Table 1.
Skin absorption of metallic NPs
| Species | Study duration | Analysis method | Observation | Reference | |
|---|---|---|---|---|---|
| ZnO NPs | Human | 2, 4 hours | ZnO NPs and NAD(P)H signals quantified by TCSPC-FLIM | No ZnO-NP penetration detected in human skin after 2 and 4 hours’ treatment; free NAD(P)H signal significantly increased in tape-stripped viable epidermis treated for 4 hours with ZnO-NP compared to vehicle control | 19 |
| TiO2 NPs | Hairless rat | 2, 4, and 8 weeks | Histopathological, TEM, EDS | Particles located only in stratum corneum layer of epidermis and follicular epithelium histopathologically; TEM and EDS analysis failed to show TiO2 NPs in viable skin areas | 20 |
| TiO2 NPs | Porcine back skin | 8, 24, 48 hours | PIXE, RBS, STIM, SEI | TiO2 NPs penetrated SC into SG but not SS within 8 hours; NPs not detected in hair follicles | 21 |
| TiO2 NPs and ZnO NPs | Human | 2, 48 hours | Nuclear microscopy, PIXE | NPs observed only in the 70%–90% depth of SC and openings of follicles | 22 |
| AuNPs | Human | 2 hours | Multiphoton microscopy | AuNPs detected up to 14 µm deep in human skin, whereas a wide range of detectable depths (20–100 µm) observed in reconstructed skin | 23 |
| AgNPs | Porcine ear skin | 24 hours | TPT-FLIM, CRM, SERS | TPT-FLIM, CRM, SERS showed depths of 12–14, 11.1±2.1, and 15.6±8.3 µm, respectively | 24 |
| AgNPs | Human | 4–6 days | Histopathological, SEM, XRD | A limited number of NPs noted histopathologically; metallic particles seen within the dermis by SEM; XRD confirmed these were AgNPs | 25 |
| AuNPs | Human | 0.5, 2, 6, and 24 hours | Franz method, multiphoton microscopy | AuNPs penetrated SC into deeper skin layers after 24 hours’ skin exposure | 26 |
| CoNPs | Human intact and abraded skin | 2, 4, 8, 16, and 24 hours | Franz method, ICP-AES | NPs able to penetrate human skin in an in vitro cell-diffusion system; cobalt concentration in damaged skin significantly greater than in intact skin | 27 |
| NiNPs | Human intact and needle-abraded human skin | 4, 8, 16, and 24 hours | Franz method, ICP-AES | NiNPs caused increase in nickel content into skin and significant permeation flux through skin in vitro, higher when damaged-skin protocol used | 28 |
Abbreviations: TCSPC, time-correlated single-photon counting; FLIM, fluorescence-lifetime imaging microscopy; TEM, transmission electron microscopy; EDS, energy-dispersive X-ray spectroscopy; SG, stratum granulosum; SS, stratum spinosum; PIXE, particle-induced X-ray emission; RBS, Rutherford back-scattering; STIM, scanning transmission ion microscopy; SEI, secondary electron imaging; TPT, two-photon tomography; CRM, confocal Raman microscopy; SERS, surface-enhanced Raman scattering; XRD, X-ray diffusion; ICP-AES, inductively coupled plasma–atomic-emission spectroscopy; NPs, nanoparticles.