TECHNICAL FIELD

The present invention relates to a method for preparing dextrin nanosponges by means of interfacial cross-linking .

BACKGROUND ART

Dextrin nanosponges are polymers of dextrins, in particular cyclodextrins , obtained by means of cross-linking with appropriate cross-linking agents.

Cyclodextrins (CD) are non-reducing cyclic oligosaccharides constituted by 6-8 glucose molecules linked with a 1,4-a- glucosidic bond, which have a characteristic frustoconical structure. The arrangement of the functional groups of glucose molecules is such that the surface of the molecule is polar, whereas the internal cavity is relatively lipophilic.

The lipophilic cavity bestows on cyclodextrins the capacity for forming stable inclusion complexes even in solution with organic molecules of a suitable polarity and dimension.

For this reason, cyclodextrins have already been studied and have numerous applications in various fields of chemistry in which the characteristics of inclusion compounds are exploited.

The documents Nos . WO03/085002, WO06/002814, and WO09/003656 describe cyclodextrin-based polymers obtained by cross-linking with cross-linking agents and used as drug carriers or for removal of pollutant agents from water. Said polymers are by now commonly known as "nanosponges".

These polymers have moreover shown interesting properties of application also in controlled-release systems in the pharmaceutical field. Currently known are various methods for preparing dextrin nanosponges, which envisage use of anhydrous dextrins, high temperatures, and high-boiling solvents that are difficult to remove. In addition, said methods enable preparation of nanosponges in the form of solid blocks that then require further treatments to enable use thereof, for example, washing, Soxhlet extraction, and grinding.

Consequently, there is felt the need to identify new methods for preparing dextrin nanosponges that are extremely fast and will not require the use of complex laboratory apparatuses.

DISCLOSURE OF THE INVENTION

The aim of the present invention is consequently to provide a new method for preparing dextrin nanosponges that will be free from the disadvantages of the methods according to the known art .

The above aim is achieved by the present invention in so far as it relates to a method for preparing dextrin nanosponges according to Claim 1 and to a nanosponge according to Claim

12.

Definitions

By the term "polyfunctional cross-linking agent" is meant a molecule having at least two reactive functional groups capable of creating a bond with different dextrin molecules.

By the term "water- immiscible organic solvent" is understood an organic solvent having a certain difference of polarity as compared to water. In particular, given that the index of polarity of water is equal to 9, all the organic solvents having a difference of index of polarity with respect to that of water of at least 5.0 are considered water- immiscible . Moreover considered water- immiscible are solvents with a miscibility number higher than 17. By the term "reactive carbonyl group" is meant a functional group characterized by a carbon atom linked with a double bond to an oxygen atom and with two simple bonds to activating groups such as halogens, imidazole, electronegative atoms.

By the term "nanosponge" is meant a highly cross-linked porous polymer obtained by polymerization of dextrins. BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, there now follows a description also with reference to the attached drawings, wherein:

Figure la illustrates the results of the DSC thermal analysis for a nanosponge obtained by cross -linking of beta- cyclodextrin with carbonyl diimidazole in a ratio 1:8 according to the methods known to the art;

Figure lb illustrates the results of the DSC thermal analysis for a nanosponge obtained by cross -linking of beta- cyclodextrin with carbonyl diimidazole in a ratio 1:8 according to the method of the present invention;

Figure 2a illustrates the results of the DSC thermal analysis for a nanosponge obtained by cross-linking of beta- cyclodextrin with hexamethylene diisocyanate in a ratio 1:4 according to the methods known to the art;

Figure 2b illustrates the results of the DSC thermal analysis for a nanosponge obtained by cross -linking of beta- cyclodextrin with hexamethylene diisocyanate in a ratio 1:4 according to the method of the present invention;

- Figure 3a illustrates the results of the DSC thermal analysis for a nanosponge obtained by cross -linking of beta- cyclodextrin with hexamethylene diisocyanate in a ratio 1:8 according to the methods known to the art;

Figure 3b illustrates the results of the DSC thermal analysis for a nanosponge obtained by cross -linking of beta- cyclodextrin with hexamethylene diisocyanate in a ratio 1:8 according to the method of the present invention;

- Figure 4 illustrates the results of the DSC thermal analysis for a nanosponge obtained by cross-linking of beta- cyclodextrin with hexamethylene diisocyanate in a ratio 1:2 according to the method of the present invention and not obtainable according to the methods known to the art;

