WO2012064179A1

WO2012064179A1 - Ph sensor

Info

Publication number: WO2012064179A1
Application number: PCT/MY2011/000135
Authority: WO
Inventors: Ahmad Mohd Rais; Alva Sagir
Original assignee: Mimos Berhad
Priority date: 2010-11-12
Filing date: 2011-06-23
Publication date: 2012-05-18

Abstract

The present invention relates to an improved pH sensor (10) for electrochemical sensing and method of preparing the same. The pH sensor (10) based on doped polypyrrole electrochemical transducer that gives stable electrical signal. The pH sensor (10) does not require the use of hy-drophilic internal layer and is easy to fabricate.

Description

Title of Invention: PH SENSOR

Technical Field

[1] The present invention relates to an improved pH sensor for electrochemical sensing and method of preparing the same .

Background Art

[2] Conventional pH measurements have been carried out by using glass electrodes. The glass electrodes have advantages in terms of selectivity, reliability and wide dynamic range. Despite the advantages, pH glass electrodes have setbacks due to its high resistance, fragility, its instability in hydrofluoric acid or fluoride solutions. Moreover due to its bulky size and fragile nature glass pH electrode is not suitable for biological, environmental and agriculture applications that require maintenance free miniaturized device.

[3] Miniaturized pH sensors based on polymeric membrane especially poly(vinyl

chloride) (PVC) sensing membrane have been introduced to replace glass electrodes. PVC membranes have been widely used due to its low electrical resistance and ease of fabrication. In addition PVC material exhibits physical strength that makes it a good choice for measuring pH in the fields for in situ agriculture, medical and environmental applications.

[4] Traditionally silver-silver chloride (Ag/AgCl) electrode has been employed as the electrochemical transducer in chemical sensors and reference electrode. While the Ag/ AgCl electrode works best in the bulky glass electrode equipped with comparatively large volume of liquid internal electrolyte, the miniaturized solid state version employs hydrophilic polymeric membrane as an internal layer. In a typical laboratory procedure, the internal layer is hydrated with certain concentration of the target analyte, to act as the reference concentration. Depending on storage condition and due to other factors like aging the concentration of the internal layer changes and thus causes the electrical signal to change. This requires frequent or daily calibration; else the sensor would transmit inaccurate data.

[5] Daily calibration is difficult for field deployed integrated sensors. Another problem caused by the hydrophilic internal layer is its incompatibility with the sensing membranes. The sensing membrane is usually highly lipophilic and its adhesion to hydrated hydrophilic internal layer is marginal and this results in peeling of the sensing membrane. [6] Conductive polymers have been introduced as electrochemical transducer and it has been shown that chemical sensors employing conductive polymer can produce fast response and stable signal without the use of additional internal layer. Polypyrrole, polythiophene and polyaniline have been used as conductive polymers for chemical sensors.

[7] Polypyrole has been the most widely used due to numerous advantages. Doping the polypyrrole increases the conductivity of the conducting polymer and this is usually achieved by electropolymerization from solution of pyrrole monomer containing electrolyte of chloride or nitrate salts of potassium. This causes a problem because the doping electrolyte is aqueous solution, whereas the pyrrole monomer is only moderately soluble in this electrolyte. Therefore, vigorous shaking and continous stirring are required to achieve homogenous mixture and to maintain homogeneity of the doped polypyrrole on a pH sensor.

[8] Hence, there is still a need in the art for an improved pH sensor for electrochemical sensing.

Disclosure of Invention

Technical Problem

[9]

Technical Solution

[10]

Summary

[11] According to a first aspect of the invention, there is provided a pH sensor 10

comprising; a substrate 18, a thick film screen printed carbon layer 16, a pH sensing membrane 12; polypyrole layer 14; characterized in that the polypyrole layer 14 is homogeneously doped on the carbon layer 16.

