A plasmonic optical fiber biosensor, developed by researchers at Jinan and Tsinghua universities, uses light-matter interaction and advanced surface chemistry to achieve ultrasensitive detection of environmental estrogens (EEs). The compact, flexible biosensor is portable and easy to implement. It could be used to detect endocrine disruptors in remote locations and spaces that are hard to reach. The biosensor platform is based on a gold-coated tilted fiber Bragg grating (TFBG). The TFBG excites high-density, narrow cladding mode spectral combs that overlap with the broad absorption of surface plasmon to achieve high-accuracy interrogation and enable the ultrasensitive monitoring of refractive index changes at the fiber surface. Through the covalent surface modification of plasmonic fiber optics, the researchers converted the interaction of nuclear estrogen receptors (nERs) and EEs into measurable signals, enabling the biosensor to easily detect EEs in environmental samples. With estrogen receptors as a model, they designed an estradiol-streptavidin conjugate and converted the specific recognition of environmental estrogens by an estrogen receptor into a surface-based affinity bioassay for protein. Using this conjugate-induced amplification approach to biosensing, the researchers were able to detect environmental estrogens down to 1.5 ng l-1 estradiol equivalent concentration. The capability to detect EEs down to the nanogram-per-liter level demonstrates the lowest limit of detection for any estrogen receptor-based detection reported to date. The sensing mechanism for the detection of broad ranges of EEs using human estrogen receptor α (hERα) as the biological recognition element. In brief, a gold-coated TFBG excites surface plasmon resonance, which enables the ultrasensitive monitoring of refractive index changes at the fiber surface. This method offers in situ detection of environmental estrogens with high sensitivity and specificity. Courtesy of Lanhua Liu, Xuejun Zhang, Qian Zhu, Kaiwei Li, Yun Lu, Xiaohong Zhou, and Tuan Guo. The in-fiber plasmonic biosensor showed a minimal cross-sensitivity to temperature, and its fabrication does not affect the structural integrity of the fiber. The biosensor could be used for the in-field, continuous detection of estrogenic endocrine disruptors. Moreover, its use could be extended to the sensing and detection of a variety of endocrine disruptors. Nuclear receptors such as androgen receptor, thyroid receptor, and progesterone receptor could be integrated with the TFBG-based surface-plasmon resonance (SPR) biosensing platform for ultrasensitive detection of other types of endocrine disruptors. The researchers also think that the biosensor could be integrated with a hypodermic needle and used as a portable measurement device, to perform on-site and in-field analyses for monitoring human health. Environmental estrogens have been designated a global environmental issue to be addressed through international collaboration. The TFBG-based, ultrasensitive, plasmonic biosensor, with its compact size and flexible shape, could support the strategic placement of sensors worldwide for the continuous assessment of global environmental endocrine disruptors, to monitor and improve environmental and human health world-wide. The research was led by professor Tuan Guo from Jinan University and professor Xiaohong Zhou from Tsinghua University. It was published in Light: Science & Applications (www.doi.org/10.1038/s41377-021-00618-2).