Sunscreen Performance of Lignin from Different Technical Resources

May 24, 2016 - Lignin types from different technical resources are proven to be effective natural broad-spectrum sun blockers and have general synergi...
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Research Article pubs.acs.org/journal/ascecg

Sunscreen Performance of Lignin from Different Technical Resources and Their General Synergistic Effect with Synthetic Sunscreens Yong Qian,†,‡ Xueqing Qiu,*,‡ and Shiping Zhu*,† †

Department of Chemical Engineering, McMaster University, 1280 Main Street West, Hamilton, Ontario L8S 4L7, Canada School of Chemistry and Chemical Engineering, State Key Lab of Pulp and Paper Engineering, South China University of Technology, 381 Wushan Road, Tianhe District, Guangzhou 510640, P. R. China

‡

S Supporting Information *

ABSTRACT: Five types of industrial lignin are blended with a pure cream and a commercial sunscreen lotion. Lignin is found to significantly boost their sunscreen performance. Photostability of the lignin-modified lotions is analyzed. The results show that hydrophobic lignin has better sunscreen performance than hydrophilic counterpart. Sun protection factor (SPF) of the pure cream containing 10% organosolv lignin (OL) reaches 8.66. Small amount of hydrophobic lignin dramatically increases SPF value of the sunscreen lotions. Adding 1% lignin almost doubles the sun lotion’s SPF. Addition of 10% OL to the lotion boosts its SPF from 15 to 91.61. However, it is also found that hydrophilic lignin tends to demulsify the lotions due to an electrostatic disequilibrium. After 2 h of UV radiation, UV absorbance of all the five lignin-modified sunscreen lotions increases up to the limit of measuring instrument. All the lignin types studied in this work are found to have a general synergistic effect with sunscreen actives in the commercial lotion. An effort is also made to elucidate radical mechanisms of the synergy. KEYWORDS: Lignin, Sunscreen, Sunscreen protect factor, Photostability, Synergistic effect

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INTRODUCTION

Lignin, generated by plants via CO2 photosynthesis, constitutes 15 to 40% dry weight of terrestrial plants. It is the most productive aromatic biopolymer in the nature.15−17 Due to existence of large amount of phenolic, ketone, and intramolecular hydrogen bond, lignin has great potential in UV defense.18−21 Furthermore, lignin from different resources has been proved safe and lignin-based capsules have no cell cytotoxicity.22,23 Lignin works so well in supporting and protecting trees and other plants in the nature. Why not make it work for human health care? It is believed to have a high potential in protecting human skins from sun radiation. Several years ago, we initiated a research program to develop lignin-based sunscreens. Alkali lignin was first used as a natural broad-spectrum sun blocker in the development, which was highlighted in Chemistry World and has attracted good attention in the research community.24,25 In this work, we expand the effort to study lignin types from various technical sources. As it is often said: fiber varies from one to another, so does lignin. Lignin has a high reputation to have ill-defined network structures and complicated chemical compositions. The properties can be very different from different technical sources. The objective of this work is to find if there is a commonality in terms of the sunscreen effect. Five

Sunscreens are commonly used in protecting skin from ultraviolet (UV) radiation. As reported, 80% of sunscreen markets in US and Western Europe are occupied by synthetic chemical products.1,2 Although side effects of active ingredients in the chemical sunscreens have not been confirmed, consumer’s concerns are inevitable and have never stopped, as the related surveys show for years.3−5 Natural sun blockers provide appealing alternatives as replacement for the synthetic ingredients in sunscreens. Natural sunscreen actives have been extracted from green coffee, soy, carica papaya, rosa kordesii, helichrysum arenarium, and other plants.6−10 In general, they are not only good sun blockers, but also natural antioxidants.6,8,9,11 However, simple blending of a natural sun blocker with skincare products cannot boost sun protect factor (SPF). In addition, they can only block a part of spectra in UV radiation. Therefore, natural sun blockers are often blended with synthetic sun blockers in commercial sunscreen products.9 It should be pointed out that most widely used sunscreen actives in the market, no matter natural or synthetic, are small organic molecules. They are usually water insoluble and have potential of penetrating into skins and causing allergies after a long time use.12−14 Finding natural macromolecular UV-block actives and developing polymeric sunscreens are clearly beneficial. © XXXX American Chemical Society

