Ligand-Sensitised LaF3 :Eu3+ and SrF2 :Eu3+ Nanoparticles and in Vitro Haemocompatiblity Studies.

Luminescent Ln3+ -doped nanoparticles (NPs) functionalised with the desired organic ligand molecules for haemocompatibility studies were obtained in a one-pot synthesis. Chelated aromatic organic ligands such as isophthalic acid, terephthalic acid, ibuprofen, aspirin, 1,2,4,5-benzenetetracarboxylic acid, 2,6-pyridine dicarboxylic acid and adenosine were applied for surface functionalisation. The modification of the nanoparticles is based on the donor-acceptor character of the ligand-nanoparticle system, which is an alternative to covalent functionalisation by peptide bonding as presented in our recent report. The aromatic groups of selected ligands absorb UV light and transfer their excited-state energy to the dopant Eu3+ ions in LaF3 and SrF2 NPs. Herein, we discuss the structural and spectroscopic characterisation of the NPs and the results of haemocompatibility studies. Flow cytometry analysis of the nanoparticles' membrane-binding is also presented.

3,5-Dihydroxy Benzoic Acid-Capped CaF2:Tb3+ Nanocrystals as Luminescent Probes for the WO4 2- Ion in Aqueous Solution.

We report a facile and effective luminescence method for the determination of the WO4 2- ion in aqueous medium at initial pH = 6.3. This is achieved using 3,5-dihydroxybenzoic acid-capped CaF2:Tb3+ (5%) nanocrystals (NCs) as a luminescent probe. This is accomplished based on the energy transfer luminescence from the WO4 2- ion to the Tb3+ ion in small-size CaF2:Tb3+ NCs. Hydroxyl groups on the surface ligand helps in binding the tungstate ion to the surface of the NCs. With the gradual addition of the WO4 2- ion, the intensity of the Tb3+ excitation and emission spectra significantly increased. The linear range of the detection was from 1 to 10 µM for the WO4 2- ion (R 2 = 0.99). The calculated detection limit was 0.4 µM (by applying the 3σ/K criterion).

Classification of Transitions in Upconversion Luminescence of Lanthanides by Two-Dimensional Correlation Analysis.

Upconversion luminescence bands from Yb3+/Er3+ codoped into a matrix such as NaGdF4 can show a very complex structure on account of multiple intra-f shell transitions occurring in the presence of random crystal fields. We demonstrate that two-dimensional correlation analysis, applied to such time-integrated luminescence spectra measured as a function of excitation power, allows us to gain substantial information about the states involved in transitions, without any additional theoretical input. The detailed correlation analysis allows us not only to identify the location of various transitions but further to club them into groups on the basis of their quantum mechanical origin, and finally subclassify the transitions with each group depending on whether they have a common initial or final state.

Ce3+ -Sensitized Tm3+ /Mn2+ -Doped NaYF4 Colloidal Nanocrystals: Intense Cool White Light from a Phosphor-Coated UV LED.

Colloidal NaYF4 :Ce3+ /Tm3+ /Mn2+ nanocrystals found to be an efficient single component phosphor to produce intense white light. The use of Mn2+ ions instead of green- and red-emitting Ln3+ ions is interesting as the later are expensive and less abundant. The single band blue emission of Tm3+ ions was combined with the broadband green-yellow emission of Mn2+ ions to produce strong white light emission using Ce3+ ions as excitation source. Interestingly, there is hardly any re-absorption of the blue emission by Mn2+ ions and the spectrum matches the commercial Ce3+ -doped yttrium aluminium garnet (YAG) phosphor coated over a blue LED. In addition, we have successfully incorporated the colloidal nanocrystals into a polymer matrix to make a nanocomposite as well as transparent films without altering the optical properties of the nanocrystals. The colloidal nanocomposite was coated over a UV LED to fabricate white LEDs. Finally, when the concentration of the dopant (Mn2+ ) ions was tuned the correlated color temperature (CCT) values varied between cool white light and daylight region.

Tuning the Energy Transfer Efficiency between Ce3+ and Ln3+ Ions (Ln=Tm, Sm, Tb, Dy) by Controlling the Crystal Phase of NaYF4 Nanocrystals.

NaYF4 is a superior host matrix to study the luminescence properties of lanthanide (Ln3+ ) ions. Ln3+ ions in hexagonal-phase NaYF4 (ß-phase) nanocrystals (NCs) exhibit strong luminescence via an upconversion process compared to cubic NaYF4 (α-phase) NCs. However, in Ce3+ /Ln3+ -doped NaYF4 NCs (Ln=Tm, Tb, Sm, Dy) the α-phase NaYF4 NCs shows strong luminescence compared to their counterpart ß-phase NCs despite the latter being much larger in size. This is attributed to comparatively large overlap between Ce3+ ions emission band with excited energy levels of those Ln3+ ions in α-phase compared to ß-phase NCs. This difference is attributed to different crystal-field splitting of Ce3+ ions 4f-5d band in different crystal environments of the α-phase (cubic crystal field environment) and ß-phase (trigonal prismatic with equatorials crystal field environment) NaYF4 NCs with respect to their barycenter. The enhanced luminescence from α-phase NaYF4 NCs is advantageous as they are prepared at a relatively lower temperature and shorter reaction times compared to ß-NaYF4 NCs.

