A set of similarly sized (Yb3+, Nd3+, Er3+)-doped upconversion nanoparticles of different architecture were spectroscopically examined in water at broadly varied excitation power at 980 nm & 808 nm to study the sensitizer dependent penetration-depth.
Ensemble and single particle studies of the excitation power density (P)-dependent upconversion luminescence (UCL) of core and core-shell β-NaYF4:Yb,Er upconversion nanoparticles (UCNPs) doped with 20% Yb3+ and 1% or 3% Er3+ performed over a P regime of 6 orders of magnitude reveal an increasing contribution of the emission from high energy Er3+ levels at P > 1 kW/cm2. This changes the overall emission color from initially green over yellow to white. While initially the green and with increasing P the red emission dominate in ensemble measurements at P < 1 kW/cm2, the increasing population of higher Er3+ energy levels by multiphotonic processes at higher P in single particle studies results in a multitude of emission bands in the ultraviolet/visible/near infrared (UV/vis/NIR) accompanied by a decreased contribution of the red luminescence. Based upon a thorough analysis of the P-dependence of UCL, the emission bands activated at high P were grouped and assigned to 2–3, 3–4, and 4 photonic processes involving energy transfer (ET), excited-state absorption (ESA), cross-relaxation (CR), back energy transfer (BET), and non-radiative relaxation processes (nRP). This underlines the P-tunability of UCNP brightness and color and highlights the potential of P-dependent measurements for mechanistic studies required to manifest the population pathways of the different Er3+ levels.
A nanoengineered
interface fabricated by self-assembly enables
the online determination of vitamin B12 via a simple luminescence
readout in serum without any pretreatment. The interplay of Tm3+-doped NaYF4 nanoparticles (UCNPs) and a gold
nanotriangle array prepared by nanosphere lithography on a glass slide
is responsible for an efficient NIR to UV upconversion. Hot spots
of the gold assembly generate local electromagnetic-field enhancement,
favoring the four-photon upconversion process at the low-power excitation
of approximately 13 W·cm–2. An improvement
by about 6 times of the intensity for the emission peaking at 345
nm is achieved. The nanoengineered interface has been applied in a
proof-of-concept sensor for vitamin B12 in serum, which is known as
a marker for the risk of cancer; Alzheimer disease; or, during pregnancy,
neurological abnormalities in newborn babies. Vitamin B12 can be detected
in serum down to 3.0 nmol·L–1 by a simple intensity-based
optical readout, consuming only 200 μL of a sample, which qualifies
as easy miniaturization for point-of-care diagnostics. Additionally,
this label-free approach can be used for long-term monitoring because
of the high photostability of the upconversion nanoparticles.
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