Self microfabrication

J. L. Wilbur, A. Kumar, E. Kim, G. M. Whitesides. Microfabrication by microcontact printing of self-assembled monolayers. Adv Mater (5 600-604, 1994. 

As the analytical, synthetic, and physical characterization techniques of the chemical sciences have advanced, the scale of material control moves to smaller sizes. Nanoscience is the examination of objects—particles, liquid droplets, crystals, fibers—with sizes that are larger than molecules but smaller than structures commonly prepared by photolithographic microfabrication. The definition of nanomaterials is neither sharp nor easy, nor need it be. Single molecules can be considered components of nanosystems (and are considered as such in fields such as molecular electronics and molecular motors). So can objects that have dimensions of >100 nm, even though such objects can be fabricated—albeit with substantial technical difficulty—by photolithography. We will define (somewhat arbitrarily) nanoscience as the study of the preparation, characterization, and use of substances having dimensions in the range of 1 to 100 nm. Many types of chemical systems, such as self-assembled monolayers (with only one dimension small) or carbon nanotubes (buckytubes) (with two dimensions small), are considered nanosystems. 

Novel microreactors with immobilized enzymes were fabricated using both silicon and polymer-based microfabrication techniques. The effectiveness of these reactors was examined along with their behavior over time. Urease enzyme was successfully incorporated into microchannels of a polymeric matrix of polydimethylsiloxane and through layer-bylayer self-assembly techniques onto silicon. The fabricated microchannels had cross-sectional dimensions ranging from tens to hundreds of micrometers in width and height. The experimental results for continuous-flow microreactors are reported for the conversion of urea to ammonia by urease enzyme. Urea conversions of >90% were observed. 

FIGURE 7.41 Picture of the microfabricated fluidic device integrated with a standard MALDI-TOF sample plate. Because of the self-activating character of the microfluidic device, the system can be introduced into the MALDI ionization chamber without any wire or tube for the sample introduction and the flow control [820]. Reprinted with permission from the American Chemical Society. 

Weigl, B.H., Kriebel, J., Mayes, K., Yager, P., Wu, C.C., Holl, M., Kenny, M., Zebert, D., Simultaneous self-referencing analyte determination in complex sample solutions using microfabricated flow structure (T-sensors). Micro Total Analysis Systems 98, Proceedings pTAS 98 Workshop, Banff, Canada, 13-16 Oct. 1998, 81-84. 

Fig. 50. A comparison of the relative sizes of structures generated in biology, synthetic chemistry, and microfabrication. The scale (left) is logarithmic and the electromagnetic spectrum (right) is included as a reference. Both biology and microfabrication provide examples of structures with dimensions ranging from 1 to 104 nm. Structures prepared by chemical synthesis are concentrated in the 0.1- to 2-nm range. The application of self-assembly in chemical synthesis may make it possible to obtain structures that have sizes of 10 to 103 nm. (G. Whitesides, et. al. [184] with permission from Science)...

Self-assembly of beads into cavities. All cavities are the same and their generation is a parallel, well-established microfabrication process. No miniaturized placement of individual beads to particular positions is necessary. 

Figure 11A shows an optical micrograph of water drops preferentially condensed on hydrophilic SAMs terminated by carboxylic (COOH) groups no water condensed on the hydrophobic SAMs terminated by methyl (CH3) groups [98]. This process shows how the functionality of a SAM influences the eondensation of water vapor on a SAM-derivatized surface. It uses self-assembly at two scales the formation of SAMs at the molecular scale and the directed condensation of water vapor at the macroscopic scale. The organization of liquids into patterned arrays illustrates one of the uses of self-assembly in microfabrication [99]. 

Two-dimensional, metal-containing grids are of particular interest as their structures suggest the basis for the construction of information storage devices. Arrays on this type, in which the metal ions act as ion dots that might prove accessible by external stimuli, would be of smaller size than quantum dots. They have the potential advantage of self-assembling spontaneously and hence may eliminate the need for microfabrication. ... 

---