Preparation and Characterisation of Polypyrrole-Iron Oxyhydroxide Nanocomposite as Sensing Material

Faculty of Defence Science and Technology, National Defence University of Malaysia, Sungai Besi Camp, Kuala Lumpur 57000, Malaysia Centre for Defence Foundation Studies, National Defence University of Malaysia, Sungai Besi Camp, Kuala Lumpur 57000, Malaysia Research Centre for Chemical Defence, National Defence University of Malaysia, Sungai Besi Camp, Kuala Lumpur 57000, Malaysia Centre for Tropicalisation, National Defence University of Malaysia, Sungai Besi Camp, Kuala Lumpur 57000, Malaysia Enzyme and Microbial Technology Research Center, Faculty Biotechnology and Biomolecular Science, Universiti Putra Malaysia, Jalan Universiti 1, Serdang, Seri Kembangan 43400, Selangor, Malaysia


Introduction
Polypyrrole (PPy) is one of the conducting polymers (CPs) that exhibit optical and electrical properties of both semiconductors and metals and are known as a distinctive group of organic materials [1,2]. Poly(3,4-ethylene dioxythiophene) (PEDOT), polythiophene, polyaniline, and polyacetylene are the other examples of commonly used CPs described in the research [3]. ese polymers are commonly synthesised through chemical or electrochemical oxidation of monomers using oxidant agents through the conjugated bond system with the polymer backbone being rendered conductive [4]. CP has attracted much attention among researchers due to its inexpensive production, thermal stability, simple synthesis process, and presence of adequate electrical conductivity [5]. Among CPs, PPy is one of the most promising of these polymers because of its remarkable chemical stability and electrical conductivity [6]. However, one of the limitations is that it is unable to be well dispersed during synthesis due to its strong interchain reactions and is somewhat lacking in mechanical properties that affect fabrication processes [7,8]. erefore, PPy in nanoparticle sizes has been explored to improve these limitations as PPy NPs have a much larger surface area and have a porous form [9].
is PPy NP can be synthesised using different oxidants such as ammonium persulfate (N 2 H 8 S 2 O 8 ), iron (III) sulfate (Fe 2 (SO 4 ) 3 ), and iron (III) chloride (FeCl 3 ) [10,11]. As described by Rao et al. [12], FeCl 3 is commonly chosen as an oxidant due to its stabilisation oxidation process with the pyrrole monomer as is shown in equation (1), and it is able to give higher conductivity properties compared to other oxidants [13]. en, to further enhance the synthesis, surfactants also play an important role in the polymerisation process by improving the physical properties of polymers such as their solubility in solvents, stability, and conductivity [14]. Sodium dodecylsulfate (SDS), polystyrene sulfonate, dodecylbenzenesulfonic acid, and sodium dodecylbenzenesulfonate (SDBS) are examples of surfactants used in the polymerisation processes of polymers [15,16]. As reported by Xing and Zhao [17], SDBS has been commonly utilised as a surfactant compared to any others as it can enhance the stabilisation of PPy NPs in terms of its dispersion state.
Over the last few years, PPy NP has been used as a sensing material for the detection of the analyte DMMP which is known as a simulant of chemical warfare nerve agents. is simulant is a chemical that is able to mimic both the physical and chemical properties of nerve agents without its toxic effects [18]. Diethyl chlorothiophosphate (DCTP), methylphosphonic dichloride (MPDC), diisopropylfluorophosphate (DFP), and dimethyl methylphosphonate (DMMP) are several types of simulants that are used to substitute actual nerve agents (CWAs) for research and training because of safety issues with the use of actual nerve agents for these purposes [19]. DMMP is a widely utilised simulant for CWAs because of its nontoxicity and organophosphorus compound elemental composition that imitates many nerve agents [20,21].
As a result of its unique properties, PPy NPs have been employed as sensing materials for various detection methods against chemicals in pesticides, air pollutants, and acidic gases [22]. In order to improve its sensing performance, surface modification of PPy NPs is needed because this would be able to increase the sensitivity of the sensor towards the analyte. In previous works, Kwon et al. [23] have shown that the introduction of carboxylic groups on the surface of PPy nanotube transistors has resulted in increased sensitivity towards DMMP via intermolecular hydrogen bonding while a combination of PPy with copper phthalocyanine was reported to be able to detect DMMP gas simulants through its high electron-donating nature [24]. Furthermore, Das and Roy [25] claimed that PPy NPs and titanium dioxide (TiO 2 ) increased the exposure of hydrogen gas due to the enhanced surface area of the composite and the increased diffusion pathway of the hydrogen molecule. Malook et al. [26] also have synthesised polypyrrole-bimetallic oxide composites (PPy-V2O5-MnO2) and found that the resistance of the sensor was increased when exposed to ammonia gas due to doping of PPy during the synthesis of lead composites that contained positive charge carrier. Other work byŠetka et al. [27] has demonstrated that zinc oxide-based polypyrrole (ZnO-based PPy) nanocomposites were able to detect ammonia better as compared to tin (IV) oxide (SnO 2 )-based PPy. is is due to the attributes that Zn 2+ possesses in which it is redox-active, thus influencing the charge density and conductivity of the nanocomposites. Recently, FeOOH NPs have shown pronounced results in detecting DMMP when the FeOOH NPs were attached to the PPy structure (by replacing H in the N atom) [28].
In this present study, PPy NPs and modified PFFs nanocomposite have been synthesised using chemical oxidative polymerisation. e chemical interaction and morphology of the synthesised PPy NPs and PFFs nanocomposite were then characterised using Fourier transform infrared (FTIR), field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM). e potential of the synthesised PPy NPs and PFFs nanocomposite as sensing materials was then investigated using cyclic voltammetry (CV).

