Research paperA functionalized nanomaterial based, new, solid state cationic-surfactant-selective sensor with fast response and low noise

- Chemically functionalized MWCNTs as surfactant sensing element.
- Excellent selectivity for cetylpyridinium cation.
- Better selectivity for cationic surfactants in comparison to anionic surfactants.
- Negligible signal drift of the sensor.
- Ability of differential titration of two cationic surfactants in a mixture.

A new selective and highly sensitive potentiometric sensor for cationic surfactants was prepared based on multi-walled carbon nanotubes (MWCNTs) functionalized with a sulfate group and cetylpyridinium ion (CP) as a sensing element.The chemical modification of MWCNTs with CP cations had positive effects on the most important sensor properties, such as greater membrane stability, considerably reduced leaching of the sensing material from the membrane, prolonged life time of the sensor and negligible noise and signal drift.The sensor exhibited a Nernstian response for CP (58.9 and 58.2 mV/decade in water and 0.01 M Na2SO4, respectively) and sub-Nernstian responses for hexadecyltrimethylammonium ion (CTA) (56.6 and 55.5 mV/decade in water and 0.01 M Na2SO4, respectively) and Hyamine (52.1 and 49.7 mV/decade in water and 0.01 M Na2SO4, respectively).The detection limit for CP was 1.2 × 10−7 M (in water and 0.01 M Na2SO4), whereas those for CTA were 2.5 × 10−7 and 3.2 × 10−7 M (in water and 0.01 M Na2SO4, respectively), and that for Hyamine was 1.5 × 10−6 (in water and in 0.01 M Na2SO4).The response time of the sensor was 5 s for cetylpyridinium chloride (CPC) concentration changes greater than 0.005 mM in water and 9 s for CPC concentration changes greater than 0.005 mM in 0.01 M Na2SO4 solution.The signal drift of the sensor was 0.3 mV/h in 4 mM CPC solution and 0.8 mV/h in a hand sanitizer solution.The new sensor exhibited excellent selectivity for CP cations compared to the inorganic and organic cations commonly present in commercial formulations, and it was successfully used as an end-point detector in the potentiometric titration of cationic surfactants in a pH range from 3 to 10. A two-component mixture containing CPC and Hyamine was successfully differentially titrated.

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