Abstract
In sunscreens, ZnO nanoparticles (NPs) are used as inorganic UV filter which have a prominent band edge emission in the UVA region (~385 nm). When applied to biological surface, this highly penetrating UVA emission from ZnO NPs would enhance the generation of reactive oxygen species resulting in oxidative stress. Therefore, the elimination of this harmful UVA emission from ZnO NPs are much sought after for the development of safer sunscreens. In this paper we introduce the use of defect-rich ZnO (D-ZnO) quantum dots (QDs) as a multifunctional active ingredient in sunscreen/cosmetic application. These D-ZnO QDs make use of their defect energy levels to emit in visible region by bypassing their harmful band edge emission at UVA region. The D-ZnO QDs also showed prominent visible luminescence which matches well with the autofluorescence of in vivo human skin. Hence, this visible luminescence could be useful for camouflaging, thereby enabling its potential as a biologically safe active ingredient for both cosmetic and UV screening applications.
Introduction
Sun protection compositions generally available as sunscreens are used to protect human skin from the deleterious effects of ultraviolet (UV) rays [1–3]. Sunscreen compositions typically contain a dispersion of inorganic compound or organic compound as UV filters (or) a combination of both inorganic and organic compounds as UV filters. Among these inorganic sunscreens are more preferred over organic sunscreens because they possess high photostablity when compared with organic sunscreens, which generally undergo photodegradation [4]. These inorganic sunscreens filter UV rays by either absorbing or scattering UV radiation. As the size of the inorganic UV filters increases, the sunscreen compositions appear white in color when applied to skin surface due to increased visible light scattering [5]. This is not desirable for the aesthetic appearance of the skin surface, thereby reducing its demand in the sunscreen market.
In order to tackle the scattering problem faced by large sized inorganic UV filters, currently nano sized inorganic UV filters are used which also have an improved absorbance in the UV range [6]. Among these nanosized inorganic UV filters, the most prominent sunscreen materials studied are TiO2 and ZnO. In this, ZnO nanoparticles (NPs) are known to have a prominent absorbance for UV radiation in the entire spectrum of UVC, UVB and UVA radiation along with good transparency in the visible region [5].
However, these transparent ZnO NPs used as an inorganic UV filter in sunscreen have a prominent band edge emission in the UVA region (~385 nm) [7]. This UVA emission from ZnO along with UVA radiation from sunlight could augment the UVA induced radiation effects on skin surface. These highly penetrating UVA emissions from ZnO NPs are concentrated on the biological surface which could eventually enhance the generation of reactive oxygen species (ROS) [8]. The ROS generated by UVA would induce oxidative stress, resulting in various skin problems such as erythema, hyperpigmentation, elastosis, photocarcinogenesis, photo aging, etc. [9]. Therefore, the elimination of this harmful UVA emission from ZnO NPs is much sought after for the development of safer ZnO NPs-based sunscreens. In this paper, we propose the potential of atomic defects in ZnO which can considerably reduce the harmful UVA band edge emission of ZnO.
It is well known that the broad visible luminescence coming from defect centers in ZnO NPs could be tuned with particle size [10–13]. Hence, this defect induced visible emission from ZnO NPs could be tuned to match the skin emission profile, which could find potential application in camouflaging skin imperfections. In general, normal human skin exhibits a substantial level of autofluorescence [14]. This autofluorescence can be observed across the different skin layers, with the epidermis showing a faint autofluorescence signal, the stratum corneum being slightly stronger, and the most intense emissions occur from dermis and subcutaneous fat. The fluorescence spectra of human skin measurable over a wide excitation wavelength exhibit a predominant emission in the green region, as a result of strong autofluorescence coming from elastin and collagen present in the dermis [15]. Several studies have proved that UV exposure as well as advancing age results in the reduction of these elements responsible for green fluorescence [16]. Therefore, formulating a sunscreen composition with ZnO having a broad green fluorescence would be an ideal multifunctional active ingredient suitable for camouflaging applications.
The problems associated with UV exposure and age restoration could be solved by using a multifunctional active ingredient that can filter UV radiation and at the same time provide visible fluorescence for brightening the skin for youthful appearance [17]. Already different patent publications have appeared in the public domain claiming the use of luminescent nanoformulations for improving the skin complexion [17–19]. All these patent publications have one or more disadvantages; having toxic heavy metal based composition, complex core shell structures, or weak emission for getting the desirable brightening effect. However, the use of ZnO defect emission for cosmetic applications is rarely touched on, despite its less intrinsic toxicity. In the current study we explored the above mentioned potential application of atomic defects in ZnO by using defect-rich ZnO QDs (D-ZnO QDs).
