Surface-Enhanced Raman Spectroscopy (SERS) Cellular Imaging of Intracellulary Biosynthesized Gold Nanoparticles

Green algae biosynthesize gold nanopartides (AuNPs) in the presence of dissolved gold, but the precise biosynthesis mechanism remains unclear. Furthermore, few surface-enhanced Raman spectroscopy (SERS) spectra and even fewer SERS cellular images have been collected of intracellularly grown gold nanoparticles, despite the detailed information SERS can provide about nanosurface-associated molecules. In this effort, SERS imaging was used to detect intracellular and extracellular gold nanopartides biosynthesized by the green algae Pseudokirchneriella subcapitata to identify surface-associated biomolecules and to evaluate the nanoparticle biosynthesis mechanism. Three-dimensional SERS spectral maps imaged AuNPs biosynthesized in the presence of 0.005-0.5 mM HAuCl4 over a variety of pH conditions. Algal growth and AuNP biosynthesis were monitored over a 72 h exposure period using UV-vis spectroscopy, electron microscopy, and elemental analysis. Principle component analysis (PCA) and cluster analysis of SERS data demonstrate reproducible trends in the SERS spectral maps and simplify peak identification analyses. SERS cellular images contain peaks consistent with glutathione, beta-carotene, chlorophyll a, hydroxyquinoline, NAD, and proteins such as a reductase enzyme. Each is a biomolecule previously thought to be involved in intracellular AuNP biosynthesis in bacteria and fungi. Little mechanistic study has been previously conducted with green algae. Identification of AuNP surface-associated biomolecules from SERS spectra requires prior knowledge of the system, but peaks not found in the SERS spectra can be used to narrow the list of potential AuNP surface-associated candidate molecules. Continued development of SERS spectral imaging will facilitate noble metal nanopartide surface analyses to elucidate biosynthesis mechanisms relevant to green synthesis, to monitor nanomaterial function and stability in complex media, and to image AuNPs employed for drug delivery applications.