DNA-stabilized silver nanoclusters (DNA-AgNCs) are a class of promising fluorophores for imaging and sensing applications. All aspects of their spectroscopic properties are not yet fully characterized, leaving this field still with a number of fundamental studies to be addressed. In this work, we studied the spectroscopic properties of red-emitting DNA-AgNCs at different pH (5 to 9) and ionic strength μ (0.005 to 0.5). The photophysical properties of high performance liquid chromatography (HPLC) purified DNA-AgNCs proved to be constant over a large range of pH and μ, with absorption, emission and fluorescence decay times only being affected at very high pH and μ values. Non-purified DNA-AgNCs were also unaffected by pH and/or μ variations, but significant differences can be observed between the rotational correlation times of purified and non-purified DNA-AgNCs.

The preparation of tailored nanomaterials able to support cell growth and viability is mandatory for tissue engineering applications. In the present work, silica nanoparticles were prepared by a sol-gel procedure and were then functionalized by condensation of amino groups and by adsorption of silver nanoparticles. Transmission electron microscopy (TEM) imaging was used to establish the morphology and the average dimensions of about 130 nm, which were not affected by the functionalization. The three silica samples were deposited (1 mg/mL) on cover glasses, which were used as a substrate to culture adult human bone marrow-mesenchymal stem cells (hBM-MSCs) and human adipose-derived stem cells (hASCs). The good cell viability over the different silica surfaces was evaluated by monitoring the mitochondrial dehydrogenase activity. The analysis of the morphological parameters (aspect ratio, cell length, and nuclear shape Index) yielded information about the interactions of stem cells with the surface of three different nanoparticles. The data are discussed in terms of chemical properties of the surface of silica nanoparticles.

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The immobilization of proteins on inorganic supports is attracting increasing interest since the realization of active surfaces finds application in enzyme-assisted catalysis, environmental sciences, and medical fields. In the present study, cytochrome c (cyt c) is adsorbed on silica nanoparticles (SNPs) and amino-functionalized silica nanoparticles (SNPs–APTES), which are prepared for this purpose and having a diameter of about 50 nm. The peroxidase activity of the protein is investigated under different experimental conditions, to evaluate the impact of differently charged surfaces on the catalytic activity of the biomolecule. The peroxidase activity of cyt c increases upon adsorption on SNPs, and it shows a linear behavior with nanoparticles concentration; on the other hand, the contact with increasing amounts of SNPs–APTES does not affect the catalytic activity of the protein. The kinetic profile of the oxidation reaction is altered for cyt c–SNPs sample, suggesting that upon adsorption, changes in the catalytic process take place. Moreover, we observe that the enhancement of peroxidase activity of cyt c–SNPs is almost completely inhibited in high-ionic-strength buffer: this indicates that the protein establishes electrostatic interactions with SNP. The spectroscopic properties of the adsorbed protein on the two different matrices are investigated by using fluorescence and Raman spectroscopies to account for the enzymatic activity of the hybrid materials. The fluorescence spectra of cyt c–silica bio-composites reveal that the adsorption on silica modifies the microenvironments of the emitting amino acid residues of the protein. Indeed, their fluorescence gains intensity and appears blue-shifted compared to that of the native protein; these modifications are more evident when cyt c is adsorbed on SNPs. Raman spectra suggest that both oxidation and spin state of heme iron change when cyt c is adsorbed on SNPs but not on SNPs–APTES. The spectroscopic data of biocomposite materials are discussed in terms of structural changes to account for the increment of peroxidase activity upon adsorption on the negatively charged surface of SNPs.

Semiconductor nanomaterials are often proposed as photocatalysts for wastewater treatment; silica nanomaterials are still largely unexploited because their photocatalytic performances need improvements, especially under visible light. The present study is a proof-of-concept that amorphous silica colloids once submitted to the proper surface modifications change into an efficient photocatalyst even under low-energy illumination source. For this reason, silica-based colloidal nanomaterials, such as bare (SiO2 NPs), aminated (NH2-SiO2 NPs), and Ag NPs-decorated (Ag-SiO2 NPs) silica, are tested as photocatalysts for the degradation of 9-anthracenecarboxylic acid (9ACA), taken as a model aromatic compound. Interestingly, upon irradiation at 313 nm, NH2-SiO2 NPs induce 9ACA degradation, and the effect is even improved when Ag-SiO2 NPs are used. On the other hand, irradiation at 405 nm activates the plasmon of Ag-SiO2 NPs photocatalyst, providing a faster and more efficient photodegradation. The photodegradation experiments are also performed under white light illumination, employing a low-intensity fluorescent lamp, confirming satisfying efficiencies. The catalytic effect of SiO2-based nanoparticles is thought to originate from photo-excitable surface defects and Ag NP plasmons since the catalytic degradation takes place only when the 9ACA is adsorbed on the surface. In addition, the involvement of reactive oxygen species was demonstrated through a scavenger use, obtaining a yield of 17%. In conclusion, this work shows the applicability of silica-based nanoparticles as photocatalysts through the involvement of silica surface defects, confirming that the silica colloids can act as photocatalysts under irradiation with monochromatic and white light. Graphic abstract Silica and Ag-decorated silica colloids photosensitize the formation of Reactive Oxygen Species with 17% efficiencies. ROS are able to oxidase aromatic pollutants chemi-adsorbed on the surface of the colloids. Silica-silver nanocomposites present a photocatalytic activity useful to degrade aromatic compounds.