Printable Colorimetric Sensors for the Detection of Formaldehyde in Ambient Air

Introduction With an annual production of 21 million tons [1], formaldehyde (CH2O) is probably one of the most produced chemicals worldwide. It serves as starting material for many other industrially produced chemicals, but also the demand of its pure form is steadily rising. In nature, CH2O originates i.a. endogenously as intermediate of metabolism in most living systems [2], however chronic exposure poses a high risk to humans' health. In June 2014, the European Union reclassified the toxicological properties of CH2O in its regulation 605/2014, as carcinogen (category 1B) and mutagen (category 2) [3]. Despite its high toxicity, but due to its disinfecting, germicidal and preserving properties, the usage of CH2O is common in the production of many everyday objects like for example adhesives, furniture, coatings, cosmetics and textile finishing. Thus, CH2O is not only a risk to workers of processing industries, but it is also becoming important for the assessment of indoor air quality, increasingly. Within the scope of our research, we present a simple method to monitor CH2O in ambient air based on a visible color change of a disposable sensor, which can be evaluated using the camera of a smartphone or even naked eye. Figure 1a illustrates the sensor principle. The sensor consists of a paper/plastic substrate on which the gas sensitive material is deposited by printing process. The integration of the gas sensitive layer into a defined QR code pattern realizes quantitative as well as an illumination and camera independent evaluation of the color change due to the contact with the target gas. To protect the gas sensitive material and the color reference against environmental influences, the sensor is packed gas-tight. Focus of this work is on the development of printable pastes based on the reaction of a primary amine with CH2O through nucleophilic addition, which takes place under change of basicity. One method, to track this chemical reaction with conventional pH indicators, and the adaption to the later scope by the choice of pH indicator is discussed. Figure 1b shows the reaction scheme by the example of the pH indicator bromocresol green. Method Starting with a white poly(p-phenylene oxide) (PPE) coated matte paper, pastes were developed for the deposition of the gas sensitive material by screen-printing. The color reference was printed by UV offset in advance. For the preparation of printable pastes, the pH indicators and the primary amine were dissolved in ethanol and embedded into an ethyl cellulose matrix (ethoxyl content 48%, ALDRICH Chemistry, USA) using tributyl phosphate (≥99.0%, SIGMA-ALDRICH, USA) as plasticizer. Furthermore, aids to support the printing process, the layer and the color formation were investigated. Two primary amines in combination with eight pH indicators (bromophenol blue, bromocresol green, methyl red, bromophenol red, bromocresol purple, bromothymol blue, phenol red and phenolphthalein) with transition ranges pH 3.0-12.0 were examined. The color change was characterized by UV/Vis spectroscopy (Lambda900, Perkin-Elmer, USA) with a setup for diffuse reflection (Praying Mantis, Harrick Scientific Products Inc., USA) as well as the by the evaluation of RGB values taken with an in-situ readout station using the camera of an iPhone 6s. General description of the used gas measurement station is given in [4]. The printed sensors were characterized to CH2O concentrations between 100 ppb to 10 ppm. Furthermore, cross-sensitivities to humidity and other interfering gases with focus on ammonia were examined. Results and Conclusions Our research evaluates and compares eight pH indicators, to monitor CH2O by the reaction with a primary amine, regarding sensitivity and selectivity. Figure 1c shows the reflection spectra of a printed gas sensitive layer to 4 ppm CH2O by the example of the pH indicator bromocresol purple. The measurement consists of 15 spectra recorded in intervals of two minutes. CH2O was exposed for 20 minutes, before and afterwards the gas measurement chamber was flushed with synthetic air for five minutes, respectively. After exposure to the target gas, a complete color change from blue to yellow occurred (see Figure 1d). Based on this evaluation and adaption of the printing parameters, the integration time of the sensors can be adjusted to the later target range e.g. to a working day of eight hours or a short-term measurement of 15 minutes and the gas concentration range