Controllable Preparation And SERS Properties Of The Tannic Acid Caped Gold And Silver Nanoparticles

Gold nanoparticles (Au NPs) and silver nanoparticles (Ag NPs) have attracted extensiveresearch interests due to their size/shape dependent optical/electric properties and potentialapplications in catalysis, biological sensing, nonlinear optics, and surface enhanced Ramanspectroscopy (SERS). During the past decades, great efforts had been devoted to control thesize and shape of Au and Ag NPs. In our previous work, Ag NPs with a size range of3070nm could be obtained by optimizing the pH-dependent activity of ascorbic acid reductant.This means that the reactivity of reduce agent is important one of the factors on control of theAg NP size. Therefore, the controlling reactivity of reduce agents is an important method oncontrol the size of Ag NPs. From the point of fundamental and application research, it is stillmeaningful to explore reaction system in which size of the Ag NPs could be tuned mucheffectively.In this dissertation, we developed a method on preparation of nearly monodispersedspherical Ag NPs by using tannic acid (TA) as reductant. The size tenability of Ag NPs ismainly attributed to the use of TA, which effectively promoted the separation of thenucleation and growth processes during preparation of Ag NPs. The size of the as-preparedAg NPs could be tuned in a range of3-66nm by changing the molar ratio of TA to silvernitrate and pH of the reaction solutions，and size of the Ag NPs was extendable up to200nm in one-pot fashion by the multi-injection approach. The size-dependent surface-enhancedRaman scattering (SERS) activity of the as-prepared Ag NPs was evaluated and the NPs withsize around100nm were identified to show a maximum enhanced factor of3.6105.Moreover, the as-prepared TA-coated Ag NPs presented excellent colloidal stability comparedto the conventional citrate-coated ones.Nearly monodispersed spherical Ag NPs were synthesized by using TA both as reductantand ligands. The size of the as-prepared Ag NPs could be tuned in a range of7-66nm bychanging the molar ratio of TA to silver nitrate and pH of the reaction solutions. It is foundthat the size of the resulting Ag NPs is decreased with the decrease of the monomerconcentration above pH8.5. When the concentration of silver nitrate was set as0.1mM andthe molar ratio of TA/AgNO3was1:10, the size decreased from9nm to3nm of the resultingAg NPs with the increased pH from8.5to11. UV-visible spectra, TEM observations andtemporal evolution of the monomer concentrations for the reactions carried out at differentexperimental conditions showed that the improved size distribution and size tunability of theAg NPs were mainly attributed to the use of TA which could promote the balance ofnucleation and growth processes of the NPs effectively. The size of the Ag NPs is primarilydependent on the number of the nuclei, which should be related to the activity of the silvermonomer and reducing power of the reductant. This fact can be explained well in term ofdiffusion limited reaction mechanism.Here, the formation of these nearly monodispersed spherical Ag NPs with different sizesindicated the good separation of the nucleation and growth processes in these reactions.Multi-injections were known to be feasible used commonly for the further growth of NPs atfixed number of nuclear. To eliminate the continuous nucleation caused in the multi-injections,system pH was set at5.5. By using the multi-injection approach, size of the Ag NPs could beextended to up to200nm in one-pot fashion by using TA as reductant. Overall, the size of AgNPs is completely tunable in a range of3-200nm by current technique. Interestingly, thePlasmon resonance of Ag NPs is ligand-dependent for a given sample. It is found that thePlasmon resonance of TA-coated Ag NPs experienced an obvious red-shift compared to thetheoretical value, while the citrate-coated ones only shifted slightly to longer wavelength. It isknown that the Plasmon resonance is related to the dielectric constant of the medium, which is proportional to the polarizability of the medium. Therefore, the difference of dielectricconstant (or polarizability) from the ligands is totally responsible for the Plasmon resonanceof Ag NPs with the same size.Apart from preparation of Ag NPs, the nearly monodispersed spherical Au NPs withdifferent sizes were also synthesized by using TA as reductant. By control the reactivity of TAnearly monodispersed Au NPs were also obtained, indicating that TA might be a generalreductant for preparation of other metal NPs. The size of as prepared Au NPs is tunable in asize range of20-65nm in diameter. In addition, flower-shaped Au NPs were successfullyprepared in one pot reaction by control of the ratio of gold precursor to TA.The colloidal stability of such TA-coated Ag NPs was compared with the conventionalcitrate-coated Ag NPs. Colloidal stability of the Ag NPs was evaluated by repeatedprecipitation and successive redispersion of the NPs in pure water. The variations inabsorbance intensity of TA-coated Ag NPs and citrate-coated ones with the cycles ofprecipitation and redispersion showed that the absorbance of the citrate-coated Ag NPsdecreased rapidly with the cycle times, and became very weak after6cycles. However, theabsorbance intensity of the TA-coated Ag NPs almost kept unchanged even after8cycles,indicating their excellent colloidal stability compared to the conventional citrate-coated ones.Colloidal stability of the Ag NPs was further evaluated by dispersing the NPs into PBSbuffers with different concentrations. Experimental results indicate that the TA-coated Ag NPsare more tolerant of ion-rich environments than the citrate-coated ones.Finally, SERS spectra of the Ag NPs with different sizes were collected by using R6G asprobe. The enhancement factor was calculated for Ag NPs with various sizes. Theexperimental results indicated that the enhancement factors first increased and then decreasedwith the increased size of the NPs. A maximum value (3.6105) was observed for the Ag NPswith an average size around100nm. The enhanced factor of Ag NPs increased with theincreased size of Ag NPs attributed to the promoted local electromagnetic enhancement.However, increased particle size would also contribute to increased scattering efficiency,which resulted in lowered enhanced factor for the NPs with larger size. It is expected that amore precious control over size and shape of the Ag NPs be helpful to further understand theirsize/shape-dependent SERS performance.