MgNi2O3 nanoparticles as novel and versatile sensing material for non-enzymatic electrochemical sensing of glucose and conductometric determination of acetone

Journal Pre-proof MgNi2O3 nanoparticles as novel and versatile sensing material for non-enzymatic electrochemical sensing of glucose and conductometric determination of acetone N. Lavanya, S.G. Leonardi, S. Marini, C. Espro, M. Kanagaraj, S. Lokeswara Reddy, C. Sekar, G. Neri PII:

S0925-8388(19)34033-2

DOI:

https://doi.org/10.1016/j.jallcom.2019.152787

Reference:

JALCOM 152787

To appear in:

Journal of Alloys and Compounds

Received Date: 21 July 2019 Revised Date:

11 October 2019

Accepted Date: 23 October 2019

Please cite this article as: N. Lavanya, S.G. Leonardi, S. Marini, C. Espro, M. Kanagaraj, S.L. Reddy, C. Sekar, G. Neri, MgNi2O3 nanoparticles as novel and versatile sensing material for non-enzymatic electrochemical sensing of glucose and conductometric determination of acetone, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.152787. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier B.V.

MgNi2O3 nanoparticles as novel and versatile sensing material for non-enzymatic electrochemical sensing of glucose and conductometric determination of acetone N. Lavanya1,2, S. G. Leonardi2, S. Marini2, C. Espro2, M. Kanagaraj1, S. Lokeswara Reddy1, C. Sekar1*, G. Neri2* 1

Department of Bioelectronics and Biosensors, Alagappa University, Karaikudi-630 004, Tamilnadu, India 2 Department of Engineering, University of Messina, Messina- 98122, Italy

Abstract In the present work, we report the synthesis of pure MgNi2O3 and MgNi2-xAxO3 (A = Co or Zn; x = 2.5, 10, 25 wt.%) nanoparticles by chemical precipitation method and its novel applications in the field of chemical sensors. Powder XRD, SEM, HRTEM and XPS characterization of the synthesized samples indicate formation of nearly phase pure undoped MgNi2O3 nanoparticles. At lower concentration (2.5 and 5 wt%), both Co and Zn loading yielded nearly phase pure Mg-123 samples. On the other hand, increase in dopant concentration, led to the segregation of small quantity of impurity phases Co3O4 and ZnO with the major phase being Mg-123. Results of electrical and electrochemical characterization studies indicated the influence of the dopants on the physical and chemical properties of MgNi2O3. Both the pure and doped MgNi2O3 nanoparticles have been applied for the fabrication of electrochemical and conductometric sensors for monitoring blood glucose and breath acetone respectively. Electrochemical tests indicated that the CoMgNi2O3 is highly effective for the electrochemical detection of glucose. Under optimal conditions, glucose was monitored at concentration from 0.05 mM to 5 mM with sensitivity of 528.6 µA mM-1cm-2. With regard to breath acetone detection, the Zn-MgNi2O3 conductometric sensor showed the best performances with high response and low detection limit (0.5 ppm at SN = 3), fast response/recovery times of about 25 s and 250 s, respectively, at the operating temperature of 200°C. The good electrochemical and gas sensing performances suggest that MgNi2O3 based nanomaterials can be considered as versatile and promising candidates for developing chemical sensors helpful in the management of acute complications of insulin dependent diabetes, such as hypoglycemia and ketoacidosis.

Keywords:

MgNi2O3nanoparticles, Co and Zn doping, non-enzymatic glucose sensor,

breath acetone sensor. *Corr. authors: C. Sekar, E-mail: [email protected]; Tel: +91 4565 223360 Giovanni Neri, E-mail: [email protected], Tel: +39 347 727 8520

1

1. Introduction The development of sensors for the management of diabetes, with the ultimate goal of achieving the long-term continuous glucose monitoring and prevention of some acute complications, e.g. hypoglycemia and ketoacidosis, is an important research issue today, with obvious social implications. Diabetes is a chronic metabolic disorder resulting from insulin deficiency and is a major health problem worldwide [1]. Frequent testing and accurate determination of blood glucose levels is essential for the diagnosis, effective management and treatment of this disease [2]. The management of long-term complications of diabetes include frequent analysis (many times in a day) for monitoring and controlling high blood glucose levels, which are actually performed by means of disposable enzymatic electrochemical sensors. A number of these sensors, which have also exploited commercially, are available for the detection of blood glucose but, despite their advantages, there are several drawbacks, such as the high cost and intrinsically poor stability which are not compatible for proposing them for continuous glucose monitoring. Indeed, some acute diabetes complications, e.g. hypoglycemia and ketoacidosis, need almost real-time diagnosis and intervention. Hypoglycemia is a critical condition which is reached when blood glucose level falls below 3.3 mM. Impairment of action and judgment usually becomes obvious below 2.2 mM, while blood glucose level below 0.55 mM can lead to coma [3]. It is the most common complication of insulin therapy. On the other hand, hyperglycemic episodes can lead to a serious condition called ketoacidosis, due to accumulation in the blood of ketone bodies produced through the breakdown of fatty acids in the liver, leading to alterations in blood pH and electrolyte composition, which are both hazardous conditions for diabetic patients [4]. Enzymatic decarboxylation of ketones, leads to formation of acetone which is able to cross the membrane barrier into the alveoli of the lung and the airway and it is usually found in breath at abnormal values [5]. Acetone concentration ranging from 1.7 ppm to 3.7 ppm could be detected in breath for those who are diabetics, while the breath of healthy human typically contains less than 0.8 ppm [6-7]. Higher level of acetone can be found during ketoacidosis in patients with insulin dependent diabetes [4,8]. As hypoglycemic and ketoacidosis symptoms can occur without much warning and/or not be properly evaluated by diabetic patients, it appears then evident that the development of simple sensors for blood glucose and breath acetone monitoring is useful for monitoring these acute complications. Recent researches on the development of electrochemical glucose sensors focused the attention on improving their lifetimes, e.g. using enzyme-free electrochemical sensors based on metal oxide nanomaterials [9-12]. Up to date, 2

