Sacrificial spacer method

Figure 7 Sacrificial spacer method, as exemplified by the imprinting of cholesterol (a) cholesteryl (4-vinyl)phenyl carbonate 8 is used as the template monomer to form a covalently imprinted polymer (b) in the polymerization step, the carbonyl group of the carbonate ester holds the functional monomer and template oxygen atoms apart by two bond distances (c) hydrolysis results in loss of the template and loss of the spacer as CO2 (d) rebinding can now occur with the cholesterol ligand occupying essentially the same space as the template cholesteryl group (adapted from Ref. 10).

Figure 8 (a) Binding of cholesterol to cholesterol-imprinted and control polymers, from a 2 mM solution of cholesterol in hexane, as a function of polymer concentration, (b) Binding of cholesterol and various cholesterol analogues (2 mM) to the cholesterol-imprinted polymer, prepared by the sacrificial spacer method. Reprinted with permission from Journal of the American Chemical Society. Copyright 1995 American Chemical Society (Ref. 10). 

Liibke et al. [23] used this method to prepare the template monomer 29 (Fig. 18), which was copolymerized with an additional monomer capable of forming a charge transfer complex with the electron-deficient aromatic dioxin analogue. This study showed that it was possible to use a combination of noncovalent and sacrificial spacer methodologies to prepare an imprinted material. 

An approach taken to overcome these deficiencies is the use of the so-called sacrificial spacers [29]. In this method the intermediate species used during the pre-polymerization of the MIP to link the monomers and the template is chosen so that it is eliminated—or in other words sacrificed—during the removal of the template. This way not only is the template attached to the monomer, the chances of steric influences of the residual moe-ities during the future rebindings with the target species are also avoided. Instances of the common sacrificial intermediate species are spacers with carbonyl groups [59-70] or the less frequent instances of salicylate (2-hydroxybenzoate) [60,71], dimethyl silyl group of silyl ethers [72] and silyl esters [73]. 

Since advanced lithography tools with resolution down to sub 100 nm may not be readily available, some novel techniques to fabricate ID nanochannels have been developed and reported. One of these techniques uses standard microfabrication process and has the potential to make integrated nanofluidic devices [2]. This technique is similar to the spacer technique developed in solid-state electronic device fabrication. A spacer is the thin side wall achieved by conformal deposition of selected thin film on the side wall of a sacrificial structure. With the fine control of the LPCVD deposition process, a spacer as thin as 10 nm can be made. After removing the sacrificial structure, a spacer as thin as 10 nm is left on the substrate, which can be subsequently used as a mask to pattern nanometer lines or used as a sacrificial line to make a nanochannel. Other novel techniques involve shrinking a larger channel made by standard micromachining to smaller sizes by methods such as filling the channel with other materials [3]. 

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