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Hierarchical porous carbon derived from waste amla for the simultaneous electrochemical sensing of multiple biomolecules
Colloids and Surfaces B: Biointerfaces 177 (2019) 529–540
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Hierarchical porous carbon derived from waste amla for the simultaneous electrochemical sensing of multiple biomolecules
T
Velayutham Sudhaa,b, Sakkarapalayam Murugesan Senthil Kumara,b, ⁎ Rangasamy Thangamuthua,b, a b
Materials Electrochemistry Division (MED), CSIR-Central Electrochemical Research Institute (CECRI), Karaikudi, 630 003, Tamil Nadu, India Academy of Scientific and Innovative Research (AcSIR), CSIR-CECRI, Karaikudi, 630 003, Tamil Nadu, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Amla fruits Carbonization Hierarchical porous carbon Simultaneous electrochemical sensing Biomolecules
For the first time, highly porous and hierarchical carbon with high surface area (∼2430 m2 g−1) has been prepared from waste amla fruits at different carbonization temperatures viz., 700 °C, 800 °C and 900 °C by a simple and eco-friendly method for the simultaneous electrochemical sensing of biologically important compounds such as ascorbic acid (AA), dopamine (DA), uric acid (UA) and nitrite. The porous carbon materials synthesized at 700 °C, 800 °C and 900 °C are denoted as ABC-700, ABC-800 and ABC-900, respectively. The structural and morphological evaluations of as-synthesized hierarchical porous carbon are carried out with advanced tools and the existence of porous morphology is ascertained. The morphology, amorphous nature, disordered nature, surface area, pore volume, thermal stability and elemental composition of the as-prepared porous carbon are investigated by SEM, HRTEM, FT-IR, Raman, TGA, BET, XPS, EDAX and CHNS analysis. Compared with ABC-700 and ABC-900, the electrochemical sensing ability was higher in the case of ABC-800. Therefore, further electrochemical sensing studies are carried out by using ABC-800. The limit of detection for the simultaneous determination of AA, DA, UA and NO2- are 13.7 μM, 3.2 μM, 1.1 μM and 3.3 μM, respectively. The sensitivity are (0.55, 0.01), (4.73, 0.11), 0.11 and 0.57 μA cm−2 μM−1 and linear ranges are (33–166, 166–26470), (1.6–72, 82–2630), 1.6–4134 and 4.9–1184 μM, respectively for AA, DA, UA and NO2−. The porous carbon based sensor also proves reliable operational stability, long time stability, selectivity and good antifouling properties. The porous carbon based sensor was successfully applied to the practical application for the detection of these biomolecules in the real samples of urine.
1. Introduction Due to the electroactive nature of neurotransmitters, numerous electroanalytical methods have been used to study their role in the brain [1,2]. Particularly, this area of analytical chemistry was first introduced by Ralph Adams in the year of 1970s and termed as “brain chemistry” [3]. Interestingly, ascorbic acid or vitamin C is dispersed in both plant and animals and widely used as an antioxidant in food for the stabilization of colour, cosmetic applications and pharmaceutical preparations [4]. The main role of AA is to prevent cancer and immunity improvement [5]. Dopamine (3, 4-dihydroxyphethylamine, DA) is one of the essential catecholamine neurotransmitters present in the body. Normal level of DA in biological systems is 10 nM to1 μM. Low or unbalanced DA concentration is responsible for the neurological disorders such as Parkinson’s disease, Schizophrenia and HIV infection
[6,7]. Uric acid is another important biomolecule in our body. Its concentration range from 207 to 444 μM, is normally co-exist with DA in biological fluids and abnormal concentration levels may lead to a number of diseases such as gout, hyperuricaemia and Lesch-Nyhan syndrome [5,8]. Nitrite ion is an essential analyte, which added as additive in food products, fertilizers in agriculture and also corrosion inhibitors [9–14]. It is one of the widely studied analytes in many fields including food and environmental studies because the concentration of NO2− in excess level in blood tends to oxidise the haemoglobin [10,11,14,15]. Also, it forms a highly carcinogenic N-nitrosamine when it reacts or interacts with amines and amides which are known to be carcinogens [16,17]. Therefore, the determination of AA, DA, UA and NO2− are important for human health. AA and UA are coexisting species along with DA. Compared to DA, AA has 100 to 1000 times higher concentration in body fluids. NO2− is also present in the biological
⁎ Corresponding author at: Materials Electrochemistry Division (MED), CSIR-Central Electrochemical Research Institute (CECRI), Karaikudi, 630 003, Tamil Nadu, India. E-mail addresses: [email protected], [email protected] (R. Thangamuthu).
https://doi.org/10.1016/j.colsurfb.2019.01.029 Received 16 October 2018; Received in revised form 1 January 2019; Accepted 14 January 2019 Available online 16 February 2019 0927-7765/ © 2019 Elsevier B.V. All rights reserved.
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dispersion of material were studied using high-resolution transmission electron microscopy (HR-TEM, 200 kV, Tecnai G2 TF20) working at an accelerating voltage of 200 kV. Thermogravimetric analysis (TGA) of ABC-700, ABC- 800 and ABC-900 was carried out in TGA/DTA analyzer (SDT Q 600) to find out the material decomposition temperature ranging from room temperature to 700 °C in an air atmosphere. Raman spectroscopic measurements were carried out using RENISHAW I via laser Raman microscope with He–Ne laser (wavelength λ = 633 nm) to understand the chemical nature of the prepared carbon materials. The C, H, N, and S contents in the ABC were estimated using CHNS analysis (elementarvario EL III). The elemental composition of the porous carbon materials were studied using X-ray photoelectron spectroscopy (XPS) with Mg Kα (1253.6 eV) as X-ray source (Thermo Scientific, MULTILAB2000). The surface areas of the ABC-700, ABC -800, ABC-900 were measured using Brunauer-Emmett-Teller (BET) surface area analyzer AutosorbiQ2.
system. Therefore, simultaneous sensing of DA, AA, UA and NO2- [18], and DA, AA and UA is essential [5,8]. Large over potential and fouling by oxidation product result in poor sensitivity and selectivity and hence is not possible to determine these analytes directly by using ordinary (carbon or metal) electrodes [18]. Therefore, the researchers proposed various modified electrodes with appreciable electrochemical sensing ability for this purpose. Bio-waste and natural waste management has always been a major problem in most of the countries including highly populated cites. Usually natural source of waste is burnt and it produces ash and functionalized porous form of carbon by man-made synthesis. Depending on the source of waste, it may contain different types of elements in various proportions and it is used for growing carbon based applications. Carbon is a well-known, naturally abundant material, existing in a variety of molecular and structural forms such as graphite, diamond, nanotubes, graphene, fullerene, nanodiamonds, amorphous carbon and porous carbon, which are used in diverse applications for the benefit of mankind [19–24]. The porous carbon has high surface area, chemically inert, cost effective and simple synthesis routes, ease of availability, environmental friendly and good conductive properties [25–28]. Highly porous carbons are prepared from various bio-waste materials, including eichhnomia crassipes [29], coconut shell [30] and various other bio-waste materials because of their availability. Such porous carbon materials have high electrical conductivity, high surface area and uniform porosity and because of these properties they enhance the accumulation of charge and removes hazardous heavy metals and ions from the water. The man-made porous carbon materials are also used for many applications including sensor, supercapacitors, batteries, adsorption and purification of toxic heavy metal ions [25–33]. Carbon activation process using alkali creates porosity, surface area related advantages [34]. Particularly, KOH activated carbon leads to creation of pores and high specific surface area and the respective electrochemical behaviour mechanism was elaborately discussed in the literature [35–37]. In this work, we synthesized ABC-700, ABC-800 and ABC-900 by chemical activation process. ABC-800 was used as an electrocatalyst for the simultaneous electrochemical oxidation of multiple bio-analytes such as AA, DA, UA and NO2−. The ABC-800 displays excellent electrocatalytic activity towards these analytes. The bio-carbon material increases the surface area and electrical conductivity of the electrode and the porous nature may enhance the electron transfer ability. The present study demonstrates the synthesis of bio-derived carbons from waste amla and their application for the simultaneous electrochemical sensing of AA, DA, UA and NO2−.
