Carbon Nanotube NIR Fluorescent Probes

In clinical tests, detection of weak signals is especially important for detection of low concentration biomolecules.  An enzyme-linked immunosorbent assay (ELISA) is often employed for antigen detection, where background fluorescence that is generated from the combination of autofluorescence from serum components and nonspecific binding serum proteins, limits detection sensitivity of an antigen by ELISA.

Near-infrared (NIR) fluorescence has attracted much more interests in diagnostic fields because of the lower background interference and the improved tissue penetration. Several NIR fluorescent molecules have been tried in enzymatic assays, DNA sequencing, capillary electrophoresis, immunoassay and so on. For example, by using NIR fluorescent nanoparticles, alpha fetoprotein was successfully detected even in whole-blood samples, which indicates a great advantage of NIR labels in clinical tests. Under these circumstances, novel NIR fluorescent molecules for biological label have so far been synthesized.

Single-walled carbon nanotubes (CNTs) have received much attention due to their superior mechanical, optical, thermal and electrical properties. In particular, CNTs have been considered as ideal labels for biological imaging in vitro and in vivo because CNTs show fluorescence in the wavelength region of 1000−1400 nm, so-called the second NIR window. In this wavelength region, indigenous tissue autofluorescence is much lower than that in the visible (400−750 nm) and shorter NIR (750−900 nm) regions, which enables clear vascular imaging with the CNT labels. This benefit is further enhanced by a large Stokes shift between emission and excitation resonances of CNTs, leading to further suppression of endogenous autofluorescence. Moreover, CNT photoluminescence does not show photobleach under prolonged duration of excitation as compared with NIR fluorescent dyes and quantum dots.

In the present work, to show the potential ability of the CNT labels in the immunological tests, we conjugated CNTs with immunoglobulin G (IgG) antibodies by using a linker molecule of phospholipid polyethylene glycol (PEG), and their immunoprecipitation using protein G magnetic beads was examined [1]. The combination of IgG and protein G was adopted because they specifically bind with each other. The present results, using CNTs as the NIR fluorescent labels for the immunoassay, will pave a new way for medical applications of CNTs.

 

1. Y. Iizumi, T. Okazaki, Y. Ikehara, M. Ogura, S. Fukata and M. Yudasaka, ACS Appl. Mater. Interfaces, 5, 7665−7670 (2013).