Fluorescent carbon nanomaterials: “quantum dots” or nanoclusters?

Abstract

Despite many efforts, the mechanisms of light absorption and emission of small fluorescent carbon nanoparticles (C-dots) are still unresolved and are a subject of active discussion. In this work we address the question as to whether the fluorescence is a collective property of these nanoparticles or they are composed of assembled individual emitters. Selecting three types of C-dots with "violet", "blue" and "green" emissions and performing a detailed study of fluorescence intensity, lifetime and time-resolved anisotropy as a function of excitation and emission wavelengths together with the effect of viscogen and dynamic fluorescence quencher, we demonstrate that the C-dots represent assemblies of surface-exposed fluorophores. They behave as individual emitters, display electronic anisotropy, do not exchange their excited-state energies via homo-FRET and possibly display sub-nanosecond intra-particle mobility.

Insight into the Excitation‐Dependent Fluorescence of Carbon Dots

“…[19][20][21] It can therefore be postulated that there is a prevalence of two types of chromophores, non-emitting and those emitting in the visible range. Such behavior is quite contrary to that of semiconductor quantum dots but has been commonly observed for CDs and also for graphene and graphene oxide nanoparticles.…”

Facile fabrication of luminescent organic dots by thermolysis of citric acid in urea melt, and their use for cell staining and polyelectrolyte microcapsule labelling

“…Thus, the resulting O-dot is a set of primary fluorophores bound by covalent or intermolecular (coordination, donor–acceptor, hydrogen) bonds, for example, forming a polymer-like structure [47], so that the particles formed can be identified as clusters of independent fluorophores [19]. Solubilization of these clusters in water leads to micellar peptization and colloidal sol formation; peptization increases with temperature and dilution.…”

“…Therefore, these nanostructures should, more precisely, be called organic dots (O-dots), or luminescent organic clusters (LOC). A semiconductor quantum dot is a single collective electronic oscillator; in contrast, an organic dot is probably a clustered pack of isolated oscillators (phosphors) [19]. According to literature data, depending on the reaction conditions, (temperature, duration of the process and the ratio of precursors, among others), the O-dots conditionally fall into two main categories.…”

Carbon nanomaterials: multi-functional agents for biomedical fluorescence and Raman imaging

“…This initial excitation is followed by a non-radiative relaxation of the electron to the conduction band and then the fluorescence event with a photon energy of E 11 -see Fig. [89][90][91][92] However, it is certain that structurally perfect, defect free bulk graphene does not show luminescence upon photo-excitation. 80 The fluorescence quantum yields of SWNTs are low, typically o0.01 for macroscopic samples of SWCNT, 81,82 and show a strong dependency on environmental factors.…”