An innovative plastic optical fiber-based biosensor for new bio/applications. The case of celiac disease

Abstract

In this work an innovative and low cost surface plasmon resonance (SPR) biosensor, based on the utilization of plastic optical fibers (POF), is presented and experimentally tested for the diagnosis and/or follow-up of celiac disease. In particular, the POF-based sensor was used to monitor the formation of the transglutaminase/anti-transglutaminase antibodies formation, a new hallmark for the diagnosis of celiac disease. In particular, on the gold layer surface of the POF sensor, the guinea pig transglutaminase was immobilized. The ability of the POF-sensor to detect the binding of anti-transglutaminase antibodies to the immobilized transglutaminase was studied.

The obtained results showed that the POF-sensor is able to sense the transglutaminase/anti-transglutaminase complex in the range of concentrations between 30 nM and 3000 nM. Importantly, the proposed sensing method could be easily expanded to different target compounds.

Introduction

Over the past years, surface plasmon resonance (SPR) biosensor technology has made great strides, and a large number of SPR sensors, biomolecular recognition elements, and measurement configurations have been developed. SPR is a very sensitive technique for determining small refractive index changes at the interface between a metallic layer and a dielectric medium (analyte) [1]. This technique is widely used as a detection principle for many sensors that operate in the areas of biological and chemical sensing. Biological targets are generally transported through a microfluidic system by a buffer fluid or a carrier fluid. With SPR sensors, when the transducing media (ligands) react with the target molecules present in the analyte, the refractive index at the surface changes, and this change is detected by optical interrogation. In many works, the SPR sensor system is based on a high refractive index prism coated with a thin metallic layer. The incidence angle of the light can change in a wide range and as a consequence the surface plasma waves (plasmons) may exist whatever the surrounding medium, i.e. a gas or a liquid. Nevertheless, the sensors are usually bulky and require expensive optical equipment, it is not easy to miniaturize them and, in addition, their remote sensing may be difficult to develop. Very recently, Jorgenson et al. replaced the prism by a multimode optical fiber [2]. The metal was deposited on the bare core of the fiber. The use of an optical fiber allows for a remote sensing and may reduce the cost and dimensions of the device. Due to the propagation of the light in the fiber, the angle of incidence on the metallic layer exceeds the critical angle which depends on the refractive indices of both core and cladding components. Therefore SPR only exists for surrounding dielectrics whose refractive index lies in a narrow range. To overcome this drawback, Jorgenson et al. used a polychromatic light source and a spectrograph. This device is low cost, easy to implement and can offer some attractive advantages such as the possibility of the use in the presence of flammable substances and human hazardous environments because of its electricity-free and remote sensing capabilities. Furthermore, because of the small size and non-invasive features, it can be used for medical (self-) diagnosis with the possibility of integration of SPR sensor platforms with optoelectronic devices, eventually leading to “lab on a chip” [3].

In particular, sensors based on bent or straight plastic optical fibers (POF) represent a simple approach to low cost bio-sensing [4], [5], [6]. Very recently, SPR biosensors have been shown to be likely able to play an important role in numerous important fields including pharmaceutical research, medical diagnostics, environmental monitoring, food safety and security, where fast, portable, low cost and rugged units are needed for early detection and identification of molecules and biological agents in the field. Anyway, detection limits for large sized analytes, such as bacteria and viruses, still need to be improved to meet today's needs [4], [5], [6].

Recently, we have developed a new geometry for low cost sensor system based on the use of SPR in POF [5] with two attractive features enabling it to be a candidate for successful biosensors implementation. It works with a planar gold layer and an external medium refractive index ranging from 1.332 to 1.418 (RIU). The planar gold layer can be employed for immobilization of the typical molecular recognition elements (MRE) used as probes for biosensor design and the refractive index range is just the right one for measurements in aqueous medium. The future development of this system for a quick multi-target detection will require significant advances toward the miniaturization of POF SPR bio-sensing platform, the development of robust bio/molecular recognition elements and the full integration of SPR sensor platform with microfluidic and optoelectronics devices.

In the present work, we show the possibility to develop a POF-based biosensor to detect the presence of the antigen–antibody complex. In particular, as a proof of principle we have developed a POF-biosensor for the detection of antibodies related to celiac disease (CD). CD is an immune-mediated disorder that is caused by the ingestion of wheat gluten and related prolamins present in barley and rye. Around 1% of the general population in developed and developing countries, with an increasing prevalence over time reported in Europe and the United States is affected by this disease. At the moment, only a life-long gluten-free diet (GFD) is mandatory to alleviate the symptoms and to normalize antibodies and intestinal mucosa. Two different types of antibodies have been associated with CD: serum antibodies against gliadin, and auto-antibodies against connective matrix proteins, in particular anti-endomysium and anti-transglutaminase (anti-tTG). The tTG is an enzyme that plays an important role in the pathogenesis of the disease and antibodies against tTG are used as serological markers for the diagnosis of CD. In scientific literature, different works show different analytical methods for the diagnosis and follow up of CD [7], [8]. We will show in the following the potentialities offered by our low cost biosensor, based on SPR in a plastic optical fiber (POF), in the detection of anti-tTG antibodies presence in the serum of patients affected by CD.