WO2013109135A1

WO2013109135A1 - A sensor for dissolved ammonia and a process of preparation thereof

Info

Publication number: WO2013109135A1
Application number: PCT/MY2013/000005
Authority: WO
Inventors: Sagir Alva; Mohd Rais Ahmad
Original assignee: Mimos Berhad
Priority date: 2012-01-16
Filing date: 2013-01-16
Publication date: 2013-07-25
Also published as: MY176885A

Abstract

The present invention relates to a sensor capable of detecting dissolved ammonia - an enviromental hazard - by the use of the change of resistance in the polymer poly-aniline. The invention provides the sensors and applications in which such a sensor can be used. Furthermore provided is a process for the fabrication of the sensor of the present invention.

Description

A Sensor for Dissolved Ammonia and a Process of Preparation Thereof Field of the Invention

The present invention relates to a sensor capable of detecting dissolved ammonia - an environmental hazard - by the use of the change of resistance in the polymer poly-aniline. The invention provides the sensor apparatus and applications in which such a sensor apparatus can be used. Furthermore provided is a process for the fabrication of the sensor of the present invention.

Background of the Invention

Ammonia (NH3) should be distinguished from its ionic form, ammonium ion (NH4+). While ammonium ion is harmless to fish at tens of ppm concentrations, dissolved ammonia is lethal to fish at less than 1 ppm. Low concentrations of dissolved ammonia cause other problems to aquaculture fish such as gill diseases, deformation and stunted growth.

Traditionally dissolved ammonia has been determined by reagent method based on Berthelot's reaction. In this method (Scheme 1) phenol or substituted phenol and hypochlorite solution is used to indicate the presence of dissolved ammonia. In this approach spectrophotometric technique can be used since the reagent is colorless whereas the presence of ammonia produces indophenol with blue appearance.

Colorless Colorless

Scheme 1: Determination of dissolved ammonia using Berthelot's reagent While the accuracy of dissolved ammonia determination based on Berthelot's reagent has been satisfactory in laboratory environment, autonomous measurement in the field is not convenient. Reagent method is practical for manual laboratory technique, and deploying devices in the field with this approach would require bulky equipment and large cost.

Furthermore, ammonia sensors are known in the art, which detect ammonia in gases by use of an ammonia permeable membrane, sensors based on semiconductor materials utilizing the change of resistance of an inorganic conductive material, and detectors using polypyrrole. However, the above mentioned approaches harbour several disadvantages, such as handling, size, sensitivity and stability of the measurement. In aquaculture ponds fish food and waste contains proteins and amino acids that decompose to ammonium ions. Under basic conditions or heat from the environment ammonium ion decomposes to ammonia, by loss of proton. Similarly ammonium ions from human waste or food proteins in waste treatment ponds also decompose to ammonia under similar conditions. This in turn pollutes the sources of fresh water, especially rivers, when the treated wastes are released to the environment. In the environment certain bacteria convert ammonia or ammonium ion to other forms of ions i.e. nitrite (N02-) and nitrate (N03-).

Another approach which is found promising in the development of ammonia sensors are detectors that are based on organic conductive polymers that may be used to analyze the dissolved ammonia based on the change of conductivity. Measurements using this method is based on modulation of resistivity of the selective material. The reciprocal of resistivity is conductivity. The general format of such an conductometry sensor type are shown in figure 1. Some material, which can change its conductivity upon interaction with chemical species is clamped between two contact electrodes and the resistance of the entire device is measured. The document US 5,252,292 describes a sensor for ammonia detection in gases based on the change of conductivity in the polymer poly-aniline. The disclosure of 5,252,292 describes a spray coating of substrate sandwiched between a pair of electrodes with a dimethylformamide solution of poly-aniline on alumina. No specific binder is used to facilitate the adhesion of the poly-aniline with the substrate. The sensor of US 5,252,292 is used to detect the concentration of ammonia in gases and therefore is not usable to monitor the content of ammonia in liquid based systems. US 2008/0093226 relates to nanoelectric sensors for detecting ammonia and environmental control systems making use of the described sensors. Specific embodiments of the disclosure described in US 2008/0093226 relate to a personnel safety system configured as a disposable batch and to a method of dynamic sampling and exposure of such a sensor.

In view of the above prior art, the problem the present invention seeks to solve is to provide an autonomous measurement of dissolved ammonia in aquaculture ponds and in the sources of fresh water, especially rivers, using ruggedized nanocomposite electrode.

