Transition Metal Ion (Mn²⁺, Fe²⁺, Co²⁺, and Ni²⁺)-Doped Carbon Dots Synthesized via Microwave-Assisted Pyrolysis: A Potential Nanoprobe for Magneto-fluorescent Dual-Modality Bioimaging

Abstract

Heteroatom-doped carbon dots (C-dots) have captured widespread research interest owing to high fluorescence and biocompatibility for multimodal bioimaging applications. Here, we exemplify a rapid, facile synthesis of ethylenediamine (EDA)-functionalized transition metal ion (Mn2+, Fe2+, Co2+, and Ni2+)-doped C-dots via one-pot microwave (MW)-assisted pyrolysis at 800 W within 6 min using Citrus limon (lemon) extract as a carbon source. During MW pyrolysis, the precursor extract undergoes simultaneous carbonization and doping of metal ions onto C-dot surfaces in the presence of EDA. The EDA-functionalized transition metal ion-doped C-dots (i.e., Mn/C, Fe/C, Co/C, and Ni/C-dots) are collectively termed as TMCDs. The water-soluble TMCDs exhibited a size of 3.2 ± 0.485 nm and were enriched with amino and oxo functionalities and corresponding metal-oxide traces on the surfaces, as revealed from Fourier transfer infrared and X-ray photoelectron spectroscopy analyses. Interestingly, TMCDs demonstrated excitation-wavelength-dependent emission with brighter photoluminescence (PL) at 460 nm. Compared to pristine C-dots with a PL quantum yield (QY) of 48.31% and a fluorescence lifetime of 3.6 ns, the synthesized Mn/C, Fe/C, Co/C, and Ni/C-dots exhibited PL QY values of 35.71, 41.72, 75.07, and 50.84% as well as enhanced fluorescence lifetimes (τav) of 9.4, 8.6, 9.2, and 8.9 ns, respectively. The TMCDs significantly exhibited enhanced biocompatibility in human colon cancer cells (SW480) for fluorescence bioimaging and showed ferromagnetic and superparamagnetic behavior with vibrant T 1-contrast ability. Interestingly, the maximum longitudinal (r 1) relaxivity of 0.341 mM–1 s–1 was observed for Mn/C-dots in comparison to that of 3.1–3.5 mM–1 s–1 of clinically used Gd-DTPA magnetic resonance (MR)-contrast agent in vitro (1.5 T). Similarly, the maximum longitudinal relaxivity (r 1) of 0.356 mM–1 s–1 was observed for Ni/C-dots (1.5 T) with respect to 4.16 ± 0.02 mM–1 s–1 attained for Gd-DTPA in vivo (8.45 T). Thus, the rapid, energy-efficient MW-assisted pyrolysis presents lemon extract derived, EDA-functionalized TMCDs with enhanced PL and efficient T 1 contrast as potential magneto-fluorescent nanoprobes for dual-modality bioimaging applications.

Emerging Dual‐Channel Transition‐Metal‐Oxide Quasiaerogels by Self‐Embedded Templating

“…The stretching vibration of OH in Fourier transform infrared (FTIR) spectrum of Co‐carbogels red‐shifts from 3450 to 3430 cm −1 compared with that of carbogels (Figure S4, Supporting Information), suggesting the efficient adsorption of Co ions on the carbogels. [ 39 ] Consequently, during the combustion process in the air at a high temperature, the Co 3 O 4 @C‐QA‐1, Co 3 O 4 @C‐QA‐2, Co 3 O 4 @C‐QA‐3, and Co 3 O 4 @C‐QA‐4 can duplicate the 3D interconnected aerogel structure of the carbogel‐1, carbogel‐2, carbogel‐3, and carbogel‐4, respectively. The X‐ray diffraction (XRD) patterns of the Co‐carbogel treated at different temperatures have been measured and presented in Figure S5 (Supporting Information).…”

Recent advance of carbon dots in bio-related applications

“…One route is treating CDs as a semiconductor nanoparticle, the bandgap of the semiconductor can be tuned by introducing heteroatom, which changes the valance band and/or conduction band position (figure 2(a)). Heteroatoms (B, N, O, P, S, Se and transition metal ions) have been employed to synthesize doped CDs with multiple color emission [41][42][43][44][45][46][47]. For examples, Yang et al [42] related the electronegativity of heteroatom to the emission wavelength of CDs where the lower electronegativity led to an increase in the [42].…”

“…In vivo MR imaging discloses a clear image of brain glioma region after injection for 30 min and 2 h. Ex vivo optical images exhibit the accumulation of Mn-CDs in the brain glioma region. In the same issue, Rub Pakkath et al synthesized TM 2+ (Mn 2+ , Fe 2+ , Co 2+ , and Ni 2+ ) doped CDs from Lemon juice, EDA and TM acetate salt by similar microwave method [46]. These TM doped CDs showed the excitation-dependent emission with decent PL QY of 35%-75% and the maximum r 1 relaxivity of 0.34 mM −1 s −1 for Mn 2+ doped CD.…”

Multifunctional Nanohybrid of Palladium Nanoparticles Encapsulated by Carbon‐Dots for Exploiting Synergetic Applications

“…Compared to other fluorescent nanomaterials, the C‐dots can be easily synthesized from different kind of sources including organic molecules, biowastes, proteins, amino acids, and so on . Moreover, the surface of C‐dots can be modified with several capping agents to improve their fluorescence properties according to the applications . That's why the C‐dots have been extensively utilized in the fields of chemo‐ and biosensors, and optoelectronic applications.…”