Synergism at the Nanoscale: Photoactive Semiconductor Nanoparticles and their Organic Ligands

Introduction

The rapid development of novel technologies and new instruments over the last years has opened up the opportunity to investigate material properties at the nanoscale level. Studies on the preparation, photophysical properties, and applicability of photoactive nanomaterials have increased exponentially since their novel properties, compared with those of bulk material, were discovered.

Photoactive nanoparticles are light-responsive nanomaterials which, depending on the nature of the material, can exhibit a broad-absorption spectrum (in the UV-visible or near infrared range) as well as narrow Stokes or anti-Stokes emission. In addition, their optical properties can depend not only on their size but also on their aggregation state. Most of nanoparticles (NPs) are capped with organic molecules, usually termed surfactants, capping agents, or ligands, which provide them solubility, processability, and functionality, as well as prevent their aggregation. The unique properties of these nanosystems can be advantageously used for their application in molecular recognition, bio-imaging, drug-delivery, photodynamic therapy, and in the manufacture of optical devices, among others.(Bechet et al., 2008; Bera, Qian, Tseng, & Holloway, 2010; Huang, Barua, Sharma, Dey, & Rege, 2011; Prabhakaran, Kim, Lee, & Prasad, 2012; Rhee, Chung, & Diau, 2013; Wolfbeis, 2015)

There are different types of photoactive NPs which intrinsic properties mainly depend on their composition, such as: i) semiconductor NPs comprising elements from the II-VI (CdSe, CdTe, CdS, ZnSe), III-V (InP, InAs), and IV-VI (PbSe) groups; ii) organometal halide perovskite NPs, which contain an anionic metal-halogen semiconducting framework and charge-compensating organic cations; iii) metal NPs, such as gold, silver, and copper NPs; iv) upconversion NPs, an unusual type of NPs consisting of a matrix doped with rare-earth ions (e.g., NaYF₄ (Yb³⁺, Er³⁺); and v) carbon-dots. Here we will focus on semiconductor NPs, specifically CdSe-based NPs (CdSe core and CdSe/ZnS core-shell NPs) and organolead halide perovskite nanomaterials. While the former are all-inorganic systems and their preparation and optical properties have extensively studied, the latter are hybrid organic-inorganic systems currently under study. Both of them will be used to explain the role of the organic ligand in their preparation and optical properties.

Semiconductor NPs based on CdSe can be decorated with organic ligands, bounded to their surface by different anchoring groups (e.g., thiolates, amine, phosphine oxides, and carboxylates), and these ligands may also have a functional group such as a photoactive chromophore, chiral compound, macrocycle, etc.(Galian & Guardia, 2009) The ligand can enhance the NP emissive efficiency, be used to detect organic analytes/drugs and oxygen, and be useful in the preparation of fluorescent organogels, among other applications. (Agudelo-Morales, Galian, & Pérez-Prieto, 2012; Aguilera-Sigalat, Casas-Solvas et al., 2012; Delgado-Pérez, Bouchet, de la Guardia, Galian, & Pérez-Prieto, 2013; Gonzalez-Carrero, Agudelo-Morales, Guardia, Galian, & Perez-Prieto, 2015; González-Carrero, de la Guardia, Galian, & Pérez-Prieto, 2014; Wadhavane et al., 2012) In addition, the organic ligand has proved to be crucial in the reversible phase transfer of CdSe NPs capped with long alkyl chain amines by making use of the interdigitation between the alkyl chain of the ligand and those of a surfactant.(Francés-Soriano, Pocoví-Martínez, González-Béjar, & Pérez-Prieto, 2014)