This paper presents a new optical fiber carbon dioxide (CO2) sensor based on the emission wavelength shift of CuInS2/ZnS quantum dots (CIS/ZnS QDs) due to changes in the absorption of a pH indicator (α-naphtholphthalein) with a changing CO2 concentration. Using an LED with a central wavelength of 375 nm as the excitation source, it is shown that using the red emission of CIS/ZnS QDs allows for the detection of CO2 concentration over the range of 0-100%, and the associated wavelength shift was found to be 630 pm/%CO2. Moreover, the observed luminescence intensity from CIS/ZnS QDs at 578 nm increases as the CO2 concentration increases. The observed luminescence intensity change by CO2 is expressed as I100/I0, which I0 is used to represent the sensitivity of the optical fiber sensor, where I100 and I0 represent the steady-state luminescence intensities in pure carbon dioxide and pure nitrogen environments, respectively. The ratio I100/I0 of this optical fiber sensor is estimated to be 100. These results of the optical sensing method can be used in practice for detection of CO2 and could offer a new approach for developing novel types of optical fiber sensor.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Carbon dioxide (CO2) sensing techniques in the atmosphere are of fundamental importantce in various fields, such as chemical, clinical analysis and environmental monitoring. CO2 sensing techniques have been developed using infrared (IR) absorptiometry , solid state electrochemical sensor devices , and optical sensors [3–9]. CO2 sensing techniques based on IR absorptiometry have some drawbacks, as they are subject to strong interference from water vapor and are very costly. On the other hand, solid-state electrochemical sensors are affected by electromagnetic fields. Recently, optical CO2 sensors based on fluorescence intensity [3–9] or changes in absorption [10–15] indicated by pH sensors have been examined and have shown several advantages, including electrical isolation, reduced noise interference, and remote sensing. Optical CO2 sensors are usually based on the CO2-induced fluorescence intensity change of luminescence dye, such as 1-hydroxy-3,6,8-pyrenetrisulfonic acid trisodium salt (HPTS) [3–9], and the colorimetric change of pH-sensitive fluorescent dye, such as α-naphtholphthalein [10–13]. Although the technique used for optical CO2 sensors based on the fluorescence intensity changes of a fluorescent dye is a simple method, fluorescent dye is extremely limited. Therefore, optical CO2 sensors based on the colorimetric change of pH-sensitive fluorescent dyes has become more attractive in recent years.
In general, optical CO2 sensors with the combination of a colorimetric change of a pH-sensitive fluorescent dye and an internal reference dye have been developed considering the wavelength overlap between the emission wavelength of the internal reference dye and the absorption wavelength of the pH-sensitive fluorescent dye. There have been some previous reports on optical CO2 sensor colorimetric changes of pH-sensitive fluorescent dyes using various pH-sensitive indicators, such as α-naphtholphthalein, thymol blue, phenol red, and cresol red [10–15], with tetraphenylporphyrin (TPP) [10–13] and europium(III) complex [14,15] as internal reference dyes. Compared with porphyrin molecules, quantum dots (QDs) are suitable for use as internal reference dyes because they undergo the effects of quantum confinement while possessing good photostability, a large Stokes shift, a narrow full width at half maximum (FWHM), and immunity to electromagnetic interferences [16,17]. Recently, CuInS2/ZnS core/shell quantum dots (CIS/ZnS QDs) have been widely investigated and used in clinical trials. The CIS/ZnS QDs were fabricated by a facile hydrothermal method and the red-emission of CIS/ZnS QDs is useful and beneficial for optical sensor or device applications. Recently, Deng et al. synthesized high-quality CIS/ZnS QDs without using conventional toxic heavy metals. These CIS/ZnS QDs exhibited improved photoluminescence properties and can be used as a versatile fluorescent probe for biomedical imaging . In more recent work, Kim et al. synthesized highly luminescent colloidal CIS/ZnS QDs and applied them in an electroluminescent-type quantum dot light emitting diode (LED). The device with an optimal emissive layer thickness exhibited a peak luminance of 1564 cd/m2 and current efficiency of 2.52 cd/A . Therefore, CIS/ZnS QDs might be a promising candidate for optical imaging and sensing to be performed in this study.
