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Oxygen Sensing Based on the Yellowing of Newspaper Jingjing Yu, Xingcai Qin, Xiaojun Xian, and Nongjian Tao ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.7b00790 • Publication Date (Web): 26 Dec 2017 Downloaded from http://pubs.acs.org on December 31, 2017
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ACS Sensors
Oxygen Sensing Based on the Yellowing of Newspaper Jingjing Yu,† Xingcai Qin,*,† Xiaojun Xian‡ and Nongjian Tao*,†,‡ †
State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering,
Nanjing University, Nanjing 210093, China ‡
Biodesign Center for Bioelectronics and Biosensors, Arizona State University, Tempe, AZ 85287, USA
ABSTRACT: Newspaper is known to turn yellow over time. We show here that this yellowing process is sensitive to oxygen when exposed to UV light, leading to oxygen sensing. Oxygen sensing is critical to many applications, including industrial process control and breath analysis, but the existing oxygen sensors have limitations, especially for breath analysis that operates at 100% humidity. The UV irradiation also triggers fluorescence emission from newspaper, and the fluorescence intensity depends on oxygen concentration, providing an additional oxygen sensing method. Newspaper is stable in ambient air, and reactive to oxygen only with UV activation, which overcomes the instability issue of a typical colorimetric sensor in ambient air. The newspaper oxygen sensor works in 100% relative humidity air, containing various interferents. These unique properties of newspaper promise low cost and reliable oxygen sensing applications. KEYWORDS breath analysis, optical sensing, paper-based sensor, yellowing of newspaper, oxygen sensing
Many chemical reactions produce specific color changes. This phenomenon has been used in chemical sensors (colorimetric sensors) by selecting a sensing material that reacts specifically with an analyte. The sensing materials of the colorimetric sensors are often highly reactive in air, and thus the sensors must be sealed in glass tubing or other containers before use.1 This has been a limitation of the colorimetric sensors. Paper has been invented for over two thousand years as printing and writing materials for books, arts, documents and currency because of its low cost fabrication and long-term stability under ambient condition. Paper turns yellow in air over time, which is due to oxidation as shown by recent experimental and theoretical studies.2-3 Here we describe an oxygen sensor based on the yellowing of newspaper. We show that newspaper is insensitive to oxygen in air (thus stable) without ultraviolet (UV) irradiation, but becomes sensitive to oxygen with UV turned on. In addition to yellowing, the UV irradiation also triggers fluorescence emission from the newspaper. To evaluate the newspaper-based oxygen sensor for breath analysis, we study both the yellowing process and fluorescence response to oxygen in humid air containing common interferents. Oxygen sensing is critical to many applications, ranging from fuel burning optimization in internal combustion engines and power plants, to quality control of biological, chemical and food processing industries.1, 4-6 A particularly important application is human respiration and metabolism analysis and diagnosis. We inhale oxygen at 20.9% from the ambient air, and exhale oxygen at a lower level that varies from people to people, and from time to time. By measuring the exhaled oxygen level, important physiological parameters, such as metabolic rate, energy ex-
penditure,7 maximum oxygen uptake and anaerobic threshold,8 can be determined. To address these needs, different oxygen sensors have been developed. The most popular one is based on electrochemistry using either liquid or solid electrolytes.6 The former faces electrolyte evaporation and other issues, and the latter requires operation of the sensor at high temperatures (hundreds of ℃). Paramagnetic oxygen sensors feature high precision and fast response time, but they are prone to humidity influence.4,9 Optical oxygen sensors have many attractive features, such as low cost and miniaturization.1,10-13 Despite the efforts, the current commercial oxygen sensors are still expensive and bulky (e.g., those used for human metabolic rate testing), and require periodic calibration, which are not suitable for many daily sensing applications. Paper-based chemical sensors have been pursued in recent years because they are cheap, environment friendly and mechanical flexible.14-21 In these previous works, paper is used as a substrate or supporting material, on which sensing materials are deposited. The present work uses the intrinsic properties of paper to overcome the instability issue of colorimetric sensors, and to detect oxygen in humid environment. We present here two oxygen-sensing principles using newspaper, one is the yellowing process (colorimetry) and the second one is fluorescence emission (Figure 1). The experimental setup consists of a piece of newspaper placed inside of a detection chamber with a gas inlet and outlet. The newspaper is illuminated with UV (365-370 nm) from a light emitting diode (LED) to activate the yellowing process and to excite fluorescence emission.
