Perpustakaan Digital ITB

Noble metal nanoparticles such as gold nanoparticles (AuNPs) are commonly used and developed in biotechnology and nanomedicine application such as biosensors due to their unique optical property called localized surface plasmon resonance (LSPR) that appears from the interaction between electromagnetic waves and conduction electron in the metal results in local enhancement field and peak in the absorbance spectra. The molecule of 3-mercaptopropionic acid (3-MPA) is commonly used for biosensors applications because of its unique structure that consists of two functional groups called thiol and carboxyl groups. The thiol group can strongly adsorb on gold surfaces and the carboxyl group can be used as a linker towards biomolecules. The important challenges in biosensor development are ensuring selectivity and stability of nanoparticles by the use of surface functionalization techniques. These research objectives are to optimize the synthesis process of gold nanoparticles capped by 3-MPA (Au-MPA) by the use of the ligand exchange method and the modified reduction method also to prepare the colorimetric assay based on Au-Citrate and Au-MPA utilizing avidin-biocytin complex for biosensors applications. Ligand exchange method uses Au-Citrate taken from citrate reduction method and then 3-MPA was injected to induce ligand exchange from citrate to 3-MPA due to the higher chemisorption energy of thiol in comparison with citrate. Simultaneous reduction method is taken and modified from a method developed by Yonezawa & Kunitake (1999) where citrate and 3- MPA are simultaneously injected into HAuCl4 as Au3+ source. A simulation was also conducted in this research by density functional theory (DFT) method implemented with Orca package version 4.2.1 by the use of B3LYP function and LANL2DZ basis set to calculate the vibrational frequency of the gold-ligand system. The system for the simulation method was prepared with a small cluster of gold attached to the ligand. Au-Citrate system was prepared by use of bicarbonate anion model to simplify the calculation and Au atom interacts with the carboxyl end, but in Au-MPA system, 3-MPA anion and Au atom interacts with the sulfur atom. Our observation found that the Au-MPA prepared by use of modified reduction method has an average size of 50.8 nm with a spherical shape that is stable for up to 6 months with slightly red-shifting and broadening in the absorbance spectra reveals a more stable colloidal gold in comparison to that of Au-MPA prepared by the exchange ligand method. It is found that the absorbance peak from Au-Citrate lies at ~519 nm with an average size of 34 nm with a spherical shape. However, the absorbance peaks from respectively Au-MPA were taken from the modified reduction method and the exchange ligand method shifted to ~527 nm and ~520 nm due to the changes in the local refractive index from adsorption of 3-MPA on gold surfaces. The vibrational peaks pe taken from the FTIR spectrometer show the effect of ligand adsorption on gold surfaces indicated by the shift of the peaks and it has a good agreement to that of the simulation results. In the case of Au- Citrate, it is found that the asymmetric stretching from citrate is shifted from 1591 cm-1 to 1632 cm-1 and the symmetric stretching is shifted from 1395 cm-1 to 1383 cm-1 due to the coordination between the COO group and gold atom. While in Au- MPA, it is found that the asymmetric stretching from 3-MPA is shifted from 1713 cm-1 to 1620 cm-1 due to the gold dipoles effect. A colorimetric assay was done on Au-Citrate and Au-MPA utilizing the avidin-biocytin complex and it is shown that the absorbance spectra are shifted and the color of the solutions are changed significantly due to the cross-linking-induced aggregation. This research can be used to develop a colorimetric assay based on metal nanoparticles, especially in this pandemic situation where a fast, reliable, and affordable sensor is needed.