M.Sc Thesis

M.Sc StudentYucknovsky Anna
SubjectPhoto-Induced Aggregation of Gold Nanoparticles by
Photoacids and Photobases
DepartmentDepartment of Chemistry
Supervisor ASSOCIATE PROF. Nadav Amdursky
Full Thesis textFull thesis text - English Version


Gold nanoparticles (AuNPs) and their assembled structures are considered valuable material due to the stability, simplicity of production and surface functionalization. Moreover, unique optical property, known as local surface plasmon resonance (LSPR), arise from their  nanoscale size.

Self-assembly of AuNPs constitutes transition from dispersed into ordered phase and can be induced by destabilization of colloid suspension. Dynamic self-assembly of nanoparticles (NPs) for the formation of ordered clusters is taking place out of thermodynamic equilibrium, and thus, sustained by external energy supply. Light is a convenient way to deliver energy in a remote, spatially precise and frequency tunable manner. Here we present light energy driven dynamic self-assembly of gold nanoparticles (AuNPs), functionalized with carboxylic acid, involving either excited state proton or hydroxide transfer phenomena. These processes are mediated by a photoacid or a photobase, which are defined as aromatic molecules that display properties of weak acid or base in their ground electronic state, but exhibit large pKa drop or increase, respectively, in their excited state. We achieve fast (seconds) control over AuNPs self-assembly by combining both photoacid (pyranine) and a photobase (6-methoxyquinoline), having a different wavelength of excitation, and in this way, a significant pKa gap arises simply by changing the wavelength of excitation. We suggest that the mechanism underlying this process is as following: upon excitation of photoacid, protonation of AuNPs ligands takes place, resulting in aggregation; upon subsequent excitation of photobase effective backward capture of protons is enabled, resulting in AuNPs dispersion.  An additional approach that we show is based on the employment of a very strong photobase (malachite green carbinol base, MGCB) that releases hydroxide, having a significant pKa gap between its excited and ground states and a fast dissociation rate. To address the recombination time aspect, we synthesized methylated derivative of MGCB with extra positive charge, MGCB, and tested the effect of enhanced Coulombic attraction between the released hydroxide anions and MGCB2 cations.

In our study we use UV-Vis spectroscopy, transmission electron microscopy (TEM) and dynamic light scattering (DLS) to characterize AuNPs and to follow their aggregation process.