The present invention proposes to solve the above problem with a dissolved ammonia sensor based on conductometric principle, using polyaniline nanocomposite to form a channel between at least two electrodes as it is exemplified in Figure 2. In a short summary, the present invention pertains to the following embodiments:

In one aspect the above problem is solved by a sensor for dissolved ammonia, comprising i. Conductive polyaniline, functions as active component to detect concentration of dissolved ammonia,

ii. Lipophilic sulfonate dopant, functions as counterion for positively charged conductive polyaniline

iii. Charged polysaccharide, to provide low impedance path across the channel and determine baseline conductivity

iv. Uncharged polysaccharide binder, provides adhesion of the doped conductive polyaniline to channel substrate

v. At least one channel in between at least two electrodes, to allow passage of current across the said channel The inventors surprisingly envisioned a dissolved ammonia sensor for determination of sub-ppm level of ammonia, specifically to be used in aquaculture ponds and in sources of fresh water. The dissolved ammonia sensor works based on conductometry principle wherein the presence of ammonia reduces the conductivity across the polyaniline nanocomposite channel, as illustrated in Scheme 2 below:

PANI-H + NH₃ ► PANI + NH₄

Scheme 2: Determination of dissolved ammonia using polyaniline nonocomposite electrode

In a preferred embodiment the electrodes are screen printed silver on printed circuit board (PCB) substrate. At least one channel, filled with sensing materials, is sandwiched between the electrodes. In the first requirement, the channel substrate should give good adhesion with the polyaniline nanocomposite materials, in another requirement, the dissolved ammonia sensor should give linear region between 0.1 to 0.001 ppm of the analyte for aquaculture and environmental applications. In yet another requirement the ammonia sensor shall be deployed in the field for autonomous operation. It is also preferred that the dissolved ammonia sensor can be reused many times without loss of sensitivity.

In another preferred embodiment the polyaniline nanocomposite is cast or printed onto the channel. The polyaniline nanocomposite ink is homogenous solution of doped polyaniline, produced by in situ polymerization of aniline monomer in polysaccharide-organic sulfonate- imidazolium electrolyte solution. The organic sulfonate dopant is produced by deprotonation of the respective sulfonic acid by polysaccharide. In the preferred embodiment the polysaccharide is the main binding materials that give good adhesion on channel substrate material. The polar polysaccharide strands such as cellulose or chitosan exhibit low impedance characteristic that facilitates transport of current across the channel. The imidazolium ion assists in increasing the rate of aniline polymerization and improves the conductivity of the polyaniline nanocomposite channel.

The dissolved ammonia sensor of the invention preferably comprises the conductive polyaniline which is cast or printed onto the substrate as a homogenous polyaniline solution of in situ polymerized aniline in polysaccharide-sulfonate dopant electrolyte buffered by imidazole- imidazolium ion solution. The polysaccharide for use in the context of the invention comprises at least one or combination of the following materials; cellulose acetate, ethyl cellulose, cellulose, chitosan, starch, dextrin, maltodextrin, beta-glucan, chitin, mannan, galactan, fructan, gum arabic, gelatin.

In another embodiment the dissolved ammonia sensor comprises at least one or combination of the following sulfonates: polystyrene sulfonate, dodecyl benzene sulfonate, toluene sulfonate, camphor sulfonate, nafion, apolate. Preferably the channel of the sensor comprises 0.1 to 30 % conductive polyaniline, 0.1 to 30% lipophilic sulfonate dopant and 40 to 99% polysaccharide, all by weight and is channel is at least one of the following materials; carbon, alumina, glass, paper, silicon dioxide, silicon nitride.

Also the problem of the invention is solved by a method for manufacturing the sensor of the invention comprising the steps of

i. Fabricating at least one channel substrate sandwiched by at least two conductive electrodes ii. Depositing conductive polyaniline solution onto the channel.

iii. Optimizing the detection limit at least to achieve 0.01 ppm level by modifying the ink composition.

iv. Measuring the current and resistance by passing at least 0.1V across the channel in different concentration of calibration solution.

v. Plot the value of current vs concentration as a calibration curve.

Preferably, the sensor of the invention is used of the dissolved ammonia sensor in at least one of the following applications; aquaculture ponds, environmental monitoring.

In another aspect the invention a dissolved ammonia conductometric setup comprises:

i. Samples as a dissolve ammonia source

ii. Dissolved ammonia electrode to detect present of dissolved ammonia in samples

iii. Readout board circuit to capture and convert electrical signal from dissolve ammonia electrode iv. Displaying screen to showing reading from readout board circuit.

