  Self-assembled monolayers (SAMs) of organic molecules are widely employed in
surface chemistry and biology, and serve as ultra-fine lithographic resists.
Due to their small thickness of only a few nanometers, the analysis of
patterned monolayer surfaces using conventional methods requires thorough
point-by-point scanning using complicated equipment. In the work reported
herein, patterned monolayers are simply and directly observed using a
bright-field optical microscope. The monolayers modify the spectral
reflectivity pattern of a silica-on-silicon thin film, and introduce a contrast
between bare and monolayer-coated regions of the substrate. The method can also
distinguish between regions of single-layer and bi-layer coatings. The
observations are supported by calculations, and by control experiments using
atomic force microscopy, scanning Raman spectrometry and scanning reflection
spectrometry. The results are useful for electro-optic devices, selective
wafer-bonding protocols and lab-on-a-chip test systems. We show here that
chemical reactions leading to the formation of a bi-layer of SAMs correspond to
an optical contrast visible to the naked eye, enabling such detection to
provide a simple, yet effective differentiation between monolayers and adsorbed
analytes with possible applications for chemical and/or biological sensing.