- Figure 5a illustrates the results of the thermogravimetric analysis (TGA) for a nanosponge obtained by cross- linking of beta-cyclodextrin with carbonyl diimidazole in a ratio 1:8 according to the methods known to the art;

- Figure 5b illustrates the results of the thermogravimetric analysis (TGA) for a nanosponge obtained by cross- linking of beta-cyclodextrin with carbonyl diimidazole in a ratio 1:8 according to the method of the present invention;

- Figure 6a illustrates the results of the thermogravimetric analysis (TGA) for a nanosponge obtained by cross- linking of beta-cyclodextrin with hexamethylene diisocyanate in a ratio 1 : 8 according to the methods known to the art ;

- Figure 6b illustrates the results of the thermogravimetric analysis (TGA) for a nanosponge obtained by cross- linking of beta-cyclodextrin with hexamethylene diisocyanate in a ratio 1:8 according to the method of the present invention;

- Figure 7a illustrates the results of the thermogravimetric analysis (TGA) for a nanosponge obtained by cross- linking of beta-cyclodextrin with hexamethylene diisocyanate in a ratio 1:4 according to the methods known to the art;

- Figure 7b illustrates the results of the thermogravimetric analysis (TGA) for a nanosponge obtained by cross-linking of beta-cyclodextrin with hexamethylene diisocyanate in a ratio

1:4 according to the method of the present invention;

- Figure 8 illustrates the results of the thermogravimetric analysis (TGA) for a nanosponge obtained by cross-linking of beta-cyclodextrin with hexamethylene diisocyanate in a ratio 1:2 according to the method of the present invention; Figure 9a illustrates the IR spectrum for a nanosponge obtained by cross-linking of beta-cyclodextrin with carbonyl diimidazole in a ratio 1:8 according to the methods known to the art ;

- Figure 9b illustrates the IR spectrum for a nanosponge obtained by cross-linking of beta-cyclodextrin with carbonyl diimidazole in a ratio 1:8 according to the method of the present invention;

- Figure 10a illustrates the results of the thermogravimetric analysis (TGA) for a nanosponge obtained by cross-linking of beta-cyclodextrin with hexamethylene diisocyanate in a ratio 1:8 according to the methods known to the art;

Figure 10b illustrates the IR spectrum for a nanosponge obtained by cross-linking of beta-cyclodextrin with hexamethylene diisocyanate in a ratio 1:8 according to the method of the present invention;

- Figure 11a illustrates the IR spectrum for a nanosponge obtained by cross-linking of beta-cyclodextrin with hexamethylene diisocyanate in a ratio 1:4 according to the methods known to the art;

- Figure lib illustrates the IR spectrum for a nanosponge obtained by cross-linking of beta-cyclodextrin with hexamethylene diisocyanate in a ratio 1:4 according to the method of the present invention;

- Figure 12 illustrates the IR spectrum for a nanosponge obtained by cross-linking of beta-cyclodextrin with hexamethylene diisocyanate in a ratio 1:2 according to the method of the present invention; and

Figure 13 illustrates a Raman spectrum of a nanosponge obtained by means of the method of the present invention as compared with that of a nanosponge obtained by means of the methods known to the art . -

DETAILED DESCRIPTION OF THE INVENTION

The method of the present invention is a method of interfacial polymerization in which the nanosponge is produced by precipitation at the interface between an organic phase and an aqueous phase that are immiscible with one another. According to the present invention, the aqueous phase is constituted by an aqueous solution of a dextrin having a pH equal to or higher than 10, in particular comprised between 12 and 13. The aqueous solution is a solution of a strong inorganic base, in particular a base of alkali metals or of alkaline-earth metals. The base preferably used is potassium hydroxide.

To obtain the present method of preparation both linear dextrins and cyclodextrins can be used, in particular, natural cyclodextrins and their derivatives, and more in particular, beta-cyclodextrins .

The organic phase is, instead, constituted by an organic solution obtained by dissolving a cross-linking agent in an organic solvent, in particular chosen in the group constituted by methylene chloride, butanone, hexane, methyl isobutyl ketone, cyclohexane, carbon tetrachloride, methyl t-butyl ether, 1, 2-dichloroethane, ethyl acetate, and chloroform.

