Singlet Oxygen Detection on a Nanostructured Porous Silicon Thin Film via Photonic Luminescence Enhancements

Because reactive oxygen species are involved in a range of pathologies, developing analytical tools for this group of molecules opens new vistas for biomedical diagnostics. Herein, we fabricate a porous silicon microcavity (pSiMC) functionalized with luminescent singlet oxygen (¹O₂) probe EuA ((Eu(III)-2,2',2″-(10-(2-((4-(2-((4-(2-((anthracen-9-ylmethyl)amino)ethyl)-1H-1,2,3-triazol-1-yl)amino)-2-oxoethyl)-2-oxo-1,2-dihydroquinolin-7-yl)amino)-2-oxoethyl)-1,4,7,10-tetraazacyclododecane-1,4,7-triyl)triacetic acid) as proof of concept of an optical sensor for reactive oxygen species. We characterize each surface modification step of the pSiMC by means of FTIR and X-ray photoelectron spectroscopy as well as by determining the optical shifts of the resonance wavelength of the pSiMC. The luminescence signal upon detection of ¹O₂ on the EuA-modified pSiMC is enhanced ∼2-fold compared to that of a single layer and a detuned microcavity. The sensing performance of the EuA probe is improved significantly on the pSiMC compared to that in aqueous solution, giving a limit of ¹O₂ detection of 3.7 × 10^-8 M.

Recent Advances on Luminescent Enhancement-Based Porous Silicon Biosensors

Luminescence-based detection paradigms have key advantages over other optical platforms such as absorbance, reflectance or interferometric based detection. However, autofluorescence, low quantum yield and lack of photostability of the fluorophore or emitting molecule are still performance-limiting factors. Recent research has shown the need for enhanced luminescence-based detection to overcome these drawbacks while at the same time improving the sensitivity, selectivity and reducing the detection limits of optical sensors and biosensors. Nanostructures have been reported to significantly improve the spectral properties of the emitting molecules. These structures offer unique electrical, optic and magnetic properties which may be used to tailor the surrounding electrical field of the emitter. Here, the main principles behind luminescence and luminescence enhancement-based detections are reviewed, with an emphasis on europium complexes as the emitting molecule. An overview of the optical porous silicon microcavity (pSiMC) as a biosensing platform and recent proof-of-concept examples on enhanced luminescence-based detection using pSiMCs are provided and discussed.

One-step synthesis of a highly homogeneous SBA-NHC hybrid material: en route to single-site NHC-metal heterogeneous catalysts with high loadings

The one-step synthesis of a mesoporous silica of SBA type, functionalized with a 1-(2,6-diisopropylphenyl)-3-propyl-imidazolium (iPr2Ar-NHC-propyl) cation located in the pore channels, is described. This material was obtained by the direct hydrolysis and co-condensation of tetraethylorthosilicate (TEOS) and 1-(2,6-diisopropylphenyl)-3-[3-(triethoxysilyl)propyl]-imidazolium iodide in the presence of Pluronic P123 as a non-ionic structure-directing agent and aqueous HCl (37%) as an acid catalyst. Small-angle X-ray diffraction measurements, scanning and transmission electron microscopies, as well as dinitrogen sorption analyses revealed that the synthesized material is highly mesoporous with a 2D hexagonal arrangement of the porous network. (13)C and (29)Si CP-MAS NMR spectroscopy confirmed that the material contains intact iPr2Ar-NHC-propyl cations, which are covalently anchored via silicon atoms fused into the silica matrix. Moreover, comparison of the latter data with those of an analogous post-synthetic grafted SBA-NHC material allowed us to establish that, as expected, (i) it is most probably more homogeneous and (ii) it shows a more robust anchoring of the organic units. Finally, elemental mapping by energy dispersive X-ray spectroscopy in the scanning electron microscope demonstrated a very homogeneous distribution of the imidazolium units within the one-pot material, moreover with a high content. This study thus demonstrates that a relatively bulky and hydrophilic imidazolium unit can be directly co-condensed with TEOS in the presence of a structure-directing agent to provide in a single step a highly ordered and homogeneous mesoporous hybrid SBA-NHC material, possessing a significant number of cationic NHC sites.

