Typical absorption and emission spectra of the dye Nile Red showing response of change in wavelength and intensity in presence of ethanol. 

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... Red dye was found to be uniformly dispersed across the volume of the core of the waveguide formed of either LS-6257 or OE-6520. Fig 8 shows the cross-section of several waveguides under a fluorescence microscope with a green filter. It was observed that after terpineol evaporation, NileRed in PDMS gave an emission peak around 590nm for both OE-6520 and LS-6257, as shown in Fig. 9. Fig. 10 shows an SEM micrograph of peeled-off Sylgard®184 with 100 μ m deep channels. The width of the waveguides depends on the design of the mask and several dimensions like 50, 100, 300 and 500 μ m have been used. The dye dispersed in the PDMS matrix was observed to be largely unaffected by continuous exposure to green light as there was only a negligible reduction to the fluorescence signal even after several hours. Further studies are needed to characterize the exact values of reliability in terms of bleaching of the fluorescent dye. As illustrated in Fig. 6, the dye-doped waveguide sample illuminated with the green LED is initially kept in a nitrogen environment for 30 minutes. Subsequently, ethanol and nitrogen are alternated with an interval of 15 minutes each. In the presence of ethanol, the fluorescence is found to have a gradual red shift as shown in Fig 1. The peaks of emission maxima are found to have a shift from 590 nm to 610 nm after four ethanol-nitrogen alternating cycles. Graphs shown in Fig 11 shows the alternating behavior of fluorescent intensity at 600 nm. Owing to the permeable nature of the PDMS, the ethanol vapours diffuse inside the core of the waveguide and interact with the fluorescent dye Nile Red. When dissolved in ethanol, as explained before, Nile Red has a much higher peak wavelength of fluorescence, at almost 650 nm. This penetration of ethanol vapours causes a redshift to the fluorescence emission of the dye. After the 15 minute interval, introduction of nitrogen causes ethanol to evaporate out of the core and the intensity of fluorescence is found to decrease. This can be attributed to the re-initiation of the environment of the dye in PDMS without ethanol. The change in fluorescent intensity at 600 nm as shown in the plots in Fig 11 is due to the combined effect of slight increase of intensity and the gradual shift in wavelength from 590 to 610 nm due to the solvatochromic effect. The response time in the forward direction (upon introduction of ethanol vapours) was in the order of 10 s for both the PDMS types. The relapse of fluorescent intensity upon introduction of nitrogen was found to be different for OE-6520 and LS-6257. In the case of OE-6520, it took 3-5 minutes for the intensity change to be visible after the turning of the valve to send nitrogen instead of ethanol. For LS-6257, the reduction in intensity upon introduction of nitrogen was more rapid with a response time in the order of a minute. However, the intensity did not fall back to the pre-ethanol value immediately, but took another 10 more minutes. This variation of time-evolution of fluorescent intensity between the two types of PDMS is possibly due to their varying values of permeability. V. C ONCLUSION The concept of mixing of fluorescent dye into the core of a highly permeable PDMS waveguide enabled the development of a sensor with a relatively fast response time to analyte vapours owing to the larger effective sensing area. It also opens up the possibility for further integration of more optoelectronic components and the presence of the dye through the entire volume of the waveguide provides flexibility in terms of illumination, either through the waveguide or in orthogo- nal position, thus enabling a novel and robust platform for vapour-sensing. Waveguides were fabricated using a cost-effective process involving soft-lithography and mixing the fluorescent dye Nile Red with two different PDMS types, i.e. OE-6520 and LS-6257. The experiments conducted using these waveguides in an elaborate gas-sensing setup showed a considerable response to ethanol vapours in a time span of 10 s. The sensor concept enables us to integrate the complete system on a stretchable foil with the waveguides, LEDs and photodiodes although more quantitative gas-sensing results need to be obtained. The use of the spectrometer for the readout can be substituted with a combination of photodiode and a filter owing to a detected shift of about 20 nm. The platform provides an option of multi-analyte sensing by adding more than one fluorescent dye to different waveguides lying parallel to each other on a foil of PDMS. The results from this study provide solid guidance towards designing a sensor for ethanol vapours on a stretchable foil ...

