Highly sensitive detection of nitrite at a novel electrochemical sensor based on mutually stabilized Pt nanoclusters doped CoO nanohybrid

Accepted Manuscript Title: Highly sensitive detection of nitrite at a novel electrochemical sensor based on mutually stabilized Pt nanoclusters doped CoO nanohybrid Author: Lu Lu PII: DOI: Reference:

S0925-4005(18)31844-6 https://doi.org/10.1016/j.snb.2018.10.074 SNB 25508

To appear in:

Sensors and Actuators B

Received date: Revised date: Accepted date:

31-5-2018 5-10-2018 12-10-2018

Please cite this article as: Lu L, Highly sensitive detection of nitrite at a novel electrochemical sensor based on mutually stabilized Pt nanoclusters doped CoO nanohybrid, Sensors and amp; Actuators: B. Chemical (2018), https://doi.org/10.1016/j.snb.2018.10.074 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Highly sensitive detection of nitrite at a novel electrochemical sensor based on mutually stabilized Pt nanoclusters doped CoO nanohybrid

IP T

Lu Lu *

SC R

State Key Laboratory of Biobased Material and Green Papermaking, College of Food Science and Engineering, Qilu University of Technology (Shandong

A

N

U

Academy of Sciences), Jinan 250353, P. R. China

*

M

Corresponding author: Lu Lu

ED

Tel.: +86 13791086029

CC E

PT

Email address: [email protected]

•

Research Highlights

A low-cost and stable Pt nanocluster doped nanoporous CoO was prepared

A

facilely.

•

The electrocatalytic activity of the nanohybrid for nitrite oxidation was evaluated.

•

The Pt nanoclusters without aggregation enhanced the performances of nitrite 1

sensor. •

The electrochemical sensor detected nitrite in real samples with acceptable

IP T

results.

Abstract

SC R

A novel, low-cost and mutually stable Pt nanocluster doped nanoporous CoO (Pt/CoO)

has been successfully designed and synthesized by facilely dealloying Al85Co14Pt1

U

ternary alloy in alkaline solution. Electronic microscope images demonstrated the Pt

N

nanoclusters are doped inside of the CoO crystal structure and they were mutually

A

stabilized with each other, resulting in a high electrocatalytical activity even if the

M

content of Pt was extremely low. A new electrochemical sensor was then constructed

ED

by modifying the Pt/CoO nanohybrid on a glassy carbon electrode, which enabled amperometric sensing of nitrite with wide linear ranges of 0.2 μM-3.67 mM and 3.67-

PT

23.7 mM, high sensitivities of 901.4 and 408.5 μA mM-1 cm-2 as well as a low detection

CC E

limit of 0.067 μM (S/N = 3). Furthermore, the sensor showed good selectivity, stability and reproducibility towards nitrite detection. The feasibility of the electrochemical

A

sensor was proved by successful detection of nitrite in real samples.

Keywords: Pt nanoclusters; Co oxides; Nanoporous hybrid; Nitrite; Electrochemical sensor

2

IP T

1. Introduction

SC R

Nitrite has been widely used in our daily lives, such as food additive, corrosion

inhibitor and fertilizer. It can not only suppress the propagation of microorganisms in

U

foods, improve the flavor and color of many meat products [1, 2], but also be used as

N

an inorganic fertilizer to provide nitrogen for the plants and vegetables [3]. However,

A

nitrite can interfere the oxygen transport system in bodies and interact with the amines

M

in stomach to generate carcinogenic N-nitrosamines, tending to damage the kidneys,

ED

spleen, nervous system and cause many cancers [4]. As a result, it is imperative to detect nitrite using rapid, sensitive, accurate and low-cost methods. Numerous techniques

PT

have been reported for nitrite determination. Compared with spectrofluorimetry [5],

CC E

chemiluminescence [6] and chromatography [7], electrochemical sensing techniques have attracted more attention for nitrite detection due to their high sensitivity, rapid response, easy microminiaturization and simple instrumentation. Because the bare and

A

plane electrodes usually tend to be poisoned by electroactive species, the electrodes modified with different nanomaterials have developed rapidly and been used in the nitrite detection. Among these nanomaterials, Pt nanoparticles have attracted extensive attention due to their high specific surface area, controllable particle size and good 3

electrocatalytic activity [8]. According to these reports, the key problem is the easy aggregation and instability of Pt nanoparticles during the fabrication and application processes. Moreover, the scarcity and high price of Pt greatly limit its large-scale application [9]. Therefore, it is very necessary to develop novel and highly

IP T

electrocatalytic nanomaterials with low Pt content to avoid the aggregation of Pt nanoparticles and reduce the dosage of noble metal Pt. Several strategies have been

SC R

developed to prepare low-Pt-content nanomaterials for modifying electrodes. For

example, alloying Pt with many transition metals (Fe, Co and Ni, etc.) via

U

electrodeposition and solvothermal synthesis [10, 11], fabrication of a Pt single layer

N

on the surface of different nanocrystals [12] and Pt-based hollow structures via

A

hydrothermal reduction or corrosion [13]. However, these synthetic strategies usually

M

involve tedious preparation steps, excessive use of organic agents and elevated

ED

operating temperatures. Recently, several experiments and theoretical studies have demonstrated that the strong-metal support interaction (SMSIs) can greatly enhance the

PT

electrocatalytic activity of Pt/metal oxide nanohybrids, and the ligand/strain effects can

CC E

promote charge transfer and change the surface chemistry of Pt on the metal oxides [14]. Therefore, Pt nanoparticles have been supported on many metal oxides, such as CoO [15], CeO2 [16], and ZrO2 [17], etc. Among these nanomaterials, Pt-Co systems

A

have attracted much attention due to their good electroactivity. But the electrocatalytic activity usually decreased because of the easy aggregation of these small Pt nanoparticles and their detachment from the metal oxides support during continuous working conditions, which is attributed to the small contacted part of Pt nanoparticles 4

contacted with the metal oxide substrate and the weak physical or electronic interaction between them. As a result, enhancing the stability of Pt/metal oxide nanohybrids is very important for maintaining their high electrocatalytic activity, which is a key factor for constructing electrochemical sensors for sensitive detection of nitrite.

