A Microdroplet-Based Colorimetric Sensing Platform on a CMOS Imager Chip

Abstract

Interest in mobile chemical sensors is on the rise, but significant challenges have restricted widespread adoption into commercial devices. To be useful these sensors need to have a predictable response, easy calibration, and be integrable with existing technology, preferably fitting on a single chip. With respect to integration, the CMOS imager makes an attractive template for an optoelectronic sensing platform. Demand for smartphones with cameras has driven down the price and size of CMOS imagers over the past decade. The low cost and accessibility of these powerful tools motivated us to print chemical sensing elements directly on the surface of the photodiode array. These printed colorimetric microdroplets are composed of a nonvolatile solvent so they remain in a uniform and homogeneous solution phase, an ideal medium for chemical interactions and optical measurements. By imaging microdroplets on the CMOS imager surface we eliminated the need for lenses, dramatically scaling down the size of the sensing platform to a single chip. We believe the technique is generalizable to many colorimetric formulations, and as an example we detected gaseous ammonia with Cu­(II). Limits of detection as low as 27 ppb and sensor-to-sensor variation of less than 10% across multiple printed arrays demonstrated the high sensitivity and repeatability of this approach. Sensors generated this way could share a single calibration, greatly reducing the complexity of incorporating chemical sensors into mobile devices. Additional testing showed the sensor can be reused and has good selectivity; sensitivity and dynamic range can be tuned by controlling droplet size.

A March to Shape Optical Artificial Olfactory System toward Ultrasensitive Detection of Improvised Explosives

“…[ 120 ] A droplet microarray is printed on the surface of complementary metal oxide semiconductor (CMOS) imager with colorimetric droplets composed of nonvolatile solvent that provides a permanent liquid environment (Figure 6b). [ 121 ] Through these nonvolatile droplets to assist in sampling and transfer the object to be measured to the surface of the CMOS, the sensor array which can realize chemical method and good optical imaging is fabricated.…”

“…Reproduced with permission. [ 121 ] Copyright 2020, American Chemical Society. c) Comparison of the inorganic explosives and high/organic explosives μPAD device before and after contacted with explosive mixtures.…”

“…b) Microarray formed by droplets with different volumes on CMOS and images before and after detection of ammonia. Reproduced with permission [121]. Copyright 2020, American Chemical Society.…”

Multiplexed Chemical Sensing CMOS Imager

“…This fast-growing demand for CMOS imagers has driven their price down to a few dollars, which allows CMOS imagers to be used even as disposable sensors. Previously we have demonstrated sub-ppm level ammonia detection can be achieved on the liquid phase colorimetric microdroplets printed on the surface of the CMOS imager 24 . However, to be compatible with modern electronics, a stable solid-state chemical sensing CMOS imager with multiplexed sensing capability is preferred, but not yet available.…”

“…Previously, we have demonstrated that sub-ppm-level ammonia detection can be achieved on the liquid-phase colorimetric microdroplets printed on the surface of the CMOS imager. 16 In this work, we describe a method to turn a CMOS imager into an integrated solid-state chemical CMOS chip (C-CMOS) for the simultaneous detection of multiple gases (Figure 1). To create different types of colorimetric sensing spots on the small imager surface, we sequentially coat droplets containing different sensing probes onto the surface of a CMOS imager and then dry the droplets to form solid microsensing spots.…”