Covalent conjugation of carbon dots with Rhodamine B and assessment of their photophysical properties

The unique photoluminescent properties of carbon dots (CDs) continue to encourage a great interest in their development for a wide range of applications in energy conversion, optoelectronics or sensing. Engaging carbon dots in resonance energy transfer processes with organic dyes could enable the design of functional materials to greatly enhance the performance of solar cells and other optoelectronic devices, or to create new types of sensors. In this work, CDs were functionalized with Rhodamine B (RhB) isothiocyanate, (CD-PEG 1500N -Rh) via a simple procedure after surface modification of bare carbon nanoparticles with poly(ethylene glycol) bis(3-aminopropyl) (PEG 1500N ). The morphology of CD-PEG 1500N was ascertained using HR-TEM while the covalent linkage of Rhodamine B at the surface of PEG 1500N capped CDs was proved by spectroscopic analysis. The overlap between the emission spectra of CDs and the absorption spectrum of RhB molecules favoured fluorescent (Förster) resonance energy transfer (FRET) from the CDs to the dye molecules. The FRET mechanism was firstly demonstrated by steady-state fluorescence measurements and its efficiency was estimated by photoluminescence lifetime measurements, using the time correlated single photon counting (TCSPC) method with the excitation of picosecond pulse lasers. The synthetic accessibility and the transfer efficiency of these conjugates make them reliable candidates for fluorescent materials to be later used in FRET based sensing platforms and photovoltaic devices. Applying the Förster formalism we studied the potential of urethane-like linkage at the surface of CDs to form donor-acceptor FRET pair in CD-Rhodamine conjugates.