also various gas sensors have been successfully developed to detect sub-ppm acetone in the breath using metal oxide nanomaterials [13-15]. Although, these sensors had achieved the purpose to detect breath acetone, the high operating temperature, higher energy cost, selectivity and the long response time still limit their further applications in practice. Great effort has been made to improve the sensing properties by choosing new materials and through material modification [16-17]. On the basis of above considerations, the search for new chemical sensors to be applied in this area is of utmost interest from both scientific and social point of view. In this paper, we report an investigation about the application of MgNi2O3 as scaffold material for sensing applications. The MgNi2O3 comopund, a relatively less known material with interesting structural and electrical properties [18], has been synthesized by sol gel method and chemically doped with Co and Zn separately (2.5, 10, 25 wt % each) at Ni site. The effect of the dopants on the structure, morphology, electrical and electrochemical properties have been investigated. Well-characterised products have been used for electrochemical detection of glucose and conductometric sensing of acetone gas for the first time. Results clearly demonstrate that the Co-doped MgNi2O3 NPs have excellent electrochemical activity towards glucose oxidation, due to higher electrocatalytic activity of Ni hydroxylated species in combination with Co ions. Zn-doped MgNi2O3 NPs were found instead to possess the highest sensitivity at low concentration (ppm or lower) for the detection of acetone. Actually, the mechanism of acetone detection in the exhaled breath and glucose detection in blood are very different in terms of basic chemistry and active sites, and, hence the fabricated devices are very different in their functioning. To the best of our knowledge, a study reporting the results for both conductometric and electrochemical sensors with the same material as sensing element has never been reported so far. Here, nanosized MgNi2O3 has been used for the first time for this dual application. From the practical point of view, the developed sensors shall be very useful complimentary devices for the management of some acute complications of diabetes.

2. Experimental 2.1 Materials synthesis and characterization Pure MgNi2O3 nanoparticles were synthesized by sol-gel method by mixing the proper quantity of Mg and Ni nitrate solution in the presence of citric acid (C6H8O7) as reducing agent and the resulting mixture was stirred for 30 min. Then, 10 ml of high-purity polyethylene glycol (PEG) was added as surfactant into the above mixture and stirred again 3

for another 30 min. The product was transferred to a glass beaker and heated at 80ºC for 30 min on a hot plate. A green-coloured gel product was obtained at the bottom of a glass beaker. The obtained dense gel was carefully filtered and gently washed with hot distilled water and acetone for removing unwanted components in the final product. Then, the obtained product was heated at 150ºC for 30 min for removing the unwanted organic product from the mixture. Finally, the product was annealed at 600ºC for 3 h to obtain high purity MgNi2O3 (MNO). MgNi2-xAxO3 (A = Co or Zn; x = 2.5, 10, and 25 wt. %) nanoparticles were synthesized by the same procedure used for preparing undoped MgNi2O3 except that the appropriate amount of metal ions, i.e. Co(NO3)2, and Zn(CH3CO2)2 were added to the starting solution. The morphology of pure and Co and Zn-doped MgNi2O3 samples was investigated by scanning Electron microscope (FE-SEM Zeiss, model 1540XB) and high resolutiontransmission electron microscopy (HR-TEM, JEOL JEM 2100). X-ray photoelectron spectroscopic analysis was carried out using ESCALAB 250XI Base system with UPS and XPS image mapping Containing XR6 Micro-focused Monochromatic (Al Kα XPS) XR4 Twin Anode Mg/Al (300/400W) X-ray source. Powder XRD analysis was performed to investigate the phase formation and crystal structure using a Bruker D8 Advance Diffractometer (CuKα1 = 1.5405 Å).Electrical characterization was made by using a multimeter data acquisition unit (Agilent 34970A).

2.2 Electrochemical glucose sensing For the preparation of the modified electrochemical sensor, 5 mg of each pure and Co-, Zn-doped MgNi2O3 powders were dispersed in 1 ml of demineralised water, then sonicated until homogenous black suspensions were obtained. Commercial screen printed carbon electrodes (SPCEs), purchased from DropSens were used for the sensors preparation. They consist of a planar substrate equipped with a 4 mm in diameter carbon working electrode, a carbon counter electrode and a silver pseudo-reference electrode. The working electrode was modified by wet impregnation method, casting 5 µl of above suspension on its surface, and successive drying at room temperature. Electrochemical experiments were performed with a DropSens µStat 400 potentiostat/galvanostat. Cyclic voltammetry (CV) measurements were performked in 0.1 M KOH solution over the potential range of 0 to 0.8 V, in presence and absence of glucose and at a scan rate of 50 mVs-1. The sensing performances were further investigated by chronoamperometric measurements in 0.1 M KOH, recording the current at fixed optimal potential (0.6 V) during successive additions of glucose. 4

