Upconversion (UC) is a powerful mulitphoton mechanism that converts low-energy photons into higher energy emission. One of the most investigated UC systems is upconverting phosphors (UCPs). Here, a new, one-pot synthetic procedure was used to prepare water dispersible, visibly emissive, rare-earth doped UCPs that were capped with the functional groups oleic acid (OA), 6-aminohexanoic acid (AHA), and 6-maleimidohexanoic acid (MHA). These synthesized UCPs were characterized by UC luminescence, dynamic light scattering (DLS), transmission electron microscopy (TEM), and powder Xray diffraction (pXRD). This study expands upon our previous

Upconverting phosphors (UCPs) convert multiple low energy photonsi nto highere nergy emission via the process of photon upconversion ando ffer an attractive alternative to organic fluorophores for use as luminescent probes. Here, UCPs were capped with functionalized silica in order to provide as urface to covalently conjugate proteins with surface-accessible cysteines.V ariants of green fluorescent protein (GFP) and the flavoenzyme pentaerythritol tetranitrate reductase (PETNR) were then attached via maleimide-thiol couplingi no rder to allow energy transfer from the UCP to the GFP or flavin cofactor of PETNR, respectively.PETNR retains its activity when coupled to the UCPs, which allows reversible detection of enzyme substrates via ratiometric sensing of the enzyme redox state. Upconvertingp hosphors (UCPs) have emerged as an important and versatile class of nanoparticles, with applications including memory storage, anti-counterfeiting measures, theranostics, and optical imaging. [1] Upconversion (UC) involves the sequential absorption of two or more lower energy photons that resultsi nt he emission of light of higher energy.T ypically, near-infrared (NIR) excitation of UCPs resultsi nv isible luminescence. [2] While UC has recently been shown in small molecule complexes, [3] them ostc ommons ystemsa re basedo nY b III !Er III or Yb III !Tm III rare-earth ionp airs dopedi ntoa ni nert matrix (e.g., NaYF 4 ,G d 2 O 2 S, etc.), [2] andm orer ecentlyN d III ions have been used in place of Yb III to enablee xcitation at 808 nm, where water and biological tissue absorb less strongly. [1f] UCPs have an umber of potential advantages over traditional fluorophores, including:alarge anti-Stokes shift;a na ssociated lack of auto-fluorescence in biological media due to their NIR excitation;n egligible photobleaching;n op hoto-blinking and generally low toxicity. [4] In addition, due to the contracted natureo ft he lanthanide(III) fo rbitals, emission wavelengths in Ln III-based UCPs are generally insensitive to particle size and environment andt heir long (typically ms-ms) lifetimes enable time-gated spectroscopic measurements to be employed if required. [5] Due to these favorable properties, UCPs have been proposed for use in ar ange of sensing and imaging applications, from heavy metal detection to image-guided photodynamic therapy. [6] An as-yet untapped application is the covalent attachment of active biomolecules to UCPs where the biomolecule can act as an acceptorf or the UC emission, although UCP biomolecule conjugates (including DNA) have been developed where the biomolecule can be electrostatically surface bound. [7] The UCP would then act as ar obust luminescentr eportero f, for example, the redox or ligand-bound state of the UC acceptor (Scheme 1, inset). Enzymea ctivity is typically monitored by following changes in concentrationo ft he substrate/product, or by directly following the enzyme in the case of single-turnover experiments.F luorescenced etection allows experiments to be performed at lower concentratio...