2.3. Synthesis of Amla porous carbon The following synthesis procedure has been followed to synthesis the carbon materials by modifying the earlier report [38]. The raw Amla (without seed) was cut into small pieces followed by washing with DI water for several times and dried in an hot air oven at 100 °C for 24 h. The dried materials were powdered well using a mortar followed by heating at 200 °C for 24 h. The sample was treated with KOH for chemical activation process. In a typical activation process, 10% KOH solution was added to the solution containing preferred amount of powdered materials in N2 atmosphere with constant stirring and heating at 60 °C for 1 h. To optimize the temperature, 10 g of the sample was heated at three different temperatures i.e., 700 °C, 800 °C and 900 °C for 2 h in inert atmosphere at 5 °C/min heating rate. Consequently, the resulting porous carbon was washed thoroughly with 1 M HCl and DI water until the pH of the sample became neutral. Afterwards the filtered sample was dried for 12 h in an hot air oven at 100 °C. 2.4. Electrochemical measurements The electrochemical activities of the ABC-700, ABC-800, and ABC900 towards the oxidation of AA, DA, UA and NO2− were studied by using three electrode system. Modified glassy carbon electrode (GCE) was used as a working electrode; platinum wire and saturated calomel electrode (SCE) were used as counter electrode and reference electrode, respectively. Cyclic voltammetry (CV) and amperometry measurements were performed by using a potentiostat of type AUTOLAB PGSTAT302 N (AU86575) analyser. Complete analysis was carried out under N2 atmosphere in 0.1 M PBS supporting electrolyte throughout the electrochemical measurements.
2. Experimental 2.1. Materials
2.5. Preparation of working electrodes Amla was collected from local land, KOH, Na2HPO4, NaH2PO4 were purchased from Hi-media. HCl (97%) from RFCL limited and H2O (18.2 MΩ cm, Millipore) were used throughout the experiments.
To prepare the working electrode, it was carefully polished with alumina on polishing cloth. Then, it was thoroughly washed with deionized water and ultra-sonication with ethanol and Milli-Q water for 10 min each to remove all the adsorbed impurities on the surface and dried in open air atmosphere. The catalyst slurry was prepared by dispersing 5 mg of the sample in a solution containing 900 μl of dimethyl formamide (DMF) and 100 μl of Millipore water. The glassy carbon electrode was coated with 3 μl of the catalyst slurry and dried in open air atmosphere. This modified electrode was used as working electrode throughout the electrochemical studies.
2.2. Apparatus The ABC materials were analysed using powder X-ray diffraction (XRD) measurements by X-ray diffractometer (BRUKER D8 ADVANCE) with Cu Kα radiation (α = 1.5418 Å). The surface functional groups of the porous carbon were exposed from Fourier transform infrared (FTIR) spectra Bruker, TENSOR 27 spectrometer ranging from 400 to 4000 cm−1. The surface morphology, porous nature and elemental composition (EDAX) of ABC samples were examined by scanning electron microscopy (SEM) using TESCAN (Supra 55 V P) operating at an accelerating voltage of 30 kV and field-emission scanning electron microscopy (FESEM) with an accelerating voltage of 30 kV (FE-SEM; Carl Zeiss AG, Supra55 V P). The particle size, nano porous nature and
3. Results and discussion The bioderived carbon materials play an important and active role in the area of electrochemical sensor due to their attractive surface chemistry and physical properties. Two types of activation methods like 530
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Scheme 1. Schematic illustration of synthesis of Activated Bio-Carbon (ABC) from waste amla and electrochemical sensing of AA, DA, UA and NO2− on ABC-800 modified electrode.
Fig. 1. (A–D) X-ray diffraction pattern, Raman spectra, FT-IR spectra and TGA results of ABC-700, 800 and 900, respectively.
DA, UA and NO2− is shown in Scheme 1.
physical and chemical activation process are available to prepare highly porous biocarbon materials [39]. However, chemical activation process is more superior because it produces porosity and high surface area. In addition to this, the technique offers cost effective, high yield, facile synthesis and less activation time [40]. A lot of chemical activating agents are available including KOH, NaOH, ZnCl2, H3PO4, K2CO3 and H2SO4. In this study KOH was used as a chemical activating agent. During the activation process the water is evaporated from the carbon surface. Synthesis of ABC from waste amla by chemical activation process and application of ABC in simultaneous electrochemical sensing of AA,
3.1. Characterization studies 3.1.1. XRD, Raman, FT-IR, and TGA studies Fig. 1A shows the typical XRD pattern of ABC-700, ABC-800 and ABC-900 in which two broad peaks are observed at around 23° and 43° corresponding to (002) and (100) planes. The examined diffraction lines are considerably broader which revealed that the ABC samples were amorphous and slightly disordered in nature [41,42]. Raman spectroscopy is a dominant tool to determine the degree of 531
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Fig. 2. XPS spectra of ABC-800: (A) survey scan; (B) C 1s; s and (C) O 1 (C) N 1s and (D) O 1s.
ABC-800 and ABC-700. It indicated that the high temperature treatment slightly enhanced the degree of graphitization and removal of surface functional groups and defect sites. These results were in good agreement with our earlier observations from FT-IR studies. The obtained results indicated that the as synthesised ABC had amorphous carbon (C - network) structure and plane defects [44,45]. Fig. 1C shows FT-IR spectrum of as-synthesized ABC-700, ABC-800 and ABC-900. The characteristic peaks were observed at around 3445 cm−1 for ABC-700 and slightly lower wavenumbers i.e., 3442 cm1 , 3435 cm−1 for ABC-800 and ABC-900, respectively. These values were attributed to OeH stretching vibrations of surface functional groups and intensity of the peaks also lowered which indicated that the number of oxygen containing surface functional groups lowered with increasing temperature. These peaks clearly indicated that three ABC materials had oxygen containing surface functional groups on their surfaces. The oxygen containing carbonyl group (C]O) stretching vibration was observed at around 1625 cm-1 for ABC-700, ABC-800 and ABC-900. The peak intensities of ABC- 800, ABC -900 were significantly reduced as compared to that of ABC-700. The above results confirmed that by increasing the carbonization temperature the extent of oxygen
Table 1 Elemental analysis of ABC. Sample Name
Sample weight (mg)
C atom (%)
H atom (%)
N atom (%)
S atom (%)
ABC-700 ABC-800 ABC-900
3.1940 1.7170 2.0020
81.62 88.22 83.61
0.026 0.000 0.000
0.940 0.790 0.877
0.000 0.000 0.000
functionalization of carbon based materials [43]. Raman spectra of the ABC-700, ABC-800, and ABC-900 samples are shown in Fig. 1B. The ABCs have two distinct peaks at 1330 cm−1 and 1580 cm−1 correspond to D and G bands of disordered and graphitic carbons, respectively. Generally, the degree of graphitization and also defective sites of carbon network can be seen from the intensity ratio of G band (IG) and D band (ID), respectively. The D and G band ratio (ID/IG) of ABC-700, ABC-800 and ABC-900 were found to be 1.05, 1.04 and 1.02, respectively. The result showed that the disorder (i.e., defect sites) nature of ABC-800 quite higher than its graphitic nature. It was also observed that the ID/IG ratio of ABC-900 was significantly lower than that of
Fig. 3. (A) BET-adsorption-desorption isotherm and (B) pore size distribution of ABC-800. 532
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Fig. 4. Representative SEM micrographs of (A) ABC-700, (B) ABC-800, (C) ABC-900 and FESEM images (D–F) of ABC-800.
Fig. 5. (A–E) HRTEM images and (F) SAED pattern of ABC-800.
carbons have predominantly contained carbon and oxygen.
containing functional groups decreases on their surfaces. TGA profile shows the thermal stability of the as-synthesized carbon materials. The profiles of ABC-700, ABC-800 and ABC-900 are shown in Fig. 1D. A weight loss of around 8.5%, 9.4% and 11.2% for ABC-700, ABC-800 and ABC-900, respectively observed at 150 °C which indicated that surface adsorbed water molecules and moisture were removed from the surface. The ABC–700 sample experienced a major weight loss of approximately 57.87% between 480 °C and 550 °C due to complete decomposition of carbon in air, whereas ABC-800, ABC-900 samples showed the major weight loss profile at slightly higher temperature range. After 650 °C, the profiles showed complete decomposition indicating that the samples contained only carbon and no other impurities were present in the samples. Figure. S1 (A–C) displays the EDAX analysis of as-synthesized ABC-700, ABC-800 and ABC-900, respectively. From this Figure it is evidently confirmed that our synthesized
3.1.2. X-ray photoelectron spectroscopy study XPS is a versatile technique known for the evaluation of surface chemical composition, binding energy of atoms and their oxidation states. The XPS survey spectra of ABC – 800 (Fig. 2A) showed a set of high intensity carbon and oxygen peaks. The C 1 s and O 1 s peaks were scanned at higher magnification and the obtained C 1 s, N 1 s and O 1 s lines were shown in Fig. 2B–D. The C 1s spectrum in Fig. 2B shows peaks at the binding energy of 284.8, 285.4, 286.3 and 289.3 eV corresponding to the presence of C]C, CeOeC/CeN, CeOH, C]O groups in the ABC-800 [46]. The N 1s spectrum (Fig. 2C) shows two major peaks at 398.7 eV and 406 eV, which are attributed to pyridinic N and pyrrolic N bonding, respectively. The third broad peak appeared at around 406 eV specifies the attachment of nitrogen atom to oxygen on 533
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Fig. 6. (A) Cyclic voltammograms of ABC-700, ABC-800 and ABC-900 modified electrodes in 0.1 M PBS (pH = 7) containing 0.25 mM AA, 0.2 mM DA, 0.1 mM UA and 0.25 mM NO2− at 25 mV s−1. (B) CV response of bare GCE in the absence (curve a) and presence (curve b) of 0.2 mM AA, 0.1 mM DA, 0.15 mM UA and 0.25 mM NO2−; ABC800/GCE in the absence (curve c) and presence (curve d) of 0.2 mM AA, 0.1 mM DA, 0.15 mM UA and 0.25 mM NO2− at 25 mV s−1.
