biosensors Article
Glucose Sensing Using Capacitive Biosensor Based on Polyvinylidene Fluoride Thin Film Ambran Hartono 1 1
2
*
ID
, Edi Sanjaya 1 and Ramli Ramli 2, *
ID
Department of Physics, Faculty of Sciences and Technology, UIN Syarif Hidayatullah Jakarta, Tangerang Selatan, Banten 15412, Indonesia;
[email protected] (A.H.);
[email protected] (E.S.) Department of Physics, Faculty of Mathematics and Natural Sciences, Universitas Negeri Padang, Padang 25131, Indonesia Correspondence:
[email protected]; Tel.: +62-813-2102-9889
Received: 10 November 2017; Accepted: 24 January 2018; Published: 30 January 2018
Abstract: A polyvinylidene fluoride (PVDF) film-based capacitive biosensor was developed for glucose sensing. This device consists of a PVDF film sandwiched between two electrodes. A capacitive biosensor measures the dielectric properties of the dielectric layers at the interface between the electrolyte and the electrode. A glucose oxidase (GOx) enzyme was immobilized onto the electrode to oxidize glucose. In practice, the biochemical reaction of glucose with the GOx enzyme generates free electron carriers. Consequently, the potential difference between the electrodes is increased, resulting in a measurable voltage output of the biosensor. The device was tested for various glucose concentrations in the range of 0.013 to 5.85 M, and various GOx enzyme concentrations between 4882.8 and 2.5 million units/L. We found that the sensor output increased with increasing glucose concentration up to 5.85 M. These results indicate that the PVDF film-based capacitive biosensors can be properly applied to glucose sensing and provide opportunities for the low-cost fabrication of glucose-based biosensors based on PVDF materials. Keywords: capacitive biosensor; glucose sensing; GOx enzyme; PVDF film
1. Introduction According to a World Health Organization (WHO) report on diabetes in 2016 [1], diabetes is a chronic and progressive disease characterized by elevated blood glucose levels. Diabetes is one of the leading causes of death and disability, such as blindness, nerve degeneration, and kidney failure [2]. Therefore, early monitoring of blood glucose levels is crucial. Developments in glucose sensing for diabetes monitoring has been reported [3]. The first generation of glucose biosensors was reported by Clark and Lyons in 1962 [4]. Various technologies have been used for glucose sensing. Among them are infrared spectroscopy [5], fluorescence spectroscopy [6], Raman spectroscopy [7], liquid chromatography [8], polarimetry [9], and impedance spectroscopy [10]. However, expensive equipment requirements and complicated operations have become some of the obstacles to accessible glucose monitoring. A promising tool for sensing glucose is a capacitive biosensor. Capacitive biosensors are electronic sensors that work based on the concept of capacitive sensing. The sensor works based on changes in the electrical energy charge that can be stored by the sensor as a result of changes in the distance plate, changes in the cross-sectional area, and volume changes in the dielectric of the capacitive material [11]. The capacitive biosensors measure the changes in dielectric properties at the interface between the electrode and electrolyte when the analyte interacts with the immobilized receptors on the isolated dielectric layer [12]. The use of capacitive biosensors in glucose detection has been reported by several researchers [13–17].
Biosensors 2018, 8, 12; doi:10.3390/bios8010012
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The construction of a capacitive sensor involves a polyvinylidene fluoride (PVDF) film The construction of of a capacitive sensorsensor involves a polyvinylidene fluoride (PVDF) film sandwiched The construction a capacitive fluoride sandwiched between two electrodes, as shown ininvolves Figure 1.aIfpolyvinylidene a voltage difference exists(PVDF) betweenfilm the between two electrodes, as shown in Figure 1. If a voltage difference exists between the two electrodes, sandwiched between two electrodes, shown Figure The 1. Ifcapacitance a voltage difference between the two electrodes, a capacitance betweenasthem willinappear. producedexists is proportional to a capacitance between them will appear. The capacitance produced is proportional to surface area two a capacitance between appear. The capacitance produced is the proportional to the electrodes, surface area of the electrodes andthem the will dielectric constant, and is inversely proportional to the of the electrodes theelectrodes dielectric constant, and is inversely proportional to the distance between the the surface area and ofthe the andDifferent the dielectric constant, is inversely proportional to the distance between two electrodes. applications ofand capacitive biosensors developed for two electrodes. Different applications of capacitive biosensors developed for different targets have been distance the two Different applications of capacitive biosensors developed for differentbetween targets have beenelectrodes. reported [11]. reported [11]. different targets have been reported [11].
