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SPI-Mark™ Colloidal Gold Reagents

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SPI Supplies maintains a ready stock of these valuable marking agents - all equivalent in quality but considerably less expensive than gold reagents from brand-name suppliers. All manufacturing is done under an ISO 9001 accreditation. Each and every product in our SPI-MARK line stands head-to-head with the competition on the following criteria:
Sharp size distribution of SPI-Mark™ Colloidal Gold products
  • Size uniformity (Variation coefficient - 18%)
  • High optical density (Strict quality control ensures consistency)
  • Purity (No "clumping" problems)
  • Long shelf life (Generally one year from date of analysis)
How are the antibodies coupled to the gold?
The proteins are attached to the gold particles by:
a). Charge attraction (Lysine)
b). Hydrophobic attraction (Tryptophan) and
c). Sulfur binding (Cysteine and Methionine)


Silver-enhanced images using SPI-Mark Silver Enhancement Kit
Is the Fc region targeted or is the orientation of the antibody attachment to the gold particle random?
We believe the strongest binding is the sulfur "hinge" joining the the two Fc regions. Certainly, it is possible to conjugate Fab fragments and the sensitivity of these is equal to the whole molecule antibodies. The Fab fragments are believed to bind at the sulfur hinge (after cleaving the Fc) leaving the binding sites exposed. The antibodies should be of the highest purity, anything else is not worth bothering with - you just won't get the sensitivity researchers expect.

Tightness of the size distribution
We are often times asked about just how "tight" is the size distribution of the different gold sizes, especially by those contemplating double or even triple labelling protocols. We first want to make clear that the different sizes are completely non-overlapping. Hence there is no problem identifying one size from another in a multiply-tagged specimen. To put this more quantitatively, we can state the following:

The coefficient of variation (c.v.) for the SPI-Mark™ colloidal gold reagents is as follows:

5 nm <12%          10-250 nm inclusive <8%

Colloidal gold in non-aqueous suspension:
As the use of the SPI-Mark™ Brand of Colloidal Gold finds more and more non-immunogold applications, we get asked what to us are really out-of-the-ordinary questions. Some we can help with and some we can not. The most often asked for question of this type has to do with being able to supply the gold unconjugated in a liquid other than water, specifically, some organic suspending medium. We have never done this before and we are unaware of anyone who has actually done this, therefore, we do not know what the results of such an effort might be likely to produce. However there may be a chance of success because there seems to be a number of metal colloids generally that are supplied suspended in organic liquids. But this would be something a customer would have to try as we could not be doing this ourselves.

Temperature considerations:
In the liquid (suspension) form, the conjugates are stable up to 36°C. Heating above this temperature will cause the conjugate to deteriorate. The conjugates are best worked with at room temperature and when the application is for rapid test kit manufacture, they can be dried in a vacuum oven. We recommend storage of conjugate at 4°C. Even in the dried form, one may see deterioration in tests at temperatures above 36°C.

However, there is no general rule. It is best for users to validate the format of any test according to the temperature range that it will experience during real use conditions. SPI Supplies can provide customers with technical and consulting services in this area, enabling customer to benefit from our extensive experience in this area of application.

Making your own gold:
As much as we would like to sell our colloidal gold probes commercially, there are always those who want to "make their own" and if you are one of those, then we could refer you to some references.

If you are a "newbie" and are not familiar personally with this rapidly growing field of immunolabelling, take advantage of our resource information: Immunogold Labelling Protocols
Summary of available products


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Wednesday May 14, 2008
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