A portable surface plasmon resonance (SPR) sensor for the detection of immunoglobulin A in plasma.

BACKGROUND: A life-threatening anaphylactic shock can occur if a patient with undiagnosed immunoglobulin A (IgA) deficiency (i.e., IgA levels <500 ng/mL) receives IgA-containing blood, hence the need for a rapid, point-of-care (POC) method for IgA deficiency screening. Enzyme-linked immunosorbent assay (ELISA) is routinely used to detect IgA, but this method requires trained specialists and ≥24 h to obtain a result. We developed a surface plasmon resonance (SPR)-based protocol to identify IgA-deficient patients or donors within 1 h. MATERIALS AND METHODS: The SPR sensor relies on the detection of IgAs captured by primary antibodies adsorbed on the SPR chip and quantified with secondary antibodies. The sensor was calibrated from 0 to 2000 ng/mL in buffer, IgA-depleted human serum, and plasma samples from IgA-deficient individuals. A critical concentration of 500 ng/mL was set for IgA deficiency. The optimized sensor was then tested on eight plasma samples with known IgA status (determined by ELISA), including five with IgA deficiency and three with normal IgA levels. RESULTS: The limit of detection was estimated at 30 ng/mL in buffer and 400 ng/mL in diluted plasma. The results obtained fully agreed with ELISA among the eight plasma samples tested. The protocol distinguished IgA-deficient from normal samples, even for samples with an IgA concentration closer to critical concentration. DISCUSSION: In conclusion, we developed a reliable POC assay for the quantification of IgA in plasma. This test may permit POC testing at blood drives and centralized centers to maintain reserves of IgA-deficient blood and in-hospital testing of blood recipients.

Cross-validation of ELISA and a portable surface plasmon resonance instrument for IgG antibody serology with SARS-CoV-2 positive individuals.

We report on the development of surface plasmon resonance (SPR) sensors and matching ELISAs for the detection of nucleocapsid and spike antibodies specific against the novel coronavirus 2019 (SARS-CoV-2) in human serum, plasma and dried blood spots (DBS). When exposed to SARS-CoV-2 or a vaccine against SARS-CoV-2, the immune system responds by expressing antibodies at levels that can be detected and monitored to identify the fraction of the population potentially immunized against SARS-CoV-2 and support efforts to deploy a vaccine strategically. A SPR sensor coated with a peptide monolayer and functionalized with various sources of SARS-CoV-2 recombinant proteins expressed in different cell lines detected human anti-SARS-CoV-2 IgG antibodies in clinical samples. Nucleocapsid expressed in different cell lines did not significantly change the sensitivity of the assays, whereas the use of a CHO cell line to express spike ectodomain led to excellent performance. This bioassay was performed on a portable SPR instrument capable of measuring 4 biological samples within 30 minutes of sample/sensor contact and the chip could be regenerated at least 9 times. Multi-site validation was then performed with in-house and commercial ELISA, which revealed excellent cross-correlations with Pearson's coefficients exceeding 0.85 in all cases, for measurements in DBS and plasma. This strategy paves the way to point-of-care and rapid testing for antibodies in the context of viral infection and vaccine efficacy monitoring.

Electrochemical enzyme-linked immunosorbent assay featuring proximal reagent generation: detection of human immunodeficiency virus antibodies in clinical samples.

We describe a simple electrochemical immunoassay for human immunodeficiency virus (HIV) antibody detection that localizes capture and detection reagents in close proximity to a microelectrode. Antigenic peptides from HIV-1 gp41 or HIV-2 gp36 were covalently attached to a SU-8 substrate that also presented a template for the deposition of three-dimensional microelectrodes. The detection of HIV antibodies was achieved with an electrochemical immunoassay where an alkaline phosphatase conjugated secondary antibody reacts with p-aminophenyl phosphate (pAPP) to produce a redox-active product, p-aminophenol. The current derived from the oxidation of the reporter group increased linearly over a wide antibody concentration range (0.001-1 µg mL(-1)), with a detection limit of 1 ng mL(-1) (6.7 pM) for both HIV-1 and HIV-2. This level of sensitivity is clinically relevant, and the feasibility of this approach for clinical sample testing was also evaluated with HIV clinical patient samples, with excellent performance observed compared against a commercially available gold standard. This approach was used to develop the first electrochemical enzyme-linked immunosorbent assay (ELISA) to detect HIV in clinical samples, and excellent performance relative to a gold standard test was achieved.

EOT or Kretschmann configuration? Comparative study of the plasmonic modes in gold nanohole arrays.

The debate is still ongoing on the optimal mode of interrogation for surface plasmon resonance (SPR) sensors. Comparative studies previously demonstrated that nanoparticles exhibiting a localized SPR (LSPR) have superior sensitivity to molecular adsorption processes while thin Au film-based propagating SPR is more sensitive to bulk refractive index. In this paper, it is demonstrated that nanohole arrays (1000 nm periodicity, 600 nm diameter and 125 nm depth), which support both LSPR and propagating SPR modes, exhibited superior sensitivity to bulk refractive index and improved detection limits for IgG sensing by using the Kretschmann configuration. The greater sensitivity to IgG detection in the Kretschmann configuration was obtained despite the shorter penetration depth of nanohole arrays excited in the enhanced optical transmission (EOT) configuration. The decay length of the electromagnetic field in EOT mode was estimated to be approximately 140 nm using a layer-by-layer deposition technique of polyelectrolytes (PAH and PSS) and was confirmed with 3D FDTD simulations, which was lengthen by almost a factor of two in the Kretschmann configuration. Spectroscopic data and field depth were correlated with RCWA and FDTD simulations, which were in good agreement with the experimental results. Considering these analytical parameters, it is advantageous to develop sensors based on nanohole arrays in the Kretschmann configuration of SPR.

