Synthesis, Characterization of Some Conductive Aromatic Polyamides/Fe 3 O 4 NPs/ITO, and Their Utilization for Methotrexate Sensing

: Here, we have synthesized four series of polyamide-conductive polymers and used them to modify Fe 3 O 4 NPs/ITO electrodes. The ability of the modiﬁed electrodes to detect methotrexate (MTX) anticancer drug electrochemically was investigated. Synthesis of the target-conducting polyamides, P1a–d, P2a–d, P3a, P3b, P3d, and P4c-d, based on different aromatic moieties, such as ethyl 4-(2-(4H-pyrazol-4-ylidene)hydrazinyl)benzoate, diphenyl sulfone, diphenyl ether or phenyl, has been achieved. They were successfully prepared in good yield via solution–polycondensation reaction of the diamino monomers with different dicarboxylic acid chlorides in the presence of N-methyl-2-pyrrolidone (NMP) as a solvent and anhydrous LiCl as a catalyst. A model compound 4 was synthesized from one mole of ethyl-4-(2-(3, 5-diamino-4H-pyrazol-4-ylidene)hydrazinyl) benzoate (diamino monomer) (3) with two moles benzoyl chloride. The structure of the synthesized monomers and polymers was conﬁrmed by elemental and spectral analyses. In addition, thermogravimetric analysis evaluated the thermal stabilities of these polyamides. Furthermore, the morphological properties of selected polyamides were examined using an scanning electron microscope. Polyamide/Fe 3 O 4 /ITO electrodes were prepared, and the electrochemical measurements were performed to measure the new polyamides’ conductivity and to detect the MTX anticancer drug in phosphate buffer saline using cyclic voltammetry. The polyamides (P3b and P4b)/Fe 3 O 4 /ITO electrodes showed the highest sensitivity and reversibility towards MTX.


Introduction
The realm of chemical analysis has demonstrated extraordinary development, especially in the medicinal and environmental fields. Of special interest is the invention of biosensors. Biosensors are devices used to detect the presence of a biological analyte or a chemical substance that combines biological components with a physicochemical detector [1][2][3]. Polymers are promising candidates that can facilitate a new generation of biosensors [4][5][6][7][8].

Characterization Techniques
All melting points of monomers, model compounds, and polymers were measured using the MEL-TEMP Π laboratory device, USA, instrument (Assiut University). Varo MACRO cube: elemental analyzer/Elementar (Egyptian Petroleum Research Institute, Cairo, Egypt) was used to record elemental analyses of all prepared compounds. Fourier transform infrared (FT-IR) spectra of all prepared materials were recorded by using Nicolet 6700, Thermo Fisher Scientific, USA, spectrophotometer, using the KBr pellet technique (Assiut University, Egypt). The UV-visible spectra were recorded in N-methyl-2-pyrrolidone (200-700 nm) using Shimadzu 2110 PC scanning spectrophotometer (Assiut University, Egypt). 1H NMR spectra were recorded using Bruker High-Performance Digital FT-NMR Spectrometer AVANCE lll 400 MHz (Cairo University), Varian EM390 (90 MHZ) Spectrometer (Assiut University), and AVANCE-III 400 MHz High-Performance FT-NMR Spectrom BRUKER-BioSpin International AG-Switzerland (Sohag University, Egypt); the chemical shifts are reported at δ values (ppm) relative to TMS as an internal reference. Mass spectra were performed on Thermo SCIENTIFIC Spectrometer (Assiut University, Egypt) and Electrospray Ionization Mass Spectrometry (ETH Zürich, Switzerland). Thermogravimetric analysis (TGA) was measured under nitrogen using the TGA-50 SHIMADZU instrument (Al-Azhar University, Assiut), at a heating rate of 10 • C/min. The morphologies of selected polyamide samples were examined by scanning electron microscope (SEM) using a Jeol JSM-5400 LV instrument (Assiut University, Egypt). SEM sample was prepared on a copper holder by placing a smooth part of the sample powder and then covering with gold/palladium alloy. SEM images were taken using a Pentax Z-50P Camera with Ilford film at an accelerating voltage of 15 kV, using a low-dose technique.
The X-ray diffraction patterns of the polyamides and polyamide/Fe 3 O 4 /ITO hybrid samples were obtained with X-ray PW 1710 control unit Philips anode material Cu (40 K.V, and then covering with gold/palladium alloy. SEM images were taken using a Pentax Z-50P Camera with Ilford film at an accelerating voltage of 15 kV, using a low-dose technique. The X-ray diffraction patterns of the polyamides and polyamide/Fe3O4/ITO hybrid samples were obtained with X-ray PW 1710 control unit Philips anode material Cu (40 K.V, 30 M.A) Optics in (flex, Holanda): Automatic divergence slit Beta filtering was performed using a graphite monochromator (Assiut University).

