Magnetic Flower-like Fe-Doped CoO Nanocomposites with Dual Enzyme-like Activities for Facile and Sensitive Determination of H2O2and Dopamine

Article

Magnetic Flower-like Fe-Doped CoO Nanocomposites with Dual

Enzyme-like Activities for Facile and Sensitive Determination of

H O and Dopamine

Jiajia Lian, Yanlei He, Ning Li, Pei Liu,* Zhenxue Liu, and Qingyun Liu*

Cite This: Inorg. Chem. 2021, 60, 1893 −1901

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sı Supporting Information

ABSTRACT: Herein, a new series of magnetic Fe-doped CoO

nanocomposites (Fe-CoO NCs) with dual enzyme-like activities

(

peroxidase and oxidase) were successfully synthesized. The molar

ratio of Fe /Co salts during the solvothermal process determined the

morphology and catalytic activity of the NCs. Among them, the ﬂower-

like 0.15Fe-CoO NCs showed high peroxidase-mimicking activity over a

wider pH range of 4−5 and a temperature range of 30−50 °C. Such

nanozymes were applied for constructing a facile and sensitive

colorimetric sensor to detect H O and dopamine (DA) in the linear

ranges of 6−20 and 2−10 μM with limits of detection (LODs) of 4.40

and 1.99 μM, respectively. The excellent magnetic separation perform-

ance and successful DA detection in human urine samples validated the

promising application of CoO-based nanozymes in medical diagnosis.

The superior catalytic behaviors of 0.15Fe-CoO NCs could be ascribed to the high surface area, open mesoporous structure,

increased surface active species, and the facile redox of Fe /Fe and Co /Co . Based on the results of the ﬂuorescent probe and

radical trapping tests, the possible mechanism that Fe doping promoted the decomposition of H O to produce hydroxyl radical

•

−

(

OH) and superoxide radical ( O ) was proposed.

INTRODUCTION

further improve the catalytic properties of CoO NPs. For

example, combining with carbon materials could eﬀectively

improve the electron-transfer eﬃciency and the dispersion of

■

Natural enzymes are widely present in living organisms and

play an important role in regulating the biological functions.

They are the most eﬀective catalysts for in vitro detection of

various biomolecules owing to the merits of high sensitivity

and excellent selectivity. However, their applications were

often limited owing to high price, low stability (>45 °C), and

13,14

CoO NPs.

The g-C N with large surface area and two-

3 4

dimensional (2D) structure could prevent the agglomeration

of CoO NPs and facilitate the separation of carriers in CoO/g-

15,16

C N .

The introduction of CoO nanodots in nitrogen-

doped porous carbon increased the electron density on the

carbon surface, thereby enhancing both radical and nonradical

the diﬃculty to reuse. To replace natural enzymes, many

kinds of artiﬁcial nanozymes have been developed, such as

noble metals, metal sulﬁdes and oxides, metal salts, and carbon

catalytic processes. Next, the construction of heterojunctions

can create an interfacial electric ﬁeld at the junction of two

materials. Among them, metal oxides, including Fe O

18,19

phases to fasten the electron transfer.

The ﬂower-like

nanoparticles (NPs),

Co O₄NPs, V₂O₅nanospheres,

CoO@Bi₂S₃heterojunction with multiple light reﬂection/

scattering channels and improved molecular diﬀusion kinetics

provided more exposed active edges and greater light

and CeO nanorods, have attracted more interest thanks

to their low cost, excellent stability, and reusability. Especially,

open porous-structured nanomaterials always exhibited supe-

rior catalytic performance, attributing to the large working

surface area and the convenient substrate transfer channels.

Therefore, it is highly desirable to develop an eﬀective metal

utilization eﬃciency. The promising catalytic activity of the

mesoporous CoO/CeO could be attributed to the abundant

,10

surface defects and oxygen vacancies (OVs) introduced by the

oxide-based nanozyme with these advantages.

Various cobalt oxide (CoO) nanostructures have been

studied in the ﬁelds of photo- and electrocatalysis because of

Received: November 12, 2020

Published: January 13, 2021

their high stability, large surface area, and facile redox of Co /

11,12

Co .

