Enabling Förster Resonance Energy Transfer from Large Nanocrystals through Energy Migration

Advances in highly doped upconversion nanoparticles

Lanthanide-doped upconversion nanoparticles (UCNPs) are capable of converting near-infra-red excitation into visible and ultraviolet emission. Their unique optical properties have advanced a broad range of applications, such as fluorescent microscopy, deep-tissue bioimaging, nanomedicine, optogenetics, security labelling and volumetric display. However, the constraint of concentration quenching on upconversion luminescence has hampered the nanoscience community to develop bright UCNPs with a large number of dopants. This review surveys recent advances in developing highly doped UCNPs, highlights the strategies that bypass the concentration quenching effect, and discusses new optical properties as well as emerging applications enabled by these nanoparticles.

A pre-protective strategy for precise tumor targeting and efficient photodynamic therapy with a switchable DNA/upconversion nanocomposite

Tumor-specific targeting based on folic acid (FA) is one of the most common and significant approaches in cancer therapy. However, the expression of folate receptors (FRs) in normal tissues will lead to unexpected targeting and unsatisfactory therapeutic effect. To address this issue, we develop a pre-protective strategy for precise tumor targeting and efficient photodynamic therapy (PDT) using a switchable DNA/upconversion nanocomposite, which can be triggered in the acidic tumor microenvironment. The DNA/upconversion nanocomposite is composed of polyacrylic acid (PAA) coated upconversion nanoparticles (UCNPs), the surface of which is modified using FA and chlorin e6 (Ce6) functionalized DNA sequences with different lengths. Initially, FA on the shorter DNA was protected by a longer DNA to prevent the bonding to FRs on normal cells. Once reaching the acidic tumor microenvironment, C base-rich longer DNA forms a C-quadruplex, resulting in the exposure of the FA groups and the bonding of FA and FRs on cancer cell membranes to achieve precise targeting. Simultaneously, the photosensitizer chlorin e6 (Ce6) gets close to the surface of UCNPs, enabling the excitation of Ce6 to generate singlet oxygen (1O2) under near infrared light via Förster resonance energy transfer (FRET). In vivo experiments indicated that higher tumor targeting efficiency was achieved and the tumor growth was greatly inhibited through the pre-protective strategy.

Precisely Tailoring Upconversion Dynamics via Energy Migration in Core-Shell Nanostructures

Upconversion emission dynamics have long been believed to be determined by the activator and its interaction with neighboring sensitizers. Herein this assumption is, however, shown to be invalid for nanostructures. We demonstrate that excitation energy migration greatly affects upconversion emission dynamics. "Dopant ions' spatial separation" nanostructures are designed as model systems and the intimate link between the random nature of energy migration and upconversion emission time behavior is unraveled by theoretical modelling and confirmed spectroscopically. Based on this new fundamental insight, we have successfully realized fine control of upconversion emission time behavior (either rise or decay process) by tuning the energy migration paths in various specifically designed nanostructures. This result is significant for applications of this type of materials in super resolution spectroscopy, high-density data storage, anti-counterfeiting, and biological imaging.

A Universal Upconversion Sensing Platform for the Sensitive Detection of Tumour-Related ncRNA through an Exo III-Assisted Cycling Amplification Strategy

Here, a sensitive and universal noncoding RNA (ncRNA) upconversion sensing nanoplatform is developed. Gold nanoparticles bearing one hairpin DNA (Hp) molecule are conjugated to the linker DNA modified NaYF₄ :Yb, Er@NaYF₄ upconversion nanoparticles by DNA hybridization, leading to quenching of the upconversion emission through fluorescence resonance energy transfer. A signal DNA (SDNA) sequence is designed to open Hp, recovering the upconversion emission. To achieve universality and high sensitivity of the nanoprobe, an exonuclease III (Exo III)-assisted cycling amplification strategy is introduced. A multifunctional hairpin DNA (mHp) containing ncRNA recognition sequence and SDNA sequence is designed to recognize ncRNA and trigger Exo III as a biocatalyst to stepwise disintegrate itself, releasing both ncRNA and SDNA. The released ncRNA can be reused to release more SDNA, which greatly improves the sensing sensitivity. By changing the recognition portion of mHp, various ncRNA can be detected. The sensitive detection of both homeobox (HOX) transcript antisense RNA segment and miR-21 is achieved with this novel strategy, even in human serum, indicating the universality and sensitivity of the proposed strategy. Additionally, the expression level of miR-21 in human breast cancer cell (MCF-7) lysate is successfully measured, suggesting its potential in clinical diagnosis.

