## 1 Introduction  
Nanoparticles are used in products for a wide variety of applications ranging from performance polymers to pharmaceuticals, food ingredients, cosmetics and coatings and pains [1; 2; 3; 4; 5; 6]. Very often, tailored nanoparticles of a specific size, shape and architecture are produced in bottom-up liquid phase syntheses [7; 8]. A detailed study of the nucleation and growth of such particles would provide valuable information on their mechanism of formation, and could furthermore be used for process optimization and scale up. For such a study, real-time measurements of particle size and concentration are required. Such real-time measurements, when coupled to industrial reactors, could also provide data that enable rational intervention in a production process, hence minimizing the number of off-spec batches and waste. The methods used in this paper to measure dispersed nanoparticles are ultrasound spectroscopy, electron microscopy and dynamic light scattering. We will discuss each shortly. An ultrasound method is particularly suitable for measuring the mean diameter and concentration of the nanoparticles [9; 10], as it is non-destructive, fast and relatively cheap, and can measure in high concentrations and opaque dispersions. Also, the ultrasound sensors can be placed inside the reactor, allowing for real-time measurements of the actual dispersion instead of samples. Most ultrasound instruments reported to date lack the ability to measure the ultrasound waves with enough precision to attribute the changes in the ultrasound signal to particle growth at a nanometer scale [11; 12]. However, the recently developed ultrasound nanoparticle size (UNPS) can measure the ultrasound signal with enough precision to relate this to particle growth. We will discuss the theory behind the UNPS in the next section. A drawback of the ultrasound method is that it is critical to know the material properties of the solvent and the particles (the most important being the speed of sound, density, and viscosity of the solvent, and the speed of sound and density of the particles) in order to relate the ultrasound signal to the particles in the dispersion. Another drawback is that an ultrasound method can measure the mean particle diameter; however, under practical circumstances a (relative) particle size distribution cannot be measured. Electron microscopy can be used to measure the average size of nanoparticles [13]. Moreover, it can measure the relative particle size distribution. However, samples are measured in dry form, and, therefore, the technique cannot be used as real-time analytics. Furthermore, the technique cannot measure particle concentration and is relatively expensive. Also the skill of the person operating the transmission electron microscope (TEM) is crucial. Dynamic light scattering (DLS) methods [14; 15] can measure the particle size and relative distribution of particles in solvents, and perform especially well on relatively low concentrations. However, measuring high concentrations (>5 w %) and opaque dispersions is still challenging since the method relies on single scattering events, meaning that each photon detected has been scattered only once in the sample. For this reason, dilution is typically necessary for highly scattering and concentrated samples [16]. Furthermore, DLS methods are not able to measure concentration. Recently, we have successfully developed a flow cell coupled DLS set-up that also allows monitoring of the growth of nanoparticles in real time [17]. In the developed DLS set-up, the sensors are not measuring directly in the reactor, but an analytical bypass loop is used for sampling purposes. As is clear, the three methods discussed above all have pros and cons and, therefore, are often used in combination to complement each other. _Theory behind the Ultrasound Nanoparticle Sizer (UNPS) Method_  
Nanoparticles change the speed and attenuation of ultrasonic waves in a dispersion.