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. Author manuscript; available in PMC: 2022 Oct 15.
Published in final edited form as: Sens Actuators B Chem. 2021 Jul 6;345:130404. doi: 10.1016/j.snb.2021.130404

Hydrogel-incorporated Colorimetric Sensors with High Humidity Tolerance for Environmental Gases Sensing

Jingjing Yu 1, Francis Tsow 1, Sabrina Jimena Mora 1, Vishal Varun Tipparaju 1, Xiaojun Xian 1
PMCID: PMC8315352  NIHMSID: NIHMS1722499  PMID: 34326572

Abstract

Humidity interferes most gas sensors, especially colorimetric sensors. The conventional approaches to minimize the humidity interference in colorimetric gas sensing require using extra components, causing unwanted analytes loss, or limiting the choices of sensing probes to only hydrophobic ones. To explore the possibility of minimizing the humidity interference in a hydrophilic colorimetric sensing system, we have developed a hydrogel-incorporated approach to buffer the humidity influence on the colorimetric gas sensing. The hydrogel-incorporated colorimetric sensors show not only high humidity tolerance but also the improved analytical performance. The accuracy and reliability of the hydrogel-incorporated colorimetric sensors have also been validated in field tests. This hydrogel-incorporated approach will open up an avenue to implement hydrophilic recipes into colorimetric gas sensors and extend the application of colorimetric sensors to humid gases detection.

Keywords: Humidity interference, colorimetric sensor, hydrogel, gas sensor, environmental gases

1. Introduction

Water vapor is one of the most common interferants to gas sensors, requiring such sensors to have a high humidity tolerance. Some sensing applications are challenged by the concentration of atmospheric water vapor, which varies between ~10 ppm to 5%[1], Water vapor is saturated in human breath, challenging biomarker detection in that medium[2]. Humidity can interfere gas sensors physically or chemically, depending on the sensing mechanisms and sensing platforms[3], ranging from semiconductor metal oxide sensors[4, 5], electrochemical sensors[6, 7], mass-based sensors[8, 9], resistive sensors[10], to optical sensors[11, 12], includes fluorescence sensors[13] and colorimetric sensors[14, 15]. Colorimetric gas sensors detect the presence and concentrations of gaseous analytes through chemical reactions that are associated with color changes. Colorimetric sensors are particularly advantageous because they offer the combination of being low-cost, selective, and multiplexed that have great potential for detectors such as optoelectronic nose, air quality monitoring, personal exposure tracking, food quality assessment, hazardous chemicals detection, and breath analysis[15-24]. However, the challenge of humidity interference and variability limits real-world applications of colorimetric gas sensors.

The conventional approaches against humidity interference in colorimetric gas sensing include sample desiccation[25, 26], humidity balancing using Nafion tubing[27], signal compensation from humidity sensor[28], and using hydrophobic substrates[16]. Among these approaches, desiccates and Nafion tubing may cause analyte molecules adsorption and sensitivity reduction, while signal compensation only limits to the reactions where the humidity effect is predictable. Hydrophobic substrates are preferred candidates since they avoid the need of extra components and prevents unwanted analytes loss, which makes the sensor more compact, cost-effective, and easy-to-produce. The commonly used hydrophobic substrates in colorimetric sensors include reverse-phase silica thin-layer-chromatography plates[29, 30], polyethylene terephthalate film[31-33], and polyvinylidene fluoride membrane[34-36]. Though using hydrophobic substrate is proved to be simple and effective in dealing with humidity interference, this approach limits the options of choosing sensing probes. Typically, this approach requires the incorporation of hydrophobic sensing probes onto hydrophobic substrates at the cost of losing the opportunity to leverage hydrophilicity to augment sensing dimension or power. For example, some colorimetric chemical reactions can be accelerated by water vapor[15], which can not only increase the sensitivity but also reduce the response time of the sensor. If the humidity interference could be addressed by a hydrophilic sensing system, it will open up an avenue to implement hydrophilic recipes into colorimetric gas sensors and extend the applications.

