Formation of Citrazinic Acid Ions and Their Contribution to Optical and Magnetic Features of Carbon Nanodots: A Combined Experimental and Computational Approach.

The molecular model is one of the most appealing to explain the peculiar optical properties of Carbon nanodots (CNDs) and was proven to be successful for the bottom up synthesis, where a few molecules were recognized. Among the others, citrazinic acid is relevant for the synthesis of citric acid-based CNDs. Here we report a combined experimental and computational approach to discuss the formation of different protonated and deprotonated species of citrazinic acid and their contribution to vibrational and magnetic spectra. By computing the free energy formation in water solution, we selected the most favoured species and we retrieved their presence in the experimental surface enhanced Raman spectra. As well, the chemical shifts are discussed in terms of tautomers and rotamers of most favoured species. The expected formation of protonated and de-protonated citrazinic acid ions under extreme pH conditions was proven by evaluating specific interactions with H2SO4 and NaOH molecules. The reported results confirm that the presence of citrazinic acid and its ionic forms should be considered in the interpretation of the spectroscopic features of CNDs.

Exploiting combined absorption and front face fluorescence spectroscopy to chase classification: A proof of concept in the case of Sardinian red wines.

We present the analysis of optical spectroscopy fingerprints of three different varieties of Sardinian red wine, Carignano, Cannonau and Monica, belonging to Italian DOC (Controlled Origin Designation) red wines family. We selected 15 commercial wines (five for each variety), produced in the middle and south regions of Sardinia form monovarietal grapes. Despite different grapes, farming and tasting, all the samples share common absorption and fluorescence features. A two steps analysis is applied to achieve good classification of the wines. We recorded typical absorbance spectra with a large absorption below 300 nm and a visible absorption band at about 520 nm. Colour attributes according to chromaticity coordinates were evaluated by means of transmittance data and coupled to wine aging data to achieve classification. Despite similar values of lightness (L*), chroma (C*ab), and hue angle (h°ab) for the three varieties, good clustering was gathered for Carignano and Cannonau wines, whilst Monica variety has wide spread chromatic coordinates. UV excited fluorescence spectra display large and composite emission bands from the near UV down to the whole visible range. We performed multivariate analysis in the framework of the principal component analysis of fluorescence spectra coupled with absorption ones being able to achieve good clustering between the three varieties. One anomaly among Cannonau wines is discussed.

Light assisted dimer to monomer transformation in heavily doped rhodamine 6G-Porous silica hybrids.

The spectral properties of postdoping prepared type I Rhodamine 6G-silica hybrids were investigated in the case of intentionally large doping (5 x 10(-2) mol/L). Beside expected concentration quenching effects, steady state and time resolved optical spectroscopy measurements displayed the presence of different kind of aggregates, both fluorescent and nonfluorescent. As the irradiation dose on the sample increases, the emission features change: the overall emission increases, the peak of the emission is blue-shifted, and the emission decay time also increases. The modifications of the spectral properties under light excitation are interpreted in terms of dimer to monomer light assisted transformation, and a thermodynamic model based on a monomolecular kinetics is presented.