© 1999 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Nanomedicine, Volume I: Basic Capabilities, Landes Bioscience, Georgetown, TX, 1999

4.2.8 Chemotactic Sensor Pads

Besides concentration sensing, chemical sensors may be used to determine the chemical characteristics of surfaces. A binding pad coated with an array of reversible, possibly reconfigurable, artificial receptors (Section 3.5.9) may be pressed against test surfaces such as cell membranes, or the perimeters of very large molecules, in order to detect the presence of specific chemical ligands. If pad receptor specificity is high for the target ligand, then the degree of pad "stickiness" to the test surface is a good measure of the population of target ligands on that surface. Probing two-dimensional surfaces with receptors may be constrained by orientation effects, since binding may require a specific relative orientation of ligand and receptor. Compliant pad receptors can reduce this problem but may impair lateral resolution -- there is a three-way tradeoff among angular tolerance, lateral resolution, and multioriented multicopy receptors. A physical contact sensor prevents confusion of surface-bound and free-solvated ligands of the same species. The selective and reversible stickiness of such pads can also be exploited for locomotion (Section 9.4.3.1).

Molecular adhesion experiments in chemical force microscopy^440,1066 and interfacial force microscopy⁴⁴¹ using AFM probes with functionalized tips (Section 2.3.3) typically detect maximum adhesive noncovalent force differentials of 40-160 piconewton (pN)/ligand for competing ligands at a peak-force distance of ~0.2 nm. Forces >10 pN are readily measured by nanosensors (Section 4.4.1), so binding pad sensors should give an exact count of the number of target ligands present per unit area of test surface if (receptor areal number density) > (ligand areal number density) > (1 ligand / pad area), and if contact time is adequate. Receptors for many different ligands can be mounted on moving rotors or belts, or employed as interchangeable tool tips (Section 9.3.2), thus allowing a single sensor pad to reconfigure its specificity or sensitivity under external control.

Last updated on 17 February 2003