Yet, pure CoO NPs exhibited poor electrical

conductivity and tend to self-agglomerate to mask the active

sites. Therefore, currently many eﬀorts have been made to

2021 American Chemical Society

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Inorg. Chem. 2021, 60, 1893−1901

Inorganic Chemistry

Article

heterojunction interface of CeO and CoO nanocrystals.

processes (Scheme 1). First, 1 mmol of Co(NO

homogeneous mixture containing glycerin and isopropanol (16 and

)

was added into a

Moreover, functional doping with a metal or metal oxide is also

an eﬀective way. For instance, the uniform doping of SiO onto

CoO nanorods signiﬁcantly improved the intrinsic peroxidase-

Scheme 1. Synthesis Route of the Flower-like Fe-CoO NCs

like activity of the latter. Suitable Ag decoration not only

increased the surface active species but also promoted the

separation of photogenerated electron−hole pairs of CoO

NPs. Nevertheless, only a few works about CoO-based

catalysts as nanozymes have been reported. For this reason, it

is still necessary to explore novel CoO-based nanozymes with

special properties by a facile and green strategy.

Fe doping has been proved to be an eﬃcient strategy to

develop low-cost and highly active catalysts. For example, the

doping of Fe endowed 2D g-C N nanosheets (NSs) with

better catalytic property toward H O in cancer cells to

generate O₂. When doped with Fe , more OVs were

generated to enhance the chemical absorption of O and

generation of active oxygen species (O⁻and O ).

−

25−27

Because of the coherent eﬀect between the electrons of the Fe

d orbitals and ZrO intrinsic electronic states, the band gap of

Fe-doped ZrO NPs was narrowed to promote the charge

transfer between the catalyst and the substrate. Fe (6%)

0 mL, respectively) and vigorously stirred for 1 h. Second, n mmol of

Fe(NO ) (n = 0.05, 0.1, 0.15, 0.2, 0.3, 0.4, 0.6, 0.8, and 1.0) was

doping obviously lowered the average size of CeO NRs and

added and continuously mixed for another 10 h. Third, the mixture

was transferred into a 100 mL Teﬂon container with a stainless-steel

autoclave and treated under 180 °C (6 h). The obtained precipitate

was puriﬁed by alternately washing with water and ethanol (6 times in

total) and then dried overnight at 60 °C. The precursor was roasted at

created more surface defects (Ce³ions and OVs). Notably,

both Fe and FeO active centers hold strong magnetic moments

and intrinsically drive the reactions through the radical

mechanism. Besides, the facile redox of Fe³⁺↔ Fe and

400 °C (2 h) with the protection of N ﬂow. The ﬁnal samples were

Co ↔ Co could provide reversible defect sites for the

catalytic decomposition of H O into free radicals.

,31,32

recorded as nFe-CoO NCs. Besides, pure CoO NPs were prepared

using the similar method without adding Fe salt. But without Co

Inspired by these reports, we attempted to prepare the open

porous-structured Fe-doped CoO NCs with a remarkable

enzyme-like activity. The doping of Fe compounds may also

endow CoO with excellent magnetic separation performance,

which has never been reported for CoO-based nanozymes.

Herein, we have synthesized a new series of magnetic Fe-

doped CoO NCs by the simple one-pot solvothermal method

followed by heat treatment. Especially, the ﬂower-like 0.15Fe-

CoO NCs possessed dual enzyme-like activities for quickly

oxidizing the substrate 3,3′,5,5′-tetramethylbenzidine (TMB)

with H O as an oxidant in 60 s. Owing to the magnetism of

salt, the Fe ions cannot crystallize to be FeO or Fe O NPs.

Detailed descriptions for the characterization, activity evaluation,

kinetic tests, determination of H O and DA, and research mechanism

processes are given in the Supporting Information .

RESULTS AND DISCUSSION

■

Material Characterization. X-ray diﬀraction (XRD)

patterns were determined to analyze the phase composition

and purity of the samples. As shown in Figure 1a, the patterns

of pure CoO NPs and 0.15Fe-CoO NCs feature ﬁve distinct

reﬂections at 2θ = 36.5, 42.4, 61.5, 73.7, and 77.6°,

corresponding to the cubic CoO (ICDD 43-1004). Note

that the characteristic peaks of doped Fe components are not

observed in 0.15Fe-CoO NCs, which may be attributed to the

low content or very close positions to those of CoO NPs

the composite catalyst, easy recyclability and reusability are

important for the sustainable development. The roles of doped

Fe compounds and CoO substrate were systematically

investigated by the control and deoxygenation experiments.

The catalytic behaviors of 0.15Fe-CoO NCs strongly relied on

the solution pH and temperature and conformed to the

Michaelis−Menten (M−M) kinetic model. According to the

better peroxidase-mimicking activity of 0.15Fe-CoO, a

colorimetric sensor was developed for rapid and sensitive

(

Figure S1). Fortunately, the XRD patterns of other

composites given in Figure S2a conﬁrm the latter hypothesis.

