The detection of ammonia over a wide range from parts per millions (PPM) to 1000’s ppm in a single sensor is of great importance for industrial applications. We have been exploring Vapochromic Coordination Polymers (VCP) specifically Zn[Au(CN)2]2, that was developed to achieve fluorescence when exposed to NH3. Upon high concentration ammonia exposure, the fluorescent peak under near-UV stimulation undergoes a spectral shift from 470nm to 530nm, while the intensity increases by 3~4X. However, at ammonia concentrations < 100 ppm, there is almost no peak wavelength spectral shift or intensity change and only subtle fluorescent spectrum alterations. Using a 405nm laser diode excitation source provides a narrow (4nm) stimulation easily separated from the emission peak. The emission is focused on a USB portable spectrometer (430 to 650 nm). First we create a method that gives unique values over the range <1000 ppm by dividing the spectrum into 20 nm bins, and integrate the emission in each bin, relative to that of 0 ppm exposure (Sum of Integrated Emissions). The key point in this analysis is to note that the way the spectrum changes in each wavelength bin varies at different ammonia ppm exposures. SIE gives excellent sensitivity between 0-50 ppm and <300 ppm, but the 100-300 ppm region has low accuracy. There we change the metric to the Spectral Region Subtraction (SRS) by separating the spectrum into (A) 430-516 nm and (B) from 516 -650 nm, integrate the spectrum and subtract A from B, giving a rapid change within 100-300 ppm.
The detection of ammonia in parts per millions range has been challenging in sensors research, and is of great importance for industrial applications. In previous literature, Vapochromic Coordination Polymers (VCP) were developed to achieve luminescence upon a targeted gas exposures. We investigate a specific VCP, Zn[Au(CN)2]2,as an ammonia sensing material. Upon high concentration ammonia exposure, the fluorescent peak under near-UV stimulation undergoes a spectral shift from 460nm to 520nm, while the intensity increases by 3~4X. However, at ammonia concentrations < 50ppm, the spectral shift becomes hidden within the overall changing fluorescent spectrum shape. Then simple methods, such as detecting the peak wavelength or subtracting post-exposure from pre-exposure spectrums do not work. We developed further excitation and data processing techniques to detect ammonia at lower concentrations. A low-cost 405nm blue-ray DVD laser diode was used as the excitation source, providing a narrow band-width (4nm) stimulation that is separated from the emission peak. We measured the emission using a portable spectrometer (Photon Control SPM-002), and processed the data by separating the spectrum into two regions; (A) from 425 nm to 460 nm and (B) from 460nm to 500nm. Next, the integrated emissions under both regions were computed, and the value of shorter wavelength region (A) was subtracted from the longer wavelength one (B). When exposed to ammonia, region (A) reduces overall intensity while region (B) increases, resulting a signal starting from negative value and gradually increases to positive values, enabling the detection of 5ppm ammonia in less than 30 seconds gas exposure.