  Observing the hybridisation kinetics of DNA probes immobilised on plasmonic
nanoparticles is key in plamon enhanced fluorescence detection from weak
emitting species, and refractive index based single-molecule detection on
optoplasmonic sensors. The role of the local field in providing plasmonic
signal enhancements for single-molecule detection has been studied in great
detail. Nevertheless, few studies have compared the experimental results in
both techniques for single-molecule studies. Here we developed the first
optical setup that integrates optoplasmonic and DNA-PAINT based detection of
oligonucleotides to compare these sub-platforms and provide complementary
insights into single-molecule processes. We record the fluorescence and
optoplasmonic sensor signals for individual, transient hybridisation events.
The hybridisation events are observed in the same sample cell and over a
prolonged time (i.e. towards high binding site occupancies). A decrease in the
association rate over the measurement duration is reported. Our dual
optoplasmonic sensing and imaging platform offers insight into the observed
phenomenon, revealing that irreversible hybridisation events accumulate over
detected step signals in optoplasmonic sensing. Our results point to novel
physicochemical mechanisms that result in the stabilisation of DNA
hybridisation on optically-excited plasmonic nanoparticles.
