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.

The versatility of the surface chemistry of carbon dots (CDs) can generate high flexibility in engineering their surface functionality for a wide and precise array of applications. In respect of this, we perform for the first time the surface passivation of carboxylated carbon nanoparticles with p-phenylenediamine to prepare water soluble photoluminescent CDs exhibiting intriguing physicochemical properties. The new strategy used to engineer the surface functionality allows us to synthesize CDs with a stable excitation independent green photoluminescence with a quantum yield of 14% and lifetime of 11.8 ns, suitable for application in bioimaging, sensing and optoelectronics.