Abstract
The origin and classification of energy states, as well as the electronic transitions and energy transfers associated with them, have been recognized as critical factors for understanding the optical properties of carbon nanodots (CNDs). Herein, we report the synthesis of CNDs in an optimized process that allows low-temperature carbonization using ethanolamine as the major precursor and citric acid as an additive. The results obtained herein suggest that the energy states in our CNDs can be classified into four different types based on their chemical origin: carbogenic core states, surface defective states, molecular emissive states, and non-radiative trap states. Each energy state is associated with the occurrence of different types of emissions in the visible to near-infrared (NIR) range and the generation of reactive oxygen species (ROS). The potential pathways of radiative/non-radiative transitions in CNDs have been systematically studied using visible-to-NIR emission spectroscopy and fluorescence decay measurements. Furthermore, the bright photoluminescence and ROS generation of these CNDs render them suitable for in vitro imaging and photodynamic therapy applications. We believe that these new insights into the energy states of CNDs will result in significant improvements in other applications, such as photocatalysis and optoelectronics.
Highlights
Introduction published maps and institutional affilOver the past few decades, carbon nanodots (CNDs) have been recognized as luminescent materials with properties related to photostability, biocompatibility, and bioavailability
The CNDs were synthesized via the oxidative carbonization of EA with a controlled amount of citric acid (CA) under ambient conditions
CA is used as the major precursor to form carbogenic cores and relatively a small amount of amine is needed to improve the colloidal stability and optical properties of CNDs [17,24,36,37]
Summary
The synthetic procedure for the synthesis of CND1 is as follows: Ethanolamine (2 mL). Was first vigorously stirred at 90 ◦ C in air for 24 h. The solution was allowed to cool to room temperature. The black sticky liquid was dissolved in water and dialyzed against water for at least 3 days using Spectra/Por Biotech Cellulose Ester dialysis tubes (Spectrum Chemical Mfg. Corp., Gardena, CA, USA) (100–500 Da). After the water was removed from the solution by lyophilization, the resulting powder was stored in a freezer for further use. The same procedure was followed for the synthesis of CND2, CND3, and CND4. The amounts of aqueous citric acid solution (1 g·mL−1 ) added to ethanolamine (2 mL) for CND2, CND3, and CND4, were 200 μL, 510 μL, and 2000 μL, respectively. The reaction yield of CNDs was ca.
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