2.3 Acetone gas sensing Devices for gas sensing tests were fabricated by printing films (about 20 µm thick) of the pure and doped MgNi2O3 powders dispersed in demineralised water in order to form a slurry, on alumina substrates (6×3 mm2) equipped with Pt interdigitated electrodes and a Pt heater located on the backside. The investigation of sensing properties was carried out by introducing the sensors in a stainless steel test chamber where the atmosphere can be controlled. Electrical and sensing measurements were carried out in the range from room temperature (RT) to 400ºC, under a dry synthetic air (20% O2, 80% N2) which constantly flowed through the test chamber. Acetone, or other gases, came from certified bottles could be further diluted at a given concentration by mass flow controllers. A multimeter data acquisition unit Agilent 34970A was used for acquiring the sensor signal, while a power supplier Agilent E3632A was employed to bias the built-in heater of the sensor to perform measurements at high temperatures. The sensor response, S, was defined as S = R/R0 where R0 is the baseline resistance in air and R is the electrical resistance of the sensor in the presence of acetone or other gases tested.

3. Results and Discussion 3.1. Structural, morphological and electrical characterization Powder X-ray diffraction data of both Co and Zn doped Mg-123 is shown in Figure 1. The major diffraction peaks for all the investigated samples correspond with those of standard XRD patterns of the cubic structure (Fm3m) of MgNi2O3, in agreement with the reported values from the standard JCPDS data (Card No. 24- 0712 and 34-04101). Close observations indicate that the small amount of Co and Zn (2.5 wt.%) got diluted completely into the Mg-123 system resulting in single phase sample. In case of higher concentration (10 wt.%), minor impurity peaks have appeared and the peak intensity increased when the Co loading was raised up to 25 wt.%. Almost similar trend was observed for Zn-doping also, except that the impurity peaks corresponding to unreacted ZnO is strong indicating lesser loading of Zn when compared to Co. These difference in loading level into Mg-123 system could be attributed to the difference in ionic radii of Co2+ (70 pm) and Zn2+ (74 pm) with respect to that of the host ion Ni2+ (70 pm). The doping also affect the crystallite size of the samples. Calculations, in accordance with the Scherrer’s equation, give values of 25, 18 and 13 nm for the pure Mg-123, 25wt. % Zn and 25 wt.% Co-doped Mg-123 respectively. Further modification in the synthesis procedure (doubling the crystallization period) yielded 5

better loading of Co into Mg-123 system as confirmed by the XRD pattern shown in Fig.1. It should be noted that the similar approach to the Zn-doping did not yield any improvement in the XRD confirming solubility limit of Zn due its larger size when compared to that of Co2+ ion. (Here in Fig. 1) The morphology of pure and Co and Zn-doped MgNi2O3 samples was investigated by

(B)

SEM (Figure 2). Almost spherical shaped particles with different sizes have been observed for both the pure and doped MgNi2O3. Chemically doped samples are smaller in size when compared to that of the undoped MgNi2O3. It is noteworthy that the size of such particles is larger than that evaluated by XRD analysis, suggesting that these samples are constituted by agglomeration of smaller crystalline grains. (Here in Fig. 2 and 3) High resolution transmission electron microscope (HRTEM) was used to investigate the morphology and structural features of undoped Mg-123 nanoparticles. A typical TEM picture [Fig. 3A] showed nearly distorted cuboid shaped particles agglomerated in different sizes. HRTEM picture [Fig. 3B] exhibits crystalline structure with dent-like defect on its surface, which could have occurred due to voids formation and its removal during post annealing process. These defects act as trapping centres for chemical and biosensing applications. Energy dispersive X-ray spectroscopy (EDS) performed on undoped Mg-123 confirmed the presence of Mg, Ni and O elements in stoichiometric ratio as shown in Fig. 3C.

3.2 X-ray photelectron spectroscopy (XPS ) study In order to confirm the presence of elements and to examine its chemical states in the obtained nanoparticles, XPS study has been carried out. Figure 4A shows the core level XPS spectra of the pure and Co loaded MgNi2O3 NPs. Photoelectron spectra of Mg (1s) shows a strong peak at 1003.3 - 1003.6 eV confirming the presence of Mg in the Mg-123 sample [Figure 4 B]. In case of XPS spectra of Ni, a sharp Ni2p peak appeared at 853.8 eV, and its satellite peak [Figure 4C] appeared with binding energy of 1.87 eV above the main line confirming the oxidation state of Ni is 2+. Co 2p spectra of Co-MgNi2O3 [Figure 4D] exhibit four Co2p peaks, 2p3/2, 2p1/2 doublet and their two satellite peaks at higher binding energies. The energy difference in binding energy between Co2p3/2 (795.2 eV) and Co 6

2p1/2 (779.6 eV) is 15.4 eV, which suggests that the Co is in 2+ oxidation state. The weak satellites peaks were observed at around 789.0 eV and 803.5 eV and the occurrence of such satellite peaks in the heavily doped Co-MgNi2O3 sample is evidence for presence of Co3O4 on the surface [19]. The high-resolution O 1s spectra revealed a strong peak at 529.15 eV followed by two weak satellite peaks at 530.4 eV and 533.25 eV corresponding to the O2ions in the Mg-123 nanoparticles [Figure 4E]. (Here in Fig. 4) The XPS and XRD investigations concurrently confirm that the Co2+ ions have been incorporated into MgNi2O3 lattice by substituting Ni2+ions. Both the pure and Co-doped samples do not contain any carbon in the composition Co-MgNi2O3 system. However, the XPS shows [Figure 4F] the carbon peak (1s) at 284.8 eV which might have originated from the carbonaceous film deposited on the samples when exposed to atmosphere. Similar experiments have been conducted on the Zn-doped Mg-123 samples and the presence of all the elements and their oxidation states have been confirmed (results not presented).

