Abstract
The burgeoning interest raised by carbon dots (CDs) is an epitome of the urgency to develop green and biocompatible alternatives to inorganic and hybrid quantum dots. The fast-paced development of synthetic approaches for CDs over the past few years has left many open questions on their characterization. Herein, we further confirm how the standardization of CDs cannot disregard the presence of fluorescent molecular byproducts. On the contrary, we show how the emissive properties of our solvothermally synthesized CDs largely stem from free molecular adducts produced during the synthesis of carbon cores. The presence of these small molecules, not detectable by electron microscopy, could be deceptive for a reliable characterization of the CDs and could lead to an overestimate of their optical properties. Hence, we propose to introduce more bias-free structure–property correlations based on spectroscopy techniques capable of giving direct insights into their structural properties. Through a combination of standard and unconventional characterization techniques, such as fluorescence correlation spectroscopy (FCS) and time-resolved electron paramagnetic resonance (TREPR), we demonstrate how molecular byproducts dominate the emission properties: these are freely moving in the solution rather than decorating a carbonaceous scaffold. Considering carefully the possibility that newly synthesized CDs are heterogeneous solutions will boost the research and optimization of CDs, thereby paving the way to the large-scale production of cheap and biocompatible light-emissive nanostructures.
Highlights
Despite the success of carbon dots (CDs), their peculiar photophysics based on substantial Stokes shifts and excitation energydependent photoluminescence (PL) is still highly debated within the scientific community.[17−21] Recently, a handful of groups, including our group, reported on the essential role played by molecular fluorophores in the emission properties of CDs.[22−25] Such molecular entities are produced during the pyrolysis of organic precursors in CD syntheses.[26−28] Among others, CDs synthesized by citric acid and ethylenediamine are the most paradigmatic examples of the inherent complexity of the emission properties of CDs
We found average diameters of 13 ± 2, 9.5 ± 3, and 11 ± 2 nm for samples synthesized from m-PD, o-PD, and p-PD, respectively
Using fluorescence correlation spectroscopy and analyzing the resulting translational diffusion coefficients for three selected CDs in solution, we can univocally demonstrate the molecular nature of the emissive species in these CDs
Summary
The quest for a cheaper and greener alternative to inorganic quantum dots lies at the heart of the recent success of carbon dots (CDs).[1,2] During the past few years, record high and excitation wavelength-dependent quantum yields (QY),[3] sizetunable optical properties,[4] and versatile surface chemistry analogous to inorganic QDs have been reported for bottomup-synthesized CDs.[5−8] Since their discovery, research has aimed to obtain highly efficient CDs emitters, thereby envisioning a wealth of applications in bioimaging, catalysis, and light-emitting devices.[9−11] The collective effort among the scientific community in the pursuit of higher QYs for CDs led to the discovery of different synthetic strategies, e.g., hydrothermal, solvothermal, and microwave-assisted syntheses.[12,13] In fulfillment of the high expectations, these systems were used to fabricate CD-based devices, reporting state-ofthe-art efficiencies, and further fueling the interest around them.[14−16]Despite the success of CDs, their peculiar photophysics based on substantial Stokes shifts and excitation energydependent photoluminescence (PL) is still highly debated within the scientific community.[17−21] Recently, a handful of groups, including our group, reported on the essential role played by molecular fluorophores in the emission properties of CDs.[22−25] Such molecular entities are produced during the pyrolysis of organic precursors in CD syntheses.[26−28] Among others, CDs synthesized by citric acid and ethylenediamine are the most paradigmatic examples of the inherent complexity of the emission properties of CDs. Albeit different models were proposed to account for the observed emission properties (i.e., energy transfer among different fluorophores embedded within a carbonaceous scaffold, surface-mediated PL, aggregationmediated PL),[30−32] the discovery of the highly emissive citrazinic acid derivative imidazo[1,2-a] pyridine-7-carboxylic acid, 1,2,3,5-tetrahydro-5-oxo (IPCA) as a byproduct of this synthesis cast doubt on the nature of this emission.[22] Recently, using fluorescence correlation spectroscopy (FCS), we demonstrated that emission of these systems originates mainly
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