Enhanced Emission of Gold Nanoparticles Due to Electron Transfer From Surface Bound Molecules and Its Use in pH Sensing

Gold nanoparticles have long been of interest in the development of SPR and SERS chemical and biological sensors. Recently, fluorescence-based sensing has been suggested as a viable alternative sensing method because of the development of emissive gold nanoparticles produced either in organic solution or inside the confines of dendrimers. These have properties of quantum dots, and the effect of environment on their emissions is not well understood. Here we report an all water synthesis of water-soluble gold nanoparticles with atypical near infrared fluorescence believed to originate via absorbance of light energy by a surface bound molecule and transfer of electrons to the metal which subsequently fluoresces. Particles were developed with mercaptooctanoic acid as a capping agent under reduction conditions 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 (Figure 1A). The gold nanoparticle solution shows bright red emission with the excitation on UV plate (Figure 1B). The fluorescence of the gold nanoparticle varies over the physiological pH range and has a possibility to be used as a nanosized pH sensor without the need to conjugate additional organic molecules to the particle surface. In comparison to the widely used quantum dots or fluorescent dye molecules which usually have broad excitation spectra and overlapped region with emission spectra, these particles have a 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. 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 around 1.7 μs. The distinct excitation (red shifted from that of pure MOA) and emission without significant overlap between them with a 200 nm gap between them is suggestive of excited electron transfer from surface bound molecules to the core gold for the emission. A computational simulation study using Gaussian03 program and b3pw91/lanl2dz functional/basis set was also performed to evaluate the change of the band gap of mercaptooctanoic acid with and without Au-S bond formation. The result showed similar red shift of the band gap. The emission intensity exhibited strong pH sensitivity and supports the proposed mechanism that excitation of surface capping MOA in specific charge states is an important determinant of emission in this system. The distinct excitation (red shifted from that of pure MOA) and emission without significant overlap between them with a 200 nm gap between them is suggestive of excited electron transfer from surface bound molecules to the core gold for the emission. A computational simulation study using Gaussian03 program and b3pw91/lanl2dz functional/basis set was also performed to evaluate the change of the band gap of mercaptooctanoic acid with and without Au-S bond formation. The result showed similar red shift of the band gap. The emission intensity exhibited a strong pH sensitivity and supports the proposed mechanism that excitation of surface capping MOA in specific charge states is an important determinant of emission in this system.