3.3 Electrical properties Electrical properties are important for the applications in electrochemical and conductometric sensors, so we have evaluated preliminary electrical characteristics of the synthesized materials. Figure 5 presents the resistance of the pure and doped MgNi2O3 NPs in dry air, as a function of temperature, in the range between 200 and 400°C.The resistance increases with decreasing temperature for pure and doped samples, according to semiconductor behaviour. The resistance values of doped samples are lower than that of pure MgNi2O3. (Here in Fig. 5) Based on the characterization data, we have chosen 25 wt % Co and Zn doped MgNi2O3 for electrochemical and gas sensing applications and the results are presented in comparison with that of undoped MgNi2O3.

3.4 Electrochemical sensing 3.4.1. Cyclic voltammetry studies of modified SPCE electrodes Electrochemical properties of pure MgNi2O3 (MNO), 25 wt % Co-doped MgNi2O3 (MNCoO) and 25 wt% Zn-doped MgNi2O3 (MNZnO) samples have been evaluated by 7

preparing the modified SPCE electrodes, as described in the experimental part, and performing cyclic voltammetry measurements in 0.1 M KOH solution in the potential window 0-0.8 V at 50 mVs-1, until a stable cycle was obtained. As an example, Figure 6 shows the first 50 cyclic voltammograms recorded during the electrochemical activation of MNO-modified electrode. During the first cycle, no relevant electrochemical process was observed, except a strong increase of the anodic current with onset potential at about 0.5 V, which has been attributed to the oxygen evolution reaction. During the first ten cycles, the appearance of an oxidation peak was observed, which, after the 10th cycle, remained located at around 0.55 V and increased until to be almost sable after about fifth cycles. Occurrence of broad reduction peak at about 0.44 V with the increase in peak intensity with the increase in number of cycles, complete the redox couple. This couple of peaks is ascribed to the growth of nickel oxide-hydroxide species during the cycling treatment under the alkaline conditions used [20-21]. (Here in Fig. 6) Therefore, we suppose that, on the pure MNO sample, a hydroxide layer MN-(OH)2 is instantaneously first formed which is further oxidized to hydroxylated MNO-OH species under the applied potential [22-23].The same finding was observed for both the MNCoO and MNZnO modified electrodes. 3.4.2 Glucose sensing: voltammetric tests Electrochemical activity of glucose with the modified electrodes was investigated using the voltammetric method (Figure 7). Modified MNO-, MNCoO- and MNZnO/SPCE, showed an increase of electrochemical activity in presence of 1 mM of glucose in 0.1 M KOH solution. Figure 7a shows the CV curves recorded for MNO/SPCE. Figure 7b reports the variation of anodic peak currents recorded in presence of 1 mM glucose, indicating the MNCoO modified SPCE sensor exhibit the best performances. The increase of the anodic peak is due to the oxidation of glucose, mediated by the MNO-OH species, and leading to formation of glucolactone, according to the eq. (1): MNO-OH + glucose

MN-(OH)2 + glucolactone + e-

(1)

The effect of glucose concentration has also been investigated. Figure 7c shows the CV curves for the MNCoO modified electrode recorded for different concentrations of glucose between 0 to 5 mM. The broad oxidation peak at 0.6 V, with onset potential at 0.4 V, increases with increasing of glucose concentration. During forward scan from 0 to 0.8 V, the 8

anodic peak current increases with a slight shift to higher potential with the increasing glucose concentration. The reason of peak potential shift can be ascribed to a prolonged oxidation cycle with accumulation of intermediate products on the electrode surface and/or a gradual change in local pH of the electrode surface [24].A cathodic peak is also observed during the reverse sweep of potential. This process is likely due to further glucose oxidation on the regenerated MNO-OH sites after first glucose oxidation process [25]. Unlike anodic peak, cathodic peak decreased with increase of glucose concentration as effect of the consumption of MNO-OH species induced by the oxidation of glucose [26]. Calibration curves for MNO, MNCoO and MNZnO sensors are shown in Figure 7d. As a confirmation of previous assessment, the MNCoO modified electrode showed the highest performances, displaying an almost linear current-glucose concentration relationship in all the hypoglycemic to normal glycemic range.

