Nanoparticles in tests, catalysts and devices

Six applications of nanoparticles outside medicine: colorimetric detection with oligonucleotide–gold probes, the lateral flow strip, plasmonic hot-electron photocatalysis, the three-way catalytic converter, quantum-dot colour conversion on blue micro-LEDs and mineral sunscreens of TiO₂ and ZnO. Schematics only, with every caption quoted from the cited papers.

1. Colorimetric gold probes: Gold nanoparticle probes carry mercaptoalkyl-oligonucleotides; a single-stranded 30-base target links them into a polymeric network, and the solution turns from red to pinkish/purple; the networks melt over a narrow temperature range, which told imperfect targets apart; spotted on a reverse-phase silica plate the mixture dried blue, and the unoptimized system detected about 10 femtomoles of an oligonucleotide (Elghanian 1997). 2. Lateral flow test: The sample moves along the strip by capillary action, with no external force, and the result shows within 5–30 min; the conjugate pad holds antibodies conjugated to coloured or fluorescent particles, most commonly colloidal gold; the nitrocellulose membrane (pore sizes 0.05–12 µm) carries the test and control lines. In the sandwich format a test line means positive; in the competitive format, used for small molecules, a missing test line means positive; the control line shows that the liquid flowed (Koczula 2016). 3. Hot-electron photocatalysis: A plasmon decays without emitting light into electron–hole pairs within about 1–100 fs, hot electrons above and hot holes below the Fermi level; a semiconductor in contact takes up the hot electrons, extending their lifetimes to nanoseconds, so that oxidation and reduction run apart on the metal and on the oxide; the Schottky barrier is the main hurdle; making H₂ from water needs more than 1.23 eV (< 1,008 nm) per photon (Kumar 2019). 4. Three-way catalytic converter: Three-way catalysts carry platinum group metals as the active phase, palladium and platinum for CO and hydrocarbon oxidation and rhodium mainly for NOₓ reduction, on a CeO₂–ZrO₂ washcoat coated on the walls of a cordierite monolith, one layer about 50–150 µm thick; a copper–palladium–rhodium nanocatalyst used up to 85 % less platinum group metal than commercial converters, with similar or better activity (Yakoumis 2021). 5. Quantum-dot colour conversion: Red and green CdSe/ZnS quantum-dot films under 17 µm thick let only about 0.1 % of the blue excitation through, so the conversion layer also suppresses the residual blue without colour filters; TiO₂ nanoparticles added for scattering raised the luminous intensity by more than 10 % (Hyun 2021). Perovskite quantum dots filled into a flexible layer by microfluidics gave a full-colour micro-LED display with a 200 µm pixel pitch (Li 2022). 6. Mineral sunscreens: Sunscreens protect against UVB (290–320 nm) and UVA (320–400 nm); TiO₂ is more effective in the UVB and ZnO in the UVA (Smijs 2011). TiO₂ and ZnO protect mainly by absorbing UV across their band gap and reflect only about 4–5 % of it (Cole 2016). Nanoparticles (< 100 nm) remove the opaque look of microsized particles; silica-based coatings are the most effective against photocatalytic effects (Smijs 2011).

Elghanian et al., “Selective colorimetric detection of polynucleotides based on the distance-dependent optical properties of gold nanoparticles”, Science 1997 (doi:10.1126/science.277.5329.1078); Koczula & Gallotta, “Lateral flow assays”, Essays Biochem. 2016 (doi:10.1042/EBC20150012); Kumar et al., “Harnessing plasmon-induced hot carriers at the interfaces with ferroelectrics”, Front. Chem. 2019 (doi:10.3389/fchem.2019.00299); Yakoumis, “PROMETHEUS: a copper-based polymetallic catalyst for automotive applications. Part I: synthesis and characterization”, Materials 2021 (doi:10.3390/ma14030622); Hyun et al., “Dual role of quantum dots as color conversion layer and suppression of input light for full-color micro-LED displays”, J. Phys. Chem. Lett. 2021 (doi:10.1021/acs.jpclett.1c00321); Cole, Shyr & Ou-Yang, “Metal oxide sunscreens protect skin by absorption, not by reflection or scattering”, Photodermatol. Photoimmunol. Photomed. 2016 (doi:10.1111/phpp.12214); Smijs & Pavel, “Titanium dioxide and zinc oxide nanoparticles in sunscreens: focus on their safety and effectiveness”, Nanotechnol. Sci. Appl. 2011 (doi:10.2147/NSA.S19419); Li et al., “Flexible quantum-dot color-conversion layer based on microfluidics for full-color micro-LEDs”, Micromachines 2022 (doi:10.3390/mi13030448).

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