Using ATTO Dyes To Probe the Photocatalytic Activity of Au−CdS Nanoparticles

Metal−semiconductor nanohybrids (or heterostructures), such as Au−CdS, have become an important class of materials because of their role in photochemical hydrogen production and in other catalytic reactions. Here we report the results of photophysical studies of the interactions of these particles with ATTO dyes (ATTO 590 and 655), which are used as fluorescent probes in a wide range of spectroscopic techniques, most notably super-resolution microscopies. The most important feature of the Au−CdS particles is that they provide the possibility of selective excitation at either their CdS or their Au domains, which absorb preferentially at wavelengths shorter or longer than 500 nm, respectively, thus making possible an excited-state charge transfer reaction from ATTO. Fluorescence quenching of ATTO is dominated by charge transfer to either the CdS domain (λex = 400 nm) or the Au domain (λex = 570 nm). This quenching is quantified by steady-state and time-resolved absorption and fluorescence measurements, and its assignment is confirmed by electrochemical measurements. The results indicate that the ATTO dyes are sensitive and useful probes for measuring the photocatalytic activity of nanoparticles. Characterizing the nonradiative processes of the ATTO dyes in the presence of these catalytically active particles provides a means of gauging their utility in the wide range of spectroscopies in which they are employed. ■ INTRODUCTION Hybrid nanoparticles have attracted considerable attention because of their applicability in many technologically important areas such as energy conversion and catalysis. Au− CdS has been reported as a photocatalyst for chemical reactions, most importantly for photochemical hydrogen production. Recently, a record 100% photon-to-hydrogen production efficiency has been reported with a Pt-tipped “CdSe@CdS” rod. Charge transfer from the semiconductor to the metal is very fast, less than 20 fs. The possibility of changing the composition, size, shape, or other parameters of the semiconductor enables one to tailor its optical properties. It has been shown, furthermore, that excitation of plasmons in metal nanostructures can induce the transfer of hot electrons into the semiconductors, making them interesting candidates for light harvesting materials because their absorption is tunable. In these systems, as the metal and the semiconductor are directly in contact, the electron injection from metal to semiconductor occurs with higher efficiency. This is because the electron-transfer rate can overcome the rate of the electron−electron scattering (approximately hundreds of femtoseconds). Alternatively, efficient electron transfer may occur via a plasmon-induced, metal-to-semiconductor, interfacial, charge-transfer-transition pathway as proposed by Lian and co-workers. As the energy associated with the excitonic transition of CdS nanoparticles is much different from that of the surface plasmonic resonance in Au, a selective excitation of either the semiconductor or the metal domain is possible. Vela, Fang, and co-workers designed Au−CdS nanoparticles (Figure 1) and demonstrated that they were efficient photocatalysts for the conversion of amplex red to resorufin in the presence of H2O2. 12 Two distinct mechanisms, depending on the excitation wavelength, were identified for inducing charge separation in the nanohybrids. Namely, either the Au or the CdS domains of the nanostructure could be selectively excited, thus changing the mechanism of resorufin production. Here, we demonstrate that there are other (simpler) means of probing this charge separation, namely, by employing ATTO dyes (for example, Figure 2), which are used extensively in single-molecule spectroscopies and superReceived: September 28, 2016 Revised: December 13, 2016 Published: December 13, 2016 Article