Future Developments

Molecular imprinting is a technique which has great potential. MIPs have found many applications, and many more are likely to be developed. One of the first applications investigated was CSPs, the subject of this chapter. A number of polymer systems have been developed and these have been used to imprint different classes of compounds.

To be able to attribute the binding of an MIP to an imprinting effect it is of the utmost importance to show that specific recognition sites were formed due to the presence of the print molecules during the polymerization. This is done by comparison with appropriate reference polymers. Polymers prepared without print molecules are not always the best choice, since the physical properties (surface area,

Figure 8 Separation of l- and D-phenylalanine anilide on TLC plates covered with po/y(methacrylic acid-co-EDMA) imprinted with (A) L-phenylalanine anilide; (B) D-phenylalanine anilide and (C) no print molecule. Elution with CH3CN-HOAc (99: 5). (Adapted from Kriz D, Berggren Kriz C, Andersson LI and Mosbach K (1994) Analytical Chemistry 66: 2636-2639, © 1994, with permission from the American Chemical Society, USA.)

Figure 8 Separation of l- and D-phenylalanine anilide on TLC plates covered with po/y(methacrylic acid-co-EDMA) imprinted with (A) L-phenylalanine anilide; (B) D-phenylalanine anilide and (C) no print molecule. Elution with CH3CN-HOAc (99: 5). (Adapted from Kriz D, Berggren Kriz C, Andersson LI and Mosbach K (1994) Analytical Chemistry 66: 2636-2639, © 1994, with permission from the American Chemical Society, USA.)

Time (min)

Figure 9 (A) Capillary electrochromatography (CEC). Separation of racemic propranolol on a po/y(methacrylic acid-co-TRIM) CSP (75 ^mx350mm capillary column) imprinted with (R)-propranolol. The sample was injected electrokinetically (5 kV, 3 s) and was separated at a constant voltage of 30 kV. The electrolyte was CH3CN-acetate buffer (4 mol L~1, pH 3.0) (8 : 2). Detection at 214 nm. The capillary was thermostated to 60°C and an overpressure of 7 bar was applied. (Adapted from Schweitz L, Andersson LI and Nilsson S (1997) Analytical Chemistry 69: 1179-1183, © 1997, with permission from the American Chemical Society, USA.) (B) Capillary electrophoresis (CE). Separation of racemic propranolol using 0.05% (w/v) polyN-acryloylalanine-co-EDMA) particles imprinted with (S)-propranolol as a chiral additive in the background electrolyte (100 ^m x 470 mm capillary column). The sample was injected by a 3 s pressure injection and was separated at a constatnt voltage of 15 kV. The electrolyte was 5 mmol L~1 phosphate buffer, pH 7.0. Detection at 210 nm. Temperature: 25°C. (Adapted from Walshe M, Garcia E, Howarth J, Smyth MR and Kelly MT (1997) Analytical Communications 34: 119-122, © 1997, with permission from the Royal Society of Chemistry, UK.)

Time (min)

Figure 9 (A) Capillary electrochromatography (CEC). Separation of racemic propranolol on a po/y(methacrylic acid-co-TRIM) CSP (75 ^mx350mm capillary column) imprinted with (R)-propranolol. The sample was injected electrokinetically (5 kV, 3 s) and was separated at a constant voltage of 30 kV. The electrolyte was CH3CN-acetate buffer (4 mol L~1, pH 3.0) (8 : 2). Detection at 214 nm. The capillary was thermostated to 60°C and an overpressure of 7 bar was applied. (Adapted from Schweitz L, Andersson LI and Nilsson S (1997) Analytical Chemistry 69: 1179-1183, © 1997, with permission from the American Chemical Society, USA.) (B) Capillary electrophoresis (CE). Separation of racemic propranolol using 0.05% (w/v) polyN-acryloylalanine-co-EDMA) particles imprinted with (S)-propranolol as a chiral additive in the background electrolyte (100 ^m x 470 mm capillary column). The sample was injected by a 3 s pressure injection and was separated at a constatnt voltage of 15 kV. The electrolyte was 5 mmol L~1 phosphate buffer, pH 7.0. Detection at 210 nm. Temperature: 25°C. (Adapted from Walshe M, Garcia E, Howarth J, Smyth MR and Kelly MT (1997) Analytical Communications 34: 119-122, © 1997, with permission from the Royal Society of Chemistry, UK.)

porosity, etc.) of these polymers are often different from those of imprinted polymers. Reference polymers prepared with the optical antipode or a racemic mixture as the print species are preferred. The selectivity will be reversed when using the optical antipode, and a racemic mixture will give a polymer incapable of separating the two enantiomers (unless the monomers are chiral).

The goal of an endeavour involving chromato-graphic separation is to achieve the best possible performance with respect to selectivity, resolution, load capacity and analysis time. Much research effort on MIPs has therefore focused on improving the chromatographic performance. The use of monodisperse spherical beads instead of irregular particles improves the efficiency and investigations in this direction with MIPs have already given promising results. Another issue that needs to be investigated further is the heterogeneity of the binding sites. A more homogeneous population of sites would improve the chromatographic performance. The load capacity of MIPs has been shown to be improved by the use of trifunctional cross-linkers such as TRIM

and PETRA instead of the bifunctional EDMA. These findings look promising for future developments of MIPs for semipreparative and preparative purifications.

Molecular imprinting is an expanding area attracting an increasing number of scientists, in both industry and academia. This is evidenced by the fact that three-quarters of all papers on molecular imprinting were published during the last decade and one-third during 1996-1997. The rapid development of this technique is likely to result in many breakthroughs within the next few years.

See also: II/Affinity Separation: Imprint Polymers.

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