Here we describe a lens free on-sensor imaging platform which is able to perform a high spatial resolution imaging over the large field of view. By measuring the geometrical shadow of the microscopic objects at different oblique illumination angles, we were able to measure the height of these objects. In addition, we were able to record bacterial bioluminescence signals directly on the CMOS chip. This is, to our knowledge, the first report of lens-less, on-sensor bioluminescence measurements.
Lens-based imaging approaches are faced with a trade-off between resolution and field-of-view (FOV). Generally, the greater the resolvable detail in a sample, the smaller the FOV we can observe. In our lab, we study the behaviour of microorganisms within confined spaces using microfluidic devices. In order to capture the full scope of their behaviour, we need to be able to discern individual microorganisms as well as observe the full microfluidic device area in real time. As such, visualizing such systems can be challenging, since we require an imaging system that can provide a resolution as high as 1 um, with a FOV large enough to fit our region of interest. To that end, we used the Nokia Lumia 1020 mobile phone, which has a 41.3 megapixel (MP) image sensor with a pixel size of 1.14 um, with an external lens attached to the camera for better focus, and we characterized the imaging system to have a spatial resolution of 1.2 um, with a FOV of 3.6 x 2.7 mm, and a working distance of 0.6 mm. Moreover, we used the screen of a Retina display Apple device as a versatile illumination source for this system. The screen is used to project various illumination patterns onto the specimen being imaged, each corresponding to a different illumination mode, with the Nokia phone capturing the resulting image. We tested our system by using it to image microorganisms such as Escherichia coli and Euglena gracilis within our microfluidic devices.
Electrochemistry shows great potential for biosensing due to its easy-operation (typically with a three-electrode system) and rapid measuring, particularly for portable substrates so they can be integrated as electrode conveniently with the system. The electrical and physical properties make metallic nanoparticles suitable materials for electrochemical biosensors. Metallic nanoparticles can not only facilitate electron transfer with high conductivity but also immobilize molecules of interest. Among metals, silver is the best conductor and silver nanoparticles (AgNPs) can be easily obtained through chemical synthesis. Conductive carbon tapes are a good candidate for deposition of metallic nanoparticles. They can not only make the substrates flexible and portable but also provide good conductivity during electrochemical measurements. They can be self-assembled with metallic nanoparticles. Therefore, in this work, double-sided adhesive carbon tapes coated with AgNPs are fabricated in a simple way. With the prepared conductive substrate as working electrode in a three-electrode system, urea and other proteins at low concentration were measured with cyclic voltammetry (CV). The results showed that carbon tapes coated with AgNPs were able to detect urea with strong signals and that different proteins on the substrates can also be identified. Moreover, urea can be distinguished from proteins or other samples such as plasma including several components on this substrate via EC biosensing.
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