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
4.2.2 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 cmax of the target ligand (Fig. 4.2). If the sensor consists of Nr receptors, choosing Kd >~ cmax Nr-1 / (1 - Nr-1) ensures that all receptors are occupied at c = cmax, thus avoiding array saturation at concentrations below cmax. At minimum sensitivity, Kd ~ cmin (1 - Nr-1) / Nr-1, and so (1 + (Dc / c)) = (cmax / cmin)(Nr-1) for the entire array. Thus the minimum detectable concentration differential Dc / c for the array is
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 Vs = AL Nr (1 + (Dx / L)) and sensor scale ~ Vs1/3. Thus a sensor with Nr = 100 receptor units achieves Dc / c = 0.10 (10%) with a (46 nm)3 sensor; Nr = 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