Received: May 2, 2016

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recorded on Bruker 500 MHz spectrometer at 300 K using DMSOd6 and D2O as solvents. The lignin concentration was 20%. A 90° pulse width, 1.7s relaxation delay, and 20 000 scan times were used. The electron spin resonance (EPR) spectra were obtained by Bruker ECS-EMX X-Band spectrometer equipped with ER4119HS cavity at room temperature. The power of the mercury lamp was 300 W.

types of lignin are selected from different technical resources. The lignin samples are blended with a pure cream or are added into a commercial SPF 15 sun lotion. Their sunscreen performance and photostability are evaluated. It should be emphasized that lignin, such as alkali lignin, lignosulfonate, and enzymatic hydrolysis lignin, is a byproduct of pulping or bioethonal producing process. Organosolv lignin is extracted by alcohol from plants. Compared with high-end natural biomacromolecule sunscreen, the cost for energy consumption of lignin extraction is not high. Lignin, generated by plants via CO2 photosynthesis, is natural and there is no carbon footprint.

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RESULTS AND DISCUSSION Sunscreen Performance of the Pure Cream Blended with Different Lignin Types. Five lignin-based sunscreens were prepared by blending the pure cream with the lignin samples from different technical resources and their SPF values were determined. Typical UV transmittance of the creams containing different OL amounts is shown in Figure 1. As

EXPERIMENTAL SECTION

Materials. Alkali lignin (95%, AL370), alkali lignin of low sulfonate content (96%, AL471), and lignin model chemicals were purchased from Sigma-Aldrich Co. Ltd. (St. Louis, MO, USA). AL370 has 95% pure lignin with 5% moisture. It is the byproduct of soda pulping process and insoluble in water (pH = 7). There is no sulfur in AL 370. AL 471 has 96% lignin with 4% organically bound sulfur. It is the byproduct of kraft pulping, also called kraft lignin, and it is watersoluble. Organosolv lignin (86.7%, OL) was purchased from Chemical Point UG (Deisenhofen, Germany). It contains 6.5% moisture, 0.2% residual sugar, 5.5% small lignin segments (acid-soluble lignin), and 1.1% ash. Enzymatic-hydrolyzed lignin (80%, EHL) was supplied by Shenquan Co. Ltd. (Jinan, Shandong, China). It contains ∼5% cellulose and ∼15% ash, residual sugar and enzyme. The enzyme used for enzymatic hydrolysis is commercial cellulose enzyme Cellic CTec2, which is provided by Novozyme (Tianjin, China). Sodium lignosulfonate (LS) was supplied by Shixian Papermaking Co. Ltd. (Tumen, Jilin, China). It was purified by filtration and ultrafiltration. There are 3.7% organically bound sulfonate groups in LS. Physicochemical properties of the lignin types from different technical resources were summarized in Table S1 in the Supporting Information. The data of elemental distributions are given in Table S2. Chemical sunscreen active avobenzone (BMDM) was purchased from Aladdin Industrial Corporation (Shanghai, China) and octinoxate (OMC) was purchased from TCI Development Co. Ltd. (Shanghai, China). Pure cream was NIVEA refreshingly soft moisturizing cream (75 mL). Sun lotion was LIFE SPF 15 sunscreen lotion (L15). Both pure cream and sunscreen lotion were bought from SHOPPERS drug market (Hamilton, Ontario, Canada). The active and inactive ingredients of purchased sunscreen lotion and pure cream can be found in Table S3 in the Supporting Information. Preparation of Lignin-Based Sunscreen Samples. Ligninbased sunscreens were prepared by blending lignin with the pure cream or the SPF 15 sun lotion for 24 h. It should be noted that the lignin amount in sunscreen is based on its purity. For example, 10 wt% lignin-based sunscreen was prepared by blending 0.1153g OL and 0.8847g pure cream (or SPF 15 lotion). The whole process was performed under room temperature in a dark room. Characterizations and Methods. The detailed SPF determination and photostability testing can be found from the previous work.24 The content of functional groups in lignins was determined by nonaqueous potentiometric titration (809 Titrando; Metrohm Corp., Herisau, Switzerland) according to Lin and Dence’s method.26 The content of methoxyl group was determined by headspace gas chromatography according to Zeisel method.27 The molecular weight of hydrophilic LS and AL471 was measured by Waters 1515 gel permeation chromatography (Waters Corp., Milford, USA) with 0.1 mol/L NaNO3 as eluent and NaPSS as standard. The flow rate kept at 0.5 mL/min. The molecular weight of hydrophobic OL, EHL, and AL370 was measured by Agilent 1100 series gel permeation chromatography (Agilent Technologies Corp., Santa Clara, USA) with PLgel 5 μm 1000 Å and PLgel 5 μm 500 Å columns. The mobile phase was THF with a flow rate of 1 mL/min and polystyrene was employed as standard. Fourier transform infrared (FTIR) spectra were recorded in the range 500−4000 cm−1 on a Vector 333 FT-IR spectrometer (Bruker, Germany). The 13C NMR spectra were

Figure 1. Typical UV transmittance of pure cream blended with different amounts of organosolv lignin (OL) in UVA and UVB areas.