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Both the colorimetric and fluorescence sensors are detected with a webcam (based on a CMOS imager) as shown in Figure 1.
the samples were characterized with a commercial electrochemical oxygen sensor (PGM-1600, RAE Systems, Inc.). The yellowing and fluorescence changing rates were monitored with a webcam (Logitech C525) with the illumination of the white LED and the UV LED. For each experiment, the UV LED, the white LED and the webcam were turned on for 30 mins for stabilization. A Matlab code was used to identify the newspaper regions in the captured image, process the image intensity (R, G, B) and analyze the results. UV-vis Absorption Spectroscopy and Fluorescence Spectroscopy. The UV-vis absorption spectra of newspapers (Figure 2a) were recorded with Agilent Cary 100 UVVis spectrophotometer, using a piece of fresh newspaper as a reference. The Fluorescence spectra of fresh or yellowed newspapers (Figures S1 and S2) were acquired with Hitachi F7000 fluorescence spectrophotometer.
RESULTS AND DISCUSSION Colorimetric sensing. The colorimetric sensor is based on the yellowing of newspaper. This phenomenon is originated from lignin, which is abundant in newspaper. Studies have revealed that the yellowing of lignin is sensitive to oxygen.22-23 The reaction of lignin in newspaper with oxygen in the presence of UV is given by,
Figure 1. Schematic illustration of newspaper oxygen sensing principle and setup. (a) Image of newspaper before and after yellowing triggered by UV light due to the reaction of lignin with oxygen in air. (b) Fluorescence image of the newspaper recorded along with yellowing process (the raw image contrast was adjusted for clarity). (c) Experimental setup of the sensor. Gas sample is delivered (0.6 L/min) from a sampling bag into the detection chamber via the inlet. A UV LED is used to trigger the yellowing and excite fluorescence of newspaper. Both the yellowing process and fluorescence emission are monitored with a webcam. To detect the yellowing process, a white LED is used to illuminate the newspaper.
EXPERIMENTAL SECTION Materials. The newspaper pieces were obtained from the blank area of the printed newspapers (Nanjing Daily, or otherwise specified.). The oxygen gas samples at different concentrations were prepared by mixing pure oxygen (99.999%) with nitrogen (99.999%) in sampling bags. Except the air sample, all gases used in the experiment were humidified to 100% RH by adding pure water in the sampling bags to simulate breath sample. The UV LED (part number N3535U-VNL1-A1J11H) was used with a current source of 600 mA (90.3 mW/cm2 on the surface of newspaper). The white LED (part number LTW-E670DS) was connected to a current source of 20 mA. For fluorescence detection, a 590 nm long-pass filter (FGL590, Thorlabs Inc.) was used. Setup and method. Gases prepared in the gas sampling bags were delivered into the detection chamber with a pump (0.6 L/min). The oxygen concentrations of
lignin hv → quinones yellow 1 where quinones may include p-quinone, o-quinone and stilbenequinones with absorption peaks at 350, 420 and 480 nm, respectively,22-24 thus producing yellow color. By measuring the rate of the yellowing process under UV irradiation, oxygen can be detected (Figure 1a). We measured UV-vis spectra of newspaper upon UV irradiation over various durations, and observed increased absorption of light between 400 - 520 nm with time (Figure 2a). This increased absorption of blue light is responsible for the yellowing of newspaper in the presence of oxygen. To detect oxygen, we monitored the yellowing process with the webcam, which has R (red), G (green) and B (blue) color channels. B-channel is most sensitive to blue light, which was monitored for oxygen sensing using a white LED to illuminate the newspaper. To eliminate the influence of the UV light on color detection, we turned the UV and white light LEDs on and off alternately, and measured the color change when the UV was off and the white light was on. Figure 2b shows an example of the B-channel intensity recorded before and after ~2.5 min UV irradiation. The decreased B-channel intensity after UV irradiation was due to the yellowing of the newspaper. From the intensity change, we determined the absorbance, which reflects the concentration of the yellow oxidation product (quinones). Figure 2c plots the absorbance change vs. time for oxygen at different concentrations, where each data point was recorded after 2.5 min UV irradiation. For a given oxygen concentration, the absorbance increases proportionally with time (Figure 2c). The absorbance change rate (the slope of Figure 2c) increases with oxygen concentration (Figure 3a), and the dependence can be fitted with an
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Figure 3. Properties of the colorimetric sensor. (a) Dependence of newspaper yellowing rate (absorbance change rate, ΔA/Δt) on oxygen concentration (where A is absorbance), where the error bars are the standard deviations of the slope, which measured the absorbance increase rate in Figure 2c. (b) Cross-sensitivity of the newspaper yellowing rate with common interferents
We examined the selectivity of the oxygen sensor based on the yellowing of newspaper by testing it in various common chemicals, including nitrogen, carbon dioxide, carbon monoxide, hydrogen, ammonia, acetone and methanol at different concentrations. The result (Figure 3b) shows that the sensor responses to these chemicals are at least 5 times smaller than that to 2% oxygen, demonstrating selective detection of oxygen in the presence of these common chemicals.