The newly emerging peaks correspond to the magnetic γ-

Fe O (ICDD 43-1004), which endows 0.15Fe-CoO NCs the

merit for easily separating from the reaction system. The area

determination of H O and DA. The possible mechanism was

ratios of peaks due to γ-Fe O and CoO in composites were

investigated via ﬂuorescent probe and radical trapping

experiments.

calculated by peak splitting and ﬁtting (Figure S2b and Table

S1 ). The increasing trend is basically the same as the adding

ratio of the Fe /Co salts. Moreover, no oxidized Co O and

EXPERIMENTAL SECTION

■

other impurities were detected in both CoO NPs and 0.15Fe-

CoO NCs, implying their high purities.

The porous structures of CoO NPs and 0.15Fe-CoO NCs

Reagents. Fe(NO ) ·9H O, Co(NO ) ·6H O, glycerol, isopropa-

nol (IPA), p-benzoquinone (PBQ), ethylenediaminetetraacetic acid

disodium salt (EDTA-2Na), and p-phthalic acid (PTA) were

purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai,

China). TMB and dopamine hydrochloride (DA) were obtained from

Macklin Biochemical Technology Co., Ltd. (Shanghai, China). All

other reagents were of analytical grade and were used without further

puriﬁcation. Solutions were prepared with ultrapure water.

were investigated by N -sorption isotherms. Their isotherms

(

shown in Figure 1b) are typical type-IV with an H3-type

hysteresis loop, indicating the coexistence of a mesoporous/

macroporous structure. The determined Brunauer−Em-

mett−Teller (BET) surface areas of CoO and 0.15Fe-CoO

are 216.2 and 205.4 m g , respectively. The BJH pore size

−1

Fabrication of Fe-CoO NCs. A series of Fe-CoO NCs were

fabricated by two-step routes including solvothermal and roasting

distribution of 0.15Fe-CoO is mainly in the range of 2−20 nm,

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Inorg. Chem. 2021, 60, 1893−1901

Figure 1. XRD patterns (a) and N -sorption isotherms (b) of CoO NPs and 0.15Fe-CoO NCs.

Article

Figure 2. SEM (a1), TEM (a2, a3) and HRTEM (a4) images of CoO NPs; and SEM (b1), TEM (b2, b3), and HRTEM (b4) images of 0.15Fe-

CoO NCs. (c) Element mapping photos of 0.15Fe-CoO NCs.

Figure 3. (a) XPS survey spectra and XPS spectra of (b) Fe 2p, (c) Co 2p, and (d) O 1s for CoO NPs and 0.15Fe-CoO NCs.

while that of CoO centers at about 5 nm and 30 nm (inset).

Therefore, 0.15Fe-CoO NCs have a more uniform pore size

distribution, which may facilitate the mass transfer for

heterogeneous catalysis. Although the surface area of CoO

NPs is a little higher, 0.15Fe-CoO NCs may have higher

porosity utilization. Notably, the surface area of 0.15Fe-CoO

NCs is obviously higher than other composites, implying the

key role of Fe /Co ratio in shaping the porous structure

(Figure S3 and Table S2 ). Besides, 0.15Fe-CoO NCs can be

easily separated from the solution using a strong magnet

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Inorg. Chem. 2021, 60, 1893−1901

Inorganic Chemistry

Figure S4). Therefore, the magnetic 0.15Fe-CoO NCs with

Article

peroxidase-mimicking activity of the composite catalysts

(24.9%), suggesting that the Fe doping inhibited the oxidation

rich mesoporous structure were successfully fabricated by

of surface Co . Also, a proper ratio of Co /Co can promote

the electron transfer between 0.15Fe-CoO and the reactants.

The O 1s spectra (Figure 3d) were split into three peaks at

529.8, 531.2, and 532.6 eV, corresponding to the lattice oxygen

(O ) in Fe-O and/or Co-O, surface oxygen (O ) associated

rationally adjusting the molar ratio of the Fe /Co salts.

The morphology of pure CoO NPs and 0.15Fe-CoO NCs

was observed by scanning electron microscopy (SEM) and

transmission electron microscopy (TEM) images. As shown in

Figure 2a1,b1, CoO NPs exhibit a nest-like structure and are

cross-linked together, which are diﬃcult to disperse well, while

the 0.15Fe-CoO particles are in ﬂower shape with high

dispersion. The open hierarchical porous structure can provide

eﬀective channels for fast mass transport and charge transfer

with the OVs, and the oxygen functional groups (O_γ).