To explore the possibility of minimizing the humidity interference in a hydrophilic colorimetric sensing system, we have introduced hydrogel, which can hold up to 90% of water, to serve as a matrix for loading the sensing probes and regulate the humidity of the microenvironment in the chemical sensing area. Hydrogel refers to hydrophilic gels that have 3-dimensional network of cross-linked polymer chains and water-swollen capabilities. The water-absorbing ability of hydrogels arises from the hydrophilic functional groups attached to the polymeric backbone[37]. According to the polymeric composition, hydrogels can be classified into homopolymeric hydrogels, copolymeric hydrogels, and multipolymer interpenetrating polymeric hydrogels. Because of their unique properties in water-swelling, mechanical performance, biocompatibility, and response to external stimuli, hydrogels have been widely applied in tissue engineering, drug delivery, biosensors, bioelectronics, molecules separation, and agriculture[37, 38].

The hydrogels have been previously used as transparent films/substrates in the colorimetric sensors to provide micro-liquid environment for analytes enrichment, sensitivity enhancement, and simulation of olfactory mucosa[39-42]. We here introduced hydrogel into colorimetric sensing systems as humidity buffer and regulator. We hypothesize that by keeping a stable water-rich system and humid microenvironment in the chemical sensing area, the colorimetric sensor will be less sensitive to humidity variation in the gas samples. We have engineered the structure of the sensing areas to study the effectiveness of the hydrogel to regulate the humidity in microenvironment. We have also fabricated the hydrogel-incorporated colorimetric sensors for detecting common environmental pollutants such as carbon monoxide (CO), hydrogen sulfide (H2S), and ozone (O3), and investigated their analytical performance under different humidity levels. Both bench test and field test results suggest the hydrogel-incorporated colorimetric sensors can reliably detect gas analytes in a broad humidity range. Our approach offers a promising method to introduce hydrophilic recipes into colorimetric gas sensors while minimizing the humidity interference.

2. Materials and methods

2.1. Materials

The chemicals used for preparing CO, O3, and H2S sensors were potassium disulfitopalladate (K2Pd(SO3)2, CAS Number: 68310-13-4), indigo carmine (CAS Number: 860-22-0), and copper sulfate pentahydrate (CuSO4·5H2O, CAS Number: 7758-99-8) respectively. Potassium disulfitopalladate was purchased from Alfa Aesar. Indigo carmine, copper sulfate, and agarose (CAS Number: 9012-36-6) were purchased from Sigma–Aldrich Co. Silica gel (with gypsum, 5-15 μm) was purchased from Sorbent Technologies, Inc. Ultrapure water (18 MΩ*cm) was produced via an ELGA Purelab Ultra RO system. All solutions were prepared with water as the solvent. The CO (1000 ppm, balanced with N2) and H2S (300 ppm, balanced with N2) were purchased from Gasco. Since the off-the-shelf CO and H2S gases were balanced with N2, we also used N2 to dilute these gases to prepared CO and H2S with different concentrations in Tedlar bags for sensor testing. O3 of different concentrations were directly produced by O3 generator (UVP Corp.) with synthetic air. To validate the concentrations of the prepared gases, the concentration of CO, H2S, O3 were confirmed with EXTECH CO10 carbon monoxide sensor, PAC-3500 H2S sensor, and O3 monitor (2B Technologies, Inc.), respectively.

2.2. Colorimetric sensor chip fabrication

The 3x3 colorimetric sensor array (Fig. 1a) was fabricated on 1/16” thick clear acrylic substrate (Laser Alliance LLC). The acrylic substrate was cut with a laser cutter (Universal Laser Systems, Inc.) to form 3x3 square array. Each square had a dimension of 1.8 mm (length) x 1.8 mm (width) x 1.6 mm (depth) and the distance between two wells was 2.5 mm (center to center). 8 μL CO sensing probe solution (8 mg/mL) with 6.25% silica was cast into the square in the center of the array. After the solution was dry (dry in the ambient air first, then vacuum dry for 2 h), 5 μL 80 °C 1% agarose was cast into the surrounding squares and cooled down to form gel.

Fig. 1.

Fig. 1.

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Effectiveness of hydrogel for regulating the humidity level in microenvironment for colorimetric detection of CO. (a) Schematic and images of a 3x3 CO sensor arrays without and with hydrogel and their corresponding color changes under 250 ppm dry and humid CO exposure for 2 min. (b) The absorbance change of 3x3 CO sensor arrays without and with hydrogel to 50 ppm CO under different humidity levels. (c) The dependence of sensor response to 50 ppm dry CO (5% RH) to the number of surrounding hydrogel-coated areas. (d) The performance of 3x3 CO sensor array with and without surrounding hydrogels for dry (5% RH) and humid (95% RH) CO detection. The error bars correspond to the standard deviations of three replicate measurements of three sensor chips.

