Can Chemical Analysis of Single Cells Be Improved?

CAN CHEMICAL ANALYSIS OF SINGLE CELLS BE IMPROVED? JOHN R. PLATT, Ph.D.* I. The Needfor Chemical Amplifiers The construction of magnifiers and amplifiers for light is an old and well-developed industry. The same is true ofamplifiers for sound. In each ofthese fields the ultimate sensitivity ofour mechanical devices is pressing close to the fundamental physical limits set by the phenomenon itself. Thus, in spatial amplification, the resolving power ofoptical microscopes is near the theoretical limit, about 2000 A for visible light. The resolving power of electron-optical microscopes is now reaching 10 A, not so far from the electron wave lengths. As for intensity amplification, cooled photomultipliers can detect a few tens ofphotons. In the field of sound, commercial amplifiers commonly reach the background noise level or even the thermal noise level of the pickup. So that our artificial devices for amplifying light and sound, although falling short ofthe human senses by many orders ofmagnitude in their ability to analyze complex patterns, do approach the performance ofthe human eye and ear in their threshold resolution and sensitivity. The same cannot be said of the field of smell. Inventors have usually been physicists and engineers, mechanical or electrical wizards only, and tardy about developing amplification devices to match our chemical senses. This may account for our slight physiological and psychological understanding of these senses. A dog, a fish, an insect, possibly even a micro-organism, follows trails mysterious to us because the subtle chemical fields to which these creatures respond are so well hidden from our human or artificial senses. Ifwe had chemical microscopes that would tell us the point-by-point and instant-by-instant molecular structure and * Department of Physics, University of Chicago. 24 John R. Piatt · Maximum-Resolution Chemistry Perspectives in Biology and Medicine · Autumn 1958 chemical-flow patterns in a cell, with as much detail as is given by optical microscopes, we might be decades ahead in our understanding of biochemistry . If the techniques of using the electron microscope could be modified so that it would show us ideally the detailed chemical structure ofa single molecule as well as it now begins to show us the spatial structure , numerous theoretical speculations about the operation of chromosomes and enzymes would be answered immediately. The purpose ofthis paper is to suggest that our failure to approach such ultimate chemical techniques may be due more to a failure ofinvention than to any fundamental physical limitation. The time may be ripe for concentrated research and development ofmaximum-resolution chemical detection systems. To support this assertion, five specific schemes will be outlined here which seem to offer increases by many orders ofmagnitude in the space and time resolution of chemical analysis in several diffèrent types ofproblem. Such schemes, like any order-of-magnitude improvements , would not be easy to work out; it took years ofeffort to bring the techniques ofchromatography and ofisotopie tracers to their present level of perfection. And, since the author is more familiar with physics than with biochemistry, these particular schemes may founder on innumerable foreseeable or unforeseeable difficulties. In putting such schemes forward, the hope is therefore partly that modifications or countersuggestions might make them more workable. But the hope is especially that they will stimulate others to dedicate thought and effort to working out individual ideas in this area. There can never be too many new techniques devised in a field as important as the maximumresolution chemistry of individual cell components or of individual molecules. A. AMPLIFICATION METHODS To consider the problem ofultimate detection is to consider what amplification methods and devices are available. Amplification methods can be grouped somewhat arbitrarily into three types: physical, chemical, and biological. The physical methods have already been mentioned. They are the optical and electron-optical microscope methods for spatial amplification and photomultiplier systems for intensity amplification. Chemical methods ofamplification are typified by the chemical development ofan AgBr crystal grain in a photographic emulsion. This process 25 is believed to start with the absorption of a single photon by a single "trigger" atom or site in the grain, supposedly a sulfur atom or a dislocation site (although the quantum efficiency is far from unity). This produces an electron...