360
L. D. Negro et al.
Presently, we are optimizing deterministic aperiodic SERS substrates for real-time
detection and discrimination of different bacteria including Staphylococcus, E-Coli,
and bacillus serus [126]. A representative SERS spectrum obtained from E-Coli
bacteria is shown in Fig. 9.16b. The proposed SERS technology is also utilized for
the optical identification of different DNA nucleobases. Fig 9.16c demonstrates the
ability to detect, using aperiodic nanogalaxy substrates, SERS signal from 0.5 μM
solution of adenine using 0.8 mW radiation at 785 nm and integrating for 10 s.
The bacterial and molecular Raman spectra shown in Figs. 9.15 and 9.16 demonstrate the potential and robustness of the DANS approach for real-time bio-chemical
sensing and identification using reproducible and engineerable SERS substrates that
leverage aperiodic order on a chip.
9.3.3 Applications to Colorimetric Optical Sensing
In current biosensing technology, two-dimensional periodic lattices, (i.e. 2D optical
gratings) provide a well-established approach for biochemical colorimetric detection, which can yield label-free sensing of various molecular analytes and protein
dynamics. Standard periodic grating biosensors provide a distinct change either in
the intensity of diffracted light or in the frequency of optical resonances in response
to variations in the refractive index of the surrounding environment. The physical
mechanism at the basis of these optical signatures is the well-known phenomenon of
Bragg scattering. While this process provides frequency selective responses that are
useful for colorimetric detection, the ability of light waves to interact with adsorbed
or chemically bound analytes present on the surface of these sensors is intrinsically limited. In fact, Bragg scattering is a first-order process in surface scattering
perturbation theory [127], and scattered photons easily escape from a periodic surface within well-defined spectral bands and without prolonged interaction with the
sensing layer. On the other hand, eengineering elastic light scattering in planar aperiodic structures provides novel opportunities for bio-chemical sensing applications.
In particular, optical sensing platforms can be boosted by developing scattering elements that simultaneously provide high sensitivity to the environmental changes and
high spectral resolution, as both factors contribute to the improvement on the sensor
detection limit.
Detector sensitivity is conventionally defined as the magnitude of the wavelength
shift induced by the change of the ambient refractive index (measured in nm/RIU),
and can be improved by enhancing the light-matter interaction. In turn, the resolution
in measuring wavelength shifts inversely depends on the linewidth of the resonant
mode supported by the structure. It has recently been shown that aperiodic photonic structures provide the necessary balance between the resonant character of the
quasi-localized critical modes, which simultaneously feature high quality factors and
large field intensity over large sensing areas, resulting in largely improved sensitivity
over periodic grating sensors and even photonic crystals cavities due to their smaller
analyte-field overlap factors [128]. Building on these results, we have recently intro-
L. D. Negro et al.
Presently, we are optimizing deterministic aperiodic SERS substrates for real-time
detection and discrimination of different bacteria including Staphylococcus, E-Coli,
and bacillus serus [126]. A representative SERS spectrum obtained from E-Coli
bacteria is shown in Fig. 9.16b. The proposed SERS technology is also utilized for
the optical identification of different DNA nucleobases. Fig 9.16c demonstrates the
ability to detect, using aperiodic nanogalaxy substrates, SERS signal from 0.5 μM
solution of adenine using 0.8 mW radiation at 785 nm and integrating for 10 s.
The bacterial and molecular Raman spectra shown in Figs. 9.15 and 9.16 demonstrate the potential and robustness of the DANS approach for real-time bio-chemical
sensing and identification using reproducible and engineerable SERS substrates that
leverage aperiodic order on a chip.
9.3.3 Applications to Colorimetric Optical Sensing
In current biosensing technology, two-dimensional periodic lattices, (i.e. 2D optical
gratings) provide a well-established approach for biochemical colorimetric detection, which can yield label-free sensing of various molecular analytes and protein
dynamics. Standard periodic grating biosensors provide a distinct change either in
the intensity of diffracted light or in the frequency of optical resonances in response
to variations in the refractive index of the surrounding environment. The physical
mechanism at the basis of these optical signatures is the well-known phenomenon of
Bragg scattering. While this process provides frequency selective responses that are
useful for colorimetric detection, the ability of light waves to interact with adsorbed
or chemically bound analytes present on the surface of these sensors is intrinsically limited. In fact, Bragg scattering is a first-order process in surface scattering
perturbation theory [127], and scattered photons easily escape from a periodic surface within well-defined spectral bands and without prolonged interaction with the
sensing layer. On the other hand, eengineering elastic light scattering in planar aperiodic structures provides novel opportunities for bio-chemical sensing applications.
In particular, optical sensing platforms can be boosted by developing scattering elements that simultaneously provide high sensitivity to the environmental changes and
high spectral resolution, as both factors contribute to the improvement on the sensor
detection limit.
Detector sensitivity is conventionally defined as the magnitude of the wavelength
shift induced by the change of the ambient refractive index (measured in nm/RIU),
and can be improved by enhancing the light-matter interaction. In turn, the resolution
in measuring wavelength shifts inversely depends on the linewidth of the resonant
mode supported by the structure. It has recently been shown that aperiodic photonic structures provide the necessary balance between the resonant character of the
quasi-localized critical modes, which simultaneously feature high quality factors and
large field intensity over large sensing areas, resulting in largely improved sensitivity
over periodic grating sensors and even photonic crystals cavities due to their smaller
analyte-field overlap factors [128]. Building on these results, we have recently intro-