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

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

4.7.3 Optical Sensing

In biosystems and nanosystems alike, the detection of an optical photon is a high-energy event. A single photon of red light at l = 700 nm carries an energy of 280 zJ, a blue photon at 400 nm conveys 500 zJ, and an ultraviolet (UV) photon at 200 nm transmits 1000 zJ to its receptor. These energies, ranging from 65-234 kT, exceed thermal noise by a wide margin, hence are all easily detectable reliably by suitable nanosensors. Note that optical photon detection does not require a nanosensor the size of one wavelength or larger -- for example, 1-nm chlorophyll molecules have no difficulty absorbing 660-nm photons.

Biological systems provide numerous examples of single-photon transducers, including photopigment molecules such as chlorophyll and rhodopsin which mediate electron transfers in <~1 picosecond by undergoing conformational changes,¹⁶⁹² triggering a series of biochemical events. An estimated 100-500 cell membrane channels are affected by the detection of a single optical photon. The first documented case of a physiological role for chlorophyll in animals has been found in the fish Malacosteus niger, which uses a chlorophyll derivative to see far-infrared light.¹⁵²⁸ Mammalian cells also act as indirect UV sensors (e.g., see Section 10.4.2.3), as for instance when energy absorption induces perturbations of membrane structure or a conformational change in membrane proteins, activating the JNK cascade below 300 pJ/micron²,⁴⁸¹ and centrosomes may serve as cellular directional IR sensors in single fibroblast cells.¹⁹⁶⁰ Integration times for photon sensors are briefly treated in Section 4.9.4.

Artificial "bioelectronic" cyanine-quinone chromophore molecules ~1 nm in size can detect optical photons of wavelengths l = 580-630 nm, and can trigger and reset in 1-3 picosec (governed by the tunneling times associated with reformation of the zwitterionic states of the inputs^21,480), in theory permitting ~100 GHz modulations to be received. Exciton-generating poly(p-phenylene vinylene) polymer switches have the potential to achieve ~THz switching speed;⁷³³ a birefringent Kerr medium also switches in a few picoseconds. Chalcogenide glass has viscosity >10¹³ kg/m-sec in darkness, but ~5 x 10¹¹ kg/m-sec under illumination.¹³⁰⁶ Proposals for specific implementations of photoergic transducers (Section 6.3.6) and optical cables (Sections 6.4.3.2 and 7.2.5.2) are elsewhere.

Last updated on 17 February 2003