Gouranga H. Debnath

Lanthanide cation-induced tuning of surface capping properties in zinc sulfide nanoparticles: an infrared absorption study

This study reveals the tuning of the vibrational characteristics of the capping ligands in lanthanide [Ln = Sm, Eu, Tb, Dy] cation-incorporated zinc sulfide nanoparticles, Zn(Ln)S, as monitored by Fourier transform infrared (FTIR) spectroscopy. Both stearate and trioctylphosphine oxide (TOPO) were found to act as surface-capping ligands for nanoparticles with multiple coordination environments. The vibrational characteristics of the capping ligands in the undoped ZnS nanoparticles exhibited noticeable differences in absorption as compared to those of the pure zinc stearate and TOPO molecules. Lanthanide cation incorporation tunes the corresponding vibrational characteristics to a significant extent, as compared to that in the undoped ZnS nanoparticles. It has been argued that the observed tuning of the capping ligand IR absorption characteristics is induced by the lanthanide cations that are located near or on the surface of the nanoparticles. The IR spectra suggest a probable change of carboxylate coordination environment in the Zn(Ln)S nanoparticles. The results obtained from the ZnS-based nanoparticles were analyzed based on a semi-quantitative analysis and compared with those from ZnSe and Zn(Tb)Se nanoparticles, in order to evaluate the effect of the constituent anion of the nanoparticles in modulating the IR signature.

Towards the realization of luminescence from visible emitting trivalent lanthanides (Sm, Eu, Tb, Dy) in polar zinc sulfide nanoparticles: evaluation of in vitro cytotoxicity

This work reports on the realization of luminescence from four visible emitting trivalent lanthanide (Ln) cations [samarium (Sm), europium (Eu), terbium (Tb) and dysprosium (Dy)] in polar zinc sulfide nanoparticles, Zn(Ln)S. Among the Zn(Ln)S nanoparticles studied, noticeable lanthanide cation centered luminescence has only been realized from the Zn(Tb)S and Zn(Eu)S nanoparticles in water, whereas no such effect has been observed in the corresponding Sm and Dy containing nanoparticles. In all the nanoparticles with characteristic lanthanide luminescence, the nanoparticles were found to be acting as an optical antenna and protector matrix, in order to realize luminescence from the respective lanthanide cations. The results have been rationalized with the lanthanide cations acting as charge (hole and/or electron) traps in the semiconductor nanoparticle matrix along with associated environmental effects. A comparison of the corresponding hydrophobic Zn(Tb)S nanoparticles reveal significant differences in photophysical properties. Finally, the hydrophilic Zn(Tb)S nanoparticles have been examined for in vitro cytotoxicity and the results indicate potential anti-cancer therapy for human mammary adenocarcinoma cells (MDA-MB-231) with the capability of cellular imaging.

How Important is the Host (Semiconductor Nanoparticles) Identity and Absolute Band Gap in Host-Sensitized Dopant Photoluminescence?

This work reports the host (semiconductor nanoparticles) sensitized dopant (lanthanides, Ln) photoluminescence in near band gap matched Sn(Ln)O2 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)O2 and Zn(Tb)S nanoparticles, the same was not observed in the Sn(Tb)O2 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.

Capping ligand infrared absorption and dopant photoluminescence spectroscopy provides a comprehensive picture to probe dopant spatial location in semiconductor nanoparticles

The spatial location of a foreign species (dopant) in a semiconductor nanoparticle matrix is probed by monitoring the infrared absorption and photoluminescence spectroscopy of the capping ligand and dopant moieties respectively at room temperature. The results have been rationalized within the domain of observables in each experiment and argued to provide a comprehensive picture in tracking the core and surface localized terbium cations that are incorporated in zinc sulfide nanoparticles. The progression of luminescence quantum yield of the core and surface localized terbium cations with doping extent induced variation has been found to be different. The generality of the experimental observations has been demonstrated with the corresponding europium incorporated nanoparticles.

Assessing inter lanthanide photophysical interactions in co-doped titanium dioxide nanoparticles for multiplex assays