Figure 1. Schematic diagram of a capacitive sensor based on polyvinylidene fluoride (PVDF) film. Figure 1. Schematic diagram of a capacitive sensor based on polyvinylidene fluoride (PVDF) film. Figure 1. Schematic diagram of a capacitive sensor based on polyvinylidene fluoride (PVDF) film.
In this paper, we present the concept of capacitive sensors for glucose sensing. The detection of In this paper, we present the concept of capacitive sensors for glucose The detectionThe of glucose works through the enzymatic catalysis reaction that produces or sensing. consumes electrons. In this paper, we present the concept of capacitive sensors for glucose sensing. The detection of glucose through the called enzymatic that produces or consumes electrons. The enzymesworks are appropriately redoxcatalysis enzymesreaction [18]. The sensor substrate usually consists of three glucose works through the enzymatic catalysis reaction that produces or consumes electrons. The enzymes enzymes arethe appropriately called redox enzymeselectrode, [18]. Theand sensor usually consists of three electrodes: reference electrode, the working the substrate counter electrode. Target analytes are appropriately called redox enzymes [18]. The sensor substrate usually consists of three electrodes: electrodes: theinreference electrode, the working counter electrode. Target analytes are involved the reactions that occur on the electrode, surface ofand the the active electrode, and the reaction can the reference electrode, the working electrode, andsurface the counter Target analytes arereaction involved in are in the reactions thatinoccur on the of can theelectrode. active electrode, the can leadinvolved to the transfer of electrons the dielectric layer or contribute to the and layer. The measured the reactions that occur on the surface of the active electrode, and the reaction can lead to the transfer of lead theistransfer of electrons the of dielectric layer or can contribute to equivalent the layer. The measured flowto rate proportional to the in flow electrons in the dielectric and is to the analyte electrons in the dielectric layer or can contribute to the layer. The measured flow rate is proportional to the flow rate is proportional to the flow of electrons in the dielectric and is equivalent to the analyte concentration [19]. flow of electrons in the dielectric and is equivalent to the analyte concentration [19]. concentration [19].the detection of glucose works as a capacitive sensor where the sensor component In principle, In principle, principle, the the detection detection of of glucose glucose works works as as aa capacitive capacitive sensor sensor where the the sensor sensor component component In consists of two electrodes (silver metal) sandwiching a dielectric where material (PVDF film). Both consists of two electrodes (silver metal) sandwiching a dielectric material (PVDF film). Both electrodes consists of are two electrodes (silver metal) as sandwiching a dielectric material film). Both electrodes connected to the I–V meter, shown in Figure 2. The initial state(PVDF before the reaction are connected to the I–V meter, as shown in Figure 2. The initial state before the reaction of measuring electrodes are connected to the I–V meter, as shown in Figure 2. The initial state before the reaction of measuring instruments showed no signal. At the first dielectric material, PVDF has a polarity P0, instruments showed no signal. At the dielectric PVDF has a polarity P0,has anda the potential of measuring instruments showed nofirst signal. At thematerial, first dielectric material, PVDF polarity P0, and the potential difference between the electrodes ΔV is 0. The chemical reaction between glucose difference between the electrodes ∆V is 0. The chemical reaction between glucose and the free air with and the electrodes is 0. The chemical glucose and the thepotential free air difference with the between GOx enzyme catalyst ΔV generates free chargereaction carriersbetween (electrons). The the GOx catalyst generates free charge carriers (electrons). concentration of the chargeThe on and the enzyme free air with the GOx catalyst generates freeThe carriers concentration of the charge on theenzyme electrodes causes the polarity ofcharge the electric field(electrons). that affects the the electrodes of causes the polarity the electriccauses field that material. concentration the charge on theofelectrodes the affects polaritythe ofdielectric the electric field that affects the dielectric material. dielectric material.
