Abstract
Tailoring conducting polymers (CPs) such as polyaniline (PANI) to deliver the appropriate morphology, electrochemical properties and processability is essential for the development of effective polymer-based electrochemical sensors and biosensors. Composite PANI electrodes for the detection of ammonium (NH4+) have been previously reported, but have been limited by their reliance on the electrocatalytic reaction between NH4+ and a metal/nano-catalyst. We report an advanced processable and nanofibrous polyaniline:polystyrene-sulphonate (nano-PANI:PSS) as a functional ink for the fabrication of catalyst-free NH4+ sensors and enzyme-coupled urea biosensors. The PSS provides both a soft-template for nanofibre formation and a poly-anionic charge compensator, enabling the detection of NH4+ based on an intrinsic doping/de-doping mechanism. The nanostructured morphology, chemical characteristics and electrochemical properties of the nano-PANI:PSS were characterised. We fabricated 3D-hierarchical sensor interfaces composed of inter-connected nano-PANI:PSS fibres (diameter of ~50.3 ± 4.8 nm) for the detection of NH4+ with a wide linear range of 0.1–11.5 mM (R2 = 0.996) and high sensitivity of 106 mA M−1 cm−2. We further demonstrated the coupling of the enzyme urease with the nano-PANI:PSS to create a urea biosensor with an innovative biocatalytic product-to-dopant relay mechanism for the detection of urea, with a linear range of 0.2–0.9 mM (R2 = 0.971) and high sensitivity of 41 mA M−1 cm−2. Moreover, the nano-PANI:PSS-based sensors show good selectivity for the detection of NH4+and urea in a urine model containing common interfering molecules. This processable and fibrous nano-PANI:PSS provides new advance on CP-based transducer materials in the emerging field of printed organic sensors and biosensors.
Highlights
Ammonium (NH4+) and urea are the main sources of nitrogen pol lutants associated with fertiliser production, oil refining, the fermenta tive food industry and anaerobic wastewater treatment (Dhamole et al, 2015; Tallaksen et al, 2015)
We further demonstrated the coupling of urease with the NH4+ + e− ↔ [nano-PANI(red): PSS− (NH4)+ sen sitive nano-PANI:PSSn as a urea biosensor with an innovative catalytic product-to-dopant relay mechanism, in which the urea is enzymaticallycatalysed by urease generating the NH4+ product, followed by the diffusion of NH4+ into nano-PANI:PSSn fibrous network triggering the doping/de-doping reaction at PANI:PSSn interface (Fig. 4a)
We have demonstrated the development of a novel nano-PANI:polystyrene sulphonate (PSS) combining nanostructured morphology with good processability and cationic NH4+ selectivity for the development of catalyst-free nano-PANI: PSS-based NH4+ sensors and enzyme-coupled urea biosensors
Summary
Ammonium (NH4+) and urea are the main sources of nitrogen pol lutants associated with fertiliser production, oil refining, the fermenta tive food industry and anaerobic wastewater treatment (Dhamole et al, 2015; Tallaksen et al, 2015). For the PANI-platinum-Nafion composite, the detection of enzymatically-generated NH4+ has been reported to be led by the pH-driven conversion of NH4+ to NH3, followed by electrocatalytic oxidation of NH3 associated with the oxidation of PANI (Strehlitz et al., 2000), while other research groups suggested that the NH3 interacts with the platinum to generate a signal response (Jia et al, 2011). The challenges of effective PANI-based NH4+ sensing lay in the chemical nature on the polymer backbone of PANI, which has poor selectivity and diffusion characteristics towards cationic molecules (i.e. NH4+) (Hirai et al, 1988), the close-structure of PANI with a limited specific surface area for PANI/NH4+ interaction, as well as the poor processability of PANI for scalable sensor fabrication. The PSS provides both a soft-template for nanofibre formation and a poly-anionic charge compensator enabling the detection of NH4+ by the nano-PANI:PSS based on a doping/de-doping mechanism. We further demonstrated the coupling of the enzyme urease with the nano-PANI:PSSn to create a urea biosensor exploiting this innovative catalytic product-to-dopant relay mechanism
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