Nanoprobe-free nanosensor with Tyndall-effect readout for colorimetric detection of alkaline phosphatase

This work initially describes a new colorimetric nanosensor for the visual detection of alkaline phosphatase (ALP) based on analyte-mediated in-situ production of silver nanoparticles (AgNPs) that lights up the Tyndall effect (TE). In the presence of ALP, 4-aminophenol phosphate (APP) is hydrolyzed to generate 4-aminophenol (AP), which in turn reduces Ag + ions to form AgNPs. The TE signal can be observed with the naked eye in the resulting colloidal mixture under the irradiation of a handheld laser pointer pen (acting as a light source). The TE intensity is proportional to the analyte concentration in the sample. The quantitative measurement of ALP is further realized by using a smartphone for reading out the TE signal. The results obtained under optimal experimental conditions show that the proposed nanosensor is capable of quantifying ALP within a concentration range of 0.0156–8 U/L. Its detection limit for the analyte is estimated to be approximately 0.002 U/L according to the 3σ rule. Moreover, the recovery of ALP in real human serum samples ranges from 94.38 to 96.67 %, demonstrating its satisfactory analytical accuracy and practicality. To our knowledge, this may be the first report of a nanoprobe-free TE-enhanced colorimetric nanosensor for the instrument-free analysis of enzymatic activity.