Label-free ultrasensitive colorimetric detection of copper(II) ions utilizing polyaniline/polyamide-6 nano-fiber/net sensor strips

A novel, ultrasensitive, selective and flexible sensor strip based on polyaniline/polyamide-6 (PANI/PA-6) nano-fiber/net (NFN) membranes for naked-eye colorimetric detection of Cu2+ ions in water is successfully prepared by a facile electro-spinning/netting (ESN) process. The sensing mechanism involves the transformations between different oxidation and doping forms of PANI. Upon exposure to Cu2+ aqueous solution, the sensors exhibit two significant reflectance intensity decreasing bands at 435 and 650 nm which induce the color changes from white to blue dramatically. This new sensor shows colorimetric response specifically to Cu2+ ions (white-to-blue color change) over other possible interfering metal cations and allows for detection of Cu2+ in aqueous solution with a low detection limit of 1 ppb observing by naked eye. Additionally, the colorimetric responses are visualized quantitative by using a color-differentiation map prepared from converted RGB (red, green and blue) values. Furthermore, the as-prepared PANI/PA-6 NFN sensor strips could successfully combine with the color map, which suggested a promising analytical method as an economical alternative to traditional Cu2+ sensors and also provided a new insight into the design and development of a novel colorimetric sensing system based on the NFN platform.