AN AMMONIA OPTICAL SENSOR SILCA MIROSPHERES DOPED WITH NICKEL(II) ION AND REFLECTANCE TRANSDUCTION

An optical sensor based on silica micromaterial doped with Ni(II) ion has been developed for quantitation of ammonia (NH3) concentration using reflectometric transduction in an aqueous environment. The as-synthesized silica nanospheres were immobilized with Ni(II) ion via electrostatic interaction between positively charged divalent transition metal ion and negatively charged terminal silanol groups at silica microspheres surface. The optical microsensor demonstrated a colour change from green to blue upon introduced with NH3 solution, as a result of the formation of blue-hued hexamminenickelate(II) ([Ni(NH3)6] ) complex on the silica microspheres surface, and a maximum reflectance response was attained at the wavelength of 615 nm. The response time of the silica microspheres-based sensor is rapid towards the reflectance determination of NH3, i.e. <1 min at optimum pH 6. The linear response range of the developed reflectometric NH3 microsensor was acquired from 0.0003 M to 0.008 M (R=0.9826) with a limit of detection (LOD) of 0.0012 M. The ammonia optical sensor response is good reproducibility with a relative standard deviation (RSD) obtained at 0.25% (n=7). The recovery test exhibited satisfactory results of 101.5% when the fabricated sensor was applied for the determination of NH3 in a polluted water sample collected from landfill wastewater.