A low-cost portable diagnostic plasmonic biosensor

Surface plasmon resonance (SPR), the term for a collective oscillation of electrons along a metal-dielectric interface, is very sensitive to the refractive index (RI) of the dielectric. When targeted biomolecules, viruses, or cells are adsorbed onto the interface, they modify the RI of the dielectric. This adsorption can be very sensitively monitored in real time without molecular labeling. As a result, SPR biosensors are useful for drug discovery, medical diagnostics, biotherapeutics, life science research, and food contamination detection. They can detect markers for cancer and cardiac conditions and test for antimicrobial susceptibility for rapid treatment.¹These investigations have made SPR biosensors an in-demand point-of-care testing tool for personalized medicine. However, the challenges of optical design and alignment precision have made existing SPR biosensors bulky and expensive.

To construct an SPR biosensor, an optical coupler (prism or optical grating) and oblique incident light beam are required because an evanescent wave is needed to excite SPR. Current designs usually use a rotational-stage configuration, a galvanometer combined with a lens, or a divergent-light-beam configuration with a line CCD. Most of these designs are expensive, and all of them unavoidably exclude phase interrogation, which transduces the SPR signal into the optical phase, and is the most sensitive technique in SPR measurement. The required high-quality optical-grade consumable target-loading glass slide is also expensive. The need to apply refractive-index matching oil between the glass slide and optical coupler for efficient light coupling is cumbersome, and the designs cannot integrate with modern microscopes for additional correlative microscopy verification. All these issues have limited progress in diagnostic development.

To overcome the shortages and efficiently translate laboratory research to clinical diagnostics, we have designed a free-form prism with 1D parabolic surfaces on both sides to target a compact SPR biosensor with matching-oil-free operation. This design is to integrate an optical coupler, a sample-loading plate, and reflectors for tuning the SPR-excitation angle (see Figure 1 for the operating principles). The side surfaces of the prism, shown as the transparent blue blocks in Figure 1, can use either of two parts of a parabola as a reflector for tuning the incident angle of the light beam to the focus, where a thin gold layer is coated for SPR biosensing or nanostructures are coated for localized SPR (LSPR) biosensing. The parabolic curve on the left-hand side of the dashed vertical line (semilatus rectum) requires an external mirror-coating layer for reflecting light to the focus: see Figure 1(a). However, when the parabolic curve on the right-hand side is employed, the total internal reflection (TIR) is used for lossless reflection: see Figure 1(b). The design requires no external mirror coating, which has significant fabrication advantages.

Figure 1. (a) For the parabolic curve (solid red curve) with the distance to the focus shorter than the semilatus rectum (dashed vertical line), incident light (solid green line) has an angle (Φ) smaller than 45°. (b) Φis larger than 45° when the distance from the curve to the focus is larger than the semilatus rectum. Total internal reflection occurs at Φ≥4°when the refractive index of the prism is larger than n=1.414. The prism is made of E48R, which has n=1.525. The transparent blue block is the prism, and the yellow layer is a thin film of gold or nanostructures for biosensing.

As shown in Figure 2, the incident angle to excite SPR on the focus is further controlled by the right-angle mirror, which at different positions directs the light beam from a laser along the different paths (1) and (2) to the parabolic surface, thus to the focus with different incident angles. The free-form prism as well as the right-angle mirror removes several positioning optomechanical components to greatly reduce the device size, to increase system robustness, and to simplify the device assembly. In addition, when nanostructures are used to perform LSPR biosensing, our study demonstrated that LSPR excited by an evanescent wave and detected by phase interrogation shows two orders of magnitude higher sensing resolution than extinction spectra,² which are commonly used in LSPR biosensors to display the shift of wavelength of light scattered by LSPR as the material surrounding nanostructures is changed by molecular binding.

Figure 2. The incident angle of the laser beam to the focus is controlled by the position of a right-angle mirror. The blue dashed line (1) and the dark blue solid line (2) indicate different light paths due to the different positions of the right-angle mirror. The focus (F) is where surface plasmon resonance (SPR) or localized SPR occurs for biosensing. No mirror coating is applied.

Furthermore, we have used a state-of-art injection-molding technique to fabricate the parabolic prism. This inexpensive and disposable plastic prism functions as a sample-loading plate, an optical coupler, and as parabolic TIR mirrors.² Using a disposable prism prevents cross-contamination, and its operation requires no matching oil, which simplifies experiment and diagnosis procedures. Because the light source and the detectors are stationary during incident angle scanning, this system can support four well-known interrogations, namely, angular, intensity, phase, and wavelength interrogation, in a reliable operation without complex optical mechanical components. The biosensor size is as little as 15×15×5cm³, which can easily be loaded onto a commercial microscope for additional correlative microscopy verification.

The phase interrogation using a common-path configuration,³ shown in Figure 3, results in high-sensitivity performance. Because the p-wave and s-wave propagate along a common path, scattering and grating effects from the imperfect prism are much less pronounced. The average sensitivity is around 2.65×10⁻⁷ refractive index unit (RIU). We experimentally verified the dynamic measurement range on RI as 1.33≤n≤1.36 and estimate that the range could be extended to slightly larger than 1.30≤n≤1.40.²

Figure 3. (a) Configuration for the common-path phase interrogation. (b) Relative orientation of the incident p-wave (E_p, polarized perpendicular to the plane of incidence), and s-wave (E_s, polarized parallel to the plane of incidence), principal axes of the waveplate, and transmission axis of the polarizer. , : Principal axes of the quarter waveplate. n_{x, y}: Refractive index along the x- and y-axes, respectively. θ: Rotation angle measured from to the transmission axis of the linear polarizer.

Fewer assembly steps, fewer optical and optomechanical components, and high sensitivity make a low-cost and compact SPR biosensor with extreme sensitivity more accessible. The injection-molding technique also keeps the price of the disposable prism less than one dollar to discourage reuse, and human errors in diagnostic procedure operations are reduced by no longer needing matching oil. Altogether, apart from molecular binding efficiency (an issue common to most, if not all, biosensors), the advantages of our SPR biosensor meet the current challenges⁴ of SPR biosensing. Our sensor has a low development cost, requires very little sample volume, and saves time in medical and clinical diagnostics. Perhaps more importantly, these advantages show that, in addition to laboratory research, medical and clinical diagnostics benefit from SPR biosensing technology.

Our proposed SPR biosensor can operate for about 3.5 hours solely on a popular mobile battery for cellphones. Our next step is to further reduce power consumption by replacing the miniPC with a microchip for data acquisition and autocontrol. We are also developing multichannel microfluidics to enhance the biosensor's capability in medical diagnostics and life science. Finally, we are working to improve the sensitivity to 1×10⁻⁷ RIU.