Colorimetric Humidity Sensors Based on Electrospun Polyamide/CoCl2 Nanofibrous Membranes
© The Author(s). 2017
Received: 12 April 2017
Accepted: 11 May 2017
Published: 19 May 2017
Humidity indicators based on composite polyamide 66/cobalt chloride (PA66/CoCl2) nanofibrous membranes (NFMs) were successfully fabricated by electrospinning. A series of NFMs with various weight percentage of CoCl2 to PA66 were prepared, and their humidity sensitivity based on color changing and quartz crystal microbalance (QCM) were studied. Due to the color change property of cobalt chloride, the as-spun composite NFMs show obviously macroscopic color change from blue to pink as relative humidity (RH) increasing from 12.4 to 97.2%. Moreover, the QCM detection showed a linear dependence on the RH changing and exhibited short response/recovery time (less than 65.4 s/11 s), small hysteresis (less than 11%), good reproducibility, and stability. Owing to the above double sensitive mechanism on RH, the PA66/CoCl2 composite NFM may show great potential applications from meticulous to coarse.
KeywordsColorimetric humidity sensor Electrospinning PA66/CoCl2 nanofibrous membranes
Relative humidity (RH) sensor is mainly used for monitoring atmospheric humidity environment and shows important applications in warehousing , environmental monitoring , instruments and meters, and meteorology . Until now, various types of humidity sensors have been reported, such as resistance type , capacitor type , field-effect-transistor type , optical type [7, 8], and so on. Among these types of sensors, the optical type has attracted a lot of interests since it is supplying a change in optical properties, easily detectable with the naked eye (visual) which is suitable for applications in daily life [8–10].
It is now believed that RH sensors based on the electrospun nanofibrous membranes (NFMs) show improved sensor sensitivity due to their large surface area to volume ratio [11–13], providing an increased number of sites for analyte interaction or signal transduction [10, 14–17]. For example, electrospun polyamide 6 nano-fiber/net modified by polyethyleneimine was investigated as RH indicator detected by quartz crystal microbalance (QCM) . It shows high sensitivity and the response and recover times are 120 and 50 s, respectively, with RH changing from 2 to 35% . Moreover, ceramic LiCl-doped ZnO electrospun fibers were also fabricated as humidity sensor, with response time and recovery time about 3 and 6 s .
While the sensitivity of the above humidity sensors based on electrospun fibers were detected by precise instrument, [14, 15] it may do not work in practical application limits to conditions. Optical humidity sensors will be a possible solution. Since RH do not have measurable intrinsic optical properties, some intermediate agent will be introduced to show a change in optical property. At present, several intermediate agents have been adapted to realize colorimetric indicator for humidity including photonic crystal [8, 9, 18], polymer electrolyte thin films , doped cholesteric liquid crystal , and crystalline covalent organic framework nanofibers . In addition, cobalt chloride has been applied as colorimetric RH indicator for its color changes when contaminated by water . Typical colorimetric RH indicator such as silica gel self indicator and CoCl2-based optical humidity sensor [7, 22] have been presented. However, colorimetric humidity sensor based on electrospun fibers was rarely investigated.
In this work, we fabricated PA66/cobalt chloride NFMs by electrospinning and investigated its humidity sensing properties by extensive color change and both fine quartz crystal microbalance (QCM). The results showed that the electrospun PA66/cobalt chloride NFM sensors exhibiting high humidity sensitivity with obviously color change, rapid response/recovery performance, small hysteresis, excellent reproducibility, and good stability.
Preparation of Polymer/Cobalt Chloride Solutions
The solutes were CoCl2·6H2O (Sinopharm Chemical Reagent Co., Ltd) and PA66 (Tianjin Heowns Biochem LLC., China). As a comparison, the concentrations of CoCl2·6H2O in these solutes were 0 (pure PA66), 10, 30, and 50 wt%, respectively. Dissolved these solutes in formic acid at 12 wt% and stirred thoroughly for 6 h at room temperature, then the uniform precursor solutions were obtained.
Fabrication of Colorimetric Nanofiber Membranes for Humidity Detection
Fabrication of Sensing Membranes on QCM
To further investigate the humidity sensitivity of the as-spun NFM, we also selected QCM for precision measurement. The process of fabricating sensing NFMs on the QCM (CHI400C, Shanghai Huachen Instruments Co., Ltd., China) chip is showed in Fig. 1b. Under the above electrospinning conditions, PA66/cobalt chloride NFMs were deposited onto the surface of the QCM chip, and then the humidity sensitivity can be measured.
The photographs of the colorimetric NFMs were recorded by a digital camera (DSC-TX9C 50i). Absorption spectra were recorded by a UV-vis spectrometer (U-4100, Hitachi) under absorption mode. The morphologies and structures of the NFMs were characterized by a scanning electron microscopy (SEM, EVO MA 10/LS 10, CARL ZEISS Co., Ltd., Germany) and a transmission electron microscope (TEM, JEM-2100PLUS, JEOL Ltd., Japan). Fourier transform infrared (FT-IR) spectra were recorded on a Thermo Scientific Nicolet In10 spectrometer. Means for humidity control are saturated salt solution humidity bottles, which have a stable humidity environments of 12.4% RH (Saturated LiCl solution, in 20 °C), 33.6% RH (Saturated MgCl2 solution, in 20 °C), 55.2% RH (Saturated Na2Cr2O7·2H2O solution, in 20 °C), 75.5% RH (Saturated NaCl solution, in 20 °C), and 97.2% RH (Saturated K2SO4 solution, in 20 °C), respectively.