The hydrogel-incorporated sensor chip (Fig. 2) was prepared by drop-casting method. Firstly, 10 μL 12.5% silica was drop-casted on the pre-defined sensing area on a plastic substrate and dried in ambient air. The plastic substrate is made of polyethylene (PE) and the pre-defined area is a 4 mm x 4 mm square with a depth of 1mm. Then the sensing probe solution was drop-casted on silica and dried in ambient air. Finally, the sensor was put in the vacuum chamber to dry for 2 h and sealed with black MylarFoil bags. For sensor covered with hydrogel, 10 μL 80 °C 1% agarose solution was drop-casted upon the sensor probe-silica layer. The hydrogel was cooled down to gel form before use.

Fig. 2.

Fig. 2.

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Hydrogel-incorporated colorimetric CO sensor. (a) Schematic of the hydrogel-incorporated colorimetric sensor chip. (b) Schematic of sensing setup. (c) Color changes of the CO sensor chips with and without hydrogel coating for 500 ppm dry (~5% RH) and humid (95% RH) CO gas detection (5 min exposure time). Absorbance changes of CO sensor chips with (d) and without (e) hydrogel coating for 500 ppm dry (~5% RH) and humid (95% RH) CO gas detection (120 s purging time and 30 s CO injection time).

The sensing solution for CO was prepared by dissolving 80 mg K2Pd(SO3)2 into 10 mL hot water. 0.2 g/mL CuSO4 in water was used for H2S detection. The sensing probe solution for O3 was prepared by dissolving 9 mg indigo carmine into 4 mL water. The volume of sensing probe solutions drop-casted on CO, H2S, and O3 sensor chips were 20 μL, 5 μL, and 5 μL respectively.

2.3. Experimental setup and methods

Sensors were operated at a constant lab temperature of around 25 °C. And relative humidity of gas was set and measured at around 25 °C. A humidity sensor (Sensirion SHT3x_85) was used to monitor the temperature and humidity of gas. Schematic of sensing platform was shown on Fig. S1.

The sensor chips with 3x3 colorimetric sensor array (Fig. 1a) were tested in a homemade optical detection chamber in transmission mode. In this detection chamber, a white LED (part number 897-1070-1-ND, Seoul Semiconductor Inc.) was used as the light source and a webcam (Logitech C520) was used as the photodetector. The webcam signal from green channel was controlled and recorded by Matlab and the sensing area was selected as the region of interest (ROI) for analysis. The inner space of this detection chamber had a volume of ~ 1 mL. This detection chamber had a gas inlet and gas outlet. A flow rate of 0.75 L/min was chosen for gas delivery during detection.

The performance of the hydrogel-incorporated sensor chips (Fig. 2b) was tested on a home-made optical detection chamber in transmission mode. The detection chamber used LED as the light source (wavelength: 625 nm, LED red clear 3528 SMD from Würth Elektronik, Part Number: 150141RS73100) and photodiode (Photodiode 780 TO 1050 NM from Vishay Semiconductor Opto Division, Part Number: VBPW34S) as the photodetector. The detection chamber also had a gas inlet and gas outlet for gas introduction and discharge. The inner space of the detection chamber had a volume of ~1 mL. The detection chamber was connected to a home-made circuit for receiving and processing the optical signal (intensity of transmission light) from the photodiode. Then the optical data was wirelessly transmitted to an external tablet via a Bluetooth Low Energy chip. Gas sample was delivered into gas chamber at 0.75 L/min.