Notably, compared with CoO, the O and O contents on

0.15Fe-CoO improved by 9.8 and 8.3%, respectively. There-

fore, the doping of Fe compounds endowed CoO NPs

excellent magnetic separation property and higher density of

surface reactive sites, thus exhibiting a better catalytic

performance.

9,34

during catalytic oxidation.

However, the excessive use of

Fe salt would destroy this special morphology and result in

larger particles (Figure S5), which is consistent with the results

Dual Enzyme-like Activities. As shown in Figure 4a,b, the

of N -sorption. The surface atomic ratio of Fe/Co was also

determined by elemental dispersive spectroscopy (EDS) and

the results are listed in Table S3. Although the actual Fe/Co

ratios in the composite catalysts are mostly higher than the

adding ratios of Fe /Co salts, their increasing trends are

basically the same as each other. As shown in Figure 2a2,a3,

the CoO NPs consist of a loose core of 400−500 nm and a

graphene-like shell of about 100 nm. Certainly, the special

morphology endowed CoO with high surface area and

developed porous structure. The doping of Fe not only

reduced the average particle size of 0.15Fe-CoO NCs but also

made the particles dense (Figures 2b2,b3 and S6). Importantly,

the ﬂower-like morphology is well maintained, and the petals

are composed of tiny nanosheets. The tight adhesion between

doped Fe compounds and CoO can eﬀectively promote carrier

transfer. The high-resolution TEM (HRTEM) image (Figure

Figure 4. (a) Evaluation of the peroxidase-mimicking activity of Fe-

CoO NCs: (1) CoO, (2) 0.05, (3) 0.10, (4) 0.15, (5) 0.20, (6) 0.30,

a4) of CoO identiﬁes the d-spacing values of 0.22 and 0.24

nm, corresponding to the (200) and (111) crystal planes of

(

7) 0.40, (8) 0.60, (9) 0.80, and (10) 1.00; (c) control experiments

4,35

CoO.

Unfortunately, the lattice spacing of 0.15Fe-CoO

for conﬁrming the dual enzyme-like activities of 0.15Fe-CoO: (1)

NCs shown in Figure 2b4 cannot be distinguished due to the

TMB, (2) H O + TMB, (3) CoO + TMB, (4) CoO + H O + TMB,

similar XRD peaks of FeO, γ-Fe O , and CoO. The

(5) 0.15Fe-CoO + TMB, and (6) 0.15Fe-CoO+ H O + TMB; (b)

and (d) are the corresponding color images.

2 2

distributions of Fe, Co, and O elements in 0.15Fe-CoO NCs

Figure 2c) are highly consistent, indicating the uniform

(

doping of Fe onto CoO NPs. Therefore, the open

mesoporous-structured Fe-CoO NCs with high dispersion

were successfully fabricated.

increases with the Fe /Co ratio increasing from 0 to 0.15

(lines 1−4), which can be attributed to the Fe doping.

Nevertheless, the activity would be reduced when the ratio

exceeds 0.20 (lines 5−7), and even lower than that of CoO

(ratios of 0.60−1.00, line 8−10), due to the excessive doping

of Fe compounds that destroyed the porous structure of the

CoO NPs. Thus, 0.15Fe-CoO NCs show the highest catalytic

The valence state of the surface element was examined by

the X-ray photoelectron spectroscopy (XPS) technique

(

Figure 3a). It is clear that no Fe element exists in pure

CoO NPs. The Fe 2p spectrum of 0.15Fe-CoO was ﬁtted into

ﬁve peaks (Figure 3b). The peaks centered at 713.6 and 726.3

eV with a satellite peak at 716.9 eV are assigned to Fe

activity and the optimal molar ratio of Fe /Co salts is 0.15.

The TMB alone cannot be oxidized by dissolved oxygen and

H O (Figure 4c,d, lines 1 and 2). In the absence of H O ,

6−38

ions,

while the other two peaks at 710.7 and 720.4 eV

correspond to the Fe ions. The relative contents of Fe /

CoO NPs still slightly catalyze the oxidation of TMB,

indicating their mild oxidase-like activity (line 3). The speciﬁc

absorbance is further improved with H O (line 4); thus, CoO

Fe were calculated to be 59.5 and 40.5% (Table S4),

respectively. However, the FeO phase was not observed in the

XRD patterns of all composites, which might be due to the

characteristic peaks of FeO (Wurstite) being very close to

those of CoO. The presence of magnetic FeO can improve the

redox cycle of Fe /Fe , thus enhancing the electron transfer

between the catalyst and the substrates. In Figure 3c, the Co

NPs also possess peroxidase-mimicking activity. Notably, the

doping of Fe hardly aﬀected the oxidase-like activity of CoO

(line 5) but signiﬁcantly improved its peroxidase-like activity

(line 6). Meanwhile, the 0.15Fe-CoO + H O + TMB system

generated the deepest color at the same time intervals. When

p spectra were deconvoluted into six peaks. Among which,

high-purity N was purged for 30 min before reaction, the

the peaks centered at 781.3 and 796.4 eV accompanied by two

satellite peaks at 786.2 and 802.9 eV are attributed to the Co

obvious decrease in the activity of CoO and 0.15Fe-CoO could

be observed as shown in Figure S7a,b, revealing that the

dissolved oxygen was also involved in the catalytic reaction.

