Receptor modified gold and silver nanoparticles: effect on interactions with oxoanions

Abstract

Herein we present a supramolecular non-covalent approach for the modification of gold nanoparticles (GNPs) and silver nanoparticles (SNPs) with porphyrin derivatives. The immobilization of porphyrin derivatives was carried out by two different procedures of ionic interaction. The first one was direct immobilization of the conjugate on nanoparticles and the second one was immobilization of the conjugate on 3-mercaptopropanoic acid (MPA) premodified gold nanoparticles. Such modified nanoparticles were used for interactions with selected oxoanions. The interactions were studied by UV-Vis absorption spectroscopy and electronic circular dichroism. The results showed a dependence of interaction with oxoanions on the immobilization procedures of porphyrin derivatives on the nanoparticle surface.

---

Introduction

Studies of non-covalent intermolecular interactions in solution represent a modern approach to understanding fundamental forces and create a prerequisite for the development of supramolecular devices. It is well known that despite a long-term scientific effort, the selective binding of anions in water solution (natural environment) still represents a challenge because anions have broad shape diversity and overall charge variability.^1,2

As a consequence of these facts, only a few systems capable of recognizing or extracting individual groups of oxoanions such as perrhenate or pertechnetate anions have been synthesized. One group of these receptors contains cationic modified porphyrins. Anion binding is based on electrostatic interaction accompanied with formation of non-covalent hydrophobic and π–π complexes between the porphyrin core and analyte together with additional binding modes like H-bonding.³

In our group, we have devoted considerable effort to design the porphyrin-based selectors of biologically important species.^4,5 These functional conjugates consist of two parts, the tetraphenylporphyrin unit and alkaloids. The tetraphenylporphyrin unit, which offers π–π and hydrophobic interactions, allows them to be studied using absorption and fluorescence spectrometry because porphyrins are strong chromophores and fluorophores, and brucine cation(s) have previously shown the ability to selectively recognize anions.⁶ The brucine cation is inherently chiral, the enantiodiscrimination of binaphthylcarboxylates has been described.^7,8 Porphyrin–brucine conjugates form chiral supramolecular nets and chiral polymers with various solvents. Prepared porphyrin–tetrabrucine quaternary salts also have a remarkable behavior in various solutions.^7,8

In this article we report an interaction study of two types of porphyrin–brucine quaternary salts with perrhenate and other anions in aqueous solutions. Used perrhenate anions are a good model sample for radioisotopes like the technetium complex which is of great interest in the radiopharmaceutical industry.^9–11

Suitable isotopes of these two metals are easily available. The radionuclides are present in their oxoanion forms ReO₄⁻ and TcO₄⁻ respectively.¹² These radioisotopes are produced in medical centers and they are immediately ready to use.¹³ A real ligand composition is chosen that included a range of important requirements as pharmacokinetics, a suitable level of lipophilicity, and the presence of a suitable recognition group that permits attachment to biomolecules for site-specific localization.¹⁴

The direct complexation of ReO₄⁻ and TcO₄⁻ represents an alternative approach for their transportation to a target. For this fact it is necessary to design receptor molecules which can complex these oxoanions selectively.¹⁵

As was mentioned before a lot of these receptor molecules have low solubility in water medium and this disadvantage can be solved by immobilization of these receptors on the surface of nanoparticles. Nanoparticles play an important role in different areas of science, such as nanoelectronics, nonlinear optics, biological labeling, oxidation catalysis and so on.¹⁶ Nanoparticles themselves also provide a pragmatic approach to multiscale engineering, functioning as ‘building blocks’ of regular shape and size for the fabrication of larger structures.¹⁷

In our work we used spherical gold nanoparticles (GNPs) with a size of 15 nm in diameter. These nanoparticles exhibit an intense red color due to surface plasmon (SP) absorption, the result of collective oscillations of GNP surface electrons upon interaction with visible light. This effect can be used as a colorimetric sensor to signify interactions of modified GNPs with analytes.¹⁷ Second type of noble metal nanoparticles (silver nanoparticles – SNPs) used in this work have similar properties.