3. Results and discussion

3.1. Effectiveness of Hydrogel for Regulating the Humidity Level in Microenvironment

CO is a well-known toxic, colorless, and odorless gas that can cause poisoning and even death. It is released mainly from incomplete oxidation during combustion, in which water is intrinsically produced alone with CO when burning fossil fuels such as petroleum, coal, and natural gas. Thus, humidity tolerance is one of the key requirements for CO sensor. To evaluate the effectiveness of using hydrogel to regulate the humidity level in microenvironment, a 3x3 sensor array was designed and fabricated on the sensor chip to study the influence of hydrogel spots to their adjacent sensing area (Fig. 1a). Since hydrogel is rich in water (water can take up to 99% of the total hydrogel volume), the water vapor released from the hydrogel spots on the sensor chip could humidify the surrounding microenvironment. To quantify this humidifying effect on colorimetric sensing, we selected a humidity-sensitive colorimetric reaction, (K2Pd(SO3)2), where the sensing probes selectively reacted with CO turning from yellow to black. Our previous work found that water molecules can accelerate the reaction between K2Pd(SO3)2 and CO[15]. We expected the water molecules released from the adjacent hydrogel area could make this colorimetric reaction rate tolerant to various humidity levels of the CO gas. The color changes of the sensors with or without the surrounding hydrogel areas to 250 ppm dry (5% RH) and humid (95% RH) CO are shown in Fig. 1a. The sensors with surrounding hydrogel showed similar color changes to dry and humid CO, while the color changes of sensors without surrounding hydrogel was highly dependent on the humidity level of CO. We then used a webcam-based Matlab software to record the real-time absorbance change of the sensor chips (Fig. 1b) for quantitative analysis. Nitrogen (N2) was introduced to the system to establish the baseline and then CO was injected to trigger colorimetric sensing. When the sensor was exposed to CO gas, the absorbance increased sharply and linearly. The slope of this linear curve under CO exposure was defined as the absorbance change rate and used to evaluate the sensitivity of the colorimetric sensors. The absorbance change rates of the sensors to 50 ppm dry CO were plotted along with the number of hydrogel coated areas around the CO sensing area, as shown in Fig. 1c. Clearly, the more hydrogel areas surrounding the CO sensing area, the higher the sensor response, suggesting the hydrogel can effectively regulate the humidity level in microenvironment. As shown in Fig. 1d, CO sensors without hydrogel surrounding showed great humidity dependence, while the CO sensors with eight hydrogel surrounding areas had almost no humidity dependence. The ability of hydrogel for humidity regulation in microenvironment can make the CO sensor immune to humidity change in the CO gas samples.

3.2. Humidity Tolerance of Hydrogel-incorporated Colorimetric CO Sensor

Since the hydrogel and the CO sensing probe are compatible, we incorporated them together in the same area to simplify the sensor chip fabrication process. We fabricated agarose hydrogel-incorporated colorimetric CO sensors (Fig. 2a) and evaluated their sensing performance on a home-made optical detection chamber in transmission mode (Fig. 2b). Agarose hydrogel was selected because its simple preparation procedure made it compatible with the colorimetric sensing probe (K2Pd(SO3)2) combination. We compared the color change of the sensor chips with and without the hydrogel layer for dry (5% RH) and humid (95% RH) CO gas detection (Fig. 2c). After exposed to the same concentration of CO gas (500 ppm), the sensor chip without hydrogel layer is much darker for humid than dry CO. We also did the same test on sensor chips with hydrogel layer and the color changes are nearly the same for both dry and humid CO, indicating the hydrogel-incorporated sensor chips have high tolerance to humidity change. Besides the visual comparison, we also recorded and compared the absorbance changes of sensor chips with and without the hydrogel layer for low- and high-humidity CO gas sensing. In the CO sensing tests, nitrogen was used as the purging gas to establish the baselines (120 s) before CO gas was delivered to the system (30 s) for colorimetric detection. As shown in Fig. 2d, sensor chips with hydrogel coating show similar absorbance change rates (slopes of the absorbance over time during CO gas injection, Fig. S2a) for 5% RH and 95% RH CO gases detection (red and black curves). In contrast, the absorbance change rates of sensor chips without hydrogel coating (Fig. S2b) vary significantly for 5% RH and 95% RH CO gases detection (blue and green curves in Fig. 2e). Moreover, the sensor chips with hydrogel coating demonstrate stable and consistent absorbance baselines during N2 injection compared to the sensor chips without hydrogel coating (Figs. 2d, 2e, and S3), which also suggests the importance of hydrogel in promoting the humidity tolerance of sensor chip.