The pH-sensitive dye (α-naphtholphthalein) has an absorption wavelength at 600 nm and the absorption intensity decreases as the CO2 concentration increases. On the other hand, the reference dye (CIS/ZnS quantum dots) has an emission wavelength around 578 nm and the emission intensity is not influenced by CO2. Therefore, α-naphtholphthalein and CIS/ZnS quantum dots can be used as the pH-sensitive dye and internal reference dye, respectively. In this work, we present a new optical foner CO2 sensor and provide its sensing properties based on the red-shift of the emission wavelength and colorimetric change of the CIS/ZnS quantum dots (QDs) due to the change in absorption of the pH-sensitive dye (α-naphtholphthalein) with a changing CO2 concentration. Utilizing a UV LED light source with a central wavelength of 375 nm, the emission wavelength shift and sensitivity characteristics of the optical fiber CO2 sensor are evaluated and compared with those of various optical CO2 sensors presented in literature based on colorimetric change.
Tributyl phosphate (TBP, ≥99%), α-naphtholphthalein, poly(isobutyl methacrylate) (polyIBM, MW∼130,000), copper(I) iodide (CuI, 99.999%), indium acetate (In(OAc)3, 99.99%), 1-dodecanethiol (DDT, ≥98%), and 1-octadecene (ODE, 90%) were all purchased from Sigma-Aldrich and used as received without any further purification. The solutions were prepared with deionized (DI) water.
CIS/ZnS QDs were synthesized using the hydrothermal method and a fluorescence image is shown in Fig. 1(a) . CuI (0.5 mmol), In(OAc)3 (0.5 mmol), and 10 mL DDT were mixed and put into a Teflon autoclave. The Teflon autoclave was heated at 180°C for 6 h. After the heating process, the Teflon autoclave was cooled to room temperature and CIS QDs were obtained. To obtain a ZnS shell coating, zinc stearate (2 mmol), 2 mL of DDT, 4 mL of ODE, and 6 mL of the untreated CIS QDs solution were mixed and put in the Teflon autoclave for another heating process at 200°C for 8 h. After this heating process, the Teflon autoclave was cooled to room temperature and CIS/ZnS QDs were obtained. The CIS/ZnS QDs were washed with DI water several times and mixed with EtOH/chloroform (2:1) to remove the unreacted reagents by centrifugation (7000 rpm, 10 min). Finally, the CIS/ZnS QDs were easily dispersible in a hydrophobic solution. Figure 1(b) shows a TEM image of the CIS/ZnS QDs at a resolution of 20 nm. Figure 1(c) presents the energy-dispersive X-ray spectroscopy (EDX) results for the composition of the CIS/ZnS QDs. The quantum dots are composed principally of Cu, Zn, S, and In elements, where the x-axis represents the energy (keV) and the y-axis represents the counts per second per electron (essentially the X-ray intensity).
The tetraoctylammonium hydroxide (TOAOH) phase transfer agent was prepared by stirring silver oxide (0.6 g) and tetraoctylammonium bromide (1.4 g) in methanol (10 mL) for 4 hours [3–5]. The resulting methanolic-free base was then decanted and stored in a refrigerator.
The CO2 sensing film had composition CIS/ZnS QDs/α-naphtholphthalein/tetraoctyl ammonium hydroxide (TOAOH)/polyIBM/tributyl phosphate/optical fiber as shown in Fig. 2. The pH dye solution was prepared by 3.7 mmol α-naphtholphthalein into 2 mL of tetraoctylammonium hydroxide in methanol solution (designated as solution “A”). The polymer matrix was prepared by 25 mg of polyIBM in 2.5 mL toluene (designated as solution “B”). The solution for CO2 sensing film consisted of 1.5 mL of solution A, 2.5 mL of solution B, 2 mL of CIS/ZnS QDs/Hexane solution and 0.5 mL of tributyl phosphate, and 7 mL of toluene.
The plastic optical fiber was cleaned by rinsing it with DI water and EtOH before the dip-coating process. The prepared CO2 sensing material with CIS/ZnS QDs/α-naphtholphthalein/tetraoctyl ammonium hydroxide (TOAOH)/polyIBM/tributyl phosphate was then deposited on the end of optical fiber end in a dip-coating process at a velocity of 0.25 mm/s. The sensing film in an average film thickness of 285 nm, as measured by ellipsometer. Finally, the coated plastic optical fiber was dried at room temperature for 24 hour and kept in the dark until use. The sensing properties of optical fiber carbon dioxide sensor are study using wavelength shift and fluorescence intensity changes. The emission light is irradiated from CIS/ZnS QDs and detected by the spectrometer.
