Development of portable and cost-effective optical imaging devices is essential for quantification of molecular bioassays at the point of care (POC). Mobile phone-based microscopy tools have shown great potential as biomedical reader platforms due to their small size, large distribution volume, and constantly improving optical properties. However, the optical detection sensitivity remains a challenge to further improve the diagnostic capabilities of microscopy and sensing devices based on mobile-phones. Here, we demonstrate a simple strategy to enhance the signal intensity of a smartphone fluorescence microscope by approximately an order of magnitude using surface-enhanced fluorescence (SEF) created by a thin silver film. This plasmonics-enhanced smartphone microscopy platform relies on an opto-mechanical attachment based on the Kretschmann configuration, where the sample is placed on a silver-coated glass coverslip and excited by a laser diode from the backside through a glass hemisphere. The fluorescence enhancement effect was systematically optimized by tuning the metal film thickness, spacer distance, excitation angle, and polarization, and experimentally validated by comparison to theoretical simulations. With this mobile device, single fluorescent beads as small as 50 nm and individual quantum dots (ca. 20 nm dia.) were detected. We further quantified the sensitivity limit of this mobile platform to be around 80 fluorophores per diffraction-limited spot by imaging DNA origami based brightness standards labeled with different numbers of fluorophores. We believe that this SEF-based mobile microscopy platform opens up various new opportunities for POC diagnostics and sensing applications in resource-limited-settings.
The dynamic concentration range is one of the major limitations of single-molecule fluorescence techniques. We show how bottom-up nanoantennas enhance the fluorescence intensity in a reduced hotspot, ready for biological applications. We use self-assembled DNA origami structures as a breadboard where gold nanoparticle (NP) dimers are positioned with nanometer precision. A maximum of almost 100-fold intensity enhancement is obtained using 100-nm gold NPs within a gap of 23 nm between the particles. The results obtained are in good agreement with numerical simulations. Due to the intensity enhancement introduced by the nanoantenna, we are able to perform single-molecule measurements at concentrations as high as 500 nM, which represents an increment of 2 orders of magnitude compared to conventional measurements. The combination of metallic NPs with DNA origami structures with docking points for biological assays paves the way for the development of bottom-up inexpensive enhancement chambers for single-molecule measurements at high concentrations where processes like DNA sequencing occur.
The dynamic concentration range is one of the major limitations of single-molecule fluorescence
techniques. Here, we show how bottom-up nano-antennas enhance the fluorescence intensity in a
reduced hot-spot, ready for biological applications. We use self-assembled DNA origami structures as a
breadboard where gold nanoparticle dimers are positioned with nanometer precision. A maximum of
almost 100fold intensity enhancement is obtained using 100 nm gold nanoparticles within a gap of 23
nm between the particles. The results obtained are in good agreement with numerical simulations. Due
to the intensity enhancement introduced by the nano-antenna, we are able to perform single molecule
measurements at concentrations as high as 500 nM which represents an increment of 2 orders of
magnitude compared to conventional measurements. The combination of metallic nanoparticles with
DNA origami structures with docking points for biological assays paves the way for the development of
bottom-up inexpensive enhancement chambers for single molecule measurements at high
concentrations where processes like DNA sequencing occur.
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