Electrochemical Fabrication and Characterization of a Gold-Polyaniline /Multi-walled Carbon Nanotubes/Manganese Dioxide Composite Electrode

Objectives: Electrochemical fabrication and characterization of a gold-polyaniline/multi-walled carbon nanotubes/manganese dioxide (Au-PANI/MWCNT/MnO 2 ) composite electrode. Methods: The MnO 2 nanoparticles (NPs) were prepared by heating 1% Mn(NO 3 ) 2 .4H 2 O at 100 0 C for 24 h and characterized by using Fourier transform infrared spectroscopy (FTIR) and Ultraviolet/visible (UV/Vis) spectrophotometry. The size and shape of the NPs were determined from the transmission electron microscopic image. MWCNTs were functionalized with carboxyl groups on their sidewalls by sonicating in H 2 SO 4 :HNO 3 (3:1, v/v) for 12 h at 40 kHz. The functionalization was further con-ﬁrmed through UV/Vis spectrophotometry. The working Au surface was ﬁrst activated and then electropolymerized by using 50 m l of 0.005% C 6 H 5 NH 2 in 01 N HCl followed by electrodeposition with 0.1% each of the c-MWCNTs and manganese oxide NPs through 20 cycles of cyclic voltammetry (-0.2-0.9 mV) at the rate of 20 mV/s. The Au-PANI/MWCNT/MnO 2 composite was then characterized by using FTIR spectra and scanning electron microscopy. Findings: An Au-PANI/MWCNT/MnO 2 composite electrode was fabricated and characterized. Novelty: The nanocomposite electrode was designed by using screen printed electrode, which is very simple to construct, portable, and economic. The composite can be used to design a sensors or sensor array in future. CHCl 2 S 2 HCl HAuCl 4 of comparable for 6 h at (RT). nanocomposite by combining 4-aminothiophenol and multiwalled nanotube (MWCNT) and potassium peroxydisulfate in HCl for 12 h Au@Pt core-shell onto the composite after it sensitivity LOD fabrication synthesis core-shell interfacial polymerization design nanocomposite supercapacitors graphite glassy


Introduction
Biosensors are nowadays used as an important tool in medical diagnostics, environmental sample analysis, molecular biotoxicity evaluation, explosive tracking (1)(2)(3) , and so on. Due to their simplicity, economic value, specificity, and sensitivity, these nanodevices are rapidly replacing other molecular detection methods. Sensitivity of a sensor mainly rely on the immobilized molecules, and nanocomposites can help for efficient immobilization (4,5) . A Nanocomposite, can be synthesized by using intercalation (6) , templet https://www.indjst.org/ synthesis (7) , ball milling (8) , three-dimensional printing (9) or other methods with electropolymerization being simple, stable, and common (10) . Several studies have employed Using polyaniline (PANI) as a linker and Au NPs or Au electrode to fabricate nanocmposites. Liu et al., (2012), used a chemical technique to prepare Au/PANI nanofiber composite for the reduction of O 2 (11) . At 0 0 C, Aniline in CHCl 3 was combined with (NH 4 ) 2 S 2 O 8 , and HCl for 12 h then added to HAuCl 4 of comparable concentration for 6 h at room temperature (RT). LI and her group created a nanocomposite by combining 4-aminothiophenol and multiwalled carbon nanotube (MWCNT) with aniline and potassium peroxydisulfate in HCl for 12 h at RT. Au@Pt core-shell NPswere coated onto the composite after it had been washed and dried (12) . Although the sensitivity and LOD of the method are satisfactory, the fabrication procedure involves multiple chemicals so also the synthesis of Au@Pt core-shell NP is quite hectic. Similarly, multiple investigations have reported fabrication of MWCNT-PANI composites. Sivakkumar et al., (2007), used interfacial polymerization to design a nanocomposite to use in supercapacitors (13) . Another such PANI/MWCNT-based ammonia sensor was developed by Maity et al., (2020) (14) while, Zhou et al., (2018) electrochemically fabricated PANI/MWCNT composite onto expanded graphite for improving the performance of supercapacitors (15) . Razak et al., (2009) suggested a PANI/MWCNT-MnO 2 ternary nanocomposite by coating MnO 2 -filled MWCNTs with PANI in HCl. The composite showed significant electrical conductance (16) . In two separate studies in 2014, Vilian et al. developed a simple strategy for fabricating MWCNT onto glassy carbon electrode followed by electrochemical conjugation with MnO 2 NPs (17,18) . Another ternary composite electrode was fabricated by Iqbal et al., (2020) by using PANI, MnO 2 , and CNT. They synthesized MnO 2 nanorods through hydrothermal approach. PANI@CNT was prepared by oxidative polymerization and subsequently polymerized onto MnO 2 nanorods (19) . However, hydrothermal method is time-consuming and requires a high temperature. Tran and coworkers (20) recently, developed an electrochemical impedance DNA sensor based on the MWCNT/MnO 2 /PANI nanowire composite.
We wanted to construct an Au-PANI/c-MWCNT/MnO 2 composite with a different conjugation than those previously described. The composite will provide significant surface area for efficient molecular immobilization and high electrical conductance. This on the other hand will impart high sensitivity if used to construct a sensor. We intended to make the composite by using a two-step electrochemical fabrication technique that is quick, simple, stable, and can allow broad types of molecules. We have used screen-printed gold electrode (SPGE) in this study, such that the composite electrode is easy-toconstruct, economic, and may be used to build a sensor or sensor array.

Synthesis of MnO 2 NPs and Carboxyl functionalization of MWCNT
A simple approach reported by Najafpour et al., (2012) (21) was employed with minor changes for the synthesis of MnO 2 NPs. In an incubator, 5 ml of MiliQ water containing 1% Mn(NO 3 ) 2 .4H 2 O was heated at 100 0 C for 24 h before being washed with MiliQ water. The particles were then dried at 100 0 C and stored at 4 0 C. FTIR spectroscopy and UV/Vis spectrophotometry were used for the characterization of the NPs whereas, size and shape of the NPs were determined through TEM imaging.
https://www.indjst.org/ Following our prior work, the MWCNTs were functionalized with carboxyl groups on their sidewalls (22) . The nanotubes (1.2 mg/ml) were sonicated for 12 h at 40 kHz in H 2 SO 4 :HNO 3 (3:1, v/v) and centrifuged at 8,000×g for 10 min at room temperature (RT). The particles were then rinsed repeatedly in MiliQ water to neutralize the pH. Subsequently, the particles were suspended in double the volume of DMF:H 2 O (1:1, v/v) and stored at 4 0 C for future use. UV/Vis spectra of the nanotubes for equal concentrations (1.2 mg/ml) were acquired both before and after the sonication to confirm the functionalization (23) .

Electrochemical fabrication of the nanocomposite
As described in our earlier article (24) , the working surface of the SPGE was first activated with H 2 SO 4 :H 2 O 2 (1:1, v/v) and then rinsed with MiliQ water, followed by 70% ethanol before drying at RT. After that, 50 µl of 0.005% C 6 H 5 NH 2 in 01 N HCl was electropolymerized onto the working surface (area: 0.126 cm 2 ) by 20 cycles of CV (-0.2 -0.9 mV) at a rate of 20 mV/s (25) . The Au/polyaniline (Au-PANI) electrode was then washed with 50 mM phosphate buffer, 0.9 % NaCl (PBS), pH 7.0±0.2 followed by 50 mM acetate buffer, pH 5.2±0.2 before drying at RT. Following that, 20 cycles of CV (-0.2 -0.9 mV) at the rate of 20 mV/s were used to electrodeposit 0.1% each of the c-MWCNTs and manganese oxide NPs onto Au-PANI surface in 50 mM acetate buffer, pH 5.2±0 (26) [ Figure 1]. Subsequently, the Au-PANI/MWCNT/MnO 2 nanocomposite electrode was washed with 50 mM acetate buffer, pH 5.2±0.2, MiliQ water, and 80% C 2 H 5 OH followed by air dying at RT. The surface topography of the bare Au, Au-PANI, and Au-PANI/MWCNT/MnO 2 composite electrodes was studied using SEM imaging. For further characterization, the FTIR spectra for both Au and Au-PANI/MWCNT/ MnO 2 composite electrodes were obtained.