Additionally, α-naphtholphthalein in polyIBM polymer matrix was used as the pH dependent acceptor because of its absorption spectrum overlaps the red emission fluorescence spectrum of CIS/ZnS QDs. When the CIS/ZnS QDs was excited by a UV LED light with a central wavelength at 375 nm, the red emission wavelength at 578 nm overlapping the absorbance of the basic form of α-naphtholphthalein paired with TOA that appeared at 578 nm. Consequently, increasing the carbon dioxide concentration, the luminescence intensity from the optical fiber CO2 sensor increased, allowing optical carbon dioxide sensing.
3. Results and discussion
Figure 3 illustrates the experimental setup used to characterize the performance of the optical fiber sensor for the detection of CO2. The optical fiber CO2 sensor was excited by an LED light source (Ocean Optics, Model LS-450, 375-nm wavelength) driven by a 10-kHz arbitrary waveform generator (Thurlby Thandar Instruments Ltd, Model TGA1240). The fiber-optic sensing system consisted of a plastic optical fiber (Fiber Diameter: 1000 µm) coated with the CIS/ZnS QDs/α-naphtholphthalein-doped polymer matrix on the fiber end. The relative luminescence intensity and wavelength were taken at a pressure of 101.3 kPa at room temperature using a USB4000 spectrometer (Ocean Optics, Inc.). Defined CO2 concentrations were adjusted by mixing CO2 and nitrogen using mass flow controllers (Aalborg Instruments & Controls Inc, Model GFC 17).
The room temperature change in the relative fluorescence spectrum of the CO2 sensing film when excited at 375 nm under different CO2 concentrations is shown in Fig. 4. The UV LED light source was irradiated from the optical fiber. As the absorption band around 600 nm of α-naphtholphthalein decreased with increasing CO2 concentrations, the observed luminescence intensity and wavelength from the CIS/ZnS QDs film layer at 578 nm increased. On the other hand, no change in the luminescence intensity and wavelength of the CIS/ZnS QDs immobilized in a polyIBM polymer matrix was observed by CO2, indicating that the fluorescence of CIS/ZnS QDs was not influenced by the presence of CO2. Thus, the sensing film containing the pH indicating α-naphtholphthalein and CIS/ZnS QDs embedded in a polyIBM polymer matrix may be applied as a new optical fiber CO2 sensor.
The observed CIS/ZnS QD fluorescence intensity change as influenced by the CO2 concentration is expressed as I100/I0. The ratio I100/I0 is used to represent the sensitivity of this optical fiber CO2 sensor, where I100 and I0 represent the steady-state luminescence intensities in the pure CO2 and pure nitrogen environments, respectively. Thus, the relationship between the observed emission intensity of the CO2 sensing film and CO2 concentration follows Eq. (1) [5–8]:
Figure 5 shows the plot of I/I0 versus the CO2 concentration. The solid line is the best fit based on Eq. (1) and indicates that the optical fiber CO2 sensor can be calibrated by Eq. (1). From inspection, it is found that the sensitivity (I100/I0) of this optical fiber CO2 sensor containing CIS/ZnS QDs and the pH indicator α-naphtholphthalein embedded in a polyIBM polymer matrix is estimated to be 100.
Figure 6(a) shows the variation of peak red emission wavelength with different CO2 concentrations. It can be seen that the peak of red emission wavelength increases as the CO2 concentration increase due to absorption change of pH indicator (α-naphtholphthalein) with carbon dioxide concentration. The CO2-dependent variation of wavelength at a peak of red emission is replotted in the form also shown in Fig. 6(b).
It has been shown that both the peak red emission wavelength and fluorescence intensity of CIS/ZS QDs are dependent on CO2 concentration ranging 0-100%. When the CO2 concentration is increased, the CIS/ZnS QD fluorescence intensity increases due to the changes in absorption of the pH indicator α-naphtholphthalein. The CIS/ZnS QD peak red emission wavelength, on the other hand, is shifted toward longer wavelengths as the CO2 concentration increases. The results show that the red emission of the CIS/ZnS QDs is capable of detecting CO2 ranging 0-100%, and the wavelength shift responding to changes in the CO2 concentration from 0% to 100% was 630 pm/%CO2. Either effect can be used to obtain CO2 concentration information. However, simple fluorescence intensity measurements are prone to error due to optical power fluctuations. Because wavelength shift is proportional to CO2 concentration and independent of the system optical power level, this method is preferable.