IP T

In this work, a novel and stable Pt nanocluster doped CoO (Pt/CoO) has been successfully designed and synthesized by dealloying AlCoPt ternary alloy in alkaline

SC R

solution. During the dealloying process, Al was rapidly dissolved and the remaining Co atoms were oxidized at the metal/electrolyte interface [18]. Simultaneously, Pt atoms

U

diffused to form Pt nanoclusters doped within the CoO lattice with matched atomic

N

structure, so they are automatically and mutually stabilized. A novel electrochemical

A

sensor was fabricated by modifying the Pt/CoO nanohybrid on a glassy carbon

M

electrode (GCE) and it exhibited a high electrocatalytic oxidation of nitrite with low

ED

detection, wide linear range, high sensitivity, stability and selectivity. The

PT

electrochemical sensor can be also used for the determination in real samples.

CC E

2. Experimental

2.1. Reagents and preparation of solutions NaNO2, NaOH, H3PO4, NaCl, MgCl2, CaCl2, NH4Cl, NaH2PO4, Na2HPO4, KCl and

A

ethanol were all analytical reagents and purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Nafion (5 wt.%) and Pt/C (Pt: 10 wt.%) were purchased from Sigma-Aldrich (St. Louis, MO, USA). All reagents were used without further purification. 0.1 M phosphate buffer with different pH values was prepared by mixing 5

stock solutions of Na2HPO4 and NaH2PO4 and then adjusting it with 0.1 M H3PO4 or NaOH solution. Nafion (0.5 wt.%) was prepared by diluting 100 µL Nafion (5 wt.%) with 900 µL ethanol. Triply distilled water (18.25 MΩ cm) was used throughout all the

IP T

experiments.

2.2. Preparation of Pt/CoO nanohybrid

SC R

Al85Co14Pt1 (at.%) alloy foils were prepared by melting pure Al, Co and Pt (>99.9%) and followed by injection onto a rotating Cu wheel with a tangent speed of 20 m s-1.

U

The Pt/CoO nanohybrid was fabricated by dealloying the Al85Co14Pt1 alloy foils in 1.0

N

M NaOH solution for 24 h at room temperature and followed rinsed with distilled water

M

A

until the flushing water was neutral. Then it was dried in a drying oven for application.

ED

2.3. Characterization of Pt/CoO nanohybrid

X-ray diffraction (XRD) characterization of the Pt/CoO nanohybrid was carried out

PT

on a Bruker D8 advanced X-ray diffractometer with CuKα radiation (λ=1.78897 Å).

CC E

The microstructure of the nanohybrid was characterized using a scanning electron microscopy (SEM, JEOL JSM-6700F) and a high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2100). The chemical composition was analyzed by

A

an energy-dispersive X-ray spectrometer (EDS, Bruker Quantax 200XF lash 6) and Xray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250). The surface area of Pt/CoO nanohybrid was determined via N2 adsorption/desorption by a Quantachrome Autosorb-3B surface analyzer. 6

2.4. Preparation of the modified electrodes A GCE (3 mm in diameter) was first polished with 0.5 and 0.05 µm alumina slurry, and then cleaned sequentially in an ultrasonic cleaner with 1:1 (V/V) nitric acid, acetone

IP T

and triply distilled water for 2 min, respectively. Finally, the GCE was dried in air. The previously dried Pt/CoO nanohybrid was ground into powder in an agate mortar. The

SC R

modifying ink was prepared via ultrasonic treatment of the mixture of 4.0 mg Pt/CoO nanohybrid powder, 4.0 mg carbon powder, 300 μL ethanol and 100 μL Nafion solution

U

(0.5 wt.%) for 30 min to form a uniform suspension solution. An appropriate volume

N

(3 μL) of the ink was dropped on the surface of the pretreated GCE (Pt/CoO/GCE)

A

followed by drying under a beaker in a ventilated environment. For comparison, the

ED

M

Pt/C catalyst modified GCE (Pt/C/GCE) was fabricated in a similar way.

2.5. Pretreatment of real samples and spectrophotometric measurement of nitrite

PT

The sausage and pure milk samples were bought from a local market. The extraction

CC E

of nitrite ions from the sausage sample was performed as follows. The sausage sample was grinded into puree and a portion of it (5.0 g) was mixed with saturated borax solution (12.5 mL). The mixture was boiled for thirty minutes, and 2.50 mL ZnSO4

A

solution (30%) was added and heated at 60 ℃ for ten minutes to precipitate protein. The mixture was then cooled to room temperature and remained for 30 min. After the upper oil layer was removed using degreasing cotton, the remained mixture was centrifuged and the separated supernatant was diluted to 50 mL. According to Chinese National 7

Food Safety standard (GB 5009.33-2010), the milk sample was pretreated with 0.1 M PBS (pH 6.0) before the test and then centrifuged for 15 min at 10000 rpm. The supernatant was used for analysis [19]. The pretreated samples were stored at 4 ℃ for the subsequent measurements.

IP T

For comparison, the nitrite content in real samples was measured using a standard spectrophotometric method [20]. In this method, a reddish-purple azo dye was formed coupling

diazotized

sulfanilamide

with

N-(1-naphthyl)-ethylenediamine

SC R

by

dihydrochloride at a pH value of 2.0-2.5 and was quantified via photometric

U

measurements. A 1-cm cuvette was used and the absorption spectra (400-700 nm) were

N

recorded on a UV-via spectrophotometer (TU-1810). A calibration curve was first

M

ED

determine the content of nitrite.

A

obtained and then the maximum absorption wavelength (546 nm) was selected to

2.6. Electrochemical measurements

PT

All the electrochemical experiments were performed on a CHI 660E

CC E

electrochemical workstation (Chenhua Instrument Co., Ltd., Shanghai, China). A threeelectrode system was used in which the bare GCE, Pt/CoO/GCE and Pt/C/GCE were the working electrodes, Pt wire was the counter electrode, and a saturated calomel electrode

A

(SCE) was the reference electrode. All the electrodes were provided by Tianjin Aidahengsheng Technology Co., Ltd. (Tianjin, China). Cyclic voltammetry and amperometric measurements were carried out under quiescent and hydrodynamic conditions, respectively. The electrochemical experiments 8

were carried out at ca. 25 oC. The supporting electrolyte was 0.1 M phosphate buffer with different pH values. The potential range was +0.2 ~ +1.2 V (vs. SCE) during the cyclic voltammetric measurement for NaNO2.

IP T

3. Results and discussion 3.1. Characterization of Pt/CoO nanohybrid

SC R

Fig. 1 shows the XRD patterns of the precursor AlCoPt alloy (a) and dealloying

product Pt/CoO (b). Before dealloying, many diffraction peaks are observed and can be

U

ascribed to fcc Al and AlxCo phases [21]. During the dealloying process, Al was

N

selectively etched in NaOH solution and Co was oxidized to form oxides due to its

A

active properties. And Pt was left in or on the Co oxides because of its low diffusivity.

