###### Abstract  
Iron (Fe) is a micronutrient that can be toxic at elevated concentrations, prompting its significance in frequent environmental monitoring. Typically analyzed using methods such as FAAS, ICP-OES and ICP-MS, the challenge of expensive instrumentation operated only in the laboratory presents a barrier for rapid and frequent testing. This study aimed to develop a silica-based smartphone-assisted on-site method for rapid detection of Fe in water using ImageJ software. Suitable conditions, including reagents and a color intensity measurement tool, were optimized for this method. Figures of merit such as detection limit, accuracy and precision were determined. The results showed that adding polyacrylic acid to detection points for silica worsened the results, in contrast to results for paper devices. It was also found that, on ImageJ, it is best to use an integrated density tool to measure color intensity, contrary to the previously reported mean gray tool. Results showed a limit of detection of 0.2 ng, a limit of quantification of 0.6 ng, a linear range of 0.6 ng to 4.5 ng and RSD of <20%. This method is therefore an alternative in field pre-testing and screening. Future studies include application of this method in the field with real samples and in the analysis of other metals. iron detection; rapid testing; toxic metals; ImageJ +
Footnote †: journal: Medical  
and sample processing further drive up the costs of testing with these instruments [13]. The turnaround time for using such instruments may range from a couple of weeks to a few months, depending on how far samples have to be transported [9; 14; 15; 16; 17]. However, in environmental monitoring, there is a need for high frequency testing to capture what is happening in or near real time [14; 17]. This means there is a need to increase the rapidity of testing. The current methods of analysis can also be expensive to operate, as significant amounts of reagents and gases are used and highly skilled personnel are required to operate. These challenges increase the turnaround time of using laboratory instruments and have significant implications on accessibility, especially for developing countries. In recent years, there has been a growing interest in improving environmental monitoring through lab-on-a-chip and point-of-need devices [9; 14; 15; 16; 17; 18; 19]. Such devices have been available since the 1990s and researchers have been advancing on their construction and development [20]. Point-of-need devices can use a micro-liter (or even lower) level of a solution of analyte and reagents [12; 21; 22]. They can be rapid in detection and allow for analysis to be carried out at locations outside the laboratory [23; 24]. Most have a designated point of introducing a solution, a pre-treatment zone and a detection zone [23; 25]. These zones are usually connected by small channels that have the attributes of driving solutions without the need for an external driver such as gas (e.g., capillary movement in paper devices) [25]. Some of the substrates commonly used in the construction of point-of-need devices include paper, carbon, alumina and silica [26; 27; 28]. The method of detection may be electrochemical, chemiluminescence or colorimetric, among many others [26; 29]. Iron (Fe) is ubiquitous, naturally occurring and the fourth most common element in the earth's crust [30]. It is also one of the most significant elements for its effect on human physiology and the environment [31; 32]. Occupational exposure of Fe has been found to be carcinogenic in Fe and steel founding and categorized in Group 1 (cancer causing substances) of the International Agency for Research on Cancer (IARC) [33]. It has been reported to specifically affect the human lungs [34]. As a micronutrient, Fe is the major component in the hemoglobin molecule that helps to transport oxygen in the body [35]. Fe at high concentrations in the environment has been reported to pollute underground water sources [36].