Table 1. SPF Values of the Pure Cream Blended with Lignin from Different Technical Resources.a lignin AL370 AL471 LS OL EHL a

1% 1.91 1.83 1.46 3.25 1.37

± ± ± ± ±

2.5% 0.09 0.09 0.05 0.56 0.03

3.68 2.76 2.06 5.52 2.24

± ± ± ± ±

0.25 0.40 0.09 0.40 0.15

5% 4.01 4.00 3.71 6.67 3.24

± ± ± ± ±

10% 0.35 0.67 0.29 0.25 0.28

6.81 6.33 5.54 8.66 4.20

± ± ± ± ±

1.15 0.37 0.96 0.25 0.50

The measured SPF value of the control cream was 0.99 ± 0.01.

shown in Table 1, SPF value of the cream containing 1% OL is 3.25, while SPF of the creams containing other lignin types are in the range of 1.37−1.91. As the lignin amount increased, SPF value increased to various degrees. OL-based creams had higher SPF increases than the other four lignin-based creams, with the maximum reaching 8.66 at 10% OL addition. The sunscreen performance of OL is clearly the best among the five studied lignin types. From FTIR measurements, we also found that the lignin aromatic ring characteristic band (1510 cm−1) and conjugated carbonyl group (1655 cm−1), which are believed to be responsible for UV adsorption, were most obvious in OL, as shown in Figure 2a. The most obvious conjugated carbonyl group of OL was also confirmed by 13C NMR results (Figure 2b). The signal at 168−166 ppm refers to the conjugated COOR structure in lignin and the signals at 157−151, 145.5− 142.5 ppm contain C4 signal in the other conjugated structures.28 Three conjugated structures in OL are all obvious. B

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larger than the model chemicals without (1) and with one (3) methoxyl groups in both UVA and UVB areas. Consistently, lignin with more methoxyl groups has better sunscreen performance. OL had most methoxyl groups (5.19 mmol/g) and its sunscreen performance was the best among the five lignin types. EHL with the least methoxyl groups (2.24 mmol/ g) had the smallest SPF values. Compared with AL and LS, the processes of extracting OL and EHL from woods are relatively mild, and no sulfur is introduced. However, the sunscreen performance of EHL is not as good as OL also due to cellulose impurities. Cellulose tends to absorb water by hydrogen bonding and the creams blended with EHL are dryer than the other four lignin-based creams. The SPF values of EHL-based creams were thus lower than the other creams, because it was difficult in film formation. AL470 and LS demulsified their creams when the added amounts were too high. Strong electrolytes destroyed electrostatic balances of the emulsions. SPF Increase of the Sunscreen Lotions with Addition of Different Lignin Types. Adding small amount of AL could lead to a dramatic SPF increase in sun lotions. The promotion of SPF in the sun lotions blended with different lignin types is investigated and summarized in the following. As shown in Table 2, adding 1% lignin almost doubled SPF of the commercial sun lotions. SPF of the sun lotion containing 10% OL reached 91.61. SPF value of the lignin modification increased with the lignin amount, except for AL471 and LS. It should be pointed out that LS and AL471 are naturally amphiphilic surfactants. With 1% LS and AL471 added, the sunscreen active ingredients dispersed nicely in the lotions. SPF increased to 34.59 and 35.56. Further addition of LS and AL471 demulsified the lotions. A phase separation occurred, which could be observed in Figure 4. This phase separation clearly damaged lignin’s sunscreen performance in the lotions. For a different reason, as the pure cream tended to dry after blended with EHL, the lotions containing 10% EHL became too dry to form smooth films. Therefore, SPF also decreased with excessive EHL amount. Radiation-Enhanced Sunscreen Effects and Synergistic Mechanisms. Lignin in the cell wall of plants not only increases strength of the trunk and lock the water, but also protects the plants from sunshine irradiation.29−31 In pulping or other technical cellulose extraction processes, the original network structures of lignin are destroyed. However, their basic phenylpropane units are retained with additional UV-absorbing functional groups exposed. As a result, industrial lignin should still process good photostability. Photostability of the five lignin-modified sunscreen lotions was examined. All the five lignin types showed significant synergistic effects with sunscreen actives in the lotions. As shown in Figure 5a, the initial absorbance of these ligninmodified lotions in the UVA and UVB areas was controlled at less than 2.5. The absorbance of LS-modified lotion was smaller due to demulsification. After 2 h of UV irradiation, the UV absorbance of the five lignin-modified lotions increased to the limit of the measurement instrument, especially in the UVB area (Figure 5b). The irradiation-induced absorbance increase in the UVA area was related to the phenolic hydroxyl content and the phenolic hydroxyl group could be transformed into conjugated quinonoid compounds.32 The UV absorbance increase in AL370- containing lotion after irradiation was most obvious, while those of LS and AL471-containing lotions were less impressive. From these observations, it can be

