Mid-infrared (mid-IR) absorption spectroscopy is a well-known and versatile analytical technique for uniquely identifying and measuring the concentrations of gases, chemicals, and biological molecules by measuring which wavelengths of mid-IR light an analyte absorbs. Existing mid-IR absorption sensors are however either bulky and expensive (e.g. benchtop spectrometers), or have poor sensitivity and specificity (e.g. LED based sensors). Miniaturising such sensors could be transformative for diverse medical, industrial, and environmental sensing scenarios. High performance, low cost, and small spectroscopic sensors could be created using mid-IR optical circuits on silicon chips. These chips would ideally combine all of the required optical functions of the sensor (i.e. light source, waveguides for routing light, interaction between the light and the analyte, and light detection), and could be fabricated at low cost in high volumes, thanks to existing silicon manufacturing infrastructure that has been developed for electronics and for near-infrared optical communications. The last few years have seen rapid development of many of the components that are needed to create these sensor systems: silicon photonic waveguides that can transmit light with low loss at almost any mid-IR wavelength have been developed, while lasers emitting high powers in the mid-IR are now readily available and have been successfully integrated with silicon waveguides. However, there remains a crippling lack of practical photodetector technologies; those that have already been integrated wilth optical circuits on silicon chips are either expensive to manufacture, are impractical because they have to be cooled to cryogenic temperatures, or do not work at all required wavelengths. This project will develop new waveguide integrated thermal photodetectors, which work by converting the incoming light into a temperature change that can be measured with an electronic circuit. They will be able to operate at room temperature at any mid-IR wavelength, and will be manufactured using low cost techniques. This project will also demonstrate that sensors employing these photodetectors can reach the sensitivities required for clinical and industrial uses, by using them to measure low concentrations of artificial sweeteners in soft drinks - an industrially important example application. These detectors will potentially transform mid-infrared sensor systems from an academic curiosity into a commercially viable technology.