Abstract
Smartphone fluorescence microscopy has various applications in point-of-care (POC) testing and diagnostics, ranging from e.g., quantification of immunoassays, detection of microorganisms, to sensing of viruses. An important need in smartphone-based microscopy and sensing techniques is to improve the detection sensitivity to enable quantification of extremely low concentrations of target molecules. Here, we demonstrate a general strategy to enhance the detection sensitivity of a smartphone-based fluorescence microscope by using surface-enhanced fluorescence (SEF) created by a thin metal-film. In this plasmonic design, the samples are placed on a silver-coated glass slide with a thin spacer, and excited by a laser-diode from the backside through a glass hemisphere, generating surface plasmon polaritons. We optimized this mobile SEF system by tuning the metal-film thickness, spacer distance, excitation angle and polarization, and achieved ~10-fold enhancement in fluorescence intensity compared to a bare glass substrate, which enabled us to image single fluorescent particles as small as 50 nm in diameter and single quantum-dots. Furthermore, we quantified the detection limit of this platform by using DNA origami-based brightness standards, demonstrating that ~80 fluorophores per diffraction-limited spot can be readily detected by our mobile microscope, which opens up new opportunities for POC diagnostics and sensing applications in resource-limited-settings.
Highlights
Miniaturized optical reader devices have been essential for quantifying rapid diagnostic assays and bringing some of the conventional biomedical tests from the bench or laboratory settings to the point-of-care (POC)
surface-enhanced fluorescence (SEF) phenomenon on a planar thin metal film is strongly dependent on several variables such as the metal thickness (t), the spacer distance (d), the excitation angle (θ), the excitation polarization and wavelength
We systematically optimized the overall performance of our platform by monitoring the angular modulation factor (AMF) as well as the enhancement factor (EF) of fluorescence due to silver-coated substrates under ~470 nm excitation on a benchtop setup (Supplementary Fig. 1)
Summary
Qingshan Wei[1,2,3,4], Guillermo Acuna[5,6,7], Seungkyeum Kim[8], Carolin Vietz[5,6,7], Derek Tseng[1], Jongjae Chae 1, Daniel Shir[1], Wei Luo[1], Philip Tinnefeld5,6,7 & Aydogan Ozcan 1,2,3,9. We quantified the detection limit of this platform by using DNA origami-based brightness standards, demonstrating that ~80 fluorophores per diffraction-limited spot can be readily detected by our mobile microscope, which opens up new opportunities for POC diagnostics and sensing applications in resource-limited-settings. We prepared a handheld SEF microscopy device installed on a smartphone, which provided a maximum fluorescence enhancement of approximately 10-fold compared to a bare glass substrate, allowing imaging of individual 50 nm fluorescent particles as well as individual QDs. the limit of detection was estimated to be around 80 fluorophores per diffraction-limited spot by using fluorescently labeled DNA origami structures as microscopy brightness standards. The limit of detection was estimated to be around 80 fluorophores per diffraction-limited spot by using fluorescently labeled DNA origami structures as microscopy brightness standards We believe that this field-portable mobile phone based SEF microscopy platform opens up various new prospects for POC sensing and molecular diagnostics in resource scarce environments
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