One-pot polycondensation and characterization of melamine-based polymer for mercury and sodium hypochlorite sensing

Abstract

A porous, low-cost, organic, amorphous and carbon-nitride functionalized polymer was synthesized using melamine and 5-bromo-2-thiophenecarboxaldehyde. One-pot polycondensation method was opted, to yield hyper-crosslinked polyaminal network which was used for the detection of NaOCl and Hg²⁺. Reaction proceeds in single step without addition of any catalyst and gives promising yield. The authenticity of the synthesized polymer MB was established using X-Ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FT-IR) and Energy-dispersive spectroscopy (EDS). However, the morphology and thermal stability of MB was studied using X-Ray diffraction (XRD) & Scanning electron microscopy (SEM) and Thermogravimetric analysis (TGA) & Differential thermal analysis (DTA) respectively. BET studies were carried out to analyze the porous structure of MB. The pore diameter is centred at 6.2851 nm with specific surface area of 24.348 m² g^–1 and pore volume of 0.038258 cm³ g^–1. The TGA curve showed that it has a good thermal stability (250°C). The synthesized compound was checked for its sensing behaviour with different analytes (metal ions: Cu²⁺, Cr²⁺, Mg²⁺, Cd²⁺, Mn²⁺, Zn²⁺, Pb²⁺, Hg²⁺; acids: HCl, H₂SO₄, HNO₃ and ROS: t-BuOOH, KO₂, NaOCl) in deionized water and was found to show significant change in intensity with NaOCl and Hg²⁺. The change in absorption spectra was observed at varied concentrations of NaOCl and Hg²⁺ solutions in deionized water using UV-Vis. Spectrophotometer.

Keywords

Melamine mercury removal hypochlorite sensor porous organic polymer one-pot synthesis

1 Introduction

Nowadays, the presence of heavy metal ions and reactive oxygen species in various water bodies is responsible for causing water pollution and major health issues in humans [1]. Out of all known heavy metal ions, mercury is of main concern which is highly toxic and responsible for originating environmental disturbances by contaminating water [2 –4] as it enters in the biological world by means of food chain [5, 6]. Heavy metal ions if present beyond permissible limits in the body may cause various health issues such as kidney damage, lung diseases, nerve failure and so on [7 –9]. Along with this, abnormally high intracellular concentration ROS also lead to wide range of harmful diseases in living beings [10] such as cellular dysfunction, and finally progression into disease states such as neurodegenerative diseases including Parkinson’s disease (PD) and Alzheimer’s disease (AD), cancer, diabetes, inflammation, arthritis, and so on [11, 12]. Additionally, the exposure of sodium hypochlorite to human eye can cause immediate effects such as edema and pain in eyes [13]. These days, the aerosols which are commercially available possess higher contents of NaOCl that may cause irritation in lungs in humans [14]. Keeping in mind these issues, melamine-based polymers could be used which have capability to detect such heavy metal ions and reactive oxygen species (ROS) as well as show significant changes with acids [15 –17].

Melamine is a non-edible [18], non-hygroscopic [19], white crystalline and a weak base compound [20]. This compound bears a very low solubility in water [21] and sublimes at a low temperature [22]. The presence of three amino groups on melamine core provides an excellent platform to interact with other functionalities [23]. The literature survey on melamine reveals that various melamine-based resins can be developed with better durability, i.e., long lasting thermosetting polymers [24]. The versatile applications of these polymers have been reported by various researchers, especially in the field of high-pressure lamination used for decoration purposes, insulation related work as well as in dry erase boards [25]. Further these polymer’s utility has been investigated for polymeric cleaning products, sound proofing, in adhesives, paints, permanent-press fabrics, paper coatings, giving finishing to textiles and so on [26, 27]. As the anthropogenic activities have resulted in an inundation of hazardous industrial and domestic waste water which is released into the environment without appropriate treatment [28]. The waste water comprises of non-biodegradable heavy metal cations, acids or reactive oxygen species (ROS) which accumulates in the biosphere and causes serious health issues. In general, some of the known physicochemical methods like sedimentation, flocculation and photocatalysis could be employed in treating polluted water. However, they show less effectiveness in removing micropollutants from water resources such as household and toxic industrial waste water. Besides using these traditional methods, new absorption or adsorption methods by using melamine-based polymers could be employed for the uptake of these micropollutants (heavy metal ions or ROS species) [29]. In these regards, polymeric materials show promising alternative materials for environmental refinement. The melamine based polymeric entities are known for their vast chemistry that allows them to form a unique combination of favourable properties for water treatment which cannot be easily achievable on using conventional water treatment materials and methods [30 –32].

