Characterization of Bare and Surface-Modified Gold Nanoparticles Department of Chemical and Environmental Engineering University of California, Riverside Thi (Kathy) Nguyen Huynh Graduate student mentor: Hyunjung N. Kim Advisor: Dr. Sharon Walker

Outline Background Objectives Experimental Approach Results -Bare Gold Nanoparticles (GNPs) -Surface-modified GNPs Conclusions to date Acknowledgements

Background Nanostructures are popular for many industrial applications Ongoing studies investigating the interactions between nanostructures with living organisms Nanostructures are source of environmental contamination By the year of 2025, 48 countries will be short of fresh water  water reuse/recycling will become standard Therefore, the ability to remove these nanostructures must be determined.

Objectives To determine what physical and chemical mechanisms control ◈ Overall project’s objective: To determine what physical and chemical mechanisms control the transport and fate of nanostructures in aquatic environments. Task 1: Synthesis and Characterization of One-Dimensional Nanostructures Task 2: Radial Stagnation Point Flow (RSPF) experiments Task 3: Filtration experiments ◈ Specific objectives – initiating task 1: To establish methods to characterize surfaces of Gold Nanoparticles (GNPs) To compare characteristics of bare and surface-modified GNPs (S-GNPs)

Experimental Approach ◈ Model nanoparticles (GNPs) - Synthesis done by SUNRISE student in Dr. Myung’s lab - Diameter: 200 nm - Length: 2.5 – 4.0 µm ◈ Surface Modification (S-GNPs) S O H - 3-Mercapto-1-Hexanol (C6H14OS) - Procedure Wash with DI water for 7 times: centrifuge at 12000 rpm for 2 minutes each time 3hrs GNPs + 1mM 3-Mercapto-1-Hexanol (1mL) (2mL)

Experimental Approach ◈ Surface Characterization Size Measurement (Inverted microscope Olympus IX70) Hydrophobicity (VCA Optima Goniometer) Electrokinetic properties (ZetaPALS)

What is Electrokinetic Property? A particle’s ability to move in the electromagnetic field ZetaPALS measures the particles’ mobility, and then calculates to give zeta potentials or the surface charge values Mechanism: Point of measurement Potential Distance from surface Stern layer

for these particles and in this condition Results – Electrokinetic Properties of GNPs ◈ Effect of concentration ◈ Effect of size pH: 5.8, DI water, 3 µm pH: 5.8, DI water Optimum concentration (OD546nm) : 0.15 - 0.30 Mobility ≠ f (size) for these particles and in this condition

Results – Electrokinetic Properties of GNPs ◈ Effect of valence and ionic strength As ionic strength increased in the presence of salt solutions, mobility became less negative (charge on particle approached neutral) Valance had an important role on mobility: in the presence of divalent cations, mobility was less negative than that in the presence of monovalent cations. pH: 5.8

What is Hydrophobicity? Hydrophobicity refers to a surface’s property of being water-repellent Task: To what degree are GNPs hydrophobic? Contact Angle Method gLG Hydrophobic: ө>90o Hydrophilic: ө<90o Water droplet ө gSG gSL Solid surface

Results – Hydrophobicity of GNPs ◈ Contact angle measurement - Solution concentration: OD546nm : 1.684 (2.5x dilution) Glass 20 μL 70 μL 100 μL 200 μL Optimum concentration Contact angle of Bare GNPs : 130.6  3.2 O  Surface of bare GNPs: Hydrophobic

Results – Electrokinetic Properties of Bare GNPs vs. S-GNPs Why surface-modified? - The mobility of S-GNPs was less negative than that of bare GNPs in the presence of KCl. However, the difference was not significant in the presence of CaCl2. - Valence played an important role on GNPs’ mobility regardless of the presence of 3-mercapto-1-hexanol groups. pH: 5.8

Results – Hydrophobicity of GNPs vs. S-GNPs Bare GNPs S-GNPs Contact angle of S-GNPs : 135.8  3.2 O Surface of S-GNPs: Hydrophobic Functional groups 3-mercapto-1-hexanol did not affect the hydrophobicity significantly.

Proposed Mechanisms Why did mobility of GNPs decrease in the presence of 3-mercapto-1-hexanol? SH end, hydrophilic with greater affinity to GNPs Modification OH end, hydrophilic end Increase in mobility of GNPs and Surface becomes more hydrophilic Decrease in mobility of GNPs and Surface becomes more hydrophobic ◈ Proposed Changes: Increase in concentration of 3-mercapto-1-hexanol Increase in amount of time suspending the GNPs in the solution Reduce the length of the GNPs when keeping the same concentration

Conclusions to date 1. Methods to characterize the surface of GNPs has been established. Mobility of GNPs was not a function of concentration nor size, over a range investigated in this study. 2. Solution chemistries (Ionic strength and valence) considerably influenced mobility of bare and surface-modified-3-mercapto-1-hexanol GNPs. 3. Mobility of S-GNPs was less negative than that of bare GNPs in the presence of KCl, while the mobility was not sensitive to the presence of 3-mercapto-1-hexanol in the presence of CaCl2. 4. The surface of bare GNPs was determined to be hydrophobic. 5. The modification of 3-mercapto-1-hexanol did not make a significant difference in hydrophobicity.

Acknowledgements The Coordinators of BRITE Program The bacterial adhesion research lab members Dr. Nosang Myung and Heather Yang

Thank you !! Questions?