Roberto Martín

Enhancement of the Catalytic Activity of Supported Gold Nanoparticles for the Fenton Reaction by Light

Laser flash photolysis of supported gold nanoparticles exciting at the surface plasmon band (532 nm) has allowed in the case of Au/CeO(2) and Au/OH-npD (OH-npD: Fenton-treated diamond nanoparticles) detection of transients decaying in the microsecond time scale that have been attributed as indicating photoinduced electron ejection from gold based on N(2)O quenching and the observation of the generation of methyl viologen radical cations. This photochemical behavior has led us to hypothesize that there could be assistance to the catalytic activity of these materials by irradiation in those cases wherein the mechanism involves electron transfer to or from a substrate to the gold. This hypothesis has been confirmed by observing that the catalytic activity of Au/OH-npD for the Fenton degradation of phenol with hydrogen peroxide can be increased over 1 order of magnitude by irradiation at 532 nm. Moreover, there is a linear relationship between the initial reaction rate and the incident photon flux. This photoenhancement allows promoting Fenton activity at pH 8 in which the catalytic activity of Au/OH-npD is negligible. The same photo enhancement activity for the Fenton degradation of phenol was observed for other supported gold catalysts including those that do not exhibit microsecond transients in the nanosecond laser flash photolysis (Au/TiO(2) and Au/SiO(2)) due to their lifetime shorter than microseconds. It is proposed that the photo enhancement should be a general phenomenon in gold catalysis for those reaction mechanisms involving positive and/or negative gold species.

Gold on Diamond Nanoparticles as a Highly Efficient Fenton Catalyst

The Fenton reaction consists of the generation of highly aggressive hydroxyl radicals from hydrogen peroxide and is widely used to degrade organic pollutants. Due to its general applicability, the Fenton reaction is employed in water and soil disinfection/remediation and for removal of non-biodegradable chemicals. The main limitation of the Fenton reaction is the consumption of stoichiometric amounts of transition metals, mostly iron. There is considerable incentive in developing a catalytic Fenton process using exclusively hydrogen peroxide and a catalyst. Herein we report that gold nanoparticles grafted on nanoparticulate diamond catalyze the formation of hydroxyl radicals from hydrogen peroxide with at least 79 % efficiency and reach a turnover number of 321 000, many orders of magnitude higher than any currently available catalysts. This extraordinary activity is derived directly from the nanometric diameters of gold and diamond (“nanojewels”) and from the remarkable inertness of the diamond surface. The Fenton reaction, in which highly aggressive hydroxyl radicals (HOC) are generated from H2O2 by reduction with Fe II ,C u II , or other transition metal salts, is a general process that can be used for the degradation/mineralization of recalcitrant organic pollutants as well as for disinfection. [1–4] In spite of the wide applicability of the Fenton reaction for decomposing almost any organic compound, its widespread use for pollution abatement and disinfection is limited by the need for stoichiometric amounts of Fe II or other transition metals. Most of the efforts to transform the Fenton reaction from a stoichiometric to a catalytic process have met with failure or at best can produce HOC with remarkably low efficiency. [5] For instance, the photo-Fenton process requires transparency of the solution (a prerequisite not frequently fulfilled in polluted waters or soils) and consumes “expensive” photons as stoichiometric reagents. A large number of iron-containing solids such as iron-exchanged zeolites and montmorillonites have also been reported as heterogeneous

Fenton-Treated Functionalized Diamond Nanoparticles as Gene Delivery System

When raw diamond nanoparticles (Dnp, 7 nm average particle size) obtained from detonation are submitted to harsh Fenton-treatment, the resulting material becomes free of amorphous soot matter and the process maintains the crystallinity, reduces the particle size (4 nm average particle size), increases the surface OH population, and increases water solubility. All these changes are beneficial for subsequent Dnp covalent functionalization and for the ability of Dnp to cross cell membranes. Fenton-treated Dnps have been functionalized with thionine and the resulting sample has been observed in HeLa cell nuclei. A triethylammonium-functionalized Dnp pairs electrostatically with a plasmid having the green fluorescent protein gene and acts as gene delivery system permitting the plasmid to cross HeLa cell membrane, something that does not occur for the plasmid alone without assistance of polycationic Dnp.

Biodistribution of amino-functionalized diamond nanoparticles. In vivo studies based on 18F radionuclide emission.

