Molecular Biomimetics: Synthesizing Gold Nanostructures with Amino Acids Alexander Chen Chemical and Environmental Engineering Department, UC Riverside

Reasons For Using Biomimetics Simple and efficient method that creates consistent nanostructures and patterns in easily controllable environments. Environmentally Safe- no harmful waste are produced in comparison to chemical methods of synthesis. Has potential for greater compatibility and efficiency for integration within biological environments during medical use.

Process of Synthesizing Gold Nanostructures with Amino Acids The nanostructures are synthesized through mixing AuCl 4 and specific amino acids within an aqueous solution and incubated (Mimicking physiological environments in which typical synthesis reactions occur). The gold structures that are formed are based on the specific binding properties of each particular amino acid and the control of concentration, contents of solution, pH, and temperature. Synthesis Conditions and Procedure: –1mL samples consisting of 940ųL water, 40ųL 0.5mM or 20mM AuCl4, and 10ųL amino acid (Alanine, Arginine, Asparagine, Aspartic acid, Cysteine, Glutamic acid, Glutamine, Glycine, Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Proline, Serine,Threonine, Tryptophan, Tyrosine, and Valine) and 1 control sample with no amino acid –Excess chloride concentration were set at 0N, 0.1N, and 0.5N for various sets. –25M NaOH used to adjust pH to 3 and 5 for 0.5mM AuCl4 and to 3.2, 4.5, 5.2, 7.5, and 11.6 for 20mM AuCl 4

- Initial pH of each individual sample is recorded to ensure uniformity between samples (±0.1) - Solutions are incubated for 3 days at 37°C - Centrifuged and washed for three cycles at 10 rpm and 10min/cycle. - Final pH after the reaction are measured to examine correlation with synthesizing and binding efficiency. - 10ppm of amino acid solution mixed with HCl(10%) to create 5mL solutionand measured gold ion concentration in solution with AAS (Atomic Absorbtion Spectrophotometer). - Photographs of gold nanostructures and speciation taken with microscope.

Amino Acids and Gold Chloride Used to Make Standard Solution

C Extremely small, granular gold structures D Medium, thin, overlapping, truncated triangle gold structures P Medium, solid, polygonal gold structures W Extremely small, granular gold structures Speciation of Gold Nanostructures (0.5mM AuCl 4 pH 3 0.0M NaCl)

Reduction Rate of Amino Acids (0.5mM AuCl 4 pH 3 0.0M NaCl)

L-PH5-0.0N NaCl Small, head and tail w/ granular, gold structures N-PH5-0.0N NaCl Small, granular, gold structures L-PH5-0.1N NaCl Large, head and tail w/ morphing into polygonal plates N-PH5-0.1N NaCl Small, head and tail w/ clusters, gold structures L-PH5-0.5N NaCl Large, solid polygonal plate, gold structures N-PH5-0.5N NaCl Very small, solid polygonal plate, gold structures 0.5mM AuCl 4 (0.0M, 0.1M, 0.5M NaCl) pH 5 Comparison (Lysine and Asparagine)

Wavelength Scans by Functional Group (pH 5)

Lysine Asparagine 0.5mM AuCl 4 (0.0M, 0.1M, 0.5M NaCl) pH 5 Color Comparison

V-PH3 Medium, solid, hexagonal gold structures V-PH5 Small, head and tail, gold structures with extremely small, granular gold structures N-PH3 Large, solid & thin, gold structures N-PH5 Extremely small, granular gold structures 0.5mM AuCl4 0.0M NaCl pH 3 and 5 Comparison of Geometry

0.0N NaCl0.1N NaCl0.5N NaCl pH Geometry Change (20mM AuCl 4 )

Reduction Rate (20mM AuCl 4 )

pH 3.2pH 4.5pH 5.2 pH 7.5pH 11.6 pHSpeciesGeometry 3.2AuCl4- (Yellow)Plates 4.5AuCl3(OH)- (Yellow)Plates 5.2AuCl2(OH)2- (Yellow)Head and Tail 7.5AuCl2(OH)2- + AuCl(OH)3-(Orange)Dark Orange Film 11.6AuC(OH)3- + Au(OH)4- (Clear)Granular Partcles

Conclusion Decrease in pH allows for greater synthesizing and bonding efficiency. Size of nanostructure and thickness is inversely related to synthesizing efficiency. Lowering the pH shifts the species towards [AuCl 4 ] – and plate structures. Increasing excess chloride concentration using NaCl has same effect as lowering pH down. Initial structure for 0.0N NaCl has a strong correlation with the specific wavelength in UV/vis spectrophotometer, color of sample, and pH: –Granular particles: 480 nm, Violet/Brown/Green/Grey, pH<3.8 –Head and tail w/ granular particles: 500nm, Black or Dark Green/Grey Precipitates with Clear-Light Grey Solutions, pH>3.8 Morphology of the nano structures from [AuCl(OH) 3 ] - to [AuCl 4 ] -, : granular->dark film->head and tail->cluster->solid plates

Morphology has a trend based on functional group of amino acids. - Hydrocarbons: small head and tail w/ granular->medium/large head and tail morphing into plates->very small/small solid plates. -Neutral: granular->small head and tail or granular w/ gold clusters->very small-very large solid plates. -Base: small head and tail/granular->plate/morphing into plate->small-medium plate -Acid: small head and tail/granular->head and tail w/ clusters->plate When morphology shifts, the amount of gold synthesized increases dramatically (Figure 4). Increasing gold concentration has a similar, but reduced, effect as increasing chloride concentration or lowering pH and increases the size of the structures formed, as well. Also increases consistency of species and variation of pH between different amino acids (Figure 2 and 6). Reduction rate increases whenever there is a shift in morphology or shape and size of geometry

Future Experiments Determine quantity of dominant species formed for each amino acid at various other pHs and concentrations. Further correlate trends between speciation, pH, solution color, wavelength, and concentration. Controlling the shape of synthesized gold structures through changes in pH levels and chloride concentration. Use specific peptides to synthesize uniform nanowires and nanoplates for use in electronic and medical sensors. Acknowledgments Thanks goes to the BRITE program and Jun Wang for organizing this wonderful opportunity in research, and to Professor Nosang Myung and Ms. Jungok Kim of the Chemical and Environmental Engineering Department for their guidance and mentoring.