Polypyrrole Polymerisation Process.
Firstly, 40 wt% of SDBS was stirred magnetically in 40 mL of distilled water at room temperature. After that, 18.5 wt% with 14.9 mmol of pyrrole monomer was added to the surfactant solution dropwise while 40.1 wt% ferric chloride (FeCl 3 ) that has been dissolved in distilled water was then added into the mixture of surfactant/ pyrrole. e chemical polymerisation proceeded for 2 h at room temperature. In order to improve the distribution of nanoparticles, a sonication method was introduced into the preparation of PPy NPs. e mixture was sonicated for an hour and three hours to obtain well-dispersed nanoparticles in the solution before being washed with an excess of methyl alcohol and finally being dried in a vacuum at room temperature. Figure 1 shows the schematic diagram for the PPy NPs polymerisation process.

Fabrication of Polypyrrole Nanoparticles-Iron Oxyhydroxide (PFFs).
e obtained PPy NPs were then mixed for two hours with 10 wt% of FeCl 3 into two separate beakers with the same process. en, both of the mixtures were dropped into sodium hydroxide (NaOH) solution with two different concentrations (1.9 and 3.7 wt%) and were left to mix for an hour. ese mixtures were then separated into two different weight percentages of FeCl 3 (5 wt% and 10 wt%). In addition, each mixture was magnetically stirred for two hours to ensure all of its components were completely mixed, followed by sonication for an hour at room temperature. Finally, ethanol was used to wash the obtained precipitate several times before being dried inside a vacuum oven at 60°C for 12 hours. e schematic diagram for the preparation of PFFs nanocomposite is shown in Figure 2.

Fourier Transform Infrared Spectroscopy (FTIR).
e polymerisation process of pyrrole to PPy NPs and the combination of PFFs nanocomposite were analysed to obtain the different functional group bands of the prepared samples. e FTIR used was a PerkinElmer Spectrometer Frontier FTIR with a frequency range of 650-4000 cm −1 , and the transmission bands of each were scanned at a spectrum scan rate of 4 cm −1 . Baseline correction was done for all the prepared samples.

Field Emission Scanning Electron Microscopy (FESEM).
e surface condition or morphology of the PPy NPs and the PFFs nanocomposite was observed using a field emission scanning electron microscope (FESEM) (Zeiss, Gemini500 SEM) with 10,000x magnification.

Transmission Electron Microscopy (TEM)
. TEM analysis was conducted using a FEI Talos L120C with a resolution of 120 kV and 120 K magnification and used to obtain the sizes of the PPy NPs and PFFs nanocomposite since the sizes of these materials play an important role in this experiment with respect to conductivity and total surface area of absorption.

Cyclic Voltammetry (CV).
Electrochemical analysis of the PPy NPs and PFFs nanocomposite was performed using the CV technique. A screen-printed carbon electrode (SPCE) was used as the electrode. e SPCE is typically composed of a carbon layer as the working and counter electrode, whereas a silver layer is used as the reference electrode that is connected to a Metrohm Autolab B.V (PGSTAT204) potentiostat. 10 μL gold nanoparticle suspension, PPy NPs, and PFFs nanocomposite were drop cast onto the SPCE to detect 10 μL of DMMP. e scan rate used was 50 mVS −1 for eight cycles. All measurements were carried out at room temperature. Figure 3 depicts the setup of the electrochemical sensing system using a screen-printed electrode.

Fourier Transform Infrared Spectroscopy of Pyrrole
Monomer and Nanoparticle Polypyrrole. FTIR analysis was conducted to study the formation of PPy NPs from its monomer. As shown in Figure 4, the pyrrole monomer gives out a sharp peak of N-H stretching vibration bands at 3397 cm −1 while the PPy NPs showed broad main absorption peaks at 3217 cm −1 due to the stacking of monomer rings of N-H groups, which indicated that polymerisation had occurred [29,30]. An obvious peak of symmetric and asymmetric C-H stretching vibrations of saturated hydrocarbons of the PPy was shown at 2887 cm −1 compared to a small peak of 40   Advances in Materials Science and Engineering aliphatic C-H bonds observed with the pyrrole monomer at 2942 cm −1 [31,32]. e wavenumber of stretching vibration for C�C bonds was found slightly shifted from 1530 cm −1 (for pyrrole) to 1538 cm −1 for the PPy NPs rings, and the peak was noted slightly broader [33] while a sharp peak of stretching vibration C-N bonds of pyrrole was observed at 1469-1417 cm −1 while deformation C-N bonds peaks of PPy NPs slightly shifted to a lower region at 1431-1409 cm −1 [13]. Furthermore, the C-N stretching bands of pyrrole at 1075 cm −1 and 1047 cm −1 were found to disappear from the PPy FTIR spectra [9,30]. e C-H band in pyrrole appeared at 1013 cm −1 and was slightly shifted towards the lower region for the PPy NPs peak at 1003 cm −1 due to the out-of-plane bending C-H bond [31,[34][35][36]. e shift of these vibration bands indicates that the PPy NPs have been successfully produced.

Fourier Transform Infrared Spectroscopy of Polypyrrole-Iron Oxyhydroxide
Nanocomposite. e characteristic band regions of organometallic compounds (500 to 2000 cm −1 ) are the main focus for observation of the interaction between FeOOH and PPy. A sharp peak of the FeOOH vibrational band was found at 691 cm −1 as shown in Figure 5(b). is observation is in concordance with the research reported by Wei et al. [37]. Fe-O vibrational modes in FeOOH were ascribed at 1640, 1363, and 1141 cm −1 in the PFFs spectra while there were no peaks in   the PPy NPs spectra [38]. Furthermore, the presence of -OH vibrational modes also could be detected at 891 cm −1 which is assigned as -OH bending modes and sharp -OH stretching mode at 3300 cm −1 [39,40]. In the PPy NPs, this region (3217 cm −1 ) is assigned as the secondary amine band (-NH). However, when the FeOOH attaches to the PPy NPs at the N atom (as shown in the schematic diagram in Figure 6), this secondary amine has changed to become a tertiary amine. e tertiary amine does not show any band in this region [41]. erefore, this indicates that the peak observed in this region as illustrated in Figure 5(a) belongs to the -OH stretching band from FeOOH [40]. ese changes and the additional peaks confirm that the PFFs nanocomposite was produced. Figure 6 illustrates the schematic interaction of PFFs nanocomposite.

Field Emission Scanning Electron Microscopy (FESEM) of Polypyrrole Nanoparticles and Polypyrrole-Iron Oxyhydroxide Nanocomposite.
e morphology and surface conditions of samples can be observed using FESEM imagery. Figure 7(a) shows the synthesised PPy NPs that did not undergo the sonication process. From the figure, it can be seen that the morphology of PPy is agglomerated and looks like linked globular subparticles, as suggested by Effati et al. [42]. Figure 7(b) shows the SEM images for the PPy that underwent the sonication process for one hour. e improvement of PPy NPs morphology can be observed by the disappearance of "linked globular subparticles" and the nanoparticles can now be clearly seen. However, the shape of the nanoparticle is irregular and shows a rough surface, which is commonly described as cauliflower morphology [43]. e morphology of PPy NPs with a longer time of sonication (three hours) is shown in Figure 7(c). It can be seen that the PPy NPs have turned back to become an agglomerate and show a clump and globular structure [40].
is indicates that a one-hour sonication time is enough to produce fine nanoparticles of PPy. e size of the nanoparticle obtained will be discussed in the TEM section as follows.
e preparation of PFFs was done by the combination of prepared PPy NPs with iron (III) chloride (FeCl 3 ) and sodium hydroxide (NaOH). e concentration of FeCl 3 was varied into 5% and 10 wt% in each preparation. e morphology of PFFs using 5 wt% of FeCl 3 indicates spherical shapes as shown in Figure 7(d). e shape of the PFFs nanocomposite is better defined compared to the PPy NPs, thus leading to the formation of a porous PPy NPs shell with FeOOH NPs.
is observation has also been reported by Guo et al. [44] after a long time of stirring (seven hours). In our finding, we were able to reduce the stirring time to only two hours together with one-hour duration of ultrasonication. e morphology of 10 wt% of FeCl 3 is depicted in Figure 7(d). It shows that, with a higher concentration of FeCl 3 , the FeOOH was covered with a spherical shape of PPy particles that resulted in multiple agglomerated particles. is finding is in concordance with the research reported by Nalage et al. [45], who showed that agglomeration tends to occur with an increasing concentration of metal ion together with the PPy.  Advances in Materials Science and Engineering

Transmission Electron Microscopy of Polypyrrole Nanoparticles and Polypyrrole-Iron Oxyhydroxide Nanocomposite.
is analysis was performed to observe the size range of prepared PPy NPs and PFFs nanocomposite and to identify if the polymer obtained is within nanoscale parameters. Figure 8(a) depicts the TEM image of the PPy NPs after onehour sonication. It shows that the particle size range obtained was between 50 and 70 nm (with the average size of 67.18 ± 12.69), which is within the size parameters to be classified as nanoparticles, and this finding corroborates with other works which also prepared PPy NPs [46,47]. In addition, these nanoparticles show linear structural shapes from the polymerisation process as described by Ahn et al. [48] while on the other hand, some of the spherical-shaped particles seen connected between the head and tail of linear PPy as seen in the image were due to irreversible aggregations of particles [45]. Figure 8(b) shows the TEM image of PFFs nanocomposite. e size of this nanocomposite was ranged between 110 and 160 nm (with the average size of 138.6 ± 26.60) which is larger than the size of the PPy nanoparticles due to the presence of a higher insulating FeOOH particle loaded within the nanocomposite as has been reported by Guo et al. [44]. is can be proved that the sizes of PPy NPs are much smaller compared to PFFs as stated in FESEM.

Electrochemical Properties of Polypyrrole-Iron Oxyhydroxide Nanocomposite in Ferricyanide Solution.
Cyclic voltammetry (CV) is a concise process for measuring the formal potential of half-reactions when both the oxidised and reduced forms are stable during the time required to obtain a current-potential graph that is commonly known as a voltammogram [49]. A screen-printed carbon electrode (SPCE) is chosen as the test electrode with the addition of PPy NPs and PFFs nanocomposite for the detection of the analyte DMMP (10 μL). SPCEs are commonly used as sensor constructs as they are easy to use and are cost-efficient in terms of their fabrication [50]. Ferricyanide [Fe(CN) 6 ] 3− is used as the redox probe to observe the electron transfer reaction which reflects the performance of the modified electrode [51]. Figure 9(a) represents the comparative CV of different types of modified electrode surfaces, specifically bare electrodes, PPy NPs, and PFFs nanocomposite with concentrations of either 5 or 10 wt% in 5 mmol of [Fe(CN) 6 ] 3− in 0.1 M of KCl solution. It can be seen that the oxidation peak (current ∼57.24 μA and potential ∼0.22 V) for the bare SPCE with a peak separation of (ΔE p ) ∼240 mV was obtained. en, the SPCE modified with PPy showed an enhancement in the oxidation peak current ∼75.88 μA with ΔEp ∼170 mV which was higher than the bare SPCE. However, the SPCE with PFFs of 5 and 10 wt% showed a decrement in their oxidation peak currents, namely, ∼70.07 μA (ΔE p ∼166 mV) and 69.16 μA (ΔE p ∼190 mV), respectively.. However, the 10 wt% showed 69.16 μA and ΔE p ∼190 mV, respectively. e increase of current signal by 32.6% for PPy NPs that is higher than only 22.4% for the PFFs nanocomposite is due to the improvement of surface area, which has been confirmed by the TEM analysis. According to Liang et al. [52], the decrease in the oxidation peak current for the PFFs nanocomposite, both 5 and 10 wt%, appears to be related to the presence of FeOOH molecules that have relatively high resistance and weak cyclic stability. Furthermore, the SPCE modified with PPy NPs (∼170 mV) appeared to demonstrate lower peak separation values compared to the bare electrode as well as the other modified electrode due to an increase in the contact surface area between the nanostructure of PPy and the electrolyte [Fe(CN) 6 ] 3− as reported by Ahn et al. [48].