Figure 2. The schematic capacitive sensor before and after the reaction. Figure 2. 2. The The schematic schematic capacitive capacitive sensor sensor before before and and after after the the reaction. reaction. Figure
The polarity of dielectric P0 transforms into P′. This polarity change, ΔP = P0 − P′, causes the 0 . This ofofdielectric into ≠P polarity change, ∆P = P0 ΔP −of P=0 ,current causes potential The polarity polarity dielectric P0 transforms into This polarity P0 − P′,the causes the 0 transforms potential difference of the Pelectrodes (ΔV 0). P′. Empirically, thechange, amount detected is difference of the electrodes (∆V 6 = 0). Empirically, the amount of current detected is dependent on the potential difference of the electrodes (ΔV ≠ 0). Empirically, the amount of current detected is dependent on the potential difference. The potential difference generated depends on the amount of potential The potential generated depends thecase, amount of on chemical reagent dependent on the concentration, potential difference. The potential generated depends the amount of chemical difference. reagent fordifference both glucose anddifference enzymes. Inon this the concentration charge concentration, for reaction both glucose andfor enzymes. In thisof case, the concentration charge ofconcentration the chemical chemical reagent concentration, boththe glucose and enzymes. In this case, charge of the chemical results cause flow electric charges that arethe detected in the reaction form of results cause the flow of electric charges that are detected in the form of voltage (V). In other words, of the chemical reaction results cause the flow of electric charges that are detected in the form of voltage (V). In other words, the magnitude of the output voltage is proportional to the amount of the magnitude the output voltage isreaction proportional the amount of charge generated to from chemical voltage (V). Inof other words, the magnitude of thetooutput voltage is proportional thethe amount of charge generated from the chemical [20]. reactiongenerated [20]. charge from the chemical reaction [20]. 2. Materials and Methods 2. Materials and Methods 2.1. Fabrication of PVDF Thin Film as a Capacitive Sensor 2.1. Fabrication of PVDF Thin Film as a Capacitive Sensor In this research, the PVDF film was fabricated using the deep coating method. The deep coating In thisisresearch, the PVDF film wasafabricated using thewith deepthe coating method. The deep technique a technique used to make thin polymer film dissolution method. Thecoating PVDF technique is a technique used to make a thin polymer film with the dissolution method. The PVDF
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2. Materials and Methods 2.1. Fabrication of PVDF Thin Film as a Capacitive Sensor Biosensors 2018, 8, x FOR PEER REVIEW
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In this research, the PVDF film was fabricated using the deep coating method. The deep coating technique is a technique used to make a thin polymer film with the dissolution method. The PVDF powder is dissolved in a dimethyl formamide (DMF) solvent, and the film is produced through a powder is dissolved in a dimethyl formamide (DMF) solvent, and the film is produced through a PVDF PVDF solution coating process on the glass substrate by using a deep coating machine. solution coating process on the glass substrate by using a deep coating machine. The initial stage of this experiment involved the manufacturing of PVDF thin films by making The initial stage of this experiment involved the manufacturing of PVDF thin films by making PVDF solutions with various concentrations. PVDF powder was dissolved in DMF solvent. The PVDF solutions with various concentrations. PVDF powder was dissolved in DMF solvent. The PVDF PVDF solution was made for three different concentrations, 10%, 15%, and 20%, referred to as S#1, solution was made for three different concentrations, 10%, 15%, and 20%, referred to as S#1, S#2, S#2, and S#3. The dissolution of PVDF powder was conducted in a beaker glass and stirred using a and S#3. The dissolution of PVDF powder was conducted in a beaker glass and stirred using magnetic stirrer. The dissolution process was performed for 60 min, and a temperature of 100 °C was a magnetic stirrer. The dissolution process was performed for 60 min, and a temperature of 100 ◦ C maintained to obtain a homogeneous solution. was maintained to obtain a homogeneous solution. The fabrication of PVDF thin films was first performed for the PVDF solution with a The fabrication of PVDF thin films was first performed for the PVDF solution with a concentration concentration of 10%. Dyeing was completed once and the duration of immersion was 30 h. Next, the of 10%. Dyeing was completed once and the duration of immersion was 30 h. Next, the PVDF film PVDF film was dried and then exfoliated from glass preparations. Thereafter, the thin PVDF films was dried and then exfoliated from glass preparations. Thereafter, the thin PVDF films were annealed were annealed for temperature variations of 70 °C, 80 °C, 90 °C, 100 °C, and 110 °C. Samples of thin for temperature variations of 70 ◦ C, 80 ◦ C, 90 ◦ C, 100 ◦ C, and 110 ◦ C. Samples of thin PVDF films were PVDF films were characterized by Fourier Transform Infrared Spectroscopy (FTIR) and X-Ray characterized by Fourier Transform Infrared Spectroscopy (FTIR) and X-Ray Diffraction (XRD). Diffraction (XRD). PVDF film that was produced and had good piezoelectric quality was used as a sensor element, PVDF film that was produced and had good piezoelectric quality was used as a sensor element, then coated with silver as an electrode, which was then assembled into a sensor instrument. The sensor then coated with silver as an electrode, which was then assembled into a sensor instrument. The design is shown in Figure 3. sensor design is shown in Figure 3.
