A downhole composition includes a carrier fluid and at least one shape-changing nanoparticle. The shape-changing nanoparticle is configured to change shape upon exposure to elevated temperature for a period of time. A method of monitoring temperature in a reservoir is provided. The method includes introducing a temperature monitoring composition into the reservoir, the temperature monitoring composition including a carrier fluid and at least one shape-changing nanoparticle. Then, after a predetermined period, producing a produced fluid from the reservoir, collecting samples of the produced fluid, measuring an optical property of the shape-changing nanoparticle, and based on the measured property of the shape-changing nanoparticle, determining a temperature of at least a portion of the reservoir.