Figure 2. IR and 13C spectra of five technological lignins.

The conjugated structures in the other lignin are partially obvious. For example, only the signal at 145.5−142.5 ppm is obvious in AL370 and AL471, while the other conjugated structure signals are very weak. The signal at 157−151 ppm is obvious, but the signal at 145.5−142.5 ppm is very weak in EHL. The methoxyl group is an important electro-donating group, which can significantly contribute to the conjugated system in lignin. As shown in Figure 3, the lignin model chemicals with (2) or with two (4) methoxyl groups have UV transmittance

Figure 3. Typical UV transmittance of pure cream blended with same amount of lignin model chemicals in UVA and UVB areas. C

DOI: 10.1021/acssuschemeng.6b00934 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering Table 2. Measured SPF Values of the Sunscreen Lotions with Addition of Different Ligninsa lignin OL AL370 AL471 EHL LS a

1% 31.58 24.89 35.56 19.15 34.59

± ± ± ± ±

2.5% 0.76 3.64 2.35 2.89 4.09

38.41 37.47 22.57 34.34 20.16

± ± ± ± ±

5%

4.23 2.01 0.82 1.79 2.66

55.72 53.67 15.33 47.11 22.02

± ± ± ± ±

10% 1.26 5.24 3.23 6.29 4.25

91.61 76.97 18.37 23.66 20.13

± ± ± ± ±

8.47 6.38 2.34 1.28 5.31

The measured SPF value of the control sunscreen lotion was 18.22 ± 0.92.

of UV absorbing performance of lignin mixed with a single chemical sunscreen active in solution. OL with best UVblocking performance was selected for the investigation. Avobenzone (BMDM) and octinoxate (OMC), two most commonly used chemical sunscreen actives, were selected as the chemical sunscreen additives. As shown in Figure 6, UV

Figure 4. Appearance of SFP 15 sunscreen lotions blended with different lignin types of 10 wt% addition.

Figure 6. UV absorbance of organosolv lignin (OL) blended with avobenzone (BMDM) or octinoxate (OMC) in (a) dioxane and (b) glycol solution. The initial concentrations of lignin, BMDM, and OMC solution are 0.05−0.1 mg/mL.

absorbances of OL, BMDM, and OMC, as a single component, are all less than 1, no matter in dioxane or glycol solution. When the same amount OL solution mixed with BMDM or OMC solution, UV absorbance of the mixed solution became stronger than superposition of the two solutions. This was particularly true in glycol solution. This UV absorbing enhancement was probably induced by π−π* stacking of the aromatic rings in lignin and chemical sunscreen active.33 For example, OMC and OL had no or a little UV absorbance in the UVA area, but very strong UV absorbance appeared after mixing. Redshift of the UV absorbing spectrum suggests that Jaggregation (one type of π−π* stacking) occurred between OL and OMC. J-aggregation between lignin and chemical sunscreen active formed larger conjugated structures and the

Figure 5. UV absorbance of the sunscreen lotion blended with different lignin types: (a) before UV radiation and (b) after UV radiation for 2 h.

concluded that the synergy of lignin with chemical actives in sunscreens is general, regardless of the lignin sources. The synergistic effect between lignin and chemical sunscreens in lotion becomes most interesting and puzzling. We have made efforts in its mechanistic elucidation. For the purpose, we simplified the system and focused on investigation D

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Figure 7. Typical energy diagram of π−π* stacking between lignin and chemical sunscreen active OMC.

