New solid state microsensors in plant physiology

Abstract

Solid state microelectrodes are being increasingly used in many areas of plant physiology, ranging from classical electrophysiology studies to new applications such as scanning tunneling microscopy or atomic force microscopy. When the size of the sensors is decreased from the millimeter to the micrometer scale, many changes occur in their behavior and use. These changes lead to dramatic improvements in the quality of physiological data and make achievable experiments previously impossible. The major areas of improvement include increased temporal resolution, increased sensitivity and the ability to make spatial resolved measurements. Electrochemical methods have significant advantages over the other techniques to monitor local concentrations of a chemical compound near plant tissues. In fact, microelectrodes (amperometric or voltammetric) can be positioned close to the cells and provide a means to estimate the local concentration. Electrochemical sensors for use in plants should display high selectivity and sensitivity, long-term calibration stability, and possess a small size. In addition, if assessing concentrations at different distances from a tissue, as in the case of the self-referencing technique (see chapters by Smith et al.; Fejio; Shabala in this book for exhaustive details on the self-referencing technique), it is essential that the tips of the microelectrodes have a planar geometry so that precise concentrations at precise distances away from the source/sink, can be determined with good spatial resolution. Another factor of crucial importance in the use of the microelectrode as a vibrating probe is a fast response time. Indeed, the sensor remains in the two measurement positions for times not longer than few seconds (usually, less than 5 s, often 2-3 s). Ideally, the electrode response time should permits measurements to be made on a subsecond time scale. In the following pages, four different new solid state microsensors to be used in plants research will be discussed: 1. The IAA-selective microelectrode 2. The O2-selective microelectrode 3. The NO-selective microelectrode 4. The Cu2?-selective microelectrode.