The sensitivity of the hydrogel-incorporated colorimetric CO sensor chip is evaluated under different humidity levels. As shown in Figs. 3a and 3b, at a consistent humidity level, the absorbance change rate of the sensor chip increases with the concentration of CO, regardless of whether there is hydrogel coating on the sensor chips or not. However, for the sensor chips without hydrogel coating, the calibration plots vary with the humidity level (Fig. 3a), meaning the sensor response depends not only on CO concentration but also the humidity level of the gas; while the calibration plots are almost the same for sensor chips with hydrogel coating under different humidity levels (Fig. 3b). These results suggest that the hydrogel-incorporated colorimetric CO sensor chips can effectively reduce the humidity influence on CO sensing, which is critical for deploying the sensors for real-world applications where humidity always varies. As for the same concentration of CO detection, the response from sensor chips with hydrogel coating is higher than the response from sensor chip without hydrogel coating at low humidity level (5% RH) but obviously lower than that at high humidity level (95% RH). We speculate this may be ascribed to the competition of two processes: on one side, water can accelerate the colorimetric reaction, as shown in Fig. 2c; on the other side, an additional diffusion process is created for the CO molecules, which need to pass through the hydrogel layer before encountering the sensing probe (Fig. S4)[43]. Nevertheless, the moderate sensitivity of hydrogel-incorporated colorimetric CO sensor chips still allows CO detection at the ppm level, as shown in Fig. 3b. Hydrogel can dehydrate over time, which may affect the lifetime and sensitivity of the hydrogel-incorporated colorimetric CO sensor. We investigated the swelling ratio (Fig. S10) and influence of water content of the agarose hydrogel on the sensitivity of the sensors (Fig. S8). We found the swelling ratio of agarose can be as high as 450%. The gas flow rate and the humidity level of the gas significantly influence the dehydration rate of the hydrogel. The moving air can dehydrate sensor chip more quickly than still air. The lifetime of the sensor chip were 8 min, 18 min, and 135 min with the humidity of the purging gas of 5%, 55%, and 95% RH, respectively (gas flow rate: 0.75 L/min). The sensitivity of hydrogel-incorporated sensor was quite stable under different relative water content in the range of 45-100%.

Fig. 3.

Fig. 3.

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Sensitivity of CO sensor chips without (a) and with (b) hydrogel coating under different humidity levels. Reproducibility of CO sensor chips without (c) and with (d) hydrogel coating under different humidity levels. The error bars correspond to the standard deviations of three replicate measurements of three sensor chips. The hydrogel layer used here is agarose containing 99% of water.

Besides the sensitivity, we also assessed the reproducibility of the CO sensor chips under different humidity levels. The response of sensor chips without hydrogel coating to a given concentration of CO changes with the humidity and therefore will not allow the distinction between 150 ppm and 500 ppm CO if the humidity level is unknown, as shown in Fig. 3c. While the hydrogel-incorporated sensor chips show reproducible response to a given concentration of CO (150 ppm or 500 ppm) under different humidity levels, as shown in Fig. 3d.

3.3. Influence of Hydrogel to the Stability of CO Sensor

It can be imagined that the hydrogel may gradually lose water trapped in its matrix over time, which could reduce the shelf-life of the colorimetric sensor. To evaluate the sensor stability along with storage time, the sensitivities of sensor chips with and without hydrogel coating were tested and compared. After preparation, a batch of sensor chips were sealed in black MylarFoil bags and stored in oven at 25 °C;. The sensitivity (absorbance change rate) of the sensor chips were periodically tested for a month, as shown in Fig. 4. We normalized the sensitivities of the sensors with and without hydrogel in Fig. 4 for easy comparison. The sensitivity of sensor without hydrogel coating slightly decreased over time, which may be caused by the sensing probe degradation. On the contrary, the sensitivity of sensor with hydrogel coating was not noticeably affected by the storage time. This result suggests that when the hydrogel-incorporated colorimetric sensor is packaged in a well-sealed bag, its structure can be preserved and the shelf-life and stability of the sensor will not be compromised. It should be mentioned that colorimetric sensors are typically for one-time-use only. The hydrogel approach is also a one-time-use method, since water loss could happen after the sensor has been exposed. This one-time-use nature is compatible with the colorimetric sensor design.

Fig. 4.

Fig. 4.

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Stability of the hydrogel-incorporated colorimetric CO sensor and sensor without hydrogel. The sensor chips were sealed in Black MylarFoil bags and stored in oven at 25 °C. The error bars correspond to the standard deviations of three replicate measurements of three sensor chips.