The polyfunctional cross-linking agent is a compound comprising a compound chosen in the group constituted by compounds comprising at least one reactive carbonyl group and epichlorohydrin, and in particular selected in the group constituted by carbonyl diimidazole, triphosgene, diphenyl carbonate, pyromellitic anhydride, epichlorohydrin, di- and polyisocyanates , and chlorides of carboxylic acids, more in particular carbonyl diimidazole and hexamethylene diisocyanate . Once prepared, the aqueous dextrin solution and the organic solution of cross-linking agent are set in close contact and possibly stirred so as to increase the surface of contact also through the use of ultrasound.

Up to now interfacial polymerization has found application only in the preparation of polymers of selected monomers. Its limited application is due also to the difficult recovery of the polymer from the reaction environment.

More widely used is, instead, emulsion polymerization, conducted in the presence of surfactants in so far as it leads to better results. Unlike the emulsion-polymerization methods known to the art and used for preparing polymers such as polycarbonates and polyamides, the method of the present invention does not require the use of a surfactant in solution. Advantageously, the method according to the invention, unlike the methods known to the art for preparing dextrin nanosponges, does not require the use of anhydrous dextrins and of extractions for a subsequent purification. This method moreover enables nanoparticles to be obtained without the use of processes of a mechanical type, enables a reduction in the amount of solvents used, a reduction in the energy used, and is very fast.

The thermogravimetric analysis (TGA) and the differential- scanning-calorimetry (DSC) analysis conducted on the nanosponges obtained by applying the method according to the invention have shown how these products present profiles different from those obtained from the corresponding nanosponges obtained with the methods currently known to the art. Said nanosponges consequently prove to be structurally different. This is evident from a comparison of the spectra appearing in Figures lb-3b and 5b-7b with those of Figures la- 3a and 5a-7a. The analysis of the infrared spectra of the nanosponges obtained (see Figures 9-12) has moreover revealed how the nanosponges obtained with the method according to the invention show characteristic peaks not present in the dextrin nanosponges obtained with known methods, which indicates the fact that a new type of products has been obtained, which show excellent chemical and thermal stability and also a higher inclusion capacity.

Consequently, according to a further aspect of the invention, a nanosponge is provided, which can be obtained with the method described above.

In accordance with the data present in the relevant literature, for interfacial polymerizations the cyclodextrin/cross-linking agent ratio (D/CL) appears to be a factor that is less determining, if indeed of no effect, for obtaining nanosponges, affecting, rather, the global process yield. Further characteristics of the present invention will emerge from the ensuing description of some examples provided merely by way of non- limiting illustration.

Examples

Example 1

Preparation of a nanosponge of beta-cyclodextrin cross-linked with N,N carbonyl diimidazole (CDI)

1.135 grams of beta-cyclodextrin were dissolved completely in 20 mL of an 0.1-M aqueous solution of potassium hydroxide under magnetic agitation or by means of a sonicator.

1.297 grams of CDI were dissolved in 20 ml of methylene chloride to obtain an organic CDI solution (CDI/CD molar ratio = 8.0).

The dextrin solution was added to the CDI solution under continuous agitation for 30 minutes. The precipitate was washed with distilled water and centrifuged at 3000 rpm for 15 minutes. The filtrate was filtered in vacuum conditions with distilled water and then with pure ethanol to remove the material that had possibly not reacted. The filtrate was collected and dried in vacuum conditions to obtain the nanosponge .

Examples 2-23

Following the method illustrated, nanosponges were prepared also with alpha-cyclodextrin and gamma-eyelodextrin, according to what appears in Table 1.

Table 1

Ex. Aqueo Organic Dextrin (D) Cross -linking D/CL molar No. s phase agent (CL) ratio

