Microwave selective heating effects

These results provide clear evidence for the existence of selective heating effects in MAOS involving heterogeneous mixtures. It should be stressed that the standard methods for determining the temperature in microwave-heated reactions, namely with an IR pyrometer from the outside of the reaction vessel, or with a fiber-optic probe on the inside, would only allow measurement of the average bulk temperature of the solvent, not the true reaction temperature on the surface of the solid reagent. 

Because this change in selectivity is difficult to explain by a classical heating effect, Langa et al. [9] consider that it is one of the most convincing examples of a possible specific microwave effect. 

Several reasons have been proposed to account for the effect of microwave heating on chemical reactions and catalytic systems. The results summarized in 1 to 7, above, show that under specific conditions microwave irradiation favorably affects reaction rates of both the liquid- and gas-phase processes. This phenomenon has been explained in terms of microwave effects, i. e. effects which cannot be achieved by conventional heating. These include superheating, selective heating, and formation of hot spots (and possibly nonthermal effects). 

It is obvious that nonisothermal conditions induced by microwave heating lead to very different results from those obtained under conventional heating conditions. In summary microwave effects like superheating, selective heating and hot spots, can all be characterized by temperature gradients ranging from macroscopic to molecular scale dimensions. 

Ultrasound can be used to supplement or replace the microwave radiation. Ultrasound effects a high frequency mechanical vibration that warms the interior of the exposed object. In this case, the sample can exposed, and heated, as a whole. However, because ultrasonic waves can be so readily focused, it is also possible to apply them in bundled form so that they act on certain selected regions of the blank, for instance by sweeping along a raster. 

These differential heating effects may also be used in solid-state reactions involving metal powders that are made into a fluidised bed by a counter stream of gas. The metal particles interact very strongly with the microwaves and rapidly heat, whereas the gases are transparent to microwaves therefore, the reaction is induced by a very selective interaction between the metal particles and microwaves. 

There are three major components of the MAMEF technique 1) plasmonic nanoparticles (i.e., silver, gold, copper, nickel, aluminum, zinc, etc.), 2) microwaves and 3) an aqueous assay medium. TTie plasmonic nanoparticles serve as (i) a platform for the attachment of one of the biorecognition partners (anchor probes) (ii) as an enhancer of the close-proximity fluorescence signatures via surface plasmons (i.e., MEF effect) [2] and (iii) a material not heated by microwaves for the selective heating of the aqueous media with microwave energy. 

The fact that different chemical species absorb microwave energy to a different extent implies that the thermal energy so-produced and imparted to the surrounding environment will also vary with the chemical species. Hence, for systems that possess inherent non-homogeneous structural characteristics, or that contain different chemical species with different dielectric properties dispersed into a homogeneous environment, it is possible to effect a selective heating of some areas, or components of the systems. 

The effect of microwave irradiation in chemical reactions can be attributed to a combination of the thermal and nonthermal effects, i.e. overheating, hot spots, and selective heating, and nonthermal effects of the highly polarizing field, in addition to effects on the mobility and diffusion that may increase the probabilities of effective collisions. These effects can be rationalized considering the Arrhenius law [36, 37] and result from modification of each of the terms of the equation k = Aexp -AGyRT)... 

Microscopic hot spots, for example temperature gradients between the metal particles and the support, which cannot be detected and measured because they are close to micro scale, i.e. have molecular dimensions, are dose to selective heating of active sites. Unfortunately, microwave radiation effects at the molecular level are not well understood. 

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