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Silver nanoparticles (NPs) with diameters less than 25 nm, and 5, 10, 15, 20-tetrakis(4-carboxyphenyl) porphyrin potassium salt (K4TCPPS) was synthesized. By adding silver colloids into the K4TCPPS solutions, the fluorescence concentration quenching of the K4TCPPS solutions was improved between 1 x 10(-6) - 5 x 10(-4) M, depending on the size of the Ag NPs, the concentration of the K4TCPPS and the excitation wavelength. Under excitation wavelengths of 360 and 414 nm, the enhancement was in the range of 0.34-22.8, and 0.71-82.46, respectively. The enhancement increased with decreasing size of the Ag NPs. At the two highest enhancements, the concentrations of the K4TCPPS solutions were 1 x 10(-4) and 1 x 10(-5) M respectively, and the size of the Ag NPs was about 5 nm. The main reasons for the fluorescence enhancing effect are due to fluorescence resonance energy transfer (FRET) between K4TCPPS molecules and Ag NPs, and decreased collision quenching between free K4TCPPS molecules, and free K4TCPPS molecules with Ag NPs. Our results showed that Ag NPs with sizes of about 5 nm can effectively improve the fluorescence concentration quenching of K4TCPPS solutions. This result also suggests that it is possible to use noble metal NPs to improve fluorescence concentration quenching.

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