3.1. Characterization of EDAS/(g-C3N4-Ag)NC
The formation of EDAS/(g-C3N4-Ag)NC was initially verified through UV-visible diffused reflectance spectra. The spectra of g-C3N4 nanosheets and (EDAS/g-C3N4-Ag)NC are depicted in Fig. 1. The presence of g-C3N4 nanosheets in the nanocomposite is confirmed from the typical UV-vis absorption edge near 430 nm. A new band appeared around 450 nm along with the typical absorption of g-C3N4 in EDAS/(g-C3N4-Ag)NC samples confirmed the formation of AgNPs on g-C3N4 nanosheets. The new band is attributed to the surface plasmon resonance (SPR) absorption characteristic of AgNPs.
Figure 2 shows the FT-IR spectra of g-C3N4 nanosheets and EDAS/(g-C3N4-Ag(2 mM))NC in the wave number range of 500–4000 cm‒1. The peak obtained in the range of 1240–1643 cm‒1 is attributed to the characteristic stretching modes of C–N heterocycles. The broad absorption peaks appeared in the range of 3040 and 3300 cm‒1 are ascribed to the N–H vibration and H2O adsorption, respectively [37, 38]. The FT-IR data inferred that the attachment of Ag nanoparticles does not affect the structural features of the g-C3N4 nanosheets.
Figure 3 shows TEM images of the prepared EDAS/(g-C3N4-Ag(2 mM))NC. The sheet-like structure of g-C3N4 suggests the presence of exfoliated g-C3N4 (Fig. 3 (A)). A large number of spherical AgNPs with size range of 10–15 nm are found on the g-C3N4 nanosheets surface, which confirmed the formation of AgNPs-loaded g-C3N4 nanosheets (Fig. 3(B)). The lattice fringes with interplanar distance of 0.235 nm assigned to Ag(111) facet was found on the AgNPs. The elemental characterization of EDAS/(g-C3N4-Ag(2 mM))NC was performed through EDX analysis (Fig. S1). The spectrum confirmed the presence of the elements C, N, O, Si and Ag by showing their X-ray lines. The elemental mapping of EDAS/(g-C3N4-Ag(2 mM))NC showed the uniform distribution of AgNPs in the nanocomposite and confirmed the formation of g-C3N4-Ag nanocomposite in EDAS sol-gel network (Fig. S1). Cyclic voltammogram of EDAS/(g-C3N4-Ag)NC modified electrode (Fig. S2) shows an anodic peak representing Ag oxidation at 0.23 V, which affirm the Ag presence in the composite material [39].
3.2. Electrocatalytic reduction of nitrobenzene (NB)
The electrocatalytic reduction of NB at the EDAS/(g-C3N4-Ag(2 mM))NC modified electrode (thin films over flat bottom of 3 mm dia GC electrode) was investigated through cyclic voltammetry. The cyclic voltammograms (CVs) recorded for 500 µM of NB at bare GC, GC/(EDAS/g-C3N4) and GC/EDAS/(g-C3N4-Ag(2 mM))NC modified electrodes at a scan rate of 50 mV s‒1 are shown in Fig. 4. In the reductive scan with the GC/EDAS/(g-C3N4-Ag(2 mM))NC modified electrode, the reduction peak for nitrobenzene was observed at ‒0.68V, whereas, the corresponding oxidation peak was observed at 0.6 V. The reduction peak of NB in the bare GC and GC/(EDAS/g-C3N4) were observed at ‒0.78 V and ‒0.74 V, respectively. The reduction peak current of nitrobenzene for GC/EDAS/(g-C3N4-Ag(2 mM))NC, GC/(EDAS/g-C3N4) and bare GC were found to be 22.22 µA, 7.2 µA and 8 µA, respectively. The above results imply that the GC/EDAS/(g-C3N4-Ag(2 mM))NC possesses enhanced electrocatalytic activity towards reduction of nitrobenzene.
