Highly sensitive detection of nucleic acids in the nanogram range has been achieved by the specific chemical reduction of silver ions. The methods for silver-staining nucleic acids employ either a histologically derived procedure that uses ammoniacal solutions of silver [142, 143, 144, 145], or a photochemically derived reaction in which silver binds to nucleic acid bases and is then selectively reduced by chemical agents or light[146, 147, 148, 149]. These silver staining protocols can be as sensitive as radioisotopic methods. However, they are complex and time consuming and require the preparation and handling of several solutions. In an attempt to simplify the routine use of silver stains to detect nucleic acids, BASSAM et. al. [150] have optimized the photochemically derived silver stain originally introduced by MERRIL et. al. [151] for protein staining and later applied to nucleic acids [146, 147, 149] which uses formaldehyde to selectively reduce silver ions to metallic silver under alkaline conditions.
Several steps were omitted and others modified. Image development was in the
presence of sodium thiosulfate as suggested by BLUM et.
al.[149] but instead using higher concentrations of formaldehyde. This
procedure is highly sensitive, avoids unspecific background staining without
loss of contrast, uses less silver and no oxidizing pretreatments, and stains
complex mixtures of DNA resolved in polyacrylamide gels bound to polyester
backing film. The originally suggested concentration of AgNO 
(12 mM) can be
halved provided the concentration of formaldehyde is increased at least two- to
threefold during image development. Reduction of silver by formaldehyde is
concentration dependent. Optimal band intensity occurs at about 0.00555% (by
volume) formaldehyde. Thiosulfate dissolves insoluble silver ions from the gel
surface which in turn decreases nonspecific staining [149]. A
concentration of 4 
M thiosulfate is sufficient to reduce nonspecific
background staining without noticeably affecting DNA image development.
Gels polymerized onto polyester backing films are handled easily during staining, and, when dried, produce a permanent record. However, the polyester film layer has the disadvantage of decreasing the surface area of the gel in contact with the liquid, slowing diffusion of solutes in and out the gel matrix. This lengthens the time required for image development during staining which can substantially increase background staining (see figure fig:cg_kpn).
The limit of detection upon visual inspection of double-stranded DNA was
approximately 1 pg/mm 
. This is about 1000 to 10,000 times more sensitive
than ethidiumbromide staining (see section EtBr).