Interaction of nitric oxide with gold nanoparticles capped with a ruthenium(II) complex

Abstract

Gold nanoparticles capped with a cis-(4-aminothiophenol)bis(bipyridyl)(chloro)ruthenium(II) complex that are able to coordinate nitric oxide, become fluorescent and then liberate it by photolabilization when irradiated at 430 nm is reported.

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Nitric oxide (NO) is a radical with important biological roles.^1,2 Any excess or defect of this essential bioregulator can provoke many disease states, including septic shock, inflammation, diabetes, etc.² Therefore, numerous studies have been oriented to find mechanisms that could permit the regulation of appropriate amounts of NO inside our bodies.

When the concentration of NO is high, scavengers are required. Several ruthenium complexes have been studied as NO scavengers with positive results.³ On the contrary, low concentrations of NO can be avoided by the use of in vivo generators of NO. In the last century, Na₂[Fe(CN)₅NO] and nitroglycerine were used with that purpose.^4,5 Compounds able to liberate NO by photoexcitation have received special attention, constituting an alternative for NO in vivo administration.^4,6,7

At present, targeting drug delivery is a very promising approach in pharmacology, which should include bioregulation of NO concentration. Moreover, smart nanodevices able to release and/or scavenge NO at specific cellular targeted sites could constitute in the future the best option for NO bioregulation.

Recently, NO-photorelease by different types of nanoparticles capped with molecules able to generate NO by photolabilization has been reported.^8-11 These papers could constitute the basis for the future preparation of smart nanodevices as NO bioregulators.

Here, we report gold nanoparticles capped with cis-(4-aminophenolthiol)bis(bipyridyl)(chloro)ruthenium(II), cis-[RuCl(ATPh)(bpy)₂]Cl, that are able to coordinate NO, become fluorescent and liberate it by photolabilization. The combination of both properties in one nanoparticle could be considered as a preliminary approach to a smart NO bioregulator nanodevice.

Dark-violet cis-[RuCl(ATPh)(bpy)₂]Cl was obtained from the interaction (1 : 1 molar ratio in methanol) of ATPh with cis-[RuCl₂(bpy)₂] (obtained by ref. 12) to be used to cap gold nanoparticles. The double set of narrow signals corresponding to the bpy ligand in the ¹³C NMR spectrum (δ 158.18, 156.52, 151.91, 151.03, 134.54, 133.29, 126.28, 125.22, 122.92, 122.07 ppm) indicates that the complex presents cis configuration. In its FTIR spectrum, a band at 2550 cm⁻¹ was observed and assigned as νSH. Therefore, the coordination of ATPh to ruthenium(II) was through the amino group, confirmed by the observed shifts in the ¹³C NMR spectrum. The major shift was observed for C4-ATPh, from δ 145.61 (free) to δ 153.14 ppm (coordinated). This complex was EPR silent as corresponds to a ruthenium(II) complex.

When cis-[RuCl(ATPh)(bpy)₂]Cl was dissolved in methanol in the presence of LiCl (as source of the required additional chloride counterion) and the resulting solution was deoxygenated and bubbled with NO dark violet-brownish cis-[Ru(ATPh)(bpy)₂NO]Cl₃ was obtained. This ruthenium(II) nitrosyl complex was also EPR silent with well defined narrow ¹³C NMR peaks, including the double set of signal of bpy (in positions very similar to those of cis-[RuCl(ATPh)(bpy)₂]Cl). The presence of an intense and sharp peak at 1922 cm⁻¹ in its FTIR spectrum indicates the linear coordination of NO⁺.¹³ Such coordination is only possible as a consequence of the interaction of the complex with NO⁺ present in solution, in equilibrium with NO (radical).^7,14 Therefore, the obtained nitrosyl complex contained a Ru(II)–NO⁺ bond. We observed that cis-[RuCl(ATPh)(bpy)₂]Cl was able to interact with NO⁺ contained in different solvents (saturated with NO) even when they do not dissolve the complex, e.g. CCl₄.

Gold nanoparticles (AuNP) were obtained by the reduction of H[AuCl₄] with NaBH₄ in the presence of cis-[RuCl(ATPh)(bpy)₂]Cl and cis-[Ru(ATPh)(bpy)₂NO]Cl₃. Both complexes self-assembled on the gold surface through the free thiol group of ATPh, upon its deprotonation. As a result each complex lost a chloride counterion.