(Here in Fig. 7) Cyclic voltammetry at different scan rates (v), ranging from 10 to 100 mVs-1 in blank 0.1 M KOH and in presence of 1 mM glucose, was performed with the aim to understand the nature of mass transfer phenomena at MNO, MNCoO and MNZnO modified SPCEs. As shown in Figures 8a-b and in related insets, both oxidation and reduction intensity peaks increased linearly with square root of scan rate. This observation reveals that the MNO-OH formation and its reduction as well as the oxidation of glucose rate-determining are diffusioncontrolled processes [27], implying that the adsorption and reaction steps of glucose and its intermediate species are faster than their diffusion to/or away from the electrode surface. (Here in Fig. 8) 3.4.3 Glucose sensing: amperometric tests The amperometric response for each electrode to different concentrations of glucose in 0.1 MKOH was evaluated at an applied potential of 0.6 V. At this potential, all the sensors showed an optimal compromise between high sensitivity, low noise and background current. Figure 9A shows the chrono amperometric response recorded during successive additions of glucose from 20 µM to 3.2 mM for MNCoO sensor, where a fast increase of current was observed when glucose was added into the solution. The related calibration curve is shown in the inset. There is a linear relation between current responses and glucose concentration in the range of 0.05 mM to 1.5 mM with a sensitivity value of 528.6 µA mM-1 cm-2, while 9

saturation occurred for concentration higher than 3 mM. The limit of detection of the sensor was calculated to be 20 µM using calibration curve (Figure 9 B) and considering the formula 3Sy/m, where Sy is standard error of y-intercept and m is the slope of calibration curve. The results are compared with the reported literatures [Table 1] for the determination of glucose using various modified electrodes [28-35] which clearly suggests the superiority of the Co loaded Mg-123 electode for glucose sensing application. (Here in Fig. 9) 3.4.3. Selectivity tests Some other electro-active species such as ascorbic acid (AC), uric acid (UA), dopamine (DA), and many electrolytes (e. g. Mg2+, and Ca2+) co-exist with glucose in blood, so they can potentially interfere with electrochemical glucose measurement. Figure 9C shows the results of amperometric response of MNCoO/SPCE based sensor to additions of 1 mM glucose and 0.1 mM of other interfering analytes in 0.1 M KOH at an applied potential of 0.6 V. This demonstrates that the determination of glucose by the reported sensor is not relevantly affected by these electro-active molecules, therefore it is suitable for monitoring glucose in blood samples.

3.5 Gas sensing 3.5.1 Sensor response to acetone To evaluate the response to target gas and to find the best operating conditions, the fabricated conductometric sensors based on pure and 25 wt.% Co, Zn doped MgNi2O3 nanoparticles were first exposed to 30 ppm acetone over an operating temperature range of 200-400ºC (Figure 10a).All the sensors show increasing responses to acetone with the decreasing of the temperature. (Here in Fig. 10) As clearly seen, the response of the Zn-MgNi2O3 sensor is much higher than that of the pure and Co-MgNi2O3 sensors, indicating that the Zn doping significantly improve the sensitivity of MgNi2O3 sensor to the acetone gas. It can be supposed that Zn doping promote surface defects formation with creation of more active sites which consequently enhances the gas sensing performances. It is worth noting that in Zn-MgNi2O3 sample the separate unreacted ZnO particles could represent a catalytic phase which favours the interaction with acetone contributing to the enhancement of the sensing performance. In addition, it is well known that ZnO generally behaves as n-type semiconductor [36]. The sensing results above 10

reported demonstrate instead that MgNi2O3 has a p-type behaviour. Therefore, on the ZnMgNi2O3 sample it is plausible to assume the formation of p-n heterojunctions at the interface between MgNi2O3 and ZnO nanoparticles. In these cases, the electronic conductivity is modified and the obtained composite show novel characteristics with respect to the base material. As a result, in our case, the response of the Zn-MgNi2O3 sensor for acetone was much higher than that of pure MgNi2O3 based sensor. The sensing mechanism occurring during acetone detection on MgNi2O3 sensor, can be described as follows; The change in resistance of metal oxide gas sensor is primarily caused by the adsorption and desorption of gas molecules on the surface of the sensing films (see reaction (1) to (3)). In air ambient, the oxygen ions adsorbed on the surface of the MgNi2O3 can form a depletion layer, withdrawing electrons from the bulk, which leads to an increase in the electrical resistance. Once exposed to reducing gas (acetone, CH3COCH3), oxygen adsorbates will react with the gas and enable the electrons injected back into the active material, causing an increase in electronic conductivity. The chemical mechanism involved can be described by the following reactions: O2 (gas) → O2 adsorbed

(1)

O2 adsorbed + e- ↔ 2O-

(2)

CH3COCH3 + 8O- ↔ 3CO2 + 3H2O + 8e-

(3)

The high response and short response/recovery times of Zn doped MgNi2O3 NPs are mainly based on large surface-to volume ratio, which is a vital factor for high sensing performance. In addition, the quantity of oxygen vacancies of Zn-MgNi2O3 NPs is also increased due to the substitution of Zn2+. Thus, owing to more oxygen species adsorbed on surface of MgNi2O3 and more surface oxygen vacancies in the Zn doped MgNi2O3 NPs which yielded a greater response. Figure 10b shows the dynamic curve acetone gas-sensing characteristic of the developed sensor based on Zn-MgNi2O3 exposed to 1-40 ppm of acetone gas at the optimal operating temperature of 200°C. The response of the fabricated sensor increases quickly upon the injection of acetone, and then decreases rapidly and returns to its initial value after the test gas is released from the chamber, indicating that the rapid and reversible response and recovery behaviours of the sensor. The response time is defined as the time required to reach 90% of the saturation value after a test gas is introduced, and the recovery time is the time necessary for the sensor to return to 10% above the origin value in air after releasing the test

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gas. The response time is about 25 s and the recovery time is 250 s for 40 ppm acetone gas at 200°C. It can be seen from Figure 10 c,d that the response of the sensor increases with the increase of acetone gas concentration. This is due to the fact that more acetone molecules participate in the surface sensing reaction, which leads to the enhancement of the sensing signal. The response of the Zn loaded MgNi2O3 sensor exhibits high sensitivity to acetone concentration between 1 and 40 ppm. The detection limit, evaluated at S/N = 3, as extrapolated by Figure 10 d, is around 300 ppb, which indicates that Zn-MgNi2O3 NPs can be used as promising material for monitoring breath acetone.

