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Authors report on a carbon nanotube (CNT)-based lateral flow immunoassay (LFI) for ultrasensitive detection of proteins. Shortened multiwalled CNTs were used as a colored (black) tag. The detection antibody was covalently immobilized on the CNT surface via diimide-activated conjugation between the carboxyl groups on the CNT surface and amino groups of antibodies. The assay involved the capture of target protein in a sandwich-type format between an immobilized capture antibody on the test zone of LFI and a CNT-labelled detection antibody. CNTs were thus captured on the test zone of the LFI and gave a black colored line to enable visual detection of protein. Quantitative results were obtained by reading the test line intensities with a portable strip reader. Rabbit IgG was used as a model target to demonstrate the proof-of-concept. Combining the advantages of lateral flow assay with the unique physical properties of CNT (color, high aspect-to-size ratio and ease of surface modification), the optimized LFI can detect of 1.3 pg mL -1 of rabbit IgG (S/N = 3). This is three orders lower than that of gold nanoparticle-based LFI. Rabbit IgG spiked into human plasma samples was successfully detected with this LFI. Conceivably, this method can be extended to various other proteins for which adequate antibodies do exist. Graphical abstract Carbon nanotubes are used as black tags in an ultrasensitive lateral flow immunoassay (LFI). The LFI was applied to the determination of rabbit IgG. The detection limit is more than 3 orders of magnitude lower than that of the conventional gold nanopaticle-based LFI.

The authors describe a rapid, low-cost and sensitive approach for the determination of carbohydrate antigen 19-9 (CA 19-9) in whole blood by using magnetized carbon nanotube (MCNT) and lateral flow strip biosensor (LFSB). MCNTs were synthesized by depositing magnetite (Fe 3 O 4 ) nanoparticles on multiwalled carbon nanotube (CNT) via co-precipitation of ferric and ferrous ions within a dispersion of shortened multiwalled CNTs. Antibody against CA 19-9 (Ab 1 ) was covalently immobilized on the MCNTs and were used to capture CA 19-9 in blood. After magnetic separation, the formed MCNT-Ab 1 -CA 19-9 complexes are applied to the LFSB, in which a capture antibody (Ab 2 ) and a secondary antibody (Ab 3 ) are immobilized on the test zone and control zone of the LFSB, respectively. The captured MCNTs on the test zone and control zone are producing characteristic brown bands, and this enables CA 19-9 to be visually detected. Quantitation is accomplished by reading the intensities of the bands with a portable strip reader. Under optimized conditions, the assay has a detection limit as low as 30 U⋅mL -1 of CA19-9 in blood. This is below the cutoff value (37 U mL -1 ) of CA 19-9. The assay duration for blood samples is 35 min. In our perception, the assay represents a rapid and low-cost tool for rapid determination of CA19-9 in blood that holds promise for clinical applications, particularly in limited resource settings.