Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Engineering Defects in Graphene Oxide for Selective Ammonia and Enzyme-Free Glucose Sensing and Excellent Catalytic Performance for para-Nitrophenol Reduction Waseem Raza* and S. B. Krupanidhi Materials Research Centre, Indian Institute of Science, Bangalore 560012, India
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S Supporting Information *
ABSTRACT: Recently, extensive attention has been given to developing an active and durable metal-free economical sensor and catalyst. Graphene oxide (GO)-based sensors and catalysts have been considered as a promising candidate in current material science research. However, the sensing and catalytic properties of GO also need to be further improved to satisfy the specific applications, such as gas detection in harsh environments, medical diagnosis based on human breath, blood glucose detection, catalytic activity, and so forth. Therefore, the effect of nitrogen in GO on the performance of glucose and ammonia sensing, and catalytic activity has been investigated. Herein, we propose a practical, high-sensitive sensor and catalyst based on high-quality defect N-enriched GO. One-step, low-cost solvothermal synthesis of N-enriched GO has been exploited for the development of high-performance sensors and excellent catalyst at room temperature. The resultant N-enriched GO (N8GO) has been studied as a promising sensing material for ammonia, glucose, and para-nitrophenol (PNP) reduction. The prevalent outstanding sensing and catalytic performance may be due to the synergistic effect of nitrogen. A probable mechanism for sensing and catalytic reduction of PNP using N8GO has been proposed. KEYWORDS: nitrogen-enriched graphene oxide, ammonia & glucose, sensor, solvothermal synthesis, high sensitivity, catalytic performance
1. INTRODUCTION Gas sensors have attracted enormous attention, are widely considered to play an important role in the quantitative detection of different toxic gases, and have found applications in many fields such as medical diagnosis, domestic and public security, as well as environmental monitoring. Particularly, the detection of ammonia has attracted a lot of attention because ammonia is one of the most commonly produced chemical in the world and the most toxic pollutant. Ammonia is widely used in the production of petrochemicals, fertilizers, explosives, water purification, manufacturing of cosmetics, fabrics, dyes, pharmaceuticals plastics, and industrial coolant.1 Ammonia is considered as one of the primary irritants to humans because it has a sharp and pungent odor, and it is harmful if present in a concentration of more than 50 ppm.2 A concentration of 25 ppm for 8 h and 35 ppm for 15 min in air has been recommended as the threshold value for human exposure.3 Therefore, there is urgent need to develop a fast, convenient, highly selective and sensitive ammonia sensor that can detect low concentrations of ammonia in air, which is critical for environmental monitoring. A lot of work has been reported in the literature for the detection of toxic and explosive gases using various oxides such as α-Fe2O3, ZnO, SnO2, WO3, NiO, © XXXX American Chemical Society
In2O3, and so forth owing to their several advantages such as high sensitivity to a large variety of gases, low detection limit, fast response and recovery, superb stability, low fabrication cost, and simple preparation.4,5 However, the oxide-based materials require external stimuli such as high temperature, ultraviolet (UV) light activation, and so forth for the sensor response and/or recovery, which restricts their further application.6−8 Hence, these materials are power-consuming sensors. Therefore, great efforts are being taken to solve the great challenge by developing advanced sensors that can effectively detect ammonia at room temperature. In this connection, a carbon-based nanomaterial, particularly GO, has been found to be gaining more attention. GO has been examined as a marvelous candidate for gas sensing. Diabetes mellitus is one of the biggest global attacking and hidden diseases of endocrine disorder marked by an increased level of glucose in the blood. Diabetes can lead to major problems such as blindness, kidney failure, cardiovascular disease,and so forth. Therefore, rapid, reliable, cheap, and Received: April 1, 2018 Accepted: July 2, 2018
A
DOI: 10.1021/acsami.8b05162 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
doping in CNTs increases the growth mechanism, metallic behavior, and sensitivity.16 Hence, nitrogen doping should also displays great impact when used for graphene modification. However, only a few studies are available for modification of graphene using pyridinic nitrogen atoms. Herein, we report the high-quality defect N-enriched GO using the solvothermal method with different weights of N in graphite flakes. The prepared materials were characterized by standard analytical techniques. The fabricated samples were examined as a sensing material for the detection of ammonia gas, glucose, and reduction of PNP in comparison to pristine GO at room temperature. The resultant N-enriched GO has been studied as a promising sensor and catalyst for ammonia, glucose, and PNP reduction.