Fig. 7. (A) Cyclic voltammograms of ABC-800/GCE in 0.1 M PB solution for the detection AA, DA, UA and NO2− in the concentration range from 60 to 220 μM, 30 to 150 μM, 30 to 120 μM and 75 to 250 μM, respectively. (B) Corresponding calibration plot.
electrolyte to the entire surface of the carbon materials and led to improved sensitivity [52]. Fig. 3B depicts the pore size distribution of ABC-800. Pore size distribution found to be in the range of 2.97–24 nm, thus confirmed that the synthesized carbon was mesoporous in nature. From the above results we could conclude that the synthesized carbon samples are hierarchical porous containing micropores, mesopores and macropores.
the surface of carbon [47,48]. There was no other peak obtained corresponding to sulphur and phosphorous. This result is in good agreement with CHNS elemental analysis (Table 1) which proves that no other hetero atom present except nitrogen. Fig. 2D demonstrated that the O 1 s spectrum of ABC-800 revealed the peaks at 531.7, 532.5, 533.5, 535.8 eV due to the presence of −OH, C–O, eC]O and adsorbed H2O, respectively [49]. From the above results we could conclude that as synthesized ABC – 800 mainly contained carbon with oxygen functionalities with very trace amount of nitrogen functional group.
3.1.4. Surface characterization studies Fig. 4A–C showed SEM images of surface morphologies of ABC-700, ABC-800 and ABC-900, which were annealed at different temperatures, 700 °C, 800 °C and 900 °C. Fig. 4D–F depicts FESEM micrographs of ABC-800. The SEM and FESEM images evidently confirmed the formation of interconnected porous nature of the carbon materials by clearly indicating the honeycomb-like porous nature of catalyst which was used like channel to diffuse the analyte throughout the catalyst. The SEM results clearly indicate that the synthesized samples (ABC700, ABC-800 and ABC-900) possess macropores. Furthermore, the HRTEM images clearly showed the formation of mesopores and micropores throughout the carbon framework in Fig. 5A–E. It is noteworthy to mention here that the H2O and CO2 gases come out during the KOH activation process which is used to create pores through the gasification process of carbon [40,53,54]. Fig. 5F showed the SAED pattern of ABC-800. It clearly revealed that the catalyst is amorphous in nature. From this above information, it is clearly explained that the
3.1.3. Brunauer-Emmett-Teller (BET) study of ABC-800 Brunauer-Emmett-Teller (BET) study is used to investigate the porosity and surface area measurement of ABC – 800 (Fig. 3A). Using BET calculation, we found very high surface area around 2430 m2 g−1 for ABC- 800 while the surface area of ABC 700 and 900 were found to be 1700 and 1887 m2/g. Pore radius of 14.8 Å was observed for ABC800. The adsorption-desorption curves showed typical behaviour of type –IV isotherm with a type H4 hysteresis loop and mesoporous nature of the synthesized ABC-800 and corresponding pore volume was 1.35 cm3 g−1. This type H4 hysteresis loop observed at (P/P0) range of 0.4–1.0 ascribed to capillary condensation and also characteristics of activated carbon [50,51]. The major advantage is that ABC samples have higher surface area because of the KOH activation, which creates porous nature of high energy adsorption sites in the sample with highly pure carbon. These properties were used to access the target ions in 534
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Fig. 8. (A, C, E and G) Effect of scan rate on the cyclic voltammograms of ABC-800/GCE in 0.1 M PB solution containing the concentration of AA, DA, UA and NO2− were 2 mM, 1 mM, 1 mM and 2 mM, respectively at different scan rates of 5, 10, 20, 30, 40, 50, 75 and 100 mV s-1; (F) peak current density versus scan rate plot of UA and (B, D and H): peak current density versus square root of scan rate plots of AA, DA, and NO2− ..
these four analytes simultaneously. ABC-800/GCE gives enhanced oxidation peak current compared to other two modified electrodes because of high surface area with suitable mesoporous nature of the material. Hence, high surface area with optimum amount of porous nature is necessary to get better electrocatalytic performance towards the simultaneous detection of multiple analytes. Therefore, further electrochemical sensor studies are carried out with ABC-800 modified electrodes. Fig. 6(B) displays the responses of bare GCE in pure PBS solution (curve a) and in the presence of 0.2 mM AA, 0.1 mM DA, 0.15 mM UA and 0.25 mM NO2− (curve b); and the modified GCE in the absence (curve c) and presence of AA, DA, UA and NO2− (curve d). The bare GCE with analyte gave poor response for the multiple analytes. It gave very broad peak at high positive potential. It is clear that the ABC-800/ GCE showed excellent electrocatalytic activity towards the electrooxidation of AA, DA, UA and NO2−.
ABC-700 to ABC-900 are highly porous nature. 3.2. Electrochemical sensor application 3.2.1. Cyclic voltammetry studies It is well known that AA, DA, UA and NO2− usually coexist in biological matrixes. Therefore, simultaneous detection of these analytes in the solution mixture is important. Fig. 6(A) displays the cyclic voltammetric response of ABC-700, ABC-800 and ABC-900 modified electrodes in 0.25 mM AA, 0.2 mM DA, 0.1 mM UA and 0.25 mM NO2− at the scan rate of 25 mV s-1. The oxidation response of AA, DA, UA and NO2− on ABC-700/GCE, ABC-800/GCE and ABC-900/GCE were observed at the potential of approximately -0.1 V, 0.15 V, 0.3 V and 0.73 V, respectively. Peak separations of AA-DA, DA-UA and UA-NO2− were 250 mV, 150 mV and 430 mV, respectively. All peaks were separated by at least 150 mV. These separations were enough to distinguish 535
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Fig. 9. Amperometric curves of ABC-800/GCE in 0.1 M PBS (A) 0.033–26.47 μM of AA; (C) 1.6–2630 μM DA; (E) 1.6–4134 μM; (G) 4.9–1184 μM NO2−. (B, D, F and H) corresponds to calibration curves of AA, DA, UA and NO2−, respectively.
displayed in Fig. 7A. The Figure clearly shows that the oxidation peaks of four analytes were well defined and the peak currents of all the four analytes increase with increasing the concentration. The linear relationship between the peak current and the concentration of AA, DA, UA and NO2− are depicted in Fig. 7B. The cyclic voltammetric results indicate that the ABC-800/GCE was appropriate for the electrochemical sensing of AA, DA, UA and NO2− with good sensitivity. The linear regression equations of AA, DA, UA and NO2− can be expressed as follows:
Table 2 Analytical parameters of the ABC-800/GCE for the amperometric determination of AA, DA, UA and NO2−. Analyte
Linear range (μM)
Sensitivity (μA cm−2 μM−1)
Correlation coefficient (R2)
(LOD μM)
AA
33–166 166–26,470 1.6–72 82–2630 1.6–4134 4.9–1184
0.55 0.01 4.73 0.11 0.11 0.57
0.989 0.988 0.998 0.989 0.996 0.989
13.7
DA UA NO2−
3.3 1.1 2.7
Δjp (AA) = 1.09 CAA + ( −13.58) (R2 = 0.9906) (Δjp (AA) ) : μA, C: μmol L−1
To investigate the sensing ability of ABC-800/GCE, electro-oxidation of four analytes, AA, DA, UA and NO2− are carried out by cyclic voltammetry technique under optimum condition and the results
Δjp (DA) = 1.89 CDA + 54.4 (R2 = 0.9910) (Δjp (DA) ) : μA, C: μmol L−1 Δjp (UA) = 2.75 C uA + 26.73 (R2 = 0.9777) (Δjp (uA) ) : μA, C: μmol L−1 536
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Table 3 Analytical performances of different carbon material modified electrodes for the simultaneous detection of AA, DA, UA and NO2− with ABC-800/GCE. Electrode
Methods
a
MCNF/PGE CNMCPEb 3D NHPCc d AC e AuNPs@MoS2-NSs/GCE La–MWCNTs modified GCE Fe(III)P/MWCNTs f CDP/GS/MWCNTs g PG/GCE h hCNTs/Au-lDA ABC-800 a b c d e f g h
DPV DPV SWV DPV DPV CA CA DPV CA CV CA
Linear range (μM)
Detection limit (μM) −
AA
DA
UA
NO2
100-10000 2-64 1-120 30-95 20-300 0.4-710 40-2500 5-480 9–2314 0-600 33-166 166-26,470
0.05-30 0.04-5.6 0.05-14.5 1.0-65.0 5-200 0.04-890 0.70-360 0.15-22 5.0-710 – 1.6-72 82.0-2630
0.5-120 0.8-16.8 2-30 2-230 20-400 0.04-810 5.8-1300 – 6-1330 0-420 1.6-4134
– – – – 5-260 0.4-710 1-1600 5-6750 – – 4.9-1184
Reference
AA
DA
UA
NO2−
50.0 2.00 0.10 4.90 3.00 0.14 3.00 1.65 6.45 15.00 13.70
0.02 0.04 0.02 0.06 1.00 0.01 0.09 0.05 2.00 – 0.30
0.20 0.20 0.40 0.70 5.00 0.01 0.30 – 4.82 15.00 1.10
– – – 0.50 0.13 0.50 1.65 – – 2.70
[57] [58] [59] [60] [61] [62] [63] [64] [65] [66] This work
Mesoporous carbon nanofiber-modified pyrolytic graphite electrode. Carbon-nanofiber-modified carbon-paste electrode. Three-dimensional N-doped hierarchically porous carbon. Activated carbon. Au nanoparticles and layered MoS2. Cyclodextrin cross-linked pre-CD / graphene sheet/MWCNTs nanocomposite. pristine graphene. herringbone carbon nanotubes/gold interdigitated microelectrodes.