In the following a detailed description of the embodiments of the invention is provided: The above problem is solved in a first aspect by a sensor for dissolved ammonium, comprising a channel between at least one pair of electrodes, wherein the electrodes are placed to allow a current to flow across said channel, and wherein the channel comprises a conductive layer composed of at least a conductive poly-aniline In a preferred embodiment the conductive layer fills the space between the at least one pair of electrodes and further comprises one or more substances selected from the group comprising of a lipophilic sulfonate, a charged polysaccharides and an uncharged polysaccharide. Preferably the conductive layer of the present invention is composed of a nanocomposite ink. The electrodes of the inventive sensor are composed of an conductive metal, preferably wherein the electrodes are screen-printed silver on a printed circuit board substrate, more preferably wherein the at least one pair of electrodes is in a interdigitated set-up.

Yet another embodiment of the invention relates to the conductive poly-aniline which is a positively charged conductive poly-aniline, preferably wherein the poly-aniline changes its electric resistance proportional to the concentration of ammonium present within and/or close to said channel.

In a further embodiment the sensor according to the invention comprises a charged polysaccharide selected from a polysaccharide that provides a low impedance path across said channel.

Preferably, the lipophilic sulfonate of the sensor of the present invention is a dopant and functions as a counterion for said positively charged poly-aniline. More preferred is that the uncharged polysaccharide binder of the sensor of the invention is selected from a polysaccharide that allows for an adhesion of the poly-aniline to said channel substrate. Preferred is also that the poly-aniline is casted or printed onto said channel; specifically the poly-aniline is not sprayed onto said channel or channel substrate. Neither sprayed is the nanocomposite ink of the invention.

The polysaccharide for use in the present inventive sensor is selected from one or a combination of the polysaccharides selected from the group comprising cellulose acetate, ethyl cellulose, cellulose, chitosan, starch, dextrin, maltodextrin, beta-glucan, chitin, mannan, galactan, fructan, gum arabic and gelatin, and the respective derivatives of these polysaccharides.

The lipophilic sulfonate for use in the sensor of the present invention is selected from at least one or a combination of polystyrene sulfonate, dodecyl benzene sulfonate, toluene sulfonate, camphor sulfonate, nafion and apolate.

The invenntion pertains in a further embodiment to a sensor channel which is composed of at least 0.1 to 30% per weight conductive poly-aniline, at least 0.1 to 30% per weight lipophilic sulfonate dopant and at least 40 to 99.8% per weight polysaccharide. In this context it is preferred that the channel substrate is composed of an insulating substrate, preferably of one or a combination of the following materials: carbon, alumina, glass, paper, silicon dioxide and silicon nitride.

The sensor of the present invention in a preferred embodiment is distinct from other prior art sensors as it detects dissolved ammonium in the range of 0.1 to 0.001 ppm. Furthermore, the inventive sensor is reusable without the loss of sensitivity.

Another embodiment of the invention pertains to the described sensor, wherein the conductive poly-aniline is produced by electrochemical polymerization of aniline. Preferably using a process as described in the examples of the present invention. The invention additionally pertains to a sensor that is equipped with a readout circuit, a microcontroller and a wireless transmitter. The inventive system is suitable to measure the dissolved ammonia in a given environment, preferably automatically, at desired times, and may send the data to a central database - the database in this embodiment may also function to provide an analysis of the data and to evaluate the results, with a subsequent decision whether to initiate a pre-programmed response or intervention at the monitored environment. The profile of the aquaculture ponds and sources of fresh water, may include other parameters such as pH, nitrite level and temperature, and the data can be accessed from any location. In a further aspect the problem of the present invention is solved by the use of the sensor of the invention for dissolved ammonium in environmental monitoring of dissolved ammonium in a aqua cultural pond, specifically in a fish pond, or environmental monitoring of fresh water sources, such as rivers, lakes, ground water or drinking-water reservoirs. Furthermore, another aspect of the invention pertains to a process for preparing a sensor for dissolved ammonia according to the present invention.