energy required for the π−π* transition became lower. According to the energy diagram of π−π* stacking in lignin,33 an energy diagram of π−π* stacking between lignin and chemical sunscreen active OMC is shown in Figure 7. The π−π* interaction between aromatic rings results in two distinct aggregation modes, H and J aggregations. J-aggregate is formed when a tilt angle between the molecular axis connecting the center of chromophores and a transition dipole moment of each chromophore, θ, is less than 54.7°. The H-aggregate is formed when θ is larger than 54.7°.33 J-aggregation usually results in energy decrease for π−π* transition, while Haggregation results in energy increase for π−π* transition. Since J-aggregation is formed between lignin and chemical sunscreen active, such as OMC, photons in UVA zone can be absorb to realize π−π* transition. Therefore, the OL/chemical sunscreen active conjugated system thus showed a substantial redshift (i.e, broadening the UV absorbing area) in the UV spectrum, and the UV absorbance increased. After exposure to UV irradiation for 2 h, the UV absorbances of OL/OMC and OL/BMDM solutions increased further, indicating formation of more conjugated chromophores, such as quinonoid compounds.32 Phenolic hydroxyl groups of OL could first form phenoxyl radicals after absorbing UV and later become quinonoid structures.34 As shown in Figure 8, there is no stretching vibration of quinonoid before blending OL and BMDM or OL and OMC, but obvious adsorption peak of quinonoid appears near 1660 cm−1 and strengthens after UV irradiation. Then, UV absorbance of OL with more quinonoid structures increased dramatically. The free radical mechanism of transforming normal lignin to conjugated chromophores under UV radiation in the presence chemical sunscreen actives is believed to be responsible for the observed synergistic effects of lignin with sunscreen lotions. As shown in Figure S1 in Supporting Information, the free radical signals of OL, BMDM, and OL/BMDM mixture increase as UV irradiation. As the experimental conditions were the same, the integral area of ESR signal is proportional to the free radical content. The increase of free radical content in OL/BMDM mixture is slower than that in OL, suggesting possible interactions of the free radicals produced by OL with those by BMDM, as shown in Figure 9. It is our hope that this preliminary work will attract attentions of academic researchers and industrial practitioners in the fields and invite efforts in further elucidating the mechanisms and the development of natural sunscreen products.

Figure 8. FTIR spectra of OL, BMDM, and OMC before and after blending and that after UV radiation.

It must also be cautioned that this work represents a first step toward the development of lignin-containing sunscreen products. The lack of cytotoxicity of lignin does not result in the material being acceptable for cosmetic preparations. Numerous vitro and vivo tests are required to grant safety for lignin-based sunscreen to finally go market. Indeed, while we are excited about the discovery of lignin’s potential use as additive for sunscreen lotions, we are also puzzled with many unknowns. Developing high-end applications for lignin is our general goal. It is a big task, which requires effort from many researchers for many years. E

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CONCLUSION Lignin types from five different technical resources are all proven to be effective natural broad-spectrum sun blockers. Compared with the lignin salts, the hydrophobic lignin samples, such as AL370 and OL, have better sunscreen performance. OL performs very best among the five, because it has most obvious conjugated structures. All the five lignin types boost performance of the sunscreen lotions and dramatically improve their photostability. A preliminary mechanistic exploration suggests that the synergistic effect in the lignin-modified sunscreen lotions could be contributed to the π−π* stacking between lignin and chemical sunscreen actives and the formation of UV radiation-induced conjugated chromophores. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b00934. Physicochemical properties and elemental distributions of lignins from different technical resources; the active and inactive ingredients of purchased sunscreen lotion and pure cream; and the ESR spectra of OL, BMDM, and OL/BMDM mixture under different UV radiation time (PDF)

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Figure 9. ESR integral area of OL and OL/BMDM mixture under different UV radiation times.

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AUTHOR INFORMATION

Corresponding Authors

*Phone: +86-20-87114722; Fax: +86-20-87114721; E-mail: [email protected]. *Phone: +1 (905) 525 9140 ext. 24962; E-mail: zhuship@ mcmaster.ca. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are thankful for the financial support of the National Natural Science Foundation of China (21436004), China Postdoctoral Science Foundation (2015M570714), and that from the Natural Sciences and Engineering Research Council (NSERC) of Canada. Y.Q. also thanks McMaster University to support the extension of his visit for completion of the work. F

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DOI: 10.1021/acssuschemeng.6b00934 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX