Article pubs.acs.org/acssensors
Flexible UV Exposure Sensor Based on UV Responsive Polymer Michele E. Lee and Andrea M. Armani* Mork Family Department of Chemical Engineering and Materials Science, University of Southern California, Los Angeles, California 90089, United States S Supporting Information *
ABSTRACT: One critical challenge facing society is balancing the positive and negative effects of UV exposure. While UV exposure contributes to Vitamin D production, in excess, UV exposure is linked to skin cancer. Therefore, methods to monitor UV exposure and help society achieve this delicate balance have been the focus of numerous research efforts. Here, we leverage advances in functional materials to create a wearable ultraviolet light sensor. The flexible, lightweight trilayer sensor, which is composed of a UV responsive polymer layer sandwiched between two transparent protective layers made from FDA-approved polymers, changes color from transparent to yellow upon UV exposure. Notably, the entire trilayer system is less than 250 μm thick, allowing it to maintain mechanical flexibility. The UV responsive material leverages the photocleavable ortho-nitrobenzyl (ONB) moiety. Because a singular ONB cleaving group is centrally located along the polymer backbone, the colorimetric response is very controlled. Using an AM1.5 solar simulator, the sensor’s linear working range is demonstrated to cover the medically relevant range of 15 min to 1 h. Additionally, we determine the sensor’s robustness to potential environmental and mechanical stress as well as long-term storage. After mechanical bending and water immersion, the performance is unchanged. Storage in ambient conditions also does not degrade the behavior. When combined with sunscreen, the sensor’s response is predictably decreased due to the attenuation of the UV light. This flexible colorimetric sensor based on a smart polymer system can greatly aid society in achieving a balance in sufficient UV exposure. KEYWORDS: ultraviolet sensor, wearable, power-free, smart polymer, ortho-nitrobenzyl
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habits (e.g., walking in the shade or near reflective surfaces like snow and sand) all play a role in cumulative UV exposure. In addition, person-specific factors such as individual skin melanin content and pre-existing medical conditions also greatly affect how much UV exposure results in risk of skin damage. Clearly, sun exposure guidelines need to be incredibly personalized in order to be effective. Implementing these guidelines necessitates a method of measuring an individual’s daily cumulative UV exposure. Current progress in wearable technologies has focused on reducing the UV sensor size by leveraging advances in fabrication techniques.6−9 However, the majority of the technologies developed require power to either generate or measure the signal, creating an inherently complex sensing system. In addition, many sensors only measure instantaneous UV light intensity, rather than providing the cumulative dose.10 More recently, color-changing UV indicators have been developed based on multistep mechanisms where UV-induced degradation triggers a dye response.11−13 In these types of indicators, there are two separate functional components to the device: a UV-responsive agent and a color indicating
kin cancer is currently the most commonly diagnosed cancer in the country, with one in five Americans predicted to develop the disease over the course of a lifetime.1 However, skin cancer is also the most preventable form of cancer. It is well established that exposure to ultraviolet light is the primary risk factor, and it is suggested that avoiding this risk factor alone could prevent more than 3 million new cases each year.2 Yet, the rates of skin cancer continue to increase annually in the United States. Compounding this challenge is that moderate sunlight exposure is important for our health. Sunlight exposure is a key step in the production of Vitamin D. Vitamin D deficiency can result in weak or brittle bones in the elderly and has been indirectly connected to heart disease, diabetes, and other forms of cancer.3 The number of Vitamin D deficient people has doubled over the past two decades, in part, due to an increase in precautionary sun protection in some populations. Therefore, the medical profession is actively debating how to best advise their patients on sun exposure.4 The main challenge is that simple guidelines for cumulative sun exposure are impractical when applied broadly over the general population. The intensity of UVA light, the wavelengths most commonly linked to skin cancer, differs greatly depending on environmental and situational factors.5 Geographic location, time of day, specific weather conditions, altitude, and individual © XXXX American Chemical Society
Received: August 9, 2016 Accepted: September 13, 2016
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DOI: 10.1021/acssensors.6b00491 ACS Sens. XXXX, XXX, XXX−XXX
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Figure 1. UV-light induced photocleavage and color change due to absorbance shift. (a) Photochemical reaction of ortho-nitrobenzyl (ONB) modified PMA, followed by dimerization of the photocleavage products. Formation of the azo group in the dimer (green) is responsible for the yellow color change of the material. (b) Change in absorbance value with increasing simulated sun exposure time. Decrease of absorbance at 269 nm (blue squares) occurs in parallel with increase of absorbance at 318 nm (black circles). Inset shows rendering of transparent bilayer sensor. (c) Representative images of a sample before (top) and after (bottom) being exposed to 2 h of simulated sun. The color change is clearly evident.
acid.18−20 Following photocleavage, the nitrosobenzaldehyde photoproduct often undergoes a dimerization reaction with itself to form an azo-linked polymer16,21 (Figure 1a). The formation of these specific photocleavage products is verified using NMR spectroscopy (SI Figure 2). The molecular weight of the resulting polymers was approximately 13 000 Da, with a narrow polydispersity index of 1.07. Additionally, because the single ONB group is located in the center of the polymer, the polymer cleaves precisely in half upon exposure to UV, providing additional control over the material response (SI Figure 3). For initial characterization measurements, bilayer samples of photocleavable poly(methyl acrylate) (PMA) deposited on polydimethylsiloxane (PDMS) were fabricated and exposed to an AM 1.5 solar simulator. The UV-induced azo generation and color intensity change is quantified using UV−vis spectroscopy (SI Figure 4, Figure 1b). The UV−vis spectrum of the polymer deposited on PDMS prior to simulated sun exposure exhibited a single peak at 269 nm which can be attributed to the ONB aromatic structure.20 As the simulated sun exposure dose is increased, the initial 269 nm peak decreased in absorbance and a second peak appeared around 318 nm, along with a broad increase in absorbance from 350 to 450 nm. This bathochromic shift indicates conversion of the ONB moiety to the photoproducts shown in Figure 1a containing nitroso and azo functionalities.20,22 Molecules containing an azo functionality in a conjugated system often absorb in the visible wavelength range, and the intensity of the absorption scales with azo concentration present in the material. The 318 nm peak appears in as little as 5 min of simulated sun exposure and reaches a steady state absorbance value at approximately 1 h of exposure (Figure 1b). We visually observed a strong color change of the sensor from transparent to orange, which corresponds well to the increased absorbance from 350 to 450 nm (Figure 1b inset, SI Figure 4). Nontoxic Sensor Fabrication and Characterization. While the bilayer structure shown in Figure 1b demonstrates a clear UV response, it also leaves the active PMA layer exposed to the environment. To create a robust and wearable UV sensor, it is necessary to shield the active PMA layer without impeding UV exposure. Based on these design criteria, a trilayer structure was designed in which the active PMA layer was placed between a thin polyethylene naphthalate (PEN) film and a PDMS coating (Figure 2a). We used a nontoxic solution of ethyl acetate and 10 wt % photocleavable PMA to drop-cast a thin film onto the flexible PEN substrate. The resulting smooth film was approximately 60
mechanism. For example, Mills et al. utilized a photoresponsive acid-release agent to trigger a pH change upon UV irradiation, which then induced protonation of a pH-responsive dye and consequent color change.11 In addition, Khiabani et al. developed a printable paper-based UV dosimeter that operates based on the degradation-induced photocatalysis of titanium dioxide on food dyes.12 In their sensor, the UV indication method is a color degradation of the dye instead of a color appearance. Another route to simplify the approach is to use a UVresponsive material that can act alone as an indicator of cumulative exposure. Polysulfone and polyphenylene oxide (PPO) were the first such materials used as UV indicators because their degree of degradation can be directly correlated to cumulative UV exposure.14,15 However, those materials require the use of a spectrometer to analyze absorbance change and are therefore impractical for portable use. An ideal material would incorporate an indicator system within the material itself, such as the color-change mechanism described in multistep sensors. In the present work, a power-free, environmentally stable, and flexible cumulative UV sensor is intelligently designed, fabricated, and demonstrated. The sensing mechanism is based on an ortho-nitrobenzyl (ONB) modified photocleavable polymer film that irreversibly cleaves and changes color upon exposure to UV light.16,17 Unlike most common photochromic dyes, this photocleavable polymer is stable across a wide range of temperatures and is not susceptible to oxygen degradation. To fabricate the flexible UV sensor, the polymer is integrated into a trilayer polymer sandwich structure. The simple, two-step fabrication process uses minimal solvent and only materials that have been approved by the U.S. Food and Drug Administration for human use and food contact. Using an AM1.5 solar simulator, the flexible UV sensor’s performance was determined, and the colorimetric index change was calculated. The sensor’s response was measured after mechanical bending, exposure to water and sunscreen, and storage for 5 weeks.
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RESULTS AND DISCUSSION
Synthesis and Characterization of Photocleavable PMA. We prepared linear homopolymers of methyl acrylate with a photocleavable ONB unit at the center using atom transfer radical polymerization with an asymmetric initiator containing a photocleavable ONB group (Figure 1a, SI Scheme 1).16 The structure of the polymer was verified using 1H NMR spectroscopy (SI Figure 1). Upon exposure to UV light, the ONB group cleaves to form an aldehyde and a carboxylic B
DOI: 10.1021/acssensors.6b00491 ACS Sens. XXXX, XXX, XXX−XXX
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Figure 2. Characterization of transparent sensor cross section and demonstration of flexibility. (a) Rendering of trilayer sensor composition. (b) Rendering illustrating simulated sun exposure experiments. Darker shades of orange color change correspond to longer exposure times. (c) Optical micrograph of sensor cross section showing smooth film attachment and layer thickness. (d) Image showing transparency of sensor. (e) Flexible sensor wrapped around a pen with radius 3.75 mm. Figure 3. Yellowness Index change (ΔYI) across multiple environments shows consistent sensor performance. (a) To test reproducibility between devices, two different samples (Trilayer 1, 2) were prepared and tested. Additionally, a third sensor was stored for 5 weeks. All devices exhibit very similar ΔYI, indicating both reproducibility between devices and a long shelf life. The control sensor without active PMA layer shows no change in ΔYI. (b) Bending and water exposure tests show similar behavior to the original sensor from part a. When SPF 100+ sunscreen was applied directly to the sensor surface, the response was attenuated. (c−e) Rendering of sample environmental test (left) with corresponding images before and after simulated sun exposure (right) for the lifetime, water immersion, and SPF100+ measurements. From left to right, sample exposure times are 10 min, 20 min, 30 min, and 1 h. Error bars are included for all data. In some cases, the error is smaller than the symbol.
μm thick. Following solvent removal, we spin coated a protective PDMS layer over the active PMA surface. This process yielded a trilayer structure consisting of PDMS/ photocleavable PMA/PEN (Figure 2a). We used crosssectional imaging to verify that each layer adhered completely to the previous layer with no air gaps or defects within the individual layers (Figure 2c). It is important to note that both PDMS and PEN are transparent, flexible, biocompatible materials (Figure 2d,e). Therefore, this trilayer structure is a nontoxic flexible UV sensor. To accurately characterize the flexible UV sensor, we used an AM1.5 solar simulator. Sample exposure area was limited by a 1.5-mm-diameter aperture and exposed for different amounts of time, resulting in varying shades of color change (Figure 2b). In addition to measuring the sensor’s response in a controlled environment, the sensor was exposed to several common environmental stresses to determine its ability to maintain performance when worn. Specifically, mechanical bend tests, water immersion and sunscreen exposure tests, and shelf life tests were performed. UV Sensor Based on Smart Material. We quantified the color of the material by calculating the yellowness index (YI). YI is calculated using the ASTM D1925 Standard Test Method for Yellowness Index of Plastics. YI = 100 ×
observed across multiple samples with active PMA layers varying in thickness from 30 to 60 μm, the rate of increase in the linear region of the sigmoid differed between samples of different thickness (SI Figure 5, Table 1). Specifically, the YI Table 1. Summary of Coefficients in Gompertz Fitting Function for ΔYI of UV-Exposed Films a (asymptote) trilayer, 5%
1.27R − 1.06B G
trilayer, 10%
