Elsevier

Sensors and Actuators B: Chemical

Volume 186, September 2013, Pages 711-717
Sensors and Actuators B: Chemical

Capillary array waveguide amplified fluorescence detector for mHealth

https://doi.org/10.1016/j.snb.2013.06.030Get rights and content

Highlights

  • We developed a simple low cost fluorescence detector for assays based on mobile phone or webcam for Mobile Health (mHealth).

  • The detector combines capillary array for optical signal amplification and LED excitation.

  • This combination enables an increase of 100 fold in fluorescence signal sensitivity to achieve a level of detection comparable to conventional fluorescence plate readers, but using 10X smaller sample volumes.

  • This approach has the potential to increase mHealth clinical utility, especially for telemedicine and for resource-poor settings in global health applications.

Abstract

Mobile health (mHealth) analytical technologies are potentially useful for carrying out modern medical diagnostics in resource-poor settings. Effective mHealth devices for underserved populations need to be simple, low cost, and portable. Although cell phone cameras have been used for biodetection, their sensitivity is a limiting factor because currently it is too low to be effective for many mHealth applications, which depend on detection of weak fluorescent signals.

To improve the sensitivity of portable phones, a capillary tube array was developed to amplify fluorescence signals using their waveguide properties. An array configured with 36 capillary tubes was demonstrated to have a ∼100× increase in sensitivity, lowering the limit of detection (LOD) of mobile phones from 1000 nM to 10 nM for fluorescein. To confirm that the amplification was due to waveguide behavior, we coated the external surfaces of the capillaries with silver. The silver coating interfered with the waveguide behavior and diminished the fluorescence signal, thereby proving that the waveguide behavior was the main mechanism for enhancing optical sensitivity.

The optical configuration described here is novel in several ways. First, the use of capillaries waveguide properties to improve detection of weak florescence signal is new. Second we describe here a three dimensional illumination system, while conventional angular laser waveguide illumination is spot (or line), which is functionally one-dimensional illumination, can illuminate only a single capillary or a single column (when a line generator is used) of capillaries and thus inherently limits the multiplexing capability of detection. The planar illumination demonstrated in this work enables illumination of a two dimensional capillary array (e.g., x columns and y rows of capillaries). In addition, the waveguide light propagation via the capillary wall provides a third dimension for illumination along the axis of the capillaries. Such an array can potentially be used for sensitive analysis of multiple fluorescent detection assays simultaneously.

The simple phone based capillary array approach presented in this paper is capable of amplifying weak fluorescent signals thereby improving the sensitivity of optical detectors based on mobile phones. This may allow sensitive biological assays to be measured with low sensitivity detectors and may make mHealth practical for many diagnostics applications, especially in resource-poor and global health settings.

Introduction

Mobile health (mHealth), which is defined as “mobile computing, medical sensor, and communications technologies for healthcare” [1] may enable the practice of medicine and public health using mobile devices. With this enhanced mobility, mHealth has the potential to provide access to medical diagnostics for underserved populations and in remote locations. Currently, most of the research in mHealth has been focused on exploiting the connectivity capabilities of mobile phones [2], [3], [4], [5], [6] and not on enhancing the analytical capabilities of mobile devices. At the same time, most analytical diagnostic technologies used today have been developed for laboratory settings in high-income countries, and in many cases are not affordable or compatible with the needs and conditions found in low and middle-income countries. The challenges and the need to develop simple, low-cost diagnostics for resource-poor settings with minimal medical infrastructure are well recognized [7], [8], [9] and mHealth analytical technologies can be potentially used to overcome the limitations of current medical diagnostics technologies by providing simple and affordable analytical diagnostic technologies that are compatible with mobile phones technologies.

Several detection technologies based on mobile phones have been developed for biodetection, including an integrated rapid-diagnostic-test reader platform for lateral flow immuno-chromatographic assays [10], capillary array based immunodetection for Escherichia coli [11], wide-field fluorescent microscopy [12], fluorescent imaging cytometry [13], lensfree microscopy [14], detection systems for melanoma or skin lesion [15], [16], [17], loop-mediated isothermal amplification (LAMP) genetic testing device [18], microchip ELISA-based detection of ovarian cancer HE4 biomarker in urine [19], surface acoustic wave enhanced immunoassay [20], a pocket-sized colorimetric reader [21], phone-assisted microarray decoding platform for signal-enhanced mutation detection [22], and mobile phone cameras for DNA detection [23]. However, all these technologies rely on the inherent sensitivity of the CMOS camera native to the mobile phone, which is less sensitive than detectors used for biodetection. The camera sensitivity, which is a limiting factor for detection, may be improved through additional hardware or image processing algorithms to improve the performance of these devices.

