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Quantum Dots

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Bioconjugation

Utilizing dPEG® Products in Quantum Dots Applications

What are Quantum Dots?

Quantum dots are nanocrystals less than 10nm in diameter comprised of a semiconductor alloy core and coated with a shell of a different alloy metal such as Ag, Cd, Hg, Ln, P, Pb, Se, Te, and Zn [1]. Depending on their size and composition, quantum dots fluoresce at different wavelengths. The smaller the quantum dot, the more blue-shifted its emission wavelength while larger quantum dots (6 nm and larger) shift to red and near-IR. Quantum dots fluorescence makes them a suitable alternative to organic dyes in specific situations. These nanoparticles provide benefits including broad excitation spectra (absorb energy at a wide range of wavelengths), narrow emission spectra (emit a specific wavelength), and they do not suffer from photobleaching (degradation due to light intensity/oxidizing agents) [2].

What is dPEG?

The term “dPEG®” is Vector Laboratories’ trademarked acronym for “discrete polyethylene glycol” or “discrete PEG“. The “discrete” portion of the dPEG trademark indicates single molecular weight PEG technology and can help simplify product analysis. Comparatively, traditional PEGs are not single compounds.

Like traditional PEGs, our products contain an amphiphilic backbone of repeating ethylene oxide units. The term “amphiphilic” means that the compound is soluble in both water (or aqueous buffer) and organic solvents. All PEG products that do not contain hydrophobic substituents are soluble in water and in a variety of organic solvents. The addition of hydrophobic groups to PEG reduces the water solubility of some PEG products.

As part of Vector Laboratories’ BioDesign™  portfolio, dPEG was developed and continues to be manufactured using our proprietary synthetic and purification processes, ensuring lot-to-lot consistency and optimal performance.

Quantum Dot Synthesis

Synthesis of quantum dots involves injecting solutions of the semiconductor metals into hot (>300°C) organic solvent, such as octadecene, and allowing the metals to nucleate (the first step of crystallization) and form alloys. For emission wavelengths of 470-720nm CdSe, CdTe, and InP alloys are used, while PbS and PbSe are utilized for emission wavelengths >900nm.

Water solubility is one of the major hurdles that must be overcome when producing quantum dots for biological applications. When the quantum dots have reached their desired size, the surface is coated so that they can be easily isolated. Originally this was done using tri-n-octylphosphine oxide (TOPO) which results in a very hydrophobic quantum dot (Figure 1).

quantum dots figure1

Figure 1: TOPO coated green quantum dot (approximately 3 nm diameter).

At this point, the TOPO must be replaced with either a water-soluble group or a more hydrophilic group, such as dPEG. The TOPO is commonly replaced by a thiol containing linker such as 2-mercaptoacetic acid, lipoic acid (also known as thioctic acid), 2-mercaptoethylamine (2-MEA), or cystamine (Figure 2).
quantum dots figure2a

thioctic acid, also known as lipoic acid
Chemical Formula: C8H14O2S2
Molecular Weight: 206.33

quantum dots figure2b

cyctamine, also known as
2,2’-disulfanediyldiethanamine
Chemical Formula: C4H12N2S4
Molecular Weight: 152.28

quantum dots figure2c

2-mercaptoacetic acid
Chemical Formula: C2H11NO2S2
Molecular Weight: 169.27

quantum dots figure2d
2-mercaptoethylamine (MEA)
Chemical Formula: C2H7NS
Molecular Weight: 77.15

Figure 2: Commonly used sulfur-containing compounds for attachment to gold surfaces such as quantum dots.

While these modifications allow for increased hydrophilicity, the water solubility can be increased even more by adding a dPEG to the acid or amine end by a simple coupling using 1-Ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC). Alternatively, a thiol-dPEG could also be used directly on the surface of the quantum dot. Figure 3 shows dPEG compounds attached to a quantum dot either directly or coupled to a thiol monomer already on the surface.

quantum dots figure3

Figure 3: Red Quantum Dot (approximately 6 nm diameter) with Attached dPEG compounds m-dPEG®8-lipoamide (PN10800), m-dPEG®8-acid (NHS ester) (PN10324, PN10260 – coupled to cystamine) and m-dPEG®8-amine (PN10278 – coupled to mercaptoacetic acid)

Reducing Quantum Dot Non-Specific Binding with dPEG

Elizabeth L. Bentzen, et al., studied the effects of PEG on nonspecific binding associated with quantum dots and found that PEG as small as PEG350 (approximately dPEG8), when coated over the surface of quantum dots, can reduce nonspecific binding in certain cells [3]. The results showed that, in the cells studied, the PEG length could possibly be shortened to 12 units without increasing the nonspecific binding. Using a dPEG in place of a traditional polydispersed PEG gives the advantages of lot-to-lot reproducibility and confidence in the identification of the final product.

Conclusion

dPEG has the potential to play an important role in quantum dots applications by addressing challenges such as water solubility and nonspecific binding. By leveraging Vector Laboratories’ proprietary dPEG products, researchers and developers can unlock new potential in quantum dots fluorescence and further expand their applications in research and medicine. For more information about dPEG products, click here.

Products

Browse our metal surface modification dPEG reagents here. For the products referenced in the caption to Figure 2, above, click on the links below:

PN10800, m-dPEG®8-lipoamide

PN10324, m-dPEG®8-acid

PN10260, m-dPEG®8-NHS ester

PN10278, m-dPEG®8-amine

References:
  1. Kesrevani, R. K., & Sharma, A. K., Nanoarchitectured Biomaterials: Present Status and Future Prospects in Drug Delivery., William Andrew Publishing (2016), 35-66. [ScienceDirect]
  2. Abramowitz, M., & Davidson, M. W. (n.d.), Overview of fluorescence excitation and emission fundamentals., Olympus Microscopy Resource Center, [OlympusConfocal]
  3. Bentzen EL, Tomlinson ID, Mason J, Gresch P, Warnement MR, Wright D, Sanders-Bush E, Blakely R, Rosenthal SJ. Surface modification to reduce nonspecific binding of quantum dots in live cell assays. Bioconjug Chem., (2005), 16(6), 1488-94. [PubMed]
  4. Hermanson, G. T. (2013). Bioconjugate Techniques, (3rd ed., pp. 455–463). Academic Press.

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