Grain size effects in polycrystalline gold nanoparticles

Abstract

We report a structure-property relationship in gold nanoparticles (NPs), grain-size effects, which not only allow material properties observed on different characteristic length scales to be engineered in a single NP but further enhance those properties due to the coupling among different-size grains. The grain size effects were achieved by creating polycrystalline gold NPs (pAuNPs) with two distinct grain-size populations (5 and 1 nm) comparable to electron mean free path and electron Fermi wavelength (EFW), respectively. Successful integration of molecular and plasmonic properties into a single nanostructure without additional fluorophores enables these highly polycrystalline AuNPs to serve as multimodal probes in a variety of optical microscopic imaging techniques.

Figure 1

pAuNPs created in glycine matrices through a solid-phase thermal reduction method. (a) pAuNPs of 20 nm were created after the reaction was completed. Inset: size distribution of pAuNPs by counting 150 particles. (b) HR-TEM image of pAuNPs, containing many grains with size down to 1 nm (scale bar: 2 nm), two ~5 nm and two ~1 nm representative individual grains are labelled with white circles; Inset: commercially available multi-twinned gold NP (mAuNP) composed of 8 nm grains (scale bar: 5 nm). (c) X-ray photoelectron spectroscopic measurements on Au 4f7/2 binding energy (BE) of pAuNPs and mAuNPs. Two peaks with maxima at 84.0 eV and 85.1 eV were observed from ~20 nm pAuNPs while only one peak at 83.9 eV was observed from the same size mAuNPs.

Figure 2

Optical properties of pAuNPs. (a) UV-Vis absorption spectra of as synthesized 20 nm pAuNPs, 20 and 5 nm mAuNPs aqueous solutions. The surface plasmon maximum of pAuNPs is located at 536 nm, which is red shifted about 16 nm compared to those of 5 and 20 nm mAuNPs. (b) Fluorescence image of individual pAuNPs. Inset: emission spectrum of pAuNPs under 532 nm laser excitation. (c) Raman spectra of pAuNPs and mAuNPs coated by p-mercaptobenzoic acid (MBA) molecules. (d) Raman image of individual MBA-pAuNPs were constructed based on the vibration at 1579±10 cm⁻¹.

Figure 3

MBA-pAuNPs-cRGD served as a multi-modality probe that enables α_vβ₃ integrin receptors on U87MG cancer cells to be imaged with four different imaging techniques at the single cell level. (a) Strong single-particle luminescence from pAuNPs enabled the integrin receptors on a single U87MG cell to be imaged using fluorescence microscopic imaging modality. (b) Raman image of a U87MG cancer cell labeled by cRGD-pAuNPs-MBA, which was constructed based on the Raman vibration of MBA at 1579±10 cm-1(fluorescence background was substracted). (c) Bright-field image of the cancer cells labeled by the NPs. Strong surface plasmons of the pAuNPs made the cells purplish. (d) Strong surface plasmon scattering of pAuNPs allow the cancer cells to be visualized using dark-field imaging modality.