Orthographic projection capillary array fluorescent sensor for mHealth

Abstract

To overcome the limited sensitivity of phone cameras for mobile health (mHealth) fluorescent detection, we have previously developed a capillary array which enables a ∼100 × increase in detection sensitivity. However, for an effective detection platform, the optical configuration must allow for uniform measurement sensitivity between channels when using such a capillary array sensor. This is a challenge due to the parallax inherent in imaging long parallel capillary tubes with typical lens configurations. To enable effective detection, we have developed an orthographic projection system in this work which forms parallel light projection images from the capillaries using an object-space telecentric lens configuration. This optical configuration results in a significantly higher degree of uniformity in measurement between channels, as well as a significantly reduced focal distance, which enables a more compact sensor. A plano-convex lens (f=150 mm) was shown to produce a uniform orthographic projection when properly combined with the phone camera's built in lens (f=4mm), enabling measurements of long capillaries (125 mm) to be made from a distance of 160 mm. The number of parallel measurements which can be made is determined by the size of the secondary lens. Based on these results, a more compact configuration with shorter 32 mm capillaries and a plano-convex lens with a shorter focal length (f=10mm) was constructed. This optical system was used to measure serial dilutions of fluorescein with a limit of detection (LOD) of 10nM, similar to the LOD of a commercial plate reader. However, many plate readers based on standard 96 well plate requires sample volumes of 100 μl for measurement, while the capillary array requires a sample volume of less than 10 μl. This optical configuration allows for a device to make use of the ∼100 × increase in fluorescent detection sensitivity produced by capillary amplification while maintaining a compact size and capability to analyze extremely small sample volumes. Such a device based on a phone or other optical mHealth technology will have the sensitivity of a conventional plate reader but have greater mHealth clinical utility, especially for telemedicine and for resource-poor settings and global health applications.

Keywords: Camera; Capillary; Fluorescence; Mobile and smart phone; Orthographic projection; Telecentric lens; mHealth.

Figure 1. The effect of orthographic projection configuration on the imaging of capillary array

To demonstrate the effect of orthographic projection optics, a sixteen tubes capillary array (12.5 cm in length) imaged with a typical cell phone camera through a plano-convex lens (4.5 cm diameter, 15 cm focal length). (A) A schematic of the object-space telecentric optics with (i) the plano-convex secondary lens and the principal rays imaged by the lens of (ii) the phone camera which focuses the image onto (iii) the camera CMOS image sensor. (B) A schematic of the capillary array. The capillary array was imaged at three different camera-to-lens distances: C = 14.5 cm, D = 30cm and E = 5 cm. The object-space telecentric condition is satisfied at (C) the ideal camera-to-lens distance so that all capillaries of the array appear parallel. (D) Too far of a camera-to-lens distance, and (E) position at too close of a camera-to-lens distance. Configurations D and E both result in uneven images of the capillary array with the center capillaries appearing brighter than those on the edges during fluorescence measurement.

Figure 2. Orthographic projection fluorescent sensor for mHealth

(A) A schematic of the orthographic projection fluorescent sensor and (B) a photograph of the device. Constituent parts are: (i) camera phone, (ii) emission filter, (iii) secondary lens, (iv) alignment fixture, (v) capillary tube array, (vi) two spaced excitation filters, and (vii) multi-wavelength LED light box. In (A), d_f is the distance between the capillaries and the camera lens.

Figure 3. The light distribution using orthographic projection optics

(A) A 3D visualization of the planar illumination source is shown as seen with the single lens system. (B) The light field seen by the single lens system when looking at an array of 16 capillary tubes, with (C) corresponding 3D visualization. (D, E, F) Corresponding results for the telecentric lens system are also shown, demonstrating a significantly more uniform illumination field.

Figure 4. The sensitivity of Orthographic Projection Fluorescent Sensor compared to the sensitivity of plate reader

A twelve capillary array was loaded with fluorescein. (A) A single fluorescence image from camera phone is shown with (B) corresponding 3D intensity plot. The six concentrations of fluorescein were measured in duplicate pairs: capillaries 1 and 2, 100 nM; capillaries 3 and 4, 10 nM; capillaries 5 and 6, 1 nM; capillaries 7 and 8, 0.1 nM, capillaries 9 and 10, 0.01 nM, and capillaries 11 and 12, 0nM (water). (C) Results for the camera phone are plotted as open circles, along with results from a commercial fluorescence plate reader plotted as open squares. Limit of detection for both systems were 10nM for the concentrations measured (LOD marked as a horizontal black line for both sets of data).