Modern computer and electronic technologies, including artificial intelligence, are becoming ever more powerful for information processing. There is a lack of proportional progress in information collection, which is performed by various sensors1–3. Sensors are an integral part of today’s electronics. For example, a smartphone is equipped with more than a dozen sensors, including magnetic sensor, accelerometer, gyroscope, microphone, thermometer, proximity sensor, barometer, and complementary metal-oxide-semiconductor (CMOS) imagers4,5. However, all these sensors measure physical quantities, and none can sense chemicals. A miniaturized and multiplexed chemical sensor would empower mobile devices to detect early signs of diseases, alert contamination of food and drinking water, and sense danger of toxic chemicals in air6–8.
Low cost and miniaturized chemical sensors have been actively pursued9,10. A promising example is metal oxide sensors, which are sensitive, miniaturized and compatible with electronics. However, they lack selectivity, and the high power consumption limits their scalability to meet the need of integrating increasing number of sensors11,12. Colorimetric sensors detect a color change originated from the specific reaction of a target analyte with a sensing material13,14. The most successful example of colorimetric gas sensing is the detection tubes, each containing a sensing material sealed in a glass tube (Fig. 1a). Breaking the tube exposes the sensing material to a chemical and leads to a color change. Millions of detection tubes are being sold each year for safeguarding workers in chemical and related plants, firefighters on duty, and preventing air pollution and chemical leakage15,16. While useful, these tubes are bulky, time-consuming, semi-quantitative, and each detects typically only one analyte17–19. Alternatively, a colorimetric sensing array can be printed on a substrate and then imaged with an optical system20–23 (Fig. 1d). This approach is, however, not compatible with integrated circuits, and difficult to miniaturize because of the large size of each printed sensing element and use of optical components.
CMOS imager is an attractive platform for multiplexed optoelectronic sensing. A today’s CMOS imager with a size of a few millimeters offers millions of pixels, each as a low noise optical sensor, yet it is fully compatible with modern electronics and widely used in every smartphone, tablet, personal computer and security camera. 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 imager24. 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. Substantial innovations are required for sensing materials coating, image processing, and sensing algorithms development.
In this work, we describe a method to turn a CMOS imager into an integrated solid-state chemical CMOS chip (C-CMOS) for simultaneous detection of multiple analytes (Fig. 1). To create different types of colorimetric sensing spots on the small and fragile imager surface, we sequentially spray droplets containing different sensing materials onto the surface of CMOS imager and then dry the droplets to form solid sensing spots. Comparing to conventional inkjet printing, the spray method enables depositing droplets with sizes much closer to a single pixel. Because the sensitivity is spot-size invariant (evidenced by experimental data) and each sensing spot is addressable through image processing, the spray method provides a simple and cost-effective approach for large-scale fabrication of C-CMOS chips. Though solid-state sensing elements are preferred because of their mechanical and chemical stability, achieving high sensitivity on the CMOS imager surface is challenging. This is because the small surface area of the sensing spot limits the number of active sensing sites, and the short optical path through the tiny sensing spot results in small optical absorbance change. We have solved these issues by introducing nanoparticles to the droplets, which enhances colorimetric sensing signals by several orders of magnitude. We have built integrated C-CMOS chips, tested their analytical performance, and demonstrated their compatibility with mobile electronics in realistic application scenarios by building and testing a C-CMOS chip-based smartphone accessory.