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Abstract
Ratiometric optical sensor gas sensing continues to develop optical sensing techniques and materials used in various industrial and environmental applications. This research focuses on a new ratiometric optical sensor using the development of new material of FAPbI3 perovskite QDs and a simple method to detect oxygen (O2) gas. FAPbI3 perovskite QDs are used as an indicator of oxygen gas, and rhodamine 110 (Rh 110) is a reference material in a ratiometric optical sensor. All of the sensing and reference materials are embedded in an ethyl cellulose (EC) matrix and coated on the surface of the filter paper. Using a UV LED with a central wavelength of 380 nm as the excitation light source, the emission spectra results show that the emission wavelengths of the oxygen-sensitive dye (O2) FAPbI3 perovskite QDs do not overlap with the Rh 110 reference signal. Thus, oxygen concentration can be measured using a ratiometric fluorescence reference-based approach. The sensing signal will be obtained in the presence of analyte gas in the ratiometric sensitivity of R0/R100, where R0 and R100 represent the luminescence intensity detected in 100% nitrogen and 100% oxygen concentrations, respectively. The experimental results show the optical oxygen sensor's sensitivity as R0/R100 = 12.7. In addition, the response time and recovery of the oxygen gas sensor produced are 75 s and 93 s, respectively. The use of a new type of FAPbI3 perovskite QDs material has been successfully developed in the optical ratiometric sensor for oxygen gas. The sensor proposed in this study has a low cost and easy fabrication process. The effect of spurious fluctuations in the excitation source intensity can be suppressed by the ratiometric optical sensing method.
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1. Introduction
Oxygen gas sensors continue to be developed with the discovery of a new type of material FAPbI3 perovskite QDs, which is sensitive to oxygen gas. Oxygen (O2) is a vital component in daily life and an essential reactant or product in various important, necessary essential reactants or products in multiple essential reactants or products in numerous chemical and biologic reactions [1–5]. Optical oxygen sensors effectively eliminate many of these limitations and are therefore widely used in chemistry [6–11], clinical [12,13], and environmental [14,15]. A colorless and odorless gas called oxygen is necessary for life. Oxygen toxicity usually begins at a partial pressure of >50 kPa causing seizures and other health problems. In addition, molecular oxygen (O2) detection is highly important in various fields, such as environmental monitoring, biological detection, aerodynamics, plant science, chemical analysis, and food packaging. Optical sensors for oxygen have attracted widespread interest in recent decades. The absorption-based visual method has good accuracy but is limited by the small oxygen cross section [16].
Recently quantum dots (QDs) have been extensively studied as they offer many advantages over conventional organic dyes due to generally high luminescence quantum yield, good photostability, broad excitation band, narrow emission band, multiphoton capability, color tunability, wavelength size-dependent emissions and a large effective stoke shift [17]. The organic components of perovskite structurally allow device portability and field applications, including narrowband devices under development. A more specific narrow band is suitable for developing a multi-analyte compact sensor array, with analytes detected in different spectral ranges, for gas phase and dissolved oxygen (DO) monitoring [18]. The design of a quenched photoluminescence probe for molecular oxygen (O2) perovskite nanocrystals has been proposed as an oxygen-responsive probe and revealed the role of O2 in the optical de-excitation process [19]. Moreover, in oxygen sensor devices, halide perovskite can display high sensitivity in relative oxygen concentrations at room temperature [20]. Ratiometric techniques such as those described for measuring excitation light either directly or indirectly overcome many problems with typical luminescence intensity measurement techniques. Most ratiometric optical sensors are based on using two different luminophores whose spectral areas exhibit a visible difference in behavior when the desired substance (e.g., oxygen, in this case) is present. Usually, one spectral zone shows a significant change in the analyte concentration while the other remains unchanged. The optical sensor membrane consists of a dye-sensitive analyte in a support matrix. When a polymer matrix is used, the photo-excited state of the trapped organic pigment can undergo photochemical reactions with the surrounding matrix, resulting in low photostability of the dopants dye and the support matrix [21].
The ratiometric method overcomes many problems associated with measuring techniques’ fluorescence intensity, such as the distribution of optical fiber probes and aberrations of LED light sources and photodetectors [22]. This study presents a ratiometric optical oxygen sensor based on FAPbI3 perovskite QDs as an indicator signal sensitive to oxygen gas and Rhodamine 110 (Rh 110) as a reference signal embedded in ethyl cellulose (EC) matrix to accommodate oxygen-sensitive fluorophores and assist oxygen entering into the support matrix. Recently, the use of ethyl cellulose (EC) has been suggested for the detection of O2 gas [23,24]. The ratiometric optical oxygen sensor experiments were coated materials on the surface of filter paper. Then, in excitation using an LED as an excitation source with a single wavelength, the optical ratiometric oxygen sensor experiment on FAPbI3 perovskite QDs emits oxygen-sensitive emission spectra. At the same time, rhodamine 110 is an oxygen-free reference signal. It can be seen that the emission spectra shown do not overlap each other. Thus, a fluorescence-based ratiometric method was used to overcome the effect of spurious fluctuations in the intensity of the excitation source [25]. In this approach, the sensor is designed to receive an additional reference signal displayed by a different fluorophore. Ratiometric sensors are based on using two different luminophores whose spectral areas exhibit other characteristics when fired by a gas (e.g., oxygen in this case) [26]. Usually, one spectral zone shows a significant change concerning the analyte concentration, while the other does not change for the cooling effect of oxygen. The ratio of the fluorescence intensity of the gas indicator dye to the reference dye at various analyte concentrations was used to calculate the sensor's sensitivity.
Table 1 shows the materials used for wavelength-ratiometric optical oxygen (O2) sensors and compares the performance of current optical sensors with different types of sensors made using various O2-sensitive fluorescent dyes. Researchers have demonstrated the fluorescence intensity for several kinds of ratiometric optical oxygen sensors based on Ru(II), Pt(II) and Pd(II) complexes. However, these fluorescent dyes are expensive and metal content. This work uses of a new type of FAPbI3 perovskite QDs material for oxygen sensing. Since FAPbI3 perovskite QDs have low cost, high brightness and easy fabrication process. The new ratiometric optical oxygen sensor developed in this study can be applied in industry and environmental oxygen concentration monitor.
Table 1. Properties of typical optical oxygen sensors
2. Experiment
2.1 Materials
Ethyl cellulose (EC) was purchased from Tokyo Chemical Industry Co., LTD (TCI) (Chuo, Japan). Other reagents, such as EtOH (99.5%), were purchased from ECHO Chemical co Ltd. (Miaoli, Taiwan). Rhodamine 110 (Rh 110) 98%, pure laser grade, was purchased from ACROS Organics (New Jersey, USA). FAPbI3 perovskite quantum dots (QDs) were prepared using a synthesized following a reference. All of the chemicals were used as received without further purification. In addition to materials carried out by the synthesis process of FAPbI3 Perovskite QDs.
2.2 Synthesis of FAPbI3 perovskite QDs
The FAPbI3 synthesis process used the method reported in Ref. [27]. Briefly, 0.521 g of a mixture of Formamidine acetate (FA-acetate 99%, Aldrich) and 10 mL of OA (90%; Sigma Aldrich) was placed in a three-neck flask at a temperature of 120°C for 1 hour. After that, it was stirred at 80°C. for 1 hour under a nitrogen stream to produce the FA-oleate precursor. In the next step, 175 mg PbI2 (99.999%; Sigma-Aldrich) and 10 mL octadecene (ODE; 90%; Sigma-Aldrich) were added to a three-necked flask and heated at 100°C for 1 hour until the temperature increased 120°C. Under nitrogen flow. OAm (90%; Sigma-Aldrich) and OA were added to three neck flasks, 1 mL each. After complete dissolution, PbI2 expands to 80°C. Next, add 1 ml of the FA-acetate precursor. After 5 seconds, the solution was refrigerated to obtain a crude explanation of FAPbI3. Pure FAPbI3 QDs (PEAI0) simple solution, prepared for the purification process. In the initial stage, the solution was centrifuged at 12,000 rpm for 10 minutes. First deposit result, added hexane (95%; Sigma-Aldrich) and ethyl acetate (99%; Alfa Aesar) in a ratio of 1:1 each 7 mL. It was followed by centrifugation at 12,000 rpm for 15 minutes. All results were stored for dispersion into 2 mL octane (98+%; Alfa Aesar) through a centrifugation process at 12,000 rpm for 15 minutes. Finally, the resulting FAPbI3 perovskite QDs was stored in a refrigerator at 4°C to form a stable colloidal solution.
Figures 1 (a) and (b) show the TEM images of the FAPbI3 perovskite QDs at resolutions of 50 nm and 20 nm, respectively. Figure 1 (c) presents the energy-dispersive X-ray spectroscopy (EDX) result for the composition of the FAPbI3 perovskite QDs nanoparticles. It is seen that the nanoparticles are composed principally of Pb, I, and Cu elements, where the x-axis represents the energy (keV) and the y-axis the counts per second per electron (basically X-ray intensity), respectively. The Cu content originates from the copper grid, while Pb and I are chemicals of perovskite quantum dots solution inside the structure FAPbI3 atom.
Fig. 1. TEM images of FAPbI3 perovskite QDs at resolution of (a) 50 nm, (b) 20 nm and (c) EDX analysis result for FAPbI3 perovskite QDs.
2.3 Preparation of ratiometric optical oxygen sensing materials
The oxygen sensitivity dye solution developed in this study was prepared by the synthesis process on FAPbI3 perovskite QDs (solution determined A). The reference dye solution was prepared by dissolving 5 mg of rhodamine 110 (Rh 110) in 2 mL of ethanol as a reference signal and dissolved ultrasonically for 20 min at room temperature (solution designated B). The support matrix was prepared by dissolving 1.25 mg of ethyl cellulose (EC) into 10 mL and adding 2.25 mL of EtOH solution (99.5%), and stirring magnetically to form a transparent glue-like solution, which is reported in the reference (designated as a solution C) [28]. Finally, the sensing material was prepared for the sensor experiment was mixed 30 µL of ethyl cellulose (EC), 20 µL of rhodamine 110 (Rh 110) and 0.5 µL of FAPbI3 perovskite QDs. The ratiometric optical sensing material was spread on the surface of the filter paper and dried at room temperature, allowed to stabilize under ambient conditions for 5-10 min. The fluorophores were excited using a 380 nm LED light source to obtain the emission spectrum.
2.4 Instrumentation
The instrumental setup for measuring the performance of ratiometric optical oxygen sensor is shown in Fig. 2. In the ratiometric optical oxygen sensing experiment, the sensing materials were excitation used an UV LED light source with center wavelength of 380 nm driven by an arbitrary wave function generator (TGA1240, Thurlby Thandar Instruments (TTi) Ltd.., Huntington, UK) at a frequency of 10 kHz pulse signal. The relative fluorescence intensities were measured using a USB 4000 spectrometer (U.S. Ocean Optics Inc., Largo, FL, USA). Defined different oxygen concentrations were adjusted by mixing the pure oxygen and nitrogen with mass flow controllers (Aalborg Instruments and Controls Inc., Orangeburg, NY, USA, Model GFC 17). On the UV-VIS spectrophotometer, the absorption spectrum of the fluorophore was obtained.
3. Results and discussion
3.1 Optical properties of ratiometric optical oxygen sensor
Figures 3 (a) and (b) present the absorption and emission spectra of the FAPbI3 perovskite QDs and Rhodamine 110 reference fluorescent dyes, respectively. Each material’s absorption and emission spectra were captured individually using a thin film. The obtained absorption spectra reveal that all fluorophores can be easily excited by UV LEDs with a central wavelength of 380 nm. A clear and distinct emission spectrum was associated with the FAPbI3 perovskite QDs O2 indicator. Using an UV LED light source with a 380 nm excitation source, the peaks observed the strong fluorescence emission spectra are at 770 nm and 560 nm, respectively. Thus, the detection of individual gases is carried out through well-completed monitoring of the emission of the sensing materials.
Fig. 3. Spectra properties of the material used in the ratiometric optical oxygen sensor: (a) absorption spectra and (b) emission spectra of FAPbI3 perovskite QDs and Rh 110.
3.2 Oxygen sensing properties of the ratiometric optical sensor
Figure 4 (a) presents the emission spectrum response of the FAPbI3 perovskite QDs at 770 nm is selectively quenched considerably; with the increase in applied O2 concentration up to 100%, the quenching of the fluorescence intensity increases continuously, confirming that intensity quenching is proportional to the applied concentration. At higher O2 concentrations, the increased number of oxygen molecules causes a larger number of molecular collisions and hence an increased extent of quenching. Moreover, the maximum intensity quenching is observed at low O2 concentrations, indicating that the sensor exhibits higher sensitivity at low O2 concentrations. Notably, the intensities of the other fluorescence signals are unaffected or show a negligible effect in the presence of O2 on rhodamine 110 emission as a reference signal at 560 nm in the presence of different oxygen concentrations. The sensitivity increases to 12.7, and with an increase in the oxygen concentration from 0% to 100%, we can observe a considerable decrease in the relative fluorescence intensity of the FAPbI3 perovskite QDs. This shows that the quenching power is proportional to the applied oxygen concentration. On the other hand, oxygen does not affect other fluorescence signals, indicating the sensor selectivity to the oxygen sensor. The result experiments of several samples have been measured on FAPbI3 perovskite QDs, to show the analytical validity of the sensor selectivity to the oxygen sensor. The obtained plot shows a good linear, as shown in Fig. 4 (b) presents a Stern-Volmer scheme for sensitivity in an optical sensor system. The optical oxygen sensor can be described by the Stern-Volmer equation [29], i.e.
Fig. 4. (a) Emission spectra of the ratiometric optical oxygen sensor under different oxygen concentrations and (b) Stern-Volmer plot.
In the ratiometric optical oxygen sensor developed in this study, the oxygen concentration is derived from the ratio of the maximum fluorescence intensity of FAPbI3 perovskite QDs to that of Rh 110 reference fluorescent dye, i.e.
Where IFAPbI3 (770 nm) and IRh110 (560 nm) represent the steady-state fluorescence intensities of the FAPbI3 perovskite QDs and Rh 110, respectively. The response of the ratiometric optical oxygen sensor can be calculated by replacing I0 and I in the Stern-Volmer equation by R0 and R, respectively, i.e.R0 and R represent the ratio of fluorescence intensity sensing signal in the absence and presence of oxygen, respectively. The sensitivity of optical oxygen sensor is defined as the ratio R0/R100, where R0 and R100 represent the ratio of fluorescence intensities in pure nitrogen and pure oxygen environments, respectively. The experimental result shows that the sensitivity (R0/R100) of the ratiometric optical oxygen sensor is 12.7.
3.3 Response time the FAPbI3 perovskite quantum dots-doped oxygen sensor
An optical sensor's fast response and recovery are required for practical applications when reliable concentration indication is needed. Figure 5 displays the oxygen response time of the ratiometric sensor for O2 sensing of FAPbI3 perovskite QDs. Response time and recovery of oxygen gas sensing are calculated individually. From closer inspection, it is clear that the response time of the optical O2 sensor is 75 seconds when switching from 100% N2 to 100% O2 with gradual steps to increase the O2 concentration. The recovery time is about 93 seconds when switching from 100% O2 to 100% N2; the process is repeated for seven cycles in 2 minutes to measure the response time in each cycle. FAPbI3 perovskite QDs returned to their initial position when the fluorescence intensity after cooling reached 100% O2 gas. The FAPbI3 perovskite QDs oxygen sensor provides a stable and reproducible signal when the environment is alternately fully oxygenated and deoxygenated.
Fig. 5. (a) Response time of the optical oxygen sensor switching between 100% nitrogen and 100% oxygen and (b) Dynamics response times of the ratiometric optical oxygen sensor.
