# Sensor Micro and Nanoparticles for Microfluidic Application
## 2 Construction Principles of Sensor Particles
Generically, they consist of three parallel inlet channels: the continuous phase flow in two side channels and the middle microchannel is for the dispersed phase. A small orifice is placed in front of the middle channel where the thread of dispersed phase breaks up into small droplets. The mechanism of breakup depends on the geometry of the junction and the ratio of flow rate and properties of the fluids [3]. The most common capillary-based devices types are: the co-flow, the cross-flow and the flow-focusing capillary devices. In the co-flow configuration, two coaxially aligned fluids (the dispersed and continuous phases) flow in the same direction. The characteristic of the cross-flow device is that the fluid of dispersed phase flows perpendicularly to the continuous phase flow's direction. In the flow focusing device, drops are created by hydrodynamic focusing, where the continuous phase pinches the stream of dispersed phase, which breaks into droplets [8]. According to the reacting phases, the microfluidic devices for synthesis are categorized as a single-phase, laminar flow or segmented flow microfluidic systems. Flow of single-phase laminar flow devices is characterized by the very low Reynolds numbers; therefore, the parabolic profile of fluid velocity vectors dominates. Due to the turbulence-free condition in continuous-flow microfluidic devices the mixing is slow and accomplished only through molecular diffusion, which is controlled in space and time. For this reason, it is possible to determine controlled reaction-diffusion (RD) experiments. In order to produce homogeneous mixtures and improve the mixing processes, so-called chaotic micromixers can be used for inducing complex flow pattern by consistently folding the laminated flow with split and recombine units [3,9]. Segmented flow microfluidic systems mix reagents via advection by a flow-induced segment-internal circular fluid motion, which can provide a fast homogenization process down to the sub-millisecond time scales. Typically, the mixing process is completed in the channel of capillary where the droplet travel length to channel width ratio is 20. Nevertheless, mixing in a segment can be increased by applying zig-zag channels, which enhance the mixing by sharp turns of the flow. Additionally, mixing can be programmed and multiple reactions can be synchronized in order to fabricate core-shell particles and crystals [10]. In addition to the variety of different conventional batch synthesis and microfluidic synthesis, the hole mask colloidal lithography method can be applied for fabricating uniform nanoparticle arrays. Additionally, for simple and rapid formation of metal structures, physical vapor deposition followed by a rapid thermal annealing (RTA) can be applied. Unfortunately, structures formed using RTA  
Figure 1: Examples of microfluidic devices, which are used for the emulsification of a monomer. Microchannel-based devices: (**A**) terrace-like device, (**B**) T-junction device, (**C**) flow focusing device. Capillary-based devices: (**D**) co-flow device, (**E**) cross-flow device, (**F**) flow focusing device. CP—continuous phase, DP—dispersed phase. Reproduced from [8]. Copyright (2008) Wiley-VCH GmbH. Reproduced with permission. feature poor homogeneity and broad size distribution. A summary of the methods used for fabricating sensor particles is presented in Table 2.