Nanoparticles have been proposed for several biomedical applications; however, in vivo biodistribution studies to confirm their potential are scarce. Nanodiamonds are carbon nanoparticles that have been recently proposed as a promising biomaterial. In this study, we labeled nanodiamonds with (18)F to study their in vivo biodistribution by positron emission tomography. Moreover, the impact on the biodistribution of their kinetic particle size and of the surfactant agents has been evaluated. Radiolabeled diamond nanoparticles accumulated mainly in the lung, spleen, and liver and were excreted into the urinary tract. The addition of surfactant agents did not lead to significant changes in this pattern, with the exception of a slight reduction in the urinary excretion rate. On the other hand, after filtration of the radiolabeled diamond nanoparticles to remove those with a larger kinetic size, the uptake in the lung and spleen was completely inhibited and significantly reduced in the liver.

Nano-jewels in biology. Gold and platinum on diamond nanoparticles as antioxidant systems against cellular oxidative stress.

Diamond nanoparticles (DNPs) obtained by explosive detonation have become commercially available. These commercial DNPs can be treated under Fenton conditions (FeSO(4) and H(2)O(2) at acidic pH) to obtain purer DNP samples with a small average particle size (4 nm) and a large population of surface OH groups (HO-DNPs). These Fenton-treated HO-DNPs have been used as a support of gold and platinum nanoparticles (≤2 nm average size). The resulting materials (Au/HO-DNP and Pt/HO-DNP) exhibit a high antioxidant activity against reactive oxygen species induced in a hepatoma cell line. In addition to presenting good biocompatibility, Au/HO- and Pt/HO-DNP exhibit about a two-fold higher antioxidant activity than glutathione, one of the reference antioxidant systems. The most active material against cellular oxidative stress was Au/HO-DNP.

Sunlight-Assisted Fenton Reaction Catalyzed by Gold Supported on Diamond Nanoparticles as Pretreatment for Biological Degradation of Aqueous Phenol Solutions

Gold nanoparticles supported on Fenton-treated diamond nanoparticles (Au/DNPs) have been reported as one of the most efficient solid catalysts effecting the Fenton reaction, achieving a turnover number (TON) as high as 321,000. However, at room temperature the main limitation for the catalytic activity of Au/DNPs is the pH of the solution, which should be less than 5. In this paper, we report that exposure of Au/DNPs to sunlight enhances the catalytic activity of Au/DNPs up to the point that it can promote the Fenton reaction at room temperature even at slightly basic pH values. Also, in addition to performing a deep Fenton treatment and considering that the excess of H(2)O(2) used in the process should be minimized, we have achieved in our study, using a mild Fenton reaction promoted by Au/DNPs under sunlight irradiation, an optimum in the biodegradability, a minimum in the ecotoxicity, and no toxicity for the Vibrio fischeri test. The results have shown that, by using an H(2)O(2) -to-phenol molar ratio of 5.5 or higher, it is possible to achieve a high biodegradability as well as a complete lack of ecotoxicity and of Vibrio fischeri toxicity. The stability of Au/DNPs was confirmed by analyzing the gold leached to the solution and by performing four consecutive reuses of the catalyst with initial pH values ranging from 4 to 8. It was observed that, after finishing the reaction and exhaustive washings with basic aqueous solutions, the initial reaction rate of the used catalyst is recovered to the value exhibited by the fresh solid. Overall, our study shows that the synergism between catalysis and photocatalysis can overcome the limitations found for dark catalytic reactions and that the reaction parameters can be optimized to effect mild Fenton reactions aimed at increasing biodegradability in biorecalcitrant waste waters.

Influence of the Preparation Procedure on the Catalytic Activity of Gold Supported on Diamond Nanoparticles for Phenol Peroxidation

The catalytic activity of diamond-supported gold nanoparticle (Au/D) samples prepared by the deposition/precipitation method have been correlated as a function of the pH and the reduction treatment. It was found that the most active material is the one prepared at pH 5 followed by subsequent thermal treatment at 300 °C under hydrogen. TEM images show that Au/D prepared under optimal conditions contain very small gold nanoparticles with sizes below 2 nm that are proposed to be responsible for the catalytic activity. Tests of productivity using large phenol (50 g L(-1)) and H(2)O(2) excesses (100 g L(-1)) and reuse gives a minimum TON of 458,759 moles of phenol degraded per gold atom. Analysis of the organic compounds extracted from the deactivated solid catalyst indicates that the poisons are mostly hydroxylated dicarboxylic acids arising from the degradative oxidation of the phenyl ring. By determining the efficiency for phenol degradation and the amount of O(2) evolved two different reactions of H(2)O(2) decomposition (the Fenton reaction at acidic pH values and spurious O(2) evolution at basic pH values) are proposed for Au/D catalysis. The activation energy of the two processes is very similar (ranging between 30 and 35 kJ mol(-1)). By using dimethylsulfoxide as a radical scavenger and N-tert-butyl-α-phenylnitrone as a spin trap under aerated conditions, the EPR spectrum of the expected PBN-OCH(3) adduct was detected, supporting the generation of HO(.), characteristic of Fenton chemistry in the process. Phenol degradation, on the other hand, exhibits the same activation energy as H(2)O(2) decomposition at pH 4 (due to the barrierless attack of HO(.) to phenol), but increases the activation energy gradually up to about 90 kJ mol(-1) at pH 7 and then undergoes a subsequent reduction as the pH increases reaching another minimum at pH 8.5 (49 kJ mol(-1)).

