Showcasing research from electrochemistry laboratory ofProfessor Masato Tominaga, Graduate School of Scienceand Engineering, Saga University, Japan.
Highly sensitive detection of phosphate using well-ordered
crystalline cobalt oxide nanoparticles supported bymulti-walled carbon nanotubes
Electrochemical sensor for the detection of phosphatewith great sensing properties has successfully developed
based on CoONPs hybridized with PBI and MWCNTs. This
work shows the CoONPs/PBI/MWCNT-modified glassy
carbon electrode exhibited high sensitivity similar to thefluorescence and spectrophotometric methods due to theeffects of morphology and microstructure resulting on better
electrochemical performance. The modified electrode wasable to detect low concentration of phosphate even below
the eutrophication threshold making the electrode suitablefor phosphate monitoring in environmental samples.

As featured in:Materials
Advances

Volume 3
Number 4
21 February 2022
Pages 1837–2236

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Easwaramoorthi Ramasamy et al.
Dual-functional inorganic CuSCN for efficient hole extraction
and moisture sealing of MAPbI3 perovskite solar cells

See Masato Tominaga et al.,Mater. Adv., 2022, 3, 2018.

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3, 2018

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Highly sensitive detection of phosphate using
well-ordered crystalline cobalt oxide
nanoparticles supported by multi-walled carbon
nanotubes†
Shaimah Rinda Sari, a Masayuki Tsushida,b Tetsuya Satob andMasato Tominaga *a
Phosphates are well-known groundwater and surface water contaminants, with even modest increases
in their concentration contributing to the eutrophication of lakes and coastal waterways and thus
potentially harming the environment. Consequently, sensors capable of detecting phosphate ions atconcentrations below the eutrophication threshold (0.1 mM) are highly sought after. Herein, cobalt oxide
nanoparticles (CoONPs) supported by polybenzimidazole (PBI)-modified multi-walled carbon nanotubes

Received 20th November 2021,
Accepted 24th December 2021

(MWCNTs) were prepared and shown to feature uniform size (3.5–5.5 nm) and limited Co phase mainlycontaining hexagonal Co3O4 and a minor amount of CoO. The synthesized NPs exhibited better phos-

DOI: 10.1039/d1ma01097b

phate sensing performance than previously reported polycrystalline Co wires, i.e., the CoONPs/PBI/
MWCNT-modified glassy carbon electrode could detect phosphate at pH 4 and 7 at levels of 0.1 to

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100 nM, that is, below the eutrophication threshold of 0.1 mM.

Graduate School of Science and Engineering, Saga University, 1 Honjomachi,
Saga 840-8502, Japan. E-mail: masato@cc.saga-u.ac.jpbFaculty of Engineering, Kumamoto University, 2-39-1 Kurokami,
Kumamoto 860-8555, Japan† Electronic supplementary information (ESI) available: Additional CoONP characterization data. See DOI: 10.1039/d1ma01097bof oxygen.3,4 The eutrophication threshold concentrations ofphosphates have been estimated to be 0.1–0.32 mM.5 Given thatthe increase in the phosphate levels in rivers over the past
50 years6 has negatively affected aquatic ecology and water
quality, phosphate ion monitoring is crucial for ensuring
environmental safety.
Phosphate is typically quantified using (i) high-performanceliquid chromatography,7 (ii) the colorimetric method of Murphy and Riley,8–10 and (iii) spectroscopic methods relying on
the molybdenum blue reaction or a series of complicatedsample pretreatments.11,12 All the above-mentioned methods
require complex processes and are therefore poorly suited forphosphate monitoring. Other classical techniques such as
gravimetric methods,13 volumetric methods,14 and instrumental methods based on chromatographic measurements15,16 also
require sample pre-treatment and can therefore be timeconsuming, expensive, and/or produce toxic wastes. Such methods also had detection ranges higher than the eutrophicationthreshold of 0.1 mM.
Recent studies using colorimetric and ECL17 and fluorescence methods18 satisfy the requirement of lower detection
range than the eutrophication threshold, however they are
diﬃcult to operate for on-site measurement of phosphateconcentration. Hence, the development of inexpensive, easy
to manufacture, and reliable sensors for monitoring phosphateconcentration in agricultural wastewater and other aqueous
systems is expected to benefit the mass production of food and2018 | Mater. Adv., 2022, 3, 2018–2025© 2022 The Author(s). Published by the Royal Society of Chemistry

Introduction
Phosphorus is involved in important physiological processes
and is therefore an essential nutrient. In natural waters,
phosphorus is typically present as phosphates, among which
the inorganic orthophosphate form is more thermodynamicallystable than other forms.1 However, phosphates are also wellknown contaminants of groundwater and surface water2 andare mainly transported from terrestrial ecosystems to aquaticones via wind erosion, surface runoﬀ, and leaching. The
eutrophication caused by this transport may be further accelerated by agricultural activities (supply of phosphorus to crops
as a fertilizer), animal husbandry, and other human activities.3
Under the right conditions, even a modest increase in phosphate concentration due to influx from agricultural land orother sources can contribute to the eutrophication of lakes
and coastal waterways. Eutrophication refers to the overenrichment of natural waters in nutrients, such as phosphates,
which leads to the excessive growth of algae and other aquaticplants and may cause the death of aquatic fauna due to the lacka

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Paper
agricultural products as well as help solve certain environmental problems.
In recent decades, considerable attention has been directed
toward the development of electrochemical phosphate sensors
in view of their simple operation, fast response, and highsensitivity.19 For example, a Co electrode has been successfully
used as a base material for phosphate sensing,20 while the firstCo-based phosphate sensor was reported in 1995 by Xiao et al.21
Co-based nanomaterials have attracted significant attention
owing to their large specific surface area, which is reported to
be 410 m2 g 1 (beneficial for increasing the number of active
sites),22 excellent conductivity, catalytic activity, and ability tosense phosphate. A phosphate sensor fabricated by coating a
glassy carbon electrode (GCE) with Co oxide nanoparticles(NPs) and reduced graphene oxide exhibited a good linear
potentiometric response.23 However, its lowest detection limit
was 1 mM, which is higher than the eutrophication threshold.
Herein, we synthesized CoONPs supported by polybenzimidazole (PBI)-modified multi-walled carbon nanotubes (MWCNTs)
to detect phosphate ions with high sensitivity. MWCNTs wereselected owing to their large surface-to-volume ratio,24 good
conductivity, which makes them suitable for catalytic andsensor applications,25–29 and relatively low cost.30 The CoONPs
featured a narrow size, well-distribution and well-defined limited crystal planes that provided sites specifically interactingwith phosphate ions and enabled their sensitive detection.

Experimental
Materials
N,N-Dimethylacetamide (DMAc), polyethylene glycol 400(PEG400), Co(CH3COO)24H2O, Na2SO4, NaOH, and HClO4 werepurchased from Fujifilm Wako Pure Chemical (Osaka, Japan).
PBI fine powder was obtained from Sato Light Industrial Co.,Ltd (Tokyo, Japan) and used as received. A polytetrafluoroethylene filter (pore size = 0.2 mm, Millipore) was used for filtration.
The GCE (outer diameter = 6 mm, inner diameter = 3 mm) was
purchased from BAS Inc. (Tokyo, Japan). MWCNTs (purity:
Z98%, length: 3–6 mm, outer diameter: 10 nm  1 nm, innerdiameter: 4.5 nm  0.5 nm) were purchased from Sigma
Aldrich (St. Louis, MO, USA). Co wire (purity: 99.99%, diameter:
0.50 mm) was sourced from Nilaco Co. (Tokyo, Japan). Phosphate solutions (phosphate-buffered saline, PBS) were preparedby mixing solutions of NaH2PO4 (Fujifilm Wako Pure Chemical)
and Na2HPO412H2O (Fujifilm Wako Pure Chemical). The
solution pH was adjusted to 4 and 7 with 0.1 M HClO4 and
0.1 M NaOH. All solutions were prepared with deionized water(resistivity 418.2 MO cm) obtained using a Milli-Q water
purification system (Millipore, Billerica, MA).

Materials AdvancesFig. 1

Schematic of the CoONPs/PBI/MWCNT composite.

supplemented with MWCNTs (10 mg) and sonicated foranother 20 min to prepare the PBI/MWCNT dispersion(Fig. S1, ESI†). This dispersion was treated with 3 mL of thesolution prepared by dissolving Co(CH3COO)24H2O (500 mg)
in deionized water (10 mL) to obtain a ratio of 3 : 1
Co(CH3COO)24H2O to PBI/MWCNT. The reaction mixture
was supplemented with PEG400 (reducing agent), heated at
130 1C for 6 h in an oil bath upon stirring, filtered, and washedwith DMAc to remove excess PBI and PEG400. The GCE waspolished with a 0.05 mm alumina suspension on a polishingpad, rinsed with Milli-Q water, and sonicated in Milli-Q waterfor 5 min. Subsequently, a 10 mL (10 mg, see ESI† for surfacearea) aliquot of the CoONPs/PBI/MWCNT dispersion was caston the polished GCE and dried under vacuum (B0.06 MPa). A
Co wire electrode (prepared by polishing with emery paper(#2000) and rinsed with Milli-Q water) was used as a reference
for comparison with the modified electrode.
Materials characterizations
Fourier transform infrared (FTIR) spectra were recorded on a
VERTEX 70v spectrometer (Bruker Corp., Billerica, MA, USA)using the KBr method. Thermogravimetric analysis (TGA) was
performed using an SII EXSTAR 6000 instrument (Seiko Instruments, Inc., Chiba, Japan) operated in conditioned air from 25
to 700 1C at a heating rate of 10 1C min 1. Transmission
electron microscopy (TEM) measurements were conductedusing an FEI Technai F20 microscope (Tokyo, Japan). X-Ray
diﬀraction (XRD) patterns were recorded using a micro/thinfilm diﬀractometer (Rigaku SmartLab) with Cu Ka radiation(l = 1.54059 Å) at 45 kV, 200 mA, a scan speed of 0.51 min 1,
and an amorphous Si plate as a sample holder. X-Ray photoelectron spectroscopy (XPS) measurements were conductedusing a Shimadzu Kratos AXIS-ULTRA instrument with Mg Ka
radiation (hn = 1253.6 eV). Peak calibration was performed
using the peaks of pure In (99.999%) at 665.2 eV and
443.7 eV (In 3d3/2 and 3d5/2, respectively).
Electrochemical measurements and ICP measurements

A schematic of the synthesis of the CoONPs/PBI/MWCNT
composite is shown in Fig. 1. PBI (40 mg) was dissolved in
DMAc (10 mL) with 20 min ultrasonication (probe-type BRANSON 5520 sonicator, Kanagawa, Japan), and the solution was

Electrochemical measurements were conducted using an electrochemical analyzer (model 700B, ALS Co., Ltd, Tokyo, Japan).
pH was recorded using a pH meter (AUT-501, DKK-TOA Corp.,
Tokyo, Japan). Cyclic voltammetry (CV) measurements were
performed using a three-electrode system with Ag|AgCl|sat.
KCl and a Pt wire as reference and counter electrodes, respectively. Prior to measurements, high-purity O2 (99.99%) was
bubbled through the solutions for at least 30 min. Opencircuit potentials (OCPs) were measured using a two-electrodesystem with the CoONP-modified electrode as the working© 2022 The Author(s). Published by the Royal Society of Chemistry

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Materials Advanceselectrode and Ag|AgCl|sat. KCl as the reference electrode.
Inductively coupled plasma (ICP) measurements were performed using a Shimadzu ICPS-8100 instrument (Kyoto, Japan)
to confirm the results of OCP measurements for real samples.
Prior to real sample measurements, the pH of the solution was
adjusted to pH 4 and 7.

Results and discussion
Characterization of CoONPs/PBI/MWCNTs
TEM imaging of the CoONPs/PBI/MWCNTs composite (Fig. 2)
revealed black dots attached to the MWCNTs (as indicated by
the red arrows), which indicated that the CoONPs were uniformly deposited on the MWCNTs without forming aggregates.
The zoomed-in image of the CoONPs/PBI/MWCNTs reveals that
the NPs were closely contacted the MWCNT surface, together
with the Co exposure over the PBI layer.
FTIR measurements were conducted to verify the MWCNT
functionalization with PBI (Fig. S4 and Table S1, ESI†). The
amount of PBI was determined to be 7.4 wt% from the weightloss diﬀerences (TGA) between MWCNTs and PBI/MWCNTs(Fig. S5, ESI†). FTIR measurements and TGA demonstrated thesuccessful incorporation of PBI onto the MWCNTs. The thin
wrapping of PBI around the MWCNTs enabled the eﬃcient andhomogeneous loading of Co on the latter. Without the presenceof PBI, well-defined crystalline CoO nanoparticles with noaggregation failed to form (Fig. S6, ESI†). Moreover, PBI is
known to eﬀectively solubilize CNTs by acting as an exfoliatorand individually wrapping CNTs based on p–p interactions.31
PBI also serves as an active binding site, as it is strongly

Paper
adsorbed at the surface of pristine graphitized carbon (e.g.,
MWCNTs), and can strongly bind metal ions32 such as Au,33
Pd34 and Pt35,36 by coordinating them via nitrogen.37
Through high-resolution TEM imaging (Fig. 2c), the average
lattice spacing of CoONPs was estimated to be B0.29 nm,
which corresponds to the (220) plane of hexagonal Co3O4. This
result agreed with the fast Fourier transform imaging anddiﬀraction simulation results (Fig. S7, ESI†), confirming thevalidity of our crystal phase assignment. The elemental compositions of the synthesized materials were probed by energydispersive X-ray spectroscopy (Fig. 3). The obtained composite
was shown to contain C (from MWCNTs), Co (from CoONPs),
and other elements such as Cu, Si, and Cl (from the gridsupport film) (Table S2, ESI†). The CoONPs appeared to havea uniform size with an average diameter of 3.5–5.5 nm (Fig. 4).
The narrow size distribution and well-defined limited crystal
surface of the CoONPs provide the advantages of a large specificsurface area (with no aggregation) and high sensitivity tophosphate. Given that the properties of metal NPs are often
strongly size-dependent, the narrow size distribution of the
CoONPs was expected to limit the effect of nanoparticle size
variation on phosphate ion detection. The crystal phases andCo valence states were further probed by XRD (Fig. 5) and XPS(Fig. 6).
Diﬀraction peaks from Fig. 5 at 2y = 25.581 and 42.851
characteristic of the (002) and (101) planes of the MWCNTs(JCPDS Card No. 75-1621) were observed for all samples, alongwith those of the CoONPs. Although the diﬀraction peaks for
CoONPs appeared rather small and noisy, at least every peak
could be distinguished and all of them agreed well with the
crystal structures of Co3O4 (JCPDS Card No. 74-1657) and cubic
CoO (JCPDS Card No. 78-0431). The peak at 36.211 was ascribedto the (111) plane of CoO, while those at 31.70, 36.55, 44.65, and59.091 were attributed to the (220), (311), (400), and (511) planesof Co3O4, respectively. Thus, the CoONPs largely contained
Co3O4 with a small amount of cubic CoO, i.e., they exhibiteda rather simple biphasic composition.
The survey X-ray photoelectron spectrum of CoONPs/PBI/
MWCNTs (Fig. 6a) showed that this nanocomposite mainlycontained Co, O, N (due to PBI), and C. The C 1s spectrum(Fig. 6b) was deconvoluted into six peaks at 283.43, 284.45,

Fig. 2 (a) TEM plane views of CoONPs/PBI/MWCNTs, (b) zoomed-in TEM
plane views of CoONPs/PBI/MWCNTs with red arrows pointing toward the
CoONPs and (c) estimation of the interplanar distance in CoONPs where
the intensity pattern shown is across the yellow line in the high-resolution
lattice fringe image (HRLFI). The inset figure in (c) shows the corresponding
fast Fourier transform (FFT) pattern.

Fig. 3 Energy-dispersive X-ray spectrum of CoONPs/PBI/MWCNTs. Inset
image shows the area measured (red circle) from the TEM microphotograph of CoONPs/PBI/MWCNTs.

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Materials AdvancesFig. 4 Size distribution of CoONPs.

Fig. 6 (a) Survey, (b) C 1s, (c) N 1s, (d) Co 2p, (e) Co 3s, and (f) Co 3p X-ray
photoelectron spectra of CoONPs/PBI/MWCNTs.

Fig. 5 XRD patterns of (black line) pristine MWCNTs and (red line)
CoONPs/PBI/MWCNTs.

285.15, 286.26, 288.14, and 291.02 eV, which were attributed to
CRC, main peak of sp2 hybridized carbon C–C/CQC,38 CQN/
C–O which may arise from the nitrogenous carbon precursors
and formation of interfacial C–O–Co bonds from combination
between carbon materials with Co oxide,39,40 C–N/CQO,
O–CQO bonds, and p–p interactions, respectively.
The N 1s spectrum (Fig. 6c) was deconvoluted into fourpeaks at 398.4, 399.5, 400.1, and 401.3 eV, which were attributed to –N = pyridinic N, –NH– pyrrolic N, metal–nitrogenbonds of Co–N, and oxidized N for the PBI, respectively.
Fig. 6d displays the related Co 2p spectrum, revealing two main
spin orbit lines at 780.1 eV (Co 2p3/2) and 795.68 eV (Co 2p1/2),
in line with the Co3+ and Co2+ states expected for Co3O4 andCoO. Each of these main peaks was deconvoluted into threecharacteristic peaks. Peaks at 780.6 and 795.9 eV were assigned
for Co–N, originated from the interactions of Co with N of the
PBI believed to generate high active sites for oxidation
reaction.41 The above results agreed well with those of XRD
and TEM. Satellite peaks near the Co 2p3/2 and Co 2p1/2 signals
were observed at 773.5, 785.16, and 802.39 eV, corresponding tospin orbit components. Satellites are commonly observed for© 2022 The Author(s). Published by the Royal Society of Chemistrytransition metals.42 Therefore, in the case of Co3O4, which contains
both Co(II) and Co(III), we expected to see some satellite features dueto these states.43 The Co 3s spectrum of the prepared composite(Fig. 6e) featured a peak at 102.08 eV corresponding to a low-spin
Co3+ state. Finally, the Co 3p spectrum (Fig. 6f) featured a peak at
61.33 eV. The ratio of 3 : 1 was calculated as amount differences
between Co3O4 and CoO obtained from each Co3+ and Co2+ peakarea from Co 2p spectra. The presence of Co3O4 and CoO was
expected to result in high sensitivity to phosphate. Transition metal(e.g., Co) oxides are widely used for sensing because of their surface
redox properties.44,45 Specifically, the Co2+ and Co3+ states of Co
found in our composite are readily accessible and thermodynamically stable.46
Electrochemical characterization
CV curves in phosphate solution were recorded for both the bulk Co
wire electrode and CoONPs/PBI/MWCNTs/GCE (Fig. 7). The actual
current vs. potential of the CV curves shown on Fig. S8 (ESI†). The
curve of the Co wire electrode recorded in 0.1 M PBS at pH 7
featured one anodic peak around 0.35 V and two cathodic peaks
around 0.8 V and 1.1 V (Fig. 7a). Given that this behavior was
similar to that previously reported by Xu et al.,20 the first cathodic
peak was identified as that observed in the absence of H2PO4 for
Co, while the second cathodic peak was attributed to the reductionof Co(H2PO4)2 to Co. The following reactions are believed to occur
on the surface of the Co electrode in the tested potential region.
Co2+ + 2e " Co,(1)

Co(OH)2 + 2e " Co + 2OH ,(2)

Co(H2PO4)2 + 2e " Co + 2H2PO4 .

(3)

The curve of CoONPs/PBI/MWCNTs/GCE featured one anodic peak around 0.35 V (the same as that observed for the Co

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Materials AdvancesFig. 7 CV curves recorded at 5 mV s 1 for (a) the bulk Co wire electrode(broken line) and CoONPs/PBI/MWCNTs/GCE (solid line) in 0.1 M PBS withpH 7 and (b) CoONPs/PBI/MWCNTs/GCE in 0.1 M Na2SO4 (broken line)
and 0.1 M PBS with pH 7 (solid line).

wire electrode) and only one cathodic peak at around 0.9 V.
Notably, this cathodic peak was not observed when the measurement was performed in 0.1 M Na2SO4 (i.e., in the absence ofphosphate) (Fig. 7b), and therefore reflected the presence of phosphate ions. Specifically, the cathodic peak was ascribed to theoccurrence of reactions (2) and (3), in which case the potential
shifted to values more positive than that of the bulk Co wire
electrode. This diﬀerent behavior was also presumably attributed
to diﬀerences in the exposed crystal planes in bulk Co wire and
CoONPs/PBI/MWCNTs, i.e., the CoONPs in CoONPs/PBI/MWCNTs
were thought to react with phosphate ions in a diﬀerent way thanpolycrystalline bulk Co. TEM, XRD, and XPS results also suggested
that the CoONPs contained Co3O4 and CoO phases, which ensured
a considerably higher sensitivity and selectivity for phosphate ionsthan that attained for polycrystalline bulk Co. We assumed Co3O4
might play the main role in sensing phosphate ion as the main
crystal phase. In particular, the detection of phosphate by the
CoONPs–based electrode was not hindered by the presence of otheranions such as SO42 , Cl , CH3COO , and NO3 , unlike that by theCo wire electrode (Fig. S9 and S10, ESI†). Fig. S11 and S12 (ESI†)
depict the performances of both electrodes at various pH values.

Paper

Fig. 8 OCPs (E) obtained for CoONPs/PBI/MWCNTs/GCE at diﬀerent
phosphate concentrations (cPBS = 1.0  10 10 to 1.0  10 7 M) at pH(a) 4 and (b) 7. The results of triplicate measurements are marked as A, B,
and C and indicate high repeatability.

The dynamic responses of the composite electrode to changes
in phosphate concentration at pH 4 and 7 were measured using
OCP in a two-electrode system (Fig. 8). These pH values wereselected because the electrode worked best under acidic conditions (i.e., pH 4–7). The practical response time was recorded

by varying the concentration of the phosphate solution from
1.0  10 10 to 1.0  10 7 M (equivalent to 0.1–100 nM) every 3 min.
A solution (pH 7) obtained by mixing 0.1 M NaOH and 0.1 M HClO4
was used as the initial solution. Measurements were conducted intriplicate to confirm the response repeatability.
Fig. 8a shows that the potential commenced at 0.05–0.15 V for a
0.1 nM phosphate solution at pH 4, increasing to 0.20–0.25 V as thephosphate concentration increased to 1 nM. This significant
potential change was observed until the phosphate level reached
B10 nM. At higher concentrations, the response curve was almost
flat, although the potential still shifted to a more positive value withincreasing phosphate concentration. Notably, phosphate detection
was impossible in the absence of Co, i.e., for MWCNTs/GCE (Fig.
S13, ESI†) and the absence of PBI for CoONPs/MWCNT/GCE showeddisproportionate relationship of potential value with the concentration change unlike the one with PBI (Fig. S14, ESI†).
The CoONPs-modified electrode could detect phosphateions at very low concentrations, with the detection range at
pH 7 determined as 0.1–100 nM (Fig. 8b). The wide detection
range at both pH 4 and 7 enabled the sensing of phosphate atconcentrations lower than the eutrophication threshold of
B0.1 mM,5 and our electrode was therefore suitable for phosphate monitoring in environmental samples. While previous
studies using similar electrochemical methods reported a responsetime of 15–60 s, our composite electrode featured a fast responsetime of B5 s. The potential response for 10 nM phosphate solution

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Table 1

Materials Advances
Comparison of the analytical performances obtained with diﬀerent detection types for phosphate ions

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Detection range Type of detection

Electrode/reagent

Response time Real sample

60 nM–3 mM
Spectrophotometric (880 nm) Molybdenum blue
Up to 60 s
50 nM–5 mM
Fluorescence (590 nm)
Enzymatic system + amplex red
15 minUp to 0.434 nM Colorimetric and ECL
CoOOH Nanosheet
10 min0.15–5 mM
Fluorescence (520 nm)
CoFe Nanoparticles
60–120 min
1 mM–10 mM
Spectrophotometric
Co3O4 Film electrode (transmitted at 620 nm) 50 s
10 mM–0.1 M
Electrochemical
Co-Based phosphate microelectrodeo60 s
10 mM–0.1 M
Electrochemical
Cobalt phosphate coated Co
30 s
a
15 s
1 mM–10 mM
Electrochemical
Co NPs/rGO/GCE
10 mM–10 mM Electrochemical
Co Microelectrode30–60 s
10 mM–0.1 M
Electrochemical
Co Electrodeo40 s
0.1 nM–0.1 mM Electrochemical
CoONPs/PBI/MWCNT/GCE
B5 s
a

Ref.

—
47
—
48
Tap and river water
17
SBF and HBS
18—
49
Microbial flocs
50—
20
Tap and well water
23Standard ATP and ADT 51
Waste water
52
Creek waterThis paper

Co nanoparticles and reduced graphene oxide/glassy carbon electrode.

pH 7 remained almost unchanged d for 1 month using same
electrode stored inside an initial mixed solution of 0.1 M NaOH
and 0.1 M HClO4 when not in use under the condition of room
temperature (Fig. S15, ESI†). The proposed sensor was comparedwith other types of detection for phosphate ions with respect to thedetection range, type of electrode/reagent, and response time
(Table 1).
To confirm the fast response of the modified electrode, we
performed CV measurements in the presence and absence ofphosphate (Fig. 9). The actual current vs. potential of the CV curves
shown on Fig. S16 (ESI†). Redox peaks at 0.15 and 0.27 V were
initially observed for the CoONPs-modified electrode in a mixed
solution (pH 4) of 0.1 M NaOH and 0.1 M HClO4 but disappeared
after the addition of 0.1 nM phosphate ions (Fig. 9a). The same
behavior was also observed for pH 7 (Fig. 9b), in which case theinitial peaks were observed at 0.1 and 0.19 V. The addition ofphosphate seemed to strongly influence the double-layer capacitance. Thus, phosphate binding occurred on the CoONPs surface
even at low phosphate concentrations, which suggested that ourcomposite electrode is capable of highly sensitive phosphate
detection.
As mentioned before, cobalt oxides react with phosphateions in solution to form cobalt phosphate precipitates at theelectrode surface, with the precipitate nature depending on pH.
Soluble phosphorus in natural water is largely present as four
orthophosphates (H3PO4, H2PO4 , HPO42 , and PO43 ), withthe proportions of these species depending on pH (Fig. S17,
ESI†).53 Hem investigated the distribution of orthophosphate
species at 25 1C, revealing that no more than two types arepresent at any pH.54 The phosphate species formed herein at
pH 4 is thought to be H2PO4 (eqn (4)), while H2PO4 and/or
HPO42 might form at pH 7 (eqn (4) and (5)).

