CONDUCTIVE NANOCOMPOSITES WHICH CAN BE FUNCTIONALIZED

Abstract

A composition includes at least one type of conductive or semiconductive nanostructures, wherein at least one conductive ligand is arranged on the surface of the nanostructures, and at least one solvent, wherein the ligand has at least one group by which functionalization is possible. This makes it possible in simple fashion to obtain functionalizable conductive structures, in particular by inkjet processes.

Claims

1. A composition for producing conductive or semiconductive layers by wet coating, comprising: at least one type of conductive or semiconductive nanostructures, wherein at least one conductive ligand is arranged on a surface of the nanostructures; at least one solvent, wherein the ligand has at least one functional group by which functionalization is possible.

2. The composition as claimed in claim 1, wherein the ligand is a conductive polymer or oligomer based on thiophene.

3. The composition as claimed in claim 1, wherein the at least one conductive ligand comprises a polymer or oligomer having at least 10 bonding sites for achieving a coordinative bond to the surface of the nanostructure.

4. The composition as claimed in claim 1, wherein at least one type of conductive or semiconductive nanostructure comprises a metallic nanostructure.

5. The composition as claimed in claim 1, wherein the at least one functional group is a functional group of a side chain.

6. The composition as claimed in claim 1 the functional group is a carboxyl group.

7. The composition as claimed in claim 1, wherein the at least one solvent comprises solvents or mixtures of solvents each having a boiling point below 120° C.

8. The composition as claimed in claim 2, wherein the conductive polymer is a thiophene having as a functional group an aliphatic chain having 3 to 8 carbon atoms and a carboxyl group, wherein the carboxyl group counts as a carbon atom.

9. A process for producing a functionalized conductive or semiconductive layer on a surface comprising: a) application of a composition as claimed in claim 1 to a surface; b) removal of the at least one solvent; and c) functionalization of the composition.

10. The process as claimed in claim 9, wherein the process comprises no treatment of a coating at temperatures of more than 60° C. after the application to the surface.

11. A functionalized conductive or semiconductive structure obtained by the process as claimed in claim 9.

12. A process for producing a composition as claimed in claim 1, comprising: provision of a dispersion of conductive or semiconductive nanostructures, wherein the dispersion is stabilized by at least one first ligand; addition of at least one conductive ligand; and substitution of the first ligand by the at least one conductive ligand to obtain a modified nanostructure.

13. The process as claimed in claim 12, wherein the modified nanostructure is purified by one or more centrifugations.

14. The process as claimed in claim 13, wherein at least one centrifugation is preceded by addition of a surfactant.

15. The process as claimed in claim 14, wherein the surfactant is added in an amount of 0.02% to 0.1% by weight.

16. A modified nanostructure obtained by the process as claimed in claim 12.

Description

[0112] Further Details and Features are Apparent from the Following description of preferred exemplary embodiments in conjunction with the subsidiary claims. The respective features may be realized alone or in a plurality in conjunction with one another. The options for solving the object are not limited to the exemplary embodiments. Thus for example, indicated ranges always comprise all—unlisted—intermediate values and all conceivable subintervals.

[0113] FIG. 1 Raman spectrum of the C.sub.6-polymer-stabilized AuNPs after the coupling reaction with a PEG amine (methoxypolyethylene glycol amine, Mw=20 000 Da). At about 1615 cm.sup.−1 a new band is formed wick is characteristic of the amide bond being formed (arrow);

[0114] FIG. 2 Detail from a micrograph of a printed circuit board having dimensions of 10 mm×1 mm (a)), detail from a circuit board having a width of about 300 μm. This corresponds approximately to the width of one pixel (b));

[0115] FIG. 3 Fibroblast cells on different conductive inks: 100% inventive ink: The cells grow on the substrate and start to spread (a)), 100% “normal” ink: Cells cannot grow and remain rounded (b));

[0116] FIG. 4 Experiments with “Neurospheres”. The concentration of the neuropheres is significantly higher on the inventive ink than on the substrate.

PRODUCTION OF THE INVENTIVE COMPOSITION

[0117] The polymers were obtained from Rieke Metals, USA. The nanostructures comprise CTAB as the ligand before the ligand substitution.

[0118] It was experimentally determined that the water solubility of the polymers reduces with increasing chain length of the side chains while simultaneously the stabilization capability increases as a result of the increasing steric demands. The tested C.sub.6-polymer was the only polymer which simultaneously has sufficient solubility in water and can also stabilize the AuNPs (diameter 80 nm) (table 1). The polymers have a molecular weight of 55 000-65 000 g/mol.

[0119] For dissolution of the polymer, preferably of the C.sub.6-polymer, in water certain conditions are preferably to be observed to ensure successful coating of the AuNPs:

[0120] Complete dissolution of the polymer without visible aggregate formation is required.