Synthesis of the Model Compound 4
A mixture of the diamino monomer 3 (1.09 g, 4.00 mmol) and LiCl (1.00 g) (as a catalyst) in 20 mL N-methyl-2-pyrrolidone (NMP) was stirred in an ice bath under nitrogen and then a solution of benzoyl chloride (1.12 g, 8.00 mmol) in 8 mL NMP was added to the reaction mixture. The stirring was continued for 6 h at room temperature. The reaction mixture was poured into iced water (100 mL) (Scheme 2). A solid product was separated, filtered off, washed well with water, dried, and recrystallized from toluene as yellow crystals; yield: 85%; m.p.

Synthesis of the Model Compound 4
A mixture of the diamino monomer 3 (1.09 g, 4.00 mmol) and LiCl (1.00 g) (as a catalyst) in 20 mL N-methyl-2-pyrrolidone (NMP) was stirred in an ice bath under nitrogen and then a solution of benzoyl chloride (1.12 g, 8.00 mmol) in 8 mL NMP was added to the reaction mixture. The stirring was continued for 6 h at room temperature. The reaction mixture was poured into iced water (100 mL) (Scheme 2). A solid product was separated, filtered off, washed well with water, dried, and recrystallized from toluene as yellow crystals; yield: 85%; m.p.: 213-215 • C. Anal. Calcd. for C 26

Polymers Syntheses
General Procedure A: For the Synthesis of Polyamides A mixture of the diamino compound (4.00 mmol), and LiCl catalyst (1.00 g) in 20 mL NMP was stirred in an ice bath under nitrogen; then, a solution of diacid chloride (4.00 mmol) in 15 mL NMP was dropwise added while stirring over 20 min; the mixture was subsequently stirred for 6 h at room temperature. The reaction mixture was poured into iced water (Scheme 3). A solid polymer was separated, then washed with hot water and ethanol and dried under reduced pressure (I mm/Hg) at 60 °C for 24 h.

Polymers Syntheses
General Procedure A: For the Synthesis of Polyamides A mixture of the diamino compound (4.00 mmol), and LiCl catalyst (1.00 g) in 20 mL NMP was stirred in an ice bath under nitrogen; then, a solution of diacid chloride (4.00 mmol) in 15 mL NMP was dropwise added while stirring over 20 min; the mixture was subsequently stirred for 6 h at room temperature. The reaction mixture was poured into iced water (Scheme 3). A solid polymer was separated, then washed with hot water and ethanol and dried under reduced pressure (I mm/Hg) at 60 • C for 24 h.