Figure 3. The scheme for glucose detection with a capacitive sensor [21]. Figure 3. The scheme for glucose detection with a capacitive sensor [21].
2.2. Glucose Sensing 2.2. Glucose Sensing The experimental stages involved preparing materials and equipment, making the GOx enzyme The experimental stages involved preparing materials and equipment, making the GOx enzyme and glucose solutions, coating the GOx enzyme, and sensor testing. The materials used in this and glucose solutions, coating the GOx enzyme, and sensor testing. The materials used in this experiment were GOx enzyme ((Sigma Aldrich, Singapore), saccharine (Sigma Aldrich, Singapore), experiment were GOx enzyme ((Sigma Aldrich, Singapore), saccharine (Sigma Aldrich, Singapore), 70% alcohol (Sigma Aldrich, Singapore), and Aqua Dest (Merck, Indonesia). The equipment used 70% alcohol (Sigma Aldrich, Singapore), and Aqua Dest (Merck, Indonesia). The equipment used included dropper drops (0.5 mL, 1 mL, and 5 mL), sample bottle, measuring cup, beaker glass, sample included dropper drops (0.5 mL, 1 mL, and 5 mL), sample bottle, measuring cup, beaker glass, box, PVDF film, and I–V meter. sample box, PVDF film, and I–V meter. 2.2.1. Preparation of GOx Enzyme Solution 2.2.1. Preparation of GOx Enzyme Solution The steps for making the solution involved 25 KU (Kilo unit, 1 KU = 100 mg) of GOx enzyme The steps for making the solution involved 25 KU (Kilo unit, 1 KU = 100 mg) of GOx enzyme dissolved in 10 mL Aqua Dest (25 KU/10 mL), called Z1 solution. The Z1 solution was then divided dissolved in 10 mL Aqua Dest (25 KU/10 mL), called Z1 solution. The Z1 solution was then divided into into two parts of 5 mL each. One part of the Z1 solution was dissolved in 5 mL of Aqua Dest (25 two parts of 5 mL each. One part of the Z1 solution was dissolved in 5 mL of Aqua Dest (25 KU/20 mL), KU/20 mL), called Z2 solution. Z2 solution was again divided into two parts of 5 mL each. One part called Z2 solution. Z2 solution was again divided into two parts of 5 mL each. One part of the Z2 of the Z2 solution was then dissolved in 5 mL Aqua Dest to obtain Z3 (25 KU/40 mL). The same solution was then dissolved in 5 mL Aqua Dest to obtain Z3 (25 KU/40 mL). The same process was process was completed until a GOx enzyme solution was obtained with 10 concentrations, as shown completed until a GOx enzyme solution was obtained with 10 concentrations, as shown in Table 1. in Table 1. The GOx enzyme acts as a bio-receptor in the sensor design. Therefore, the PVDF films should be The GOx enzyme acts as a bio-receptor in the sensor design. Therefore, the PVDF films should integrated with enzymes, with a coating of GOx enzyme solution on the PVDF film surface. The coated be integrated with enzymes, with a coating of GOx enzyme solution on the PVDF film surface. The film was allowed to sit for 1 h until the enzyme solution dried. This step was repeated for all enzyme coated film was allowed to sit for 1 h until the enzyme solution dried. This step was repeated for all solutions with different concentrations that had been made. enzyme solutions with different concentrations that had been made.
