Abstract
Methods to diagnose malaria are of paramount interest to eradicate the disease. Current methods have severe limitations, as they are either costly or not sensitive enough to detect low levels of parasitemia. Here we report an ultrasensitive, yet low-resource chemical assay for the detection and quantification of hemozoin, a biomarker of all Plasmodium species. Solubilized hemozoin catalyzes the atom transfer radical polymerization of N-isopropylacrylamide above the lower critical solution temperature of poly(N-isopropylacrylamide). The solution becomes turbid, which can be observed by naked eye and quantified by UV-visible spectroscopy. The rate of turbidity increase is proportional to the concentration of hemozoin, with a detection limit of 0.85 ng mL−1. Malaria parasites in human blood can be detected down to 10 infected red blood cells μL−1. The assay could potentially be applied as a point-of-care test. The signal-amplification of an analyte by biocatalytic precipitation polymerization represents a powerful approach in biosensing.
Highlights
Methods to diagnose malaria are of paramount interest to eradicate the disease
Natural hemozoin, which is insoluble under physiological conditions, was obtained from Plasmodium falciparum cultures
Solubilized Hz can catalyze the polymerization of NIPAAm to PNIPAAm in the presence of the ATRP reagents sodium ascorbate and 2-hydroxyethyl bromoisobutyrate (HEBIB) (Fig. 1b)
Summary
Methods to diagnose malaria are of paramount interest to eradicate the disease. Current methods have severe limitations, as they are either costly or not sensitive enough to detect low levels of parasitemia. Microscopy in combination with Giemsa staining is the most widely used technique to diagnose malaria[4] It allows the identification of the malaria species and has a detection threshold of ~50 infected red blood cells (iRBC) μL−1 5. This method requires welltrained microscopists to correctly identify the parasite in a blood smear. There are more than 200 MRDTs currently available;[8] most of them are based on immuno-chromatography strips which perform antigen–antibody complexation[8] Their sensitivity to malarial parasites is relatively low (detection limits around 200 iRBC μL−1)[5,9]. To our knowledge, solubilized hemozoin has not been explored as a catalyst for radical polymerization, neither for synthesis purposes nor for biosensing applications, even though heme-containing enzymes and hemin[34,35,36] are known to initiate free radical polymerizations[37,38,39] or to catalyze reversibledeactivation radical polymerizations[40,41,42,43,44,45,46,47]
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