Molecular Insights of Carbon Nanodots Formation and Their Two‐Photon Emission Properties

Abstract

The structure formation of carbon nanodots (C-dots) prepared from three different organic precursors is discussed at the molecular level. During microwave synthesis, organic chromophores associated with C-dot structures are formed that exhibit distinct optical features. The molecular structure of these fluorophores is elucidated and their optical properties with and without the C-dots are investigated. The emergence of two-photon emission is observed and correlated with the hybridization state of the carbon atoms within the C-dot as well as the formation of the fluorophores. Varying contents of sp 2 -and sp 3 -hybridization in different C-dots also affect their one-photon and two-photon emission characteristics. Understanding the molecular structure of the carbon nanocore and the organic fluorophores formed in C-dots would enable rational design of C-dots with improved optical features, which would be of great relevance for their applications, for example, in bioimaging.

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“…In addition, a Fourier-transform infrared spectroscopy (FTIR) analysis previously reported clearly showed the presence of carboxylic acid groups on its surface. 43 Moreover, the negative ζ potential of Cdots (−3.4 ± 0.32 mV) corroborates the presence of acid groups on its surface.…”

“…Compared to the bulk solution (10.1 ns), this means a significant reduction of the fluorescence lifetime for Cdots inside cells, which can be plausibly explained by the many new opened-up nonradiative interaction paths inside the cellular matrix. 43 Similarly, Cdots attached to the HSA-Ru-Cdot conjugate showed a decreased mean lifetime of 670 ps (Figure 4B,D). The typical FRET efficiency inside the cells was calculated to be 54%, which is comparable to the outside cellular environment (see section 4.7, eq 3).…”

“…Properties. The Cdots were synthesized via microwave synthesis according to a procedure reported recently, 43 using citric acid (CA) as a carbon source and o-phenylenediamine (PDA) as a passivating agent. The Cdots showed a maximum two-photon excited PL emission maximum at around 480 nm under 810 nm as the two-photon excitation wavelength (Figure 2A).…”

“…The Raman spectrum clearly showed the presence of a mixture of sp 2 and sp 3 hybridization states within the Cdots, as reported previously. 43 For a detailed analysis of the surface composition of the Cdots and the chemical state of the elements, we have performed X-ray photoelectron spectroscopy (XPS) measurements. The survey spectrum showed the expected three prominent characteristic peaks of C 1s (284.6 eV), N 1s (399.9 eV), and O 1s (531.1 eV) with no measurable amounts of impurities containing other elements (Figure S3H−J).…”

“…A 5 mL portion of an aqueous solution containing 0.5 g of CA and 0.22 g of PDA was heated for 20 min at 150 °C (8 bar, 70 W) with a microwave reaction system (Biotage Initiator+, Fourth Generation Microwave Synthesizer) in a 2−5 mL microwave vial in organic synthesis mode. 43 The precursor solution was initially colorless, and after 20 min of microwave treatment it changed to greenish yellow. The Cdot solution was filtered using a cellulose-membrane-based syringe filter (CHROMA-FILRC) with 0.2 μm pore size.…”