Poly(N-vinylimidazole) assisted formation of bright far-red/near infrared gold nanocluster aggregates

Abstract

Up to the present date, positive reports on aggregates of gold nanoclusters (Au NCs) are rare. In this work, water-soluble Au NCs aggregates were facilely synthesized by heating the mixture of poly(N-vinylimidazole) (PVIm), mercaptosuccinic acid (MSA) and HAuCl4. The obtained PVIm-MSA@Au NCs always showed bright far-red/near infrared emission (centered at ca. 715 nm) no matter of UV or visible light excitation. The emission was mainly phosphorescence supported by the facts: (1) a large Stokes shift (>450 nm) and (2) a long life time of τ = 2.53 μs. PVIm-MSA@Au NCs did not experience the normal serious aggregation-caused quenching upon increasing the concentration. Moreover, PVIm-MSA@Au NCs exhibited higher quantum yields in solid state than in solution. Characterizations of transmission electron microscope and dynamic light scattering proved that both dilute and concentrated Au NCs in water were in the aggregated state. X-ray photoelectron spectroscopy measurements demonstrated that the strong emission of PVIm-MSA@Au NCs aggregates originated from the Au° formed Au NCs, not the Au(I)-thiolate complex. Control experiment with the replacement of PVIm by imidazole (Im)-containing molecules showed that Im-MSA@Au NCs at dilute concentration were also in aggregate state, however, its emission was very weak. This implied that for the formation of bright Au NCs, simple packing together of Au NCs was not enough. The important function of PVIm might act as glue to bridge together many discrete Au NCs to form densely packed aggregates that were responsible for the strong emission.