A sensitive colorimetric label-free assay for trypsin and inhibitor screening with gold nanoparticlesElectronic supplementary information (ESI) available: the absorption spectra of Au-NP containing only trypsin; pH interference. See DOI: 10.1039/c1an15224f

Gold nanoparticles (Au-NPs) with negative charges aggregate in the presence of Arg6due to electrostatic interactions resulting in the red-shift of the plasmon absorption. But, after incubation of Arg6with trypsin the aggregation of Au-NPs can be prohibited. Accordingly, a newly designed-Au-NPs based colorimetric assay method for trypsin activity is established. Trypsin with a concentration as low as 1.6 ng mL−1can be assayed with this new colorimetric assay. This colorimetric label-free assay for trypsin can be performed in aqueous solution and both Au-NPs and Arg6are easily accessible. Thus, this assay method is useful for screening inhibitors of trypsin. [ABSTRACT FROM AUTHOR]