Functionalized graphene sensors for real time monitoring fermentation processes Chinnathambi, Selvaraj
IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from it. Please check the document version below.
Document Version
Publisher's PDF, also known as Version of record
Publication date: 2020
Link to publication in University of Groningen/UMCG research database
Citation for published version (APA):
Chinnathambi, S. (2020). Functionalized graphene sensors for real time monitoring fermentation processes: electrochemical and chemiresistive sensors. University of Groningen.
Copyright
Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).
Take-down policy
If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.
Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum.
Nitrogen and boron doped reduced graphene oxide chemiresistive dissolved
oxygen sensor-A new approach towards dissolved oxygen sensing
Abstract
A dissolved oxygen (DO) micro sensor is an important analytical device for process control
during fermentation processes in a micro reactor-screening platform. The widely used
clark-type electrodes pose a problem for an integration with other sensors because of its size.
Optical fiber based DO sensors require reactors with proprietary sensor dots integrated in the
wall of the reactor. It is with this background that we developed a small and cheap DO sensor
based on chemiresistive sensing material. Here, we report our new method to construct a DO
sensor using Nitrogen and Boron doped reduced graphene oxide as chemiresistive material.
Our study shows that N,B-HRGO changes its conductivity with respect to dissolved oxygen
concentration, and it has high response below 1 mg/L. We tested the feasibility of sensor to
monitor DO concentration during a bacterial growth experiment with the aerobic organism
Amycolatopsis methanolica.
Key words: Dissolved oxygen, Nitrogen and Boron doping, Reduced graphene oxide, chemiresistive sensor, Amperometric sensor, Oxygen reduction.
---Part of this chapter has been published as:
Selvaraj chinnathambi, G. J. W. Euverink , Nitrogen and Boron doped hydrothermally reduced graphene oxide amperometric dissolved oxygen sensor, IMCS 2018 (DOI 10.5162/IMCS2018/P2EC.2)
5. 1. Introduction
Dissolved oxygen (DO) is an important parameter in various biological and
environmental processes like aquatic system, water quality maintenance and fermentation
processes [1]. There are several techniques available to measure dissolved oxygen including
Clark-type polarographic electrode [2], luminescence based optical [3-7], fiber optics
chemical sensor [8,9] electron magnetic resonance (EPR) [10], nuclear magnetic resonance
[11-12], functional magnetic resonance imaging [13-14]. The choice for selecting one of
these sensor techniques depends on the ease of use, invasiveness of the sensor, sensitivity,
selectivity, and resolution [15]. Among them, Clark-type polarographic electrode and optical
sensor are widely used for measuring dissolved oxygen in biological processes. However, the
consumption of oxygen, the large size, and complex assembly of Clark-type electrodes make
it difficult to use them in micro bioreactors. Optical sensors have an advantage over
polarographic electrodes for measuring dissolved oxygen during biological processes [15-16].
The need for small and cheap optoelectronic sources, detectors and data read out make it a
challenge to use them in integrated miniature applications [17]. Many of the problems
associated with these sensing techniques are based on the principle how the sensor measures
dissolved oxygen and the components needed to transduce the signals. In this chapter, we
report our new approach towards the development of a dissolved oxygen sensor based on
chemiresistive material. An advantage of these sensors is the simple electrode structure,
possibility of constructing micro and nano sensors. Moreover, the compatibility with existing
modern electronic components makes chemiresistive sensors attractive for application in
miniaturized bio reactor systems [18,35].
Graphene is a two-dimensional material that has a high electronic conductivity and is
thermally and chemically stable [19]. Graphene properties can be tuned by functionalizing
graphene have been studied as electrocatalyst for oxygen reduction reaction. The presence of
a hetero-atom makes the graphene electron-deficient, which increases the catalytic activity
towards the oxygen reduction reaction (ORR) [28-29]. The nitrogen and boron (N,B)-doped
graphene has been studied extensively as a metal-free electrocatalyst for oxygen reduction
reaction. However, it has not been studied to detect oxygen dissolved in a solution. The main
advantage of these metal-free hetero-atom doped electrocatalysts as a dissolved oxygen
sensor material is that these materials are cheap and have high chemical stability to withstand
harsh environments.
Chemiresistive sensors transduce the signal by means of a conductivity change as a
result of charge transfer reaction with an analyte. In the present case, chemiresistive material
changes the conductivity due to absorption and desorption of oxygen molecules dissolved in
the electrolyte. There are several published reports available for the construction of
chemiresistive O2 gas sensors [30,31]. However, there are no attempts published to develop
chemiresisive sensors for oxygen dissolved in solution. A few studies reported the sensing of
analytes dissolved in highly conductive aqueous electrolytes using chemiresistors [32-35]. To
the best of our knowledge, there are no reports available on the development of sensors to
detect dissolved oxygen using chemiresistive material. We used N,B-doped reduced graphene
oxide prepared by hydrothermal method, as a chemiresistor for dissolved oxygen sensing.
One of the reasons to use N,B-doped reduced graphene oxide is that it has been reported to
facilitate oxygen adsorption on a carbon surface and improves the electron transfer process.
For our sensor we need a material that effectively adsorbs oxygen molecules that are present
in a solution and at the same time act as a chemiresistor. As N,B-doped reduced graphene
oxide satisfy both requirements we used them to construct a chemiresistive sensor to detect
For the better understanding of the N,B-doped reduced graphene oxide, the electrocatalytic
activity and amperometric DO sensing also investigated and the results are discussed here.
The cyclic voltammetric techniques was used to investigate electrocatalytic activity. Though,
the electrocatalytic activity of N,B-doped graphene in alkaline and acidic medium [36,37] was
reported previously. Here the electrocatalytic studies and amperometric DO sensing were
carried out in neutral pH electrolyte.
5. 2. Experimental details 5. 2.1. Material preparation
N,B-doped graphene (N,B-HRGO) is synthesised by a hydrothermal method. In a
typical procedure, 50 mg of graphite oxide (GO) is dissolved in 50 ml water. The dispersion
was sonicated for four hours and unexfoliated graphite oxide flakes (pellet material) are
removed by centrifugation [38,39]. The resultant GO dispersion is used for functionalization.
For nitrogen and boron doping, 5 ml NH3.H2O and 5 ml of boric acid were added to the 50
ml GO dispersion and autoclaved for 12 hours at 120 oC (0.12 MPa). Finally, the prepared
N,B-HRGO is collected by filtration and washed several times with water.
Table 5.1: Mixing of N2 and O2 saturated 0.1 M KNO3
Air sat. (ml)
N
2. Sat (ml)
Air:N
2(%)
10 0 100:0
7.5 2.5 75:25
5.0 5.0 50:50
2.5 7.5 25:75
5.2.2. Electrode preparation
For the amperometric sensor, a 2 mm gold disc, a platinum wire, and Ag / AgCl were used as
working, counter, and reference electrode, respectively. The electrochemical measurements
were performed with a potentiostat (CH760, CH instruments, Austin, Texas, USA). A
threecompartment electrochemical cell was used for measurements. For amperometric sensing, a
-0.4 V constant potential was applied to the electrode and the current was measured against
time. For dissolved oxygen sensing, the amperometric response was obtained by placing the
electrodes in 0.1 M KNO3 containing different dissolved oxygen concentrations. The different
DO concentrations were achieved by mixing different volumes of nitrogen-gas and
oxygen-gas saturated solutions (Table 1). For continuous DO sensing, 0.1 M KNO3 was saturated
with pure O2 and N2 gas bubbling. The flow of O2 and/or N2 gas was stopped at various
intervals to maintain stable and varying DO concentration. For comparison, t DO
concentration of 0.1 M KNO3 solutions were also measured with a commercial DO sensor
(Greisinger GOX20 Oxymeter).
For chemiresistive sensing, 2 µl of N,B-HRGO (1 mg/ml) is drop-casted on an interdigitated
Au electrode and dried at 101 °C for 12 hrs. To measure the DO, a constant bias voltage of
-0.4 V is applied between the gold fingers and the resulting current is converted to resistance according to Ohm’s law. For testing the sensor in fermentation medium, a silicon-based
oxygen permeable membrane is dip-coated on the electrode [40-42]. The membrane is used,
because our previous study involving a chemiresistive pH sensor indicated that metabolic
products in the fermentation medium interfere with the sensor surface [33].
X-ray Photoelectron Spectroscopy (XPS) was performed using a Surface Science SSX-100
ESCA instrument with a monochromatic Al Kα X-ray source (hν = 1486.6 eV). The constant
electrons with respect to the surface normal was 37º. The diameter of the analyzed area was 1
mm yielding a total experimental energy resolution of 1.1 eV. Binding energies are reported ±
0.1 eV and referenced to the Au 4f7/2 photoemission peak originating from the substrate,
centered at a binding energy of 84 eV. All the samples were prepared in water by mild
sonication and a small drop of the suspension was left to dry in air on the 100 nm thick gold-
on-mica substrate. The spectral analysis included a Shirley background subtraction and peak deconvolution employing mixed Gaussian− Lorentzian functions, in a least squares
curve-fitting program (WinSpec) developed at the LISE, University of Namur, Belgium.
Perkin Elmer Raman station equipped with 785 nm laser was used for Raman
measurements. Samples are deposited on microscopic glass slide and dried in the oven at 100
C. Transmission electron microscopy (TEM) was carried out at 120 keV (CM12, Philips, The
Netherlands). The samples were prepared on carbon-coated 400 mesh copper grid and the
images were recorded with a slow-scan CCD camera.
5.3. Results and discussion
5.3.1. N,B-HRGO synthesis and characterization
Ammonia and Boric acid was used as nitrogen and Boron source for doping. After the
hydrothermal reduction process brown colour dispersion changed to black color solid as
shown in Fig. 5.1. The colour change indicates the removal of oxygen functional groups and
the restoration of -C=C- conductive network.
Figure 5.1: Images of GO dispersion (a), N,B-HRGO (b)
X-ray Photoelectron Spectroscopy (XPS)
The incorporation of nitrogen and boron in HRGO doping was confirmed by X-ray
photoelectron spectroscopy (Fig. 5.2). The C1s core-level region of GO and N,B-HRGO is
shown in Fig. 5.2 (a), and 5.2 (b). The three main peaks for GO at 284.4 eV [43], 286.3 eV
[44], and 288.4 eV [45] correspond to aliphatic C-C, C=O/C-O, and COO. The additional peak at 292.5 eV is indicative of π-π* . In the case of N,B-HRGO, the peak intensity at 286.3
eV decreased due to the reduction of C=O/C-O. Two additional peaks at 283.9 eV [46] and
285.4 eV [43] appeared in the spectrum of N,B-HRGO, corresponding to C-B and C-N.
Figure 5.2: XPS spectra of GO and N,B-HRGO. Deconvoluted (a) C1s spectra of GO and C1s, N1s, and B1s spectra of N,B-HRGO (b,c, and d)
N1s and B1s deconvoluted spectra in Fig 5.2(c) and Fig 5.2(d) show the different binding
states of N and B in N,B-HRGO. The peaks at 398.8 eV [43], 400.0 eV [43,47], and 401.7 eV
[47] in N1s spectra correspond to pyridinic, pyrrolic, and graphitic nitrogen, respectively. The
peaks at 191.1 eV [44], 191.7 eV [48], and 192.3 eV [49] attributed to the presence of B-C3,
B-C-O, and B-O, respectively.
Raman spectrum
Raman spectrum of GO, HRGO, and N,B-HRGO is shown in Fig. 5.3. The Raman
spectra indicates that the D to G ratio of N,B-HRGO increased compared to the D to G ratio
of GO due to doping of nitrogen and boron in the carbon lattice [50].
Figure 5.3: Raman spectrum of GO, HRGO and N,B-HRGO. Transmission electron microscopy (TEM)
The morphology of N,B-HRGO was investigated using TEM. The N,B-HRGO showed thin
layer of graphene sheets with wrinkled morphology. The TEM image of GO and N,B-HRGO
Fig. 5.4: TEM images of GO and N,B-HRGO (a, b)
5.3.2. Amperometric sensing
Electrochemical oxygen reduction reaction
Electrocatalytic activity of N,B-HRGO for oxygen reduction reaction in 0.1 M KNO3 neutral
electrolyte was evaluated by cyclic voltammetry (CV) (Fig 5.4). The voltage over the
electrodes was cycled between 0 and -1.0 V at a scan rate of 50 mV/s. The CV of N,B-HRGO
showed a sharp oxygen reduction peak at - 0.3 V with an onset potential lower than - 0.1 V.
This peak completely disappeared when the dissolved oxygen was purged with nitrogen gas.
The CV of ERGO in oxygen-saturated solution showed a different behaviour. The peak
around -0.3 V was less intense with an onset potential higher than -0.1V. The oxygen
reduction peak appeared at -0.8 V with an onset potential of -0.6 V which indicated that
N,B-HRGO reduces oxygen at a more positive potential compared to ERGO.
Figure. 5.5: Cyclic voltammetry of N,B-HRGO (a) and ERGO (b) in 0.1 M KNO3 in
Nitrogen saturated (black) and oxygen saturated (blue)
Amperometric detection of dissolved oxygen
The reduction potential for oxygen was determined by cyclic voltammetry. For
amperometric sensing, -0.4 V is applied and the reduction current was monitored as the sensor
response. The DO sensing response of N,B-HRGO and ERGO was measured in 0.1 M KNO3
solutions with different percentages of oxygen. The current values were plotted against the
percentage of maximal dissolved oxygen concentration (Fig. 5.6). The N,B-HRGO deposited
electrode showed a sensitivity of 10 nA / % of oxygen and the ERGO deposited electrode,
showed a 50 % lower sensitivity of 4.8 nA / % of oxygen.
Figure 5.6: Current response of N,B-HRGO and ERGO at varying percentages of dissolved oxygen concentration in 0.1 M KNO3.
An effect of stirring on the sensor response was observed for the N,B-HRGO electrode. In
Figure 5.7, the amperometric curve of the electrode at various applied voltages with (Fig. 7a)
and without (Fig. 5.7(b)) stirring is shown. The current response of the electrode under
stirring is higher due to increased mass transport of oxygen molecules to the electrode
surface. In order to understand the effect of stirring on the electrode response, the
response of the N,B-HRGO electrode in a solution with 8.8 mg.L-1 O2 at 22 ºC that was
stirred at 0, 150, 300 and 500 rpm stirring rates is shown. The current response was more or
less constant at a stirring rate above 300 rpm.
Figure 5.7: Amperometric response of the N,B-HRGO electrode in an unstirred (a) and a stirred solution at 100 rpm (b). Effect of stirring on the oxygen reduction current at different
stirring rates: 0, 150, 300, and 500 rpm, respectively (c).
In order to detect the dissolved oxygen in real-time, the amperometric response was
continuously recorded while the DO concentration of the solution was changed by purging the
solution with N2 gas. The continuous current measurement of the N,B-HRGO electrode in 0.1
M KNO3 during N2 bubbling was performed. As a reference, the DO concentration was also
of N2 gas started. The initial DO concentration was 8.8 mg.L-1 O2 at 22 ºC. The DO
concentration at the end of the N2 bubbling was 1.5 mg.L-1 O2. The sensitivity of the
N,B-HRGO electrode at various DO concentration was studied. In Fig. 8a the amperometric
measurement of N,B-HRGO electrode at different DO concentration is shown. To obtain a
specific DO concentration, O2 bubbling and N2 bubbling was performed as necessary. Before
measuring the current, the bubbling was stopped at the desired DO concentration as measured
with the commercial DO sensor. A slope of 0.198 µA / mg.L-1 O
2 was calculated when the O2
concentration was in the range of 1.5 to 10.0 mg L-1 (Fig. 5.8(b)).
Figure 5.8: Continuous DO sensing of N,B-HRGO electrode. (a) Amperometric response of N,B-HRGO at varying DO concentration and corresponding calibration curve for N,B-HRGO
electrode (b).
Previously, other researchers identified glucose, ascorbic acid, and urea as substances that can
interfere with these type of amperometric sensors [51]. Therefore, the interference of these
molecules with DO response of N,B-HRGO electrode was tested. There was no interference
observed for glucose (10 mM) and urea (10 mM). However, in the presence of 1 mM ascorbic
Figure 5.9: Sensor response after the subsequent addition of glucose, urea, and ascorbic acid (each 1 mM).
Real-time monitoring of dissolved oxygen during the growth of Amycalotopsis methanolica
The amperometric response of the N,B-HRGO electrode was tested in an A.
methanolica fermentation. A. methanolica is an obligate aerobic organism and consumes
oxygen during its growth. This means that the presence of oxygen in the growth medium is
essential for the growth of A. methanolica. Fig. 5.10(a) shows the electrode response during
the growth of this bacterium in liquid medium. The sensor response is stable during the lag
phase in which growth is not yet started, and decreases exponentially during the subsequent
exponential growth phase. When the bacteria completely consumed the oxygen, the growth
ceased, and the sensor response stabilized at 0 mg.L-1 O2. Subsequently, air was bubbled
through the growth medium until the DO concentration reached 6.6 mg.L-1 O2. The bacteria
immediately consumed the newly supplied O2 (Fig. 5.10(b)). This procedure was repeated
two more times and the electrode responded reproducibly to this alternating O2 supply and O2
Figure 5.10: Real-time DO measurement with the N,B-HRGO electrode during an 18 hour
Amycolotopsis methanolica fermentation (a). N,B-HRGO electrode response during repeated
sparging of air in the growth medium to supply oxygen to the bacteria (b).
Significance of this experiment is that DO detection during A. Methanolica fermentation do
not require any membrane. Therefore the membrane less N,B-HRGO sensor can be used to
monitor DO during the A. Methanolica fermentation process. This is contrast to the pH
sensing of polyaniline functionalized electrochemically reduced graphene oxide where
5.3.3. Chemiresistive sensing
Sensor characterization
The I-V characteristics of the N,B-HRGO-deposited electrode was carried out under a N2
atmosphere. In air, water molecules and other gas species were adsorbed on the surface.
When the sensor was exposed to N2 gas, the unwanted adsorbed gas species and water
molecules were desorbed from the surface and the resistance of the electrode was decreased
(Fig. 5.11(a)). The I-V characteristics of N,B-HRGO-deposited electrode with and without a
silicon membrane under N2 atmosphere is shown in Fig. 5.11 (b). Linear I-V curve was
obtained which indicated that good ohmic contact was formed between N,B-HRGO and Au
electrode.
Figure 5.11: Change of resistance during N2 purging (a) and I-V curve of the sensor with and
Effect of humidity and ionic adsorption
The effect of humidity and ionic adsorption on the conductivity of N,B-HRGO was studied by
following the differences in resistance over time.(Fig. 5.12). For this measurement, 50 ml of
0.1 M KNO3 solution was injected into the container where the sensor was placed under a N2
atmosphere. For the humidity test, the solution level was kept below the electrode. In other
words, the electrode stand on top of the solution without touching the solution level. To study
effect of ionic adsorption, the electrode was slowly immersed into the solution. To remove the
effect of oxygen throughout the experiment, the solution was purged with pure N2 gas before
the experiment was started. The resistance of the electrode was monitored continuously
during the experiment.
The effect of humidity and ionic adsorption on the resistance of the N,B-HRGO sensor is
shown in Fig. 5.12. The resistance of the N,B-HRGO sensor was decreased when the
surrounding atmosphere becomes more humid due to adsorption of water molecules from the
KNO3 solution. The low resistance in the presence of water is due to the doping of the
material. This is indicative of a p-type behavior of the N,B-HRGO sensor film [52-53].
Figure 5.12: Effect of humidity and ionic adsorption of the sensor in N2 saturated 0.1 M
Chemiresistive DO sensing
For chemiresistive oxygen sensing N,B-HRGO deposited electrode was immersed in a
50 ml of 0.1 M KNO3 solution. Varying concentration of oxygen in this solution were
obtained by sparging with pure nitrogen (0% oxygen), air (20% oxygen), and pure oxygen
(100 % oxygen), respectively. A DO concentration of 0 mg/L, 7 mg/L, and 24-30 mg/L was
obtained under saturating conditions with nitrogen, air, and oxygen, respectively. In order to
create a real-time online measurement data, the sensor response of the N,B-HRGO was
measured continuously. During the measurements, dissolved oxygen concentration was also
monitored using a commercial Clark-type dissolved oxygen sensor. Fig. 5.13 shows the
sensor response of N,B-HRGO electrode in nitrogen or oxygen saturated solutions. The
resistance of the sensor was decreased when the oxygen concentration in the solution was
increased. Similarly, the resistance increased when the oxygen was removed from the solution
upon sparging with nitrogen gas (Fig. 5.13 (a)). A similar sensor response was obtained when
the solution was subsequently sparged with nitrogen, air or oxygen. Notably, more than 90%
of the maximal sensor response was obtained when the solution was sparged with air. Further
saturation of the solution with pure oxygen showed a limited increase of the sensor response
while the commercial Clark-type electrode showed that the DO was above 25 mg/L. It
indicates that the sensor has a saturation point, after that the resistance is not changing even
the DO concentration is increased. The N,B-HRGO electrode coated with silicon membrane
also shown similar behaviour, but the differences in resistance between N2 and O2 saturated
solution were low (Fig. 5.13 (b) ). In order to study the sensitivity of the N,B-HRGO
electrode with membrane, the solution was saturated with mixtures of N2 and O2. A gas-flow
controller was used to mix different ratio of N2 and O2 gas, the dissolved oxygen
concentration of the mixtures were measured using commercial Clark type electrode. Fig.
resistance of the sensor changed correspondingly with the changes in DO concentration. The
sensor has good sensitivity for DO concentrations below 1 mg/L.
Figure 5.13: Sensor response without membrane (a) and with membrane (b) in N2/air/O2
saturated 0.1M KNO3 solution. The sensor response of N,B-HRGO with membrane in 0.1 M
KNO3 saturated with N2:O2 mixtures (c). Fig. 3 (d) shows the effect of oxygen diffusion from
the atmospheric air after saturating with pure N2 and pure O2 gas.
The mechanism for the sensor response for dissolved oxygen can be explained by p-type
doping of adsorbed O2 molecules at the N,B-HRGO surface [54]. The p-type doping increases
the hole conduction resulting in a lower resistance of N,B-HRGO. The reversibility of the
sensor response suggest that oxygen molecules that are dissolved in the solution, diffuse
desorb and diffuse out of the solution during N2 sparging. The response is similar to that of
the oxygen gas sensing mechanism reported earlier [19,20] except that oxygen is dissolved in
the solution and that the oxygen molecules have to diffusion through the double layer before
reaching the active surface of the sensor. Further understand the effect of oxygen diffusion to
the sensor surface, the electrolyte was allowed to come to equilibrium with atmospheric
oxygen after saturating with pure oxygen and Nitrogen (Fig. 5.13(d)). The resistance of the
sensor slowly increased because of diffusion of oxygen molecule from atmosphere into the
solution. The resistance was continuously increased until electrolyte saturated with oxygen.
Figure 5.14: N,B-HRGO sensor response at temperature 25 0C (a) and 37 0C (b). Sensor
response at 37 0C at different DO concentration (c) and corresponding calibration curve (d).
(b) (a)
(d) (c)
The chemiresistor performances affected by the temperature changes. The effect of
temperature on the chemiresistive performance of N,B-HRGO electrodes was investigated.
Fig. 5.14 shows the response of N,B-HRGO electrode at 25 0C and 37 0C. Initially the
electrode response at 25 0C was recorded (Fig 5.14 (a)) in N2 and O2 saturated 0.1 M KNO3
electrolyte. The electrode showed a similar response to the DO concentration changes as
mentioned before. After the solution saturated with N2, the temperature of the solution slowly
risen to 30 0C (Fig. 5.14 (b)). The resistance of the electrode decreased with increasing
temperature. After the temperature of the electrolyte and the electrode was stabilized, the
solution was again saturated with O2 and then N2. The sensor responded to the change of the
oxygen concentration as expected. Subsequently, the electrode subjected to different DO
concentration and the electrode response was recorded (Fig. 5.14 (c)). The calibration curve
was obtained for that which is shown in Fig. 5.14 (d). The slope of 0.011 MΩ / mg L-1.O2 was
obtained with the intercept 0.270 MΩ. This value can be fit in the equation, O2 = 0.011 x +
0.271. One of the important observation of this study was that the sensitivity of the electrode
did not change with temperature, but only the baseline of the electrode resistance has
changed. It indicates that the simple temperature correction is sufficient to use the sensor at
different temperature.
Impedance spectroscopy measurements
In order to determine if the resistance change was a result of charge transfer due to
adsorption / desorption of oxygen molecules, the impedance of the N,B-HRGO film with
silicon membrane was measured. Impedance spectroscopy analysis was performed in the
frequency range 1 MHz to 0.01 Hz at different DC potentials (EDC 0, -0.4, -0.6 V) with an AC
amplitude, EAC -10mV (Fig. 5.15 (a-d)). The Nyquist-plot of nitrogen-saturated solution in
Fig. 5.15(a) shows one big impedance arc at lower frequencies without any defined
Figure. 5.15: Impedance spectra of an N,B-HRGO with membrane in N2 and O2 saturated
solutions. (a) Nyquist -plot at EDC -0.6 V in N2 and O2 saturated solution (b) Nyquist plot at
different applied EDC in O2 and N2 saturated solution. Bode Z and Bode phase (c and d)
impedance plot of N,B-HRGO with membrane in O2 and N2 saturated electrolyte
In a solution saturated with oxygen, the diameter of the semicircle in the Nyquist-plot was
decreased and a clear semicircle appeared. This indicates that there is a charge transfer
process occurred at the N,B-HRGO / solution interface between N,B-HRGO and oxygen. The
Nyquist-plot of N,B-HRGO in nitrogen and oxygen saturated solutions at different EDC is
shown in Fig. 5.15(b). Nitrogen saturated solutions showed a higher impedance arc than
the resistance of N,B-HRGO is also seen in the Bode Z and Bode phase plots measured at -0.6
V (Fig. 5.15 (c, d)).
Real time DO measurement during growth of Amycolatopsis methanolica
We used the sensor to monitor dissolved oxygen in real time during the growth of
Amycolatopsis methanolica. Fig. 5.16 shows the sensor response of N,B-HRGO during the
growth of A. Methanolica. Initially, the DO concentration was declined as the temperature of
the culture increased to 37 ºC. The DO concentration at room temperature (RT) and at 37 ºC
before and after adding A. methanolica is shown in Table 2.
Table 5.2: DO concentration in the culture medium at different temperature Medium Temp. (°C) DO (mg/L) Cult. med. 24.4 7.6 Cult. med. 37 6.0 Cult. med. + A. methanolica 37 5.4
The sensor response was recorded continuously during the growth, while the real DO
concentration was monitored using Commercial clark type dissolved oxygen sensor. The
resistance of the sensor decreased when the culture temperature increased as the dissolved
oxygen diffused out of the solution. The resistance of the sensor was stable after the culture
medium reached the thermal equilibrium. Afterwards, the resistance of the electrode started
decreasing continuously during the exponential growth and the oxygen was consumed by
growing A. methanolica. Once all the oxygen was consumed the resistance of the sensor
reached a plateau indicated no more oxygen in the media. The results suggests that
N,B-HRGO chemiresistive sensor can be used to monitor DO during fermentation process in
plate based miniaturised bioreactor as a high throughput screening platform for bacterial
fermentation processes.
Figure 5.16: The resistance of the N,B-HRGO sensor during an A. methanolica fermentation process. The inset shows expanded graph of the sensor response in the first hour of the growth.
Even though, we added the same amount of N,B-HRGO dispersion onto the electrode, the
resistance varies between electrodes. Hence, a proper calibration by exposing the electrode to
oxygen free (0 %) and oxygen saturated electrolytes (100 %) is necessary. Although, this
electrode is simple to manufacture and has the possibility of turning them into nanosensors,
each electrode needs to be calibrated before use.
5.4. Conclusion
N,B-HRGO was studied as an ORR electrocatalyst for dissolved oxygen sensing.
Cyclic voltammetry was used to evaluate the electrocatalytic activity of N,B-HRGO in a
neutral electrolyte. Amperometric sensing method was used to detect dissolved oxygen in 0.1
between 1.5 – 10.0 mg.L-1 O2. The measurement of DO was affected by slow stirring. At a
stirring rate above 300 rpm, stable measurements were obtained with the N,B-HRGO
electrode. The applicability of the sensor in microbial fermentations was tested using the
aerobic bacterium A. methanolica. The response of the sensor correlated nicely with the
oxygen consumption of the bacteria during the growth. The components in the fermentation
medium did not interfere with the measurements of DO using the N,B-HRGO electrode.
We have shown that chemiresistive sensor can be fabricated using N,B-HRGO as
chemiresistive material to measure DO in real time. The sensor responded well with the
changing DO concentration. The resistance of the N,B-HRGO electrode showed temperature
dependency, but the sensitivity of the sensor has not changed. We also shown that the
resistance changes in the solution is not due to water or any other ionic adsorption at all. The
sensor response due to interaction of oxygen with N,B-HRGO surface. This sensor was able
measure DO in real time during the A. Methanolica fermentation. The simplest electrode
structure of this chemiresistive DO sensor means that this N,B-HRGO electrode can be used
in miniaturised bioreactor system or any other applications where the miniaturised sensor is
needed.
The amperometric sensing study shown that N,B-HRGO electrode can be used as membrane
less sensor to monitor DO during fermentation process. This is contrast to the pH sensor
(chapter 3) study where the protection of sensor surface is necessary for pH measurement in
the fermentation process. The contrasting result could be due to different culture medium used
for the growth. To avoid potential problem with other culture medium, the DO measurement
in A. methanolica fermentation was carried out with N,B-HRGO electrode coated with
oxygen permeable silicon membrane. Hence, the N,B-HRGO electrode can be used in any
Acknowledgement
The authors would like to thank the University of Groningen for financial support. We also
thank Prof. Dr. Wesley Browne for Raman spectroscopic analysis, Dr. Marc D. A. Stuart for
TEM analysis.
5.5. References
1. A. Mills, Oxygen indicators and intelligent inks for packaging foods, Chem. Soc.
Rev., 34 (2005) 1003.
2. L. C. Clark, G. Misrahy, and R. P. Fox, Chronically implanted polarographic
electrodes, J Appl Physiol, 13 (1958) 85.
3. H. Kautsky, Quenching of luminescence by oxygen, Trans Faraday Soc. 35 (1939)
216.
4. S. M. Borisov and I. Klimant, Ultrabright oxygen opcodes based on cyclometalated
Iridum (III) coumarin complexes, Anal. Chem. 79 (2007) 7501.
5. X. Wang, Xi Chen, Z. Xie, and X. Wang, Reversible optical sensor strip for oxygen,
Angew. Chem. 120 (2008) 7560.
6. C. Wu, B. Bull, K. Christensen, and J. McNeill, Ratiometric single nanoparticle
oxygen sensor for biological imaging, Angew. Chem. 121 (2009) 2779.
7. J. N. Demas, B. A. DeGraff, P. B. Coleman, Oxygen sensor based on Luminescence
sensing, Analytical Chemistry News & Features 1 (1999) 793A.
8. C. McDonagh, C. S. Burke, and B. D. MacCraith, Optical chemicals sensor, Chem.
Rev. 108 (2008) 400.
9. X. D. Wang, O. S. Wolfbeis, Fiber optic-chemical sensors and biosensors, Anal.
Chem. 85 (2013) 487.
10. R. Ahmad and P. Kuppusamy, Theory, instrumentation and applications of electron
11. J. Pekar, L. Ligeti, Z. Ruttner, R. C. Lyon, TM. Sinnwell TM, P. Gelderen, D. Fiat, C.
T. W. Moonen, A. C. McLaughlin, In vivo measurement of cerebral oxygen
consumption and blood flow using 170 magnetic resonance imaging, Magn Reson
Med, 21 (1991) 313.
12. X. H. Zhu, N, Zhang, Y. Zhang, X. Zhang, K. Ugurbil, W. Chen, In vivo 170 NMR
approaches for brain study at high field, NMR Biomed, 18 (2005) 83.
13. S. G. Kim, K. Ugurbil, Comparison of blood oxygenation and cerebral flood flow
effects in FMRI: estimation of relative oxygen consumption Magn Reson Med, 38
(1997) 59.
14. S. G. Kim, K. Ugurbil, Functional magnetic resonance imaging of the human brain, J.
Neurosci Methods, 74 (1997) 229.
15. O. Ndubuizu, J. C. LaManna, Brain tissue oxygen concentration measurements,
Antioxid. Redox Signaling, 9 (2007) 1207.
16. G. Xudong, M. Hanson, H. Shen, Y. Kostov, K. A. Brorson, D. D. Frey, A. R.
Moreira, G. Rao, Validation of optical sensor based high throughput bioreactor
system for mammalian cell culture Journal of Biotechnology, 122 (2006) 293.
17. N. Opitz, D. W. Lübbers, Theory and developemnt of fluorescence based
optochemical oxygen sensor: optodes, Int Anesthesiol Clin, 25 (1987) 177.
18. J. M. Azzarelli, K. A. Mirica, J. B. Ravnsbæk, T. M. Swager, Wireless gas detection
with a smartphone via rf communication, PNAS, 111 (2014) 18162.
19. A. K. Geim and K. S. Novoselov, The rise of graphene, Nature Materials, 6 (2007)
183.
20. L. Qu, Y. Liu, J. B. Baek, and L. Dai, Nitrogen-Doped Graphene as Efficient
21. Z. Yang, Z. Yao, G. Li, G. Fang, H. Nie, Z. Liu, X. Zhou, X. Chen, and S. Huang,
Sulfur-Doped Graphene as an Efficient Metal-free Cathode Catalyst for Oxygen
Reduction, ACS Nano, 1 (2012) 205.
22. Z. W. Liu, F. Peng, H. J. Wang, H. Yu, W. X. Zheng, and J. Yang, Phosphorus-doped
graphite layers with high electrocatalytic activity for the O2 reduction in an alkaline
medium, Angew. Chem. Int. Ed. 50 (2011) 3257.
23. Z. Yao, H. Nie, Z. Yang, X. Zhou, Z. Liu and S. Huang, Catalyst-free synthesis of
iodine-doped graphene via a facile thermal annealing process and its use for
electrocatalytic oxygen reduction in an alkaline medium, Chem. Commun., 48 (2012)
1027.
24. Z. H. Sheng, H. L. Gao, W. J. Bao, F. B. Wang and X. H. Xia, Synthesis of boron
doped graphene for oxygen reduction reaction in fuel cells, J. Mater. Chem., 22 (2012)
390.
25. J. Liang, Y. Jiao, M. Jaroniec, and S.Z. Qiao, Sulfur and nitrogen dual-doped
mesoporous graphene electrocatalyst for oxygen reduction with synergistically
enhanced performance, Angew. Chem. Int. Ed. 51 (2012) 11496.
26. S. Wang, L. Zhang, Z. Xia, A. Roy, D. W. Chang, J. B. Baek, and L. Dai, BCN
graphene as efficient metal-free electrocatalyst for the oxygen reduction reaction,
Angew. Chem. Int. Ed. 51 (2012) 4209.
27. Y. Zheng, Y. Jiao, L. Ge, M. Jaroniec, and S. Z. Qiao, Two-step boron and nitrogen
doping in graphene for enhanced synergistic catalysis, Angew. Chem. 52 (2013) 3110.
28. L. R. Radovic, M. Karra, K. Skokova, P. A. Thrower, The role of substitutional boron
29. T. Ikeda, M. Boero, S. F. Huang, K. Terakura, M. Oshima, J. Ozaki, Carbon Alloy
Catalysts: Active Sites for Oxygen Reduction Reaction, J. Phys. Chem. C, 38 (2008)
14706.
30. R. K. Joshi, H. Gomez, F. Alvi, and Ashok Kumar, J. Phys. Chem. C, 2010, 114,
6610.
31. C. W. Chen, S. C. Hung, M. D. Yang, C. W. Yeh, C. H. Wu, G. C. Chi, F. Ren, S. J.
Pearton, APPLIED PHYSICS LETTERS, 99 (2011) 243502.
32. P. Gou, N. D. Kraut, I. M. Feigel, H. Bai, G. J. Morgan, Y. Chen, Y. Tang, K. Bocan,
J. Stachel, L. Berger, M. Mickle, E. Sejdic, A. Star, Scientific reports, 4 (2014) 4468.
33. S. chinnathambi, G. J. W. Euverink, Sensors and Actuators, 264 (2018) 38.
34. B. Raguse, E. chow, C. S. Barton, L. wieczorek, Anal. Chem. 79 (2007) 7333.
35. B. Raguse, C. S. Barton, K. H. Mueller, E. chow, L. wieczorek, J. Phys. Chem. C, 113
(2009) 15390.
36. K. Gong, F. Du, Z. Xia, M. Durstock, L. Dai, Nitrogen-Doped Carbon Nanotube
Arrays with High Electrocatalytic Activity for Oxygen Reduction, Science 323 (2009)
760.
37. S. Wang, E. Iyyamperumal, A. Roy, Y. Xue, D. Yu, Liming Dai, BCN Graphene as
Efficient Metal-Free Electrocatalyst for the Oxygen Reduction Reaction, Angew.
Chem. Int. Ed. 50 (2011) 11756.
38. W. S. Hummers J, and R. E. Offeman, Preparation of Graphitic oxide, J. Am. Chem.
Soc. 80 (1958) 1339.
39. C. Selvaraj, S. Kumar, N. Munichandraiah, L. G. Scanlon, Reduced Graphene
Oxide-Polypyrrole Composite as a Catalyst for Oxygen Electrode of High Rate Rechargeable
40. Gi-Ja Lee, S. K. Kim, S. W. Kang, O. Kim, S. J. Chae, S. Choi, J. H. Shin, H. K. Park,
J. H. Chung, Analyst, 137 (2012) 5312.
41. J. H. Shin, B. J. Privett, J. M. Kita, R. M. Wightman, M. H. Schoenfisch, Anal.
Chem., 80 (2008) 6850.
42. Jae Ho Shin, Stephen W. Weinman, and Mark H. Schoenfisch, Anal. Chem. 77 (2005)
3494.
43. J. Li, X. Li, P. Zhao, D. y. Lei, W. Li, J. Bai, Z. Ren, X. Xu, Searching for magnetism
in pyrrolic N-doped graphene synthesized via hydrothermal reaction, CARBON 84
(2015) 460.
44. H. Wang, Y. Zhou, D. Wu, L. Liao, S. Zhao, H. Peng, and Z. Liu, Synthesis of
Boron-Doped Graphene Monolayers Using the Sole Solid Feedstock by Chemical Vapor
Deposition, small 9 (2013) 1316.
45. K. Spyrou, M. Calvaresi, E. K. Diamanti, T. Tsoufi, D. Gournis, P. Rudolf , and F.
Zerbetto, Graphite Oxide and Aromatic Amines: Size Matters, Adv. Funct. Mater. 25
(2015) 263.
46. S. Li, Z. Wang, H. Jiang, L. Zhang, J. Ren, M. Zheng, L. Dong and L. Sun,
Plasma-induced highly efficient synthesis of boron doped reduced graphene oxide for
supercapacitors, Chem. Commun., 52 (2016) 10988.
47. R. Yadav, C.K. Dixit, Synthesis, characterization and prospective applications of
nitrogen-doped graphene: A short review, Journal of Science: Advanced Materials and
Devices 2 (2017) 141.
48. A. Z. Yazdi, H. Fei, R. Ye, G. Wang, J. Tour, and U. Sundararaj, Boron/Nitrogen
Co-Doped helically unzipped multiwalled Carbon Nanotubes as efficient electrocatalyst
49. Z. Jiang, X. Zhao, X. Tian, L. Luo, J. Fang, H. Gao, and Z.J. Jiang, Hydrothermal
Synthesis of Boron and Nitrogen Co-doped Hollow Graphene Microspheres with
Enhanced Electrocatalytic Activity for Oxygen Reduction Reaction, ACS Appl.
Mater. Interfaces, 7 (2015) 19398.
50. C. Selvaraj, S. Kumar, N. Munichandraiah, L. G. Scanlon, J. Electrochem. Soc., 161 (2014) A554.
51. L. P.H. Saravia, A. Sukeri, J. M. Gonçalves, J. S. Aguirre-Araque, B. B.N.S. Brandão,
T. A. Matias, M. Nakamura, K. Araki, H. E. Toma, M. Bertotti, CoTRP/Graphene
oxide composite as efficient electrode material for dissolved oxygen sensors,
Electrochimica Acta 222 (2016) 1682.
52. F. Schedin, A.K. Geim, S.V. Morozov, E.W. Hill, P. Blake, M.I. Katsnelson, K.S.
Novoselov, Detection of individual gas molecules adsorped on graphene, Nat. Mat., 6
(2007) 652.
53. T.O. Wehling, M.I. Katsnelson, A.I. Lichtenstein, Adsorpates on graphene: impurity
states and electron scattering, Chemical Physics Letters 476 (2009) 125.
54. C. W. Chen, S. C. Hung, M. D. Yang, C. W. Yeh, C. H. Wu, G. C. Chi, F. Ren, S. J.
Pearton, Oxygen sensor made by monolayer graphene under room temperature,
