<![CDATA[Novel Planar Electromagnetic Sensors for Detection of Nitrates and Contamination in...

1440 IEEE SENSORS JOURNAL, VOL. 11, NO. 6, JUNE 2011

Novel Planar Electromagnetic Sensors forDetection of Nitrates and Contamination in

Natural Water SourcesMohd Amri Md. Yunus, Student Member, IEEE, and Subhas Chandra Mukhopadhyay, Fellow, IEEE

Abstract—This paper introduces novel sensors based on the com-bination of planar meander and interdigital electromagnetic sen-sors for monitoring the level of contamination in water sources. Aseries of experiments was conducted to determine the sensors char-acteristics. Two nitrates forms, namely sodium nitrates and ammo-nium nitrates, were mixed in several different ratios dissolved in 1L of distilled water and were used to observe the response of thesensors. Initial results show that the sensors can detect very wellthe presence of nitrate added in distilled water. Furthermore, thecontamination level of water samples which were taken from var-ious sources and locations in New Zealand have been determinedusing the sensor, and the results was compared with the results ob-tained using nuclear magnetic resonance (NMR) technique. Theoutcomes show a very good correlation of contamination level withthe output response of the sensor. The work and improvement forfuture consideration are discussed in this paper.

Index Terms—Natural water sources, nitrate detection, planarelectromagnetic sensors and water contamination.

I. INTRODUCTION

W ATER supplies come from two main sources: groundwater and surface water. Nitrate is one of the pollu-

tants in natural water sources which can render the natural watersources unsuitable for both human and animal consumption aswell as the environment putting it into a great risk. Most of theground water is contaminated by nitrate in which the sourcesare from decaying plants and animal material, agricultural fer-tilizer and domestic sewage [1]. As for the surface water sup-plies, the contaminations are influence by runoff and dischargeof domestic waste water such as septic tank and refuse pumps[2], [3].

According to the international guidelines, the normal nitrateconcentration of surface water is 5 mg L [4], [5], but it has in-creased over the past 20 years [6]. Some of the contributing fac-

Manuscript received April 21, 2010; revised October 04, 2010; accepted Oc-tober 31, 2010. Date of publication November 11, 2010; date of current versionApril 20, 2011. The associate editor coordinating the review of this paper andapproving it for publication was Prof. Michiel Vellekoop.

M. A. Md. Yunus is with the School of Engineering and Advanced Tech-nology, Massey University, Manawatu 5301, New Zealand, on leave from theControl and Instrumentation Engineering Department, Faculty of ElectricalEngineering, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor DarulTakzim, Malaysia (e-mail: [email protected]).

S. C. Mukhopadhyay is with the School of Engineering and Advanced Tech-nology, Massey University, Palmerston North, Manawatu 5301, New Zealand(e-mail: [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JSEN.2010.2091953

tors are increase in nitrogenous fertilizer, change in land use likeincrease of recycling of sewage influent in low land drivers [2].

Nitrate concentration in excess of 10 mg L in drinkingwater can cause methemoglobinemia in infants less than sixmonths and most significantly less than three months. Methe-moglobinemia is a resulting condition from the conversion ofhemoglobin which is unable to transport oxygen [2], [3], [7],[8]. There is still a little information to come to a conclusionabout the relationship of high nitrate intake and other humandiseases like gastric cancer [8]. However, the link cannot beruled out due to inadequacy of data available. Nevertheless,nitrate removal through physical chemical or biological treat-ment not only removes nitrate but can change the overallcomposition of water that may increase treatment cost [9].The level of Nitrate consumed by human must not exceeded10 mg NO -N L [10] and for most farms, drinking water isaround 2 to 3 mg NO -N L and should not exceed 100 mgNO -N L .

II. EXISTING METHODS OF NITRATES DETECTION

According to Hassan et al. [11], nitrate detection methodscan simply be divided into indirect and direct methods. Indirectmethods are often expensive due to many components require-ment. It also require tedious measurement steps which consumea lot of time, involve controlled working condition, and prepara-tion of extra reagent or chemical. Such applications of indirectmethods are reduction of nitrate into nitrite of nitric oxidegas followed by gasometry or potentiometry [11], detectionof ammonioum ion resulting from the reduction of nitrate bytrichloride in hydrochloric acid [12], spectrophotometric [13],polarography and voltmetric [14]. The direct methods offera more intuitive solution; such applications are chromatog-raphy [15] and biosensors [16]. The most popular direct methodwhich is also considered as low cost is potentiometry [17].Hassan et al. [11] have demonstrated the advantageous ofpotentiometry technology such as uncomplicated fabricationprocess, simple monitoring instrumentation, fast response,accurate, small size thus require small amount of sample, andsuitable for continuous measurement. However, the magnitudeof voltage potential depends on the strength or concentrationof the ions and often requires amplification. Moreover, a largenumber of reagents which might be harmful to the environmentfor the detection of nitrate have been used to overcome theinterference from not targeted ions [12], [18].

1530-437X/$26.00 © 2010 IEEE

YUNUS AND MUKHOPADHYAY: NOVEL PLANAR ELECTROMAGNETIC SENSORS 1441

In New Zealand, the farmers are blamed for contaminatingwater with lots of nitrates due to urinals of cows and are chargedfor penalty by the city council. The current sensors are not goodenough for the particular application; the sensors are based onabsorption spectrophotometry system using a specific reagent.This system established for laboratory standards thus makingit expensive due to many components requirement, cannot pro-vide fast results and not portable [19], [20]. Therefore, there isa need of a new sensor which will provide reliable data of thelevel of contamination. This is the motivation of the project todevelop a new sensor. Therefore, this research aims to developa sensor which can be integrated as a low cost, convenient, andsuitable for in situ measurement system for water quality moni-toring, particularly nitrate detection. The present work describesthe development, experiments, and analysis of a new nitrate de-tection method using new planar electromagnetic sensors. Thesensors were tested with various kinds of prepared samples andnatural water samples taken from natural sources around NewZealand.

III. NOVEL PLANAR ELECTROMAGNETIC SENSORS

A. Principle of Operation

Applications of planar electromagnetic sensors have been re-ported in [21]–[23]. In this paper, sensors with meander and in-terdigital in series and parallel combination are introduced. Thesensor was designed using Altium Designer 6, and it has beenfabricated using simple printed circuit board (PCB) fabricationtechnology where the thickness of the PCB is 0.25 mm.

A few sensors have been designed and fabricated. Thesensors providing the best results are described in this paper.Fig. 1(a) and (b) illustrates the top view and bottom view ofthe schematic and picture of the fabricated parallel combina-tion sensors, SCP5. The meander sensors part consists of twocoils. The exciting coil [Fig. 1(b)] is driven by high-frequencysinusoidal current to create a high-frequency magnetic field.To increase the magnetic field strength, the number of meanderturns was increased by introducing several spiral rectangularturns toward the inner section. When the sensor (bottom layer)is placed near a material, an electromagnetic field is inducedon the material. The induced field will modify the createdfield, and the sensing coil [Fig. 1(a)] which is placed above theexciting coil, is used to pick up the resultant field.

The interdigital sensor employs a conventional configurationhaving consecutive negative and positive electrode placed apartat a fixed distance where the negative electrodes are of 3 mmwidth, as shown in Fig. 1(b). The operating principle behind theinterdigital sensor is very similar to the one served in a parallelplate capacitor [24]. The positive electrodes are driven by an acelectric potential (same source as the meander) to create an acelectric field between the positive and negative electrodes. Thesefields pass through material under test; thus, material dielectricalproperties as well as the electrode and material property af-fect the capacitance and the conductance between (impedance)the two electrodes. The penetration depth of the electric fieldis determined from the wavelength, i.e., the distance betweenthe consecutive positive and negative electrode [23]–[26]. A

Fig. 1. Schematic diagram and picture of (a) SCP5: top layer, (b) SCP5: bottomlayer, (c) SCS3: top layer, and (d) SCS3: bottom layer.

grounded backplane was deposited on the other side of the in-terdigital sensor to provide shielding from external perturba-tion, define ground potential, and confine the electric field tothe upper half-space [27]. The total impedance is used as thecharacterization parameter for the parallel combination sensor.

Fig. 1(c) and (d) shows the top and bottom layers of theschematic and picture of the fabricated planar electromagneticsensors combined in series, SCS3. The meander type of coil isconnected in series with the interdigital coil, and an ac voltageis applied across the combination of the coils. The meander coil


Fig. 2. Electrical equivalent circuits of (a) SCP5 and (b) SCS3.

will produce a magnetic field, and the interdigital coil will pro-duce an electric field. So the combination of meander and inter-digital coils will produce a combination of electric and magneticfield.

B. Principle of Measurement

Electrical equivalent circuits of parallel and series combina-tion sensor are shown in Fig. 2. The sensor is connected to afunction generator where is the output resistance with nom-inal value of 50 .

For the parallel combined sensor, a series surface mount re-sistor resistance is connected as shown in Fig. 2(a), whereas

and can be calculated by

(1)

where is the rms value of total current that flows throughthe sensor, and is the voltage across the surface mount re-sistor. is considered as the reference so that its phase angleis 0 . is the series surface mount resistor used to measurethe total current through both sensors. Introducing (1) into (2)giving (the total impedance),

(2)

is the phase difference between with in degree,taking as reference. Therefore, and aregiven by

(3)

(4)

where and are the rms values of input voltage. is the phasedifference between with in degree, taking asreference.

For the series combination, denotes the series surfacemount resistor connected to the sensors as shown in Fig. 2(b).Hence, and can be calculated from:

(5)

where is the rms value of current through the sensor, isthe voltage across the surface mount resistor. is consideredas the reference so that its phase angle is 0 . Introducing (5) into(6), giving (the total impedance)

(6)

is the phase difference between with in degree,taking as reference. Therefore, and aregiven by

(7)

(8)

IV. EXPERIMENTAL SETUP AND ANALYSIS

The sensor characteristic of the sensor was determined bymeasuring the total impedance (absolute) of the sensor at dif-ferent frequencies between 10 kHz and 100 MHz when no ma-terial is placed near the sensor (air). Our initial target is to testthe response of the sensors with different concentration of Ni-trates samples in the form of sodium nitrate (NaNO ) and am-monium nitrate (NH NO ) mix. Five different ratio mixes wereprepared where the total weight of every single mix is givenin 20 mg. Then, each mix is diluted in 1 L of distilled waterinto a container. The ratios of NaNO to NH NO were 1:1(Ratio 1), 1:2 (Ratio 2), 2:1 (Ratio 3), 1:3 (Ratio 4), and 3:1(Ratio 5). Beforehand, the sensor was sprayed with Wattyl Kill-rust Incralac to form an acrylic resin-based protective coating.The sensitivities of the sensors are calculated from the real partand imaginary part as governed by the following equations:

Real part sensitivity %

(9)

Imaginary part sensitivity %

(10)

The experimental setup has a frequency waveform generatorwhere the standard sinusoidal waveform with 10 V peak-to-peakvalue was set as the input signal for the sensors. An old mi-croscope was used as a platform for the sample container andthe sensors were fully immersed into the sample. The Agilent54622D mixed signal oscilloscope was interfaced to a PC wherethe output signals and the sensor’s impedance was recorded andcalculated consecutively using developed programs using Lab-View software. The pH value for each solution was measuredusing Orion pH meter 420A. The picture of the experimental


Fig. 3. (a) Experimental setup for the experiment. (b) The front panel of LabView program. (c) A part of the block diagrams of the LabView program.

Fig. 4. Impedance characterization of the sensors from the experiment.

setup and the caption of the developed LabView program areshown in Fig. 3.

V. EXPERIMENTAL RESULTS AND DISCUSSION

A. Impedance Characterization

Fig. 4 illustrates the impedance characterization of the sen-sors. The impedance of SCP5 increases with the increasing fre-quency and drops after 50 MHz making the meander sensorwith inductive characteristic as dominant, and the capacitivepart starts to appear after 50 MHz. As for the SCS3, a U-shapedimpedance characteristic was obtained as shown in Fig. 4. Theright-half and left-half curves show that SCS3 is capacitive atlow frequency range and inductive at high frequency range, re-spectively. It is seen that within the range of around 50 kHz to10 MHz, the sensor shows the combined effect of inductive andcapacitive response.

B. The Response of SCP5 and SCS3 to the Solutions WithDifferent Ratio Mix of NaNO and NH NO

The operating frequencies for SCP5 and SCS3 during watersample experiments were set at 10 MHz and 500 kHz, respec-tively. The temperatures of all solutions were recorded between21 and 22 C. The response of the sensors to distilled water was

Fig. 5. Sensitivity of SCP5 with nitrate solutions.

used to calculate the sensitivity in (9) and (10). SCP5 gives amoderate response given by the sensitivity values ranging justbetween 1% and 10% when tested with the nitrate solutions, ascan be seen in Fig. 5. Better detection of nitrates was given bySCS3 where the imaginary part sensitivity reaches the range be-tween 300% to 400%, as shown in Fig. 6(b) where the changeof resistive part is limited to only just below 45%, as shown inFig. 6(a). This makes SCS3 as the best sensor and will be usedfor further experiments.

C. Effect of Experimental Variables (pH) on SCS3

The effect of pH on SCS3 response was determined using aset of distilled water samples with different pH values, whichvaried from 3.5 to 12. The pH value of the sample was ad-justed with a dilute HCl or NaOH solution. SCS3 response to thevarying pH values can be seen in Fig. 7. SCS3 is independent ofpH in the range of between 6.5 to 10. In the pH range below 6.5where the total amount of H is basically increasing, the rela-tively bigger effective area of negative electrodes, as comparedto positive electrodes associated with SCS3, attempt to balancethe increase of H and try to put back the sensor response into


TABLE ICONTENT OF WATER SAMPLES FROM NMR TEST RESULTS

Fig. 6. Sensitivity of SCS3 with nitrate solutions: (a) real part and (b) imagi-nary part.

the previous equilibrium state. It is seen that in Fig. 7, a signifi-cant change of SCS3 response was observed when the pH valueis over 10, largely due to the increased of total amount of OH .

The electrical conductivity of the solution of water is highlydependent on its concentration of dissolved salts where NaNOand NH NO were added in the distilled water. In the other con-dition, the conductivity of the solution of water may also be af-fected by other chemical species that ionize in the solution. Thepurer the water samples, the lower the conductivity (the higherthe resistivity), and vice versa. Therefore, the real part calcu-lated from SCS3 decreases with the increasing total

Fig. 7. Sensitivity of SCS3 values plotted against pH value between the rangeof 3.5 and 12.

amount of NaNO and NH NO concentration due to greaterelectricity flow in the sensor. This explains why the real partsensitivity of SCS3 is in negative values. However, it is seenthat the value from SCP5 has not significantly changedsince the electricity flow in the sensor is already good and betterthan SCS3 (complete closed circuit of meander exciting coil).

D. Detection of Contamination in Water Samples Using SCS3

Sensor SCS3 has been used to observe the response of thesensor with natural water mixed with contaminants. The sam-ples of different bottles were taken from previous research byKarunanayaka in [28], which related to magnetic filtration tech-nique of drinking and waste waters. The water samples wereanalyzed using nuclear magnetic resonance (NMR) analysis tocheck the amount of organic material in the sample and summa-rized in Table I. Table I shows the amount of mineral in g mincluding the pH value. The experimental results as shown inFig. 8 suggested that the water samples to be divided into threegroups of “unsafe to consume,” “use with caution,” and “safeto consume.” The “unsafe to consume” water sources are of570-5 and 570-7; this can be explained from the lowest real partsensitivity [Fig. 8(a)] and the highest imaginary part sensitivity[Fig. 8(b)]. The total amount of mineral in 570-5 and 570-7, asgiven in Table I, validate the decision. As for the “safe to con-sume” water samples, 570-8, 570-9, and 570-10 are included. Ithas been postulated that the low imaginary sensitivity, i.e., justbelow 2000% [Fig. 8(b)], is the main reason for the selection


Fig. 8. Sensitivity of SCS3 with water samples: (a) real part and (b) imaginarypart.

where the total amount of mineral for each sample in Table Iconfirms the selection.

Finally, 570-1, 570-2, 570-3, 570-4, and 570-6 are all groupedtogether in the “consume with caution” group as the imaginarysensitivity has just passed 2000% and just below 4000%. Al-though some of the samples appear to be in a low total amount ofmineral, the visual tests experienced by the researcher indicatethat there are some discolorations caused by a certain kind ofalgae or living organism inside the water samples, thus changingthe electrical properties of the water samples. This may causethe sensitivity to be significantly high, as compared to the totalamount of mineral given in Table I.

