Designing Heteroatom Doped Carbon Dots for Sensing Applications

Abstract

In CHAPTER 1, introduced the production, development, and molecular recognition mechanisms of small molecule fluorescent probes, reviewed by the research frontier, present the basic design ideas and research methods of this paper. In CHAPTER 2, nitrogen and boron-incorporated carbon dots (NBCDs) were fabricated using a hydrothermal process with ascorbic acid (AA) and 4-aminobenzenebornic acid hydrochloride as novel precursors. The size and morphology were examined using the appropriate spectrochemical characterization. The NBCDs emission was examined in the presence of different metal ions. The fluorescence was quenched selectively in the presence of the Fe3+ ions and with increasing Fe3+ concentration in the reaction medium. From this fluorescence “Turn Off” phenomenon, Fe3+ could be detected in aqueous solution with a limit of detection (LOD) of 7.50 M. This quenched fluorescence was recovered in the presence of AA selectively. This “Turn On” phenomenon was AA concentration dependent and AA could be detected in aqueous solution with a LOD value of 7.72 M. Analysis of a real sample was conducted for the detection of Fe3+ ions in solution, which was collected from iron sheet reacted over the pH range of 27 for set times. In CHAPTER 3, the water-soluble N-doped carbon dots (NCDs) are designed by simple hydrothermal treatment of aqueous mixture of citric acid and urea. The initially prepared NCDs are further been passivated by L-glutathione (GSH) through carbodiimide-activated coupling reaction to enhance the emissive properties of the NCDs. The as-obtained passivated carbon dots (GSH-NCDs) show intense blue emission at 440 nm when excited at 355 nm. However, this emission is observed to be quenched selectively in the presence of dopamine (DA) leading to design a platform for DA detection. The quenching of fluorescence of GSH-NCDs is dependent on DA concentration. The limit of detection is 1.01 nM, with a linear detection ranges from 20 nM to 10 M (R2 = 0.992). The average lifetime values and the spectral profile of GSH-NCDs in absence and presence of DA indicate the possibility of photoinduced electron transfer assisted inner filter effect mechanism for the DA induced quenching of GSH-NCDs’ fluorescence. Further experiments show that the as-designed DA detection technique works successfully for biological fluids. In CHAPTER 4, we report the green, blue, and orange color emitting N-doped carbon dots (CDs), which are being synthesized from ascorbic acid (AA) and o-/m-/p-phenylenediamine (o-PDA, m-PDA, and p-PDA, respectively). The effects of the solvent polarity and solution pH on the PL emission properties of the as-synthesized CDs have been systematically investigated. It has been observed that the PL emission of the as-synthesized CDs decreases with the increase in solvent polarity due to the greater agglomeration. The surface charge of CDs also shows prominent effects on the pH-dependent PL emission properties. In CHAPTER 5, high quantum yield 3-aminophenylboronic acid-functionalized nitrogen-doped carbon dots (GAAP-CDs) were fabricated using a simple hydrothermal route and used as a sensing probe for toxic hypochlorite (ClO). The as-synthesized GAAP-CDs showed absorption peaks at 252, 297, and 370 nm and an emission peak at 375 nm with an excitation wavelength of 310 nm. The quantum yield of GAAP-CDs reached 58.28%, with no noticeable fluorescence change observed under high ionic strength conditions and a three-month long-term test. GAAP-CDs-based ClO sensing was carried out by UVvis absorbance and fluorescence spectroscopy; the detection limit was as low as 0.77 M (linear range of 0100 M), and 0.50 M (linear range of 0.1100 M), respectively. In addition, the as-synthesized GAAP-CDs showed excellent selectivity towards ClO ions in the presence of various interfering chemicals. The satisfactory results from the proposed method of ClO detection in tap water and drinking water samples, suggesting promising application of GAAP-CDs for ClO detection. In CHAPTER 6, the low-cost and abundant nitrogen and boron-doped high quantum yield carbon dots (CPAP-CDs) were successfully synthesized through the facile hydrothermal process. The obtained CPAP-CDs showed wide absorption and strong fluorescence, the quantum yield can be achieved to 64.07%. The obtained carbon dots showed pH-dependent behavior and fluorescent response for hazardous anions nitrite in solutions. The fluorescence of CPAP-CDs was quenched in the presence of nitrite anion. The detection limit was 6.6 nM with a linear detection range of 2 M1 mM. It is speculated that a diazotization between nitrites and CPAP-CDs, which induces photoinduced electron transfer process from the excited state of CPAP-CDs to the diazo compounds on the surface, quenches the inherent fluorescence of the as-synthesized carbon dots. The sensitivity was analyzed with different water samples, such as tap, drinking and sea water. In CHAPTER 7, summary.