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Superior Surface Protection of Gold Nanoparticles With Short-Chain PEG

Surface protection of gold nanoparticles is improved by using short-chain, alcohol-terminated dPEG® linkers rather than (2-mercaptopropanoyl)glycine (tiopronin) or mercapto-undecyl-tetraethyleneglycol, according to research findings from the lab of David E. Cliffel, Department of Chemistry, Vanderbilt University. Short-chain dPEG®s increase water solubility, are non-toxic, and show no immune response to anti-PEG antibodies at low concentrations.(1)

Surface protection and opsonization

Tiopronin and mercapto-undecyl-tetraethyleneglycol (Figure 1) have been used for monolayer surface protection of gold nanoparticles, but both have problems associated with their use. Using tiopronin as the monolayer for a gold nanoparticle surface above 40μM causes severe renal damage that ultimately kills test animals.(2) Mercapto-undecyl-tetraethyleneglycol has been shown to have poor water solubility when added to the cluster as the monolayer. To reduce the damage caused by tiopronin in vivo mercapto-undecyl-tetraethyleneglycol is added to the monolayer in high concentrations.(2) These high concentrations create anti-PEG antibodies to attack the cluster and make it unreactive by the mechanism known as opsonization.(1) Opsonization (Figure 2) occurs when anti-PEG antibodies react with the monolayer and render the cluster unreactive and is removed from the body.


Chemical structures of tiopronin and mercapto-undecyltetraethyleneglycol used in surface protection of gold nanoparticles.
Figure 1: Tiopronin (left) and mercapto-undecyltetraethyleneglycol (right) used in surface protection of gold nanoparticles


Process of Opsonization. Picture copyright 2011 by The Board of Trustees of the University of South Carolina. Used with permission.
Figure 2: Process of Opsonization. Picture copyright 2011 by The Board of Trustees of the University of South Carolina. Used with permission.

Short-chain dPEG® compounds enhance surface protection while avoiding opsonization

Short-chain, thiol-dPEG® compounds used in a mixed monolayer with tiopronin increase the water solubility of the tiopronin short-chain monolayer as compared to a mixed monolayer of tiopronin and mercapto-undecyltetraethyleneglycol. This increase in water solubility is attributed to the elimination of the akyl chain of the mercapto-undecyltetraethyleneglycol. A mixed monolayer containing 10% short-chain, alcohol-terminated dPEG® (Figure 3a) on the gold nanoparticle showed no renal damage or other toxicity.(1) The short-chain dPEG® apparently shields the cluster from opsonization and allows for fluid movement of the monolayer, which is thought to be the mechanism that thwarts opsonization.(1) Note that Quanta BioDesign sells the S-acetyl protected version of this alcohol as product number 10156 (see also Figure 3c).


Compounds used for surface protection studies. A. Thiol-PEG4-alcohol. B. Thiol-dPEG®4-acid (Quanta BioDesign PN10247). C. S-acetyl-thiol-dPEG®4-alcohol, which was not used in the study but is the S-acetyl-protected version of 3a. 3c is Quanta BioDesign product number 10156.
In studying surface protection of gold clusters, Thiol-PEG4-alcohol and Thiol-dPEG®4-acid were compared. A. Thio-PEG4-alcohol. B. Thiol-dPEG®4-acid (Quanta BioDesign PN10247). C. S-acetyl-dPEG®4-alcohol (Quanta BioDesign PN10156) which is the S-acetyl-protected version of 3a). This product was not used in this study, but it can be used for future, similar applications.


The short-chain dPEG® mixed monolayer showed no immune response in vivo. At a 10% molar exchange ratio using an alcohol-terminated short-chain dPEG® mixed monolayer, no immune response occurred in animal models. Red blood cell count increased at a 65-70% molar exchange ratio using a thiol-dPEG®-carboxy-terminated short chain dPEG® (Figure 3b), but again, no immune response occurred. This is Quanta BioDesign's product number 10247. Surface protection of the gold nanoparticles was gained without the complication of anti-PEG antibodies or the serious problem of renal damage. These results favor use of short-chain dPEG® compounds in mixed monolayer with tiopronin instead of mercapto-undecyltetraethyleneglycol.(2)


PEG chain length affects in vivo residence time

PEG chain length directly affects residence time in the body. Short-chain dPEG®s have been shown to move through the body much faster (24 hours) than longer chains (2-4 weeks)(1) For applications where a short residence time is desirable, short-chain dPEG® compounds are a strong asset, because they provide high water solubility, no immune response, and no toxicity issues.

