Force Functionalisation

Chemical force microscopy (CFM) [26] is a progression from the physicochemical based detection of LFM to specific chemical detection. CFM performs the nanoscale chemical analysis of the sample, through the measurement of forces related to specific chemical interaction between a chemically functionalised tip (e.g., with carbon nanotubes or oligonucleotides) and a surface that is chemically functionalised with complementary (or non-complementary) chemical species, e.g., complementary oligonucleotides. 

Over the last few years, chemical modification of the tips has attracted a lot of attention because of their application for chemical force microscopy. Different methods have been developed to functionalise a SFM-tip with either hydrophobic or hydrophilic molecules. Most of them are based on the monolayer self-as-... 

In chapters 19 (1,3-diCO) and 21 (1,5-diCO) we were able to use an enol(ate) as the carbon nucleophile when we made our disconnection of a bond between the two carbonyl groups. Now we have moved to the even-numbered relationship 1,2-diCO this is not possible. In the simple cases of a 1,2-diketone 1 or an a-hydroxy-ketone 4, there is only one C-C bond between the functionalised carbons so, while we can use an acid derivative 3 or an aldehyde 5 for one half of the molecule, we are forced to use a synthon of unnatural polarity, the acyl anion 2 for the other half. We shall start this chapter with a look at acyl anion equivalents (d1 reagents) and progress to alternative strategies that avoid rather than solve the problem. 

Hwang et al use the second order cumulant expression to examine the potential of mean force for ion transport in a channel—specifically the transport of Na and ions in a cyclic peptide nanotube in water. They show how the de-solvation of the ions on entering the tube produces a free energy barrier and an attractive interaction between the ions and the functionalised tube provide free energy minimum in the tube. 

A version of AFM is chemical force microscopy (CFM), where an AFM tip is coated with a thin chemical layer that interacts selectively with the different functional groups exposed on the sample surface. For example, gold-coated tips functionalised with a thiol... 

Following the work on metal-phosphine coordination polymers, the group started investigating mechanical dissociation of silver(I)-coordination complexes with A-heterocyclic carbene (NHC) functionalised polymers [81]. It has been shown that polymers with an Ag(NHC)2 coordination complex in the pTHF main chain have significantly lower values. pTHF has an M around 40 kg moP whereas the for Ag(NHC-pTHF)2PF6 is lower than 13 kg mol [75]. Thus external force selectively breaks Ag-NHC bonds and yields fi-ee NHC, which was used to catalyse the transesterificatiOTi of benzyl alcohol and vinyl acetate under sonication [82, 83] (Fig. 15). The complex form of the carbene displayed no activity, proving the latency of the catalyst. Control experiments confirmed that the catalyst was activated mechanically. 

The advantage of non-covalent functionalisation is that it does not alter the structure of the nanotubes and therefore both the initial electrical and mechanical properties should also remain unchanged. However, the efficiency of the load transfer might decrease as the forces between the wrapping molecules and the nanotube surface may be relatively weak. 

The driving force for this reaction is the relief of ring strain in cyclic olefins (e.g., norhornene or cyclopentene). If more than one type of strained, unsaturated ring is present in the reaction medium, a copolymer is formed. This phenomenon is the first limitation of ROMP reactions the availability of this mandatory strained cyclic structure. Various hackhones can be created through monomer functionalisation, but such alterations can negatively affect ring strain (and hence the success of the corresponding ROMP). 

Latex with hydroxyl functionalised cores of a methyl methacrylate/butyl acrylate/2-hydroxyethyl methacrylate copolymer, and carboxyl functionalised shells of a methyl methacrylate/butyl acrylate/methacrylic acid copolymer was prepared by free radical polymerisation. The latex was crosslinked using a cycloaliphatic diepoxide added by three alternative modes with the monomers during synthesis dissolved in the solvent and added after latex preparation and emulsified separately, then added. The latex film properties, including viscoelasticity, hardness, tensile properties, and water adsorption were evaluated as functions of crosslinker addition mode. Latex morphology was studied by transmission electron and atomic force microscopy. Optimum results were achieved by introducing half the epoxide by two-step emulsion polymerisation, the balance being added to the latex either in solution or as an emulsion. 8 refs. 

Sterically bulky N-substituents are often incorporated into NHCs to aid in the stabilisation of resulting complexes. The effect of this may also force C-H bonds within close proximity of the metal which can encourage activation. Most often, C-H bond activation of the wingtip substituents of an NHC occurs via C(sp )-H functionalisation of aryl substituted NHC complexes, with an early example documented by Lappert in 1977. Here, the 16-electron complex, [Ru(PPh3)3Cl2], was heated at reflux in the presence of an electron-rich carbene dimer in xylene, to deliver a Ru -NHC complex with a metalated iV-aryl side-arm. Later work suggested that the excess dimer acts as an efficient proton acceptor to promote the C-H activation process. ... 

Measurements can be conducted both in ambient conditions and liquid, although liquid is more commonly used to avoid capillary forces and allow for environmental control by changing buffers [32], enabling the effect of ionic strength to be determined [75-77]. Where naked tips are preferred for imaging, chemical modification may be required to increase the likelihood of interaction [78,79] and polymer pick-up for force-distance work. "Chemical force microscopy may be used to describe the apphcation of a functionalised tip in AFM to map surface properties [80-82]. 

These adhesion forces measurements between functionalised tips and surfaces were then used as routine operation in order to validate the grafting of functional groups on tips. It is worth noting that the standard deviation is more important in N2 (20 to 25%) than in water (10 to 15%) due to capillary forces, which are not completely suppressed in N2. 

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