3.5.2 Selectivity, reproducibility and stability In order to investigate the stability of sensor, the sensing response of the sensor was measured in the presence of 10 ppm acetone gas at its optimal working temperature of 200°C (Figure 11a). The results clearly indicate that the response remains almost constant with slight fluctuations during the test, indicating a good stability of the 25wt.% Zn loaded MgNi2O3 based sensors and excellent reproducibility. (Here in Fig. 11) In order to further validate the performance of the Zn-MgNi2O3 based sensor, the selectivity towards different gases was investigated at the operating temperature of 200ºC. Figure 11b shows the sensor responses to test gases at various concentrations. It is clear that the sensor response to acetone is much higher than that to NO2, H2, CO, CO2, NO and NH3 implying that Zn loaded MgNi2O3 NPs based sensor has good selectivity to acetone. Further, Table 2 shows the comparison of the performance of previously developed sensors for breath acetone based on different metal oxides [37-45]. It is well known that methanol and ethanol could act as interferents for the conductometric sensing of acetone. However, methanol (a toxic substance for humans!) is not present in the breath, whereas ethanol is present in appreciable amount only in peoples drinking alcoholic beverages. Therefore, under normal physiological conditions, they are absent in the breath of humans and then does not alter acetone signal. In view of the overall results, it can be concluded that the sensor based on the as-prepared Zn loaded MgNi2O3 possesses superior acetone-sensing performance when compared to pure and Co loaded MgNi2O3.

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4. Conclusions Novel sensors have been developed using MgNi2O3nanoparticles for detection of breath acetone and blood glucose. The effect of dopants (Zn and Co) on electrical and electrochemical properties of the pure and doped-MgNi2O3 nanoparticles have been investigated. Among these, 25 wt.% Zn loaded MgNi2O3 based chemo-resistive sensor showed a remarkable response for breath acetone detection and 25 wt % Co loaded MgNi2O3 based sensor exhibited a maximum electrocatalytic effect towards the detection of blood glucose. In particular, Zn-MgNi2O3 based acetone sensor exhibited good sensor response for acetone with high stability and selectivity as well as comparatively rapid response-recovery times, which could be attributed to large surface-to-volume ratio and the possible formation of p-n heterojunctions at the interface between MgNi2O3 and ZnO nanoparticles. On the other hand, Co-MgNi2O3 modified electrode showed the best electrochemical performance towards glucose detection in alkaline media, likely due to the synergic effect of the electrocatalytic activity of Co ions in combination with Ni hydroxylated species. In summary, this work demonstrated the excellent and versatile chemical sensing performances of new electrodes based on magnesium-nickel oxide, whose electrical and electrochemical properties can be effectively tuned by proper doping with different ions, for developing high performance biomedical sensors. These preliminary results will be exploited in the monitoring of breath acetone and blood glucose in human subjects to test their practical applicability.

Acknowledgement Authors

NL

and

CS

acknowledge

the

DST

(SERB–SB/S2/LOP-027/2013;

SR/PURSE Phase 2/38 (G) dated:21.02.2017) and MHRD-RUSA 2.0 (No. F.24-51/2014- U, Policy (TNMulti-Gen), dated .09.10.2018) for the financial assistance. NL thanks Council for Scientific Industry and Research for Research Associateship (CSIR-RA).

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34. R. Ponnusamy, B. Chakraborty, C. S. Rout, Pd-Doped WO3 Nanostructures as Potential Glucose Sensor with Insight from Electronic Structure Simulations, J. Phys. Chem. B2018122102737-2746 35. Y. Zhou, X. Ni, Z. Ren, J. Ma, J. Xub, X. Chen, A flower-like NiO–SnO2 nanocomposite and its non-enzymatic catalysis of glucose, RSC Adv., 2017, 7, 45177 36. Miller, R. Derek, A. Akbar Sheikh, A. Morris Patricia, Nanoscale metal oxidebased heterojunctions for gas sensing: A review, Sens. Actuators B, 204 (2014) 250272.38. 37. F. Gu, H. Chen, D. Han, Z. Wang, Metal–organic framework derived Au@ZnO yolk-shell nanostructures and their highly sensitive detection of acetone. RSC Adv. 6 (2016) 29727–29733. 38. M.S. Khandekara, N.L. Tarwal, I.S. Mulla, S.S. Suryavanshi, Nanocrystalline Ce doped CoFe2O4 as an acetone gas sensor. Ceram. Int. 40 (2014) 447–452. 39. H. Zhang, H. Qin, C. Gao, G. Zhou, Y. Chen, J. Hu, UV light illumination can improve the sensing properties of LaFeO3 to acetone vapor. Sensors 18, (2018) 1990. 40. M. Karmaoui, S.G. Leonardi, M. Latino, D.M. Tobaldi, N. Donato, R.C. Pullar, M.P. Seabra, J.A. Labrincha, G. Neri, G. Pt-decorated In2O3 nanoparticles and their ability as a highly sensitive (<10 ppb) acetone sensor for biomedical applications. Sens. Actuators B Chem. 230 (2016), 697–705. 41. G. Geng, S. Ge, X. He, S. Zhang, J. Gu, X. Lai, H. Wang, Q. Zhang, Q. Volatile organic compound gas-sensing properties of bimodal porous alpha-Fe2O3 with ultrahigh sensitivity and fast response. ACS Appl. Mater. Interfaces 10 (2018) 13702–13711. 42. S.M. Li, L.X. Zhang, M.Y. Zhu, G.J. Ji, L.X. Zhao, J. Yin, L.J. Bie, Acetone sensing of ZnO nanosheets synthesized using room-temperature precipitation. Sens. Actuators B Chem. 249 (2017) 611–623. 43. Q. Wang, N. Yao, D. An, Y. Li, Y. Zou, X. Lian, X. Tong, Enhanced gas sensing properties of hierarchical SnO2 nanoflower assembled from nanorods via a one-pot template-free hydrothermal method. Ceram. Int.42 ( 2016) 15889–15896. 44. X. Zhou, B. Wang, H. Sun, C. Wang, P. Sun, X. Li, X. Hu, G. Lu, G. Template-free synthesis of hierarchical ZnFe2O4 yolk-shell microspheres for high-sensitivity acetone sensors. Nanoscale 8 (2016) 5446–5453. 45. H. Xu, J. Gao, M. Li, Y. Zhao, M. Zhang, T. Zhao, L. Wang, W. Jiang, G. Zhu, X. Qian, Y. Fan, J. Yang, W. Luo, Mesoporous WO3 Nanofibers With Crystalline Framework for High-Performance Acetone Sensing, Front. Chem. 7 266 (2019).