Figure 2. UV trigged newspaper yellowing. (a) UV-vis spectra of newspaper in air under UV irradiation over different durations. (b) Newspaper yellowing process detected with the blue (B) channel of the webcam. (c) Absorbance (B channel) increases with time at different oxygen concentrations, where the error bars are the standard deviations of the data over 5 seconds.
Fluorescence emission. In addition to yellowing, newspaper fluoresces under the UV irradiation, which is originated from lignin and related materials in newspaper.25-28 We observed that the fluorescence emission decreased with a rate related to oxygen concentration, thus providing a second signal transduction principle to sense oxygen with newspaper. We first examined the fluorescence spectrum of the newspaper excited by UV at 365 nm and found that the most sensitive wavelength range of fluorescence response to oxygen is fluorescence with wavelengths longer than 590 nm (Figure S1, see Supporting Information for details). To demonstrate oxygensensing capability, we illuminated the newspaper with the
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UV LED, and measured fluorescence emission with the CMOS imager (webcam) by placing an optical filter that passes light with wavelengths longer than 590 nm in front of the CMOS imager. We note that the UV source used to excite fluorescence was the same as that used to trigger the yellowing of the newspaper, which allows the oxygen sensing by detecting both yellowing process and fluorescence change (Figures 1a and 1b)
Figure 4. Fluorescence intensity change of newspaper under UV irradiation. Fluorescence intensity detected from the red (R) channel of the webcam over time with different concentrations of oxygen. The curves were obtained by switching between reference gas (20.9% O2, ambient air oxygen concentration) and sampling gas (oxygen at different concentrations).
For breath oxygen analysis, the baseline (inhaled oxygen level) is the ambient oxygen concentration of 20.9%, and the oxygen level in exhaled air can decrease to ~16%. To examine the oxygen sensing capability for breath analysis, we tested the fluorescence emission by varying oxygen from 0% to 24% at 100% relative humidity. Figure 4 shows repeated measurement of fluorescence intensity by changing the oxygen concentration from 20.9% to 8%, 16% and 24% oxygen, respectively. 20.9% oxygen serves as the reference or background because it is the oxygen level of ambient air (inhaled oxygen). The data reveals linear dependence of the fluorescence intensity on time for a given oxygen concentration, similar to the yellowing process. The slope of the linear dependence measures the rate of fluorescence intensity change, which is negative, reflecting decreasing fluorescence emission when exposed to oxygen. This decrease in fluorescence can be attributed to the degradation of lignin in the presence of oxygen and UV irradiation. The slope, the fluorescence intensity over time, varies with oxygen concentration, allowing extraction of oxygen concentration. Figure 5a shows the slope of normalized fluorescence intensity vs. oxygen concentration (%). Similar to that of the yellowing of newspaper, the relationship between the fluorescence intensity response and oxygen concentration is nonlinear, which can be fit with an empirical equation (see Supporting Information for details).
Figure 5. Properties of the fluorescence sensor. (a) Dependence of fluorescence changing rate of the newspaper with oxygen concentration, where ΔI/Δt is the fluorescence intensity changing rate, and Δ(ΔI/Δt) is the difference in the fluorescence changing rate between sampling gas and reference gas (oxygen at ambient concentration). The error bars were determined from 20 measurements using 4 regions of newspaper (5 continuous measurements/region). (b) Crosssensitivity of fluorescence emission with common interferents in breath, where the concentration of interferents is shown as the concentration difference between the sampling gas and reference gas. The error bars of interferents were calculated from 3 measurements.