E. Stability Test

Long-term response stability has been identified as one of theimportant factors of a chemical sensor performance. In order toevaluate the response stability of the SCS3, one reference re-sponse value of distilled water was recorded. Then, the exper-iments involving the solutions with different ratio mix as wellas the water samples were repeated separately where the sen-sors’ output response to each solution was recorded at an equalinterval between approximately 550 s to determine the real partand imaginary part sensitivities. The outcomes from Figs. 9 and10 are the extensions of those that have been obtained in Figs. 6and 8, respectively. As a whole, the previous results are stillvalid and significant. Moreover, the real part sensitivities re-mained constant, as shown in Figs. 9(a) and 10(a), while the

Fig. 9. Sensitivity of SCS3 with nitrate solutions: (a) real part and (b) imagi-nary part.

imaginary part sensitivities moderately varied as can be seen inFigs. 9(b) and 10(b).

VI. CONCLUSION AND FUTURE WORK

Initial studies show that a very distinct detection of nitratesand contamination in water can be achieved using the SCS3sensor. It can be used as a tool for water sources monitoringin farm where the nitrate level should not exceed 100 mgNO -NL . SCS3 sensor has also been tested with drinkingwater samples and has shown very promising results to bea sensing tool of contamination detection in drinking water.For further improvement, a gold base electrode is suggestedas a report has shown that a gold base electrode is capable ofattracting nitrates ion [29]. To further improve the selectivity,a nitrate selective membrane is proposed as reported in [30].Substrate with higher dielectric constant will be considered toreduce the amount of electric fields penetrating to the other sideof the sensor, which have been reducing the sensor sensitivityto material [22]. The concept of a low-cost system is alsoconsidered where the function generator will be replaced witha voltage controlled oscillator, auxiliary circuits will be built to


Fig. 10. Sensitivity of SCS3 with water samples: (a) real part and (b) imaginarypart.

measure the sensor impedance, and a microcontroller will beused as display.

ACKNOWLEDGMENT

The authors would like to thank K. Betteridge fromAgResearch Grasslands, New Zealand, for valuable insightsand suggestions.

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Mohd. Amri Md. Yunus (S’08) received the B.Eng.(Hons.) degree in electrical (control and instrumenta-tion) and the M.Eng. degree in electrical engineeringfrom the Universiti Teknologi Malaysia, Skudai,Malaysia, in 2002 and 2005, respectively. He iscurrently working towards the Ph.D. degree at theSchool of Engineering and Advanced Technology,Massey University, Palmerston North, New Zealand.

In 2006, he joined the Universiti TeknologiMalaysia, Skudai, Johor, Malaysia, as a Lecturerunder the Control and Instrumentation Engineering

Department. His research interests include process tomography, planar electro-magnetic sensors, and sensing technology.

Subhas Chandra Mukhopadhyay (F’11) was bornin Calcutta, India, in 1965. He graduated from theDepartment of Electrical Engineering, JadavpurUniversity, Calcutta, India in 1987 with a Goldmedal and received the Master of Electrical Engi-neering degree from the Indian Institute of Science,Bangalore, India, in 1989 and the Ph.D. (Eng.)degree from Jadavpur University, India, in 1994 andthe Doctor of Engineering degree from KanazawaUniversity, Japan, in 2000.

From 1989 to 1990, he worked almost two years inthe Research and Development Department of Crompton Greaves, Ltd., India.

In 1990, he joined the Electrical Engineering Department, Jadavpur Univer-sity, India, as a Lecturer and was promoted to Senior Lecturer of the samedepartment in 1995. He was awarded a Monbusho fellowship by the Govern-ment of Japan in 1995. He worked with Kanazawa University, Japan, as a Re-searcher and Assistant Professor until September 2000. In September 2000, hejoined the Institute of Information Sciences and Technology, Massey University,New Zealand, as Senior Lecturer. He is currently working as an Associate Pro-fessor with the School of Engineering and Advanced Technology, Massey Uni-versity, New Zealand. His fields of interest include smart sensors and sensingtechnology, wireless sensors network, electromagnetics, control, electrical ma-chines, and numerical field calculation, etc. He has published over 240 papersin different international journals and conferences and has written a book and abook chapter and edited nine conference proceedings. He has edited nine books,seven of which are with Springer-Verlag.

Dr. Mukhopadhyay is a Fellow of the Institution of Electrical Engineers(IEE), London, U.K., an Associate Editor of the IEEE TRANSACTIONS ON

INSTRUMENTATION AND MEASUREMENTS. He is Co-Editor-in-Chief of the In-ternational Journal on Smart Sensing and Intelligent Systems (www.s2is.org).He has also edited eight special journal issues, three for the IEEE SENSORS

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