A “fluid cantilever” to detect amphiphilic biomolecules

Label-free techniques are now identified as relevant tools to detect any biological material which cannot be grafted by a fluorescent tag, for instance. In this growing context, a new detection strategy based on the resonance of micrometric capillary waves is proposed to detect biomolecules. The sensitivity of the surface rheology to the (bio)chemical content of a liquid surface is found to behave as an original transduction means to convert any slight change in the surface composition into a detectable change in the surface geometry. Micrometric deformations are promoted steadily along a functionalised liquid surface by generating linear meniscus waves from a capillary boundary layer which develops all around the outer circular edge. The geometry of the subsequent nearly standing waves net is accurately characterised using refractometry and interferometry techniques. This paper focuses on this last technique which is especially developed to characterise the waves amplitude at the surface centre with a sensitivity of a few hundreds of nanometers. As such, local interferometry allows us to follow in real-time the resonance frequencies of the wave net and finally, to detect any slight change in surface tension induced during the trapping of solubilised biomolecules at the functionalised surface. In order to clearly illustrate the potentialities of a resonant wavy meniscus, use is made of two complementary oligonucleotides. This label-free technique is demonstrated to deliver useful information on the adsorption rate of initially solubilised DNA strands to a lipidic matrix. By following the time-dependent frequency spectrum of the surface waves, it is possible to discriminate between single-stranded DNA strands and double-stranded DNA strands. This new label-free sensor can therefore be considered as a liquid analogy of the well-known cantilever technique.