Experimental

Materials

Water (de-ionized water prepared by Ionex (R = 10 MΩ)), methanol (99.5%, PENTA), sodium sulphate (99.8%, PENTA), sodium perchlorate (98%, Lachema), sodium nitrite (99.8%, Lachema), sodium dihydrogen phosphate (98%, Lachema), potassium perrhenate (99%, Aldrich), potassium tetrachloroaurate(III) (98%, Aldrich), trisodium citrate dihydrate (99%, PENTA), 3-mercaptopropanoic acid (99%, Fluka), and silver nitrate (99%, Sigma) were used. Porphyrin derivatives (1 and 2) used here were prepared similar to the procedure described elsewhere.⁷

Preparation of gold nanoparticles (GNPs)

To 100 mL of boiling water, 1 mL of a 1% aqueous solution of potassium tetrachloroaurate(III) and 2.5 mL of a 1% aqueous solution of trisodium citrate dihydrate were added. Heating was continued for 10 min during which time the solution changed the colour from pale yellow to gray-blue, to purple and then to wine-red. The reaction vessel was allowed to cool to room temperature.

Preparation of silver nanoparticles (SNPs)

To 50 mL of boiling water, 1 mL of a 1.06 mmol L⁻¹ aqueous solution of silver nitrate and 1 mL of a 1% aqueous solution of trisodium citrate dihydrate were added. Heating was continued for 20 min during which time the solution changed the colour to yellow. The reaction vessel was allowed to cool to room temperature and nanoparticles were diluted 10 times.

Nanoparticle derivatization with MPA

A solution of 3-mercaptopropanoic acid (MPA) (6.3 μL) in methanol (0.5 mL) and water (0.5 mL) was added to 100 mL of gold and silver nanoparticles. The flask was capped and left to stand for a day in the dark at room temperature.

GNPs modification with 1 and 2

Stock solutions of 1 and 2 were prepared in DMSO at concentrations of [1] = 4.0 mmol L⁻¹ and [2] = 4.1 mmol L⁻¹. The stock solution was added to 10 mL of citrate stabilized or MPA modified nanoparticles. These mixtures were kept in the dark overnight. Unbounded 1 and 2 were removed by repeated (three times) centrifugation. The final concentration estimated as a difference between added porphyrin amounts and amounts removed in supernatants was 2.6 mol L⁻¹.

SNPs modification with 1 and 2

DMSO solutions of 1 and 2 were prepared at concentrations of [1] = 39.8 mmol L⁻¹ and [2] = 40.6 mmol L⁻¹. These solutions were diluted to a final concentration of 2.8 μmol L⁻¹ in 10 mL of citrate stabilized or MPA modified nanoparticles. These mixtures were kept in the dark overnight.

Titration of nanoparticles with anions

All titrations were performed at fixed concentrations of 1 and 2 by dissolving an appropriate amount of anion in solutions of modified nanoparticles. Titration curves were measured after the addition of anion solutions into solutions of modified nanoparticles in a measuring cell (a quartz cuvette). Individual additions were realized in molar ratios of porphyrin to anion ranging from 1 : 25 to 1 : 500.

UV-Vis spectrometry

UV-Vis spectra were recorded on a Cary-400 UV-Vis spectrophotometer equipped with a 1 cm path length quartz cell; absorbances between 300 and 800 nm were recorded.

ECD spectrometry

ECD spectra were recorded on a J-850 ECD spectrometer (Jasco, Japan) over a spectral range of 300–800 nm. The optical path length of the cuvettes used was 1 cm.

Results and discussion

Prepared gold and silver nanoparticles were characterized by transmission electron microscopy (TEM) and UV-Vis absorption spectroscopy.^18,19 The average diameter of the spherically shaped GNPs electrostatically stabilized with citrate was about 15 nm. The wavelength of the surface plasmon absorbance at 518 nm corresponds well with the average diameter estimated by TEM.²⁰ The average diameter of SNPs electrostatically stabilized with citrate was about 45 nm and the wavelength of the surface plasmon absorbance was 419 nm. Porphyrin derivatives 1 and 2 were used in this study. They are quaternary salts based on the alkylation of different tertiary amines with 5,10,15,20-tetrakis(bromomethylphenyl) porhyrin, where the bromomethyl group is in the meta or para position on the phenyl periphery. Besides perrhenate, other anions (i.e. sulfate, perchlorate, dihydrogenphosphate and nitrite) were involved in interactions with 1 and 2 in water, by monitoring the absorption of the Soret band (418 nm for 1 and 415 nm for 2) of the two porphyrins and of their mixtures with anions. From previously published results,²¹ it is known that both 1 and 2 interact with perrhenate as the absorbances of the Soret band drop down to about 40% of their original value. While 1 interacts only with perrhenate, 2 interacts also with sulfate and slightly with the other anions. The interaction mode and stoichiometry of the complexes were studied by absorption spectroscopy and electronic circular dichroism (ECD) was also applied because of the inherent chirality of 1 and 2, which are able to form chiral supramolecular polymers in solution.^7,8

Stock solutions of gold and silver nanoparticles modified by porphyrin derivatives were prepared in water. A given stock solution was also used for the dissolution of the oxoanions so the concentration of the porphyrin derivatives remained constant during the course of titration. Individual spectra were measured after the addition of the dissolved oxoanion. Unfortunately, the results with porphyrin derivatives immobilized on SNPs via MPA (SNP-MPA-1 and SNP-MPA-2) cannot be compared with the results obtained with modified GNPs (GNP-MPA-1 and GNP-MPA-2) due to aggregation of SNPs and their subsequent sedimentation at the bottom of the cuvette.

Interaction of 1 with perrhenate

In the absorption spectra after the addition of perrhenate to GNP-1 (immobilized compound 1 to GNPs), there is a decrease in the Soret band (418 nm) and also a significant decrease in the 580 nm band (Fig. 1A). This suggests an aggregation of nanoparticles. Immobilization of 1 was therefore carried out via MPA. In the absorption spectra, the addition of perrhenate to GNP-MPA-1 resulted in a decrease in the Soret band (at 418 nm) and simultaneously a slight decrease of the new band at about 580 nm (Fig. 1C). The former wavelength can be tentatively attributed to the monomeric form of H-aggregates and the latter to the monomeric aggregated form of 1.^22–25 In contrast to high selectivity, the broad band at 580 nm indicates higher aggregates.²⁶ Even though the tetraphenylporphyrin core is not chiral, the inherent chirality of 1 can be observed in the Soret band wavelength of the ECD spectra (Fig. 1B and D). As can be seen from the ECD spectra of GNP-MPA-1 (Fig. 1D), the addition of perrhenate causes a decrease of ellipticity at this wavelength. This indicates the formation of aggregates without a supramolecular chirality. If there is a supramolecular chirality, it would be demonstrated by an increase of the ECD band intensity. The ECD spectra of GNP-1 (Fig. 1B) after addition of perrhenate show a slight decrease of the Soret band. All these experiments revealed a complexation between 1 and perrhenate based on “monomers” of 1 surrounded by perrhenate that is based on an anion-exchange process.

Fig. 1 (A) Absorption and (B) ECD spectra of GNP-1 after the addition of 0–500 equiv. of KReO₄ ([1] = 2.5 μmol L⁻¹ for the absorption and ECD spectra). (C) Absorption and (D) ECD spectra of GNP-MPA-1 after the addition of 0–500 equiv. of KReO₄ ([1] = 2.2 μmol L⁻¹ for the absorption and ECD spectra).