[12] Accordingly, the pH sensor 10 wherein the doped polypyrrole layer 14 having

doping electrolyte of the following structure;

[13]

[14] X= CI, Br, I, BF₄, PF₆, OAc, CF₃C0₂, N0₃, Fe(CN)₆, oxalate, tosylate

[15] n= 1, 2, 3

[16] R= H, methyl, ethyl, butyl, allyl

[17] Accordingly, the pH sensing membrane 12 is photo-cured co-polymer of methyl methacrylate and tetrahydrofurfuryl acrylate containing photoinitiator, diacrylate crosslinker, lipophilic borate salt and hydrogen ionophore.

[18] Accordingly, the pH sensing membrane 12 is bulk co-polymer of methyl

methacrylate and tetrahydrofurfuryl acrylate containing photoinitiator, diacrylate crosslinker, lipophilic borate salt and hydrogen ionophore.

[19] Accordingly, the use of the pH sensor 10 in the manufacture of an ion selective

electrode (ISE) pH sensing device.

[20] Accordingly, the use of the pH sensor 10 in the manufacture of an ion sensitive field effect transistor (ISFET) pH sensing device.

[21] According to a second aspect of the present invention there is provided a method of depositing doped polypyrrole electrochemical transducer layer comprising;

[22] 1. a) preparing a pyrrole monomer doped electrolyte solution and a hydrophilic dopant in a polar solvent; D

[23] 1. b) immersing a carbon electrode, a counter electrode and a reference electrode into the doped pyrrole electrolyte solution; and D

[24] 1. c) depositing the doped polypyrrole electrochemically . D

[25] Accordingly, in step (a) the pyrrole monomer doped electrolyte solution having concentration ranges from 0.1M to 3M of pyrrole monomer and the hydrophilic dopant in a polar solvent having a concentration ranges from 0.1M to 3M.

[26] Preferably, the hydrophilic dopant is choline chloride.

[27] Accordingly, the depositing step (c) is by electrochemical polymerization of constant current method with current density of 0.1 to 10 mA per square centimetre.

[28] Accordingly, the depositing step (c) is by electrochemical polymerization utilising cyclic voltammetry method with scans between - IV and +1V.

[29] Advantageously, the method of depositing doped polypyrrole electrochemical

transducer layer increases the potential of obtaining homogeneously doped polypyrrole layer on the pH sensor.

[30] Advantageously, the preparation of the pH sensor takes shorter time by utilising the method of depositing doped polypyrrole electrochemical transducer layer.

Description of Drawings

[31] Figure 1 : illustrates pH Sensor with doped polypyrrole

[32] Figure 2 : iillustrates hydrophilic organic salts for polypyrrole doping

[33] Figure 3 : illustrates doped polypyrrole conducting polymer.

[34] Figure 4 : illustrates molecular structures of methyl methacrylate (2) and tetrahydrofurfuryl acrylate (3) monomers

[35] Figure 5 : illustrates cyclic voltammatograms of Ppy(Cl) from ethanol solvent on Pt in 0.1M KC1 with 90 and 150 sec electropolymerisation time.

[36] Figure 6 : illustrates pH sensor response with various electropolymerisation times

[37] Figure 7 : illustrates cyclic voltammatograms of Ppy(Cl) from 50% ethanol solvent on Pt in 0.1M KCl with 0.5, 1, 1.5, 2, 3, 4 and 5 mA cm ² electropolymerisation current density.

[38] Figure 8 : illustrates pH sensor response (50% v/v ethanol platform) with various electropolymerisation current densities.

[39] Figure 9 : illustrates cyclic voltammatograms of Ppy(Cl) from 25% ethanol solvent on Pt in 0.1M KCl with 1, 2, 3, 4 and 5 mA cnr² electropolymerisation current density.

[40] Figure 10 : illustrates pH sensor response (25% v/v ethanol platform) with various electropolymerisation current densities.