Table 4 Interference of some co-existing substance for 100 μM of AA, 30 μM DA, 30 μM UA and 100 μM NO2−. Interferents
Tolerance level (μM)i
NaCl, CaCl2, (NH4)2CO3 MgSO4, NaNO3, Cu(CH3COO)2 H2S, N4H4 Glucose Citric acid L-Lysine
700 700 500 200 150 50
Table 5 Determination of AA, DA, UA and NO2− by the proposed sensor in different urine samples. Sample
Analyte
Detected (μM)
Added (μM)
Found (μM)
Recovery (%)
Human urine 1
AA DA UA NO2− AA DA UA NO2−
– – 6.3 – – – 8.1 –
50 50 50 50 100 100 100 100
50.2 50.3 55.2 49.5 99.7 102.0 108.7 99.8
100.4 100.7 110.5 99.0 99.7 102.0 108.7 99.6
Human urine 2
Fig. 10. (A) Linear sweep voltammetry response of ABC-800/GCE in the presence of 0.6 mM, 0.3 mM, 0.3 mM and 0.6 mM of AA, DA, UA and NO2−, respectively. Scan rate: 25 mV s-1; (B) Corresponding current density vs potential plot and (C) Storage stability of ABC-800/GCE. 537
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response of AA, DA, UA and NO2− was decreased by only 5.50%, 5.66%, 1.70% and 1.3%, respectively from its original response. Finally, it can be concluded that the proposed sensor possesses excellent reproducibility, storage stability, long-time stability and anti-interferent ability.
Δjp (NO2−) = 1.97CNO2− + 2.8 (R2 = 0.9907) (Δjp (NO2−) ) : μA, C: μmol L−1 The kinetics of electrode reaction was examined by assessing the effect of scan rate. As shown in Fig. 8(A, C, E and G), the anodic peak current of all the analytes increases linearly with scan rate (υ) and the oxidation peak current gradually shifts to the positive value. As shown in Fig. 8B, D and H, Ip of the ABC-800/GCE for the AA, DA and NO2− is linear with the square root of the scan rate (υ1/2) suggesting that the electron transfer at ABC-800/GCE is diffusion controlled process. Fig. 8E depicts the effect of scan rate on UA oxidation in the range of 5–100 mV s-1. As shown in Fig. 8F, anodic peak current increases linearly with the scan rate, which indicates that the electron transfer was confined to the electrode surface [55]. Further, the logarithm of the oxidation peak current of UA (log Ip) was proportional to the logarithm of the scan rate (log υ) in the sweep range of 5–100 mV s−1. Similarly, the logarithms of the oxidation peak currents (log Ip) of AA, DA, and NO2− were proportional to the logarithm of the scan rate (log υ) in the sweep range of 5-100 mV s-1. The slope values of AA, DA and NO2− were nearly equal to 0.5 which indicates that the oxidation of these three species on ABC-800/GCE are diffusion controlled processes [56]. The slope value of UA was found to be nearly 1 which suggests that the oxidation of UA was adsorption controlled process.
3.2.4. Analytical application In order to explore the reliability of our modified electrode for practical application, urine samples from people of different age groups were selected as a real sample to examine the analytes by standard addition method. The urine samples were centrifuged and the supernatant was collected. After that, the supernatant was filtered to remove large-size protein before the experiment. All the samples were diluted with supporting electrolyte (PBS pH = 7) and preferred amount of the diluted sample was added to the stirring PB solution. It was observed that no AA, DA and NO2− were detected in the practical urine samples then the standard addition method was used for the detection of the analytes. The results are summarized in the Table 5. The recovery of the spiked samples are in the range of 92 to 110.5 which shows that ABC800/GCE can be used to detect the analytes (AA, DA, UA and NO2−) in the urine samples for practical application. 4. Conclusion Hierarchical porous bio-carbons were prepared by simple KOH activation process and used as an electroactive material for the simultaneous sensing of AA, DA, UA and NO2−. The morphological studies, elemental analysis, surface area and pore size were performed using physical characterization techniques. The electrocatalytic responses were performed with ABC-700, ABC-800 and ABC-900 modified electrodes. Among these, ABC-800 modified electrode exhibits excellent catalytic activity towards the electrooxidation of AA, DA, UA and NO2− with well separation in their oxidation peak potentials in CV measurement, which may be due to the high surface area (around 2430 m2 g-1) of ABC-800. The sensitivity and detection limit of AA, DA, UA and NO2− were found to be (0.55, 0.01), (4.73, 0.11), 0.11 and 0.57 μA cm−2 μM−1 and 13.7, 3.2, 1.1 and 3.3 μM, respectively. The highly porous bio-carbons produced from waste amla with high surface area could act as a novel platform for electrochemical sensing of neurotransmitters and biological compounds. Furthermore, the practical application of bio-carbon based sensor was demonstrated.
3.2.2. Amperometry studies To evaluate the analytical feasibility of ABC-800 modified electrode, the amperometric measurements were performed as shown in Fig. 8. Detection of AA, DA, UA and NO2− were carried out by amperometry technique at the optimized applied potentials of -0.1, 0.15, 0.3 and 0.73 V, respectively. Fig. 9A, C, E, G displays amperometric currenttime response of AA, DA, UA and NO2− where current increases linearly with an increase in the concentration of AA from 0.033-0.166 and 0.166–26.47, DA from 1.6 to 72 and 82–2630, UA from 1.6 to 4134 and NO2− 4.9–1184 μM, respectively as shown in Fig. 9(B, D, F and H). Detection limit of AA, DA, UA and NO2− were 13.7, 3.28, 1.07 and 3.29 μM, respectively. Table 2 displays the amperometric results of AA, DA, UA and NO2−. These results clearly showed that the proposed sensor has excellent oxidation ability towards the detection of AA, DA, UA and NO2−. As shown in Table 3, the analytical parameters such as linear range, sensitivity and limit of detection towards the simultaneous detection of AA, DA, UA and NO2− at ABC-800/GCE are compared with previous reports on carbon based nanomaterials [57–66]. It can be noticed that the ABC-800/GCE shows wide linear range. From the above table we can conclude that ABC-800 is a good alternative material for the simultaneous detection of the four compounds.
Conflicts of interest There are no conflicts of interest to declare Acknowledgements One of the authors, Mrs.V. Sudha thanks UGC, New Delhi for the award of UGC-SRF for financial support.