accurate detection of glucose is of immense scientific importance especially for clinical diagnostics in diabetes mellitus control, food industries, fermentation analysis, and analytical applications in biotechnology.9−11 A lot of methods have been reported for the detection of glucose in blood, such as high-performance liquid chromatography, fluorimetry, and electrochemical analysis.12−15 Among all, amperometric glucose sensors are promising because of their simple equipment and can reach clinic glucose measurement accuracy. However, amperometric sensors have the greatest drawbacks such as instability, high cost, complicated immobilization techniques, and rigorous operating conditions.16,17 Hence, it is critical to explore and develop a highly sensitive and selective nonenzymatic glucose sensor with low detection limit for getting convincing clinical measurements. Recently, various nanomaterials such as noble metals Pd, Pt, and Au; metal oxides NiO, CuO, Co3O4, and Cu2O; and metal alloys have been given significant attention as electrode modifications for enzyme-free glucose sensing.18−25 Among these, GO has been explored as an outstanding alternative material. Recently, the reduction of PNP has become a vital topic because the product para-aminophenol (PAP) is of great commercial importance as a useful intermediate for the manufacture of pharmaceuticals (paracetamol, acetaminophen, analgesics, and antipyretic drugs), dyes, papers, polymers, corrosion inhibitors, and so forth.26 Hence, many efforts have been made for the reduction of PNP to PAP, such as catalytic reduction, chemical oxidation, photocatalytic degradation, solvent extraction, biodegradation, and so forth.27 Among all the approaches, catalytic reduction is of great significance for sustainable development because of its low cost, environmental friendliness, and compatibility with industrial processes. GO is an allotrope of carbon consisting of a single flat atomic layer of sp2 hybrid two-dimensional honeycomb configuration.28,29 Therefore, its entire carbon is exposed to the environment and provides a higher surface area, resulting in better sensitivity to various gases, glucose, and superb catalysis.30,31 In addition to this, graphene exhibits higher surface to volume ratio, high carrier mobility, thermal conductivity, good porosity, high mechanical strength, tunable band gap, and high elasticity, thus enabling GO to be a promising application in electricity-based sensors and catalysis.32,33 GO particles display better aspect ratios if intercalated or exfoliated and are cheaper than their counterpart, carbon nanotubes (CNTs).34−36 Graphene is the mother of all graphitic forms of carbon including OD fullerenes and ID CNTs. However, the electronic application of pristine graphene is limited because of its zero band gap. The exceptional sensitivity of pristine GO for the detection of toxic gases at ppt level, blood glucose, and catalytic performance is still a challenge, which is necessary for industrial and environmental safety. To improve its sensitivity toward reducing gases such as ammonia, blood glucose, and catalytic activity, GO needs to be functionalized through doping. Doping of graphene with heteroatoms has been receiving a lot of attention in the field of electronics and sensing area because it can modify the band structure and properties of GO.37−40 There are several potential heteroatoms used for functionalization of GO, among which nitrogen has been attracting great attention and is considered as a potential candidate for doping in GO because of its comparable atomic size and having five valance electrons which can form a strong bond with carbon atoms. It has been reported that nitrogen
2. EXPERIMENTAL SECTION 2.1. Reagents and Chemicals. Graphitic flakes, sodium borohydride (NaBH4), PNP, and urea were obtained from SigmaAldrich. Ethylene glycol, ethanol, methanol, chloroform, benzene, acetone, isopropanol, cyclohexane, liquor ammonia, sulfuric acid, sodium nitrate, and hydrogen peroxide were taken from s d Fine Chemical Limited (SDFCL). Potassium permanganate (KMnO4), glucose, sucrose, dopamine, mannose, cysteine, mannose, Na2SO4, NaCl, ascorbic acid, and fructose were supplied by Fisher Scientific. Milli-Q water was used in all experiments. 2.2. Preparation of GO. GO was synthesized by natural graphite flakes using a modified Hummer’s method as reported in the literature.41 In a typical procedure, natural graphite flakes (2 g) and sodium nitrate (1 g) were mixed in previously cooled sulfuric acid (50 mL) and stirred in an ice bath, and the suspension was cooled down to 0 °C. Subsequently, KMnO4 (6 g) was gradually added to the suspension with continuous stirring and cooling, maintaining the temperature at 5−10 °C. After that, the reaction suspension was stirred at 40 °C for 1 h to form a thick paste. Thereafter, Milli-Q water (100 mL) was slowly added, and the paste was stirred at 90 °C for 2 h. Finally, more Milli-Q water (100 mL) was added slowly followed by the addition of hydrogen peroxide (10 mL) to remove the excess KMnO4 indicated by change in color. The reaction paste was filtered and washed with HCl (0.1 M) to remove the sulfate followed by the addition of Milli-Q water. The obtained GO paste was dissolved in methanol and sonicated for 30 min for getting an exfoliated GO sheet. The GO sheet was dried in oven at 60 °C. 2.3. Synthesis of Nitrogen-Enriched GO. The nitrogenenriched GO was prepared using the solvothermal method. First, 100 mg of dry GO was ultrasonically redispersed in 100 mL N,N dimethyl formamide to form a homogenous solution. The solution was transferred to a 100 mL Teflon-lined stainless steel hydrothermal bomb containing the desired amount of urea as a nitrogen source. The hydrothermal bomb was heated at 150 °C for 16 h. After that, the autoclave was naturally cooled down to room temperature. The resulting product was separated by a centrifuge and washed several times with Milli-Q water and ethanol and dried at 60 °C in an oven. The molar ratios of nitrogen-enriched GO were synthesized using the same procedure by varying the urea content and are named N2GO, N4GO, and N8GO. The X-ray diffractometry (XRD), Raman spectroscopy, scanning electron microscopy (SEM), elemental, Xray photoelectron spectroscopy (XPS) analysis and their related details are given in the Supporting Information. 2.4. Characterization. Phase analysis of the fabricated samples was employed using an X-ray diffractometer (Philips PANalytical) with Cu Kα radiation (λ = 0.154 nm) at a scanning rate of 5° min−1. The surface morphologies of the as-prepared samples were characterized using SEM (FEI Inspect F-50). The chemical composition was analyzed using energy-dispersive X-ray spectroscopy by JEOL JEM-2100F. XPS was performed using Kratos Axis Ultra XPS to detect the bonding and for elemental analysis. Raman spectra of the as-prepared materials were studied using a WITec system with a 355 nm excitation wavelength. B
DOI: 10.1021/acsami.8b05162 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces 2.5. Gas-Sensing Experiment. The gas-sensing study of the assynthesized materials has been performed using a Keithley meter (model 2410) under ambient conditions. For sensing studies, a film was prepared by drop-casting the material paste which was prepared as follows. The as-prepared samples (7 mg) were initially dispersed in ethanol (1 mL) and Milli-Q water (1 mL) mixture to form a suspension, which was then coated on a soda lime cover glass (1 cm × 1 cm). The change in conductivity of the film as a function of time was measured in fresh air and in gas atmosphere. For the conductivity measurement, the film was attached with electrodes using a silver paste for contacts, which was connected to a Keithley meter, serving as a current and dc voltage source. The gas-sensing experiments were employed in a very simple homemade testing chamber, and the distance between the film and aqueous ammonia solution was 1 cm as shown in Figure 1. The ammonia vapors of different concentrations
under sonication. The required amount of prepared pristine and Nenriched GOs were dispersed in water and ethanol by ultrasonication. Finally, a 10 μL sample in 5 wt % Nafion dispersion was drop-casted on to the GCE and air-dried before electrochemical experiments. All glucose-sensing experiments were carried out in 0.1 M KOH aqueous solution with successive addition of a 0.5 mM glucose solution. 2.7. Catalytic Reduction of PNP. To evaluate the catalytic activity of pristine and N-enriched GOs, the reduction of PNP to PAP in the presence of excess NaBH4 at room temperature was carried out. Typically, an aqueous solution of PNP (3 mL, 0.1 mM) and excess amount of NaBH4 (100 mg) was mixed in a quartz cuvette, leading to a color change from light yellow to deep yellow. After that, 100 μL of catalyst was added to the above solution and time-dependent absorption spectra were recorded. The deep yellow color of the PNP solution gradually vanished, indicating the reduction of PNP. The reaction progress was monitored using a UV−visible (vis) spectrophotometer (Hitachi, U-2900) until the deep yellow solution became colorless.
3. RESULTS AND DISCUSSION 3.1. Evaluation of Gas-Sensing Performance. NH3 is very toxic and harmful especially in high concentrations and poses an immense risk to human health. Therefore, there is urgent need to develop a highly sensitive and selective ammonia gas sensor to monitor the leakage of NH3 gas. Hence, to prove the sensing properties of the as-synthesized GO and N-enriched GO-based sensor, we used NH3 as the analyte. The sensor films are prepared by the drop-cast method on a glass slide, and solutions of different ammonia concentrations were prepared by diluting with Milli-Q water. The sensing of water was also investigated by exposing the water in the chamber, and the sensor film revealed some change in current; to neutralize the sensing effect of water, we subtracted the water vapor from the output obtained by the exposure to the water−ammonia solution as shown in Figure S4. Figure 2A−D exhibits the reversible dynamic response of GO and N-enriched GO-based sensor toward different ammonia concentrations ranging from 500 ppt to 1000 ppm. The current of the device increases significantly upon exposure of NH3 in the chamber and returns back to the initial value upon removal of NH3 or exposure to air in the case of GO, as
Figure 1. Home-built schematic setup for NH3 sensing. evaporate spontaneously from aqueous ammonia solution, which gives some change to the film and are detected by a change in current. The sensor sensitivity (S) was evaluated by the response using the following relation. S % = I − I0/I0 × 100
(1)
where I and I0 are the electrical resistance of the sensor film in test gas and fresh air at room temperature, respectively. 2.6. Electrochemical Testing. All electrochemical measurements were examined on a CHI750E electrochemical workstation using a conventional three-electrode system at room temperature. A commercial glassy carbon electrode (GCE, 3 mm diameter, 0.07 cm2) was used as the working electrode, Pt wire as the counter electrode, and Ag/AgCl as a reference electrode. Prior to use, the bare GCE was polished with alumina powder and cleaned with water
Figure 2. Room-temperature dynamic sensing response of (A) GO, (B) N2GO, (C) N4GO, and (D) N8GO toward different concentrations of ammonia. C
DOI: 10.1021/acsami.8b05162 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 3. (A) Selectivity histogram of the N8GO film with different volatile organic compounds and (B) response and recovery curve of the N8GO film to 1 ppm NH3 at room temperature.
Figure 4. (A) Relationship between S as a function of different % of N-doping, (B) I−V curves of N8GO, (C) corresponding log−log plot of sensitivity vs different ammonia concentrations (ppm), and (D) plot between adsorption intensification (1/n) and N-doping.
seen from Figure 3A that the as-prepared N8GO sensor exhibits a significantly high response to NH3 as compared to other analytes. The results indicate that the as-synthesized N8GO-based sensor was found to be highly selective toward ammonia and can be considered as an excellent sensor for the detection of NH3. The response and recovery time is also a key factor to exhibit the reliability and feasibility of the sensor for target molecules. Therefore, we conducted experiments to evaluate the response and recovery time using 1 ppm NH3 in the presence of N8GO sensor, as shown in Figure 3B. On exposure of NH3 to the sensor, the current decreased rapidly and recovered to the original value after the removal of NH3 or exposure to air. The response is defined as the time required for the current to decrease from 10 to 90% of the total change, and the recovery is the time needed for the recovery of current to the initial value from 90 to 10% of the total time. The response and recovery times were found to be 37 and 35 s, respectively, as shown in Figure 3B. The results indicate that the sensor exhibited excellent stability and reproducibility. Therefore, this type of sensor with fast response and steady recovery properties is particularly useful for gas leak detection of NH3.
shown in Figure 2A. However, the current of the device decreases upon exposure of NH3 in the chamber and returns back to the initial value upon removal of NH3 or exposure to air in the case of N-enriched GO-based sensors, as shown in Figure 2A−C. All N-enriched GO-based films showed reversible behavior because only weak interactions took place between sensors and the targeted gas, which are responsible for the fast recovery during consecutive cycles. The results indicate that an N8GO-based sensor presents a high response to NH3 and the response value of GO increases with the nitrogen amount. It could be seen from Figure 2A−D that GO shows the opposite trend in current as compared to N-enriched GO. This behavior indicates that GO treated with urea affects the electronic behavior of GO. The transport and electronic properties of activated carbon strongly depend upon specific surface chemistry, type, and defects in the carbon matrix. The results indicate that N8GO exhibited the highest sensing response to ammonia gas compared to other previously reported papers, as shown in Table S1. It is well known that selectivity is one of the most important criteria for a reliable and useful sensor. Therefore, various volatile organic compounds such as chloroform, ethanol, methanol, isopropanol, acetone, cyclohexane, and benzene along with ammonia have been tested as an analyte. It could be D
DOI: 10.1021/acsami.8b05162 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
oxide (or nitrogen) and water. The electrons transferred to the material recombine with the hole, resulting in decrease in hole concentration and net surface negative charge, which decreases the overall current of the p-type sensing film. The prevalent outstanding sensing performance was due to the synergistic effect of nitrogen. The N8GO or RGO has oxygen-containing functional groups and defects in N8GO, which increase gas adsorption capacity of the material, resulting in improvement in the sensitivity of NH3. 3.3. Electrocatalytic Activity toward Glucose Oxidation. In this study, the electrochemical activities of GO and Nenriched GOs were investigated using glucose as a probe to determine a possible application in nonenzymatic glucose sensing by cyclic voltammetry (CV) in 0.1 M KOH electrolyte at a scan rate of 50 mV s−1. Figure 5A exhibits the CV curves obtained of bare GCE in comparison to GO and N-enriched GOs in the presence of 0.5 mM glucose. It could be seen from Figure 5A that the bare GCE does not show any response for glucose oxidation in the potential range (0.0−0.6 V vs Ag/ AgCl). However, the electrochemical activity of N8GO is much higher than those of GO and other NGOs, which may be due to the synergistic effect of nitrogen. On the other hand, GO, N2GO, and N4GO electrodes also show a noticeable increase in current, indicating their electrocatalytic activity for glucose sensing. CV curves were recorded for the N8GO electrode in N2 and O2 saturated 0.1 M KOH solution at a scan rate of 50 mV s−1. It could be seen from Figure 5B that the bare GCE does not show any response and the N2 and O2 saturated CV curves are almost similar, indicating that N8GO is not much affected by oxygen. To further characterize the sensitivity of the N8GO electrode, we performed experiments by successive addition of glucose from 0 to 3 mM at a scan rate of 50 mV/s in 0.1 M KOH solution, as shown in Figure 5C. The results indicate that there is linear correction between glucose concentration and output current as shown in the inset of Figure 5C. The linear relationship suggests the workability of the working electrode toward glucose sensing. The glucose-sensing experiments by successive addition of glucose from 0 to 3 mM at a scan rate of 50 mV/s in 0.1 M KOH solution for GO, N2GO, and N4GO are shown in Figure S5. Moreover, the effect of scan rate on glucose oxidation was investigated to get the reaction kinetics in 0.1 M KOH electrolyte in the presence of 0.5 mM glucose by using the N8GO electrode. Figure 5D shows the CV curves of glucose sensing at various scan rates ranging from 2 to 200 mV s−1. It is observed that anodic and cathodic peak currents were found to increase with increasing scan rate. The inset of Figure 5D indicates that the redox peak current changed linearly with the scan rate with linear regression values (R2) of 0.980 and 0.988 for the anodic and cathodic peak potentials, respectively. This confirms that electrochemical kinetics of glucose oxidation by the N8GO electrode is a surface control electrochemical process. 3.4. Amperometric Detection of Glucose at the N8GO Electrode. The sensitivity of the N8GO electrode toward glucose was examined using a chronoamperometry experiment in 0.1 M KOH solution at a constant potential of 0.5 V with the consecutive stepwise addition of 1 mM glucose. The testing vial containing electrolyte and glucose was continuously stirred after the successive addition of glucose to ensure the homogenous distribution of glucose. With further addition of 1
Furthermore, the variation of sensitivity as a function of NH3 concentration has been studied as shown in Figure 4A. The change in sensitivity increases with nitrogen loading and follows a linear behavior. Figure 4B shows the room temperature I−V curves of N8GO and exhibits excellent linearity, indicating good ohmic contact between the sensing material and electrodes. Figure 4C demonstrates the correlation between the logarithm of sensitivity and the logarithm of ammonia concentration, which corresponds to the Freundlich adsorption isotherm. It is reported that the Freundlich adsorption isotherm equation gives the straight line between the adsorption of a concentration of the solute on the surface of a solid adsorbent and is given by eqs 2 and 3. S = αC1/n
(2)
ln S = ln α + 1/n ln C
(3)
where S is the sensitivity, C is the concentration of the targeted gas, α is the Freundlich adsorption coefficient, and 1/n is the adsorption intensification. The log α tells about the intercept, which is found to be 1.5, 2.1, 2.2, and 2.4, and 1/n tells about the slope which is 0.16, 0.24, 0.25, and 0.27 for GO, N2GO, N4GO, and N8GO, respectively. It could be seen from Figure 4D that the value of 1/n increases with the nitrogen content and shows linear behavior. 3.2. Gas-Sensing Mechanism. The gas-sensing mechanism of a p-type semiconductor such as reduced graphene or N-enriched GO (N8GO) can be explained on the basis of change in electron and hole concentration of N8GO. It could be seen from Figure S1A that after the addition of nitrogen, the GO changed to reduced graphene oxide (RGO), that is, of ptype. It has been reported that after doping of nitrogen (pyridine type as confirmed from XPS), the GO becomes electron-deficient compared to pristine GO and converts to ptype GO or RGO.42 Pyridine-type nitrogen has localized electrons owing to electronic cyclization because of the resonance hybrid in pyridine, which creates more holes and results in p-type conductivity of N8GO.43 Overall, we can say that the transport and electronic properties of carbon strongly depend on the distribution and the type of defects (nitrogen) in the carbon matrix. However, in the p-type material, the majority carriers are holes and vacancies. When the N8GO sensor was exposed to air, it absorbed oxygen and was partially ionized into O−, O2−, and O2− ions by withdrawing electrons from the valance band and created HAL near the surface, as shown in Scheme 1. It has been reported in the literature that when the sensor was exposed to reducing gases such as ammonia, then ammonia reacts with the adsorbed oxygen and gives electrons back to the material surface, resulting in nitrous Scheme 1. Schematic Diagram for the Gas-Sensing Mechanism of the N8GO Sensor
E
DOI: 10.1021/acsami.8b05162 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 5. (A) CV curves of bare GCE, GO, and N-enriched GOs, (B) CV curves obtained of bare GCE and N8GO in O2 and N2 saturation, (C) CV curves of the N8GO at different glucose concentrations and the inset plot showing a linear correlation between j and glucose concentration, and (D) CV curves obtained of N8GO at different scan rates in 0.1 M KOH with a 0.5 mM glucose concentration and the inset plot showing linear correlation between j and the scan rate for anodic and cathodic peaks.
Figure 6. (A) Amperometric response of N8GO to glucose at an applied potential of +0.5 V, (B) fitting of the amperometric response, (C) antiinterference study of N8GO in the presence of various interfering species (1: glucose, 2: D-sucrose, 3: dopamine, 4: mannose 5: urea; 5: L-cysteine, 6: Na2SO4, 7: NaCl, 8: ascorbic acid, 9: fructose) with a glucose concentration of 1 mM and the interfering concentration of 2 mM, and (D) stability of N8GO in the presence of 1 mM glucose at +0.5 V.
sensitivity of N8GO is compared with previously reported catalysts are given in Table S2. Furthermore, the interference in nonenzymatic glucose sensing is a major challenge for selective detection of glucose. Human blood always contains many biomolecules such as sucrose, urea, ascorbic acid, dopamine, and NaCl, and it can also be easily oxidized at the applied potential on the sensor electrode. However, the concentration of glucose (0.125 and 0.33 mM) in blood is about 10 times higher than that in the interfering species.44 Therefore, the anti-interfering experiment
mM glucose in each step, the N8GO exhibits a fast response with a distinct increase in the current with an interval of 50 s; afterward, it reaches saturation within 3 s, indicating a rapid response of the N8GO electrode toward glucose sensing, as shown in Figure 6A. The corresponding current concentration calibration plot clearly indicates well-defined linearity with successive addition of glucose up to 1 mM, as shown in Figure 6B. The lower detection limit of glucose is found to be 0.66 μM with a good sensitivity of 670 μA mM−1 cm−2 and the correlation coefficient of 0.9965. The calculated glucose F
DOI: 10.1021/acsami.8b05162 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 7. Time-dependent UV−vis spectrum for the reduction of PNP by NaBH4 in the presence of (A) N8GO, (B) N4GO, (C) N2GO, and (D) GO as catalysts.
