Chemical characterization at the nanoscale is of significant importance for many applications in physics, analytical chemistry, material science, and biology. Despite the intensive studies in the infrared range, high-spatial resolution and high-sensitivity imaging for compositional identification in the visible range is rarely exploited. In this work, we present a gap-plasmon-enhanced imaging approach based on photothermal-induced resonance (PTIR) for nano scale chemical identification. With this approach, we experimentally obtained a high spatial resolution of similar to 5 nm for rhodamine nanohill characterization and achieved mono-layer sensitivity for mapping the single-layer chlorophyll-a islands with the thickness of only 1.9 nm. We also successfully characterized amyloid fibrils stained with methylene blue dye, indicating that this methodology can be also utilized for identification of the radiation-insensitive macromolecules. We believe that our proposed high-performance visible PTIR system can be used to broaden the applications of nanoscale chemical identification ranging from nanomaterial to life science areas.