We evaluated the selectivity of the fluorescence sensing of oxygen with the newspaper by testing its response to carbon dioxide, carbon monoxide, hydrogen, ammonia, acetone and methanol. These analytes were selected for test because of their presence in breath. Figure 5b shows that the influences of these interferents on oxygen sensing are relatively small. Concentrated CO2 (6%) can cause some effect on the signal, however, the CO2 concentration in breath is typically fixed between 4-5.3%,29 and its effect on metabolic rate is small. These results indicate that the newspaper-based fluorescence sensor is capable of selectively detecting oxygen in human breath. We evaluated oxygen sensing of different newspapers as they have different textures, compositions and lignin contents. Different papers have different sensitivities for both
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ACS Sensors the colorimetric and fluorescence sensors (Figures 6a and 6b), but the colorimetric sensitivity (yellowing rate) is linearly correlated with the fluorescence response (Figure 6c). We believe that the different responses are mainly due to the variability in the lignin contents in different papers. We have thus determined lignin in different newspapers from fluorescence spectra (see Supporting
Information for details).25 Figure 6d shows that sensor sensitivity is linearly correlated with lignin, which further confirm that lignin plays a key role in yellowing process and fluorescence emission. Although different newspapers show different sensitivities, they are also sensitive to oxygen. For sensor applications with newspapers, calibration may be performed before use.
Figure 6. Colorimetric sensing and fluorescence sensing of different papers. Different papers used for (a) colorimetric sensing and (b ) fluorescence sensing, where Δ(ΔA/Δt) and Δ(ΔI/Δt) are the difference in the sensor response (absorbance change rate and fluorescence change rate) between N2 and 20.9% O2. (c) Correlation between colorimetric sensor sensitivity (horizontal axis) and fluorescence sensor sensitivity (vertical axis). (d) Dependence of colorimetric sensor sensitivity (left vertical axis) and fluorescence sensor sensitivity (right vertical axis) on lignin amount.
Both the yellowing process and fluorescence response (Figures 3a and 5a) slow down at high oxygen concentrations. At first glance, this appears to be due to the depletion of lignin in the newspaper. However, the newspaper used in the experiment has a thickness of ~70 μm, which should provide sufficient lignin to react with oxygen. Additionally, we observed that despite the slowing-down, the color and fluorescence intensity continued to change when exposed to oxygen at high concentrations (Figures 2c and 4), indicating sufficient lignin in the newspaper. Another possible reason for the slowing down is the limited diffusion rate of oxygen in newspaper. The diffusion rate of oxygen in paper is more than ~10-6 m2/s (Supporting Information),30-32 so that the mass transport due to diffusion alone is ~3 ms, which is fast compared to the yellowing process. To examine this slowing-down the
yellowing process and fluorescence response, we measured UV light penetration through the paper (see Supporting Information for details), and found it decayed by ~10 times over a distance of ~30 μm (Figure S4). This result shows that after oxygen reaction with lignin near the newspaper surface, the reaction slows down because of the attenuated UV intensity in the interior, which explains the observation of the decreasing yellowing process and fluorescence response. Quantitative modeling of the reaction requires considering diffusion, adsorption, reaction of oxygen with lignin in the newspaper, and UV intensity distribution, as well as detailed structural information of the newspaper. Although the overall responses of the yellowing and fluorescence to oxygen are similar, the former saturates more quickly with increasing oxygen than the latter. This indicates different chemicals in paper are responsible for
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the yellowing and the degradation in fluorescence intensity. Considering lignin in paper is a class of complex polymers,22,33 some produce quinones with the distinct yellow color, while others give rise to fluorescence emission, which degrades upon UV irradiation in the presence of oxygen. Despite the difference in the mechanism, both the UV-induced yellowing of newspaper and UV-induced degradation of the fluorescence emission allow selective detection of oxygen at high humidity. For most applications, including breath oxygen analysis, oxygen levels are high, so that the precision of oxygen sensing is more important than the detection limit. Response time is another key parameter of oxygen sensing, and its requirement also depends on applications. For measuring human metabolic rate based on indirect calorimetry, response time on the order of minutes is sufficient because these measurements typically