Relevance. The need for sensitive determination of the content of inorganic thiocyanate in water and water-oil emulsions for solving hydrological problems and tracer studies of oil fields. Aim. The search for an environmentally friendly and sustainable non-laboratory method, the determination of thiocyanate and the minimization of sample preparation, while increasing the specificity of its detection, and the study of a new method for detecting a digital signal by changing the color of the sensor. Methods. Digital colorimetry, solid-phase spectrophotometry, sensor detection. Results and conclusions. The authors have developed a new colorimetric sensor based on gold nanoparticles immobilized in a polymethylmethacrylate matrix to detect thiocyanate in the reservoir waters of oil fields. Obtaining Au nanoparticles in the solid polymer phase by reducing metal salts is environmentally sustainable and corresponds to the concept of green chemistry. The requirements for the gold content in polymethylmethacrylate matrix at the level of 10^–6 wt % removes the cost restrictions for colorimetric optical sensors. The method combines solid-phase extraction and digital colorimetry, which minimizes sample preparation and increases the sensitivity of the analysis. Optimal extraction and detection conditions were established using single-factor and multifactorial analysis. The sensor demonstrates a linear response in the range of thiocyanate concentrations 0.3–25.0 mg/l, with a detection limit of 0.1 mg/l and high selectivity. The method is resistant to interfering impacts and remains stable for 8 months. The use of a polymer matrix makes it possible to visualize and quantify the color change associated with the presence of thiocyanate. The method was successfully tested on real samples of reservoir water with good accuracy and reproducibility. The development opens up prospects for the creation of portable express monitoring systems.