Tony Jun
Huang
Department of Engineering Science and Mechanics, The Pennsylvania State University, USA. E-mail: junhuang@psu.edu
First published on 16th January 2015
In this issue of Lab on a Chip, Whitesides et al. (DOI: 10.1039/C4LC01161A) present their efforts on developing new signals for POC immunoassays. They used density as a measurable target to reflect the quantity of molecular analytes in solution. Therefore, they demonstrated immunoassays that can be read using only the naked eye and a ruler. The use of density to identify objects can be dated back to the third century BC, when Archimedes used density as a marker to determine the purity of King Hiero's new crown. Whitesides et al. revitalize the role of this old parameter in analysis by introducing the Magnetic Levitation (MagLev) method for high-sensitivity density determination. By placing two permanent magnets coaxially facing each other, the density of objects can be obtained by observing the ‘levitation height’ of the object. Binding of analytes is designed to occur on the surface of polystyrene beads and the signal is amplified by gold-labeled antibody and electroless gold or silver deposition. As a result, the presence and the amount of target analytes will result in a change in density of the polystyrene beads, which is reported as a change in the levitation height of the bead. This platform is demonstrated for both competitive and indirect immunoassays through probing neomycin in a milk sample and Hepatitis C virus NS3 protein and syphilis T. pallidum p47 protein in serum, respectively.
This new readout strategy minimizes the requirements of external resources to measure an immunoassay. It does not need any complex equipment or electricity to achieve accurate quantitative measurements. Thus it is ideal for POC applications in remote sites and resource-poor areas. For future development, if this technique allows convenient and accurate quantification of target analytes, many of the current POC immunoassays can be further simplified and improved, and made more amenable to portable applications.
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