sensors Article
Highly Sensitive and Wide-Dynamic-Range Multichannel Optical-Fiber pH Sensor Based on PWM Technique Md. Rajibur Rahaman Khan and Shin-Won Kang * School of Electronics Engineering, Kyungpook National University, 80 Daehakro, Bukgu, Daegu 41566, Korea;
[email protected] * Correspondence:
[email protected]; Tel.: +82-53-950-6829 (ext. 940-8609); Fax: +82-53-950-7932 Academic Editor: W. Rudolf Seitz Received: 24 September 2016; Accepted: 4 November 2016; Published: 9 November 2016
Abstract: In this study, we propose a highly sensitive multichannel pH sensor that is based on an optical-fiber pulse width modulation (PWM) technique. According to the optical-fiber PWM method, the received sensing signal’s pulse width changes when the optical-fiber pH sensing-element of the array comes into contact with pH buffer solutions. The proposed optical-fiber PWM pH-sensing system offers a linear sensing response over a wide range of pH values from 2 to 12, with a high pH-sensing ability. The sensitivity of the proposed pH sensor is 0.46 µs/pH, and the correlation coefficient R2 is approximately 0.997. Additional advantages of the proposed optical-fiber PWM pH sensor include a short/fast response-time of about 8 s, good reproducibility properties with a relative standard deviation (RSD) of about 0.019, easy fabrication, low cost, small size, reusability of the optical-fiber sensing-element, and the capability of remote sensing. Finally, the performance of the proposed PWM pH sensor was compared with that of potentiometric, optical-fiber modal interferometer, and optical-fiber Fabry–Perot interferometer pH sensors with respect to dynamic range width, linearity as well as response and recovery times. We observed that the proposed sensing systems have better sensing abilities than the above-mentioned pH sensors. Keywords: pH; pH sensor; side-polished optical-fiber; sensing element; optical-fiber pulse width modulation; sensing membrane
1. Introduction Over the last several decades, the development and applications of chemical and biosensors have grown very rapidly [1]. Among chemical sensors, pH sensors have received the most attention because of the importance of pH measurement in biomedical, scientific, and chemical research; the food and beverage industries; diagnostic centers; the agricultural sector; etc. In the biomedical research field, a pH sensor is used to monitor the pH in a microcell culture [2]. pH sensors are used in the food industry for quality control of food processing; in the fermentation process [3]; to determine the freshness of meat [4], milk quality [5], and the quality of drinking water [6]; and to measure microbial growth. In the medical field and at diagnostic centers, pH sensors are widely used to determine the acidity of body fluids [7,8] as well as blood pH [9]. In industrial sectors, pH sensors are used to process control in bioreactors [10] and to determine the presence of heavy metal ions in industrial wastewater. pH sensors are also used in environmental monitoring to determine the acidity of rain water [11]. Research studies have attempted to detect pH in the following areas: potentiometric [12], micro-electro-mechanical systems (MEMS) [13], capacitive [14], surface acoustic waves [15], complementary metal–oxide–semiconductor (CMOS) [16], carbon nanotubes [17,18], chemiresistors [19,20], red green
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blue (RGB) colors array [21], fluorescent [22], surface plasmon resonance (SPR) [23,24], localized surface plasmon resonance (LSPR) [25,26], etc. Optical-fiber sensors have become popular because of their acceptable cost, light weight, high accuracy, small dimensions, geometrical versatility, capability of monitoring remote sensing, nonexplosivity, immunity to electromagnetic interference, freedom from strong magnetic fields, and completely electrical isolation. Therefore, optical-fiber sensors are very safe, can detect multiple parameters using a single optical fiber without crosstalk, and can measure at inaccessible sites in harsh environments as well as from long distances. Many optical-fiber pH sensors have been developed based on different optical-fiber architectures, including the following: without/removed cladding [27], optical-fiber U-shaped pH sensor [28–30], side-polished optical-fiber pH sensor [31], D-type optical-fiber pH sensor [32], optical-fiber Bragg-grating pH sensor [33,34], and optical-fiber Fabry–Perot pH sensor [35]. An optical-fiber sensor that detects the pH of a fluid can be based on the principles of intensity modulation or wavelength modulation. For optical-fiber intensity modulation pH sensors, different types of pH-sensitive materials/dyes are used on the optical-fiber core or polished cladding. They measure the pH value of a given fluid by observing optical parameters such as absorption, reflection, transmission of optical power, and fluorescence. On the other hand, wavelength-modulated pH sensors use a pH-sensitive hydrogel with an optical-fiber Bragg grating [36], a long period grating [37], or a reflective diffraction grating [38]. Constructing intensity-modulation pH sensors is easy, but their sensitivity is low, and fluctuations in the light intensity affect the pH measurement. Wavelength-modulated pH sensors also have some disadvantages including a complex fabrication process, low sensitivity, and high complexity. A potentiometric pH sensor utilizing different pH-sensitive polymers coated on wire electrodes was proposed by Santiago et al. [39]. The main feature of this sensor is its highly sensitive (approximately −43 mV/pH) and simple construction and linear sensing response. However, the proposed pH sensor also has disadvantages such as its bulk, a low pH measurement range of 3 to 10, and a high response time of approximately 3 to 5 min on average. Mishar et al. proposed an optical-fiber SPR sensor to detect the pH of fluid [23]. In this study, the researchers deposited different metal layers on an unclad optical-fiber using the thermal evaporation method. Then, they deposited a hydrogel layer on the deposited metal layer to fabricate an optical-fiber SPR sensor. Although the sensor’s construction is very simple and it can detect pH levels from 3 to 11, it has several disadvantages including high cost owing to different metal depositions, bulk, nonlinear response properties, and difficulty in determining the resonance wavelength. A pH sensor using a shear horizontal surface acoustic wave was proposed by Haekwan et al. in 2013 [15]. In their study, they prepared input and output interdigitated transducers on a piezoelectric substrate and placed a ZnO film containing nanoparticles as a pH-sensitive material between the input and output transducers. The construction and working principle of this sensor is simple, and it also gives a linear sensing response. However, its main disadvantage is that the pH measurement range is very low (from 2 to 7). Lei et al. proposed a graphene chemiresistor sensor to measure the pH of fluid [20]. The sensor has some advantages including ease of fabrication, small size, and good sensing ability. However, the proposed sensor has a low dynamic range of 4–10, and its performance is less stable. Gu et al. proposed and developed an optical-fiber sensor to measure pH in a range of 2 to 11 using a modal interferometer method [40]. They applied a layer-by-layer electrostatic self-assembly technique to deposit two kinds of polyelectrolytes on the side surface of a thin-core fiber modal interferometer. The sensors have several features including a wide pH sensing range, linear sensing response, and good stability. However, this sensor also has several disadvantages including a complex fabrication process, low sensitivity, and long response and recovery times (about 60 s and 80 s, respectively). An optical-fiber Fabry–Perot interferometer sensor for measuring pH was presented by Zheng and coworkers in 2016 [35]. The interferometer was prepared by placing a small section of a hollow-core
