Nanozyme catalysis-assisted ratiometric multicolor sensing of heparin based on target-specific electrostatic-induced aggregation

Monitoring the level of heparin in clinical matrices is significant because of its pivotal role in preventing thrombosis. Compared to traditional single-signal sensors, multi-signal ratiometric detection can provide anti-interference results especially in complicated environments. However, fabricating an easy-to-operation, low-cost and robust sensor for the ratiometric detection of heparin still remains challenging. Here we propose a novel nanosensor for the ratiometric multicolor sensing of heparin with high performance. The sensor is based on the specific electrostatic interaction between the target and a positively charged species generated from nanozyme catalysis. FeMoO4 nanorods are explored as an oxidase mimic for the first time, showing a high activity at neutral pH to catalyze the colorless 3,3',5,5'-tetramethylbenzidine (TMB) oxidation to blue TMBox with an absorbance at 652 nm. Heparin can induce the rapid aggregation of the produced TMBox intermediate with rich positive charges due to their strong electrostatic interaction, leading to the formation of a purple Heparin-TMBox complex providing a signal at 565 nm. With the increase of heparin, the color changes from blue to indigo and further purple, enabling the multicolor sensing of the target. As a result, ultrasensitive determination of heparin was obtained with a very low detection limit. The fabricated nanosensor could differentiate heparin from complex species with no interferences, and it provided reliable analytical results for heparin in both serum and plasma. With robust performance, low cost and facile fabrication, the sensor holds great potential in monitoring heparin for clinical applications.