Reliable quantification by surface enhanced Raman scattering (SERS) highly depends on the development of a reproducible substrate with excellent anti-interference capability. In this work, an intrinsic internal standard (IS) SERS substrate based on a diblock copolymer, polystyrene-block-poly-4-vinylpyridine (PS-b-P4VP), as a template was developed by a modified block copolymer lithography (BCL) technique of reserving the diblock copolymer template. This substrate generated a stable vibration peak at 1066 cm−1 attributed to poly(4-vinylpyridine) (P4VP) which could be used as an intrinsic IS for precise SERS quantification. A series of characterization results showed that the diblock copolymer template endowed this substrate with homogeneous distribution of Au nanoparticles, excellent stability and reproducibility. A minor relative standard deviation (RSDs) of SERS responses at 1066 cm−1 was measured and calculated to be 4.3% from ten different substrates in the same batch, which suggested good substrate-to-substrate reproducibility of this substrate. Finally, this intrinsic IS substrate was successfully applied for the rapid quantification of a trace banned dye, chrysoidine, in food samples with complicated matrices by SERS. It was proved that chrysoidine could be found and quantified to be 0.50 and 0.41 mg L−1 in a positive dried bean curd stick and chilli powder sample respectively with good recoveries by this substrate coupled with SERS. The development of intrinsic IS substrates would benefit rapid and accurate quantification of trace targets in complex samples coupled with SERS analysis.
Block copolymer (BCP) brush-like layer absorbed on optical fibers are shown to address the longstanding challenge of forming dense and well-dispersed gold nanorods (AuNRs) films on highly curved surface, allowing the development of real-time localized surface plasmon resonance (LSPR) biosensors for antigen-antibody interaction. This method requires only solution-based manufacturing processes and a plasma cleaning step to self-assemble AuNRs with various aspect ratios on the optical fibers. Because AuNRs capped with cetyltrimethylammonium bromide (CTAB) have been problematic for biosensing applications due to their cytotoxicity and displacement difficulty in the construction of biosensors, citrate-stabilized AuNRs were prepared from CTAB-stabilized AuNRs via surfactant exchange. For comparison, both a poly(styrene)- b -poly(acrylic acid) (PS- b -PAA) BCP template with CTAB-stabilized AuNRs and a poly(styrene)- b -poly(4-vinyl pyridine) (PS- b -P4VP) BCP template with citrate-stabilized AuNRs were employed for the fabrication of the optical fiber biosensor. The LSPR spectral analysis results demonstrate that the sensitivity of the sensor produced by the PS- b -P4VP templating method was 1.5 times better than that of the sensor produced by the PS- b -PAA templating method, which was due to the improved surface coverage and reduced aggregation of nanoparticles. The detection limit of the PS- b -P4VP-templated sensor for human IgG was 0.6 nM. Furthermore, for citrate-stabilized AuNRs, the PS- b -P4VP templating method also led to a better surface morphology and higher sensitivity as compared to the standard particle absorption methods using alkoxysilanes or polyelectrolytes on identical fibers. The simplicity of the fabrication process and the far superior optical performance of the PS- b -P4VP-templated LSPR optical fiber sensor illustrate the ability to regulate the detection range by tuning the aspect ratios of the AuNRs, and demonstrate its application prospects as a real-time biosensor without cytotoxicity.
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