Figure 8. (A) Change in the concentration of PNP as a function of time in the presence of GO and N-enriched GOs, (B) plot of ln(Ct/C0) vs reaction time for the reduction of PNP by NaBH4 in the presence of GO and N-enriched GOs, and (C) conversion efficiency of PNP using N8GO for 10 repeated cycles.
The stability of the N8GO sensor was systematically monitored by performing multiple amperometric experiments at 0.5 V in the presence of 1 mM glucose, as shown in Figure 6D. We checked the sensor response toward 1 mM glucose in 0.1 M KOH solution daily for a week. The N8GO electrode was stored in a refrigerator when not in use. The results suggested that the sensor retains good stability of the initial sensitivity, indicating that the nonenzymatic glucose sensor possesses long-term stability. The excellent durability mainly results from the robust mechanical stability of graphene.
of N8GO was studied with successive addition of 1 mM glucose and 2 mM sucrose, mannose, urea, cysteine, ascorbic acid, dopamine, Na2SO4, NaCl, and fructose as shown in Figure 6C. These interfering species were also examined amperometrically at +0.5 V with glucose. The initial current response was increased after the addition of 1 mM glucose; after that no significant response was observed for the interfering species. Finally, after studying the anti-interference test, the electrode again responded to the addition of 1 mM glucose, indicating excellent selectivity of the sensor for glucose detection in the presence of common interfering species. G
DOI: 10.1021/acsami.8b05162 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
3.6. Approaching the Catalytic Mechanism of the N8GO Sheet for PNP Reduction. To get more insight into the reaction process, we explored a synergic mechanistic pathway to elucidate the promising catalytic activity of the N8GO sheet for the reduction of PNP. The proposed mechanism for the reduction of PNP in the presence of the N8GO sheet is shown in Scheme 2. The reduction of PNP by
3.5. Catalytic Activity for the Reduction of PNP to PAP. The catalytic performances of the as-synthesized GO and N-enriched GOs were quantitatively examined by the reduction of PNP into PAP in the presence of an excess amount of NaBH4 as a model catalytic reaction. Figure 7A−D exhibits that PNP has a maximum absorption peak at around 315 nm, and after addition of NaBH4 the absorption peak is immediately shifted to 400 nm, indicating the formation of the para-nitrophenolate ion. Figure 7A−D shows the timedependent UV−vis absorption spectra, showing the gradual decrease in the absorption peak at 400 nm along with fading of yellow color and ultimately bleaching to colorless, after the addition of the as-synthesized GO and N-enriched GOs, indicating the reduction of PNP. There is a concomitant increase in the intensity of the new peak at 296 nm corresponding to the reduced product PAP, indicating successful conversion of PNP to PAP.45 It could be seen from Figure 7A that the peak of PNP at 400 nm disappears completely after 50 s using N8GO, implying the complete reduction of PNP to PAP. In contrast, the absorbance peak at 400 nm (Figure 7B) for PNP disappears after a longer time of 80 s, suggesting the lower catalytic property of N4GO than that of N8GO. However, the complete reduction of PNP was achieved after 100 s in the presence of N2GO, as shown in Figure 7C. In addition to this, a much longer time is needed for full reduction of PNP using GO (600 s) as the catalyst, as shown in Figure 7D. Interestingly, the quick full reduction of PNP in the presence of the as-synthesized GO and N-enriched GOs with an increase in the content of nitrogen is clearly seen in Figure 8A. It could be seen from Figure 8A that the reduction of PNP increases with the nitrogen content from GO to N8GO. For better understating of the catalytic performance of the as-synthesized GO and N-enriched GOs, kinetic analysis for the reduction of PNP to PAP is also discussed in terms of the Langmuir Hinshelwood model.46−50 As the initial concentration of NaBH4 is much higher than that of PNP, pseudofirst-order kinetics could be applied for the evaluation of the catalytic rate with respect to the concentration of PNP.51,52 Therefore, the kinetic equation can be written as
Scheme 2. Schematic Diagram of PNP Reduction in the Presence of the N8GO Catalyst
(4)
sodium borohydride is very difficult without a catalyst because the energy barrier between two negative ions is very high.53,54 Therefore, the presence of a catalyst is very important-to release the energy barrier, as well as for reaction being taking place on the surface of N8GO. First, the sodium borohydride ionizes in aqueous solution to borohydride ions (BH4−) which absorb on the surface of the catalyst. Thereafter, absorption of PNP also takes place on the surface of the catalyst in a babywipe manner because direct contact between the catalyst and reactant is necessary for the catalytic reaction and formation of the para-nitrophenolate ion. The catalyst brings down the kinetic barrier created because of mutual repulsion between the negatively charged para-nitrophenolate ion and BH4− ion. The N8GO stimulates the catalytic reduction by an electron relay system and transfer of electrons from BH4− to paranitrophenolate ion, and then the transfer of active hydrogen ions via the cleavage of the B−H bond over N8GO takes place to the para-nitrophenolate ion. Finally, PNP reduced to PAP as shown in Scheme 2.
