Synthesis Of Gold Nanoparticles And Their Applications In The Development Of New Antimicrobial Substances

Gold nanoparticles(AuNPs)play a vital role in the development or designing of new antimicrobial substances against a number of foodborne pathogenic microorganisms.These AuNPs have been reported for their most desirable properties(stable,noncorrosive,and bactericidal activity)as compared to any other metal-based nanoparticles,which wider their applications in various fields including catalysis,bio-imaging,biosensors,medicine,biology,food safety,and material chemistry.Currently,AuNPs have been actively investigated in the areas of biology,medicine,and food that help to cope with the eradication of existing and emerging pathogenic bacteria.These AuNPs are also helpful to destroy the multidrug-resistant bacteria,which is usually impossible with routinely used antibiotics.These health threaten bacterial strains harm the human population directly or through the consumption of contaminated food products,which is one of the major risks at present.By combining AuNPs with other metal-based nanoparticles and antimicrobial compounds,the overall stability of the combined products can be enhanced.Moreover,the antimicrobial potential spectrum of combined products can be wider against several antibiotic-resistant bacteria.This dissertation covers three basic goals,which also give an analytical understanding of the antibacterial potential of AuNPs depending on their shapes.Secondly,this work gives away an environmental friendly approach to synthesize AuNPs,which are potentially acting as a catalyst,antibacterial,and antioxidant agent.Thirdly,this work involves the conjugation of antimicrobial peptides with AuNPs and their nanocomposites with silver nanoparticles(Ag NPs).The main research contents and results of this thesis are as follows:AuNPs are biochemically active because of their smaller size and specific shapes,which improved the antibacterial activity majorly through cell wall damage and lysis.In the first study,the shape-dependent antibacterial activity of AuNPs: nanospheres(AuNSps),nanostars(AuNSts),and nanocubes(AuNCs)were investigated against Escherichia coli,Pseudomonas aeruginosa and Staphylococcus aureus at lower concentrations.Shapedependent antibacterial qualitative and quantitative analyses revealed an effective bactericidal activity of AuNCs against the tested bacteria with a 100% inactivation rate,followed by AuNSps and AuNSts.The visual analysis confirmed the antibacterial activity of AuNCs and AuNSps by showing physical mutilated bacterial cells which involved cell loss,loosening of the cell wall,loss of flagella,and cellular matrix.Finally,released nucleic acid was measured for the treated bacterial cells which support the physical mutilation by releasing 38 μg/m L(Pseudomonas aeruginosa)of cellular material after treating with AuNCs.It is concluded from this study that AuNPs showed significant antibacterial property at lower concentrations.More application fields can be explored including anti-infections,decontamination,and food safety.Biological synthesis of nanoparticles using environmental friendly systems have the potential to wider the new possibilities in their applications by avoiding the involvement of hazardous chemicals.In the second study,we used Lactobacillus acidophilus for the environmental friendly production of AuNPs extracellularly which were confirmed by various physicochemical techniques.The surface-bound proteins and major carbon functional groups(C-H,C-C,and C-N)were involved in the formation and stabilization of AuNPs.These AuNPs exhibited strong catalytic activity by reducing various dyes including methylene blue,Congo red,methyl orange,and malachite green,which were reduced under 5 minutes for methylene blue and Congo red,10 minutes for methyl orange and 3 minutes for the malachite green in the presence of AuNPs.These AuNPs proved to be a moderate antioxidant agent by oxidizing 1,1-diphenyl-2-picrylhydrazyl(DPPH)compound within 40 minutes.A thin layer of AuNPs on the disposable carbon electrode showed their electrochemical sensitivity,which can be applied for biosensing protocols.The bactericidal effect of these AuNPs was tested against Escherichia coli and Listeria monocytogenes which exhibited their antibacterial properties by losing cellular integrity through membrane damage and leaky cells.This method gives a natural approach to synthesize biochemically active metal-based nanoparticles without various environmental hazards.The third study covers two major concerns regarding the synthesis of nanoparticles and improved antibacterial efficacy of nisin peptide that affect their application in medicine and food.This present work characterized the synthesis nanoparticles(NPs)and nanocomposites(NCs)of gold(Au)and silver(Ag)with face cubic centered,which are predominantly composed of(Au 100%),(Ag 74.42%)and(Au 75.51 and Ag 13.11%)respectively as major elements analyzed by using FE-EDS.Later,nisin assembly was materialized onto the interface of nanoparticles and nanocomposites.The nisin inconjunction with NPs and NCs have improved antibacterial efficacy against Grampositive bacteria(Staphylococcus aureus).Meanwhile,additional studies were done that showed nisin-NP conjunction,which includes UV-visible absorption spectroscopy,and attenuated total reflection infrared(ATR-FTIR)spectroscopy.FTIR and ATR-FTIR study showed the presence of amide Ⅰ group in NPs and NCs,while in nisin-conjugates it showed amide Ⅰ and amide Ⅱ groups due to the presence of nisin peptide.Apart from physicochemical characterization,these nisin conjugates actively inhibited the growth of nisin-resistant bacteria.The minimum inhibitory concentration(MIC)values for S.aureus was shown by Ag NPs-nisin conjugates with a concentration of 4.02 μg/m L followed by Au Ag NCs(4.8 μg/m L)and Au Ag NCs-nisin(4.83 μg/m L).This study provides a nontoxic approach for the synthesis of metal nanoparticles and their interfacial conjugation with nisin peptide that enhances its antibacterial activity against enzymebased resistance of bacteria,whereas the intact nisin peptide was completely ineffective for the studied concentrations.Overall the studies of this thesis document indicated(1)the significant shape-dependent antibacterial potential of AuNPs against Gram-positive and Gram-negative foodborne pathogenic bacteria at specific lower concentrations,(2)the environmental friendly approach to synthesize biochemically active metal-based nanoparticles,which proved to be substantial during various chemical and antimicrobial tests and(3)this document gives away a biological and environmental friendly way to synthesize metallic and bimetallic nanocomposites with the interfacial assembly of nisin,which are significant antibacterial agents against nisin-resistant bacteria.