Inline Spectroscopic Investigation of Metal Patch Growth on Spherical Nanoparticles in Continuous Flow

Plasmon resonant nanoparticles have been extensively investigated over the last decades due to their promise in a wide range of applications (e.g. theranostics, pigments, sensors, photocatalysts). As the tunability of the resonant frequency of isotropic plasmonic nanoparticles (NPs) is limited, anisotropic particles can be used to expand the available property space. One example of anisotropic plasmonic NPs are patchy particles, where noble metal patches with controlled size and morphology are deposited onto a dielectric core particle. We have demonstrated these particles to have a wide optical tunability and scalable production in a continuous flow process. However, there is still a lack of predictive models for patch formation, hindering further process optimization. To obtain the kinetic parameters for such models, in situ characterization of the growing patches is required. Addressing this, the present contribution will give details of a purpose-built inline spectroscopic setup and the results obtained with it.<br/>Inline measurements to understand the continuous flow growth of various types of nanoparticles have been reported in the literature. In the case of noble metal nanoparticles, a change in size and morphology can be detected via a change in the optical spectrum. Consequently, inline extinction spectroscopy offers an easily accessible tool to follow the particles’ structural evolution in time.<br/>To investigate the patch growth under well-mixed conditions, we use a continuous inline spectroscopy setup which consists of a polyamide T-mixer connected to a rectangular borosilicate glass tube. The latter is mounted on a linear stage on an optical pegboard. Via an automated protocol, the glass tube is moved through a fixed measurement position of a fiber spectrometer. Thus, each position along the glass tube corresponds to a specific reaction time. An adjustable spacer tube can be used to obtain data at later residence times. <br/>Using measurements at different reaction conditions, we collect kinetic data for model development. To validate our measurements, we utilize a quenching approach which freezes the metal patches at various growth stages and thus enables them to be investigated via ex situ characterization techniques. Correlating in situ and ex situ data, we show that a kinetic model for the formation of patchy particles valid for a wide range of reaction conditions can be established, paving the way for further process optimization and automation.