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Table 1. Concentration of glucose oxidase (Gox) enzyme solution. GOx Solution
Concentration (U/L)
Z1 Z2 Z3 Z4 Z5 Z6 Z7 Z8 Z9 Z10
2,500,000 1,250,000 625,000 312,500 156,250 78,125 39,062.5 19,531.25 9765.625 4882.813
2.2.2. Preparation of Glucose Solution The glucose solution to be measured by the sensor was made with various concentrations, as shown in Table 2. Table 2. Concentrations of the glucose solutions. Glucose Solution
Concentration (M)
C1 C2 C3 C4 C5 C6 C7 C8 C9 C10
5.85 2.92 1.46 0.73 0.37 0.18 0.09 0.05 0.025 0.013
2.2.3. Sensor Measurement Sensory testing was performed by dripping a glucose solution over a PVDF film coated with the GOx enzyme. The biochemical reaction will occur, resulting in free charge carriers (electrons). A number of free electrons in the PVDF films result in the emergence of a field affecting the polarity of the PVDF films. Consequently, this field causes electrical displacement and produces a potential difference that can be read on I–V meters. The amount of potential read on the I–V meter is influenced by the amount of reactant concentration, including glucose solution and enzyme concentration. 3. Results and Discussion Glucose Sensing Based on the results of FTIR and XRD characterization, as shown Figures S1 and S2, the PVDF film used as a glucose sensor was sample S#5. The theory that biochemical reactions in the sensors are strongly influenced by the concentration of the catalyst and reactants was demonstrated by the curve of the potential measurement results from given enzyme and glucose concentration conditions. Next, we compared the potential versus time curve for various enzyme and glucose concentrations. Figure 4a shows the potential versus time curve of the sensor for various enzyme concentrations with a glucose concentration of 5.85 M. Figure 4b shows the potential versus time curve of the sensor for various glucose concentrations with a GOx concentration of 2,500,000 units/L. According to Figure 4a,b, the resulting curves have an almost identical shape. The potential increases, followed by a constant potential moment at a certain value. These parameters will be reviewed for further
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According to Figure 4a,b, the resulting curves have an almost identical shape. The potential increases, followed by a constant potential moment at a certain value. These parameters will be reviewed for further as they relate to the reaction rate and activity ofreactions the biochemical analysis as they relate analysis to the reaction rate and enzyme activity of enzyme the biochemical occurring. reactions occurring. The relationship between potential and time shows a relationship of exponential The relationship between potential and time shows a relationship of exponential equations up to a certain equations up to a shows certainthe time. This potential shows the reaction dynamics of the enzymatic reactionand between time. This potential dynamics of the enzymatic between the GOx enzymes glucose. the GOx enzymes and glucose. The reaction between GOx enzymes and glucose will produce The reaction between GOx enzymes and glucose will produce electrons. In an oxidized state, the enzyme electrons. In an oxidized state, the enzyme and glucose reactions produce gluconic acid and release and glucose reactions produce gluconic acid and release two electrons and two protons. The electron of two electrons and two protons. The electron of this reaction changes the concentration of the charge this reaction changes the concentration of the charge on the PVDF film as a dielectric material. on the PVDF film as a dielectric material.
(a)
(b)
Figure 4. Potential versus time for (a) various enzyme concentrations with a glucose concentration of Figure 4. Potential versus time for (a) various enzyme concentrations with a glucose concentration of 5.85 M, and (b) various glucose concentrations with an enzyme concentration of 2,500,000 units/L. 5.85 M, and (b) various glucose concentrations with an enzyme concentration of 2,500,000 units/L.
The number of these electrons depends on the amount of hydrogen peroxide. The amount of The number of these electrons on theconcentration. amount of hydrogen peroxide. amount hydrogen peroxide depends on depends the glucose Consequently, the The higher the of hydrogen peroxide depends the glucose thethe higher the of concentration concentration of glucose, theonmore electronsconcentration. are generated.Consequently, This is shown by amount potential of produced glucose, the electrons generated. This is shown by the amount of potential produced for for more various glucose are concentrations. relationship between the sensor output potential and the glucose concentration is shown in variousThe glucose concentrations. Figure The sensor between output increased with increasing glucose up to 5.85 M. In The 5. relationship the sensor output potential andconcentration the glucose concentration is theory, shown in the rate of enzyme will increase with the concentration of the reactingup substrate. 5 is Figure 5. The sensor reaction output increased with increasing glucose concentration to 5.85 Figure M. In theory, analogous to the saturation curve of the enzyme reactions, as shown in Figure 6, in which the 5 the rate of enzyme reaction will increase with the concentration of the reacting substrate. Figure parameter that states the rate of reaction is the potential. The rate of reaction states the number is analogous to the saturation curve of the enzyme reactions, as shown in Figure 6, in whichofthe reactions that that states occur with every of time. In potential. this case, time expressed by the concentration of of parameter the rate of unit reaction is the The israte of reaction states the number solute in the reaction produced every second of the reaction. reactions that occur with every unit of time. In this case, time is expressed by the concentration of Electrochemical biosensors are usually based on enzymatic catalysis reactions that produce or solute in the reaction produced every second of the reaction. consume electrons. The enzyme involved is aptly called the redox enzyme. Substrate sensors usually Electrochemical biosensors are usually based on enzymatic catalysis reactions that produce or contain three electrodes: reference electrode, working electrode, and counter electrode. The target consume electrons. The enzyme involved is aptly called the redox enzyme. Substrate sensors usually analyte is involved in the reaction occurring on the surface of the active electrode, and the reaction contain three electrodes: reference electrode, working electrode, and counter electrode. The target may cause electron transfer in the dielectric layer, producing current, or it may contribute to the analyte is involved in the reaction occurring on the surface of the active electrode, and the reaction may coating, producing voltage. The measured current is proportional to the electron flow rate in the cause electron transfertointhe theconcentration dielectric layer, producing dielectric equivalent of the analyte. current, or it may contribute to the coating, producing voltage. The measured current is proportional to the electron flow rate in the dielectric equivalent to the concentration of the analyte.