Polymers Syntheses
General Procedure A: For the Synthesis of Polyamides A mixture of the diamino compound (4.00 mmol), and LiCl catalyst (1.00 g) in 20 mL NMP was stirred in an ice bath under nitrogen; then, a solution of diacid chloride (4.00 mmol) in 15 mL NMP was dropwise added while stirring over 20 min; the mixture was subsequently stirred for 6 h at room temperature. The reaction mixture was poured into iced water (Scheme 3). A solid polymer was separated, then washed with hot water and ethanol and dried under reduced pressure (I mm/Hg) at 60 °C for 24 h.   Preparation of Polyamide/Fe 3 O 4 /ITO Electrodes and Polyamide/ITO Electrodes The polyamide/ITO electrodes were prepared to measure the conductivity of polyamides. A (0.001 g, 1 mg/1 mL) of the polyamide was dissolved in 1 mL NMP; then, the ITO electrode was immersed in this solution for 6 h, taken out, rinsed with DIW, and dried. To prepare polyamide/Fe 3 O 4 /ITO electrodes, Fe 3 O 4 /ITO electrodes were firstly prepared by the solvothermal method [42]. Secondly, the Fe 3 O 4 /ITO electrode was immersed in a solution of (0.001 g) of the polyamide, dissolved in 1mL NMP for 6 h, rinsed with DIW and dried.

Electrochemical Measurements
All the electrochemical measurements were performed under ambient laboratory conditions using the electrochemical workstation Metrohm (AUTOLAB) (Netherlands) instrument (Faculty of Science-Assiut University, Egypt) by using Ag/AgCl as a reference electrode, the polymer/Fe 3 O 4 /ITO electrode (1 cm × 1 cm) as a working electrode, and Pt as a counter electrode, based on cyclic voltammetry (CV) technique. All the used working electrodes were fabricated under the same conditions. The active geometrical area of the working electrode was 1 cm 2 ; the electrolyte volume was 20 mL; and the scan rate was 50 mV/s.

Monomer Synthesis
The diamino monomer 3 was synthesized in good yield by the reaction of one mole of the dicyano compound 2 with 1.1 moles of hydrazine hydrate, as described in Scheme 1. The chemical structures of the synthesized dicyano compound 2 and the diamino monomer 3 were confirmed via elemental analysis and spectral data. The FT-IR spectra displayed new characteristic absorption bands at 2230 cm −1 (CN) for the dicyano compound 2 and displayed characteristic absorption bands at 3300 & 3390 cm −1 (NH 2 ) for the diamino monomer 3 (see supporting information).

Synthesis of the Model Compound 4
Before attempting the polymerization process, model compound 4 was synthesized by the reaction of one mole of diamino monomer 3 with two moles of benzoyl chloride in NMP using anhydrous LiCl as a catalyst under nitrogen, as described in Scheme 2. The chemical structure of the synthesized model compound was confirmed via elemental analysis and spectral data. The FT-IR spectrum of the model compound displayed characteristic absorption bands at 1714, and 1666 cm −1 due to (C=O ester) and (C=O, amide), respectively (see Supporting Information).

Polymerization and Polymer Characterization
The target four new series of polyamides P1a-d, P2a-d, P3a,b,d, and P4c,d, were synthesized using the polycondensation technique [43] by the interaction of one mole from the diamines with one mole of the dicarboxylic acid chlorides, as illustrated in Scheme 3. Polyamides P3c, P4a, and P4b were synthesized at a very low yield. The FT-IR spectra of all synthesized polyamides showed new characteristic bands at 1640-1690 cm −1 for (C=O, amide) and 3200-3460 cm −1 for (N-H, amide) groups, along with the presence of other characteristic absorption bands for specific groups present in the various polymers (see Supporting Information). Additionally, the 1 H NMR spectra of all polyamides in DMSO were compatible with the proposed structures (see Supporting Information). The various characteristics of the prepared polymers, including solubility, X-ray diffraction analysis, TGA, SEM, and electrochemical study with Fe 3 O 4 nanoparticles, were determined, and all the data are discussed below.