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core photonic crystal fiber between a sensing and lead-in single-mode optical-fiber. The other end of the sensing fiber was a polyvinyl hydrogel The containing a sensing photonic crystal fiberdeposited between ainsensing and alcohol/poly-acrylic lead-in single-modeacid optical-fiber. other end of the membrane. proposedalcohol/poly-acrylic sensor was good, about 11 nm/pH, and offered good sensing fiberThe wassensitivity depositedof inthe a polyvinyl acid hydrogel containing a sensing repeatable performance and sensing stability. The sensor had several disadvantages including a membrane. The sensitivity of the proposed sensor was good, about 11 nm/pH, and offered good complex fabrication process, a low dynamic range of pH from approximately 4.1 to 6.9, nonlinear repeatable performance and sensing stability. The sensor had several disadvantages including a response properties,process, and high response and recovery of approximately 606.9, s and 90 s, complex fabrication a low dynamic range of pH times from approximately 4.1 to nonlinear respectively. response properties, and high response and recovery times of approximately 60 s and 90 s, respectively. In our study, study,we weproposed proposeda fast, a fast, highly sensitive, wide-dynamic-range side-polished opticalIn our highly sensitive, wide-dynamic-range side-polished optical-fiber fiber pH sensor array that is based on optical-fiber pulse width modulation (PWM) [41,42] principles. pH sensor array that is based on optical-fiber pulse width modulation (PWM) [41,42] principles. According optical-fiber PWM received sensing According to to the the optical-fiber PWM technique, technique, the the pulse pulse width width of of the the received sensing signal signal from from the the optical-fiber optical-fiber pH-sensing pH-sensing element element of of the the array array changes changes if if the the sensing sensing element element of of the the array array comes comes into buffer solution. solution. This into contact contact with with aa pH pH buffer This can can occur occur because because of of changes changes in in the the refractive refractive index index of of the sensing membrane of the optical-fiber sensing element. In our experiment, five different kinds of the sensing membrane of the optical-fiber sensing element. In our experiment, five different kinds pH sensitive dyes (methyl red,red, methyl orange, thymol blue,blue, Nile Nile red, and were were used of pH sensitive dyes (methyl methyl orange, thymol red, rhodamine-B) and rhodamine-B) as the principal materials of the sensing membrane. These dyes were individually mixed with N,Nused as the principal materials of the sensing membrane. These dyes were individually mixed with dimethylacetamide (DMAC) and Polyvinyl chloride (PVC) and deposited on side-polished opticalN,N-dimethylacetamide (DMAC) and Polyvinyl chloride (PVC) and deposited on side-polished fiber devices to fabricate five optical-fiber sensing elements of an array. To observe the sensing ability optical-fiber devices to fabricate five optical-fiber sensing elements of an array. To observe the of the proposed optical-fiber pH sensor array, we used buffer solutions of different pH (from 2 to 12), sensing ability of the proposed optical-fiber pH sensor array, we used buffer solutions of different pH and obtained a linear sensinga response with highly stable properties. Theproperties. proposed (fromwe 2 to 12), and we obtained linear sensing response withresponse highly stable response optical-fiber PWM pH sensor has several other features: simple construction, ease of fabrication, low The proposed optical-fiber PWM pH sensor has several other features: simple construction, ease of cost, reusability, and light weight. The sensor used electronic circuitry prepared from inexpensive fabrication, low cost, reusability, and light weight. The sensor used electronic circuitry prepared from available electronic at local electronic shops. Weshops. compared different inexpensive availablecomponents electronic components at local components electronic components We compared sensing parameters of the proposed optical-fiber PWM sensor array with different pH sensors, and different sensing parameters of the proposed optical-fiber PWM sensor array with different pH sensors, found that that the proposed sensing system has ahas better sensing ability. and found the proposed sensing system a better sensing ability. 2. Theory Theoryand andWorking WorkingPrinciple Principleof ofthe theOptical-Fiber Optical-FiberPWM PWMpH-Sensing pH-Sensing System System We proposed an optical-fiber optical-fiber PWM-based PWM-based pH-sensing pH-sensing system. Generally, Generally, an electrical PWM system consists consists of two inputs and an output. One One input input is is called called the pulse input, input, which is used for the electrical pulse entering into the system. The other input is called the control input, which is used to change the pulse width width of the input input signal signal without without changing changing the the time time period period of of the the input input signal. signal. The output output is isused usedto toobtain obtainaadesired desiredpulse pulse width with same time period of the input signal. In width with thethe same time period of the input signal. In our our experiment, we deposited a polymer waveguide that contained pH-sensitive on a sideexperiment, we deposited a polymer waveguide that contained pH-sensitive dye on adye side-polished polished optical-fiber prepare an optical-fiber pH-sensing element, in Figure 1. optical-fiber device to device preparetoan optical-fiber pH-sensing element, as shown as in shown Figure 1.
Figure 1. Schematic Schematic diagram diagram of of the the side-polished side-polished optical-fiber with a sensing membrane. Figure
In the proposed optical-fiber PWM system, a light pulse is passed through a fiber-optic-based waveguide. When the sensing membrane of the optical-fiber sensing element makes contact with the pH buffer solution, then the optical properties (such as the refractive index of the sensing membrane) change. This in turn changes the pulse’s peak value as well as the fall time. This occurs as a result of the absorption of light in the waveguide. As a result, the pulse width of the received sensing signal
In the proposed optical-fiber PWM system, a light pulse is passed through a fiber-optic-based waveguide. When the sensing membrane of the optical-fiber sensing element makes contact with the pH buffer solution, then the optical properties (such as the refractive index of the sensing membrane) change. in 1885 turn changes the pulse’s peak value as well as the fall time. This occurs as a 4result SensorsThis 2016, 16, of 17 of the absorption of light in the waveguide. As a result, the pulse width of the received sensing signal changes. The pulse width of the received light pulse depends on light absorption into the polymer changes. The pulse width of the received light pulse depends on light absorption into the polymer waveguide, which can be considered a pulse control input. The width of the light pulse is a result of waveguide, which can be considered a pulse control input. The width of the light pulse is a result the variation in the refractive index of the overlay waveguide, which corresponds to the change in of the variation in the refractive index of the overlay waveguide, which corresponds to the change pH of the buffer solution. Therefore, some analogies can be established between the electrical as well in pH of the buffer solution. Therefore, some analogies can be established between the electrical as as optical-fiber PWM sensing systems with regard to their operating principles and structures. These well as optical-fiber PWM sensing systems with regard to their operating principles and structures. analogies are presented in Tablein1Table and Figure 2. These analogies are presented 1 and Figure 2. Table 1. Three levels electricaland andoptical-fiber optical-fiber PWM sensing systems. Table 1. Three levelsofofanalogy analogywithin within the the electrical PWM sensing systems.