3.2.3. Selectivity, stability, and reproducibility of the ABC-800/GCE The anti-interference ability of ABC-800 was examined towards the detection of 100 μM of AA, 30 μM DA, 30 μM UA and 100 μM NO2− from the co-existing biological and other interferents such as NaCl, CaCl2, (NH4)2CO3, MgSO4, NaNO3, Cu(CH3COO)2, H2S, N4H4, glucose, citric acid and L-Lysine. No significant change in the current response was observed in the presence of interferents (Table 4). The above result indicates that the ABC-800/GCE was selectively detecting these analytes in the presence of interferents. The reproducibility of the modified electrode was investigated by linear sweep voltammetry. Fig. 10A displays the intra-electrode response (repeatability measurements) and corresponding current variation were recorded 10 times with 3 min interval time. The current variation response was shown in Fig. 10B (flow chat). The relative standard deviation (RSD) of the oxidation peak current of AA, DA, UA and NO2− was found to be 5.8%, 4.2%, 6.7% and 4.3%, respectively. The storage stability of the fabricated sensor was examined by storing the ABC-800/GCE at 4 °C in a refrigerator (Fig. 10C). After 8 days, the
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Hierarchical porous carbon derived from waste amla for the simultaneous electrochemical sensing of multiple biomolecules
T
Velayutham Sudhaa,b, Sakkarapalayam Murugesan Senthil Kumara,b, ⁎ Rangasamy Thangamuthua,b, a b
Materials Electrochemistry Division (MED), CSIR-Central Electrochemical Research Institute (CECRI), Karaikudi, 630 003, Tamil Nadu, India Academy of Scientific and Innovative Research (AcSIR), CSIR-CECRI, Karaikudi, 630 003, Tamil Nadu, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Amla fruits Carbonization Hierarchical porous carbon Simultaneous electrochemical sensing Biomolecules
For the first time, highly porous and hierarchical carbon with high surface area (∼2430 m2 g−1) has been prepared from waste amla fruits at different carbonization temperatures viz., 700 °C, 800 °C and 900 °C by a simple and eco-friendly method for the simultaneous electrochemical sensing of biologically important compounds such as ascorbic acid (AA), dopamine (DA), uric acid (UA) and nitrite. The porous carbon materials synthesized at 700 °C, 800 °C and 900 °C are denoted as ABC-700, ABC-800 and ABC-900, respectively. The structural and morphological evaluations of as-synthesized hierarchical porous carbon are carried out with advanced tools and the existence of porous morphology is ascertained. The morphology, amorphous nature, disordered nature, surface area, pore volume, thermal stability and elemental composition of the as-prepared porous carbon are investigated by SEM, HRTEM, FT-IR, Raman, TGA, BET, XPS, EDAX and CHNS analysis. Compared with ABC-700 and ABC-900, the electrochemical sensing ability was higher in the case of ABC-800. Therefore, further electrochemical sensing studies are carried out by using ABC-800. The limit of detection for the simultaneous determination of AA, DA, UA and NO2- are 13.7 μM, 3.2 μM, 1.1 μM and 3.3 μM, respectively. The sensitivity are (0.55, 0.01), (4.73, 0.11), 0.11 and 0.57 μA cm−2 μM−1 and linear ranges are (33–166, 166–26470), (1.6–72, 82–2630), 1.6–4134 and 4.9–1184 μM, respectively for AA, DA, UA and NO2−. The porous carbon based sensor also proves reliable operational stability, long time stability, selectivity and good antifouling properties. The porous carbon based sensor was successfully applied to the practical application for the detection of these biomolecules in the real samples of urine.
1. Introduction Due to the electroactive nature of neurotransmitters, numerous electroanalytical methods have been used to study their role in the brain [1,2]. Particularly, this area of analytical chemistry was first introduced by Ralph Adams in the year of 1970s and termed as “brain chemistry” [3]. Interestingly, ascorbic acid or vitamin C is dispersed in both plant and animals and widely used as an antioxidant in food for the stabilization of colour, cosmetic applications and pharmaceutical preparations [4]. The main role of AA is to prevent cancer and immunity improvement [5]. Dopamine (3, 4-dihydroxyphethylamine, DA) is one of the essential catecholamine neurotransmitters present in the body. Normal level of DA in biological systems is 10 nM to1 μM. Low or unbalanced DA concentration is responsible for the neurological disorders such as Parkinson’s disease, Schizophrenia and HIV infection
[6,7]. Uric acid is another important biomolecule in our body. Its concentration range from 207 to 444 μM, is normally co-exist with DA in biological fluids and abnormal concentration levels may lead to a number of diseases such as gout, hyperuricaemia and Lesch-Nyhan syndrome [5,8]. Nitrite ion is an essential analyte, which added as additive in food products, fertilizers in agriculture and also corrosion inhibitors [9–14]. It is one of the widely studied analytes in many fields including food and environmental studies because the concentration of NO2− in excess level in blood tends to oxidise the haemoglobin [10,11,14,15]. Also, it forms a highly carcinogenic N-nitrosamine when it reacts or interacts with amines and amides which are known to be carcinogens [16,17]. Therefore, the determination of AA, DA, UA and NO2− are important for human health. AA and UA are coexisting species along with DA. Compared to DA, AA has 100 to 1000 times higher concentration in body fluids. NO2− is also present in the biological
⁎ Corresponding author at: Materials Electrochemistry Division (MED), CSIR-Central Electrochemical Research Institute (CECRI), Karaikudi, 630 003, Tamil Nadu, India. E-mail addresses: [email protected], [email protected] (R. Thangamuthu).
https://doi.org/10.1016/j.colsurfb.2019.01.029 Received 16 October 2018; Received in revised form 1 January 2019; Accepted 14 January 2019 Available online 16 February 2019 0927-7765/ © 2019 Elsevier B.V. All rights reserved.
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dispersion of material were studied using high-resolution transmission electron microscopy (HR-TEM, 200 kV, Tecnai G2 TF20) working at an accelerating voltage of 200 kV. Thermogravimetric analysis (TGA) of ABC-700, ABC- 800 and ABC-900 was carried out in TGA/DTA analyzer (SDT Q 600) to find out the material decomposition temperature ranging from room temperature to 700 °C in an air atmosphere. Raman spectroscopic measurements were carried out using RENISHAW I via laser Raman microscope with He–Ne laser (wavelength λ = 633 nm) to understand the chemical nature of the prepared carbon materials. The C, H, N, and S contents in the ABC were estimated using CHNS analysis (elementarvario EL III). The elemental composition of the porous carbon materials were studied using X-ray photoelectron spectroscopy (XPS) with Mg Kα (1253.6 eV) as X-ray source (Thermo Scientific, MULTILAB2000). The surface areas of the ABC-700, ABC -800, ABC-900 were measured using Brunauer-Emmett-Teller (BET) surface area analyzer AutosorbiQ2.
system. Therefore, simultaneous sensing of DA, AA, UA and NO2- [18], and DA, AA and UA is essential [5,8]. Large over potential and fouling by oxidation product result in poor sensitivity and selectivity and hence is not possible to determine these analytes directly by using ordinary (carbon or metal) electrodes [18]. Therefore, the researchers proposed various modified electrodes with appreciable electrochemical sensing ability for this purpose. Bio-waste and natural waste management has always been a major problem in most of the countries including highly populated cites. Usually natural source of waste is burnt and it produces ash and functionalized porous form of carbon by man-made synthesis. Depending on the source of waste, it may contain different types of elements in various proportions and it is used for growing carbon based applications. Carbon is a well-known, naturally abundant material, existing in a variety of molecular and structural forms such as graphite, diamond, nanotubes, graphene, fullerene, nanodiamonds, amorphous carbon and porous carbon, which are used in diverse applications for the benefit of mankind [19–24]. The porous carbon has high surface area, chemically inert, cost effective and simple synthesis routes, ease of availability, environmental friendly and good conductive properties [25–28]. Highly porous carbons are prepared from various bio-waste materials, including eichhnomia crassipes [29], coconut shell [30] and various other bio-waste materials because of their availability. Such porous carbon materials have high electrical conductivity, high surface area and uniform porosity and because of these properties they enhance the accumulation of charge and removes hazardous heavy metals and ions from the water. The man-made porous carbon materials are also used for many applications including sensor, supercapacitors, batteries, adsorption and purification of toxic heavy metal ions [25–33]. Carbon activation process using alkali creates porosity, surface area related advantages [34]. Particularly, KOH activated carbon leads to creation of pores and high specific surface area and the respective electrochemical behaviour mechanism was elaborately discussed in the literature [35–37]. In this work, we synthesized ABC-700, ABC-800 and ABC-900 by chemical activation process. ABC-800 was used as an electrocatalyst for the simultaneous electrochemical oxidation of multiple bio-analytes such as AA, DA, UA and NO2−. The ABC-800 displays excellent electrocatalytic activity towards these analytes. The bio-carbon material increases the surface area and electrical conductivity of the electrode and the porous nature may enhance the electron transfer ability. The present study demonstrates the synthesis of bio-derived carbons from waste amla and their application for the simultaneous electrochemical sensing of AA, DA, UA and NO2−.
