Fluorescent Polycytosine-Encapsulated Silver Nanoclusters
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Small silver nanoclusters are synthesized using polycytosines as matrices. Different size silver nanoclusters ranging from Ag1 to Ag7 exhibit bright emission maxima at blue (480nm), green (525nm), red (650nm), and IR (720nm) wavelengths with varying the excitation wavelengths. With electrophoresis, correlation of emission with mass spectra, the Ag cluster sizes are identified with blue emitters as Ag5, green emitters as Ag4, red emitters as Ag3, and IR emitters as Ag2. Ag4 and Ag5 appear to be partially oxidized while Ag2 and Ag3 are likely fully reduced. Silver cluster stability and their dynamics are observed from silver clusters encapsulated by polycytosine (Cm:Agn). From length study of polycytosine, the longer the polycytosine is, the more stable the larger clusters such as Ag5 are. In time-dependent optical measurements, isosbestic points are observed from Cm:Agn by converting red and IR species into blue and green species, while in the case of temperature-dependent optical properties, with increasing temperature, the blue (oxidized Ag5) and green (oxidized Ag4) emitters convert into the red (Ag3) and IR (Ag2) emitters. NaCl-dependent optical measurements support the assignments of oxidized and fully reduced silver emitters. Circular dichroism (CD) is used to investigate conformational changes in Cm and Cm:Agn with varying conditions (time, temperature and NaCl) and the studies indicate that no conformational changes in Cm:Agn are observed from the time and temperature, while the conformational changes in Cm:Agn are observed from the NaCl studies. From pH-dependent emission study of Cm:Agn, the silver nanocluster dynamics slow down at high pH. Using confocal microscopy technique, single molecules on IR species, C12:Ag2 are investigated and demonstrate that C12:Ag2 is brighter and more photostable than Cy5 which is known to be one of the best IR dyes. With low excitation power, molecules can be monitored for hours, giving bright blinking free, stable fluorescence. The photophysics of this new dye make it a promising candidate for single molecule studies in biological applications.