This work develops a rationale for effective sensitization of trivalent lanthanide cation (Ln3+) luminescence in a semiconductor nanoparticle by examining the luminescence characteristics of Ln3+ dopants in titanium dioxide nanoparticles [Ti­(Ln)­O2] [Ln = praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), or ytterbium (Yb)], as a representative model system. For excitation of the TiO2 host at 350 nm the intraconfigurational 4f–4f sharp luminescence bands are observed for the Nd, Sm, Eu, Ho, Er, Tm, and Yb incorporated (doped) nanoparticles, and no such luminescence is observed for the Pr, Gd, Tb, and Dy containing nanoparticles. While host sensitized luminescence of lanthanide ions dominate the emission in the Nd and Sm incorporated nanoparticles, the host sensitization effect is less pronounced for the Eu and Yb containing systems, and for the Ho, Er, and Tm doped nanocrystals only a subset of the dopant ions’ luminescence bands is sensitized. The experimental observations of the host sensitized Ln3+ luminescence properties in the [Ti­(Ln)­O2] nanoparticles can be rationalized by considering that the dopant ions act as charge traps in the host lattice and associated environment induced luminescence quenching effects. Using these results, an energy offset between the trap site and the nanoparticle’s band edge that will generate an optimal host sensitized dopant emission is proposed. The approach presented necessarily improves over a combinatorial approach to select the host and dopant moieties, with the benefit of providing physicochemical insight regarding the nature of photophysical processes in a given host (semiconductor nanoparticle)–guest (Ln3+) composite system.

This study develops water dispersible trivalent terbium cation incorporated zinc sulfide nanoparticles for potential anti-cancer therapy and cellular imaging.

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The lanthanide photoluminescence in the trivalent terbium (Tb 3+ ) incorporated zinc sulfide nanoparticles [Zn(Tb)S] has been reported with the nanoparticle size varying from 2.0 ± 0.3 to 14 ± 3 nm in diameter as a function of reaction temperature. In all the nanoparticles, the Tb 3+ luminescence has been sensitized by the nanoparticle acting as an optical antenna. The relative contribution of different excitation bands in sensitizing Tb 3+ luminescence in the Zn(Tb)S nanoparticles has been found to be dependent on the size of the nanoparticles. The observed Tb 3+ luminescence efficiency in the Zn(Tb)S nanoparticles has been rationalized by competing factors: (i) the sensitization efficiency that is guided by the relative energy level position of the Tb 3+ ground and excited states with respect to the valence and conduction bands of the ZnS and (ii) the extent of incorporation of Tb 3+ in the nanoparticles. Additionally, it has been argued that the spectral overlap between the nanoparticle (donor) emission and Tb 3+ (acceptor) absorption is not a prerequisite in determining the Tb 3+ emission in the Zn(Tb)S nanoparticles studied. ■ INTRODUCTIONDoped semiconductor nanoparticles are useful due to the possible modulation of chemical, optical, electrical, and magnetic properties of the materials. In the perspective of optically active doped semiconductor nanoparticles, trivalent lanthanide (Ln 3+ ) incorporated semiconductor nanoparticles find wide attraction. 1−6 The luminescence that originates from the intraconfigurational 4f−4f transitions in Ln 3+ is unique compared to that in the conventional luminophores and finds usage in various luminescence-based applications. 7−14 The luminescence of lanthanide cations exhibit sharp emission bands spanning the entire visible and near-infrared spectral region allowing multiplex assays; longer (microseconds to milliseconds) lifetime which makes time-gated measurements feasible; and resistance to photobleaching mechanisms, hence allowing longer experiment time thereby increasing the signal-to-noise ratio.In order to detect the Ln 3+ -based luminescence, two significant challenges need to be overcome. First, the molar extinction coefficient of these cations is at least 1000 times smaller than the corresponding values for the conventional organic fluorophores, allowing the direct excitation of the lanthanide cations to the higher lying excited state difficult. In addition to this, the Ln 3+ luminescence is quenched by the vibrational overtones of various chemical bonds present in the solvent and ligand molecules of the immediate environment. 15 Incorporating Ln 3+ in a semiconductor nanoparticle matrix offers a way to overcome these challenges. 1 In such a system, the nanoparticle matrix acts as an optical antenna and protective matrix simultaneously in order to realize the Ln 3+ luminescence. Specifically, the nanoparticles with high molar extinction coefficient absorb light and transfer the energy nonradiatively to the lanthanide center, and concomitantly an incorporation of ...

Trivalent lanthanide (Ln3+) doped semiconductor nanoparticles (NPs) provide an avenue for developing unique luminophores, which combine the properties of Ln3+ and NPs. Realizing their promise requires a thorough understanding of their underlying photophysical processes. This article summarizes experimental findings and uses them to sketch a framework for understanding the important NP core and surface properties that affect Ln3+ luminescence. A charge trapping mediated Ln3+ emission sensitization mechanism is shown to operate in both the synthetically doped and postsynthetically modified NPs, and it is strongly affected by the energy offsets between the lanthanide and NP electronic states. While both the host (semiconductor NP) and guest (Ln3+) properties must be considered in designing and controlling the photophysical outcome, the predictable trends in Ln3+ energetics make it possible to predict trends and behaviors semiempirically. The role of spectral overlap mediated energy transfer mechanisms are shown to contribute negligibly to the sensitization. Proof of principle experiments to uncover the roles of surface localized dopants and surface capping ligands in governing the dopant emission, and the realization of emission from distinct Ln3+ in a codoped assembly, are discussed.

Yb] photoluminescence in doped tin dioxide [Sn(Ln)O 2 ] and doped titanium dioxide [Ti(Ln)O 2 ] nanoparticles shows that the emission efficiency of trivalent lanthanide cations (Ln 3+ ) in an oxide matrix can be improved by change of the cation site symmetry. An analysis of Ln 3+ emission quantum yield and asymmetry ratio is used to identify the importance of symmetry breaking around the dopant site for enhancing the Ln 3+ emission intensity. These findings identify an important criterion for engineering the luminescence intensity of dopant ions in semiconductor nanoparticle-based luminophores, which goes beyond the primary criterion of engineering the relative positions of the dopant energy levels with respect to the band edges of the host nanoparticle matrix.

This work reports the host (semiconductor nanoparticles) sensitized dopant (lanthanides, Ln) photoluminescence in near band gap matched Sn(Ln)O and Zn(Ln)S [Ln = Sm, Tb] nanoparticles to address the importance of the nanoparticle identity and absolute band gap in the underlying process. While the sensitization was evident in the Sn(Sm)O and Zn(Tb)S nanoparticles, the same was not observed in the Sn(Tb)O and Zn(Sm)S nanoparticles. This observation stresses the importance of nanoparticle identity as the determining factor in realizing the host-sensitized dopant photoluminescence and provides important insight into developing novel doped inorganic nanoparticle-based optical materials.

This work reports the photophysical properties of 1-thioglycerol capped hydrophilic terbium cation incorporated (doped) regime compared to in bulk environments. The results presented provide an avenue for the detection of heavy metal ions in general and Pb 2+ in particular, with a limit of detection that is at least in the range of sub-ppm, using either the broad ZnS or sharp Tb 3+ emission, respectively. This strategy provides an avenue to combine (i) the extremely sensitive and easily accessible analytical technique of photoluminescence spectroscopy, (ii) post-synthetic modification reactions in semiconductor nanoparticles that can be performed with less experimental demand, (iii) time-gated measurement related to the longer luminescence lifetime of terbium cations and (iv) the simultaneous use of broad ZnS nanoparticle and sharp Tb 3+ emission from the same assembly, helping eliminate false positive results.