CC BY
14B-I-13
BIOMEDICAL PHOTONICS
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ALT'22
Gold concentration in colloids from extinction at 400 nm: universality for nanoparticles of various shape, morphology, and nanoparticle clusters
N. Khlebtsov1'2. B. Khlebtsov1, S. Zarkov3
1- Institute of Biochemistry and Physiology of Plants and Microorganisms, Saratov Scientific Centre of the Russian Academy of Sciences (IBPPM RAS) 2- Saratov State University
3- Institute of Precision Mechanics and Control, Saratov Scientific Centre of the Russian Academy of Sciences
khlebtsov@ibppm.ru
The gold concentration in colloids represents important information for biomedical applications, including bioimag-ing, photothermal therapy, and nanotoxicology. Similarly, the formation and aggregation studies of gold nanoparticles (GNPs) in colloids need an accurate Au concentration as a crucial input parameter. There are several well-established methods to evaluate the GNP concentration in colloids such as ICP-MS, AAS, OES, SAXS, XANES, XAFS, NAA, and voltammetry. However, an optimal procedure would be that requires only standard laboratory equipment such as common UV-Vis spectrophotometry. In particular, the size and concentration of spherical GNPs can be determined from the extinction and the plasmon resonance (PR) wavelength position or the extinction ratio ApR/A450. As the PR amplitude/ position strongly depend on the particle shape/morphology or clustering, the most attractive approach seems to be the measurement of 400-nm extinction (absorption) because it corresponds to the interband transition energies from 5d to 6sp in bulk gold and should not be sensitive to the particle details. Specifically, it has been shown that the 400-nm extinction method is suitable for spherical particles of different size and Au nanorods with aspect ratio 3, 4, and 5. Now a reasonable question arises: how universal the 400-nm extinction method is? In other words, can we use it for particles of any shape/morphology or for clusters made of spherical particles? To the best of our knowledge, this point has not been addressed yet. Here, we present a detailed numerical investigation of the method for several models: (1) gold nanorods and nanodiscs with aspect ratios from 2 to 6; (2) gold nanostars and primitive nanostars such as cone, bicone, bicone on a sphere, etc; (3) 2D nanotriangles and nanoplates; (4) ballistic cluster aggregates made of bare GNPs and two-layered GNPs with a dielectric coating. Our main conclusion is that the 400-nm extinction (absorption) does allow the Au(0) concentration to be determined accurately (Fig. 1), as the extinction cross-section scales as the gold volume with a universal prefactor. Therefore, the extinction A400 is proportional to the mass-volume gold concentration regardless of the particle morphology and cluster structure. To validate the method, we have fabricated a representative set of samples to analyze them with AAS and ICP-MS techniques. These experiments are ongoing and preliminary data are encouraging.
Fig. 1. Evidence for the linear dependence of the extinction cross section of randomly oriented Au nanorods, nanodiscs, nanostars, and clusters on their volume. Simulations were made for 400-nm, the external medium is water. In panels (a) and (b), for each set of 6 points, the aspect ratio varies from 1 to 6 at a constant volume for all 6 aspect ratios. NT stands for nanotriangles (c), different points correspond to models with different volumes; in panel (d), the number of cluster particles varies from 2 to 1000. For all models, the linear fits are very close.
This research was supported by the Russian Scientific Foundation (project no. 18-14-00016-n).