In a preferred embodiment the process of the invention for preparing a sensor for dissolved ammonia, comprises the steps of

i. Fabricating at least one channel substrate, wherein the channel is in between of at least one pair of electrodes,

ii. Depositing a conductive layer onto said channel, wherein said conductive layer is composed of a nanocomposite ink comprising at least a conductive poly-aniline solution, wherein the deposition is performed by printing or casting said poly-aniline solution onto said channel substrate,

iii. optionally, adding to said nanocomposite ink one or more substances selected from the group comprising of a lipophilic sulfonate, a charged polysaccharides and an uncharged polysaccharide.

iv. drying said deposited nanocomposite ink, preferably in a continuous flow of nitrogen or in an oven at 70 to 90 °C. In another embodiment the inventive process comprises further the steps of

iv. Optimizing the detection limit at least to achieve 0.01 ppm level by modifying the ink composition,

v. Measuring the current and resistance by passing at least 0.1V across the channel in different concentration of calibration solution,

vi. Plot the value of current vs concentration as a calibration curve.

Yet a further embodiment of the inventive process pertains to a process, wherein the poly-aniline solution is prepared by in-situ electrochemical polymerization of anilin monomers in an electrolyte solution. Most preferred is that the electrolyte solution is a polysaccharide-organic sulfonate-imidazolium electrolyte solution.

In another embodiment it is provided that said lipophilic sulfonate is produced by deprotonation of the respective sulfonic acid by the polysaccharide.

While the present invention has been described with specificity in accordance with certain of its preferred embodiments, the following examples serve only to illustrate the invention and are not intended to limit the invention within the principles and scope of the broadest interpretations and equivalent configurations thereof.

Description of the Drawings

Figure 1: shows the general format of a typical state of the art conductometry sensor. Figure 2: shows a schematic of a sensor for dissolved ammonia according to the invention.

Figure 3: shows a flow chart of dissolved ammonia sensor fabrication

Figure 4: shows a polyaniline-camphor sulfonic acid-cellulose acetate nanocomposite ink Figure 5: shows a cyclic voltammetry plot of cast polyaniline nanocomposite on a screen printed carbon electrode in a 0.1M KG solution

Figure 6: shows a sensor of the invention from cast polyaniline nanocomposite: Figure 7: shows a plot of current response versus ammonia concentration of dissolved ammonia sensor with 0.1, 0.05 and 0.01 ppm of analyte and IV of biasing across the channel.

Detailed Description of the Invention

The present invention pertains to a sensor for dissolved ammonia based on conductomery for the detection of the respective analyte of in the concentration range of between 0.1 to 0.01 ppm. The herein described sensor is of particular use for an autonomous operation in the field, for example in monitoring the ammonia content in aquaculture ponds and/or in sources of fresh water, specifically drinking water.

Fabrication of the proposed dissolved ammonia sensor involves five main steps: in the first step the conductometric device is fabricated to form narrow rectangular channels sandwiched by conductive electrodes.

In the second step poly-aniline nanocomposite ink is dispensed (cast) or printed onto the channel substrate. The deposited ink is dried under continuous flow of nitrogen or in the oven at 70 to 90 °C.

In the third step the linear range of the dissolved ammonia response is optimized by changing the polyaniline nanocomposite ink.

In the fourth step DC voltage of about IV is applied across the channel and the resulting current is recorded. Finally plots of resistance or current versus concentration of ammonia are produced to confirm required linearity over the desired analyte range.

Example I: Preparation of Polyaniline Nanocomposite Ink Polyaniline nanocomposite ink was prepared by the following procedure: 500 mg cellulose acetate (CA) was added into 374.8 mg camphor sulfonic acid (CSA) in 15 mL tetrahydrofuran (THF) and 7 mL ethanoi. The mixture was continuously stirred for 1 hour until homogeneous viscous solution is produced.

If a white cloudy solid precipitates, 1 mL portions of ethanoi are added until colorless solution is achieved. Then, 146.9 μΐ aniline is added to the cellulose acetate-sulfonate solution and the mixture solution is stirred for 1 hour until homogeneous colorless solution is achieved. Again, if a white cloudy solid precipitates, 1 mL portions of ethanoi are added until colorless solution is achieved. Then, 500 μΐ methyl immidazolium-methyl immidazolium solution buffered at pH 8 and two portions of 250 μΐ hydrogen peroxide (H₂0₂) were added and the final solution mixture remains colorless. Heating of the colorless mixture in the oven at 90°C until 14 hours produced dark brown polyaniline solution (Figure 4). Cyclic voltammetry plot of cast poly-aniline nanocomposite on screen printed carbon electrode in 0.1M KC1 solution shows 2 large peaks, oxidation peak at 0.4V and reduction peak at 0.2V, as shown in Figure 5.