Fig. 9 CV curves of CoONPs/PBI/MWCNTs/GCE recorded in a mixed
solution of 0.1 M NaOH and 0.1 M HClO4 (broken line) and in 0.1 nM PBS(solid line) at (a) pH 4 and (b) pH 7 using a scan rate of 5 mV s 1.

The CoONPs/PBI/MWCNT electrode was used to monitor phosphate ions in creek water samples collected around Saga City,
Japan. Before measurements, the sample pH was adjusted to
pH 4 or 7 to ensure that the electrode worked under optimal

conditions, and phosphate levels were determined by matching
the potential values to the OCPs obtained at these pH values.
The final concentration of phosphate was determined by adding phosphate to the analyzed samples to obtain a total
concentration of 10 mM, and the validity of the obtained
concentration was confirmed by ICP measurements. The phosphate additions were performed because the exceptionally low
concentration of phosphate in the analyzed samples complicated its analysis by ICP with limited range detection forcomparison and validation.
An OCP of 0.28 V was obtained at pH 4, indicating that thephosphate concentration was between 1.0  10 2 and 10 mM.
Furthermore, the OCP obtained at pH 7 (0.31 V) indicated thatthe phosphate concentration was between 1 and 10 mM. The© 2022 The Author(s). Published by the Royal Society of Chemistry

Mater. Adv., 2022, 3, 2018–2025 | 2023

3CoO + 2H2PO4 + 2H+ " Co3(PO4)2 + 3H2O,(4)

3CoO + 2HPO42 + H2O " Co3(PO4)2 + 4OH .

(5)

Phosphate ion monitoring in real water samples

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Table 2Paper

Concentrations of phosphate ions in real water samples

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SampleIntended conc./ ICP result/ ICP conc./ EOCP/ OCP conc./
no.
pH mM
ppm
mM
V
mM

Notes and references

We are grateful to Professor Keisuke Ohto (Saga University) for
help with ICP measurements. This work was supported by the
Nanotechnology Platform Program (Molecule and Material
Synthesis) of the Ministry of Education, Culture, Sports, Scienceand Technology (MEXT), Japan (Grant no. JPMXP09
S20KU0040, JPMXP09 S21KU0041). We also acknowledge support in the form of research equipment sharing by the MEXT
Project for Promoting the Public Utilization of Advanced
Research Infrastructure (Program for Supporting the Introduction of the New Sharing System; Grant no. JPMXS0422400020).

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2024 | Mater. Adv., 2022, 3, 2018–2025© 2022 The Author(s). Published by the Royal Society of Chemistry

1
2

4
7

5–55
5–55

0.45
0.36

9.4
7.5

0.28
0.31

B10
B10

combined results suggested that the level of phosphate in theanalyzed sample was approximately 10 mM. This result was ingood agreement with that of the ICP measurements, indicating
the suitability of our electrode for purely electrochemical
phosphate quantitation (Table 2).

Conclusions
A new phosphate ion sensor based on CoONPs hybridized with
PBI and MWCNTs was developed. The synthesized CoONPs
were shown to be well dispersed on the MWCNTs and exhibit auniform size of 3.5–5.5 nm, mainly comprising hexagonal
Co3O4 with a fraction of CoO. The effects of morphology andmicrostructure on electrochemical performance were investigated to reveal that the CoONPs–containing electrode exhibiteda considerably higher sensitivity than that of the bulk Co wire
electrode. Specifically, the former electrode could sense phosphate at pH 4 and 7 and at concentrations (0.1–100 nM) lower
than the eutrophication threshold. The CoONPs/PBI/MWCNT
electrode was successfully used to detect phosphate in actual
creek water samples.

Author contributions
Shaimah Rinda Sari: data curation, formal analysis, investigation, methodology, writing – original draft; Masayuki Tsushida:
resources, formal analysis, investigation, validation; Tetsuya
Sato: resources, formal analysis, investigation, validation;
Masato Tominaga: conceptualization, investigation, funding
acquisition, writing – review & editing, supervision.

Conflicts of interest
There are no conflicts to declare.

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Paper

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