The results suggest that the oxidase- and peroxidase-like

activities of 0.15Fe-CoO NCs are mainly derived from CoO

ions, and the peaks located at 779.6 and 794.7 eV are

contributed to the Co³⁺ions.⁴⁰^,⁴¹The presence of Co might

have originated from the oxidation of surface Co by oxygen

during preparation. Interestingly, the Co³content of 0.15Fe-

CoO NCs (21.1%) is obviously lower than that of CoO NPs

and the doped Fe compounds, respectively. The remarkable

dual enzyme-like activities of 0.15Fe-CoO NCs may be

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Inorg. Chem. 2021, 60, 1893−1901

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Figure 5. Inﬂuences of pH (a) and temperature (b) on the catalytic activity of 0.15Fe-CoO NCs; and (c) change of TMB under diﬀerent pH

values and temperatures.

Figure 6. Steady-state kinetics for 0.15Fe-CoO as peroxidase mimics: (a) varying H O from 2 to 20 mM and keeping TMB of 0.1 mM; (c) varying

TMB from 10 to 100 μM and keeping H O of 20 mM. (b) and (d) are the corresponding linear double reciprocal curves of (a) and (c).

ascribed to the highly dispersed ﬂower-like structure and the

synergistic eﬀect between Fe compounds and CoO. For

convenience, subsequent experiments were performed on the

basis of their peroxidase-mimicking activity.

Similarly, too high temperature (>50 °C) would also cause

excessive oxidation of TMB. Figure 5c clearly shows the

change of TMB under diﬀerent pH values and temperatures.

Therefore, pH of 5 and temperature of 37 °C were selected for

the following reactions. Besides, the 0.15Fe-CoO catalyst also

exhibits excellent durability and reusability (Figure S8a,b),

which are essential for its practical applications.

The inﬂuences of pH and temperature were systematically

investigated. As shown in Figure 5a, the 0.15Fe-CoO catalyst

exhibits higher catalytic activity under a wider pH range of 4−

. However, at strong acidic conditions (pH<4), TMB would

Steady-State Kinetics of 0.15Fe-CoO NCs. As shown in

44,45

be directly oxidized to the yellow benzidine.

Under the pH

Figure 6a,c, the steady-state kinetics of H O and TMB as

range of 6−8, the activity of the 0.15Fe-CoO NCs is

substrate conform to the typical Michaelis−Menten (M−M)

signiﬁcantly inhibited owing to the direct decomposition of

curves. From the corresponding double reciprocal curves

most H O into H O and O . The 0.15Fe-CoO NCs can

(Figure 6b,d), the M−M constants (K ) of the 0.15Fe-CoO

retain larger than 80% of the highest activity at higher

temperatures (30−50 °C), revealing their excellent temper-

ature resistance (Figure 5b). Especially, they display the

highest activity around body temperature (37 °C), which is

essential for their practical application in medical diagnosis.

NCs toward H O and TMB were calculated to be 11.2 and

0.170 mM, respectively. They are comparable to or lower than

those of horseradish peroxidase (HRP) (3.7 and 0.434 mM)

and many nanozymes such as Fe O MNPs (154 and 0.098

mM), WS nanosheets (15.2 and 0.805 mM), and so on

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Inorg. Chem. 2021, 60, 1893−1901

Article

Figure 7. (a) H O detection limit tests: varying H O concentration in 1−100 μM; (b) selectivity of 0.15Fe-CoO to H O (100 μM) and

interferences (500 μM). (c) ΔA responses of the sensor toward DA in 1−50 μM; and (d) selectivity of the sensor to DA (50 μM) and interferences

−

(

500 μM). Conditions: nanozymes 30 μg mL , TMB 0.1 mM, H O 20 mM, pH of 5, T of 37 °C, and 60 s.

