# Preparation of Nanoparticle-Loaded Microbubbles via an Electrohydrodynamic Atomization Process
## 1 Introduction  
Micron-sized bubbles, also known as microbubbles, are widely used in micro reactor engineering [1], sewage treatment [2], food processing [3] and many other fields due to their superior ultrasonic response performance and efficient gas-liquid interface mass transfer. Since the discovery of microbubbles with good acoustic properties and encapsulation ability, microbubbles have shown ideal application prospects in the research fields of ultrasound contrast agents, drugs and gene delivery carriers [4; 5; 6; 7; 8; 9; 10]. Microbubbles are further developed towards the integration of diagnosis and treatment from drug loading to drug delivery system with targeted functions [7; 11; 12; 13]. Due to the excellent performance of nanoparticles in the biomedical field [14; 15; 16], microbubbles loaded with nanoparticles have become the focus of attention in the microbubble research field [17; 18; 19; 20; 21]. Conventional preparation methods of microbubbles include mechanical vibration/stirring, ultrasonication, a freeze-drying method, and an adsorption method, among others [22]. However, the defects of these methods in precise control of the bubble size and the complexity of the operating procedure limit their applications and further development [23]. In recent years, newly developed technologies for the preparation of microbubble, such as a microfluidic method [24; 25; 26; 21] and an electrohydrodynamic atomization method [27], have attracted increasing attention. The microfluidic method has great potentials in precise control of bubble size, while the production rate and channel blockage issues still need to be addressed, especially for highly viscous fluids and solid particle contained flows. The electrohydrodynamic atomization method as an extension of electrospray [28] uses an electric field to intensify the microbubble formation process. The microbubble size range can be controlled by adjusting the applied voltage and flow rate. This can overcome the drawbacks of the microfluidic method,which usually adjusts the bubble size by changing the microchannel size, which is costly and associated with the channel blockage issue when small microbubbles are desired and a complex shell layer of the microbubble is required [29; 30]. [PERSON] et al. [27; 31] first studied the preparation of microbubbles using electrohydrodynamic atomization technology. [PERSON] et al. [32] took this as an example to prepare microbubbles with better monodispersity by varying coaxial nozzle configurations. However, only conventional microbubbles, simply consisting of a shell layer and a gas core, were produced. It is still unclear whether microbubbles with functional substances can be formed by the electrohydrodynamic atomization method. On the other hand, due to the poor mechanical properties of the shell layer and the fast diffusion rate of the gas core of conventional microbubbles, microbubble stability is usually low, which limits the practical applications of microbubbles [29; 33; 34]. It has been reported that loading nanoparticles into the microbubbles will not only increase the mechanical strength of the shell layer but also prevent gas diffusion from the core region of microbubble [18; 19]. Therefore, it is of great significance to produce nanoparticle-loaded microbubbles. Unfortunately, no study on the preparation of nanoparticle-loaded microbubble via electrohydrodynamic atomization method has been reported to date. In this study, the electrohydrodynamic atomization technology was first used to prepare nanoparticle-loaded microbubbles. Bovine serum albumin (BSA) and air was selected as the template material for the shell layer and the gas core of the microbubbles. Silica (SiO\({}_{2}\)) nanoparticles were used as the template nanoparticles. The effects of nanoparticle loading on microbubble stability were studied in terms of bubble size and number variation.