Biotin/(strept)avidin self-assembly is a powerful platform for nanoscale fabrication and capture with many different applications in science, medicine, and nanotechnology. However, biotin/(strept)avidin self-assembly has several well-recognized drawbacks that limit performance in certain technical areas and there is a need for synthetic mimics that can either become superior replacements or operational partners with bio-orthogonal recognition properties. The goal of this tutorial review is to describe the recent progress in making high affinity synthetic association partners that operate in water or biological media. The review starts with a background summary of biotin/(strept)avidin self-assembly and the current design rules for creating synthetic mimics. A series of case studies are presented that describe recent success using synthetic derivatives of cyclodextrins, cucurbiturils, and various organic cyclophanes such as calixarenes, deep cavitands, pillararenes, and tetralactams. In some cases, two complementary partners associate to produce a nanoscale complex and in other cases a ditopic host molecule is used to link two partners. The article concludes with a short discussion of future directions and likely challenges.

Self-assembly of ligand 1 and Pd(NO3)2 delivers Fujita-type metal organic polyhedron (MOP) 3 which bears 24 covalently attached methyl viologen units on its external surface as evidenced by 1H NMR, DOSY NMR, electrospray mass spectrometry, TEM, and AFM measurements. MOP 3 undergoes non-covalent complexation with cucurbit[n]urils to yield MOPs 4 – 6 with diameter ≈ 5–6 nm. MOP 5 can be fully loaded with doxorubicin prodrug 2 via hetero ternary complex formation to yield 7. The MOPs exhibit excellent stability toward neutral to slightly acidic pH in 10 mM sodium phosphate buffer mitigating the concern of disassembly during circulation. The results of MTS assays show that MOP 7 is 10-fold more cytotoxic toward HeLa cells than equimolar quantities of doxorubicin prodrug 2. The enhanced cytotoxicity can be traced to a combination of enhanced cellular uptake of 7 and DOX release as demonstrated by flow cytometry and confocal fluorescence microscopy. The confluence of properties imparted by the polycationic MOP architecture and plug-and-play CB[n] complexation provides a potent new platform for drug delivery application.

Mixed self-assembly of ligand 1, 2, 1,6-hexanediamine (HDA) and Pd(NO3)2 afforded Fujita-type metal organic polyhedron MOP1 (diameter ≈ 8.2 nm) which is covalently functionalized with an average of 18 cucurbit[7]uril (CB[7]) units as evidenced by 1H NMR, diffusion ordered spectroscopy NMR and transmission electron microscopy measurements. By virtue of the host-guest properties of CB[7], the inner cavity of MOP can be rendered hydrophobic by using octadecyl HDA (3) as guest during the self-assembly process. The hydrophobic cavity was successfully utilized to trap the hydrophobic dye Nile Red (NR) and the anticancer drug Doxorubicin (DOX). The stimuli responsive release of encapsulated NR or DOX occurs: 1) upon addition of a competitive binder (e.g. adamantane ammonium (ADA)) for CB[7], 2) by a dual pH-chemical stimulus involving the protonation state change of adamantane carboxylate at pH 5.8, and 3) a dual pH-photochemical stimulus involving photoisomerization of trans-6 to cis-6 at pH 5.8. NR is released from NR@MOP2 within HeLa cancer cells. This body of work suggests that the covalent attachment of cucurbit[n]uril to metal organic polyhedra constitutes a promising vehicle for the development of both diagnostic and therapeutic nanoparticles.

A family of supramolecular four-component nanorotors was quantitatively self-assembled from two different zinc(II) porphyrins: one representing the stator and the other the rotator with DABCO as an interconnecting axle and copper(I) ions. Rotational spinning in ROT-1' occurs at 97,000 s(-1) at 25 °C but is virtually stopped at -75 °C. The activation vs binding data suggest that spinning is an intrasupramolecular process occurring to >99.9% without dissociation. Addition and removal of two further equiv of Cu(+) reversibly switches the mode of the stochastic rotation between pure 180° and mixed 90°/180° steps and reversibly regulates the speed between 97,000 and ~80,000 s(-1).

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The heteroleptic supramolecular double-decker porphyrin 1 was synthesized with DABCO as a guest between two cofacial porphyrin units as characterized by (1)H NMR and ESI-MS. While DABCO is not seen to tumble inside the cavity, even at higher temperatures (80 °C), such motion was triggered upon addition of various coordinating ligands (quinuclidine, 4-bromopyridine, or excess of DABCO). Different stoichiometric amounts were needed depending on the n donor quality of the added ligands to initiate tumbling of the "inside" DABCO. As demonstrated in an example with excess DABCO, the tumbling was stopped by lowering the temperature to -50 °C.

A highly selective receptor, circular dichroism and chiral competition are combined in this versatile method for d-glucose analysis.

A five-component supramolecular nanorotor with reversibly acting brakes has been prepared from a four-component nanorotor by adding the photo- and heat-responsive 2,2'-diazastilbene as a signal transducer. The rotational speed was reversibly switched between 86 and 38 kHz.