Optimal sensitivity for molecular recognition MAC-mode AFM

Molecular recognition force microscopy (MRFM) using the magnetic AC mode (M AC mode) atomic force microscope (AFM) was recently investigated to locate and probe recognition sites. A flexible crosslinker carrying a ligand is bo und to the tip for the molecular recognition of receptors on the surface of a sample. In this report, the driving frequency is calculated which optimi zes the sensitivity (S). The sensitivity of MRFM is defined as the relative change of the magnetically excited cantilever deflection amplitude arising from a crosslinker/antibody/antigen connection that is characterized by a very small force constant. The sensitivity is calculated in a damped oscill ator model with a certain value of quality factor Q, which, together with l oad, defines the frequency response (unloaded oscillator shows resonance at Q > 0.707). If Q < 1, the greatest value of S corresponds to zero driving frequency Omega (measured in units of eigenfrequency). Therefore, for Q < ! , MAC-mode has no advantage in comparison with DC-mode. Two additional extr emes are found at Omega(L) = (1 - 1/Q)(1/2) and Omega(R) = (1 + 1/Q)(1/2), with corresponding sensitivities S-L = Q(2)/(2Q - 1), S-R = Q(2)(2Q + 1) Th e L-extreme exists only for Q > 1, and then S-L > S-R, i.e. the L-extreme i s the main one. For Q > 1, S-L > 1, and for Q > 2.41, S-R > 1. These are th e critical Q-values, above which selecting driving frequency equal to Omega (L) or Omega(R) brings advantage to MAC mode vs. DC mode. Satisfactory qual ity of the oscillator model is demonstrated by comparison of some results w ith those calculated within the classical description of cantilevers. (C) 2 000 Elsevier Science B.V. All rights reserved.