the present invention relates to a method for signal amplification for detecting target substances . in particular , the invention provides for amplification of signal and enhancement of assay sensitivity for detection of minute quantities of target molecules , e . g ., nucleic acids , that are based on silver stain detection . in practicing this invention , the inventive method is used after the application of conventional silver staining where it leads to the formation of silver - gold ( oligonucleotide )- silver assemblies , termed here silver - gold ′- silver sandwich assemblies . the prior steps leading up to and including conventional silver staining can , for example , involve : capture of a target oligonucleotide by an oligonucleotide capture probe bound to a glass surface , washing , addition of nanoparticle oligonucleotide conjugates complementary to an unhybridized segment of the bound target oligonucleotide , washing , addition of a silver staining solution ( ag + plus a reducing agent , available through sigma ), washing , drying , and viewing , either with the naked eye or with aid of pictures obtained using a simple flat - bed scanner . this procedure was described by taton et al ., science , 289 , 1757 ( 2000 ), who showed that although the gold spot obtained was too weak to observe directly for target concentrations of 100 nm in oligonucleotide , conventional silver deposition affords a strong dark spot . the limit of detection for this system using conventional silver staining is 50 fm concentrations , for which an extremely faint spot is observed . re - exposure to the silver solution did not noticeably further enhance the intensity of the spot . for amplification of the silver signal by the inventive silver - gold ′- silver sandwich method , the glass plate containing silver stain from a prior application of conventional staining is further exposed to an aqueous amplification solution of nanoparticles having oligonucleotides bound thereto , preferably gold nanoparticle - oligonucleotide conjugates or a gold - nanoparticle oligonucleotide complexes . the oligonucleotide sequence does not need to be related to the target oligonucleotide sequence or the sequences on the initial probes . for gold nanoparticle - oligonucleotide conjugates , the amplification solution generally contains between about 1 nm and about 5 nm nanoparticle oligonucleotide conjugates . the amplification solution may include salts , such as buffer salts , and detergents such as phosphate , citrate , hepes , and nes and is preferably has a ph of about 7 . in practicing the invention , an aqueous amplification solution including aqueous 0 . 1m nacl and 10 mm phosphate ( ph 7 . 0 ) was found to be particularly useful . for gold nanoparticle oligonucleotide complexes , the amplification solution is the same . any suitable mode of exposing the silver stain to the amplification solution may be used . representative examples include spraying , dipping , and the like . after about 5 to 30 minutes , preferably about 10 minutes , the unbound nanoparticles conjugates or complexes are removed by washing the plate with a suitable aqueous solution , preferably water . optionally , the plate is dried by any suitable method such as air drying . the plate is then re - exposed to a silver staining solution for a suitable time period , generally for about 5 to about 10 minutes , preferably about 5 minutes . it will be understood that any suitable silver staining method may be used . suitable , but non - limiting , examples of useful silver staining methods and compositions are described , for instance , in m . a . hayat , ed ., “ immunogold - silver staining ,” crc press ( 1995 ). generally , silver staining solutions include silver ion in the form of a silver salt such as silver acetate ( preferred ), silver lactate , and silver nitrate . these solutions also include a reducing agent that is admixed into the solution just prior to use . suitable , but non - limiting , examples of reducing agents include hydroquinone , n - propyl galate , p - phenylenediamine , and formaldehyde . if desired , other agents such as gum arabic may be used to mediate the silver stain process . any suitable mode of contacting the substrate with the silver stain solution may be used . representative examples include spraying , dipping , and the like . following washing with water ( preferred ) or other suitable solution ( e . g ., aqueous solution containing 0 . 1m nacl and 10 mm phosphate ( ph 7 . 0 ) to remove unreacted silver stain solution , the re - exposed plate is observed visually or copied by a flat - bed scanner . this treatment leads to a great increase in the darkness of the spot . the process may be repeated as often as desired to further enhance the amount of deposited silver and the darkness of the spot . with this inventive amplification method , one can readily observe a dark silver spot for an assay using 25 fm target concentration . with two cycles of the new nanoparticle - silver sandwich procedure , 1 fm target solutions can be recognized , and with three cycles , 0 . 1 fn solutions give positive though weak spots . these experiments were carried out using 1 microliter of the target solution in each case . for the assay with 0 . 1 fm solution this corresponds to ˜ 60 target molecules in the applied sample . any substrate can be used which allows observation of a silver stain as the detectable change . suitable substrates include transparent solid surfaces ( e . g ., glass , quartz , plastics and other polymers . the substrate can be any shape or thickness , but generally will be flat and thin . preferred are transparent substrates such as glass ( e . g ., glass slides ) or plastics ( e . g ., wells of microtiter plates ). the silver stain signal amplification method of the invention depends on the use of nanoparticles - oligonucleotide conjugates or complexes that satisfy certain characteristics . first , the nanoparticles do not stick to the surface of the chip being tested . ordinary nanoparticles prepared by the citrate reduction method of frens ( frens , g ., nat . phys . sci ., 241 , 20 - 22 ( 1973 ) are not satisfactory since they bind indiscriminately to the oligonucleotide - derivatized glass plate used as the substrate for these assays . subsequent silver enhancement then gives false positives as dark areas . second , the nanoparticles bind to a deposited silver surface such that on subsequent washing , the attached nanoparticles remain bound to the silver area while nanoparticles suspended in solution are cleanly removed . third , the nanoparticles function as agents to reduce silver ions under silver staining conditions . in practicing this invention , useful nanoparticles are nanoparticles coated with oligonucleotides linked through sulfur to the surface ( nanoparticle oligonucleotide conjugates ) such as the ones described in j . j . storhoff et al ., j . am . chem . soc ., 120 , 1958 ( 1999 ) ( for a specific example , see conjugate i in example 1 below ) or with natural type oligonucleotides adsorbed to the surface ( nanoparticle oligonucleotide complexes ) such as conjugate iii described in example 1 . both types of nanoparticles work well in low or moderate salt solution ( e . g . up to 0 . 1 m ), but the conjugates containing the sulfur anchor are particularly preferred for tests conducted at high salt concentration , at which the complexes formed by simple adsorption of oligonucleotides are unstable and aggregate . it will be understood by the ordinary skilled artisan that any nanoparticle preparation that meets the criteria listed above are useful as intermediary agents in forming the sandwich assemblies , and the methodology can be applied for the amplification of the silver signal for any target visualized by an initial silver deposition . while gold nanoparticles are particularly preferred , any nanoparticle that catalyze the reduction of silver can be used including silver and platinum nanoparticles . the preparation of nanoparticles suitable for use in the practice of the invention , the attachment of oligonucleotides to them , the flatbed scanner technique , and various assays formats for the detection of nucleic acids using conventional silver staining are described in co - pending applications serial nos . 09 / 760 , 500 , filed jan . 12 , 2001 ; 09 / 603 , 830 , filed jun . 26 , 2000 ; 09 / 344 , 667 , filed jun . 25 , 1999 ; 09 / 240 , 755 , filed jan . 29 , 1999 ; 60 / 031 , 809 , filed jul . 29 , 1996 ; 60 / 176 , 409 ; and 60 / 200 , 161 , filed apr . 26 , 2000 ; and international application nos . pct / us97 / 12783 , filed jul . 21 , 1997 ; pct / us00 / 17507 , filed jun . 26 , 2000 ; and pct / us01 / 01 190 , filed jan . 12 , 2001 , entitled “ nanoparticles having oligonucleotides attached thereto and uses therefor ,” the entire contents of which are incorporated herein by reference . oligonucleotides and 5 ′- mercaptoalkyl - oligonucleotides were prepared using phosphoramidite chemistry as described by storhoff et al . [ j . am . chem . soc . 120 , 1959 - 1964 ( 1998 )]. gold nanoparticles (˜ 13 nm diameter ) were prepared as described by grabar , k . c ., et al . [ j . analyt . chem ., 67 , 735 - 743 ( 1995 ); frens , g ., nat . phys . sci ., 241 , 20 - 22 ( 1973 )]. for preparation of the nanoparticle conjugate ( i ) used in the sandwich silver - gold ′- silver amplification scheme , 5 ′- mercaptoalkyl - oligonucleotide ii was prepared and joined to gold nanoparticles by the general linking procedure described by storhoff et al ., [ j . am . chem . soc . 120 , 1959 - 1964 ( 1998 )]. the nanoparticle - oligonucleotide complex ( iii ) was prepared by mixing 1 μl of an aqueous solution containing 0 . 26 a 260 units of oligonucleotide iv with 100 μl of citrate stabilized gold colloid ( prepared as described by grabar et al . [ j . analyt . chem ., 67 , 735 - 743 ( 1995 )) and allowing the solution to stand overnight . ii . 5 ′- hs ( ch 2 ) 6 op ( o )( o ) o -( a ) 20 tgggtagcagacctc ( seq id no . : 1 ) in this example , four separate experiments were conducted using the nanoparticle oligonucleotide conjugate ( i ) or complex ( iii ) prepared as described in example 1 , and glass slides containing silver spots from oligonucleotide assays carried out using the silver staining method described by t . a . taton et al , science , 289 , 1757 - 60 ( 2000 ). the glass plate bearing the silver spots was exposed to a solution of gold nanoparticle conjugate i ( fig2 , and 5 ) or gold nanoparticle complex iii ( fig3 ) for 10 minutes , washed with 1 m nano 3 in nanopure water , and exposed for five minutes at room temperature to a 1 : 1 mixture of freshly mixed sample of the two commercial silver enhancer solutions ( catalog nos . 55020 and 55145 , sigma corporation , st . louis , mo .) for 5 minutes , following the sigma protocol for the silver staining step , including final washes with nanopure water , sodium thiosulfate solution , and nanopure water . the plate was dried and observed , both visually and by copying for records using a hewlett packard scanner [ model no . 5200c ]. the direct visual observations of spots corresponded to the prints obtained using the scanner . the figures are enlarged . [ 0051 ] fig2 illustrates the results for the first experiment using nanoparticle oligonucleotide conjugate i . in this experiment , two plates , each containing very faint silver spots from previous hybridization and silver staining of a dilute target solution , were used . plate ( a ) was subjected to a second round of conventional silver staining in an attempt to further enhance the spot . slight but very little signal enhancement took place . plate ( b ) was subjected to conditions for the gold sandwich amplification . this entailed exposing the plate containing the weak silver spots to the solution of nanoparticle conjugate i for 10 minutes , followed by washing with water and conventional silver staining for 4 minutes . the darkness of the spots in plate ( b ) relative to those in plate ( a ) clearly demonstrate the power of the new metal sandwich or signal enhancement method . [ 0052 ] fig3 illustrates the results for an experiment using nanoparticle oligonucleotide complex iii . plate ( a ) indicates the signal obtained for a sample using conventional silver staining . plate ( b ) shows the enhancement achievable by the gold sandwich technology . note that two sample spots are shown in each case . for plate ( b ), a plate containing a weak silver spot corresponding to that in plate ( a ) was exposed to a solution of nanoparticle oligonucleotide complex iii , and following washing , was subjected to conventional silver staining . in this case , three parts of colloid iii , prepared as described above , was mixed with seven parts of 10 mm phosphate buffer at ph 7 . the plate was then exposed to this colloid solution for 10 minutes , washed with nanopure water , and exposed to the silver staining mixture for 4 minutes as before . [ 0053 ] fig4 illustrates the results for an experiment using gold nanoparticle oligonucleotide i to enhance the weak silver signal resulting from an assay of a 63 nucleotide target oligonucleotide 5 ′ gta ggc gaa ccc tgc cca ggt cga cac ata gg t gag gtc tgc tac cca cag ccg gtt agg tgc 3 ′ ( seq id no . : 3 ) at 25 fm concentration . note that our standard assay using just silver amplification gives a very weak spot ( plate ( a )). plate ( a ) shows the results of conventional silver staining for a test in which a 25 fm solution of the target oligonucleotide was used . plate ( b ) shows the results for the same test in which the initial silver staining was following by 10 minute exposure to the colloidal solution of nanoparticle conjugate i , followed by silver staining . in this case , the gold solution had been diluted six - fold with 0 . 1 m nacl - 10 mm phosphate buffer . note the strong signal for a target , captured at 25 fm concentration , after the metal sandwich enhancement procedure . [ 0054 ] fig5 illustrates the results for an experiment using nanoparticle oligonucleotide conjugate i to enhance the silver signal obtained from an assay carried out using a 63 - mer target [ see seq id no . : 3 ] at a concentration of 1 fm . in this case , our standard procedure for silver staining failed to show any spot . double enhancement using the gold - silver treatment , however , showed a strong signal , as shown . this experiment was carried out by treating the plate that had been exposed to silver amplification by the stand procedure successively with : ( 1 ) gold nanoparticle conjugate i ( 1 . 5 nm in nanoparticles in 0 . 1 m nacl and 10 mm phosphate buffer at ph 7 . 0 ) for 10 minutes ; ( 2 ) wash with 1 m nano 3 ; ( 3 ) treat with a mixture of silver enhancer solutions ( catalog nos . 5020 and s 5145 , sigma corporation , by the sigma protocol ; ( 4 ) wash with 1 m sodium nitrate ; ( 5 ) repeat steps 1 - 4 ; ( 6 ) wash with sodium thiosulfate ; and ( 7 ) wash with water .