High Resolution Imaging of Quantum Dots

Jared Fialkoff

Optics 307/407

B.S Chemical Engineering '15

University of Rochester

Abstract:

Quantum dots are fluorescing nanoparticle produced from various heavy metals, which range from 2-8 nm in diameter. These nanoparticles are used throughout biomedical research for their long-term photostability, and narrow emission spectra making them ideal labeling and imaging makers. For the purpose of this study, the Quantum Dots of interest are Cadmium/Selenium (Cd/Se) cored Quantum Dots, which are known for their long-term stability. Originally synthesized with a hydrophobic coating, one must prepare Quantum Dots for biological applications by coating the Quantum Dot with a ligand, making the Quantum Dot hydrophilic. The chemical technique to perform this coating is called ligand exchange, where the ligand of interest, usually containing a thiol group, exchanges with the outer zinc shell of the quantum dot, yielding a nanoparticle with different properties. Besides making the quantum dot hydrophilic the coating contributes significantly to surface charge which effects particle aggregation/agglomeration. In this study Quantum Dots coated with Glutathione (GSH) were imaged to visualize and to quantify agglomeration/aggregation.


Sample Preparation:

Gluthathione Quantum Dots (GSH-QD henceforth) were synthesized following the protocol introduced in “Thiol antioxidant-functionalized CdSe/ZnS quantum dots: Synthesis, Characterization , Cytotixcity” [1] and stored in 4°C until needed. GSH-QDs were suspended in methanol and then applied to a TEM grid for transmission electron microscopy and a silicon substrate for atomic force microscopy.

The image above is of GSH-QD placed on a UV lamp .The left two vials are GSH-QD, while the right vial is GSH-QD suspended in methanol.

Interaction Modeling:

To understand how the electron beam interacted with GSH-QD film on TEM grid, electron flight simulation was conducted. Below looking solely at the horizontal line at 0.0 microns (Y-axis), representative a 100-angstrom QD film on an organic substrate, one can observe that the beam is relatively unperturbed as it interacts with the surface of the sample.

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TEM Micrographs:

TEM micrographs were collected using The Institute of Optics' FEI Tecnai F20 at a 200Kv accelerating voltage


From Figures 1 and 2 one can observe that GSH-QDs are relatively stable and do not instantaneously agglomerate. These observation can be understood by quantifying the zeta potential of GSH-QDs. Zeta potential is a measure of colloidal stability. “If all the particles in suspension have a large negative or positive zeta potential then they will tend to repel each other and there will be no tendency for the particles to come together.” [2], GSH QDs have a zeta potential of about -23.8 mV indicative of a stable nanoparticle.

In Figure 3 and 4 one can observe the peptide corona, GSH, surrounding the Cd/Se core.

In Figures 5 and 6 one can observe the atomic lattices composing the GSH-QDs.

 

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Additional Micrographs:

Atomic Force Microscopy:

TEM provided great resolution of the GSH-QDs. To confirm the collected micrographs using a different technique, the Atomic Force Microscope was used. The AFM images below are of single GSH-QDs on a silicon substrate. Looking at the image, the sample appears to have a Z-axis height of 8-10nm and a width in the X and Y-axis of approximately 50nm. This is inaccurate when compared to the hydrodynamic diameter, diameter of particle measured when a thin film of fluid surrounds the nanoparticle, measured at 13-16nm (Malvern Zetasizer/Dynamic Light Scattering) and the true diameter, Cd/Se core and GSH, measured at 12-13nm (ImageJ). The images collected suggests these particles are quantum dots however, it is hard to reconcile the difference in size between TEM and Malvern measurements.

A single Quantum Dots imaged using the AFM.

Second Order Subtracted Images: Corrected for the pitch of the sample stage .

STEM and X-Ray Mapping:

Energy Dispersive X-Ray using the EDAX detector was used to map the GSH-QD based on their composition. GSH is peptide, which is composed mostly of carbon, nitrogen, sulfur, and oxygen. It was my hope that this peptide surrounding of Cd/Se core would be visible when mapped. Unfortunately the carbon signal with respect to the other signals was significantly larger diminishing the resolution of the smaller nitrogen, sulfur, cadmium, and selenium peaks.

Above is the X-Ray map of the GSH-QD of the Cd(K) and Se(K) lines. One can observe a faint outline of the GSH-QDs

ImageJ Analysis:

The image below is a micrograph recorded and then analyzed using ImageJ. Particle analysis was used to determine the number of GSH-QDs in the micrograph. Below in the colorized section, ImageJ was used to compute the diameter of the QD core which was found to be 4.89nm. This measurement accurately reflects the true QD core size for which the manufacturer,NN-Labs, indiciates to be 4.3-4.7nm. Finally ImageJ was used to determine the diameter of the GSH coated QD which was calculated to be 12-13nm.

Colorization:

Conclusions:

From the collect micrographs one can conclude that GSH-QD are stable Quantum dots and do not readily agglomerate.


Acknowledgements:

I would like to thank Brian L. McIntyre for assisting me throughout this project and for teaching me every early monday morning at 8am over the course of the semester. I would also like to thank Dr.Lisa DeLouise for the opportunity to assist in research at the University of Rochester Medical Center.


References:

[1]-Zheng, Hong, Luke J. Mortensen, and Lisa A. DeLouise. "Thiol Antioxidant-Functionalized CdSe/ZnS Quantum Dots: Synthesis, Characterization, Cytotoxicity." Journal of Biomedical Nanotechnology 9.3 (2013): 382-392

[2]-Hunter, R.J. (1988) Zeta Potential In Colloid Science: Principles And Applications, Academic Press, UK.



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