III.             IV.  THE CHEMICAL PROPERTIES OF ELEMENTAL AND NANOCRYSTAL SILVER

Silver is a noble metal meaning that it can be found free in nature. The chemical symbol or abbreviation for silver is derived from the Latin word for silver (argentium).  Of all the metals, silver is the number one conductor of heat and electricity.  Silver is a soft metal second only to gold in terms of malleability.  Pure silver is relatively inactive chemically.  It does not react with water or oxygen at normal temperatures and is not soluble in dilute acids or bases.  These properties have contributed to its historic use in coinage.  However, silver will react with concentrated acids, e.g. nitric acid or hydrochloric acid to form salts which are chemically active.⁽²⁵⁾

Silver salts and other chemically active silver compounds are used for their antimicrobial properties as previously described.  When placed in aqueous solution the silver ion, responsible for antimicrobial effects, is released.  Silver, because of its antimicrobial activity, has been impregnated into the plastic of vascular catheters, toothbrushes, air conditioning systems and may other products.

However silver, when used as a topical antimicrobial, with the exception of burn injury, has not been the agent of choice in other wounds due to the rapid deactivation of the silver by tissue proteins or chloride anions.  Most silver products are rapidly deactivated.  A continuous silver release would be a much more effective use of silver ions.  In addition, release of other silver radicals, not so readily neutralized by tissue protein, would be of benefit.  Until recent nano-technologic advances, such a silver delivery system was not available.

Benefits of Silver Nanocrystals

Nanotechnology is a general term referring to a new frontier in technology.  The prefix “nano” refers to one-billionth.  A nanometer is one billionth of a meter, a nanogram one billionth of a gram.  Ten hydrogen atoms placed side by side would equal one nanometer in length.^(23-25)    

Nanotechnology is concerned with building products very precisely in very small sizes.  One advantageous property is that as crystals get smaller, their surface area to volume ratio increases.    

Decreasing the volume of a crystal i.e., a nanocrystal increases the proportion of atoms on the surface compared to internal atoms. Increasing surface area of materials, such as a silver membrane, is important as chemical reactions take place between molecules on the surface, thus the larger the surface area, the more reactions can take place in a shorter time.  The properties of the material are markedly amplified in nanocrystalline metals.

Nanotechnology has provided a method to achieve the ideal delivery system for silver to tissues for antimicrobial activity.  Prior to nanocrystals, silver had to be delivered in a chemically combined form.  A method of depositing pure silver as a nanoparticle or crystal cation onto a high density polyethylene mesh has provided a way to deliver the pure silver cation and its radicals.    

The nanocrystal silver is much more reactive to water and oxygen due to the increased surface area for reaction, thereby releasing silver cations very rapidly and for long periods, i.e. days.  This rapid and sustained release has considerable advantages over the other available silver products, especially the presence of silver radicals.

Reactive Species of Silver Released from Silver Products

* concentration of silver in solution (one square inch of surface / 5 ml water)

With nanocrystals there is an increase in the speed of silver delivery to the bacterial wall resulting in a much more rapid kill. In addition, the improvement in antibacterial properties, compared to other available silver products, strongly suggest the nanocrystals are providing new reactive silver species. To date several silver radicals have been identified with silver nanocrystals and more are likely. One appears to be a cluster of silver cations and radicals identified using a scanning electron microscope. These silver radicals which are much more reactive than silver cations have only been observed with the nanocrystalline silver delivery.^(23-28)

In summary, based on advances made in nanotechnology, a new nanocrystalline silver delivery system product has been produced which has more potent antimicrobial properties than other silver products. This advance may also produce other effects of silver, which have been described, mainly pro-healing and anti-inflammatory properties.