Solubility
The solubility of all the synthesized polyamides P1a-d, P2a-d, P3a,b,d, and P4c,d was tested at room temperature using various solvents, including dimethylformamide (DMF), dimethylsulfoxide (DMSO), tetrahydrofuran (THF), chloroform, dichloromethane (DCM), N-methyl-2-pyrrolidone (NMP), and formic acid. A 0.5% (w/v) solution was taken as the criterion for solubility; the solution was then analyzed by visual inspection (see Table 1). All polymers are completely soluble in NMP and insoluble in CHCl 3 and DCM. It was noticed that the polymers in the first series, P1a-d, were slightly more soluble than in the other series. This behavior could be attributed to the steric hindrance effect of the substituted phenyl rings in polyamides P1a-d, which increases the chain packing distances, decrease the inter-chain interactions like hydrogen bonding, and thereby makes solvation easier [44].

X-ray Analysis
The X-ray diffractograms of the prepared polyamides P1a-d, P2a-d, P3a,b,d, and P4c,d available in the Supporting Information, showed a few reflection peaks intermediate between crystalline and amorphous phases in region 2θ = 5-80 • . This indicates that there is a large class of structures in the ordered states between crystalline and amorphous phases (with a pronounced long-range order) in the arrangement of their atoms and molecules [45,46]. Polyamides P1a, P1b, P2b, P2c, and P2d showed amorphous characters (i.e., no reflection sharpness peaks), while, in the polyamides P1c, P1d, P2a, P3a, P3d, P4c, and P4d, the reflection sharpness increased, and the polymers became semicrystalline. The presence of polar and hydrogen bonding groups in the polymer main chains increases the possibility of dipole-dipole and hydrogen bonding intermolecular forces, which increase the crystallinity of these polymers [47].

Scanning Electron Microscope (SEM) Measurements
The morphology of selected examples from the polyamides P1a, P1c, P2a, P2c, P3a, P3d, and P4c were examined by SEM using a low-dose technique [48]. The tested polymer samples were prepared as mentioned in the experimental section. Figure 1a (X = 2000) showed that polyamide P1a has a sponge shape with big pores; higher magnification (X = 5000) (Figure 1b) showed a cavity shape. Figure 1c,d (X = 2000 & 5000) showed that polyamide P1c has a spongy shape at different magnifications. Figure 1e (X = 2000) showed that polyamide P2a has a cavity shape with big pores; higher magnification (X = 10,000) ( Figure 1f) showed a sub-globular structure. Figure 1g (X = 2000) showed that polyamide P2c has a spongy shape with big pores; higher magnification (X =10,000) ( Figure 1h) showed a cauliflower structure. Figure 1i,j (X = 2000 & 5000) showed that polyamide P3a has aggregates of layers structure at different magnifications. Figure 1k,l (X = 5000 & 10,000) showed that polyamide P3d has a spongy structure at different magnifications. It is worth noting that the formed polymers showed non-uniform particle size, which means it is difficult to obtain the average particle size. ij153 software was used to study the particle sizes of the fabricated polymers. Twenty-five particles were selected randomly from each SEM image to calculate the average particle size. The distribution of the particle sizes of each polymer is represented in Supplementary Data. The average particle size of the P1a, P1c, P2a, P2c, P3a, and P3d was 640.000 nm, 512.192 nm, 268.651 nm, 203.978 nm, 233.090 nm, and 191.535 nm, respectively. Furthermore, the morphology of the Fe 3 O 4 NPs-modified ITO electrode was studied using SEM. Figure 2 shows the SEM image of the Fe 3 O 4 NPs-modified ITO electrode, which indicated the formation of nanoparticles. 5000) (Figure 1b) showed a cavity shape. Figure 1c,d (X = 2000 & 5000) showed that polyamide P1c has a spongy shape at different magnifications. Figure 1e (X = 2000) showed that polyamide P2a has a cavity shape with big pores; higher magnification (X = 10,000) ( Figure 1f) showed a sub-globular structure. Figure 1g (X = 2000) showed that polyamide P2c has a spongy shape with big pores; higher magnification (X =10,000) ( Figure 1h) showed a cauliflower structure. Figure 1i,j (X = 2000 & 5000) showed that polyamide P3a has aggregates of layers structure at different magnifications. Figure 1k,l (X = 5000 & 10,000) showed that polyamide P3d has a spongy structure at different magnifications. It is worth noting that the formed polymers showed non-uniform particle size, which means it is difficult to obtain the average particle size. ij153 software was used to study the particle sizes of the fabricated polymers. Twenty-five particles were selected randomly from each SEM image to calculate the average particle size. The distribution of the particle sizes of each polymer is represented in Supplementary Data. The average particle size of the P1a, P1c, P2a, P2c, P3a, and P3d was 640.000 nm, 512.192 nm, 268.651 nm, 203.978 nm, 233.090 nm, and 191.535 nm, respectively. Furthermore, the morphology of the Fe3O4 NPsmodified ITO electrode was studied using SEM. Figure 2 shows the SEM image of the Fe3O4 NPs-modified ITO electrode, which indicated the formation of nanoparticles.

