The recent ability to manipulate and visualize single atoms at atomic level has given rise to modern bottom-up nanotechnology. Similar exquisite degree of control at the individual polymeric chain level for producing functional soft nanoentities is expected to become a reality in the next few years through the full development of so-called "single chain technology". Ultra-small unimolecular soft nano-objects endowed with useful, autonomous and smart functions are the expected, long-term valuable output of single chain technology. This review covers the recent advances in single chain technology for the construction of soft nano-objects via chain compaction, with an emphasis in dynamic, letter-shaped and compositionally unsymmetrical single rings, complex multi-ring systems, single chain nanoparticles, tadpoles, dumbbells and hairpins, as well as the potential end-use applications of individual soft nano-objects endowed with useful functions in catalysis, sensing, drug delivery and other uses.

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We present an investigation, by combining small-angle neutron scattering (SANS) and coarsegrained molecular dynamics (MD) simulations, on the conformational properties of single-chain nano-particles (SCNPs) in crowded macromolecular solutions. By using linear chains as crowders SANS shows a crossover from almost unperturbed SCNP conformations in dilute conditions toward a continuous collapse of the macromolecule with increasing crowding. This collapse starts when the total concentration of the solution reaches the value of the overlap concentration of the pure SCNP solutions. MD-simulations suggest the generalizability of these experimental findings and extend them to the case when the SCNPs themselves are used as crowders-a situation which in real systems leads to unavoidable formation of aggregates, as shown here by SANS and DLS. Exploiting the simulations we have calculated the contact probability and the distance between monomers as functions of the contour distance between them; the results suggest that crumpled globular conformations are generally adopted by SCNPs in crowded macromolecular solutions. In the case of linear crowders, the SCNPs show, at fixed monomer concentration, a non-monotonic dependence of their collapse on the length of the crowders.

Controlling the spatial distribution of catalytic sites in metallo-folded single-chain nanoparticles (SCNPs) is a first step toward the rational design of improved catalytic soft nano-objects. Here an unexplored pathway is reported for tuning the internal structure of metallo-folded SCNPs. Unlike the conventional SCNP synthesis in good solvent (protocol I), the proposed new route (protocol II) is based on the use of amphiphilic random copolymers and transfer, after SCNP formation, from selective to good (nonselective) solvent conditions. The size and morphology of the SCNPs obtained by the two protocols, and the corresponding spatial distribution of the catalytic sites, have been determined by combining results from size exclusion chromatography with triple detection, small-angle X-ray scattering and molecular dynamics (MD) simulations. Remarkably, the use of these protocols allows the tuning of the internal structure of the metallo-folded SCNPs, as supported by MD simulations results. While the conventional protocol I yields a homogeneous distribution of the catalytic sites in the SCNP, these are arranged into clusters in the case of protocol II.