J. Eur. Opt. Society-Rapid Publ. 21, 26( 2025) 265
Figure
26.( a) Optoplasmonic single-molecule sensor scheme, experimental setup, and resonance profiles with shifts due to the TOP effect.( b) extinction spectrum of the GNR used in the experiments;( c) Examples of measured resonance wavelength under the attachment of 3-phosphoglycerate kinase( 3PGK) molecules. Red and blue shifts are recorded for low and high intensities inside the WGM, respectively. Reproduced from [ 104 ].
optoplasmonic WGMR based sensors upon the attachment of single protein [ 104 ] and novel single molecule dynamometer( force gauge) combining optical tweezer and a hybrid WGMR [ 105 ]. The authors used the thermo-optical effect initiated by single molecules binding to a plasmonic nanorod [ 104 ]. Instead of injecting WGMs with low power, the authors opted to inject high power into the WGMs, generating thermo-optoplasmonic( TOP) effects. With their optoplasmonic single molecule detection setup, the author observed red-shifts for low intensities within the WGM, and blue-shifts for high intensities. These measurements were then used to estimate the absorption crosssection of single molecules. The authors reported on the experimental investigation of seven molecules and complexes. They observed blue shifts for dye molecules, amino acids, and anomalous absorption of enzymes in the nearinfrared spectral region. The optoplasmonic experiment is summarised in Figure 26. Figure 26a shows the optoplasmonic single-molecule sensor scheme, the experimental setup, and the resonance profiles together with their TOP shifts. Figure 26b shows the extinction spectrum of the GNR used in the experiments. Finally, Figure 26c shows the measured resonance wavelength under the attachment of 3-phosphoglycerate kinase( 3PGK) molecules: one can observe red-shifts for low intensities into the WGM, and blue-shifts for high intensities. The same group has published very recently an optoplasmonic WGM sensor with DNA origami structures [ 106 ]. The origami structures are a scaffold for the precise assembly of plasmonic dimers( GNRs), which generate strong near-field enhancements in the nanogap between the them. The authors have thoroughly investigated the hybridization kinetics under varying environmental conditions such as salt concentration.
Xu et al. [ 107 ] proposed a microbubble with GNRs adsorbed into the inner surface as a DNA sensor( see Figs. 27a and 27b). The authors detected DNA strands with a molecular weight of 8kDa in a volume of 10 pL. The authors used the interface mode, which enabled a detection limit as low as 0.3 pg / cm 2( see Fig. 27c).
5.3 Physical sensors
The mode shift mechanism can be also applied for sensing physical parameters such as force, mass, pressure and temperature. Hollow polymeric microspheres [ 108 ] were first proposed as pressure sensors and then as force sensors [ 109 ]. In the case of MBRs, the first demonstration of force sensors was given by Sumetsky et al. [ 110 ]. In that paper, they tuned the MBR over one FSR by stretching it through a piezo-activated clamp, but they could not give an