[0121] The dissolution operation of the C.sub.6-polymer requires a duration of (12-24 h). Overnight dissolution is recommended. The temperature should be around room temperature or slightly above room temperature (20-35° C.).

[0122] Elevated temperatures above 50° C. lead to incomplete dissolution. Polymer aggregates can still be observed in solution even after 24 h. The elevated temperature apparently results in increased mobility and diffusion of the individual polymer strands. This favors interaction between the polymers and the formation of aggregates.

[0123] Ligand Substitution

[0124] In addition to the dissolution of the C.sub.6-polymer, further conditions should be met to ensure complete coating of the AuNPs:

[0125] The coating of the AuNPs is carried out in aqueous solution with stirring for 7 days at a temperature >30° C. Excessively low temperatures favor agglomeration of the AuNPs during the coating operation.

[0126] An excessively short time (<7 days) does not allow full substitution of the original ligand with the C.sub.6-polymer. The original ligand CTAB is still detectable by spectroscopic methods. This can lead to instabilities (agglomeration) of the AuNPs during workup of the finished ink.

[0127] By contrast, longer coating durations (>7 days) again favor the interaction between the free polymer strands that are not bound to the particles. The polymer strands in turn assemble into aggregates. This in turn has an adverse effect on the stability of the coated particles. In addition, aggregate formation hampers the removal of the excess (unbound polymer) from the solution by centrifugation.

[0128] Purification

[0129] Centrifugation may be use to remove the excess C.sub.6-polymer that remains in the solution after the coating process and is not bonded to the AuNPs. This is decisive because excess C.sub.6-polymer would reduce the conductivity of the resulting nanoparticle inks. Centrifugation also allows the ink to be concentrated and the solvent to be changed. The centrifugation parameters (time and speed) were optimized to avoid agglomeration of the particles. It has been found that excessive speeds result in agglomeration of the particles. The C.sub.6-polymer-stabilized particles were found to be stable during centrifugation up to a speed of 1000 rpm. Long centrifugation times (4-15 h, depending on rpm <1000 rμm) were required to ensure complete separation of the particles from the excess C.sub.6-polymer.

[0130] However, the stability of the C.sub.6-Polymer-stabilized AuNPs during centrifugation was improved by addition of 0.05% by weight of the surfactant (Tween 20). This made it possible to centrifuge at faster speeds and thus substantially reduce the centrifugation time.

[0131] A speed of 2000 rpm for 3 h were found to be optimal centrifugation parameters. Obtaining a fully purified (no excess free C.sub.6-polymer in solution) and highly concentrated (100 mg/ml) ink requires five centrifugation steps. In the last centrifugation step no further surfactant Tween 20 is added, only pure solvent. The last centrifugation step thus not only removes further C.sub.6-polymer but also washes out the surfactant which would otherwise have adverse effects on the conductivity of the finished ink. A filter operation (PES filter, pore size Ø=0.22 μm) after the first centrifugation step can facilitate the separation of the excess C.sub.6-polymer.

[0132] Measurement of the coating of the nanoparticles with the polymer C.sub.6-polymer (PTEBS) was done using thermogravimetric Analysis (TGA). A proportion of 1.38% by weight (about 13% by volume) was determined. This results in a ligand density of 3.6 mg/m.sup.2. Other higher molecular weight conductive polymers applied to the same gold particles resulted in a ligand density of 1.8-1.9 mg/m.sup.2. The concentration ratio of polymer to gold during the ligand substitution was the same at 0.8:1 (based on mass concentration) for all polymers during the ligand substitution. The calculated thickness of the polymer layer for the inventive particles is on average 1.76 nm which is markedly greater than the thickness of the polymers of 0.9 nm. The pi-pi interaction distance of polythiophenes is 0.37-0.39 nm. This has the result that in the case of PTEBS 4-5 polymer layers surround the particles. The first polymer layer is directly bonded to the gold particle. The remaining layers are attached via pi-pi interactions.

[0133] Before the ligand substitution the nanoparticles have a zeta potential of +31.6 mV in water. After the ligand substitution the zeta potential is −36.1 mV (in water). The C.sub.6-polymer is soluble only in water alone and not in alcohols. Surprisingly, the nanoparticles functionalized with the polymer are stable in alcohols. Alcohols are preferred solvents for inks since the reduced surface tension results in improved wetting and thus facilitates inkjet printing. The best results were obtained with n-propanol. Structures were obtained with isopropanol and ethanol but the print quality was lower. No gaps must be formed by dewetting during printing, since otherwise conductivity is reduced.

[0134] The particles are colloidally stable in alcohols. The zeta potential is −24.4 mV in n-propanol.

[0135] The stores the particles may be kept in water which reduces the risk of drying out.