Figure 5 depicts the CVs of GC/EDAS/(g-C3N4-Ag(1 mM))NC, GC/EDAS/(g-C3N4-Ag(2 mM))NC and GC/EDAS/(g-C3N4-Ag(3 mM))NC modified electrodes in the presence of 0 µM to 600 µM nitrobenzene. Upon increasing the concentration of nitrobenzene, linear increase in the cathodic peak current is observed. For comparing the electrocatalytic reduction of nitrobenzene in different electrodes, the plots of cathodic peak current (Ipc) vs concentration of nitrobenzene obtained for various electrodes are shown in Fig. 5(d). It can be clearly visible that, the GC/EDAS/(g-C3N4-Ag(2 mM))NC modified electrode showed improved electrocatalytic activity than those of GC/EDAS/(g-C3N4-Ag(1 mM))NC and GC/EDAS/(g-C3N4-Ag(3 mM))NC. Since the GC/EDAS/(g-C3N4-Ag(2 mM))NC modified electrode exhibited better activity, it was used for further electrochemical experiments.
Fig. S3 demonstrates the effect of pH on the electro-reduction of 0.5 mM nitrobenzene at GC/EDAS/(g-C3N4-Ag(2 mM))NC modified electrode. The peak potential for nitrobenzene reduction is shifted towards negative potentials on increasing the pH of the solution. It is evident that the negative shift of peak potential advocates the electron transfer and proton transfer occurs simultaneously. An increasing trend is observed in the cathodic peak current upto to pH 7.4 and tend to drop beyond that pH (pH>7.4). As the maximum peak current was attained at pH 7.4, it was fixed for sensor applications.The variation of scan rate on the electrocatalytic reduction of 0.5 mM nitrobenzene at GC/EDAS/(g-C3N4-Ag(2 mM))NCelectrode is displayed in Fig. S4.While increasing the scan rate from 10 to 300 mV s‒1, well-defined cathodic peak appears at ‒0.64 V. The value of current increased with increasing scan rate. Due to the irreversible nature of nitrobenzene reduction, the cathodic peak shifts toward more negative potential with increasing scan rate. A linear plot is obtained between the peak current and square root of the scan rate, which is depicted in Fig. S4 and it suggests that the reduction of nitrobenzene was a diffusion controlled process.
The mechanism of electroreduction of nitrobenzene may be understood from the scrutiny of CV recorded for the reduction of NB (500 µM) on the GC/EDAS/(g-C3N4-Ag(2 mM))NC modified electrode shown in Fig. S5. The peak obtained at ‒0.65 V is corresponding to the reduction of –NO2 group to –NHOH group and an anodic peak at + 0.05 V is due to the oxidation of –NHOH to –NO group. A new peak observed at ‒0.11 V in the second cycle may be ascribed to the reduction of –NO to –NHOH (Fig. 6) [17, 40]. The electrocatalytic activity of GC/EDAS/(g-C3N4-Ag(2 mM))NC was also evaluated in the presence of nitrobenzene derivatives. The well resolved peaks and very good electrochemical responses observed for the electroreduction of nitrobenzene derivatives at the modified electrode indicated that this modified electrode is capable of sensing nitrobenzene derivatives by its electroreducing property. The present GC/EDAS/(g-C3N4-Ag(2 mM))NC showed peak potential of ‒0.8 V, ‒0.67 V, 0.78 V and ‒0.7 V for the reduction of nitroaniline, nitrobenzoic acid, nitrophenol and nitrotoluene, respectively (Fig. S6). Upon increasing the concentration of analytes, linear increase in peak current is observed, revealing good sensing response of the GC/EDAS/(g-C3N4-Ag)NC electrode.
