br The recent exploitation reveals that D layered materials
The recent exploitation reveals that 2D-layered materials could modulate light with superior performance for realistic applications in ultrafast laser, light processing, chemical and biomedical applications (Sun et al., 2016; Liu et al., 2014). Bao et al. reported the use of atomic layer graphene as saturable absorber for ultrafast pulsed fiber laser and in-line fiber-to-graphene coupler for the broadband polarizer (Bao et al., 2009, 2011). Albert et al. demonstrated gold nanofilm-coated tilted fiber grating (TFG) as a plasmonic sensor and gold nanoparticles (AuNPs) based optical fiber sensor for protein detection (Caucheteur et al., 2016; Lepinay et al., 2014). With the advancement of material science and lab-on-fiber technology (Cusano et al., 2014; Wang and Wolfbeis, 2013), various materials including thin metal film, carbon nanotubes, titanium dioxide, zinc oxide, graphene and graphene oxide (GO) have been deposited on short-period fiber gratings and long-period gratings (LPGs) for the enhancement of performance (Erdogan, 1997; Zhou et al., 2006; James and Tatam, 2013; Del Villar et al., 2005; Coelho et al., 2016). Among those materials, the large surface area of 2D-layered materials maximize the eﬀect of adsorbed Ambuic Acid on the host layer resulting in high sensitivity (Tan et al., 2014; Jiang et al., 2016; Liu et al., 2017, 2018). The use of fiber based optical biosensing platform takes the intrinsic merits of the optical fibers, such as elec-tromagnetic interference immunity, compactness, lightness, and high compatibility with optoelectronic devices. Moreover, in-fiber grating technology provides advantages of high sensitivity, real-time, multi-plexing and in-line determination as the optical signal is spectrally modulated (Cusano et al., 2014; Wang and Wolfbeis, 2013; Chiavaioli et al., 2017). However, the lack of biocompatibility and coating ap-proach restricts the usage of BP for fiber optic biosensing and biome-dical applications (Choi et al., 2018).
In this paper, for the first time to our knowledge, we propose a BP-fiber optic biosensor for ultrasensitive diagnosis of NSE cancer bio-marker. As illustrated in Fig. 1, a largely tilted fiber grating acting as optical transducer couples the light from fiber core to cladding yielding evanescent field at cladding/surrounding boundary. BP nanosheets are synthesized by a liquid-phase exfoliation method and deposited on the fiber by an in-situ layer-by-layer (i-LbL) technique. BP overlay is bio-functionalized by poly-L-lysine (PLL) to provide a biocompatible en-vironment for the immobilization of anti-NSE, leaving active binding sites free for bioaﬃnity recognition of target NSE biomarkers. The perturbation caused by aﬃnity binding changes local analyte con-centration hence local refractive index (RI), alerting the optical signal which can be monitored in real-time as a change of local RI, thereby eliminating the need of ligand labeling. The BP-TFG has been im-plemented to detect the specific human NSE biomarkers as well as non-specific biomolecules, demonstrating ultrahigh sensitivity, specificity, real-time, and label-free sensing performance.
2. Materials and methods
The (3-Aminopropyl)triethoxysilane (APTES), Sodium hydroxide (NaOH), Poly-L-lysine (PLL), N-(3-Dimethylaminopropyl)-N′-ethylcar-bodiimide hydrochloride (EDC), N-Hydroxysuccinimide (NHS), Biosensors and Bioelectronics 137 (2019) 140–147
Fig. 1. Schematic of biofunctionalized black phosphorus based optical biosen-sing platform. The BP nanosheets deposited tilted fiber grating (BP-TFG) pro-vides an ultrasensitive bioanalytical platform for the detection of aﬃnity in-teractions between bioreceptor anti-NSE and target NSE biomarkers.
Phosphate buﬀered saline (1 × PBS, pH 7.4), Bovine serum albumin (BSA), immunoglobulin G (IgG) and prostate specific antigen (PSA) were purchased from Sigma-Aldrich (United Kingdom). Anti-Human neuron-specific enolase (anti-NSE) was purchased from Medix Biochemica Inc. (Finland). Human neuron-specific enolase (NSE) was purchased from Abcam Ltd. (United Kingdom). Isopropanol, methanol, ethanol, acetone, and deionized (DI) water were purchased from Thermo Fisher Scientific Inc. (United Kingdom).
All chemical and biochemical reagents were of analytical grade and were used as received without further purification. All aqueous solu-tions were prepared with DI water.
2.2. Fabrication of largely tilted fiber grating
The 82°-TFG with grating length of 8 mm was inscribed in a hy-drogenated BGe single mode fiber by the use of a frequency-doubled Ar laser at 244 nm wavelength and mask scanning technique. After UV fabrication, the TFG was annealed at 80 °C for 48 h to remove the re-sidual hydrogen and to stabilize the optical properties.