Near-infrared light triggered superior photocatalytic activity from MoS2-NaYF4:Yb(3+)/Er(3+) nanocomposites.

A near infrared (NIR) responsive photocatalyst, composed of a narrow band gap semiconductor (i.e. MoS2) and an optical material possessing upconverting ability (i.e. NaYF4:Yb(3+)/Er(3+)) has been successfully prepared via a simple hydrothermal method. The latter has the ability to convert NIR light into visible light while the MoS2 uses the light to degrade organic pollutants. Upon near infrared (NIR) excitation of the MoS2-NaYF4:Yb(3+)/Er(3+) nanocomposites, the energy of the strong green and the red emissions along with the weak violet emissions from the NaYF4:Yb(3+)/Er(3+) nanocrystals (NCs) is transferred to MoS2. This results in enhanced NIR light triggered photocatalytic performance, as verified by studying the degradation of Rhodamine B (RhB) dye under 980 nm laser excitation. The strong photocatalytic activity of MoS2-NaYF4:Yb(3+)/Er(3+) composites is attributed to the layered nature of the photocatalyst which leads to the efficient separation of photogenerated carriers (electron-hole pairs) and excellent upconversion properties of NaYF4:Yb(3+)/Er(3+) NCs. The study also shows the importance of the composite formation, as the physical mixture leads to only very low photocatalytic activity. Our results can be helpful in the structural design and development of high-performance photocatalysts.

Enhanced visible and near infrared emissions via Ce(3+) to Ln(3+) energy transfer in Ln(3+)-doped CeF3 nanocrystals (Ln = Nd and Sm).

We report the enhancement of both visible and near infrared (NIR) emissions from Nd(3+) ions via Ce(3+) sensitization in colloidal nanocrystals for the first time. This is achieved in citrate capped Nd(3+)-doped CeF3 nanocrystals under ultraviolet (UV) irradiation (λex = 282 nm). The lasing transition ((4)F3/2 → (4)I11/2) at 1064 nm from Nd(3+)-doped CeF3 nanocrystals has much higher emission intensity via Ce(3+) ion sensitization compared to the direct excitation of Nd(3+) ions. The nanocrystals were prepared using a simple microwave irradiation route. Moreover, the study has been extended to Sm(3+)-doped CeF3 nanocrystals which show strong characteristic emissions of Sm(3+) ions via energy transfer from Ce(3+) ions. The energy transfer mechanism from Ce(3+) to Nd(3+) and Sm(3+) ions is proposed.

Highly Selective and Sensitive Detection of Cu(2+) Ions Using Ce(III)/Tb(III)-Doped SrF2 Nanocrystals as Fluorescent Probe.

We report a green synthetic approach to the synthesis of water dispersible Ce(3+)/Tb(3+)-doped SrF2 nanocrystals, carried out using environment friendly microwave irradiation with water as solvent. The nanocrystals display strong green emission due to energy transfer from Ce(3+) to Tb(3+) ions. This strong green emission from Tb(3+) ions is selectively quenched upon addition of Cu(2+) ions, thus making the nanocrystals a potential Cu(2+) ions sensing material. There is barely any interference by other metal ions on the detection of Cu(2+) ions and the detection limit is as low as 2 nM. This sensing ability is highly reversible by the addition of ethylenediaminetetraacetic acid (EDTA) with the recovery of almost 90% of the original luminescence. The luminescence quenching and recovery cycle was repeated multiple times without much effect on the sensitivity. The study was extended to real world water samples and obtained similar results. In addition to the sensing, we strongly predict the small size and high luminescence of the Ce(3+)/Tb(3+)-doped SrF2 nanocrystals can be used for bioimaging applications.

Strong Single-Band Blue Emission from Colloidal Ce(3+) /Tm(3+) -Doped NaYF4 Nanocrystals for Light-Emitting Applications.

An intense single-band blue emission at λ=450 nm is observed from Tm(3+) ions through Ce(3+) sensitization, for the first time, in colloidal Ce(3+) /Tm(3+) -doped NaYF4 nanocrystals. The intense Tm(3+) emission through broad-band excitation is advantageous for developing luminescent nanocomposites because they can be easily incorporated into polymers. The composites can easily be coated over UV light-emitting diodes (LEDs) to develop phosphor-based blue LEDs.

Intense NIR emissions at 0.8 µm, 1.47 µm, and 1.53 µm from colloidal LiYbF4:Ln(3+) (Ln = Tm(3+) and Er(3+)) nanocrystals.