UV-Vis Absorption Study
The electronic absorption (UV-vis) of the polyamides P1a-d, P2a-d, P3a,b,d, and P4c,d were measured in NMP at (0.125 mg/mL) concentration at room temperature. The UV-vis spectra of all polyamides showed absorption bands at λmax 297-419 nm, due to the π-π* transition within the benzenoid system and n-π* transition of C=O groups, in addition to the extended conjugation in the polymers' main chains. In addition, the polymers P2c,d showed absorption bands with λmax near 360-365 nm, which was attributed to the presence of the N=N group in the polymer backbone [49][50][51][52] (Figure 3).

UV-Vis Absorption Study
The electronic absorption (UV-vis) of the polyamides P1a-d, P2a-d, P3a,b,d, and P4c,d were measured in NMP at (0.125 mg/mL) concentration at room temperature. The UV-vis spectra of all polyamides showed absorption bands at λ max 297-419 nm, due to the π-π* transition within the benzenoid system and n-π* transition of C=O groups, in addition to the extended conjugation in the polymers' main chains. In addition, the polymers P2c,d showed absorption bands with λ max near 360-365 nm, which was attributed to the presence of the N=N group in the polymer backbone [49][50][51][52] (Figure 3).

UV-Vis Absorption Study
The electronic absorption (UV-vis) of the polyamides P1a-d, P2a-d, P3a,b,d, and P4c,d were measured in NMP at (0.125 mg/mL) concentration at room temperature. The UV-vis spectra of all polyamides showed absorption bands at λmax 297-419 nm, due to the π-π* transition within the benzenoid system and n-π* transition of C=O groups, in addition to the extended conjugation in the polymers' main chains. In addition, the polymers P2c,d showed absorption bands with λmax near 360-365 nm, which was attributed to the presence of the N=N group in the polymer backbone [49][50][51][52] (Figure 3).

TGA Studies
The thermal behavior of all synthesized polyamides, P1a-d, P2a-d, P3a,b,d, and P4c,d, was evaluated by TGA under a nitrogen atmosphere at a heating rate of 10 °C/min (see Supporting Information). The temperature values, corresponding to 10%, 20%, 30%, 40%, and 50% weight loss in addition to char yield and LOI (limiting oxygen index) at 800

