Effect of Conformational Entropy on the Nanomechanics of Microcantilever-Based Single-Stranded DNA Sensors

Abstract

An entropy-controlled bending mechanism is presented to study the nanomechanics of microcantilever-based single-stranded DNA (ssDNA) sensors. First; the conformational free energy of the ssDNA layer is given with an improved scaling theory of thermal blobs considering the curvature effect; and the mechanical energy of the non-biological layer is described by Zhang's two-variable method for laminated beams. Then; an analytical model for static deflections of ssDNA microcantilevers is formulated by the principle of minimum energy. The comparisons of deflections predicted by the proposed model; Utz-Begley's model and Hagan's model are also examined. Numerical results show that the conformational entropy effect on microcantilever deflections cannot be ignored; especially at the conditions of high packing density or long chain systems; and the variation of deflection predicted by the proposed analytical model not only accords with that observed in the related experiments qualitatively; but also appears quantitatively closer to the experimental values than that by the preexisting models. In order to improve the sensitivity of static-mode biosensors; it should be as small as possible to reduce the substrate stiffness.

Theoretical Analysis for Bending of Single-Stranded DNA Adsorption on Microcantilever Sensors

“…In computation, the geometrical parameters of the microcantilever are taken as: l = 800 , b = 100 , h Si = 0.9 , h Cr = 2.5 nm, h Au = 25 nm [ 24 ]; the material parameters are taken as: E Si = 169 GPa [ 9 ], E Cr = 279 GPa, E Au = 78 GPa; the other parameters: a = 0.22 nm [ 22 ], k B = 1.38 × 10 −23 J K −1 , T = 298 K.…”

“…The dimensionless scaling variable and dimensionless grafting density is introduced as follows: where (=0.6) is the Flory exponent [ 22 , 23 ].…”

“…According to Daoud and Cotton’s blob model for cylindrical polymer brushes; Hagan et al [ 21 ] presented a simplified two-layered model to study the influence of conformational entropy on such motion. Subsequently, Tan and Zhang expanded upon this method by considering the effects of coating thin layers on the mechanical energy of biosensors [ 22 ]. Utz and Begley [ 23 ] established a relationship between molecular properties and adsorption-induced stresses in polymer brushes using the thermal blob.…”