Upconversion Luminescence Sensitized pH-Nanoprobes

Photon upconversion materials, featuring excellent photophysical properties, are promising for bio-medical research due to their low autofluorescence, non-cytotoxicity, low photobleaching and high photostability. Upconversion based pH-nanoprobes are attracting considerable interest due to their superiority over pH-sensitive molecular indicators and metal nanoparticles. Herein, we review the advances in upconversion based pH-nanoprobes, the first time in the seven years since their discovery in 2009. With a brief discussion on the upconversion materials and upconversion processes, the progress in this field has been overviewed, along with the toxicity and biodistribution of upconversion materials for intracellular application. We strongly believe that this survey will encourage the further pursuit of intense research for designing molecular pH-sensors.

Heterodimers Made of Upconversion Nanoparticles and Metal-Organic Frameworks

Creating nanoparticle dimers has attracted extensive interest. However, it still remains a great challenge to synthesize heterodimers with asymmetric compositions and synergistically enhanced functions. In this work, we report the synthesis of high quality heterodimers composed of porphyrinic nanoscale metal-organic frameworks (nMOF) and lanthanide-doped upconversion nanoparticles (UCNPs). Due to the dual optical properties inherited from individual nanoparticles and their interactions, absorption of low energy photons by the UCNPs is followed by energy transfer to the nMOFs, which then undergo activation of porphyrins to generate singlet oxygen. Furthermore, the strategy enables the synthesis of heterodimers with tunable UCNP size and dual NIR light harvesting functionality. We demonstrated that the hybrid architectures represent a promising platform to combine NIR-induced photodynamic therapy and chemotherapy for efficient cancer treatment. We believe that such heterodimers are capable of expanding their potential for applications in solar cells, photocatalysis, and nanomedicine.

Cooperative and non-cooperative sensitization upconversion in lanthanide-doped LiYbF4 nanoparticles

Lanthanide (Ln³⁺)-doped upconversion nanoparticles (UCNPs) have attracted tremendous interest owing to their potential bioapplications. However, the intrinsic photophysics responsible for upconversion (UC) especially the cooperative sensitization UC (CSU) in colloidal Ln³⁺-doped UCNPs has remained untouched so far. Herein, we report a unique strategy for the synthesis of high-quality LiYbF₄:Ln³⁺ core-only and core/shell UCNPs with tunable particle sizes and shell thicknesses. Energy transfer UC from Er³⁺, Ho³⁺ and Tm³⁺ and CSU from Tb³⁺ were comprehensively surveyed under 980 nm excitation. Through surface passivation, we achieved efficient non-cooperative sensitization UC with absolute UC quantum yields (QYs) of 3.36%, 0.69% and 0.81% for Er³⁺, Ho³⁺ and Tm³⁺, respectively. Particularly, we for the first time quantitatively determined the CSU efficiency for Tb³⁺ with an absolute QY of 0.0085% under excitation at a power density of 70 W cm^-2. By means of temperature-dependent steady-state and transient UC spectroscopy, we unraveled the dominant mechanisms of phonon-assisted cooperative energy transfer (T > 100 K) and sequential dimer ground-state absorption/excited-state absorption (T < 100 K) for the CSU process in LiYbF₄:Tb³⁺ UCNPs.

Optical nanoprobes for biomedical applications: shining a light on upconverting and near-infrared emitting nanoparticles for imaging, thermal sensing, and photodynamic therapy

Shining a light on spectrally converting lanthanide (Ln³⁺)-doped nanoparticles: progress, trends, and challenges in Ln³⁺-nanoprobes for near-infrared bioimaging, nanothermometry, and photodynamic therapy.