Materials and methods
Chemicals
All the precursors for synthesis of D-ZnO QDs are commercially available. The absolute ethanol (Assay: 99.99 %, Analytical Reagent, China) was the solvent used for D-ZnO QDs synthesis. The raw precursors Zn(OAc)2 (purity ~ 99.99 %), NaOH (purity ≥ 98 %) and Polymethyl Methacrylate (PMMA) procured from Sigma Aldrich (USA), were used as received without further purification. The ZnO NPs purchased from Sigma Aldrich (USA) was used as the control ZnO NPs.
Synthesis of D-ZnO QDs
The D-ZnO QDs were synthesized as per our previous report with some modifications [13]. For a typical synthesis 0.05 M Zn(OAc)2 was mixed with 0.05 M NaOH (3 mL each) in a quartz reaction vial and the reaction was initiated by using a CEM Discover microwave reactor with 2.45 GHz operating frequency. The reaction temperature (75 ± 5 °C) was maintained for 1 min under high magnetic stirring followed by cooling to room temperature using compressed air flow. The obtained stable suspension containing D-ZnO QDs were used for further studies.
Preparation of the skin emission model
The PMMA/D-ZnO QDs nanocomposite was used to mimic skin emission profile, which emits in the yellowish-green range of in vivo human skin emission under UV excitation. For a typical preparation, a stable suspension of 0.5 mg/mL of D-ZnO QDs in ethanol was precipitated using oleic acid and the precipitate was re-dispersed into a chloroform solution. The PMMA dissolved in chloroform (10 mg/mL) was added to the solution containing D-ZnO QDs and mixed via ultrasonication. The resultant mixture was a solution cast in a Petri dish to obtain a transparent nanocomposite film.
Characterization
The normal- and high-resolution transmission electron micrographs were taken using a JEOL JEM-2100F high-resolution transmission electron microscope (HR-TEM) operating at 200 kV. Powder X-ray diffraction (XRD) data were collected using a Philips (PANalytical) powder diffractometer, with Cu Kα radiation as the X-ray source. Photoluminescence (PL) spectra were recorded at room temperature using a HORIBA Jobin Yvon Fluorolog 3 spectrofluorometer and the absorption spectra were collected using an Agilent Cary 100 UV-Vis spectrophotometer.
Results and discussion
Physico-chemical properties of control ZnO NPs (C-ZnO NPs) and D-ZnO QDs
A less defective ZnO NPs procured from Sigma Aldrich (USA) was used as a control for normal ZnO NPs used in sunscreens. Near-spherical and mono-dispersed D-ZnO QDs were prepared by using microwave assisted sol–gel chemistry [13]. Figure 1 shows the transmission electron micrograph (TEM) of C-ZnO NPs of size ~50 ± 15 nm (Fig. 1a) and D-ZnO QDs of size ~5.16 ± 0.3 nm (Fig. 1b).
a) TEM images of C-ZnO NPs, the bottom right inset shows the HR-TEM of C-ZnO NPs and b) TEM image of D-ZnO QDs, the bottom right inset shows the HR-TEM of D-ZnO QDs (the circle shows the location of defects, arrow is for eye guide).
A distinct lattice ordering was observed in the high-resolution transmission electron microscopy (HR-TEM) investigation of control ZnO NPs (Fig. 1a, bottom right inset), while the D-ZnO QDs (Fig. 1b, bottom right inset) shows a strained lattice with point defects. This confirmed that the D-ZnO QDs were rich in defects when compared with C-ZnO NPs. Further, the C-ZnO NPs and D-ZnO QDs were characterized for purity and phase using X-ray diffraction (Fig. 2) studies, which show peaks associated with ZnO wurtzite crystal structure (JCPDS card No. 36-1451). A broadening in the X-ray diffraction peaks were observed for D-ZnO QDs, which could be attributed to the strained lattice due to reduced crystal size and increased surface defects (surface defects like oxygen vacancy will impart coordination loss for D-ZnO QDs surface atoms, thereby contributing to its lattice strain).
X-ray diffraction of D-ZnO QDs and C-ZnO NPs.