Angle-dependent resonance of localized and propagating surface plasmons in microhole arrays for enhanced biosensing.

The presence of microhole arrays in thin Au films is suited for the excitation of localized and propagating surface plasmon (SP) modes. Conditions can be established to excite a resonance between the localized and propagating SP modes, which further enhanced the local electromagnetic (EM) field. The co-excitation of localized and propagating SP modes depends on the angle of incidence (θ(exc)) and refractive index of the solution interrogated. As a consequence of the enhanced EM field, enhanced sensitivity and an improved response for binding events by about a factor of 3 to 5 was observed with SPR sensors in the Kretschmann configuration for a set of experimental conditions (λ(SPR), θ(exc), and η). Thus, microhole arrays can improve sensing applications of SPR based on classical prism-based instrumentation and are suited for SP-coupled spectroscopic techniques.

Nanohole arrays in chemical analysis: manufacturing methods and applications.

Since the last decade, nanohole arrays have emerged from an interesting optical phenomenon to the development of applications in photophysical studies, photovoltaics and as a sensing template for chemical and biological analyses. Numerous methodologies have been designed to manufacture nanohole arrays, including the use of focus ion beam milling, soft-imprint lithography, colloidal lithography and, more recently, modified nanosphere lithography (NSL). With NSL or colloidal lithography, the experimental conditions control the density of the nanosphere mask and, thus, the aspect of the nanohole arrays. Low surface coverage of the nanosphere mask produces disordered nanoholes. Ordered nanohole arrays are obtained with a densely packed nanosphere mask in combination with electrochemical deposition of the metal, glancing angle deposition (GLAD) or etching of the nanospheres prior to metal deposition. A review of these methodologies is presented here with an emphasis on the optical properties of nanoholes interesting in analytical chemistry. In particular, applications of these novel plasmonic materials will be demonstrated as substrates for a localized surface plasmon resonance (LSPR), Surface Plasmon Resonance (SPR), surface enhanced Raman spectroscopy (SERS), and in electrochemistry with nano-patterned electrodes.

Propagating surface plasmon resonance on microhole arrays.

Metallic thin films patterned with micrometer size triangle or hole arrays present plasmonic properties when excited in the Kretschmann configuration, that are improved in comparison to conventional thin film surface plasmon resonance (SPR). These optical properties can be tuned by varying the physical aspects of the microplasmonic structures. Triangles and microhole arrays were prepared with modified nanosphere lithography (NSL) using latex spheres of 0.65, 0.82, 1.0, 1.5, or 3.2 microm in diameter. This allowed the preparation of triangles with edge lengths between 275 to 2000 nm and microhole arrays of various periodicities, diameters, and hole depths. These microstructures were studied to understand the relationship between the physical aspects and the optical properties, such as the sensitivity, working refractive index range, spectral width of the plasmonic peaks, spectral noise, and refractive index resolution. Microhole arrays with a hole diameter equal to half the periodicity were found to combine the advantages of both localized surface plasmon resonance (LSPR) on nanoparticles and SPR on a thin film. These microhole arrays exhibited high sensitivity to refractive index (>3000 nm/RIU), sensitivity to monolayer formation (2-fold improvement compared to thin films), and excellent refractive index resolution (10(-6) RIU). Finally, a biosensor for the detection of 10 nM of immunoglobulin G (IgG) exhibited a greater response with microplasmonic materials compared to conventional thin Au films. Hence, these novel plasmonic materials exhibit a strong potential as an SPR sensing platform. They can be implemented on existing instrumentation and use detection protocols developed for current SPR sensors.

High-resolution surface plasmon resonance sensors based on a dove prism.

Wavelength interrogation surface plasmon resonance (SPR) spectroscopy using a dove prism combines a simple and inexpensive optical design with high-resolution refractive index monitoring and biosensing. A BK7 dove prism inverts an optical image with a total internal reflection angle of 72.8 degrees , an angle active in SPR. Hence, a unique system can accomplish SPR biosensing using wavelength interrogation and also perform SPR imaging. This optical configuration advantageously uses a single axis optical path between each optical component, simplifying the optical design of SPR instruments without compromise of the analytical performance. Fluidics were also incorporated to the instrument design for efficient sample delivery. The SPR instrument is characterized in terms of refractive index (RI) sensitivity, RI resolution, reproducibility, and application for monitoring low concentration biological events. Data analysis methodologies are compared for improved resolution of the measured response. Raw data analyzed using a minimum hunting procedure results in RI resolution in the 10(-6) range, while pre-treating data with singular value decomposition improves the resolution by one order of magnitude. Depending on the spectrophotometer employed, the RI range accessible can be easily tuned; examples with a 550-850 nm and a 550-1100 nm spectrophotometers are shown and results respectively in RI ranges of 1.32-1.39 RIU and 1.32-1.42 RIU. Monitoring of microM concentration of beta-lactamase is performed using the wavelength interrogation configuration of the biosensor. Finally, a SPR image of a surface with a water droplet (volume=500 nL) was obtained using the dove prism SPR with a band pass filter and a CCD camera. SPR using a dove prism configuration combines advantages of portable SPR instruments, SPR imagers and research-grade SPR instruments in a unique platform.