Partially reduced graphene oxide based FRET on fiber-optic interferometer for biochemical detection

Fluorescent resonance energy transfer (FRET) with naturally exceptional selectivity is a powerful technique and widely used in chemical and biomedical analysis. However, it is still challenging for conventional FRET to perform as a high sensitivity compact sensor. Here we propose a novel ‘FRET on Fiber’ concept, in which a partially reduced graphene oxide (prGO) film is deposited on a fiber-optic modal interferometer, acting as both the fluorescent quencher for the FRET and the sensitive cladding for optical phase measurement due to refractive index changes in biochemical detection. The target analytes induced fluorescence recovery with good selectivity and optical phase shift with high sensitivity are measured simultaneously. The functionalized prGO film coated on the fiber-optic interferometer shows high sensitivities for the detections of metal ion, dopamine and single-stranded DNA (ssDNA), with detection limits of 1.2 nM, 1.3 μM and 1 pM, respectively. Such a prGO based ‘FRET on fiber’ configuration, bridging the FRET and the fiber-optic sensing technology, may serve as a platform for the realization of series of integrated ‘FRET on Fiber’ sensors for on-line environmental, chemical, and biomedical detection, with excellent compactness, high sensitivity, good selectivity and fast response

Microscope image of the GSMS in the buffer. The interferometric section packaged in the buffer is fixed straightly to avoid bending. Here the bar is 100 μm. a, Fabrication process. A singlemode-multimode-singlemode structure (SMS) was fabricated by using two section of single mode fiber (SMF-28e, Corning) and a section of multimode fiber (MMF, core diameter 105 µm, Corning), with multimode cavity length of ~3.2 cm. Then, the silica clad of the SMS was etched off by HF, with keeping a 90 μm core.
Firstly, GO was fabricated as following: Graphite powder (2 g) and NaNO 3 (1 g) were mixed, then add into concentrated H 2 SO 4 (80 mL) with an ice bath. Under vigorous stirring, KMnO 4 (8 g) was added slowly to keep the temperature of the mixture below 20℃. After removing the ice bath, the mixture was stirred at 35 ℃ in a water bath for 2 h.
Successively, 240 mL of H 2 O was slowly added to the pasty and brown mixture.
Addition of water into the concentrated H 2 SO 4 will release a large amount of heat; therefore, water should be added slowly so that the temperature of the mixture in the ice bath was below 50 ℃. After adding 240 mL of H 2 O, 5 mL of 30% H 2 O 2 was added to the mixture, then the diluted mixture color changed to brilliant yellow. After continuously stirring for 2 h, the mixture was filtered and washed with 10% HCl aqueous solution(250 mL), DI water, and ethanol (anhydrous) to remove other ions. Finally, the resulting solid was dried by vacuum. Then, the dried GO was dissolved in 40mL DI water with sonication for 2 h to form a uniform dispersion. Then the etched SMS was immersed in the GO dispersion, on a substrate. The water of the GO dispersion was evaporated 4 naturally in air at room temperature after 24h therefore the thin GO film coated on the fiber was formed. Then the fiber coated by the GO thin film were immersed in 100 mL VC aqueous solution (30g/L), which was heated in a water bath to 80 o C. After reduced by the hot VC solution for 20 min, the fiber was washed by DI water for several times, and finally dried on a hotplate at 50 o C.
b, By using a 633 nm laser, it is obvious that the light energy transmits from the core of the SMF to the surface of the etched MMF. During this process, multimode interferences occur. By launching the 633 nm laser from left side and right side of the SMS respectively, the evanescent light transmitting out of the fiber is obvious, which makes it sensitive to local environment. By using this method, the length of the MMF section could also be conveniently characterized. c, Picture of the GO dispersion (yellow) and the partially reduced GO (prGO) dispersion reduced by VC (black).   Then, DA with different concentration is added in, afterwards the fluorescence and spectra are measured. c, Using Sensor 3 samples to detect ssDNA. Firstly, the GSMS 8 samples are immersed in 5% Na 2 CO 3 solution for 10 min. Then they are cleaned by using enough distilled water to remove excess Na + . The samples are measured first by immersing it in Rh6G. Then, Cd 2+ with different concentration is added in, afterwards the fluorescence and spectra are measured.  with producing H + . In e, similar to d, Rh6G binds on the prGO first, when DA added in, binding competition occurring, Rh6G-prGO turns to be DA-prGO and Rh6G, with fluorescent recovery. In f, the prGO is functionalized by Na + first, the Rh6G binds the functionalized prGO-Na + via ionic bond, with fluorescent quenching. However, the binding of the Rh6G and DNA would be much stronger on the prGO, so that the Rh6G will be took away from the prGO by DNA, with restoring the fluorescence. In Rh6G + DI water (0), in Rh6G + analytes (1), washed by water (2), in Rh6G + analytes again (3), washed by water again (4), in Rh6G + analytes again (5).