this research was partially supported by grants from rfbr ( nos.05 - 02 - 16776 , 04 - 04 - 48224 ) , the targeted program research of cooperative and non - linear phenomena in light transport through mesascopic media as applied to development of diagnostical techniques in biology , medicine and industry bk was supported by grants from the president of russian federation ( mk 961.2005.2 ) , crdf ( brhe annex bf4m06 y2-b-06 - 08 ) , and intas young scientist fellowship grant 06 - 1000014 - 6421 . vz was supported by grants from the national institute of biomedical imaging and bioengineering ( nih / nibib , nos . eb000873 and eb0005123 ) .
we report on the first application of silica - gold nanoshells to a solid - phase dot immunoassay . 
 the assay principle is based on staining of a drop ( 1 l ) analyte on a nitrocellulose membrane strip by using silica / gold nanoshells conjugated with biospecific probing molecules . 
 experimental example is human igg ( higg , target molecules ) and protein a ( probing molecules ) . for usual 15-nm colloidal gold conjugates , 
 the minimal detectable amount of higg is about 4 ng . by contrast , for nanoshell conjugates ( silica core diameter of 70 nm and gold outer diameter of 100 nm ) we have found significant increase in detection sensitivity and the minimal detectable amount of higg is about 0.5 ng . 
 this finding is explained by the difference in the monolayer particle extinction .