(337t) Enhanced Emission of Gold Nanoparticles with Atypical Electron Transfer From Surface Bound Molecules to Gold Surface

Fluorescent system has long been used for the bio-imaging agent. 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 on UV plate. The fluorescence of the gold nanoparticle is dependent on the physiological pH range and has a possibility to be used as a nano-pH sensor due to its carboxylic acid containing capping molecules.In comparison to the widely used quantum dots or fluorescent dye molecules 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 (Figure 1). 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. A 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. The fluorescence of the gold nanoparticle is more efficient than other reported fluorescent gold nanoparticles with quantum yield (QY) of 0.015 measured with fluorescein dye molecule as a standard (QY=0.95) and the life time of the fluorescence is a lot longer than other reported fluorescent systems with 1.7 μs of half life. Along with these findings, the distinct excitation and emission without any overlaps between them with 200 nm gap suggested us a complex excited electron transfer from surface bound molecules to the core gold for the emission. This gold nanoparticle system 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.