Gold Nanoparticles Supported on Nanoparticulate Ceria as a Powerful Agent against Intracellular Oxidative Stress

Ceria-supported gold nanoparticles are prepared exhibiting peroxidase activity and acting as radical traps. Au/CeO(2) shows a remarkable biocompatibility as demonstrated by measuring cellular viability, proliferation, and lack of apoptosis for two human cell lines (Hep3B and HeLa). The antioxidant activity of Au/CeO(2) against reactive oxygen species (ROS) is demonstrated by studying the cellular behavior of Hep3B and HeLa in a model of cellular oxidative stress. It is determined that Au/CeO(2) exhibits higher antioxidant activity than glutathione, the main cytosolic antioxidant compound, and its CeO(2) carrier. Overall the result presented here shows the potential of implementing well-established nanoparticulated gold catalysts with remarkable biocompatibility in cellular biology.

Two-photon chemistry in ruthenium 2,2'-bipyridyl-functionalized single-wall carbon nanotubes.

Ruthenium polypyridyl complexes are widely used as light harvesters in dye-sensitized solar cells. Since one of the potential applications of single-wall carbon nanotubes (SWCNTs) and their derived materials is their use as active components in organic and hybrid solar cells, the study of the photochemistry of SWCNTs with tethered ruthenium polypyridyl complexes is important. A water-soluble ruthenium tris(bipyridyl) complex linked through peptidic bonds to SWCNTs (Ru-SWCNTs) was prepared by radical addition of thiol-terminated SWCNT to a terminal C=C double bond of a bipyridyl ligand of the ruthenium tris(bipyridyl) complex. The resulting macromolecular Ru-SWCNT (approximately 500 nm, 15.6% ruthenium complex content) was water-soluble and was characterized by using TEM, thermogravimetric analysis, chemical analysis, and optical spectroscopy. The emission of Ru-SWCNT is 1.6 times weaker than that of a mixture of [Ru(bpy)(3)](2+) and SWCNT of similar concentration. Time-resolved absorption optical spectroscopy allows the detection of the [Ru(bpy)(3)](2+)-excited triplet and [Ru(bpy)(3)](+). The laser flash studies reveal that Ru-SWCNT exhibits an unprecedented two-photon process that is enabled by the semiconducting properties of the SWCNT. Thus, the effect of the excitation wavelength and laser power on the transient spectra indicate that upon excitation of two [Ru(bpy)(3)](2+) complexes of Ru-SWCNT, a disproportionation process occurs leading to delayed formation of [Ru(bpy)(3)](+) and the performance of the SWCNT as a semiconductor. This two-photon delayed [Ru(bpy)(3)](+) generation is not observed in the photolysis of [Ru(bpy)(3)](3+); SWCNT acts as an electron wire or electron relay in the disproportionation of two [Ru(bpy)(3)](2+) triplets in a process that illustrates that the SWCNT plays a key role in the process. We propose a mechanism for this two-photon disproportionation compatible with i) the need for high laser flux, ii) the long lifetime of the [Ru(bpy)(3)](2+) triplets, iii) the semiconducting properties of the SWNT, and iv) the energy of the HOMO/LUMO levels involved.

Single- and multi-walled carbon nanotubes covalently linked to perylenebisimides: synthesis, characterization and photophysical properties

Covalent functionalization of single- and multi-walled carbon nanotubes, both at the tips and on the walls, by perylenebisimides (PBIs) is reported and the resulting PBI-CNT hybrid materials have been characterized by spectroscopic (UV-Vis, Raman, 1H-NMR), thermal (TGA) and microscopy techniques (TEM). Photoexcitation resulted in the one-electron reduction of the PBI moiety with the simultaneous one-electron oxidation of CNT, yielding (PBI)˙−-(CNT)˙+, as revealed by transient absorption measurement.