Minute Paper #1 (09/04/2007) - Christy L. Haynes
Title: In Vivo Detection of Gold-Imidazole Self-Assembly Complexes: NIR-SERS Signal Reporters
By: G. R. Souza et al.
Journal: Analytical Chemistry
In this paper, the authors attempt to control aggregation of Au nanoparticles by adding imidazole in order to shift the nanoparticle optical properties so that high surface-enhanced Raman scattering (SERS) enhancement factors can be achieved with near-IR laser excitation. The techniques used in this work include: nanoparticle synthesis, UV-vis extinction spectroscopy, transmission electron microscopy, dynamic light scattering, and near-IR SERS.
The authors claim that adding specific concentrations of imidazole (with 2 available nitrogens for covalent attachment to Au) to Au nanoparticles generates nanoparticle aggregates with discrete sizes and optical properties. Individual Au nanoparticles absorb and scatter light at 520 nm; however, the authors need the optical properties to shift to the NIR in order to achieve large SERS enhancement factors in the "water window" (700 - 900 nm) for biological imaging. SERS enhancement factors are largest when the laser excitation wavelength matches the surface-enhancing substrate's optical properties. After aggregating the nanoparticles, the authors calculate their SERS enhancement factors to be as large as 10^9. This is a much larger than expected enhancement factor for spherical gold nanoparticles. The authors then inject the nanoparticle aggregates into mouse tumors and demonstrate that they can collect SERS spectra through the skin of the mouse.
The authors leave many questions unanswered in this work. In figure 1a, you see that the normal Raman scattering and SERS spectra are very different. This is expected because normal Raman scattering reports the vibrational structure of the bulk imidazole in solution while the SERS spectrum reveals the molecules covalently bound to the Au surface. In figure 1b, you can not see the red-shift in the optical properties very easily because the post-aggregation spectrum has very low intensity. This suggests that the nanoparticles are aggregating to the point where they precipitate out of solution. Also, the authors continually report the nanoparticles extinction at 520 nm (the absorption band for the original unaggregated Au nanoparticle solution) even though the most useful information would be the extinction efficiency at 785 nm (since this is the laser excitation they plan to use). Further, the calculated enhancement factors seem way too high based on previous literature precedent - this leads me to believe that their molecular coverage calculations are not correct. They should do a quantitative coverage measurement in order to conclusively demonstrate the claimed enhancement factor. Finally, while the authors show that you can collect SERS spectra through skin, the signals are very small and they do not explain how this will be used in actual biological imaging.