Bottom-up assembled photonic crystals for label-free sensing: Evaluation between photonic band gap and Fabry−Pérot fringes

Abstract

The major challenge of the application of photonic crystals (PhCs) biosensors is the test sensitivity, selectivity, and reliability of the target analytes in complex real samples, like bodily fluids. Herein, we introduce the concept of utilizing the optical thickness (OT), which came from Fabry−Pérot fringes (F−P fringes), of bottom-up assembled PhCs to replace the traditional photonic band gap (PBG) as a parameter for biosensing process monitoring and quantification of analytes. The bottom-up assembled silica PhCs films are specialized to own a thickness with good interferometric properties and an ordered porous interconnecting and mechanically stable structure as a scaffold for biological reactions. By selecting a specific diameter (190 nm) of the silica PhCs building block, both PBG and F-P fringes can be observed on the spectrum simultaneously at the range of white light, and during detection, mutual interference between the two can be minimized to a great extent. This study finds that nearly 14 times lower low limit of detection (LLoD) can be achieved with F−P fringes than the PBG in ethanol concentration benchmark test on the same silica PhCs film. The thrombolysis experiment and human immunoglobulin G content evaluation in human plasma also show the sensitivity and reliability of F−P fringes in real samples, which PBG property can not perform well. This is the first systematic comparison study on PBG and F−P fringes in silica PhCs films.