Abstract
The objective of this study was to benchmark the different signal enhancements found in a recently exploited fluorescence ratiometric optode design used for nano-molar imaging of ammonium and ammonia. The sensing scheme of these optodes are based on a mediated transfer of the analyte together with a fluorescent dye in a two-phase system consisting of a gold nanoparticle (GNP) doped ether (organic phase), emulsified in a hydrogel (hydrous phase). The coextraction of the ion dye pair causes changes in fluorescence in relation to the analyte concentration. Performances of optodes with and without GNPs using the gradually improved instrumentation and signal processing were evaluated and normalized to be comparable. Signal to noise was enhanced due to signal processing based on ensemble averaging (1.7× - 3.2×), CCD sensitivity (2×), and plasmon assisted fluorescence (10× - 100×), which altogether with the ratiometric treatment of the fluorescence contributed to the great sensitivity for ammonium and ammonia. The study shows that GNP doped sensors are relatively more sensitive to matrix effects but if they are isolated by a protective layer they will dramatically increase in sensitivity. Proper isolation of the active chemical components from the matrix will make the sensor design one of the most powerful and versatile concepts for chemical imaging and single point detection in complex environments as the optodes likely can be constructed for most ions that have selective ionophores.
Highlights
The idea with this paper was to illustrate and discuss the remarkable sensitivity associated with a versatile fluorescence ratiometric plasmon assisted imaging optode design
In order to compare the sensitivities for the two different CCDs that used 3 and 10 images in the image average, the CCD system using 10 images was normalized to 3 images by making use of the principles for ensemble averaging [17] i.e. signal to noise (S/N) (n images )0.5 (Table 1)
Even if this is far from the demonstrated more than 5000 times improvement in the spectrofluorometer, it is believed that the imaging optode was hampered by the matrix in this particular study of porcine tissue degradation
Summary
The idea with this paper was to illustrate and discuss the remarkable sensitivity associated with a versatile (applicable to many ions) fluorescence ratiometric plasmon assisted imaging optode design. In the emulsion the negatively charged fluorescent dye (MC540) and a carrier molecule (in their case Valinomycin for potassium) coextract the ion from the hydrous phase into the organic phase that resulted in an increased fluorescence emission and redshift from ~570 nm to ~590 nm due to the solvatochromic dye This concept makes it possible to detect more than 40 different ions in the same way (same excitation and emission) by just changing the ionophore or ionophore dye pair for negative ions, which was done in several of the following publications [1,2,3,4,5]. Even if the sensing concept was good and general, it was far from functioning for imaging and questionable to function for single point measurements The argument for this is that the fluorescent signal was not normalized and sensitive for changes in excitation light intensities, dye leakage and photo bleaching, making accurate detection of ions difficult in reality. The normalization technique make use of the fluorescence signals from both the dye in hydrous and organic phase in a fluorescence
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