid powders will exhibit a greater extinction performance than the bare TiO2

nanoparticles. To demonstrate this assumption, the

depend-ence of the UV extinction efficiency on the particle

concentration was investigated using the hybrid powders

produced by the TiO2colloidal method. In this experiment,

to optimize the synthetic process, the hybrid powder was prepared in different conditions from the previous experi-ment. The volume ratio between TTIP and 1 M HCl, i.e., TTIP:HCl was reduced to 1 : 10 from 1 : 4 and the reaction was conducted at 80°C for 30 min. Therefore a lower amount of TTIP was used and the reaction time was shortened. The shape and crystalline structure were examined by TEM

(Energy Filtering TEM, Carl Zeiss, 200 kV, Germany). TiO2

nanoparticles located in the halloysite can be hardly seen; most TiO2nanoparticles appear to cover the outer surface of

halloysite (Fig. 7). This implies that the majority of the TiO2

nanoparticles lie on the outer surface of halloysite when synthesized by the colloidal method. The selected area diffraction (SAD) pattern clearly shows a highly crystallized rutile phase TiO2 (Inset of Fig. 7). At 0.0025 mass%, the

absorbance of the hybrid powder synthesized at a lower TTIP concentration (Fig. 8) is lower than that of the previous sample powder (Fig. 4). This decrease in the absorbance is attributed to a lower TiO2nanoparticle proportion in the same

total solid concentration of 0.0025 mass%. Nevertheless, the

hybrid powder at 0.01 mass%shows higher absorbance in the

UVB wavelength range than the bare TiO2 nanoparticles at

0.0025 mass%. The hybrid powder at 0.02 mass% shows

a significantly higher absorbance over the whole UV light

wavelength range. Notably, the increase of the hybrid powder concentration may not cause any problems in the cosmetic

Fig. 4 Ultraviolet (UV) extinction efficiency of the hybrid powders prepared by the two processes compared with raw halloysite and bare TiO2nanoparticles. The particle concentration is 0.0025 mass%.

Fig. 5 Scanning electron microscopy (SEM) micrographs of (a) raw halloysite, (b) hybrid powders produced by the titanium tetraisopropoxide (TTIP)-HCl solution method, and (c) hybrid powders produced by the TiO2colloidal method.

formulation, as the allowance on the concentration of sunscreening agents are based on the effective ingredients, i.e., TiO2. The sunscreen active concentration allowed by the

US Food and Drug Administration (FDA) is 25 mass% for

TiO2, and a typical formulation of a sunscreen product

contains approximately 5 mass%of TiO2nanoparticles.33)

4. Conclusion

As TiO2nanoparticles smaller than 30 nm are being widely

used in sunscreen products, toxicity concerns such as ROS generation and penetration into skin tissue are being raised. Here, we have developed two synthetic routes to immobilize the TiO2 nanoparticles on halloysite, a naturally occurring

benign clay mineral. Theex situTiO2 nanoparticle

immobi-lization, following the synthesis of a separate batch of nanoparticles, resulted in the adsorption of the nanoparticles

on the halloysite surface. On the other hand, the in situ

generation of TiO2nanoparticles inside halloysite resulted in

a clean surface, implying that the largest portion of TiO2

is located inside the halloysite lumen. The former TiO2

colloidal method showed higher UV extinction efficiency

than the latter TTIP-HCl solution method, probably because

of the higher TiO2 proportion in the same total solid

concentration. On the other hand, the latter method provides better results if the direct contact between the sunscreening agent and the skin tissue needs to be avoided, e.g., for

damaged or sensitive skins. It should be noted that TiO2

nanoparticles of pure rutile phase were synthesized to prevent the ROS generation. The nanoparticles were synthesized in the absence of any surfactants, eliminating any possibility

of inclusion of harmful ingredients in the final sunscreen

products. An increase in the hybrid powder concentration led to a sunscreening efficiency equivalent to that of the bare TiO2nanoparticles. Although this increase in the total solid

concentration can limit the usability, the adoption of a natural clay mineral in cosmetics is preferable for some applications. We conclude that this study provides two methods for producing nanoparticle-halloysite hybrid powders that are suitable for UV light screening while reducing the potential toxicity concerns.

Acknowledgments

This work was supported in part by the Brain Korea 21 PLUS project in the Division of Creative Low Impact

Development and Management for Ocean Port City

Infrastructures, the PM2.5 research center supported by Ministry of Science, ICT, and Future Planning (MSIP) and National Research Foundation (NRF) of Korea (NRF-2014M3C8A5030890), and the Basic Research Project of the Korea Institute of Geoscience and Mineral Resources.

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