The mechanism for red upconversion luminescence of Yb–Er codoped materials is not generally agreed on in the literature. Both two-photon and three-photon processes have been suggested as the main path for red upconversion emission. We have studied β-NaYF4:Yb3+,Er3+ nanoparticles in H2O and D2O, and we propose that the nanoparticle environment is a major factor in the selection of the preferred red upconversion excitation pathway. In H2O, efficient multiphonon relaxation (MPR) promotes the two-photon mechanism through green emitting states, while, in D2O, MPR is less effective and the three-photon path involving back energy transfer to Yb3+ is the dominant mechanism. For the green upconversion emission, our results suggest the common two-photon path through the 4F9/2 energy state in both H2O and D2O.
Upconversion nanoparticles (UCNPs) have been frequently applied in Forster resonance energy transfer (FRET) bioanalysis. However, the understanding of how surface coatings, bioconjugation, and dye-surface distance influence FRET biosensing performance has not significantly advanced. Here, we investigated UCNPto-dye FRET DNA-hybridization assays in H 2 O and D 2 O using ∼24 nm large NaYF 4 :Yb 3+ ,Er 3+ UCNPs coated with thin layers of silica (SiO 2 ) or poly(acrylic acid) (PAA). FRET resulted in strong distance-dependent PL intensity changes. However, the PL decay times were not significantly altered because of continuous Yb 3+ -to-Er 3+ energy migration during Er 3+ -to-dye FRET. Direct bioconjugation of DNA to the thin PAA coating combined with the closest possible dye-surface distance resulted in optimal FRET performance with minor influence from competitive quenching by H 2 O. The better comprehension of UCNP-to-dye FRET was successfully translated into a microRNA (miR-20a) FRET assay with a limit of detection of 100 fmol in a 80 μL sample volume.
<p>In central and southern Europe, <em>Aphanizomenon</em> spp., <em>A. gracile</em> Lemmermann in particular, have been associated with paralytic shellfish toxin (PST) production. In western Poland, <em>A. gracile</em> is very common, and <em>Cylindrospermopsis raciborskii</em> (Woloszyńska) Seenayya & Subba Raju, another potentially PST-producing species, is often found as well. To date it is, however, unknown if the cyanobacterial populations in this area harbour the genetic capability to produce PSTs, and to what extent toxin biosynthesis occurs. The objective of this study was to survey the prevalence of potentially PST-producing cyanobacteria by measuring paralytic shellfish toxin biosynthesis gene <em>sxtB</em> copy numbers, <em>sxtA</em>, <em>sxtG</em> and <em>sxtS</em> gene presence, and PST concentrations in Polish freshwater lakes. In total, 34 lakes in western Poland were sampled twice during summer 2010. The presence of PST biosynthesis genes <em>sxtA</em>, <em>sxtG</em> and <em>sxtS</em> was determined using conventional qualitative PCR. Quantitative PCR (qPCR) was used to measure <em>sxtB </em>copy numbers, and the samples were analysed for PSTs using ion-pair high performance liquid chromatography with post-column oxidation and fluorescence detection (HPLC-FLD). Cyanobacteria carrying the <em>sxtB</em> gene were present in 23.5% of all samples (n=16) and in 14 lakes of the studied 34. Gene copy numbers ranged from 8.2×10<sup>4</sup> to 5.1×10<sup>7</sup> <em>sxtB</em> copies L<sup>-1</sup> (mean 3.8×10<sup>6</sup>). The median was 4.5×10<sup>5</sup> <em>sxtB</em> gene copies L<sup>-1</sup> and the majority of results clustered at the lower end of the <em>sxtB</em> qPCR linear range. In 12 out of the 16 samples positive for <em>sxtB</em> the gene co-occurred with the other three targeted PST biosynthesis genes <em>sxtA</em>, <em>sxtG</em> and <em>sxtS</em>. However, five additional samples lacked one or two of the targeted four genes. Thirteen samples contained PSTs, of which 12 samples at levels <0.072 µg L<sup>-1</sup>, <em>i.e.</em>, close to or below the quantitative detection limit of the HPLC-FLD method (0.01 µg L<sup>-1</sup>). One sample contained 0.57 µg L<sup>-1</sup> saxitoxin, co-occurring with all four <em>sxt</em> genes studied. No correlation between PST and <em>sxt</em> gene occurrence or copy numbers was observed. <em>A. gracile</em> and <em>C. raciborskii</em> occurred in 92% and 50% of samples, respectively, containing PSTs, <em>sxt</em> genes or both. In conclusion, the results confirm that potential PST producers constitute an established subpopulation of cyanobacteria in Polish freshwater lakes. However, none of the <em>sxt</em> genes targeted in this study could serve as a reliable marker for active PST biosynthesis.</p>
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