This review highlights the potential of using liposomes in bioassays. Liposomes consist of nano- or micro-sized, synthetically constructed phospholipid vesicles. Liposomes can be loaded with a number of reporting molecules that allow a dramatic amplification of the detection threshold in bioassays. Liposome-based sensors bind or react with the biological components of targets through the introduction of properly tailored vectors anchored on their external surface. The use of liposome-based formulations allows the set-up of bioassays that are rapid, sensitive, and often suitable for in-field applications. Selected applications in the field of immunoassays, as well as recognition/assessment of corona proteins, nucleic acids, exosomes, bacteria, and viruses are surveyed. The role of magnetoliposomes is also highlighted as an additional tool in the armory of liposome-based systems for bioassays.

Gd-L1 is a macrocyclic Gd-HPDO3A derivative functionalized with a short spacer to a trisulfonated pyrene. When compared to Gd-HPDO3A, the increased relaxivity appears to be determined by both the higher molecular weight and the occurrence of an intramolecularly catalyzed prototropic exchange of the coordinated OH moiety. In water, Gd-L1 displayed a relaxivity of 7.1 mM^-1 s^-1 (at 298 K and 0.5 T), slightly increasing with the concentration likely due to the onset of intermolecular aggregation. A remarkably high and concentration-dependent relaxivity was measured in human serum (up to 26.5 mM^-1 s^-1 at the lowest tested concentration of 0.005 mM^-1 s^-1). The acquisition of ¹H-nuclear magnetic relaxation dispersion (NMRD) and ¹⁷O-R₂ vs T profiles allowed to get an in-depth characterization of the system. In vitro experiments in the presence of human serum albumin, γ-globulins, and polylysine, as well as using media mimicking the extracellular matrix, provided strong support to the view that the trisulfonated pyrene fosters binding interactions with the exposed positive groups on the surface of proteins, responsible for a remarkable in vivo hyperintensity in T_1w MR images. The in vivo MR images of the liver, kidneys, and spleen showed a marked contrast enhancement in the first 10 min after the i.v. injection of Gd-L1, which was 2-6-fold higher than that for Gd-HPDO3A, while maintaining a very similar excretion behavior. These findings may pave the way to an improved design of MRI GBCAs, for the first time, based on the setup of weak and dynamic interactions with abundant positive groups on serum and ECM proteins.

Digadoglucitol is an extracellular macrocyclic dinuclear gadolinium-based contrast agent (GBCA) based on the association of two [Gd­(HP-DO3A)] units conjugated through a spacer containing the glucamine moiety. It displays a relaxivity per Gd that is 2 to 3 times higher than the most currently used GBCAs, allowing the use of reduced doses while ensuring a noninferior image contrast. Its high relaxivity is the result of a rational design aimed at exploiting the intramolecular catalysis of the prototropic exchange of the coordinated –OH groups as well as the second sphere contribution brought about by the presence of the hydroxyl functionalities on glucamine. Digadoglucitol maintains the excellent kinetic and thermodynamic properties of the parent [Gd­(HP-DO3A)] with an SAP/TSAP ratio of 2/3. A HPLC workup yielded three fractions of diastereoisomers based on the chirality of the 2-hydroxypropyl pendants with similar relaxometric and stability properties. From pH 5 to 9, the deprotonated glucamine nitrogen acts as base to catalyze the prototropic exchange of the coordinating –OH group bringing an enhancement of 1.5–2.0 mM^–1 s^–1 of the observed relaxivity with respect to the expected value for a q = 1 complex of a similar formula weight. Biodistribution and the Magnetic Resonance Imaging pharmacokinetics of digadoglucitol resulted very similarly to those found for [Gd­(BT-DO3A)].

A one-carbon extension of the hydroxyl-bearing arm enables Yb(HnP-DO3A) to bind lactate and generate a ratiometric ParaCEST response, allowing concentration-independent lactate quantification.