TGA Studies
The thermal behavior of all synthesized polyamides, P1a-d, P2a-d, P3a,b,d, and P4c,d, was evaluated by TGA under a nitrogen atmosphere at a heating rate of 10 • C/min (see Supporting Information). The temperature values, corresponding to 10%, 20%, 30%, 40%, and 50% weight loss in addition to char yield and LOI (limiting oxygen index) at 800 • C, are shown in Table 2. TGA curves showed a small weight loss within the range of 0.5 to 5%, starting at 49-290 • C, which may be attributed to the loss of moisture and trapped solvents [53]. TGA curves showed the initial decomposition temperature (IDT) of these polyamides (10% weight loss), which is the start of polymer decomposition and occurred within the range of 150 to 305 • C for all polyamides [54,55]. The temperature value of 50% weight loss (T 50 %) is one of the main criteria for determining the relative thermal stability of a polymer [56,57], which occurred within the range of 630 • C to 800 • C for these polyamides (see supporting information). The high thermal resistance of these aromatic polyamides, with decomposition temperatures above 500 • C, is a result of high-bond dissociation energies of C-N and C-C bonds [11] Some of these polyamides, P1b, P2a, P2d, P3d, P4c, and P4d, showed one broad degradation step in their decomposition patterns, while, in the case of polyamides P1a, P1c, P1d, P2b, P2c and P3b, two degradation steps were observed, and the curves of some of these are slightly overlapped. P3a showed three overlapped degradation step curves.
The thermal cracking curve of P4a showed one broad degradation step. In this polymer, there are at least three possible positions where the fission can occur (as examples, see Supporting Information). Thus, the broad degradation step of this polymer could be attributed to the overlap between the degradation steps as the degradation temperatures of these steps are closed.

Qualitative Assessment of the Polymers' Conductivities
To shed more light on the conductivity of the prepared polyamides P1a-d, P2a-d, P3a,b,d, and P4c,d [61][62][63], they have been used to coat ITO electrodes. The thickness of the polymer layer was investigated using SEM. The standard SEM measurements provided an indicative layer thickness of around 1-2 microns (see The Supplementary Material). The electrochemical conductivity of the prepared polyamide/ITO electrodes in [Fe(CN) 6 ] 3−/4 -a solution based on the CV technique [64][65][66][67], revealed that most of these prepared polyamide polymers are conducting polymers and most of them have higher conductivity than the bare ITO electrode (Figure S40a-d). The results indicated that the modification of the bare ITO with all prepared polyamide polymers led to enhancement of the electroconductivity, excepting three polymers (P2a, P2b, and P2d), which showed lower electrochemical conductivity compared with the bare ITO electrode.

