The concentration of TMB required to obtain copper oxide nanoparticles with an absorption peak at 650 nm will depend on the specific synthesis method and reaction conditions used. In general, the concentration of the reducing agent can affect the size and shape of the nanoparticles, as well as their optical properties such as the absorption peak wavelength.
It's important to note that the choice of reducing agent and synthesis conditions can have a significant impact on the size, shape, and properties of the resulting nanoparticles. Therefore, it's recommended to consult the literature or conduct experiments to optimize the synthesis conditions for the desired properties of the nanoparticles.
Here are some references that you may find helpful for further reading:
K. Saha, S. Agasti, C. Kim, X. Li, and V. M. Rotello, "Gold nanoparticles in chemical and biological sensing," Chemical Reviews, vol. 112, no. 5, pp. 2739-2779, 2012.
N. T. Thuy, N. H. Thao, N. T. K. Thanh, N. P. T. Thuy, N. T. Phuong, and T. H. Nguyen, "Green synthesis of copper oxide nanoparticles and their application for colorimetric sensing of glucose," Materials Science and Engineering: C, vol. 110, 110672, 2020.
J. Zhou, T. Zhang, L. Du, X. Wang, and H. Yang, "A review on the synthesis and applications of copper oxide nanoparticles," Journal of Materials Science, vol. 55, no. 22, pp. 9337-9370, 2020.
S. K. Saha, S. Dasgupta, A. Karak, and D. Das, "Green synthesis of copper oxide nanoparticles and its application for adsorption of toxic metal ions from water," Journal of Environmental Chemical Engineering, vol. 7, no. 1, 102915, 2019.
Y. Yin, Y. Wang, C. Zhu, and Y. Huang, "Colorimetric biosensors based on DNA-modified nanoparticles for cancer detection," Analytical Chemistry, vol. 87, no. 1, pp. 238-252, 2015.