Single-molecule chemistry, probes

Abstract Chemists may find it difficult to admit that their concepts and opportunities have always been strongly influenced by the available methods for characterization and analysis. Physics, has, of course, the lead when it comes to the visualization of single molecules in real space and to the detection of their specific, not ensemble-averaged properties. The challenge for chemistry is to provide molecules as objects of study which really disclose new concepts of structure and function. This chapter presents a chemical approach toward nanosciences which comprises (i) design and synthesis, (ii) immobilization, often using principles of self-assembly, (iii) visualization, e.g. by scanning probe... 

In principle, there are two methods for the AFM probing of amorphous layers, namely (i) an active probing, which probes the surface chemistry, and (ii) a passive probing, which probes the surface physics. The active AFM probing, which is essentially an extension of the AFM-based single molecule studies, relies on the measurement of LF by an AFM tip that is fimctionahsed with molecular recognition-active molecules (e.g., a potentially complementary DNA). 

Advances in nanochemistry have been fuelled by advances in instrumentation for visualisation and manipulation on the nanoscale, particularly scanning probe microscopic techniques such as STM and AFM, and observing and manipulating the chemistry of single molecules is now a reality. 

Key questions ahead of us concern new concepts for addressing individual molecular switches and the construction of more complex systems which incorporate several switchable functions. Advances in scanning - probe techniques and single molecule spectroscopy as well as supramolecular chemistry will play an important role in this endeavor. 

Here, further exemplary results are given to illustrate the diverse approaches to study supramolecular chemistry with scanning probe techniques. As mentioned previously, one way to study molecules is to build self-organized or self-assembled monolayers [138], but also single molecules or supermolecules as well as supramolecular assemblies were studied in the past. Only some of many examples can be mentioned here. 

Single molecule studies. All the techniques discussed so far provide knowledge about the behaviour of nanocrystals at an ensemble level. Unlike molecules that are identical to each other, several key parameters such as surface chemistry can vary from nanocrystal to nanocrystal. This makes it essential to supplement ensemble information with other types of spectroscopic tools that can convey information regarding speciflc members. Due to the relatively small absorption cross sections of individual members, fluorescence based optical techniques have been frequently used to probe individual members or populations. 

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