Short-chain dPEGs® make a difference in the biological aspect of surface protection chemistry. Quanta BioDesign is the inventor of, and world leader in, dPEG® technology with a vast range of products with varying length and terminal groups with high purity (>90%) for your convenience. If you do not see a product that you want, please call or e-mail for custom synthesis! We want to help you get the best out of your scientific application!



Simpson, C. A.; Agrawal, C. A.; Balinski, A; Harkness K. M.; Cliffel, D. E. Short-Chain PEG Mixed Monolayer Protected Gold Clusters Increase Clearance and Red Blood Cell Counts. ACS Nano, 2011, 5 (5), 3577–3584.

Simpson, C. A.; Huffman, B. J.; Gerdon, A. E.; Cliffel, D. E. Unexpected Toxicity of Monolayer Protected Gold Clusters Eliminated by PEG-Thiol Place-Exchange Reactions. Chem. Res. Toxicol. 2010, 23, 1608–1616.


Additional surface protection and surface modification products

Click here for general surface modification products from Quanta BioDesign.

Click here for metal surface modification products from Quanta BioDesign.

PN10156, S-acetyl-dPEG®4-alcohol

PN10160, S-acetyl-dPEG®8-alcohol

PN10939, S-acetyl-dPEG®12-alcohol

PN10247, Thiol-dPEG®4-acid

PN10183, Thiol-dPEG®8-acid

PN10850, Thiol-dPEG®12-acid


Lipoic acid (1,2-dithiolane-3-pentanoic acid), because of its strained 5-member dithiolane ring, provides superior dative bonding to gold surfaces and, hence, superior surface protection to gold surfaces. Quanta BioDesign, Ltd. offers several lipoic acid-functionalized dPEG® derivatives. A partial list of our line of such products is below:

PN10806, Lipoamido-dPEG®4-acid

PN10641, Lipoamido-dPEG®4-TFP ester

PN10807, Lipoamido-dPEG®8-acid

PN10642, Lipoamido-dPEG®8-TFP ester

PN10808, Lipoamido-dPEG®12-acid

PN10814, Lipoamido-dPEG®12-TFP ester

PN10811, Lipoamido-dPEG®24-acid

PN10643, Lipoamido-dPEG®24-TFP ester


You can see all of our lipoic acid derivatives in our Metal Surface Modification Reagents list.

Ian Hotham, B.S., received his B.S. in Chemistry from The Pennsylvania State University in Spring of 2013. Ian is a Process Development Chemist involved in process development and scale-up activities. You can connect with Ian on LinkedIn at

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Thiol Reactive Crosslinkers for Bioconjugation

Thiol reactive crosslinkers are one of the most common classes of crosslinkers in bioconjugation (1). The popularity of conjugation to a thiol is due in part to its presence in many proteins, but they are not as prevalent as amines, which are another site for conjugation. This will allow for greater control of the conjugation. Even greater control of the conjugation process is afforded if a thiol reactive compound is combined with an amine reactive compound to create a heterobifunctional crosslinker.

Thiol reactive crosslinkers from Quanta BioDesign, Ltd.


1. Maleimide crosslinkers

Quanta BioDesign offers a variety of homo- and heterobifunctional thiol reactive crosslinkers for bioconjugation with dPEG® spacers of several different lengths. Among those we offer are maleimide crosslinkers. Near neutral pH, the double bond of the maleimide reacts preferentially and very rapidly with a thiol to form a thioether bond that is not susceptible to reduction (2). Quanta BioDesign offers homobifunctional bis-maleimides as well as a few heterobifunctional maleimide bioconjugation crosslinkers with the other end being an amine reactive active ester. One of our most popular products is Mal-dPEG®4-NHS ester, product number 10214 (shown in Figure 1). It contains a 22 atom (24.8 Å) tetraethylene glycol spacer functionalized on one end with a thiol reactive maleimidopropyl group and on the other end with an amine reactive propionic acid-N-hydroxysuccinimide (NHS) ester.


Thiol reactive PN10214, MAL-dPEG®4-NHS ester
Figure 1: Thiol reactive crosslinker PN10214, one of Quanta BioDesign's most popular PEGylation reagents, has a thiol reactive maleimide group on one end and an amine reactive NHS ester on the other end of a tetraethylene glycol linker.


Quanta BioDesign also offers this product with dPEG®2, dPEG®6, dPEG®8, dPEG®12, dPEG®24, and longer dPEG® linkers. You can view them all on our website. They are listed below in this post.  Another version of this product is PN10551, where the NHS ester is replaced by the 2,3,5,6-tetrafluorophenyl (TFP) ester. In-house research by Quanta BioDesign, Ltd., demonstrates that the TFP ester is much less susceptible than the NHS ester to hydrolysis.


2. Pyridyl disulfide crosslinkers

Quanta BioDesign also offers pyridyl disulfide (SPDP) bioconjugation crosslinkers (see also here), and with these, the thiol reacts with the SPDP moiety to produce a new disulfide bond, as illustrated in Reaction 1. Pyridine-2-thione is generated, but it cannot react with any remaining SPDP crosslinker because it does not contain a thiol (3, 4). If desired, the newly-formed disulfide bond can be cleaved with a reducing agent. It can also be oxidized back to the disulfide bond, which provides a flexibility not available with the maleimide crosslinkers. Like the maleimides, the SPDP crosslinkers are also offered as the NHS and TFP esters.


The thiol reactive pyridyl disulfide (SPDP) group is used in bioconjugation.
Reaction Scheme for the thiol reactive pyridyl disulfide (SPDP) group in bioconjugation


Thiol reactive crosslinkers are available now from Quanta BioDesign

Both the maleimide and SPDP crosslinkers with dPEG® are available from Quanta BioDesign with a variety of PEG spacer lengths, ranging from four to twenty-four ethylene oxide units (and in some cases, even longer) Whatever the length and functionality you need for your thiol reactive crosslinking PEGylation reagent, Quanta BioDesign can provide it for you. If you do not see what you want in our catalog, contact us about a custom synthesis. We can provide you with what you are looking for.



1.  Hermanson, Greg T. Bioconjugate Techniques, 3rd Edition. Waltham, MA: Elsevier (Academic Press), copyright 2013, 1146 pages. (A copy of the 2nd edition of Greg's phenomenal work is available from Quanta BioDesign, Ltd., for $75 plus shipping, or for free with any order of $500 or more, excluding tax and shipping. Look here for more details.)

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2.  Smyth, Derek G., Blumenfeld, O. O., and Konigsberg, W. Reactions of N-ethylmaleimide with peptides and amino acids. Biochem J. (1964), 91, 589-595.

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3.  Carlsson, J., Håkan, D., and Axén, R. Protein thiolation and reversible protein-protein conjugation. Biochem J. (1978), 173, 723-737.

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4.  Myers, D. A., Murdoch, W. J., and Villemez, C. L. Protein-peptide conjugation by a two-phase reaction, Biochem J. (1985), 227(1), 343.

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Additional Products from Quanta BioDesign, Ltd.

MAL-dPEG®2-NHS ester

MAL-dPEG®4-NHS ester

MAL-dPEG®6-NHS ester

MAL-dPEG®8-NHS ester

MAL-dPEG®12-NHS ester

MAL-dPEG®24-NHS ester

Please call or email us and ask about our MAL-dPEG®x-TFP ester derivatives. If you want a longer length dPEG® spacer than you see here, please call or email us about that also. We will be glad to discuss them with you!

Dan Dawson, M.S. received his BS in Chemistry from the University of Indianapolis in 2006, and his M.S. in Organic Chemistry from the University of Michigan in 2008. Dan is a Process Development Chemist involved in process development and scale-up activities. You can connect with Dan on LinkedIn at

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