[41] Figure 11 : illustrates plot of response versus Hydrogen Ion Activity for MT28 pH

Sensor

[42]

[43] Detailed description of the present invention

[44] The present invention relates to a miniaturized solid state pH sensor as illustrated in Figure 1 based on doped polypyrrole wherein the doping electrolytes are hydrophilic organic saltsl as illustrated in Figure 2 dissolved in polar organic solvent, mixture of polar solvents or mixture of polar solvent and deionized water. In one embodiment, the miniaturized solid state pH sensor 10 based on doped polypyrrole electrochemical transducer that gives stable electrical signal. The pH sensor 10 does not require the use of hydrophilic internal layer and is easy to fabricate.

[45] Figure 1 shows a pH sensor 10 comprising; a substrate 18, a thick film screen

printed carbon layer 16, a pH sensing membrane 12; polypyrole layer 14; wherein the polypyrole layer 14 is homogeneously doped on the carbon layer 16. Figure 1 also shows that the polypyrrole conducting polymer 14 is deposited on top of screen printed thick film carbon 16 electrode. The carbon 16 surface must first be cleaned by sonication, and the pyrrole monomer is electropolymerized from the doping electrolyte.

[46] In one embodiment of the pH sensor 10, the doped polypyrrole layer 14 having

doping electrolyte of the following structure;

[48] X= CI, Br, I, BF₄, PF₆, OAc, CF₃C0₂, N0₃, Fe(CN)₆, oxalate, tosylate

[49] n= l, 2, 3

[50] R= H, methyl, ethyl, butyl, allyl [52] In one embodiment of the pH sensor 10, the pH sensing membrane 12 is photo-cured co-polymer of methyl methacrylate and tetrahydrofurfuryl acrylate containing pho- toinitiator, diacrylate crosslinker, lipophilic borate salt and hydrogen ionophore.

[53] In another embodiment of the pH sensor 10, the pH sensing membrane 12 is bulk copolymer of methyl methacrylate and tetrahydrofurfuryl acrylate containing pho- toinitiator, diacrylate crosslinker, lipophilic borate salt and hydrogen ionophore.

[54] The pH sensor 10 of the present invention is usable in the manufacture of an ion selective electrode (ISE) pH sensing device. Also, the pH sensor 10 is usable in the manufacture of an ion sensitive field effect transistor (ISFET) pH sensing device.

[55] The polymerization process of the pyrrole monomer releases two moles of electrons for each mole of the monomer. In the oxidized state polypyrrole exists as a polyradical cation, and at this state anions such as chloride, are attracted electrostatically into the polymerized film as counter ions or dopants as illustrated in Figure 3.

[56] The electrolytes of hydrophilic organic salts are prepared by dissolution in polar organic such as ethanol, methanol, 2-methoxy ethanol, dimethyl sulfoxide, acetonitrile or tetrahydrofuran. Mixture of polar organic solvents or mixture of the organic solvent and deionized water can also be used to dissolve the salts and the pyrrole monomer. Pyrrole exhibits very high solubility in these solvents and vigorous shaking or stirring is not required for mixing or in keeping homogeneity during electropolymerization.

[57] A method of depositing doped polypyrrole electrochemical transducer layer

comprising;

[58] 1. a) preparing a pyrrole monomer doped electrolyte solution and a hydrophilic dopant in a polar solvent; D

[59] 1. b) immersing a carbon electrode, a counter electrode and a reference electrode into the doped pyrrole electrolyte solution; and D

[60] 1. c) depositing the doped polypyrrole electrochemically . D

[61] In one example, in step (a) the pyrrole monomer doped electrolyte solution having concentration ranges from 0.1M to 3M of pyrrole monomer and the hydrophilic dopant in a polar solvent having a concentration ranges from 0.1M to 3M. Preferably, the hydrophilic dopant is choline chloride.

[62] In one example, the depositing step (c) is by electrochemical polymerization of

constant current method with current density of 0.1 to 10 mA per square centimetre.

