  The growth of supermassive black holes (SMBHs) through merging has long been
predicted but its detection remains elusive. However, a promising target has
been discovered in the Seyfert-1 galaxy J1430+2303. If a binary system truly
lies at the center of J1430+2303, the usual symmetry expected from pole-on
views in active galactic nuclei (AGNs) responsible for the observed low ($\le$
1\%) optical linear polarization in the continuum of these objects is expected
to be broken. This should lead to higher-than-usual polarization degrees,
together with time-dependent variations of the polarization signal. We used the
specialized photopolarimeters RoboPol mounted on the 1.3m telescope at the
Skinakas Observatory and the Alhambra Faint Object Spectrograph and Camera
(ALFOSC) mounted on the 2.56m Nordic Optical Telescope (NOT) at the "Roque de
los Muchachos" Observatory to measure the B-, V-, R-, and I-band polarization
of J1430+2303. Observations were complemented using the FORS2
spectropolarimeter mounted on the VLT to acquire 3500 -- 8650 Angs polarized
spectra. We compared our set of observations to Monte Carlo radiative-transfer
predictions to look for the presence of a SMBH binary. The observed linear
continuum polarization of J1430+2303 in the V and R bands is $\sim$ 0.4\% with
an associated polarization angle of slightly larger than 0$^\circ$. We detected
no significant changes in polarization or photometry between May, June, and
July of 2022. In addition, there is no significant difference between the
polarization of H$\alpha$ and the polarization of the continuum. A single SMBH
at the center of an AGN model is able to reproduce the observed spectrum and
polarization, while the binary hypothesis is rejected with a probability of
$\sim$85\%.
