Low Light CMOS Contact Imager with Integrated Dual Band Emission Filters for Fluorescence Detection

In recent times, the scope of optical imaging systems has been widened substantially to address the need for inexpensive, portable and miniaturized sensory system for applications such as in situ biomedical and environmental monitoring. Contact imaging is a relatively new and low-cost technique that offers significant advantages over conventional imaging. Conventional imaging systems often require bulky and expensive magnifying objectives along with photodetector devices. In contrast, contact imaging lacks the need of optical elements such as lenses between the sample and the sensor array, and therefore provides better light collection efficiency without optical loss and a significant improvement in sensitivity. Such unique features are achieved by contact imaging systems mainly due to direct coupling of the sensor array with the sample of interest via a thin-film filter.

Fluorescence imaging is a powerful and highly sensitive tool that is widely used in diverse areas including diagnostic purposes, bioassays, pharmaceutical research and sensor development. Recently, contact image sensors fabricated using complementary metal-oxide semiconductor (CMOS) technology have been foreseen as an alternative to conventional fluorescence detection systems. Owing to the ability of CMOS imagers to integrate possibly all functions required for timing, exposure control, analog-to-digital (ADC) conversion, color processing, image enhancement and image compression on the same platform, they are considered key components of future micro-total-analysis-system (µTAS). CMOS imagers consume low power and require low voltage as well as facilitate a vast range of flexibility. However, fabrication of a high quality emission filter, also known as absorption filter, remains one of the major challenges in developing high sensitive CMOS-based fluorescence detection system. In particular, the integration of a CMOS image sensor with different types of filters that possess discrete cut-off wavelengths has so far been unexplored.

Here, we demonstrate the fabrication of polyvinyl acetate (PVAc) based two distinct emission filters integrated with a single CMOS contact imager for fluorescence detection of two different analytes. PVAc was used as an adhesive for the emission filter due to its optical transparency, inexpensiveness and biocompatiblity properties. The fabrication process of emission filters involve dissolving PVAc in methanol, mixing it with absorbing specimens and finally drying up in air to harden the filter. Two distinct emission filters that possess discrete spectra are integrated onto a single CMOS image sensor. The integrated sensory system thus incorporates dual bandpass emission filters that are capable of selectively isolating fluorescence emission from two probes simultaneously. The thickness of the filters is optimized by calculating the desired signal-to-noise ratio (SNR) using Beer-Lambert’s law for liquids, Quantum Yield of the fluorophore and the Quantum Efficiency of the sensor array. Finally, the efficacy of the sensory system is tested by examining two different analytes i.e., calcium and pH. Overall, CMOS image sensor that combines dual bandpass emission filters holds much potential for low-cost and on-site deployable sensor development.