  Fluorescent nanodiamonds (FNDs) have been exploited as sensitive quantum
probes for nanoscale chemical and biological sensing applications, with the
majority of demonstrations to date relying on the detection of single FNDs.
This places significant limits on the measurement time, throughput and
statistical significance of a measured result as there is usually marked
inhomogeneity within FND samples. Here we have developed a measurement platform
that can report the T1 spin relaxation time from a large ensemble of FNDs in
solution. We first describe a refined sensing protocol for this modality and
then use it to identify the optimal FND size for the detection of paramagnetic
targets. Our approach is simple to set up, robust and can be used for rapid
material characterisation or a variety of in-situ quantum sensing applications.
