Photoluminescent crystalline-assembly of gold nanoclusters: Facile preparation with the mediation of hydroxyl-terminated hyperbranched polyethylenimine and its reversible response to CO2 adsorption and desorption

Stimuli-responsive metal nanoclusters (NCs) are intriguing materials due to their wide applications in photoluminescence (PL) sensors, bioimaging and catalysis. CO2 gas as a green stimulus could arouse the PL variation of certain metal NCs only under high-pressure condition. Metal NCs responding to the normal-pressure CO2 have not been reported yet. In this work, hydroxyl-terminated hyperbranched polyethylenimine (HPEI-OH) was used to mediate the preparation of water-soluble Au NC assemblies (Au NC-As) with crystallization-induced emission (CIE) nature. HPEI-OH played a multifunctional role, such as reducing Au ions, endowing water solubility to Au NC-As, packing thiolated Au NCs closely and enabling CO2-responsiveness to Au NC-As. Controlling the preparation pH at 2.0 or 3.2, red or yellow-emitting Au NC-As with quantum yield of ca. 4.6% or 5.8% were achieved. The as-prepared CIE-type Au NC-As possessed pH-responsive characteristic and strong PL emission was achieved under acidic condition. The CIE nature rendered not only strong PL emission to Au NC-As, but also strong photoelectron emission verified by the X-ray photoelectron spectroscopy. The introduction of Zn2+ ions extended the application range of Au NC-As from acidic condition fill to weak basic condition. Under neutral condition, the formed complex of Zn2+/Au NC-As showed reversible PL response to the normal-pressure CO2/N-2 cycling through adsorption and desorption process. Finally, the application of the complex in measuring the CO2 concentration of carbonated beverages was presented.