Table S5 ), indicating the high aﬃnity of 0.15Fe-CoO toward

sensor shows strong response only to DA (50 μM), compared

to various interferences (500 μM). Therefore, the developed

colorimetric sensor also shows excellent sensitivity and

selectivity for the determination of DA.

H O and TMB. The Fe /Fe and Co /Co balances as

well as the special mesoporous structure may contribute to a

high binding aﬃnity to the substrates. Besides, the maximum

reaction rates (V ) with H O and TMB as substrate are 24.1

To evaluate the practical performance of the colorimetric

−

−8

−1

10 and 50.9 × 10 M s , respectively, much higher than

sensor, the H

determined by the standard addition method. As shown in

Table 1, for H O detection, the recovery values are between

2 2

O and DA in human urine samples were

those of the contrast catalysts except WS NSs, well explaining

the fast reaction rate of the 0.15Fe-CoO + H O + TMB

system in just 60 s.

Determination of H O and DA. A colorimetric sensor

Table 1. Results of H

Urine Samples

O and DA Detection in Human

2 2

(

0.15Fe-CoO + H O + TMB) was constructed based on the

2 2

.15Fe-CoO NCs for the determination of H O . As depicted

2 2

samples

H O

spiked (μM)

found (μM)

recovery (%)

RSD (%)

in Figure 7a, the feature absorbance ﬁrst rapidly increases and

then slightly increases over a broad H O concentration range

5.89

10.06

14.89

17.57

19.73

1.89

4.29

5.89

8.08

10.33

98.2

100.6

106.4

97.6

98.6

94.6

107.2

98.2

102.0

103.3

13.3

9.24

2.99

5.54

2.85

0.17

0.14

0.05

0.08

0.06

of 1−100 μM. The absorption maxima show a linear

relationship with H O concentration in the range 6−20 μM

(

inset, R = 0.9910). As a result, the limits of detection (LOD)

of this sensor toward H O was calculated to be 4.40 μM (3σ/

k), where σ represents the standard deviation of night blank

groups without adding H O , and k is the slope of the linear

ﬁtting curve. The LOD is comparable to or lower than that of

many reported works (Table S6). Although ﬂuorescence and

electrochemical methods are more sensitive, the colorimetric

strategy exhibits unique merits including simple operation and

the much shortened reaction time. Besides, the selectivity of

the sensor toward H O was studied in comparison with metal

7.6 and 106.4% with the relative standard deviations (RSDs)

of 2.85−13.3%. As for the determination of DA, the recovery

values are 94.6−107.2% with RSDs ranging from 0.06 to

ions (K , Na , Ca , Mg ), sugars (Fru, Mal), and amino acids

Arg, His, Ser). As can be seen from Figure 7b, 0.15Fe-CoO

exhibits a strong response only to H O , even though the

.17%. Therefore, the sensing platform based on 0.15Fe-CoO

(

NCs is highly accurate and has great potential for H O and

DA detection in real samples.

Possible Mechanism. After reacting for 30 min, the

0.15Fe-CoO + PTA + H O system produces bright blue

concentrations of interferences are 5 times higher than that of

H O . These results demonstrate the good sensitivity and

selectivity of 0.15Fe-CoO NCs for H O sensing.

ﬂuorescence, which has the maximum emission (λ ) at around

Furthermore, the proposed sensor was applied for DA

sensing by determining the absorbance diﬀerence (ΔA) at 652

nm before and after adding DA (Figure 7c). The ΔA values are

linearly related to the DA concentrations in the range of 2−10

33 nm (Figure 8a). No obvious ﬂuorescence can be observed

in the pure PTA system (line 1). Interestingly, the λ of the

PTA+0.15Fe-CoO (30 μg mL ) system appears at about 387

nm (line 2). This may be because, in the absence of H O ,

0.15Fe-CoO NCs act as oxidase mimics to react with dissolved

−

μM (inset, R = 0.9912). As a result, the LOD was determined

•

−

as 1.99 μ M, which is comparable to that of many previous

oxygen, producing O to combine with PTA. Note that the

reports (Table S7). Importantly, as shown in Figure 7d, the

λ_mintensity increases with the catalyst concentration

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Inorg. Chem. 2021, 60, 1893−1901

The Supporting Information is available free of charge at

Article

−

Figure 8. (a) Fluorescence probe tests: (1) PTA, (2) PTA+0.15Fe-CoO (30 μg mL ), (3−9) PTA + H O + 0.15Fe-CoO (0, 5, 10, 15, 20, 25,

−

•

−

and 30 μg mL ); (b) radical trapping experiments using IPA, PBQ, and EDTA-2Na to scavenge OH, O , and h , respectively. Each experiment

was repeated three times.