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Figure 5. Sensor output voltage compared to glucose concentrations for various enzyme concentrations. Figure 5. Sensor output voltage compared to glucose concentrations for various enzyme concentrations. Figure 5. Sensor output voltage compared to glucose concentrations for various enzyme concentrations.
Figure6.6. Sensor Sensor output compared to enzyme concentrations for variousfor glucose concentrations. Figure outputvoltage voltage compared to enzyme concentrations various glucose Figure 6. Sensor output voltage compared to enzyme concentrations for various glucose concentrations. concentrations.
In the experiment, the substrate in Figure 6 is glucose. The relationship between the potential In the experiment, the substrate in Figure 6 is glucose.i.e., The relationship between the potential and of reactionthe is seen from in theFigure current amount ofbetween charge flowing per time, Inthe therate experiment, substrate 6 definition, is glucose. Thethe relationship the potential and the rate ofCoulomb/second. reaction is seen from the current definition, i.e., the amount of charge flowing per time, in units of Therefore, the current represents the reaction rate. The more electrical and the rate of reaction is seen from the current definition, i.e., the amount of charge flowing per time, in charge units ofthat Coulomb/second. Therefore, the current represents the reaction rate.The Thereaction more electrical flows into the dielectric material, the greater the rate of reaction. rate of the in units of Coulomb/second. Therefore, the current represents the reaction rate. The more electrical charge that flows into the dielectric material, the greater the rate of reaction. The reaction rate of the biochemical reaction between the material, GOx enzyme and glucose (v)of is reaction. a functionThe of potential (V), of and charge that flows into the dielectric the greater the rate reaction rate thecan biochemical reaction between the GOx enzyme and glucose (v) is a function of potential (V), and can be expressed by Equation (1): biochemical reaction between the GOx enzyme and glucose (v) is a function of potential (V), and can be expressed by Equation (1): v=τV (1) be expressed by Equation (1): v=τV (1) where τ is a constant. v=τV (1) rate per mol is obtained from the generated saturation potential value. Based on the where τThe is areaction constant. where τ is a constant. experimental results, themol increase in saturation potential resembles the exponential equation The reaction rate per is obtained from the generated saturation potential value. Basedand on is The reaction rate per mol is obtained from the generated saturation potential value. Based on by Equation (2). increase in saturation potential resembles the exponential equation and theexpressed experimental results, the the experimental results, the increase in saturation potential R0 · x resembles the exponential equation and V=V (2) 0·e is expressed by Equation (2). is expressed by Equation (2). R0 ⋅ x where V 0 is the initial potential, and A andVR=0 V are⋅ econstants. (2) R0 ⋅ x 0 V = V ⋅ e (2) 0 The second part of the right-hand side of Equation (2) requires further study because it involves where V0 is the initial potential, and A and R0 are constants. various affecting the of biochemical reactions as described by the Arrhenius equation. where V0 isparameters the initial potential, andrate A and R0 are constants. The secondwe part of the right-hand side associated of Equationwith (2) requires further study because it involves In The this second paper, examine the parameters reaction further rates due to changes and part of the right-hand side of Equation (2) requires study becauseinitglucose involves various parameters affecting the rate of biochemical reactions as described by the Arrhenius equation. enzyme concentrations. various parameters affecting the rate of biochemical reactions as described by the Arrhenius equation. In this paper, we examine the parameters associated with reaction rates due to changes in glucose In this paper, we examine the parameters associated with reaction rates due to changes in glucose and enzyme concentrations. and enzyme concentrations.
