What nanoparticles are made of: STEM, ICP-MS and magnetometry

Six principles for finding what nanoparticles are made of and how their elements are arranged: preparing a TEM grid, HAADF-STEM with its Z-contrast, EDS detectors around the specimen, ICP-MS from acid digestion to the plasma, single-particle ICP-MS and the vibrating-sample magnetometer. Instruments and principles only, with every caption quoted from the cited papers.

1. Preparing a TEM grid: A drop of the dispersion on a 3 mm Au grid coated with a holey carbon film, the grid on a tissue that absorbed the liquid flowing through the film, left to dry in air for a few minutes; smaller particles tend to settle on the film, biasing TEM sizes toward smaller particles, and aggregates can come from drying (Tuoriniemi 2014). 2. HAADF-STEM: Combined with EDX mapping, the atomic-number (Z) contrast of HAADF-STEM characterises how the two metals are arranged in bimetallic Pd–Au particles, and HAADF-STEM tomography shows them in 3D (Tran 2012); Z-contrast can read nanoparticle barcodes built from different elements (Chen 2015). 3. Mapping the elements: X-ray (XEDS) detectors can be arranged symmetrically around the specimen; XEDS adds information where Z-contrast is low (Genc 2013). 4. ICP-MS: Au nanoparticles and Au–Ag nanoshells were mineralised in aqua regia with HF at 65 °C for 4 h (Faucher 2018); CTAB-capped gold nanorods were decomposed in a microwave with reverse aqua regia and H₂SO₄ (Godoy 2021); tissues were homogenised and lysed with aqua regia (Sonavane 2008). After dilution the liquid is nebulised with argon; the aerosol passes a spray chamber (90–98 % ends as waste) to the plasma (≈ 7500 K), where it is desolvated, vaporised, atomised and ionised, and the ions are separated by mass-to-charge ratio (Resano 2022). 5. Single-particle ICP-MS: Each particle that reaches the plasma gives a burst of ions, detected as a short pulse above the background; the pulse intensity is proportional to the mass of the analyte in the particle and the number of pulses to the number concentration; transport efficiency is never 100 % (Resano 2022). 6. Vibrating-sample magnetometry: VSM and SQUID magnetometry measure the magnetisation of magnetic nanoparticles under varying magnetic fields and temperatures (Amorim 2025); in lymph nodes the iron content was quantified from the magnetic moment curve with the Langevin model, the tissue fixed tightly and the vibration amplitude kept small (Visscher 2013).

Starting materials: Nanoparticles drop-cast on a grid, digested in acid, or held still in a magnetometer. Product: Arrangement of the elements in each particle, the metal content, particle mass and number, and the magnetisation.

Tuoriniemi et al., “Intermethod comparison of the particle size distributions of colloidal silica nanoparticles”, Sci. Technol. Adv. Mater. 2014 (doi:10.1088/1468-6996/15/3/035009); Tran et al., “Configuration of microbially synthesized Pd–Au nanoparticles studied by STEM-based techniques”, Nanotechnology 2012 (doi:10.1088/0957-4484/23/5/055701); Genc et al., “XEDS STEM tomography for 3D chemical characterization of nanoscale particles”, Ultramicroscopy 2013 (doi:10.1016/j.ultramic.2013.03.023); Resano et al., “Living in a transient world: ICP-MS reinvented via time-resolved analysis for monitoring single events”, Chem. Sci. 2022 (doi:10.1039/d1sc05452j); Amorim, “A compendium of magnetic nanoparticle essentials: a comprehensive guide for beginners and experts”, Pharmaceutics 2025 (doi:10.3390/pharmaceutics17010137); Chen et al., “Rare earth core/shell nanobarcodes for multiplexed trace biodetection”, Anal. Chem. 2015 (doi:10.1021/acs.analchem.5b00944); Faucher et al., “Gold and silver quantification from gold-silver nanoshells in HaCaT cells”, J. Trace Elem. Med. Biol. 2018 (doi:10.1016/j.jtemb.2018.01.017); Godoy et al., “Evaluating the total gold concentration in metallic nanoparticles with a high content of organic matter through microwave-assisted decomposition platform and plasma-based spectrometric techniques (ICP-MS and ICP OES)”, Talanta 2021 (doi:10.1016/j.talanta.2020.121808); Sonavane, Tomoda & Makino, “Biodistribution of colloidal gold nanoparticles after intravenous administration: effect of particle size”, Colloids Surf. B 2008 (doi:10.1016/j.colsurfb.2008.07.004); Visscher et al., “Quantitative analysis of superparamagnetic contrast agent in sentinel lymph nodes using ex vivo vibrating sample magnetometry”, IEEE Trans. Biomed. Eng. 2013 (doi:10.1109/TBME.2013.2261893).

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