Figure 7 shows the photostability of proposed optical fiber CO2 sensor using CIS/ZnS QDs/α-naphtholphthalein/tetraoctyl ammonium hydroxide/polyIBM/tributyl phosphate sensing film. The optical fiber CO2 sensor was continuously illuminated using an LED light source with a 375 nm wavelength in air. After continuous illumination for around 1 hour, the relative fluorescence intensity of the CO2 sensing signal in the air was determined to be 39.6 ± 4.3.
The optical parameters of the colorimetric change sensing methods are summarized in Table 1. Extensive studies have been performed on the fluorescence intensity-based [10–15] CO2 sensing method. The currently available optical sensing methods utilize the fluorescence intensity changes of an internal reference dye due to the absorbance change of the pH indicator. A major limitation of the fluorescence intensity-based sensing method is that the signal output can be affected by factors such as the excitation source intensity, environmental conditions, sensor distribution, and instrumental efficiency. By contrast, the wavelength shift measurement performed in this study provides a more precise and sensitive evaluation of CO2 concentration changes. In this work, red emission CIS/ZnS QDs used as an internal reference dye were synthesized by a one-step hydrothermal method and their CO2 sensing properties based on the pH fluorescent dye α-naphtholphthalein and CIS/ZnS QDs embedded in a polyIBM polymer matrix were studied.
Figure 8 presents the results of an operational stability test conducted by alternating the atmosphere in the gas chamber between 100% N2 and 100% CO2 gas and monitoring the resulting changes in the detected relative fluorescence intensity. Note that in acquiring the measurement results, the integration time of the CCD spectrometer was set to 5 ms in order to track the dynamic behavior of the optical fiber CO2 sensor. From inspection, it can be seen that the response time of the optical fiber CO2 sensor (t90; the time required for a 90% change in the fluorescence intensity reading) is 23 s when switching from 100% N2 to 100% CO2 and 71 s when switching from 100% CO2 to 100% N2. Figure 8 shows that the optical fiber CO2 sensor provides stable, reproducible signals as the atmosphere is repeatedly changed between 100% N2 and 100% CO2. On the other hand, the response and recovery signal changes are fully reversible and hysteresis of the CO2 sensing film was very small and can be ignored during the measurements.
Figure 9 shows the photoluminescence spectra of optical CO2 sensors at various temperatures in air environment. It is observed that the fluorescence intensities of the optical CO2 sensor reduce by approximately 65%, as the temperature increases from 25.3°C to 54.8°C.
This study presented a new optical fiber sensor for CO2 based on the emission wavelength shift of CIS/ZnS quantum dots (QDs) due to changes in absorption of the pH indicator (α-naphtholphthalein) with the CO2 concentration. The red emission CIS/ZnS QDs were capable of detecting CO2 in the range 0-100% and the wavelength shift responding to changes in CO2 concentration the range 0-100% was found to be 630 pm/%CO2. Otherwise, the luminescence intensity of the CIS/ZnS QDs at 578 nm increased with increasing CO2 concentration. The sensor has a sensitivity of approximately 100 for CO2 concentrations in the range 0-100%. The sensitivity of the optical fiber CO2 sensor was quantified in terms of the ratio I100/I0, where I0 and I100 represent the detected fluorescence intensities in pure nitrogen and pure carbon dioxide environments, respectively. Overall, the experimental results demonstrated that the pH indicator (α-naphtholphthalein) and CIS/ZnS QDs embedded in a polyIBM polymer matrix possess both the wavelength shift and fluorescence intensity characteristics required for a CO2 sensor and therefore provide a feasible alternative to existing optical CO2 sensors.
Ministry of Science and Technology, Taiwan (MOST) (107-2221-E-131-016-MY2).
The authors gratefully acknowledge the financial support provided to this study by the Ministry of Science and Technology of Taiwan under Grant No. MOST 107-2221-E-131-016-MY2.
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