M

It can be seen that the previous diffraction peaks disappear, demonstrating the AlCoPt

ED

alloy has been dealloyed completely. It can be also found that some weak and broad diffraction peaks (ca. 37°, 43° and 60°) are formed on the dealloying product, which

PT

are assigned to (111), (200) and (220) planes of CoO. The weak and broad peaks

CC E

indicated the formation of metal oxides in nanoscale. There are no obvious peaks observed from Pt phase, indicating that no large Pt crystals were formed and the small amount of Pt was uniformly distributed in the dealloyed product. In other words, the

A

left Pt in or on the CoO lattice was in the main form of clusters or single atoms [21]. Fig. 2 shows the SEM (A) and HRTEM (B and C) images of the obtained Pt/CoO nanohybrid. It is seen from Fig. 2A that the dealloyed product has relatively big pores and thin ligaments, which were ascribed to the corrosion of large amount of Al and the 9

self-growth of the interconnected CoO nanosheets. Fig. 3 gives the EDS spectrum and the composition analysis of the Pt/CoO nanohybrid. It can be seen that the content of Pt is very low, reducing the cost of material preparation. Moreover, the EDS elemental mapping analysis shows that the nanomaterial is mainly composed by Pt, Co and O

IP T

elements (Fig. S1), whose distribution is quite uniform. HRTEM image (Fig. 2B) shows an obvious contrast between darker Pt nanoclusters and brighter Co oxides due to the

SC R

higher atomic numbers of Pt. Some Pt nanoclusters had been marked using red arrows.

In Fig. 2C, the lattice distance in the brighter area was about 0.22 and 0.15 nm,

U

corresponding to that of the (200) and (220) crystal plane of Co oxides; in the darker

N

area, the lattice distance was measured to be about 0.23 nm, which can be assigned to

A

be (111) plane of Pt. The parallel diffraction pattern indicated the coherent relationship

M

between Pt nanoclusters and Co oxides. The Brunauer-Emmett-Teller (BET) surface

ED

area of the Pt/CoO nanohybrid was determined to be about 40.2 m2 g-1 via N2 adsorption/desorption method.

PT

XPS was used to further characterize the Pt/CoO nanohybrid. In Fig. 4A, the XPS

CC E

spectrum indicates the Pt/CoO nanohybrid contains Pt, Co and O elements. The highresolution XPS spectra of Pt, Co and O are shown in Figs. 4B-C. Two Pt 4f peaks (Fig. 4B) can be assigned to Pt 4f7/2 and Pt 4f5/2 at 71.3 and 74.4 eV, and O 1s peak is observed

A

at 531.3 eV (Fig. 4D). In Fig. 4C, the binding energies of 780.7 eV for Co 2p3/2 and 796.6 eV for Co 2p1/2 with a satellite peak at 786.1 eV demonstrate the formation of CoO [22]. These characterization results illustrated that a novel Pt nanocluster doped CoO nanohybrid with very low Pt content was successfully fabricated via a facile 10

dealloying method.

3.2. Electrochemical behaviors of the modified electrodes 3.2.1. Electrochemical oxidation of nitrite

IP T

Fig. S2 shows the cyclic voltammogram of Pt/CoO/GCE in 0.5 M H2SO4 solution. The well-defined hydrogen adsorption/desorption peaks are observed obviously. By

SC R

measuring the coulombic charge and assuming 210 μC cm-2 for a hydrogen monolayer

adsorption [23], the active surface area of Pt/CoO/GCE was calculated to be 21.7 cm2,

U

which was about 305.6 times the area of the bare GCE (0.071 cm2). The result

N

demonstrated the distinct advantage of Pt/CoO for electrochemical analysis.

A

The electrochemical behaviors of the different electrodes were first investigated via

M

cyclic voltammetric technique using the analysis NaNO2 as the probe. Fig. 5 shows the

ED

cyclic voltammograms of Pt/CoO/GCE (a and d), bare GCE (b) and Pt/C/GCE (c) in 0.1 M phosphate buffer (pH 6.0) without (a) or with (b-d) 10.0 mM NaNO2 at a scan

PT

rate of 100 mV s-1. In the blank phosphate buffer (curve a), only a background cyclic

CC E

voltammetric curve without any peaks can be observed during the detected potential range, indicating a small background current at the Pt/CoO/GCE. When NaNO2 was added, it is seen that nitrite can be oxidized on the three electrodes. But the

A

electrochemical oxidative behaviors of nitrite were much different. The onset potentials of nitrite oxidation were +0.55, +0.63 and +0.71 V (vs. SCE) at the Pt/CoO/GCE, Pt/C/GCE and GCE, respectively, indicating nitrite is more easily oxidized at the novel Pt/CoO nanocomposite modified electrode. It can also be found that the 11

electrooxidative signal of nitrite is much higher and the oxidative peak is sharper at the Pt/CoO/GCE than that at the bare GCE and Pt/C/GCE, since the nitrite ions can diffuse sufficiently into the large numbers of nanopores of Pt/CoO nanohybrid and the electroactive area was equivalently increased. In addition, the high electrocatalytic

IP T

activity of Pt/CoO nanohybrid toward nitrite is mainly due to the strong metal-support-

aggregation of ultra-small Pt nanoparticles [21].

U

3.2.2. Effect of pH on nitrite oxidation at Pt/CoO/GCE

SC R

like interaction between Pt nanoclusters and CoO substrate, which prevents the

N

To explore the electrooxidative mechanism of nitrite and improve the performance

A

of the sensor, the effect of pH value on electrochemical response of 10.0 mM nitrite at

M

the Pt/CoO/GCE was investigated. Fig. 6A shows the cyclic voltammograms of

ED

Pt/CoO/GCE in phosphate buffer containing 10.0 mM NaNO2 with different pH values (from 4.0 to 8.0). With the change of the pH value, the sensor exhibited different cyclic

PT

voltammetric behaviors. Both the oxidative peak current and peak potential are obvious

CC E

influenced by the pH of buffer solution. It can be observed that the maximum oxidative peak current appears at pH 6.0. Additionally, with the increase of the pH value from 4.0 to 7.0, the oxidative peak potential (Epa) shifts positively. A linear relationship between

A

Epa and pH is found and the linear equation is expressed: Epa = 0.0587pH + 0.603 (R2 = 0.9950) (curve a in Fig. 4B). The slope value of 58.7 mV/pH was close to the theoretical value of 59 mV/pH in the Nernst system at 25 ℃, indicating the number of electrons and protons taking part in the electrochemical reaction was the same [24]. For 12

convenient comparison, the plot of peak current density versus the solution pH value is depicted in Fig. 6B (curve b). As is shown, the oxidative peak current increases with the increase of pH until a maximum value is reached at pH 6.0 and the current decreases when the pH value is further enhanced. This phenomenon can be attributed to the

IP T

instability of NO2- in solution with pH < 6.0, where it was easily oxidized to NO3- [25]. The corresponding transformation can be expressed by the following equation (1): (1)

SC R

− + 3NO− 2 + 2H → NO3 + H2 O + 2NO

When the pH > 6.0, it was difficult for nitrite oxidation due to the lack of protons [26].

