Advanced diagnostic procedures are required to satisfy the continuously increasing demands of modern biomedicine while also addressing the need for cost reduction in public health systems. The development of infrared luminescence-based techniques for in vivo imaging as reliable alternatives to traditional imaging enables applications with simpler and more cost-effective apparatus. To further improve the information provided by in vivo luminescence images, the design and fabrication of enhanced infrared-luminescent contrast agents is required. In this work, we demonstrate how simple dopant engineering can lead to infrared-emitting rare-earth-doped nanoparticles with tunable (0.1-1.5 ms) and medium-independent luminescence lifetimes. The combination of these tunable nanostructures with time-gated infrared imaging and time domain analysis is employed to obtain multiplexed in vivo images that are used for complex biodistribution studies.

Upconversion nanoparticles (UCNPs), typically converting near-infrared (NIR) light into visible luminescence, are promising for bioimaging applications. However, optical multiplexed in vivo upconversion experiments have long been hampered by the exceptional rarity of available luminescence bands in UCNPs that can penetrate deep in tissues. Herein, we describe an approach to accomplish multiplexed upconversion in vivo imaging through time-domain discrimination of tissue-penetrating NIR luminescence at 808 nm (from thulium ions) with a multitude of distinct lifetimes. A tetradomain nanostructure design enables one to regulate energy migration and upconverting processes within confined nanoscopic domains in defined ways, thus yielding high quantum yield upconversion luminescence (maximum ≈ 6.1%, 0.11 W/cm2) with precisely controlled lifetimes that span 2 orders of magnitude (from 78 to 2157 μs). Importantly, intravenous and subcutaneous administration of aqueous form UCNPs into a Kunming mouse demonstrates high-contrast lifetime-colored imaging of them in liver and two abdomen subcutis. Moreover, optical patterns of these UCNPs allow multicolour presentation of a series of deciphered images that are hued with precisely defined lifetimes. The described temporal multiplexed upconversion approach, demonstrated in in vivo imaging and multilevel anticounterfeiting, has implications for high-throughput biosensing, volumetric displays, and diagnosis and therapy.

importance to unravel the complexity of these systems and to impact a multitude of technological applications ranging from integrated photonic devices to precise medicine. [1-3] Thermocouples and thermistors dominate the market but are inappropriate to probe temperature in live biological systems as physical contact with measured samples is a prerequisite, which disturbs the measurements at sub-millimeter scales. [4] Alternatively, luminescence nanothermometry is emerging as a noninvasive spectroscopic method that allow to probe temperature variation at nanometric spatial resolution and in remote distance, spurring wide interests. [5,6] The contactless and high-resolution nature makes them ideal candidates for temperature evaluation in the early diagnosis of several diseases as well as for providing real-time temperature feedback in thermal (hypothermia or hyperthermia) therapies of malignant cancers. [7-9] Indeed, the potential clinical and preclinical applications fuel a fast development of luminescence nanothermometers particularly for in vivo studies in the nearinfrared (NIR) range. [10] Light in the first biological window (NIR-I, 750-950 nm) or the second biological window (NIR-II, 1000-1700 nm) is known to have minimized scattering and absorption, thus allowing for maximized light Luminescence nanothermometry is promising for noninvasive probing of temperature in biological microenvironment at nanometric spatial resolution. Yet, wavelength-and temperature-dependent absorption and scattering of tissues distort measured spectral profile, rendering conventional luminescence nanothermometers (ratiometric, intensity, band shape, or spectral shift) problematic for in vivo temperature determination. Here, a class of lanthanide-based nanothermometers, which are able to provide precise and reliable temperature readouts at varied tissue depths through NIR-II luminescence lifetime, are described. To achieve this, an inert core/ active shell/inert shell structure of tiny nanoparticles (size, 13.5 nm) is devised, in which thermosensitive lanthanide pairs (ytterbium and neodymium) are spatially confined in the thin middle shell (sodium yttrium fluoride, 1 nm), ensuring being homogenously close to the surrounding environment while protected by the outmost calcium fluoride shell (CaF 2 , ≈2.5 nm) that shields out bioactive milieu interferences. This ternary structure enables the nanothermometers to consistently resolve temperature changes at depths of up to 4 mm in biological tissues, having a high relative temperature sensitivity of 1.4-1.1% °C −1 in the physiological temperature range of 10-64 °C. These lifetime-based thermosensitive nanoprobes allow for in vivo diagnosis of murine inflammation, mapping out the precise temperature distribution profile of nanoprobes-interrogated area.

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Photodynamic therapy (PDT) is a minimally invasive cancer modality that combines a photosensitizer (PS), light, and oxygen. Introduction of new nanotechnologies holds potential to improve PDT performance. Upconversion nanoparticles (UCNPs) offer potentially advantageous benefits for PDT, attributed to their distinct photon upconverting feature. The ability to convert near-infrared (NIR) light into visible or even ultraviolet light via UCNPs allows for the activation of nearby PS agents to produce singlet oxygen, as most PS agents absorb visible and ultraviolet light. The use of a longer NIR wavelength permits light to penetrate deeper into tissue, and thus PDT of a deeper tissue can be effectively achieved with the incorporation of UCNPs. Recent progress in UCNP development has generated the possibility to employ a wide variety of NIR excitation sources in PDT. Use of UCNPs enables concurrent strategies for loading, targeting, and controlling the release of additional drugs. In this review article, recent progress in the development of UCNPs for PDT applications is summarized.