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The reaction rate per mole was obtained based on the slope of the graph in Figure 6. Based on the result of the graph curve, the value of each constant could be obtained by matching the value of the sensor output to the glucose concentration for various enzyme concentrations, as listed in Table 3. Table 3. Value of V0 and R0 in Equation (2). Enzyme Concentration (Unit/L)
Constants V0
(×10−6
2,500,000.00 1,250,000.00 675,000.00 337,000.00 168,750.00 84,375.00 42,187.5 21,093.75 10,546.88 5273.44
volt)
R0 (×10−9 mol/s)
−0.359 −0.430 −0.418 −0.623 −0.986 −0.942 −0.928 −0.948 −0.922 −1.007
6.360 5.416 4.720 3.992 3.607 3.384 3.496 3.054 2.869 2.699
Furthermore, the effect of the reactant concentration on the polarization change from the PVDF film will be presented. To analyze its influence, we began by reviewing experimental results and theoretical studies. Based on the experimental data and theoretical study of the working principle of capacitive sensors, the values of stress and charge for various concentrations of glucose at fixed enzyme concentrations are 2,500,000 units/L, as shown in Table 4. Table 4. The value of current, charge, and potential for various concentrations of glucose with a fixed enzyme concentration of 2,500,000 units/L. Glucose Concentration (M)
V (×10−6 volt)
Q (×10−14 C)
I (×10−15 A)
5.85 2.92 1.46 0.73 0.37 0.18 0.09 0.05 0.025 0.013
6.04 5.25 4.75 4.20 3.78 3.42 3.08 2.86 2.42 2.12
4.83 4.20 3.80 3.36 3.02 2.74 2.46 2.29 1.94 1.70
1.61 1.40 1.27 1.12 1.00 0.91 0.82 0.76 0.65 0.57
Table 4 shows that the greater the glucose concentration, the greater the potential produced. Theoretically, this can be understood as the greater the concentration of glucose, the greater the charge released from the reaction, so the greater the potential difference. According to Table 4, the value of the electric field (E) arising from various glucose concentrations can be calculated, and the number of dipole moments can be determined. Those quantities were used to obtain the polarization changes in PVDF films, as shown in Figure 7. The capacitive biosensor based on the PVDF film for glucose detection was performed, having a dynamic range of 0.013–5.85 M and limit of detection of 0.013 M. This result is larger than the range and limit of detection of the capacitive biosensors that have been developed by other researchers, as shown in Table 5.
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Table 5. Dynamics range, limit of detection, and sensitivity of capacitive biosensors for glucose detection. Table 5. Dynamics range, limit of detection, and sensitivity of capacitive biosensors for glucose detection.
Limit of Sensitivity Ref. Detection (M) Limit of Sensitivity Ref. Sensor Method Immobilization of Concanavalin A (ConA) on Dynamic Range (M) Detection (M) A negligible loss in sensitivity 1.0 × 10−6 [15] gold nanoparticles incorporated on the 1.0 × 10−6–1.0 × 10−2 after 10 cycles (7.5%) Immobilization of Concanavalin A (ConA) on tyramine-modified gold electrode A negligible loss in sensitivity [15] gold nanoparticles incorporated on the 1.0 × 10−6 –1.0 × 10−2 1.0 × 10−6 after 10 cycles (7.5%) Immobilization of Concanavalin A (ConA) and tyramine-modified gold electrode 1.0 × 10−6 N/D [16] replacement of small glucose with the large 1.0 × 10−5–1.0 × 10−1 Immobilization of Concanavalin A (ConA) and glucose polymer replacement of small glucose with the large N/D [16] 1.0 × 10−5 –1.0 × 10−1 1.0 × 10−6 Glucose solution over a PVDF film coated with This glucose polymer 1.3 × 10−2–5.85 1.3 × 10−2 N/D the GOx enzyme work Glucose solution over a PVDF film coated with N/D This work 1.3 × 10−2 –5.85 1.3 × 10−2 the GOx enzyme Sensor Method
Dynamic Range (M)
Figure Figure7. 7.The Thechange changeininPVDF PVDFfilm filmpolarization polarizationversus versusglucose glucoseconcentration concentrationwith withananenzyme enzyme concentration of 2,500,000 units/L. concentration of 2,500,000 units/L.