U

As a result, pH 6.0 was selected for the subsequent nitrite measurements. The

N

electrocatalytic oxidation process of nitrite can be proposed as follows [27]. It can be

A

seen that the electrochemical reaction was a one-electron and one-proton process. (2)

NO2− ↔ NO2 + 𝑒 −

(3)

ED

M

NO2− + H + ↔ HNO2

(4)

PT

− 2NO2 + H2 O ↔ 2H + + NO− 2 +NO3

CC E

3.2.3. Effect of scan rate on nitrite oxidation at Pt/CoO/GCE The kinetics of electrode reaction was explored via the investigation of the effect of

scan rate on nitrite oxidation at Pt/CoO/GCE. As shown in Fig. 7A, with the increase

A

of the scan rate from 50 to 350 mV s-1, a good linearity (R2 = 0.9979) can be obtained between the oxidative peak current and the scan rate (the inset in Fig. 7A), indicating the nitrite oxidation is dominated by surface-controlled process. This result was in accordance with that obtained at nanoporous PdFe alloy/GCE [28] and Pt-graphene 13

modified electrode [24]. Fig. 7B shows the linear relationship between the oxidative peak potential (Ep) and the logarithm of scan rate (logν). For an irreversible process, the electron transfer coefficient (α) and electron transfer rate constant (k) of nitrite

2.303RT (5) logν + K 2(1 − α)nF (1 − α)nFD RT 2.303 ′ K = E0 + [0.78 + log (1 − α)nF 2 k 2 RT Ep =

IP T

oxidation can be evaluated from the following equations (5-6) [20]:

(6)

SC R

Where n is the number of electron transfer in the rate-determining step (n=1), R is the thermodynamic constant (8.314 J mol-1 K-1), T is thermodynamic temperature (298 K),

U

F is Faraday constant (96485 C mol-1), E0’ is standard electrode potential (0.9 V) and D

N

is diffusion coefficient (1.7 × 10-4 cm2 s-1). Therefore, α value was estimated to be 0.85

A

by assuming one-electron in rate-determining step. The k value was calculated to be

M

0.70 cm s-1, which is much higher than that obtained at the Pt/C/GCE (0.21 cm s-1) and

ED

GCE (0.11 cm s-1).

PT

3.3. Amperometric determination of nitrite

CC E

The electrochemical detecting ability of three electrodes for nitrite was evaluated by measuring the response current versus time at respective optimal potentials. According to Fig. 5, the oxidative peak potentials of nitrite at bare GCE, Pt/C/GCE and

A

Pt/CoO/GCE were +1.0 V, +0.89 V and +0.9 V (vs. SCE), which were selected as the detecting potentials. Fig. 8A records the current responses to successive addition of nitrite from 0.2 μM to 23.7 mM at the three electrodes. For clarity, the amplified amperometric curves at Pt/CoO/GCE of 160-500 s and 460-1250 s are shown in Fig. 14

8B and its inset. It is clear that the current responses generated at the Pt/CoO/GCE (c) are obviously higher than those on bare GCE (a) and Pt/C/GCE (b) when the same amount of NaNO2 is added into the stirred phosphate buffer, indicating much higher sensitivity of the Pt/CoO modified electrode. This result was in good agreement with

IP T

that obtained from the cyclic voltammograms. At the Pt/CoO/GCE, a response step is clearly observed when the concentration of added nitrite is as low as 0.2 μM (see the

SC R

inset in Fig. 8B). And after adding nitrite, the response current reached about 96% of the steady-state current value within about 3 s, which was similar to that obtained at

U

Au-Pt bimetallic nanoparticles/N-doped graphene (3.4 s) [29], but lower than those

N

obtained at Pt nanoparticles/reduced graphene oxide sheets (4 s) [24], gold

A

nanoparticles/chitosan@N,S co-doped multiwalled carbon nanotubes (5 s) [30] and

M

graphene/Pt nanocomposite (5 s) [31]. These results suggested that the Pt/CoO/GCE

ED

responded quickly.

Fig. 8C shows the calibration curves of steady-state current obtained from

PT

Pt/CoO/GCE. The sensor detected nitrite with two linear regions of concentration in

CC E

0.2 μM-3.67 mM (R2 = 0.9949) (curve a) and 3.67-23.7 mM (R2 = 0.9936) (curve b) with the sensitivity of 901.4 and 408.5 μA mM-1 cm-2, respectively. A two-stage linear ranges were also found by Rao et al. [30] and Wang et al. [32]. The detection limit was

A

estimated to be 0.067 μM (S/N = 3) at the Pt/CoO/GCE. Table 1 shows the comparison of sensing performances at the Pt/CoO/GCE with other nitrite sensors. It is seen that this Pt/CoO-based nitrite sensor has a relatively wide linear range, high sensitivity and a low detection limit. The improved performance for 15

nitrite detection at the Pt/CoO/GCE should be attributed to its high electrocatalytic activity from the synergistic effect [33, 34] of Pt nanocluster and CoO structure as well as unlimited diffusion of nitrite into the 3D porous Pt/CoO nanohybrid.