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... I NTRODUCTION L AST electronic few noses, decades utilizing have witnessed an array of a chemical great interest sensors in with different specificities responding to the volatile organic compounds present in the gas analyte. The development was driven by the need for a swift and portable device capable of analyzing complex gases in a wide range of applications, including medicine, food-packaging, chemical analysis and defense systems [1]–[6]. For these purposes, various types of gas sensors have been reported, and owing to its inher- ent advantages there has been a lot of focus on optical sensing principles, based on either absorption, reflectance or fluorescence [7]–[13]. Fluorescent dyes in particular are found to have a strong advantage in terms of selectivity and sensitivity [2], [7], [14]. On the other hand, implementing the sensing principle using fiber-optics allows for an excellent light delivery, long interac- tion length, and the ability not only to excite but also to capture the emitted light [15], [16]. It has been reported that fiber- optic chemical sensors (FOCS) can detect traces of xylene and gasoline in temperature-stabilized environments of leakage- monitoring using the properties of fluorescent dyes [17]. Those in situ, continuous measurements of volatile organic compounds in groundwater and soil samples show the potential of fluorescence sensing in the field of artificial olfaction and gas-sensing [17]. Furthermore, a wide range of dyes have been reported as indicators in determining concentrations of gases like oxygen, carbon dioxide and other analytes [18]. Nile red, a solvatochromic dye, has been described as a promising candidate in the class of indicator molecules for empirical parameter determination of solvent polarity [19]. This photochemically stable dye has a very high partition coefficient from water to hydrophobic solvents [20]. It has been proven that Nile red has quantitative sensing capabilities in detecting a continuous stream of analyte vapours like butane-2-ol, hexane and methanol, and dimethylmethylphos- phonate [21] by incorporating the dye in polymer matrices. Furthermore, dissolved in a matrix of ethyl cellulose, this dye has been developed as a sensor film to determine the concentration of gaseous CO 2 and dissolved CO 2 (in water) by using an additive (a hydrophobic amidimine) which is capable of reversibly binding dissolved carbon dioxide [22]. This reversible binding is accompanied by a large change in hydrophilicity. Finally, Nile red can also be used to probe the amount of hydrophobic sites for a wide range of proteins in buffered aqueous media [20]. In this paper we present a gas sensor realized using stretchable optical waveguides doped with the dye Nile Red. Although a multitude of chemical sensors using fluorescent dyes in combination with fiber-optic bundles are available [16]–[18], integration into planar polymer optical waveguides provides several additional advantages. The presence of adjacent waveguides makes the design compatible for multianalyte sensing. Moreover, the use of stretchable technologies has enabled an emerging range of applications involving curved surfaces that are impossible to achieve using conventional rigid or flexible technologies, for example wear- able on-body sensing systems. The use of low-cost PDMS materials and replication strategy used in fabrication makes the sensor cost-effective in comparison with existing technologies. Integration of optical sources and detectors can be realized at a later stage in the same technology platform of stretchable optoelectronic systems, for example by embedding non- stretchable islands in a stretchable material and joining them with electrical interconnections, patterned as meander-shaped structures [23], [24]. II. S ENSING P RINCIPLE The shift in emission wavelength of the spectra of a fluorescent dye is utilized in the conceptualization of the sensor. Nile Red which naturally absorbs in the green and emits in the orange-red is found to have a wavelength shift in the presence of analyte vapours as shown in Fig. 1. It was found that the emission spectrum of Nile Red, with a lineshape as shown in Fig. 2, undergoes a shift in wavelength and also a slight increase in intensity upon variations in the environment. Nile Red is a fluorophore that has polar substituents on the aromatic rings, and hence are found to be sensitive to the chemical and physical environment of surrounding solvent molecules. Fig. 3 shows the fluorescence properties of Nile Red in different solvents. In the solvents analyzed here, the emission maxima reveal a shift over a span of 70 nm upon excitation with a green LED giving colors ranging from golden yellow to deep red. In the sensor described, Nile Red is mixed uniformly in different types of Polydimethylsiloxane (PDMS) polymers. This dye-mixed PDMS forms the core of a waveguide that is used in the sensing of the analyte. Nile Red inside the PDMS matrix has more of an orange colour and in the presence of ethanol turns into more deep red. Fig. 4 shows two glass plates coated with the dye-mixed PDMS, and the one to the right after immersion in ethanol for about 30 minutes. In the presence of an analyte gas (ethanol in our instance), the vapour molecules diffuse into the PDMS. PDMS is widely known for its permeability to gases and vapours due to a large free volume appearing between the macromolecular chains owing to the rotational freedom of the siloxane bond [25]. In an ethanol environment, Nile Red has a different emission wavelength and this change is found to be relatively rapid upon introduction of ethanol owing to the high permeability of PDMS. The change of emission wavelength is also found to be reversed back when ethanol vapours slowly evaporate out. PDMS mixed with the dye forms the core of the waveguide acting as the ethanol vapour sensor. As recently reported, similar waveguides in combination with light sources and photodiodes have been found to be able to be embedded in a flexible package resulting in a truly bendable, stretchable or mechanically deformable optical link [23], giving direction to a lot of research in developing a gas-sensor on flexible and stretchable ...