Spectroscopic imaging and time-resolved spectroscopy are used to study the surface plasmon polariton (SPP) enhanced infrared to visible upconversion luminescence from NaYF 4 :Tm:Yb nanoparticles embedded in polymethyl methylacrylate (PMMA) supported on Au nanopillar arrays. The arrays have a lattice resonance associated with the SPP near 980 nm, near-resonant with the peak absorption of the Yb 3+ ion, while a local surface plasmon resonance (LSPR) associated with the individual pillars is seen to enhance the near-infrared emission of Tm 3+ ions near 780 nm. The two combined channels of enhancement result in a significantly higher enhancement of the near-infrared emission when compared to the visible upconversion lines of the Tm 3+ ion, consistent with the interpretation of sequential surface plasmon assisted absorption and emission at two separate and disparate energies. The presence of SPP and LSPR was confirmed by spectrally resolved reflectivity, and the mechanisms for luminescence enhancement were further confirmed by time-resolved measurements of the upconversion luminescence.

Legume-Rhizobium symbiosis results in root nodules where rhizobia fix atmospheric nitrogen into plant usable forms in exchange for plant-derived carbohydrates. The development of these specialized root organs involves a set of carefully orchestrated plant hormone signalling. In particular, a spatio-temporal balance between auxin and cytokinin appears to be crucial for proper nodule development. We put together a construct that carried nuclear localized fluorescence sensors for auxin and cytokinin and used two photon induced fluorescence microscopy for concurrent quantitative 3-dimensional imaging to determine cellular level auxin and cytokinin outputs and ratios in root and nodule tissues of soybean. The use of nuclear localization signals on the markers and nuclei segmentation during image processing enabled accurate monitoring of outputs in 3D image volumes. The ratiometric method used here largely compensates for variations in individual outputs due to sample turbidity and scattering, an inherent issue when imaging thick root and nodule samples typical of many legumes. Overlays of determined auxin/cytokinin ratios on specific root zones and cell types accurately reflected those predicted based on previously reported outputs for each hormone individually. Importantly, distinct auxin/cytokinin ratios corresponded to distinct nodule cell types indicating a key role for these hormones in nodule cell type identity.

The spatial variations in upconversion luminescence from NaYF4:Er3+,Yb3+ nanoparticles embedded in PMMA on Au nanocavity arrays are investigated over a wide range of excitation powers, spanning the nonlinear and saturation power-dependence regimes. Spatially resolved upconversion spectra on these arrays show a minimum of ≈3× luminescence enhancement compared to the adjacent smooth Au surface under high-intensity excitation, with progressively higher enhancement ratios, up to 30×, at excitation intensities below 100 W/cm2. It is found that the average upconversion luminescence enhancement, obtained by spectroscopic imaging and far-field measurements, can be almost entirely accounted for by an effective multiplicative shift in the excitation intensity, P eff = F·P, which is robust over 5 orders of magnitude variation in excitation intensity. We reconcile this constant excitation enhancement factor, F = 4.46, with the wide range of observed luminescence enhancement factors, ranging from 3× to 30×, using an analytical model for a three level system, and by numerically solving a system of coupled rate equations for the Yb3+, Er3+ system. By analyzing the statistical distributions of luminescence intensities in the spectroscopic images on and off the nanocavity arrays, estimates of the luminescence enhancement factor independent of fluctuations in nanoparticle density are obtained. The results clearly relate observed enhancement factors to the kinetics of the energy-transfer upconversion process, suggesting the primary upconversion enhancement from these substrates is in the Yb3+ absorption channel, and demonstrate these self-assembled enhancing substrates as a low-cost and scalable route toward efficient near-infrared to visible upconversion at low excitation intensities.

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