IP T

3.4. Selectivity of Pt/CoO/GCE and real sample analysis In the practical applications, it is very necessary to evaluate the selectivity of the

SC R

sensor because some possible coexisting inorganic compounds may interfere the

detection of nitrite. Fig. 9 shows amperometric response of the Pt/CoO/GCE with

U

successive additions of nitrite and the interferential species (NaCl, MgCl2, CaCl2,

N

NH4Cl and NaH2PO4) in a stirred phosphate buffer (0.1 M, pH 6.0) at +0.9 V. In addition,

A

the result of the influences of more species (NaS2O3, NaNO3, NaAc, Na3PO4, Na2SO4,

M

Na2CO3, KCl and ZnCl2) is shown in Fig. S3. It is seen that with the first addition of

ED

0.04 mM nitrite, the sensor responds rapidly. However, the addition of the interferences doesn’t cause obvious current response. With the addition of 0.04 mM nitrite again, the

PT

sensor exhibits a rapid and same amount of current response, indicating its good anti-

CC E

poisoning ability. The result indicated that the Pt/CoO was very favorable to construct a nitrite sensor with good selectivity. For practical applications, the feasibility of the proposed Pt/CoO-based sensor was

A

evaluated by determining nitrite in real samples. The pretreatment of the real samples and a standard spectrophotometric method for comparison were mentioned in the experimental section. The concentration of nitrite in the sausage, tap water and pure milk samples were found to be (2.78 ± 0.14) and (2.64 ± 0.10) μM, 0, (0.42 ± 0.02) and 16

(0.51 ± 0.03) μM via spectrophotometry and the Pt/CoO-based electrochemical method, respectively, showing the good accuracy of the proposed method. The recovery measurement was also carried out using the Pt/CoO/GCE sensor and the results were listed in Table 2. It can be seen that the recovery was found to be 95.7-107.0%,

IP T

indicating the present sensor was feasible to determine nitrite in real samples.

SC R

3.5. Stability and reproducibility of the Pt/CoO/GCE

The sensing stability of the Pt/CoO modified electrode was essential for reliable

U

detection of nitrite, which was evaluated by investigating its successive measurements,

N

steady-state current response stability and long-term stability. For 0.04 mM NaNO2 at

A

the Pt/CoO/GCE, five successive measurements gave an average current of (2.14 ± 0.08)

M

μA (n = 5) with a relative standard deviation (RSD) value of 3.7%. Fig. 10A presents

ED

the amperometric response of the Pt/CoO/GCE towards 0.04 mM NaNO2 in a stirred phosphate buffer for 3000 s. As the addition of NaNO2, the current responded rapidly

PT

and the current maintained 91.2% of the initial signal even after 3000 s. However, the

CC E

response current of nitrite decreased greatly at Pt nanoparticles modified GCE (the inset in Fig. 10A), demonstrating the catalytic durability for nitrite detection of Pt/CoO nanohybrid was much better than that of Pt nanoparticles. The stability of Pt/CoO/GCE

A

was further explored by detecting nitrite every two days for a period of 14 days. As shown in Fig. 10B, the amperometric response of the sensor towards 0.04 mM NaNO2 maintains 91.1% of the initial signal after 14 days. An average current of (2.21 ± 0.10) μA (n = 5) with an RSD value of 4.5% in the response to 0.04 mM NaNO2 was obtained 17

for five different and freshly prepared Pt/CoO/GCEs under the same conditions. These results demonstrated that the Pt/CoO/GCE had good stability and reproducibility for nitrite detection, which can be attributed to the good stability between Pt nanoclusters

IP T

and CoO crystal container.

4. Conclusions

SC R

Pt nanocluster doped nanoporous CoO modified GCE was applied for the

electrochemical sensing of nitrite. The nanohybrid with low Pt content showed

U

excellent electrocatalytic activity for nitrite oxidation and the sensor exhibited wide

N

linear range, high sensitivity, low detection limit, good selectivity and long-term

A

stability. Benefiting from the excellent performances for nitrite sensing and its easy

M

fabrication, this novel nanohybrid is a promising candidate for developing low-cost

PT

Acknowledgements

ED

electrochemical sensors for the determination of nitrite in real samples.

CC E

This work was supported by the Shandong Provincial Natural Science Joint Found with Universities and Scientific Research Institution (ZR2018LB022), A Project of Shandong Province Higher Educational Science and Technology Program (J18KA097)

A

Special Funds for Taishan Scholars Project and Ten Thousand Talent Program of China.

References [1] G. Sriram, M.P. Bhat, P. Patil, U.T. Uthappa, H.Y. Jung, T. Altalhi, T. Kumeria, T.M. Aminabhavi, R.K. Pai, Madhuprasad, Paper-based microfluidic analytical devices for colorimetric detection of toxic 18

ions: a review, TrAC Trends Anal. Chem., 93(2017)212-227. [2] Q.H. Wang, L.J. Yu, L. Yang, L. Lan, R.G. Lu, J.P. Zhu, H. Lan, Z.L. Lu, Methods for the detection and determination of nitrite and nitrate: A review, Talanta, 165(2017)709-720. [3] M. Ghanei-Motlagh, M.A. Taher, A novel electrochemical sensor based on silver/halloysite nanotube/molybdenum disulfide nanocomposite for efficient nitrite sensing, Biosens. Bioelectron.,

IP T

109(2018)279-285. [4] H. Zhang, S. Qi, Y. Dong, X. Chen, Y. Xu, Y. Ma, X. Chen, A sensitive colorimetric method for the

determination of nitrite in water supplies, meat and dairy products using ionic liquid-modified methyl

SC R

red as a colour reagent, Food Chem., 151(2014)429-434.

[5] B. Wang, Z. Wu, W. Qin, Polyamidoamine dendrimer-armed fluorescent magnetic nanoparticles for sensitive and selective determination of nitrite in beverages, Sens. Actuators, B, 247(2017)774-779.

U

[6] J. Wu, X. Wang, Y. Lin, Y. Zheng, J.M. Lin, Peroxynitrous-acid-induced chemiluminescence

N

detection of nitrite based on Microfluidic chip, Talanta, 154(2016)73-79.

A

[7] A. Antczak-Chrobot, P. Bąk, M. Wojtczak, The use of ionic chromatography in determining the contamination of sugar by-products by nitrite and nitrate, Food Chem., 240(2018)648-654.

M

[8] J. Liu, X. Bo, Z. Zhao, L. Guo, Highly exposed Pt nanoparticles supported on porous graphene for

ED

electrochemical detection of hydrogen peroxide in living cells, Biosens. Bioelectron., 74(2015)71-77. [9] Y. Nie, L. Li, Z. Wei, ChemInform abstract: recent advancements in Pt and Pt‐free catalysts for

PT

oxygen reduction reaction, Chem. Soc. Rev., 44(2015)2168-2201. [10] S. Guo, S. Zhang, D. Su, S. Sun, Seed-mediated synthesis of core/shell FePtM/FePt (M = Pd, Au)

CC E

nanowires and their electrocatalysis for oxygen reduction reaction, J. Am. Chem. Soc., 135(2013)1387913884.