2.3. Synthesis of Amla porous carbon The following synthesis procedure has been followed to synthesis the carbon materials by modifying the earlier report [38]. The raw Amla (without seed) was cut into small pieces followed by washing with DI water for several times and dried in an hot air oven at 100 °C for 24 h. The dried materials were powdered well using a mortar followed by heating at 200 °C for 24 h. The sample was treated with KOH for chemical activation process. In a typical activation process, 10% KOH solution was added to the solution containing preferred amount of powdered materials in N2 atmosphere with constant stirring and heating at 60 °C for 1 h. To optimize the temperature, 10 g of the sample was heated at three different temperatures i.e., 700 °C, 800 °C and 900 °C for 2 h in inert atmosphere at 5 °C/min heating rate. Consequently, the resulting porous carbon was washed thoroughly with 1 M HCl and DI water until the pH of the sample became neutral. Afterwards the filtered sample was dried for 12 h in an hot air oven at 100 °C. 2.4. Electrochemical measurements The electrochemical activities of the ABC-700, ABC-800, and ABC900 towards the oxidation of AA, DA, UA and NO2− were studied by using three electrode system. Modified glassy carbon electrode (GCE) was used as a working electrode; platinum wire and saturated calomel electrode (SCE) were used as counter electrode and reference electrode, respectively. Cyclic voltammetry (CV) and amperometry measurements were performed by using a potentiostat of type AUTOLAB PGSTAT302 N (AU86575) analyser. Complete analysis was carried out under N2 atmosphere in 0.1 M PBS supporting electrolyte throughout the electrochemical measurements.
2. Experimental 2.1. Materials
2.5. Preparation of working electrodes Amla was collected from local land, KOH, Na2HPO4, NaH2PO4 were purchased from Hi-media. HCl (97%) from RFCL limited and H2O (18.2 MΩ cm, Millipore) were used throughout the experiments.
To prepare the working electrode, it was carefully polished with alumina on polishing cloth. Then, it was thoroughly washed with deionized water and ultra-sonication with ethanol and Milli-Q water for 10 min each to remove all the adsorbed impurities on the surface and dried in open air atmosphere. The catalyst slurry was prepared by dispersing 5 mg of the sample in a solution containing 900 μl of dimethyl formamide (DMF) and 100 μl of Millipore water. The glassy carbon electrode was coated with 3 μl of the catalyst slurry and dried in open air atmosphere. This modified electrode was used as working electrode throughout the electrochemical studies.
2.2. Apparatus The ABC materials were analysed using powder X-ray diffraction (XRD) measurements by X-ray diffractometer (BRUKER D8 ADVANCE) with Cu Kα radiation (α = 1.5418 Å). The surface functional groups of the porous carbon were exposed from Fourier transform infrared (FTIR) spectra Bruker, TENSOR 27 spectrometer ranging from 400 to 4000 cm−1. The surface morphology, porous nature and elemental composition (EDAX) of ABC samples were examined by scanning electron microscopy (SEM) using TESCAN (Supra 55 V P) operating at an accelerating voltage of 30 kV and field-emission scanning electron microscopy (FESEM) with an accelerating voltage of 30 kV (FE-SEM; Carl Zeiss AG, Supra55 V P). The particle size, nano porous nature and
3. Results and discussion The bioderived carbon materials play an important and active role in the area of electrochemical sensor due to their attractive surface chemistry and physical properties. Two types of activation methods like 530
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Scheme 1. Schematic illustration of synthesis of Activated Bio-Carbon (ABC) from waste amla and electrochemical sensing of AA, DA, UA and NO2− on ABC-800 modified electrode.
Fig. 1. (A–D) X-ray diffraction pattern, Raman spectra, FT-IR spectra and TGA results of ABC-700, 800 and 900, respectively.
DA, UA and NO2− is shown in Scheme 1.
physical and chemical activation process are available to prepare highly porous biocarbon materials [39]. However, chemical activation process is more superior because it produces porosity and high surface area. In addition to this, the technique offers cost effective, high yield, facile synthesis and less activation time [40]. A lot of chemical activating agents are available including KOH, NaOH, ZnCl2, H3PO4, K2CO3 and H2SO4. In this study KOH was used as a chemical activating agent. During the activation process the water is evaporated from the carbon surface. Synthesis of ABC from waste amla by chemical activation process and application of ABC in simultaneous electrochemical sensing of AA,
3.1. Characterization studies 3.1.1. XRD, Raman, FT-IR, and TGA studies Fig. 1A shows the typical XRD pattern of ABC-700, ABC-800 and ABC-900 in which two broad peaks are observed at around 23° and 43° corresponding to (002) and (100) planes. The examined diffraction lines are considerably broader which revealed that the ABC samples were amorphous and slightly disordered in nature [41,42]. Raman spectroscopy is a dominant tool to determine the degree of 531
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Fig. 2. XPS spectra of ABC-800: (A) survey scan; (B) C 1s; s and (C) O 1 (C) N 1s and (D) O 1s.
ABC-800 and ABC-700. It indicated that the high temperature treatment slightly enhanced the degree of graphitization and removal of surface functional groups and defect sites. These results were in good agreement with our earlier observations from FT-IR studies. The obtained results indicated that the as synthesised ABC had amorphous carbon (C - network) structure and plane defects [44,45]. Fig. 1C shows FT-IR spectrum of as-synthesized ABC-700, ABC-800 and ABC-900. The characteristic peaks were observed at around 3445 cm−1 for ABC-700 and slightly lower wavenumbers i.e., 3442 cm1 , 3435 cm−1 for ABC-800 and ABC-900, respectively. These values were attributed to OeH stretching vibrations of surface functional groups and intensity of the peaks also lowered which indicated that the number of oxygen containing surface functional groups lowered with increasing temperature. These peaks clearly indicated that three ABC materials had oxygen containing surface functional groups on their surfaces. The oxygen containing carbonyl group (C]O) stretching vibration was observed at around 1625 cm-1 for ABC-700, ABC-800 and ABC-900. The peak intensities of ABC- 800, ABC -900 were significantly reduced as compared to that of ABC-700. The above results confirmed that by increasing the carbonization temperature the extent of oxygen
Table 1 Elemental analysis of ABC. Sample Name
Sample weight (mg)
C atom (%)
H atom (%)
N atom (%)
S atom (%)
ABC-700 ABC-800 ABC-900
3.1940 1.7170 2.0020
81.62 88.22 83.61
0.026 0.000 0.000
0.940 0.790 0.877
0.000 0.000 0.000
functionalization of carbon based materials [43]. Raman spectra of the ABC-700, ABC-800, and ABC-900 samples are shown in Fig. 1B. The ABCs have two distinct peaks at 1330 cm−1 and 1580 cm−1 correspond to D and G bands of disordered and graphitic carbons, respectively. Generally, the degree of graphitization and also defective sites of carbon network can be seen from the intensity ratio of G band (IG) and D band (ID), respectively. The D and G band ratio (ID/IG) of ABC-700, ABC-800 and ABC-900 were found to be 1.05, 1.04 and 1.02, respectively. The result showed that the disorder (i.e., defect sites) nature of ABC-800 quite higher than its graphitic nature. It was also observed that the ID/IG ratio of ABC-900 was significantly lower than that of
Fig. 3. (A) BET-adsorption-desorption isotherm and (B) pore size distribution of ABC-800. 532
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Fig. 4. Representative SEM micrographs of (A) ABC-700, (B) ABC-800, (C) ABC-900 and FESEM images (D–F) of ABC-800.
Fig. 5. (A–E) HRTEM images and (F) SAED pattern of ABC-800.
carbons have predominantly contained carbon and oxygen.
containing functional groups decreases on their surfaces. TGA profile shows the thermal stability of the as-synthesized carbon materials. The profiles of ABC-700, ABC-800 and ABC-900 are shown in Fig. 1D. A weight loss of around 8.5%, 9.4% and 11.2% for ABC-700, ABC-800 and ABC-900, respectively observed at 150 °C which indicated that surface adsorbed water molecules and moisture were removed from the surface. The ABC–700 sample experienced a major weight loss of approximately 57.87% between 480 °C and 550 °C due to complete decomposition of carbon in air, whereas ABC-800, ABC-900 samples showed the major weight loss profile at slightly higher temperature range. After 650 °C, the profiles showed complete decomposition indicating that the samples contained only carbon and no other impurities were present in the samples. Figure. S1 (A–C) displays the EDAX analysis of as-synthesized ABC-700, ABC-800 and ABC-900, respectively. From this Figure it is evidently confirmed that our synthesized
3.1.2. X-ray photoelectron spectroscopy study XPS is a versatile technique known for the evaluation of surface chemical composition, binding energy of atoms and their oxidation states. The XPS survey spectra of ABC – 800 (Fig. 2A) showed a set of high intensity carbon and oxygen peaks. The C 1 s and O 1 s peaks were scanned at higher magnification and the obtained C 1 s, N 1 s and O 1 s lines were shown in Fig. 2B–D. The C 1s spectrum in Fig. 2B shows peaks at the binding energy of 284.8, 285.4, 286.3 and 289.3 eV corresponding to the presence of C]C, CeOeC/CeN, CeOH, C]O groups in the ABC-800 [46]. The N 1s spectrum (Fig. 2C) shows two major peaks at 398.7 eV and 406 eV, which are attributed to pyridinic N and pyrrolic N bonding, respectively. The third broad peak appeared at around 406 eV specifies the attachment of nitrogen atom to oxygen on 533
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Fig. 6. (A) Cyclic voltammograms of ABC-700, ABC-800 and ABC-900 modified electrodes in 0.1 M PBS (pH = 7) containing 0.25 mM AA, 0.2 mM DA, 0.1 mM UA and 0.25 mM NO2− at 25 mV s−1. (B) CV response of bare GCE in the absence (curve a) and presence (curve b) of 0.2 mM AA, 0.1 mM DA, 0.15 mM UA and 0.25 mM NO2−; ABC800/GCE in the absence (curve c) and presence (curve d) of 0.2 mM AA, 0.1 mM DA, 0.15 mM UA and 0.25 mM NO2− at 25 mV s−1.