where R, G, and B represent the standard primary color values. By varying the total exposure time, the change in YI (ΔYI) as a function of sun exposure can be determined. As shown in Figure 3, the ΔYI increased linearly with exposure time until it reached a maximum value around 30 min of simulated sun exposure. Reproducibility between different devices is a key concern with any sensor. To study this metric, multiple samples were made, and the same trend was observed across multiple fabricated sensor samples (Trilayer 1, Trilayer 2). Once exposed, the UV sensor samples retained their change in color over the course of several weeks indicating that the photoreaction is irreversible and stable. Control experiments using PDMS−PEN samples without the UV-active PMA layer verified that the color change mechanism is entirely due to the active PMA layer (Figure 3a). In addition, samples were made with a higher concentration of the PMA-ONB solution to increase the layer thickness (SI Figure 5). While the same overall sigmoidal response was
lifetime bend test water suncreen
23.278 28.238 28.792 33.000 30.322 31.896 30.870 9.047
± ± ± ± ± ± ± ±
1.028 0.457 0.400 1.221 0.474 2.603 0.279 2.121
k (rate, h−1) 9.697 6.172 8.622 8.180 7.073 5.962 8.267 2.049
± ± ± ± ± ± ± ±
1.571 0.338 1.009 0.850 0.370 1.538 0.259 0.731
xc (h) 0.212 0.222 0.185 0.202 0.218 0.171 0.229 0.393
± ± ± ± ± ± ± ±
0.011 0.006 0.006 0.014 0.005 0.016 0.003 0.118
increase rate was higher for thicker samples than thinner samples. This strategy provides one route to tune the response rate and consequently device sensitivity. Alternatively, it is possible to shift the rate in the opposite direction by adding UV filters to reduce the response. Lifetime measurements were also performed on nonexposed samples that had been stored for 5 weeks in an ambient environment, and, within error, identical responses to freshly prepared samples were observed (Figure 3a,c). These measurements indicate that the sensor performance was not affected with extended storage time. C
DOI: 10.1021/acssensors.6b00491 ACS Sens. XXXX, XXX, XXX−XXX
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ACS Sensors Stability to Environmental Changes. In order for the device to function as a wearable sensor, it must be flexible enough to bend with the skin surface and maintain its performance when exposed to water and sunscreen. To test these criteria, bending, water immersion, and sunscreen resistance tests were performed on the sensor strips. For the bending test, the sensor strip was wrapped around a pen with a 3.75 mm bend radius, and then the response was characterized as previously described. As observed in Figure 3b, the impact of the mechanical bending on the sensor’s behavior was minimal, indicating that the active PMA layer was not damaged. For the water-resistance test, the samples were rinsed with DI water for 1 min, and then dried using an air gun before performing the simulated sun exposure measurements. Similar to the bending tests, these sensors also did not deviate from the original sensor behavior, indicating that the PDMS and PEN layers protect the active PMA layer from water damage (Figure 3b,d). To determine the sensor’s compatibility with sunscreen, we sprayed a commercially available SPF 100+ sunscreen over the sensor strip and measured the ΔYI. As expected, the change in yellowness occurred at a reduced rate in the sunscreen-treated sensor strip than in the unprotected strip (Figure 3b,e). There was, however, still a slight increase in the YI as compared to the control. These results indicate that an optical filter can be added on top of the device to modify the sensitivity and rate of the active layer response. To more quantitatively compare the data across samples and measurements, the results of the samples containing PMAONB were fit to a Gompertz sigmoidal function, which is a special case of a generalized logistic function:
y = ae−e
−k(x − xc)
through the addition of attenuating coatings, like sunscreen. This flexible UV sensor will play a key role in the development and implementation of personalized UV exposure guidelines, enabling society to find a solution to the challenge of balancing sufficient UV exposure to prevent Vitamin D deficiency with excess UV exposure that can cause skin cancer.