This study assesses the inter lanthanide photophysical interactions in trivalent lanthanide cations (Ln3+) co-doped titanium dioxide nanoparticles. As a case study, incorporation of neodymium (Nd3+) and samarium (Sm3+) to generate Ti(NdSm)O2 nanoparticles has been considered. The presence of co-doping offers a promising avenue for multiplex assays. The co-doped nanoparticles have characteristic visible emission at 584, 612, 664 and 726 nm respectively from Sm3+ and near infrared (NIR) emission at 912 and 1094 nm respectively from Nd3+, thus presenting composite doped nanoparticles with six distinct emission wavelengths spanning both the orange-red and NIR spectral window, using a single excitation wavelength. The photophysical properties of the Ti(NdSm)O2 nanoparticles have been compared with that observed in the singly doped Ti(Nd)O2 and Ti(Sm)O2 nanoparticles. Remarkable differences in the Ln3+ emission have been observed in the singly and doubly doped nanoparticles. Both the Nd3+ and Sm3+ emissions have been found to decrease in the Ti(NdSm)O2 nanoparticles, compared to those observed in the singly doped Ti(Nd)O2 and Ti(Sm)O2 nanoparticles. However, the extent of decrease in emission was found to be unequal for Nd3+ and Sm3+, with a decrease being marginally more prominent in Nd3+. The results have been rationalized by considering the Ln3+ as charge traps in the nanoparticles and associated relaxation pathways that are dictated by the spin selection rule. This photophysical rationalization was further tested and verified by performing experiments with the Ti(NdEr)O2 nanoparticles. The results presented provide important physical insight on the design criteria of co-doped semiconductor nanoparticles.

Role of reactant concentration and identity of added cation in controlling emission from post-synthetically modified terbium incorporated zinc sulfide nanoparticles: an avenue for the detection of lead(II) cations

This work reports the photophysical properties of 1-thioglycerol capped hydrophilic terbium cation incorporated (doped) zinc sulfide [Zn(Tb)S] nanoparticles, which have been post-synthetically modified using Pb2+ [Zn(Tb)S/Pb] under ambient conditions with [Zn(Tb)S] : [Pb2+] = 1 : 10−5–1 : 10, essentially providing a scenario with low to heavy co-doping and ultimately the possibility of forming a material of different chemical identity. The effects of selected concentrations of [Zn(Tb)S] : [Mn+] = 1 : 1 and 1 : 10−2 have also been evaluated for the post-synthetic addition of Hg2+, Cd2+, Ca2+, Mg2+, Na+ and K+. The broad zinc sulfide nanoparticle and sharp Tb3+ emission have different dependence on the relative reactant concentration, with cation identity playing a significant role. The underlying photophysical processes have been rationalized based on the interplay among the (i) cation exchange, (ii) modification of the structural properties of the nanoparticles without necessarily exchanging the cations and (iii) emission enhancement of terbium dopants. In cases where Tb3+ emission is apparent, all the nanoparticles studied demonstrate an optical antenna effect, thus accessing a lower Tb3+ concentration regime compared to in bulk environments. The results presented provide an avenue for the detection of heavy metal ions in general and Pb2+ in particular, with a limit of detection that is at least in the range of sub-ppm, using either the broad ZnS or sharp Tb3+ 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 Tb3+ emission from the same assembly, helping eliminate false positive results.

Host-sensitized sharp samarium emission from doped titanium dioxide nanoparticles as non-cytotoxic photostable reporters for live-cell imaging

Lanthanide-incorporated semiconductor nanoparticle luminophores are attractive alternatives to conventional organic fluorophores for live-cell imaging due to their non-overlapping sharp emission bands, longer luminescence lifetimes and resistance to photobleaching. These materials provide avenues to use the unique luminescence properties of lanthanides, allowing the incorporation of multiple luminophores into the host matrix generating an enhanced emission, with additional benefits from the surface functionalization of the nanoparticles. Here, we report the first demonstration of host-sensitized samarium (Sm3+) emission, from (3-aminopropyl)trimethoxysilane (APTMS) functionalized Sm3+ doped titanium dioxide [Ti(Sm)O2] nanoparticles, and its usefulness as a live-cell imaging probe in human mammary adenocarcinoma (MDA-MB-231) and human embryonic kidney (HEK 293T) cell lines. Photoluminescence spectroscopic experiments with the lysates, obtained from cells treated with APTMS-Ti(Sm)O2 nanoparticles, displaying the host-sensitized sharp samarium (Sm3+) emission and the images acquired from both the bright field and emission channels in epifluorescence microscopy experiments of nanoparticle treated cells clearly indicate the interaction of the nanoparticles with the cells and their effectiveness as an imaging probe. Additionally, the non-cytotoxicity and photostability of this probe provide further benefits in the context of (i) increasing the probe concentration without compromising cellular viability and (ii) longer experimental times without encountering photobleaching, thus essentially improving the signal-to-noise ratio; challenges that are often associated with organic moiety based imaging probes.

What Is Beyond Charge Trapping in Semiconductor Nanoparticle Sensitized Dopant Photoluminescence

A systematic comparison of the Ln3+ [Ln = Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb] photoluminescence in doped tin dioxide [Sn(Ln)O2] and doped titanium dioxide [Ti(Ln)O2] nanoparticles shows that the emission efficiency of trivalent lanthanide cations (Ln3+) in an oxide matrix can be improved by change of the cation site symmetry. An analysis of Ln3+ emission quantum yield and asymmetry ratio is used to identify the importance of symmetry breaking around the dopant site for enhancing the Ln3+ 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.