Room temperature synthesis of PbSe quantum dots in aqueous solution: stabilization by interactions with ligands

Abstract

An aqueous route of synthesis is described for rapid synthesis of lead selenide quantum dots (PbSe QDs) at room temperature in an attempt to produce water-soluble and stable nanocrystals. Several thiol-ligands, including thioglycolic acid (TGA), thioglycerol (TGC), 3-mercaptopropionic acid (MPA), 2-mercaptoethyleamine hydrochloride (MEA), 6-mercaptohexanoic acid (MHA), and L-cysteine (L-cys), were used for capping/stabilization of PbSe QDs. The effects of the ligands on the stability of PbSe QDs were evaluated for a period of two months at room temperature under normal light conditions and at 4 °C in dark. The TGA- and MEA-capped QDs exhibited the highest stability prior to purification, almost two months when kept in dark at 4 °C. However, the stability of TGA-capped QDs was reduced substantially after purification to about 5 days under same conditions, while MEA-capped QDs did not show any significant instability. The stabilization energies of Pb-thiolate complexes determined by theoretical DFT simulations supported the experimental results. The PbSe QDs capped with TGA, MPA and MEA were successfully purified and re-dispersed in water, while those stabilized with TGC, MHA and L-cys aggregated during purification attempts. The purified PbSe QDs possess very susceptible surface resulting in poor stability for about 30 – 45 min after re-dispersion in water. In the presence of an excess of free ligand, the stability increased up to 5 days for TGA-capped QDs at pH 7.19, 9 –12 days for MPA-capped QDs at pH 7.3–7.5 and 45–47 days for MEA-capped QDs at pH 7.35. X-Ray Diffraction (XRD) results showed that the QDs possess a cubic rock salt structure with the most intense peaks located at 2θ = 25.3° (200) and 2θ = 29.2° (100). TEM images showed that the size of the QDs ranges between 5 and 10 nm. ICP-MS results revealed that Pb:Se ratio was 1.26, 1.28, 3.85, 1.18, and 1.31 for the QDs capped with TGA, MPA, MEA, L-Cys, and TGC, respectively. The proposed method is inexpensive, simple and utilizes environmentally friendly chemicals and solvents.

Cytocompatibility of direct water synthesized cadmium selenide quantum dots in colo-205 cells

“…The moieties detection is a clear indicator of the successful deprotonation process of the carboxylic acid in TGA. The presence of these bands in the FT-IR spectra confirms the existence of carboxylate moieties on the QDs surface, as reported in previous studies on the absorption of carboxylic acids on metal surfaces (Primera-Pedrozo et al 2012). It was also observed a low-frequency vibrational mode located at 669 cm −1 , which was assigned to the C–S stretching mode, further confirming the TGA bonding onto the QDs surface.…”

“…However, the spectra of the QDs show that such vibrational modes vanish when TGA-capped CdSe regardless of the Se(IV) volume were used, indicating the successful formation of S–Cd bonds between the ligand and QDs (Jiang and Ju 2007). It was also observed that the characteristic mode of the C=O functional group of TGA was shifted from ~1700 cm −1 to 1560 cm −1 for both suspensions (Primera-Pedrozo et al 2012). No strong OH − vibrations were detected.…”

“…The synthesis was performed following our previous synthetic methodology (Arslan et al 2011; Primera-Pedrozo et al 2012) with some modifications. Briefly, the glassware was roughly cleaned with aqua regia for 12 h and then rinsed thoroughly with ultra-high pure water (dH 2 O).…”

“…Worse yet, it has been reported that such ions tend to be accumulated into the nervous system, kidneys, and specifically in the liver (Arslan et al 2011). To tackle this drawback, some alternative stabilizing surface coating approaches to both substantially reduce the leeching of Cd 2+ ions and effectively protect the core materials from oxidation, as well as new synthetic procedures able to control the [Metal:Se:ligand] ratios without causing any optical detriment, have been proposed (Arslan et al 2011; Primera-Pedrozo et al 2012; Xue et al 2011). CdSe QDs can be in fact capped with different ligands or biomolecules, such as glutathione (GSH) or thioglycolic acid (TGA), thus relatively reducing their cytotoxicity (Aldeek et al 2008; Gaponik et al 2002).…”

Synthesis of Air-stable PbSe Quantum Dots Using PbCl2-oleylamine System

“…Either diphenylphosphine (DPP) or 1,2-hexadecanediol (HDD) was used to increase the yield and quality of the PbSe QDs. Also, alternative attempts to use phosphine free reagents such as selenium-octadecylamine as Se-precursor, [22] alkyl selenide and acidic thioligands--the 2-mercaptoethylamine hydrochloride as capping ligand and diffusion controlled lead halide using bis(trimethylsilyl) selenide (TMS 2 Se) in OLA have been reported with short term air stability [23][24]. Alkylselenide as capping ligand either to prepare PbSe QDs during synthesis or to replace the oleate ligands at the post reaction stage were reported.…”