Detection of l-histidine using DNAzymes that do not bind l-histidine directly. Reprinted (adapted) from P. Gu, G. Zhang, Z. Deng, Z. Tang, H. Zhang, F. Y. Khusbu, K. Wu, M. Chen and C. Ma, A novel label-free colorimetric detection of l-histidine using Cu2+-modulated G-quadruplex-based DNAzymes, Spectrochim. Acta, Part A, 2018, 203, 195–200, with permission from Elsevier.268