As per literature survey, many researchers have reported absorption-based sensors for removal of different heavy metal ions (Hg²⁺) [33], acids and ROS species (sodium hypochlorite) which follow the process of absorption by porous melamine-based polymers [34]. In such covalent organic framework, Sulphur-mediated groups are generally present [35]. Because of the affinity of sulfur for mercury ions, triazine based compounds are modified with sulfur-based substituents in order to develop detection and removal properties [36]. In our course of work, we have developed a derivatized thiophene and melamine-based polymeric system that is used to detect NaOCl and Hg²⁺ at varied concentrations in water.

Melamine undergoes extensive cross-linking on reacting with aldehydes, ketones or acids which results in the formation of many widely used porous organic polymers [37, 38]. Such polymers usually possess lighter elements like carbon, hydrogen, oxygen or nitrogen which are involved in strong covalent bond formation [39]. It results in generation of high stability (chemical and thermal) [40], high specific surface area and low density [41]. Numbers of synthetic protocols have been listed in literature on design and development of melamine-based polymers by using various aldehydes such as terephthalaldehyde [42, 43], isophthalaldehyde [44], dialdehyde [45], formaldehyde [46], benzaldehyde [47], 5-bromo-2-thiophenecarboxaldehyde etc. Liu et al has used dicyandiamide and 5-bromo-2-thiophenecarboxaldehyde in a one-pot reaction which passed through various stages, grinding and moderate to high temperature heating for the in-situ generation of melamine-linked polymeric assembly [48]. In another synthetic protocol, a Schiff-base type of reaction has been proposed which involves melamine and targeted aldehydes. It can be seen in the work done by Hou et al and Amirilargani et al who used terephthalaldehyde and isophthalaldehyde as a base unit to react with melamine. The basic difference between both synthetic protocol is a very important crucial factor for generating specific property in proposed polymeric assemblies. In the former reaction, there is generation of imine bond(C = N) while in the latter case C-N takes place [49, 50]. In the present work, we have followed second synthetic protocol. Besides aldehydes, melamine also reacts with acids such as trimesic acids [51], poly- (glutamic acid) [52], formic acid, glyoxylic acid etc. Shao et al used trimesic acid and melamine as monomers in their synthesis under high temperature conditions for 72 hours in DMSO [51]. In another work, cleaner leather production was done by Pradeep et al by utilizing both formic acid and glyoxylic acid along with melamine as starting materials [53].

Firstly, the reaction of melamine and 5-bromo-2-thiophenecarboxaldehyde was carried out for the formation of highly stable polymeric product which has the least solubility in all solvents at room temperature. This compound shows a very high thermal and mechanical properties due to extensive crosslinking. The present approach ensures good yield of 79.5% with least wastage of chemicals. The characterization and analysis of MB was performed by various analytical techniques such as X-Ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FT-IR), Thermogravimetric analyser (TGA), Differential thermal analysis (DTA), X-Ray diffraction (XRD), Scanning electron microscopy (SEM), Energy-dispersive spectroscopy (EDS), Brunauer-Emmett-Teller (BET) and Ultraviolet-visible (UV-vis) spectroscopy. The absorption spectra of MB was studied with various analytes (Cu(II), Cr(II), Pb(II), Zn(II), Mn(II), Mg(II) and Ni(II)) where it was found to show significant changes with Hg²⁺ and NaOCl, however, minute or no changes were observed with all other analytes.

2 Experimental section

2.1 Materials and instrumentation

Melamine and 5-bromo-2-thiophenecarboxaldehyde were purchased from Loba chemicals and used for the synthesis purpose without any further purification. The solvents used in the synthesis were analytically pure, no further purification/drying was performed. FT-IR analysis was done using a device named Perkin Elmer spectrum IR version 10.6.1 from 400–4000 cm^–1. The texture of MB was analysed through Bruker D8 Advance X-Ray diffractometer where high-speed energy-dispersive LYNXEYE XE-T detector uniquely combines both fluorescence and Kβ radiations. Elemental composition and morphology of the compound was obtained through scanning electron microscope (SEM) images by using instrument named JSM-IT500 along with an energy dispersive spectrometer (EDS). The thermogravimetric analysis (TGA) and Differential thermal analysis (DTA) was performed through a TGA 2950 instrument. XPS study was performed through a K-alpha X-ray photoelectron spectrometer while UV-visible absorption spectra were collected on UV-1900 UV- Visible Spectrophotometer by Shimadzu. Brunauer-Emmett-Teller (BET) results were obtained by using Autosorb iQ-x.

2.2 Synthesis and characterization

The synthesis of MB was performed by reacting melamine (100 mg, 0.79 mmol) with 5-bromo-2-thiophenecarboxaldehyde (0.151 g, 0.79 mmol) (Scheme 1). Both reactants were mixed in 1 ml of DMSO and acetic acid was added to it in catalytic amount. The reaction mixture was heated in furnace at 180°C for 8 hours. After those resultant precipitates were filtered on Whatman filter paper and washed with 100 ml of water as well as 50 ml of acetone. The washed precipitates were dried in oven at 110°C for 6 hours. The dried compound was used for analysis such as XPS, FT-IR, TGA, DTA, XRD, SEM-EDS, BET and UV-Vis spectroscopy. (Brownish-Black solid, Yield = 79.5%, FT-IR: 1416, 1313, 1150, 1036, 767 and 549 cm^–1).

Scheme 1

Synthesis of melamine-based polymer MB.

2.3 Characterization

The synthesized polymer MB was characterized using X-Ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FT-IR) and Energy-dispersive spectroscopy (EDS). However, the morphology of MB was studied using X-Ray diffraction (XRD) & Scanning electron microscopy (SEM). To check the thermal stability of MB Thermogravimetric analysis (TGA) and Differential thermal analysis (DTA) studies were carried out. BET (Brunauer-Emmett-Teller) analysis was performed to analyze the porous structure of MB by finding the pore diameter, specific surface area and pore volume.

2.4 Absorption studies

The stock solution of 50 mg of MB in 100 ml of DI water was prepared which was used for making different concentrations by dilution. 100 mg/L MB was used to perform the absorption studies. 1×10^–3 g/ml stock solutions were prepared for all the metal analytes i.e., Hg²⁺, Pb²⁺, Zn²⁺, Cu²⁺, Cr²⁺, Mn²⁺, Mg²⁺ and Co²⁺ in DI water. 1×10^–3 M solutions of various ROS (t-BuOOH, KO2 and NaOCl) and 1×10^–2 M solutions of hydrochloric acid, sulphuric acid and nitric acid were prepared in DI water. The absorption spectra were recorded by titrating MB solution with various analytes.

3 Results and discussion

The melamine-based polymer MB was developed via one pot condensation. The resulting hyper-crosslinked structure is insoluble in any solvent under normal temperature and pressure conditions. The analysis of MB was performed through XPS, FT-IR, TGA, XRD, SEM-EDS, BET and UV-Vis spectroscopy. In this section all the characterization techniques opted will be discussed to authenticate the identity and the photo-physical property of the polymer will be analysed.

3.1 X-Ray photoelectron spectroscopy

The confirmation of presence of thiophene group (as electron donor) and cyano groups (as electron acceptor) was done by X-Ray photoelectron spectroscopy (XPS) (Fig. 1). In our work, for MB, the peaks presence for the N 1 s spectra was observed at 398.83 eV (bi-coordinated N(C-N = C)), 399.7 eV (tri-coordinated N-(C)₃) and 400.42 eV (amino N(C-NH_x)) respectively which is having very close similarity with the literature report [54 –57]. In C1 s spectra, three peaks related with C-C/C = C, C-NHx and N-C = N bonds [58, 59] with their respective binding energies were located at 284.41 eV, 285.67 eV and 288.67 eV. In the case of O 1 s spectra, peaks at 530.6 eV (C-O-H bonds), 531.32 eV (C-O-C bonds), 532.5 eV (O = C-O bonds) and 533.3 eV (ring) are providing needed information of present functionalities [60, 61]. The surface S atoms could be divided into 2 major peaks of binding energies 162.8 and 163.8 eV are showing the presence of S 2p1/2 and S 2p3/2 respectively. Also, the peaks at 164.72 and 167.12 shows the presence of C-S-C bonds and C-SOx-C bonds respectively [62 –65]. Moreover, the peaks show similar 1:2 intensity ratio for both S 2p 1/2 and S 2p 3/2 which was previously reported for compounds like MB. Hence, on the basis of above mentioned XPS spectrum, it could be concluded that the donor-acceptor structure of MB was established successfully.

