###### Abstract  
Contamination of trace levels of volatile organic compounds (VOCs) in enclosed spaces is not usually a significant cause for concern; however, it can be relevant in the case of canine scent detection training as a canine's superior sense of smell makes them highly likely to detect low levels of contamination, contributing to inefficient training. Thus, herein, we address the need for a simple, low-cost, robust, vaporochromic sensor to determine the cross-contamination of VOCs within closed containers, such as canine training aid kits. This study focuses on the development of a vapor sensor, which produces a rapid colorimetric change when a target chemical vapor is present. A pH indicator is used as the colorimetric dye and its incorporation into a sol-gel matrix on a paper substrate is confirmed via SEM characterization. The sensor's stability and performance is tested against exposure to various levels of sunlight and temperature. The design allows the sensor to present a clear and unambiguous visible response to the release of the volatile target within a closed container. It can be readily incorporated into existing training kits and functions as a straightforward reminder of when training aids need to be changed or a new containment system should be considered. vapochromic sensor; vapor cross-contamination; headspace pH; colorimetric sensor; sol-gel +
Footnote †: journal: _Analytica_  
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coating infused with a colorimetric dye to detect potential vapor cross-contamination in closed containers. The development of chemical vapor sensors--which can produce a fluorescent, electric, or colorimetric response based on chemical stimuli originating from their surrounding environment--has previously been of particular interest in the detection of toxic vapors [9]. However, many of such sensors can be costly and/or involve further complicated steps for final analysis [1, 10, 11, 12]. Colorimetric sensors have a unique advantage of inducing a response that can be easily visible to the human eye [13]. In most cases, they are also cost-effective and versatile. The proposed sensor utilizes a paper substrate because it is robust, widely available, of low-cost, and easy to use and dispose. Its vapochromic capabilities are derived from chemresponsive compounds, such as pH indicator dyes. The pH indicator dyes are compounds that change color due to the acidity or basicity of their surroundings [11, 12]. Similarly, vapochromism is a change in color when a substance is exposed to different vapors [14]. The ensuing color changes following solvent or vapor exposure are to be conveniently observed by the naked eye. Previous work on vapor sensors using similar colorimetric compounds has noted a lack of response when exposed to the target vapor [15, 16] and difficulty in immobilizing the indicator to the substrate. To address this shortcoming, in the research herein, the colorimetric indicator was incorporated into a polymer matrix created via sol-gel synthesis which increases its vapochromic response [15]. Sol-gel synthesis has the additional advantage of being physically robust with high thermal, solvent, and chemical stability [17]. Sol-gel synthesis involves the formation of a three-dimensional cross-linked structure starting from a colloid solution known as the \"sol\", which is subsequently gelled (\"gel\") through a cascade of catalytic reactions at room temperature [18]. The resulting polymer matrix is then treated to remove any leftover solvents. The colorimetric indicator used should remain contained within the sol-gel layer due to its the porous nature. In previous work during 2015, researchers used a color-changing reagent to produce a colorimetric sol-gel-based sensor for the detection of 3,4-Methylenedioxymethamphetamine (MDMA) [19].