A facile route to diatoms decorated with gold nanoparticles and their optical properties

This paper presents a facile route for the decoration of biogenic silica from diatoms with gold nanoparticles (AuNPs) with dimensions from 10 to 40 nm. The AuNP decoration on the diatoms is performed through initial attachment of colloidal AuNPs (2 nm) followed by enhancement of the size of the AuNPs by mild reduction of tetrachloroauric (III) acid from solution. The samples were characterised by transmission electron microscopy (TEM), energy-dispersive X-ray analysis, Fourier transform infrared spectroscopy and light spectroscopy in the ultraviolet and visible (UV/Vis) range. The size of the isolated AuNPs on unmodified acid-cleaned diatoms was 10 nm according to TEM micrograph analysis. The measured UV/Vis spectrum showed an absorption peak at 525 nm, which corresponds well to the optical properties of AuNPs of the same size. The extent of decoration with AuNPs of the diatoms was significantly improved after functionalisation with 3-aminopropyltriethoxysilane of the biogenic silica. The absorption peak in the visible range was redshifted to 590 nm due to enlargement of the AuNPs up to 40 nm and clustering of the AuNPs. The AuNP decoration on the hierarchical and complex structured biogenic silica of diatoms might be interesting for biodiagnostic and surface-enhanced Raman spectroscopic applications.