No. of Stage
Port
1 2 3
In
No. of Stage 1 2 3
Port
In Control Control Out Out
PWM System PWM System Electrical Optical-Fiber Electrical Optical-Fiber Electrical pulse Light pulse Electrical pulse Light pulse Voltage/current index index Voltage/current Refractive Refractive Electrical Light pulse Electricalpulse pulse Light pulse
Figure 2. Functional analogybetween betweenthe the two two PWM PWM systems: and (b)(b) optical-fiber PWM Figure 2. Functional analogy systems:(a) (a)electrical; electrical; and optical-fiber PWM sensing systems. sensing systems.
purpose Table11isis to to understand understand easily thethe electrical andand the the TheThe purpose ofofTable easily the theanalogies analogiesbetween between electrical proposed optical-fiber PWM sensing system. For example, the proposed optical-fiber PWM sensing proposed optical-fiber PWM sensing system. For example, the proposed optical-fiber PWM sensing system three ports(in, (in,control controland and out), out), which which is PWM system. Moreover, in in system hashas three ports is similar similartotoelectrical electrical PWM system. Moreover, electrical PWMsystem, system,aa voltage/current voltage/current isisapplied itsits control portport to change the pulse widthwidth of an an electrical PWM appliedtoto control to change the pulse the output electrical signal, which is obtained at the out port. This is similarly found in the proposed of the output electrical signal, which is obtained at the out port. This is similarly found in the optical-fiber PWM system. In the proposed PWM system, the control port is the change of refractive proposed optical-fiber PWM system. In the proposed PWM system, the control port is the change of index of the sensing membrane of the sensing element. Therefore, if the refractive index of the sensing refractive index of the sensing membrane of the sensing element. Therefore, if the refractive index of membrane change due to the change of pH value, then the out port light pulse’s width change without the sensing membrane change due to the change of pH value, then the out port light pulse’s width changing the time period of the in port light pulse. change In without changing the timedown period in on port light our study, we polished to of thethe core one sidepulse. of the optical-fiber. Then a sensing In our study, we polished down to the core on one side of thetwo optical-fiber. Then a sensing membrane was deposited on this side-polished optical-fiber to create waveguides. Therefore, membrane was deposited side-polished optical-fiber to create waveguides. when light passed throughon thethis side-polished optical-fiber, a fraction of the two radiation extended Therefore, a small when light (called passedan through the side-polished a fraction of the radiation extended a small distance evanescent field) from the optical-fiber, polished region. This evanescent field entered the upper distance (called an the evanescent field)optical-fiber from the polished region. evanescent entered the upper waveguide from side-polished waveguide. TheThis evanescent field’sfield energy may change
waveguide from the side-polished optical-fiber waveguide. The evanescent field’s energy may change owing to absorption or scattering of light into the overlay waveguide, or from changes in the
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owing to absorption or scattering of light into the overlay waveguide, or from changes in the refractive index of the overlay waveguide. Mathematically, the evanescent field can be represented by [43] z , E (z) = E0 exp − dP
(1)
where E0 is the electric field amplitude of light at the interface of core–cladding and z is the distance of electric field in the cladding from the interface. dp is the penetration depth, and the sensitivity of the side-polished optical-fiber sensor depends on the penetration depth. The penetration depth can be defined mathematically as [44] λ dp = 2πn1
( 2
sin θ −
n2 n1
2 )−0.5 ,
(2)
where λ is the wavelength of the transmitted light, and θ is the angle of incidence to the normal at the interface. n1 and n2 are the refractive index of the fiber cladding and the material of the overlay waveguide, respectively. Now, if a light pulse with time period T is transmitted through the side-polished optical-fiber waveguide, then the received light pulse width TH can be written mathematically as [41,42] T TH = (3) 0.5 , n2 γL α where L is the length of the polished cladding, γ is the evanescent wave absorption coefficient, and α is the phenomenological ion-specific parameter. Therefore, by observing the pulse width of the received sensing signal, the pH-value can be determined. The pulse width is proportional to the pH, which corresponds to changes in the refractive index of the overlay waveguide as well as the absorption of the evanescent field into the waveguide. 3. Experimental Details 3.1. Fabrication of the Side-Polished Optical-Fiber Device In our study, we fabricated a side-polished optical-fiber device for preparing the optical-fiber pH sensing element of the array. To do this, we chose a quartz block of approximately 25 × 10 × 5 mm3 , and made a V-groove of approximately 160 µm in width using a mechanical slicer. Then, we collected a single-mode optical-fiber of about 1 m in length and removed its jacket approximately 20 mm at the middle. The radius of the optical-fiber core was 3 µm, and the cladding radius was 125 µm. The removed jacket portion was bent with a radius approximately 60 cm and placed in the V-groove of the quartz block. Then, we applied and dried epoxy so the bent cladding portion was strongly attached to the quartz block. The surface of the cladding attached to the quartz block was polished using l000-µm and 8000-µm polishing powders on polishing pads to fabricate a side-polished optical-fiber device. Figure 3a–e shows the step-by-step fabrication procedure for the side-polished optical-fiber device, and Figure 3f shows a photograph of the prepared side-polished optical-fiber device.
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Figure Figure 3. 3. Fabrication Fabrication process process for for side-polished side-polished optical-fiber optical-fiber device: device: (a) (a) quartz quartz block; block; (b) (b) making making the the V-groove the optical-fiber; (d)(d) bending thethe portion of V-groove in in the the quartz quartzblock; block;(c) (c)removing removingthe thejacket jacketofof the optical-fiber; bending portion the optical-fiber with jacket removed, placing it it into the of the optical-fiber with jacket removed, placing into theV-groove V-grooveofofthe thequartz quartzblock blockand and apply apply epoxy; epoxy; (e) (e) polishing polishing the the optical-fiber optical-fiber quartz quartz block; block; and and (f) (f) photograph photograph of of fabricated fabricated side-polished side-polished optical-fiber device. device. optical-fiber
3.2. 3.2. Fabrication Fabrication of of the the Sensing Sensing Membrane Membrane and and Optical-Fiber Optical-Fiber Sensing Sensing Element Element
To To make make optical-fiber optical-fiber pH pH sensing sensing elements elements of of the the array, array, we we needed needed some some pH pH sensitive sensitive chemicals chemicals which will change their optical properties such as refractive index and chemical structures which will change their optical properties such as refractive index and chemical structures when when those those dyes/chemicals dyes/chemicalscontaining containingpolymer polymerwaveguide waveguideofofsensing sensingelements elementsin in contact contact with with the the pH pH buffer buffer solution. solution. Those Those dyes/chemicals dyes/chemicalschange changetheir theirmolecular molecularand andelectronic electronicstructures structuresin in acidic, acidic, neutral, and alkali alkalisolutions, solutions, and of change their molecular and electronic is neutral, and and thethe wayway of change their molecular and electronic structurestructure is different. different. Asthe a result, dipole of moment of the molecule in turn changes the dielectric As a result, dipolethe moment the molecule changes, changes, which in which turn changes the dielectric constant constant well as the index, refractive sincesensing the overly sensingcontains membrane contains dye. pH as well asaschange thechange refractive sinceindex, the overly membrane pH sensitive sensitive dye. Therefore, when the sensing of pH optical-fiber pH sensing element of the array Therefore, when the sensing membrane ofmembrane optical-fiber sensing element of the array contact with contact with asolution, pH buffer theindex refractive of the sensing membrane changes. Asthe a result, a pH buffer thesolution, refractive of theindex sensing membrane changes. As a result, pulse the pulse of the received sensingalso signal also changes. In our we study, wefive used five different width of width the received sensing signal