phase

2 0.1 M CH ₂ C1 ₂ β-CD CDI 1:8

KOH

3 0.1 M CH ₂ C1 ₂ β-CD CDI 1:8

KOH

4 0.1 M methyl β-CD CDI 1:8

KOH isobutyl

ketone

5 0.1 M CH ₂ C1 ₂ β-CD CDI 1:8

NaOH

6 0.1 M hexane β-CD PMDA 1:8

KOH

7 0.1 M CH ₂ C1 ₂ β-CD TPh 1:2

KOH

8 0.1 M CH ₂ C1 ₂ β-CD TPh 1:4

KOH

9 0.1 M DMC β-CD DMC 1:8

KOH

10 0.1 M CH ₂ C1 ₂ a-CD CDI 1:8

KOH 11 0.1 M CH ₂ C1 ₂ Y-CD CDI 1:8 KOH

12 0.1 M C¾C1 ₂ β-CD CDI 1:8

LiOH

13 0.1 M CH ₂ C1 ₂ β-CD DPC 1:8

KOH

14 0.1 M CH ₂ C1 ₂ a-CD CDI 1:8

KOH

15 0.1 M CH ₂ C1 ₂ Y-CD CDI 1:8

KOH

16 0.1 M hexane β-CD PMDA 1:8

KOH

17 0.1 M CH ₂ C1 ₂ β-CD TPh 1:2

KOH

18 0.1 M CH ₂ C1 ₂ β-CD TDI 1:8

KOH

19 0.1 M CH ₂ C1 ₂ β-CD HDI 1:8

KOH

20 0.1 CH ₂ C1 ₂ β-CD SSC 1: 8

KOH

21 1 M CH ₂ C1 ₂ β-CD CDI 1:8

KOH

22 0.1 M CH ₂ C1 ₂ β-CD epichlorohydrin 1:8

KOH

23 1 M CH ₂ C1 ₂ β-CD epichlorohydrin 1:8

KOH

PMDA: pyromellitic anhydride, TPh: triphosgene, CDI: carbonyl diimidazole, SSC: sebacoyl chloride, DCM: dichloromethane , DPC: diphenyl carbonate, CD: cyclodextrin, TDI: toluene diisocyanate, DMC: dimethyl carbonate, HDI: hexamethylene diisocyanate.

The reaction of interfacial polymerization for the production of cross- linked cyclodextrin polymers hence appears generally applicable .

Example 24

Solubility of the nanosponges

The solubility in various solvents of some of the nanosponges appearing in Table 1 was measured. The results obtained are given in Table 2.

Table 2

Nanosponge Diethyl DMF Ethanol Petroleu DMSO Water ether m ether

β-NS-CDI - - - - - - θί-NS-CDI - - - - - - γ-NS-CDI - - - - - - β-NS-HDI - - - - - - β-NS-TPh - - - - - - -NS-HDI - - - - - - β-NS-SSC - - - - - - β-NS-CDI: nanosponge obtained by cross-linking of β- cyclodextrin and CDI; a-NS-CDI: nanosponge obtained by cross- linking of a-cyclodextrin and CDI; γ-NS-CDI: nanosponge obtained by cross-linking of γ-cyclodextrin and CDI; β-NS-HDI: nanosponge obtained by cross-linking of β-cyclodextrin and HDI (hexamethylene diisocyanate ) ; β-NS-TPh: nanosponge obtained by cross-linking of β-cyclodextrin and TPh; α-NS-HDI: nanosponge obtained by cross-linking of a-cyclodextrin and HDI; β-NS-SSC: nanosponge obtained by cross-linking of β-cyclodextrin and sebacoyl chloride.

The insolubility observed of the products obtained in solvents with different polarity is in accordance with the formation of a cross -link in the synthesis of nanosponges.

Example 25

Charging of 4-nitrophenol in nanosponges

100 mg of nanosponges were set in contact with 2 ml of a solution of 4-nitrophenol (molecular weight 139.11). The nanosponge samples thus obtained were analysed under the UV spectrophotometer after 30 minutes and after 1 day. The results of the encapsulation appear in Table 3. Table 3

It should be noted that the majority of nitrophenol is encapsulated in a few minutes . Example 26

The nanosponges obtained by means of the method according to the invention with reactive carbonyl groups were subjected to IR analysis after prior formation of tablets with KBr: all the analyses show a peak at 1752 cm ^"1 , which is characteristic of the carbonyl group present in the bond between the molecules of cyclodextrin as illustrated in Figures 9-12. From said figures it is also evident that the peak around 1700 cm ^"1 typical of cross-linked cyclodextrins is absent in the IR spectrum of the native cyclodextrins .

As a whole, the FTIRs show small differences in the signals, whereby it may be stated that the main bonds are the same in the nanosponges obtained with interfacial polymerization and those obtained with the methods known to the art. However, the fine structure appears significantly different, as a demonstration of the different interactions present and as a confirmation of what was already observed with the thermal analyses . In addition, the so-called Boson Peak in the Raman spectra (Figure 13) is located at approximately 28-29 cm ^"1 in the nanosponges obtained with the method according to the invention, a value that is a little higher than the average value of 26 cm ^"1 obtained for nanosponges synthesized according to the methods known to the art. Also this experimental observation shows the peculiar and different molecular structure of the nanosponges according to the invention as compared to those already known.