Coiled Optical Nanofiber for Optofluidic Absorbance Detection.

A challenge for optofluidic absorbance detection is the high concentration limit of detection due to the short optical path length. Herein, we introduce a concept of utilizing the coiled optical nanofiber for highly sensitive and robust optofluidic absorbance detection. Investigated by measuring the absorbance of FeCl solutions, the sensor shows a detection limit down to 10 μM with excellent reversibility in a concentration range of 0-5 mM. The sensitivity is 10-fold higher than that of standard absorbance measurement by using a 1 cm cuvette. Also, highly sensitive chloramphenicol sensing was demonstrated by using the enzyme-linked immunosorbent assay (ELISA) method, achieving a detection limit below 0.5 ng/L. The higher sensitivity and lower detection limit are caused by the large fractional power of evanescent field outside the nanofiber and the long detection length, which can effectively improve the absorption of the evanescent field, while the excellent reversibility is caused by the support of a polydimethylsiloxane (PDMS) pillar rather than by suspending the nanofiber in the microchannel. We envision that the present work may open up new opportunities for ultrasensitive chemical and biological sensing.