br RI sensitivity of BP TFG was evaluated
RI sensitivity of BP-TFG was evaluated by immersing the device in a set of aqueous sucrose solutions with RIs ranging from 1.332 to 1.4135. Fig. 4c plots the spectral VH298 of BP-TFG against RI. It is a clear appearance of intensity reduction of TM and TE resonances while they both move to long wavelength side with increase of external RI. For BP-TFG, the cladding guided modes are partially radiated to the overlay behaving as leaky modes. By increasing external RI, the leaky radiation Biosensors and Bioelectronics 137 (2019) 140–147
could reduce the coupling coeﬃcient between core and cladding hence decrease the resonant intensity while the eﬀective RI of cladding could be changed resulting in wavelength shift with respect to the phase-matching condition Eq. (1). It can be seen from Fig. 4d that the RI sensitivity exhibits a nonlinear characteristic and increases dramati-cally. The RI sensitivities of TM resonance (blue symbols) achieve 198.3 nm/RIU, 373.7 nm/RIU, and 1515.3 nm/RIU for low (1.332–1.356), medium (1.374–1.391) and high (1.398–1.413) RI regions, respec-tively, while those of TE mode (red symbols) are 170.0 nm/RIU, 336.6 nm/RIU and 1483.3 nm/RIU for the corresponding RI regions. The value of coeﬃcient of variation of wavelength shift at diﬀerent RI is on the order of 10−6. TM resonance shows higher RI sensitivity than that of TE mode, hence it has been selected for further biosensing experi-ments. As bioassays and biological events are usually carried out in low RI region (∼1.33), it is noteworthy that RI sensitivity of BP-TFG in low RI region is seven times higher than that of the conventional LPGs (Lee, 1997), indicating BP-TFG could be a good candidate for biosensing applications. In addition, TFG owns narrow band resonance giving higher Q-factor (∼343) and higher accuracy than LPG (Zhou et al., 2006).
3.3. Biofunctionalization of BP and immobilization of anti-NSE
Construction of bio-nano-photonic interface is a critical role for optical biosensor. It has been reported that poly-L-lysine has various advantages including plentiful active amino groups, flexible molecular backbone, and good biocompatibility and solubility (Kumar et al., 2016; Shan et al., 2009). Functionalization of nanomaterials by bio-compatible PLL, as a cross-linker through amide groups, can facilitate the active treatment and provide the opportunities for bioactive mole-cular attachment.
Fig. 5 plots the schematic of biofunctionalization of BP-fiber optic biosensor. (i) The BP-TFG was incubated in 0.01% PLL solution for 4 h at room temperature. The positively charged PLL attached on BP via electrostatic interaction to form a cross-linker between BP and bioactive molecules. (ii) Then 12 mg/mL EDC, 24 mg/mL NHS and 2 mg/mL anti-Human neuron-specific enolase (anti-NSE) were mixed in 1 × PBS to activate the carboxyl groups. The PLL-functionalized BP-TFG was in-cubated in such EDC/NHS/anti-NSE mixture for 2 h to immobilize anti-NSE molecules. EDC/NHS coupled reactions were highly eﬃcient and usually increased the bioconjugation significantly (Hermanson, 2013). The carbodiimide EDC was used to form active ester functionalities with carboxylate groups using the compound NHS. The advantage of adding NHS to EDC reactions was to increase the solubility and stability of the active intermediate, which ultimately reacted with the attacking amine. NHS esters were hydrophilic reactive groups that coupled ra-pidly with amines on anti-NSE, which were used to react with the amine groups of PLL to form a covalent immobilization of anti-NSE on host BP, leaving the binding sites free for target NSE recognition. (iii) After that, the anti-NSE bound BP-TFG was washed by 1 × PBS buﬀer and immersed into 1% BSA solution for 30 min. The non-bound anti-NSE was washed away by PBS buﬀer. The unreacted sites on BP surface were passivated by BSA to block the remaining active carboxylic groups and to prevent non-specific adsorption. Here, the anti-NSE immobilized BP-TFG was ready as a biosensor for the detection of target NSE bio-markers.
The optical responses of BP-TFG from surface modification to bio-functionalization were determined by monitoring the TM-mode spectra (Fig. S3). The resonance has been shifted ∼1.40 nm, 1.40 nm, 0.46 nm and 0.59 nm after APTES silanization, BP deposition, PLL biofunctio-nalization, and anti-NSE immobilization, respectively. Due to the multiple modifications applied on device surface, the eﬀective RI of cladding was modified yielding a total resonant red-shift of 3.85 nm while the leaky radiation reduced the coupling coeﬃcient hence de-creased the peak intensity by 5.6 dB.
Fig. 4. BP-induced strong optical modulation and enhanced light-matter interaction. (a) TFG spectral evolution during BP deposition processes (C0: non-coating, C1-