where C0 is the initial concentration of PNP in the absence of a catalyst; C is the concentration of PNP at any time, “t” after catalyst was added; and “k” is the rate constant. The plots between ln(C/C0) and reaction time (t) for all as-synthesized GO and N-enriched GOs are shown in Figure 8B. The reaction rate constants (k) can be calculated from the slope of the linear fit of ln(C/C0) versus t, which is summarized in Table S3. To explore the practical applications of a catalyst in aqueous solution, reusability is the top priority for the catalyst. Therefore, the represented sample N8GO was examined for stability and recyclability for the reduction of PNP in 10 successive cycles under the same reaction conditions, as shown in Figure 8C. Generally, for the repeated cyclic run, the catalyst is collected, washed, and dried, which is an extremely difficult task. Here, we used a simple method for the cyclic purpose without wasting time and chemical for collecting and washing. We added PNP repeatedly into the stock solution of PAP, NaBH4, and the catalyst. As seen, N8GO was reused for 10 successive cycles with a stable conversion efficiency of more than 96% of its original catalytic activity, indicating that N8GO is a potential catalyst for application in PNP reduction.
4. CONCLUSIONS In summary, we report on the synthesis of GO and NGOs by a facile cost-effective modified Hummer’s and solvothermal method, exploitation of the sheets for glucose & ammonia sensing and PNP reduction. The N-enriched sheets exhibit excellent glucose- and gas-sensing performance toward different concentrations of glucose and NH3, in terms of ultrahigh sensitivity, wide linear range with lower detection limit, longterm stability, reproducibility, fast response and recovery times, and selectivity, suggesting that the N-enriched sheets are a promising material for detecting glucose and toxic chemicals even at room temperature. The sensitivity is found to exhibit Freundlich-like behavior with the ammonia concentration. Moreover, the N-enriched GOs exhibit opposite trends in the electrical signal compared to pristine GO; the decrease in conductivity in the case of NGOs was attributed to the increased contribution of pyridine-type impurities after addition of urea. Pyridine-type nitrogen has localized electrons owing to electronic cyclization because of the resonance hybrid in pyridine, which creates more holes and results in p-type
ln(C /C0) = −kt
H
DOI: 10.1021/acsami.8b05162 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
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conductivity of N8GO. They also exhibit excellent catalytic activity toward the hydrogenation of PNP by NaBH4 with very good recyclability and stability. Therefore, NGO sheets may strongly encourage the practical application of glucose- and ammonia sensing as well as PNP reduction because of easy accessibility and high performance. This will open a new door for the designing of excellent sensors and catalysts for practical application in the field of sensing of glucose and ammonia as well as PNP reduction and also beneficial for energy conversion and environmental protection.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b05162. XRD, Raman spectra, SEM, elemental mapping, XPS analysis of the as-synthesized materials, ammoniasensing graph of water−ammonia solution, glucosesensing graphs by successive addition of glucose from 0 to 3 mM at a scan rate of 50 mV/s in 0.1 M KOH solution for GO, N2GO, and N4GO, comparative ammonia and glucose sensitivity, and reaction rate constants (k) (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected],
[email protected]. ORCID
Waseem Raza: 0000-0002-1660-3231 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Dr. Waseem Raza acknowledge the financial support to Department of Science and Technology (DST), Government of India, through the National Postdoctoral Fellowship (NPDF) no. PDF/2016/001471. The authors also acknowledge our Institute for providing the TEM facility. Dr. K. K. Nanda is kindly acknowledged for help.
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