changes. In our study, used different kinds kinds of pH of pH sensitive dyes (methyl red, methyl blue, Nile and rhodamine-B) [45,46] to sensitive dyes (methyl red, methyl orange,orange, thymolthymol blue, Nile red, andred, rhodamine-B) [45,46] to prepare prepare five optical-fiber pH sensing of anWe array. used five different dyes containing five optical-fiber pH sensing elementselements of an array. usedWe five different dyes containing sensing sensing of the array tothe observe theability sensing ability ofdyes different dyes containing elementselements of the array to observe sensing of different containing optical-fiberoptical-fiber pH sensing pH sensing We have chosen these five dyes because are lowtocost, easy to make sensing element. Weelement. have chosen these five dyes because they are lowthey cost, easy make sensing membrane, membrane, and in the chemical markets which also saveAll our time. Alldifferent dyes have different and available inavailable the chemical markets which also save our time. dyes have pKa values. pKa values. the To prepare the optical-fiber sensing of the array,dyes different with different To prepare optical-fiber pH sensingpH element ofelement the array, different withdyes different amounts amounts were individually mixed withand DMAC PVC to create five different types of pHsolution. sensing were individually mixed with DMAC PVCand to create five different types of pH sensing solution. The chemical compositions of the five different pH-sensing solutions for different optical-fiber The chemical compositions of the five different pH-sensing solutions for different optical-fiber pH-sensing to to S5)S5) of the array are tabulated in Table 2. The2.preparation procedure for the pH-sensingelements elements(S1 (S1 of the array are tabulated in Table The preparation procedure pH solution solution was as follows: we mixed different dye individually with 4 mL for sensing the pH sensing was as first, follows: first, we mixedamounts differentofamounts of dye individually of DMAC and sonicated for approximately 10 min to make thetodye solution. we with 4 mLsolution of DMAC solution anditsonicated it for approximately 10 min make the dyeThen, solution. added 0.30added g of PVC to make a pH-sensing All chemicals collected Then, we 0.30togthe of dye PVCsolution to the dye solution to make asolution. pH-sensing solution. were All chemicals from Sigma-Aldrich Chemical Corporation Korea)(Seoul, and used without further purification. were the collected from the Sigma-Aldrich Chemical(Seoul, Corporation Korea) and used without further We cleaned We the cleaned polishedthearea of thearea optical-fiber device properly with methanol, ethanol, and purification. polished of the optical-fiber device properly with methanol, ethanol, Next, we deionized (DI) water. Then,Then, we dried the side-polished optical-fiber device using N2 gas. and deionized (DI) water. we dried the side-polished optical-fiber device using N2 gas. Next, deposited different sensing solutions individually on the optical-fiber device using a spin coater with we deposited different sensing solutions individually on the optical-fiber device using a spin coater speed 1000 rpm, and dried them them by placing them them on a hotplate at 50 at °C50for◦ C20for min make five with speed 1000 rpm, and dried by placing on a hotplate 20 to min to make optical-fiber pH-sensing elements of the In our experiment, we used the the PVC polymer to five optical-fiber pH-sensing elements of array. the array. In our experiment, we used PVC polymer immobilize the pH sensitive dye properly on the surface of side-polished optical-fiber device. The to immobilize the pH sensitive dye properly on the surface of side-polished optical-fiber device. thickness of the membrane of the pH sensing element of the array waswas about 22 The thickness ofsensing the sensing membrane of optical-fiber the optical-fiber pH sensing element of the array about µm andand waswas measured by aby scanning electron microscope (SEM) (S-4800, Hitachi, Ibaraki, Japan). The 22 µm measured a scanning electron microscope (SEM) (S-4800, Hitachi, Ibaraki, Japan). relative standard deviation (RSD) of thickness of the membranes was was about 0.001. The relative standard deviation (RSD) of thickness ofsensing the sensing membranes about 0.001.
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Table 2. Composition of the pH-sensing solution to prepare different optical-fiber sensing elements Table 2. Composition of the pH-sensing solution to prepare different optical-fiber sensing elements of of an array. an array. Sensor ID S0 S1 S2 S3 S4 S5
Sensor ID Dye Polymer Dye Polymer S0 PVC S1 Methyl red (0.05 g) PVCPVC red (0.05orange g) S2 MethylMethyl (0.05 g) PVCPVC Methyl orange (0.05 g) S3 Thymol blue (0.05 g) PVCPVC Thymol blue (0.05 g) PVC S4 Nile red (0.01 Nile g) red (0.01 g) PVCPVC S5Rhodamine Rhodamine B (0.01 g) B (0.01 g) PVCPVC
Solvent DMAC Solvent DMAC DMAC DMAC DMAC DMAC DMAC DMAC DMAC DMAC DMAC DMAC
3.3. System 3.3. Detection Detection Mechanism Mechanism of of the the Proposed Proposed Optical-Fiber Optical-Fiber pH pH Sensing Sensing System
The The experimental experimental setup setup of of the the proposed proposed optical-fiber optical-fiber PWM PWM pH-sensing pH-sensing system system is is shown shown in in Figure 4 and consists of three units: a pulse modulation unit, an optical-fiber pH sensor array used Figure 4 and consists of three units: a pulse modulation unit, an optical-fiber pH sensor array used as as a transducer unit, a signal processing We designed the pulse modulation unitthe and the a transducer unit, andand a signal processing unit.unit. We designed the pulse modulation unit and signal signal processing unit using low-cost and easily available components from local electronics processing unit using low-cost and easily available components from local electronics suppliers. suppliers.
Figure 4. Schematic diagram of the experimental setup of the proposed optical-fiber PWM pH sensing Figure 4. Schematic diagram of the experimental setup of the proposed optical-fiber PWM pH system. sensing system.
The pulse generator/modulation [47–49] unit consists of three units: a square-wave generator, a buffer amplifier, and a laser diode (LD) driver with LD. The square-wave generator produces a The pulse generator/modulation [47–49] unit consists of three units: a square-wave generator, square with aand frequency of 10 kHz a 50% duty In our experiment, we produces designed a a bufferwave amplifier, a laser diode (LD)and driver with LD.cycle. The square-wave generator square-wave using a well-known timer (IC duty NE555) and In associated electronic components. square wave oscillator with a frequency of 10 kHz and a 50% cycle. our experiment, we designed a This square-wave generator a squaretimer wave(IC with a frequency of 20 kHz withoutcomponents. a 50% duty square-wave oscillator usingoffers a well-known NE555) and associated electronic cycle. Our target is generator to obtain 10 kHzawith a 50% duty cycle. Therefore,of the of the square-wave This square-wave offers square wave with a frequency 20output kHz without a 50% duty oscillator fed tois the input 10 of kHz a T-flip-flop, which of a CD4027 (JK flip-flop in cycle. Ouristarget to obtain with a 50% dutyconsists cycle. Therefore, the output of the employed square-wave toggle mode) a perfect 10-kHz signal with a 50%ofduty cycle.(JK flip-flop employed in toggle oscillator is fedtotoobtain the input of a T-flip-flop, which consists a CD4027 The the T-flip-flop connected theduty inputcycle. of the buffer amplifier. The purpose of the mode) tooutput obtain of a perfect 10-kHz is signal with ato 50% buffer amplifier is to reduce loading effects because the buffer amplifier has low input impedance
