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

Antonio Cappai,Pier Carlo Ricci,Daniele Chiriu,Carlo Maria Carbonaro,Marcello Salis,Riccardo Corpino,Francesca Mocci,Chiara Olla

doi:10.3390/ma14040770

Abstract

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.

Highlights

Carbon nanodots (CNDs) are carbon-based nanoparticles which represent a new emerging field of research for optoelectronic and photonics purposes due to their unique optical properties
Optical properties in terms of pH conditions are modelled by calculating the potential energy surface of the interacting molecular system
In the table we reported for each species the tautomer and the rotamer with the lowest Self Consistent Field (SCF) solvation free energy

Summary

Introduction

Carbon nanodots (CNDs) are carbon-based nanoparticles which represent a new emerging field of research for optoelectronic and photonics purposes due to their unique optical properties. Among them stands out a stable and tunable photoluminescence in the visible region combined with cost-effective synthesis routes [1,2,3,4] The origin of their fluorescent properties is still a debated topic, mainly because of the peculiar excitation dependence that in general causes a red shift of the emission as the excitation wavelength increases. The luminescence is currently interpreted for the most within the core-shell picture, assuming a crystalline carbon core and a disordered surface that could host different functional groups and molecular moieties [2,5,6,7] These emitting core states, surface states and molecular states are expected to contribute to the observed luminescence and its excitation dependence. The precise determination of each contribution is challenging, in agreement with the variable order-disorder ratio reported depending on the synthesis conditions, such as, for example, precursors and temperature [8,9]

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Results

Discussion

Conclusion