[0136] Functionalization

[0137] To demonstrate the covalent bonding of bioactive molecules to the AuNPs stabilized with C.sub.6-polymer (table 1) an exemplary amine (methoxypolyethylene glycolamine, Mw=20 000 Da) was coupled to the particles of bio-ink in solution. The covalent bonding is effected by the formation of an amide bond. This newly formed amide bond may be detected by Raman spectroscopy. Amides have a characteristic band in the range of 1500-1700 cm.sup.−1. The Raman spectrum of the ink particles coupled with PEG amines exhibit a new band in this range which was not detectable prior to the coupling reaction (see FIG. 1). The results of Raman spectroscopy thus confirm the covalent bonding of the amine to the C.sub.6-polymer-stabilized AuNPs.

[0138] Printing

[0139] It was possible to produce inks having a high concentration of gold nanoparticles which were processable by inkjet printing. Solvents may be selected from various inorganic and organic substances such as for example water, ethanol, propanol etc. The solvent should be selected according to the substrate, thus allowing sufficient wetting to be achieved. Substrates may be employed include various materials such as paper, glass or various plastic films such as PET, PDMS, TPU etc.

[0140] A glass slide coated with a thin layer of hydrogel was used as the substrate in the experiments. A diamine was then attached to the hydrogel via a chemical reaction, said diamine being bonded to the hydrogel via an amide bond.

[0141] The ink was then applied to the substrate using an ink jet printer. The printing properties of the ink depend on the solvent, wherein the use of n-propanol has proven particularly suitable since it markedly reduces blocking of the nozzle. The use of n-propanol is also suitable having regard to the substrate used. The inkjet printer makes it possible to produce any desired patterns. FIG. 2 shows detail views of various printed lines.

[0142] The ink may subsequently be coupled to the substrate via an amide bond provided that the substrate has —NH.sub.2 groups. This prevents possible detachment of the ink from the substrate during subsequent cell experiments.

[0143] The conductivity in aqueous cell cultivation media was investigated. In a slightly acidic environment (MES buffer, pH=6.1) and in a slightly basic environment (PBS buffer, pH=7.4). The measured layer resistance (typically 100-500Ω/□) changed by less than 3% compared to the dry film over 8 hours.

[0144] The inks used were formulated in alcohols, mostly n-propanol, and had a gold content of 100-150 mg/ml.

[0145] Cell Experiments

[0146] In order to check to what extent the interaction between the inventive ink and cells differs from the “normal” ink and cells, the following experiments were performed:

[0147] The ink was mixed with different amounts of the normal ink (100:0; 80:20; 50:50, 20:80, 0:100). The ink was applied to the substrate by drop-casting. Once the ink had dried, RGD, a peptide that enables adhesion of fibroblast cells and has the sequence Arg-Gly-Asp (SEQ ID No. 1), was applied to the ink. RGD can be covalently bonded to the inventive ink but not to the conventional ink. The specimen was subsequently washed and cell experiments performed. The results showed (FIG. 3) that the fibroblasts can only grow (visible by the spreading of the cells) on inks comprising a high proportion of the inventive ink (at least 80%). The cells did not grow on the other inks. This results shows that the growth of the fibroblasts on the inventive ink is brought about by the peptide RGD that is covalently bonded to the ink and does not just take place on account of an unspecific interaction (for example adsorption processes).

[0148] In a further experiment the peptide IK-19 (CSRARKQAASIKVAVSADR, SEQ-ID No. 2) was covalently bonded to the ink instead of RGD. In contrast to RGD, fibroblasts showed no response to IK-19. Neurons, on the other hand, responded to the IK-19 with improved growth.

[0149] A first experiment with so-called “neurospheres” was also carried out. To this end, 100% bio-inks based on different solvents were tested: a) n-propanol and b) water comprising 0.05% by volume of Tween 20. The peptide IK-19 was coupled to both inks after drop casting. The cell experiments showed a markedly higher concentration of neurons on the ink spot relative to the substrate (see FIG. 4). This suggests that the neurons are present to a greater extent where the IK-19 is located. To summarize, the experiments show that the neurons grow onto the printed ink to a much greater extent in the presence of the peptide IK-19. It was shown for both cell types (fibroblasts and neurons) that covalent bonding of a peptide to the ink is decisive for the differentiation of the cells.

TABLE-US-00001 TABLE 1 water Stabilization Name/Abbreviation Structure solubility of AuNPs Poly[3-(potassium-4- butanoate)thiophene- 2,5-diyl]/C.sub.4- polymer [00001] + − Poly[3-(potassium-5- pentanoate)thiophene- 2,5-diyl]/C.sub.5- polymer [00002] + − Poly[3-(potassium-6- hexanoate)thiophene- 2,5-diyl]/C.sub.6- polymer [00003] + + Poly[3-(potassium-7- heptanoate)thiophene- 2,5-diyl]/C.sub.7- polymer [00004] − + [00005]