To improve sensitivity of low cost but high noise detectors (e.g., $10 webcams) for mHealth applications, a computational approach was developed based on an image stacking algorithm to remove the noise and enhance weak signals for fluorescent detection to improve sensitivity [24]. While the computational approach increases the sensitivity of fluorescent detection, our unpublished data suggests that this approach may not be suitable for many portable phones because the native camera hardware filters out weak signals in an effort to reduce image noise, which renders computational approaches for weak signal enhancement useless. Since the computational improvement of low signals to increase detection sensitivity was not practical, optical amplification of the signals is needed to increase the level of light intensity from a fluorescent sample to one that is high enough that the phone camera hardware will not reject it as noise.

Improved limit of detection can be achieved using capillaries, which enable both assay fluid handling and waveguide illumination [25]. While a capillary array has been used previously to performing immunoassay for detection with a mobile phone [11], the capillary array was not used for optical signal amplification of the assay. Instead, a high level of detection sensitivity (5–10 cfu mL−1 for E. coli) was obtained using quantum dots.

Capillaries have been used in several types of optical biosensors utilizing various excitation and emission modalities, optical path geometries and configurations shown in Fig. 1 (adapted from Ref. [26]) including: (1) vertical (to the long axis) at 90° angle spot excitation and detection of emitted light [27] (Fig. 1(1)), (2) vertical excitation (e.g., the entire length of the capillary) detected at one end [26], [28] (Fig. 1, Fig. 2), (3) horizontal (or angular) excitation through the end of the capillary with the excitation light propagation through the capillary (or planar) walls enabling evanescent excitation and the propagation of the emitted light along the capillary, utilizing grating to couple the light out of the waveguide to the detector [29], such configuration is also used by evanescent fiber optic biosensors [30] (Fig. 1, Fig. 2, Fig. 3). Horizontal excitation (in most cases with an angle) generating evanescent wave illumination and vertical detection is used mainly for planar (not capillary) detection [31], [32], [33], [34] (Fig. 1, Fig. 2, Fig. 3, Fig. 4).

However, most published capillary configurations described above are not suitable for mHealth because they require costly components such as laser illumination, photomultiplier tubes (PMTs) or cooled CCD detectors. These devices utilize complex optical configurations, have limited portability or are high cost, whereas effective mHealth devices should be simple, low cost and portable.

To enable the use of portable phones in mHealth applications involving highly sensitive fluorescent detection, we present an alternative optical enhancement approach which uses a standard portable phone with a CMOS imaging sensor combined with LED excitation in a very simple optical configuration: in-line horizontal excitation and horizontal detection. This optical configuration has been used effectively in previous work [24], [35], [36], [37], [38], [39], [40], [41], [42], however such configuration was used in planar (e.g., microtiter plate) mode and did not take advantage of the waveguide properties of capillaries for signal amplification.

In this work, we developed a new capillary waveguide optical configuration for multiplexed fluorescence detection. While the conventional utilization of evanescent waves for fluoresce detection requires coherent light in the form of lasers [26], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52] as the excitation source illuminated in a critical angle and a grating to couple the light out of the waveguide and into a detector [29], in the simple configuration described here the capillaries were illuminated by the multi-wave length LED light emitted horizontally to the capillary axis (Fig. 1, Fig. 2, Fig. 3, Fig. 4, Fig. 5). The light-wave energy propagating through the capillary walls can interact directly with and excite the fluorescent molecules via evanescent waves. The fluorophores emit light that is detected at the end of the capillary to provide higher detection sensitivity when compared to the same volume of sample being detected in a standard microtiter plate format. This is the basis for using the term amplification in this work: the measured fluorescent signal is increased substantially through the use of evanescent wave excitation. This simple capillary array platform is capable of measuring multiple florescent-detected assays simultaneously without the need of dedicated laboratories and complex equipment. The approach described here has the potential to form the basis for high sensitivity, low cost medical diagnostics in resource-poor settings for mHealth.

