Catalytic activities of metal laurates with different coordination capabilities and sodium salts with different ligand alkalinities toward the transesterification between isosorbide (ISB)-based polycarbonate (I c C-PC) and bisphenol-A polycarbonate (BPA-PC) are investigated to accelerate asymmetric chain exchange without aggravating chain degradation. Only catalysts capable of direct reaction with PC chains to form metal alkoxide containing active species (PC–O-M) can initiate and thus accelerate chain transesterification . Sodium salts exhibit the highest catalytic activity and can be further improved by complex formation of 15-crown-5, demonstrating the key role played by alkalinity of reactive ligands. Different from weak catalytic selectivity and serious chain scissions by tin-based catalysts, it is found that, for sodium-catalyzed systems, BPA-PC chains prefer to bond to ISB segments in endo position and the high reactivity of sodium-based catalysts not only significantly reduces essential loading but also suppresses chain degradation. We confirm that these differences must arise from whether active PC-O-M acts as a direct nucleophilic attacker or works with coordination process.
In this paper, DNA containing six cytosines as the formation site for silver nanoclusters (Ag NCs) was adopted as a template for preparing fluorescent DNA-Ag NCs. For the first time, it was found that the fluorescence of DNA-Ag NCs could be quenched after hybridization with their complementary sequence. On the basis of this new phenomenon, we designed a sequence C1 that was completely complementary to human immunodeficiency virus (HIV) DNA, and probe DNA which was partially complementary to C1 for the synthesis of DNA-Ag NCs. The fluorescence of DNA-Ag NCs was quenched after hybridization with C1 and the DNA-Ag NCs/C1 composite was formed, while C1 could be dissociated away from the DNA-Ag NCs by HIV DNA through a strand exchange reaction due to the stronger affinity between HIV DNA and C1, which could switch on the quenched Ag NCs, thus a new “off–on” fluorescence method for HIV DNA detection was developed. In the experiment, the Ag NCs formation site of DNA, the number of base pairs, and the pH and salt concentration of binding buffer were optimized. Under the optimum conditions, the limit of detection for HIV DNA was obtained to be 3.18 nM (3σ/N, n = 7) with the linear range of 15–150 nM for the 150 nM DNA-Ag NCs/C1 probe. Besides, the probe showed excellent specificity to HIV DNA, and even distinguished one nucleotide mismatched HIV DNA.
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