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

Abstract

Green algae biosynthesize gold nanoparticles\n(AuNPs) in the presence\nof dissolved gold, but the precise biosynthesis mechanism remains\nunclear. Furthermore, few surface-enhanced Raman spectroscopy (SERS)\nspectra and even fewer SERS cellular images have been collected of\nintracellularly grown gold nanoparticles, despite the detailed information\nSERS can provide about nanosurface-associated molecules. In this effort,\nSERS imaging was used to detect intracellular and extracellular gold\nnanoparticles biosynthesized by the green algae Pseudokirchneriella\nsubcapitata to identify surface-associated biomolecules\nand to evaluate the nanoparticle biosynthesis mechanism. Three-dimensional\nSERS spectral maps imaged AuNPs biosynthesized in the presence of\n0.005–0.5 mM HAuCl₄ over a variety of pH conditions.\nAlgal growth and AuNP biosynthesis were monitored over a 72 h exposure\nperiod using UV–vis spectroscopy, electron microscopy, and\nelemental analysis. Principle component analysis (PCA) and cluster\nanalysis of SERS data demonstrate reproducible trends in the SERS\nspectral maps and simplify peak identification analyses. SERS cellular\nimages contain peaks consistent with glutathione, β-carotene,\nchlorophyll <i>a</i>, hydroxyquinoline, NAD, and proteins\nsuch as a reductase enzyme. Each is a biomolecule previously thought\nto be involved in intracellular AuNP biosynthesis in bacteria and\nfungi. Little mechanistic study has been previously conducted with\ngreen algae. Identification of AuNP surface-associated biomolecules\nfrom SERS spectra requires prior knowledge of the system, but peaks\nnot found in the SERS spectra can be used to narrow the list of potential\nAuNP surface-associated candidate molecules. Continued development\nof SERS spectral imaging will facilitate noble metal nanoparticle\nsurface analyses to elucidate biosynthesis mechanisms relevant to\ngreen synthesis, to monitor nanomaterial function and stability in\ncomplex media, and to image AuNPs employed for drug delivery applications.