FUNCTIONALIZED PHOTOLUMINESCENT CARBON DOTS AND THEIR NANOCOMPOSITES WITH METAL OXIDE NANOPARTICLES: APPLICATIONS IN SENSING AND PHOTOCATALYSIS

Abstract

The thesis comprises of 7 chapters and it deals with developing suitable photoluminescence (PL) probes for detection of potential water pollutants and to develop efficient photocatalyst for sunlight driven photocatalytic degradation of emerging contaminants. On a broader perspective, the goal of the thesis is to develop a sustainable approach to ensure clean water for improving the quality of life in the earth. The thesis comprises of three main verticals, (a) discussing the fundamental concepts involved in carbon dot based PL probe for detection of pollutants and the semiconductor metal oxides as photocatalyst for degradation of the pollutants; (b) literature review in the above areas to identify the research gap and research objectives; and (c) developing suitable functionalized carbon dots as PL probe and p-type semiconductor based metal oxide nanocomposites with some of these functionalized carbon dots as solar photocatalyst for degradation of some selected water pollutants. In view of this, two batches of functionalized carbon dots are prepared using different carbon precursors, e.g., (i) sodium citrate, and (ii) 1,4 Butanediol, and both are functionalized by siloxane group. The first batch is prepared by hydrothermal heating of sodium citrate and (3-Aminopropyl)triethoxysilane (APTES) in an autoclave at 170 ºC for 12 h. The second batch is prepared by solvothermal method by heating 1,4-Butanediol and APTES in an autoclave at 170 ◦C for 12 h. These two batches are named as N-SiO2@CD1 and N-SiO2@CD2 respectively. The TEM studies revealed formation of network structure of polysiloxane with carbon dots of sizes less than 10 nm in them. Both exhibited bluish PL corresponding to emission wavelength of λ= 440 nm and 443 nm, respectively. The PL quantum yield of N-SiO2@CD1 is 23.1% and N-SiO2@CD2 is 27.0% measured against quinine sulphate. The obtained PLQY for both the batches are found to be higher than several of the nitrogen doped carbon dots reported in literature. Furthermore, the PLQY of the batch prepared with butanediol and APTES by solvothermal method is 10% higher than the batch prepared by hydrothermal method using sodium citrate and APTES. The chemical composition of both the batches has been studied by Fourier transformed infrared spectroscopy (FT-IR) and by X-ray photoelectron spectroscopy (XPS). Both the batches of N-SiO2@CD1 and N-SiO2@CD2 exhibited quantitative PL quenching by Cr(VI) and satisfied the Stern-Volmer relation. The linear region of PL quenching is found to be at least in the range of 1-75 μM. The limit of detection (LOD) of Cr(VI) by N-SiO2@CD1 and N-SiO2@CD2 are determined as 0.78 μM and 0.20 μM, respectively. Notably, the batch that exhibited higher PLQY, correspond to lesser LOD. In addition to Cr(VI), the better performing PL probe, i.e., N-SiO2@CD2 also exhibited linear PL quenching when treated with another water pollutant, e.g., 4-nitrophenol (4-NP), in the region 0.3 and 200 µM. The LOD for 4-NP detection is found to be much better (i.e., 0.07 µM) than iii that of Cr(VI). The PL quenching mechanism has been attributed to ground state complexation. The detection of Cr(VI) and 4-NP are not interfered by the common cations and anions that are present in water. The detection of 4-NP is mildly interfered by structural analogues of other nitrophenols. The feasibility of using N-SiO2@CD1 and N-SiO2@CD2 as probe for detecting Cr(VI) and 4-NP has been tested in real samples by estimating the recovery concentration analysis of spiked concentration in the real water samples. Furthermore, N-SiO2@CD2 has been used for imaging the lung cancer cells (A549 cells).