Discrete complexes immobilized onto click-SBA-15 silica: controllable loadings and the impact of surface coverage on catalysis

Azidopropyl functionalized mesoporous silica SBA-15 were prepared with variable azide loadings of 0.03-0.7 mmol g(-1) (~2-50% of maximal surface coverage) through a direct synthesis, co-condensation approach. These materials are functionalized selectively with ethynylated organic moieties through a copper-catalyzed azide alkyne cycloaddition (CuAAC) or "click" reaction. Specific loading within a material can be regulated by either the azide loading or limiting the alkyne reagent relative to the azide loading. The immobilization of ferrocene, pyrene, tris(pyridylmethyl)amine (TPA), and iron porphyrin (FeTPP) demonstrates the robust nature and reproducibility of this two-step synthetic attachment strategy. Loading-sensitive pyrene fluorescence correlates with a theoretically random surface distribution, rather than a uniform one; site-isolation of tethered moieties ~15 Å in length occurs at loadings less than 0.02 mmol g(-1). The effect of surface loading on reactivity is observed in the oxygenation of SBA-15-[Cu(I)(TPA)]. SBA-15-[Mn(II)(TPA)]-catalyzed epoxidation exhibits a systematic dependence on surface loading. A comparison of homogeneous, site-isolated and site-dense complexes provides insight into catalyst speciation and ligand activity.

Direct synthesis and fluorescent imaging of bifunctionalized mesoporous iodopropyl-silica

The cocondensation of 3-iodopropyltrimethoxysilane and tetraethoxysilane with an additional substituted trimethoxysilane (RTMS) in the presence of Pluronic P123 and hydrogen iodide yields bifunctionalized mesoporous silica. The pore-size distribution of these materials depends on the nature of the RTMS additive. Excellent results in terms of a narrow pore-size distribution were obtained with methyltrimethoxysilane. A particularly interesting bifunctionalized mesoporous material is formed by the coinclusion of iodopropyl and aminopropyl moieties. The complementary reactivity of these two functional groups is demonstrated by the selective labeling of the amino-iodo-functionalized mesoporous silica with 2-hydroxy-substituted Nile red and fluorescein isothiocyanate, allowing further characterization of the functional group distribution by confocal laser scanning microscopy.

Controlled loadings in a mesoporous material: click-on silica

Hybrid mesoporous SBA-15 silicas were synthesized directly with variable alkylazide loading representing 2-50% surface coverage. These hybrid silica materials retain the favorable physical attributes of the parent SBA-15 materials and allow efficient covalent attachment of ethynylated organic moieties through a copper catalyzed 3 + 2 Huisgen cycloaddition reaction. Three distinctly different examples are provided demonstrating the efficiency and robust nature of this attachment synthetic strategy. The direct syntheses provide predefined loadings of randomly distributed organics within the materials, from site-dense to site-isolated. Such control over loadings along with simply implemented analytic procedures should facilitate the translation of homogeneous chemistries to heterogeneous supports.

Imprinted nanoporous organosilicas for selective adsorption of nitroenergetic targets

Periodic mesoporous organosilicas incorporating diethylbenzene bridges in their pore walls were applied for the adsorption of nitroenegetic targets from aqueous solution. The materials were synthesized by co-condensing 1,4-bis(trimethoxysilylethyl)benzene (DEB) with 1,2-bis(trimethoxysilyl)ethane to improve structural characteristics. Molecular imprinting of the pore surfaces was employed through the use of a novel target-like surfactant to further enhance selectivity for targets of interest (tri- and dinitrotoluenes) over targets of similar structure ( p-cresol and p-nitrophenol). The headgroup of the commonly used alkylene oxide surfactant Brij76 was modified by esterification with 3,5-dinitrobenzoyl chloride. This provided a target analogue which was readily miscible with the Brij76 surfactant micelles used to direct material mesopore structures. The impact of variations in precursor ratios and amounts of imprint molecule was evaluated. The use of 12.5% of the modified Brij surfactant with a co-condensate employing 30% DEB was found to provide the best compromise between total capacity and selectivity for nitroenergetic targets.