Page 60 - Nanotechnology an introduction

The introduction of binary (i.e., cell–no cell) spacial inhomogeneity in the cover medium C broadens the incoupling peaks (Section 5.5.3). The
greatest inhomogeneity must necessarily occur when exactly half the surface is covered with objects (i.e., cells) whose refractive index differs from
that of the bathing medium. If the locally engendered effective refractive indices are sufficiently different two peaks may be observed. If the
penetration depth is rather small, as the cell–substratum contact area a increases from zero (corresponding to the perfectly spherical initially
attached state of the cell), some measure of peak width w (such as full width at half maximum) should first increase pari passu with effective
refractive index N, up to the point at which the cells cover half the detected area, and then decline; the highest value attained by w depends on the
optical contrast between the two materials (i.e., cells and culture medium) constituting the covering medium.
Microexudate
Very commonly, living eukaryotic cells respond to the solid part of their environment (e.g., their substratum) by excreting extracellular matrix (ECM)
molecules (mostly large glycoproteins), which adsorb onto the substratum, modifying it to render it more congenial. Even the most carefully
prepared nanostructure may thereby rapidly lose its initial characteristics. The prokaryotic equivalent is the biofilm, which constitutes a kind of
microbial fortress defending the population against external attack. These adsorbed secretions, whose refractive index is greater than that of the
culture medium, cause the effective refractive index N to increase additionally to the increase caused by the cell shape changes (spreading).
However, the coupling peak widths w will be negligibly affected because the discrete objects (glycoproteins) comprising the secreted microexudate
are much smaller than the wavelength of light, and can in general be assumed to be adsorbed uniformly over the entire waveguide surface. Hence,
by simultaneously comparing the evolutions of N and w, two processes contributing independently to the increase of N, namely the increase of a
and the accumulation of ECM glycoproteins at the substratum surface, can be separately accounted for.
5.6. Summary
Metrology at the nanoscale imposes stringent requirements of accuracy and precision. A particular challenge is the fact that the size of the
measuring instrument and the feature being measured become comparable. Should the nanoscale feature be absolutely small in the quantum
sense, then we have the potential problem that its state is destroyed by the measurement. Given the identity of quantum objects, however, this does
not usually pose a real practical difficulty—one merely needs to sample (and destroy) one of many. Regardless of the absolute size of the object
being measured, if the measurement instrument has the same relative size as the object being measured, the measurement is subject to distortion.
Previously this could always be resolved by shrinking the relative features of the measurement instrument. If, however, the object being measured is
already at the lower size limit of fabrication (i.e., in the nanoscale) no further shrinkage of the measurement instrument is possible. Nevertheless,
this does not pose an insuperable difficulty. Indeed, it has already been encountered and largely overcome whenever light has been used to
measure features of an object smaller than the wavelength of the light.
The ultimate goal of nanometrology is to provide a list of the coordinates and identities of all the constituent atoms of a nanoscale structure, and all
practical metrology is aimed at that goal. Approaches can be categorized as imaging or nonimaging, each of which category contains methods
that can in turn be categorized as contact or noncontact. As one approaches the nanoscale, however, the distinction becomes harder to make. In
another dimension, one can distinguish between topographical and chemical features; some techniques yield both. In particular, commercial
scanning transmission electron microscopy is now able to yield the atomic coordinates of arbitrary three-dimensional structures.
Techniques able to make time-resolved measurements in situ are very useful for monitoring actual processes.
The representation of structure by a list of the atomic coordinates and identities will usually lead to vast, unmanageable data sets. Therefore, there
is considerable interest in capturing the salient features of topography and chemistry by identifying regularities, possibly statistically.
The nano/bio interface presents perhaps the greatest challenges to nanometrology, not least because the introduction of living components into the
system under scrutiny moves the problem into territory unfamiliar to most metrologists. However, important advances are being made in this
domain, which has already allowed it to assume a far more quantitative nature than hitherto.
Perhaps the greatest current challenge is to devise instrumentation able to combine two or more techniques, enabling the simultaneous
observation of multiple parameters in situ on the same sample.
5.7 Further Reading
Leach, R., Fundamental Principles of Engineering Nanometrology. (2009) Elsevier, Amsterdam.
Kunz, R.E., Miniature integrated optical modules for chemical and biochemical sensing, Sens. Actuators B 38–39 (1997) 13–28.
Ramsden, J.J., High resolution molecular microscopy, In: (Editor: Dejardin, Ph.) Proteins at Solid-Liquid Interfaces (2006) Springer-Verlag,
Heidelberg, pp. 23–49.
Ramsden, J.J., OWLS—a versatile technique for drug discovery, Front, Drug Des. Discovery 2 (2006) 211–223.