last for 10-30 min. For measuring maximum oxygen intake, however, oxygen concentration should be tracked breath by breath, which requires response time on the order of one second. The precision of oxygen sensing based on yellowing of newspaper is determined by how precisely the color change can be measured over a certain time interval (response time). Therefore, the precision and response time are related. Using the setup presented here, the noise level (standard deviation) in the absorbance over five seconds for the color-based oxygen sensing is ~2.3x10-4 (Figures 2b and 2c), corresponding ~0.3% oxygen level (precision)(Figure 3a). Similarly, for fluorescence oxygen sensing, the noise level in the slope of the response curve over one minute is ~2x10-4 min-1 (Figure 4), which corresponds to ~0.3% oxygen (precision)(Figure 5a). Compared with other oxygen sensors, such as electrochemical oxygen sensors and paramagnetic oxygen sensors, the newspaper-based sensors are simple, low cost and irreversible, which can be used as disposable sensors. A distinct advantage of the newspaper-based sensors is that they can be pre-calibrated in batches, thus reducing the user burden for performing routine calibrations. Commercial breath oxygen analyzers, such as metabolic carts ($10-50 k) that track energy expenditure, require calibration for accurate measurement, which prevents broader applications of the technology, especially for fitness and weight management use at home. Different newspapers have different composition, and this precalibration strategy solves the variability issue of different newspapers. Unlike the existing oxygen sensors that often require removal of water from the sample or preconditioning of the sample before detection, the newspaper-based sensors work at high humidity (100%), which simplifies the product design, and is particularly attractive for breath analysis. A major component of the present newspaper-based sensor is a CMOS imager and associated signal-processing unit that detects and analyzes color changes. High-performance CMOS imagers (used in smartphones) cost a few dollars each. We anticipate an affordable (reusable) breath analyzer using pre-calibrated disposable paper sensors for home-based breath and metabolism analysis.7
Compared to traditional colorimetric sensors, newspaper is stable under ambient condition, and can be activated to detect oxygen with UV-irradiation when needed, thus helping overcome the long standing shelf-life issue of colorimetric sensors. While newspaper has many advantages for practical applications, its chemical composition and microscopic structure are complex, making it difficult to elucidate the detailed reaction rate and mechanism on the microscopic scale, a task requiring further investigations.
CONCLUSIONS Paper is one of the most important human inventions with broad applications because of its unique properties, including stability under ambient environment. We have studied newspaper color changes (yellowing) upon exposure to UV in the presence of oxygen at different concentrations. The yellowing process arises from the oxidation of lignin under UV irradiation, which produces yellow quinones. The rate of the yellowing process correlates with oxygen concentration, allowing for oxygen sensing with newspaper. In addition to yellowing, we have studied fluorescence emission from the newspaper, and found that it decreases with time with a rate depending on oxygen concentration. We attribute the decrease of the fluorescence emission to the degradation of lignin due to oxidation. Both the color and fluorescence signals allow oxygen sensing. To examine the suitability of the newspaperbased oxygen sensors for breath analysis, we have studied the yellowing and fluorescence degradation processes in humid air containing various interferents commonly found in breath, and the results show selective oxygen sensing in humid air. In addition to low cost and simplicity, the UV-activation of the newspaper sensor overcomes the instability issue of colorimetric sensors, which are reactive to ambient air and must be stored in oxygen-free environment.
ASSOCIATED CONTENT Supporting Information. Fluorescence spectra of newspapers and their responses to oxygen； Calibration curves of newspaper yellowing and fluorescence-changing rates vs. oxygen concentration； Estimation of lignin amount in newspaper； Diffusion rate of oxygen in paper； Sensor sensitivity and UV light power； UV light penetration into paper; Humidity effect.
AUTHOR INFORMATION Corresponding Author *E-mail:
[email protected] *E-mail:
[email protected] ORCID Jingjing Yu: 0000-0001-8985-8502
Notes The authors declare no competing financial interest.
ACKNOWLEDGMENT This project was supported by National Natural Science Foundation of China (NSFC, Grants 21327008, 21575062), and
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ACS Sensors the Natural Science Foundation of Jiangsu Province (BK20150574).
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