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The output of the T-flip-flop is connected to the input of the buffer amplifier. The purpose of the buffer amplifier is to reduce loading effects because the buffer amplifier has low input impedance and high output impedance. Then, its output is fed to the input of the LD driver circuit, whose output is connected to an LD. The function of the LD driver circuit is to turn on/off the LD according to the pulse of its applied input signal. As a result, the LD emits a light pulse with a 10-kHz frequency at 850 nm. The LD is connected to the optical-fiber pH-sensing element. In our study, we used six LDs to transmit light pulses through the sensor array, which consists of five optical-fiber pH-sensing elements and one reference optical-fiber element. The opposite terminal of the pH sensor array is connected to six photodiodes. The signal processing unit consists of a photodetector (PD), amplifier, pulse-shaping circuit, and peak detector. The photodiode circuit converts the optical light pulse coming from the optical-fiber pH sensor array into an electrical pulse. The output of the PD circuit is connected to the input of the operational amplifier, which is connected to the current-follower configuration for desired signal amplification. The pulse-shaping circuit is used to shape the pulse as a square without distorting the signal, and its output is the input of the peak detector. The peak value of the signal is obtained from the peak detector. The six outputs of the peak detector are connected to the six inputs of the data acquisition (DAQ) module (NI USB-6216 BNC, National Instruments, Debrecen, Hungary). A computer is connected to the DAQ module. To observe the sensing performance of the optical-fiber pH sensor array and store the data in the computer, we developed a LabVIEW program. An amplitude modulation based side-polished optical-fiber sensor cannot detect a very small change in the light due change of refractive index of the overlay sensing membrane. However, in the case of the proposed optical-fiber PWM sensing system, the received light pulse width depends on the light pulse amplitude as well as fall time. The effect on the pulse width amplitude and the fall time caused by a small change of refractive index of the overlay sensing membrane. As a result, the proposed optical-fiber PWM pH sensing system offers a good linear dynamic range and can effectively detect low pH. A laser light source with higher wavelength is required to obtain better sensitivity and we obtained better sensing performance using the 850 nm laser source, which is why in our experiment we have selected the laser diode of 850 nm wavelength. The sensing response of a specific sensing element is the pulse width differences between the signals which is received from the reference element and that particular sensing element of the array. In our experiment, before measuring the pH of the buffer solution, we calibrated the system so that the same amount of light pulse passes through all optical fiber-sensing elements as well as a reference element. As a result, we obtain a pulse width of zero. The relative pulse width (∆TH ) of a given pH is the difference between the reference pulse width and the sensing pulse width. Then, we slowly inject pH buffer solution by a syringe into the test chamber, and observe the sensing response at room temperature. The fabricated optical-fiber sensor array carries different pH-sensitive compounds. Therefore, when the sensor array makes contact with any buffer solution, the refractive index of the sensing membrane changes. As a result, the width of the light pulse corresponds to changes in the received electrical signal pulses as well as changes in the output voltages of the peak detectors. The pulse width of the received signal depends on the amount of pH and the properties of the sensing membrane’s materials. The relative pulse width increases as the pH of the buffer solution increases. An oscilloscope (OWON, VDS3104, Zhangzhou, China) is used to measure the pulse width of the received sensing signals. The thickness of the dye containing PVC polymer sensing membrane of the optical fiber sensing element was very thin and we also used the modulated light of frequency about 10 kHz, which increase the fast sensing response to changes in pH. In our experiment, we obtained the higher sensitivity and faster sensing response at 22 µm thickness of the sensing membrane and the modulated light of frequency about 10 kHz. Rhodamine-B is a xanthene dye and its optical properties change under different pH values [50]. To observe the pH sensing ability of Nile red and rhodamine-B under different pH, we prepared Nile red and rhodamine-B containing pH solutions. To do this, Nile red and rhodamine-B were individually dissolved into different pH (2–12) of buffer solution to make
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10 mM of Nile red and rhodamine-B containing pH solution, then we observed the absorption peak of
is discussed detail inpH [48]. This occurs owing a change in of the refractive index of dye containing those dyesin containing solutions and we foundtothat as the pH a Nile red/rhodamine-B containing pHpH solution. These results indicate that the Nile red and rhodamine-B can produce responses for a buffer solution increases, the absorption peak of the Nile red/rhodamine-B containing pH solution range of pH.asThe absorption (a.u.) peak of of rhodamine-B containing pH in 2, pH 4, in pH[48]. 8, pH 10, and pH increases well. The experimental setup this experiment is discussed detail This occurs owing to a change in the refractive index of dye containing pH solution. These results indicate that a 12 solutions was about 4676.17, 5442.27504, 5691.28, 6042.16, and 6475.09, respectively. Moreover, the Nile redpH andsensor rhodamine-B can produceNile responses for areported range of pH. The absorption peakof of the colorimetric array including red was in [45]. The main(a.u.) feature rhodamine-B containing pH 2, pH 4, pH 8, pH 10, and pH 12 solutions was about 4676.17, 5442.27504, proposed sensor is its high sensitivity, stability, fast response time, and its principle of operation is 5691.28, 6042.16, and PWM 6475.09, respectively. Moreover, a colorimetric including called the optical-fiber technique, which was proposed by uspH andsensor it wasarray reported first Nile time to red was reported in [45]. The main feature of the proposed sensor is its high sensitivity, stability, develop pH sensing system using the PWM technique by this paper. The proposed optical-fiberfast PWM response time, and its principle of operation is called the optical-fiber PWM technique, which was pH sensing system is suitable to detect pH of any sample solution in agriculture, environment, proposed by us and it was reported first time to develop pH sensing system using the PWM technique medical, research sectors and so on. by this paper. The proposed optical-fiber PWM pH sensing system is suitable to detect pH of any sample solution in agriculture, environment, medical, research sectors and so on.
4. Results and Discussion
4. The Results and Discussion waveform response of the proposed optical-fiber PWM pH-sensing system is shown in
Figure 5a. were response no pulse-width differences between PWM the reference and system sensingissignals TheThere waveform of the proposed optical-fiber pH-sensing shown when in there was no buffer solution in the test chamber. However, when we injected a buffer solution Figure 5a. There were no pulse-width differences between the reference and sensing signals wheninto was no buffer in theof test chamber. However, wedecreased. injected a buffer intorelative the thethere test chamber, the solution pulse width the received sensingwhen signal As a solution result, the test chamber, the pulse width of the received sensing signal decreased. As a result, the relative pulse pulse width between the sensing and the reference signals increased. This is shown in Figure 5b. The widthpulse-width between the sensing and the reference increased. is shown Figure 5b. The relative an relative difference of the buffersignals solution at pHThis 7 for methylinred, which contained pulse-width difference of the buffer solution at pH 7 for methyl red, which contained an optical-fiber optical-fiber sensing element of the array, is about 1.4 µs. We also observed the response of the sensingPWM element of the system array, isafter about 1.4 µs. We also observed the response of the PWM proposed sensing removing the buffer solution of pH 7 and weproposed found that as the sensing system after removing the buffer solution of pH 7 and we found that as the buffer solution is buffer solution is removed from the test chamber the pulse width of the received sensing returns to removed from the test chamber the pulse width of the received sensing returns to its original state in its original state in less than 12 s and we observed that there is no pulse width difference between the less than 12 s and we observed that there is no pulse width difference between the sensing and the sensing and the reference signal after removing the buffer solution from the test chamber. This result reference signal after removing the buffer solution from the test chamber. This result indicates that the indicates that the sensing performance of the designed signal processing unit is excellent, and that it sensing performance of the designed signal processing unit is excellent, and that it has the ability to hasdetect the ability detect small differences in the pulse width. small to differences in the pulse width.
Figure 5. 5. Waveform optical-fiberPWM PWMpH-sensing pH-sensing system: without Figure Waveformresponse responseofofthe the proposed proposed optical-fiber system: (a) (a) without buffer solution; and (b)(b)with pH 77 for formethyl methylred redcontaining containing optical-fiber sensing buffer solution; and withbuffer buffersolution solution at at pH optical-fiber sensing element of of thethe array. element array.
