Long-Range Surface Plasmon-Polariton Waveguide Biosensors for Human Cardiac Troponin I Detection

Abstract

Straight long-range surface plasmon-polariton (LRSPP) waveguides as biosensors for label-free detection are discussed. The sensors consist of 5-μm-wide 35-nm-thick gold stripes embedded in a low-index optical-grade fluoropolymer (CYTOP^TM) with fluidic channels etched to the Au surface of the stripes. This work demonstrates the application of the LRSPP biosensors for the detection of human cardiac troponin I (cTnI) protein. cTnI is a biological marker for acute myocardial infarction (AMI), often referred to as a heart attack, which can be diagnosed by elevated levels of cTnI in patient blood. Direct and sandwich assays were developed and demonstrated over the concentration range from 1 to 1000 ng/mL, yielding detection limits of 430 pg/mL for the direct assay and 28 pg/mL for the sandwich assay (1 standard deviation), the latter being physiologically relevant to the early detection or onset of AMI. In addition, a novel approach for data analysis is proposed, where the analyte response is normalized to the response of the antibody layer.

Keywords: fluoropolymer; human cardiac troponin I; long-range surface plasmon-polariton; optical biosensor; waveguide.

Figure 1

Schematic representation of the sensor on a metal base with a fluidic lid; the volume of the fluidic cell is 20 µL.

Figure 2

Schematic of the optical setup for biosensing using a straight long-range surface plasmon-polariton (LRSPP) sensor.

Figure 3

Functionalization strategy and sandwich bioassay developed for the detection of cTnI.

Figure 4

Real-time interaction of Protein G and goat polyclonal anti-human cTnI IgG (AT) as revealed by an LRSPP biosensor. The PG/IgG functionalized surface was tested by the periodic injection of AT (200 μg/mL) until the difference between baselines disappeared. The inset shows a different experimental run where the first injection of AT was carried out for 90 min straight.

Figure 5

Example sensorgram of a sandwich bioassay for the detection of cTnI.

Figure 6

Summary plot of the normalized LRSPP biosensor responses over the cTnI concentration range of 1 to 20,000 ng/mL using direct and sandwich bioassays. All the responses were normalized with respect to the surface mass load of the first adlayer of goat anti-human cTnI IgGs (AT1), and multiplied by a factor of 100 for clarity. The error bars are significantly smaller than the response values, therefore are not visible in the plot. The superimposed color gradient and timescale show how the concentration of cTnI in blood increases as an AMI progresses in time [28].