16

Figure Captions

Figure 1 Powder X-ray diffraction patterns of both (A) Co- and (B) Zn -doped MgCu2O3 Figure 2 FE-SEM images of (A) MgNi2O3, (B) Co-MgNi2O3 and(C) Zn-MgNi2O3 Figure 3 (A) TEM, and (B) HR-TEM image and (C) Compositional analysis of undoped MgNi2O3 Figure 4 XPS spectra of Co doped NPs (A) full scan, (B) high resolution spectra of Mg 1s, (C) Ni2p, (D) Co2p, (E) O1s and (F) C1s . Figure 5 Temperature dependence of resistance for the pure MgNi2O3 and Co, Zn doped MgNi2O3 NPs based sensors in air. Figure 6 Evolution of the CV curves with number of cycles of MNO modified SPCE in 0.1 M KOH at scan rate of 50 mVs-1.

Figure 7 (a) Cyclic voltammograms of MNO modified SPCE in absence and presence of 1 mM glucose in 0.1 M KOH recorded at scan rate of 50 mVs-1; (b) Current variation in presence of 1 mM glucose for MNO, MNCoO and MNZnO the modified SPCE electrodes; (c) CVs of MNCoO modified SPCE for different concentrations of glucose (0-5 mM) in 0.1 M KOH at scan rate of 50 mVs-1; (d) Calibration curves for the MNO, MNCoO and MNZnO modified SPCE sensors as a function of glucose concentration.

17

Figure 8 CVs of MNCoO modified SPCE in the absence (A, B) and the presence of 1 mM glucose (C,D) in 0.1 M KOH at different scan rates between 10 and 100 mVs-1.

Figure 9 Chronoamperometric response of (A) MNCoO sensor during different additions of glucose in 0.1 M KOH at 0.6 V, (B) shows the calibration curve of the sensor and (C) Chronoamperometric response of MNCoO/SPCE in 0.1 M KOH at 0.6 V after successive additions of 1 mM glucose and 0.1mM of interfering analytes.

Figure 10 (a) Operating temperature dependence of the sensing response to 30 ppm of acetone gas for the pure and Co and Zn doped MgNi2O3 based sensors; (b) Dynamic response to different concentrations of acetone at the working temperature of 200ºC for Zn-MgNi2O3 sensor and (c,d) calibrationcurves.

Figure 11 Reproducibility of response to (a) 10 ppm acetone and (b) selectivity towards common gases of Zn-MgNi2O3 at 200°C.

18

Intensity (a.u.) 10 20

(111)

(111)

(111)

(220) (111)

(111)

40 (200)

(200)

(311) (222)

(311) (222)

(511) (220) (440)

(311) (222)

(200)

25% Co-Mg123

(220)

10 wt % Co-Mg123

19

80

(222)

(220)

MgNi2O3

70 (311)

(200)

(200)

2.5 wt % Co-Mg123

60

(220)

JCPDS carde No 00-034-0410

50

2 Theta (degree)

30

(220) (111)

(511) (220) (440)

(311) (222)

Intensity (a.u.) 10 20 (100)

(002)

(101) (111)

(111)

(111)

40 (200)

(200)

(200)

(110) (220) (212) (311) (222)

(110) (220) (212) (311) (222)

25 wt% Zn-Mg123

(311) (222)

(200) (102)

10 wt.% Zn-Mg123

80

Figure 1

(222)

2.5 wt.% Zn-Mg123

(311)

(220)

MgNi 2O3

70 (311) (222)

JCPDS carde No 00-034-0410

60

(220)

(220)

(101) (111)

(200) (102)

50

2 Theta (degree)

30

(002) (111)

(A)

(B)

1 µm

1 µm

(C)

1 µm

Figure 2

20

C

Figure 3

21

Figure 4

22

4

Resistance (Kohm)