Interaction of 2 with perrhenate and sulphate

The screening of interactions of porphyrins with anions showed the lower selectivity of 2 compared to 1. The recorded data, on the other hand, are very interesting. The absorption and ECD spectra measured after individual additions of perrhenate to GNP-2 are shown in Fig. 2A and B and for GNP-MPA-2 spectra are shown in Fig. 2C and D. In the case of GNP-2 (Fig. 2A), there is a decrease in both the Soret band (415 nm) and the 580 nm band. Besides the absorbance changes of GNP-MPA-2 (Fig. 2C) at 415 and 425 nm, which are similar to those upon addition of perrhenate to 1,²¹ another new band at about 580 nm was observed after larger amount of perrhenate has been added. The ECD spectra of GNP-2 (Fig. 2B) after adding a perrhenate show a slight decrease of the Soret band. A rather complicated situation, completely different from that of 1, can be observed in the ECD spectra. The addition of perrhenate to GNP-MPA-2 causes a change of sign of the band at 414 nm and its shift to 417 nm. Simultaneously, a band at 427 nm arises, and the process of spectrum development is completed by the band at 432 nm. The reasons for the different results of the interactions of 1 and 2 with perrhenate can be explained by supramolecular complexation rather than a simple anion exchange mechanism. The latter is only part of the entire process.

Fig. 2 (A) Absorption and (B) ECD spectra of GNP-2 after the addition of 0–500 equiv. of KReO₄ ([2] = 2.3 μmol L⁻¹ for the absorption and ECD spectra). (C) Absorption and (D) ECD spectra of GNP-MPA-2 after the addition of 0–500 equiv. of KReO₄ ([2] = 2.4 μmol L⁻¹ for the absorption and ECD spectra).

The changes in absorption (Fig. 3A and C) and ECD (Fig. 3B and D) spectra can also be observed after the addition of sulfate to systems containing 2 immobilized on nanoparticles. Significant changes of the ECD spectrum of GNP-MPA-2 after the addition of perrhenate and sulfate indicate a conformational change of 2 during the ion-exchange. The existence of a supramolecular polymer based on J-aggregates²³ formed by 2 surrounded by sulfates is indicated by the strong increase in ellipticity of the new band at 425 nm in the ECD spectrum after sulfate addition; the bidentate anion can behave like a paperclip between adjacent porphyrins.

Fig. 3 (A) Absorption and (B) ECD spectra of GNP-2 after the addition of 0–500 equiv. of Na₂SO₄ ([2] = 2.3 μmol L⁻¹ for the absorption and ECD spectra). (C) Absorption and (D) ECD spectra of GNP-MPA-2 after the addition of 0–500 equiv. of Na₂SO₄ ([2] = 2.5 μmol L⁻¹ for the absorption and ECD spectra).

Conclusions

We observed the selective interaction of perrhenate with meta-substituted derivate 1 immobilized on MPA modified gold nanoparticles. Interaction of perrhenate with meta-substituted derivative 1 immobilized on the surface of gold nanoparticles directly leads to aggregation of nanoparticles. In the case of para-substituted derivate 2 no selective interaction which can relate with the lower conformational flexibility of the brucine isomer was observed. Experiments with systems containing silver nanoparticles and derivatives 1 and 2 showed lower stability in water solution that leads to faster nanoparticle sedimentation. SNPs seem to be inappropriate for such kind of time-consuming measurements without further better stabilization. On the other hand, SNPs modified with 1 and 2 have only one strong absorption band in visible spectra that can be useful in further spectroscopic applications.

Acknowledgements

Financial support from the Grant Agency of the Czech Republic (203/09/1311, P303/11/1291), Grant Agency of the Academy of Sciences of the Czech Republic (KAN200100801), Ministry of Industry and Trade of the Czech Republic (FR-TI3/521) and the grant A1 FCHI 2012 003 is gratefully acknowledged.

