Metal nanoclusters (NCs) as a new class of phosphors have attracted a great deal of interest owing to their unique electronic structure and subsequently molecule-like optical properties. However, limited successes have been achieved in producing the NCs with excellent luminescent performance. In this paper, we demonstrate the significant luminescence intensity enhancement of 1-dodecanethiol (DT)-capped Cu NCs via self-assembly strategy. By forming compact and ordered assemblies, the original nonluminescent Cu NCs exhibit strong emission. The flexibility of self-assembly allows to further control the polymorphism of Cu NCs assemblies, and hence the emission properties. Comparative structural and optical analysis of the polymorphic NCs assemblies permits to establish a relationship between the compactness of assemblies and the emission. First, high compactness reinforces the cuprophilic Cu(I)···Cu(I) interaction of inter- and intra-NCs, and meanwhile, suppresses intramolecular vibration and rotation of the capping ligand of DT, thus enhancing the emission intensity of Cu NCs. Second, as to the emission energy that depends on the distance of Cu(I)···Cu(I), the improved compactness increases average Cu(I)···Cu(I) distance by inducing additional inter-NCs cuprophilic interaction, and therewith leads to the blue shift of NCs emission. Attributing to the assembly mediated structural polymorphism, the NCs assemblies exhibit distinct mechanochromic and thermochromic luminescent properties. Metal NCs-based white light-emitting diodes are further fabricated by employing the NCs assemblies with blue-green, yellow, and red emissions as phosphors.

Aggregation/assembly induced emission (AIE) has been observed for metal nanoclusters (NCs), but the origin of the enhanced emission is not fully understood, yet. In this work, the significant contribution of metal defects on AIE is revealed by engineering the self-assembly process of Cu NCs using ethanol. The presence of ethanol leads to a rapid assembly of NCs into ultrathin nanosheets, promoting the formation of metal defects-rich surface. Detailed studies and computer simulation confirm that the metal defects-rich nanosheets possess increased Cu(I)-to-Cu(0) ratio, which greatly influences ligand-to-metal-metal charge transfer and therewith facilitates the radiative relaxation of excitons. Consequently, the Cu NCs self-assembly nanosheets exhibit obvious emission enhancement.

Photothermal nanoplatforms with small size, low cost, multifunctionality, good biocompatibility and in particular biodegradability are greatly desired in the exploration of novel diagnostic and therapeutic methodologies. Despite Fe3O4 nanoparticles (NPs) have been approved as safe clinical agents, the low molar extinction coefficient and subsequent poor photothermal performance shed the doubt as effective photothermal materials. In this paper, we demonstrate the fabrication of polypyrrole (PPy)-enveloped Fe3O4 NP superstructures with a spherical morphology, which leads to a 300-fold increase in the molar extinction coefficient. The basic idea is the optimization of Fe3O4 electronic structures. By controlling the self-assembly of Fe3O4 NPs, the diameters of the superstructures are tuned from 32 to 64 nm. This significantly enhances the indirect transition and magnetic coupling of Fe ions, thus increasing the molar extinction coefficient of Fe3O4 NPs from 3.65 × 10(6) to 1.31 × 10(8) M(-1) cm(-1) at 808 nm. The envelopment of Fe3O4 superstructures with conductive PPy shell introduces additional electrons in the Fe3O4 oscillation system, and therewith further enhances the molar extinction coefficient to 1.12 × 10(9) M(-1) cm(-1). As a result, the photothermal performance is greatly improved. Primary cell experiments indicate that PPy-enveloped Fe3O4 NP superstructures are low toxic, and capable to kill Hela cells under near-infrared laser irradiation. Owing to the low cost, good biocompatibility and biodegradability, the PPy-enveloped Fe3O4 NP superstructures are promising photothermal platform for establishing novel diagnostic and therapeutic methods.

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