Nanocrystals - Fluorescence Microspectroscopy of Nanocrystals Using QDI 2010 Microspectrophotometer from CRAIC Technologies

Topic List

Background
What are Nanocrystals
Applications of Quantum Dots
UV-Visible Microspectroscopy of Quantum Dots
Results
Conclusions

Background

CRAIC Technologies is the worlds leading developer of UV-visible-NIR range scientific instruments for microanalysis. These include the QDI series UV-visible-NIR microspectrophotometer instruments designed to help you non-destructively measure the optical properties of microscopic samples. CRAIC's UVM series microscopes cover the UV, visible and NIR range and help you analyze with sub-micron resolutions far beyond the visible range. CRAIC Technologies also has the CTR series Raman microspectrometer for non-destructive analysis of microscopic samples. And don't forget that CRAIC proudly backs our microspectrometer and microscope products with unmatched service and support.

What are Nanocrystals

Nanocrystals, also called Quantum dots, are inorganic crystals that exhibit very strong fluorescent emissions. These materials are considered semiconductors and range from one to fifty nanometers in diameter. Their unique optical characteristics are due to their size in addition to the material of which they are made. Quantum confinement is a direct result of their small size and leads to discrete energy levels. As such, changing their size varies their optical properties including the wavelength of absorbed and emitted light. Additionally, these semiconductor nanocrystals are remarkably durable.

Applications of Quantum Dots

Because of such features as durability and optical tunability, there are many uses for quantum dots. They have been a mainstay as fluorescent markers in biological and medical research for years. As such, they are usually incorporated into polystyrene beads, mixed into a solution and then incorporated into the biological system. Additional uses have been found in homeland security where these nanocrystals are used as anti-counterfeit materials and even for counter espionage when used as "quantum dust". There is also some exciting work where quantum dots are being used to develop high efficiency light emitting diodes (LED). As they are so small, these quantum dot LEDs could potentially be fabricated in any form and even painted on to surfaces.

UV-Visible Microspectroscopy of Quantum Dots

Quantum dots were acquired from CrystalPlex (Pittsburgh, PA) and were embedded in polystyrene beads approximately 2 microns across. The polystyrene beads were dispersed in an aqueous PBS solution. The sample was prepared for microspectral analysis by placing a drop of the solution onto a glass slide. No other preparation was required.

The instrument used was a QDI 2010™ UV-visible-NIR microspectrophotometer from CRAIC Technologies, Altadena, California. See Figure 1. The instrument was configured for transmission and fluorescence microspectroscopy. This instrument has a spectral range of 200 to 1000 nm and is able to detect the fluorescence of quantum dots down to sub-micron sampling areas.

Sample spectra were acquired from individual beads with a 1.5x1.5 micron sampling area. Each bead was excited at 365 nm and the emission from 400 to 900 nm was observed. In each case, 5 spectra were averaged. Each full-range spectrum was acquired with an integration time of 2 seconds. No post-processing or smoothing has been applied to any of the data shown here.

Figure 1. QDI 2010™ microspectrophotometer

Results

Individual beads were located and then excited at 365 nm and the fluorescence emission was observed. There were three types of beads. The first was an blank polystyrene bead and this was measured to quantify the background fluorescence of the polymer bead itself. The second and third beads were tagged with nanocrystals and fluoresced quite brightly but at different wavelengths. The picture on the left is the untagged bead, the center and the right images are of two different tagged beads. The black square in the center of the right-most picture is the entrance aperture of the microspectrophotometer and is 2x2 microns.

Below are the spectra from all three samples taken under identical conditions. The green trace is the un-tagged polystyrene bead and corresponds with the left-most picture. As can be seen, this bead has a substantial amount of autofluorescence. The center image corresponds to the red trace that has a maximum of emission at 581 nm. A substantial amount of the polymer emission is also present. The blue trace corresponds to the righthand picture. It has an emission maximum of 527 nm.

Conclusions

The purpose of this paper is to show how fluorescence Microspectroscopy can be used to analyze individual quantum dot beads as commonly used as biological markers. The spectra and images show that this type of instrumentation can easily detect the emissions from these beads and even from multiple beads if required. Microspectroscopy has been used both in vivo and in vitro and by combining the technique with such markers, it can be used to track biological changes.

Source: "Microfluorometry of Nanocrystals" by CRAIC Technologies.

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