Metal-organic framework-based ratiometric point-of-care testing for quantitative visual detection of nitrite

Nitrite (NO₂^-) is categorized as a carcinogenic substance and is subjected to severe limitations in water and food. To safeguard the public's health, developing fast and convenient methods for determination of NO₂^- is of significance. Point-of-care testing (POCT) affords demotic measurement of NO₂^- and shows huge potential in future technology beyond those possible with traditional methods. Here, a novel ratiometric fluorescent nanoprobe (Ru@MOF-NH₂) is developed by integrating UiO-66-NH₂ with tris(2,2'-bipyridyl)ruthenium(II) ([Ru(bpy)₃]²⁺) through a one-pot approach. The special diazo-reaction between the amino group of UiO-66-NH₂ and NO₂^- is responsible for the report signal (blue emission) with high selectivity and the red emission from [Ru(bpy)₃]²⁺ offers the reference signal. The proposed probe shows obviously distinguishable color change from blue to red towards NO₂^- via naked-eye. Moreover, using a smartphone as the detection device to read color hue, ultra-sensitive quantitative detection of NO₂^- is achieved with a low limit of detection at 0.6 μΜ. The accuracy and repeatability determined in spiked samples through quantitative visualization is in the range of 105 to 117% with a coefficient of variation below 4.3%. This POCT sensing platform presents a promising strategy for detecting NO₂^- and expands the potential applications for on-site monitoring in food and environment safety assessment.