[63] In another example, the depositing step (c) is by electrochemical polymerization

utilising cyclic voltammetry method with scans between -IV and +1V.

[64] In one example, pyrrole electropolymerization process takes place at the screen

printed carbon 16 working electrode while conventional double-junction reference electrode and platinum or glassy carbon electrode counter electrode complete the chronopotentiometry setup. [65] Co-polymer of acrylates, methyl methacrylate (2) and tetrahydrofurfuryl acrylate (3) has been employed as the pH sensing membranes 12 based on doped polypyrrole. The monomers can be photo-polymerized from a cocktail containing the monomers, pho- toinitiator, crosslinker, lipophilic borate salt and hydrogen ionophore.

[66] Likewise, bulk co-polymer of the above monomers can also be used as the sensing membrane 12. In another example of preparation, the bulk co-polymer is first synthesized by refluxing the desired ratios of the monomers and benzoyl peroxide in benzene. After purification and drying, samples from the bulk polymer are dissolved with methylene chloride or tetrahydrofuran, along with the above components for pH sensing membrane 12. A few microliters of the bulk cocktail is dispensed coated on the sonicated polypyrrole electrode and the solvent air dried before testing for pH response can be conducted.

[67] Polypyrole is a conducting polymer due to its conjugated network of double bonds.

In this manner it acts as a molecular wire and thus employed to conduct electrical current. Polypyrrole has another property that is of interest in chemical sensing - it undergoes reversible oxidation-reduction cycle at well-defined potentials. This property allows the use of polypyrrole as electrochemical transducer in chemical and biosensors.

[68] Advantageously, the method of depositing doped polypyrrole electrochemical

[69] Advantageously, the preparation of the pH sensor takes shorter time by utilising the method of depositing doped polypyrrole electrochemical transducer layer.

[70] The invention now being generally described, the same will be better understood by reference to the following detailed examples which are provided for purposes of illustration only and are not to be limiting of the invention unless so specified.

[71] Example 1

[72] Preparation of pH Sensor with Choline Chloride Doped Polypyrrole Transducer - Different Polymerization Times

[73] The screen printed electrodes (SPE) with 4 mm diameter were cleaned ultrasonically with deionised water for 1 min. The electrochemical polymerisation was performed in a conventional three-electrode cell with a Pt as counter electrode and the Ag/AgCl double junction as reference electrode using Autolab PGSTAT MODEL 128N for 90 and 150 sec. The polypyrrole (Ppy) films were generated with current density of 2 mA cm-² in aqueous solution of 0.5M pyrrole containing 1M choline chloride dopant and 50% v/v ethanol solvent. After electropolymerisation, cyclic voltammetry experiments were conducted between -1.0 V and +1.0 V with a potential sweep rate of 100 mV sec ¹ in 0.1M potassium chloride (KC1) solution. The plots were shown as illustrated in Figure 5.

[74] pH cocktail 12 was prepared by mixing 37 mg poly(vinyl) chloride (PVC), 3 mg sodium tetrakis[bis-3,5(trifluoromethyl)phenyl] borate (NaTFPB), 10.6 mg Tridodecyl amine (Hydrogen Ionophore I), 67 mg Bis(2-Ethylhexyl) Sebacate (DOS) and 600 DL tetrahydofuran (THF) solvent. Then, the homogenous cocktail 12 was deposited on top of Ppy film 14 with 50% v/v ethanol as solvent formed on top of SPE and dried overnight at room temperature. This pH sensor 10 was tested using commercial Ag/ AgCl double junction reference electrode with 0.1M LiOAc as outer solution. The results were shown in Table 1. The plots of emf response versus activity of hydrogen ion 9 have shown acceptable Nernstian response and linearity.