3.3. SWVs of NB on GC/EDAS/(g-C3N4-Ag (2 mM))NC
The sensing ability of the GC/EDAS/(g-C3N4-Ag(2 mM))NC modified electrode (thin films over flat bottom of 3 mm dia GC electrode) towards the detection of nitrobenzene and its derivatives was investigated by SWV. The square wave voltammograms (SWVs) were recorded by applying a step potential of 4 mV, amplitude of 25 mV and a frequency of 15 Hz. The SWV curve obtained for each addition of 5 µM nitrobenzene at the GC/EDAS/(g-C3N4-Ag(2 mM))NC modified electrode is shown in Fig. 7(A). The cathodic peak for NB reduction appears at ‒0.58V. While increasing the concentration of nitrobenzene, linear increase in the current is observed. The modified electrode also exhibits better responses for the addition of 2 µM nitrobenzene which is shown in Fig. 7(B).
The SWV signals obtained for the determination of nitrobenzoic acid (NBA), nitroaniline (NA), nitrophenol (NP) and nitrotoluene (NT), employing the GC/EDAS/(g-C3N4-Ag (2 mM))NC modified electrode, at each increase of 5 µM nitrobenzene derivatives are shown in Fig. 8. The modified electrode performed well with moderate concentrations of NBA, NA, NP and NT (Fig. 8). Due to the adsorption of redox analytes at higher concentration of analytes, a split in square wave voltammograms is observed [41, 42].
Table 1 summarizes the results obtained with various electroanalytical techniques and different metal nanocomposite electrodes including EDAS/(g-C3N4-Ag)NC modified electrode for the determination of NB and its derivatives [8, 24, 43–53]. The abbreviations of the modified electrodes in the table are given as provided in the appropriate references. The prepared EDAS/(g-C3N4-Ag)NC showed relatively low LOD and wide sensing range, mainly owing to the synergy of AgNPs and g-C3N4 nanosheets.
Table 1
Comparison of LOD for nitrobenzene at EDAS/(g-C3N4-Ag)NC with other reported modified electrodes.
Electrode material
|
Analyte
|
Analytical Technique
|
Detection limit
|
Ref
|
GC/EDAS-AgNPs
|
Nitrobenzene
|
Amperometry
|
2.5 nM
|
[8]
|
GC/TPDT-SiO2/AgNPs
|
Nitrobenzene
|
SWV
|
0.5 µM
|
[24]
|
GC/GO
|
p-Nitrophenol
|
LSV
|
0.02 µM
|
[43]
|
GC/nano-gold
|
p-Nitrophenol
Semi-derivative
|
Voltammetry
|
8 µM
|
[44]
|
CPE/CD-SBA
|
p-Nitrophenol
|
DPV
|
0.01 µM
|
[45]
|
GC/Pt-PdNPs/CNTs-rGO
|
Nitrobenzene
|
DPV
|
0.42 ppb
|
[46]
|
GC/Pt-PdNPs/CNTs-rGO
|
p-Nitroaniline
|
DPV
|
0.62 ppb
|
[46]
|
CPE/DTD/AgNPs
|
p-Nitroaniline
|
DPV
|
0.23 µM
|
[47]
|
GC/OMCN-800
|
Nitrobenzene
|
DPV
|
0.18 µM
|
[48]
|
GC/po-MWCNTs
|
p-Nitrotoluene
|
SWASV
|
0.28 µM
|
[49]
|
GC/pn-MWCNTs
|
p-Nitrotoluene
|
SWASV
|
0.442 nM
|
[49]
|
GC/γ-Al2O3
|
Nitrobenzene
|
DPV
|
0.15 µM
|
[50]
|
GC/AuNPs
|
Nitrobenzene
|
DPV
|
0.016 µM
|
[51]
|
OMC/DDAB/GC
|
Nitrobenzene
|
LSV
|
10 µM
|
[52]
|
β-CD1.2mg/GO/SPCE
|
Nitrobenzene
|
LSV
|
0.184 µM
|
[53]
|
GC/EDAS/(g-C3N4-Ag)NC
|
Nitrobenzene
|
SWV
|
2 µM
|
Present work
|