Notes and references

1. E. A. Katayev, Y. A. Ustynyuk and J. L. Sessler, Coord. Chem. Rev., 2006, 250, 3004.
2. P. D. Beer and P. A. Gale, Angew. Chem., Int. Ed., 2001, 40, 486.
3. K. Záruba, V. Setnička, J. Charvátová, O. Rusin, Z. Tománková, J. Hrdlička, D. Sýkora and V. Král, Collect. Czech. Chem. Commun., 2001, 66, 693.
4. V. Král, J. Králová, R. Kaplánek, T. Bříza and P. Martásek, Physiol. Res., 2006, 55, S3.
5. O. Rusin and V. Král, Tetrahedron Lett., 2001, 42, 4235.
6. K. Záruba and V. Král, Tetrahedron: Asymmetry, 2002, 13, 2567.
7. V. Král, S. Pataridis, V. Setnička, K. Záruba, M. Urbanová and K. Volka, Tetrahedron, 2005, 61, 5499.
8. V. Setnička, M. Urbanová, S. Pataridis, V. Král and K. Volka, Tetrahedron: Asymmetry, 2002, 13, 2661.
9. R. Alberto, R. Schibli, D. Angst, P. A. Schubiger, U. Abram, S. Abram and T. A. Kaden, in Application of Technetium and Rhenium Carbonyl Chemistry to Nuclear Medicine, Springer, Heidelberg, 1997, vol. 22, p. 597.
10. H.-J. Biersack and L. M. Freeman, in Clinical Nuclear Medicine, Springer, Heidelberg, 2007, p. 39.
11. K. Hashimoto and K. Yoshihara, in Technetium and Rhenium Their Chemistry and its Applications, Springer, Heidelberg, 2006, vol. 176, p. 275.
12. W. A. Volkert and T. J. Hoffman, Chem. Rev., 1999, 99, 2269.
13. M. J. Heeg and S. S. Jurisson, Acc. Chem. Res., 1999, 32, 1053.
14. C. Bolzati, A. Boschi, L. Uccelli, F. Tisato, F. Refosco, A. Cagnolini, A. Duatti, S. Prakash, G. Bandoli and A. Vittadini, J. Am. Chem. Soc., 2002, 124, 11468.
15. E. A. Katayev, N. V. Boev, V. N. Khrustalev, Y. A. Ustynyuk, G. Tananaev and J. L. Sessler, J. Org. Chem., 2007, 72, 2886.
16. M.-C. Daniel and D. Astruc, Chem. Rev., 2004, 104, 293.
17. U. Drechsler, B. Erdogan and V. M. Rotello, Chem.–Eur. J., 2004, 10, 5570.
18. P. Řezanka, K. Záruba and V. Král, Tetrahedron Lett., 2008, 49, 6448.
19. P. Žvátora, P. Řezanka, V. Prokopec, J. Siegel, V. Švorčík and V. Král, Nanoscale Res. Lett., 2011, 6, 366.
20. D. I. Gittins and F. J. Caruso, J. Phys. Chem. B, 2001, 105, 6846.
21. L. Veverková, K. Záruba, J. Koukolová and V. Král, New J. Chem., 2010, 34, 117.
22. P. Kubát, K. Lang, P. Anzenbacher, Jr, K. Jursíková, V. Král and B. Ehrenberg, J. Chem. Soc., Perkin Trans. 1, 2000, 933.
23. K. Procházková, Z. Zelinger, K. Lang and P. Kubát, J. Phys. Org. Chem., 2004, 17, 890.
24. V. Král, F. P. Schmidtchen, K. Lang and M. Berger, Org. Lett., 2002, 4, 51.
25. A. Synytsya, A. Synytsya, P. Blafkova, K. Volka and V. Král, Spectrochim. Acta, Part A, 2007, 66, 225.
26. K. Kano, M. Takei and S. Hashimoto, J. Phys. Chem., 1990, 94, 2181.

---