[11] M. Ammam, E.B. Easton, Quaternary PtMnCuX/C (X=Fe, Co, Ni, and Sn) and PtMnMoX/C (X=Fe,

A

Co, Ni, Cu and Sn) alloys catalysts: Synthesis, characterization and activity towards ethanol electrooxidation, J. Power Sources, 215(2012)188-198. [12] C. Koenigsmann, A.C. Santulli, K. Gong, M.B. Vukmirovic, W.P. Zhou, E. Sutter, S.S. Wong, R.R. Adzic, Enhanced electrocatalytic performance of processed, ultrathin, supported Pd-Pt core-shell nanowire catalysts for the oxygen reduction reaction, J. Am. Chem. Soc., 133(2011)9783-9795. [13] X.Y. Lang, G.F. Han, B.B. Xiao, L. Gu, Z.Z. Yang, Z. Wen, Y.F. Zhu, M. Zhao, J.C. Li, Q. Jiang, 19

Mesostructured intermetallic compounds of platinum and non ‐ transition metals for enhanced electrocatalysis of oxygen reduction reaction, Adv. Funct. Mater., 25(2015)230-237. [14] S.A. Park, H. Lim, Y.T. Kim, Enhanced oxygen reduction reaction activity due to electronic effects between Ag and Mn3O4 in alkaline media, ACS Catalysis, 5(2015)3995-4002. [15] C. Meng, T. Ling, T.Y. Ma, H. Wang, Z. Hu, Y. Zhou, J. Mao, X.W. Du, M. Jaroniec, S.Z. Qiao,

IP T

Atomically and electronically coupled Pt and CoO hybrid nanocatalysts for enhanced electrocatalytic performance, Adv. Mater., 29(2017)1604607.

[16] G.N. Vayssilov, Y. Lykhach, A. Migani, T. Staudt, G.P. Petrova, N. Tsud, T. Skála, A. Bruix, F. Illas,

SC R

K.C. Prince, Support nanostructure boosts oxygen transfer to catalytically active platinum nanoparticles, Nat. Mater., 10(2011)310-315.

[17] N. Cheng, M.N. Banis, J. Liu, A. Riese, X. Li, R. Li, S. Ye, S. Knights, X. Sun, Extremely stable

N

the oxygen reduction reaction, Adv. Mater., 27(2015)277-281.

U

platinum nanoparticles encapsulated in a zirconia nanocage by area-selective atomic layer deposition for

A

[18] C. Xu, R. Wang, Y. Zhang, Y. Ding, A general corrosion route to nanostructured metal oxides,

M

Nanoscale, 2(2010)906-909.

[19] L. Zhou, J.-P. Wang, L. Gai, D.-J. Li, Y.-B. Li, An amperometric sensor based on ionic liquid and

ED

carbon nanotube modified composite electrode for the determination of nitrite in milk, Sensors Actuators B: Chem., 181(2013)65-70.

PT

[20] M.R. Majidi, S. Ghaderi, Hydrogen bubble dynamic template fabrication of nanoporous Cu film supported by graphene nanaosheets: A highly sensitive sensor for detection of nitrite, Talanta,

CC E

175(2017)21-29.

[21] H.J. Qiu, J.J. Gao, Y.R. Wen, B. Shang, J.Q. Wang, X. Lin, Y. Wang, Platinum cluster/nanoparticle on CoO nanosheets with coupled atomic structure and high electrocatalytic durability, ACS Applied

A

Energy Materials, 1(2018)1840-1845. [22] C. Meng, T. Ling, T.Y. Ma, H. Wang, Z. Hu, Y. Zhou, J. Mao, X.W. Du, M. Jaroniec, S.Z. Qiao, Atomically and electronically coupled Pt and CoO hybrid nanocatalysts for enhanced electrocatalytic performance, Advanced Materials, 29(2017)1604607. [23] H.H. Wang, Z.Y. Zhou, Q. Yuan, N. Tian, S.G. Sun, Pt nanoparticle netlike-assembly as highly durable and highly active electrocatalyst for oxygen reduction reaction, Chem. Commun., 47(2011)340720

3409. [24] B. Yang, B. Duan, H. Wang, M. Zhu, P. Yang, Y. Du, High quality Pt–graphene nanocomposites for efficient electrocatalytic nitrite sensing, Colloids Surf., A, 481(2015)43-50. [25] X. Xing, D.A. Scherson, A. Chem, Electrochemical oxidation of nitrite on a rotating gold disk electrode: a second-order homogeneous disproportionation process, Anal. Chem., 60(1988)1468-1472.

IP T

[26] B.R. Kozub, N.V. Rees, R.G. Compton, Electrochemical determination of nitrite at a bare glassy carbon electrode; why chemically modify electrodes?, Sens. Actuators, B, 143(2010)539-546.

[27] S. Zhang, B.Q. Li, J.B. Zheng, An electrochemical sensor for the sensitive determination of nitrites

SC R

based on Pt–PANI–graphene nanocomposites, Anal. Methods, 7(2015)8366-8372.

[28] J. Wang, H. Zhou, D. Fan, D. Zhao, C. Xu, A glassy carbon electrode modified with nanoporous PdFe alloy for highly sensitive continuous determination of nitrite, Microchim. Acta, 182(2015)1055-

U

1061.

N

[29] Z. Li, Z. An, Y. Guo, K. Zhang, X. Chen, D. Zhang, Z. Xue, X. Zhou, X. Lu, Au-Pt bimetallic

A

nanoparticles supported on functionalized nitrogen-doped graphene for sensitive detection of nitrite, Talanta, 161(2016)713-720.

co-doped

multi-walled

carbon

nanotubes

sensor:

fabrication,

ED

nanoparticle/chitosan@N,S

M

[30] H. Rao, Y. Liu, J. Zhong, Z. Zhang, X. Zhao, X. Liu, Y. Jiang, P. Zou, X. Wang, Y. Wang, Gold

characterization and electrochemical detection of catechol and nitrite, ACS Sustainable Chem. Eng., 5(2017)10926-10939.