[75] Table 1 : pH response (50% v/v ethanol platform) with varied polymerisation time [Table 1]

[Table ]

[76] Example 2

[77] Preparation of pH Sensor with Choline Chloride Doped Polypyrrole Transducer - Different Current Densities

[78] The screen printed electrodes (SPE) with 4 mm diameter were cleaned ultrasonically with deionised water for 1 min. The electrochemical polymerisation was performed in a conventional three-electrode cell with a Pt as counter electrode and the Ag/AgCl double junction as reference electrode using Autolab PGSTAT MODEL 128N fori 50 sec. The polypyrrole (Ppy) films 14 were generated with current density of 0.5, 1, 1.5, 2, 3, 4 and 5 mA cnr² in aqueous solution of 0.5M pyrrole containing 1M choline chloride dopant and 50% v/v ethanol solvent. After forming electropolymerisation, cyclic voltammetry experiments were conducted between -1.0 V and +1.0 V with a potential sweep rate of 100 mV sec ¹ in 0.1M potassium chloride (KC1) solution. The plots were shown as illustrated in Figure 7.

[79] pH cocktail 12 was prepared follow by example 1. The homogenous cocktail 12 was deposited on top of Ppy film 14 with 50% v/v ethanol as solvent formed on top of each SPE with current density various and dried overnight at room temperature. This pH sensor 10 was tested using commercial Ag/AgCl double junction reference electrode with 0.1M LiOAc as outer solution. The results were shown in Table 2. The plots of emf response versus activity of hydrogen ion 11 have shown acceptable Nernstian response and linearity.

Table 2: pH response (50% v/v ethanol platform) with varied polymerisation current density

[Table 2]

[Table ]

[81] Example 3

[82] Preparation of pH Sensor with Choline Chloride Doped Polypyrrole Transducer - 25% Ethanol/Water, Different Current Densities

[83] The screen printed electrodes (SPE) with 4 mm diameter were cleaned ultrasonically with deionised water for 1 min. The electrochemical polymerisation was performed in a conventional three-electrode cell with a Pt as counter electrode and the Ag/AgCl double junction as reference electrode using Autolab PGSTAT MODEL 128N for 90 sec. The polypyrrole (Ppy) films were generated with current density of 1, 2, 3, 4 and 5 mA cm ² in aqueous solution of 0.5M pyrrole containing 1M choline chloride dopant and 25% v/v ethanol solvent. After electropolymerisation, cyclic voltammetry experiments were conducted between -1.0 V and +0.4 V with a potential sweep rate of 100 mV sec¹ in 0.1M potassium chloride (KC1) solution. The plots were shown as Figure 9.

[84] pH cocktail 12 was prepared follow by example 1. The homogenous cocktail was deposited on top of Ppy film 14 with 25% v/v ethanol as solvent formed on top of each SPE with current density various and dried overnight at room temperature. This pH sensor 10 was tested using commercial Ag/AgCl double junction reference electrode with 0.1M LiOAc as outer solution. The results were shown in Table 3. The plots of emf response versus activity of hydrogen ion 13 have shown acceptable Nernstian response and linearity.

Table 3: pH response (25% v/v ethanol platform) with varied polymerisation current density

[Table 3]

[Table ]

[86] Example 4

[87] Preparation of pH Sensor with Methyl Methacrylate-Tetrahydrofurfuryl

Acryate (MT28)

[88] Sensing Membrane

[89] MT28 cocktail (lOOuL) was first prepared by mixing 2 parts of methyl methacrylate monomer and 8 parts of tetrahydrofurfuryl acrylate monomer - by volume. The MT28 cocktail was transferred into a 5mL vial and lmg

2,2-Dimethoxyl-2-phenylacetophenone (DMPP), 3 mg sodium

tetrakis[bis-3,5(trifluoromethyl)phenyl] borate (NaTFPB) and 10.6mg Hydrogen Ionophore I were added into the cocktail. The vial was capped tightly and wrapped with paraffin film to avoid evaporation of volatile components. The mixture was sonicated at room temperature for 2 minutes to get homogenous mixing of the components. The MT28 pH cocktail (luL) 12 was dispensed over the freshly fabricated polypyrrole-carbon electrode covered by thin layer of PVC film containing the pH sensing components. The PVC layer acts as an interface layer that helps cover the porous polypyrrole layer 14. The PVC interface layer was prepared using solvent cast technique using high molecular weight PVC. The dispensed MT28 cocktail 12 was photocured under UV radiation in nitrogen ambient for 180sec. The MT28 pH sensor 10 was characterized for response to hydrogen ion at pH 4, 7 and 10 (Table 4). The plot of the pH response is provided in Figure 11.

