Organic-to-Aqueous Phase Transfer of Cadmium Chalcogenide Quantum Dots using a Sulfur-Free Ligand for Enhanced Photoluminescence and Oxidative Stability

Abstract

This paper describes a procedure for transferring colloidal CdS and CdSe quantum dots (QDs) from organic solvents to water by exchanging their native hydrophobic ligands for phosphonopropionic acid (PPA) ligands, which bind to the QD surface through the phosphonate group. This method, which uses dimethylformamide as an intermediate transfer solvent, was developed in order to produce high-quality water soluble QDs with neither a sulfur-containing ligand nor a polymer encapsulation layer, both of which have disadvantages in applications of QDs to photocatalysis and biological imaging. CdS (CdSe) QDs were transferred to water with a 43% (48%) yield using PPA. The photoluminescence (PL) quantum yield for PPA-capped CdSe QDs is larger than that for QDs capped with the analogous sulfur-containing ligand, mercaptopropionic acid (MPA), by a factor of four at pH 7, and by up to a factor of 100 under basic conditions. The MPA ligands within MPA-capped QDs oxidize at E_ox ~ +1.7 V vs. SCE, whereas cyclic voltammograms of PPA-capped QDs show no discerible oxidation peaks at applied potentials up to +2.5 V vs. SCE. The PPA-capped QDs are chemically and colloidally stable for at least five days in the dark, even in the presence of O₂, and are stable when continuously illuminated for five days, when oxygen is excluded and a sacrificial reductant is present to capture photogenerated holes.

Figure 1

A) Scheme of the organic-to-aqueous ligand exchange procedure, in which the yellow shading represents the portion of the sample that contains the QDs: 1) add PPA to oleate-capped QDs in hexanes, 2) centrifuge and decant the supernatant, 3) redisperse QDs in DMF, 4) add 10 mM KOH solution in H₂O, 5) wash with CHCl₃ to remove DMF and evaporate the remaining chloroform with N₂. B) ¹H NMR spectrum of CdS QDs in C₆D₆ before addition of PPA. The spectrum contains a peak corresponding to the vinyl protons (“H_a” and “H_b”) of bound oleate at 5.67 ppm and a smaller peak corresponding to those protons on freely diffusing oleate at 5.49 ppm. All of the peaks between 1 ppm and 3 ppm correspond to the alkyl protons in oleate. C) ¹H NMR spectrum of the PPA-capped QDs in DMF-d7. The two multiplets at 1.92 ppm (“H_c”) and 2.56 ppm (“H_d”) correspond to protons of free PPA. The spectrum also contains a small feature corresponding to residual free oleate (5.42 ppm, 15.7 ± 0.5 oleates per QD) and a small broad feature, not visible on this scale, corresponding to residual bound oleate (5.55 ppm, 2.5 ± 0.4 oleates per QD). The Supporting Information contains an NMR spectrum of the supernatant obtained after step 2.

Figure 2

A) Normalized absorption spectra (solid lines, left axis) and PL spectra (dotted lines, right axis) of oleate-capped CdS QDs in hexanes (black), PPA-capped CdS QDs in water (red), and MPA-capped QDs in water (blue). The small scattering baseline in the spectrum of the PPA-capped CdS QDs is due to the formation of small QD aggregates during the phase transfer. The PL spectrum of oleate-capped CdS QDs has been multiplied by 0.025. The pH of the aqueous CdS samples is 8. B) Analogous spectra for CdSe QDs. The pH of the aqueous CdSe samples is 11. All PL spectra have been scaled by the respective absorbances of the sample at the excitation wavelength (350 nm for CdS and 500 nm for CdSe), and by the integrated fluorescence intensity of a standard (anthracene for CdS or Rhodamine B for CdSe), to convert the y-axis to quantum yield in nm^-1. C) Photographs of oleate-capped CdSe QDs in hexanes, PPA-capped CdSe QDs in DMF, and PPA-capped CdSe QDs in water, under illumination with a UV lamp.

Figure 3

CVs of films of Cd-bound PPA (dashed red), Cd-bound MPA (dashed black), PPA-capped CdS QDs (solid red), and MPA-capped CdS QDs (solid black). The solvent background is in blue. All measurements were done with 0.1 M NBu₄PF₆ as the supporting electrolyte, at a scan rate of 0.1 V/s, with a glassy carbon working electrode, a Pt wire counter electrode and Ag wire reference electrode. CVs were shifted to the SCE scale using Fc/Fc⁺ (= 0.342 V vs. SCE) as a standard, see the Supporting Information. The black arrows show the scan direction used for all samples.

Figure 4

A, B) Absorbance at the energy of the first excitonic peak as a function of time over 5 days, scaled by its value at time zero (A₀), for PPA-capped CdS (A) and CdSe (B) QDs. The QDs were stored in room light under Ar or air, and in the prescence or absence of 50 mM TEAO as sacrificial reductant, as labeled. C,D) FWHM of the first excitonic peak in the absorbance spectra of the same samples of PPA-capped CdS (C) and CdSe (D) QDs under the same set of conditions as in A,B. E,F) PL spectra, acquired over 5 days, of the same samples of PPA-capped CdS (E, excited at 350 nm) and CdSe (F, excited at 450 nm) stored under Ar with 50 mM TEOA. The quantities in A-D were obtained by fitting the absorbance spectrum with a set of Gaussian functions, see the Supporting Information, Figure S10-S17. The PL spectra are scaled by their absorbances at the excitation wavelength, relative to that at 0 h.