We report on the synthesis of diamond shaped Ln(3+)-doped LiYbF4 (Ln = Tm and Er) nanocrystals with flat edges via the thermal decomposition method. Strong near-infrared emissions at 0.8 µm, 1.47 µm and 1.53 µm are observed from colloidal dispersions of Tm(3+)-doped and Er(3+)-doped LiYbF4 nanocrystals, respectively, under 0.98 µm diode laser excitation. The NIR emission intensities for Tm(3+)-doped and Er(3+)-doped LiYbF4 nanocrystals are comparable with those of the sodium counterpart NaYbF4, suggesting that LiYbF4 is also an excellent host matrix for lanthanide ions to obtain strong NIR emissions in colloidal solutions of LiYbF4 (Tm(3+) or Er(3+)) nanocrystals.

Methyl oleate-capped upconverting nanocrystals: a simple and general ligand exchange strategy to render nanocrystals dispersible in aqueous and organic medium.

We report a simple and general ligand exchange strategy to functionalize the nanocrystals with both hydrophobic and hydrophilic ligands. This is achieved by first capping the Er/Yb-doped NaYF4 nanocrystals with a weak ligand such as methyl oleate and subsequently ligand exchanged with various organic ligands which can strongly coordinate to the surface of the nanocrystals. The method involves only a simple stirring or sonication of the nanocrystals dispersion with the ligands of interest. Dicarboxylic acids such as sebacic acid, adipic acid, succinic acid, and malonic acid-functionalized nanocrystals which are difficult to achieve via thermal decomposition method were easily prepared by this ligand exchange strategy. In addition, low boiling point ligands like hexanoic acid can easily be coated over the surface of the Er/Yb-doped NaYF4 nanocrystals. Both size and shape of the nanocrystals were preserved after the ligand exchange process. The methyl oleate-capped Er/Yb-doped NaYF4 nanocrystals display strong upconversion emission after ligand exchanged with hydrophobic and hydrophilic molecules. The high stability of the nanocrystals after ligand exchange process is verified by performing time-dependent luminescent measurements at different pH, buffers, etc.

3,5-Dinitrobenzoic acid-capped upconverting nanocrystals for the selective detection of melamine.

In this Research Article, we report for the first time the use of upconverting nanoparticles to detect melamine up to nanomolar concentration. Detection of melamine is important as it is one of the adulterant in protein rich food products due to its high nitrogen content. In this work, we have shown how the electron deficient 3,5-dinitrobenzoic acid (DNB)-coated Er/Yb-NaYF4 nanocrystals can specifically bind to electron rich melamine and alter the upconverting property of the nanocrystals. This selective binding led to the quenching of the upconversion emission from the nanocrystals. The high selectivity is verified by the addition of various analytes similar in structure with that of melamine. In addition, the selective quenching of the upconversion emission is reversible with the addition of dilute acid. This process has been repeated for more than five cycles with only a slight decrease in the sensing ability. The study was also extended to real milk samples, where the milk adulterated with melamine quenches the emission intensity of the DNB coated NaYF4:Er/Yb nanocrystals, whereas hardly any change is noted for the unadulterated milk sample. The high robustness and the sharp emission peaks make Er(3+)/Yb(3+)-doped NaYF4 nanocrystals a potential melamine sensing material over other organic fluorophores and nanocrystals possessing broad emissions.

Microwave synthesis, photoluminescence, and photocatalytic activity of PVA-functionalized Eu3+-doped BiOX (X = Cl, Br, I) nanoflakes.

We report a facile microwave-assisted green synthetic route for colloidal poly(vinyl alcohol) (PVA)-coated europium (Eu(3+))-doped luminescent heavy metal bismuth oxyhalide (BiOX; X = Cl, Br, I) nanoflakes at low temperature and examine their structural, optical, and photocatalytic characteristics. PVA coating onto the surface of the nanoflakes endows them with hydrophilic nature. Both Eu(3+)-doped BiOCl and BiOBr nanoflakes exhibit strong optical properties related to Eu(3+) and Bi(3+) which are quenched in case of Eu(3+)-doped BiOI matrix. These results are supported by Eu(3+) photoluminescence lifetime values of 0.61 ms, 0.59 ms, and 8.9 µs, respectively. The former two matrices have quite similar crystal field environments as deduced from the asymmetric ratios of (5)D0 → (7)F2 (614 nm) and (5)D0 → (7)F1 (591 nm) transitions. In addition to possessing interesting photoluminescence properties, a comparison of the photocatalytic activity of Eu(3+)-doped BiOX (X = Cl, Br, I) nanoflakes, with corresponding estimated band gaps of 3.36, 2.74, and 1.67 eV has been evaluated using Rhodamine B (RhB) dye under visible light irradiation. The nanoflakes exhibited 100% dye degradation under visible light irradiation. Eu(3+)-doped BiOCl nanoflakes manifested higher photocatalytic efficiency compared to the other matrices following apparent first-order kinetics. Such a boost in efficiency is attributed to their high surface area to volume ratios, layered crystalline structures, indirect band gap nature, and ability to utilize broad bands in the solar spectrum.