Nucleic acid reactions investigated by cantilever-based sensors

Cantilever-based surface stress sensing has been shown sensitive enough to detect interactions of monolayers of biomolecules. This has a direct application for making a new generation of mechanical biosensors for label-free detection of Deoxyribose Nucleic Acids (DNA) and antigens/antibody recognition. The goal of this project has been to detect the melting of DNA as a first step toward the in-situ monitoring of Polymerase Chain Reaction (PCR).

All works published about the detection of biomolecular recognition use the optical leverage read-out technique for detecting the bending of the cantilever. In this project, cantilever based sensors with integrated piezoresistive read-out were used. A sensor sustaining the conditions of salt concentration and temperature required for in situ PCR monitoring was build by packaging a silicon cantilever chip into polymer. The sensor was characterized by recording the output voltage during PCR-like heat cycles and during repeated two-temperatures heat cycles.

In parallel, the immobilization of thiol-modified DNA on gold surfaces for PCR conditions was developed and characterized. First, the thermal stability of DNA immobilized on gold in a micro-array format was characterized during PCR heatcycling by fluorescence scanning and radio-labeling. Thereafter, the method was successfully used to locally immobilize DNA on one side of a cantilever inside a fully packaged cantilever-based sensor.

Moreover, measurements of DNA hybridization in a flow were performed. Those measurements are a pre-requisite to detecting non-isothermal hybridization or melting of DNA. They were realized using two different set-ups, one at Cantion A/S and one at MIC. The results of the measurements are discussed. The measurements using one gold coated cantilever as reference yield a tensile surface stress signal of about 16 mN/m but non-specific signals in the same order of magnitude were also obtained. Further control experiments are being pursued.

Attempts of detecting the melting of DNA by monitoring the output signal of the sensor while successively heating and cooling the sensor in presence of the complementary DNA was performed. The results of those attempt are presented and discussed.