The ratiometric optical oxygen sensor's dynamic response and recovery data on FAPbI3 perovskite QDs were captured in the same pattern as the response and recovery time data retrieval from the ratiometric oxygen sensor. Figure 5 (b) shows the dynamic response and recovery to O2. By switching the oxygen sensor from 100% N2 to 100% O2, the response time and recovery of the sensor oxygen are calculated to be 60 s and 73 s, respectively.
3.4 Selectivity of the ratiometric optical oxygen sensor
The presence of certain other gases under experimental conditions sometimes interferes with the proper operation of the gas sensor. Therefore, the selectivity of the gas sensor is also a key property to consider. In this test, the response of the ratiometric optical oxygen sensor in the presence of carbon dioxide (CO2) and ammonia (NH3) gas was observed. The proposed ratiometric optical oxygen sensor was alternately exposed to CO2 and NH3 gases for 20 minutes to watch the fluorescence intensity change. Figure 6 (a) shows the experiment results obtained under 100% CO2; the fluorescence intensity shows no difference and effect. Likewise, Fig. 6 (b) shows the results on exposure to 500 ppm NH3 gas, no change in the emission spectrum is also shown. These results confirm that the proposed ratiometric optical oxygen sensor does not interfere with the presence of NH3 and CO2 gases.
3.5 Photostability of the ratiometric optical oxygen sensor
Figure 7 shows the photostability of proposed ratiometric optical oxygen sensor using FAPbI3 perovskite QDs as sensing material and rhodamine 110 (Rh 110) as reference material embedded in the ethyl cellulose (EC) matrix and coated on the surface of filter paper. Then, it is excited by using an UV LED with a central peak wavelength of 380 nm. After continuous illumination for about 1 hour under room temperature, it was seen that the fluorescence intensity of the FAPbI3 perovskite QDs oxygen sensing signal and the rhodamine 110 (Rh110) reference signal were 108 ± 97 and 454 ± 39, respectively.
3.6 Humidity effect of the ratiometric optical oxygen sensor
The ratiometric optical oxygen sensor is tested in different humid environments to see the change in sensitivity. Figure 8 shows the humidity effect on the sensitivity of ratiometric optical oxygen sensor. The results in Fig. 8 reveal a decrease in the sensitivities with the decrease in the RH value from 25% to 67%.
4. Conclusion
This work presents a new ratiometric optical oxygen sensor based on the fluorescence intensity ratiometric method. The ratiometric optical oxygen sensor FAPbI3 perovskite QDs as an indicator dye sensitive to oxygen gas and rhodamine 110 (Rh 110) fluorescent dye as the reference signal. All sensing materials were excited using an UV LED with a central wavelength of 380 nm as the light source. The fluorescence intensity did not overlap between the respective emission spectra. The experimental results show a significant decrease in peak fluorescence intensity to oxygen contention (0%–100%) in FAPbI3 perovskite QDs with sensitivity (R0/R100) to the ratiometric optical oxygen sensor, which was 12.7. The response and recovery times are 75 s and 93 s, respectively. The ratiometric intensity-based method of the optical oxygen sensor proposed in this study can eliminate measurement errors caused by fluctuations in the power of the external light source or the excitation light source of the UV LED and the transmission properties of optical sensors. The photostability experiment results show that the sensor is stable at room temperature. In medical, industrial and environmental monitoring applications, the optical oxygen sensor still has challenges in a humid environment. The new ratiometric optical oxygen sensor based on perovskite quantum dots and Rh 110 embedded in an ethyl cellulose matrix can be fluorescence intensity characteristics required for an O2 sensor to provide a feasible alternative to existing optical O2 sensors.
Funding
Ministry of Science and Technology, Taiwan (109-2221-E-131-005-MY2, 111-2221-E-131-016).
Disclosures
The authors declare no conflict of interest.
Data availability
The authors confirm that the data supporting the findings of this study are available within the article. Data underlying the results presented in this paper are available in Ref. [27].
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