10

MNO MNZnO MNCoO

3

10

2

10

1

10

0

10

200

250

300

350

400

Temperature (°C)

3

Resistance (Kohm)

10

200 C Baseline resistance increase

2

10

Baseline resistance decrease

1

10

0

10

MNO

MNZnO Sensor

MNCoO

Figure 5

23

Figure 6

24

200

40 Current variation (µA)

Current (uA)

MNO/SPCE 0.1 M KOH MNO/SPCE 1mM glucose

100

0

-100 0.0

a) 0.2

0.4

0.6

0.8

b) 30 20 10 0

MNO

Potential (V)

m

M

Gl u

co se

400

0-

5

200 0

c) 0.2

0.4 0.6 Potential (V)

Current variation (µ A)

300 MNCoO

Current (uA)

MNZnO

Sensor

600

-200 0.0

MNCoO

MNO MNCoO MNZnO

Normal glycemia

Hypoglycemia

200

100

d)

0

0.8

0

1

2

3

4

5

Glucose (mM)

Figure 7

25

MNCo in KOH

m Vs

200

Current (µA)

-1 00

200

10

Current (uA)

400

-1

MNCoO

400

0 -200

0 -200

-400 0,0

a)

0,2

0,4

0,6

b)

0,8

3

4

Potential (V)

7

8

9

MNCo in glucose 1 mM -1

400 Current (µ A)

10 0m Vs

0 -200

200 0 -200

c)

0,0

10 11

Scan rate (mVs )

10 -

Current (uA)

200

6

-1 1/2

MNCoO - 1 mM glucose

400

5

0,2

0,4

0,6

d)

0,8

3

4

5

6

7

8

9

10 11

-1 1/2

Potential (V)

Scan rate (mVs )

Figure 8

26

Figure 9

27

ο

2

20 ppm

8,0x10

4

10 ppm

6,0x10

b

4

4,0x10

1 ppm

3

40 ppm

MNZnO- Acetone 200 C

4

Resistance (Ω )

Response (Rg/Ra)

5

1,0x10

a

MNO MNCoO MNZnO

30 ppm

4

4

2,0x10

1 200

250 300 350 Temperature (°C)

400

4000

6000

8000

Time (sec)

4

4

ο

MNZnO- Acetone 200 C

ο

MNZnO- Acetone 200 C

Response

Response

3 3

2

c

1 0

20

2

d

1 0,1

40

1

10

Concentration (ppm)

Concentration (ppm)

Figure 10

28

4

7x10

ο

MNZnO- Acetone 200 C

10 ppm

2,5

a

Acetone 10 ppm

b

4

Response

Resistance (ohm)

6x10

4

5x10

4

4x10

2,0

1,5

4

3x10

H2 NO2 5 ppm

4

2x10

6000

100 ppm CO 10 ppm

1,0

9000

NH3

CO2 1000 ppm

50 ppm NO 100 ppm

Gas

Time (sec)

Figure 11

29

Tables

Table 1. Comparison of glucose electochemical performance of Co-MgNi2O3 with previous electrodes.

Materials/ Electrodes Co3O4/GCE

Liner range (mM) 0.005-0.8

Detection limit (mM) 0.13

Sensitivity (µA/mM) 520.7

Ref.

CuO/GCE

0.006-2.5

0.8

431.3

[29]

Cu2O/GCE

0-6

49

1620

[30]

ZnO/GCE

0.001-0.01

0.5

5.601

[31]

Mn3O4/GCE

0.015-8

0.3

726.9

[32]

AgO/Cu

0.2-3.2

0.01

298.2

[33]

Pd-WO3/GCE

0.005-0.375

4.2

5.6

[34]

NiO-SnO2/GCE

0.01-26

1

0.14

[35]

Zn-MgNi2O3/SPCE

0.05 – 5

0.02

528.6

This work

30

[28]

Table 2. Comparison of acetone sensing performance of Zn-MgNi2O3 with previous sensing materials. Materials

Synthesis method MOF template

Conc. ppm 10

Temp. ◦C 280

Rres (s) Rrec (s) sec sec 15 12

Ref.

Au/ZnO Ce/CoFe2O4

Molten salt

2000

200

1.7

38

61

[38]

LaFeO3 NPs

Sol gel

10

200

7.8

21

6

[39]

Pt/In2O3 NPs

Sol gel

1.56

200

18

25

120

[40]

Fe2O3 NPs

Hard template

100

300

15

6

-

[41]

ZnO

Precipitation

200

300

44

18.7

13.7

[42]

SnO2

Hydrothermal

50

170

29

3

28

[43]

ZnO/ZnFe2O4 Template

20

140

14

5.2

12.8

[44]

WO3

Sol gel

50

350

22.1

24

27

[45]

Zn-MgNi2O3

Sol gel

10

200

2.3

25

250

This work

31

Response R0/R or R/R0 43

[37]

Highlights A new class of pure and doped MgNi2O3 nanomaterials has been synthesized by a simple chemical precipitation method and proposed for the fabrication of glucose and acetone sensors. Co-doped MgNi2O3was highly effective for the electrochemical sensing of glucose with the high sensitivity of 528.6 µA mM-1cm-2 over a wide linear range of 0.05 mM to 5 mM. Zn doping into MgNi2O3 was proved to be useful for conductometric acetone sensing with high response and low detection limit of 0.5 ppm (S/N = 3) at 200°C.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: