The in-line monitoring of ethanol concentration in liquids is a crucial part of process monitoring in breweries and distilleries. Current methods are based on infrared spectroscopy, which is time-consuming and costly, making these methods unaffordable for small and middle-sized companies. To overcome these problems, we presented a small, compact, and cost-effective sensing method for the ethanol content, based on a nanostructured, plasmonically active sensor substrate. The sensor substrate is coated with an ethanol-sensitive hydrogel, based on polyacrylamide and bisacrylamide, which induces a change in the refractive index of the substrate surface. The swelling and shrinking of such hydrogels offer a means to measure the ethanol content in liquids, which can be determined in a simple transmittance setup. In our study, we demonstrated the capability of the sensor principle for the detection of ethanol content ranging from 0 to 30 vol% ethanol. Furthermore, we determined the response time of the sensor substrate to be 5.2 min, which shows an improvement by a factor of four compared to other hydrogel-based sensing methods. Finally, initial results for the sensor’s lifetime are presented.
Abstract. The continuous monitoring of industrial and environmental processes is becoming an increasingly important aspect with both economic and societal impact. So far, spectral-optical sensors with their outstanding properties in terms of sensitivity and reliability have not been considered as a potential solution because of the cost-intensive and bulky readout hardware. Here we present a card-size, inexpensive, and robust readout platform based on a wavelength-sensitive photodiode. In test and characterization experiments we achieved a wavelength shift resolution of better than 0.1 nm and a detection limit of 0.001 AU for ratiometric measurements. We furthermore discuss the capability and current limitations of our readout unit in context with interrogation experiments we performed with a photonic crystal-based fluid sensor. In sum we expect the presented readout platform to foster the exploitation of spectral-optical sensor technology for gas monitoring, chemical analytics, biosensing and many others fields.
Abstract. The inline monitoring of parameters in aqueous liquids is facing an increasing demand in many different application areas. Hydrogels with pH-induced swelling and deswelling behavior offer a means to measure pH in such liquids. Here we investigate the optical interrogation of a pH-sensitive hydrogel which can be applied in the physiological pH range. For this, a nanostructured gold substrate supporting surface plasmon oscillations is coated with a HPMA/DMAEMA/TEGDMA/EG hydrogel. The gel swells in the pH range under investigation (here 4.5 to 6.5), and the resulting refractive index changes subsequently lead to a spectral shift of the plasmon resonance of the gold nanostructure. The spectral resonance position is determined from optical transmittance spectra of the sensor substrates, and the initial results for our hydrogel reported here indicate a nearly linear dependence between the swelling state and the plasmon resonance wavelength.
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