Fig. 7. (A) Cyclic voltammograms of ABC-800/GCE in 0.1 M PB solution for the detection AA, DA, UA and NO2− in the concentration range from 60 to 220 μM, 30 to 150 μM, 30 to 120 μM and 75 to 250 μM, respectively. (B) Corresponding calibration plot.
electrolyte to the entire surface of the carbon materials and led to improved sensitivity [52]. Fig. 3B depicts the pore size distribution of ABC-800. Pore size distribution found to be in the range of 2.97–24 nm, thus confirmed that the synthesized carbon was mesoporous in nature. From the above results we could conclude that the synthesized carbon samples are hierarchical porous containing micropores, mesopores and macropores.
the surface of carbon [47,48]. There was no other peak obtained corresponding to sulphur and phosphorous. This result is in good agreement with CHNS elemental analysis (Table 1) which proves that no other hetero atom present except nitrogen. Fig. 2D demonstrated that the O 1 s spectrum of ABC-800 revealed the peaks at 531.7, 532.5, 533.5, 535.8 eV due to the presence of −OH, C–O, eC]O and adsorbed H2O, respectively [49]. From the above results we could conclude that as synthesized ABC – 800 mainly contained carbon with oxygen functionalities with very trace amount of nitrogen functional group.
3.1.4. Surface characterization studies Fig. 4A–C showed SEM images of surface morphologies of ABC-700, ABC-800 and ABC-900, which were annealed at different temperatures, 700 °C, 800 °C and 900 °C. Fig. 4D–F depicts FESEM micrographs of ABC-800. The SEM and FESEM images evidently confirmed the formation of interconnected porous nature of the carbon materials by clearly indicating the honeycomb-like porous nature of catalyst which was used like channel to diffuse the analyte throughout the catalyst. The SEM results clearly indicate that the synthesized samples (ABC700, ABC-800 and ABC-900) possess macropores. Furthermore, the HRTEM images clearly showed the formation of mesopores and micropores throughout the carbon framework in Fig. 5A–E. It is noteworthy to mention here that the H2O and CO2 gases come out during the KOH activation process which is used to create pores through the gasification process of carbon [40,53,54]. Fig. 5F showed the SAED pattern of ABC-800. It clearly revealed that the catalyst is amorphous in nature. From this above information, it is clearly explained that the
3.1.3. Brunauer-Emmett-Teller (BET) study of ABC-800 Brunauer-Emmett-Teller (BET) study is used to investigate the porosity and surface area measurement of ABC – 800 (Fig. 3A). Using BET calculation, we found very high surface area around 2430 m2 g−1 for ABC- 800 while the surface area of ABC 700 and 900 were found to be 1700 and 1887 m2/g. Pore radius of 14.8 Å was observed for ABC800. The adsorption-desorption curves showed typical behaviour of type –IV isotherm with a type H4 hysteresis loop and mesoporous nature of the synthesized ABC-800 and corresponding pore volume was 1.35 cm3 g−1. This type H4 hysteresis loop observed at (P/P0) range of 0.4–1.0 ascribed to capillary condensation and also characteristics of activated carbon [50,51]. The major advantage is that ABC samples have higher surface area because of the KOH activation, which creates porous nature of high energy adsorption sites in the sample with highly pure carbon. These properties were used to access the target ions in 534
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Fig. 8. (A, C, E and G) Effect of scan rate on the cyclic voltammograms of ABC-800/GCE in 0.1 M PB solution containing the concentration of AA, DA, UA and NO2− were 2 mM, 1 mM, 1 mM and 2 mM, respectively at different scan rates of 5, 10, 20, 30, 40, 50, 75 and 100 mV s-1; (F) peak current density versus scan rate plot of UA and (B, D and H): peak current density versus square root of scan rate plots of AA, DA, and NO2− ..
these four analytes simultaneously. ABC-800/GCE gives enhanced oxidation peak current compared to other two modified electrodes because of high surface area with suitable mesoporous nature of the material. Hence, high surface area with optimum amount of porous nature is necessary to get better electrocatalytic performance towards the simultaneous detection of multiple analytes. Therefore, further electrochemical sensor studies are carried out with ABC-800 modified electrodes. Fig. 6(B) displays the responses of bare GCE in pure PBS solution (curve a) and in the presence of 0.2 mM AA, 0.1 mM DA, 0.15 mM UA and 0.25 mM NO2− (curve b); and the modified GCE in the absence (curve c) and presence of AA, DA, UA and NO2− (curve d). The bare GCE with analyte gave poor response for the multiple analytes. It gave very broad peak at high positive potential. It is clear that the ABC-800/ GCE showed excellent electrocatalytic activity towards the electrooxidation of AA, DA, UA and NO2−.
ABC-700 to ABC-900 are highly porous nature. 3.2. Electrochemical sensor application 3.2.1. Cyclic voltammetry studies It is well known that AA, DA, UA and NO2− usually coexist in biological matrixes. Therefore, simultaneous detection of these analytes in the solution mixture is important. Fig. 6(A) displays the cyclic voltammetric response of ABC-700, ABC-800 and ABC-900 modified electrodes in 0.25 mM AA, 0.2 mM DA, 0.1 mM UA and 0.25 mM NO2− at the scan rate of 25 mV s-1. The oxidation response of AA, DA, UA and NO2− on ABC-700/GCE, ABC-800/GCE and ABC-900/GCE were observed at the potential of approximately -0.1 V, 0.15 V, 0.3 V and 0.73 V, respectively. Peak separations of AA-DA, DA-UA and UA-NO2− were 250 mV, 150 mV and 430 mV, respectively. All peaks were separated by at least 150 mV. These separations were enough to distinguish 535
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Fig. 9. Amperometric curves of ABC-800/GCE in 0.1 M PBS (A) 0.033–26.47 μM of AA; (C) 1.6–2630 μM DA; (E) 1.6–4134 μM; (G) 4.9–1184 μM NO2−. (B, D, F and H) corresponds to calibration curves of AA, DA, UA and NO2−, respectively.
displayed in Fig. 7A. The Figure clearly shows that the oxidation peaks of four analytes were well defined and the peak currents of all the four analytes increase with increasing the concentration. The linear relationship between the peak current and the concentration of AA, DA, UA and NO2− are depicted in Fig. 7B. The cyclic voltammetric results indicate that the ABC-800/GCE was appropriate for the electrochemical sensing of AA, DA, UA and NO2− with good sensitivity. The linear regression equations of AA, DA, UA and NO2− can be expressed as follows:
Table 2 Analytical parameters of the ABC-800/GCE for the amperometric determination of AA, DA, UA and NO2−. Analyte
Linear range (μM)
Sensitivity (μA cm−2 μM−1)
Correlation coefficient (R2)
(LOD μM)
AA
33–166 166–26,470 1.6–72 82–2630 1.6–4134 4.9–1184
0.55 0.01 4.73 0.11 0.11 0.57
0.989 0.988 0.998 0.989 0.996 0.989
13.7
DA UA NO2−
3.3 1.1 2.7
Δjp (AA) = 1.09 CAA + ( −13.58) (R2 = 0.9906) (Δjp (AA) ) : μA, C: μmol L−1
To investigate the sensing ability of ABC-800/GCE, electro-oxidation of four analytes, AA, DA, UA and NO2− are carried out by cyclic voltammetry technique under optimum condition and the results
Δjp (DA) = 1.89 CDA + 54.4 (R2 = 0.9910) (Δjp (DA) ) : μA, C: μmol L−1 Δjp (UA) = 2.75 C uA + 26.73 (R2 = 0.9777) (Δjp (uA) ) : μA, C: μmol L−1 536
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Table 3 Analytical performances of different carbon material modified electrodes for the simultaneous detection of AA, DA, UA and NO2− with ABC-800/GCE. Electrode
Methods
a
MCNF/PGE CNMCPEb 3D NHPCc d AC e AuNPs@MoS2-NSs/GCE La–MWCNTs modified GCE Fe(III)P/MWCNTs f CDP/GS/MWCNTs g PG/GCE h hCNTs/Au-lDA ABC-800 a b c d e f g h
DPV DPV SWV DPV DPV CA CA DPV CA CV CA
Linear range (μM)
Detection limit (μM) −
AA
DA
UA
NO2
100-10000 2-64 1-120 30-95 20-300 0.4-710 40-2500 5-480 9–2314 0-600 33-166 166-26,470
0.05-30 0.04-5.6 0.05-14.5 1.0-65.0 5-200 0.04-890 0.70-360 0.15-22 5.0-710 – 1.6-72 82.0-2630
0.5-120 0.8-16.8 2-30 2-230 20-400 0.04-810 5.8-1300 – 6-1330 0-420 1.6-4134
– – – – 5-260 0.4-710 1-1600 5-6750 – – 4.9-1184
Reference
AA
DA
UA
NO2−
50.0 2.00 0.10 4.90 3.00 0.14 3.00 1.65 6.45 15.00 13.70
0.02 0.04 0.02 0.06 1.00 0.01 0.09 0.05 2.00 – 0.30
0.20 0.20 0.40 0.70 5.00 0.01 0.30 – 4.82 15.00 1.10
– – – 0.50 0.13 0.50 1.65 – – 2.70
[57] [58] [59] [60] [61] [62] [63] [64] [65] [66] This work
Mesoporous carbon nanofiber-modified pyrolytic graphite electrode. Carbon-nanofiber-modified carbon-paste electrode. Three-dimensional N-doped hierarchically porous carbon. Activated carbon. Au nanoparticles and layered MoS2. Cyclodextrin cross-linked pre-CD / graphene sheet/MWCNTs nanocomposite. pristine graphene. herringbone carbon nanotubes/gold interdigitated microelectrodes.
