Gadolinium(III) complexes have been widely utilised as magnetic resonance imaging (MRI) contrast agents for decades. In recent years however, concerns have developed about their toxicity, believed to derive from demetallation of the complexes in vivo, and the relatively large quantities of compound required for a successful scan. Recent efforts have sought to enhance the relaxivity of trivalent gadolinium complexes without sacrificing their stability. This review aims to examine the strategic design of ligands synthesised for this purpose, provide an overview of recent successes in gadolinium-based contrast agent development and assess the requirements for clinical translation.

Reported immunoadjuvants still have many limitations, such as inferior cellular uptake capacity and biocompatibility, overly large particle sizes, single function, and unsatisfactory therapeutic efficacy. Here, large‐pore mesoporous‐silica‐coated upconversion nanoparticles (UCMSs) with a size of less than 100 nm are successfully prepared by a typical silica sol–gel reaction using mesitylene as a pore‐swelling agent and are applied as a novel immunoadjuvant. The obtained UCMSs not only show significantly higher loadings for the photosensitizers merocyanine 540 (MC540), model proteins (chicken ovalbumin (OVA)), and tumor antigens (tumor cell fragment (TF)), but also are successfully employed for highly efficient in vivo vaccine delivery. The prepared UCMSs–MC540–OVA under 980 nm near‐infrared irradiation shows the best synergistic immunopotentiation action, verified by the strongest Th1 and Th2 immune responses and the highest frequency of CD4+, CD8+, and effector‐memory T cells. Additionally, nanovaccines UCMSs–MC540–TF can more effectively inhibit tumor growth and increase the survival of colon cancer (CT26)‐tumor‐bearing BALB/c mice compared with either photodynamic therapy or immunological therapy alone, suggesting the enhanced immunotherapy efficacy and clinical potential of UCMSs as immunoadjuvants for cancer immunotherapy.

Lanthanide-doped upconversion nanocrystals (UCNCs) have recently become an attractive nonlinear fluorescence material for use in bioimaging because of their tunable spectral characteristics and exceptional photostability. Plasmonic materials are often introduced into the vicinity of UCNCs to increase their emission intensity by means of enlarging the absorption cross-section and accelerating the radiative decay rate. Moreover, plasmonic nanostructures (e.g., gold nanorods, GNRs) can also influence the polarization state of the UC fluorescence—an effect that is of fundamental importance for fluorescence polarization-based imaging methods yet has not been discussed previously. To study this effect, we synthesized GNR@SiO2@CaF2:Yb3+,Er3+ hybrid core–shell–satellite nanostructures with precise control over the thickness of the SiO2 shell. We evaluated the shell thickness-dependent plasmonic enhancement of the emission intensity in ensemble and studied the plasmonic modulation of the emission polarization at the single-particle level. The hybrid plasmonic UC nanostructures with an optimal shell thickness exhibit an improved bioimaging performance compared with bare UCNCs, and we observed a polarized nature of the light at both UC emission bands, which stems from the relationship between the excitation polarization and GNR orientation. We used electrodynamic simulations combined with Förster resonance energy transfer theory to fully explain the observed effect. Our results provide extensive insights into how the coherent interaction between the emission dipoles of UCNCs and the plasmonic dipoles of the GNR determines the emission polarization state in various situations and thus open the way to the accurate control of the UC emission anisotropy for a wide range of bioimaging and biosensing applications.

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A water-soluble azobenzene-containing functional monomer 4-[(4-methacryloyloxy)phenylazo] benzenesulfonic acid (MAPASA) was developed for the fabrication of a photoresponsive molecularly imprinted hydrogel material that can function in the biocompatible aqueous media. Cross-linking the functional monomer with various bisacrylamide and bismethacrylamide cross-linkers produced polyacrylamide hydrogels with enough optical transparency in the aqueous media for spectroscopic characterization and measurements. Paracetamol, (N-(4-hydroxyphenyl)acetamide), a common analgesic and antipyretic drug, was adopted as the molecular template for the imprinting. Reorientation of the hydrated sulfonated azobenzene chromophores in the course of their trans−cis photoisomerization was found to be sterically demanding. When incorporated into the relatively confined and rigid hydrogel environment generated by the most commonly adopted polyacrylamide cross-linker, N,N′-methylenebisacrylamide (1-C), these chromophores were not able to undergo photoisomerization. Lengthening the spacers separating the polymerizable acrylamide/methacrylamide functionalities at both ends of the cross-linkers from ethylene (2-C) to octylene (8-C) enhanced the flexibility of the resultant hydrogel matrices and resumed the photoisomerization properties of the chromophores. The rate of photoisomerization gradually increased with spacer length. On the other hand, substrate binding strength of the imprinted receptors dropped with the increasing flexibility of the hydrogels. Balancing these factors, the cross-linker N,N′-hexylenebismethacrylamide (6-C) was subsequently selected as the optimal cross-linker for the fabrication of the photoresponsive imprinted hydrogel. Scatchard analysis revealed the specific and nonspecific binding strength of the resultant imprinted hydrogel to be 1.96 × 105 and 747.0 M−1, respectively. The density of the imprinted receptors in the hydrogel was 0.47 µmol g−1. The affinity of the hydrogel for paracetamol can be photoregulated. Upon irradiation at 353 nm, 83.6% of receptor-bound paracetamol was released from the imprinted hydrogel. Subsequent irradiation at 440 nm caused 94.1% of the released paracetamol to be rebound by the hydrogel again. Such a photoregulated release and uptake process is repeatable. Results of our work demonstrated the potential of stimuli-responsive molecularly imprinted materials as biocompatible smart chemicals and drugs transfer systems.