Multimodal Imaging and Lighting Bias Correction for Improved μPAD-based Water Quality Monitoring via Smartphones

Katherine E Mccracken,Kelly A Reynolds,Scott V Angus,Jeong-Yeol Yoon

doi:10.1038/srep27529

Katherine E Mccracken, Kelly A Reynolds + Show 2 more

Open Access

https://doi.org/10.1038/srep27529

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Abstract

Smartphone image-based sensing of microfluidic paper analytical devices (μPADs) offers low-cost and mobile evaluation of water quality. However, consistent quantification is a challenge due to variable environmental, paper, and lighting conditions, especially across large multi-target μPADs. Compensations must be made for variations between images to achieve reproducible results without a separate lighting enclosure. We thus developed a simple method using triple-reference point normalization and a fast-Fourier transform (FFT)-based pre-processing scheme to quantify consistent reflected light intensity signals under variable lighting and channel conditions. This technique was evaluated using various light sources, lighting angles, imaging backgrounds, and imaging heights. Further testing evaluated its handle of absorbance, quenching, and relative scattering intensity measurements from assays detecting four water contaminants – Cr(VI), total chlorine, caffeine, and E. coli K12 – at similar wavelengths using the green channel of RGB images. Between assays, this algorithm reduced error from μPAD surface inconsistencies and cross-image lighting gradients. Although the algorithm could not completely remove the anomalies arising from point shadows within channels or some non-uniform background reflections, it still afforded order-of-magnitude quantification and stable assay specificity under these conditions, offering one route toward improving smartphone quantification of μPAD assays for in-field water quality monitoring.

Highlights

IntroductionMicrofluidic paper analytical devices (μPADs; essentially microfluidic devices fabricated on paper substrates) have emerged separately as a favorably portable and low-cost means for detecting customizable classes of waterborne contaminants[17]
As a sensing platform, microfluidic paper analytical devices have emerged separately as a favorably portable and low-cost means for detecting customizable classes of waterborne contaminants[17]
The auto-exposure and auto-focus were locked on the central paper channel surface at 90° for Cr(VI), total chlorine (TC), and caffeine assays, and at 65° for E. coli K12 assays – the optimum scattering detection angle determined from previous work[30] (Fig. 1d)

Summary

Introduction

Microfluidic paper analytical devices (μPADs; essentially microfluidic devices fabricated on paper substrates) have emerged separately as a favorably portable and low-cost means for detecting customizable classes of waterborne contaminants[17]. The benefits and limitations of this technique were assessed by its performance in handling changes in the lighting field (lighting angle φ;lighting source/quality) and in the imaging field (height of light source; background absorbance/reflectance characteristics) when comparing an array of channels under identical assay conditions Further assessments of this technique were made by comparing the limits of detection and distinction between signals for multiple colorimetric- and fluorescence-based assays (detected at 90° from the channel surface) and a Mie scattering-based assay (detected at 65°) under varying lighting conditions (Fig. 1d). Each assay – evaluating the interactions between diphenylcarbazide (DPC) and Cr(VI), N,N-diethyl-p-phenylenediamine (DPD) and total chlorine (TC), 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS) and caffeine, as well as IgG-microparticle immunoagglutination with Escherichia coli K12 – has been previously demonstrated by laboratory-based spectroscopic quantification using precisely controlled lighting[27,28,29,30] These targets allowed comparison between our smartphone algorithm and established levels of detection

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Conclusion