Electrochemical Behaviour of Polypyrrole-Iron
Oxyhydroxide Nanocomposite (PFFs) towards DMMP. In order to apply the PPy NPs and PFFs nanocomposite as sensing materials for the detection of DMMP, the electrochemical behaviour of the PFFs nanocomposite was investigated. Figure 9(b) shows the CV of PPy NPs and PFFs nanocomposite-modified SPCE, with and without DMMP, respectively. It can be seen that the presence of DMMP affects the current scan for both PPy NPs and PFFs nanocomposite-modified SPCE compared to the modified electrodes without DMMP exposure. Figure 9(c) summarises the oxidation peak currents for PPy NPs and PFFs  Advances in Materials Science and Engineering nanocomposite-modified electrodes exposed to DMMP. e oxidation peak current for PPy NPs (∼75.88 μA) decreased to ∼57.83 μA while the PFFs nanocomposite peak current decreased from ∼70.07 μA to ∼55.30 μA in the presence of DMMP. Moreover, peak separation values (ΔE p ) for PPy NPs and PFFs nanocomposite-modified electrodes when exposed to DMMP were also observed ( Figure 9(b)). e peak separation for the SPCE with PPy NPs was significantly lower (ΔE p ∼230) compared to the PFFs nanocomposite-modified SPCE (ΔE p ∼310).
e reduction of the oxidation peak and increment of peak separation was due to a longer electron path where the electrons need to move from DMMP towards FeOOH before being transferred to PPy NPs [42]. Both the PPy NPs and PFFs nanocomposite contribute towards the enhancement of surface contact between the electrode and electrolyte. us, it shortens the path for ion diffusion and access to rich accessible redox reaction sites [53]. e sensitivity in detecting the analyte DMMP is improved as the vertically aligned FeOOH on PPy NPs directly increases the DMMP active sensing sites [28].

Conclusion
PPy NPs and PFFs nanocomposite were successfully synthesised using a chemical oxidative polymerisation technique with two different periods of sonication time (one and three hours). e one-hour sonication time produced finer nanoparticles compared to three hours of sonication. e polymerisation of the pyrrole monomer was confirmed by the broadening of the sharp peak of -NH stretching at 3217 cm −1 while the interaction between FeOOH and PPy NPs was seen through the disappearance of the N-H band and the existence of a new -OH sharp band at around 3500 cm −1 . FESEM imagery showed that the 5 wt% FeOOH nanocomposite possessed iron metals that were well distributed without agglomeration in the PPy NPs compared to the 10 wt% FeOOH. e size of the prepared PPy NPs and PFFs nanocomposite was between 50 and 70 nm and between 110 and 160 nm, respectively. e potential of PPy NPs and PFFs nanocomposite as sensing materials in detecting DMMP was demonstrated using cyclic voltammetry (CV). A reduction of oxidation peak current has been observed in the CV measurement when DMMP was introduced to both the PPy NPs and PFFs nanocompositemodified electrodes. erefore, PPY NPs and PFFs nanocomposite can be promising sensing materials in the construction of an electrochemical sensor for the detection of DMMP. A further study is recommended in terms of determining the sensitivity of the electrochemical sensing system in detecting different analytes with varying concentration range. In addition, the PPy NPs and PFFs nanocomposite have the potential to be utilised as sensing materials in gas sensors.

Data Availability
e data used to support the study are available within the article.

Conflicts of Interest
e authors declare that they have no conflicts of interest.