Dissimilatory iron-reducing bacteria (DIRB) play an important role in controlling the redox chemistry of Fe and other transition metals and radionuclides in the environment. During bacterial iron reduction, electrons are transferred from the outer membrane to poorly soluble Fe(III) minerals, although the precise physiological mechanisms and local impact on minerals of these redox processes remain unclear. The aim of this work was to use a range of microscopic techniques to examine the local environment of Geobacter sulfurreducens grown on thin films of Fe(III)-bearing minerals, to provide insight into spatial patterns of Fe(III) reduction and electron transfer. Confocal fluorescence microscopy showed that sparse biofilms formed on the mineral coatings, while the selective Fe(II) probe RhoNox-1 revealed Fe(II) patches on the minerals sometimes co-located with cells. Atomic force microscopy highlighted thin filamentous structures extending radially from the cell surface. Further analysis using fluorescent redox dyes showed redox-active, linear nanowires that formed cell to cell connections, although they were not implicated in playing a dominant role in direct electron transfer to the Fe(III) minerals. Overall this paper provides new methods and insights on studying Fe(III) reduction and other redox transformations in situ.

The use of organic based fluorophores has been firmly established as a key tool in the biological sciences, with many biological-sensing methods taking advantage of Förster Resonance Energy Transfer (FRET) between different fluorescent organic based dyes following one photon excitation. Nevertheless, the employment of UV-visible absorbing dyes as fluorescent tags and markers typically suffer from several drawbacks including relatively high energy of excitation wavelength, photobleaching and competitive autofluorescence, which often limits their effectiveness and longevity both in vitro and in vivo. As an alternative, lanthanide doped upconverting phosphors (UCP) have emerged as a new class of materials for use in optical imaging and RET sensing; they exhibit high photo- and chemical stability and utilise near infrared excitation. Approaches to sensing a given analyte target employing upconverting phosphors can be achieved by engineering the UCP to operate analogously to fluorescent dyes via Luminescence Resonance Energy Transfer (LRET) and such systems are now becoming central to optically sensing low concentrations of biologically important species and performing distance measurements. Similarly to FRET, the LRET process is distance dependent and requires spectral overlap between the absorption of the acceptor luminophore and the emission of the donor moiety, yet essential measures of the relationship between spectral overlap and the degree of quenching have not yet been established. To address this, we have investigated the Stern-Volmer relationship for a set of six commonly functionalised organic dyes and seven biomolecules that contain key chromophoric co-factors with Gd2SO4:Yb:Er (PTIR545) and Gd2SO4:Yb:Tm (PTIR475) UCPs under low power nIR excitation, and found that for the organic dyes a linear relationship between spectral overlap and degree of quenching is observed. However, this linear relationship is observed to break down for all the biomolecules investigated.

The use of organic-based fluorophores has been firmly established as a key tool in the biological sciences, with many biological-sensing methods taking advantage of Förster Resonance Energy Transfer (FRET) between different fluorescent organic-based dyes following one photon excitation. Nevertheless, the employment of UV-visible absorbing dyes as fluorescent tags and markers typically suffer from several drawbacks including relatively high energy of excitation wavelength, photobleaching and competitive autofluorescence, which often limit their effectiveness and longevity both in vitro and in vivo. As an alternative, lanthanide-doped upconverting phosphors (UCP) have emerged as a new class of materials for use in optical imaging and FRET sensing; they exhibit high photo- and chemical stability and utilise near infrared (nIR) excitation. Approaches to sensing a given analyte target employing upconverting phosphors can be achieved by engineering the UCP to operate analogously to fluorescent dyes via Luminescence Resonance Energy Transfer (LRET) and such systems are now becoming central to optically sensing low concentrations of biologically important species and performing distance measurements. Similarly to FRET, the LRET process is distance dependent and requires spectral overlap between the absorption of the acceptor luminophore and the emission of the donor moiety, yet essential measures of the relationship between spectral overlap and the degree of quenching have not yet been established. To address this, we have investigated the Stern-Volmer relationship for a set of six commonly functionalised organic dyes and seven biomolecules that contain key chromophoric co-factors with Gd2SO4:Yb:Er (PTIR545) and Gd2SO4:Yb:Tm (PTIR475) UCPs under low power nIR excitation, and found that for the organic dyes a linear relationship between spectral overlap and degree of quenching is observed. However, this linear relationship is observed to break down for all the biomolecules investigated.