Carbon dots (CDs) are well-known for their wide applications; however their structures and chemistry remain unresolved due to highly complex nanostructured framework. This is the first report of trapping seven stable intermediates of CDs generated during the pyrolysis of citric acid with dopant Ruthenium (Ru) (III), which are identified even by naked eyes where Ru(III) acts as an indicator. The highly fluorescent amine functionalized CDs undergo photoinduced electron transfer with carcinogenic quinone derivatives. A sensing platform has been developed to quantify the toxicity of the quinone using CDs in live HeLa cells which can be used for drug screening.  The paper is published in the journal, Chemistry of Materials, 28, 7404-7413, 2016 [DOI: 10.1021/acs.chemmater.6b03008, Publication Date (Web): September 26, 2016] with Kallol Bera, Abhishek Sau, Pritiranjan Mondal, Rukmini Mukherjee§, Debdatto Mookherjee§, Amaresh Metya‡, Asish K. Kundu‡, Debranjan Mandal†, Biswarup Satpati‡, Oishee Chakrabarti§, and Samita Basu* of Chemical Sciences Division, Surface Physics & Material Science Division‡ and Biophysics & Structural Genomics Division§ of SINP.
 
Carbon quantum dots (CDs) have increasing applications in modern biotechnology since these are used for development of medicines and biomarkers as well as for fabrication of biosensing, bioimaging and light-emitting devices. However their physical, chemical and structural properties still remain unveiled because of their highly complex nanostructured framework. One of the unusual features of CDs is their excitation-dependent photoluminescence (PL). The question regarding the origin of PL still persists and becomes one of the subjects of scientific debates. The ambiguity regarding PL of CDs may be revealed through the study of gradual evolution of their electronic structure. We have been able to trap the seven stable intermediates of CDs during the pyrolysis of citric acid with dopant Ru(III) (top panel of Figure 1), which can be identified even by naked eyes where Ru(III) acts as an indicator as shown in the centre of the Figure 1. The metamorphosis of Ru:CDs has been monitored by high-resolution transmission electron microscopy (HR-TEM) (Figure 2), dynamic light scattering (DLS), ¹H-NMR, FT-IR, UV-visible and fluorescence spectroscopy.


                                      Figure 2. TEM images of metamorphic steps

A chemical cocktail comprising of multiple fluorescent dye with different absorption and emission features has been prepared to mimic the optical property of Ru:CDs, which helps to investigate the origin of high PL in CDs, This cocktail contains dopamine: tryptophan: acridine: fluorescein: pyronine-Y units with the molecular concentration ratio of 6.5: 1.9 : 0.06 : 0.004 : 0.0002 respectively (Figure 3).The different fluorophore units are connected on the surface or interior of the carbon backbone and produce PL emission directly.


 Figure 3. Photoluminescence of cocktail dyes

We have found that amine functionalized Ru:CDs (Ru:CNDEDAs)  and  their derivatives chemically modified by a linker, homocysteinthiolactone (HCTL) (Ru:CNDEDAHCTLs) undergo photoinduced electron transfer with a quinone derivative, menadione (MQ) which acts as an electron acceptor (bottom panel of Figure 1). As electron transfer is a distance dependent phenomenon, its efficiency is enhanced almost twice with Ru:CNDEDAHCTLs. By exploiting the electron transfer process we have developed a new technique to quantify excess toxic and carcinogenic quinone in live cells (HeLa). We have also searched out a few pathways which reveal Ru:CNDEDAs as endosomal marker in HeLa cells.


Figure 4. Characterisation of PL of Ru:CNDEDAHCTLs in live cells and detection of toxic quinone. a. Colocalization study with Ru:CNDEDAs and Rab5-mcherry labeled endosomal marker [after 24 h] in HeLa cells. b. Cytotoxicity assay of Ru:CNDEDAs at different time points after the treatment c. Relative fluorescence response (λ_ex= 350 nm) of Ru:CNDEDAHCTLs toward various component and (Inset: Relative fluorescence response to two quinoid dopamine (dopamine) at pH 8.0, p-amino phenol (PAP) and non-quinoid form of dopamine at pH 5.0) d. High photo stability to LASER (405 nm) with Ru:CNDEDAHCTLs e. Quenching of PL of Ru:CNDEDAHCTLs  at different time points after treatment with MQ. f. Fold change in PL intensity of Ru:CNDEDAHCTLs  in absence and presence of MQ.