Smartphone flashlight-triggered covalent organic framework nanozyme activity: A universal scheme for visual point-of-care testing

The spread of point-of-care testing (POCT) undoubtedly has an important significance for food safety, environmental conservation and disease diagnosis, particularly for resource-limited regions. However, there are still challenges in fabricating an affordable, portable and reliable POCT sensor. Addressing this challenge, a smartphone-based visual POCT platform was designed by integrating the flashlight illumination and camera imaging functions. 2,4,6-triformylphloroglucinol (Tp) and 4,4′-(benzo[c][1,2,5]thiadiazole-4,7-diyl) dianiline (BTD) were used as monomers to construct a covalent organic framework (COF) nanozyme (termed as TpBTD). And this is the first attempt to demonstrate the as synthesis TpBTD owns outstanding nanozyme property under the much safer and green flashlight illumination. The mechanism of TpBTD as flashlight-responsive oxidase was detailedly clarified. Significantly, TpBTD can be served as an excellent colorimetric probe for visual POCT since it can rapidly induce the allochroic reaction of 3,3′,5,5′-tetramethylbenzidine (TMB). Taken organophosphorus pesticides (OPs) as model, the typical acetylcholinesterase (AChE)-acetylthiocholine (ATCh) based reaction was introduced. As a result, red-green-blue (RGB) recognition of disparate TMB colored products induced by OPs can be performed by utilizing the high-resolution of the same smartphone camera. Trichlorfon residues in actual samples were compared by UV–vis and RGB colorimetry with reasonably agreeable results. With the ubiquitous smartphone, this work provides a convenient and feasible strategy for the application of COF with photo-catalytic property in POCT fields.