Bottom-up routes

Well, probably a lot. If they exist. What does exist for sure, though, is the challenge to understand in detail how the human heart works. And, similar to the above scenario, among the many different ways to advance this venture, there are at least two main directions the top-down and the bottom-up route. Accordingly, bio-scientists tend to get pigeonholed into two schools of thought. 

Nonetheless, the bottom-up route has turned into an engineering approach to synthetic biology [150], The strategy is to combine predefined DNA modules, so-called bio-bricks that can be combined to bio-circuits, designed to be implementations of biological functions [151], In that sense, synthetic biology is seen as the successor of molecular cloning, in particular, with respect to safety issues. 

The synthetic methods for both inorganic and organic QCNs can be broadly classified into two categories viz. top-down and bottom-up routes as shown schematically in Figure 3.5. 

Clearly, the connection of predeposited, low-molecular-weight precursors in a controlled manner, directly onto a surface through irreversible covalent bonding, offers the means to overcome the limitations of solution synthesis defined above. In fact, the present bottom-up route for the creation of atomically precise graphene nanoribbons and nanographenes from appropriate polyphenylene... 

CDs can also be prepared through bottom-up routes, including solution chemistry, cyclodehydrogenation of polyphenylene precursors, carbonization of some special organic precursors, or the fragmentation of suitable preeursors, e.g. Cgg. Compared with the top-down routes, reports concerning the bottom-up routes are relatively scarce. Bottom-up methods offer opportunities to control the CDs with well-defined moleeular size, shape, and thus properties. Nevertheless, these methods always involve eomplex synthetic procedures, and the speeial organic precursors may be diffieult to obtain. 

Control over silicon particle shape, porosity, and polydispersity could provide structural control of color of powders in the future. Of relevance here are the so-called silicon colloids made by bottom-up routes. Porous silicon microspheres of 0.5-5 5 pm diameter scattered yellow, orange, and red colors when under white light illumination (Fenollosa et al. 2010). 

Porous silicon has been fabricated by both top-down techniques from solid silicon and bottom-up routes from silicon atoms and silicon-based molecules. Over the last 50 years, electrochemical etching has been the most investigated approach for chip-based apphcations and has been utilized to create highly directional mesoporosity and macroporosity. Chemical conversion of porous or solid silica is now receiving increasing attention for applications that require inexpensive mesoporous silicon in powder form. Very few techniques are currently available for creating wholly microporous silicon with pore size below 2 nm. This review summarizes, from a chronological perspective, how more than 30 fabrication routes have now been developed to create different types of porous silicon. 

Figure 1.4. Illustrations for the top-down and bottom-up approach to materials synthesis, (a) The top-down route is often used to transform naturally occurring products into useful materials. Representations shown above include the conversion of wood into paper products, as well as certain golf ball covers. (b) The bottom-up route of materials synthesis is most prevalent. The representation shown above is the fabrication of plastics and vinyl found in common household products and automotive interiors, through polymerization processes starting from simple monomeric compounds (see Chapter 5).

As mentioned above, nanoparticles can be ptrejjared via a combination of "top-down" and "bottom-up" routes. First, free metal atoms are obtained via a chosen physical method and their subsequent agglomeration is controlled by a stabilizer. Below we give a short overview of some of these physical methods, which found attention in the past or recent hterature. 

Zhong, C., Cooper, A., Kapetanovic, A., Fang, Z., Zhang, M., and Rolandi, M. (2010]. A Facile Bottom-Up Route to Self-Assembled Biogenic Nanofibers, Soft Matter, 6,5298-5301. 

The bottom-up route, on the other hand, arrives at nanostructures by assembhng either atoms or molecules. Whereas physicists prefer using atoms as building blocks, chemists are well acquainted with the synthesis, modihcation, and study of molecules. Given that molecules are stable assemblies of atoms covalently linked to one another, we can expect the chemists molecule-by-molecule bottom-up approach to nanostructures to be more effechve and promising than that of manipulating sticky atoms one at a hme. 

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