PT

[31] K. Vijayaraj, S.H. Jin, D.S. Park, A sensitive and selective nitrite detection in water using

CC E

graphene/patinum nanocomposite, Electroanalysis, 29(2016)345-351. [32] J. Wang, N. Hui, A nanocomposite consisting of flower-like cobalt nanostructures, graphene oxide and polypyrrole for amperometric sensing of nitrite, Microchim. Acta, 184(2017)1-8. [33] L. Lu, Recent advances in synthesis of three-dimensional porous graphene and its applications in

A

construction of electrochemical (bio)sensors for small biomolecules detection, Biosens. Bioelectron., 110(2018)180-192. [34] L. Lu, J. Kang, Amperometric nonenzymatic sensing of glucose at very low working potential by using a nanoporous PdAuNi ternary alloy, Microchim. Acta, 185(2018)111. [35] G. Bharath, R. Madhu, S.M. Chen, V. Veeramani, D. Mangalaraj, N. Ponpandian, Solvent-free 21

mechanochemical synthesis of graphene oxide and Fe3O4–reduced graphene oxide nanocomposites for sensitive detection of nitrite, J. Mater. Chem. A, 3(2015)15529-15539. [36] S.J. Li, G.Y. Zhao, R.X. Zhang, Y.L. Hou, L. Liu, H. Pang, A sensitive and selective nitrite sensor based on a glassy carbon electrode modified with gold nanoparticles and sulfonated graphene, Microchim. Acta, 180(2013)821-827.

IP T

[37] S. Jiao, J. Jin, L. Wang, One-pot preparation of Au-RGO/PDDA nanocomposites and their application for nitrite sensing, Sens. Actuators, B, 208(2015)36-42.

[38] H. Wang, F. Wen, Y. Chen, T. Sun, Y. Meng, Y. Zhang, Electrocatalytic determination of nitrite based

SC R

on straw cellulose/molybdenum sulfide nanocomposite, Biosens. Bioelectron., 85(2016)692-697.

[39] P. Viswanathan, R. Ramaraj, Bi-functional sensing capability of gold multi-pod network nanostructures towards nitrite and guanine, Sens. Actuators, B, 270(2018)56-63.

U

[40] Q. Sheng, D. Liu, J. Zheng, A nonenzymatic electrochemical nitrite sensor based on Pt nanoparticles

N

loaded Ni(OH)2/multi-walled carbon nanotubes nanocomposites, J. Electroanal. Chem., 796(2017)9-16.

A

[41] C. Duan, W. Bai, J. Zheng, Non-enzymatic sensors based on a glassy carbon electrode modified with Au nanoparticles/polyaniline/SnO2 fibrous nanocomposites for nitrite sensing, New J. Chem.,

M

42(2018)11516-11524.

ED

[42] V. Sudha, S.M.S. Kumar, R. Thangamuthu, Simultaneous electrochemical sensing of sulphite and nitrite on acid-functionalized multi-walled carbon nanotubes modified electrodes, J. Alloy. Compd.,

CC E

PT

749(2018)990-999.

Biographies

Lu Lu is a teacher in School of Food Science and Technology, Qilu University of

A

Technology (Shandong Academy of Sciences), China. Her research interests are design and synthesis of nanoporous materials, electrocatalysis and electrochemical (bio)sensors.

22

Figure captions: Fig. 1. XRD patterns of Al85Co14Pt1 before (a) and after (b) completely dealloying in

IP T

0.1 M NaOH solution. Fig. 2. SEM image (A) and HRTEM images (B and C) of the Pt/CoO nanohybrid.

SC R

Fig. 3. EDS spectrum of the Pt/CoO nanohybrid. Inset: the atomic and weight ratio (%) of Pt, Co and O in the nanohybrid.

U

Fig. 4. XPS spectra of scan (A), Co 2p (B), Pt 4f (C), and O 1s (D) for Pt/CoO

N

nanohybrid.

A

Fig. 5. Cyclic voltammograms of Pt/CoO/GCE (a and d), bare GCE (b) and Pt/C/GCE

M

(c) in 0.1 M phosphate buffers (pH 6.0) without (a) or with (b-d) 10.0 mM NaNO2. Scan

ED

rate: 100 mV s-1.

Fig. 6. (A) Cyclic voltammograms of Pt/CoO/GCE in 0.1 M phosphate buffers with

PT

10.0 mM NaNO2 (pH: 4.0, 5.0, 5.6, 6.0, 6.5, 7.0 and 8.0). Scan rate: 100 mV s-1. (B)

CC E

Effect of the buffer pH on the oxidative current density of 10.0 mM NaNO2 at the Pt/CoO/GCE.

Fig. 7. (A) Cyclic voltammograms of Pt/CoO/GCE in 0.1 M phosphate buffer (pH 6.0)

A

containing 10.0 mM NaNO2 at different scan rates (a-j: 50, 100, 150, 200, 250, 300 and 350 mV s-1). Inset: plot of the oxidative peak current density versus scan rate. (B) Plot of the oxidative peak potential versus the logarithm of scan rate. Fig. 8. (A) Amperometric response of the bare GCE (a), Pt/C/GCE (b) and 23

Pt/CoO/GCE (c) to the successive addition of different concentrations of NaNO2 into the stirred phosphate buffer (0.1 M, pH 6.0) at respectively optimal potential (bare GCE: +1.0 V, Pt/C/GCE: +0.89 V and Pt/CoO/GCE: +0.9 V). (B) The amplified curves from 460 to 1250 s and from 160 to 500 s (the inset). (C) The corresponding calibration

IP T

curves obtained from (A) and (B). Inset: amplified calibration curves of NaNO2 with low (a) and high (b) concentrations.

SC R

Fig. 9. Amperometric response of the Pt/CoO/GCE with successive additions of 0.04 mM NaNO2, 1 mM NaCl, 1 mM MgCl2, 1 mM CaCl2, 1 mM NH4Cl, 1 mM NaH2PO4

U

and 0.04 mM NaNO2 in a continuously stirred phosphate buffer (0.1 M, pH 6.0) at +0.9

N

V (vs. SCE).

A

Fig. 10. (A) Amperometric response of the Pt/CoO/GCE with the addition of 0.04 mM

M

NaNO2 for 3000 s in a continuously stirred phosphate buffer (0.1 M, pH 6.0) at +0.9 V

ED

(vs. SCE). Inset: amperometric response of Pt nanoparticles modified GCE obtained under the same condition. (B) Long-term (14 days) stability of the Pt/CoO/GCE for the

A

CC E

PT

detection of 0.04 mM NaNO2 every two days.

24

Figures Fig. 1

Al85Co14Pt1

SC R

a

IP T

Intensity (a.u.)

