Detection of copper ions in an aqueous solution by a dual-peak long period fiber grating functionalized with a polypyrrole-chitosan composite.

A dual-peak long period fiber grating (DP-LPFG) sensor functionalized by polypyrrole-chitosan composite was proposed for sensitive detection of Cu ions in aqueous solution. The nitrogen atom on the polypyrrole ring and the amino group on the chitosan chain in the complex matrix can chelate the Cu ions. Thus, the refractive index of the overlay changed and further modulated the transmission spectrum of DP-LPFG. After special design, the double peaks can move in opposite directions with the increase of Cu ion concentration, thereby greatly improving detection sensitivity. The linear sensitivity of the fabricated sensor was measured to be 9.12 and 2.14 nm/ppm (0.61 and 0.14 nm/µM) for concentrations of 0.1-0.5 (1.5 µM-7.5 µM) and 0.5-2 ppm (7.5 µM-30 µM), respectively. In addition, the Langmuir isothermal model was used to evaluate the overall response of the sensor to Cu ions quantitatively, and the detection limit was determined to be 0.05 ppb (0.75 nM). This ingenious sensor offers a new solution for sensitive detection of heavy metal ions in environmental water.