Water-Based Route to Ligand-Selective Synthesis of ZnSe and Cd-Doped ZnSe Quantum Dots with Tunable Ultraviolet A to Blue Photoluminescence

Abstract

A water-based route has been demonstrated for synthesizing ZnSe and Cd-doped ZnSe (Zn(x)Cd(1-x)Se, 0 < x < 1) quantum dots (QDs) that have tunable and narrow photoluminescence (PL) peaks from the ultraviolet A (UVA) to the blue range (350-490 nm) with full-width at half-maximum (fwhm) values of 24-36 nm. Hydrazine (N(2)H(4)) was used to maintain oxygen-free conditions, allowing the reaction vessel to be open to air. The properties of the QDs were controlled using the thiol ligands, 3-mercaptopropionic acid (MPA), thiolglycolic acid (TGA), and l-glutathione (GSH). On the basis of optical spectra, linear three-carbon MPA attenuated nucleation and growth, yielding small ZnSe QDs with a high density of surface defects. In contrast, TGA and GSH produced larger ZnSe QDs with lower surface defect densities. The absorption spectra show that growth was more uniform and better controlled with linear two-carbon TGA than branched bifunctional GSH. After 20 min of growth TGA-capped ZnSe had an average diameter of 2.5 nm based on high-resolution transmission electron microscopy images; these nanocrystals had an absorbance peak maximum of approximately 340 nm (3.65 eV) and a band gap PL emission peak at 372 nm (3.34 eV). Highly fluorescent Zn(x)Cd(1-x)Se QDs were fabricated by adding a Cd-thiol complex directly to ZnSe QD solutions; PL peaks were tuned in the blue range (400-490 nm) by changing the Zn to Cd ratio. The Cd-bearing nanocrystals contained proportionally more Se based on X-ray photoelectron spectroscopy, and Cd-Se bonds had ionic character, in contrast to primarily covalent Zn-Se bonds.

Facile synthesis of high-quality water-soluble N-acetyl-l-cysteine-capped Zn1−xCdxSe/ZnS core/shell quantum dots emitting in the violet–green spectral range

“…In 2007, Ying reported the aqueous synthesis of glutathionecapped Zn 1Àx Cd x Se QDs with a maximum QY of 50% via a two-step synthetic method, which posses the efficient emission from 370 to 500 nm [21]. Subsequently, the similar aqueous approaches were further developed by Chiang and Muscat [22,23]. These results represented the best optical quality of the violet-blue fluorescent QDs in aqueous media.…”

“…Compared with the previous works, our synthetic route was greener and simpler: (1) the ligand NAC chosen is a derivative of L-cysteine that is inodorous, inexpensive, stable and environmentally friendly. So the resulting NAC-capped QDs should have excellent biocompatibility; (2) the favorable PL quantum yield of QDs are obtained without using microwave (high temperature) and UV or white light irradiation [18][19][20]22]; (3) more importantly, Cd 2+ ions used for tuning the PL emission peak of Zn 1Àx Cd x Se QDs are less and loaded in the inner core of the QDs, leading to the less release of toxic Cd 2+ ions and lower cytotoxicity, as compared to the Zn 1Àx Cd x Se QDs obtained via a two-step synthetic approach [21,23].…”

UV-enhanced toxicity of water-stable quantum dots in human pancreatic carcinoma cells

“…This minor emission peak at 485 nm would suggest that MPA-capped QDs would host more structural defects than the TGA-capped ones. Related reports have evidenced that ligands with shorter chains, such as TGA, could promote fewer structural flaws on the surface of the QDs [23].…”

Synthesis of Glutathione Coated Quantum Dots

“…Deng et al examined two other thiol ligands beside GSH, MPA and TGA, for stabilization of ZnSe and Zn x Cd 1-x Se QDs synthesized by water-based route (Deng et al 2009). A typical synthetic procedure for ZnSe QDs started with mixing Zn(NO 3 ) 2 , thiol molecule and N 2 H 4 (hydrazine), which was used to maintain oxygen-free conditions, allowing the reaction vessel to be open to air.…”