AlxCo Al

nanoporous Pt/CoO (111)

(220)

(200)

20

30

40

U

b

50

60

70

80

A

N

2-Theta/degree

A

CC E

PT

ED

M

Fig. 2

25

A ED

PT

CC E

IP T

SC R

U

N

A

M

Fig. 3

Fig. 4

26

IP T SC R U N A M

Fig. 5

ED

8

d

4

c

A

CC E

PT

j/(mA cm-2)

6

b

2

a

0 0.2

0.4

0.6

0.8

1.0

1.2

E/V (vs.SCE)

Fig. 6

27

8

A

pH = 4 pH = 5 pH = 5.6 pH = 6 pH = 6.5 pH = 7 pH = 8

j/(mA cm-2)

6 4 2

IP T

0 -2 0.2

0.4

0.6

0.8

1.0

1.2

E/V (vs.SCE)

1.00

b

5

2

Fig. 7

A

9

5.0

0.80 5.5

6.0

6.5

7.0

pH

j

10

2

R = 0.9979

8

-2

j/(mA cm-2)

0.85

j/(mA cm )

A

PT

12

4.5

ED

4.0

M

3

CC E

0.90

R = 0.9950

4

A

0.95

U

a

N

j/(mA cm-2)

6

Epa/V(vs.SCE)

B

SC R

7

6

6

a

0.05 0.10 0.15 0.20 0.25 0.30 0.35

/(V s-1)

3 0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

E/V (vs.SCE)

28

B 2

R = 0.9974 0.95

0.90

0.85 1.6

1.8

2.0

2.2

2.4

j/(mA cm-2)

A

CC E

Fig. 8

PT

ED

M

A

N

U

SC R

log

2.6

IP T

Epa/V(vs.SCE)

1.00

c

A

10

4 mM

8 6

2 mM

4

1 mM 0.5 mM

2 0.05 mM

0 0

500

1000

b a

0.2 mM 0.1 mM

t/s

1500

2000

2500

29

0.15

B

25

0.2 M

j/(mA cm-2)

0.12

20

j/(A cm-2)

0.5 M

20 M

15

0.09 200

300

400

t/s

0.06

10 M 5 M

18

C

3.0 2.5

a

2.0 1.5

2

12

R = 0.9949

1.0 0.5 0.0

1

2

c/mM

6

3

4

b

10

5

10

2

R = 0.9936

6 4 2

4

15

c/mM

8

c/mM

12

20

16

20

24

25

A

CC E

Fig. 9

PT

ED

0

M

a

0

b

8

A

-2

3

1200

N

0

j/(mA cm )

9

-2

j/(mA cm-2)

1000

t/s

j/(mA cm )

15

800

IP T

600

SC R

1 M

U

0.03

2 M

30

NaNO2

0.10

j/(mA cm-2)

0.08 NaNO2 MgCl 2 NH 4 Cl

0.06 CaCl 2 NaH 2PO4

NaCl

0.02

0

100

200

300

400

500

600

SC R

t/s Fig. 10 0.06

A

U N

0.04 NaNO2

A

0.03

0.008 mA cm

0.02 0.01

-2

M

j/(mA cm-2)

0.05 NaNO2

t/s 200

ED

0.00

500

1000

1500

400

2000

600

800

2500

1000

3000

t/s

B

A

CC E

Current percent/%

PT

120

IP T

0.04

100

80 60 40 20

0

2

4

6

8

t/day

10

12

14

31

32

A ED

PT

CC E

IP T

SC R

U

N

A

M

Table 1 Comparison of the sensing performances of Pt/CoO/GCE with other nitrite sensors. Sensing materials

Technique

Linear range/mM

Sensitivity/ μA mM-1 cm-2

Detection limit/μM

Refs.

Fe3O4/RGO

DPV

0.01-2.882

196

0.1

[35]

0.2

[36]

AM

0.01-3.96

45.44

AM

0.5-25.5

-

0.8

DPV

0.05-8.5 b

-

0.04

IP T

AM

0.001-7

759.5

0.2

[30]

AM

0.002-0.425

89.9 a

0.7

[3]

TOSC-MoS2

AM

0.006-3.14, 3.14-4.20

2

[38]

Pt-PANIgraphene

AM

0.0004-0.99 0.99-7.01

485.5 154.3

0.13

[27]

AM

0.0004-0.8

7.4 a

0.4

[39]

AM

ED

145

0.13

[40]

0.00025-2.4

0.08314

0.08

[41]

DPV

0.1-0.7

209.9

0.565

[42]

AM

0.0002-3.67, 3.67-23.7

901.4 408.5

0.067

This work

GC/AuMNN Ss Pt/Ni(OH)2/ MWCNTs/ GCE Au/PANI/Sn O2/GCE

CC E

HOOCMWCNT

AM

Pt/CoO/GCE

0.0004-5.67

N

U

SC R

[28]

A

[37]

μA mM-1. b μM.

A

a

-

M

Nanoporous PdFe alloy Au-RGO/ PDDA AuNPs/CS@ N,S co-doped MWCNTS Ag/HNTs/ MoS2-CPE

PT

Au NPs/SG

a

DPV: different pulse voltammetry. AM: amperometry. RGO: reduced graphene oxide. Au

NPs/SG:

gold

nanoparticles/sulfonated

graphene.

PDDA:

poly(diallydimethylammonium chloride). AuNPs/CS@N,S co-doped MWCNTS: Au nanoparticles deposited on chitosan@N,S co-doped multiwalled carbon nanotubes. 33

HNT: halloysite nanotube. CPE: carbon paste electrode. TOSC: TEMPO oxidized straw cellulose. AuMNNSs: Au multi-pod network nanostructures. PANI: polyaniline. Table 2 Determination of nitrite in sausage, tap water and pure milk samples using Pt/CoObased electrochemical sensor*

0

0.42 ± 0.02

3.05 ± 0.11

3.6

1.00

3.74 ± 0.16

4.3

1.50

4.10 ± 0.13

3.2

0.50

0.53 ± 0.02

3.7

106.0

1.00

1.07 ± 0.03

2.8

107.0

1.50

1.47 ± 0.06

4.1

98.0

0.50

0.88 ± 0.03

3.4

95.7

1.00

1.47 ± 0.05

3.4

103.5

1.85 ± 0.05

2.7

96.4

A

97.1

IP T

0.50

1.50

102.7 99.0

Each datum was an average of five replicate determinations

A

CC E

PT

ED

M

*

Recovery/%

SC R

2.64 ± 0.10

Tap water

Pure milk

Added/μM Total found/μM RSD/%

U

Sausage

Value found in sample/μM

N

Sample

34