Silver nanoclusters (AgNCs) generated on DNA-templates belong to a new class of fluorescent tags showing excellent brightness, photostability as well as biocompatibility. Thus, AgNCs-DNA have been applied in various applications, from the detection of DNA/RNA, environmental monitoring to bioimaging and cancer therapy. In this work, we report fluorescent AgNCs synthesized using two 1,3-diaza-2-oxophenothiazine (tC)-modified oligonucleotides that contain RET-related sequence CCCCGCCCCGCCCCGCCCCA. The communication compares the absorption and emission properties of the obtained systems with silver nanoclusters synthesized on the unmodified oligonucleotide. First, we showed the optimal conditions for AgNCs-DNA synthesis on three DNA templates: (1) RET20 with the sequence 5'-CCC CGC CCC GCC CCG CCC CA-3'; (2) 19tC with the sequence 5'-CCC CGC CCC GCC CCG CCC tCA-3'; (3) 14tC with the sequence 5'-CCC CGC CCC GCC CtCG CCC CA-3'. Next, the silver nanoclusters were characterized by UV/Vis absorption, fluorescence and circular dichroism spectroscopy. Silver nanoclusters RET19tC-AgNCs and RET14tC-AgNCs indicated the several times higher fluorescence intensities in the long-wave emission spectra as compared to RET20-AgNCs. Moreover, silver nanoclusters on tC-modified oligonucleotides showed the higher stability over time. The possibility of using of the silver nanoclusters RET19tC-AgNCs and RET14tC-AgNCs for monitoring pH changes would be also demonstrated.