Abstract :
[en] Noble metal nanoparticles (NPs) can be used as a robust tool for optical bio-sensing. These NPs are known for their strong interactions with light through their surface plasmon resonance (SPR), which corresponds to the collective oscillations of the conduction electrons on the particles [1].
Among metals, silver and gold NPs are of particular interest not only because they are air-stable but also because their SPR absorption bands are in the visible and near ultra-violet spectral regions, that appear as the most appropriate for technological applications [2].
The first advantage of such optical SPR biosensors is their ability to measure complex formation in real time. Indeed, the SPR absorption spectrum band of the NPs is sensitive to the shape, size, inter-particle distance and composition of the NP as well as the dielectric properties of the surrounding medium [2]. Because of the sensitivity of SPR to the local dielectric environment, plasmonic NPs can act as transducers that convert small changes in the local refractive index and the inter-particle distance into spectral shifts and broadening in the absorption spectra bands [3].
Biotin is a water-soluble B complex vitamin necessary for the production of fatty acids and the metabolism of fats and amino acids. The avidin is a tetrameric protein which can react with biotin to form the strongly bonded biotin-avidin complex.The prototypical biotin-avidin interaction forms the basis of a simple sol-based diagnostic technique for biological analytes. We focused on this well-known couple of bio-molecules to compare optical properties of silver and gold colloidal NPs.
Gradual changes with time in the absorption spectra bands of biotinylated 10 nm silver and gold NPs were studied as a function of added avidin. After avidin addition, an increased red-shift of the SPR wavelength and a broadening of the absorption band with time are observed. These changes in the optical properties of colloidal NPs are due to the biomolecular recognition process between biotin and avidin which leads to aggregation of these NPs arising from cross-linking by the tetrameric protein. Moreover, the recognition process induces a variation of the local refractive index around these NPs and thus induces a red-shift of SPR also.
The maximum SPR red-shift was reached after 45 minutes and was equal to 25 nm and 15 nm for silver NPs and gold NPs respectively. We concluded that the dielectric sensitivity of gold NPs is smaller than the silver NPs one for a same geometry and for an equivalent concentration of avidin. Therefore, the silver sol is more adapted to detection of avidin than the gold sol. The detection limit, described as the lowest concentration for clear identification of wavelength shift [4] due to biomolecular recognition is determined to be 4 nM for both silver and gold NPs. In this case, the corresponding wavelength shift is about 3 nm. The specificity of the interaction between biocytin and avidin was checked by replacing avidin by BSA. When BSA was added, we observed a SPR shift which was smaller than the detection limit of 3 nm.
Future works will be devoted to transpose this kind of biomolecular recognition experiments on gold nanorods in order to improve the dynamic phototherapy efficiency of cancers.
