  Silicon-vacancy (SiV) centers in diamond are a promising candidate for
all-optical nanoscale high-sensitivity thermometry because they have sufficient
sensitivity to reach the subkelvin precision required for application to
biosystems. It is expected that nanodiamonds with SiV centers can be injected
into cells to measure the nanoscale local temperatures of biosystems such as
organelles. However, the smallest particle size used to demonstrate thermometry
using SiV centers is a few hundred nanometers. We recently developed
SiV-center-containing nanodiamonds via a detonation process that is suitable
for large-scale production. Here, we investigate the spectral response of
SiV-center-containing detonation nanodiamonds (SiV-DNDs) to temperature. We
used air-oxidized and polyglycerol-coated SiV-DNDs with a mean particle size of
around 20 nm, which is the smallest size used to demonstrate thermometry using
color centers in nanodiamond. We found that the zero-phonon line for SiV-DND is
linearly red-shifted with increasing temperature in the range of 22.0 to 40.5
$^\circ C$. The peak sensitivity to temperature was 0.011 $\pm$ 0.002 nm/K,
which agrees with the reported high sensitivity of SiV centers in bulk diamond.
A temperature sensitivity analysis revealed that SiV-DND thermometry can
achieve subkelvin precision. All-optical SiV-DND thermometry will be important
for investigating nanosystems such as organelles in living cells.