Synthesis of MnO 2 NPs and functionalization of MWCNT
After heating the Mn(NO 3 ) 2 .4H 2 O solution at 100 0 C for 24 h, a black precipitate formed which was washed and dried at 100 0 C to result in black NPs. The FTIR spectra of the NPs revealed a broad peak at 3066.82 cm -1 may be corresponding to the H-O-H stretching vibration, indicating towards the hydrate nature of the particles (27) . The band at 1078.21, 1398.39, and 1670.35 cm -1 might have been contributed by the bending vibrations of O-H combined to Mn (28) . The absorption bands at 415.88 and 567.07 cm -1 may be attributed to the O-Mn-O stretching (29) [Figure 2a]. In the UV/Vis study, the particles produced an absorption peak (λ max ) at 315 nm, which may be corelated to the surface plasmon resonance of the manganese oxide (O→Mn) NPs (30) [ Figure 2b]. The results obtained with FTIR and UV/Vis study confirmed the NP thus synthesized to be MnO 2 . Shape of the MnO 2 NPs thus synthesized were found to be roughly spherical under TEM [ Figure 2c] by using ImageJ software analysis. Further, average size (µ) and standard deviation (σ ) of the NPs were found to be 40.92 nm ±0.63, and 3.74, respectively (31) [ Figure 2d]. The UV/Vis spectra of the MWCNT before sonication showed no specific λ max in between 190-320 nm whereas, after sonication for 12 h, two clear and distinguished peaks appeared at 209 and 265 nm [ Figure 2e], might be indicating towards the O=C-O-H functionalization (32) . https://www.indjst.org/

Fabrication of the nanocomposite
The multi-layer coating [ Figure 3a] of the working Au electrode with PANI, changed the surface into green (33) while electrodeposition of the c-MWCNT [ Figure 3b] and MnO 2 NPs rechanged the surface into black (34) . With increasing scan repeat, both CV cathodic (I pa ) and anodic (I pa ) peaks of electrodeposition increased. The I p value of Au-PANI/MWCNT/MnO 2 nanocomposite electrode was higher than that of Au-PANI electrode [ Figure 3b inset] indicating that the former has higher immobilization surface area (25) . As a result, a sensor if synthesized by using this nanocomposite will impart a higher sensitivity through enhancing the immobilization efficiency.
https://www.indjst.org/  Figure 4a, b, and c depicts the SEM images of the Au, Au-PANI, and Au-PANI/MWCNT/MnO 2 nanocomposite electrode. The first had a smooth layer (35) , whereas the second one showed a clear and distinct uniform layer that may be attributed to the PANI layer (36) . Porous tubular intercrossed network-like structures impregnated with spherical granules was seen in the final electrode. MWCNTs may be responsible for the network, whereas granules may indicate MnO 2 NPs (36,37) . The functionalization was further confirmed through FTIR analysis. The FTIR spectrum of Au didn't showed any specific absorption peak while FTIR spectrum of the composite electrode revealed peaks at 33392.78, 2951.08 and 2355.08, 1651.06, 1172.72 and 1064.74, 852.32 and 588.29 cm -1 . The broad peak at 33392.78 cm -1 may correspond to the O-H stretching vibration of the surface -COOH functional groups (38,39) while the bands at 2951.08 and 2355.08 cm -1 could signal towards the asymmetric and symmetric methyl stretching, respectively on the defective regions of the MWCNTs (40) . The peak at 1651.06 cm -1 perhaps indicates for C-O bands of -COOfunctional groups on the sidewalls of MWCNTs (41) whilst, the band at 1172.72 and 1064.74 cm -1 may be attributed to Mn-O vibrations. (25) [ Figure 4d]. The peaks at 852.32 and 588.29 may correspond to the C-C stretch of the MWCNTs (42) .

Conclusion
We have electrochemically polymerized aniline onto the Au surface of a SPGE followed by electrochemical deposition of MWCNT/MnO 2 to construct a nanocomposite electrode. Since SPGE is small electrode, therefore, the composite electrode fabricated by using this electrode can be used construct a sensor or sensor array easily. A portable potentiostat can be designed for onsite detection of an analyte by using the sensor. In addition, the current technique includes NM electro-polymerization, which is a simple and cost-effective method that may be used to immobilize a variety of molecules for the fabrication of electrochemical sensors. Furthermore, because electro-polymerization is a stable fabrication technique, the electrodes can be stored at 4-8 0 C for more than six months. The antimicrobial properties of MnO 2 NMs as well as physicochemical resistance of both MnO 2 and c-MWCNTs, added to the electrode's stability. However, immobilization of biomolecules on the other hand, may change its stability, which will be calculated on real-time in future during the development of any sensor that uses it.