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

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

4.2.2 Narrowband Receptor Arrays

If the concentration of the target ligand varies only slightly within known limits, similar results may be obtained using a narrowband sensor that employs a repeating array of a single receptor type tuned to the maximum expected concentration c_max of the target ligand (Fig. 4.2). If the sensor consists of N_r receptors, choosing K_d >~ c_max N_r^-1 / (1 - N_r^-1) ensures that all receptors are occupied at c = c_max, thus avoiding array saturation at concentrations below c_max. At minimum sensitivity, K_d ~ c_min (1 - N_r^-1) / N_r^-1, and so (1 + (Dc / c)) = (c_max / c_min)^(Nr-1) for the entire array. Thus the minimum detectable concentration differential Dc / c for the array is

{Eqn. 4.4}

If A = receptor unit active area (~50 nm²), L = receptor unit length (~10 nm), and receptor arrays are packed in parallel sheets with separation Dx (~10 nm, for small-molecule diffusion in ~10^-6 sec) to allow sample access, sensor volume V_s = AL N_r (1 + (Dx / L)) and sensor scale ~ V_s^1/3. Thus a sensor with N_r = 100 receptor units achieves Dc / c = 0.10 (10%) with a (46 nm)³ sensor; N_r = 1400 receptor units achieves Dc / c = 0.01 (1%) with a (112 nm)³ sensor. A dedicated micron-scale nanodevice employing a (1.44 micron)³ sensor array having 3 million receptor cells could at best distinguish 0.001% concentration differentials in ~ 3 sec (from Eqn. 4.6) for common molecules like serum glucose.

The ion channel switch (ICS) biosensor (aka. the "Australian sensor") can detect small-molecule concentrations as low as ~10^-14 nm^-3 in a ~600 sec measurement time.^3039,3040

Last updated on 16 February 2003