The pulse-width differences between the sensing signal and the reference signal in a buffer solution of pH 2 to 12 for methyl red containing optical-fiber sensing element S1 of the array is shown in Figure 6. In Figure 6, it is seen that the relative pulse width increases as the pH-value of the buffer solution increases.
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The pulse-width differences between the sensing signal and the reference signal in a buffer solution of pH 2 to 12 for methyl red containing optical-fiber sensing element S1 of the array is shown in Figure 6. In Figure 6, it is seen that the relative pulse width increases as the pH-value of the buffer solution increases. Sensors 2016, 16, 1885 10 of 17 Sensors 2016, 16, 1885
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Figure 6. Pulse width response of the proposed optical-fiber PWM pH sensor array for methyl red
Figure 6. Pulse width response of the proposed optical-fiber PWM pH sensor array for methyl red containing sensing element S1. containing sensing element S1. Figure 6. Pulse width response of the proposed optical-fiber PWM pH sensor array for methyl red
To observe and determine containing sensing element S1. the performance of each sensing element of the optical-fiber sensor array with respect to differentthe pH-value of the buffer solution, we slowly injected buffer solution To observe and determine performance of each sensing element of thethe optical-fiber sensor To observe and determine the performance of each sensing element of the optical-fiber sensor with different pH-value individually in the test chamber, and took measurements at room array with respect to different pH-value of the buffer solution, we slowly injected the buffer solution array withpH-value respect to individually different pH-value oftest theproposed buffer solution, we slowly injected the solution temperature. The sensing performance the optical-fiber PWM pH-sensing system of five with different in theof chamber, and took measurements at buffer room temperature. with different pH-value individually in solution the test of chamber, andis took measurements at room sensing elements of the array for a buffer pH 2 to 12 shown in Figure 7. This figure The sensing performance of the proposed optical-fiber PWM pH-sensing system of five sensing temperature. The sensing performance ofbuffer the proposed optical-fiber PWM pH-sensing system of five shows that as the amount of pH in the solution increases, the relative pulse width increases elements of the array for a buffer solution of pH 2 to 12 is shown in Figure 7. This figure shows sensing the array a buffer solution of determine pH 2 to 12the is shown in Figure 7. This figure linearly.elements A linear of curve fittingfor is used in Figure 7 to slope, i.e., the sensitivity of the that as the amount of pH in the buffer solution increases, the relative pulse width increases linearly. shows that as the amount of pH in the buffer solution increases, the relative pulse width increases proposed optical-fiber PWM pH-sensing systems. According to our experimental observations, it is A linear curve is used in Figure 7intoFigure determine the slope, the i.e., sensitivity of the proposed linearly. A fitting linear curve fitting is used 7 to determine thei.e., slope, the sensitivity the found that the sensing element of the array offers a linear sensing performance over theofwide optical-fiber PWM pH-sensing systems. According to our experimental observations, it is found proposed pH-sensing to our experimental observations, it is that dynamic optical-fiber range. It alsoPWM indicates that thesystems. highest According detection corresponds to thymol blue containing the sensing element of array a detection linear sensing performance over red the containing wide dynamic found that the sensing element of the array offers linear sensing over thesensing widerange. sensing element S3,the while theoffers lowest ratea corresponds to performance Nile It also indicates that the highest detection corresponds to thymol blue containing sensing element dynamic range. It also indicates that the highest detection corresponds to thymol blue containing element S4 of the optical-fiber sensor array. The hysteresis response of the proposed optical-fiber PWM S3, while the lowest detection rate corresponds to Nile red containing sensing element S4 of the sensing element S3, while the lowest detection rate corresponds to Nile red containing sensing pH sensing system for methyl red containing sensing element of the array is shown in Figure 8. element S4 of the optical-fiber sensor array. The hysteresis response of the proposed optical-fiber PWM optical-fiber sensor array. The hysteresis response of the proposed optical-fiber PWM pH sensing pHfor sensing system methyl red containing sensing of the array isin shown in Figure 8. system methyl red for containing sensing element of element the array is shown Figure 8.
Figure 7. pH response of the proposed optical-fiber sensor array. Figure pH responseof ofthe the proposed proposed optical-fiber sensor array. Figure 7. 7. pH response optical-fiber sensor array.
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Figure 8. 8. The The hysteresis response of of methyl methyl red red containing containing sensing sensing element element of of the the array. array. Figure array. The hysteresis hysteresis response
The proposed proposed highly highly sensitive sensitive pH pH sensing sensing system system is is based based on on the the optical-fiber optical-fiber PWM PWM principle principle The The proposed highly sensitive pH sensing system is based on the optical-fiber PWM principle and the received sensing signal pulse width change due to a small change in refractive index of the the and the the received received sensing sensing signal signal pulse pulse width width change change due due to to aa small small change change in in refractive refractive index index of of and the sensing membrane. This refractive index of the sensing membrane changes due to changes in the sensing membrane. membrane. This changes in in the the sensing This refractive refractive index index of of the the sensing sensing membrane membrane changes changes due due to to changes optical properties properties as as well well as as the the chemical chemical structure structure of of dye. dye. For For example, example, thymol thymol blue blue shows shows red, red, optical optical properties as well as the chemical structure of dye. For example, thymol blue shows red, yellow, yellow, and and blue blue color color at at pH pH 2, 2, pH 7, 7, and pH pH 12, 12, respectively. respectively. From From this this response, response, we we can can see see that that yellow, and blue color at pH 2, pH 7, andpH pH 12,and respectively. From this response, we can see that thymol blue thymol blue changes its refractive index at different pH of buffer solutions, which corresponds to thymol blue changes index its refractive index atofdifferent pH of buffer solutions, which to changes its refractive at different pH buffer solutions, which corresponds tocorresponds changes in the changes in in the the optical optical properties properties as as well well as as the the chemical chemical structure. structure. Therefore, when when the the thymol thymol blue blue changes optical properties as well as the chemical structure. Therefore, whenTherefore, the thymol blue (as well as other (as well as other pH sensitive dyes) containing sensing membrane of the optical-fiber sensing element (as well as other pHcontaining sensitive dyes) containing sensing of the optical-fiber element pH sensitive dyes) sensing membrane of themembrane optical-fiber sensing elementsensing in contact with in contact contact with with any any pH pH buffer buffer solution, solution, the the refractive refractive index index of of the the sensing sensing membrane membrane changes changes as as a in any pH buffer solution, the refractive index of the sensing membrane changes as a result of the lighta result of the light pulse amplitude as well as fall time changes, which correspond to changes in the result amplitude of the lightaspulse well as which fall time changes, which correspond to changes in the the pulse well amplitude as fall timeas changes, correspond to changes in the pulse width of pulse width of the received sensing signal and offer a linear and wide sensing range. pulse width of the received signaland andwide offersensing a linearrange. and wide sensing range. received sensing signal and sensing offer a linear The radar radar chart chart in in Figure Figure 9a 9a shows shows the the sensitivities sensitivities of of the the five five optical-fiber optical-fiber sensing sensing element element of of The The radar chart in Figure 9a shows the sensitivities of the five optical-fiber sensing element of the proposed PWM pH-sensing systems under different pH of the buffer solution. According to the the proposed proposed PWM PWM pH-sensing pH-sensing systems systems under under different different pH pH of of the the buffer buffer solution. solution. According According to to the the the results, the the proposed proposed sensing sensing systems systems offer offer the the highest highest sensitivity sensitivity for for thymol thymol blue blue containing containing sensing sensing results, results, the proposed sensing systems offer the highest sensitivity for thymol blue containing sensing element S3, S3, whose whose sensitivity sensitivity is is about about 0.46 0.46 µs/pH. µs/pH. The lowest lowest sensitivity is is offered by by the the Nile Nile red red element element S3, whose sensitivity is about 0.46 µs/pH. The The lowest sensitivity sensitivity is offered offered by the Nile red containing sensing element S4, with a sensitivity of about 0.21 µs/pH. containing sensing sensing element element S4, S4, with with aa sensitivity sensitivity of of about about 0.21 0.21 µs/pH. µs/pH. containing
Figure 9. 9. Performance of of the proposed proposed optical-fiber pH-sensing pH-sensing system: system: (a) (a) sensitivity; sensitivity; and and (b) (b) linearity. linearity. Figure Figure 9.Performance Performance ofthe the proposed optical-fiber optical-fiber pH-sensing system: (a) sensitivity; and (b) linearity.
