Optical and Fluorescent Sensors

Optical and fluorescence based sensors have a broad range of applications such as in R&D, in medical diagnostics, and for environmental monitoring. Challenging is the design and optimization of an optical sensor system for a particular application, which needs a detailed knowledge in material physics, optics, and simulations.

Our research is focused on the development of optical and fluorescence sensor-platforms for biomedical and environmental purposes based on high sensitive fluorescence detection and thermal-lens spectrometry for quantification of microorganisms/water pathogens and trace substances. All aspects for the development of an optical sensor platform are covered by the group, starting by the optimal detection technology and sensor configuration, data signal processing, and the system integration together with fluidics and microfluidics. Each step in the development of optimized optical sensor systems is supported by analytical methods and high performance numerical simulations.

Developed sensors 

Oil sensor, Optical sensor for detecting oil films on the surface of the water 

Oil spills in wastewater and surface waters pose a significant risk to ecosystems and drinking water production. In particular, diffuse inputs from traffic areas, tunnels or parking lots are difficult to locate, which is why early detection is of particular importance. The aim was to develop a compact, cost-effective and contactless (optical) sensor system for continuous real-time monitoring of oilpollution, whichcan be used as an early warning system. 

Laboratory tests show reliable detection of both mineral and vegetable oils. For mineral oils such as petrol, diesel and motor oil, a detection limit of less than 0.01 μl/cm² water surfacewasreached. The sensor system delivers stable results even at varying ambient temperatures and moving water surfacesand can be implemented in a compact, waterproof housing. It thus represents a practicalsolutionfor continuous watermonitoring in the sense of the EU Water Framework Directive. 

Sensor for online measurement of nitrate and nitrite 

Nitrate and nitrite pollution in surfaces and groundwater pose a considerable environmental and health risk, especially in regions with intensive agricultural use. Excess nitrogen from fertilizers can enter water bodies and exceed the limits set in the European Drinking Water Directive. Conventional monitoring methods rely on manual sampling and laboratory-based analysis, which is time-consuming and prevents high temporal resolution of the data. In this thesis, an automated, cost-effective system for the in-situ measurement of nitrate and nitrite concentrations in water bodies is presented. 

Experimental results show reliable nitrate detection in the range of 0–50 mg/l with a detection limit (LOD) of 2.003 mg/l as well as nitrite detection with an LOD of 0.611 mg/l (measuring range 0–10 mg/l) and 0.013 mg/l (measuring range 0–1 mg/l). The measurement accuracy achieved meetsthe requirements of the EuropeanDrinking Water Directive andpredestines the system forlong-term autonomous water qualitymonitoring. 

Reference: 

M. Brandl and K. Kellner, "Automatic Measurement System for Nitrite and Nitrate in Water Bodies," in IEEE Sensors Journal, vol. 22, no. 14, pp. 14531-14539, 15 July15, 2022, doi: 10.1109/JSEN.2022.3182785   

Optical sensor for dissolved organic substances (DOM) and fluorescence indices 

Dissolved organic matter (DOM) plays a central role in biogeochemical processes of aquatic ecosystems and influences the carbon cycle, microbial activity and water quality, among other things. Optical fluorescence indices are often used to characterize the origin and freshness of DOM; however, their determination is usually carried out with stationary laboratory fluorometers, which limits flexibility and temporal resolution. In this thesis, a compact and portable sensor system for the rapid on-site determination of the fluorescence index (FIX) and the biological index (BIX) in water samples is presented. 

The developed instrument uses UVLED excitation in combination with spectrally selective optical filters and photomultiplicative detection to detect the fluorescence emissions required for FIX and BIX calculation. To suppress interference from ambient light and to improve signal stability, the light sources are modulated and the detector signal is evaluated by means of lock-in amplification. The water samples are measured directly in a cuvette and enable fast and user-friendly application under field conditions. 

The sensor system was characterized using fluorescent reference standards and validated with native water samples from various flowing waters. A comparison with a stationary laboratory fluorescence spectrometer shows a good agreement of the measurement results, with the typical deviations for both indices being less than ±10%. Due to the lightweight, battery-powered design and the low system costs, the presented device isparticularly suitable forfield use. The sensor system is thus an effective tool for the in-situ characterization of DOM and supports the high-resolution monitoring of temporal and spatial changes in water quality. 

Reference:  

M. Brandl, T. Posnicek, R. Preuer and G. Weigelhofer, "A Portable Sensor System for Measurement of Fluorescence Indices of Water Samples," in IEEE Sensors Journal, vol. 20, no. 16, pp. 9132-9139, 15 Aug.15, 2020, doi: 10.1109/JSEN.2020.2988588 

Contact:  Martin Brandl