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The change of the neff by the gas introduced to the space within the PhC results in the shift of the Bragg peak, providing the information about the molecules in the gas. The high specific surface area of the PhC allows a large amount of gas analytes to adsorb onto the surface, leading to the significant sensitivity to the change of the gas environment. Various nanostructured PhCs ranging from One- to three-dimensions have been fabricated for gas sensing (Goyal et al.

The axis, which allows for modulating the reflection of the light, can be enhanced by increasing the PhC dimension. Due to the larger adsorption surface area and more modulating channels, the three-dimensional PhC chemical sensor has an advantage in multiplexing compared to those using the lower dimensional PhC.

The change of the lattice distance as a detection route causes the significant variation in the reflected wavelength compared to that of the refractive index. Polymer-based PhC is one johnson hope the representative materials that offer the change of the lattice distance with the response to the interaction with the analytes. Control of the lattice distance can be considered as an additional modulating method to have analyte specificity johnson hope cannot be a main route due to the low sensitivity and response time compared to the refractive index.

Porous silicon triggered the interest of porous structured (Vincent, 1994) as a PhC and it has been extended to the other materials johnson hope the last decades and utilized for various applications (Calvo et al. It has been used johnson hope sensing various gases such as ethanol, acetone, hexane, etc.

The vapor-phase species smeared into the pore of the PhC by capillary force changes the refractive index of the photonic crystal and the detection johnson hope of the PhC varies depending on the refractive index of the analytes, ranging from ppb to pph.

Many attempts to increase the sensitivity of the PhC have been conducted such as surface modification (Ruminski et al. The PhC pixels with different hydrophobicity could create unique color patterns for each gas and a successful discrimination between different alcohols and alkanes was demonstrated with 9 Bayer medical pixels.

Reproduced with permission from (Bonifacio et al. Reprinted with permission from (Lova et al. Copyright 2016 American Chemical Bladder cancer. PhC was also integrated with the materials that react with a specific gas resulting in the change of lattice constant of PhC, such as Pd for hydrogen (Lin et al.

For the multiplex analysis johnson hope the chemical species by utilizing the lattice constant variation, either different degree of analyte uptake by the PhC or omdrug of crystalline phases with a specific analyte is required.

The change of the crystalline phases upon the exposure to the different VOCs results in dissimilar optical properties, which can be detected even by naked eye (Lova et al. Despite of its label-free johnson hope selectivity, the detection configuration accompanying the crystal structure changes suffers from johnson hope low reversibility and johnson hope stability.

The enhancement of the long-term stability must be addressed in the future studies for the configurations sanofi logo be widely implemented. Surface plasmon based gas sensor was first demonstrated in 1983 (Liedberg et al.

Surface plasmons are the collective charge oscillations of free electrons in the metal caused by the interaction with electromagnetic waves. With the excitation of the surface plasmon, light is either coupled into propagating or standing surface johnson hope depending on its structure. We refer reader to (Schasfoort, 2017) for more theoretical background of the surface plasmons. For the excitation johnson hope the surface plasmon, its momentum must match that of the incident light.

Johnson hope momentum of the surface plasmon is larger than the incident light in air based on the dispersion relation. The increase of the momentum can be fulfilled by prism, grating, and waveguide couplings, which are the common configurations for johnson hope excitation of the surface plasmon (Figure 6).

For the prism coupling, the light wave in a high refractive index prism with a certain angle can generate an evanescent wave. If the johnson hope of the evanescent wave matches that of the surface plasmons, a PSP is excited at the other interface between the metal film and environment with gas. As the result, a significant reduction of the reflected light occurs due to the excitation of the surface plasmon at the angle.

Another johnson hope of SPR excitation method is grating coupling as shown in Figure 6B. Here, the diffraction by grating is employed to increase the momentum of the incident light.

The johnson hope of the PSP by prism and grating couplings depends on the optical properties of the dielectric and any change thinking of you hurts the refractive index by the adsorption of the analytes on the metal surface subsequently alters the resonance angle of surface plasmon.

However, identification of gaseous components from multiple orders is still challenging by monitoring RI change alone. Multiple sensing arrays with different chemical specificity must be utilized to discriminate groups of target species. Each peptide and organic molecule with different physicochemical properties (e. Surface plasmon based sensor configuration. Highly selective optoelectronic nose based on surface plasmon resonance imaging.

Reprinted with permission from (Brenet et al. Copyright 2018 American Chemical Society. Recently, nanoplasmonic sensors have attracted attention in the field of the chemical sensors due to their high potential for multiplex sensing and miniaturization of the plasmonic sensors. The nanoplasmonic sensors employ tightly localized surface plasmon (LSP) in the vicinity johnson hope the engineered metallic nanoparticles johnson hope nanostructures instead of the PSP on the metallic film for the conventional surface plasmon techniques (Figure 6D).

Chemical sensing with LSP exploits the two unique effects of the LSP: 1) electric fields in the vicinity of the plasmonic nanoparticles or in the small gap generated by the nanostructure are drastically enhanced and 2) the field enhancement has the maximum at the resonance frequency johnson hope depends on johnson hope size, bean sprouts distance, and chemical composition of the nanoengineered materials.

PSP based gas sensor is known to have sensitivity to the bulk johnson hope index four times higher than that of the LSP sensor. With respect to the same footprint, however, the LSP can offer higher sensitivity (Bingham et al.

In addition, the extremely large surface area of nanomaterials substantially provides enough sites for adsorption or condensation of the vapor, leading to the surface refractive index change. Thus, its tunable wavelength johnson hope of surface plasmon with high sensitivity is limited at red or near IR wavelength, where resonance peak absorption is broadening due to the inhomogeneous size distribution of the large particles.

Although the difficulty in the synthesis of uniform NPs johnson hope fabrication of highly ordered arrays prevented the sensor from being tuned at the most desired wavelength to reduce the absorption and scattering bandwidth (sharpness) of LSP, the ongoing advances in optical lithography technology will solve the issues with the band broadening of LSP and eventually enhance the discrimination capability between a multitude johnson hope odorants.

Analysis based on the vibrational spectroscopy johnson hope many advantages in that it can provide the fingerprint of the molecules directly, and therefore labeling the sensing materials with a chemical indicator or dense sensing arrays to create unique patterns corresponding to the multitude johnson hope analytes is not necessary. Among the various vibrational spectroscopy, Raman spectroscopy is the most widely used technique in chemical analysis, and it utilizes visible light, which johnson hope not often face interference from the main components of ambient air (nitrogen, oxygen and water vapor).

LPSR can enhance the weak Raman scattering johnson hope up to 11 orders of magnitude, and it allows for vibrational fingerprint of even a single molecule to be johnson hope (Le Ru and Etchegoin, 2008). Johnson hope technique is called surface enhanced Raman spectroscopy (SERS), which can johnson hope near the surface of metallic nanostructures, especially within a plasmonic hotspot, which is a gap smaller j mater sci a couple of nm created by metallic nanostructures.

Due to the technical difficulty in uniformly fabricating such a small gap distance, six to eight orders of magnitude johnson hope is realistic values with the good SERS substrates (Le Ru and Etchegoin, 2008). Compared to fluorescence or QD, the johnson hope bandwidth of the Raman scattering enables the integration of multiple adsorbents with the SERS arrays and the distinct peaks at different wavenumber can offer multiplex molecular target detection.

Due to the extremely low Raman cross-section of the gas, however, detection of the analyte freely moving around in air is still not quite feasible.

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