Fig. 1

(a) XPS spectrum of MB. (b-e) High resolution XPS spectra of (b) O 1 s (c) C 1 s (d) S 2p (e) N 1 s.

3.2 Fourier-transform infrared spectroscopy

FT-IR analysis was carried out to verify the completion of reaction. FT-IR spectra of melamine, 5-Bromo-2-thiophenecarboxaldehyde, and MB is depicted in Fig. 2. The peaks of primary amine group at 3468 cm^–1 and 3418 cm^–1 of melamine disappeared in the final product indicating the consumption of primary amine [66]. The absence of C = N strain band at 1625 cm^–1 in MB, indicates the formation of aminal bonds i.e. (-NH-CH-NH) instead of imine bonds. The peak for C-N-C asymmetric stretch appeared at 1159 cm^–1 [67]. The stretching bands at 2800 cm^–1 and 3039 cm^–1 confirms the formation of N-H and C-H bonds respectively [46]. Secondly, the spectra of 5-Bromo-2-thiophenecarboxaldehyde shows absorbance at 1659 cm^–1 [68] which cannot be noted in the spectra of MB. Hence, from the resulting spectra of polymer, it can be concluded that there is no residual melamine and reacting aldehyde left. Similarly, the spectra showed the bands at 1151 cm^–1 and 1159 cm^–1 attributed to the aliphatic C-N [69]. Moreover, the peaks at 1313 cm^–1, 1416 cm^–1 and 1415 cm^–1 corresponds to the triazine rings [70]. Furthermore, a band at 767 cm^–1 is noted in spectra of MB which also confirms the presence of triazine rings [71]. Besides this, some bands at 1036 cm^–1 and 1040 cm^–1 are also there corresponds to the presence of C-S vibrations of thiophene ring [72, 73]. Also, the peak at 549 cm^–1 depicts the presence of C-Br bond [74] in the resulting compound.

Fig. 2

(a) FT-IR spectra of Melamine (b) FT-IR spectra of 5-bromo-2-thiophenecarboxaldehyde (c) FT-IR spectra of MB.

3.3 Thermogravimetric analyser and differential thermal analysis

The thermogravimetric analyser (TGA) and differential thermal analysis (DTA) (Fig. 3) were used to check the thermal stability of synthesized polymeric sample. The MB showed a very slight curve representing mass loss from 50–250°C related with adsorbed water molecules with higher heat resistant capacity involving exothermic process. After that, a continuous weight loss from 300 to 580°C was related to melamine degradation [46]. While, at 580°C a very small hump was visible which showed the start of the polymer backbone decomposition and it continued till 850°C with around 30% residual mass [75, 76]. On the basis of DTA, it can be concluded that the decomposition of MB involves two exothermic events.

Fig. 3

(a) XRD curve of MB (b) TGA and DTA curves of MB.

3.4 X-Ray diffraction

X-Ray diffraction (XRD) pattern of MB was recorded in powdered form (Fig. 3). The resulted pattern reveals a wide peak positioned around 2θ equals to 20° and no sharp peak was observed which proves that obtained compound is with very less crystallinity (2%) [48]. More or less, it can be predicted as a polymeric amorphous material. Also, the absence of any sharp peaks clarifies the complete consumption of melamine during the course of reaction.

3.5 Scanning electron microscopy and energy-dispersive spectroscopy

With the help of SEM images (Fig. 4), the morphology of MB was investigated. Few changes were noticed in the structure of melamine on reacting with 5-Bromo-2-thiophenecarboxaldehyde. Instead of porous structure, aggregation of particles was observed on the surface of melamine [77, 78]. Beyond this, gaps were also located on the interface which proves there is less contact between the reacting molecules which results in the formation of cross-linked network structure of polymer [79].

Fig. 4

SEM images of MB.

Also, EDS results confirms the presence of carbon, nitrogen, sulphur and bromine in the structure of MB (Table 1). As per elemental analysis (EA), very high N content was expected which was verified by our results as well [80, 81]. The MB showed 41.91% of S and very few traces of Br were also noted.

Table 1

Elemental analysis results of MB

Element	Line	Mass%	Atom%
C	K	11.9±2.54	23.97±5.13
N	K	5.12±3.25	8.85±5.61
O	K	13.97±2.56	21.16±3.88
S	K	55.48±5.82	41.91±4.40
Br	L	13.55±5.35	4.11±1.62
Total	100.00	100.00

3.6 Brunauer-Emmett-Teller (BET) analysis

The curve for N₂ adsorption-desorption isotherm at temperature 77.35 K is presented in Fig. 5. The obtained isotherm belongs to type IV as average pore diameter value falls in between 2 nm-30 nm which is 6.2851 nm indicating the presence of mesopores in MB. Moreover, the BET surface area and pore volume of MB reached 24.348 m² g^–1 and 0.038258 cm3 g^–1 respectively which was higher than that of parent polyaminal network of melamine [82].

Fig. 5

N₂ adsorption-desorption isotherm curves.

3.7 Ultraviolet-visible (UV-Vis) spectroscopy

Absorption spectra of MB (100 mg/L) was recorded in DI water which was then titrated with different heavy metals (Hg²⁺, Pb²⁺, Zn²⁺, Cu²⁺, Cr²⁺, Mn²⁺, Mg²⁺ and Co²⁺) 100μL each. The absorption curve of MB indicated a significant enhancement in the intensity with the addition of Hg²⁺ ions. However, an insignificant change was observed with all other metal ions except Mg²⁺ which showed some intensity enhancement but the same was found to be much lesser in comparison to Hg²⁺. The absorption studies of MB with varied concentrations of Hg²⁺ was also carried out and it was found that with the increasing concentration the absorption intensity also increased.

Besides this, the absorption change was also studied with acids i.e. hydrochloric acid, nitric acid and sulphuric acid but no change or insignificant change was observed with all the three acids. The study of ROS (t-BuOOH, KO₂ and NaOCl) detection by MB pointed that sodium hypochlorite showed comparatively much high response towards MB. MB was found to show exceptionally higher selectivity and sensitivity for sodium hypochlorite even in the presence of other ROS species (Fig. 7). Initially, 20μL of all ROS were added one by one to MB solution where sodium hypochlorite shows exceptionally higher absorption (Fig. 2A). Furthermore, the MB solution was quantified by titration method. Titration was performed against lower to higher concentrations of sodium hypochlorite i.e., 1μL, 2μL, 3μL, 4μL, 5μL, 6μL, 8μL, 10μL and 12μL with MB solution. It was observed that the added concentration of sodium hypochlorite and is directly proportional to absorption shown by MB stock solution, i.e., absorption increases with elevation in concentration of sodium hypochlorite.

Fig. 6

(A) UV-vis spectra of MB solution (100 mg/L) with 100μL of different metal ions (Mn²⁺, Cr²⁺, Mg²⁺, Cd²⁺, Zn²⁺, Cu²⁺, Pb²⁺ and Hg²⁺) (1×10^–3 M) and (B) Titration curve of Hg²⁺ solution (1×10^–3 M) of different concentration (20μL, 40μL, 60μL, 80μL, 100μL, 200μL and 300μL) against MB solution (100 mg/L).

Fig. 7

(A) Absorption spectra of acids and 20μL of both ROS species (t-BuOOH, KO₂ and NaOCl) (1×10^–3 M) and acids (HCl, H₂SO₄ and HNO₃) (1×10^–2 M) against MB solution (100 mg/L). (B) Titration curve of different concentration of sodium hypochlorite (1μL, 2μL, 3μL, 4μL, 5μL, 6μL, 8μL, 10μL and 12μL) against MB solution (100 mg/L).

4 Conclusion

To summarise, in our work we introduced a facile and single step method to synthesize hierarchical donor-acceptor type process. In this nitrogen-rich and porous structured melamine combines with 5-bromo-2-thiophenecarboxaldehyde, where thiophene group of latter donates electrons to the cyano group of former. The synthesized MB possessed multiple functional groups due to which it had a scope in detection of heavy metal ions and ROS. So, MB showed “turn-ON” type behavior with NaOCl and Hg²⁺. In the present work not only an easy and economical method of preparing MB is suggested but also its capability for sensing Hg²⁺ and NaOCl is elaborated. The chemical structure and interactions were revealed by X-Ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FT-IR) techniques. The thermal and mechanical stability were proven via Thermogravimetric analyser (TGA) and Differential thermal analyser (DTA) analysis. The described system of MB offers flexibility in design i.e., may reacts with commercially available di-aldehydes, tri-aldehydes, ketones or acids. Large scale production of this amorphous MB could be done due to cheap and easy availability of its monomeric units.

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