Unique nanomaterials called quantum dots have inherent characteristics that are changed by quantum phenomena. As a result, they have several potential applications in solar technology, biology, and sensing. Since their discovery in 2004, carbon dots (CDs) have attracted much scientific attention.
Advantages of Carbon Dots
From the standpoint of ingredients and processing techniques, CDs are more straightforward and less expensive to acquire. Different raw materials, including carbon, are used as precursors to create CDs using different preparation techniques. Petroleum coke, fruit, grass, hair, coal, plants, graphite and its byproducts, glucose, citric acid, amino acids, and even household garbage are examples of raw materials. Preparation techniques include microwave, pyrolysis, hydrothermal, chemical oxidation, and so on.
The many surface groups provide CDs with outstanding water stability and dispersion. Selecting suitable precursors may readily control their chemical polarity, encouraging dispersion in more solvents. However, despite significant advancements in CD manufacturing, the thorough investigation of their PL mechanism is still up for controversy because of the diverse structures and surface activities of CDs. It is crucial to have a deeper grasp of the PL origin to advance the applications that depend on PL developments.
Previous Research on Carbon Dots
Over the last ten years, research work has led to significant advancements in knowledge of the PL mechanism of CDs. The key points of view are the surface state, carbon core state, molecular state, and their synergistic impact.
The PL characteristics of CDs are very susceptible to external factors, such as pH and the type of the solvent medium, since the surface fluorophore of CDs often comprises acidic/basic functional groups, such as -NH2, -COOH, etc. Therefore, the degree of protonation on the surface of the CDs will vary when the pH value changes, which in turn influences their PL characteristics.
In this study, researchers created a very simple experimental setup for synthesizing CDs to reduce the interference effects and investigate the luminescence mechanism of the as-prepared CDs.
How the Research was Conducted
The researchers used the model carbon source molecule m-phenylenediamine (mPD) to create the CDs using the solvothermal approach. By altering the solution's pH before the reaction, it is simple to protonate and deprotonate the amino group in m-PD. The surface state of the resultant CDs and the associated fluorescence emission properties are governed by the protonation degree of m-PD.
Altering the pH value only affects how much the functional groups of the carbon source m-phenylenediamine (mPD) molecule are protonated, not the molecule's primary skeletal structure. As a result, this study not only helps explain the PL mechanism of CDs but also produces several CD materials with adjustable PL features that may be used explicitly in chemical sensors and PL anti-counterfeiting.
CDs-PVA films preparation
To thoroughly dissolve 0.20 g of m-PD, 50 mL of ethanol was added, and the mixture was then sonicated for 10 minutes. Next, the m-PD ethanol solution was put in a reaction kettle lined with Teflon and heated to 180 °C for 15 hours while being treated with hydrochloric acid and sodium hydroxide to change its pH to 2, 7, 10, or 14.
To create a carbon dot solution, contaminants or insoluble particles were eliminated. The resulting CDs were given the names 2, 7, 10, and 14 CDs, respectively, based on the pH of the solution before to the reaction. To create thin films, CDs (1 mL) prepared under various pH settings were combined with PVA (2 mL), agitated at room temperature for an hour, and then the solvent was evaporated at 45 °C.
The experimental procedures conducted in this study include high-resolution transmission electron microscopy (HRTEM), X−ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), fluorescence spectrophotometry, spectrophotometry, and Fourier transform infrared (FTIR) spectrometry.
The degree of protonation of the carbon source mPD was altered by changing the pH of the reaction's solution, which impacted the CDs' surface states. As a result, the CDs' as-prepared characteristics varied in terms of luminescence.
The mechanism analysis demonstrates that the C-N functional groups on the surface of CDs progressively decrease and the C=N functional groups gradually rise with an increase in the protonation degree of mPD, which accounts for the variable PL characteristics of CDs obtained at various pH levels.
CDs in ion sensors and PL anti-counterfeiting were made possible because of the variations in luminescence characteristics brought about by various surface states. The suggested approach will provide a practical way to produce CDs controllably with various PL features.
Hao Yi, Jing Liu, Jian Yao, Ruixing Wang, Wenying Shi and Chao Lu (2022) Photoluminescence Mechanism of Carbon Dots: Triggering Multiple Color Emissions through Controlling the Degree of Protonation. Molecules. https://www.mdpi.com/1420-3049/27/19/6517