Point-of-need quantitation of 2,4-dichlorophenoxyacetic acid using a ratiometric fluorescent nanoprobe and a smartphone-based sensing system

This study reports a new ratiometric fluorescence probe (referred to as rQD@PS@bCDs) and a smartphone-based sensing system for on-site quantitation of Fe³⁺, ascorbic acid (AA), alkaline phosphatase (ALP) and 2,4-dichlorophenoxyacetic acid (2,4-D). rQD@PS@bCDs with well-resolved dual emissions is facilely synthesized by covalent conjugation of red-emissive quantum dots (rQDs), the internal standard, with blue-emissive carbon dots (bCDs), the Fe³⁺-specific fluorescent indicator. 2,4-D can specifically inhibit the enzymatic activity of ALP, leading to suppress the production of AA which can reduce Fe³⁺ to Fe²⁺ and thereafter trigger the fluorescence response of rQD@PS@bCDs. Taking advantages of these highly specific cascade catalytic reactions, ratiometric fluorescence schemes are established for sensing Fe³⁺, AA, ALP and 2,4-D both in aqueous solution and on paper-based microarray. The smartphone-based fluorescence detector integrated with the rQD@PS@bCDs-based microarray demonstrated good sensing performance with a wide linear range and low detection limit, and is successfully applied to quantitate 2,4-D in cabbage and tap water samples. The reported method which enables point-of-need quantitation in a sample-in-result-out manner and highlights its features of cost-effectiveness and ease-of-use, is ideally for food safety inspection, environment surveillance, health monitoring at point-of-need, especially in low resource settings.