Thermally Activated Upconversion Near‐Infrared Photoluminescence from Carbon Dots Synthesized via Microwave Assisted Exfoliation

Abstract

and reduced autofluorescence; this calls for development of luminophores featuring both the absorption and emission in the NIR region. [1] Comparing with the Stokes-shifted photoluminescence (SPL) commonly observed for the most of luminescent materials, upconversion photoluminescence (UCPL) represents short-wavelength emission, which occurs under long-wavelength excitation; it avoids the background from the Stokes fluorescence interference and thus offers a higher signal-to-background ratio in luminescence imaging. [2] Moreover, UCPL in the NIR spectral region is preferred for in vivo luminescence imaging as compared with upconversion to the visible region because of the greater penetration depth of both excitation and emission light into and from the biological tissue. UCPL can take place in a number of systems, such as lanthanide (Ln 3+ )-doped nanoparticles, [3] organic dyes, [4] and inorganic quantum dots [5] by utilization of additional photons or heat. Ln 3+ -doped nanoparticles represent an established class of UCPL materials operating on the basis of the unique electronic Upconversion near-infrared (NIR) fluorescent carbon dots (CDs) are important for imaging applications. Herein, thermally activated upconversion photoluminescence (UCPL) in the NIR region, with an emission peak at 784 nm, which appears under 808 nm continuous-wave laser excitation, are realized in the NIR absorbing/emissive CDs (NIR-CDs). The NIR-CDs are synthesized by microwave-assisted exfoliation of red emissive CDs in dimethylformamide, and feature single or few-layered graphene-like cores. This structure provides an enhanced contact area of the graphene-like plates in the core with the electron-acceptor carbonyl groups in dimethylformamide, which contributes to the main NIR absorption band peaked at 724 nm and a tail band in 800-850 nm. Temperature-dependent photoluminescence spectra and transient absorption spectra confirm that the UCPL of NIR-CDs is due to the thermally activated electron transitions in the excited state, rather than the multiphoton absorption process. Temperature dependent upconversion NIR luminescence imaging is demonstrated for NIR-CDs embedded in a polyvinyl pyrrolidone film, and the NIR upconversion luminescence imaging in vivo using NIR-CDs in a mouse model is accomplished.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.

Optical Properties of Carbon Dots in the Deep‐Red to Near‐Infrared Region Are Attractive for Biomedical Applications

“…Li and coworkers produced CDs with single-to-few layer graphene-like cores by exfoliating the multilayered CDs and demonstrated thermally activated ASPL at 784 nm from the resultant CDs under excitation of 808 nm cw laser. [57] In this case, one-photon absorption occurred during ASPL process, while the influence of temperature on the ASPL spectra (Figure 9c) revealed a mechanism involving thermally activated electron transitions in the excited state, as demonstrated in Figure 9d. Electrons were excited to S 1 of edge state (so-called "S 1 -edge") under 808 nm cw excitation and then followed two pathways for their radiative deactivation: i) relaxation from S 1 -edge to S 0 via Stokes-shifted PL (SPL), or ii) thermally activated upconversion from S 1 -edge to S 1 of intrinsic state (so-called "S 1 -int") followed by radiative relaxation from S 1 -int to S 0 via UCPL, which became more predominant compared with path (i) with rising temperature to thermally facilitate high electron population on S 1 -int.…”

“…Li and coworkers synthesized CDs through a microwave-assisted exfoliation of red emissive multilayered CDs in DMF. [57] The resulting NIR-CDs possessed either single-layer or few-layered graphene-like cores, which offered an enhanced contact area with electron-acceptor carbonyl groups from the DMF molecules. As a result, the exfoliated NIR-CDs had both the absorption and emission bands in the NIR region (λ abs = 724 nm, λ em = 770 nm, PL QY 11%) and showed a thermally activated upconversion PL (UCPL) at 784 nm under 808 nm continuous-wave (cw) laser excitation.…”

“…Bonding of these electron-accepting groups on the basal planes of the outer layers of CDs shifted down the LUMO of the outer layers, thereby yielding the NIR absorption and emission of the surface-modified CDs (see Figure 1). In a related study, [57] multilayered π-conjugated cores of red emissive CDs were exfoliated into single-to-few-layer CDs by microwave heating, which facilitated stronger interaction of DMF molecules with outer layers of CDs, leading to the appearance of the absorption band at 724 nm and PL band at 770 nm. Li and coworkers reported on small (2 nm) spherical CDs with dual emission at 430 and 642 nm, [98] where in presence of glutathione, the long-wavelength emission of CDs was efficiently quenched due to the glutathione-induced aggregation, but blue emission remained.…”

Chlorine Modulation Fluorescent Performance of Seaweed-Derived Graphene Quantum Dots for Long-Wavelength Excitation Cell-Imaging Application

“…We obtain the specific lattice spacing value because the Cl-doping increases the interplanar space, indicating that the Cl atoms are doped inside the GQDs [ 13 , 27 ]. The results of Figure 1 e,f show that the average thicknesseses of GQDs and Cl-GQDs are 3.47 and 3.95 nm, respectively, which are higher than the thickness of GQDs obtained using the bottom-up method [ 28 , 29 , 30 ].…”

NIR-quantum dots in biomedical imaging and their future

“…Li and co-workers synthesized NIR-emitting CQDs by combining the solvothermal method and microwave methods (Li et al, 2019b). In a first step, red-emitting CQDs are synthesized by the solvothermal route by mixing citric acid and urea in dimethylformamide (DMF) in an autoclave at 160 C for 6 h. In a second step, the synthesized CQDs were then mixed with DMF and heated by microwave irradiation at 100 C for 70 min at atmospheric pressure.…”