Controllable Synthesis Of Water-Soluble, Quasi-Spherical Silver Nanocrystals And Their Application

Silver nanocrystals (Ag NCs) have attracted considerable attention due to their remarkable optical properties and potential applications in surface plasmon resonance (SPR) and surface enhanced Raman scattering (SERS). Since the properties of Ag NCs are dependent on their sizes and shapes, Ag NCs with different sizes and shapes such as spheres, cubes, prisms, plates, disks, rods, and wires have been synthesized. Currently, spherical Ag NCs are widely studied in many areas for applications, such as SERS, single-molecule labeling and recognition, antimicrobial agents and so on.Although monodisperse Ag NPs can be prepared in an organic solvent, the organic ligand surface coating has largely limited their technical applicability in biomedicine and catalysis. In the myriad of synthetic methods for spherical Ag NCs, the chemical reduction of silver salt by reducing agents, such as NaBH4, ascorbic acid and citrate, is the facile and most commonly used one. Although it has been developed for decades to reduce silver ions to small Ag NCs by NaBH4in water, the resulting NCs usually have broad size distributions. For instance, citrate is commonly used for the preparation of noble metal NCs due to its simple protocol, nontoxicity and easy to exchange ligands, and flexibility to tune their size. However, most of citrate approach easily results in Ag NCs with fairly broad size and shape distribution. To date, no reliable techniques are available to directly produce Ag NCs with monodisperse sizes and truly quasi-spherical shapes in water.In this dissertation, a given number of sodium citrate, AgN03, and anions (Cl-, Br-, I-, SO42-, CO32-, PO43-or S2-) were consecutively added to water with stirring, and then the mixture solution was added into the boiling water of ascorbic acid (AA). The monodisperse, quasi-spherical Ag NCs were synthesized in water. The factors of AA, citrate and various anions were studied systematically. The reaction mechanism of the size change of Ag NCs due to adding anions in mixture was studies in detail. Then high quality and yield Au nanocages were synthesized in water used monodisperse, quasi-spherical Ag NCs as sacrifice template by galvanic replacement reaction. The performance of H2O2detection and4-nitro phenol catalysis of Au/Ag nanocages were also studied. The main content is as follows:(1) We synthesized monodisperse, quasi-spherical Ag NCs via AA/citrate reduction protocol and discussed the reaction mechanism in detail. In the conventional citrate reduction protocol, the NCs with different shapes such as spheroid, rod, and triangle coexist, and the NC sizes are in the range of38-84nm. Due to their fairly broad size and shape distribution, the resulting Ag NCs exhibit a broad and considerably asymmetric SPR band, centered at ca.460nm; the full width at half-maximum (fwhm) is ca.150nm. It has been demonstrated that, during citrate reduction of Ag+ions to Ag0, citrate can form relatively stable complexes with positively charged Ag2+ions, which suppresses the formation of Ag42+ions. The precursors, Ag42+ions, are essential for nucleation during Ag NC growth. Thus, the nucleation rate at the early stage during Ag NC growth via citrate reduction is rather slow. Owing to the slow nucleation, the concentration of Ag+ions remaining at the NC growth stage is expected to be fairly high, so the secondary nucleation is unavoidable, thus leading to a broad size and shape distribution. In order to rapidly consume a large amount of Ag+ions for fast nucleation, we added additional reducing agent, AA to water. Monodisperse, quasi-spherical Ag NCs were synthesized via AA/citrate protocol; the size of Ag NCs is31±3nm. This significantly improved size uniformity and shape sphericity makes the SPR band of the resulting Ag NCs fairly narrow and symmetric; its maximum absorption is centered at405nm and its fwhm is ca.50nm. AA is known to have a stronger reducing ability but weaker complexation ability than citrate. Thus, the presence of AA allows fast reduction of a considerable amount of Ag+ions to Ag0with a limited amount of Ag2+ions formed. This should not only facilitate the formation of Ag42+ions for nucleation but also significantly reduce the amount of Ag+ions leftover, thus suppressing secondary nucleation and accelerating the growth of Ag NCs.(2) Monodisperse, quasi-spherical Ag NCs with the size range from16to30nm were synthesized by adding different anions (Cl-, Br-, I-, SO42-, CO32-,PO43-and S2) into premixture solution via AA/citrate reduction protocol. Here we calculated the solubility product constants (Ksp) of different silver compounds at100℃and the maximum concentrations of the corresponding anions in water at the AgNC>3concentration used. The concentrations of SO42-, CO32-, PO43-, or Cl-ions are sufficiently low so as not to precipitate Ag+ions. In the AA/citrate reduction protocol, Ag NCs were31nm. Ag NCs obtained in the presence of SO42-, CO32-, PO43-, and Cl-ions were30nm,27nm,25nm and23nm, respectively. The sizes of as-prepared Ag NPs decrease with the following potential decrease in the order of anions:NO3-> SO42-> CO32-> PO43-> Cl-. According to the LaMer model of NP growth, the sizes of final NPs ought to be determined dominantly by the numbers of nuclei formed in the nucleation stage. As such, silver compounds with small potentials are easily reduced, allowing fast nucleation and the formation of a large number of nuclei upon nucleation, thus leading to the small size of final Ag NPs. The concentration of I", Br-, and S2" ions were far larger than the maximum concentration needed to form silver precipitates as a result of the exceedingly low solubility of silver compound in water, which makes heterogeneous nucleation inevitable during the growth of Ag NPs. Thus, the sizes of Ag NCs in the presence of I-, Br-, and S2-ions were decreased from23nm,21nm to16nm, respectively.(3) The optimal citrate concentration in AgNO3/citrate mixture was found in the range of0.58-0.85mM for formation of monodisperse, quasi-spherical Ag NCs. Polydisperse, elongated Ag NPs are obtained when the citrate concentration is adjusted to below0.34mM, which should be attributed to the fact that there are not sufficient citrate ions to stabilize the growing NPs. However, polydisperse, elongated Ag NPs are also obtained at citrate concentrations greater than1.02mM. This is possibly because the citrate reduction of Ag+ions to Ag0at that high concentration inevitably takes place, thus leading to undesirable nucleation in the reaction. To guarantee the formation of monodisperse, quasi-spherical Ag NPs, one ought to strengthen the stabilizing role of citrate and, at the same time, weaken its reducing role.(4) Monodisperse, spherical Ag nanocrystals are used as sacrificial template to synthesis of high quality and yield Au/Ag nanocages by galvanic replacement reaction with HAuCl4in water at room temperature. The UV-vis spectrum of Au/Ag nanocages is in NIR. region and the peak is820nm. The linearity of the Au/Ag nanocages biosensor for the detection of H2O2spans from0.2mM to26.5mM (R2=0.999) with a detection limit of11uM (S/N=3). Due to the high surface-to-volume radios, Au/Ag nanocages can be used in biosensor, and show more excellent performance than solid Au nanocrystals. Besides, the enzyme-free Au/Ag nanocages H2O2biosensor has a short response time (<6s), and good anti-interference and stability. In order to explore the effect of the structure of Au/Ag nanocages, Au/Ag nanocages are also found to serve as an effective catalyst for the reduction of4-nitrophenol to4-aminophenol in the presence of NaBH4. The results show that the activity parameter of Au/Ag nanocages is4000s-1g-1, which is better than catalytic performance of other structure NCs. Compared with the catalysis of the same number of solid Au NCs, the activity parameter is only283s-1g-1. Thus, Au/Ag nanocages had the superior performance in catalysis.