Residual imprints

In addition to indentation curves, the residual imprints after the application of a defined load force can be images using intermittent contact mode AFM. This is shown below for indentation tests performed on PE, in which the size of the imprint was shown to be reduced to the level of individual lamellae (Fig. 4.23). 

Fig. 4.25 AFM phase images of the residual imprints in HDPE unveil the complex morphology evolution for nanoindentations performed at 0.7, 1.4, 2.1, 2.8, 3.5, 4.1 pN from top right clockwise respectively after 6 (a), 400 (b), and 1,400 (c) min. (Reprinted with permission from [74]. Copyright 2006. American Chemical Society)...

Malzbender et al. conducted nanoindentation tests and analysis of the fracture cracks surrounding the residual imprint, analyzing in this way the influence of the nanoflller content on the final fracture energy of the coating. Some interesting conclusions were obtained from this study, which explain the different tendencies observed by other researchers. 

Several selective interactions by MIP membrane systems have been reported. For example, an L-phenylalanine imprinted membrane prepared by in-situ crosslinking polymerization showed different fluxes for various amino acids [44]. Yoshikawa et al. [51] have prepared molecular imprinted membranes from a membrane material which bears a tetrapeptide residue (DIDE resin (7)), using the dry phase inversion procedure. It was found that a membrane which contains an oligopeptide residue from an L-amino acid and is imprinted with an L-amino acid derivative, recognizes the L-isomer in preference to the corresponding D-isomer, and vice versa. Exceptional difference in sorption selectivity between theophylline and caffeine was observed for poly(acrylonitrile-co-acrylic acid) blend membranes prepared by the wet phase inversion technique [53]. 

Fig. 6. A Chromatogram of a mixture containing the print molecule (oxacillin), two other p-lactam-antibiotics (penicillin G and penicillin V) and a non- 3-lactam-antibiotic (bacitracin) on an oxacillin imprinted MIP containing 4-vinylpyridine residues, cross-linked with TRIM. The analysis was performed in organic mobile phase (ACN/AcOH,99 l).B Same conditions but using the respective non-imprinted control polymer. C Structures of penicillin V, penicillin G, and oxacillin. Reprinted with permission from Skudar K, Briiggemann O, Wittelsberger A, Ramstrom O (1999) Anal Commun 36 327. Copyright 1999 The Royal Society of Chemistry...

Recent trends in pesticide analysis in food aims for reduced sample pretreatments or simplified methodologies (as QuEChERS approaches), the use of online purification processes, the use of new adsorbents (such as molecular imprinted polymers (MIPs) and nanomaterials) for the extraction and clean-up processes, and focused on the development of large multiresidue methods, most of them based on LC-MS/ MS. In spite of the relevant role of LC-MS/MS, GC-MS-based methods still play an important role in pesticide analysis in food. Despite the development achieved in the immunochemical approaches, the need for multi-residue methods has supported the development and use of instrumental techniques. 

In the molecular imprinting technique, a cross-linked polymer matrix is formed around a target analyte (the template). The precursor mixture contains a functional monomer which can interact with the template molecule by covalent or non-covalent bonding. After the polymerisation process, the functional groups are held in position by the polymer backbone and the template molecule is removed. The residual binding sites are complementary to the target molecules in size and shape. 

The development of the concept of reactive immunization yielded more effective antibody aldolases.119-120 In this new approach, rather than raise antibodies against an unreactive hapten designed to mimic the transition state, the antibodies were raised against a reactive moiety. Specifically, a p-diketone that serves as a chemical trap to imprint a lysine residue in the active site of the Ab (Scheme 5.65) was used.340 A reactive lysine is a requirement of the type I aldolase mechanism. By this method two aldolase catalytic antibodies, 38C2 and 33F12 were identified.119... 

Where the template has been an aromatic amine of low basidty, interaction with carboxylic acid groups has proved too weak to allow selective imprinting. However, Sherrington et al. [112] have shown that the sulphonic add residue in 2-acrylamido-2-methylpropane sulphonic acid is very useful under these circumstances the much higher acidity of this add allows protonation of the weak base template, and hence sufficiently strong electrostatic interaction for imprinting to be achieved. 

Cross-linked beads made by vinylic polymerisations usually have some residual polymerisable functionalities [8], which allows the secondary imprinting polymerisation to covalently attach to the bead structure giving very stable particles. Where glass or silica is used a pretreatment with vinyl [9] or methacryloxy [2] silane derivatives ensures stable covalent attachment, while at the same time blocking surface silanol groups which might otherwise interfere with imprinting. 

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