Water insoluble, self-binding viologen functionalized ionic liquid for simultaneous electrochemical detection of nitrophenol isomers

Highlights

• Hydrophobic, gel-type viologen functionalized ionic liquid was synthesized.

• Vio-IL utilized as self-binding mediator and immobilized on MWCNT coated SPE.

• Vio-IL/MWCNT/SPE shown excellent electrocatalytic activity towards the reduction of 2-NP and 4-NP.

• Fabricated sensor offered promising performance for simultaneous detection of 2-NP and 4-NP.

Abstract

Herein, a water insoluble viologen functionalized ionic liquid (Vio-IL) was designed for the simultaneous electrochemical detection of 2-nitrophenol and 4-nitrophenol. Carboxyl functionalized benzimidazolium based ILs were hierarchically synthesized and the water insoluble carboxylic IL was covalently attached with aminopropylmethyl viologen through DCC coupling to obtain water insoluble Vio-IL gel. Vio-IL was immobilized by simple dropcasting over a multiwalled carbon nanotube deposited screen printed carbon electrode to obtain Vio-IL/MWCNT/SPE. The Vio-IL/MWCNT/SPE exhibited two sets of well-resolved redox peaks corresponding to V²⁺ to V^+• and V^+• to V⁰ redox couple. Furthermore, Vio-IL/MWCNT/SPE has shown excellent electrocatalytic activity towards the reduction of 2- and 4-nitrophenol individually, and also for the simultaneous determination of 2- and 4-nitrophenol. The linear range, sensitivity and detection limit for simultaneous detection of 2-nitrophenol at Vio-IL/MWCNT/SPE were found to be 4–494 μM, 0.884 μA μM⁻¹ cm⁻², 1.5 μM and that for 4-nitrophenol were 2–259 μM, 2.615 μA μM⁻¹ cm⁻², 0.70 μM. The Vio-IL/MWCNT/SPE established excellent sensitivity and selectivity towards the simultaneous determination of 2- and 4-nitrophenol together with impressive stability and reproducibility. The exemplary analytical parameters achieved are due to the rational immobilization of the mediator covalently in the highly conducting IL/MWCNT backbone which maintained the mediator characteristics effectively.

Introduction

The detection and monitoring of trace levels of potential toxic chemical species like aromatic nitro compounds has always been a major concern, because of the widespread existence of these pollutants in water, air and soil, and thus rising as a more complicated environmental and biological threat [[1], [2], [3], [4]]. Among various aromatic nitro compounds, both 2-nitrophenol (2-NP) and 4-nitrophenol (4-NP) are considered to be most priority hazardous wastes contaminating soil and ground water, since they have been extensively used in different domains for the manufacture of drugs, dyes, fungicides and pesticides [[5], [6], [7]]. These nitroaromatics are poorly degradable due to strong electron-withdrawing groups which produces undesirable and irritating odor, and would cause enormous adverse effects in living organisms [8,9]. Therefore, development of a simple and efficient technique for the detection of 2- and 4-NP is strongly encouraged. Although various analytical methods concerning the determination of 2- and 4-NP including electrophoresis [10], spectrophotometry [11], high performance liquid chromatography [12] and flow injection analysis [13] are reported with reasonably good performances, other approaches are being explored in parallel in order to improve the analytical characteristics. These two compounds are isomers of nitrophenols and thus possess similar chemical structure and properties, making them challenging for determination due to signal overlapping. Hence, an uncomplicated precise analytical tool to distinguish the signals of 2- and 4-NP which often interfere with each other is of great demand.

