Ecological method for separation of inulin from phytoextracts using molecularly imprinted poly (vinyl alcohol) films.

Patachia, Silvia ; Croitoru, Catalin ; Scarneciu, Ioan 等

1. INTRODUCTION

Phytoextracts (aqueous or hydroalcoholic extracts from plants) are a natural source of a various class of compounds such as carbohydrates, alkaloids, steroids, flavonoids, terpenes and complex acids often used in traditional medicine as drugs, nutritional supplements, dyes and so forth. However, separation of a certain compound from the complex aqueous or hydroalcoholic mixture is a tedious task, often implying time and energy consuming procedures or exposing the biologically active substance to thermal degrading (Hart & Shea 2001).

Molecular imprinting is a novel method for designing materials with molecular memory, which consists of cavities that bear the shape and dimensions of a template molecule. The cavities are highly specific towards the molecule that imprints the polymer, making molecularly imprinted materials suitable for use in isomer separations, catalysis, biosensor assays or in controlled drug delivery (Rechichi et al. 2006; Dickert 2006). Recognition and separation of saccharides have been the focus of much recent attention. Boronic acid is a powerful tool for this purpose since it can form a stable complex (cyclic ester) with saccharides, especially with those comprising a cis-diol group. However; these supports have problems in mechanical and/or chemical stability. Hydrogels are reported to be rather weak under pressure, while glass beads exhibit poor stability under basic conditions (Hayden & Dickert 2002; Wulff & Karsten 2002). Such polymers obtained from bulk or solution polymerization, however, necessitate some tedious treatments, such as grinding and sieving before use (Bolman & Revsbech 2005). In this work, a new method of alternative molecular imprinting to design imprinted poly (vinyl alcohol) [PVA] stable hydrogelic films crosslinked with boric acid has been proposed (Patachia 2003; Patachia 2006). An oligomer of fructose with average molecular weight of 17000- inulin- has been used as template molecule. Inulin is widely used in diabetes treatment, as dietary fiber, blood pressure regulator and glomerular filtration rate measuring.

2. EXPERIMENTAL

2.1 Materials

PVA 120-98 (1200 polymerization degree and 98% hydrolysis degree) was purchased from Chemical Enterprises Rasnov, Romania. Inulin oligomer with average molecular mass of 17000, boric acid, vanillin and concentrated sulphuric acid have been purchased from Sigma-Aldrich, and were of reagent grade. All reagents have been used without further purification.

2.1 Imprinted films obtaining

PVA solution has been prepared by dissolving the polymer powder in Milli-Q deionized water, under magnetic stirring at room temperature, followed by heating at 80 [degrees]C for 4h. The solid content of the obtained solution was 10%.

PVA- IN mixtures have been prepared, by adding the weighted IN powder to 25 mL of PVA solutions with different solid contents.

Mixtures of IN to PVA ratios from 18 % to 38% (reported to 1g of dry PVA [xerogel]) have been obtained. The PVA films have been obtained by polymer solution (with the respective amount of IN added) casting and solvent evaporation in a vacuum drying stove (T = 28[degrees]C; p = 120 mbar) for 24 hours.

The films have been immersed in a 0.8M boric acid solution for 3 hours to achieve crosslinking. Reference probe of PVA crosslinked film without IN has been prepared using the same procedure.

The template IN molecule has been removed from the polymer matrix in order to form the active cavities with the shape and size complementary with that of the template by curing the obtained films in distilled water for 3 days.

2.2 Imprinted films testing

The imprentation of the PVA films has been tested by comparing the amount of IN absorbed in the imprinted polymeric matrix with the amount of IN absorbed in the non-imprinted polymer.

IN absorption has been studied by immersing weighted film samples in a determined amount of 0.1 g/L IN solution.

At certain time intervals, 2 mL of IN solution were drawn and analyzed and the film samples have been immersed in a fresh IN solution of 0.1 g/L.

IN has been determined by the spectrophotometric method using a SPECOL 10 Karl-Zeiss Jena spectrophotometer, due to its ability to form a colored complex (absorption maximum at 520 nm) with vanillin in the presence of concentrated sulphuric acid.

IN desorption from the imprinted films has been studied by immersing the samples subjected earlier to absorption studies in a determined amount of distilled water. Solution samples have been drawn and analyzed as above, and after each determination the films have been reimersed in a fresh amount of distilled water.

3. RESULTS AND DISCUSION

Amount of IN absorbed in the imprinted PVA films (in terms of absorbed IN amount (g) reported to 1g of xerogel is plotted in Fig. 1 (absorption kinetic). Amount of absorbed IN as a function of IN: PVA ratio is presented in fig.2:

[FIGURE 1 OMITTED]

As it can be seen from Fig. 1, the imprinting of PVA with IN template was successful. All imprinted films absorb a higher amount of IN than the reference non-imprinted sample. A maximum of absorption is observed (Fig.2) at 30.46 % IN: PVA ratio. After this maximum, the absorbed IN amount decreases, probably due to the lower polymer content in the films, which could lead to insufficient formation of active cavities. The absorption rates are also higher for the imprinted samples, comparative with the non-imprinted one. The desorption kinetic is presented in Fig.3 in terms of desorbed IN amount/g xerogel:

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

From the desorption kinetic (Fig.3) it could be seen that the desorption rate of IN from the polymeric matrix is higher in the films with optimum IN: PVA ratio, probably due to the presence of more non-deformed active cavities in the polymeric matrix, which leads to an increase in desorption. Also it could be seen that the IN molecule is retained in a higher amount in the imprinted PVA samples. Elimination of all IN amount from the polymeric matrix was not possible.

4. CONCLUSION

Imprinted PVA 120-98 films with inulin as template molecule have been obtained and characterized. Sorption and desorption analysis have been performed. The results indicated that the imprentation of PVA 120-98 with IN was successful.

All the imprinted films absorb a higher amount of IN than the non-imprinted PVA.

The optimum IN: PVA ratio has been found to be 30.46%. The imprinted films are transparent, homogenous and have good resistance to solvent action, due to crosslinking.

Taking into account the non-toxicity, biocompatibility, biodegradability of PVA and the possibility of obtaining molecular imprinted materials based on PVA/IN, new applications in special fields (medicine, pharmacy, separation processes) could be developed.

5. ACKNOWLEDGEMENTS

We would like to be thankful to The National University Council from Romania (CNCSIS) for funding of the research through the national grants CEEX10/2005, CEEX18/2005 and CEEX 148/2006.

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