Hypoxia occurs when limited oxygen supply impairs physiological functions and is a pathological hallmark of many diseases including cancer and ischemia. Thus, detection of hypoxia can guide treatment planning and serve as a predictor of patient prognosis. Current methods suffer from invasiveness, poor resolution and low specificity. To address these limitations, various hypoxia-responsive probes (HyPs) for photoacoustic imaging are disclosed. The emerging modality converts safe, non-ionizing light to ultrasound waves, enabling acquisition of high-resolution 3D images in deep tissue. The HyPs feature an N-oxide trigger that is reduced in the absence of oxygen by haem proteins such as CYP450 enzymes. Reduction of HyPs produce a spectrally distinct product, facilitating identification via photoacoustic imaging. HyPs exhibit selectivity for hypoxic activation in vitro, in living cells and in multiple disease models in vivo. HyPs are also compatible with NIR fluorescence imaging, establishing its versatility as a multimodal imaging agent.