Journal of the Korean Magnetic Resonance Society (한국자기공명학회논문지)

Korean Magnetic Resonance Society (한국자기공명학회)
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NMR analysis of organic ligands on quantum-dots

  • Received : 2019.06.19
  • Accepted : 2019.06.20
  • Published : 2019.06.20
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Abstract

Quantum dot (QD) is an emerging novel nanomaterial that has wide applicability and superior functionality with relatively low cost. Nuclear magnetic resonance (NMR) spectroscopy has been contributed to elucidate various features of QDs and to improve their overall performance. In particular, NMR spectroscopy becomes an essential analytical tool to monitor and analyze organic ligands on the QD surface. In the present mini-review, application of NMR spectroscopy as a superb methodology to appreciate organic ligands is discussed. In addition, it was recently noted that ligands exert rather greater influence on diverse features of QDs than our initial anticipation, for which contribution of NMR spectroscopy is briefly reviewed.

Keywords

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Figure 1. Structure of ligands on PbS QDs. (A) The simulated atomic structure of 5-nm diameter PbS QD with oleic acid as passivating ligands. (B) Detailed view showing interaction between oleic acid (upper layer; C: small black, H: teal, O: red) and the surface (bottom layer; Pb: black, S: yellow) of PbS nanocrystals. Adapted with permission from Boles et al.4

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Figure 2. QD-bound ligand can be studied with 1H NMR. (A) 1D 1H NMR spectra of QD-bound octylamine (the upper black spectrum) and of its free form (the lower red spectrum) are shown. The structure of octylamine is shown in the upper left side of the panel. Signals assigned to the certain nuclei at the octylamine structure are marked with their designated numbers. Signals originating from CDCl3 and methanol are marked with ‡ and ◆, respectively. (B) Typical 2D NOESY spectrum of QD-bound octylamine. (C) 2D DOSY spectrum of QD-bound (blue) and free (red) octylamine. Adapted with permission from Hassinen et al.10 Copyright (2010) American Chemical Society.

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Figure 3. Ligand exchange can be directly monitored with NMR spectroscopy. (A) NMR spectroscopic data proved that oleic acid (blue) on CdSe QDs can be exchanged bidirectionally with the ligand having a carboxylic acid group at its end, while, upon being replaced by the ligand having phosphonic acid or thiol group at its end, oleic acid cannot replace it back. (B) The 1H NMR spectra of a series of samples having different ratios of ligands are taken to estimate bound vs. free ligand population and follow ligand exchange processes. The relative ratios of undec-10-enoic acid (UDA) to the amount of QD are shown on the upper left side. The original ligands of QD was oleic acid (OA). Adapted with permission from Knauf et al.14 Copyright (2016) American Chemical Society.

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