_i_n_c_r_e_a_s_i_n_g_f_r_o_m₀_t_o₃₀_μ_g_m_L₋1 (lines 3−9), implying that the

Fe + H O → Fe + O + 2H⁺

(3)

•

higher catalyst concentration creates more OH radicals.

Besides, radical trapping experiments were performed using

Co²+ O → Co + O

• −

•

−

(

IPA, PBQ, and EDTA-2Na to scavenge OH, O , and hole

(

h ), respectively. As shown in Figure 8b, the addition of IPA

and PBQ signiﬁcantly reduced the absorbance but not for

−

TMB + O + 2H → ox − TMB + H O

(5)

(6)

•

EDTA-2Na. Therefore, it can be concluded that both OH and

•

−

O₂species participated in the oxidation of TMB catalyzed by

Co³+ TMB → ox − TMB + Co²⁺

.15Fe-CoO with H O as an additive.

Based on the results and discussion of the whole work, the

feasible catalytic mechanism of 0.15Fe-CoO NCs was

preliminarily investigated (Scheme 2). First, the catalytic

CONCLUSIONS

■

In summary, a series of magnetic Fe-CoO NCs were

successfully synthesized and were found to possess dual

enzyme-like activities (oxidase and peroxidase). Control of the

Scheme 2. Possible Catalytic Mechanism of 0.15Fe-CoO

NCs with Dual Enzyme-Like Activities

Fe /Co ratio in hydrothermal process determined not only

the morphology but also the catalytic activity of the composite

catalysts. Especially, the ﬂower-shaped 0.15Fe-CoO NCs with

open mesoporous structure were highly dispersed, due to the

Fe doping eﬀectively inhibiting the agglomeration of CoO

NPs. The superior catalytic activity could be attributed to the

high speciﬁc surface area, special ﬂower-like structure, and

increased surface active species. The TMB oxidation wa₋s

achieved by catalyzing the generation of ^•OH and ^•O₂

radicals. A colorimetric sensor was developed based on

.15Fe-CoO NCs for rapid and sensitive determination of

H O and DA. The good magnetic separation characteristic

and the accurate real-sample detection ability prove that this

catalyst has a strong practical performance. This work provides

a general strategy for synthesizing high-eﬃciency nanozymes

through functional doping.

reactions mainly occurred on the doped Fe compounds (FeO

and γ-Fe O ). Fe ions with excellent Fenton reactivity

•

catalyzed the decomposition of H O , generating OH radicals

to oxidize the TMB. The oxidized Fe ions would be restored

back to Fe by H O , meanwhile, producing a molecule of O .

ASSOCIATED CONTENT

sı Supporting Information

■

So, in the presence of H O , the composite catalyst exhibited

peroxidase-mimicking activity. Then, the reactions moved to

_t_h_e_s_u_p_p_o_r_t_C_o_O_N_P_s_._C_o2+ ions ﬁrst reacted with dissolved

https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c03355.

•

−

O and the O from the decomposition of H O to create O

•

−

^r^a^dⁱ^c^a^l^s^.^T^h^eⁿ^,^T^M^B^w^o^u^l^d^b^e^o^xⁱ^dⁱ^z^e^d^b^y^O2 under weakly

Detailed descriptions for the characterizations, activity

evaluation, kinetic tests, determination of H O and DA,

•

−

acidic conditions. However, the oxidation capacity of O is

•

signiﬁcantly lower than that of OH. Thus, 0.15Fe-CoO NCs

and mechanism research processes; XRD patterns; N₂

adsorption/desorption isotherms; magnetic separation

photos; N₂bubbling experiments; tables of the

parameters of N₂adsorption/desorption and XPS

analysis; and tables of the kinetic parameters; and

H O and DA sensing parameters compared with

showed weak oxidase-like activity. At last, Co³ions could

immediately oxidize TMB, indicating that CoO NPs also

possess a certain degree of oxidation.

_F_e₂+ + H O → Fe + OH + OH

^•OH + TMB → ox − TMB

−

•

(1)

(2)

previous reports (PDF)

899

Inorg. Chem. 2021, 60, 1893−1901

Inorganic Chemistry

Article

(

8) Korschelt, K.; Schwidetzky, R.; Pfitzner, F.; Strugatchi, J.;

AUTHOR INFORMATION

Corresponding Authors

■

Schilling, C.; von der Au, M.; Kirchhoff, K.; Panthofer, M.;

Lieberwirth, I.; Tahir, M.; Hess, C.; Meermann, B.; Tremela, W.

