Following this experimental scheme SiGNPs decorated with a ssDNA sequence (called Utag) or streptavidin were synthesized. Absorption spectra for functionalized SiGNPs are reported in Figure The performance of the so-obtained functionalized AuNPs was assessed using SP-IRIS prototype that is able to count individual nanoparticles bound to the surface of a microarray chip as described in Sections 2.15 and 2.16. Briefly, chips were functionalized with two sequences of DNA, namely Probe2 (the negative control) and Probe7. The chips were incubated with Utag-Tag7 that binds to Probe7. Then, SiGNPs binds to Utag-Tag7 directly (in the case of Utag-functionalized SiGNPs) or through Utag-Biotin sequence (when streptavidin-functionalized SiGNPs are used). Results (see Figure ) show that both SiGNPs (functionalized with either Utag or streptavidin) can bind selectively to positive spots (i.e. Probe7) with low nonspecific signals on negative spots. It can be also appreciated how streptavidin-functionalized SiGNPs perform better that Utag-functionalized ones (7-fold improvement in the signal). We hypothesize that this divergent performance stems from the necessity for high-affinity interactions to bind bulky objects such as 80 nm AuNPs. For this reason, streptavidin-functionalized SiGNPs are more likely to be immobilized on the surface of positive spots than ssDNAfunctionalized ones.