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functionalization of GQDs would stimulate further investigations for their applications in biomedical and optoelectronics fields in a larger scale.
The highlights of the present thesis are presented below.
A. Tuning of the photoluminescence intensity of GQDs through the functionalization with SWCNTs
Undoped GQDs with high PL quantum yield are prepared by a solvothermal approach. We demonstrated a new approach to tune of PL intensity of undoped GQDs (U-GQDs) through the functionalization with SWCNTs over a concentration range of 2–60 g/mL. We reported an anomalous quenching behavior of U-GQDs in the presence of SWCNTs. In the very low concentration region (SWCNTs: 2–8 g/mL), PL intensity of U-GQDs is observed to be enhanced systematically, while a systematic quenching of PL intensity of U-GQDs is monitored with the higher concentration of SWCNTs (≥10 g/mL), following a non-linear Stern-Volmer equation.
The enhancement of PL intensity of U-GQDs in the low concentration region is governed by the dominating metallic nature of SWCNTs at the low concentrations, which increases the incident local field on the U-GQDs by the plasmonic effect. On the other hand, faster than exponential quenching in the high concentration region is attributed to the combined effect of ground state composite formation and excited-state charge transfer from fluorescent U-GQDs to SWCNTs due to the dominating semiconducting nature of SWCNTs when they are bundled. This work has been published in “Phys. Chem. Chem. Phys. 20 (2018), 4527-4537”.
B. Elucidating the origin of high photoluminescence quantum yield in N-GQDs and their applications as SESR sensor and white light convertor
We have presented a comparative study of the structural and optical features of various types of GQDs synthesized in water, DMF, and DMSO medium by a top-down approach with GO as the precursor. We proposed that in DMF and DMSO medium, N-GQDs and S-GQDs are formed mainly with the nucleophilic reaction, while in water the GO sheet is cut into pieces due to the strain caused by the epoxy groups on the basal planes. Among different types of GQDs, N-GQDs show the highest PL QY (~34%) due to the reduction of non-radiative sites at the time of solvent reaction and the presence of electron-donating N atoms. In contrast, the as-synthesized S-GQDs do not yield high PL intensity primarily due to the presence of electron-withdrawing S=O
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functional groups, which act as a non-radiative trap center. Next, we demonstrated N-GQDs as a very efficient SERS substrate with a chemical enhancement factor (EF) of ~3.2103 at 1648 cm-1 in the presence of 10-4 M RhB as a target molecule under 488 nm laser excitation, which is the highest among the reported values, and consequently, this SERS substrate is able to detect RhB as low as 0.1 nM. For the first time, the individual contributions of interaction and FRET process in the SERS enhancement are evaluated by comparing with different target molecules, laser excitation wavelengths, and controlling the functional groups of N-GQDs through vacuum annealing. Besides, the highly fluorescent N-GQDs are successfully implemented in developing a liquid phase white LED with the help of a low-cost UV LED and RhB solution. This work has been published in “Carbon 160 (2020), 273-286”.
C. Ultrasensitive Dopamine sensing in human serum by GO/WS2 hybrid
We have explored a unique hybrid system of GO and WS2 QDs for the ultrasensitive and selective fluorometric detection of DA as low as 10 pM in a basic medium, which is lowest among the reported values. In the GO/WS2 hybrid, the interaction of WS2 QDs with GO via van der Waals interaction, and the defect states/functional groups lead to the excited state charge transfer from fluorescent WS2 QDs to GO. With the addition of DA in GO/WS2, DA is first adsorbed on the GO surface due to the strong interaction, enabling the easy charge transfer from GO to DA.
Additionally, in the basic medium, the conversion of DA to DQ (a strong electron acceptor) promotes the efficient electron transfer from GO to DA, and as a result, the PL intensity of WS2
QDs quenches more rapidly in the presence of DA. To explain the nature of PL quenching in a wide concentration range, here we have developed a modified model by considering the combined effect of surface adsorption in the ground-state complex formation following the Freundlich isotherm and excited-state charge transfer. From a comparative study, we have also experimentally elucidated that compared to GQD based hybrid systems, GO/WS2 is highly efficient for the DA sensing due to a higher possibility of interaction with DA and a high charge transporting ability of GO. Furthermore, the proposed fluorescence-based sensor is successfully implemented as an efficient DA sensor spiked in the Brahmaputra river water and the human serum samples.
This work has been published in “J. Mater. Chem. C 8 (2020),7935-7946”.
D. Dual-mode dopamine sensing with Au@N-GQDs by the unique core-shell structure formation
We presented in-situ green synthesis of Au@N-GQDs hybrid and it is demonstrated as a dual- mode sensor of DA in the human serum through the formation of a unique core-shell structure with DA as a shell. In the presence of DA, the enhancement of UV-Vis absorption intensity and the quenching of PL intensity of Au@N-GQDs have been used for the colorimetric and fluorometric detection of DA, respectively, in the range of 0.04–100.0 M. Primarily electrostatic interaction and - stacking between N-GQDs and DA facilitate the N-GQDs/DA ground state complex formation, and the presence of Au NPs accelerates this ground-state complex formation by making a core-shell structure with phenoxide-enolate. It is also highlighted that the core-shell structure helps in higher quenching of PL intensity of Au@N-GQDs through electron transfer.
This work demonstrates the efficacy of Au@N-GQDs as a more efficient DA sensor than the bare N-GQDs. This work has been published in “Appl. Surf. Sci. 490 (2019),318-330”.
E. Implementation of Au@N-GQDs as a metal ion (Fe3+) sensor and a high-speed photodetector
Besides a biomolecule sensor, we have also established Au@N-GQDs as an efficient metal ion sensor as well as fast photodetector without using any charge transporting layer. Au@N-GQDs are implemented as a label-free sensor of Fe3+ ions with ultra-high sensitivity (< 1 nM) and selectivity by exploring the PL quenching of Au@N-GQDs in presence of Fe3+ ions. The quenching behavior does not follow the known laws of quenching and the unusual quenching of the PL intensity of Au@N-GQDs in the presence of Fe3+ ions is modeled by solving the analytical rate equations, incorporating Langmuir's law of adsorption and non-radiative charge transfer to the acceptor ions, for the first time. Further, the newly developed sensor is successfully executed for the detection of spiked Fe3+ ions in human serum and real water samples, including Brahmaputra river water with satisfactory recovery.
Next, we have demonstrated the Au@N-GQDs film based planar photodetector with fast photoresponse and high photoresponsivity. With the incorporation of plasmonic Au NPs in the Au@N-GQDs system, Au NPs generate hot electrons and transfer these to N-GQDs to achieve high photocurrent. Additionally, through the attachment of the Au NPs on the basal plane of N- GQDs, the reduction of the defect states along with the modification of the edge functional groups