Gold nanoparticles in biological optical imaging

•Gold nanoparticles (AuNPs) are greatly revolutionizing the field of biological optical imaging due to their unique plasmonic properties.•Direct visualization of AuNPs inside the biosystems with high resolution, and the rotational and orientation tracking of AuNPs are discussed.•Monitoring biological processes using surface-enhanced Raman spectroscopy (SERS) and plasmon-enhanced fluorescence (PEF) are discussed.•In vivo deep tissue imaging using AuNPs for disease diagnoses is discussed.•Current challenges and future directions are highlighted.