The maximum of the surface plasmon resonance (SPR) of the gold nanoparticles capped with cis-[RuCl(ATPh-H)(bpy)₂], AuNP-Ru, was observed at 547 nm while that of gold nanoparticles capped with cis-[Ru(ATPh-H)(bpy)₂NO]Cl₂, AuNP-RuNO (Scheme 1), was observed at 546 nm. TEM micrographs permitted to determine the average size of the capped gold nanoparticles, AuNP and AuNP-NO.¹⁵ Both systems presented practically the same size and dispersion, with an average diameter of 24.9 ± 2.2 nm. An image of a single AuNP-RuNO nanoparticle is represented in Fig. 1.

Fig. 1 TEM image of a single AuNP-RuNO nanoparticle. Scale bar corresponds to 10 nm. Inset: general view of a group of AuNP-RuNO.

Scheme 1 Representation of AuNP-Ru.

The bpy → Ru(II) MLCT bands of the cis-[RuCl(ATPh)(bpy)₂]Cl complex (361–535 nm) could not be observed when they capped AuNP. On the other hand, the Ru(II) → NO⁺ MLCT band was observed at 388 and 391 nm for cis-[Ru(ATPh)(bpy)₂NO]Cl₃ and AuNP-RuNO, respectively. This latter result, plus the presence in the FTIR spectrum of the ν(NO) peak at 1920 cm⁻¹, confirmed the presence of coordinated NO⁺ in AuNP-RuNO.

Both types of gold nanoparticles, AuNP-Ru and AuNP-RuNO, were soluble in DMSO and methanol and slightly soluble in water. They were also EPR silent as their capping ruthenium(II) complexes. AuNP-RuNO could also be obtained by the interaction of AuNP-Ru with NO. Therefore, cis-[RuCl(ATPh)(bpy)₂]Cl was able to coordinate NO⁺, capping or not AuNP.

The four obtained species dissolved in DMSO, cis-[RuCl(ATPh)(bpy)₂]Cl, cis-[Ru(ATPh)(bpy)₂NO]Cl₃, AuNP-Ru and AuNP-RuNO, were studied by spectroflourimetry. As can be observed in Fig. 2, the species not containing coordinated NO⁺ (cis-[Ru(bpy)₂(ATPh)Cl]Cl and AuNP-Ru) were quenched while cis-[Ru(bpy)₂(ATPh)NO]Cl₃ and AuNP-RuNO fluoresced when irradiated at 430 nm. Consequently, these latter two species, cis-[Ru(bpy)₂(ATPh)NO]Cl₃ and AuNP-RuNO behaved as fluorescent probes for the detection of NO.

Fig. 2 Emission fluorescent spectra (in DMSO, irradiation at 430 nm) of: (a) cis-[Ru(bpy)₂(ATPh)Cl]Cl, (b) AuNP-Ru, (c) AuNP-RuNO, (d) cis-[Ru(bpy)₂(ATPh)NO]Cl₃.

What resulted, most interestingly, was that AuNP-RuNO liberated NO by a photo-stimulated process when irradiated with visible light (430 nm). The NO-photorelease from AuNP-RuNO was complete after 30 min (Fig. 3). Therefore, AuNP-Ru resulted to be a nanodevice able to coordinate NO⁺ to become fluorescent, and then release it upon photolabilization.

Fig. 3 Emission fluorescent spectra (in DMSO, irradiation at 430 nm) of AuNP-RuNO recorded at different intervals of time (0, 15 and 30 min).

In conclusion, gold nanoparticles capped with cis-[RuCl(ATPh)(bpy)₂]Cl (AuNP-Ru) and cis-[Ru(ATPh)(bpy)₂NO]Cl₃ (AuNP-RuNO) were obtained through the self-assembly of the deprotonated thiol groups of ATPh. Only cis-[Ru(ATPh)(bpy)₂NO]Cl₃ and AuNP-RuNO resulted in fluorescent by the absorbance of visible radiation (430 nm). The fact that gold nanoparticles capped with cis-[Ru(ATPh)(bpy)₂NO]Cl₃ (AuNP-RuNO) were, not only fluorescent, but also able to liberate NO by photolabilization constitutes an important step forward in the development of nanodevices that are able to regulate the concentration of NO within the human body.

Notes and references

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