Gold nanostars co-coated with the Cu(ii) complex of a tetraazamacrocyclic ligand

Abstract

The twelve-membered tetraazamacrocyclic ligand L1 bears an appended lipoic acid unit, whose disulphide ring is an efficient grafting moiety for the surface of gold nanostars (GNS). The GNS that were used featured a localized surface plasmon resonance (LSPR) absorption at ∼800 nm, i.e. in the near infrared (NIR). We investigated different approaches for coating them with the Cu²⁺ complex of L1. While the direct reaction of [CuL1]²⁺ with as-prepared GNS led to aggregation, an initial coating step with polyethyleneglycol–thiol (PEG–SH) was found to be advantageous. Displacement reactions were carried out on pegylated GNS either with [CuL1]²⁺, directly generating [Cu_n(L1@GNS)]²ⁿ⁺, or with void L1, thus obtaining L1@GNS that coordinates Cu²⁺ in a second step. In both cases, even with a large excess of the competing disulphide moiety, only partial displacement of PEG–SH is observed, obtaining ca. 500–1500 [CuL1]²⁺ per GNS depending on the conditions, with PEG–SH remaining in the [Cu_n(L1@GNS)]²ⁿ⁺ hybrids and imparting them with remarkable stability. Comparison of the photothermal and two-photon luminescence (TPL) properties of the GNS between the pegylated GNS and [Cu_n(L1@GNS)]²ⁿ⁺ revealed that the grafted copper complex does not change them to any extent. Finally, the stability against demetallation and transmetallation of the complexes, as well as the fast kinetics of complexation of the monodispersed macrocycle and of L1@GNS, have been examined, suggesting [Cu_n(L1@GNS)]²ⁿ⁺ as a device capable of TPL optical tracking and NIR photothermal therapy and as a possible agent for PET imaging.