Protein-modified silver nanoplates for the complementary analytical method of localised surface plasmon resonance and matrix assisted laser desorption/ionisation mass spectrometry

Abstract

A complementary analytical methodology combining localised surface plasmon resonance (LSPR) sensing and matrix assisted laser desorption/ionisation-mass spectrometry (MALDI-MS) has been developed by using triangular silver nanoplates (Ag NPLs). Ag NPLs with near-IR LSPR absorbance were prepared via a two-step photo-mediated growth process from Ag nanoparticles. These could be utilised as a common platform for LSPR/MALDI-MS, as they were suitable for LSPR sensing via the analysis of surface plasmon absorption bands and as the assisting material for LDI-MS. For practical use, the detection of analytes by LSPR sensing can be achieved by using specific biomolecular recognition techniques that involve the surface modification of metal NPs with large molecules such as proteins. We investigated the effect on LSPR sensitivity (i.e. refractive index unit, RIU) of modifying the Ag surface with molecules of different sizes, and found that the RIU values were proportional to the amplitude of the cube root of the molecular weight, M_w^1/3. We demonstrated the utility of this complementary LSPR and MALDI-MS analysis methodology by evaluating the binding of a soybean trypsin inhibitor to the Ag NPL substrate covalently modified with trypsin. This specific biomolecule recognition phenomenon was first detected using the LSPR technique, and then successfully identified by MALDI-MS.