Polyamides/Fe 3 O 4 NPs/ITO Electrodes for Analysis of MTX
To improve the electrochemical conductivity of the prepared polyamides, organicinorganic hybrid polyamides/Fe 3 O 4 NPs/ITO electrodes were prepared. The formation of the organic-inorganic hybrids electrode was confirmed based on the XRD ( Figure S37). Then, all the polyamides/Fe 3 O 4 NPs-modified ITO electrodes were applied for the MTX detection [68][69][70]. Figure 4 shows the CV responses of selected polyamides/Fe 3 O 4 NPs/ITO electrodes in the presence and the absence of 1 mM of MTX anticancer, which showed the highest CV response towards the MTX. Figure   To study the stability of the developed polyamides/Fe3O4 NPs-modified ITO trodes towards the detection of MTX, P4d/Fe3O4/ITO electrode was used to detect over 80 cycles. Figure 5a shows cyclic voltammograms of 10 −9 mol L −1 of MTX a P4d/Fe3O4/ITO electrode (80 cycles), which demonstrates that the intensity of the o tion current peak was decreased with the cyclic numbers. The relationship betwee oxidation current peak and the cyclic number was studied. Figure 5b illustrates the tionship between the oxidation current peak (µA) and the cyclic number. Figu demonstrates that the current peak decreased by about 25.6% of its value after 80 cy which means that the developed electrode retained about 74.5% of its conductivity To study the stability of the developed polyamides/Fe 3 O 4 NPs-modified ITO electrodes towards the detection of MTX, P4d/Fe 3 O 4 /ITO electrode was used to detect MTX over 80 cycles. Figure 5a shows cyclic voltammograms of 10 −9 mol L −1 of MTX at the P4d/Fe 3 O 4 /ITO electrode (80 cycles), which demonstrates that the intensity of the oxidation current peak was decreased with the cyclic numbers. The relationship between the oxidation current peak and the cyclic number was studied. Figure 5b illustrates the relationship between the oxidation current peak (µA) and the cyclic number. Figure 5b demonstrates that the current peak decreased by about 25.6% of its value after 80 cycles, which means that the developed electrode retained about 74.5% of its conductivity after 80 cycles. These results indicated good stability after use for 80 cycles. To study the stability of the developed polyamides/Fe3O4 NPs-modified ITO electrodes towards the detection of MTX, P4d/Fe3O4/ITO electrode was used to detect MTX over 80 cycles. Figure 5a shows cyclic voltammograms of 10 −9 mol L −1 of MTX at the P4d/Fe3O4/ITO electrode (80 cycles), which demonstrates that the intensity of the oxidation current peak was decreased with the cyclic numbers. The relationship between the oxidation current peak and the cyclic number was studied. Figure 5b illustrates the relationship between the oxidation current peak (µA) and the cyclic number. Figure 5b demonstrates that the current peak decreased by about 25.6% of its value after 80 cycles, which means that the developed electrode retained about 74.5% of its conductivity after 80 cycles. These results indicated good stability after use for 80 cycles. Moreover, the sensitivity of the developed polyamides/Fe3O4 NPs-modified ITO electrodes towards the detection of MTX was also investigated. P4d/Fe3O4/ITO electrode was used to detect different a wide range of MTX concentrations. Figure 6a shows cyclic voltammograms of different MTX concentrations (10 −9 :10 −14 mol L −1 ) at the P4d/Fe3O4/ITO electrode, which demonstrated that the intensity of the oxidation current peak was increased with increasing the MTX concentration. The relationship between the oxidation current Moreover, the sensitivity of the developed polyamides/Fe 3 O 4 NPs-modified ITO electrodes towards the detection of MTX was also investigated. P4d/Fe 3 O 4 /ITO electrode was used to detect different a wide range of MTX concentrations. Figure 6a shows cyclic voltammograms of different MTX concentrations (10 −9 :10 −14 mol L −1 ) at the P4d/Fe 3 O 4 /ITO electrode, which demonstrated that the intensity of the oxidation current peak was increased with increasing the MTX concentration. The relationship between the oxidation current peak and the MTX concentration was studied. Figure 6b illustrated the relationship between the oxidation current peak (µA) and the logarithm (log) of the MTX concentration (nmol L −1 ). The log function was used because of the very low values of the MTX concentrations (10 −9 :10 −14 mol L −1 ). Figure 6b demonstrates a linear curve within the MTX concentrations, from 10 −9 to 10 −14 mol L −1 with an R 2 = 0.9772 and slope of 0.34 ± 0.03. The calibration curve showed the ability of the developed sensor for detecting very low concentration values. The limit of detection of the developed sensor was found to be about 4.2 nmol L −1 .
Surfaces 2023, 5, FOR PEER REVIEW 12 peak and the MTX concentration was studied. Figure 6b illustrated the relationship between the oxidation current peak (µA) and the logarithm (log) of the MTX concentration (nmol L −1 ). The log function was used because of the very low values of the MTX concentrations (10 −9 :10 −14 mol L −1 ). Figure 6b demonstrates a linear curve within the MTX concentrations, from 10 −9 to 10 −14 mol L −1 with an R 2 = 0.9772 and slope of 0.34 ± 0.03. The calibration curve showed the ability of the developed sensor for detecting very low concentration values. The limit of detection of the developed sensor was found to be about 4.2 nmol L −1 .

Conclusions
Here, we reported on the design, synthesis, and characterization of some new aromatic polyamides. The synthesized compounds were used to enhance the electrochemical conductivity of the Fe3O4 NPs/ITO electrode. Some of these polyamides/Fe3O4 NPs/ITO electrodes showed higher electrochemical conductivity compared with the bare Fe3O4 NPs/ITO electrode. The polyamides-modified Fe3O4 NPs/ITO electrodes were used as

Conclusions
Here, we reported on the design, synthesis, and characterization of some new aromatic polyamides. The synthesized compounds were used to enhance the electrochemical conductivity of the Fe 3