Linearity is is an an important important parameter parameter of of the the sensor sensor and and it it is is represented represented by by the the correlation correlation Linearity Linearity is an important parameter of the sensor and itthat is represented by the correlation coefficient 22 value. 22 value value. If the R value is near 1 it indicates the performance of sensing system is is coefficient R If the R is near 1 it indicates that the performance of sensing system coefficient R R2 value. If the R2 value is near 1 it indicates that the performance of sensing system is more linear. more linear. The linear performance of the proposed PWM pH-sensing system is presented in Figure more linear. The linear performance of the proposed PWM pH-sensing system is presented in Figure The linear performance of the proposed PWM pH-sensing system is presented in Figure 9b. According 9b. According According to to the the experiment, experiment, the the fifth fifth sensing sensing element element S5 S5 of of the the array array that that contains contains rhodamine-B rhodamine-B 9b. to the experiment, the fifth sensing element S5 of the array that contains rhodamine-B with a sensing of with a sensing membrane offers the highest linearity, with a correlation coefficient R22 of with a sensing membrane offers the highest linearity, with a correlation coefficient R approximately 0.997. The lowest linearity is offered by the third sensing element S3 of the opticalapproximately 0.997. The lowest linearity is offered by the third sensing element S3 of the optical-
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membrane offers the highest linearity, with a correlation coefficient R2 of approximately 0.997. The lowest linearity is offered by the third sensing element S3 of the optical-fiber sensor array, and its R2 is approximately 0.988. In our experiment, the third sensing element, S3, which contains thymol blue, shows the highest sensitivity and lowest linearity of the sensing elements in the array. If we look at Figure 7, we can see that, in the case of thymol blue, some measuring points are shifted from the mean value, which increased the variance as a result the linearity, i.e., R2 value decreased. In our experiment, we determined the precision/reproducibility performance of the proposed optical-fiber PWM pH-sensing system. Therefore, we prepared five samples of methyl red containing an optical-fiber sensing element and observed the sensing ability of these optical-fiber sensing elements in a buffer solution with a pH of about 7. According to our experiment, we found that the five methyl-red-containing optical-fiber sensing elements had almost the same sensing performances. Table 3 lists the statistical data for those five measurements. Therefore, we can say that the sensing elements have excellent precision/reproducibility performance, and their relative standard deviations (RSDs) were about 0.019. Table 3. Statistical data of the proposed optical-fiber PWM pH-sensing elements with methyl red, including sensing membrane in a buffer solution with pH of about 7 for the five observations. Observation Number
Relative Pulse Width (µs)
Standard Deviation of the Relative Pulse Width
1 2 3 4 5
1.4 1.423 1.389 1.433 1.399
0.019
In our study, we used five different dyes containing optical-fiber sensing element of the array and we observed the precision/reproducibility performance of all sensing elements of the array and the performance were almost same. Therefore, in this study, we only represent the reproducibility performance of the methyl red containing pH sensing element S1. The reproducibility performances of methyl orange, thymol blue, Nile red, and rhodamine-B containing pH sensing elements were 0.020, 0.0195, 0.021 and 0.0199, respectively. The performance of the proposed optical-fiber PWM pH sensing system was tested for four months and used many times to observe the change of the properties of membrane as well as sensing element’s properties. We found from the collected data after this extended period that the sensing element offered almost the same sensing response. Therefore, we can say that the sensor has long-term stability and there are no sensing membrane/dye degradations of time. The standard deviation of stability of the proposed optical-fiber PWM pH sensing system was about 0.018 and the resolution of the proposed optical-fiber PWM pH sensing system was about 0.012 pH. The pulse width of the proposed optical-fiber PWM sensing system change due to change the refractive index of sensing membrane and organic fluorophores photobleach is not an issue for the sensing system. The relative voltage response of the proposed optical-fiber PWM pH-sensing system in a buffer solution of pH 2 to 12 is shown in Figure 10a. It is found that the relative voltage of the proposed pH-sensing system increases as the pH of the buffer solutions increases. In Figures 7 and 10, we have presented the relative pulse width and relative output voltage of the proposed pH sensing system under different pH of buffer solutions, respectively. Since the reference element of the array does not show any sensing response under a pH of buffer solution, it is used to compensate for common error sources such as environmental temperature and pressure. Therefore, in Figures 7 and 10, we did not present the response of the reference element of the array.
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Figure 10. 10. Sensing Sensing performance performance of of the the proposed proposed optical-fiber optical-fiber pH-sensing pH-sensing system: system: (a) Figure (a) sensing sensing ability; ability; Figure 10. Sensing performance of the proposed optical-fiber pH-sensing system: (a) sensing ability; and (b) sensitivity. and (b) sensitivity. and (b) sensitivity.
Figure 11a shows the response and recovery times of the proposed optical-fiber PWM sensing Figure 11a shows shows the the response response and and recovery recovery times times of of the the proposed proposed optical-fiber optical-fiber PWM PWM sensing sensing system for sensing element S1, which contains a methyl-red sensing membrane. It is observed that system for sensing element S1, which contains a methyl-red sensing membrane. It is observed sensing element S1, which contains a methyl-red sensing membrane. observed that that the response and recovery times were approximately proportional to the increase in pH of the buffer the response response and recovery recovery times were approximately approximately proportional proportional to the increase in pH of the buffer solution. This occurs because at a high pH, the active sites on the surface of the sensing element are solution. This occurs because at a high pH, the active sites on the surface of the sensing sensing element are saturated. Therefore, in a buffer solution with a high pH, the reaction on the surface gradually gained saturated. Therefore, in a buffer solution with a high pH, the reaction on the surface gradually gained saturated. Therefore, in buffer solution with control and slowed down the response and recovery processes. The response and recovery times of control recovery processes. The response andand recovery times of the control and andslowed sloweddown downthe theresponse responseand and recovery processes. The response recovery times of the proposed optical-fiber pH-sensing system were 8 s and 9 s, respectively. The response versus proposed optical-fiber pH-sensing system were 8were s and89ss,and respectively. The response versus recovery the proposed optical-fiber pH-sensing system 9 s, respectively. The response versus recovery times of the proposed pH-sensing system are presented in Figure 11b. times of the proposed pH-sensing system aresystem presented in Figure 11b. recovery times of the proposed pH-sensing are presented in Figure 11b.
