Precise separation and photoluminescence enhancement of hydrophilic ultrasmall CdTe nanoparticles

Hydrophilic semiconductor nanoparticles (NPs) with sizes of ∼2 nm are attractive for sensing applications because their high surface-to-volume ratios enhance their interactions with target species. However, systematic experimental access to this ultrasmall regime remains limited because of the difficulty in isolating uniform NPs. Herein, we employed polyacrylamide gel electrophoresis (PAGE) to precisely separate a series of ultrasmall CdTe NPs. The PAGE-separated CdTe NPs exhibited gradual blue shifts in the excitonic absorption peaks, as is characteristic of colloidal NPs, rather than the discrete peak progression observed in nanoclusters (NCs), while retaining sharp excitonic absorption features that are reminiscent of NCs. These observations indicate that ultrasmall CdTe NPs exhibit optical characteristics of both colloidal NPs and NCs. While excitonic absorption features evolve continuously as the size decreases, the photoluminescence (PL) properties are strongly influenced by the surface and defect states. The PL intensity decreased markedly for smaller CdTe NPs, whereas thermal annealing enhanced the PL efficiency of larger CdTe NPs. These results demonstrate that PAGE fractionation provides practical access to the ultrasmall regime and offers insights into the optical properties at the boundary between NCs and NPs.