Microsensors, selectivity

Recently, we have used microsensors selective for peroxide to monitor the in vivo formation of peroxide in rat striatum following electrical and pharmacological manipulations of the dopamine system [22]. These studies reveal a biphasic increase in peroxide in striatum upon brief (10 s) electrical stimulation of the MFB (Fig. 10), demonstrating that our sensors are able to monitor the real-time formation of peroxide in vivo. The combination of selective and sensitive hydrogen peroxide microsensors with selective axonal stimulation provided a valuable new approach to the investigation of the neurodegenerative mechanisms associated with Parkinson s disease. 

Kolesar ES, Brothers CP, Howe CP, et al. 1992. Integrated-circuit microsensor for selectively detecting nitrogen-dioxide and diisopropyl methylphosphonate. Thin Solid Films 220(1-2) 30-37. 

S. Descroix and F. Bedioui, Evaluation of the selectivity of overoxidized polypyrrole/superoxide dismutase based microsensor for the electrochemical measurement of superoxide anion in solution. 

Nevertheless, the major issue in the miniaturization of this kind of assays continues to be the strong non-selective interactions with the polymers. This problem is often avoided, or at least minimized, on large scale experiments such as chromatographic separations. However, it is not easy to solve in the case of microsensors or microchips and direct or indirect assays, where competition can be strongly affected by unspecific binding. 

A multi-microsensor array of potentiometric MIP chemosensors has been devised for determination of a serotonin neurotransmitter [180]. In the toluene porogenic solvent solution, the MAA functional monomer and the EGDMA cross-linker were polymerized in the presence of the serotonin hydrochloride template (Table 6). Subsequently, the resulting MIPs were immobilized on a plasma polymer layer by swelling and polymerization. Plasma polymerization was performed using styrene or ethylbenzene as the monomer. The chemosensor fabricated that way was appreciably responsive to serotonin while selectivity to serotonin analogues, like acetaminophen... 

Conducting polymers have already been well documented in conjunction with the classical ionophore-based solvent polymeric ion-selective membrane as an ion-to-electron transducer. This approach has been applied to both macro- and microelectrodes. However, with careful control of the optimisation process (i.e. ionic/electronic transport properties of the polymer), the doping of the polymer matrix with anion-recognition sites will ultimately allow selective anion recognition and ion-to-electron transduction to occur within the same molecule. This is obviously ideal and would allow for the production of durable microsensors, as conducting polymer-based electrodes, and due to the nature of their manufacture these are suited to miniaturisation. There are various examples of anion-selective sensors formed using this technique reported in the literature, some of which are listed below. 

Realization of two-terminal microsensors like that represented in Scheme I depends on the discovery of viable reference and indicator molecules that can be confined to electrode surfaces. Described herein are three different microelectrochemical sensing systems. The first is a three-terminal microelectrochemical sensing system for CO based upon an indicator molecule, ferrocenyl ferraazetine, that selectively reacts with CO (2). The second microelectrochemical system is based upon a disulfide functionalized ferrocenyl ferraazetine that can be adsorbed onto Au or Pt via monolayer self-assembly techniques. Efforts to make a two-terminal CO sensor based upon the... 

J. E. Shin, and J. M. Wiseman, Integrated-Circuit Microsensor For Selectively Detecting Nitrogen Dioxide And Diisopropyl Methylphosphonate , Thin Solid Films 220, 30 (1992). 

Microsensors have the potential for selective GC detectors and also as remote sensors when combined in arrays often referred to as electronic noses . Promising microsensors include surface acoustic wave (SAW) detectors normally coated with different semi-selective polymeric layers and microelectromechanical systems (MEMS) including microcantilever sensors. The hope is that, in the future, hundreds of such microcantilevers, coated with suitable coatings, may be able to achieve sufficient selectivity to provide a cost-effective platform for detecting explosives in the presence of potentially interfering compounds in real environments. This array of... 

Acetylcholineesterase and choline oxidase A Cross-linkable polymer, poly (vinyl pyridine) derivatized at the N atoms with a combination of iron-linking and redox functionalities was used to immobilize the enzymes and to shuttle electrons. Enzymes were deposited with the polymer and deposited onto C electrode. For peroxide selectivity over ascorbate is achieved by incorporation of Nafion. The microsensors if they can be successfully used in vivo will provide valuable information for brain diseases (Parkinsion s and Alzheimer s). [88]... 

Electrochemical microsensors, which measure the concentration of special substances, typically in macroscopic volumes. The microcell is often separated from the environment by a selective barrier, e.g., a membrane. 

Wire that senses pH changes such as antimony metal can be coated with urease (65) and used to determine urea in pure blood (66). Covering the antimony metal with a gas perm-selective membrane (67) improves the selectivity. The microsensors respond from 0.1 to 10 mM urea in 30-45 s. This ammonia sensor has a faster baseline recovery than commercial gas membrane electrodes. 

The newest generation of electrochemical-based biological detection devices is the biosensors. These devices combine the high spatial and temporal resolution of an electrochemical microsensor with a biologically selective recognition element. In... 

Phillips PEM, Wightman RM. Critical guidelines for validation of the selectivity of in-vivo chemical microsensors. Trac-Trends Anal Chem. 2003 22 509-514. 

It should be noted that GC mode experiments with amperometric tips may contain a feedback component to the current if the electrochemical process at the tip is reversible and the tip-to-specimen distance is less than about 5a. However, at greater distances or when employing a potentiometric tip, the tip acts approximately as a passive sensor, i.e., one that does not perturb the local concentration. This situation is quite distinct from feedback mode, where the product of the electrolysis at the tip is an essential reactant in the process at the specimen surface. This interdependence of tip and specimen reactions in feedback mode ensures that the biochemical process is confined to an area under the tip defined by the tip radius and diffusional spreading of the various reagents (20). In contrast, the biochemical process in GC mode is independent of the presence of the tip and may therefore occur simultaneously across the whole surface. In addition, the tip signal often does not directly provide information on the height of the tip above the surface methods to overcome this limitation are described in Sec. I.D. Finally, since the tip process and the biochemical reaction at the specimen are independent, a wide range of microsensors may be employed as the tip, e.g., ion-selective microelectrodes, which are not applicable in feedback experiments. 

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