Example II: Fabrication of Conductometric Electrode

Interdigitated multiple channels are defined by screen printing comb structures of silver electrodes onto prefabricated screen printed carbon channel substrate. The distance across the channel is 1mm and the length of the channel is 3mm. The entire surface of the device is covered with screen printed solder mask or epoxy insulating materials, exposing only the contact windows to silver electrodes. The screen printed layers are oven cured at 120 °C under blanket of nitrogen gas for 30 minutes. The cocktail composition is dispensed to cover the channel windows and dried under continuous flow nitrogen gas for 30 minutes and further air dried overnight (Figure 6).

Example III: Fabrication and Characterization of Dissolved Ammonia Sensor The screen-printed conductometric carbon substrate channel and silver electrodes were cleaned ultrasonically with deionized water for 1 min. Polyaniline nanocomposite ink was drop coated onto the screen printed carbon conductometric channels, sandwiched between silver electrodes (Figure 6). The deposited polyaniline nanocomposite was dried under continuous flow of nitrogen for 30 minutes. The dissolved ammonia sensor was tested with Agilent Semiconductor Analyzer B 1500A by measuring the current response caused by 0.1, 0.05 and 0.01 ppm of standard ammonia solutions when I V of DC voltage is applied across the channel, while one of the electrodes is grounded. The plot of resistance current response versus concentration of ammonia shows good linearity (Table 1 and Figure 7).

Table 1: urrent response o sso ve ammon a sensor w t .1 , . an 0.01 ppm analyte and IV of biasing across the channel

Claims

A sensor for dissolved ammonium, comprising a channel between at least one pair of electrodes, wherein the electrodes are positioned to allow a current to flow across said channel, and wherein the channel comprises a conductive layer composed of at least a conductive poly-aniline

The sensor according to claim 1, wherein the conductive layer fills the space between the at least one pair of electrodes and further comprises one or more substances selected from the group comprising of a lipophilic sulfonate, a charged polysaccharide and an uncharged polysaccharide.

The sensor according to claim 1, wherein the conductive poly-aniline is a positively charged conductive poly-aniline, preferably wherein the poly-aniline changes its electric resistance proportional to the concentration of ammonium present within and/or in close proximity to said channel.

The sensor according to claim 2, wherein the charged polysaccharide is selected from a polysaccharide that provides a low impedance path across said channel.

The sensor according to claim 2, wherein the lipophilic sulfonate is a dopant and functions as a counterion for said positively charged poly-aniline.

The sensor according to claim 2, wherein the uncharged polysaccharide binder is selected from a polysaccharide that allows for an adhesion of the poly-aniline to said channel substrate.

7. The sensor according to claim 1 wherein the conductive poly-aniline is casted or printed onto said channel.

8. The sensor according to claim 2, wherein the polysaccharide is selected from one or a combination of the polysaccharides selected from the group comprising cellulose acetate, ethyl cellulose, cellulose, chitosan, starch, dextrin, maltodextrin, beta-glucan, chitin, mannan, galactan, fructan, gum arabic and gelatin, and the respective derivatives of these polysaccharides.

9. The sensor according to claim 2, wherein the lipophilic sulfonate is at least one or a combination of substances selected from the group consisting of polystyrene sulfonate, dodecyl benzene sulfonate, toluene sulfonate, camphor sulfonate, nafion and apolate.

10. The sensor according to claim 2, wherein said channel is composed of at least 0.1 to 30% per weight conductive poly-aniline, at least 0.1 to 30% per weight lipophilic sulfonate dopant and at least 40 to 99.8% per weight polysaccharide.

1 1. The sensor according to claim 2, wherein the channel substrate is composed of an insulating substrate, preferably of one or a combination of the following materials: carbon, alumina, glass, paper, silicon dioxide and silicon nitride.

12. The sensor according to claim 1 , wherein the sensor detects dissolved ammonium in the range of 0.1 to 0.001 ppm.

13. A use of the sensor for dissolved ammonium in environmental monitoring of dissolved ammonium in an aqua cultural pond, specifically in a fish pond, or environmental monitoring of fresh water sources, such as rivers, lakes, ground water or drinking-water reservoirs.

14. A process for preparing a sensor for dissolved ammonia, comprising i. fabricating at least one channel substrate, wherein the channel is in-between of at least one pair of electrodes,

iii. optionally, adding to said nanocomposite ink one or more substances selected from the group comprising of a lipophilic sulfonate, a charged polysaccharide and an uncharged polysaccharide.

iv. drying said deposited nanocomposite ink,

15. The process according to claim 14, wherein the electrolyte solution is preferable a polysaccharide-organic sulfonate-imidazolium electrolyte solution.