The enzymatic Thepresence presenceofof enzymaticreactions reactionsbetween betweenGOx GOxand andglucose glucoseenzymes enzymesresults resultsininfree freecharge charge carriers. that causes a change the dielectric polarity ofof the carriers.This Thisfree freecharge chargecreates createsananelectric electricfield field that causes a changeinin the dielectric polarity the PVDF film. Consequently, an electric movement occurs inoccurs the PVDF The greater concentration PVDF film. Consequently, an electric movement in film. the PVDF film.the The greater the ofconcentration glucose, the greater the change in PVDF polarity. This phenomenon that PVDF films of glucose, the greater the change in PVDF polarity. This indicates phenomenon indicates that function well as capacitive sensors. We suspect that this is related to the phenomena PVDF films function well as capacitive sensors. We result suspect that this result is relatedcalled to the chemico-electrical mechanisms. This mechanism canThis be explained as follows. The electric field from phenomena called chemico-electrical mechanisms. mechanism can be explained as follows. The the GOx enzyme andthe glucose the polarization the dipole orientation of electric field from GOxreaction enzymeaffects and glucose reaction in affects the polarization in the thePVDF dipole film. This polarization is related to changes in chemical reactions that occur betweenreactions the GOxthat enzyme orientation of the PVDF film. This polarization is related to changes in chemical occur and glucose, Equation (3). between thevia GOx enzyme and glucose, via Equation (3). dP =
∂P ∂P ∂P P ∂P ∂ S ∂P + dS ∂P dP = Sχ ε 0 dE + ∂P dC + dZ dS + χ ε dE + dC + ∂ ∂ ∂ ∂ Z C T S 0 E ;S ∂C ∂Z dZ ∂S E=0;T E =0;T ∂T E;S
(3) (3)
where P is polarization, S is strain, T is temperature, E is the electric field, χ is susceptibility, ε0 is the where P is polarization, S is strain, T is temperature, E is the electric field, χ is susceptibility, ε0 is the vacuum dielectric permittivity, C is glucose concentration, and Z is the enzyme concentration. vacuum dielectric permittivity, C is glucose concentration, and Z is the enzyme concentration. The change in polarization due to the change of chemical reactions is expressed in Equations (4) and The change in polarization due to the change of chemical reactions is expressed in Equations (4) (5). and (5). ∂P ∂P ∂P ∂P dP = dP = dC ++ dZ dZ (4) dC (4) ∂C ∂C ∂Z ∂Z P −PP−r =Prc(C −C = c(C − 0C) 0+) +z(Z z(Z−−ZZ00))
(5) (5)
where the reactant constant and z is thethe enzyme constant. wherec is c is the reactant constant and z is enzyme constant.These Theseconstants constantsassociated associatedwith withthe the reaction rate are due toto changes inin the reactant concentration. reaction rate are due changes the reactant concentration.
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4. Conclusions In summary, we discovered a glucose sensing technique using a capacitive biosensor based on a PVDF film. The sensor output potential increases with increasing concentrations of glucose and GOx enzymes. The reaction rate between the GOx enzyme and glucose increases with increasing glucose and enzyme concentrations. The concentration of enzymes and glucose influences the change in PVDF film polarization through chemico-electrical mechanisms. This event shows the relationship of the enzyme and glucose concentration parameters to the polarization, following an exponential equation. We concluded that the PVDF film-based capacitive biosensors can be properly applied to glucose sensing. The dynamics range and detection limits of the capacitive biosensors that we developed are low, but still warrant further development. The results of this study provide opportunities for the fabrication of glucose-based biosensors based on PVDF materials in the future. Supplementary Materials: The following are available online at www.mdpi.com/2079-6374/8/1/12/s1: Figure S1. The Fourier Transform Infrared (FTIR) results for the PVDF film with different solution concentrations. Figure S2. The X-ray Diffraction (XRD) results for the PVDF film with different annealing temperatures. Acknowledgments: The authors would like to thank the Institute for Research and Community Service (LP2M) at Universitas Islam Negeri (UIN) Syarief Hidyatullah Jakarta 2017, Indonesia, for technical support of this work. Author Contributions: A.H. and R.R. performed all of the experiments and analyzed the data; A.H. wrote the manuscript draft; E.S and R.R. reviewed and finalized the manuscript. All authors approved the final version of the manuscript. Conflicts of Interest: The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.
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