Ultrasound is an acoustic wave with a frequency
Ultrasound is an acoustic wave with a frequency >20 kHz that needs a medium to propagate . Accompanied by the spread of an ultrasonic wave, a series of alternating cycles of compression and rarefaction emerge in the liquid medium. During the rarefaction cycle, microbubbles are formed because of the reduced pressure. Consequently, acoustic cavitation is generated owing to the formation and subsequent dynamic life of microbubbles. During cavitation, the pressure and temperature inside the bubble can rise to >1000 atm and 5000 K, respectively. Acoustic cavitation can be classified into stable and transient cavitation based on whether the microbubbles break up. In stable cavitation, microbubbles can be expanded and compressed stably for some cycles. The motion of the stable microbubbles leads to microstreaming, which, in turn, produces strong eddy currents in the medium around the cavitation bubbles. In addition, the diffusion of dissolved gases into and out of the bubbles also creates microcurrents around them [21, 22]. During transient cavitation, the microbubbles collapse violently when they grow to a certain size or implode during the compression part of the ultrasonic wave. The significant release of energy during the collapse produces outward propagating shockwaves and shear forces, causing strong turbulence within the surroundings. These mechanical effects accelerate the porcine of reactants and enhance heat and mass transfer during the process [, , ]. The collapse of microbubbles near the particle surface generates high-speed micro-jets of liquid that damage the material surface. The aforementioned phenomena constitute physical effects. In addition to the thermal and physical phenomena, cavitation also brings about chemical effects. The collapse of acoustic bubbles can dissociate water and dissolved oxygen molecules and create highly reactive free radicals (OH, OOH). Subsequently, reactive species can recombine or react with other molecules .
Ultrasound is used for a growing variety of purposes in diverse areas because it is highly efficient and energy saving, has low instrument requirements and produces no pollution. Ultrasound is known to provoke changes in product properties and accelerate some chemical reactions and industrial processes due to its mechanical and radical effects [22, , , ].
For many years, ultrasound has been employed as an enzyme inactivation method, whereas some works have stated that ultrasound did not inactivate all enzymes under mild conditions . Ultrasound has positive effects on enzyme activity and can be used to accelerate enzymatic reactions. The involvement of ultrasound in enzymatic reactions shows great potential for industrial applications. In this work, ultrasonic treatment used throughout the whole process of enzymatic reactions will be reviewed (as shown in Fig. 1). How ultrasound promotes enzymatic reactions will also be discussed.
Ultrasonic modification of enzymes
Ultrasound assisted enzyme immobilization Enzyme immobilization can be simply explained as immobilization of the free enzyme on an insoluble phase (carrier or support) against substrates and products. This approach overcomes the restrictions of poor stability and recovery of the free enzyme, conduces to the product separation, and makes the complete process more economically viable. Thus, immobilized enzymes are adaptable for multifarious types of industrial processes, including multienzyme and chemoenzymatic cascade reactions [52, 53]. There are numerous methods to prepare immobilized enzymes using a variety of matrices. Immobilization carriers can be natural polymers, synthetic polymers or inorganic materials . Enzyme immobilization techniques can be divided into two methods: physical and chemical. Physical immobilization methods are achieved via physical forces involving van der Waals forces, hydrophobic interactions and hydrogen bonding. Chemical methods involve the attachment of enzymes onto different matrices using covalent or ionic bonds via an irreversible process .