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Abstract
The research reported herein integrates a generic holographic sensor platform and a smartphone-based colour quantification algorithm in order to standardise and improve the determination of the concentration of analytes of interest. The utility of this approach has been exemplified by analysing the replay colour of the captured image of a holographic pH sensor in near real-time. Personalised image encryption followed by a wavelet-based image compression method were applied to secure the image transfer across a bandwidth-limited network to the cloud. The decrypted and decompressed image was processed through four principal steps: Recognition of the hologram in the image with a complex background using a template-based approach, conversion of device-dependent RGB values to device-independent CIEXYZ values using a polynomial model of the camera and computation of the CIEL*a*b* values, use of the colour coordinates of the captured image to segment the image, select the appropriate colour descriptors and, ultimately, locate the region of interest (ROI), i.e. the hologram in this case, and finally, application of a machine learning-based algorithm to correlate the colour coordinates of the ROI to the analyte concentration. Integrating holographic sensors and the colour image processing algorithm potentially offers a cost-effective platform for the remote monitoring of analytes in real time in readily accessible body fluids by minimally trained individuals.
Highlights
Holographic sensors provide a real-time colour, alphanumeric or image response to the analyte of interest which is readable by the human eye [1,2,3,4,5,6,7]
The mechanism of colour change in holographic sensors is driven by the kinetics of volume changes in the smart hydrogel [37]
The colour of the smart pH hydrogel used in the work described in this paper reversibly changes from blue to green to red in response to changing proton concentrations
Summary
Holographic sensors provide a real-time colour, alphanumeric or image response to the analyte of interest which is readable by the human eye [1,2,3,4,5,6,7]. The visual inspection of a holographic sensor by an untrained operator is often adequate in providing only semi-quantitative categorical interpretation of analyte concentrations such as positive, negative, high or low. Improving the sensitivity and standardisation of colour quantification of the holographic sensors requires the use of measurement instruments, which in this report, is a cameraenabled mobile phone. Colour digital imaging is an active research area for quantification in colorimetric instruments. The scope of the work reported in the literature is diverse and covers a wide range of sensors and digital image-based colour analysers.
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