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.

pH sensing IR-excited dye-linked upconversion nanoparticles, via inner field effect quenching at 540 nm referenced to emission at 650 nm.

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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Light-mediated remote control of stem cell fate, such as proliferation, differentiation, and migration, can bring a significant impact on stem cell biology and regenerative medicine. Current UV/vis-mediated control approaches are limited in terms of nonspecific absorption, poor tissue penetration, and phototoxicity. Upconversion nanoparticle (UCNP)-based near-infrared (NIR)-mediated control systems have gained increasing attention for vast applications with minimal nonspecific absorption, good penetration depth, and minimal phototoxicity from NIR excitations. Specifically, 808 nm NIR-responsive upconversion nanomaterials have shown clear advantages for biomedical applications owing to diminished heating effects and better tissue penetration. Herein, a novel 808 nm NIR-mediated control method for stem cell differentiation has been developed using multishell UCNPs, which are optimized for upconverting 808 nm NIR light to UV emission. The locally generated UV emissions further toggle photoswitching polymer capping ligands to achieve spatiotemporally controlled small-molecule release. More specifically, with 808 nm NIR excitation, stem cell differentiation factors can be released to guide neural stem cell (NSC) differentiation in a highly controlled manner. Given the challenges in stem cell behavior control, the developed 808 nm NIR-responsive UCNP-based approach to control stem cell differentiation can represent a new tool for studying single-molecule roles in stem cell and developmental biology.

Upconversion nanoparticles (UCNPs) should be particularly well suited for measurement inside cells because they can be imaged down to submicrometer dimensions in near real time using fluorescence microscopy, and they overcome problems, such as photobleaching, autofluorescence, and deep tissue penetration, that are commonly encountered in cellular imaging applications. In this study, the performance of an UCNP modified with a pH-sensitive dye (pHAb) is studied. The dye (emission wavelength 580 nm) was attached in a polyethylene imine (PEI) coating on the UCNP and excited via the 540-nm UCNP emission under 980-nm excitation. The UC resonance energy transfer efficiencies at different pHs ranged from 25 to 30% and a Förster distance of 2.56 nm was predicted from these results. Human neuroblastoma SH-SY5Y cells, equilibrated with nigericin H + /K + ionophore to equalize the intra-and extracellular pH‚ showed uptake of the UCNP-pHAb conjugate particles and, taking the ratio of the intensity collected from the pHAb emission channel (565-630 nm) to that from the UCNP red emission channel (640-680 nm), produced a sigmoidal pH response curve with an apparent pK a for the UCNP-pHAb of~5.1. The UCNP-pHAb were shown to colocalize with LysoBrite dye, a lysosome marker. Drug inhibitors such as chlorpromazine (CPZ) and nystatin (NYS) that interfere with clathrin-mediated endocytosis and caveolae-mediated endocytosis, respectively, were investigated to elucidate the mechanism of nanoparticle uptake into the cell. This preliminary study suggests that pH indicator-modified UCNPs such as UCNP-pHAb can report pH in SH-SY5Y cells and that the incorporation of the nanoparticles into the cell occurs via clathrinmediated endocytosis.

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