CeO ₂ _‑_x nanorods with intrinsic urease-like activity. Nanoscale 2018,

Pei Liu − College of Chemistry and Chemical Engineering,

Analysis and Testing Center, Henan Polytechnic University,

Jiaozuo 454000, China; Email: skdpeiliu@163.com

Qingyun Liu − College of Chemical and Biological

Engineering, Shandong University of Science and Technology,

Qingdao 266590, China; orcid.org/0000-0003-1178-

9) Cheng, H.; Lin, S.; Muhammad, F.; Lin, Y.; Wei, H. Rationally

0, 13074−13082.

modulate the oxidase-like activity of nanoceria for self-regulated

bioassays. ACS Sens. 2016, 1, 1336−1343.

(10) Zhao, Y.; Ding, B.; Xiao, X.; Jiang, F.; Wang, M.; Hou, Z.; Xing,

B.; Teng, B.; Cheng, Z.; Ma, P.; Lin, J. Virus-like Fe O @Bi S

2 3

232 ; Phone: +86 0532 86057757; Email: qyliu@

nanozymes with resistance-free apoptotic hyperthermia-augmented

nanozymitic activity for enhanced synergetic cancer therapy. ACS

Appl. Mater. Interfaces 2020, 12, 11320−11328.

sdust.edu.cn

Authors

(11) Ling, T.; Yan, D.; Jiao, Y.; Wang, H.; Zheng, Y.; Zheng, X.;

Jiajia Lian − College of Chemical and Biological Engineering,

Shandong University of Science and Technology, Qingdao

Mao, J.; Du, X.; Hu, Z.; Jaroniec, M.; Qiao, S. Engineering surface

atomic structure of single-crystal cobalt (II) oxide nanorods for

superior electrocatalysis. Nat. Commun. 2016, 7, No. 12876.

66590, China; orcid.org/0000-0002-8298-4669

Yanlei He − College of Chemical and Biological Engineering,

Shandong University of Science and Technology, Qingdao

(12) Cao, K.; Jiao, L.; Liu, Y.; Liu, H.; Wang, Y.; Yuan, H. Ultra-high

capacity lithium-ion batteries with hierarchical CoO nanowire clusters

66590, China

as binder free electrodes. Adv. Funct. Mater. 2015, 25, 1082−1089.

Ning Li − College of Chemical and Biological Engineering,

Shandong University of Science and Technology, Qingdao

(13) Jin, W.; Guo, X.; Zhang, J.; Zheng, L.; Liu, F.; Hu, Y.; Mao, J.;

Liu, H.; Xue, Y.; Tang, C. Ultrathin carbon coated CoO nanosheet

arrays as efficient electrocatalysts for the hydrogen evolution reaction.

Catal. Sci. Technol. 2019, 9, 6957−6964.

66590, China; orcid.org/0000-0003-0551-5479

Zhenxue Liu − College of Chemical and Biological

Engineering, Shandong University of Science and Technology,

Qingdao 266590, China

(14) Shi, W.; Guo, F.; Wang, H.; Han, M.; Li, H.; Yuan, S.; Huang,

H.; Liu, Y.; Kang, Z. Carbon dots decorated the exposing high-

reactive (111) facets CoO octahedrons with enhanced photocatalytic

activity and stability for tetracycline degradation under visible light

irradiation. Appl. Catal., B 2017, 219, 36−44.

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.inorgchem.0c03355

(15) Guo, F.; Shi, W.; Zhu, C.; Li, H.; Kang, Z. CoO and g-C N

3 4

Notes

complement each other for highly efficient overall water splitting

The authors declare no competing ﬁnancial interest.

under visible light. Appl. Catal., B 2018, 226, 412−420.

16) Mao, Z.; Chen, J.; Yang, Y.; Wang, D.; Bie, L.; Fahlman, B.

ACKNOWLEDGMENTS

Novel g-C N /CoO nanocomposites with significantly enhanced

3 4

■

visible-light photocatalytic activity for H evolution. ACS Appl. Mater.

This work was supported by the National Natural Science

Foundation of China (21971152) and Natural Science

Foundation of Shandong Province (ZR2018MB002 and

ZR2018MEE003).

Interfaces 2017, 9, 12427−12435.

(17) Ma, L.; Zhou, D.; Zhan, J.; Xie, M.; Tang, J.; Fang, G.; Zhang,

M.; Kong, L.; Zhu, F. Biomass Schiff base polymer-derived N-doped

porous carbon embedded with CoO nanodots for adsorption and

catalytic degradation of chlorophenol by peroxymonosulfate. J.

Hazard. Mater. 2020, 384, No. 121345.

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