The PL emission spectra and absorption spectra of C-ZnO NPs are given in Fig. 3a. The absorption spectra of C-ZnO NPs showed the characteristics of a broad band UV filter with onset of absorption ~382 nm, however, a prominent band edge emission at 385 nm was observed. Figure 3b shows the schematics of UV attenuation imparted by C-ZnO NPs coating on skin, where the UVA emission coming from C-ZnO NPs could penetrate deep into the dermis. This is not desirable for an ideal sunscreen, as UVA was reported as a harmful radiation which can induce skin cancer.
a) Illustrates the absorption and emission spectra of C-ZnO NPs, b) schematics show the penetration of intense UVA emission from C-ZnO NPs to dermis and penetration of UVC, UVB and UVA without sunscreen.
In the case of D-ZnO QDs, absorption spectra showed a broad band absorption characteristic similar to C-ZnO NPs with a blue shift in the absorption onset (~378 nm), which could be attributed to quantum confinement effect. It could be observed from the emission spectra that D-ZnO QDs make use of their defect energy levels to emit in the visible region (Fig. 4a) by bypassing their harmful band edge emission at UVA region, otherwise observed in control ZnO NPs with fewer defects (Fig. 3a). The potential of D-ZnO QDs as a safe UV filter is schematically shown in Fig. 4b. Therefore from the optical characterization results it could be inferred that by replacing the normal ZnO NPs used in sunscreen with D-ZnO QDs would offer better photo protection due to its reduced UVA band edge emission when compared with the normal ZnO NPs.
a) Illustrates the absorption and emission spectra of D-ZnO QDs, b) schematics showing the UV attenuation in the presence of D-ZnO QDs as a sunscreen and penetration of UVC, UVB and UVA without sunscreen.
The cosmetic potential of D-ZnO QDs was explored by coating D-ZnO QDs on the surface of a skin emission model which could mimic the broad green fluorescence of in vivo human skin. Figure 5a and b shows the photograph (taken under UV lamp excitation) of D-ZnO QDs (0.1 mg/cm2) and ZnO NPs (0.1 mg/cm2) paste (formulated in Pluronic® P-123), coated on the surface of a skin emission model. It can be noted that the D-ZnO QDs showed a bright emission in the yellowish-green region, while the C-ZnO NPs emits in the bluish-violet range. In addition to this, an excellent transparency was observed for oleic acid capped D-ZnO QDs coated on a glass substrate (Fig. 5c) under room light. This is an important sensory attribute required for a UV filter to formulate aesthetically pleasing sunscreens. Under UV excitation this transparent coating of D-ZnO QDs also showed a bright yellowish-green fluorescence (Fig. 5d). This bright fluorescence in the yellowish-green region matches well with the in vivo skin autofluorescence, which further establish the cosmetic value of D-ZnO QDs.
a) The photograph of C-ZnO NPs coated on a skin emission model (PMMA/D-ZnO QDs nanocomposite) under a 365 nm UV lamp excitation, b) a photograph of D-ZnO QDs coated on a skin emission model under a 365 nm UV lamp excitation, c) shows the transparency of oleic acid capped D-ZnO QDs coated on a glass substrate and d) its yellowish-green fluorescence under a 365 nm UV lamp excitation.
Figure 6 shows the integrated PL intensity collected from the skin emission model and skin emission model coated with C-ZnO NPs and D-ZnO QDs. The integrated emission collected from the surface coated with C-ZnO NPs shows ~39 % reduction in the visible emission of skin emission model. However, the UVA emission from the C-ZnO NPs coated surface was substantially increased when compared with D-ZnO QDs coated surface. The ratio of intensity of UVA emission (IUVA) to intensity of visible emission (Ivisible), IUVA/Ivisible = 0.35 for C-ZnO NPs coated surface, while for D-ZnO QDs IUVA/Ivisible = 0.071, which is insignificant when compared with UVA emission of C-ZnO NPs. This increase in the UVA emission is a result of band edge emission from the C-ZnO NPs coated on the surface of skin emission model. However, the integrated visible emission of D-ZnO QDs coated skin emission model shows an increase of 18 % when compared with the skin emission model.
The integrated PL emission intensity of skin emission model, skin emission model coated with D-ZnO QDs and C-ZnO NPs collected at 330 nm excitation.
Therefore, from the above results it can be inferred that, the D-ZnO QDs in properly formulated sunscreen composition can form a transparent layer on the skin, and under solar UV radiation it emits in the visible range giving a bright youthful appearance to skin. This visible defect luminescence of D-ZnO QDs can also find utility for camouflaging skin defects. Hence, the sunscreen composition containing D-ZnO QDs is beneficial for certain age groups and for age-conscious men and women who wish to use a multifunctional cosmetic product that can provide safer sun protection and can also restore skin’s youthful look.