Section snippets

Materials and reagents

The 36-channel capillary arrays used for analysis were fabricated using glass capillaries (Drummond Scientific, Broomall, PA) held in a square array by black poly(methyl-methacrylate) (PMMA), also known as acrylic (Piedmont Plastic, Inc., Beltsville, MD). For bonding the black acrylic with the polycarbonate, 3 M 9770 adhesive transfer double sided tape was used (Piedmond Plastics Inc., Beltsville, MD). To block the waveguiding properties of the glass capillaries some were coated with high purity

Capillary array fluorescence detectors

The main challenge of using a mobile phone for low light fluorescence detection in mHealth applications is the low sensitivity of the phone camera. Wave guiding capillaries were used in this work for evanescent excitation to increase sensitivity.

Detector configuration: The basic configuration of the fluorescence capillary array detectors as shown schematically in Fig. 2(1) are: (A) a fluorescence excitation source, (B) excitation filter, (C) sample holder array, (D) emission filter, (E)

Conclusions

Many detection technologies based on mobile phones have been developed for biodetection [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]. However, the sensitivity of the mobile phone camera is a limiting factor: it is currently too low to be effective for many of the diagnostic applications of these devices. To enable the use of mobile phones for mHealth diagnostics involving highly sensitive optical detection, we developed an optical detection approach which

Acknowledgments

This work was supported by the FDA's Center for Devices and Radiological Health, Division of Biology, the National Cancer Institute and the NIH. The views expressed are those of the authors and do not represent those of the U.S. Government.

Joshua M. Balsam studied Mechanical Engineering at the University of Maryland (College Park, MD) and in 2009 received his BS in the field. In 2010 he began research in microfluidic biological sensors with Dr. Avraham Rasooly at the US Food and Drug Administration (FDA) in White Oak, MD. Currently he is an engineering PhD candidate at the University of Maryland under Dr. Hugh A. Bruck, as well as a research assistant at the FDA in the Office of Science and Engineering Laboratories (OSEL),

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    Joshua M. Balsam studied Mechanical Engineering at the University of Maryland (College Park, MD) and in 2009 received his BS in the field. In 2010 he began research in microfluidic biological sensors with Dr. Avraham Rasooly at the US Food and Drug Administration (FDA) in White Oak, MD. Currently he is an engineering PhD candidate at the University of Maryland under Dr. Hugh A. Bruck, as well as a research assistant at the FDA in the Office of Science and Engineering Laboratories (OSEL), Division of Biology (DB). His work involves the development of low cost optical point of care biosensors for global health and low resource settings.

    Prof. Hugh A. Bruck received BS and MS in Mechanical Engineering in 1988 and 1989, respectively, and his PhD in Materials Science from Caltech in 1995. After working at Idaho National Engineering Laboratories, he came to the Department of Mechanical of Engineering at the University of Maryland as an Assistant Professor in 1998 and is currently Professor and Associate Chair for Academic Affairs working on functional materials and nanoscale property characterization. His current research has led to the development of a new combinatorial technique for formulating polymer nanocomposites and highly filled polymers, new processing-structure–property models for multifunctional polymer nanocomposites, new label-free biosensors based on electrical percolation and enhanced chemiluminescence in bio-nanocomposites, and new multiscale characterization approaches for hierarchically structured polymer composites and biological materials. He has authored or co-authored 9 book chapters, 78 journal papers, and 81 conference papers through support from ONR, AFOSR, NSF, FDA, ARO, ARL, NAVAIR, and NAVSEA. His work during that time has been recognized with many honors and awards, including the Best Paper Award at the 2011 ASME Mechanisms & Robotics Conference, A.J. Durelli Award, ONR Young Investigator Program Award, and Fulbright Scholar Award. He currently serves on the International Advisory Board for the journal Experimental Mechanics, and was named a fellow of ASME in 2008.

    Dr. Avraham Rasooly joined the Food and Drug Administration (FDA) as a Microbiologist in 1995, where he developed microbial detection approaches for foodborne pathogens and their toxins using biosensors and DNA microarrays. In 2002, Dr. Rasooly joined the National Cancer Institute (NCI). He currently serves as a Special Assistant for Cancer Technologies and Translational Research at the NCI, overseeing research in technology related to cancer, and as a researcher at the FDA's Center for Devices and Radiological Health (CDRH) directing a research lab studying new technologies for rapid bio-detection including nanotechnology, biosensors and lab-on-a-chip.

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