Results and Discussion
Colorimetric Property of the As-Spun NFM upon Exposure to Humidity
In order to ensure the colors of PA66/cobalt chloride NFM exposed to different relative humidity conditions, visible absorption spectra (380–780 nm) of the 50 wt% CoCl2·6H2O-doped NFM following exposure to variable RH were examined as displayed in Fig. 2b. It was found that the absorbance of the hybrid NFM lying in the range of 410–550 nm corresponds to the spectral characteristics of CoCl2·6H2O . Similarly, there is a weak absorption bands located at 580–750 nm (yellow and red light) with absorbance peak at ~670 nm, which may be caused by a small amount of residual anhydrous cobalt chloride inside the fibers. As the humidity decreases, the absorption bands of 580–750 nm are gradually sharpening, which correspond to the pink faded and turn to blue by degrees. Until 12.4% RH, the absorbing peak from 410 to 550 nm of the NFM disappears, and the absorption peak of 710 nm reaches a maximum value, which corresponds to the spectral characteristics of anhydrous CoCl2 . It suggests that in different RH, the PA66/CoCl2·6H2O NFM shows different colors, indicating the composite NFM have potential application in humidity visualization hygrometer.
Humidity Sensing Characteristics by QCM
Sensors based on QCM technique has been attracted a lot of interests due to its advantages of high sensitivity and reliability [14, 23, 24]. The operating principle of these sensors is primarily associated with the adsorption of the water molecules on the surface of the QCM electrode, inducing variation in the molar response of a quartz crystal, which lead to the resonance frequency shift [14, 25]. Consequently, we also selected QCM for further humidity sensitivity measurement.
Repeatability and Humidity Hysteresis Characteristic
Sensor hysteresis is examined to obtain some information about the reliability [14, 34]. Figure 4d shows the PA66/CoCl2·6H2O (50 wt%) NFM sensors’ humidity hysteresis characteristic. The black line in the figure was measured from low RH to high RH (from 12.4 to 97.2%), i.e., for the adsorption process, and the red line represents desorption process (measured in the opposite direction). The curves between adsorption process and desorption process have good coincidence for the as-spun sensor. The maximum humidity hysteresis was 9.9% (at about 75.5% RH) for PA66/CoCl2·6H2O NFM sensor with concentration of 50 wt% CoCl2·6H2O, indicating a good reliability of these humidity sensors, and the other humidity hysteresis curves of electrospun PA66/CoCl2·6H2O NFM with different CoCl2·6H2O concentration were shown in Additional file 1: Figure S2. Moreover, the stability of the sensor was also examined showing in Additional file 1: Figure S3.
Morphology of the As-Spun Membranes
FT-IR Spectra and Humidity Sensing Mechanism
The humidity sensitive characteristics of the hybrid PA66/cobalt chloride NFMs mainly come from the cobalt chloride. When exposed to dry atmosphere, the cobalt chloride loses its crystalline water, conversely, in a wet environment it absorbs water molecules and turns into hydrated cobalt chloride. In this process, the color of PA66/cobalt chloride NFM changes. Moreover, in the process of dehydration and hygroscopic, the mass of the NFMs also changes, which leads to an according change of resonant frequency. In addition, it is noted that the PA66 has very small contribution to the mass change because the pure PA66 NMF can only cause a small frequency shift when RH changes.
In summary, hybrid PA66/cobalt chloride humidity sensitive colorimetric nanofibrous membranes have been fabricated successfully by electrospinning. The influences of CoCl2 concentrations on morphology, color, and humidity sensitivity of the NFMs have also been systematically studied. The hybrid membranes exhibit different colors in different humidity conditions, indicating that PA66/cobalt chloride NFMs have promising application in visual hygrometer. Furthermore, PA66/cobalt chloride NFMs deposited on QCM also exhibit interesting humidity sensing properties. Since the nanofibrous structure may increase the sensing area and surface activity, the QCM-based humidity sensor shows high sensitivity, fast response, and recovery time, good reproducibility in moisture-sensitive and longtime stability in a stable humidity environment with a little frequency shift. These results indicate that the PA66/cobalt chloride NFM sensor has potential applications in humidity detection.
This work was supported by the National Natural Science Foundation of China (51373082 and 51673103), the Taishan Scholars Programme of Shandong Province, China (ts20120528), the Shandong provincial key research and development plan (2016GGX102011), the Shandong Provincial Natural Science Foundation, China (ZR2016EMB09), and the Research Fund for the Doctoral Program of Higher Education of China (20133706110004).
MHY, XY, and YZL designed the experiments. MHY, XY, and XXH prepared the colorful NFMs. MHY, XY, and XXW collected and analyzed the data of the scanning electron microscopy, transmission electron microscope, and color measurement. MHY and JZ collected and analyzed the QCM humidity characteristics. JZ, MY, and XN analyzed the IR spectra data and absorption spectra. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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