Imaging with Planar Optodes for Investigating Spatio-Temporal Gradients

Methods to control and investigate concentration gradients or inhomogeneity are for example:

• simultaneous measurement at several measurement positions (A. Flohr et al. "Assessing the Impact of pH Fluxes", M. Köster "Monitoring Oxygen Dynamics at the Sediment-Water Interface")
• analyte profiling by moving one sensor in different positions (K. Koop-Jakobsen and M. Gutbrod "O₂, pH and CO2 Dynamics in Salt Marsh Tidal Ponds")
• or the use of 2D read-out (K. Vopel et al. "Benthic Disturbance Recovery")

Multipoint measurements need manual repositioning of the read-out unit, multiple read-out units, or a multi-channel device to cover and record a multitude of measurement spots in the same experiment. For the user this is often a matter of time, free space around the sample and the complexity of the read-out system. Microprofiling is discussed in a separate basics section (see Microprofiling). Here we want to point out the beneficial use of 2D read-out with luminescent chemical optical sensors.

2D read-out of optical sensors or so-called analyte Mapping or Imaging uses a 2D array detector to record a multitude of signals or measurement points, respectively, at the same time and within the very same experimental read-out (O. S. Wolfbeis, BioEssays 2015, 37, 8). Special luminescence camera detectors instead of single channel photodetectors are used to perform the 2D signal reading of planar optode sensors. Such a system for 2D reading of optical sensors consists of three major parts:

A camera detector unit is used for two main purposes, the generation of excitation light signals and the collection of luminescence emission signals in a 2D manner. Second part is at least one luminescent sensor element that interacts with the sample in a reversible way and generates analyte dependent luminescent changes upon excitation. The third element is a software that transforms the recorded raw light signal data into an analyte dependent data array or image.

Imaging enables to detect thousands of measurement spots at the same time and to generate 2D analyte maps over a region of interest. Furthermore, automated time series recordings allow to analyze spatio-temporal changes of the respective analyte. The data results in a visual representation (image, map or video slide-show) of the analyte distribution that can be used to quickly identify spatial and temporal differences or hot spots of metabolic or reaction activity. This gives a quick and easy overview on what is going on in the sample. Besides this, each measurement element (each pixel) contains information on the analyte at that specific position as a second level of information. Freely selected Regions of Interest (ROIs) allow analyzing and comparing regions. Several gradients can also be analyzed within one image, or can be compared at different time points.

Besides this 'picture filling' 2D imaging approach, where the whole field of view contains sensor information, imaging offers more advanced possibilities: Multiple sensors can be combined in one field of view., so that either various positions in one experiment or even several samples can be recorded simultaneously. Moreover, several sensor types for one analyte, e.g. with different ranges, can be combined in one sample to extend the measurement range. Furthermore, sensors for different analytes can be combined in one field of view and read out with the same device in the same experiment, or even whole sensor arrays can be recorded at the same time (Y. Reinders et al. "Imaging of pH and pO₂ on Irradiated Fibroblasts").