Conclusions
In conclusion, the potential of D-ZnO QDs as a multifunctional active ingredient in sunscreen and cosmetics is established in this study. The D-ZnO QDs make use of their defect energy levels to emit in visible region by bypassing their harmful band edge emission at UVA region, otherwise seen in defect-free materials with fewer defects. Thus D-ZnO QDs can find potential application as a safer multifunctional active ingredient for sunscreen and cosmetics. As a rule of thumb in photocatalysis, an efficient radiative emitter will be always less photoactive. This knowledge is rather interesting which stimulated further experimental investigations on the photo activity of D-ZnO QDs, which is an efficient visible light emitter. The results of these studies will be reported in a separate paper.
Article note
A collection of invited papers based on presentations at the 16th International Congress on Photobiology (ICP-16), Córdaba, Argentina, 7–12 September 2014.
Acknowledgments
We express our thanks to SAIF, IIT Bombay for UV-Vis absorption and HR-TEM studies. A. Asok thanks travel grant (SB/ITS-Y/03055/2014-15) from SERB, Government of India and support from the 16th International Congress of Photobiology for presenting this work in the Congress.
References
[1] Y. Matsumura, H. N. Ananthaswamy. Toxicol. Appl. Pharmacol. 195, 298 (2004).Search in Google Scholar
[2] B. Diffey. Phys. Med. Biol. 49, R1 (2004).10.1088/0031-9155/49/1/R01Search in Google Scholar PubMed
[3] L. H. Kligman, F. J. Akin, A. M. Kligman. J. Am. Acad. Dermatol. 3, 30 (1980).Search in Google Scholar
[4] N. Serpone, A. Salinaro, A. V. Emeline, S. Horikoshi, H. Hidaka, J. Zhao. Photochem. Photobiol. Sci. 1, 970 (2002).Search in Google Scholar
[5] S. R. Pinnell, D. Fairhurst, R. Gillies, M. A. Mitchnick, N. Kollias. Dermatol. Surg. 26, 309 (2000).Search in Google Scholar
[6] P. Singh, A. Nanda. Int. J. Cosmet. Sci. 36, 273 (2014).Search in Google Scholar
[7] C. Hariharan. Appl. Catal. A 304, 55 (2006).10.1016/j.apcata.2006.02.020Search in Google Scholar
[8] H. Ou-Yang, G. Stamatas, C. Saliou, N. Kollias. J. Invest. Dermatol. 122, 1020 (2004).Search in Google Scholar
[9] R. Lavker, K. Kaidbey. J. Invest. Dermatol. 108, 17 (1997).Search in Google Scholar
[10] H. M. Xiong, Y. Xu, Q.-G. Ren, Y. Y. Xia. J. Am. Chem. Soc. 130, 7522 (2008).Search in Google Scholar
[11] A. Asok, M. N. Gandhi, A. Kulkarni. Nanoscale 4, 4943 (2012).10.1039/c2nr31044aSearch in Google Scholar PubMed
[12] X. Tang, E. S. G. Choo, L. Li, J. Ding, J. Xue. Chem. Mater. 22, 3383 (2010).Search in Google Scholar
[13] A. Asok, A. Kulkarni, M. N. Gandhi. J. Mater. Chem. C 2, 1691 (2014).10.1039/C3TC32107JSearch in Google Scholar
[14] M. J. Fellner. Arch. Dermatol. 112, 667 (1976).10.1001/archderm.1976.01630290017003Search in Google Scholar
[15] H. Zeng, C. MacAulay, B. Palcic, D. I. McLean. Photochem. Photobiol. 61, 639 (1995).Search in Google Scholar
[16] D. J. Leffell, M. L. Stetz, L. M. Milstone, L. I. Deckelbaum. Arch. Dermatol. 124, 1514 (1988).Search in Google Scholar
[17] D. Reinehr, H. Luther, G. Metzger. US6818205 B2 (2004).Search in Google Scholar
[18] L. George, J. Gubernick, G. Cioca, A. J. Bevacqua. US20020012681 A1 (1999).Search in Google Scholar
[19] L. Vidal, E. Huguet, P. Barbarat. WO2011089571 A2 (2011).Search in Google Scholar
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