Abstract
This study presents the fabrication of a low cost poly-acrylic acid (PAA) based emission filter integrated with a low light CMOS contact imager for fluorescence detection. The process involves the use of PAA as an adhesive for the emission filter. The poly-acrylic solution was chosen due its optical transparent properties, adhesive properties, miscibility with polar protic solvents and most importantly its bio-compatibility with a biological environment. The emission filter, also known as an absorption filter, involves dissolving an absorbing specimen in a polar protic solvent and mixing it with the PAA to uniformly bond the absorbing specimen and harden the filter. The PAA is optically transparent in solid form and therefore does not contribute to the absorbance of light in the visible spectrum. Many combinations of absorbing specimen and polar protic solvents can be derived, yielding different filter characteristics in different parts of the spectrum. We report a specific combination as a first example of implementation of our technology. The filter reported has excitation in the green spectrum and emission in the red spectrum, utilizing the increased quantum efficiency of the photo sensitive sensor array. The thickness of the filter (20 μm) was chosen by calculating the desired SNR using Beer-Lambert’s law for liquids, Quantum Yield of the fluorophore and the Quantum Efficiency of the sensor array. The filters promising characteristics make it suitable for low light fluorescence detection. The filter was integrated with a fully functional low noise, low light CMOS contact imager and experimental results using fluorescence polystyrene micro-spheres are presented.
Highlights
IntroductionFluorescence spectroscopy will be a key component of future micro-total-analysis-systems (μTASs) [1,2], which will integrate the capabilities of entire laboratories onto compact devices consisting of microchips and other micro-fabricated elements (Lab-on-a-chip) [3,4,5,6,7]
Fluorescence spectroscopy will be a key component of future micro-total-analysis-systems [1,2], which will integrate the capabilities of entire laboratories onto compact devices consisting of microchips and other micro-fabricated elements (Lab-on-a-chip) [3,4,5,6,7]
The room temperature vulcanizing (RTV) sealant is applied with a fine brush to the exposed parts covering the wire bonds and overlapping part of the CMOS Imager chip, but it does not cover any part of the sensor array
Summary
Fluorescence spectroscopy will be a key component of future micro-total-analysis-systems (μTASs) [1,2], which will integrate the capabilities of entire laboratories onto compact devices consisting of microchips and other micro-fabricated elements (Lab-on-a-chip) [3,4,5,6,7]. For objects in close proximity with the sensor surface, the contact imager subtends nearly 2π of the total solid angle, so the collection efficiency can be as high as 50% for samples that emit light [12]. Salama et al [13] estimated that the optical efficiency of a contact imaging system is improved by 35 dB in comparison with camera-based imaging system [14] This makes it possible to use a low power LED as an illumination source for dark objects because the improvement in collection efficiency allows the detection of a weak signal. The fundamental requirement of the filter is that it be bio-compatible with the object of interest This requires minimizing the impact on cell physiology while protecting the sensor array from damage by exposure to the biological environment.
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