[90] Table 4: Response of pH Sensor with Protective Glue Layer [Table 4]

[Table ]

[91] The invention being thus described, it will be apparent that the same may be varied in many ways. Such variations are to be regarded as within the scope of the invention, and all such modifications as would be apparent to one skilled in the art are intended to be within the scope of the following claims.

Best Mode

[92]

Mode for Invention

[93]

Industrial Applicability

[94]

Sequence List Text

[95]

Claims

[Claim 1] 1. A pH sensor 10 comprising;

a substrate 18,

a thick film screen printed carbon layer 16,

a pH sensing membrane 12;

polypyrole layer 14;

characterized in that the polypyrole layer 14 is homogeneously doped on the carbon layer 16.

[Claim 2] 2. The pH sensor 10 according to Claim 1, characterized in that the doped polypyrrole layer 14 having doping electrolyte of the following structure;

X= CI, Br, I, BF₄, PF₆, OAc, CF₃C0₂, N0₃, Fe(CN)₆, oxalate, tosylate n= 1, 2, 3

R= H, methyl, ethyl, butyl, allyl

[Claim 3] 3. The pH sensor 10 according to Claim 1, characterized in that the pH sensing membrane 12 is photo-cured co-polymer of methyl methacrylate and tetrahydrofurfuryl acrylate containing photoinitiator, diacrylate crosslinker, lipophilic borate salt and hydrogen ionophore.

[Claim 4] 4. The pH sensor 10 according to Claim 1, characterized in that the pH sensing membrane 12 is bulk co-polymer of methyl methacrylate and tetrahydrofurfuryl acrylate containing photoinitiator, diacrylate crosslinker, lipophilic borate salt and hydrogen ionophore.

[Claim 5] 5. Use of the pH sensor 10 according to any preceding claim in the manufacture of an ion selective electrode (ISE) pH sensing device.

[Claim 6] 6. Use of the pH sensor 10 according to claim 1 to 4 in the manufacture of an ion sensitive field effect transistor (ISFET) pH sensing device.

[Claim 7] 7. A method of depositing doped polypyrrole electrochemical

transducer layer comprising;

a) preparing a pyrrole monomer doped electrolyte solution and a hy- drophilic dopant in a polar solvent;

b) immersing a carbon electrode, a counter electrode and a reference electrode into the doped pyrrole electrolyte solution; and c) depositing the doped polypyrrole electrochemically .

[Claim 8] 8. A method of depositing doped polypyrrole electrochemical

transducer layer according to Claim 7, characterized in that in step (a) the pyrrole monomer doped electrolyte solution having concentration ranges from 0.1M to 3M of pyrrole monomer and the hydrophilic dopant in a polar solvent having a concentration ranges from 0.1M to 3M.

[Claim 9] 9. A method of depositing doped polypyrrole electrochemical

transducer layer according to Claim 8, characterized in that the hydrophilic dopant is choline chloride.

[Claim 10] 10. A method of depositing doped polypyrrole electrochemical

transducer layer according to Claim 7, characterized in that the depositing step (c) is by electrochemical polymerization of constant current method with current density of 0.1 to 10 mA per square centimetre.

[Claim 11] 11. A method of depositing doped polypyrrole electrochemical

transducer layer according to Claim 4, characterized in that the depositing step (c) is by electrochemical polymerization utilising cyclic voltammetry method with scans between -IV and +1 V.