What are label free assays?

July 02, 2012

Label free assays measure the interaction of molecules (ligand-receptors, antibodies-antigens, proteins-proteins, etc.) without the use of tags such as radioisotopes or fluorescent dyes.

Image of Dr EJ Dell
Dr EJ Dell
PhD, Sales Manager Northwest

There are a variety of methods to perform these assays and they usually require highly specialized instruments such as NMR or BIACORE. One form of label-free technology, Surface Plasmon Resonance (SPR), measures the resonance of valence electrons in a substance (usually gold or silver) that occurs upon light absorption at a specific wavelength. When molecules bind to the gold or silver surface, a change in absorption can be measured which is directly proportional to the amount and size of molecule that is binding.


Recently a label-free assay has been developed by PharmaDiagnostics in Belgium which can work on all of BMG LABTECH’s spectrometer-based microplate readers. The SoPRanoTM assay works on a variation of SPR known Localized Surface Plasmon Resonance (LSPR) and it consists of Gold Nano Rods (GNRs) that can be conjugated to proteins via the free amine. Then when increasing amounts of a second molecule is added to the protein conjugated GNRs, a dose-dependent shift in the absorbance maximum (ODλmax) is observed. The SoPRanoTM assay is perfect for all BMG LABTECH spectrometer-based instruments because this full spectrum shift in ODλmax can be measured in <1 second per well using the CCD-based spectrometer.  No other microplate reader technology can measure a full aborbance spectrum that quickly.