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METHODS
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ASSOCIATED CONTENT
Materials. Teonex PEN films were purchased from Teijin DuPont Films. A commercially available spray-on sunscreen (Neutrogena Ultra Sheer SPF 100+) was used. Dichloromethane (DCM), ethyl acetate, and chloroform were purchased from BDH. Tetrahydrofuran (THF) (stabilized with 250−350 ppm inhibitor) was purchased from Alfa Aesar. Sylgard 184 elastomer base and curing agent were purchased from Dow Corning. All other reagents and solvents were purchased from Sigma-Aldrich. Photocleavable Polymer Synthesis and Characterization. The bifunctional ONB initiator and photocleavable PMA were prepared according to a previously reported procedure.16 Proton NMR measurements were conducted in THF-d8 on a Varian 600 MHz spectrometer. UV exposure of the NMR sample was conducted in a Luzchem ICH-2 photoreactor with 16 Hitachi FL8BL-B UVA bulbs. Size exclusion chromatography (SEC) was conducted on a Waters GPC system with refractive index detector and four Waters Styragel columns (Styragel HR1, HR4, HR4E, HR5E). GPC experiments were performed at room temperature under a THF flow rate of 1 mL/min. Relative molecular weights were determined using a polystyrene calibration standard. Bilayer and Trilayer Sensor Fabrication. Solutions of 5 and 10 wt % photocleavable PMA (ONB-PMA) in ethyl acetate were prepared and mixed for 30 min at room temperature. The PEN sheets were first stripped of adhesive, then ozone treated to form a more hydrophilic surface. PDMS was prepared according to standard procedure using Sylgard 184 elastomer base:curing agent ratio of 10:1, then ozone treated. For the bilayer fabrication, PMA solutions were drop-cast directly onto PDMS and dried in a vacuum oven overnight. For the trilayer sensors, the ONB-PMA solutions were deposited onto PEN via drop casting in a closed, evacuated chamber. By conducting the drop-casting in a closed, evacuated chamber, we created an environment that was internally cooled via rapid solvent evaporation and saturated with solvent gas. This environment facilitated controlled evaporation of the solvent across the entire sample surface by balancing polymer mobility in solution with pinned edge solvent evaporation. The chamber was kept under vacuum for 3 min. Resulting films were dried for 5 min on a hot plate at 50 °C, then in a vacuum oven overnight. We incorporated PDMS as the top protective layer of the trilayer device by spin coating it over the dried films at 2000 rpm for 1 min, followed by thermal curing at 75 °C for 30 min. Unless explicitly mentioned, all results presented are from 10% ONB-PMA films. Sample cross section imaging measurements were performed on a Keyence VHE 5000 Digital Microscope. Simulated Solar Exposure Experiments. Samples were placed on a stage with a 1.5 mm diameter aperture for light illumination. Solar simulation was conducted with an Oriel 300W lamp with AM 1.5G filters. Prior to each experiment, the lamp intensity was calibrated to be equal to one sun. The color change of sun-exposed samples were analyzed via two methods: absorbance change using an Agilent 8453 UV−vis spectrometer and colorimetric change. The colorimetric change was determined by sampling 3 points per sun exposure time and calculating the yellowness index as detailed above.
