Synthesis of Cd-free InP/ZnS Quantum Dots Suitable for Biomedical Applications

Summary

In this protocol, the synthesis of Cd-free InP/ZnS quantum dots (QDs) is detailed. InP-based QDs are gaining popularity due to the toxicity of Cd²⁺ ions that may be released through nanoparticle degradation. After synthesis, QDs are solubilized in water using an amphiphilic polymer for use in biomedical applications.

Abstract

Fluorescent nanocrystals, specifically quantum dots, have been a useful tool for many biomedical applications. For successful use in biological systems, quantum dots should be highly fluorescent and small/monodisperse in size. While commonly used cadmium-based quantum dots possess these qualities, they are potentially toxic due to the possible release of Cd²⁺ ions through nanoparticle degradation. Indium-based quantum dots, specifically InP/ZnS, have recently been explored as a viable alternative to cadmium-based quantum dots due to their relatively similar fluorescence characteristics and size. The synthesis presented here uses standard hot-injection techniques for effective nanoparticle growth; however, nanoparticle properties such as size, emission wavelength, and emission intensity can drastically change due to small changes in the reaction conditions. Therefore, reaction conditions such temperature, reaction duration, and precursor concentration should be maintained precisely to yield reproducible products. Because quantum dots are not inherently soluble in aqueous solutions, they must also undergo surface modification to impart solubility in water. In this protocol, an amphiphilic polymer is used to interact with both hydrophobic ligands on the quantum dot surface and bulk solvent water molecules. Here, a detailed protocol is provided for the synthesis of highly fluorescent InP/ZnS quantum dots that are suitable for use in biomedical applications.

Introduction

Quantum dots (QDs) are semiconducting nanocrystals that exhibit fluorescent properties when irradiated with light¹. Due to their small size (2-5 nm), which is similar to many larger biomolecules, and ease of biofunctionalization, QDs are an extremely attractive tool for biomedical applications. They have found use in biological labeling, single-molecule live-cell imaging, drug delivery, in vivo imaging, pathogen detection, and cell tracking, among many other uses^2-8.

Cd-based QDs have been most commonly used in biomedical applications because of their intense fluorescence and narrow emission peak widths^9<....

Protocol

1. Synthesis of Indium Phosphide/Zinc Sulfide (InP/ZnS) Quantum Dots

1. Synthesis of Indium Phosphide (InP) Quantum Dot Cores
   1. Fit a 100 ml round bottom, 3-neck, flask with a 12-inch condenser. Add 30 ml oleylamine (OLA), 0.398 g indium (III) chloride (InCl₃), 0.245 g zinc (II) chloride (ZnCl₂) and stir while evacuating at RT using a vacuum for 1 hr. The solution should appear colorless with a white precipitate.
   2. Using a heating mantle with a thermocouple and proportional-integral-derivative (PID) temperature controller, increase the temperature of the solution to 120 °C. Evacuate the solution u....

Results

The uncoated InP cores do not demonstrate substantial visible fluorescence by eye. However, InP/ZnS (core/shell) quantum dots appear to fluoresce brightly by eye under UV irradiation. The fluorescence of InP/ZnS QDs was characterized using fluorescence spectroscopy. The fluorescence spectrum of QDs in hexanes (Figure 1) excited at 533 nm demonstrates one major peak centered at 600 nm with a full width at half maximum (FWHM) of 73 nm. While absorbance (0.2) offset in

Discussion

This protocol details the synthesis of highly fluorescent InP/ZnS QDs that can be used in many biological systems. The QD products synthesized here exhibited a single fluorescence emission peak centered at 600 nm with a FWHM of 73 nm (Figure 1), which is comparable to other previously described syntheses¹². Reaction time and reaction temperature are extremely crucial steps due to their profound effect on QD synthesis quality and repeatability. After solubilization in water, the QDs were determ.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Oleylamine	Acros	129540010
Zinc(II) chloride	Sigma	030-003-00-2
Indium(III) chloride	Chem-Impex	24560
Tris(dimethylamino)phosphine	Encompass	50-901-10500
1-dodecanethiol	Acros	117625000
Hexanes	Fisher Sci	H292-4
Acetone	TransChemical	UN 1090
Zinc Stearate	Aldrich Chem	307564-1KG
Tetrahydrofuran	Acros	34845-0010
Molecular Water	Fisher Sci	BP2470-1
Poly(maleic anhyrdride-alt-1-tetradecene), 3-(dimethylamino)-1-propylamine derivative	Sigma	90771-1G
Boric acid	Fisher Sci	BP168-500
Sodium Tetraborate Decahydrate	Fisher Sci	BP175-500
Rhodamine B	Aldrich Chem	R95-3
Nitrogen gas	Airgas	UN1066
Trypan blue	Thermo Sci	SV30084.01
3 ml plastic Luer-lock syringe	BD	309657
Luer-lock Needle	Air-Tite	8300014471	4 inch, 22 gauge
50 ml polypropyene centrifuge tube	Falcon	352098
250 ml centrifuge bottle	Thermo Sci	05-562-23	Nalgene PPCO
5 ml centrifuge tubes	Argos-Tech	T2076
1.5 ml microcentrifuge tubes	Bio Plas	4150
0.1 μm Syringe filter	Whatman	6786-1301	Puradisc 13 mm nylon filter
Slide-A-Lyzer MINI Dialysis Unit	Thermo Sci	69590	20,000 MWCO
Rotary Evaporator	Heidolph
Centrifuge 5072	Eppendorf		Swinging Bucket with 50 ml tube adapters
Lambda 650 UV/VIS Spectrometer	Perkin Elmer		UV-Vis Spectrophotometer
LS 55 Fluorescence Spectrometer	Perkin Elmer		Fluorometer
Axio Observer.A1	Zeiss		epifluorescence microscope
AxioCam MRm	Zeiss		CCD Camera
Tecnai TF20 Microscope	FEI		Transmisison Electron Miscroscope
TEM Eagle CCD	FEI		TEM CCD Camera
NanoBrook Omni DLS	Brookhaven		Dynamic Light Scattering Instrument