The Effect Of Functional Molecular Modification On The Aggregation Behavior Of Gold Nanoparticles And Its Mechanism

Gold nanoparticles(AuNPs),exhibiting unique photophysical properties such as surface plasmon resonance and easy surface modification,have been widely applied in bio-sensing,drug release,catalysis,etc.Sensors based on the aggregation behavior of AuNPs have drawn increasing attention.In the reported literature,researchers have paid more attention to developing sensors based on the aggregation behavior of functionalized AuNPs.However,some explanations for the aggregation mechanisms behind the color change are inconsistent and not verified in the reported literature.As a result,there are many contradictory results in similar study systems.If there is no experimental study to support the theoretical hypotheticals,it will inevitably be misleading in the design and applications of functionalized AuNPs.Hence,it is necessary to explore the reasonable mechanisms behind the experimental phenomena in the applications based on functionalized AuNPs composites.Generally,researchers expect the aggregation behavior of functionalized AuNPs as same as that of their desired design,and give an explanation without any experimental supports.In our opinion,lack of understanding for the complexity of functionalized AuNPs sensing systems is the main cause to result in the inconsistent results and explanations in the literature.To confirm the above analytical judgment,this dissertation aimed at clarifying the contradictory interpretations in the reported literature about the aggregation behaviors of AuNPs respectively modified by several typical functional molecules.These functional molecules include 4-mercaptophenyboronic acid(4-MPBA),cetyltrimethylammonium bromide(CTAB),poly(N-Isopropylacrylamide)(PNIPAM)and polyacrylic acid(PAA).Based on the results obtained from systematical studies of the four typical modified AuNPs systems above mentioned,the following conclusions could be drawn.Firstly,the aggregation behavior of AuNPs involved in 4-MPBA and H2O2 has been investigated due to the inconsistent results regarding this system reported in literature.The results indicate that(1)the aggregation of citrate capped-AuNPs caused by 4-MPBA mainly ascribes to the change in the charge density of AuNPs rather than the condensation between citric acid and 4-MPBA or 4-MPBA molecules as reported in literature;(2)The reaction of H2O2 with free 4-MPBA is different from that of H2O2 with 4-MPBA immobilized onto AuNPs.For free 4-MPBA((4-MPBA+H2O2)+AuNPs),with an increase of the amount of H2O2,H2O2 oxidizes 4-MPBA not only to form 4-HTP and boric acid but also to form BHPD with bigger steric hindrance and peroxoboric acid with stronger acidity.However,for 4-MPBA immobilized onto AuNPs((4-MPBA+AuNPs)+H2O2),BHPD cannot form except for the formation of similar products as((4-MPBA+H2O2)+AuNPs)system.These differences make the mixture((4-MPBA+AuNPs)+H2O2)more sensitive to the aggregation of AuNPs caused by H2O2 than the mixture((4-MPBA+H2O2)+AuNPs);(3)The formation of peroxoboric acid can markedly reduce the pH of the medium,which affects the distribution of species with weak acidity or basicity.In 4-MPBA,H2O2 and AuNPs involved system,the effect of this change in pH on the aggregation of AuNPs should not be ignored.The second work is regarding the effect of CTAB on the aggregation of citrate capped-AuNPs and the mechanism on the assembly structure variation of CTAB on the surface of AuNPs.The following main findings are included in our investigations:(1)There is a strong interaction between positively charged CTAB micelles and negatively charged AuNPs.CTAB micelles in AuNPs solutions are prone to disassemble and reassemble on the surface of AuNPs.In the beginning,the positively charged heads of CTAB molecules adsorb on the negatively charged AuNPs via electrostatic interaction,forming a CTAB monolayer with outward hydrophobic tails.Then,the formed hydrophobic CTAB monolayer interacts with disassembled CTAB micelles in the solution through hydrophobic interaction,resulting in a CTAB bilayer with outward hydrophilic heads.This variation process of CTAB structure on AuNPs corresponds to the change in the ? potential of AuNPs from negative to zero and then to positive,achieving a transition of AuNPs from a negatively charged stable state to a positively changed stable form.As long as the numbers of CTAB are enough to formation of bilayers on the surface of AuNPs,the abovementioned assembly could occur regardless of the CTAB concentration being lower or higher than its CMC.This finding overturns the traditional cognition that CTAB micelles stabilize citrate capped-AuNPs.(2)The formation of the CTAB bilayer on the surface of AuNPs has advantages over that of CTAB micelles.When enough amount of CTAB exists in the AuNPs solution,only after forming a bilayer of CTAB on the surface of AuNPs could CTAB micelles form around AuNPs.(3)The mixing sequence of the positively charged CTAB and the negatively charged AuNPs can directly impact the reversibility of the aggregation of AuNPs induced by the CTAB assembly.The aggregates formed by the gradual addition of AuNPs into a CTAB solution can be re-dispersed through the further addition of CTAB,whereas the aggregated AuNPs induced by the dropwise addition of a CTAB solution into an AuNPs solution are difficult to be re-dispersed.The third work reveals the reason for the temperature-sensitive failure of PNIPAM anchored on the surface of AuNPs.The results indicate that(1)the failed thermo-responsiveness of AuNPs-PNIPAM composites is due to the strong interaction among PNIPAM molecules anchored on the surface of AuNPs to hinder the conformation change of PNIPAM;(2)The aggregation of AuNPs-PNIPAM caused by HCl and salt attributes to changes in Zeta potential and local refractive index of AuNPs rather than the conformational change of anchored PNIPAM;(3)AuNPsPNIPAM system containing free PNIPAM shows a visible thermo-responsiveness,which ascribes to the conformation change of free PNIPAM rather than that of the PNIPAM tethered on the surface of AuNPs.Moreover,this effect of free PNIPAM in AuNPs-PNIPAM solution is only manifested in the turbidity change,and has little effect on SPR of the AuNPs;(3)The reversible color variation of Au-PNIPAM composite caused by HCl relates to the Zeta potential change,which attributes to reversible adsorption and desorption of protons by PNIAPM.The last work reveals the essential reason for the aggregation behavior responding to pH for AuNPs modified with PAA.The following main findings are included.(1)The strong interaction among PAA molecules anchored on the surface of AuNPs hinders the conformation change of PAA to respond to pH change.Its aggregation behavior causing a reversible change of solution color with pH attributes to the surface charge change of AuNPs-PAAs.(2)All factors that can increase the charge density of AuNPs-PAAs,such as CTAB,salt and sodium polystyrene sulfonate,could enhance the dispersion of AuNPs-PAAs.(3)Dielectric properties of solvent can affect the pH responsiveness of AuNPs-PAAs.The aggregation state of AuNPs-PAAs has opposite pH responsiveness in aqueous and ethanol solutions,which relates to the effect of dielectric properties on the electrostatic interaction among AuNPs-PAAs composites.All the above results are obtained from systematically designed contrast experiments.These findings are very valuable not only to clarify the inconsistent results reported in the literature but also to design sensing systems based on the change of aggregation behavior of AuNPs assembly for various applications.