Passive probe

In principle, there are two methods for the AFM probing of amorphous layers, namely (i) an active probing, which probes the surface chemistry, and (ii) a passive probing, which probes the surface physics. The active AFM probing, which is essentially an extension of the AFM-based single molecule studies, relies on the measurement of LF by an AFM tip that is fimctionahsed with molecular recognition-active molecules (e.g., a potentially complementary DNA). 

Potentiometric ion-selective electrodes are passive probes, which in contrast to voltammetric sensors do not convert the analyte in the sample. The response of an ISE depends linearly on the logarithm of the activity (concentration) of a potential determining ion (primary ion) in the presence of other ions. The schematic layout of a complete potentiometric cell including an ion-selective electrode is shown in Figure 2. The electrochemical notation of the cell assembly is given as ... 

The tips discussed so far have been amperometric probes, typically of Pt-Ir, that produce faradaic currents reflecting redox processes at an exposed surface. However, it is also possible to use potentiometric tips, such as ion selective electrodes based on micropipets, in an SECM (28, 29). These produce potentials (with respect to a reference electrode) that depend logarithmically on the solution activity of a specific ion. Tips of this type that can detect H, Zn, NH4, and with a resolution of a few pirn have been described. Tips of this type are especially useful for detection of nonelectroactive ions, like many of those of interest in biological systems. However, such tips are passive probes, in that they detect the local activity of a given species but do not sense the presence of the substrate. They cannot be used to determine d, so they must be positioned with respect to a substrate, such as, in studying a concentration gradient at an electrode, by visual observation with a microscope, by resistance measurements, or by using a double-barrel tip that contains both an amperometric element and a potentiometric one. 

We examine here the passive probe model [71], which ignores the effect of the probe on the SNOM image and assumes that the signal detected is proportional to the near-field intensity at the nanostructure surface in the absence of the probe. This hypothesis may be valid either if the field scattered by the tip is very small or if it is not reflected back by the sample. Thus, from this qualitative analysis, we may expect the probe to be passive either if the tip is very small or if the sample has a low reflectivity. Therefore, a metallic tip close to a metallic sample may not satisfy the assumption of a passive probe, whereas a tiny metallic tip above a dielectric (or magnetic) might be considered as a passive probe. 

A passive probe model simplifies calculation substantially. Indeed, such an approach enables us to work in the first Born approximation, while for calculation of the near field we only need to calculate the scattered field [i (z,to, n) by equation (19) with the Fourier transform chosen to be compati-... 

The system is composed of the gas chamber (probe) and the monitor, by employing optic fiber between them to provide a remote and on-line measurement. The oxygen monitoring unit is often placed in a control room. The probe is installed at the monitored location. The fiber cable is used to transmit signals between the probe and monitoring unit. The passive probe makes it more suitable for being deployed in the hazard environment. The monitor unit can be connected up to multiple optic fiber oxygen probes. 

Passive probe arrays do not offer the possibility to write the arbitrarily complex and heterogeneous layers of material that would be needed in such applications. 

SECM with AC impedance relies on measuring the conductance of the solution between the microdisc tip and the counterelectrode in the bulk of the solution. It was shown that the conductance depends on the tip-substrate distance, in the same way as the diffusion-controlled current, and this can be exploited to accurately position the tip [135]. This approach has the merit of being applicable in the absence of a redox mediator, and is convenient when the tip is operated as a passive probe [136]. This methodology has been exploited by Wipf and coworkers to construct an AC-SECM capable of harnessing AC and DC amperometric signals to record feedback images at constant distance with respect to... 

Fluorescent probes. Fluorescence probes are materials which indicate, by fluorescence, the presence of some specific chemical species or environment. There are two types passive and active. Passive probes indicate the presence of the material of interest simply by physically or chemically binding to it. They thus indicate the presence of an analyte substrate by their own fluorescence (although this may also be altered by the presence of the analyte), the intensity of which can be used to determine the presence of the analyte either qualitatively or in some cases quantitatively. A typical use of such probes is in fluorescence microscopy, and there are a wide range of commercially available probes for this purpose (e.g. 4.3). Active probes undergo a photochemical change in the presence of the chemical or environmental feature to which they are sensitive. This results in a change in some emission property such as wavelength, intensity, polarisation, or lifetime (e.g. 1.4, 4.8). Optical probes are discussed in detail in Chap. 12. 

At smaller values of the tip-substrate separation distance, d, feedback and hindering effects, as those observed in the feedback modes at amperometric tips, perturb the transport processes. The tip then starts to interfere with the source, which complicates the quantitative data analysis without numerical modeling.Such perturbing effects are not observed when passive probes such as potentiometric " or biosensor microelectrodes are used, but then the positioning of these substance-selective sensors is difficult. An alternative would be to consider the use of the scanning ion conductance microscopy (SICM). Indeed, recently, the SICM afforded the opportunity to image and quantify precisely local K+ and Cl ionic fluxes. ... 

---