Table 4 Interference of some co-existing substance for 100 μM of AA, 30 μM DA, 30 μM UA and 100 μM NO2−. Interferents
Tolerance level (μM)i
NaCl, CaCl2, (NH4)2CO3 MgSO4, NaNO3, Cu(CH3COO)2 H2S, N4H4 Glucose Citric acid L-Lysine
700 700 500 200 150 50
Table 5 Determination of AA, DA, UA and NO2− by the proposed sensor in different urine samples. Sample
Analyte
Detected (μM)
Added (μM)
Found (μM)
Recovery (%)
Human urine 1
AA DA UA NO2− AA DA UA NO2−
– – 6.3 – – – 8.1 –
50 50 50 50 100 100 100 100
50.2 50.3 55.2 49.5 99.7 102.0 108.7 99.8
100.4 100.7 110.5 99.0 99.7 102.0 108.7 99.6
Human urine 2
Fig. 10. (A) Linear sweep voltammetry response of ABC-800/GCE in the presence of 0.6 mM, 0.3 mM, 0.3 mM and 0.6 mM of AA, DA, UA and NO2−, respectively. Scan rate: 25 mV s-1; (B) Corresponding current density vs potential plot and (C) Storage stability of ABC-800/GCE. 537
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response of AA, DA, UA and NO2− was decreased by only 5.50%, 5.66%, 1.70% and 1.3%, respectively from its original response. Finally, it can be concluded that the proposed sensor possesses excellent reproducibility, storage stability, long-time stability and anti-interferent ability.
Δjp (NO2−) = 1.97CNO2− + 2.8 (R2 = 0.9907) (Δjp (NO2−) ) : μA, C: μmol L−1 The kinetics of electrode reaction was examined by assessing the effect of scan rate. As shown in Fig. 8(A, C, E and G), the anodic peak current of all the analytes increases linearly with scan rate (υ) and the oxidation peak current gradually shifts to the positive value. As shown in Fig. 8B, D and H, Ip of the ABC-800/GCE for the AA, DA and NO2− is linear with the square root of the scan rate (υ1/2) suggesting that the electron transfer at ABC-800/GCE is diffusion controlled process. Fig. 8E depicts the effect of scan rate on UA oxidation in the range of 5–100 mV s-1. As shown in Fig. 8F, anodic peak current increases linearly with the scan rate, which indicates that the electron transfer was confined to the electrode surface [55]. Further, the logarithm of the oxidation peak current of UA (log Ip) was proportional to the logarithm of the scan rate (log υ) in the sweep range of 5–100 mV s−1. Similarly, the logarithms of the oxidation peak currents (log Ip) of AA, DA, and NO2− were proportional to the logarithm of the scan rate (log υ) in the sweep range of 5-100 mV s-1. The slope values of AA, DA and NO2− were nearly equal to 0.5 which indicates that the oxidation of these three species on ABC-800/GCE are diffusion controlled processes [56]. The slope value of UA was found to be nearly 1 which suggests that the oxidation of UA was adsorption controlled process.
3.2.4. Analytical application In order to explore the reliability of our modified electrode for practical application, urine samples from people of different age groups were selected as a real sample to examine the analytes by standard addition method. The urine samples were centrifuged and the supernatant was collected. After that, the supernatant was filtered to remove large-size protein before the experiment. All the samples were diluted with supporting electrolyte (PBS pH = 7) and preferred amount of the diluted sample was added to the stirring PB solution. It was observed that no AA, DA and NO2− were detected in the practical urine samples then the standard addition method was used for the detection of the analytes. The results are summarized in the Table 5. The recovery of the spiked samples are in the range of 92 to 110.5 which shows that ABC800/GCE can be used to detect the analytes (AA, DA, UA and NO2−) in the urine samples for practical application. 4. Conclusion Hierarchical porous bio-carbons were prepared by simple KOH activation process and used as an electroactive material for the simultaneous sensing of AA, DA, UA and NO2−. The morphological studies, elemental analysis, surface area and pore size were performed using physical characterization techniques. The electrocatalytic responses were performed with ABC-700, ABC-800 and ABC-900 modified electrodes. Among these, ABC-800 modified electrode exhibits excellent catalytic activity towards the electrooxidation of AA, DA, UA and NO2− with well separation in their oxidation peak potentials in CV measurement, which may be due to the high surface area (around 2430 m2 g-1) of ABC-800. The sensitivity and detection limit of AA, DA, UA and NO2− were found to be (0.55, 0.01), (4.73, 0.11), 0.11 and 0.57 μA cm−2 μM−1 and 13.7, 3.2, 1.1 and 3.3 μM, respectively. The highly porous bio-carbons produced from waste amla with high surface area could act as a novel platform for electrochemical sensing of neurotransmitters and biological compounds. Furthermore, the practical application of bio-carbon based sensor was demonstrated.
3.2.2. Amperometry studies To evaluate the analytical feasibility of ABC-800 modified electrode, the amperometric measurements were performed as shown in Fig. 8. Detection of AA, DA, UA and NO2− were carried out by amperometry technique at the optimized applied potentials of -0.1, 0.15, 0.3 and 0.73 V, respectively. Fig. 9A, C, E, G displays amperometric currenttime response of AA, DA, UA and NO2− where current increases linearly with an increase in the concentration of AA from 0.033-0.166 and 0.166–26.47, DA from 1.6 to 72 and 82–2630, UA from 1.6 to 4134 and NO2− 4.9–1184 μM, respectively as shown in Fig. 9(B, D, F and H). Detection limit of AA, DA, UA and NO2− were 13.7, 3.28, 1.07 and 3.29 μM, respectively. Table 2 displays the amperometric results of AA, DA, UA and NO2−. These results clearly showed that the proposed sensor has excellent oxidation ability towards the detection of AA, DA, UA and NO2−. As shown in Table 3, the analytical parameters such as linear range, sensitivity and limit of detection towards the simultaneous detection of AA, DA, UA and NO2− at ABC-800/GCE are compared with previous reports on carbon based nanomaterials [57–66]. It can be noticed that the ABC-800/GCE shows wide linear range. From the above table we can conclude that ABC-800 is a good alternative material for the simultaneous detection of the four compounds.
Conflicts of interest There are no conflicts of interest to declare Acknowledgements One of the authors, Mrs.V. Sudha thanks UGC, New Delhi for the award of UGC-SRF for financial support.
3.2.3. Selectivity, stability, and reproducibility of the ABC-800/GCE The anti-interference ability of ABC-800 was examined towards the detection of 100 μM of AA, 30 μM DA, 30 μM UA and 100 μM NO2− from the co-existing biological and other interferents such as NaCl, CaCl2, (NH4)2CO3, MgSO4, NaNO3, Cu(CH3COO)2, H2S, N4H4, glucose, citric acid and L-Lysine. No significant change in the current response was observed in the presence of interferents (Table 4). The above result indicates that the ABC-800/GCE was selectively detecting these analytes in the presence of interferents. The reproducibility of the modified electrode was investigated by linear sweep voltammetry. Fig. 10A displays the intra-electrode response (repeatability measurements) and corresponding current variation were recorded 10 times with 3 min interval time. The current variation response was shown in Fig. 10B (flow chat). The relative standard deviation (RSD) of the oxidation peak current of AA, DA, UA and NO2− was found to be 5.8%, 4.2%, 6.7% and 4.3%, respectively. The storage stability of the fabricated sensor was examined by storing the ABC-800/GCE at 4 °C in a refrigerator (Fig. 10C). After 8 days, the
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