(5bd) Development of Novel Water-Soluble Gold Nanoparticles with Atypical Fluorescence and Study On Its Mechanism

I have received BS and MS degrees in Chemical Engineering department, Seoul National University and have worked on enzyme entrapment in silica micro-particles and covalent immobilization on the surface of hydrophilic chitosan micro-particles. Before the graduation of the MS program, I have worked in Pacific Northwest National Laboratory as a graduate fellow to work in single enzyme nanoparticle development and characterization. I am currently a PhD candidate at the Bioengineering department, University of Utah, expected to graduate by Fall 2009, working on the development and characterization of water-soluble gold nanoparticle with enhanced emission and studying the mechanism of the atypical fluorescence due to electron transfer from surface bound molecules. The summary of my current PhD work is given below.

Water-soluble gold nanoparticles with atypical fluorescence have been developed with mercaptooctanoic acid as a capping agent under reduction condition with sodium borohydride. The product was purified with ethanol trituration for further application or characterization. TEM analysis of the nanoparticle showed its diameter to be 2.2 ± 0.6 nm. The gold nanoparticle solution shows bright red emission with the excitation with UV.

In comparison to the widely used quantum dots which usually have broad excitation spectra and overlapped region with emission spectra, it showed distinct excitation and emission spectra (excitation spectra peak @ 280 nm and emission spectra peak @ 610 nm) with 200 nm gap between them with large Stokes shift.

It shows the possibility of two band gap modalities with responsible for excitation and emission and electron transfer between them. The loss during the electron transfer is considered to be the reason of distinct excitation and emission peak location with large Stokes shift. In this hypothetical diagram, excitation occurs in the left hand side band gap from surface covering mercaptooctanoic acid (MOA) for 280 nm excitation and transferred to the surface of the gold nanoparticles and the emission process occurs in the right hand side with relatively smaller band gap responsible for 610 nm emission.

This mercaptooctanoic acid stabilized gold nanoparticle also showed strong relationship with pH on its emission. The fluorescence intensity weakened at lower pH while it remained high at higher pH. The dependency of the fluorescence on the pH change also supports the proposed mechanism of excitation on surface capping mercaptooctanoic acid molecules. The charge state of carboxylic acid groups of mercaptooctanoic acid is believed to be responsible for this phenomenon and the hypothesis is proposed.

This electron energy density distribution change eventually results higher energy level distribution of ground state for the charged carboxylic acid group comparing to neutral carboxylic groups.

In addition to the pH effect, computation simulation study was also performed for the change of the band gap of mercaptooctanoic acid with and without Au-S bond formation. Gaussian03 program was used with b3pw91/lanl2dz functional/basis set combination. The result showed similar trend of band gap change as it is experimentally confirmed by absorption band edge measurement. Detailed result and its hypothesis will be discussed in the presentation.

As a future research plan, I'd like to develop different types of optically active nanoparticles by understanding its intrinsic fluorescent mechanisms and apply these novel fluorescence systems to many applications including bio-imaging agents and nano-pH sensors. My work philosophy is collaboration and maximize its synergy effect. There are always many branching projects coming from a single specified work and it is hard for one person to execute professionally in all the branching projects efficiently. By collaborating with other professionals on the project together, it would be more efficient to achieve the break-through results and get professionalized on that field during the collaboration. I understand that providing specialties from my side is also essential key factor for the successful collaborations. In my case, I am specialized in enzyme/protein engineering, organic synthesis and bio-nanotechnology based on previous experiences during my education and professional life.