Nanomedicine, Volume I: Basic Capabilities

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

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

 

4.2.3 Counting Rotors

Concentration may be determined by varying exposure time to the sample. One energy-efficient design (Fig. 4.3) uses an input sorting rotor running at varying speeds (according to target concentration) synchronized with a counting rotor (linked by rods and ratchets to a data storage register device) to assay the number of molecules of the desired type that are present in a known volume of fluid. The counting rotor uses a steric probe, moved up and down in synchrony with the arrival of a binding site under the test probe, to count the number of molecules transferred by the sorting rotor. The fluid sample is drawn from the environment into a 4000 nm3 reservoir (Lreservoir ~ 16 nm) with the equivalent of 104 refills/sec on a flow-through basis, using paddlewheel pumps or reciprocating pistons (Section 9.2.7; not shown) operated slowly enough to avoid damaging molecules in the sample volume. This sensor, which measures 45 nm x 45 nm x 10 nm comprising ~500,000 atoms (~10-20 kg), should count 'N ~ 104 - 105 molecules/sec of small common molecules present at typical human serum concentrations of 10-3 - 10-2 nm-3, such as glucose, with the rotor spinning at ~1% of normal speed (Section 3.4.2). Clogging of intake apertures in vivo is addressed in Sections 3.3.1 and 9.2.3, and in Section 15.3.6.6.

For rare molecules with c ~ 10-11 nm-3, the count rate for a single sensing rotor falls to 'N ~ 10-4 molecules/sec. However, a (340 nm)3 reservoir (a ~104 larger chamber volume) serviced by a bank of 104 rotors (4 x 10-18 kg additional mass) improves the count rate for rare molecules to 'N ~ 1 molecule/sec; that is,

{Eqn. 4.5}

Sample preconcentration may allow further improvement by a factor of up to 10-100. Energy requirements are comparable to those for sorting rotors (Section 3.4.2) except for the need to retrieve ~all target molecules from a known sample volume, which slows the sorting process, requiring ~10 zJ/count for common molecules and ~100 zJ/count for the rarest molecules in the human body. Large-molecule concentrations may be measured using a shuttle pump (Section 3.5.9) and a counting device.

 

Last updated on 16 April 2004