Biothiols Detection with Nanomaterials and the Synthesis of the Noble Metal Nanoclusters for Antibacterial Application

Author:Zhu Hai Shuang

Supervisor:wang jin


Degree Year:2019





Metal nanoclusters bridge between the metal atoms and plasmonic nanoparticles.When the size decreases to<2 nm,metal nanoclusters behave discrete electronic structure,strong quamtum size effects come into play.These ultrasmall nanoclusters become very sensitive to particle size,even a single atom makes a difference.The stacking and structure of the core and the outside motifs have an original influence on the stability,catalysis,photoluminescence and biomedicine application.Acquisition of pure and atom precise single crystal is the pivotal step before further XRD structure analysis.Assortment of crystal structures has been successfully resolved by the material scientists,whereas the pyrimidinethiol capped nanoclusters is a blind spot maybe due to the water-soluble property with the difficulty of crystallization.Herein,we have successfully obtained the single crystal and further structure is analyzed.The antibacterial effect of the metal nanoclusters aroused widespread interest after Xie’s research of Au nanoclusters behaved antibacterial effect due to its size.However,the further application was hampered by the relatively low quantum yield and stability.Herein,protamine(Prot)functionalized Au NCs(Prot/MTU-Au NCs)with high stability was achieved through a simple mixing with 6-methyl-2-thiouracil capped Au NCs(MTU-Au NCs)due to the hydrogen bonding between MTU and the guanidine groups from Prot.Interestingly,a distinctly enhanced photoluminescence from Prot/MTU-Au NCs(ca.28 folds)was observed due to the formation of rigid host-guest assemblies.We inferred that the cross-linked structure and supramolecular hydrogen bonds both contributed to the fluorescence enhancement and stability.The extra small size of the NCs and the efficient antibacterial capability from the capping shell of Prot promoted us to probe its antibacterial performance systemically.It was found that the Prot/MTU-Au NCs with highly-stable loading of positive charged antibacterial reagents were prone to penetrate into the bacteria and thus enhanced the ability to kill both gram-negative bacteria(Escherichia coli)and gram-positive bacteria(Multiple-resistant Staphylococcus aureus).The synergetic effect between the unique size and the capping layers enabled the minimal inhibitory concentration of as-derived Prot/MTU-Au NCs reduced by~100 folds compared to that with the individual Au nanoparticles.The antibacterial mechanism further revealed that membrane injury occurred and reactive oxygen species were generated after the incubation of the bacteria with Prot/MTU-Au NCs.Moreover,the highly luminescent fluorescence and positive surface charge of Prot/MTU-Au NCs could image the bacteria easily,which held great potential for imaging-guided antibacterial platform.Inspired by the antimicrobial effect of a peptide from bovine neutrophils called Bactenecin(Pepl),we designed a new kind of antimicrobial peptide(AMP)with abundant arginine and leucine(CCLLLLRRRRRR,Pep2)with similar antibacterial property.Employed the new AMP as protectant,another kind of Ag nanoclusters was synthesized with pink luminescence.Confirmed by TEM and mass spectra,1.25 nm pricise and uniform Ag19 nanoclusters were obtained.The reliance on traditional extracting AMP from natural organism was relieved and the expense of AMP was greatly reduced by decreasing its dosage via decreasing the minimum inhibitory concentration(MIC)after chelating with Ag.After careful investigation,we attributed the excellent antibacterial effect to three aspects including size effect,which led to higher level release of Ag+,capping agent with antibacterial effect and the luminescence.MIC determination indicated that the bacterial-killing efficiency was improved by three orders of magnitudes compared to individual commercial Ag NPs.Further biocompatibility test and animal model also proved its practical application safety issue and antibacterial efficiency.Thus the presented Ag nanoclusters can be an excellent alternative with the threat of resistance and tolerance of antibiotics.L-tyrosine methyl ester capped carbon dots(Tyr-CDs)were employed and prepared as the fluorescent probes.The as-prepared Tyr-CDs displayed narrow size distribution and distinct blue fluorescence with high quantum yield(12.9%)compared with the unmodified CDs.Moreover,Tyr-CDs exhibited higher quenching efficiency due to the efficient energy transfer between Tyr-CDs and the quinone products in the presence of tyrosinase.When the targeted biothiols was present,the catalytic reaction of the tyrosinase to the formation of quinone was inhibited and the fluorescence signal was recovered in a biothiols-concentration-dependent manner which provided the basis for the analysis of biothiols.The practical application of the present system was demonstrated by testing the biothiols in human plasma samples and good recovery was obtained,indicating that the sensing platform we proposed hold great promise in the accurate detection of biothiols in complex biosystems.Moreover,the present system avoided the inputs of heavy metals as quenchers,which was beneficial for the bioanalysis.