CdTe/CdS QUANTUM DOTS AND DIFFERENT TYPES OF CARBON DOTS AS PHOTOLUMINESCENT PROBES FOR SENSING APPLICATIONS

Abstract

The present thesis work is based on developing CdTe/CdS quantum dots and non-graphitic nitrogen doped carbon dots as photoluminescent probes for sensing selected metal ions, e.g., Cu2+, Ni2+ and chromates; substituted phenols, e.g., 4-nitrophenol and some biomolecules, e.g., dopamine, epinephrine and norepinephrine. The research objectives were chosen on the basis of gaps identified in rigorous literature search. Emphasis has been to develop novel methodologies in synthesis and sensing application. Unlike most literature, we have developed a novel method of synthesizing CdTe/CdS core-shell quantum dots by refluxing the precursor at 100 oC. No inert medium was required for synthesis. The sizes varied between 2.8±0.8 nm to 3.9±1.1 nm, which depended on the refluxing time. Exogenous sulphide was not used. Instead, reduced glutathione (GSH) was used as in-situ sulphide source as well as capping agent. The thoroughly characterized QDs exhibited size dependent photoluminescence property and the photoluminescent quantum yield was drastically improved due to core-shell structure. The photoluminescence (PL) of the batches of CdTe/CdS QD synthesized at different refluxing time was drastically quenched selectively by Cu2+ ion. Systematic analytical studies based on Stern-Volmer plot suggested that the batch of CdTe/CdS QD synthesized during 30 min corresponded to the best limit of detection (3.0 ppb). The mechanism for Cu2+ detection is discussed in the light of structural changes in the QDs and by electron transfer phenomenon, as reflected from steady state and PL decay kinetic results. Furthermore, the relative change in the PL lifetime of CdTe/CdS QDs i.e., (avg)probe/(avg)probe+metal ion has been developed as an analytic signal for detecting Ni2+. In this method, Ni2+ could be detected while Cu2+ could not be detected, owing to the differences in the detection mechanism. The Ni2+ detection was accounted by collision-based electron transfer phenomenon, while Cu2+ detection was complex in nature which is based on both electron transfer and ion binding effect. The best result was obtained for the batch of CdTe/CdS QDs synthesized during 60 min refluxing time, which also exhibited maximum photoluminescence quantum yield (45.8%). Next, we have developed some high ordered non-graphitic carbon dots doped with nitrogen containing groups. One such batch was synthesized via oligomerization of ethylene glycol by heating with β-alanine at 170 oC for 30 min. It was thoroughly characterized by arrays of characterization techniques, which collectively suggested oligomerization of β-alanine doped ethylene glycol chains, systematically self-assembled via hydrogen bonding. The highly ordered photoluminescent carbon dot with PLQY= 14.3%, exhibited selective response towards detection of Cr(VI) species at different pH and 4-nitrophenol via PL quenching. The PL quenching mechanism has been attributed to static quenching and inner filter effect. The developed method was validated and robustness of the detection of Cr(VI) species and 4-nitrophenol in spiked real water sample has been demonstrated. Another batch of nitrogen doped carbon dots in non-aqueous medium were synthesized via heating alanine anhydride with ethylene glycol at 170 oC for 120 min. The as synthesized carbon dots were quenched by MnO4 ̶ and the quenched PL was restored selectively by dopamine, norepinephrine and epinephrine. Practicability of the detection strategy by our method was assessed by analyzing spiked real samples, e.g., river water, tap water and waste water for MnO4- and human urine sample for dopamine, norepinephrine and epinephrine. Again, PL property of nitrogen doped carbon dots was enhanced by functionalized with CdS QDs. The material was synthesized by one-pot approach where the precursors e.g., ethylene glycol, L-cysteine and cadmium chloride solution was refluxed together at 170 oC for 2 h. The PL intensity and the PLQY of the probe was significantly enhanced when it is left in the mother liquor for aging and the maximum PL intensity was recorded after 50 days of aging. This photoluminescent material was used as probe for selective detection of tetracycline hydrochloride (TC) via PL quenching. Practicality of the detection was assessed by analyzing spiked real samples, e.g., river water, tap water and human urine sample for TC. The thesis work has been summarized and possible future studies are identified.