Photoluminescence of Carbon Dots and Their Applications


Student thesis: Doctoral Thesis

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Awarding Institution
Award date24 Sep 2019


Carbon dots (CDs) as a new class of fluorescent materials exhibit low toxicity, good biocompatibility, and chemical inertness compared to semiconductor quantum dots. Thus they act as potential replacement to semiconductor quantum dots in many fields, such as optoelectronics, biomedicine, sensing, and catalysis. Fluorescent CDs have been intensively studied due to their efficient low-cost preparation approach and well developed bright fluorescent emission. Currently, the solid-state fluorescence of CDs is limited due to the aggregation-induced quenching; moreover, the unclear photoluminescence (PL) mechanism of CDs hinders their further exploration and novel applications. In this dissertation, the PL mechanism of CDs was investigated, which indicated the dominant influence from graphitization degree and surface functional groups trapping effect. Furthermore, novel strain sensor, namely flexible CD-polyurethane (CD-PU) composite film was found to overcome the aggregation-induced quenching by mechanically-driven uniaxial tension. Moreover, non-conjugated polymer dots (NCPDs) synthesized through electrochemical anodization method exhibited blue fluorescence both in the aqueous solution and solid-state powder, displaying significant potential for the nanoprobe to metal ion detection.

Chapter 1 provides an overview of the synthesis methods, PL emission mechanism, and applications of CDs.

In Chapter 2, study of fluorescent nitrogen-doped CDs obtained through pyrolysis synthesis for the PL mechanism is presented. With the increase in the pyrolysis time, the PL peak shows apparent red shift, caused by the increased conjugated sp2-domain and surface functional groups, whereas the increased degree of carbonization results in a decreased PL quantum yield (QY) due to the non-radiative relaxation channels created in the carbon structures (e.g., the interlamination of graphite). Moreover, high content of graphitic-N and pyridinic-N bestows CDs with electrocatalytic activity toward the oxygen reduction reaction (ORR) performance with the electron transfer number of 2.7, indicating 2-electron (2e-) oxygen reduction pathway.

The fluorescence of aqueous solution of CDs has been widely investigated. However, solid-state fluorescence of CDs, which is promising for optoelectronics, faces the serious problem of aggregation-induced quenching. In Chapter 3, flexible CD-PU composite films were conveniently fabricated with solid-state fluorescence, and their use as low-cost nontoxic fluorescent strain sensors is also discussed. The PL intensity and QY are responsive to the wide range of applied strain (up to 250% strain). The observed PL responses to strain are ascribed to the enlarged inter-particle distance of CDs along the tensile direction, thus pointing to a novel anisotropic anti-quenching mechanism of fluorescent CDs.

NCPDs are a type of fluorescent organic material different from CDs and possess only sub-fluorophores (such as C=O, C-N) instead of typical conjugated fluorophore groups, thus displaying fluorescence in solid state. Chapter 4 presents the facile synthesis of NCPDs by electrochemical method using citric acid and ethylenediamine as precursors at anodic voltage. NCPDs display blue fluorescence both in the aqueous solution and solid state, which can serve as the nanoprobe in metal ion sensing of Fe3+.

In summary, the PL properties of fluorescent quantum dots including CDs and NCPDs were investigated herein. The carbonization degree and surface functional groups play a vital role in determining the PL property of CDs. By investigating the PL response of novel CD-PU strain sensors simply constructed by dispersing CDs into PU, a new anti-quenching mechanism was discovered herein: PL can be controlled based on the anisotropic interplays of the fluorescence dots for overcoming the aggregation-induced quenching. Moreover, NCPDs exhibit blue fluorescence both in the aqueous solution and solid-state powder, which is attributed to sub-fluorophores such as C=O and C-N, and they can be utilized as effective nanoprobe for the detection of Fe3+.