Electrochemical sensors have emerged as an efficient way to distinguish multiple or overlapping signals with wide separation to offer selective, simultaneous and accurate quantitative detection of these isomers even when they are located in complex environmental or biological systems [[14], [15], [16]]. Diverse materials including carbon nanomaterials, metal nanoparticles, metal oxides have been developed towards the simultaneous determination of important analytes. These materials afford larger surface area and conductivity which facilitate resolved electrochemical response of individual analytes [[17], [18], [19]]. Meanwhile, development of electrochemical devices using electroactive mediators which can selectively oxidize/reduce the target analyte is a subject of intense research activity and this has created an evolution in widespread design of electrochemical sensors and biosensors [[20], [21], [22], [23], [24]]. Among variety of mediators, methyl viologen (1,1′-Dimethyl-4,4′-bipyridinium dichloride) and its derivatives have attracted immense interest in materials research thanks to their unique photochemical and electrochemical properties [[25], [26], [27], [28]]. This organic dye has great potential as mediator due to the highly desirable redox behavior with two successive well distinguished reversible redox peaks corresponding to one electron reduction process each (V²⁺ to V^+• and V^+• to V⁰) [[29], [30], [31]]. In spite of its distinctive redox characteristics, viologen and its derivatives are less investigated in electrochemical sensing and this could be due to their high water solubility. This requires specific matrix/immobilization strategy to prevent the leakage of viologen from the electrode surface which would otherwise lead to highly unstable electrochemical system making it unsuitable for reliable and reproducible measurements.

Attempting to circumvent the solubility problem and to generate a stable electrochemical response, viologen based sensing platforms were developed with great efforts using specific binders to attach viologen molecule on electrode surface [[32], [33], [34], [35]]. All these approaches have demonstrated the stable anchoring of viologen on the electrode surfaces during which they have focused only on the first redox peak (V²⁺ to V^+•) and the second redox couple (V^+• to V⁰) was not addressed, which makes these sensor systems unsuitable for the determination of multiple analytes [33,34]. Establishment of both the redox peaks of viologen would facilitate the simultaneous detection of 2- and 4-NP. Therefore, it is inevitable to synthesis water insoluble viologen, supported with highly conducting architecture in order to establish both the redox peaks with high stability and fast electron transfer. Ionic liquids (ILs) are considered as promising candidates in electrochemistry, as they satisfy the requirement of versatility and conductivity [[36], [37], [38]]. ILs have proven to enhance electrocatalytic behavior with higher peak current, larger faradaic to capacitive current ratios and faster response time [39,40]. The hydrophobicity/hydrophilicity of these molecules could be tuned by changing the cations and anions and thus they can be judiciously tailor-made for desired purpose. Furthermore, these ILs can be synthesized with suitable functionality in order to covalently immobilize the redox mediators and biomolecules, resulting in augmented stability [41,42]. Thus, designing a water insoluble IL with suitable functional group would be of great interest to immobilize viologen and to explore its sensing ability towards the detection of multiple analytes.

In the present work, we intended to design a novel water insoluble viologen containing IL and to employ it towards the simultaneous determination of 2- and 4-NP. In order to serve the purpose, it was decided to synthesize a water insoluble carboxylic acid functionalized benzimidazolium IL (Bim-IL) and to couple with aminopropylmethyl viologen (APM-Vio) through amidation. Accordingly, Bim-ILs containing hexanoic acid (HA) on quaternary nitrogen and alkyl group with increasing carbon chain length from C₂ to C₁₆ on the tertiary nitrogen were synthesized. Bim-ILs comprising C₂–C₁₄ carbon chain lengths were found to be water soluble and to our delight the Bim-IL with C₁₆ carbon chain was obtained water insoluble (C₁₆-Bim-HA-IL). Thus, we have subjected C₁₆-Bim-HA-IL and APM-Vio to undergo a coupling reaction in presence of N,N′-dicyclocarbodiimide (DCC), which resulted in the desired water-insoluble viologen functionalized IL (Vio-IL). The resultant Vio-IL was obtained as a gel, which was dropcasted over multiwalled carbon nanotube (MWCNT) deposited on screen printed carbon electrode (SPE) to obtain Vio-IL/MWCNT/SPE modified electrode. MWCNTs have superior conductivity, larger surface area, high mechanical stability, distinctive electronic properties and has the ability to blend with other materials, which makes it as a suitable candidate for electrode fabrication [43,44]. Therefore, MWCNT has been used in the electrode modification to afford augmented conductivity which would result in high sensitivity and signal resolution [45,46]. The Vio-IL/MWCNT/SPE modified electrode exhibits highly stable electrochemical behavior with two well resolved reversible redox peaks with higher peak currents, which were further applied towards selective and simultaneous electrochemical detection of 2- and 4-NP.