New trends in nonconventional carbon dot synthesis

Abstract

Carbon dots (CDs) are currently one of the hot topics in the nanomaterial world. Until recently, their preparation has been mostly based on solvothermal or hydrothermal syntheses requiring high temperatures, long reaction times, or toxic solvents. Moreover, the resulting materials are often affected by low reproducibility and difficult purification. A potential solution to these problems could be represented by innovative fields of chemistry, such as mechanochemistry, flow chemistry, and laser synthesis in the liquid phase. Machine learning could also be applied to go beyond the trial-and-error approach commonly used to explore the CD chemical space. In this review, we explore these recent approaches and their future potential to address some of the CD limitations, widening the range of properties and applications of these highly promising nanomaterials. Synthesis of carbon dots: current challengesCarbon dots (CDs) are currently one of the most studied carbon-based materials, due to their highly promising properties. CDs are carbon-based nanoparticles presenting dimensions <10 nm, emissions ranging over the whole visible spectrum, low toxicity, and long-term photo and colloidal stability [1,2]. Generally, CDs are synthesized following either a top-down or bottomup approach, with the latter more commonly used due to its superior versatility and accessibility [3]. In fact, CD synthesis mostly relies on hydrothermal or solvothermal treatment of relatively small molecular precursors (citric acid, amino acids, carbohydrates, anilines) involving either high temperatures/pressures, long reaction times, or toxic organic solvents [4][5][6][7][8][9][10][11]. This approach is highly appreciated for the simple and economic set up and for the wide range of precursors that can be employed. An improvement in solvothermal synthesis has been achieved by using microwaves. This resulted in a great reduction of reaction times and solvent amount, mainly arising from a more controllable thermal transport [4,12]. Despite these advantages, the high temperatures needed (>150°C) result in multiple reaction pathways that are not clearly predictable, leading to extensive formation of by-products, which are often highly emissive (Figure 1) [13][14][15][16][17]. As a result, various papers identified the presence of molecular fluorophores in CD samples, highlighting the urgent need for more stringent purification protocols [18][19][20][21]23,25]. This aspect, together with irregular heat and mass transfer typical of batch synthesis, often contributes to the low reproducibility observed in CD preparation [4,22]. Furthermore, the procedures used to tailor the obtained materials are still based on a trial-and-error strategy. This approach is limited by the absence of predictability, resulting in long and difficult optimizations of the target properties and extensive time consumption. To go beyond these limitations, approaches able to increase the reproducibility of the synthesis and finely tailor the material characteristics are required. A poss...

High-Yield, Stable, and Ultrasensitive Carbon Dots for pH Sensing and Fe2+ Detection Are Synthesized at Room Temperature

“…Xie et al used a one-step hydrothermal method to synthesize nitrogen and silicon codoped fluorescent CDs with a product yield of 52.56% . However, the product yield of these CDs prepared by them is not high enough, and the disadvantages of the hydrothermal method are also obvious; the reaction temperature of the hydrothermal method was usually higher than 150 °C during the use of high-pressure reactors . Undoubtedly, high temperature and high pressure will cause great security risks.…”

Cross‐linking enhanced room‐temperature phosphorescence of carbon dots

“…Carbon dots (CDs), as emerging luminescent nanomaterial, are now widely applied in biological, [18][19][20] optoelectronic, [21][22][23] and energy-related fields, [24][25][26] owing to their impressive optical properties, small size, low toxicity, easy preparation, and biocompatibility. [27][28][29] At present, most reported studies on the optical properties of CDs focus on fluorescence (FL). Few reports investigate the RTP properties of CDs, especially in matrix-free systems.…”

Nitrogen-Doped Carbon Nanodots Produced by Femtosecond Laser Synthesis for Effective Fluorophores

“… 29 − 31 Besides, recent interest in unconventional strategies for carbon nanodot fabrication attracted attention to this synthetic method. 32 To date, pulsed laser synthesis was only applied to homocyclic aromatic compounds, benzene or toluene, and produced nitrogen-free nanodots, hence the understanding of the nitrogen effect on the properties of CNDs produced by this method was lacking. In this work, we for the first time produced carbon nanodots by laser synthesis from a heterocyclic aromatic compound (pyridine), and also, to the best of our knowledge, synthesized nitrogen-doped CNDs from a compound where all carbon and nitrogen atoms belonged to the aromatic system.…”

“…Simple aromatic molecules are especially attractive in this context since their aromatic ring is a natural building block for constructing nanoparticles with a graphitic crystalline structure. A number of works demonstrated that intense optical fields of pulsed lasers are an effective instrument for the assembly of aromatic molecules into CNDs with characteristic excitation-dependent visible fluorescence. − Besides, recent interest in unconventional strategies for carbon nanodot fabrication attracted attention to this synthetic method . To date, pulsed laser synthesis was only applied to homocyclic aromatic compounds, benzene or toluene, and produced nitrogen-free nanodots, hence the understanding of the nitrogen effect on the properties of CNDs produced by this method was lacking.…”