(1)
In the present case, a is the maximum value (asymptote), k is the response rate of the PMA-ONB, and xc is the time when the sensor has responded to half its maximum value. The Gompertz function was selected as it allows for the different cleavage rates at the beginning and end of the reaction to be captured. The fits are shown with dashed lines in Figure 3 and SI Figure 4, and the parameters determined from the fits are summarized in Table 1. All fits had adjusted R2 values above 98%. With the exception of the sunscreen-treated sample, all of the samples had similar values for a, k, and xc. This similarity is especially notable as it quantitatively verifies that the response rate, total signal, and half response signal remained constant even if a sample was bent, stored, or immersed in water. For the sunscreen coated samples, all values changed. Specifically, the response (or k) changed by a factor of approximately 4 due to the sunscreen’s attenuation of the UV. This shift resulted in the xc changing. The decrease in a was unexpected, but it could be an artifact of the fit because there is insufficient data to accurately fit a sigmoid. In this work, we report the development of a novel, inexpensive, and wearable cumulative UV sensor that is fabricated using nontoxic solvents and biosafe materials. The polymeric sensor exhibits a long shelf life, while also maintaining the functional criteria of flexibility, water resistance, and sunscreen compatibility. The sensor color changing response time at AM 1.5 spans 5 min to 1 h; a biologically relevant range for both vitamin D production and potential UVinduced skin damage. We also demonstrate that the sensor response can be optimized on-the-fly for multiple skin types
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssensors.6b00491. D
DOI: 10.1021/acssensors.6b00491 ACS Sens. XXXX, XXX, XXX−XXX
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(11) Mills, A.; McDiarmid, K.; McFarlane, M.; Grosshans, P. Flagging up Sunburn: A Printable, Multicomponent, UV-Indicator That Warns of the Approach of Erythema. Chem. Commun. 2009, 1345−1346. (12) Khiabani, P. S.; Soeriyadi, A. H.; Reece, P. J.; Gooding, J. J. Paper-Based Sensor for Monitoring Sun Exposure. ACS Sensors 2016, 1, 775−780. (13) Lee, S. K.; Sheridan, M.; Mills, A. Novel UV-Activated Colorimetric Oxygen Indicator. Chem. Mater. 2005, 17, 2744−2751. (14) Parisi, A. V.; Kimlin, M. G. Personal Solar UV Exposure Measurements Employing Modified Polysulphone with an Extended Dynamic Range. Photochem. Photobiol. 2004, 79, 411−415. (15) Lester, R. a; Parisi, a V; Kimlin, M. G.; Sabburg, J. Optical Properties of poly(2,6-Dimethyl-1,4-Phenylene Oxide) Film and Its Potential for a Long-Term Solar Ultraviolet Dosimeter. Phys. Med. Biol. 2003, 48, 3685−3698. (16) Lee, M. E.; Gungor, E.; Armani, A. M. Photocleavage of Poly(methyl Acrylate) with Centrally Located O-Nitrobenzyl Moiety: Influence of Environment on Kinetics. Macromolecules 2015, 48, 8746−8751. (17) Gungor, E.; Armani, A. M. Photocleavage of Covalently Immobilized Amphiphilic Block Copolymer: From Bilayer to Monolayer. Macromolecules 2016, 49, 5773−5781. (18) Bochet, C. Photolabile Protecting Groups and Linkers. Journal of the Chemical Society, Perkin Transactions 1 2001, 125−142. (19) Pelliccioli, A. P.; Wirz, J. Photoremovable Protecting Groups: Reaction Mechanisms and Applications. Photochem. Photobiol. Sci. 2002, 1, 441−458. (20) Il’ichev, Y. V.; Schwörer, M. A.; Wirz, J. Photochemical Reaction Mechanisms of 2-Nitrobenzyl Compounds: Methyl Ethers and Caged ATP. J. Am. Chem. Soc. 2004, 126, 4581−4595. (21) Zhou, H.; Lu, Y.; Qiu, H.; Guerin, G.; Manners, I.; Winnik, M. A. Photocleavage of the Corona Chains of Rigid-Rod Block Copolymer Micelles. Macromolecules 2015, 48, 2254−2262. (22) Schumers, J.; Gohy, J.; Fustin, C. A Versatile Strategy for the Synthesis of Block Copolymers Bearing a Photocleavable Junction. Polym. Chem. 2010, 1, 161.
Polymer synthesis scheme, NMR and GPC characterization of uncleaved and cleaved polymers, kinetics of absorbance change in bilayer device, and YI change with varying PMA concentrations (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Northrop Grumman-Institute for Optical Nanomaterials and Nanophotonics and by the Office of Naval Research [N000141410374, N000141110910]. The authors thank M. Siron, D. Armani, M. Thompson, E. Gungor, and X. Shen for helpful discussions.
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ABBREVIATIONS DCM, dichloromethane; GPC, gel permeation chromatography; NMR, nuclear magnetic resonance; ONB, ortho-nitrobenzyl; PDMS, polydimethylsiloxane; PEN, polyethylene naphthalate; PMA, poly(methyl acrylate); PPO, polyphenylene oxide; SEC, size exclusion chromatography; THF, tetrahydrofuran; UV, ultraviolet; YI, yellowness index
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REFERENCES
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