Fluid Embeddable Coupled Coil Sensor for Wireless pH Monitoring in a Bioreactor

IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 63, NO. 5, MAY 2014 1337

Fluid Embeddable Coupled Coil Sensor forWireless pH Monitoring in a Bioreactor

Sharmistha Bhadra, Student Member, IEEE, Warren Blunt, Chris Dynowski, Michael McDonald,Douglas J. Thomson, Senior Member, IEEE, Michael S. Freund, Nazim Cicek,

and Greg E. Bridges, Senior Member, IEEE

Abstract— A passive embeddable coupled coil sensor forremote bioprocess pH monitoring is described. The sensor issterilizable, able to operate in a fluid medium, and small enoughto fit inside a small bioreactor or test tube. It consists of aplanar spiral inductor connected parallel to a varactor formingan LC resonant circuit. A pH combination electrode made ofan iridium/iridium oxide sensing electrode and a silver/silverchloride reference electrode is connected parallel to the varactor.A potential difference change across the electrodes due topH variation of the medium changes the voltage-dependentcapacitance and shifts the resonant frequency of the sensor.An interrogator coil is inductively coupled to the sensor coiland remotely tracks the resonant frequency of the sensor. Forin-fluid monitoring, the sensor is encapsulated in a mannerthat reduces the influence of the permittivity and conductivityof the medium. The sensor, calibrated over 2–12 pH range,exhibits a rapid response with 2.477-MHz/pH sensitivity. Thesensor was used for remote pH monitoring of Yarrowia lipolyticaand Saccharomyces cerevisiae fermentation in a shake flask over63 and 25 h, respectively. The experiments demonstrate that themedium pH can be monitored repeatedly with an accuracy of0.08 pH.

Index Terms— Bioreactor, coupled coil, in-fluid, passive,pH sensor, Saccharomyces cerevisiae, wireless, Yarrowia lipolytica.

I. INTRODUCTION

IN RECENT years, numerous efforts have been dedicated todevelop pH sensors for fields such as industrial and chem-

ical processing, environmental and bioprocess applications,food quality monitoring, structural health monitoring, andbiomedical sensing [1]–[7]. Bioprocesses are most commonlycarried out in bioreactors to produce many commodities andchemicals. During bioprocess, optimal cell growth dependson pH control and many cells produce acids as a metabolicbyproduct. Therefore, monitoring and controlling medium

Manuscript received June 5, 2013; revised October 9, 2013; acceptedOctober 31, 2013. Date of publication December 6, 2013; date of currentversion April 3, 2014. This work was supported by the Natural Sciences andEngineering Research Council of Canada. The Associate Editor coordinatingthe review process was Dr. Deniz Gurkan.

S. Bhadra, C. Dynowski, D. J. Thomson, and G. E. Bridges are withthe Department of Electrical and Computer Engineering, University ofManitoba, Winnipeg, MB R3T 5V6, Canada (e-mail: [email protected]; [email protected]).

W. Blunt and N. Cicek are with the Department of Biosystems Engineering,University of Manitoba, Winnipeg, MB R3T 2N2, Canada.

M. McDonald and M. S. Freund are with the Department of Chemistry,University of Manitoba, Winnipeg, MB R3T 2N2, Canada.

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

Digital Object Identifier 10.1109/TIM.2013.2292279

pH during bioprocess is important [1], [8]. Conventionally,sterilizable electrochemical pH probes are used for bioprocessmonitoring. They, however, need wired connections for dataexchange and are inherently invasive. Small bioreactors, suchas shake flasks and test tubes, are regularly used for bio-process development. The use of wired pH probes for multiplereactors (i.e., simultaneous monitoring of several shake flasks)requires a proportional increase of wiring, cost, and complex-ity. Fiber-optic probes are an alternative, but are fragile whenmechanically stressed and not sterilizable [9]. Embeddablewireless sensors are an attractive alternative for many of thesesituations [10]–[13].

Although a large number of studies have been done ondeveloping wireless pH sensors, only a small fraction canbe used inside bioreactors. Many sensors do not endurein the harsh bioprocess environment as the medium canpermeate through and damage the sensor. Often the fluidmedium culture is not well defined and interferes with sensorreadings. Moreover, sensors used in bioprocess need to besterilizable and their components should not leach out into themedium to avoid medium contamination and interference withmetabolism [1]. Another challenge is that the sensors have toperform measurement without recalibration, replacement, ormanual attention over the operational period [8]. Noninvasiveoptical sensors based on absorbance or fluorescence frompH-sensitive dyes have been successfully used inside bioreac-tors. They can be affixed to sterile vessels [8], [10], [14]–[16].Several commercially available systems [17]–[19] use theseoptical-based sensors for online pH monitoring of cultures.The advantages they offer over conventional electrochemicalprobes are high sensitivity and accuracy, ease of miniatur-ization, embeddable in a wide range of vessel types, andmost importantly wireless monitoring [20]. However, opticalsensors suffer from a narrow operating range and drift due tophotobleaching over time [1], [14], [17], [20].

In this paper, we present a coupled coil pH sensor thatcan be integrated with and embedded inside a bioreactor. Itemploys a varactor-based passive LC resonator whose resonantfrequency changes with the pH of the medium. This voltage-dependent frequency shift type sensor has been previouslyproposed for pH measurement [21], [22] as well as biopo-tential transmission [23]. We show how the varactor-basedLC sensor can be embedded in a fluid so that the resonantfrequency is insensitive to the electrical properties of the

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1338 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 63, NO. 5, MAY 2014

Fig. 1. Conceptual block diagram of the coupled coil pH sensor.

Fig. 2. Equivalent circuit diagram of the coupled coil pH sensor.

lossy medium. As the sensor is passive, it requires no internalpower source. Therefore, once embedded inside the bioreactor,there is no need to remove the sensor for battery chargingor replacement. An interrogator coil is inductively coupled tothe sensor inductor and the change in the sensor’s resonantfrequency is remotely detected by measuring the inducedchange in the impedance of the interrogator coil (Zin, as shownin Fig. 2). The design of the sensor is simple and inexpensiveto manufacture, making it a cost-effective solution to remotebioprocess pH monitoring.

II. PH SENSOR OPERATION

A block diagram of the coupled coil pH is shown inFig. 1. In the remote sensor, a spiral inductor is connected inparallel with a voltage-dependent capacitor (varactor)-basedvoltage sensing circuit. A pH combination electrode is con-nected in parallel with the varactor and provides a biasingvoltage to the varactor. In the sensor shown in Fig. 1, LS

is the inductance of the spiral inductor (sensor coil). Cf isthe interwinding parasitic capacitance between the traces ofthe spiral inductor, C(VpH) is the capacitance of the voltagesensing circuit, and VpH is the potential difference developed atthe pH combination electrode when in contact with a solution.The capacitance, C(VpH), changes in response to the low-frequency change of the biasing voltage, VpH. The spiralinductor and capacitors form a resonant circuit with a resonantfrequency, f0, approximately given by

f0 = 1

2π√

Ls(C(VpH) + C f ). (1)

The sensor inductor is inductively coupled to an interrogatorcoil, whose impedance is monitored using a swept frequencyimpedance analyzer. The resonant frequency of the weaklycoupled sensor coil, f0, is obtained by measuring the pertur-bation of interrogator coil impedance. In this manner, when

the interwinding capacitance is small and fixed (C f � C), theresonant frequency is directly related to the potential differencedeveloped at the pH combination electrode.

An equivalent circuit diagram of the fabricated coupled coilpH sensor is shown in Fig. 2. In the circuit, LS and RS are theseries inductance and resistance of the sensor coil, respectively.L1 and R1 are the series inductance and resistance of theinterrogator coil, respectively. M is the mutual inductancebetween interrogator and sensor coil. Cf is the interwindingcapacitance between the traces of the sensor coil. C is thesmall signal junction capacitance of the voltage-dependentcapacitor (varactor diode) in the sensing circuit. In the reversebias state, C is approximated by

C(Vc) = C0(1 − Vc/ϕ)−1/2 (2)

where C0 is the junction capacitance at zero bias, ϕ is thejunction built in potential, and VC is the bias voltage appliedacross the varactor. RpH is the cell resistance developed at thepH combination electrode when in contact with a solution.In the circuit, R3 and C2 act as a low-pass filter with acutoff frequency of 13 Hz. This isolated the resonator fromthe load impedance of the electrode cell and the varactorresponds to only low-frequency variations in VpH. For a smallsource oscillation amplitude, small M , and RpH � (R2 + R3),VC ∼= VpH. Assuming the losses are small, Cf is small andfixed, and C1 � C(VC), the pH of the contact solution, whichis shown by VpH, can be monitored by tracking the resonantfrequency of the sensor [24].

Referring to Fig. 2, near and at the resonant frequency, theimpedance, Zin, observed by the impedance analyzer is givenas follows:

Z in( f ) = Z1 + ZT = R1 + j2π f L1 + (2π f )2 M2

ZS(3)

where f is the interrogator source frequency andZS ∼= RS + j2π f LS + 1/( j2π f (C + C f )) is the sensor seriesimpedance. In our system, the resonant frequency was obtainedfrom the maximum of the real part of the impedance usinga quadratic curve-fitting algorithm [25]. The impedance, Zin,in (3) consists of two components: 1) Z1 = R1 + j2π f L1,due to the self-impedance of the interrogator coil and2) ZT = (2π f )2 M2/ZS , due to the sensor coupling.To remove the self-impedance of the interrogator coil, abackground subtraction, using the measured impedanceof the interrogator coil when the sensor was absent, wasimplemented prior to measuring the sensor response [26].

III. PH SENSOR DESIGN AND FABRICATION

A. pH Combination Electrode

A pH combination electrode generally consists of a sensingand a reference electrode. The sensing electrode providesa potential that is dependent on pH of the sample whilethe reference electrode ideally provides a pH-independentpotential. Thus, the potential difference across the sensingand reference electrode provides a measure of pH. Glass pHcombination electrodes are widely used for pH measurement.However, they are easily affected by alkaline solutions, have

BHADRA et al.: FLUID EMBEDDABLE COUPLED COIL SENSOR 1339

high cell resistance (in the order of 107 to 109 �) and aredifficult to miniaturize and planarize. Because of their brittlenature, they have a risk of breakage [3], [27], [28]. Dueto these disadvantages, iridium/iridium oxide (Ir/IrOx) andsilver/silver chloride (Ag/AgCl) electrodes were chosen assensing and reference electrodes, respectively, for the com-bination electrode in our sensor. An Ir/IrOx oxide electrodehas the advantages of easy preparation, small size, contin-uous detection, fast and stable response to pH in aqueous,nonaqueous, conductive, and even corrosive media. It exhibitsa linear response to pH and low cell resistance (3.16 M�)with respect to a silver/silver chloride electrode over a widepH range [29], no requirement for pretreatment and negligibleinterference of ions and complexing agents. Ir/IrOx electrodeshave been used for pH measurements in technical media, suchas fuels, food applications, and biological media. Ag/AgClelectrode is the most widely used reference electrode in theindustry and research due to its simple construction and cheapmanufacturing cost [7], [27], [30], [31].

The iridium/iridium oxide sensing electrode was preparedby a direct oxidation method. Ir metal wire (0.5 mm indiameter, 99.8% purity, obtained from Alfa AESAR) of 10 mmin length was ultrasonically cleaned with 6M HCl solutionfollowed with deionized (DI) water. The clean wire wasthen oxidized by bringing it to a temperature of 800 °C inan electric oven for 45 min after wetting its surface with1 M NaOH solution. The wetting and heating process wasrepeated six times until a blue–black coating was formed onthe surface [30]. The electrode was immersed in boiling DIwater for an hour and then in DI water at room tempera-ture for 30 days to reduce aging effects. A small area of(about 2 mm) iridium oxide film at one end was scrapped offand connected to an insulated wire using silver epoxy. High-temperature epoxy was applied over the connection area forelectric insulation.

The silver/silver chloride reference electrode was preparedfrom silver wire by electroplating method; 10 mm of Agwire (1 mm in diameter, 99.8% purity, obtained from AlfaAESAR) was polished using sandpaper. An insulated wirewas connected to a small area of the polished Ag wire (about2 mm in length) using silver epoxy. High-temperature epoxywas applied over the connection area for electric insulation.A layer of AgCl was formed on the remaining 8 mm of thepolished Ag wire by applying 0.5 V for 50 s in a 0.1 Mpotassium chloride (KCl) solution. An immobilized electrolytewas freshly prepared by saturating 12 mL of tetrahydrofuranwith KCl at room temperature and then adding 0.4 g ofpolyvinyl chloride. The Ag/AgCl wire was dip coated in theimmobilized electrolyte solution and dried in a house vacuum-evacuated desiccator for 48 h. After drying, the electrode wasdip coated with a protective nafion layer to prevent the leakageof chloride ions. Following the dip coating, the electrodewas cured in a pump-evacuated oven for 1 h at 140 °C,stored in a desiccator for seven days, placed in DI waterfor 24 h, and then stored in a desiccator until it was used.Nafion was selected because it is a cation-exchange polymerthat prevents anion exchange. Thus, transport of chlorideions through the nafion film is blocked and the leakage

Fig. 3. Nafion-coated Ag/AgCl quasi-reference electrode potential withrespect to a commercial glass Ag/AgCl reference electrode (CH Instru-ments 111) as a function of time and medium pH. An initial period ofapproximately 3.5 h is required for stabilization in buffer solution afterremoval from a desiccator.

Fig. 4. (a) Prototype coupled coil sensor for bioprocess pH monitoring.(b) Interrogator coil.

is eliminated. Nafion stability was greatly enhanced by thermalcuring at a high temperature, which protects the referenceelectrode and provides increased reproducibility and stabil-ity [32]. High-temperature (120 °C–150 °C) curing also makesthe nafion membrane stronger and more resistant to crackingthan nonheat treated nafion [33]. Fig. 3 shows the potentialof the nafion-coated Ag/AgCl quasi-reference electrode withrespect to a commercial glass Ag/AgCl reference electrode(CH Instruments 111). It can be observed that an initialstabilization time of around 3.5 h was required for the nafionto act as an ion exchanging membrane after the electrode wasremoved from the desiccator and placed in the fluid [32]. Theresponse for different pH shows that, after initial stabilization,the potential has a very small deviation, maximum 6 mV,over the 4–10 pH range (see inset of Fig. 3 where pH 4, 7,and 10 exhibit potentials of 166 ± 0.53, 168.07 ± 1.7, and170.58 ± 0.95 mV, respectively).

B. Sensor and Interrogator Description

A prototype sensor, as shown in Fig. 4(a), was constructedusing the Ir/IrOx and Ag/AgCl electrodes described previously.


Fig. 5. (a) Measured junction capacitance of the varactor at 77 MHz as a function of the bias voltage and (b) potential of IrOx electrode with respect tonafion-coated Ag/AgCl quasi-reference electrode, VpH, versus pH of the contact solution.

For our study, the prototype was fabricated as individual com-ponents. The electrodes, sensor electronics, and encapsulationcan later be integrated in a more compact manner. Thesensor was designed to have a resonant frequency, f0, near77 MHz. The sensor inductor and the voltage sensing circuitwere fabricated on a 20 mm × 6 mm single-sided FR4printed circuit board (PCB) with surface mount capacitors andresistors. The encapsulation is described latter. The inner andouter dimensions of the rectangular, four-turn planar spiralinductor were 6.5 mm × 1.2 mm and 12 mm × 5.5 mm,respectively, producing LS = 0.13 μH and RS = 1.25 � andC f = 0.98 pF at 77 MHz in air and fres = 445.89 MHz(measured with a network analyzer HP 8753). The junctionbuilt-in potential of the varactor (NXP BB202) used in thevoltage sensing circuit is 0.7 V. The junction capacitance ofthe varactor, C , as a function of bias voltage was measuredat 77 MHz and is shown in Fig. 5(a). It provides a voltagesensitivity of dC/dV = 0.02 pF/mV at a bias voltage of111 mV (corresponding to a pH of 7). Fig. 5(b) shows theIrOx electrode potential with respect to the nafion-coatedAg/AgCl quasi-reference electrode, VpH, for pH solutions inthe range of 2–12 (the nafion coated Ag/AgCl quasi-referenceand Ir/IrOx electrode potential was measured with a high-impedance voltmeter). VpH has a slope of 58 mV/pH, whichis very close to the ideal slope (∼59 mV/pH). The electroderesistance, RpH, was approximately 3.16 M�. The Ag/AgCland Ir/IrOx electrodes were connected to the positive andnegative terminals of the voltage sensing circuit (Fig. 2),respectively. The sensor along with the electrodes is normallystored in a desiccator to prevent leakage. Since an initializationstabilization time of around 3.5 h is required, as observed fromFig. 3, the electrodes were stored overnight in DI water beforethe experiments.

The interrogator coil was fabricated on a 32 mm × 32 mmsingle-sided FR4 PCB. The inner and outer dimensions of thesquare, eight-turn planar spiral inductor were 3 mm × 3mmand 13 mm × 13 mm, respectively, producing LS = 0.82 μH,RS = 2.2 � at 77 MHz and a self-resonant frequency,

Fig. 6. Effect of lossy fluid on sensor coil and sensor response. Illustrationsof (a) fluid influence for a thin passivation layer (interwinding capacitancechange and conductance addition) and (b) removal of that effect by thickdielectric spacer. (c) Response of the sensor for both cases.

fres = 97.3 MHz. The sensor’s resonant frequency wasdetermined by measuring the real part of the impedance ofthe interrogator coil, Re{Z in}, when inductively coupled tothe sensor coil. The interrogator inductor coil impedance wasmeasured using an impedance analyzer (Agilent 4294A) withthe voltage source level of the analyzer set to 25 mV.

IV. EXPERIMENT AND RESULT

A. In-Fluid Monitoring

For bioprocess pH monitoring, the sensor needs to beimmersed in a fluid medium inside the bioreactor. Weassume the medium inside bioreactor is a nonferrous fluidwith relative permeability of the fluid, μr = 1 for all cases.One issue for in-fluid monitoring is the protection of theelectronics from the fluid. This is typically solved by putting athin passivation layer or encapsulation material on the sensorelectronics. Another important issue is the effect that the lossyfluid has on the sensor coil fields. Wireless sensors embed-ded in lossy media can experience loss in the interrogator–sensor coupling, especially in the ultrahigh frequency bandof operation [34]. The loss is significantly less at the lower


Fig. 7. Encapsulated coupled coil sensor immersed in a fluid medium.

frequencies our sensor operates at, where the coupling isthrough magnetic field induction [34]. Further to this, thesensor coil field can impinge into the lossy fluid. As shown inFig. 6(a), for a thin passivation layer, this results in a changeof interwinding capacitance, Cf , and an added conductanceloss. The resonant frequency becomes medium dependant, andthe quality factor, Q, of the sensor resonator is dramaticallyreduced. This parasitic effect is often advantageously usedin many sensors that monitor the dielectric properties of themedium [26]. In our sensor, it is undesirable and a thickencapsulation layer (a low-loss dielectric spacer between thesensor coil and the passivation layer), as shown in Fig. 6(b),can be used to remove the effect of the fluid medium. Fig. 6(c)shows the responses of the sensor for a thin layer of passivation(0.5-mm thick layer of polypropylene, εr = 2.1) and thickdielectric spacer when the sensor was immersed in a standardfermentation medium of pH 5.3 (measured with a commercialpH probe) at 30 °C. The dielectric spacer [see Fig. 4(a)] wasconstructed by hermetically sealing the sensor coil/electronicsin a polypropylene vial, which provided a 2.5-mm air gapbetween the sensor coil and the thin polypropylene layer.Locations where the electrodes came out of the vial weresealed with high-temperature epoxy. The experimental set upis shown in Fig. 7. The interrogator coil was concentricallyplaced outside of the beaker at a distance d = 3 cm from thesensor coil. Even though the fermentation medium is a fairlylow-loss fluid (εr = 74.48, σ = 0.04 S/m), it can dramaticallyaffect the response of the thin passivation layer sensor. It canbe observed that the thick dielectric spacer improves the Q ofthe sensor by removing the effect of the fluid medium.

The sensitivity of the prototype sensor (with thick dielec-tric spacer) to the electrical parameters of the medium wastested using the sensor in fluids with different permittivityand conductivity at 30 °C (shown in Fig. 7). All the mediahave a pH of 5.3 (measured with a commercial pH probe).The interrogator coil was placed concentrically outside of thebeaker at a distance d = 3 cm from the sensor coil. Fig. 8shows the impedance frequency response when the sensor wasin the air (only the electrodes are in contact with the mediumof εr = 74.48, σ = 0.04 S/m at the bottom of the beaker)and when it was fully immersed in different fluids. Resultsshow that the Q and f0 of the sensor did not depend onthe relative dielectric permittivity of the fluid. Only a veryhighly conductive medium reduces the Q and changes the f0

Fig. 8. Impedance frequency response when the prototype sensor (encap-sulated with thick dielectric spacer) is in air and completely immersed indifferent fluids for a constant pH of 5.3.

Fig. 9. Resonant frequency of the sensor as a function of measured pH ofthe prepared solutions.

of the sensor. In this worst case, this corresponds to an errorof 0.11 pH. This prototype sensor with thick dielectric spacerwas used for the rest of the experiments in this paper.

B. Calibration

The sensor along with the electrodes was immersed in aseries of varying pH solutions (values between 2 and 12) ina 250-mL Schott bottle at 30 °C. For the calibration test,solutions of different pH were prepared by adding KOH toHCl. During the tests, the pH of the solutions was continuouslymonitored using a commercial pH probe. The sensor coilwas aligned concentrically with the interrogator coil, whichis placed outside of the bottle, with a separation distanced = 2 cm. Fig. 9 plots the resonant frequency, f , for the


Fig. 10. Fermentation experiment with the sensor mounted inside thebioreactor.

Fig. 11. Example impedance frequency responses for two different mediumpH values.

different pH solutions as determined from the peaks of theirimpedance response. A linear fit given by

f0(MHz) = −2.477pH + 92.139 (4)

over the 2–12 pH dynamic range shows a sensitivity of2.477 MHz/pH.

C. Fermentation Experiments

Fermentations of Yarrowia lipolytica and Saccharomycescerevisiae were conducted in a 250-mL Schott bottle, whichserved as a bioreactor in this paper. The sensor was embeddedin the bioreactor containing the medium. A commercial pHprobe (connected to a data logger) was also used to monitorand log the pH inside the bioreactor. Fig. 10 shows the

Fig. 12. Resonant frequency of the sensor and medium pH as measuredwith a commercial pH probe and with the sensor using (4) and the resonantfrequency for Y. lipolytica fermentation.

experimental setup. It should be noted that the same sensor wasused for both fermentation experiments. Media preparation,experiment, and results for each fermentation are described inthe following.

1) Yarrowia Lipolytica Fermentation: The medium was pre-pared from 10-g/L yeast extract, 20-g/L peptone, and glycerol(13 mL of 50% glycerol solution to give a final concentrationof 440 mm). Glycerol was used as the carbon source. Thetotal volume of the medium and carbon source was 200 mL.The shake flask and the sensor were both sterilized usingan autoclave. The pH probe and an oxygen tubing weresterilized separately and inserted into the bioreactor to trackthe pH of the medium and supply oxygen, respectively; 40 mLof Y. lipolytica cells were harvested during midexponentialgrowth stage. This was centrifuged to separate the cells fromthe supernatant, and the liquid volume was reduced to 10 mL,concentrating the biomass in the inoculum by a factor offour. Then, 2 mL of Y. lipolytica cells were used to inoculatethe medium for fermentation. The medium was stirred ata constant speed of 100 r/min at 30 °C. The resonant fre-quency, f0, of the sensor was monitored at half hour intervalsat a distance of 2 cm using the interrogator coil. Fig. 11 showsexamples of the impedance frequency response of the sensorfor two different medium pH values.

Variation of the sensor’s resonant frequency and medium pH(measured using the sensor and commercial pH-probe) over63 h is shown in Fig. 12. The medium pH measuredby the sensor was obtained using the resonant frequency andthe calibration (4). It can be observed that the pH monitoredby the sensor was always in good agreement with that mea-sured by the commercial pH probe over the course of thefermentation period, with a maximum deviation of 0.08 pH.

2) Saccharomyces Cerevisiae Fermentation: The mediaused in this paper was prepared using a purchased mix-ture of yeast extract, peptone, and dextrose (YPD) (Bactoobtained from Becton, Dickinson and Company). To preparethe media, 12.5-g YPD was dissolved into 250 mL of DI water(50 g L−1). The pH of the media was adjusted to 6.5 by theaddition of potassium hydroxide. A volume of 200 mL of YPD


Fig. 13. Resonant frequency of the sensor and medium pH as measuredwith a commercial pH probe and with the sensor using (4) and the resonantfrequency for S. cerevisiae fermentation.

media was placed in the bioreactor. The media and sensorinside the bioreactor were sterilized using an autoclave. Thecommercial pH probe was also sterilized and inserted into thebioreactor to monitor the pH. The organism used was a recom-binant strain of the yeast S. cerevisiae, a well-studied organismthat exhibits ability to produce high yields of ethanol fromfermentation of glucose. Five grams of pelletized S. cerevisiaeyeast was added to the reaction vessel with a 200-mL workingvolume just prior to the test. The reaction was kept at 30 °Cand gentle stirring was applied using magnetic stir plate fora period of 25 h. The sensor’s resonant frequency, f0, wasobserved at a distance of 2 cm with the interrogator coilFig. 13 plots the variation of the sensor’s resonant frequencyand medium pH as measured with the commercial pH probeand the sensor using the resonant frequency and calibration (4).Over the 25-h fermentation period, the sensor pH measurementconforms with the commercial probe pH measurement withmaximum deviation of 0.07 pH.

D. Repeatability and Cyclic Stability

To test the repeatability of the sensor, base was addedto the medium after 63 h in the case of the Y . lipolyticafermentation. In this manner, the pH of the medium wasincreased from 5.26 to 5.94 and then 6.43 in two steps.The resonant frequency obtained for medium pH 5.94 and6.43 after 63 h was then compared with the resonant frequencyobtained for medium values of pH 5.94 and 6.43 measured at21.25 and 1 h, respectively, during fermentation. The results,shown in Table I, shows that the sensor produced repeatableresults (<0.05 pH) after the 63-h test. This also shows that thesensor was not affected or damaged inside bioreactor by themedium or the bioreactor stirring process.

The cyclic stability of the sensor was determined by repeat-edly immersing the sensor in solutions of pH 2, 7, and12 at 25 °C in the 250-mL Schott bottle. The sensor’s resonantfrequency was measured at each step for a separation distanced = 2 cm from the interrogator coil. pH was obtained usingthe resonant frequency and calibration (4). The results, shownin Fig. 14, show that the sensor has high cyclic stability

TABLE I

VARIATION OF RESONANT FREQUENCY WITH TIME

FOR SAME MEDIUM PH

Fig. 14. Resonant frequency and pH as measured with the sensor using (4)and the resonant frequency when exposed to repeated cycles of pH 2, 7, and12 solutions.

with a maximum deviation of 148.62 kHz (corresponding toa maximum deviation of 0.06 pH).

E. Effect of Interrogator–Sensor Separation Distance

As the separation distance between the sensor and theinterrogator coil increases, the mutual inductance betweenthe sensor and interrogator coil M decreases, reducing thereceived signal amplitude, ZT , from the sensor. In addition,the position of the sensor inside the fluid may affect theresonant frequency. The dependence on location in fluid anddistance to the interrogator was measured using a setup shownin Fig. 7. For all tests, the sensor was immersed in a medium(εr = 74.48, σ = 0.04 S/m) of pH 5.3 (measured with a com-mercial pH probe) at 30 °C in the beaker. The sensor coil wasaligned concentrically with the interrogator coil, which wasplaced on the surface of the beaker. Only the sensor was movedinside the beaker to obtain different separation distances.Theoretical values for the resonant frequency were calculatedby (3) using measured values of L1, R1, Ls , Rs , C f , C(VpH),and an approximation for coupling factor, M . The valueof M was calculated using the theoretical coupling factor fortwo concentric coils [35]. Table II shows the measured andtheoretical resonant frequency, f0, along with the theoreticalcoupling, M , values for different distances. For separationdistances ranging from 1.5 to 4 cm, the theoretical f0 value didnot change since the coupling factor was small in all the cases.


TABLE II

VARIATION OF RESONANT FREQUENCY AND MUTUAL INDUCTANCE WITH INTERROGATOR–SENSOR DISTANCE

The measured f0 varied 245 kHz (corresponding to 0.1 pH).This is due to the noise of the system, which sets an upper limitfor the operating distance of this type of sensor. As the sep-aration distance increases, the signal-to-noise ratio decreasesrapidly reducing the measurement accuracy. It should be notedthat placing the sensor too close to the interrogator coil caninduce a large voltage in the sensor coil and push the varactorinto a nonlinear operating region [29]. It is also important tohave the two coils concentrically oriented to achieve a optimalcoupling (the experiments described in this paper do this). Theupper limit for the separation distance can be improved byincreasing the power at the interrogator coil, or by increasingthe number of turns and/or radius of the interrogator coil [26].Table II also shows that the circuit behaves the same wayas the model shown in Fig. 2. The differences between thetheoretical and measured values of f0 are due to the PCBparasitic effects on the varactor [the varactor capacitance inFig. 5(a) was measured before placing it on the PCB].

V. DISCUSSION AND CONCLUSION

This paper presented a coupled coil pH sensor capable ofreal-time and repeatable in-fluid pH measurement inside thebioreactor. This simple sensor is based on an LC resonatorwith pH of the medium monitored by measuring the changeof sensor’s resonant frequency. This sensor has the advantagesof requiring no power source in the sensor, and no physicalconnection between the sensor and the data acquisition system.The sensor was calibrated over a 2–12 pH dynamic range.Using the calibration, the sensor was successfully able toprovide continuous remote pH measurement of Y. lipolyticaand S. cerevisiae fermentation. The pH recorded by thesensor was repeatable and in good agreement with the val-ues measured with a commercial pH probe. The maximumdeviations were 0.08 pH over a 6.83–5.26 pH range and0.07 pH over a 6.2–4.7 pH range from the commercial pHprobe during Y. lipolytica and S. cerevisiae fermentation,respectively. Although the accuracy of noninvasive optical pHsensors (optical pH sensors have accuracy of 0.05 pH [17]) isslightly better than the accuracy of the proposed sensor, narrowoperating range and long-term drift due to photobleachingare the two concerns [14], [20]. Since the proposed sensor isbased on a pH combination electrode, it operates over a widerdynamic range and does not have a long-term drift problem.

For general use, temperature should be considered whenusing Electrode-based pH sensors. The potential differ-ence developed at the pH combination electrode, VpH, is

affected by temperature. Since bioprocesses are carried out atdifferent constant temperatures, the sensor has to be calibratedevery time the process temperature changes to overcome pHmeasurement error due to temperature dependence of theelectrodes [29]. Another concern for the sensor is the sensingelectrode made from iridium, which is fairly costly. Thissensing electrode can be easily replaced by commerciallyavailable, low-cost, and pH sensitive mixed metal oxide elec-trodes [36]. The robust, sterilizable, and encapsulated sensorwas not affected by the electrically lossy fluid medium. Itssize is suitable for pH monitoring inside a shake flask ortest tube. In practice, simultaneous monitoring of severalbioreactors would need multiple sensors to be embedded.In this situation, the proposed sensor requires the ability touniquely identify one sensor’s response from its neighbor. Thisissue can be solved by designing sensors to have differentresonant frequencies [37]. The nafion-coated Ag/AgCl quasi-reference electrode used in this paper has a very small devia-tion in potential over a 4–10 pH range. The simple designof the sensor makes it suited to integration using printedcircuit technology and thus it can be a potential cost-effectiveway to obtain reliable pH information remotely from abioprocess.

