1. Introduction

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 [1], solid state electrochemical sensor devices [2], 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 [18]. 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 [19]. 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.

2. Experimental

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) [20]. 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).

 

Fig. 1. (a) CIS/ZnS QDs fluorescence image (b) TEM image showing CIS/ZnS QDs at a resolution of 20 nm, and (c) EDX analysis results for CIS/ZnS QDs.

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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.

 

Fig. 2. Construction of optical fiber carbon dioxide sensor.

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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).

 

Fig. 3. Schematic diagram of the experimental setup for the optical fiber carbon dioxide sensor system.

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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.

 

Fig. 4. Emission spectra of optical fiber carbon dioxide sensor under various CO₂ concentrations.

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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.

 

Fig. 5. Variation of I/I₀ with 0-100% CO₂ concentration.

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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).

 

Fig. 6. (a) Normalized luminescence intensity under different CO₂ concentration and (b) variation of wavelength with CO₂ concentration.

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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.

 

Fig. 7. Photostability of optical fiber carbon dioxide sensor.

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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.

Table 1. Comparison of sensing characteristics of current sensor and representative colorimetric change CO₂ sensors presented in the literature.

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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.

 

Fig. 8. Response time of the optical fiber carbon dioxide sensor when switching between 100% N₂ and 100% CO₂.

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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.

 

Fig. 9. Emission spectrum of optical fiber CO₂ sensor in the air at four different temperatures.

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4. Conclusion

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.