Ultra-low humidity detection is significant in some important industrial fields, such as integrated circuit manufacturing, chip packaging. However, the traditional ionic-type humidity sensor is not competent for the detection of ultra-low humidity, but can only realize the detection of medium humidity to high humidity. Here, moisture-induced discoloration material Nickel(II) iodide (NiI₂) is found to be highly sensitive to moisture, and both its electrical properties and color will change with environmental humidity variation. Especially, NiI₂ exhibits the largest impedance change to date in the ultra-low humidity range compared with state of the art. The impedance variation is up to 2 orders of magnitude in the range of 0.4% - 11% RH, which means that NiI₂ can realize the detection of ultra-low humidity. The excellent performance of NiI₂ in ultra-low humidity is attributed to the material transition characteristics susceptible to humidity rather than the traditional surface mechanism of water adsorption. Moreover, combining the reversible black-to-transparency moisture-induced discoloration behavior with humidity sensing properties, a flexible wristband based on NiI₂ is developed to realize the dual-mode detection of wide range environmental humidity. These results demonstrate that NiI₂ has great application prospects in professional ultra-low humidity detection and wearable humidity detection equipment.

超低湿度检测在某些重要的工业领域中非常重要，例如集成电路制造，芯片封装。但是，传统的离子型湿度传感器不能胜任超低湿度的检测，而只能实现中等湿度到高湿度的检测。在此，发现水分引起的变色材料碘化镍（III）（NiI ₂）对水分高度敏感，其电性能和颜色都会随环境湿度的变化而变化。尤其是，与现有技术相比，NiI ₂在超低湿度范围内迄今为止表现出最大的阻抗变化。在0.4％-11％RH的范围内，阻抗变化高达2个数量级，这意味着NiI ₂可以实现超低湿度的检测。NiI ₂在超低湿度下的出色性能归因于易受湿度影响的材料转变特性，而不是传统的水吸附表面机理。此外，将可逆的黑色到透明的水分引起的变色行为与湿度感测特性相结合，开发了基于NiI ₂的柔性腕带，以实现宽范围环境湿度的双模式检测。这些结果表明，NiI ₂在专业的超低湿度检测和可穿戴湿度检测设备中具有广阔的应用前景。