3.4. Influence of Flow Rate on Hydrogel-incorporated Colorimetric CO Sensor

Flow rate is one of the most important parameters that can affect the sensitivity of colorimetric sensors. The color change of the sensor should only come from the chemical reaction between the analyte and the sensing probes. But the speed of the color change, or in other words, the chemical reaction rate between the analyte and the sensing probes, is also influenced by the mass transportation. We investigated the sensor performance at different flow rates. As shown in Fig. 5, the absorbance changes of sensor chips without the hydrogel coating for 500 ppm CO (75% RH) were strongly influenced by the flow rate of gas delivered to the detection chamber. However, the sensor chips with hydrogel coating showed very consistent response for 500 ppm CO (75% RH) without a noticeable flow rate dependence in the range of 0-2.5 L/min. We speculate this could be due to the presence of hydrogel in the sensing probes changes the gas-solid phase reaction in a conventional colorimetric sensing system to a gas-liquid phase reaction (Fig. S4, S5). In gas-solid phase reaction (Fig. 5a), only the active sites on the surface of the sensing probes can react with analyte molecules so the chemical reaction rate is easily influenced by the mass transportation process in the gas phase. However, in the gas-liquid phase reaction (Fig. 5b), all sensing probes can diffuse to the hydrogel layer and react with the analyte molecules. Considering the diffusion in the liquid phase is slower than the gas phase, the chemical reaction rate is dominated by the mass transportation process of gas analyte in hydrogel matrix, which is a relatively stable process. The insensitivity of the hydrogel-incorporated colorimetric CO sensor to flow rate is very helpful for sensor precision and reproducibility.

Fig. 5.

Fig. 5.

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Influence of Flow rate on CO Sensor chips without (a) and with (b) hydrogel coating. The error bars correspond to the standard deviations of three replicate measurements of three sensor chips.

3.5. Comparison of the balance gases: N2 vs. air

Considering the CO detection in real-world applications is in ambient air, we have also tested the hydrogel-incorporated CO sensor with air-balanced CO samples, as shown in Fig. 6. We have found that the hydrogel-incorporated CO sensor shows almost identical sensing behaviors to N2-balanced and air-balanced CO samples. This is not surprising since the colorimetric sensing principle of the CO sensor is based on the selective reduction of K2Pd(SO3)2 into Pd by CO. Major chemical components in the air, such as O2 (~21%) will not interference this specific colorimetric reaction. Similar results have also been observed on the H2S and O3 colorimetric sensors (Figs. S7a and S7b).

Fig. 6.

Fig. 6.

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Influence of N2 vs. air as balance gas on hydrogel-incorporated CO sensors. Purging gases (white periods in the figure, nitrogen or air) and sampling gases (gray periods in the figure, CO in balance gas) were delivered to sensor alternately. The humidity of gas was around 95 %RH.

3.6. Validate the Performance of Hydrogel-incorporated Colorimetric CO Sensor in Field Tests

We chose a common combustion-related application scenario, monitoring CO level in car exhaust, to validate the performance of the hydrogel-incorporated colorimetric CO sensor. CO concentration in the car exhaust (generated by Pontiac g6 2008, at the engine idle speed of ~700 RPM) was monitored by both our hydrogel-incorporated colorimetric CO sensor and a commercial electrochemical CO monitor (EXTECH CO10) simultaneously. Reference temperature and humidity sensors were also used to track the temperature and humidity changes during the test. The configuration of the test setup was illustrated in Fig. 7a. The concentration of CO in the sampled air could be changed by adjusting the distance of sampling inlet to the car tailpipe (Fig. 7b). As shown in Fig. 7c, the black curve showed real-time absorbance of the hydrogel-incorporated colorimetric CO sensor during the continuous CO monitoring. The red and blue curves showed the humidity and temperature changes during the test, respectively. The humidity and temperature sensors were connected closely to the colorimetric sensor in order to monitor the temperature and humidity of gas delivered to colorimetric sensor, as shown on Fig. 7a. Though the temperature of the exhaust gas emitted from the car tailpipe was high, it cooled down to room temperature after the gas passing through the gas sampling system (long tubing and pump). According to our measurement, the temperature of the exhaust gas was around 26□ and the humidity was 95% RH at the testing point (Fig. 7a) during the 450s measurement time, as shown on Fig. 7c. The real-time CO concentration was calculated by applying the calibration factors from the calibration plot (Fig. 3b) to the real-time absorbance change recorded by the hydrogel-incorporated colorimetric CO sensor. The CO concentrations measured by hydrogel-incorporated colorimetric CO sensor (black curve) and commercial electrochemical CO monitor (red curve) are plotted in Fig. 7d for comparison. The profiles of CO concentrations measured by these two sensors were highly correlated with each other, and even detailed features of the CO profiles matched very well. The field test results not only validated the accuracy of the hydrogel-incorporated colorimetric CO sensor for reliable CO monitoring in highly humid environment, but also demonstrated the compatibility of these sensors for real-world applications.