Figure 11. Performance of the fiber-optic PWM pH-sensing system: (a) response and recovery times; Figure 11. Performance Performance of the the fiber-optic fiber-optic PWM PWM pH-sensing pH-sensing system: system: (a) Figure (a) response response and and recovery recovery times; times; and (b)11. response versus of recovery times of different pH of buffer solution. and (b) (b) response response versus versus recovery recovery times times of of different differentpH pHof ofbuffer buffersolution. solution. and
In our study, we measured the unknown pH of buffer solutions using the proposed optical-fiber In our study, we measured the unknown pH of buffer solutions using the proposed optical-fiber In our study, we measured the unknownavailable pH of buffer solutions the proposed optical-fiber PWM sensing system and the commercially pH-Ion meterusing (pH/Ion S220, Seven Compact, PWM sensing system and the commercially available pH-Ion meter (pH/Ion S220, Seven Compact, PWM sensingON, system and the commercially available meter (pH/Ion Seven Compact, Mississauga, Canada). First, we measured the pHpH-Ion of the unknown bufferS220, solution using a pHMississauga, ON, Canada). First, we measured the pH of the unknown buffer solution using a pHMississauga, ON, Canada). First, measured the pH the unknown buffer solution using a Ion meter S220. Then, we used ourwe proposed system andof compared the performance of the PWM Ion meter S220. Then, we used our proposed system and compared the performance of the PWM pH-Ion meter S220. Then, we used our proposed system and compared the performance of the PWM system using the pH-Ion meter S220. To do this, we connected a voltmeter (Keithley, 2002, Cleveland, system using the pH-Ion meter S220. To do this, we connected a voltmeter (Keithley, 2002, Cleveland, system using meter S220. To do this, we connected a voltmeter PWM (Keithley, 2002,system. Cleveland, OH, USA) to the pH-Ion output terminal of the peak detector of the optical-fiber sensing We OH, USA) to the output terminal of the peak detector of the optical-fiber PWM sensing system. We OH, USA) the to the output terminal of pH the 7peak detector the optical-fiber PWM sensing calibrated system using a known in the buffer of solution. The voltmeter read 7 mV,system. which calibrated the system using a known pH 7 in the buffer solution. The voltmeter read 7 mV, which We calibratedtothe a known pH We 7 inalso thecalibrated buffer solution. Themeter voltmeter read 7 mV, corresponds the system pH 7 of using the buffer solution. the pH-Ion S220 at a pH 7 of corresponds to the pH 7 of the buffer solution. We also calibrated the pH-Ion meter S220 at a pH 7 of which corresponds the pH of the buffermeasurements solution. We also calibratedpH theof pH-Ion S220 The at a the buffer solution.to Then, we7took several of unknown buffermeter solutions. the 7buffer Then, we took measurements of unknown pH ofpH buffer solutions. The pH of are thesolution. buffer solution. Then, weseveral took several measurements of unknown of buffer solutions. results tabulated in Table 4. From this measurement, we can say that the performance of the results are tabulated in Table 4. From this measurement, we can say that the performance of the The resultsoptical-fiber are tabulated in Table 4. From this measurement, we can say study, that thewe performance of the proposed PWM pH-sensing system was excellent. In our tested the pH of proposed optical-fiber PWM pH-sensing system was excellent. In our study, we tested the pH of body fluid (such as, urine) using the proposed optical-fiber PWM pH sensing system and we obtained body fluid (such as, urine) using the proposed optical-fiber PWM pH sensing system and we obtained an excellent response. We also have future plans to measure the pH of sea water. The proposed an excellent response. We also have future plans to measure the pH of sea water. The proposed
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proposed optical-fiber PWM pH-sensing system was excellent. In our study, we tested the pH of body fluid (such as, urine) using the proposed optical-fiber PWM pH sensing system and we obtained an excellent response. We also have future plans to measure the pH of sea water. The proposed optical-fiber PWM pH sensing system is highly sensitive and two point calibrations are needed to obtain more accurate responses. Table 4. pH measurement data using the commercially available pH-Ion meter S220 and the proposed optical-fiber PWM pH-sensing system under different pH of the unknown buffer solutions. Observation Number
pH-Ion Meter S220
Proposed Optical-Fiber PWM pH-Sensing System
1 2 3
7 4.6 10
7 mV 4.53 mV 10.09 mV
We compared the performance of the proposed optical-fiber PWM pH-sensing systems with different pH sensors: potentiometric, optical-fiber modal interferometer, and optical-fiber Fabry–Perot interferometer with respect to several different sensing parameters. These parameters include pH detection range/dynamic range width, linearity, and response and recovery times. We observed that the proposed sensing systems have better sensing ability than the above mentioned pH sensors. Moreover, the dynamic ranges of the proposed optical-fiber PWM, potentiometric [39], optical-fiber modal interferometer [40], and optical-fiber Fabry–Perot interferometer [35] pH sensors were 2–12, 3–10, 2–11 and 4.1–6.9, respectively. Therefore, the dynamic ranges of the proposed pH-sensing system was wider than those of the other above-mentioned pH sensors. The response and recovery times of the proposed optical-fiber PWM sensor array were 8 s and 9 s, respectively. On the other hand, the response and recovery times of the potentiometric [39], optical-fiber modal interferometer [40], and optical-fiber Fabry–Perot interferometer [35] pH sensors were 3 and 5 min, 60 s and 80 s, and 60 s and 90 s, respectively. In addition, the linearity (i.e., the correlation coefficient R2 ) of the proposed optical-fiber PWM pH-sensing system was 0.997, which was higher than those of the above-mentioned pH sensors. 5. Conclusions In this paper, we presented a highly sensitive sensor array with a wide dynamic pH range. The array is based on an optical-fiber PWM technique. According to this technique, the pulse width of the received sensing signal changes as the pH changes. Five different kinds of pH sensitive dyes were used. Methyl red, methyl orange, thymol blue, Nile red and rhodamine-B containing optical-fiber sensing elements were used to fabricate the sensor array. To observe the sensing ability of the proposed sensor array, we measured the buffer solution’s pH from 2 to 12, and we obtained satisfactory and excellent results. The sensitivity of the proposed sensor array was about 0.46 µs/pH, with linear sensing properties over a wide pH range and a correlation coefficient (R2 ) value of approximately 0.997. The proposed optical-fiber sensor array also offered several features including low fabrication costs, high reproducibility performance with a relative standard deviation (RSDs) of about 0.019, highly stable sensing response, reusability, and the capability of remote sensor monitoring. Moreover, the cost of the electronic components used to design the electronic circuitry is low, and the components are available at local electronic suppliers. In future studies, we will use other pH indicators to fabricate optical-fiber sensing elements to increase the number of sensing elements in the array. We also plan to design an optical-fiber probe-type pH sensor, an interdigitated capacitor-based pH sensor array, and an optical-fiber PWM taste sensor array. Acknowledgments: This study was supported by the BK21 Plus project funded by the Ministry of Education, Korea (21A20131600011).
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Author Contributions: Md. Rajibur Rahaman Khan is the main author of the manuscript and research. He proposed the detection idea, designed the methodologies of the fiber-optic PWM pH sensing system, performed the experiments, draw the schematic diagrams, wrote the text of the manuscript and revision of the manuscript. Shin-Won Kang provided insightful comments and suggestions. Conflicts of Interest: The authors declare no conflict of interest.
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