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Sharmistha Bhadra (S’12) received the B.Sc.degree in computer engineering from the Universityof New Brunswick, Fredericton, NB, Canada, in2008, and the M.Sc. degree in electrical engineeringfrom the University of Manitoba, Winnipeg, MB,Canada, in 2010, where she is currently pursuingthe Ph.D. degree.

She is a Research Assistant with the Departmentof Electrical and Computer Engineering, Universityof Manitoba. Her focus is to develop wirelesspassive sensors for structural health monitoring and

chemical sensing.

Warren Blunt received the B.Sc. degree in biosys-tems engineering from the University of Manitoba,Winnipeg, MB, Canada, in 2011, where he is cur-rently pursuing the M.Sc. degree in biosystems engi-neering.

His current research interests include the devel-opment and implementation of real time analyticaltechniques to monitor microbial fermentation forproduction of second-generation biofuels.

Chris Dynowski is currently pursuing the B.Sc.degree in electrical engineering with the Universityof Manitoba, Winnipeg, MB, Canada.

He is a Summer Research Assistant with theDepartment of Electrical and Computer Engineering.He has been funded in large part by the National Sci-ence and Engineering Research Council of Canada.His current research interests include wireless pas-sive pH sensors.

Michael McDonald was born in Winnipeg, MB,Canada. He received the Undergraduate degree inchemistry from the University of Manitoba, Win-nipeg, where he is currently pursuing the Ph.D.degree with the Department of Chemistry.

His current research interests include materials forphotoelectrochemical conversion to hydrogen.


Douglas J. Thomson (SM’09) received the Ph.D.degree in electrical engineering from Stanford Uni-versity, Stanford, CA, USA, in 1986.

He has been a Professor with the Department ofElectrical and Computer Engineering, University ofManitoba, Winnipeg, MB, Canada, since 1987. Heis a pioneer in the development of scanning probemicroscopy and the co-inventor of a number ofinstrumentation systems, including a record settingcapacitance sensor and a passive wireless sensor foruse in monitoring civil structures. He has published

over 80 journal papers, is co-inventor on seven patents, and has given invitedtalks at a number of international meetings. His current research interestsinclude passive wireless sensors for civil structures monitoring, scanning probemicroscopy, and biomaterial sensing in micro-fluidics systems.

Michael S. Freund received the B.S. degree inchemistry from Florida Atlantic University, BocaRaton, FL, USA, in 1987, and the Ph.D. degree inanalytical chemistry from the University of Florida,Gainesville, FL, USA, in 1992.

He was a Post-Doctoral Fellow with the Depart-ment of Chemistry, California Institute of Technol-ogy, Pasadena, CA, USA, and he became an Assis-tant Professor of chemistry with Lehigh University,Bethlehem, PA, USA. He is currently a Professorwith the Department of Chemistry and an Adjunct

Professor of electrical and computer engineering with the University ofManitoba, Winnipeg, MB, Canada, and a Tier 2 Canada Research Chair ofconducting polymers and electronic materials.

Nazim Cicek received the M.Sc. degree in chemicalengineering and the Ph.D. degree in environmen-tal engineering from the University of Cincinnati,Cincinnati, OH, USA, in 1995 and 1999, respec-tively.

He is a Professional Engineer and Professor withthe Department of Biosystems Engineering, Uni-versity of Manitoba, Winnipeg, MB, Canada. Hecurrently serves as an Associate Department Head.After over two years of experience in the privatesector, he returned to academia in 2001. He has

authored or co-authored over 70 peer-reviewed journal articles and pre-sented his research results in numerous international conferences. His cur-rent research interests include the biological waste treatment, more specif-ically membrane bioreactor technologies, biofuel and bioenergy productionprocesses, anaerobic digestion, and nutrient removal from wastewater.

Greg E. Bridges (M’82–SM’04) received the Ph.D.degree in electrical engineering from the Universityof Manitoba, Winnipeg, MB, Canada, in 1989.

He is currently a Professor with the Departmentof Electrical and Computer Engineering, Univer-sity of Manitoba, and is a Principal Investigatorfor the Advanced RF Systems Laboratory, NationalMicroelectronics and Photonics Test Collaboratory,Winnipeg. He holds several patents related to RFICprobing methods and wireless sensors. His currentresearch interests include computational electromag-

netics, wave interactions with transmission lines, bio-microfluidics, RF sen-sors, and high-frequency applications of nanoprobe-based instrumentation.

Documents

Fluid Embeddable Coupled Coil Sensor for Wireless pH Monitoring in a Bioreactor