Fig. 7.

Fig. 7.

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Hydrogel-incorporated colorimetric CO sensor for continuous car exhaust monitoring. (a) Schematic of test setup. (b) Changing CO concentration in the gas sample by adjusting the distance between the sampling inlet and the car tailpipe. (c) Real-time absorbance (black curve), humidity (red curve) and temperature (blue curve) during the field test. (d) Profiles of CO concentrations measured by hydrogel-incorporated colorimetric CO sensor (black curve) and commercial electrochemical CO monitor (red curve).

4. Conclusions

By introducing water-rich hydrogel into colorimetric sensors, the humidity of the microenvironment in the chemical sensing area could be effectively regulated. The water molecules released from the surrounding hydrogel areas make the response of the colorimetric sensor independent of the humidity level of the gas sample. Through the combination of agarose and potassium disulfitopalladate (K2Pd(SO3)2), we have fabricated a hydrogel-incorporated colorimetric CO sensor. We have systematically evaluated the influences of humidity variation on the sensitivity, reproducibility, and stability of the hydrogel-incorporated colorimetric CO sensor. Test results suggest that while the humidity tolerance of sensor has been greatly enhanced, the key analytical performance of the sensor still remains or even improved. Further tests indicate that the presence of hydrogel in the colorimetric sensor changes the gas-solid phase reaction in a conventional colorimetric sensing system to a gas-liquid phase reaction, which makes the sensor insensitive to the gas flow rate and thus improves the sensing precision and reproducibility. The accuracy of the hydrogel-incorporated colorimetric CO sensor has also been validated against a commercial electrochemical CO monitor in monitoring CO concentration in car exhaust. The CO profile tracked by the hydrogel-incorporated colorimetric CO sensor highly agrees with the CO profile from reference CO sensor, demonstrating its capability for real-world applications. Hydrogel could be instable under dry, hot or very cold environments. This drawback can be addressed by designing the colorimetric sensors to be one-time-use only and shortening the detection time by applying proper gas sampling system. We believe this hydrogel approach will open up an avenue to implement hydrophilic recipes into colorimetric gas sensors and extend the application of colorimetric sensors to humid gases detection. The hydrophilicity could add another dimension for the colorimetric sensing array to expand its sensing power.

Supplementary Material

1

Highlights.

  • A complementary method to minimize humidity interference of colorimetric gas sensor with the use of hydrophilic sensing system other than conventional hydrophobic system.

  • Water-rich hydrogel incorporated colorimetric gas sensor with high humidity tolerance.

  • Real-time detection of environmental gases in high humid conditions.

Acknowledgments

This work was supported by National Institutes of Health (NIH) (Grant Number 5R44ES029006-03).

Biographies

Author Biographies

Jingjing Yu is a postdoctoral research scholar of the Center for Bioelectronics and Biosensors, the Biodesign Institute, at Arizona State University. She received her Ph.D. in analytical chemistry from Nanjing University, China. Her current research interests focus on environmental air quality monitor and mobile health device.

Francis Tsow is affiliated with the Center for Bioelectronics and Biosensors, the Biodesign Institute, at Arizona State University. He received his doctoral degree in electrical engineering with a focus on sensor science. His research interests include sensor and engineering technologies, and is interested in combining artificial intelligence with sensing science.

Sabrina Jimena Mora received her Ph.D. in chemistry from the National University of Rio Cuarto, Argentina. She is an assistant research scientist at the Center for Bioelectronics and Biosensors, at Arizona State University. Her research interest is focused on the development of biosensors and aerosol barrier systems.

Vishal Varun Tipparaju is affiliated with the Center for Bioelectronics and Biosensors at the Biodesign Institute, Tempe, AZ. He received his Ph.D. in electrical engineering from Arizona State University. His research interests include sensing technologies, wearable systems, and their applications in physiological & pervasive sensing.

Xiaojun Xian is an associate research professor in the Biodesign Institute at Arizona State University. His research interest ranges from exploring advanced sensing materials, developing wearable sensing technologies, to building integrated wearable health care system.

Footnotes

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Declaration of Competing Interest

The authors declare no competing financial interests.

Declaration of interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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