Whether in the monitoring of critically ill patients such as shock, respiratory failure, brain injury, or in major anesthesia surgeries, it is necessary to evaluate the patient’s pO 2 and pH. An optical fiber sensor presented is capable of monitoring the presence of oxygen partial pressure (pO2) and pH in the real-time. The sensor is based on fluorescence sensing of polymer immobilized in the oxygen/pH-sensitive membranes and covalently attached to the optical fiber probe. The design of this sensor us es LED as light source, which is a n excitation light source, induc ing specific wavelengths of fluorescence on the oxygen/pH-sensitive membrane. The intensity and lifetime of fluorescence are related to the pO2 and pH. So the pO2 and pH can be measured by the relationship between the pO2/pH values and the intensity and lifetime of fluorescence. The signal conditioning system based on DSP and STM32 was used to store and process data, and display test values. The response of the sensor for pO2 and pH monitoring with nitrogen (N2) as a balancing gas in the laboratory was performed. Finally, the oxygen/pH sensing scheme presented in this work is intended for using in biological, medical and environmental applications.
In the current clinical medical diagnosis process, it is usually necessary to monitor many physiological parameters to assess the pathological and physiological health status of patients. The monitoring of pH and pO2 (oxygen partial pressure) in clinical diagnostics of blood are the two most important physiological parameters. However, most detection of pO2 and pH are used in Water Quality Monitoring [
The traditional sensor used to detect pH and pO2 is based on optical sensing techniques [
The optical fiber sensors [
Fluorescent sensors have been reported recently [
Therefore, on the basis of the above, this paper proposes a new multifunctional polymer fluorescent sensor material synthesis and preparation, and its application in the study of pO2/pH dual sensor.
According to fluorescence quenching principle, the oxygen concentration (or pO2) is inversely proportional to the intensity of the fluorescence exhibited. The calibration curve can be linearized by applying Stern-Volmer Equation (1):
I 0 I = τ 0 τ = 1 + K [ pO 2 ] (1)
where I0 and τ 0 are the respective fluorescence intensity and fluorescence lifetime without oxygen present, and I & τ are the respective fluorescence intensity and fluorescence lifetime with oxygen present, k is the Stern-Volmer constant, which is dependant on the chemical composition of the sensor.
Optical pH sensors are based on pH indicator dyes that are immobilised on sensing layers. Analogous to pH indicators applied to aqueous solutions, the basic and acidic forms of these dyes have distinct spectral properties. Hence, a pH change leads to a change in the protonation degree of these molecules, which in turn can be detected as a shift of absorbance or fluorescence signal.
When fluorescein reagent is in acidic condition, its molecule will appear protonation (obtaining h) in the form of ring opening, and the fluorescence intensity will be weakened. When it is in alkaline condition, its fluorescence molecules will diproton (deprotonation loses h) and exist on lipid form, and the fluorescence intensity will be enhanced.
Therefore, the properties of the above-mentioned carboxy fluorescein under different acid and alkali conditions can be used as fluorescent materials for pH detection.
The frequency domain method is a technique that allows precise determination of decay time. Depending on its decay time, the luminescence of the indicator has the same waveform but is phase-shifted at a given frequency fmod. The decay time ( τ ) can be determined via the phase shift Φ according to [
τ = tan Φ 2 ⋅ π ⋅ f mod (2)
where fmod is the modulation frequency of the excitation light and τ is the luminescent decay time and Φ is the modulation phase shift of the excitation light. Through the calibration measurement in the air and pure nitrogen environment, the coefficient value K that meets the stern Volmer equation can be determined, and the relationship between the phase shift and the content of the tested substance can be obtained. Finally, the concentration or content of the quencher substance to be tested can be determined through the relevant calculation.
It is proposed a double lifetime reference technology [
cot Φ m = cot Φ r e f + 1 sin Φ r e f ⋅ A i n d A r e f (3)
where Φ m is the phase shift of the overall fluorescence , A i n d is the amplitude of the measured Fluorescence signal, and Φ r e f , A r e f is the phase shift and amplitude of the reference Fluorescence signal. When applying this technique to dual sensing, the long-lived fluorescent (such as oxygen) indicator acts as a reference for a short-lived fluorescent indicator (such as pH) so that the observed phase shift depends on the concentration of both analytes. Though the long-lived fluorescent indicator sensitive to oxygen is selected as reference, but keeping its response as oxygen indicator is not used as inert reference, and the phase shift of oxygen sensitive indicator is no longer constant. So the total phase shift ( Φ m ) which can be adjusted by changing the ratio of the two parameter fluorescent indicator materials is between 0 and Φ .
Then the phase shift of oxygen indicator was calculated according to formula (2) as follows:
Φ pO 2 = arctan ( 2 π f τ ) (4)
Then the fluorescence intensity of pH was calculated according to formula (1) and (3) as follows:
I PH = I PO 2 sin Φ PO 2 ( cot Φ − cot Φ PO 2 ) (5)
where Φ is the total phase shift. This can be used for designing dual sensors capable of simultaneous detection of two analytes (pO2 and pH).
The membrane for dissolved oxygen is prepared using tris(4,7-diphenyl-1,10-phenanthroline) ruthenium (II) dichloride complex [Ru(dpp)3Cl2] as an indicator. 3-(Trimethylsilyl)-1-propanesulfonic acid, Poly (styrene) (PS), sodium dodecyl benzene sulfonate(SDS), dichloromethane (CH2Cl2) and ethanol are as supportive materials. The pH membrane is prepared using fluoresceinisothiocyanate (FITC) as an indicator and sol-gel film as supportive material with chemicals such as Poly(styrene)-amine (PS-NH2), N,N-dimethyl-Formamide (DMF), Tetrahydrofuran(THF)and ethanol.
3-(Trimethylsilyl)-1-propanesulfonic acid, fluoresceinisothiocyanate (FITC) and Poly(styrene)-amine (PS-NH2) are purchased from Sigma-Alorich. sodium dodecyl benzene sulfonate(SDS), N,N-dimethyl-Formamide(DMF), Tetrahydrofuran (THF) and ethanol are purchased from Sinopharm Chemical ReagentCo., Ltd. tris(4,7-diphenyl-1,10-phenanthroline) ruthenium (II) dichloride complex [Ru(dpp) 3Cl2] is purchased from Alfa Aesar. Poly (styrene) (PS) is purchased from Huge Biotechnology. Dichloromethane (CH2Cl2) is purchased from Shanghai Lingfeng Chemical Reagent Co., Ltd. The chemicals purchased are of analytical research grade and used without any further purification.
The Ru(dpp)3Cl2 was used as Fluorescent indicator molecule. Poly (styrene) (PS) microsphere was used as carrier material of oxygen sensitive indicator. 1.3 mg of the RuCl2 was added to 500 uL of CH2Cl2 solution. This solution was mixed with the solution of 500 uL of PS microspheres added to 5 mL of 0.25% sodium dodecyl benzene sulfonate(SDS) solution. The mixed solution was fixed on the magneton mixer and stirred vigorously for 3 h. The microsphere solution was evaporated to remove CH2Cl2 at room temperature. Finally, the supernatant was centrifuged at 13,000 r for 10 min and washed repeatedly with ethanol and water until the supernatant was colorless.
In order to verify whether the fluorescence performance of oxygen sensitive fluorescent indicator changes pre and post filling to polystyrene, and the stability of polystyrene microspheres pre and post filling with oxygen sensitive fluorescent indicator. In order to verify the stability of fluorescence emission spectrum, the scanning spectrum pre and post filling was tested on fluorescence spectrophotometer. Field emission electron microscopy (SEM) was used to scan the embedded polystyrene fluorescent microspheres and observe whether the shape of the microspheres changed pre and post swelling.
The fluoresceinisothiocyanate (FITC) was used as Fluorescent indicator molecule. 2.2 mg of FITC and 9.0 mg of PS-NH2 was added to 1 mL of ethanol DMF and well-mixed. 3 mL of THF homogeneously was added to the mentioned solution
by vigorous stirring for 4 h. After the mixture was stirred, 4 mL of ultrapure water stirred while dropping was added to the solution. The mixed solution was centrifugally washed with ethanol and water until the supernatant was colorless, and pH sensitive fluorescent microsphere was prepared completely.
Both fluoresceinisothiocyanate (FITC) dissolved in ultrapure water and PSNH-FITC dissolved in ultrapure water were detected by fluorescence scanning. Contrast diagrams were shown in
Polyurethane hydrogel was used as carrier material of Fluorescent microsphere. For the preparation of the sensor membranes, 25 mg of polyurethane hydrogel
was added to 475 mg of alcohol solution in an ethanol/water (9:1, v:v) mixture. The mixture was vigorously stirred on the magnetic mixer until the polyurethane hydrogel is completely soluble at room temperature, and so a 5% wt/wt solution of Polyurethane hydrogel was obtained.
1 mg of oxygen-sensitive fluorescent microspheres and 2 mg of PH-sensitive fluorescent microspheres were added to 1 mL of ethanol. 2 mL of polyurethane hydrogel solution was added to the fluorescent microspheres solution, and the mixture was vigorously stirred overnight at room temperature. Finally, the front end of the optical fiber probe with a core diameter of 1 mm was immersed in the mixed solution, and then dried for 24 h at room temperature and in ambient air. Then the optical fiber probe detected the pO2/pH was prepared completely. The photo of fluorescence optical fiber probe excited by excitation light after coating was shown in
The performance of dual fluorescence optical fiber sensing system was validated with test solutions of pH 7.53, 6.50 and 4.46, respectively. They were equilibrated with a mixture of nitrogen and oxygen of different proportions to give a pO2 of 5 and 15 kPa, respectively. Then the driving frequency of LED is 30 kHz and 60 KHZ respectively, the measured values of pH and pO2 of solution were processed and displayed in the DSP & STM32 system.
It can be seen from
In this work, we have developed an optical fiber sensing system which is capable
| Solution number | Analyte | Actual value | Test value | Absolute error |
|---|---|---|---|---|
| 1 | pO2 | 5 | 5.5 | 0.5 |
| pH | 7.53 | 7.36 | 0.17 | |
| 2 | pO2 | 15 | 16.2 | 1.2 |
| pH | 7.53 | 7.68 | 0.15 | |
| 3 | pO2 | 5 | 5.6 | 0.6 |
| pH | 6.86 | 6.96 | 0.1 | |
| 4 | pO2 | 15 | 15.9 | 0.9 |
| pH | 6.86 | 6.8 | 0.06 | |
| 5 | pO2 | 5 | 5.7 | 0.7 |
| pH | 4 | 4.07 | 0.07 | |
| 6 | pO2 | 15 | 16.4 | 1.4 |
| pH | 4 | 3.73 | 0.27 |
of simultaneous measurement of two important parameters such as pH and pO2. Comparing the measurement results of the system with the actual values, the dual pO2/pH sensing system designed and manufactured by the combination of nano fluorescent microsphere sensing material and DSP & STM32 fluorescent signal conditioning system show good stability, anti-electromagnetic interference and high-precision. The new technique can be used for minimal-invasive measurements of high spatial resolution and high-throughput bioprocess optimization.
This research is supported by the National Nature Science Foundation of China (Nos. 61501526; 61178087), the Fundamental Research Funds for the Central Universities, South-Central MinZu University (No. CZQ22006).
The authors declare no conflicts of interest regarding the publication of this paper.
Fu, B.R., Zhang, X.W., Cao, H.M. and Zhang, Z.S. (2023) An Optical Fiber Sensor for Simultaneous Measurement of pO2 and pH. Open Journal of Applied Sciences, 13, 579-590. https://doi.org/10.4236/ojapps.2023.134046