  Monitoring chemical reactions in solutions at the scale of individual
entities is challenging: single particle detection requires small confocal
volumes which are hardly compatible with Brownian motion, particularly when
long integration times are necessary. Here, we propose a real-time (10 Hz)
holography-based nm-precision 3D tracking of single moving nanoparticles. Using
this localization, the confocal collection volume is dynamically adjusted to
follow the moving nanoparticle and allow continuous spectroscopic monitoring.
This concept is applied to the study galvanic exchange in freely-moving
collo{\"i}dal silver nanoparticles with gold ions generated in-situ. While the
Brownian trajectory reveals particle size, spectral shifts dynamically reveal
composition changes and transformation kinetics at the single object level,
pointing at different transformation kinetics for free and tethered particles.
