Synthesised Fluorescent Silver Nanoparticles For Environmental Analysis

Noble metal (Au, Ag) nanoparticles exhibited unique physical and chemical properties. With small sizes approached the Fermi-wavelength, these nanoparticles had clearly identifiable, size-tunable and stable fluorescence properties due to radiative relaxation from electronically excited states. Silver nanoclusters have high fluorescence quantum yields because their sparser density of states favors radiation relaxation from electronically excited states. The high fluorescence quantum yield and the photostability make silver nanoparticles as good candidates for environmental analysis after Au nanoparticles.Chapter 1, we include a brief discussion on the chemical properties and the preparation of silver nanoparticles. We probe into the analysis of environmental pollutants in order to achieve the desired selectivity and sensitivity for analysis of environmental pollutants.Chapter 2, the N-Acetyl-L-cysteine protected silver nanoparticles were synthesized by the NaBH4 reduction of AgNO3. The morphology was characterized by TEM. The TEM images show that small spherical Ag NPs with 6 nm in diameter. The UV-Vis absorption of NAC-Ag NPs showed an extinction band around 500 nm and stable fluorescence emission at 662 nm when excited at 480 nm. Upon the addition of biothiol, the fluorescence emission of NAC-Ag NPs was steadily enhanced. NAC-Ag NPs fluorescent sensing behavior toward biothiol was investigated in buffered media. A linear range of homocysteine was obtained from 2.5×10-6 M to 1.25×10-5 M with the limit of detection of 2.8×10-7 M and a linear range of glutathione was obtained from 2.5×10-6 M to 1.25×10-5 M with the limit of detection of 3.3×10-7M.Chapter 3, DNA oligonucleotides could offer a convenient scaffold to synthesize Ag NCs which could directly self-assemble on single-stranded DNA with sequence dependent fluorescence. Motivated by this theory, developing suitable DNA sequence could make fluorescence emission peak of Ag NCs in different region to improve fluorescence sensitivity.The DNA hairpins stabilized Ag nanoclusters showed a strong extinction band with a maximum at a wavelength of 567 nm and stable emission at 632 nm (λex= 578nm). The resulting silver nanoclusters featured a QY of 37%. An excellent selectivity toward the detection of CN- in aqueous solution over a wide range of tested anions was observed. The fluorescence intensity is linear with the CN- concentration in the range of 2.5×10-9 to 7.5×10-8 M. The detection limit is 2.5×10-9M.Oligonucleotide stabilized Ag nanoclusters (DNA-Ag NCs) with near-infrared-emitting have been designed and synthesized. The UV-Vis absorption of DNA-Ag NCs showed a strong extinction band at 724 nm and stable fluorescence emission at 768 nm when excited at 712 nm. A linear range of Hg2+was obtained from 1.90×10-9 M to 2.4×10-8 M with limit of detection (S/N=3) of 1.90×10-9 M.Chapter 4, chitosan stabilized Ag nanoparticles were prepared by hydrothermal synthesis. Reaction conditions were investigated in the process of reaction temperature, reaction ratio and reaction time. The chitosan with silver ions of mole ratio of 1:1, at 160℃ reaction 3 h as preparation conditions of silver nanoparticles. A linear range of Hg2+was obtained from 5.0×10-8 M to 5.0×10-7 M with limit of detection (S/N=3) of 2.4×10-8 M.Chapter 5, A facile and simple one-step method for the synthesis of fluorescent Ag NCs, using bovine serum albumin (BSA) without preprocessing as scaffold was developed. BSA assembled Ag NC (BSA-Ag NCs) were first found to exhibit ratiometric fluorescence with blue emission at 480 nm and red emission at 634 nm. Upon the addition of Hg2+, the emission of BSA-Ag NCs at 480 nm gradually increased and at 634 nm steadily decreased. The presence of Hg2+induced its color change from light yellow to colorless and a bright red fluorescence to blue fluorescence. The determination of Hg2+based on ratiometric fluorescence was developed. The linear dynamic range was from 5.0×10-8 M to 2.5×10-5 M with a detection limit of 4.87×10-8 M. The relative standard deviations (RSD) at 0.30μM for Hg2+of six replicates determination was 2.8%. The BSA-Ag NCs were synthesized conveniently and facilely, which could be served as a "naked-eye" fluorescent probe for Hg2+. It might be great candidate for the application of sensing Hg2+ in environmental analysis and fluorescence imaging.Chapter 6, the research results of this paper was summarized and the future research direction was speculated.